Symposium BM6 : Fabrication, Characterization and Applications of Bioinspired Nanostructured Materials

Detecting target analytes with high specificity and sensitivity in any fluid is of fundamental importance to analytical science and technology. Surface-enhanced Raman scattering (SERS) has proven to be capable of detecting single molecules with high specificity, but achieving single-molecule sensitivity in any highly diluted solutions remains a challenge. Here we demonstrate a universal platform that allows for the enrichment and delivery of analytes into the SERS-sensitive sites in both aqueous and nonaqueous fluids, and its subsequent quantitative detection of Rhodamine 6G (R6G) down to ∼75 fM level (10⁻¹⁵ mol/L). Our platform, termed slippery liquid-infused porous surface-enhanced Raman scattering (SLIPSERS), is based on a slippery, omniphobic substrate that enables the complete concentration of analytes and SERS substrates (e.g., Au nanoparticles) within an evaporating liquid droplet. Combining our SLIPSERS platform with a SERS mapping technique, we have systematically quantified the probability, p(c), of detecting R6G molecules at concentrations c ranging from 750 fM (p > 90%) down to 75 aM (10⁻¹⁸ mol/L) levels (p ≤ 1.4%). The ability to detect analytes down to attomolar level is the lowest limit of detection for any SERS-based detection reported thus far. We have shown that analytes present in liquid, solid, or air phases can be extracted using a suitable liquid solvent and subsequently detected through SLIPSERS. Based on this platform, we have further demonstrated ultrasensitive detection of chemical and biological molecules as well as environmental contaminants within a broad range of common fluids for potential applications related to analytical chemistry, molecular diagnostics, environmental monitoring, and national security.

Keywords: SERS | slippery surfaces | nanoparticles

References
1. S. M. Nie & S. P. Emory, Probing single molecules and single nanoparticles by surface enhanced Raman scattering. Science 275, 1102 – 1106 (1997).
2. T.-S. Wong, S. H. Kang, S. K. Y. Tang, E. J. Smythe, B. D. Hatton, A. Grinthal & J. Aizenberg, Bioinspired self-repairing slippery surfaces with pressure-stable omniphobicity. Nature 477, 443 – 447 (2011).
3. S. Yang, X. Dai, B. B. Stogin, & T.-S. Wong, Ultrasensitive surface-enhanced Raman scattering detection in common fluids. Proc. Natl. Acad. Sci. USA 113, 268 – 273 (2016).

Nature-inspired liquid-repellent surfaces are primarily modeled after two classes of biological surfaces – leaves of lotus¹ and pitcher plant². Lotus leaves rely on air-infused textured surfaces to repel impinging liquid droplets¹; while the leaves of pitcher plant utilize liquid-infused textured surface to maintain a highly slippery interface³. Natural and synthetic surfaces that can switch between these two liquid-repellent states are rare due to the distinctive repellent mechanisms. Here, we demonstrated a magnetically shape-shifting surface that can reversibly transform the liquid-repellent states between the modes of lotus leaves and pitcher plant. The surface property change can be programmed on-demand by external magnetic field. The ability to alter surface interfacial properties dynamically will open up new opportunities for smart liquid-repellent skin, programmable fluid control and transport, adaptive drag reduction, and controlled-release devices.

References
1. Barthlott, W. & Neinhuis, C. Purity of the sacred lotus, or escape from contamination in biological surfaces. Planta 202, 1-8, doi:DOI 10.1007/s004250050096 (1997).
2. Bohn, H. F. & Federle, W. Insect aquaplaning: Nepenthes pitcher plants capture prey with the peristome, a fully wettable water-lubricated anisotropic surface. Proc. Natl Acad Sci USA 101, 14138-14143, doi:10.1073/pnas.0405885101 (2004).
3. Wong, T. S. et al. Bioinspired self-repairing slippery surfaces with pressure-stable omniphobicity. Nature 477, 443-447, doi:10.1038/nature10447 (2011).

To a certain degree, it is possible to control the macroscopic wetting properties of a surface by its nano- and microstructure. In particular, super liquid-repellant-surfaces have received interest due to their many potential applications, such as anti-fouling for for example. Super liquid-repellency can be achieved by nano- and microstructuring a low energy surface in a way, that the structure can entrap air underneath
the liquid. The common criteria for super liquid-repellency are a high apparent advancing contact angle and a low contact angle hysteresis.
For a better understanding of how a drop advances and recedes on such a structured surface, we imaged the motion of a water drop on a superhydrophobic array of micropillars by laser scanning confocal microscopy (LSCM). With LSCM, we imaged an advancing water front on a superhydrophobic surface at a resolution of 1 µm. The results give a qualitatively new picture of how water advances on the microscopic
scale. We demonstrate that in contrast to traditional goniometer measurements, the advancing contact angle is close to 180° or even higher.
In contrast, the apparent receding contact angle is determined by the strength of pinning. We propose that the apparent receding contact angle should be used for characterizing super liquid-repellent surfaces [1,2].

[1] F. Schellenberger et al., Phys. Rev. Lett. 116, 096101 (2016)
[2] P. Ball, Nature Materials 15, 376 (2016)

Multiscale hierarchical structures show engineered interfacial properties that are important for controlled wetting, structural color, and selective filtration. In particular, bioinspired three-dimensional (3D) substrates have achieved such properties with superior mechanical stability over large areas (>cm² ). The fabrication of 3D patterns with length scales spanning several orders of magnitude (e.g., nm to μm), however, is usually done with complex top-down processes such as multistep photolithography or imprinting. Moreover, these tools cannot easily manipulate order/disorder of multiscale features over large areas. Here we found that memory-based, sequential wrinkling process can transform flat polystyrene (PS) sheets into bioinspired, three-dimensional hierarchical textures. Multiple cycles of plasma-mediated skin growth followed by directional strain relief of the substrate resulted in hierarchical architectures with characteristic generational (G) features. Independent control over wrinkle wavelength and wrinkle orientation for each G was achieved by tuning plasma treatment time and strain-relief direction for each cycle. As a practical application, we demonstrated stretchable superhydrophobicity on elastomeric hierarchical wrinkles monolithically formed by high fidelity pattern trasfer of PS templates designed by the sequential wrinkling. The poly(dimethysiloxane) (PDMS) wrinkles consisting of three different length scales showed wetting properties characteristic of static superhydrophobicity with water contact angles (>160°) and very low contact angle hysteresis (<5°). To examine how superhydrophobicity was maintained as the substrate was stretched, we investigated the dynamic wetting behavior of bouncing and splashing upon droplet impact with the surface. The substrate remained superhydrophobic up to 100% stretching with no structural defects after 1000 cycles of stretching and releasing. Stretchable superhydrophobicity was possible because of the monolithic nature of the hierarchical wrinkles as well as partial preservation of nanoscale structures under stretching.

The ability to control the repellent properties of bio-inspired immobilized liquid layers is of interest for a wide range of applications. Liquid layers created using infused polydimethyl siloxane (PDMS) polymers offer a potentially simple way of accomplishing this goal through the adjustment of nanoscale parameters such as cross-linker ratio and infused oil viscosity. In this work, we examine how tuning these parameters affects the material properties of the infused polymer, the stability of the liquid overlayer, and finally the overall performance of this system against bacterial adhesion and biofilm formation. We find that cross-linker density appears to have the greatest impact on the system, with a lower cross-linker:base ratio resulting in both an increased liquid overlayer stability and improved performance against bacteria. We further demonstrate how this finding may be exploited to produce patterns of slippery/sticky areas on the surface of the infused polymers for controlling the spatial arrangement of proteins and bacteria. These results demonstrate a new degree of control over immobilized liquid layers and may help facilitate their use in new applications.

Tree and torrent frogs are able to adhere and move about their wet or even flooded environments without falling. The secret of their outstanding adhesive performance is the complex hierarchical structure of their attachment pads, including microchannels of different length scales, anisotropically fiber-reinforced micropillars and different constitutional materials. Understanding the design principles behind this original surface design opens the door to novel adhesion strategies for reversible attachment in artificial systems. Over the last years, strategies to prepare frog-like microstructured surfaces of different soft materials have been reported [1-4]. However, hybrid structured surfaces with oriented fibers embedded in soft microstructures represent a fabrication challenge. I will present new fabrication strategies to obtain hierarchical, microstructured surfaces containing aligned nanofibers. The role of the anisotropic morphology in mechanical stabilisation, adhesion/friction properties and detachment will be described. Our results will clarify the role that oriented keratin fibers might have for directional and reversible attachment of frogs in wet environments.

Torrent-frog inspired adhesives: attachment to flooded surfaces. J. Iturri, L. Xue, M. Kappl, L. García-Fernández, W.J.P. Barnes, H.J. Butt, A. del Campo. Adv. Funct. Mater. 2015, 25(10), 1499-1505
Morphological studies of the toe pads of the rock frog, staurois parvus (family: Ranidae) and their relevance to the development of new biomimetically inspired reversible adhesives. D.M. Drotlef, E. Appel, H. Peisker, K. Dening, A. del Campo, S.N. Gorb, W.J.P. Barnes, Interface Focus 2015, 5(1), 1-11
Bioinspired orientation dependent friction. L. Xue, J. Iturri, M. Kappl, H.J. Butt, A. del Campo*. Langmuir 2014, 30(37), 11175–11182
Insights into the adhesive mechanisms of tree-frogs using artificial mimics. D.-M. Drotlef, L. Stepien, M. Kappl, W. J. P. Barnes, H.-J. Butt, A. del Campo. Adv. Funct. Mater. 2013, 23(9), 1137-1146

Many natural surfaces such as butterfly wings, beetles’ backs, and rice leaves exhibit directional liquid adhesion or transport; this is of fundamental interest as well as for applications including self-cleaning surfaces, microfluidic devices, and phase change energy conversion. For example, the intricate scales on the wings of the Morpho aega give rise to hydrophobicity and anisotropic droplet roll-off behavior. Previous studies have explained anisotropic roll-off, for example, via the directionality of a rigid rachet surface or the re-arrangement of nanoscale tips. Inspired by the butterfly wing, we demonstrate the fabrication of flexible synthetic scale surfaces from arrays of thin carbon nanotube (CNT) microstructures. Uniform centimeter-scale arrays of CNT scales are synthesized by a strain-engineered chemical vapor deposition (CVD) technique, using an offset-patterned catalyst layer that imparts a spatial gradient in the CNT growth rate, causing the scales to curve during growth. The scale height and curvature is controlled via the CNT growth parameters. After growth, the scales are conformally coated by a thin ceramic layer (i.e., Al₂O₃, by atomic layer deposition) followed by a hydrophobic polymer (divinylbenzene, by CVD) to tune their compliance and surface wettability. We demonstrate that the CNT scales exhibit anisotropic droplet roll-off, and via high-resolution optical imaging we observe how the droplet pinning and motion are influenced by the scale geometry and flexibility. The electrical conductivity and mechanical robustness of the CNTs, and the ability to fabricate complex multi-directional patterns, suggest further opportunities to create engineered scale surfaces.

The term biofouling describes the agglomeration of microorganisms on surfaces mainly in contact with liquid [1]. Free-floating cells freely swim and approach surfaces until they undergo irreversible attachment. At this point, thanks to the quorum sensing effect a bacterial colony will start to grow and disseminate along the surface. These bacterial layers can be found on pipelines, hulls of boats or food packaging, leading to corrosion, increase on fuel consumption due to friction and food poisoning [2]. Furthermore, when they form in medical devices nosocomial infections and failure due to clogging arise. Accounting to the economical losses and mortality related to biofilm formation [3], new approaches battling this field have been proposed in the past years. However, the increased resistance (up to a factor of 1000) of enclosed bacteria compared to free-floating cells as well as the possibility to restore films within hours inspired me to focus on hindering or delaying the first adhesion events.
On this behalf, we focused on super-liquid repellent surfaces as a platform to prevent biofilm formation. Candle-soot based superamphiphobic coatings were proved to prevent wetting by both water and low surface tension liquids thanks to the existence of a mobile air layer (Cassie state) between solid features and liquid [4,5]. By means of X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) we confirmed an adsorption of proteins (bovine serum albumin and human serum plasma) below the instrument detection limit of 2 ng/cm², provided by the synergy between topography in the nano-scale and chemistry [6]. Furthermore the stability of the air layer and ability to hinder bacterial adhesion was visualized by the study through laser scanning confocal microscopy (LSCM) of E. coli (GFP expressed) biofilm formation.
[1] Costerton, J. W.; Stewart, P. S.; Greenberg, E. P. Science 284, 1318-1322, 1999.
[2] Klevens, R. M. et al.; Public Health Rep 122, 160-166, 2007.
[3] Davies D. Nat. Rev. Drug Discov. 2, 114, 2003.
[4] Deng X., Mammen L., Butt H.-J., Vollmer D., Science 335, 67-70, 2012.
[5] Paven M., Papadopoulos P., Schöttler S., Deng X., Mailänder V., Vollmer D., Butt H.-J., Nature Communication 4, 2013.
[6] Schmüser L., Encinas N., Paven M., Graham D., Castner D.G., Vollmer D., Butt H.-J., Weidner T. (submitted ).

Semiaquatic water bugs and plants efficiently move and breathe while submerged underwater due to an air film retained on their superhydrophobic hair-covered surfaces. Air entrapped between the hairs forms a shear-free air-water interface, resulting in a non-zero velocity and reduced drag at such surfaces. Artificial polymeric nanofur covered with dense layer of nano- and microhairs is fabricated using a hot pulling technique in which softened polymer is locally elongated during separation from a heated sandblasted steel plate [1]. Similarly to natural surfaces, artificial bioinspired superhydrophobic polymeric nanofur film submerged underwater traps air between its hairs, forming a fixed air film on the surface. The trapped air significantly reduces the pressure drop across the microchannels lined with bioinspired polycarbonate nanofur compared to unstructured flat polymer, indicating reduction in fluid drag. Additionally, high stability of the retained air film under external stimuli is required for underwater applications. The robustness of the underwater retained air film on the bioinspired nanofur against pressure was estimated by analyzing the air-water-interface at different applied pressures. Furthermore, by perforating the nanofur and applying additional pressure to support the air-water interface, we demonstrate a fourfold increase of the air layer stability against pressure. The response of the air-water-interface to varying pressure difference between the hydrostatic pressure and the pressure of the retained air layer was analyzed in order to estimate the stability of the nanofur under pressure fluctuations. Moreover, we observed a significant increase in lifetime of the air-water interface retained by the perforated nanofur under different hydrostatic pressures.
[1] Kavalenka et al., ACS Appl. Mater. & Interfaces 7, 1065 (2015)

Antimicrobial surfaces have been critical for many areas including medical and industry. There are two main strategies for antimicrobial surfaces; to reduce bacterial adhesion or kill them by using antibacterial agents. Bioinspired soft polydimethylsiloxane (PDMS) shark skin structures show reduced bacterial attachment due to highly rough microstructured surface design. However, they wear off by the time and are not good enough to prevent bacterial adhesion in the long term. Herein, we combine antibacterial and antifouling properties by incorporating antibacterial titanium dioxide (TiO2) nanocomposite material with shark skin structure. We demonstrated that shark skin patterns prevented bacterial attachment and also induced 80-95% bacteria death in an hour. Improved mechanical properties help them to be durable in the long term. Our method is solution processable, robust and roll to roll compatible method.

Mother nature provides the ultimate inspiration for various topologically ordered patterns, structures, and flexible motion from one-dimensional (1D) linear structures such as actin filaments and muscle fibers, two-dimensional (2D) arrayed compound eyes of insects, Morpho butterfly wings composed of three-dimensional (3D) hierarchical complex structures, etc. With self-assembly and self-organization, which are the driving principles in the formation of these natural structures, a number of biologically inspired artificial materials have been prepared.
Surface wrinkling is an inventive and unconventional technique that is also fast and inexpensive for various types of surface patterning involving sinusoids (ripples), herringbones, labyrinthine designs, etc. It is especially suited for large-area surfaces of poly(dimethylsiloxane) (PDMS) elastomers based on mechanical (buckling) instability. This self-organization buckling phenomenon is widely observed in natural systems such as humanskin, brain cortex, fruits, and plants. Owing to the periodic structure and dynamically tunable wrinkles, it has been used in many applications.
Previously, we have succeeded in fabricating ultrasmall attoliter-sized (10⁻¹⁸ L) 1D metallic nanocup arrays embedded in PDMS films by colloidal soft-lithography and wrinkle processing (H. Endo et al., Langmuir 2013, 29, 15058). Moreover, we described the fabrication of various topological 1D colloidal arrays, including single, helical, zigzag, triple-line, and random arrays integrated in sinusoidal wrinkle grooves, through simple spin-coating (H. Endo et al., Coll. Surf. A 2014, 443, 576). The particles in these arrays can be connected using plasma etching, forming beaded, robust, and long (>100 mm) colloidal chains.
In this study, we succeeded in the fabrication of a large-area ultra-water-repellent film on which water drops can be flexibly controlled by utilizing original 3D-streching method. It found that surfaces with different properties—an ultra-water-repellent and high-adsorption area and an ultra-water-repellent area—can be generated on the basis of two different pattern structures by applying water-repellent coating to the wrinkle film. Moreover, we succeeded fabrication of film with highly adhesive superhydrophobic surface and SERS activity. The results of this study will not only contribute to resolving issues of conventional top-down lithography techniques but will also be applicable to environmental, water-saving, medical and many other fields. In addition, we propose fabrication of 3D microobjects using controlled folding/bending of wrinkled-thin films besed on elastocapillary force toward new type of 3D-imprinting technology.

Combining high optical transmission with self-cleaning and water-repellency is of great interest for optical systems, especially for those operating in outdoor conditions such as solar cells. Natural surfaces of water plants Salvinia Cucullata and Pistia Stratiotes combine these functionalities in a transparent layer of dense microhairs. This layer renders their surface superhydrophobic without affecting the absorption of sunlight necessary for photosynthesis. Inspired by these natural surfaces, we introduce superhydrophobic flexible thin nanofur films made from optical grade polycarbonate, which can be used as a transparent coating on optoelectronic devices. Thin nanofur films are fabricated using a highly scalable cost-effective hot pulling technique, in which heated sandblasted steel plates are used to locally elongate softened polymer resulting in a surface covered in microcavities surrounded by randomly distributed high aspect ratio micro- and nanohairs. The superhydrophobic nanofur exhibits high water contact angles (166±6°), low sliding angles (< 6°) and is self-cleaning against various contaminants. Additionally, subjecting the nanofur to argon plasma reverses the film wettability to underwater superoleophobic, enabling its use as an underwater oil-re