Symposium EN16 : Advanced Materials, Fabrication Routes and Devices for Environmental Monitoring

Ultra-High-Sensitive gas detection in ppt level have been proposed by using pulse-heating of MEMS attached with Pd-loaded SnO2. My group reported three important factors, receptor, transducer functions and utility factor, for gas sensor material designs and their integration in 2003 and 2006, respectively. In 2014, the gas sensor using Pd-loaded SnO2 clusters based on the idea of the above integration could successfully detect toluene in ppb level. To enhance the sensor response more, we investigated the combination of utility factor and pulse-heating of MEMS. The MEMS-type gas sensors are repeatedly heated and allowed to cool by the application of voltage to the microheater; the target gas can penetrate into the interior of the sensing layer (Pd-loaded SnO2 clusters) during its unheated state. In 2018, we reported that such sensor responded to toluene in 0.1 ppb. In addition, the sensor response was found to increase by considering the oxygen adsorption state in the preheating and waiting-time before pulse-heating for measurement. The response of MEMS-type gas sensors showed a linearity to toluene concentration. It is found that the sensor response depends on the waiting-time between pre-heating and measure-heating. For example, in relationship of sensor response to gas concentration, the short waiting-time gave a steep slope, but the long waiting-time gave a gentle slope with lower detection limit. In the presentation, I will show the details as such Ultra-High Sensitive gas sensor.

New developments in consumer electronics, especially in the rapidly evolving field of wearable electronics require the use of different types of sensor devices. However, the use of gas sensors within this paradigm remains incipient, yet shows potential for developing wide spectrum of applications with enormous associated markets. Gas sensitive nanomaterials such as single crystalline (pure or metal loaded) metal oxide nanowires (NWs) will find commercial application provided they are produced employing scalable techniques that enable the mass production of high quality materials at affordable costs. Solution processing methods and additive fabrication techniques are seen as interesting enabling technologies for coupling nanomaterials to their application substrates. Here we show how metal oxide NWs can be integrated in flexible polymeric substrates for achieving chemoresitors employing either direct growth or transfer techniques. In addition, we discuss the results of careful studies on the reliability of the resulting flexible sensors having undergone stringent, automated bending tests.
Results:
The transducers for chemoresistive sensors were produced via the inkjet printing of silver or gold electrodes and heating elements on a Kapton substrate. The electrode area was then coated with mats of pure or Pd-loaded tungsten oxide NWs. Two approaches were used for integrating the nanomaterials on the transducers: 1) The direct growth of the nanomaterial via a single step aerosol assisted chemical vapor deposition (AACVD) process, which was run at 380^oC, for ten minutes. 2)The ex-situ growth of metal oxide nanowires via AACVD, ink preparation and screen-printing onto the flexible transducer. For the bending tests, an AGS-X 10 kN, Shimadzu universal testing machine was used. A compression force was exerted and the sample buckled achieving a maximum deflection. The loading speed of the crosshead displacement was high to reproduce the real situation of a sudden bending (i.e. buckling). Moreover, the curvature radii achieved (<4 mm) represent very harsh conditions, well beyond what can be expected to happen to a wearable during its normal operational fife. When directly grown onto flexible substrates, NW coatings showed better adhesion than their screen-printed counterparts. However, the direct growth of Pd-loaded NWs onto silver electrodes suffered from delamination failures because Pd reacts with Ag during the AACVD process. This was solved by inkjet printing Au, instead of Ag tracks. For tungsten oxide NW sensors, the permanent increase in electrical resistance of the active layer after bending was small (about 3%). The sensitivity to hydrogen remained unchanged. The differences observed before and after the bending process, resulted mainly from cracks that developed both in the gas sensitive layer and tracks. Such cracks generate permanent changes in the electrical resistance of the active layer, and therefore, modify the baseline and sensor response. Also, during the bending tests, NWs may change orientation and their NW to NW distance, which possibly modifies the binding energy and charge transfer between the gas sensing layer and the target gas molecules, producing slight variations (positive or negative) in sensor response. However, our results show that it is possible to produce reliable, flexible chemoresistors employing scalable, additive fabrication. After 50 cycles under tensile strain, the response of the sensors remains almost unchanged, which means that the sensing properties of the material are preserved despite the bending. This demonstrates that WO3 chemoresistive sensors on flexible substrates can withstand different harsh mechanical conditions and afterwards still be usable. In particular, inkjet-printed gold electrodes demonstrated to withstand the AACVD growth process and the bending tests, which allowed us to obtain truly flexible sensors that remained functional after over 200 cycles of harsh bending.

Sensing with tenuous volatile organic compounds (VOC) gas which contain in human exhalation is important issue for next generation healthcare. In recent years, gas sensors are much attention research topics to use synthesis nano-materials and fabricate conventional device product process. In the previous study, MoO₃ nanorod arrays growth on the silica substrate with fixed diameters of 10 nm but controllable length of 20 - 600 nm has been successfully synthesized by a simple solution metal organic decomposition (MOD) method.
Here, we are demonstrating fabrication of some oxide nanostructure gas sensor device for VOC gas with a very simple printed coating process. The sensor devices are evaluated gas sensing properties such as sensitivity, responsivity through morphology of the oxide nanorod arrays with each EtOH, MeOH, IPA and ACE volatile gas in this study.
The gas sensors show good response performance with quick response and recovery time, as well as various selectivity of each VOC gas at 573 K. The relationship between adsorption/desorption abilities and morphology of the nanorod arrays will be discussed in detail on the symposium.

Following a research trend for mobile sensors being miniaturized and consuming negligible power, a chemo-resistor consisting of semiconductor and metal catalyst has become a promising platform as a gas sensor. However, a requirement of the continuous power supply into the device limits the miniaturization of the entire sensing system owing to the demand of energy storage system bulkier than the main body of the sensor. Moreover, gradual deterioration of the sensor arisen from aggregation or surface contamination of the metal catalyst, and humidity-interference on the sensing performance remain as major barriers. Herein, we fabricated a self-powered H₂ sensor based on Pd-decorated n-indium gallium zinc oxide (Pd-IGZO)/p-silicon (Si) photodiode operated under white light. To induce significant photocurrent change before and after an H₂ exposure, a novel diode architecture consisting of finger-type electrodes interposed between the Pd-IGZO film and Si substrate were contrived. The device surface was covered by zeolitic imidazolate framework-8 (ZIF-8) in which micropores not only enabled amplifying the photocurrent change by accumulating gas (H₂ or O₂) molecules at the vicinity of the Pd surface but also prevent instability issues of the Pd nanoparticles mentioned above. The sensor exhibited remarkable sensing response of 1.57×10⁴ % at 1 % H₂ under white light at an irradiance of 5 mW cm^-2 without external bias, a limit of detection of 35 ppm, and outstanding durability indicated as negligible change after 3 months. Besides, a self-powered humidity sensor with identical architecture to the H₂ sensor except the Pd was separately fabricated and exploited to correct the humidity-interference of the H₂ sensor, resulting in marking an accurate H₂ concentration even at ever-changing humidity. In conclusion, the dual sensing platform paves the way for resolving the main drawbacks of the traditional chemo-resistor.

Nitrogen oxides (NOX) is considered as toxic molecule giving harmful influences not only on the human body but also on the environment [1]. A highly selective and sensitive NOX gas sensor operating at room temperature (25 °C) based on flower-like ZnO (FZO) microstructures were successfully fabricated by a CTAB-assisted hydrothermal process at 90 °C. We report the hydrothermal synthesis of ZnO microstructures with a gradual increase of CTAB concentration forming nanorods (0M), nanorods assembled structure (0.001M) to flower-like (0.005M) and studied morphology dependent gas sensing behavior to NOx gas at low temperature (25 °C). The characterizations of the as-prepared samples were done in detail by using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, photoluminescence (PL) spectroscopy and Brunauer-Emmett-Teller (BET). The gas sensing performance of sensors fabricated with as-prepared ZnO structures to various concentrations of NOX, ammonia, toluene, carbon monoxide, acetone, and ethanol, at various operation temperatures (25 to 150°C) was noted. A clear trend showing the effects of the morphology on the sensing behavior with temperature was demonstrated. The flower-like ZnO exhibited stability (up to 140 days) and excellent sensitivity with a high gas response of 29 which drops sharply with the increase of temperature and is highly selective towards 0.74 ppm of NOX at 25 °C without additional use of UV irradiation or doping materials such as Pt or Pd. The ZnO sample with 0.001M of CTAB shows a similar trend but with much lower sensitivities at low temperatures. For the sample without CTAB, the temperature behavior switches to a volcano-type one, in which the sensitivity is low at a low temperature and increases gradually with the temperature increasing. Thus, we proposed a possible reason for the evolution of overall characteristics by suggesting changing reactions between ZnO and reacting gases in various operating temperatures.

Gallium oxide (β-Ga₂O₃) is a wide band gap semiconductor material used in the high power, high temperature and gas sensing fields. It is widely reported that Ga₂O₃ in form of thin films is sensitive towards oxygen and reducing gases at temperatures above 600 °C. To decrease the working temperature and, consequently the power consumption, different modifications for its use as gas sensor have been introduced. The innovations proposed to overcome the material limitations concern surface functionalization, material doping or the increase of the surface-to-volume ratio using the nanowire (NW) morphology. In this work, single β-Ga₂O₃ NWs grown via vapor-liquid-solid (VLS) mechanism are studied as gas sensors.
β-Ga₂O₃ nanowires are fabricated via a metal-assisted VLS mechanism using carbothermal reduction and the synthetized NWs, after a structurally and optically characterization, are individually contacted using Focused Electron Beam Induced Deposition techniques for their use as gas sensors. The fabricated devices are tested towards different concentrations of relevant gases in air quality monitoring, from ambient temperature and up to 200 °C, resulting in fast and reproducible responses towards water vapor at room temperature (25 °C), giving rise to a very low power consumption, in the range of nW.
Different features of the response towards moisture were studied, revealing similarities with carbon-based materials. Furthermore, the summarized response of the sensor, resembles an isotherm type V, typically from charcoal. As-grown β-Ga₂O₃ NWs were studied using Electron Energy Loss Spectroscopy, confirming the presence of a carbon layer at the surface of the nanowires forming a core-shell structure. The carbon layer, result of the growth process, is crucial to understand the sensing behaviour of the fabricated devices and should be considered in other gas sensing studies.

Human breath contains numerous volatile organic compounds (VOCs). Several studies have demonstrated a strong correlation between exhaled breath components and specific diseases. For instance, the acetone concentration in exhaled breath of diabetes is much higher than that of healthy people (~0.39 to 1.09 ppm). So accurate detection of specific VOCs in exhaled breath can thus provide essential information for the diagnosis of particular diseases. To develop highly sensitive gas sensors for detecting low concentrations of specific VOCs, a series of three-dimensionally assembled hierarchical oxides nanostructures with a high surface-to-volume ratio and porosity will be specially designed and synthesized as the base sensing materials. For further improving the selectivity while detecting specific VOCs, the synthesized three-dimensionally assembled hierarchical oxides nanostructures will be subsequently decorated with polymers with desired mass fractions and chemical procedures to form a novel organic-inorganic heterogeneous nanocomposite. The sensing performance, as well as the sensitivity of the fabricated organic-inorganic nanocomposite sensors, will be systematically characterized by a variety of VOCs for investigating the potential of diagnosis of diseases with the exhaled breath.

Up to date, we have successfully synthesized a series of porous three-dimensional metal oxides nanostructures having a high specific surface area by optimizing a variety of synthesis parameters. The underlying physical and chemical properties, as well as the critical parameters of spin and drop coating, are still under investigation for the subsequent decoration procedures. Also, we have designated and are modifying a unique gas sensing system which could safely, rapidly, and precisely create an atmosphere with single or multiple targeting gases for sensing characterizations. It was found that the prepared three-dimensionally assembled hierarchical oxides nanostructures, organic-inorganic heterogeneous nanocomposites exhibit similar tendency in selectivity for these target gases, and the addition of organic polymers improved the sensitivity of the three-dimensionally assembled hierarchical oxides nanostructures as our expectation.

Zinc peroxide (ZnO₂) is a semiconductor material with a wide band gap in the vicinity of 4.5 eV. Due to its oxidative capacity, this compound has been explored in various applications from medicine to photocatalysis. Its main application, however, is to be used as a precursor of zinc oxide (ZnO), a promising material for the semiconductor industry. Although several authors have reported the synthesis of ZnO₂ nanoparticles, little effort has been given to the deposition and growth of ZnO₂ thin films. Furthermore, several authors report that ZnO₂ nanoparticles decompose in ZnO and O₂ in temperatures higher than 230°C. Within this context, we present XRD, XPS, and SEM characterization of ZnO₂ thin films deposited by RF-magnetron sputtering with a reactive atmosphere of oxygen. Our results suggest that ZnO₂ thin films do not exhibit long-range order and are stable at temperatures as high as 500°C. The gas sensing properties of this material were also investigated by monitoring the electrical resistance of these films during exposure to ozone. Preliminary results show that when operating at 300°C the response (R_O3/R_air) is higher than 500 for 40 ppb of O₃ with response time in the vicinity of 50 s.

Many oxides of the perovskite structure not only exhibit fascinating properties originating from the bulk defect chemistry, but also show intricate behaviour at the solid/gas or solid/liquid interface leading to applications such as resistive gas sensing.¹
One particular example is donor doped SrTiO₃ (STO), which shows gas sensing behaviour similar to Taguchi type materials², albeit at higher temperatures. This is usually explained by the formation of a space charge layer, however the underlying mechanisms and defect chemistry are yet to be fully understood: The space charge layer may either be formed by charged adsorbate molecules on the surface, or by defect chemical rearrangements at the surface in the form of Sr vacancies as observed in the bulk defect chemical model.^3,4
As the space charge region extends only a few nanometres form the surface into the bulk, this state is virtually impossible to quench in order to access it with conventional ultra-high vacuum spectroscopic techniques. With the advent of near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), this shortcoming is alleviated, allowing us to access the surface electronic and ionic structure of any material under conditions close to technologically relevant ones, often times requiring high temperatures and gas pressures far from UHV.
Lab- and Synchrotron-based NAP-XPS conducted on n-SrTiO₃ thin films under applied thermodynamic bias (T, pO2) reveals the presence of a variable surface potential and a corresponding negative surface charge.⁶ pO₂-dependent surface potential profiles were mapped by spectroscopic depth profiling, revealing an electron depletion layer present at the surface of n-SrTiO₃ thin films in both nominally reducing (ultra-high vacuum) and oxidizing conditions. Sr Core level XPS indicated Sr vacancy formation in the perovskite by SrO precipitation as the governing process, resulting in a resistance change at the surface. Depth resolved spectra furthermore revealed that a reversible conversion of the charge-neutral strontium oxide surface termination into strontium oxide clusters upon oxidation takes places, rendering the mechanism of resistive gas sensing in STO more complex as nanoscopic morphology changes also need to be considered.⁷

References:
¹Meyer, R, Waser, R, Resistive donor-doped SrTiO₃ sensors: I, Basic Model for a Fast Sensor Response. Sensors and Actuators B: Chemical. 101 (2004) 335-345.
²Seiyama, T., Kato, A., Fujiishi, K., Nagatani, M., A New Detector for Gaseous Components Using Semiconductive Thin Films. Anal. Chem. 34 (1062) 1502-1503.
³Ohtomo A., Hwang, H. Y. Surface Depletion in Doped SrTiO₃ Thin Films. Appl. Phys. Let. 84 (2004) 1716-1718
⁴Marchewka, A. et al. Determination of the electrostatic Potential Distribution in Pt/Fe:SrTiO₃/Nb:SrTiO₃ Thin Film Structures by Electron Holography. Sci. Rep. 4 (2014) 6975
⁵Meyer, R., Zurhelle, A., De Souza, R. A., Waser, R., Gunkel, F. Phys. Rev. B. 94 (2016) 115408.
⁶Andrä, M. et. al. Oxygen partial pressure dependence of surface space charge formation in donor-doped SrTiO₃. APL Materials 5 (2017) 056106.
⁷Andrä, M. et. al. Chemical control of the electrical surface properties in donor-doped transition metal oxides. Phys. Rev. Mat. 3 (2019) 044604.

Copper oxide Cu₂O is an important and well known p-type transition metal oxide semiconductor material which has the advantages of direct band gap 2.1 eV at 300 K, a high absorption coefficient in the visible spectral range. This material has already been employed in the fabrication of electronic devices, thanks to its low cost, non-toxicity and fairly good carrier mobility. For example Cu₂O has been used in thin photovoltaic devices, resistive switching, transistors, gas sensors or catalysts.
In our work the either epitaxial Cu₂O (110), Cu₂O(100) or nanocrystalline Cu2O films has been fabricated by Pulsed Laser Deposition on MgO(100), MgO(110) and fused silica substrates, respectively. The oxygen pressure in the chamber was varied between 10^-5 Pa and 1 Pa, while the substrate temperature was held between 200 and 750 °C. The crystalline quality and out-of-plane orientation of the films were characterised by means of X-ray diffractometers. The epitaxial single crystalline films were fabricated on MgO substrate at substrate temperature above 500 °C, contrary on amorphous substrates (fused silica, alumina) the films were polycrystalline. The surface morphology was characterised by Atomic Force Microscopy. The role of the deposition conditions on crystalline structures and following to structural properties were examined in detail by XPS, SEM and TEM. The attention was focused also on determination of defect especially cooper and oxygen vacancies by means of Electron Paramagnetic Resonance and Positron Annihilation Spectroscopy. Because we focused on utilization of the Cu2O films as gas sensors, the powerful technique near ambient pressure photoelectron spectroscopy (NAP-XPS) was carried out to investigation of surfaces composition in the presence of gasses and vapours such as ethanol, hydrogen, NO₂ and acetaldehyde. The idea was to observe the reaction of species on the surface of Cu₂O while exposed to atmosphere as a function of temperature as well as in-situ- resistivity measurements. The following chemical responses of the films on the gasses mentioned above were measured at the same condition as NA-XPS.

Methanol is poisonous when ingested or inhaled, resulting in devastating consequences including organ failure, blindness and death.¹ This is a potential hazard as methanol is commonly used as solvent and feedstock in laboratories and chemical plant, but increasingly also as alternative fuel.² Also, alcoholic beverages adulterated with methanol are a threat, commonly leading to poisoning outbreaks in developing countries with hundreds of victims. While chemical sensors, such as nanostructured metal-oxides, are portable, inexpensive and can detect alcohols down to ppb concentrations,³ they cannot differentiate methanol from ethanol and other interferants.⁴ To improve selectivity between ethanol and methanol, sensor can be combined with filters.⁵
Here, we present a detector capable to selectively detect methanol in the presence of high ethanol background. It consists of a filter placed upstream of a highly sensitive, but non-specific Pd-doped SnO₂ microsensor. The filter is a small packed bed (150 mg) of an adsorbent that separates analytes based on their chemical interaction. After passing the filter, analytes are detected by the sensor consisting of a highly porous network of Pd-doped SnO₂ nanoparticles fabricated by flame spray pyrolysis (FSP).⁶ The resulting methanol detector can selectively detect methanol without interference of simultaneous ethanol and other interferants. In specific, methanol is detected within 2 min in a large concentration range from 1–1,000 ppm at ambient 50% RH. The detection of methanol is thereby independent of the interfering ethanol concentration up to 95% relative saturation (~60,000 ppm). As a proof-of-concept, the detector was tested on methanol-spiked breath samples and rum where it could reliably detect and quantify toxic methanol concentrations. It shows thus great promise for detection of toxic methanol concentrations in ambient air or in the headspace of beverages, as well as for fast screening of methanol poisoning from breath.

1. J. A. Kraut, I. Kurtz, Clinical Journal of the American Society of Nephrology, 2008, 3, 208-225.
2. A. K. Agarwal, Progress in Energy and Combustion Science, 2007, 33, 233-271.
3. N. J. Pineau, J. F. Kompalla, A. T. Güntner, S. E. Pratsinis, Microchim Acta, 2018, 185, 563.
4. A. T. Güntner, S. Abegg, K. Königstein, P. A. Gerber, A. Schmidt-Trucksäss, S. E. Pratsinis, ACS Sens, 2019, 4, 268-280.
5. J. van den Broek, A. T. Güntner, S. E. Pratsinis, ACS Sens, 2018, 3, 677-683.
6. A. T. Güntner, V. Koren, K. Chikkadi, M. Righettoni, S. E. Pratsinis, ACS Sens, 2016, 1, 528-535.

Advanced materials have a huge potential of providing sensing data on air quality environment and individual health. The need for low-cost sensor used continuously to monitor air quality around people has sharply growing up with human activities and world industrialization. Nowadays, environment and health concerns drive the sensor market. Chromatography, optical, and spectroscopy approaches are accurate methods to obtain sensing data however they are generally expensive and non embedded equipment. Metal OXide (MOX) materials offer an alternative way to develop miniaturized resistive gas sensors with repeatable, reliable and reproducible sensing results. Since the fifties, it is well-known that the material conductivity changes with the adsorption or the desorption of a gas on a metal oxide surface. It has been proved first with Zinc Oxide (ZnO) thin layer in 1962 [1]. Then, tin oxide (SnO₂) and ZnO have been demonstrated to be good gas-sensing materials for several gases such as O₃, NO₂, NH₃, CO, and ethanol.
This talk will present the work being carried out in Microsensors and Instrumentation group in the Institute of Materials Microelectronic Nanoscience of Provence (IM2NP) at Aix-Marseille University (France). The design and the conception of the transducer is one of an important aspect. In 2013, a new design patterned in our group allows us to integrate heaters and reach low powered operation as low as 25mW [2]. Furthermore, the measurement metrology needs to be taken account. Recently, studies on the chamber design based on microfluidic simulations lead to improve our measurement reliability [3]. Concerning the sensing layer, MOX material synthesis is one of the key challenge to control the morphologies and the sensing performances. Hydrothermal and microwave hydrothermal synthesizes have been used to prepare metal oxide nanoparticles and will be presented. The deposition method plays also a major part in the sensing layer properties. Technics from solid target like Radio Frequency sputtering and from solution like spin coating or ultrasonic spray will be discuss. Repeatability, fast response and recovery times have been demonstrated. Thanks to a high surface/volume ratio, the nanostructuration for MOX like SnO₂ and ZnO improves the sensing properties at temperatures around 200 or 300°C depending on the target gases. Due to the optical properties of these materials, our studies demonstrate that these temperatures are reduced to room temperature by considering sensing activity under continuous light activation [4]. However, selectivity is still challenging with MOX materials. The combination of several sensors based on different nanomaterials is one possible way to overcome this lake of selectivity. By changing the heterojunction composition and the light wavelength, the gas sensing properties have been enhanced and a selectivity have been reached. Access to toxic gaseous detection by such resistive micro-sensors and multi-sensors is promising for applications in individual environmental monitoring devices.

References
[1] T. Seiyama et al., Anal. Chem., 34, pp.1502-150, 1962
[2] K. Aguir et al., patent N° FR 13 59494, 2013, international extension in 2016
[3] F-E. Annanouch et al., Sensors and Actuators B: Chemical, 290, pp. 598-606, 2019
[4] L. F. da Silva et al., Sensors and Actuators B: Chemical 240, pp. 573-579, 2017

The high surface-to-volume ratio as well as the typically high crystallinity and directional charge carrier transport of metal oxide NWs are beneficial for the use in chemo-resistive gas sensors. Semiconducting n-type SnO₂ nanowire networks have been site-selective deposited on gas sensor platforms by chemical vapor deposition approach based on a catalyst-mediated vapor-liquid-solid (VLS) growth mechanism. Compared to classical screen-printed SnO₂ sensors showed the direct integrated nanowire network superior sensing performance in terms of sensitivity and response time. The selective detection of target gas molecules in complex gaseous composition or high humidity is quite challenging. We improved the lack of selectivity toward gaseous species through surface decoration with metal oxides (VO₂) and noble metals (Rh) nanoparticles by a second CVD step. For this approach, we developed tailored volatile molecular precursors based on bidentate heteroarylalkenolate and enaminonate ligands. An enhanced gas detection capability under high humidity was achieved by deploying hydrophilic zeolite microcrystals embedded in nanofiber meshes on top of the sensing material.
Moreover, we fabricate ternary metal oxide (BiFeO₃, LaFeO₃ and Sn₂Nd₂O₇) nanofibers meshes by electrospinning. which exhibit excellent selective and sensitive sensing characteristics. High-resolution transmission electron microscopy in conjugation with 3D tomographic analysis confirmed an interwoven network of hollow and porous (surface) LaFeO₃ nanofibers. Owing to their high surface area and p-type behavior, the nanofiber meshes showed high chemoselectivity toward reducing toxic gases (SO₂, H₂S) that could be reproducibly detected at very low concentrations (< 1 ppm) well below the threshold values for occupational safety and health. In contrast, pyrochlore Sn₂Nd₂O₇ nanofibers showed a highly selective detection towards Hydrogen (H₂). The interdependence between structure, porosity and surface chemistry were evaluated in detail to achieve a high-detection performance.

Metal Oxide Semiconductors (MOS) have been the subject of decades of investigation directed towards integrating them in multi-sensorial electronic-nose systems¹. Despite tremendous research efforts, a key limitation persisting to date is the lack of diversity in response to analytes, especially among the materials that reached the stage of commercialization. By and large represented by n-type MOS, predominantly doped SnO₂, the materials comprising this limited set show high output correlation upon exposure to many gaseous species.
Interestingly, it has been shown^2,3 that the formation of a heterojunction between n- and p-type MOS can be leveraged to enhance and tune sensing performance. Thanks to the differentiation in surface interactions and charge transport mechanism, such materials combination gives access to a different range of gas selectivities and can mitigate the deleterious effect of humidity on sensitivity, as well as improve the sensor recovery time. It remains to be seen, however, whether highly-reproducible and cost-effective manufacturing processes can be developed to realize the untapped potential of this new class of sensing materials.
Here, we report on the fabrication and characterization of SnO₂/NiO n-p heterojunctions thin films and analyze their response to Volatile Organic Compounds (VOCs) at sub-ppm concentrations. Films of varying thickness (20 – 200 nm) are deposited by conventional magnetron sputtering and subject to different annealing protocols. We show that in optimal processing conditions, the response of the films to VOCs can be greatly increased and its dependence on temperature, typically described in the context of a Diffusion-Reaction model, altered. In addition, we show that p-type NiO layers of given thickness can trigger a reversal in the response pattern of ultra-thin n-type SnO₂ underlayers. The chemophysical and transport mechanisms leading to the emergence of such behavior are discussed.
Our results pave the way for the practical realization of n-p heterojunction MOS sensors of orthogonal response to VOCs compared to their n-type counterpart, thus expanding the range of functionalities of electronic noses.


1) Persaud K. and Dodd G., “Analysis of discrimination mechanisms in the mammalian olfactory system using a model nose” Nature 299 (1982) 352
2) Kim H.-J. and Lee J.-H., “Highly sensitive and selective gas sensors using p-type oxide semiconductors: Overview” Sens. Act. B 192 (2014) 607– 627
3) Moseley P., “Progress in the development of semiconducting metal oxide gas sensors: a review” Meas. Sci. Technol. 28 (2017) 082001

Tin dioxide (SnO₂) enables us to detect various low concentration gases for the application of environment monitoring and healthcare. Therefore, a great number of research and development has been carried out. However, relationships between surface responsivity and the other properties such as device structure and fine structure have not been completely clarified. We conducted basic researches on surface responsivity in order to understand SnO₂ thin film more deeply, as well as in order to achieve the higher sensitivity. In this study, SnO₂ thin film-based gas sensing devices were fabricated on SiO₂/Si substrate. Length and width of SnO₂ film for gas sensing were 500 μm and 100 μm, respectively. Pt catalyst was placed on SnO₂ film to enhance the gas sensitivity. It was confirmed by Raman spectroscopy that the structure of the sputtered thin film was SnO₂ although it contained SnO and SnO_2-δ slightly. As a basic property, sensitivities for acetone gas (concentration: approx. 200 ppm - 800 ppm) and ethanol gas (concentration: approx. 10 ppm - 40 ppm) were increased with a decrease in thickness of SnO₂ film. Thickness of SnO₂ thin film on device were 120 nm, 200 nm and 340 nm. This indicates that the proportion occupied by the gas sensitive region increases with decrease of thickness. Then, we carried out researches on the gas sensitive region. Considering the mechanism of gas sensing, the presence or absence of oxygen in an atmosphere around the device is important to discuss the gas response on the surface. Therefore, the relationships between the electrical property, thickness, operating temperature and atmosphere were evaluated systematically using the device with thickness of 100 nm and 700 nm under the atmosphere of pure air and N₂. As a distinctive result, we found that the both devices had almost the same resistance under the N₂ atmosphere, even though the thickness was different. Resistance at the operating temperature of 300 deg. C was approximately 40 kΩ. It seems that this was caused by the removal of the influence of oxygen that SnO₂ surface was responding to. This is an interesting property for elucidating the depth of the sensitive region because it may imply that the sensitive region can be made to appear under N₂ atmosphere. There is also a possibility that surface fine structure is different from the inner structure and this difference may be a key for the higher sensitivity. Although the fine structure and surface condition need to be clarified, fabrication of the higher sensitive devices can be expected if the sensitive region can be structured effectively. The relationships between surface responsivity, material fine structure, and device structure will be reported in detail.

Chemical sensing is an active area of research with wide implications in environmental monitoring including air quality in urban areas, inside air-conditioned buildings, near natural gas pipelines and confined underground areas. Advancing the limits of detection as well as designing new mechanisms for sensor response to external stimuli is therefore of interest. In this presentation, we will report on a class of rare-earth nickelate systems as gas sensors, with specific results on hydrogen and ozone monitoring. Typical oxide semiconductor sensors such as In₂O₃ operate based on surface or near-surface redox processes that change their electrical resistance depending on the nature of chemical species encountered. In quantum materials such as nickelates, one can exploit instead strong electron correlations from electrode-mediated electron transfer from the chemical gas to induce change in electrical resistance or optical transmittance. The change in resistance is non-linear with electron filling due to Coulomb repulsion within the oxide lattice opening the possibility for trace gas detection. We will compare and contrast single crystal perovskite nickelates such as NdNiO₃ films grown on LaAlO₃ substrate versus their amorphous counterparts (deposited at room temperature on glass and polymers) with respect to both dynamics of resistance modification under dilute hydrogen or ozone exposure and the self-limited sensor response. Our initial results show > 800% change in electrical resistance in the amorphous mixed Ni oxides to hydrogen in forming gas near room temperature, while the crystalline counterparts show few orders of magnitude change in conductivity. Both materials show reversible modulation of resistance once the sensor is exposed to air and can be cycled multiple times. Finally, we will contrast the nickelates with better-studied wide gap oxide semiconductor sensors to discuss future materials challenges in this field.

Hydrogen gas has attracted significant attention as a promising next-generation clean energy source[1]. For sustainable hydrogen energy generation systems, safe, repeatable, and cost-effective hydrogen gas-leakage monitoring is urgently needed. In this study, transparent and flexible hydrogen gas sensor membranes that satisfy the above requirements were prepared utilizing two techniques; ultraviolet light-induced graft polymerization (UV grafting) of hydroxyethyl acrylamide (HEAAm) and platinum-catalyst-loaded tungsten trioxide (Pt/WO₃) deposition using a novel sol-gel method.

Pt/WO₃ thin film coated materials render promising hydrogen gas sensing[2]. Pt/WO₃ particles show clear colorimetric changes from yellow to blue in response to hydrogen gas without the need for heating[3]. This color change is reversible and repeatable, enabling the detection of colorless hydrogen gas with the naked eye. In addition, due to the complex shape of pipes and joints for hydrogen gas transportation, flexible and easy-to-attach sensors are desirable. The general coating methods of Pt/WO₃ thin film on substrates require heat treatments above 350°C for WO₃ crystallization on the solid substrates; however, such heat treatments prevent soft organic materials from being used as substrates. Still, the use of soft organic materials is preferable owing to their superior flexibility, cost effectiveness, and lightweight.

To achieve Pt/WO₃ thin film coating on soft organic materials, a novel Pt/WO₃-thin film coating protocol was developed. The present protocol using only a sol–gel method involves acid and mild heat treatments; thus, soft organic materials with acid- and mild heat-resistant-hydrophilic surfaces can be employed as substrates. In this process, the acid acts as the catalyst to enhance the sol–gel reaction. Polydimethylsiloxane (PDMS) membranes were used as the substrate for Pt/WO₃-thin film coating, because PDMS exhibits high gas permeability, high optical transparency, and cost effective in addition to the typical properties of soft organic materials. These properties are ideal for hydrogen gas-leakage monitoring. However, because the surface of bare PDMS is hydrophobic, the sol–gel method could not be applied directly. Thus, to obtain an acid- and mild heat-resistant hydrophilic surface, UV grafting of HEAAm was performed. We selected UV grafting and HEAAm because, unlike other modification techniques the technique can easily provide stable hydrophilic surfaces.

The UV light irradiation time, the reaction temperature, and monomer concentration for UV grafting of HEAAm were optimized. Stability of the obtained hydrophilic surface was confirmed by water contact angle. The presence of a Pt/WO₃ film on the HEAAm-modified PDMS surface was confirmed by X-ray diffraction analysis. The stability of the prepared WO₃ H₂O structure was demonstrated because the membrane retained 80% color response even after the heat treatment at 250°C for 30 min. The thickness of the thin film was ca. 100 nm, as determined from the cross-sectional scanning electron microscopy image. The fabricated Pt/WO₃ thin film-coated PDMS membranes were transparent, flexible, easy-to-attach and exhibited a clear and selective colorimetric change soon after exposure to 1.0 vol% hydrogen gas, which is below the concentration range for the rapid combustion of hydrogen-containing air (4–75 vol%). The membrane color returned to transparent immediately after switching back to air flow. The prepared Pt/WO₃-thin film-coated PDMS membrane is likely to play an important role in the future hydrogen economy.

This work was partly supported by JSPS KAKENHI Grant Number 16K12901 and 18K18404. We would like to thank Profs. Akihiko Kikuchi and Kenjiro Fujimoto for their help.

[1] T. Sinigaglia et al., Int. J. Hydrogen Energ., 42 (2017) 24597.
[2] Y. Yamaguchi et al., Sensor. Actuat. B-Chem., 216 (2015) 394.
[3] H.W. Kohn and M. Boudart, Science 145 (1964) 149.

The development in industrial and agricultural activities, mainly in emerging economies, has resulted in an increase of toxic gases dumped into the atmosphere, such as CO, NO_x, NH₃, O₃, and VOC’s (volatile organic compounds). Among the VOC’s, BTEX (benzene, toluene, ethylbenzene, xylene) is the most harmful gas; being used as organic solvents in the production of paints, dyes, etc. It becomes prejudicial to human health may cause some sickness, such as respiratory irritative symptoms, headaches, rashes, and fatigue. Gas sensors based on metal semiconducting oxides (MOX) have drawn the attention due to their potential for detection of various gases. Among the MOX, hematite (α-Fe₂O₃) has been investigated as resistive gas sensors, mainly, for detection of acetone, and ethanol. Based on these motivations, we present herein an investigation on the gas-sensing properties of α-Fe₂O₃ nanocrystals towards BTEX gas. The nanocrystals were synthesized by the hydrolysis of iron (II) chloride and then crystallized via hydrothermal treatment. The structural, surface and morphological properties were characterized by X-ray diffraction (XRD), X-ray absorption near-edge structure and, X-ray photoelectron spectroscopies, and high-resolution transmission electron (HR-TEM) analyses. The XRD and Fe K-edge XANES measurements indicated that pristine hematite phase was obtained, where iron ions are in octahedral coordination. In addition, XPS spectroscopy indicated the presence of Fe³⁺ ions on the sample surface. HR-TEM analyses showed that hematite crystals exhibit a rhombus-like shape with an average size of approximately 80 nm. Gas-sensing measurements revealed that α-Fe₂O₃ nanocrystals are sensitive towards lower BTEX levels, i.e., from 3 till 400 ppb (parts-per-billion) when kept at 280^oC. Additionally, the experiments showed that hematite nanocrystals presented total reversibility, good repeatability and stability, and high selectivity (towards ethanol and acetone). These findings demonstrate the potential of hematite nanocrystals as sensing material to manufacture BTEX gas sensor devices for practical applications.

Metal oxides such as zinc oxide (ZnO) are potential candidates for gas sensors for environmental monitoring. Oxygen-deficient ZnO, in particular, has attracted much attention due to the possibility to adapt their electrical response to sensing oxidative and reducing gases. The introduction of oxygen vacancies (V_O) modifies the surface by creating active sites where the molecular or atomic gas can interact with environmental gases of concern. They can be used to modify the band structure changing the electrical properties of ZnO. In the preparation of oxygen-deficient ZnO nanomaterials, it is essential to understand how the growth parameters promote the formation of V_O. Thus, this works focuses on studying the influence of plasma processing parameters on the formation of Vo and testing oxygen-deficient ZnO thin films as gas sensors for environmental monitoring. The thin films were deposited on interdigitated transducers at room temperature by reactive magnetron sputtering at different O₂/Ar flow ratios The influence of the flow ratio on the composition and sensing response of the thin films was examined by X-ray Photoelectron Spectroscopy and by measuring the change in electrical current in argon and hydrogen atmosphere. The results show that films prepared at low percent of oxygen in the sputtering gas exhibit the best response, at working temperatures of 200 °C and 1 % of hydrogen in Ar, with an increase in electrical current from 0.17 μA to 48.3 μA. The results will be discussed in terms of the oxygen partial pressure key role in the formation of oxygen vacancies during plasma deposition, which in turn produce excellent sensing responses.

Scientific investigation for the development of extremely sensitive nanohybrid sensors and precise detection of internal climatic conditions has always remained a challenging task. It deals with template synthesis of 3D-cubic mesoporous silica with in-situ loading of metal oxides to obtain a nanocomposite material with huge number of surface active sites. The humidity sensing behavior of synthesized nanostructures divulge the high sensitivity, fast response/recovery (4.5/3.3s), negligible hysteresis and high stability to relative humidity (%RH) in the 11–98%RH range. The structural insights have been reported by analyzing the results obtained for HRTEM, FESEM, LAXRD, HAXRD, BET and FTIR absorption analysis. X-ray diffraction confirmed 3D-cubic structure of silica with Iad symmetry and existence of anatase metal oxide species and a decrease in surface area on loading of metal oxide nanoparticles is exposed via BET surface area analyzer. The present work highlights an efficient scheme in designing of high performance %RH sensors and suggested their promising applicability in futuristic humidity/gas sensing applications.

Gas sensors are extensively used in various fields, such as medical, automotive, automation, agriculture, biogas, and mining industries for safety and environmental monitoring. This includes also the detection of toxic compounds to prevent hazardous emission and explosion along with an air quality monitoring. The increasing development of various technologies and processes signify the increase of monitoring demands for the human’s health impact.^[1,2]

1D nanostructured sensing materials have shown potential in the miniaturization of the physical size of a sensor. The high surface-to-volume ratios provide large absorption sites, fast diffusion of gas molecules. In addition, the 1D orientation facilitates the charge transport process. All in all, high sensitivity, short response and recovery times and low detection limits can be achieved with these materials.^[3,4]

Self-organized anodic TiO₂ nanotube layers have garnered considerable scientific and technological attention over the past 15 years, particularly for photo-catalysis, solar cells, hydrogen generation and biomedical uses.^[5] The synthesis of 1D TiO₂ nanotube structure is carried out by conventional electrochemical anodization of a Ti sheet. This technique has matured into a well-controlled synthesis process with the flexibility to tailor the tube dimensions such as diameter and layer thickness.^[6] On the other hand, Atomic Layer Deposition (ALD) enables a conformal and homogeneous deposition of a thin layer of various chemical compositions, with precise control of the coating thickness, according to the deposition cycles. Therefore, very thin films matching the thickness of the Debye length of a material can be deposited by ALD. The Debye length on the scale of nanometers is the key factor of the sensitivity of a semiconductor gas sensor.^[7] The ALD was shown to conformally coat the high-aspect-ratio TiO₂ nanotube layers with ultrathin coatings of secondary materials^[8] such as Al₂O₃,^[9,10] ZnO^[11] and TiO₂.^[12,13]

This presentation will focus on the coating of the nanotube layers by various secondary materials using ALD. Experimental details and some very recent sensing results based on ZnO^[14] and SnO₂ modified TiO₂ nanotubes will be presented and discussed.

Reference
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[2] J. Bai, B. Zhou, Chem. Rev. 2014, 114, 10131.
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[11] A. Ghobadi et al. Sci. Rep. 2016, 6, 30587.
[12] H. Sopha et al. Appl. Mater. Today 2017, 9, 104.
[13] I. Turkevych et al. J. Ceram. Soc. Japan 2014, 122, 393.
[14] S. Ng et al. Adv. Eng. Mater. 2018, 20, 1700589.

1D nanostructures- nanowires (NWs) are promising material for electronic, magnetic applications, spintronics, sensors. One-component homogeneous NWs were investigated the first. Two and many-component NWs are more interesting due to possibility to vary the properties and to obtain the new ones. Matrix synthesis (MS) is known as one of the way of obtaining of such structures. In this work MS was used for electrodeposition of metals (Cu and metals of iron group- Fe, Co and Ni) into the pores of polymer track membrane (PET film with thickness of 10 mcm, pores diameter 30-200 nm and density approx. 10⁸ per sq.sm.). or the alloyed NWs or layered NWs were obtained, corresp.
Using the regime of constant voltage homogeneous alloys (Fe-Ni, Fe-Co and Fe-Ni) were obtained. Pulse deposition was used for synthesis of heterogeneous layered NWs (Ni/Cu and Co/Cu) using “one-bath” technique. “Double-bath” method was used for synthesis of another layered NWs- Fe/Ni and Fe/Co. Specific features of galvanic process for processes mentioned above were investigated. Topography and crystal structure were estimated by SEM, TEM and X-ray methods. Magnetic properties were also investigated by magnetometry and MFM methods.
For ALLOY NWs it was found: non-linear dependence of grooving process on time, dependence of NWs composition on deposition voltage. It was also found that ratio of elements (Fe to Co or Fe to Ni) was changed along the wires length. Effect of anomalous co-deposition of Fe was detected- it was much higher for FeNi alloys (than for FeCo NWs). The difference of NWs elemental composition from the same in electrolyte also was higher for FeNi NWs.
For LAYERED NWs deposition parameters (voltages in pulse-regime for deposition in “single-bath” regime) were determined. Element mapping (in TEM experiments) determined the concentration of elements: for example, in Ni/Cu NWs there were layers of pure Cu and Ni-based alloy (83%Ni and 17%Cu). Using of the “constant time” leads to formation of layers with variable length (due to the diffusion limitations for ions in the narrow pores). In order to obtain the NWs with regular layer length the regime of “constant charge” should be used. TEM with diffraction analysis demonstrated the polycrystalline structure of NWs . Symmetry of Ni (Fm3m), Cu (Fm3m) and oxides (Cu₂O and CuO) were determined. It was shown that decrease of time of each pulse effectively leads to decreasing of layer length down to 20 nm. For shorter time the mixed structure or the “stick-shell” structures could be obtained.
Magnetic properties data for both types of NWs are given and discussed. The effect of application of external magnetic field on grooving process (acceleration of electrodeposition) and obtained NWs (texture, formation of cavities) also presented.
It is expected that layered NWs could be used as effective gas sensors.

Acknowledgements
Grants 16-29-11763 OFI-M, RFBR grant No.18-32-01066, State Task of FNIC Crystallography and Photonics.

Cultural methods based on bacterial growth rates have played a key role in all spheres related to biosafety—including food industry and medical areas—as long regarded as the 'gold standard'. Despite profitable advantages such as low cost, manifestations of biochemical traits in a specimen, and less susceptibility to food matrix effects, still are limitations associated with the requirements of facilities and expertise workforces, and specific laborious procedures such as plating and colony counting. Paper substrates have been remarked as a platform material with high efficacy, an array of applications and other beneficial features like non-powered fluid delivery, facile sensor fabrication, and biocompatibility when used as a sensing platform. Under the supportive conditions with these traits, numerous applications have been released into the microbiological field with majorly incorporating with biosensing molecules such as antibody and aptamer. Chromogenic agar substrates have been recently involved in paper-based microbiological assessments; the paper-based diagnostics with this detecting motif compensates the drawbacks of the conventional gold standard microbiological assessment, as it diminishes time spends, the number of steps and total cost for analysis.

Herewith, we present a device aiding a rapid report of specific enzymatic phenotypes in microbiological analytes. The device was devised with stratifying paper layers that each contains chemical reagents for cell lysis, oxidation, and coloration. Optimal colors charging each detection zone on the array were respectively determined with either single uses of chromogenic substrates or blended mixtures for a selective coloration. Time length for an enzymatic assay and an enrichment step were optimized for shorted total analysis bringing about enhanced its feasibility. Provided respected sensitivities of each assay, specimens at low bacterial concentrations below 10 cfu/mL were readily sensed expending time around four hours. The robust sensing capability, when situated with food matrices, was validated by real-food spiking tests. Sophisticated sensor designs and analysis of RGB channel pattern enabled an evolution toward to a multiplex reporting for predominant hazardous pathogens in a simultaneous manner.

The multiplex biosafety assessment was miniaturized based on visually reporting respective enzymatic phenotypes of bacterial analytes. Its potentials in practical uses are appreciated when considering the high applicability to the current standardized methodology for water- and foodborne pathogen testing. Furthermore, the diagnostic device can benefit particularly in dealing with an outbreak setting by the outstanding features, low-cost, ready-to-use, and feasible traits, to name but a few.

The investigation of metal oxide semiconductors (MOX) is a fundamental point in the area of physics, chemistry, and material science, in particular nanostructured compounds. Despite the potential application of MOX, the electronic effect adverse during the charge separation process reduces the efficiency of the isolate semiconductors. Thus, great efforts have been made to further improve the MOX properties, for example, the coupling of junctions between semiconductors has been a promising option to hinder the charge recombination and thus enhancing their performance. In this way, heterostructures such as NiO/ZnO, Co₃O₄/ZnO, ZnO/CuO and ZnO/SnO₂ obtained by different methodologies, have exhibited potential properties to be applied as sensing layer. The photoactivation process has been an efficient way for the operation of sensors devices at room temperature. Based on these motivations, we present herein an investigation on the UV-assisted gas-sensing properties of the ZnO, Fe₂O₃, and SnO₂ as well as their respective heterostructures applied as ozone and nitrogen dioxide sensors operating at room temperature. The samples were prepared via microwave-assisted method. The structural, surface, and morphological properties were characterized by X-ray diffraction (XRD), X-ray absorption near-edge structure and, X-ray photoelectron spectroscopies, and high-resolution transmission electron (HR-TEM) analyses. The XRD patterns indicated the presence of ZnO, α-Fe₂O₃ and SnO₂ crystalline phases, without the evidence of solid solution formation. HR-TEM analysis reinforced the incorporation of α-Fe₂O₃ and SnO₂ nanostructure onto ZnO nanorods. The gas sensing properties were investigated for several concentrations of ozone (O₃), carbon monoxide (CO), and nitrogen dioxide (NO₂) gases. The experiments showed that nanocomposites presented total reversibility, good repeatability and selectivity toward oxidant gases. Furthermore, we also observed an enhancement of the gas-sensing properties of the heterostructures when they were kept under UV-illumination, suggesting a reduction in the charge carriers recombination. These findings demonstrate the potential of as sensing material to manufacture gas sensor devices operating at room temperature.

A liquid resistant/repellent surface is beneficial to improving human health and safety when people are exposed to harsh or harmful environment. The surface can protect people from a direct contact with toxic liquids or enhance the hydrophobic filters’ functionality, purifying air or water. The lotus leaf in nature repels water and is characterized by superhydrophobic and self-cleaning effects. And, the features are attributed to its hierarchical structure and wax coating on the surface. Particularly, roughness on the lotus leaf’s surface contributes to creating the liquid-vapor interface, reducing the liquid contact area to a solid surface, and this plays an important role in maximizing the liquid resistance/repellency. The surface roughness is varied in size and geometric structure and those affect its performance.

In this study, creating nano-scale roughness is considered in nanoweb by incorporating nano particles into an electrospinning process. To identify the most efficient geometric structure, a geometric model is developed by extending the Cassie-Baxter model and the newly developed model is justified through evaluating liquid resistance/repellency in nanoweb with nano-scale roughness. It is expected that the works including modeling, modifying and evaluating the nano-scale surface will contribute to developing a liquid resistant/repellent material, highly performing for sensing and controlling fine/ultrafine particle (PM 2.5) pollutants, which can cause lungs and heart diseases and threaten human life.

Recently, in air quality management has gained great attraction as PM2.5 has been shown to have a close association to casuing health problems^1,2,3. The Global Burden of Disease Study reported that exposure to outdoor PM2.5 was responsible for around 3.2 million premature deaths in 2010 worldwide⁴. Such outdoor air pollution can enter the indoor environment through ventilation and infiltration processes.
Various nanofiber filters have been developed for the removal of PM2.5^5,6,7. These filters exhibit high fine dust removal efficiency and relatively low air resistance. However, these filters have a problem in that their filtration performance degrades in long-term operation. To grant the previous fine dust filters with recyclability, self-cleaning effect via superhydrophobic surface can be considered, though few studies have been conducted on the application of superhydrophobicity to air filters.
In this study, polyacrylonitrile (PAN) nanofibrous membrane was fabricated on aluminum mesh and Polydimethylsiloxane/Polyvinylidene fluoride (PDMS/PVDF) microbeads were coated to make a superhydrophobic air filter. The filter exhibited high PM2.5 removal efficiency and superhydrophobicity. The filter was washed with water after a long-term filtering operation as the running water removed fine dust on the filter. The filter with the contaminants removed was tested and it was confirmed that PM2.5 removal performance was restored.

Reference
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2. Pope CA, Burnett RT, Thun MJ, Calle EE, Krewski D, Ito K. Thurston GD. Lung cancer, cardiopulmonary mortality, and long-term exposure to fine particulate air pollution. Jama-J Am Med Assoc. 2002, 287, 1132-1141.
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5. Liu C, Hsu P-C, Lee H-W, Ye M, Zheng G, Liu N, Li W, Cui Y. Transparent air filter for high-efficiency PM2.5 capture. Nat Commun. 2015, 6, 6205.
6. Xu J, Liu C, Hsu PC, Liu K, Zhang R, Liu Y, Cui Y. Roll-to-roll transfer of electrospun nanofiber film for high-efficiency transparent air filter. Nano Lett. 2016, 16, 1270-1275.
7. Zhang R, Liu C, Hsu PC, Zhang C, Liu N, Zhang J, Lee HR, Lu Y, Qiu Y, Chu S, Cui Y. Nanofiber air filters with high-temperature stability for efficient PM2.5 removal from the pollution sources. Nano Lett. 2016a, 16, 3642-3649.

Impurities of graphene during fabrication process increase doping level and decrease electron mobility. To fabricate gas sensor with high sensitivity, cleaning process is essential. In this poster, we present a mechanical cleaning for barristor-based gas sensors with high quality graphene. We adopted mechanical cleaning method based on contact mode of atomic force microscopy (AFM) to remove residues. During the cleaning processes, 2D/G intensity ratio by Raman spectroscopy increased and Dirac voltage of graphene decreased. Finally, the sensibility was improved significantly.

Also, we demonstrated a barristor-based gas sensor. Barristor junctions, graphene and tungsten disulfide (WS₂) junctions, were cleaned using contact-mode AFM. Then the cleaned sensor exhibits sensitivity of 10⁴ % at 1 ppm of nitrogen dioxide (NO₂). The sensitivity could be projected to sub ppb concentration of NO₂.

We studied a facile and practical 1D PC for real-time monitoring humidity sensor with remarkable color change, sensitivity and reversibility. High refractive index polymer and four different quaternization degree of low refractive index polymers were synthesized. Multilayers polymeric 1D photonic crystal(PC) films were fabricated by alternating low/high refractive index polymer by a simple spin-coating. 1D films were prepared 5 films of similar thickness and four samples of varying thickness, and the color change and photonic stop band are observed relative humidity from 10% to 90%. Depending on the degree of quaternization of the low refractive polymer, the color change and response time which are sensitive to the relative humidity are changed. A film having a relatively low hydrophilicity can be applied as an anti-counterfeiting film, and a film having a high hydrophilicity and remarkable sensitivity, rapid response time, various color change can be applied as a humidity sensor.

A graphene-based or carbon-based aerogel is a three-dimensional (3D) solid material in which the carbon atoms are arranged in a sheet-like nanostructure. In this study, we report the synthesis of low-density polymer-modified aerogel monoliths by 3D macro-assemblies of graphene oxide sheets that exhibit significant internal surface areas (982 m²/g) and high electrical conductivity (∼0.1 to 1 × 10² S/cm). Different types of materials were prepared to obtain a single monolithic solid starting from a suspension of single-layer graphene oxide (GO) sheets, and a polymer, made from the precursors 4-carboxybenzaldehyde and polyvinyl alcohol. These materials were used to cross-link the individual sheets by covalent bonds with polyacetylene (resulting from the transformation of polyvinyl alcohol). The obtained wet-gels were supercritically dried and then, in some cases, thermally reduced to yield graphene aerogel composites. The average densities were approaching 15-20 mg/cm³. This approach allowed for the modulation of the distance between the sheets, pore dimension, surface area, and related properties. This specific GO/Polymer ratio has suitable malleability making it a viable candidate for use in conductivity 3D printing; it also has other properties suitable for energy storage, catalysis, sensing and biosensing applications, bioelectronics, and superconductors. An electro-catalytic prototype device for environmental monitoring and remediation is under development for the municipality of Beijing in China.

Humidity sensing devices have attracted significant attention for their numerous potential applications, such as structural health monitoring, precise control for living systems, hospital apparatus, and agricultural fields, etc. Recently, various transduction mechanisms and sensing materials for a humidity sensor have been widely studied to provide high sensitivity, fast response/recovery time, excellent linearity, and low hysteresis by reading out electrical or optical signals (e.g., capacitance, resistance, photoluminescence, and gate effect in semiconductor devices).
Recently, metal halide perovskites (MHP) have received great attention in optoelectronic application fields including solar cells, down-converting materials, light-emitting diodes, and photodetectors, due to their superior optical properties. Particularly, CH₃NH₃PbI_3-xCl_x and CH₃NH₃PbBr₃ have been shown by converting the moisture exposure with increasing relative humidity using photoluminescence intensity or resistance, resulting in an ultrasensitive performance. Nevertheless, major problems of halide perovskites were shown up by the dissolution or chemical reaction with water molecules in humid environments. It is inevitably weaker in their reproducibility and stability as a long-term humidity sensor. For overcoming stability and response/recovery issues by water-MHP reaction, the latest research suggested Cs₂BiAgBr₆ with outstanding stability in the ambient environment. From this research, an excellent stability below 70 RH% and superfast response/recovery time (1.78 sec/0.45 sec) was obtained, whereas the sensing performance still exhibited high attenuation above 80 RH%. Therefore, the sole use of the MHP is recognized as a disposable sensor and is not preferable in a humidity sensing material. In order to prevail all previous shortcomings, our pilot study demonstrated a novel capacitive-type humidity sensor combining aerosol deposition (AD) process and MHP-based ceramic composite materials employed on fully room-temperature to achieve high sensing performance as well as good stability.
Herein, by keeping the room temperature during the process, we fabricated the composite films with CsPb₂Br₅ nanocrystals embedded in ceramic matrices for novel sensing characteristics with ultra-sensitivity, fast response/recovery time, and long-term stability. The humidity sensor exhibited the high sensitivity of 17773.5 pF/RH%, linearity of 0.9954, and hysteresis of 2.98%.

The detectors in the short wavelength infrared (SWIR: 1 - 2.5 um) light are applied to imaging and sensing such as astronomical observation, remote detection of minerals and chemical process monitoring [1]. Commercially available SWIR light detectors are used toxic substances as their constituent elements like mercury-cadmium telluride-MCT and indium-gallium-arsenide (InGaAs). The Mg₂Si is inexpensive and nontoxic semiconducting material consisting of abundant elements. The Mg₂Si has attracting attentions as a novel, mass-consumable SWIR photodetector with low environmental impact and low cost because its band gap of about 0.6 eV at room temperature corresponds to the cut-off wavelength at 2.1 um[2-4].However, Mg₂Si substrates has not been sufficiently advanced and is not commercially available, so far. One of the significant problems to overcome the developing of high quality Mg₂Si substrate is a sticking of Mg₂Si crystal to common crucibles used in melt growth. The high purity Mg₂Si single crystals which were grown in the PG-coated graphite crucible by the vertical Bridgman (VB) method contained the small angle tilted grains [3] due to the sticking of the crystal to the crucible. In this report, we grew the Mg2Si crystals in a pBN or BN crucibles and investigate the influence of contaminated boron on the device performance of Mg2Si pn-junction photodiode. The crystals grown in the pBN crucible was unstuck to the crucible and had a nearly convex crystal tail. The contamination of B was found to be about 30 ppm in the grown crystals by the GDMS analysis. In addition, increase of electron concentration was also found in the crsytals grown using pBN. The value of (6.8-7.0) 10¹⁶ [cm^-3] in the pBN-crystal is more than one order of magnitude higher than that of PG-crystals (3×10¹⁵ [cm^-3]). We also investigated the I-V characteristics and photosensitivity of the Mg2Si pn-junction photodiodes. Those results will be discussed in the presentation.
[1] D. Cohen-Elias et al., Infrared Physics & Technology 85 (2017) 81–8
[2]D.Tamura et al., Thin Solid Films, 515(2007)8272.
[3] H.Udono et al., Jpn.J.Appl.Phys., 54(2015)07JB06.
[4] H.Udono et al., J.Phys.Chem.Sol., 74(2013)311.

A high sensitive and high responsive short wave infrared detector (SWIR-detector) is expected to be use in a variety of industrial and scientific applications such as environmental monitoring, remote sensing of resource, growing observation in agriculture and night vision systems in social safety. We have been developing the Mg₂Si photodiode (PD) [1-4] as an alternative low-cost SWIR-photodetector suited for mass-consumption compared with the commercially available MCT and InGaAs photodetectors in the SWIR region. In this paper, we investigated the spectral photosensitivity of ring-electrode type Mg₂Si pn-junction PDs of which the band alignment between the p-Mg₂Si and metal ohmic contact was varied using Au/Ti or Au/Ni electrode. The spectral photosensitivity of the PDs was measured using a conventional lock-in technique with a monochromatic incident light which passed through a 10 cm- monochromator from a silicon carbide light source. The temperature of the PDs was varied between 77K and 300K in the cryostat (Oxford, Optistat DN2). The photosensitivity of the PD with the Au/Ti/p-Mg₂Si electrode was increased approximately 400 % compared with that of the PD with the Au/Ti/p-Mg₂Si electrode due to the favorable band alignment of Au/Ni/p-Mg₂Si electrode, where the band off-set could not form between the p-Mg₂Si and Ni. The photosensitivity was raised below about 2.1 um in both Au/Ni and Au/Ti electrode type Mg₂Si PDs and increased with decreasing the wavelength. The maximum photosensitivity under zero bias condition in the Au/Ni electrode type PD was about 0.17 A/W at about 1.42 um. The maximum photosensitivity was increased with decreasing the temperature and reached to 0.67 A/W at 1.3 um at 100K. Decrease of cut-off wavelength due to the band gap widening was also observed with decreasing the measurement temperature. The value of cut-off wavelength was about 1.85 um at 100K.
[1]H.Udono et al., Jpn.J.Appl.Phys.,54,(2015)07JB06.
[2]H.Udono et al., J. Phys. Chem. Sol., 74(2013)311.
[3]T.Akiyama et al.,JJAP Conf. Proc. , 5(2017)011102.
[4]Y.Onizawa et al.,JJAP Conf. Proc. , 5(2017)011101.

For decades, hydrogen has been considered the most renewable and sustainable energy source which can be alternative of fossil fuel, owing to its environmentally friendly and perfect combustion without carbon included emissions.[1, 2] Despite these advantages, it is required to ensure safety regarding the use of hydrogen gas due to its colorless, odorless properties and especially its explosive behavior in wide range of concentration (from 4% to 75%) in ambient air.[3] Therefore, most of research efforts have been focused on the development of hydrogen gas sensor with high sensitivity and fast response time to prevent unexpected risks.
Among various hydrogen gas sensor devices, thermoelectric based hydrogen sensors, which are composed of a thermoelectric layer that generates voltage due to temperature difference from exothermic reaction of hydrogen oxidation by catalyst layer, have shown a great potential as a next generation of hydrogen gas sensor, owing to low power consumption, fast response time and room temperature operability.[4, 5]
In this research, we have developed an ultra-fast thermoelectric hydrogen gas sensor composed of tellurium nanowires-based thermoelectric layer and graphene-supported Pt catalysts layer. The tellurium nanowires which is a p-type semiconductor were synthesized by solvothermal method without any additives. The thermoelectric layer was fabricated by tape casting method using tellurium nanowire paste and exhibited excellent Seebeck coefficient of 428 µV/K. Graphene supported Pt catalysts were synthesized by anchoring effect and confirmed the impact of the amount of Pt precursor. The device showed great sensing performance due to the high Seebeck coefficient of tellurium at room temperature and excellent exothermic performance of graphene-supported Pt catalysts. The device can detect with a wide range from 50 ppm to 3% of H₂ concentrations and which showed relatively fast response and recovery time at room temperature. In addition, interference of relative humidity was greatly minimized by using the graphene-supported Pt catalysts.

References
1. P. Chandran, A. Ghosh, S. Ramaprabhu, High-performance Platinum-free oxygen reduction reaction and hydrogen oxidation reaction catalyst in polymer electrolyte membrane fuel cell. Scientific Reports 8, 3591 (2018).
2. M. Z. Jacobson, W. G. Colella, D. M. Golden, Cleaning the air and improving health with hydrogen fuel-cell vehicles. Science 308, 1901-1905 (2005).
3. W.-T. Koo et al., Accelerating Palladium Nanowire H2 Sensors Using Engineered Nanofiltration. ACS Nano 11, 9276-9285 (2017).
4. M. Nishibori et al., Thermoelectric hydrogen sensors using Si and SiGe thin films with a catalytic combustor. J Ceram Soc Jpn 118, 188-192 (2010).
5. S. Kim et al., Fabrication and characterization of thermochemical hydrogen sensor with laminated structure. International Journal of Hydrogen Energy 42, 749-756 (2017).

There have been increasing demands on high-performance sensors with the development of technologies in data acquisition and processing. Especially, those for gas detection can give various helpful information about human health and life. For example, exhaled breath of human has many biomarkers for potential diseases, and indoor air monitoring can alert any harmful gaseous species before exposure to human. In order to collect gaseous information from various sources in real-time, the gas sensors should be small-sized for portability and exhibit low power consumption to be embedded on mobile devices. To date, various types of gas sensors have been developed including electrochemical sensors, optical sensors, mass-sensitive sensors, or chemoresisitve sensors. Among them, chemoresistive gas sensors are the most suitable candidates for future commercialization into mobile devices due to their simple structure and relatively cheap fabrication processes. Previously reported chemoresistive gas sensors have widely adopted metal oxide semiconductors (MOS) like SnO₂ or WO₃, and they are actually commercialized. However, MOS gas sensors need a relatively high operating temperature over 300°C for reliable operation and this leads to inevitable high power consumption. Although some manufacturers have been optimizing off-state current and on-state current for reduced power consumption, still lower power consumption is desired for mobile application. Therefore, two-dimensional (2D) materials have been proposed as an alternative to MOS for their high surface-to-volume ratio beneficial for operation at room temperature. Among various 2D materials, transition metal dichalcogenides (TMDs) have attracted a great amount of attention due to superior mechanical properties and remarkable electronic properties. Herein, we synthesized tungsten disulfide (WS₂) on a porous SiO₂ nanorods template for highly sensitive NO₂ sensors operating at room temperature. The SiO₂ nanorods with 500 nm thickness were prepared by the glancing angle deposition method using an e-beam evaporator. The precursor solution for WS₂ was spin-coated on SiO₂ nanorods and sulfurization was conducted using a chemical vapor deposition system. Uniformly coated WS₂ on SiO₂ nanorods exhibited the gas response of 152% to 5 ppm NO₂ at room temperature with a theoretical detection limit of 13.7 ppb. This promising gas sensing properties with excellent recovery at room temperature can offer a new perspective towards the real application of TMDs-based gas sensors.

Heavy metal ion contamination is a significant, invisible threat to public health largely presupposed as eliminated by governmental supervision. Government agencies actively survey and maintain established regulatory limits on heavy metal contamination, but only prioritized to localized hotspots in high-risk communities. The characterization techniques of heavy metal ions are typically expensive and time consuming, requiring specialized training and extended sample shipping times, with a purview restricted to public infrastructure. As residential plumbing systems corrode and contaminants leach into private water systems, independent means of oversight are becoming increasingly vital in effectively identifying and mitigating toxic exposure. In this work, a bismuth nanoflower structure has been developed as a potential nontoxic sensor material for detection of heavy metal ions. The high surface area nanostructure was synthesized via aqueous chemical reduction of bismuth chloride salt in a micellar environment of sodium dodecyl sulfate, designed for high sensitivity by consideration of the crystallization mechanism in optimization of associated parameters. The nanoflower structure was deposited onto a glassy carbon electrode and coated with a protective Nafion layer, prior to cyclic voltammetry analysis of analyte ion reactivity, achieving a lower detection limit for lead and zinc, respectively. These results indicate the viability of bismuth-based nanoflower structures for portable, accessible detection of heavy metal ions, with enhanced sensitivity.

Thin-film gas sensors utilizing oxide and nitride semiconductors have attracted interests according to the materials high thermal and chemical stability as well as to their sensitivity improved by nanostructures and surface effects. Gallium oxide (Ga₂O₃) and zinc oxide (ZnO) are anticipated wide-bandgap semiconductors for gas sensing applications, which have been researched detection of reactive gases (e.g. H₂, O₂, CO, CH₄) utilizing the oxides with their conductivity modified by doping technique [1–3]. On the other hand, epitaxial thin films with high crystal orientation would contribute to gas sensing characteristics in β-Ga₂O₃ and ZnO by modifying surface chemistry and carrier mobility of crystal materials to obtain sufficient on/off ratio in the devices. The β-Ga₂O₃ and ZnO epitaxial thin films generally have been grown at high-temperature above 400^°C, though reduction of epitaxy temperature advances development of refined devices according to improved surface flatness, interfacial sharpness, and compositional reproducibility [4]. The increased specific surface due to nanoparticulation would also enhance the gas detection properties. In this study, room-temperature syntheses of heteroeptiaxial β-Ga₂O₃ and ZnO thin films using excimer laser processing, and the effect of structural and morphological modification of the films on physical properties were investigated.
The ZnO and precursor amorphous Ga₂O₃ thin films were prepared by pulsed laser deposition technique equipped with a KrF excimer laser (λ=248 nm, d=20 ns, and E~1.5 J/cm²). The films were deposited on atomically stepped α-Al₂O₃ (0001) substrates with NiO (111) buffer layers at room-temperature. The obtained amorphous Ga₂O₃ thin film were subsequently solid-phase crystallized by atmospheric excimer laser annealing (ELA) process, in which KrF excimer laser beam (not focused, E~250 mJ/cm²) was irradiated on the film at room-temperature circumstances. As a result, room-temperature formation of both β-Ga₂O₃ (-201) / NiO (111) / α-Al₂O₃ (0001) and ZnO (0001) / NiO (111) / α-Al₂O₃ (0001) heteroepitaxial thin films was obtained. The β-Ga₂O₃ (-201) and ZnO (0001) films revealed ultra-flat surface well reflecting morphology of the substrates. The gas sensing properties of the heteroepitaxial ZnO thin films were evaluated with pure H₂ gas at room-temperature, in which the result revealed fast response that sheet resistivity was reduced by 30% from ~1×10^–3 to ~7×10^–4 Ωcm. In addition, the optical bandgap of the obtained β-Ga₂O₃ (-201) film was 4.9 eV comparable to high-temperature grown films. Structural analyses of β-Ga₂O₃ crystallization process by ELA, and conduction property modification of both materials would also be presented.
[1] T. Schwebel, M. Fleischer, and H. Meixner, Sens. Actuators B 65 (2000) 176-180.
[2] M. Ogita, K. Higo, Y. Nakanishi, and Y. Hatanaka, Appl. Surf. Sci. 175-176 (2001) 721-725.
[3] S, Park, G, Sun, and C, Lee, J. Ceram. Process. Res. 16 (2015) 367~371.
[4] M. Yoshimoto, R. Yamauchi, D. Shiojiri, G. Tan, S. Kaneko, and A. Matsuda, J. Ceram. Soc. Japan 121 (2013) 1–9.

Oxygen-related defects are dominant to the properties of Zinc Oxide nanoparticles (NPs) and thus making ZnO NPs a potential candidate for oxygen sensing applications. A solution-processed and highly sensitive oxygen partial pressure sensor based on ZnO NPs thin film was fabricated and characterized. The sensor was capable of measuring the oxygen partial pressure in air from 10^-5 mBar to 10³ mBar and it also featured a heat-erasing capability. In the meanwhile, it was observed that the annealing temperature has a notable effect on sensor sensitivity and the highest sensitivity was realized at 600°C. In order to explain the annealing effect and understand the fundamental physics behind it, ZnO NPs were investigated on their trap states and dielectric properties. In the end, mould-guided drying technique was introduced to pattern ZnO NPs into lines in micrometer and even nanometer scale directly from its dispersion, and an oxygen partial pressure sensor was fabricated based on the patterned ZnO NPs lines, which makes it extremely promising in miniaturized and integrated sensing applications.

This contribution brings connection of two impressive kinds of oxide semiconducting materials for using as enhanced sensitive material for chemical gas sensing. The first kind of material is nanostructured tin dioxide (SnO₂) in form of thin film consisting of nanowires and nanoporous structures. SnO₂ and its nanostructures are well known for years as an effective material for chemiresistive gas sensor for detection of gases with redox features (H₂, hydrocarbons, vapor of alcohols etc.). The second kind of material is transition metal oxide single molecule nanocrystal (polyoxometalate) with catalytic features. Polyoxometalates consist of large number of Mo, W, V, Ta, Nb, P, As atoms with oxygen and form lacunary (vacant) molecules with features of high chemical stability and catalytic activity. They can accommodate/incorporate transition metal ions or transition metal clusters at specific sites of the rigid structure, leading to interesting molecules with specific topologies and highly symmetric environments.
In this work we present sensor structure consists of nanostructured SnO₂ decorated with polyoxometalates. As sensor substrates we use alumina plates with interdigitating platinum electrodes (50 microns between electrodes) and platinum heater element. SnO₂ thin films are prepared by thermal CVD from tin monoxide and tin acetylacetonate source material. Different polyoxometalates are subsequently applied by using wet-techniques from water solution onto the SnO₂ nanostructures. Morphology studies are performed by electron microscopy (SEM) and chemical composition analysis is done by EDX and XPS. Sensor behavior of such structures was tested as responses to basic analytes (H₂, alkanes, alcohols and aldehydes) in concentration of up to 100 ppm in synthetic air. Sensor results from polyoxometates decorated sensors are compared to bare SnO₂ sensors. We demonstrate that polyoxometalates significantly improve sensitivity of presented sensor structures as well as they decrease the detection limit to ppb concentrations region.

The use of titanium dioxide nanotubes in the powder form (TNTP) has been a hot topic for the past decades in many applications. The high quality of the fabricated TNTP by various synthesis routes may meet the required threshold of performance in a plethora of fields such as drug delivery, sensors, supercapacitors, and photocatalytic applications. In this research work, the TNTP were used as an antibacterial agent for water microbial purification with different synthesis techniques of the powder form as hydrothermal and rapid breakdown anodization methods. The TNTP were characterized by SEM, X-Ray Diffraction (XRD), Raman, FTIR, and Uv-Visible Spectroscopy to study the effect of the structure, surface morphology, chemical composition, crystallinity and the optical properties of them. The antibacterial activity test was carried out using a viable count method against Escherichia. coli as a model of gram-negative bacteria in the presence and absence of solar radiation. Upon UV irradiation, hydrothermally synthesized-TNTP showed higher efficiency in deactivating E. coli (∼96.7%) than the anodized TNTP. Several factors play a significant role in determining the antibacterial activity of TNTP. These factors including the crystalline phase, surface hydroxyls, physicochemical properties of TiO₂ as well as the experimental conditions.

Nowadays transistors are used for the production of lightweight, transparent, flexible, portable and biocompatible electronics for a variety of applications, such as flexible displays, chemical or biological sensors, wearable and textile integrated systems, medical implants and artificial skins. Metal oxide ion-gated transistors (IGTs) are attractive for chemo- and bio-sensing platforms as they operate at a low voltage (e.g. below 1 V). Metal oxides are of interest as transistor channel materials in IGTs due to chemical stability and ease of processability in the air at a lower temperature. These include oxides of tin, indium, zinc, tungsten, titanium, and iron. Indium oxide-based channel material shows superior transistor performance, but the availability of indium is limited in the earth crust, so there is a necessity to explore alternative materials. Titanium dioxide is an abundant material, whose conductivity can be strongly modulated by ion-gating and is useful as an n-type semiconducting channel material for IGTs. IGTs use ionic liquids, polymer electrolytes or aqueous saline solutions as gating media to modulate the charge carrier density in the transistor channel. In this work, we investigate the role of TiO₂ film morphology (porous versus dense films) and cation size in the gating medium ([EMIM]⁺ versus Li⁺) on the electrical characteristics of IGTs. The solution processed film yields a porous structure with a mixture of anatase and rutile phases, whereas e-beam evaporated TiO₂ yields a dense and smooth film with mostly anatase phase. Electrical characteristics of dense-film IGTs gated by large-size cation gating media are relatively unaffected by scan rates in current-voltage measurements, as the ions cannot permeate the films. However, in these same films, small-size cations can permeate the films such that reducing the scan rate increases drain current and ON/OFF ratio.

References
Valitova, I., George, B., Subramanian, A., Bobbara, S.R., Ruggeri, I., Soavi, F., Santato, C. and Cicoira, F., Charge carrier density modulation of polycrystalline TiO₂ films in electrolyte gated transistors, in preparation.
Valitova, I., Kumar, P., Meng, X., Soavi, F., Santato, C. and Cicoira, F., Photolithographically patterned TiO₂ films for electrolyte-gated transistors, ACS Applied Materials & Interfaces, 2016, 8(23),14855-14862.

Volatile organic compounds (VOCs) are organic chemicals having a high vapor pressure at ordinary room temperature, and they are potentially dangerous to human health once chronically breath in. Daily exposure to VOCs has serious harmful effects including a wide range of sensory irritation and chronic diseases (e.g., nervous system impairment, asthma, or potential to cancer). On the other hand, severe injury and lethal death could occur in the gas explosion and consequent fire since the VOCs concentration gets above the lowest explosion limit in air. Monitoring of VOCs and delivering of the alarm message in the very early stage therefore becomes important for modern society, especially in chemical factory management. In order to guard our homeland and eliminate accidental factors, a gas sensor is urgent to be developed as a precaution tool. High-performance VOCs sensor should be a device with real-time detection, high sensitivity, and high reproducibility. In this study, we build a high-performance freestanding aligned Ag/CdSe-CdS/poly(methyl methacrylate) (PMMA) texture to detect VOCs. The insight of this new VOCs sensing materials is based on electrospinning techniques, ultraviolet (UV)/ozone treatments, and nano-optics. The incorporation of CdSe-CdS core-shell quantum rods (QRs) and silver nanocrystals in the PMMA nanofibers amplifies the polarization response of long rods from VOCs detection so as to increase the sensitivity of VOCs sensing materials. Further, the uniaxial aligned Ag/QR/PMMA was treated by UV-ozone etching to a certain increment of surface absorption. Metal frame substrate is employed for 15 min UV-ozone etching on double-side. The higher specific surface area was successfully made on the freestanding Ag/QR/PMMA sensing materials enabling advanced VOCs sensing efficient. The advanced UV-ozone etching on the double-side of uniaxial aligned Ag/QR/PMMA efficiently enhances the extinction changes of VOCs. The freestanding Ag/QR/PMMA sensing materials has characteristics of polarization at 90^o and 270^o. It amplifies extinction changes in light to a significant level for the VOCs sensitivity judgment. Moreover, when the freestanding Ag/QR/PMMA sensing materials was exposed in high concentrations (10,000 ppm) of volatile organic gases, the alcohols, esters, and the aromatic can be detected instantly. The results showed that even under a low concentration (100 ppm) n-butanol atmosphere can still be detected within one minute. The freestanding aligned Ag/QR/PMMA texture is a newly designed nanocomposite device for environmental risk monitoring. It could be accepted by the market compared to the other commercial highly sensitive VOCs sensing materials.

In this presentation, we report highly selective and sensitive ammonia gas sensors using highly electron deficient naphthalene diimide (NDI)-based organic field-effect transistors (OFETs). In order to control π-electron deficiency of NDI core, we designed the NDI derivatives by introducing various electron withdrawing groups (EWGs). In contrast to conventional FET gas sensors that operate through resistive mechanism, NDI-based OFETs exhibited high responsivity by showing high current increase when exposed to NH3 gas with excellent selectivity. From the optical and electrical characterizations, we revealed that the origin of high selectivity and sensitivity were the efficient radical formation between the NDI derivative and gas molecules, and degree of radical formation, respectively. Through the morphology optimization with device engineering, we achieved outstanding responsivity up to 250% current increase and sensitivity down to sub-ppm gas concentration. We suggest that employing different types of EWGs on organic n-type semiconductors can be a potential design strategy for developing highly selective and sensitive gas sensors.

The miniaturization of sensors plays a significant role in the development of wireless and complex detection systems. We suggest continuous-wave laser-induced forward transfer (CW-LIFT) combined with photolithography as a suitable manufacturing process for the creation of micro-chemiresistors. The laser deposition system consisted of a CW laser with a 405 nm operating wavelength, 50 mW power source and 4x microscope objective. A donor substrate was composed of borosilicate glass, a thin layer of sputtered gold and a layer of zinc phthalocyanine. The interface of the glass substrate and sacrificial gold layer absorbed the laser energy resulting in a local thermal transfer of the zinc phthalocyanine from the donor substrate to a receiving substrate with golden electrodes. The absorbed laser energy was controlled by altering a laser scan speed in a range of 0.1 to 5 mm/s. At the speed that produced the best results, the transferred phthalocyanine had a spatial resolution of less than 5 µm and a very high surface-to-volume. SEM scans showed the transferred zinc phthalocyanine structure with confirmation by FTIR. The response of microchemiresistors was tested on a wide variety of gasses with the highest response above 200 to 10 ppm of nitrogen dioxide. Our results showed CW-LIFT as a suitable high-resolution deposition technique for creation of micro-chemiresistors.

Phthalocyanine-fullerene thin films, a promising material for solar cell technology and chemical sensing, are typically formed by evaporation and PLD. However, such deposition techniques require vacuum systems and do not offer high-resolution. Here, we investigate the use of continuous wave laser-induced forward transfer (CW-LIFT) as a suitable deposition technique for the formation of zinc phthalocyanine-fullerene nanocomposites. In particular, we seek to determine the laser energy range at which nanocomposites are formed with suitable morphology and composition. The laser deposition system consisted of a CW laser with a 405 nm operating wavelength, 50 mW power source and 4x microscope objective. The laser energy was controlled by altering the laser scan speed in the range of 0.05 – 5 mm/s. A donor glass substrate/sputtered gold layer interface was used to absorb the laser energy for the local thermal transfer of thin phthalocyanine and/or fullerene films to a receiver substrate. SEM scans showed that microcrystals were formed in a wide laser scan speed range. At the speed that produced the best results, the transferred nanocomposite had a spatial resolution of less than 5 µm and a very high surface-to-volume ratio. In addition, AFM showed that the surface roughness of the donor layers was homogeneous. Collectively, our results suggest that CW-LIFT is a suitable deposition technique when operated with the appropriate laser scan speed.

Molecularly imprinted polymers (MIPs) are an intriguing class of synthetic materials capable of selectively recognizing both biological and chemical molecules for a variety of high-value applications in chemical sensors, catalysis, drug delivery, antibodies, and receptors. The concept of molecular imprinting is widely known to perform a tailored recognition process by the selective adsorption of the certain molecules often observed in natural biological systems such as enzyme catalysis or antigen-antibody interactions. In principle, the MIP technique involves the imprinting process of specific molecules in a polymer matrix by chemical synthetic approaches, which provide stable cavity configurations after the simple removal of the template molecules. Here, we report a simple one-step technique to craft a set of highly ordered MIP structures in micro- and nanoscale generated from the drying-mediated self-organization in a confined geometry where the polymer solution containing molecular templates is trapped by capillary force. MIP solutions containing a crosslinking agent, photoinitiator, and molecular templates (e.g., 2,4-D) was constrained within a roll-to-flat geometry, and the capillary held solution readily induced repetitive stick-slip motions of the contact line as the roll was slowly moved over the flat substrate. As a result, we observed highly organized MIP nanopatterns with unprecedented regularity over large areas. This self-organization of MIPs by the controlled roll-print process under UV exposure can be fully utilized to produce layered architectures with the enhanced surface area. The gravimetric quartz crystal microbalance was used to evaluate the relative selectivity of the resultant MIP sensors towards 2,4-D against 2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine, 4-N-ethyl-6-methylsulfanyl-2-N-propan-2-yl-1,3,5-triazine-2,4-diamine, and benzoic acid. Our self-organization technique is simple and cost-effective, offering a promising route to organize molecularly imprinted polymeric materials into nanoscopic sensing devices for the selective detections of endocrine disrupting chemicals.
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Suitable on-surface film deposition methods are prerequisite to incorporating metal-organic frameworks (MOFs) into functional platforms and microdevices. Heretofore, the development mostly relies on the solvent-based procedures, typically altered from powder synthesis routes, which are obstacles to the nanofabrication process. Besides, although few vapor-phase growth approaches were presented, the proposed procedures are devious and involved with a relative high-end instrument, atomic layer deposition (ALD). Here, we report a steam-aided chemical vapor deposition (CVD) approach for directly producing ZIF-67 and associated zeolitic imidazolate frameworks (ZIFs). The introduction of solvent steam, as a mediator, thoroughly prompts ZIF crystal ripening. The introduction of solvent steam, as a mediator, thoroughly prompts ZIF crystal ripening. Also, the straightforward depositing method enables the growth of highly oriented ZIF thin films by properly adjusting the growing window. With the technique, we seek to prove that the steam-assisted CVD is a microelectronics-compatible MOF thin film synthesis route by demonstrating miniature ZIF-67 chemiresistors based on a conventional microfabricating process. The ZIF-67 chemiresistors exhibit certain response to the molecules which are equipped with the ability to form hydrogen bond and penetrate into the cages of ZIF-67. The proposed steam-assisted CVD approach not only opens new avenues to explore various MOFs preparing ways but also expedite the integration of MOF materials in microelectronics and related applications.

The Layer-by-Layer (LbL) technique is a versatile method to modify surfaces by spontaneous adsorption of charged molecules. This technique allows assembling highly ordered nanostructured films with specific tunable properties, which makes them suitable for various applications, including catalysis, optics, biomedicine and sensors. It has been demonstrated that both sensitivity and selectivity of sensors based on polyelectrolyte LbL films can be significantly enhanced by doping them with metal nanoparticles (NPs).^1,2 The underlying mechanisms for this enhancement are, however, unclear today, notably because important parameters such as nanoparticle concentration are difficult to control in wet-chemical assembly. We propose here a different approach to sample fabrication, based on the implementation of surfactant-free silver nanoparticles in prefabricated LbL films, giving us full and independent control over all sample parameters. We can then study the influence of individual parameters like NP size, composition and concentration on optical and electric properties of the nanostructured films before implementing them in sensor prototypes. LbL films composed of polyethylenimine (PEI) and poly(acrylic acid) (PAA) in (PEI/PAA)_n architecture, were grown on quartz substrates and interdigitated gold electrodes. Bare Ag NPs around 3 nm diameter, fabricated in the gas phase in a magnetron cluster source are then implanted at controlled deposition energy. The first prerequisite is to verify the correct spatial dispersion of the NPs in the polymer. This is achieved by optical spectroscopy of the localized surface plasmon resonance of Ag NPs, which allows distinguishing between isolated and coupled NPs, up to the formation of (discontinuous) metallic films. Further experiments comprise in situ impedance measurements, which were performed using an impedance meter developed in our research group. The impedance measurements allow keeping track of the hybrid material’s electrical properties during NP deposition. Thus, we observe that Ag NP deposition causes a competition between capacitive and resistive behavior. Following the electrical ac response during continuous accumulation of metal NPs within the polymer matrix gives us crucial information, not only about the NP dispersion but also on how the electrical properties of the nanostructured film can be tuned for future implementation in electrochemical sensors.
References
1. A. de Barros et al., Electrochim.Acta. 235, 700–708 (2017).
2. L. A. Mercante et al., J. Nanomater. 2015 (2015).
Acknowledgments
The authors are grateful to FAPESP (2007/01722-9, 2017/19169-6), Capes and CNPq for financial support, Microfabrication Laboratory and Electron Microscopy Laboratory in Brazilian Nanotechnology National Laboratory (LNNano/CNPEM) for the experimental support.

Humidity sensors have been studied for decades. Most sensing elements are either ceramic or polymeric and the sensing platform is either capacitive of resistive. Recently 2D material, in particular MoS₂, based capacitive relative humidity sensors have shown much higher sensitivity than existing relative humidity (RH) sensors based on ceramics. The other major performance characteristics of RH sensors is the range that should ideally cover the entire range of non – condensing humidity percentage and the response time. Typical range for modern sensors is now 10 – 97% and response time is larger than 1 second (typically 10s).
In order to integrate the sensors in real-life applications, such as health monitoring system, the sensing needs to exhibit sub-second response time to obtain real-time information and be able to measure an extended range of humidity with high resolution in order to detect when humidity surpasses normal operating levels but also sense human presence in proximity with the system.
In this work we have studied the application of 2D materials such as MoS₂ or WS₂ for humidity sensing. For this purpose, we have developed the sensing platform based on TMDCs/metal nanoparticles composite sensing element. Both MoS₂ and WS₂ have been identified as promising candidates for sensing application and nanoparticles of metals such as Ag, Au and Cu were added to improve the sensitivity and stability of the sensor.
Solution-exfoliated MoS₂ and WS₂ nanosheets were obtained using a probe sonicator system and nanosheets with different sizes were extracted by a vacuum filter system. Humidity sensors with different nanosheet sizes were assembled and tested against the commercial humidity sensor. The devices showed good response to RH in the range of 5 - 75% and response time below 1 s. Moreover, they show excellent stability in time.
MoS₂ nanosheets with smaller size show better humidity sensing behavior than the original MoS₂ nanosheets without filtering and centrifugation, mainly due to their higher surface-to-volume ratio. We have also observed different resistance response to humidity between the MoS₂ and WS₂. Finally, the influence of metal nanoparticles on the sensing capabilities was studied.

With growing demand for fast and accessible testing device for health and environment monitoring, easy-to-use visual sensors has been gaining wide attention. Especially, visual based detection system for organic liquids has been gaining interest due to increase in awareness of the negative effects of the organic liquids to the health and the environment, and is increasing in demand because use in a wide variety of applications from food inspection and fuel quality control to synthesis monitoring. Currently, the most conventional visual sensors are based on colorimetric system, which produces its signal based on color change to external stimuli. However, several challenges are present for colorimetric sensors to detect liquid-phase chemicals, and needs to meet strict requirements for the sensing medium to perform liquid sensing capabilities. Therefore, development of a new visual sensing platform for detecting target liquids is highly desired.
In order for effective detection of organic liquid based on visual sensing, we took a different approach and used transparency as the visual que for the organic liquid sensing platform. The idea is to use a light scattering polymer medium that changes its transparency in response to the presence of the organic liquid. The sensing medium was designed to produce three transparency state: transparent, semi-transparent, and opaque, according to the interaction between the organic liquid and the sensing medium. The sensing medium was fabricated by engineering the shrinkage stress relaxation between the polymer matrix and the embedded particle, which produces a porous structure with micro-sized voids with embedded hollow particles. This porous structure is essential for producing the three transparency state, which is crucial for effective differentiation of various organic liquids. The advantage of the fabricated porous sensing medium is that the structure is produced by controlling polymer dynamics rather than polymer assembly, allowing use of a wide selection of polymer and control of the thickness. An array of the transparency-switching scattering medium using different polymers is able to differentiate a wide variety of organic liquids, and even liquids with similar structures and chemical properties. Furthermore, by varying the thickness of the responsive medium, the transparency-based sensing medium is able to identify the liquid composition in ternary mixture. This work shows the effectiveness of a newly designed sensing material using transparency as a visual que for organic liquid sensing.

Earth's atmosphere is a complex and dynamic system that directly impacts, and is impacted by human activity. Levels of water vapour in air and their spatial distribution determine major weather events such as the Indian monsoon. Human activity generated aerosols (especially PM_2.5) have lasting impacts on human health, and their presence is in turn affected by relative humidity through the hygroscopic growth factor. Space-based sensors such as MODIS (specifically bands 20 - 22), and VIIRS (specifically I4 and M12 bands) regularly image the Earth's atmosphere in an interesting short wave infrared range (2.5-4 μm) that contains fundamental band for water (~ 2800 nm) and absorption bands for several other atmospheric gases/pollutants - O₃ (low absorption), N₂O(significant absorption), O₂(low absorption), CO₂ and CH₄ (significant absorption for both these greenhouse gases), CO (low absorption), etc. In this work, we have designed and fabricated a bowtie nanoantenna (400 nm arm length, apex angle = 60^○, thickness = 80 nm) reflectance-based sensor exhibiting a strong response in the wavelength range 2.55-2.85 μm. An array of these nanoantennas (apex to apex gap = 100 nm) was designed using finite difference time-domain (FDTD) simulations (Lumerical, using polarized light parallel to antenna axis). A hundred fold electric field enhancement in the gap, tuned to match this wavelength range (averaged efficiency ~60%, predicted using HFSS, with a 1 kΩ source impedance), resulting in maximal absorption was predicted. To fabricate the antennas, a positive photoresist (A4 (450K)) was deposited on a cleaned (RCA) silicon substrate (University Wafers). A pre-bake step was carried out at 180^○C for 20 minutes. The photoresist was patterned using electron-beam lithography (eLine Plus, Raith) with a dose of 120μC/cm² to write a layout of the antenna array (Cadence Virtuoso). A post-bake step (110^○C, 30 minutes) was employed before pattern development in a methyl isobutyl ketone (MIBK): isopropanol (IPA)=1:1 developer for 60 seconds. Gold (Sigma-Aldrich, 4N pure) was then deposited on the substrate through thermal evaporation (Advanced Process Technologies) at a base pressure of 4x10^-6 Torr, followed by lift-off (acetone). Scanning electron microscopy (SEM) scans of the bowtie array were found to agree with the designed periodicity of 1000 nm. Fourier Transform Infrared spectroscopy (Thermo Electron Scie) measurements in the range 2.55 μm to 2.85 μm revealed a broad minimum in reflectance at ~2.75 μm, in reasonable agreement with the simulation results, which are being refined to achieve a better match with the measurements. This nanoantenna array can be tuned to a different band by changing the geometry of the antenna, and thus provides significant flexibility when compared to narrow band detectors based on InSb and HgCdTe. Space-based light-weight sensors based on these nanoantenna arrays can potentially be used for addressing several bands of interest in short wave infrared for mapping water vapour and derived aerosol content in the air column beneath a sattelite in conjunction with sensors working in visible and near infrared regimes.

Despite the early promise of high sensitivity and detection limit of carbon nanotube-based gas sensors, the practical implementation has been delayed. The primary reason is the imperfection of commercially available carbon nanotube material and the variabilities involved in the sensor fabrication procedure. A key longstanding issue has been the production of tightly controlled nanotube properties such as chirality, length and diameter. In addition, placing the nanotube in a controlled volume and orientation is another challenge. As a result, the device responses often vary from device to device and batch to batch. Therefore, a successful transition of the promise of nanotechnology into practical implementation depends on how to design a device that is tolerant to inherent material and process variations and other imperfections. Herein, we present a variation-tolerant sensor design where the sensor response is defined by a statistical value of sub-sensors array in contrast to a traditional deterministic value of a single sensor. Thanks to recent development in microfabrication or inkjet printing technology, one can easily fabricate a few hundreds of sensors in a stamp size chip. With the printed sensor chip containing multiple sub-sensor array, the sensor response can be obtained by stochastic decision, i.e. single input and multiple output (SIMO). In this work, we present the inkjet printed SIMO gas sensor. The data processing method to remove outlier data point from signal distribution is explained and the origin of outliers is investigated. The repeatability and reproducibility of SIMO sensor is demonstrated. An analytical model to support the hypothesis is addressed.

Surface Enhanced Raman Scattering (SERS) is a powerful tool for detection of analytes at low concentrations. To enhance the Raman signal, the analyte must be in close proximity to the substrate. This often requires customization of substrate, making it useful for the detection of only a specific subset of molecules. We have produced hybrid carbon-gold nanoparticles for the detection of a broad spectrum of molecules using Surface Enhanced Raman Scattering (SERS). Carboxyl-terminated carbon black (CB) nanoparticles were coated with the cationic polyelectrolyte poly-L-lysine (PLL), and exposed to a tetrachloroauric acid solution. Gold-carbon black (Au-PLLCB) particles were formed by the reduction of gold chloride ions that concentrate on the surfaces of the PLL-coated CB templates. The Au-PLLCB particles produced strong SERS signals for 4-nitrobenzene thiol (4-NBT) in ethanol, and for Congo red, crystal violet, and nitrate ions in water. The fractal morphology of the carbon black template and the presence of PLL promote the formation of sharp gold spikes on the surface, yielding hot spots for Raman enhancement. The underlying carbon acts as an absorbent for organic molecules, allowing analytes with poor affinity for the gold surface to concentrate in regions close enough to the particle surfaces to enable detection by SERS. The morphology and chemical nature of the underlying template make the Au-PLLCB a broadly applicable type of particle for detecting of a wide range of analytes in solution.

The development of a practical gas sensor is of great interest for the monitoring of toxic and non-toxic gases that might endanger our safety and wellbeing in different settings. Hereby we present a practical gas sensor based on hydrogenated graphene for CO₂ & CO monitoring. Hydrogenated graphene was synthesized by “non-conventional” method, directly, onto an Au-pattern insulating substrate. Hydrogenated graphene was then characterized using RAMAN spectroscopy, X-ray photoelectron spectroscopy (XPS) and X-ray Diffraction (XRD). Then, the interaction of the materials with CO₂ & CO was systematically study at different PPM to determine our sensor’s detection limits. Some preliminary research reveals that the hydrogenated graphene-based sensor is capable of sensing CO₂ at the mid ppm level, while the detection limit for CO could be as low as low-ppb.

As an atomically thin electric conductor, graphene is an ideal transducer of electrical signals from a molecular receptor. Concerning small gas molecules, graphene itself is a poor receptor, supporting only physical, non-specific adsorption. Monolayer graphene functionalized with nano-layers or –clusters of metal oxides (MOX) or noble metals using pulsed laser deposition (PLD) produces a sensitive chemiresistive sensor for air pollutants. The deposition of even a sub-nanometer amount on the CVD-grown graphene increases the gas sensitivity up to 2 orders of magnitude due to the formation of effective adsorption centers. Also, the selectivity can be introduced by the selection of the deposited material. For example, a layer of ZrO₂ or similar oxide lead to high selectivity towards NO₂ or O₃ with low (<10%) cross-sensitivity to other tested gases (CO, SO₂, NH₃, H₂S), whereas V₂O₅ layer leads to a similar result for NH₃ gas.
Further improvement of sensitivity and recovery rate occurs when the sensor material is exposed to UV light (< 0.2 mW/mm², 365 nm). The sensitivity level of 1%/ppb was achieved. The increase of signal amplitudes in the UV light, compared to the conductivity change in the dark state, indicates that the adsorption is photo-chemically promoted. Potential light absorbers – graphene itself, MOX, adsorbed gas molecule – were considered to establish the most plausible photochemical mechanisms. In a different approach, CVD monolayer graphene was successfully transferred to and functionalized by PLD on microheater platforms. It will be demonstrated how the pulsed or modulated heating allows achieving further gas selectivity or significantly reduced response times.
The proposed sensor materials have additional merits, such as insensitivity to air humidity and long-term (> 3 years) stability in ambient air. First principle calculations of hybrid nanomaterials consisting of single-layer graphene with MOX clusters on top of it are performed for clarifying the origin of the adsorption sites.

Population growth and urbanization has increased the vulnerability of urban water systems. To maintain the quality of water supplied in the face of increased pollution, there is a need for a more proactive monitoring of contaminants within water distribution systems. Incorporating sensing capabilities into technologies that are already part of the water treatment process, such as water filtration membranes, can help meet this need. The use of water filtration membranes as simultaneous sensors also benefits from the high surface area that is inherent in membranes and reduction in mass transfer limitations that restrict the effectiveness of traditional submerged sensors. While traditional polymeric membranes cannot be exploited for sensing, recent advances in the fabrication of electrically conductive membranes present an unexplored opportunity to incorporate in situ electrochemical sensing into membrane filtration. This study discusses the design of conductive membranes for in situ electrochemical sensing using laser induced graphitization (LIG), a facile and scalable method to form porous graphitic surface on a wide range of substrates under ambient conditions. LIG is used to fabricate conductive membranes directly from polyethersulfone (PES) and graphene oxide (GO) membranes. The conductive membrane is then used for the detection of heavy metals using anodic stripping voltammetry. The membrane acts as the working electrode in a three-electrode system while filtering water spiked with Pb²⁺ standard solution. The performance of two types of conductive membranes, namely, PES and GO derived conducive membranes, is evaluated. The effect of solution conductivity, deposition time, as well as filtration rate on the detection limit are investigated. The results of this study will introduce novel sensing architectures for monitoring contaminants within water treatment systems.

We report on recent investigations of signal readout concepts and advantageous biasing conditions for suspended single walled carbon nanotube field effect transistors (SWNT FETs) as functional building blocks in ultra-low power NO₂ sensors for environmental applications [1]. Advances in fabrication processes as well as better understanding of the signal characteristics of SWNT FETs has enabled the vision of using individual tube devices directly for NO₂ gas sensors. Suspended devices, which have the potential for upscaling in an industrial process flow, and the optimization of device contact architecture by selective Pt ALD help maintaining the cleanliness of the carbon nanotubes for low hysteresis, low contact resistance and improvement of the noise performance [2],[3], as well as improvement in the cross-sensitivity to humidity. Suspended devices are also attractive for self-heated, low-power architectures [4]. We will discuss signal readout options for ultra-low power sensing with carbon nanotubes. The comparison of transient (e.g. signal rise or change) and steady-state (i.e. working point related) parameters in the presence of 1/f noise allow the conclusions that due to the Langmuir binding behavior of NO₂ on carbon nanotubes, sensing using the considered transient parameters decreases response time relative to steady-state sensing. However, noise analysis further shows that with current devices, transient sensing has a lower SNR relative to steady-state sensing [5]. With improving control over fabrication processes gas sensor functional devices operating at extremely low power can be envisioned.

Acknowledgments:
Miro Haluska, Kiran Chikkadi, Matthias Muoth, Wei Liu, Moritz Mattmann, Laura Jenni, Lalit Kumar, Stefan Nedelcu and Seoho Jung for their contributions to SWNT-FET NO₂ sensors research, Support from ETH Zurich (TH 18/03-1, TH 13/05-3), Swiss National Science Foundation (20021-108059/1 and 200021_153292/1), KTI/CTI (8885.2 PFDP-NM), SNF-FLAG ERA CONVERGENCE (20FE-1 170224) and SFA-AM, a strategic initiative of the ETH Board, is gratefully acknowledged.

References:
[1] Eberle, S., Roman, C., Hierold, C., Microelectron. Eng., vol. 193, pp. 86-90, 2018
[2] Chikkadi, K., Muoth, M., Roman, C., Haluska, M., Hierold, C., Beilstein Journal of Nanotechnology, 2014, 5, pp 2179
[3] Jenni, L.V., Haluska, M., Hierold, C., in Proc. of 2018 IEEE Micro Electro Mechanical Systems (MEMS), 2018, pp. 479-482
[4] Chikkadi, K., Muoth, M., Maiwald, V., Roman, C., Hierold, C., Appl. Phys. Lett. 2013, 103, 223109
[5] Satterthwaite P.F., Eberle S., Nedelcu S., Roman C., Hierold C., Sens. Actuators B, 2019, (accepted)

Conjugated polymers have been widely investigated in thin film devices because of their low cost, flexibility, and room-temperature operation.1 Recently, a variety of solution-processed polymer materials have been explored for sensing toxic gases such as nitrogen dioxide based on thin-film transistors and it is particularly relevant for health and safety in work and ambient environments when NO2 concentrations reach the 1−10 ppm range.2 Recent efforts have been directed towards increasing sensitivities by adjusting film thickness, tailoring device structures and judicious tailoring of conjugated polymer backbones and sensing functional groups.3 Nevertheless, the role of molecular designs the mechanistic principles behind the interactions between the polymers backbones and the gases is less understood. In this work, ammonia and nitrogen-dioxide responsive polymer-based organic field-effect transistors (OFETs) are fabricated from diketopyrrolopyrrole (DPP)-fluorene (F) based polymers and room-temperature detection with high sensitivity and minimal current drifts entirely from the semiconductor was achieved. Six polymers (PF1-PF6) were employed as active layers to detect NO2 gas of concentrations 0.5-30 ppm. The polymer backbones were designed by coupling DPP to fluorene containing the linear n-octyl side-chains, spiro-fluorene, thiophene and N-methyl-N-phenylnaphthalen-1-amine bridges. The proportional on-current change of OFETs using these polymers reached over 200% for the DPP-fluorene polymer with thiophene-bridges over a concentration spanning 0.5-30 ppm of nitrogen-dioxide. From cyclic voltammetry studies, we found that the introduction of thiophene into the backbone raises the highest occupied molecular orbital (HOMO) levels. This work demonstrates the capability of increasing balancing current drifts with good sensitivity by modulating redox and aggregation properties of polymer semiconductors.
1. Benjamin, N.-Y.; Jung, A.-Ra; Yung, Y.; Ryu, Gi-Seong; Tabi, G.D.; Lee, K.-K.; Kim, B.S.; Noh, Y.-Y. ACS Mater. Interfaces 2017, 9, 7322-7330.
2. Li, H.; Dailey, J.; Kale, T.; Besar, K.; Koehler, K.; Katz, H.E. ACS Applied Mater. Interfaces 2017, 9, 20501-20507.
3. Zhou, J.; Cheng, X.-F.; Gao, B.-J.; Yu, C.; He, J.-H.; Xu, Q.-F.;Li, H.; Li, Na-Jun.;Chen, D.-Y.;Lu,J.-M. Small 2019, 15, 1803896.

Peptide based molecules have shown to interact with charged molecules, like DNA and various polar molecules. However, charge transfer through peptide thin films is not favourable. As a result, fabrication of OFET using peptide molecules are not explored. We have attempted to fabricated peptide based organic field-effect transistors combining Pentacene molecules as one of the semiconductor channel materials. In this work, we fabricated ^ArTAA_Py-Py Pentapeptide - pentacene based bilayer organic thin film transistor for volatile organic compound (VOC) vapours sensor with selectivity and enhanced sensitivity. Ultra-thin ^ArTAA_Py-Py Pentapeptide molecules of different thickness were grown using spin coating on pentacene semiconducting films. The device designs are optimized to enhance the detection performance. We have studied the morphology of the films using atomic force microscopy (AFM) and also optical properties of the films were studied using UV-Vis spectroscopy. The gas sensing performances of different thickness devices were tested for different volatile organic compound (VOC) vapours relevant to environmental monitoring, such as, ethanol, 2 propanol and acetone at room temperature (RT). It was observed that sensitivity of OFET increased when reduced the thickness of peptide film. From literature we have seen that these VOCs are generally detected at higher temperature but it is indeed interesting to emphasize that we could able to sense those gases efficiently at RT. Additionally, the OFET based devices exhibit higher selectivity, enhanced sensitivity with comparably fast response time (~ 3sec) and recovery time (~4sec). Gas sensing results confirmed that ^ArTAA_Py-Py Pentapeptide based OFETs show excellent response towards ethanol gas at room temperature.

Detection and control of petroleum contaminants resulting from oil spillage pollution activities to the environment remain a major challenge to both developed and developing countries. Generally, the crude oil spill discharge to the environment is regulated and one of the key parameters used for compliance monitoring is the measure of oil concentration. Environmental laws require oil companies to keep oil spill total hydrocarbons concentration at 50 mg/kg soil (50 ppm). Mostly, soil analyses for oil spillages use the standard gas chromatographic methods, but these methods are expensive, require high expertise and not suitable for in-situ analysis. This study was conducted to evaluate the possibility of developing low power and inexpensive chemiresistive material based sensors for in-situ detection of hydrocarbons in soils. Contaminated soils obtained at different depths from different oil spillage sites were extracted using Soxhlet technique and analysed by gas chromatography-flame ionization detector to determine types of hydrocarbons and their concentrations, as they are required to be detected by chemiresistors. The results recorded carbon numbers ranging from C₈ – C₃₆ with varying concentrations up to 20125 mg/kg at different sampling sites. Composites of non-conducting polymers (Poly(methyl methacrylate) (PMMA) and Polyvinyl chloride (PVC)) and conductive filler (carbon black) were prepared to make chemiresistors. The films were dried to evaporate the solvent and the morphology of the films was characterised using scanning electron microscopy (SEM). The impact of carbon concentration and geometry on the measured resistance of the polymer composite to hydrocarbons was determined. The optimum response was found to be with 10% w/w carbon black (CB) with 90% w/w polymer. Three sets of chemiresistors CB-PMMA, CB-PVC and CB-PMMA+PVA were constructed by depositing thin films of a carbon black/polymer onto interdigitated electrodes and investigated. The CB-PMMA sensors showed much higher responses when exposed to a range of hydrocarbons with varying sensitivities compared to the other two, however, the three sensors detected diesel range hydrocarbon concentrations up to Eicosane (C₂₀) more than the existing devices [3]. The sensors responses to the EPA maximum concentration (50mg/kg soil) limits are large (resistance changes), fast (90% in less than 1s), reversible and selective, hence validating high adaptability of this method. The underlying mechanism of this high sensitivity of sensors might be due to the strength of the hydrophobic interactions between the polymer and the hydrocarbons. The findings reported here expand the potential applications for inexpensive composite thin-film chemiresistor for oil spillage monitoring.
References
Thanh-Hai Le, Yukyung Kim, and Hyeonseok Yoon, ‘Electrical and Electrochemical Properties of Conducting Polymers’, Polymers, 9.4 (2017), 150 <https://doi.org/10.3390/polym9040150>.
Nathan S. Lewis and others, ‘Detection of Organic Vapors and NH3(g) Using Thin-Film Carbon Black-Metallophthalocyanine Composite Chemiresistors’, Sensors and Actuators, B: Chemical, 134.2 (2008), 521–31 <https://doi.org/10.1016/j.snb.2008.05.047>.
B. J. Doleman, E. J. Severin, and N. S. Lewis, ‘Trends in Odor Intensity for Human and Electronic Noses: Relative Roles of Odorant Vapor Pressure vs. Molecularly Specific Odorant Binding’, Proceedings of the National Academy of Sciences, 95.10 (2002), 5442–47 <https://doi.org/10.1073/pnas.95.10.5442>.

Endocrine disruptors, pollutants that interfere with the normal function of hormones in the body, are found nearly everywhere. These compounds are components of plastics, medications, and pesticides, and many have been found in high concentrations in water sources. Endocrine disruptors have been implicated in health problems, ranging from cancer to diabetes. However, due to the chemical dissimilarity of compounds that interact with a single hormone receptor, detecting endocrine disruptors in complex solutions has remained a challenge. We have developed a novel detection strategy for estrogenic compounds that is both rapid and portable. Our platform is based on E. coli engineered to surface express a native estrogen receptor construct. These E. coli enable the detection of many dissimilar compounds with inherent signal amplification from impedance measurements due to their binding to a modified electrode. We have detected sub-ppb levels of the native hormone estradiol and ppm levels of the endocrine disruptor bisphenol A (BPA) in complex solutions. As our system reports the total estrogenic activity of a sample rather than the concentration of specific components, we can measure the activity of unknown compounds, including chemicals released from a BPA-free plastic baby bottle following microwave heating. Importantly, this approach is broadly applicable to the detection of chemically diverse compounds that bind to a single receptor.

Our research of the last 10 years has been focused on the implementation of new approaches to improve power consumption. In this presentation we will review our main contributions in the context of the state-of-the-art.

First, we will show how power consumption in semiconductor devices can be lowered to just a few microwatts by means of the self-heating effect occurring in nanomaterials [1]. Only a decade ago, this principle was proved with fully hand-made devices [2]. Today, it is possible to achieve comparable efficiencies with devices produced in mass scale, using widely spread micro and nanofabrication techniques.

Second, we will move to light activated chemical sensors [3], where dramatic power savings can be achieved by combining the power efficiency of light emitting diodes (LED) with aggressive miniaturization efforts. Using industry standard technologies, it is possible to offer sub-milliwatt power demands in monolithic integrated microLED devices that can be produced in large amounts. We will also show how optical activation opens the door to complementary operation approaches, based on light energy harvesting that can enable virtually zero-power devices in the near future [4].

References
1. Fàbrega, C.; Casals, O.; Hernández-Ramírez, F.; Prades, J. D. A review on efficient self-heating in nanowire sensors: Prospects for very-low power devices. Sensors Actuators, B Chem. 2017, 256, 797–811, doi:10.1016/j.snb.2017.10.003.
2. Prades, J. D.; Jimenez-Diaz, R.; Hernandez-Ramirez, F.; Barth, S.; Cirera, A.; Romano-Rodriguez, A.; Mathur, S.; Morante, J. R. Ultralow power consumption gas sensors based on self-heated individual nanowires. Appl. Phys. Lett. 2008, 93, 123110, doi:10.1063/1.2988265.
3. Markiewicz, N.; Casals, O.; Fabrega, C.; Gràcia, I.; Cané, C.; Wasisto, H. S.; Waag, A.; Prades, J. D. Micro light plates for low-power photoactivated (gas) sensors. Appl. Phys. Lett. 2019, 114, 053508, doi:10.1063/1.5078497.
4. Hoffmann, M. W. G.; Gad, A. E.; Prades, J. D.; Hernandez-Ramirez, F.; Fiz, R.; Shen, H.; Mathur, S. Solar diode sensor: Sensing mechanism and applications. Nano Energy 2013, 2, 514–522.

Modern gas-monitoring requirements for numerous demanding applications push the limits of existing detection concepts to the point where we may reach their fundamental performance limits. Thus, without violating the laws of physics, chemistry, and electronics and without crossing the border into the 21st century science fantasy, we need to develop new analytical concepts and instruments. This talk will stimulate your scientific and engineering senses by (1) posing several fundamental and practical questions on principles of gas sensing and (2) by demonstrating on how modern multidisciplinary research addresses these questions in the developments of sensors with previously unthinkable capabilities. We will discuss new sensor-design criteria that allow multi-gas detection with individual sensors and the key roles of advanced sensing materials, transducers, and data analytics to achieve desired sensor performance. These developed multi-gas sensors are attractive when selectivity advantages of classic analytical instruments are cancelled by requirements for no consumables, low power, low cost, and unobtrusive form factors. We will conclude with a perspective for future needs in fundamental and applied aspects of gas sensing and with the 2030 roadmap for ubiquitous gas monitoring.

The capability of monitoring, collecting, transferring, analyzing various environmental information in nature enables hyper-connected society around the rural and urban area. Among the related device fabrication technologies, the development of membrane-type electronic devices and transfer printing onto desired surfaces is very important to provide electronic functions to the desired surfaces, which is so-called ‘Stick-&-Play’ system. The transfer printing naturally confronts the controversial issues between sufficient interfacial adhesion and environmental friendliness. For example, the use of too much glue to accommodate the surface mismatch enhances the adhesion but tends to lose the latter whereas simple laminating of a planar substrate with no chemical suffers from easy delamination. In this study, we introduce a nanotubular cilia structure underneath the substrate to allow conformal wrapping on complex surfaces. After transfer printing on the polymer substrate, the nanotubular structures undergo omnidirectional conformal wrapping by flattening motion to have a large contact area and thereby enhance the interfacial adhesion of the polymer substrate. The mechanical experiments with simulations confirm that the polymer substrate with highly deformable nanotubular structure strongly adheres to the target surfaces under harsh conditions which may occur in nature, such as wind and rain. Finally, we demonstrate successful transfer printing with a temperature sensor and thin film transistors on various surfaces including an eggshell, textile, and a stone.

The current concern about toxic and harmful gases is substantiate in a great number of related health problems and in the unceasing global warming. Gas sensors systems are used to survey these gas species, avoiding dangerous situations and controlling the emitting sources. A low-cost choice that offers high sensitivity performances towards several types of gases of interest are the solid-state gas sensors, including devices based in metal oxide semiconducting nanowires, presenting remarkable performance in terms of sensitivity and response time, but lacking in selectivity.
To compensate the lack of selectivity of this type of sensors to discriminate between various dangerous gases, the electronic nose configuration is used and, in this work, the use of site-selective growth and in-situ integration nanowires of different materials, is proposed. Tin oxide, tungsten oxide and germanium nanowires are grown on different membranes of a single chip surface using a chemical vapor deposition method modification to characterize them later towards carbon monoxide, nitrogen dioxide and water vapor diluted in dry synthetic air. To study the sensing behavior of the sensor arrays, with various sensing patterns arising from the differences between materials, the sensors are exposed to mixtures of the mentioned gases, mimicking real ambient scenarios.
Each of the studied nanowires are sensitive to all the exposed gases, compromising the ability of the fabricated sensing systems to distinguish the composition of gas mixtures. However, the measured set of responses from the different materials was studied using the well-known principal component analysis representation that allows to distinguish between all three studied analytes by pattern recognition. Moreover, the system is able to work under high relative humidity concentration (70 %), still maintaining the sensitivity towards nitrogen dioxide, proving the potentiality of the here-proposed approach.

Triboelectric nanogenerators (TENGs) based on the coupling effect of triboelectrication and electrostatic induction have been developedto be a promising strategy to harvest mechanical energy and convert them to electricity. Traditional TENG based self-powered sensing systems have been demonstrated by measuring the triboelectric effect of the sensing materials altered by the external stimulus. However, the limitations of triboelectric sensing materials and instable outputs caused by ambient environment significantly restrict their practical applications. In this talk, I will illustrate the impedance matching effect based self-powered sensing process which can avoid the external impacts. Since a TENG is intrinsically a capacitor, while connecting with an external load, the output current from the TENG will decrease with the increment of the load resistance. By using a traditional sensor as the external load of TENGs, a self-powered sensing system can be achieved through monitoring the variation in the current or voltage signals. Several powered systems will be introduced in this talk, such as real-time ultraviolet photodetector, vehicle emission testing system, self-powered weighing/pressure system, on-line ion concentration monitor, etc. This novel self-powered sensing system is not affected by working frequency and requires no external power supply, which is favorable to improve the stability and reliability for practical application.

We report on an inexpensive and very selective gas sensor implemented by simply combining colorimetric indicators cast on top of Scotch tape, with a commercial microchip adapted here to measure optical reflectance. This sensor can be easily reproduced (leading to quantitatively consistent results), refreshed and reconfigured to sense different target gases (CO₂, NH₃) just replacing the colorimetric tape. Colorimetric methods provide many compounds and reaction mechanisms to detect a wide variety of gaseous molecules. These techniques offer unprecedented levels of selectivity and specificity towards the target species. Regarding their readout, spectrophotometers have been traditionally the technique to measure the light absorption spectra of these indicators, but they are costly and bulky. To achieve a continuous readout and compact form factors, development focused on miniaturized systems that measure color changes at specific spectral ranges. Many different configurations have been proposed so far, but most are based on confronting a narrow-spectrum light source with a photodetector. To maximize their sensitivity, enlarging the optical path from the emitter to the detector through the indicator medium is the preferred option. Therefore, the integration of the different components is challenging. Furthermore, the efforts in miniaturization complicate refreshing the indicator substances, which are prone to degradation over time. In summary, the continuous readout of colorimetric indicators still raises a set of inconveniences.
Here, we propose an approach to colorimetric detection of gases with the following advantages: it is based on easily accessible, off-the-shelf commercial components; it is compatible with a wide range of colorimetric indicators, operating at different wavelengths; it is easily resettable/refreshable with good stability in a time frame of weeks, and offers an excellent repeatability among devices.
Our gas sensor is based on the MAX30105 component. This integrated microsystem encloses a set of 3 internal LEDs (red, green and infrared), a broadband photodiode, and the corresponding control, driving, acquisition and communication modules. We use this device to excite and monitor the indicator in a reflection configuration.
The sensor has a cost of less than 4$ and can be easily replicated, with excellent reproducibility. The device operates at different wavelengths and with moderate power requirements (starting from a few mW). The approach has also the advantage of being easily resettable/refreshable, as the gas sensitive color indicator layer can be easily removed and replaced by a new one. This also allows for addressing other target gases, as simple as just replacing the indicator, and reusing the readout chip. Therefore, the proposed principle offers a good trade-off between cost, simplicity, convenience, and good gas sensing performance; circumventing some of the limitations of colorimetric indicators in long term operation.

Earthquakes are lethal natural disasters frequently burying people alive under collapsed buildings. Tracking entrapped humans from their unique volatile chemical signature with hand-held devices would accelerate urban search and rescue (USaR) efforts.¹ Here, a compact and orthogonal sensor array has been designed to detect the breath- and skin-emitted metabolic tracers acetone, ammonia, isoprene, CO₂ and RH, all together serving as sign of life. It consists of three nanostructured metal-oxide sensors (Si-doped WO₃², Si-doped MoO₃³ and Ti-doped ZnO⁴), each specifically tailored at the nanoscale for highly sensitive and selective tracer detection along with commercial CO₂ and humidity sensors. When tested on humans enclosed in plethysmography chambers to simulate entrapment, this sensor array rapidly detects tracers of human presence with low parts-per-billion (ppb) level accuracy and precision, unprecedented by portable detectors but required for USaR.⁵ These results were validated by bench-top selective reagent ionization time-of-flight mass spectrometry (SRI-TOF-MS). As a result, an inexpensive nanostructured sensor array is presented that can be integrated readily into hand-held or even drone-carried detectors for first responders to rapidly screen affected terrain.


(1) Mochalski, P.; Unterkofler, K.; Teschl, G.; Amann, A., Trac-Trend Anal Chem 2015, 68, 88-106.
(2) Righettoni, M.; Tricoli, A.; Gass, S.; Schmid, A.; Amann, A.; Pratsinis, S. E., Anal Chim Acta 2012, 738, 69-75.
(3) Güntner, A. T.; Righettoni, M.; Pratsinis, S. E., Sensor Actuat B-Chem 2016, 223, 266-273.
(4) Güntner, A. T.; Pineau, N. J.; Chie, D.; Krumeich, F.; Pratsinis, S. E., J. Mater. Chem. B 2016, 4 (32), 5358-5366.
(5) Güntner, A. T.; Pineau, N. J.; Mochalski, P.; Wiesenhofer, H.; Agapiou, A.; Mayhew, C. A.; Pratsinis, S. E., Anal. Chem. 2018, 90 (8), 4940-4945.

Föster Resonance Energy Transfer (FRET) is a powerful technique used to probe close-range molecular interactions. Physically, the FRET phenomenon manifests as a dipole-dipole interaction between closely juxtaposed fluorescent molecules (10–100 Å). For instance, after photoexcitation, a fluorophore may de-excite through direct emission with a bathochromic spectral shift. However, in the presence of a nearby acceptor, the donor non-radiatively transfers energy to the acceptor molecule, resulting in quenched donor fluorescence. With the advent of genetically encoded fluorescent molecules, this method has found widespread biological applications as a spectroscopic atomic-scale ruler, biochemical reaction kinetics and chemical sensor. [1] Our effort is to employ this FRET technique to make a prototype device for highly sensitive detection of environmental pollutants.
Among the most common environmental pollutants, nitroaromatic compounds (NACs) are of particular interest because of their durability and toxicity. That’s why, sensitive and selective detection of small amounts of nitroaromatic explosives, in particular, TNP, DNT and TNT has been a key challenge due to the increasing threat of explosive-based terrorism and the need of environmental monitoring of drinking and wastewater. In addition, the excessive utilization of TNP in several other areas such as burn ointment, pesticides, glass and the leather industry resulted in environmental accumulation and is eventually contaminating the soil and aquatic systems. To date, A great number of elegant methods, including fluorimetry, gas chromatography, mass, ion-mobility and Raman spectrometry have been successfully applied for explosive detection. Among these efforts, fluorescence-quenching methods based on the mechanism of FRET show good assembly flexibility, high selectivity and sensitivity. [2]
Here, we report a FRET-based sensor system for the highly selective detection of NACs, such as TNP, DNT and TNT. The sensor system is composed of a copolymer Poly[(N,N-dimethylacrylamide)-co-(Boc-Trp-EMA)] (RP) bearing tryptophan derivative in the side chain as donor and dansyl tagged copolymer P(MMA-co-Dansyl-Ala-HEMA) (DCP) as an acceptor. Initially, the inherent fluorescence of RP copolymer is quenched by non-radiative energy transfer to DCP which only happens once the two molecules are within Förster critical distance (R₀). The excellent spectral overlap (J_λ= 6.08×10¹⁴ nm⁴M^-1cm^-1) between donors’ (RP) emission profile and acceptors’ (DCP) absorption profile makes them an exciting and efficient FRET pair i.e. further confirmed by the high rate of energy transfer from RP to DCP i.e. 0.87 ns^-1 and lifetime measurement by time correlated single photon counting (TCSPC) to validate the 64% FRET efficiency. This FRET pair exhibited a specific fluorescence response to NACs such as DNT, TNT and TNP with 5.4, 2.3 and 0.4 μM LODs, respectively. The detection of NACs occurs with high sensitivity by photoluminescence quenching of FRET signal induced by photo-induced electron transfer (PET) from electron-rich FRET pair to electron-deficient NAC molecules. The estimated stern-volmer constant (K_SV) values for DNT, TNT and TNP are 6.9 × 10³, 7.0 × 10³ and 1.6 × 10⁴ M^-1, respectively. The mechanistic details of molecular interactions are established by time-resolved fluorescence, steady state fluorescence and absorption spectroscopy confirmed that the sensing process is of mixed type, i.e. both dynamic and static quenching as the lifetime of FRET system (0.73 ns) is reduced to 0.55, 0.57 and 0.61 ns DNT, TNT and TNP, respectively.
In summary, the simplicity and sensitivity of this novel FRET sensor open up the possibility of designing an optical sensor of various NACs in one single platform for designing a multimodal sensor for environmental monitoring and future field-based study.
Reference
1. Jares-Erijman, E.A. et al. Nat. Biotechnol. 2003, 21, 1387.
2. Sun, X. et al. Chem. Soc. Rev. 2015, 44, 8019.

Analyte sensitivity for gas sensors based on semiconducting metal oxides should be highly dependent on the film thickness, being particularly high when that thickness is on the order of the Debye length (few- to tens- nanometres); this thickness dependence has previously been demonstrated for SnO₂. The use of ultra-thin films also has potential for understanding the fundamental sensing properties of a particular material, minimising the thermal degradation of analytes likely when using commercial thick film sensors. We have prepared ultra-thin films of TiO₂ by atomic layer deposition (ALD) and NiO using chemical vapour deposition (CVD) to compare fundamental sensing properties of these prototypical examples of n-type and p-type semiconductors. The depositions were performed on standard alumina gas sensor platforms. The films were exposed to different concentrations of CO, CH₄, NO₂, NH₃ and SO₂ to evaluate their gas sensitivities. These experiments showed that the TiO₂ film thickness played a dominant role within the conduction mechanism and the pattern of response for the electrical resistance towards CH₄ and NH₃ exposure indicated typical n-type semiconducting behaviour. The effect of relative humidity on the gas sensitivity has also been demonstrated, with NiO showing little interference.

Micro and nanofabricated structures with very low thermal mass and large surface-to-volume ratio offer many new approaches for chemical and biological sensing with high sensitivity and high selectivity. Fabricating the structures with wide band gap materials allows manipulation of surface states for sensor applications. Wide band gap materials have high density of surface states in their band gap, which are filled up to the Fermi level. By introducing surface defects, it is possible to control and manipulate the chemical, electrical, optical, and magnetic properties of nanostructures. Together with the very low thermal mass of the system, surface defects offer a unique opportunity for modulating the physical properties for developing multi-physics, multi-modal sensors with very high selectivity. Recently we have demonstrated nanowire chemical sensors where signal transduction and readout exploit modulation of surface states. While the science underlying role of surface states on chemical-to-thermal-to-electrical energy transduction is still being understood, the technological potential can be truly realized if multiple interactions can be detected simultaneously. This talk will focus on both the scientific understanding as well as the technological progress in the development of micro/nanostructures for physical, chemical and biological sensing. Recent results for achieving high selectivity in chemical and biological detection using multi-physics approach will be presented.

The growing demand for environmental control for a variety of chemical molecules has led to considerable interest in the research devoted to the development of new materials for sensor devices. Since humidity is a very common component in our environment, measurements and/or control of humidity are important not only for human comfort but also for a broad spectrum of industries and technologies. The constructive design of a good humidity sensor is a rather complicated topic, because high performance humidity sensors claim many requirements, including linear response, high sensitivity, fast response time, chemical and physical stability, wide operating humidity range and low cost.
Zeolites and zeo-type materials are known for their well-defined porous structure, moisture holding capacity and ionic conduction properties. Therefore, a study is done to investigate humidity sensing properties of microporous titanosilicate and vanadosilicate thin films. Two different zeo-type materials with different type of quantum wires in their structures (i.e.,–Ti-O-Ti-O-Ti- and –V-O-V-O-V- in the structures of ETS-10 and AM-6, respectively) were produced and the humidity sensors by using these types of films were fabricated for the first time. Developed humidity sensors are examined by impedance spectroscopy method. By this means, conductive properties of the films dependent on the relative humidity will be revealed. The results showed that stable, sensitive, and cheap resistive type humidity sensors that operate in a wide range of relative humidity can be designed by using titanosilicate ETS-10 and Vanadosilicate AM-6 films.

Acknowledgment: This study is supported by Scientific and Technological Research Council of Turkey (TUBITAK), with the project number 118M631.

Semiconductor nanowires have received much recent attention in the fields of chemical sensing and catalysis due to their high aspect ratio. The confinement of charge carriers to one dimension is ideal for environmental charge-based detection since no classical conducting paths exist where local perturbations at the surface are not felt by the electron gas, provided the nanowire diameter is similar to or less than the electronic screening length. Such a system is realized in InAs nanowires where the high electron mobility, low effective mass, and dielectric confinement of the electron gas leads to weakened screening and high charge sensitivity [1]. Indeed, InAs nanowires have demonstrated extraordinarily high charge sensitivity at room temperature [2], and thus present an excellent material system in which to probe and study the behaviour of molecules at their surfaces. In low-dimensional systems such as InAs nanowires, surface states play a dominant role in dictating transport properties. InAs nanowires have been shown to contain high densities (~ 10¹³ cm^-2) of slow surface traps originating from the native surface oxide [3]. These dynamic surface state capture and emission processes exhibit time constants on the order of seconds to several hours and dictate the time-dependent conductivity of the material in response to external stimuli [4]. Dynamic capture and emission from surface states is thus the primary mechanism determining the transport and chemical sensing behaviour.

So far, nanowire-based environmental gas sensors have relied on slow, activated processes restricting their applicability to high temperatures and macroscopic adsorbate coverages (> 1 ppm). Here, we demonstrate the ppb-level detection capabilities of InAs nanowire-based chemical sensors towards volatile organic compounds (VOCs) at room temperature. The sensors are based on globally back-gated field-effect transistors with channels consisting of parallel arrays of thousands of MBE-grown InAs nanowires transferred by a mechanical contact printing method [5]. We present a model for non-equilibrium carrier dynamics based on thermally activated capture and emission from surface states to provide insight into the chemical sensing behaviour observed in our devices and discuss factors that can be used to tune the sensitivity and dynamic range.

References:
[1] J. Salfi et al. PRB 85, 235316 (2012)
[2] J. Salfi et al. Nature Nano 5, 737 (2010)
[3] D. Lynall et al. Nano Letters 16, 6028 (2016)
[4] D. Lynall et al. Nano Letters 18, 1387 (2018)
[5] Z. Fan et al. Nano Letters 8, 20 (2008)

Owing to their high efficiency, bandgap tunability, and potential to be fabricated at low-cost on many different substrate types, perovskite photovoltaic cells present an exciting opportunity as power sources in many autonomous systems. In this work, we evaluate their suitability as power sources for wireless sensors located in buildings and harvesting only ambient light. Wide-bandgap 1.63 and 1.84 eV perovskite photovoltaic cells are fabricated for indoor-light harvesting with measured efficiencies of 21% and 18.5%, respectively, under low-intensity compact fluorescent lighting. We increase the Br content in our (Rb_0.01Cs_0.05)(MA_xFA_1-x)_0.94Pb(Br_xI_1-x)₃ composition to produce an 1.84 eV cell, that achieves a high open-circuit voltage of 0.95 V cell under a light intensity as low as 0.16 mW/cm². To demonstrate the application of these cells as power sources for Internet of Things (IoT) nodes, three perovskite photovoltaic cells are connected in series to create a module that produces 14.5 mW output power under 0.16 mW/cm² of compact fluorescent illumination, with an efficiency of 13.2%. We create a self-powered sensor, by using this indoor IoT power module as an external power source for a semi-passive RFID temperature sensor. The combination of perovskite indoor photovoltaic modules and backscatter radio-frequency sensors is further discussed as a route to ubiquitous sensing in buildings, given the potential of all components to be manufactured in an integrated manner on plastics.

Atomically thin black phosphorus (BP), a new family member of the two-dimensional (2D) material, has attracted considerable attention in these years due to its exceptional electrical, mechanical, and surface properties offering great potential in a variety of applications. However, synthesis, dimensional control, particularly the thickness, and even the preservation of BP remain the challenges since BP is extremely sensitive to humidity, which would lead to severe structural damage. In this work, extremely-high-energy ultrasonic exfoliation was performed not only for effectively fabricating dimension-controlled BP nanosheets (~1 nm to 250 nm and ~8 nm to 10 μm in thickness and width, respectively) but for substantially reducing the risk of overexposure to harmful organic solvents. The well-clarified BP nanosheets were then made into single-nanosheet and nanoassembled chemical sensors for evaluating their performance and stability in detecting humidity.

Rare earth elements (REEs) are critically important to numerous advanced materials, including electronics, magnets, catalysts, phosphors, and others, leading to their use in applications including catalytic converters, permanent magnet based motors, turbines, rechargeable batteries, petroleum refinement, and lighting displays, among others. The attainment of a stable supply of REEs is also a vital goal of national security policies due to their use in advanced technologies, including defense systems. Yet factors including monopolistic conditions, expensive separation of co-mined REE ores, and environmental concerns as a result of REE extraction have hindered the attainment of this goal. Consequently, significant effort has been devoted to increased REE domestic production, including the extraction of REEs from coal, coal combustion byproducts, and their associated waste streams such as acid mine drainage. Analytical techniques for rapid quantification of REE content in aqueous phases can facilitate REE recovery through rapid identification of high-value waste streams. Here, the metal-organic framework BioMOF-100 is used as a fluorescent-based sensitizer for emissive REE ion detection in water, providing rapid (<10 minute) analysis times and sensitive detection (part-per-billion detection limits) for terbium, dysprosium, samarium, europium, ytterbium, and neodymium, even in the presence of acids or secondary metals in aqueous conditions or following REE extraction into organic solvents. Taken together, the BioMOF-100 system is a promising step towards the rapid, sensitive detection of valuable REEs in real-world waste streams.

Water quality monitoring is essential for providing clean and safe drinking water to the public as well as for ecological safety. Recent advances in miniaturization and microbial fuel cells (MFCs) have shown great promise for the monitoring of toxic compounds in water. In this work, we demonstrate a novel paper-based, single-use, MFC-based biosensor for rapid detection of formaldehyde in water. The MFC-based biosensor was created by combining two layers of paper for a low-cost, disposable, and simple structure. Shewanella oneidensis MR-1 was pre-inoculated in the anodes and was left to be air-dried at room temperature for bacterial accumulation and for long-term storage and portability. When a drop of water sample (100mL) with formaldehyde was introduced to this MFC biosensor, rapid and sensitive voltage responses were obtained over a concentration range of 0.001% to 0.02%. To compensate for any external factors and variations among different devices, the inhibition ratio of each device before and after drying were obtained and compared. The inhibition ratio increased proportionally with the formaldehyde concentration giving R² of 0.908.

Due to the recent advances in rapid urbanization, industrialization, and excessive farming, providing access to clean water has become one of the greatest challenges in the coming decades for both developing and developed countries. To maintain safe water quality, there is an urgent need for a rapid and portable sensor for on-site and real-time measurements of toxic components in water. Conventional techniques for water monitoring allow for accurate and sensitive detection of various chemicals, but they are time-consuming, costly, and require a wide range of external equipment which may not be easily portable. Recently, MFCs have shown great potential as generic biosensors for water quality monitoring. MFCs utilize microorganisms to breakdown organic substrates to produce electrical energy. When these microorganisms are exposed to toxic components, their metabolic activities can be inhibited, thus decreasing their electron transfer reactions. Therefore, the changes in the output voltage or current of an MFC can be used to qualitatively or quantitatively measure toxic components in water. The paper-based MFCs proposed here have several advantages over the traditional MFCs in that they are cost-effective, flexible, biodegradable, and easily portable. They do not require any other external equipment, such as external pumps, which make these paper-based MFCs a truly stand-alone and self-sustainable device. Additionally, pre-inoculated and air-dried bacteria on the paper anode allow for long-term storage and on-site measurements. The dried bacteria can easily resume their biological activities by rehydrating them with a drop of water sample to be tested.

Formaldehyde was chosen as a model toxin because it has distinct biological toxicity and stability with other chemicals in media. The EPA has set the threshold limit for formaldehyde to be 10 ppm (equal to 0.001% v/v). As such, the proposed MFC-based biosensor would be suitable for the monitoring of formaldehyde in water. The MFC-based biosensor was made by combining two layers of paper: one layer for the anode and the other layer for cathode and proton exchange membrane (PEM). The PEM and the boundaries of the anode and cathode were defined by printing wax onto the papers. The functional and hydrophilic areas of anode and cathode were made conductive by injecting poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS). Furthermore, the cathode was treated with Ag₂O to act as a solid electron acceptor.

The extensive use of chromium in steel manufacturing, leather tanning, metal finishing, electroplating, and alloy manufacturing has made the element commercially important. The biologically stable and environmentally non-toxic trivalent chromium is considered to be an essential nutrient in humans as it regulates the effect of insulin in lipid and carbohydrate metabolism. The other kinetically stable form of chromium, Cr(VI) is recognized as a toxic substance. However, its thermodynamic instability results in spontaneous reduction in presence of reducing agents to the ultimate trivalent chromium ion.
In the recent years, several electroactive materials have been employed to probe Cr(VI) ion. 4-dimethylaminoazobenzene, 1-(2-(1H-imidazole-1-yl)-1-(4-methoxyphenyl)ethylidene)-2-phenyl hydrazine, 2-acetylpyridine, p-(4-Acetanilidazo)calix[4]arene, glyoxal bis(2-hydroxyanil), Aurin TCA, tributyl phosphate are some of them. Poly-L-histidine has been successfully used to detect Cr(III) and Cr(VI) by electrochemical methods. Despite the presence of the above molecular designs, the quest for an efficient , portable and miniature sensing device remains unaccomplished.
The present work reports the synthesis of a guanidine based small organic molecule which has exhibited potential to detect hexavalent chromium from contaminated water samples. The chemiresistor fabricated herein has shown selectivity towards chromium over other interfering anions such as carbonate, sulfate, phosphate, fluoride and chloride. The biguanide molecule also reveals fluorescence quenching in the presence of hexavalent chromium.
The bio-mimicking nature of the organic molecule presented herein is manifested by the nature of complexation of hexavalent chromium to the guanidine derivative. The thermodynamically stable chromium-histidine complex in mammalian cell shows the use of hydroxyl and amine groups acting as active sites to co-ordinate to the chromium ion. The guanidine derivative being a small molecule requires the use of conductive fillers such as multi-walled carbon nanotubes (MWCNTs) to be used as an electroactive sensing material in the chemiresistor fabricated in this work. The chemiresistor evaluation shows repeatability, reproducibility, sensitivity, selectivity and a lower limit of detection of 1 ppm.

Nanomaterials occur as a result of natural events such as forest fires, ocean spray, volcanic activity, and dust storms. Engineered nanomaterials were created to exploit their enhanced properties. They have found a myriad of applications as components of electronic devices, sensors, commodity chemicals, therapeutics and even food packaging. Their increased use on consumer products has raised health and safety concerns. However, studies on the short and long term of nanomaterials toxicity are limited due to the large variety of nanomaterials prepared and commercialized. Furthermore, their physicochemical properties have shown to largely influence their interactions with living organisms, their toxicity, accumulation and elimination pathways. The large variety of existent nanomaterials turns the investigation of their specific adverse health effects into an insurmountable task. Thus, unintentional exposure to nanomaterials is a concern for workers involved in the manufacturing and handling of nanomaterials.
The development of methodologies to protect workers health is now a necessity. Our work has focused on the development of analytical methodologies to assess the presence of engineered nanomaterials on workplace surfaces. Methodologies for detection using X-ray fluorescence spectroscopy and inductively coupled plasma mass spectrometry were used to detect traces of nanomaterials on laboratory countertops. The results of our research are intended to inform and enable scientist to continue innovating in the field of nanomaterials in a safe manner. We have shown that protecting workers from unintentional exposure can be achieved by adopting simple modifications to existent hygiene habits.

Aerosols consist of two systems, solid and particles which are suspended in gas. These days, there are many harmful aerosols, which badly affect to human health such as yellow dust, air pollution, bioaerosol like a fungus, even fine particle which can deposit human’s lung. These aerosol particles have a many toxic factors which can affect to the health of humans, animal even plants. Especially, in urban areas, many pollution factors that from automobile and buildings are exist almost everywhere. Because of this, monitoring and analysis of these harmful aerosols are very important for human health in urban areas. In order to measure and monitored these aerosols, we were researching about aerosol sampling technology and analysis using by simulation. There are many aerosol sampling methods such as particle measurement using optical principle which is using optical aerodynamic to measure the relaxation time, also, scattering size of particle about light sources and measurement of scattering and absorption. In this study, Autodesk Inventor and FLUENT 16.1 will use for the design of the new aerosol sampler. The basic design of designing an aerosol sampling device is cyclone sampler which generates a cyclone inside the tube. The basic theory of this aerosol sampling device is Stokes number and Reynolds number. Using these theories, we can derive an equation which is can get a proper size of the aerosol sampling device. Also, we will use a FLUENT 16.1 program for numerical analysis and simulation for designing an aerosol sampling device as a proper size. In the experiment, we can measure the particle use a Raman spectroscopy, Ion trap mass spectroscopy, Induced Fluorescence. In this study, we will collect aerosols on the outside using by existing aerosol sampling device before the development of aerosol monitoring device, and we are planning to use an aerosol sampling device for aerosol collection, which is such as ELPI (Electrode Low Pressure Impactor), CPC (Condensation Particle Counter) and SMPS (Scanning Mobility Particle Sizer). These aerosol sampling devices are not only collecting various aerosol, but also, can evaluate a collected aerosol in real time. Therefore, we expect to collect various aerosol samples in outside conditions like an environmental condition. After this process, we expect we can get an aerosol number concentration, aerosol particle size, aerosol flow rate and so on that can use for evaluating which aerosol suspended in the environmental air. Also, it can classify sampled aerosol which aerosol is more harmful, and more badly affects to human health. Furthermore, it can not only classify sampled aerosol, but also can conjecture which aerosol is coming from which factor such as automobile or building and so on. In our plan, we expect can show the sampled aerosol in outside condition and results of classified various sampled aerosols. Through these experiments we can show various results of aerosol sampling and monitoring data on the environmental condition. In addition, according these results we will development of aerosol monitoring systems for monitoring air quality of urban areas.