Fluorescence and luminescence are detection techniques that possess important advantages for bioanalytical applications and biologic imaging: high sensitivity, versatility and low costs of instrumentation. A common characteristic of biologic analytes is their presence in small quantities among complex matrices such as blood, cells, tissue and organs. These matrices emit significant background fluorescence (autofluorescence), limiting detection sensitivity.
The luminescence of lanthanide cations has several complementary advantages over the fluorescence of organic fluorophores such as sharp emission bands for spectral discrimination from background emission, long luminescence lifetimes for temporal discrimination and strong resistance to photobleaching. In addition, several lanthanides emit near-infrared (NIR) photons that can cross deeply into tissues for non-invasive investigations and that result in improved detection sensitivity due to the absence of native NIR luminescence from tissues and cells (autofluorescence). The main requirement to generate lanthanide emission is to sensitize them with an appropriate chromophore (“antenna effect”).
An innovative concept for such sensitization of NIR-emitting lanthanides is proposed herein; the current limitation of low quantum yields experienced by most mononuclear lanthanide complexes is compensated for by using a large number of lanthanide cations and by maximizing the absorption of each discrete molecule, thereby increasing the number of emitted photons per unit of volume and the overall sensitivity of the measurement. To apply this concept, we have created several types of nanomaterials and metal-organic frameworks and succeeded in generating highly emissive NIR biological reporters. We will discuss their structures, photophysical properties and their applications for biological imaging in cells with NIR microscopy.

Doped nanocrystals can address key problems in applications such as light emitting devices, photovoltaics, spintronics, and bioimaging, since doping enables further control over the electrical, optical, transport and magnetic properties of nanomaterials. This has stimulated a vast research effort on the synthesis of doped colloidal nanocrystals [1]. Incorporation of dopants into nanocrystals is, however, not straightforward and is often the limiting factor in the development of new materials. Therefore, new and more sophisticated doping strategies are of high interest. In this work we introduce a single source precursor approach that successfully leads to monodisperse Ce3+ and Eu2+ doped CaS and SrS colloidal nanoparticles with diameters smaller than 10 nm. We found that the use of host and dopant precursors with similar decomposition temperatures is one of the key factors to successful doping. Instead of simply thermally decomposing the precursors of the host crystal and the dopant together in a coordinating solvent, we also applied growth-doping and nucleation-doping strategies [2]. This allowed us to achieve enhanced dopant incorporation and better control over the dopant concentration and location. For the growth-doping strategy, an additional shell of the host material is grown around a doped nanocrystal. This internalizes surface adsorbed Ce3+ or Eu2+ ions, hence greatly improving the luminescent properties of the doped nanocrystals. The nucleation-doping strategy can be applied when the decomposition temperatures of the precursors of the host material and of the dopant are largely different. For example, EuS cores (< 4 nm) can be first synthesized at 320 C, and subsequently overcoated with a SrS shell at 250 C. Thermal annealing leads to diffusion and intermixing of Eu2+ and Sr2+ ions, which results in a pronounced increase of the Eu2+ emission. The single-source precursor approach proposed here allows for a flexible design of synthesis strategies and has the potential to be widely applicable to the doping of sulfide-based colloidal nanoparticles with transition metal and lanthanide ions.
1. D. Norris et al. Science 2008, 319, 1776.
2. D. Chen et al. J. Am. Chem. Soc. 2009, 131, 9333.

Achieving efficient visible-to-UVC upconversion by Pr³⁺-doped phosphors requires appropriate edge energy of the host-dependent 4f5d emitting band. In many oxyanion-type host crystals, the 4f5d band is of optimum energy for two-step excitation by photons in the violet-blue range; however, the high phonon cutoff frequencies of these systems results in substantial non-radiative losses. On the other hand, the weak crystal field strengths of the low-phonon energy fluoride hosts results in 4f5d band energies that are too high. Herein, we describe the use of rare-earth oxyfluoride host systems which have both low phonon cutoff energy and desirable Pr³⁺ energy level structure. Optical properties of Pr³⁺-doped oxyfluoride phosphors and synthesis challenges will be discussed.

A Zn2+-responsive bimodal MRI and luminescence imaging probe GdL was synthesized. The relaxivity and luminescence properties were examined. In the presence of 0.5 equiv. Zn2+, the longitudinal relaxivity is increased from 3.8 mM-1 s-1 to 5.9 mM-1 s-1 at 23 MHz with 55% enhancement, whereas the fluorescence exhibits a 7-fold increase. The Zn2+ responsive imaging probe shows favorable selectivity and tolerance over a variety of biologically relevant anions and metal ions in physiological pH range for both relaxivity and luminescence. In vitro phantom images and confocal fluorescence images in living cells show that the bimodal Zn2+ probe can effectively enhance T1-weighted imaging contrast and luminescence imaging effect through Zn2+ coordination with excellent cell membrane permeability and biocompatibility. Spectral and ESI-MS studies indicate that two different Zn2+-bound species, (GdL)2Zn and GdLZn, are formed when 0.5 and 1 equiv. Zn2+ is bound to GdL complex, respectively. Crystal structural determination and 17O DIS experiment demonstrate that the increased molecular weight and the improved molecular rigidity upon complexation of Zn2+ with GdL is the primary factor for relaxivity enhancement. Significant enhancement of the luminescence is due to heavy atom effect and much increased molecular rigidity upon Zn2+ binding to 8-sulfonamidoquinoline chromophore.

The mechanisms that influence the spectroscopic properties and luminescence dynamics of lanthanide ions in nanomaterials are discussed based on recent experimental results and theoretical developments. The 4f states of lanthanide ions in solids are localized and do not exhibits any quantum confinement effects. Many nano phenomena observed as a function of particle size are due to two mechanisms that are sensitive to local environment of the lanthanide ions. First, ion-phonon interaction depends on the density of phonon states which become discrete with a cutoff at the low energy side in crystals of nanometer size. As a result, such processes as phonon-assisted energy transfer and non-radiative relaxation that directly influence the luminescence dynamics are realized to change as a function of crystal size and temperature. Secondly, the crystal field strength and symmetry may change between bulk materials and nano crystals. Such a change may not induce dramatic variation in energy levels, but its impact to the intensities of electronic transitions can be very significant. Many experimental results of upconversion emission (UCE) in Er3+ and Tm3+ crystals show that the high energy UCE in the blue and green region increases while the red UCE decreases as a function of reducing crystal size into nanometers. Among possible mechanisms that lead to such an increase/decrease of UCE in different energy regions, symmetry change and increased electrostatic field in nanocrystals are expected to have a significant contribution. They induce 4f-5d configuration mixing, which is more profound for Er3+ and Tm3+ ions in high lying excited states than in the low-lying excited states. A small amount of 5d mixed into the 4f states in high energy region would significantly enhance UCE. Calculations will be compared with experimental results for Er 3+ and Tm3+ systems.

Since the discovery of TTF in early 1973, many interesting materials have been obtained as organic metals, semi-conductors, superconductors, magnets, and so on. In the last decade, TTF derivatives have been associated to d metal ions to elaborate multifunctional materials which possess magnetic and electrical properties. Lanthanide ions are well-known to exhibit large magnetic moments and strong magnetic anisotropy and therefore they are considered as good candidates for the elaboration of Single Molecule Magnets (SMMs) [1].
We have associated the tetrathiafulvalene-3-pyridine-N-oxide ligand to the Dy(III) ion producing a centrosymmetric dinuclear complex in which the magnetic moments are antiferromagnetically coupled. Surprisingly, the complex behaves as a SMM with both strong frequency dependent out-of-phase signal of the ac magnetic susceptibility and magnetization loop [1] A new Single Ion Magnet (SIM) has been also recently discovered in the Dy(III) dimers containing a TTF-based Acceptor-Donor-Acceptor bridging ligand [3].
Dinuclear complexes of lanthanides associating both 4,5-Bis(thiomethyl) -4&’-carboxylictetrathiafulvalene and 4,5-Bis(thiomethyl) -4&’-ortho-pyridyl -N-oxide -carbamoytetrathiafulvalene ligands have been elaborated. Dc magnetic susceptibility measurements highlight ferromagnetic interactions between the metallic centres. The two Dy(III) and Yb(III)-based analogues display SMM behaviour. Experimental and theoretical magnetic and photo-physical investigations have been realized to confirm that a multi-electroactive luminescent SMM is obtained in the case of the Yb(III) analogue [4].

References
1. J. C. G. Bünzli, C. Piguet Chem. Rev. 2002, 102, 1897.
2. F. Pointillart, et al., Chem. Eur, 2011, 17, 10397
3. F. Pointillart, et al., JCS Chem Commun., 2012, 48(5), 714
4. F. Pointillart, et al., submitted

Upconverting nanocrystals (UCNCs) have a wide variety of potential applications ranging from security printing to bio-labels. One of the most efficient UCNCs known is β-NaYF4 co-doped with Yb3+ and Er3+ ions. This type of UCNC has been synthesized using three main methods: hydrothermal, thermal decomposition and co-precipitation. Co-precipitation is the most convenient of these methods as it requires no special reaction apparatus as needed for the hydrothermal methods, nor does it require expensive and hazardous triflouroacetate precursors needed for the thermal decomposition method. Using real-time monitoring of the upconversion luminescence during UCNC synthesis, we were able to control the size and phase of the resulting nanoparticles, as well as elucidate key factors regarding the nucleation and growth mechanisms.

An oxygen sensor based on the complex [Tb(3,5-dcba)₃] chemically bonded to a phosphine oxide modified-polydimethylsiloxane (pdms) and its optical characterization is presented in this work. The complex was obtained by the mixture of terbium(III) and 3,5-dichlorobenzoate (3,5-dcba) aqueous solution under stirring in a proportion metal:ligand 1:3. Further the pdms was functionalized with allyldiphenylphosphine oxide in 1% ratio of the Si-H bonds of the siloxane through the hydrosilylation reaction in toluene. The complex was added to this functionalized siloxane and after its coordination by the phosphine oxide, the siloxane chain was crosslinked using tetravinylsilane. FT-IR and Raman spectroscopies were used to confirm the formation and anchoring of complex in the functionalized-pdms. The profile of the emission spectrum is characteristic of the intraconfigurational f → f transitions presenting narrow bands attributed to Tb(III) ions being the ⁵D₄ → ⁷F₅ the most intense one. The coordination of phosphine oxide to the complex leads to a broader π → π* excitation band of aromatic ring but does not significantly alter the emission spectrum profile of Tb(III) emission. The quenching of the terbium(III) emission by molecular O₂ was determined monitoring the intensity of the ⁵D₄ → ⁷F₅ transition by setting the excitation wavelength at 350 nm under several mixtures of O₂ + N₂ using an O₂ increment of 10 wt% per step. The ratio I_N2/I_O2 founded to this material is 8.4, higher compared with similar systems. The Stern-Volmer and the I₀(I₀-I) versus 1/[O₂] plot shows a non-linear and linear response respectively, indicating the inhomogeneity of the sites for O₂ diffusion in the membrane. The oxygen sensor is reversible by switching the O₂ concentration among 100% and 0% .The material is photostable against 350 nm and its response time and recuperation values of the luminescence quenching are 13 and 61 seconds, respectively. These data combined with the high oxygen sensibility indicate that this hybrid material may be applied as a potential oxygen sensor.

Novel molecular structure of organic/inorganic hybrid glass based on highly fourinated alkyl-chains was designed for optical device materials. Organic/ inorganic hybrid glasses can be provided by a ‘molecular-composite level mixing technique&’ and then often creates new optoelectronic properties, which individual component doesn&’t show. The molecular mixing technique also results no significant phase separation and thus produces high quality optical media. By modifying the Si-O-Si polymeric network, we produced a fluoroalkylene-bridged polysilsesquioxane with controlled porosities for facile and uniform doping of various ions, metals or semiconductor particles to increase the dopant levels. We then doped lanthanide, “Erbium ions,” for laser amplifier applications. In photoluminescent experiments, a significant enhancement in fluorescent intensity at 1540 nm has been obtained from the fluoroalkylene-bridged hybrid glass. Analysis by the nuclear magnetic resonance (NMR) also indicates a dramatically enhanced the degree of condensation and a low level of hydroxyl-environment in the fluoroalkylene-bridged hybrid glass matrix.

Lately, up-conversion materials doped with rare earth ions became very interesting due to the wide range of applications, such as in solid state lasers, screen displays, sensors, optical data storage, etc. Powders of gadolinium oxide doped with different concentration of Yb3+ (2, 4 and 6 at %) and Ho3+ (0.5 and 2 at %) were synthsized through combustion method at 800 °C using PEG as fuel and as nucleation agent for crystallization process. The phase composition was performed by X-ray powder diffraction (XRPD) and confirmed cubic bixbyite phase. The particle morphology was analyzed using electron microscopy and revealed agglomerated particles typical for this synthesis process. Purity of samples was investigated with Fourier transform infrared emission spectroscopy (FT-IR). Optical properties that comprised photoluminescent measurements and lifetime determinationwere investigated using 980 nm laser diode excitation. Three emission bands were observed: a green emission band in the range of 530-560 nm(5F4, 5S2 → 5I8), a weak red emission band in the range of 650-670 nm (5F5 → 5I8), and near infrared emission band in the range of 750-770 nm (5F4, 5S2 → 5I7). Influence of different concentration of dopants was further correlated with luminescent properties in order to examine the most appropriate ratio of dopants.

Up-conversion (UPC) luminescent materials convert infrared radiation into visible light and synthesis and optical characterisation of these materials, particularly the trivalent rare-earth ions doped UPC luminescent materials, have become a attractive and very active area of research in recent times. Among the trivalent rare-earth ions Er3+-Yb3+ codoping is very useful for up-conversion. Yb3+ ions are excellent sensitizers and have simple energy structure with only two states, while Er3+ ions posses many long-lived metastable energy levels that can result in emissions at various wavelengths in UV and visible region.
Here we present polymer complex solution (PCS) method for preparation of Yb and Er doped Y2O3 powders together with powder up-conversion emission properties and particle morphology. PCS is a modified combustion method where instead of classical fuel (urea, glycine, carbohydrazide) organic water-soluble polymer (in our case polyethylene glycol (PEG) is used. The utility of this polymeric approach comes from the coordination of metal cations on the polymer chains during gelation process, resulting in very low cation mobility. Polymer precursor works both as a chelating agent and as an organic fuel to provide combustion heat for the calcination process. By this way, PCS provides mixing of constituting elements at the atomic level and allows homogeneous control of very small dopant concentration. Samples are prepared with different Yb3+-Er3+ ratios (10:1, 5:1 and 2:1) to check red to green up-conversion emissions ratio. The X-ray diffraction analysis confirmed that powders obtained using PCS method possess a cubic bixbyte crystal structure (space group Ia-3) and electron microscopy analysis shows aggregates formed of individual crystalline nanoparticles with size of about few tens of nm. Luminescence measurements showed intense up-conversion emission from Er3+ 2H9/2→4I15/2, (2H11/2, 4S3/2) → 4I15/2 and 4F9/2→4I15/2 transitions.

In this report we present cost and time effective method for synthesis of Y2O3:Yb3+,Er3+ nanoparticles based on self-propagating room temperature reaction between metal nitrates and sodium hydroxide. In brief, appropriate amounts of yttrium nitrate hexahydrate, ytterbium nitrate pentahydrate and erbium nitrate pentahydrate are mixed with sodium hydroxide. Reaction proceeds at room temperature after the mixture of reactants is mechanically activated by hand mixing in alumina mortar. After being exposed to air for 3h the mixture is washed in centrifuge with distilled water and ethanol and dried for 12h at 70°C. Series of samples are prepared with calcinations on different temperatures (600°C, 800°C and 1100°C) and also with different Yb3+-Er3+ ratios (10:1, 5:1 and 2:1).Particle size and crystallite size of powders obtained at different calcinations temperatures are evaluated through X-ray diffraction analysis and transmission electron microscopy.In all samples up-conversion emissions and corresponding lifetimes are measured after excitation at 978 nm in the wide temperature range (10-300 K). The most intense emission originate from the following Er3+ transitions: [2H9/2→4I15/2] in blue (407-420 nm); [2H11/2, 4S3/2) → 4I15/2] green: 510-590 nm; and [4F9/2→4I15/2] in red (640-720 nm) spectral region.We showed that ratio of red to green emissions may be tuned with Yb3+-Er3+ dopant ratio and that intensity of up-conversion emissions and lifetimes are strongly influenced by powder particle size and crystallinity.

Thermographic phosphors are oxides doped with rare-earth or transition metal ions that will emit visible, infrared, or UV light upon excitation from an external energy source. This materials have received significant attention due to the potential application as optical temperature sensor. In this report, we have investigated yttrium oxide co-doped with changeable ytterbium to erbium ratio (Y1.94Yb0.05Er0.01 and Y1.97Yb0.02Er0.01) fabricated through hydrothermal synthesis. Process conditions (2h, 200 °C) and additional thermal treatment (3h, 1100 °C) allows obtaining nanoparticles of appropriate composition and morphology which further affect on improved photoluminescent characteristics. The fluorescence intensity ratio (FIR) technique is used to examine potential usage of samples as low temperature sensors.This optical method is based on ratio between two emission lines or areas in photoluminescence spectrum which show temperature dependence. Photoluminescent measurements (PL) are recorded in the temperature range from 10 K to 300 K under 978 nm exciting wavelength observing changes in following transitions: blue 2H9/2→4I15/2, green (2H11/2, 4S3/2) → 4I15/2 and red 4F9/2→4I15/2. Obtained experimental results implay that the fluorescent intensity ratio of the blue, green and red lines and areas show significant temperature sensitivity and can be used as low temperature sensor.

In the synthesis of nanostructured materials one of the most promising and rapidly developing areas are the use of soft templates. The soft templates are composed of soft compound such as biomolecules, polymer gels, emulsions and lyotropic liquid crystals (LLC). LLC have particular interest: different types of lyotropic mesophases - lamellar, hexagonal and cubic allow obtaining 1D, 2D and 3D ordered arrays exhibit size and shape depended properties. Lyotropic metallomesogens containing trivalent rare-earth metal ions have unique attractive behavior due to combination of some unique optical and magnetic properties of the lanthanide ions with anisotropic supramolecular organization liquid crystal. These multifunctional materials provide new promises in biomedical science and materials science. In this work presents the synthesis and study of physicochemical properties of the LLC on the basis of non-ionic surfactants - decaethylene glycol monododecyl ether C12H25O(CH2CH2O)10H (C12EO10), tetraethylene glycol monododecyl ether C12H25O(CH2CH2O)4H (C12EO4), zwitterionic surfactant - N,N- dimethylaminooxide (CH3(CH2)11N(O)(CH3)2) (C12DMAO) and hydrated lanthanide nitrates Ln(NO3)36H2O, Ln = La, Eu, Tb, Gd in water and water-decanol environments. Using complex methods (POM, SAXS, NMR, viscosity) the phase diagrams were constructed; the temperature and concentration limits and the type of supramolecular organization were defined. According from the IR spectroscopy data the formation of the LC complex of the lanthanide ion with the oxyethylated groups of the surfactant was established. The hydration extent of the first coordination sphere of the Eu(III) and Tb(III) ions in the liquid-crystal state were estimated from the luminescence lifetime data.
Using ability to easily orientation of Eu(III) and Tb(III)- containing lyotropic mesophases the approach to creation of polarized media was developed. For orientation of samples the shift deformation and polymer orientants were used. The orientation properties were investigated by ACM, SAXS, TEM. According luminescence analyses the angular dependence of the luminescence and lifetime was observed.
The LLC mesophases containing ions Eu(III) - C12EO10/Eu/H2O has been using as template for creation hybrid silica materials with efficiency luminescence. The synthesis conditions were established, the optimal composition was found. The role of liquid-crystal template was shown using the changes of the surface topography of the film during the transition to LC template, compared with simple silicate matrix. The influence of composition on luminescence properties of hybrid films was investigated. The increase of the lifetime in hybrid silica films containing europium ions compared to the LLC system C12EO10/Eu(III)/H2O on 48% was observed.
The work was supported by the RFBR 11-03-00679-a

Traditionally, the Judd-Ofelt (JO) parameters are calculated by fitting the JO parameters to obtain a match between the observed oscillator strengths of the absorption spectrum (or emission spectrum) with the calculated Judd-Ofelt oscillator strength. The radiative lifetimes (rate constants) and branching ratios of radiative emission can be estimated using the optimized JO parameters. In the poster, we report a new method for calculating the JO parameters from the measured emission branching ratios. The Er3+ emission in Er3+, Yb3+: NaYF4 is measured in order to calculate the JO parameters of Er3+: NaYF4, which have never been reported. Furthermore, the radiative rates constants and the branching ratios are simulated as well.

Full-color lanthanide (Ln) photoluminescent materials have attracted considerable interests due to their potential applications in display systems and lighting technologies. Herein, the full-color photoluminescent films have been designed and fabricated facilely with a fixed-component Ln-based (Ln= Tb and Eu) polymer hybrid doped with a proton sensitive amide-type b-diketonated photosensitizer N-(2-pyridinyl)benzoyl-acetamide (HPBA). The tunable photoluminescence emissions of the films are achieved by changing the amounts of OH- in the hybrid, rather than varying the relative concentrations of the lanthanide ions and photosensitizers, representing a new paradigm for full-color displays. The emission color can also be finely tuned through the variation of the excitation wavelength, and white-light emission can be achieved when the given film is excited at the visible region (405 nm). The photophysical properties and the mechanisms of the intra- and inter-molecular energy transfer before and after deprotonation have been investigated in detail. It is worthy to note that, the luminescence of the film can efficiently transform from green to red even after several cycles when the film is exposed to base-acid vapor. The films may display as vapoluminescent sensors which due to the good stability, sensitivity, reversibility, and quick response triggered by a base-acid vapor. The results of this work might have great theoretical and practical significance for the design of multifunctional lanthanide materials. Further investigations of these materials and other candidates are still ongoing in our lab.

Nanocrystals that have unusual or exceptional optical properties have shown promise as transformative probes for biological imaging. Phosphorescent upconverting nanoparticles (UCNPs) have proven to be especially promising as biological labels, and single-particle studies of UCNPs have shown that they exhibit nearly ideal properties as single molecule imaging probes. UCNPs absorb two or more photons in the near infrared and emit one at shorter wavelengths in the visible or nIR, an unusual characteristic that distinguishes them from all luminescent chemicals in the cell, and one that suggests background-free cellular imaging. We have shown that UCNPs do not blink on and off as most other probes do, and that they posses remarkable photostability, resisting photobleaching under continuous irradiation long after organic dyes, proteins, and even quantum dots are extinguished. We have recently developed synthetic methods for control of UCNP size, and completed a combinatorial lanthanide scan in order to tune emission wavelengths for multicolor upconverted imaging. We have also developed methods for studying single nanocrystal lifetimes and emission spectra, which has allowed us to understand lanthanide-lanthanide communication within the nanocrystal.

We describe the preparation of monodisperse, multicolor upconverting luminescent nanoparticles (UCLNPs) based on lanthanide-doped NaYF₄ with controllable diameters of ~26 nm for protein conjugation. In order to make them amenable to bioanalytical applications, surface modification was performed. First, a thin silica shell (~5 nm thick) was deposited on the inherently hydrophobic UCLNPs, followed by a poly(ethylene glycol) spacer coating containing N-hydroxysuccinimide groups.
The nanoparticles were characterized by transmission electron microscopy, Raman spectroscopy, X-ray diffraction, zeta potential measurements and dynamic light scattering. The N-hydroxysuccinimide ester functionalization renders them highly reactive towards amine nucleophiles (e.g., proteins). The protein-reactive UCLNPs and their conjugates to streptavidin and bovine serum albumin display multicolor visible emissions upon 980-nm continuous wave laser excitation. The binding properties were verified using a label-free surface plasmon resonance technique. The non-specific and specific binding of protein-reactive UCLNPs towards proteins immobilized on a gold substrate was evaluated.
We believe that such protein-reactive multicolor UCLNPs can be employed as luminescent labels for low-background imaging and bioanalytical applications, and that numerous bioassays will strongly benefit from the use of nanoparticles with photon upconversion capability.

Inorganic erbium-doped glasses are widely used in telecommunications due to the sharp intra-atomic 4I13/2 → 4I15/2 transition in the 4f orbital of erbium resulting in an emission at ~ 1.5 mu;m, which is the low loss window of silica optical fibres. The limited erbium concentration of about 10^20 ions/cm3 in inorganic erbium-doped glasses and the low absorption coefficient of the Er3+ ions, imply that relatively long lengths of fibre are required. Planar optical amplifiers at 1.5 mu;m would find many applications in photonic integrated circuits. The challenge is then to fabricate such devices with high gain for a small size and low power optical pumping. Nevertheless the thin film deposition of inorganic erbium doped glasses is not fully compatible with the fabrication planar optical waveguide amplifiers. The organic erbium complexes are materials of interest because of their higher absorption cross sections due to the photosensitization of erbium by organic conjugated ligands and broader emission bands than those of the free Er3+ ion in inorganic matrix. Nevertheless most of organic materials behave as efficient NIR luminescence quenchers due to the coupling between the electronic transition of the erbium ion and the vibrationnal overtones of O-H, N-H and C-H bonds of the ligand itself. The consequences are a decrease in the emission quantum yield and the luminescence lifetime. The substitution of hydrogen atoms by fluorine on the organic ligand is usually proposed. Moreover, fluorinated ligand confer to the complex volatility under reduced pressure allowing the complex to be sublimated.
In this work, we present the synthesis and the photophysical characterization of two organic erbium complexes based on N-(P,P-ditetrafluorophosphinoyl-P,P-ditetrafluorophenylphosphinimidates and N-(P,P-dipentafluorophosphinoyl)-P,P-dipentafluorophenylphosphinimidates ligands. We show that the substitution of a fluorine atom in para-position by an hydrogen atom on each phenyl ring of the perfluorinated organic ligand results in a dramatic decrease in luminescence lifetimes from 980 mu;s to 70 mu;s when measured under vacuum on sublimed powder state. For the first time, we are able to quantify the link between the drop in luminescence lifetime and the presence of C-H bonds located at a well defined distance from the Er3+ ion.
The authors gratefully acknowledge the French National Research Agency (ANR) for the financial support through the VERSO program METAPHOTONIQUE.

Bioimaging is an important tool allowing the visualization and understanding of biological and cellular processes. Commonly, fluorescent proteins and organic dyes are used as markers in optical bioimaging. However, their main disadvantage is color fading restricting their temporal use. Further problems are auto-fluorescence of the biological tissue and phototoxicity as well as scattering when ultraviolet (UV) light is used as excitation source. Consequently, the penetration depth is limited. In contrast to UV and visible light, the use of near-infrared (NIR) light reduces phototoxicity and scattering due to the so-called biological window in the absorption spectrum of biological tissue. This results in deeper penetration of the exciting and emitting light into and from the tissue, and imaging of deeper tissue areas becomes possible. Therefore, NIR absorbing and emitting compounds are promising candidates to overcome the disadvantages of commonly used organic dyes. Herein, lanthanide-containing inorganic nanostructures are well known for their outstanding optical properties. Gadolinium oxide nanorods and nanoparticles doped with Er³⁺ and Yb³⁺ ions were synthesized by hydrothermal and precipitation processes. Variation of additives and surfactants during the synthesis allows the control over particle size and nanorod aspect ratio. Their suitability as biomarkers for 980-nm excited NIR emission was investigated. In-vitro tests revealed a cytotoxic effect in case of incubation of macrophages with bare nanostructures, which was deduced from the poor chemical stability of Gd₂O₃ under the acidic conditions found inside macrophages, but it could be overcome with surface modification with PEG-b-PAAc. Surface modification with PEG-b-PAAc further improves the dispersion stability in water and buffer solutions. The in-vivo distribution in mouse organs was investigated with an over-1000-nm NIR in-vivo fluorescence bioimaging system, which was sensitive to wavelengths between 800 and 1700 nm. This system allows the time-resolved observation of the distribution of the nanostructures in the mouse body. Further, the use of Gd₂O₃ nanostructures doped with Ho³⁺ besides Er³⁺ allows multicolor imaging based on their 1.2 and 1.5-mu;m emission.

The nascent field of plasmon-enhanced upconversion has shown promising progress in recent years. By coupling the absorption and emission of upconverting nanoparticles to metallic nanostructures, enhancements ranging from 4-50x have been achieved. These improvements are encouraging, but for upconversion to become viable in applications such as photovoltaics and bioimaging, larger enhancements are necessary.

We explore the limits of plasmon-enhanced upconversion with controlled studies of single upconverting nanoparticles coupled to single plasmonic nanostructures. In particular, we investigate NaYF4:Yb3+,Er3+ nanoparticles near Au metallic nanorods, and correlate the specific geometry and orientation of the metal/emitter pair to its emission spectra and radiative rate. We synthesize colloidal upconverting nanoparticles of sizes ranging from 30 to 90 nm in octadecene with oleic acid as a ligand. We also synthesize colloidal Au nanorods (80 nm x 17 nm) with longitudinal and transverse plasmon resonances at 950 nm and 521 nm, respectively - tailored to match the absorption and emission frequencies of the nanoparticle. The rod geometry enables enhancement of both the absorption and anti-Stokes shifted emission from an upconverting nanoparticle. Both the particles and rods are deposited on transparent silicon nitride TEM membranes, enabling correlated optical spectroscopy and high-resolution electron microscopy. Polarization sweeps of the optical input are performed to excite both the longitudinal and transverse modes of the dispersed nanorods, tailoring enhancements of absorption and emission. Additionally, lifetime studies of upconversion and photoluminescence emission enable a direct mapping of the local density of states.

Lanthanide ion-doped upconverting nanoparticles (UCNPs) have attracted great attention owing to their unique optical properties. The photoluminescence of UCNPs lies in the visible spectral range upon NIR (980 nm) excitation and does not exhibit photoblinking on the millisecond time scale and photobleaching under long-time continuous illumination. Moreover, the advantages of using UCNPs for live-cell imaging include the absence of autofluorescence and low degree of photodamage to cells thanks to the NIR excitation. We carried out live-cell imaging study with biocompatible PEG-lipid coated UCNPs in living HeLa cells. We observed that the UCNPs were internalized and transported to the perinuclear region along the microtubules from the snapshot images taken over 12 hours. We also visualized the movement of the vesicle-encapsulated UCNPs in real-time (20 frames/s) as long as 6 hours without interruption. The particle tracking analysis showed that the internalized UCNPs were actively transported presumably by motor proteins, such as dynein or kinesin, moving along the microtubules.

Lanthanide ion-doped upconverting nanoparticles (UCNPs), which emit in the visible range upon absorption of NIR photons, have attracted great attention in the area of biological imaging owing to their unique properties. First, two-photon upconversion of NIR excitation to the emission of a visible photon is so efficient that a tiny CW laser with the output of tens of milliwatts is sufficient as the excitation source. Second, by employing NIR excitation, one can suppress cellular autofluorescence, hardly induce photo-damage to cells, and achieve relatively deep penetration into tissues. Finally, UCNPs exhibit neither photoblinking nor photobleaching, and their cytotoxicity is very low. As a result, UCNPs are recognized as one of the most promising nanoparticle systems for biological imaging and there are continuing efforts to improve their properties (e.g., increasing luminescent intensity and reducing the particle size) by designing new synthetic strategies. In this study, we demonstrated the benefits of using UCNPs as the probe for real-time imaging and particle tracking in living HeLa cells. Combined with the low cytotoxicity and photostability of UCNPs, NIR excitation enabled uninterrupted long-term imaging of living cells. For the first time, we obtained real-time images of endocytosed UCNPs at the single vesicle level for 6 h continuously at the rate of 20 frames sec-1. The dynamics of particle transport was composed of multiple phases within a single trajectory including the active transport by motor proteins such as dyneins and kinesins. Furthermore, we imaged the spatiotemporal distribution or intracellular pathway of UCNPs from endocytosis to exocytosis for individual single cells and provided fundamental insights into the cellular interaction of nanomaterials, which is crucial for biomedical applications thereof.

NIR-emitting lanthanide (Er3+, Nd3+, Yb3+) quinolinolates are currently receiving widespread attention as low-cost functional materials for their applications in telecom technology. The quinolinolato ligands are conveniently employed as light-harvesting “antennas” for resonance energy transfer (RET) to lanthanide ion&’ higher levels (sensitized emission), in order to overcome the very small absorption coefficients of the lanthanide-based line-like absorption transitions, which are related to parity-forbidden f-f transitions.
Despite the Er3+ complex has been firstly applied as emitting center for OLED application in the NIR (1540 nm), since recently, the structures of lanthanide quinolinolates were not available. A reinvestigation by our group and others on synthetic methods of these complexes has shown that they exhibit a rich structural chemistry. Depending on reaction conditions, species with high coordination numbers and differing for nuclearity and stoichiometry where the ligands coordinate either as chelating terminal or bridging ligand or also as a zwitterions, have been isolated and fully characterized [1].
Photophysical studies, performed on samples in solutions, as crystals or as sol-gel glasses, have demonstrated that quinolinolato ligands are particularly effective as “light harvesting antennae”, transferring absorbed energy to the lanthanide ion (Er3+, Yb3+) with nearly 100% efficiency and allowing metal emission enhancement. In fact these complexes exhibit, in solution and in the solid state, dual radiative emission in the visible (ligand-centered) and in the NIR (metal-centered) spectral region. Time-resolved studies on the excited-state dynamics of these systems, provide evidence that ligand-to-metal sensitization occurs through a two-step mechanism involving ligand triplet excitons and that inter-system crossing (ISC) from ligand singlet excitons and resonance energy transfer (RET) to metal upper levels take place with time constants in the ps range. NIR decay times are in the mu;s range and the well-established Förster theory allows to evaluate quantitatively the luminescence quenching processes via vibronic coupling with the high frequency oscillators present in the coordinated ligand, such as CH. A good agreement with the NIR decay times, falling in the 2mu;s range, is obtained for Er-complexes, whereas a less satisfactory agreement is found for the Yb case. Further studies, both experimental and theoretical, are required to clarify this discrepancy [2]
First experiments on some of these complexes incorporated as dopant into silica sol-gel glasses, afford homogeneous and transparent materials displaying dual visible and NIR emission. The obtained doped material can be considered a “solid state solution” whose spectroscopic and photophysical properties are very close to those observed in solution.
[1] F. Artizzu, et al., Advanced Functional Materials 2007, 17, 2365
[2] F. Artizzu et al., ChemPlusChem 2012, 77, 240

A gold cavity array is designed and fabricated to enhance NIR-to-visible upconversion luminescence from a monolayer of β-NaYF4: 17%Yb, 3%Er upconversion nanocrystals in poly(methyl methacrylate). The upconversion luminescence spectrum is used to characterize the nature of the enhancement by comparing the luminescence intensity of the upconversion monolayer on the array area with that on the flat gold area. Under 12 W/cm2 980 nm excitation, ~7 times enhancement was observed for green upconversion and ~48 times enhancement was observed for red upconversion. It is also shown that as the excitation power increases to 40 W/cm2, the enhancement factors decrease to ~4 times for green upconversion and ~25 times for red upconversion. Power dependence experiments reveals that, over the range of excitation power densities used, the power dependence of the upconversion monolayer on the array area is of lower order compared to that on the flat gold area and also decreases as function of the excitation power. This would support the intensification of the excitation field by the gold cavity array surface as being the mechanism of enhancement.

The organization, complexing, and aggregation of biomolecules in solution and in lipid membranes lie at the foundation of many critical processes in metabolism, signaling, and disease. But our understanding of these phenomena remains limited by our ability to probe small aqueous volumes under physiological conditions. Here we present progress toward a high-brightness, rapidly scannable, nanoscale light source, which can interrogate biomolecular dynamics in highly concentrated aqueous environments below the diffraction limit. This light source consists of a thin film of cathodoluminescent (CL) material that can be activated by a low energy, tightly focused electron beam, easily obtained in a scanning electron microscope (SEM). By integrating the CL film into a liquid sample cell, a biological sample can be isolated from the SEM&’s vacuum environment.
To fabricate the liquid cell, we use pulsed laser deposition (PLD) to grow smooth, highly crystalline films of cerium-doped yttrium aluminum perovskite (YAP:Ce), an efficient CL material, on silicon substrates prepared with adhesion layers of strontium titanate and lanthanum aluminate. Extensive characterization of our films reveals that, at a thickness of 10 nm, their spectral properties are identical to bulk YAP:Ce; they withstand prolonged exposure to electron bombardment; and their CL brightness is within an order of magnitude of the bulk material, as confirmed with the optical detector in our SEM. Furthermore, we use x-ray diffractometry to confirm the high crystallinity of our films, and atomic force microscopy to show that their mean roughness is on the order of 0.2 nm, resulting in highly uniform CL emission over large areas (also confirmed via SEM). To expose one side of the CL film to the electron beam and the other side to the aqueous region within the liquid cell, a portion of the silicon substrate will be etched away, making a “window.” By working at low (~1 keV) electron energies, the electron beam will be converted to an optical excitation instead of impinging directly on the biological sample. Because the CL films are only 10 nm thick, it should be possible to non-radiatively transfer optical excitations to adjacent fluorescently labeled molecules in the encapsulated sample while preserving a resolution also on the order of 10 nm. Thus, we anticipate achieving a spectrally-specific biocompatible scanning optical microscopy with at least 20 nm lateral resolution and 10 nm axial resolution, with video frame rates, excellent background rejection, and no moving parts.

The synthesis, structures and properties of a remarkable new class of discrete self-assembled molecules which feature both lanthanide (4f) and d-block transition metals will be described. The use of specifically designed ligands results in the formation of cage-like supramolecular architectures which possess drum or barrel like structures. So far the compounds feature stoichiometries in which there is a Ln:Tm ratio of 1:3 (Ln = lanthanide ion; Tm = d-block transition metal). Examples include [Ln8Cd24L12(OAc)48] and [Ln6Cd18L9Cl9(OAc)27]. The ligands "L" are derived from a Schiff-base ligand of the "salen" type but which contain 6 or more methylene (CH2) units in the backbone linking the metal binding N,O terminals. The potential applications in host-guest encapsulation and sensing will be discussed.

In recent years, bottom up strategies for the synthesis of upconverting luminescent nanoparticles (UCLNPs) based on lanthanide-doped NaYF4 have become increasingly more elaborate. Currently, monodisperse, oleic acid-coated nanocrystals with diameters less than 20 nm can be prepared reproducibly on a large scale. Therefore, this class of nanocrystals is highly attractive for in vitro and particularly in vivo applications, where a narrow particle size distribution is crucial. UCLNPs can be excited with near-infrared light, which reduces background luminescence and photodamage to biological tissue. Moreover, they display anti-Stokes emission (upconversion luminescence) in the visible spectrum and are highly photostable.
The greatest challenge lies with proper surface modification. The inherently hydrophobic UCLNPs require sophisticated surface engineering to render them water dispersible and amenable to bio(analytical) applications. Various solutions are reported in the literature, including silica coating, polymer encapsulation, ligand exchange, and the utilization of amphiphilic molecules and surfactants. For many applications, it is imperative that the UCLNPs are biocompatible after surface modification and that they do not aggregate under physiological conditions.
In this study UCLNPs coated with various amphiphilic molecules were compared with silica-coated UCLNPs. The nanocrystals are characterized by transmission electron microscopy, X-ray diffraction, Raman spectroscopy, zeta-potential measurements, dynamic light scattering, and inductively coupled plasma atomic emission spectroscopy. The photophysical properties of the respective UCLNPs with regard to the ratio of upconversion emissions and emission lifetimes in water were investigated. The binding properties were verified using a label-free surface plasmon resonance technique. The specificity and the binding kinetics of surface-engineered UCLNPs towards biomolecules immobilized on a gold substrate were evaluated. Furthermore, cell incorporation and cell cytotoxicity with respect to the coated UCLNPs were compared. These results aid in the engineering of an appropriate UCLNP surface for further bio-conjugation and bio(analytical) applications.

Photon upconversion is a promising approach to surpass the Shockley-Quiesser limit of photovoltaic cells. Upconverting materials convert sub-band gap light to higher energy photons that can then be utilized by the cell. However, the field is incipient and current materials require substantial optimization if upconverters are to reach industrial relevance. In nanostructured upconverters, the upconversion efficiency is critically dependent on the nanocrystal phase, size, and shape (and of course the choice of materials). Analyzing the influence of pressure represents a simple and powerful means of probing the influence and optimal values of the aforementioned parameters while simultaneously providing crucial insight into the structure of these poorly understood materials.
In this work, we investigate the pressure dependence of upconverted luminescence (UCL) from hexagonal-phase NaYF₄:Er³⁺,Yb³⁺ nanoparticles. We synthesized nanoparticles with diameters of 25, 40, and 90 nm using a facile procedure which employs diffusion-limited growth of nanocrystals coated in an organic solvent, effectively suppressing nucleation of cubic-phase partiles. Phase-purity was verified via x-ray diffractometry. Particles were loaded in a diamond-anvil cell using silicone oil as the pressure medium within a stainless steel gasket. The cell also contained a small ruby, the fluorescence peak of which was used to determine the pressure within the cell. Ruby fluorescence was achieved via excitation with an Ar-ion laser emitting at 488 nm, while a diode laser emitting at 980 nm was used to excite UCL. The ruby fluorescence and UCL were quantified using an optical spectrometer while pressure was varied from ambient up to approximately 25 GPa.
Particles of each of the three sizes studied exhibit a negative correlation between pressure and UCL intensity. Additionally, noticeable shifts in emission peak positions are observed; whereas peaks initially located at 550 nm redshift by as much as 20 meV, peaks at 654 nm exhibit 3-meV blueshifts while the 540-nm emission shows no spectral change. The peaks return to their initial positions upon pressure reduction. A decrease in the average erbium-erbium interionic distance (which would increase the prevalence of cross-relaxation) or a detuning of the energy resonance between the Yb³⁺ sensitizing state and the Er³⁺ emitting states are the two most likely hypotheses explaining the observed decrease in UCL with pressure. The shifts in peak positions are attributed to changes in the surrounding crystal field environments of the ions due to lattice distortion at high pressures. These results provide insight into the as-yet-unknown positions of dopant Er³⁺ and Yb³⁺ ions within the hexagonal NaYF₄ host lattice. Additionally, our results probe the relative importance of interionic separations and energy resonances in this system, offering an increased depth of understanding which will aid in the optimization of these materials.

Lanthanide-doped upconversion nanoparticles (UCNPs) have attracted growing attention due to their unique optical property of emitting visible light when being excited by NIR radiation. These UCNPs have been popularly applied in a variety of fields such as bioimaging, three-dimensional displays, and solid-state lasers. In our work, we incorporated UCNPs into polymeric nanocomposites by either layer-by-layer (LbL) assembly or electrospinning technique. Both types of upconversion polymeric nanocomposites were characterized by a variety of microscopic techniques. The embedded UCNPs preserved their excellent UC properties and the nanocomposites were uniform and mechanically robust. The methods we developed in preparing upconversion polymeric nanocomposites with controlled morphologies, structures, and properties may offer new possibilities for applying UCNP in broad applications.

Lanthanide ion emission arises from f-f transitions and, due to the core nature of the f orbitals, leads to line-like emission spectra. This translates into high color purity, which is interesting for display as well as imaging applications. We recently started working towards monomers incorporating lanthanide ions capable of forming luminescent polymers. The chelating and sensitizing moiety is a pyridine-bis(oxazoline), while different pendent groups are utilized, which will enable polymerization of the compounds. We discuss here progress made towards monomers with methylmethacrylate, norbornene and terthiophene as the polymerizable functional groups, as well as their characterization and the formation of lanthanide-containing polymers obtained from these monomers.

Multifunctional luminescent nanomaterials can be defined as a nanostructured luminescent materials which also have other functions such porous structures for controlled absorption and release of drug molecules, magnetic properties for targeting and MRI (magnetic resonance imaging) besides the luminescent properties (down-conversion and up-conversion luminescence) for optical imaging. These materials are promising for realizing the diagnosis and therapy in one material system, which has aroused great interests in biomedical fields in recent years. As good alternatives for optical imaging agents, upconversion (UC) luminescence materials are emerging as a new class of fluorescent biolabels. Compared with conventional fluorescent labels, such UC nanomaterials can convert low-energy light to high-energy light through a two-photon or multiple photon process. These UC nanomaterials thus combine all the advantages of traditional two-photon imaging such as weak autofluorescence from biological samples, remarkable light penetration depth in tissue, high signal-to-noise ration and no photobleaching and photoblinking. So far, how to realize the controllable fabrication of multifunctional luminescent nanomaterials that meet the requests of biomedical application (such as suitable size less than 250 nm, water dispersable, low cytotoxicity etc) is still a great challenge for chemical researchers.
In this presentation, we will highlight the research work in our group for the controllable preparation of multifunctional nanomaterials together with their properties and application in biomedical fields. The obtained materials include: (i) hollow structured and spherical luminescent materials, such as CaF2: Ce3+/Tb3+ and CaF2: Ce3+/Tb3+-poly(acrylic acid) hybrid microspheres, GdPO4:Eu3+, GdVO4: Yb3+/Er3+, NaYF4:Yb3+/Er3+ etc; (ii) core-shell structured nanocomposite materials such as SBA-15@YVO4:Eu3+, Yb(OH)CO3@YbPO4:Er3+, Fe3O4@nSiO2@mSiO2@YVO4:Eu3+ (NaYF4:Yb3+, Er3+/Tm3+), NaYF4:Yb3+/Er3+ UCNs/Hydrogel, Gd2O3:Eu3+@P(NIPAm-co-AAm) @HMS etc; (iii) fiber like and porous structured SiO2, Ca10(PO4)6(OH)2:Eu3+ and NaYF4:Yb3+/Er3+@Silica nanocomposites, rod-like and porous Sr5(PO4)3OH etc. All these materials have been investigated as drug carriers and/or cell imaging agents with IBU and anticancer drug (DOX) as models. Temperature and pH dependent controlled releasing properties, emission intensity dependence of the system on the cumulative released amount of the model drug have been found in these multifunctional luminescent nanomaterials, which are promising for future biomedical applications.
References
[1] Piaoping Yang, Shili Gai and Jun Lin, Chem. Soc. Rev., 41(9) (2012), p. 3679.
[2] Yunlu Dai, Ping&’an Ma, Jun Lin et al., ACS Nano, 6(4) (2012), p. 3327.
[3] Zhiyao Hou, Chunxia Li, Jun Lin et al., Adv. Funct. Mater., 22(13) (2012), p.2713.
[4] Xiaojiao Kang, Ziyong Cheng, Jun Lin et al., Adv. Funct. Mater., 22(7) (2012), p. 1470.

Spectroscopic imaging was used to study the surface plasmon polariton (SPP) enhanced upconversion luminescence of NaYF₄ :Tm:Yb nanoparticles embedded in PMMA supported on Au nanopillar arrays. Spatially-resolved upconversion spectra show enhancement in both the visible and near-infrared region of the spectrum, clearly associated with the plasmonic resonances of an engineered periodic array of nanopillars. The array has a lattice resonance associated with the SPP near 980nm, at the peak absorption of the Yb³⁺ ion, while the local surface plasmon resonance (LSPR) of the individual pillars is seen to enhance the near-infrared emission of Tm³⁺ near 800nm. The combined effect results in a significantly higher enhancement of the near-infrared emission when compared to the visible upconversion lines of Tm³⁺, consistent with the interpretation of sequential surface plasmon assisted absorption and emission at two separate and disparate energies. The presence of SPP and LSPR were confirmed by spectrally resolved reflectivity, and the mechanism for luminescence enhancement was investigated by time resolved measurements of the luminescence decay. Reflectivity measurements are compared to finite difference time domain simulations (FDTD).

Water-soluble nanoparticles are introoduced as scaffolds for multimodal imaging applications in the nanoscale. Thiol-active lanthanide complexes have been used to coat gold nanoparticles and their cell uptahe was monitored together with their toxicity. Dithiol-modified peptides have also been used in coating nanoparticles for targeting their delivery in cells. pH Controlled delivery of lanthanide nanoparticles in platelets has been achieved with a co-coating of lanthanide probes and a pH low insertion peptide. The rapid delivery of the luminescent nanoparticles is achieved only upon pH change from 7.3 to 6.5. Imaging techniques based on transmission electron microscopy, luminescence and confocal reflectance microscopy have been employed throughout to monitor the delivery of the nanoparticles.

Due to their unique properties in converting low energy light into higher energy electronic transitions, upconverting nano-materials have garnered considerable interest in bio-imaging, photovoltaic, and opto-electronic applications. In particular, lanthanide-doped upconverting nanoparticles (UCNPs) have demonstrated a host of functionalities due to their nanoscale dimensions and wide range in transition-metal doped compounds. Unlike quantum dots and other single emitters that exhibit luminescence blinking and bleaching, UCNPs undergo upconverting energy processes regardless of material size and provide high photostability in ambient environments. However, while much as been reported on the optical properties of UCNPs, a fundamental understanding of the photo-physics remains unclear, particularly at nanoscale dimensions where surface properties can dominate energy transfer processes. Moreover, the emissive properties of single UNCPs with diameters less than 10nm, ideal for cellular uptake in bio-imaging, are still relatively unknown.
Here we report on the luminescence properties of Er³⁺, Yb³⁺-doped NaYF₄ UCNPs with diameters ranging from 5 - 150 nm. Optical characterization of the luminescence lifetime and spectral emission of UCNPs on a single particle level reveal a strong dependence on UCNP size and surface passivation¹, which provides a basis for investigating the enhanced optical properties of single particles coupled with functional architectures. In addition, a quantitative model based on Yb and Er distances inside the NaYF₄ crystal is presented to explain how over 90% of the total luminescence originates from only a few atomic emitters. Details from this model will be used to describe how significant enhancements in the optical properties of nanoscale upconverting materials based on dopant proximity can be realized.
[1] - A. Ostrowski, D.J. Gargas, et al ACS Nano 6, 2686, 2012

Lanthanide-doped NaYF₄ upconversion nanophosphors (UCNPs) have been extensively studied in recent years due to their high chemical stability, photostability (no photobleaching) compared with conventional organic fluorophores, and environmentally benign nature (Cd-free composition). The NaYF₄ has two different polymorphs such as cubic α phase and hexagonal β phase. Among these, β phase NaYF₄ with hexagonal structure is known as the most efficient host material for upconversion luminescence. The β-NaYF₄:Yb,Er UCNPs emit bright green light under illumination of 980 nm infrared (IR) light. Utilizing IR light as an excitation source makes the UCNPs a promising material for applications to bio imaging and transparent display.
In this study, we systematically controlled morphology of β-NaYF₄:Yb,Er UCNPs by manipulating synthetic conditions. When the ratio of coordinating solvent [oleic acid (OA)] to non-coordinating solvent [1-octadecene (ODE)] was increased, nanorod morphology evolved from spherical shape. This is due to preferred adsorption of OA ligand on {10-10} planes. Below critical concentration of OA, the size of UCNPs decreased with increasing OA amounts. However, above critical concentration of OA, aspect ratio of the β-NaYF₄:Yb,Er UCNPs increased from 1.0 to 2.7 depending on the ratio of OA to ODE. In addition, rounded end-tip of the UCNP nanorods was flattened by using an additive. When Cl^- ions were present in reaction solution, they inhibited the growth along <0001> direction and stabilized {0001} planes of β-NaYF₄:Yb,Er nanorods. As a consequence, hexagonal prism morphology bounded with {10-10} and {0001} planes was formed, and aspect ratio of hexagonal prism was also adjusted with varying the amount of Cl^- ions. However, too much Cl^- ions in the reaction solution resulted in formation of α-NaYF₄:Yb,Er. The β-NaYF₄:Yb,Er UCNPs showed green light peaking at 542 nm under 980 nm excitation. However, the UCNPS with different morphologies showed different color coordinates. In addition, the synthesized UCNP nanorods and hexagonal prisms were blended with polydimethylsiloxane (PDMS) polymer and both nanorod-PDMS and hexagonal prism-PDMS composites showed bright green light under IR laser pointer.
In summary, morphology of β-NaYF₄:Yb,Er UCNPs was rationally controlled from sphere to rod and from rod to hexagonal prism by adjusting the concentration of ligand and an additive. They emit green light with different color coordinates according to their morphologies. Their composites with PDMS were highly transparent and showed bright green light under 980 nm IR excitation, implying that they are promising materials for transparent three-dimensional volumetric displays.

Nitride phosphors have been reported to be of potential for applications in solid state lighting. However, presently available methods for the synthesis of nitride phosphors all require relatively severe conditions such as high temperature, high pressure, difficulty in handling(due to using moisture or oxygen sensitive reactants) and long processing time. In this presentation, we report the development of a new method for the synthesis of a nitride phosphor (i.e., CaSiAlN3) based on self-propagating high temperature synthesis (SHS) reactions. The reactants used for the synthesis include calcium, silicon, aluminum, silicon nitride, sodium azide, ammonium chloride, europium oxide, neodymium oxide, gallium oxide, dysprosium oxide, manganese oxide and cerium oxide. These powders were mixed and pressed into compacts. The reactant compacts were then wrapped up with an igniting agent (e.g., Mg+Fe3O4). The reactant compact was ignited by electrical heating under a N2 atmosphere of ~6 atm. The CaSiAlN3 phosphor could thus be produced under a low N2 pressure and a short reaction time. We also investigated the effects on the properties of the product of several important experimental parameters including reactant compositions, compact pressure, N2 pressure and different type of activator. The luminescence properties were also compared with a commercial phosphor. The results showed that the luminescence intensity and wavelength of the CaSiAlN3 phosphor are affected by several experimental parameters such as reactant composition, N2 pressure, compact pressure and different type of activator. A typical product has an excitation spectrum in the range of 220-500nm and a single broadband emission in the range of 400-670nm centering at 562nm upon excitation at 380nm.The luminescence intensity is 99.8% of a corresponding commercial phosphor.

I will describe our recent results on colloidal Ln3+ doped nanoparticles for application as deep-tissue optical and magnetic resonance imaging contrast agents. With respect to the optical properties we have an emphasis on the biological window, i.e. the near-infrared range roughly from 700 to 1300 nm, where tissue is more transparent than to UV-visible light, thus providing an opportunity for deep-tissue imaging. The general strategy to improve the optical properties is to make core-shell structures in order to reduce quenching processes. In addition, we develop better T1 and T2 contrast agents for magnetic resonance imaging (MRI). The synthesis and basic characterisation of the nanoparticles will briefly be discussed, with some emphasis on the challenges to prove the actual formation of core-shell structures. To this end, advanced high-resolution electron microscopy and energy-dependent X-ray photo-electron spectroscopy, the latter at the Canadian Light Source, will be discussed. Preliminary results on targeted imaging and cytotoxicity and biodistributions will also be included.

In lanthanide-doped materials, energy transfer (ET) between codopant ions can populate or depopulate excited states, giving rise to spectrally pure luminescence that is valuable for the multi-color imaging and simultaneous tracking of multiple biological species. Here, we use high throughput methods to synthesize NaYF4 nanocrystals co-doped with every combination of three lanthanide ions. We used the spectra from this combinatorial library to theoretically investigate the ET mechanisms that selectively enhance or suppress visible upconversion luminescence under near-infrared excitation. Using an experimentally validated population balance model and using a path-tracing algorithm to objectively identify transitions with the most significant contributions, we isolated networks of pathways that divert energy away from undesirable radiative transitions and concentrate energy in manifolds that participate in desired energy transfer upconversion (ETU) processes. In efficient and spectrally pure upconverting nanoparticles, such as NaYF4:Er3+/Tm3+, the strength of these ETU processes is a consequence of strong donor-acceptor coupling, optimum energy level alignment, and the concentration of excited state population in donor manifolds. Ultimately, these ET pathways and others elucidated by our combinatorial screening and theoretical modeling will enable the programming of physical properties in lanthanide-doped materials for a variety applications that demand strong and precisely defined optical transitions.

Lanthanide doped fluorides are interesting hosts for efficient luminescence, thus opening the door to their use in important technological applications, especially in biomedical diagnostics. Er3+/Yb3+ and Tm3+/Yb3+ doped MF2 (M=Ca, Sr) nanoparticles (NPs) very recently have received attention, due to their strong upconversion (anti-Stokes) emission [1-3]. A facile hydrothermal one-step procedure was used to prepare citrate capped MF2 NPs triply doped with Er3+, Gd3+ and Yb3+ or Tm3+, Gd3+ and Yb3+ ions. The present NPs are easily dispersible in saline solutions, essential properties for their potential use in biological fluids. The obtained nanoparticles are cubic single phase and are well size monodispersed, with average sizes that can be easily tuned by changing the preparation conditions. The obtained transparent saline colloidal dispersions show strong upconversion emission in the red (around 650 nm) and in the NIR (around 800 nm) for the Er3+ doped and Tm3+ doped nanoparticles, respectively, upon laser excitation at 980 nm in the 2F5/2 level of Yb3+. Both the excitation and the emitted radiation are close to or inside the biological window, suggesting the possible use of the NPs for in-vitro and in-vivo biological optical imaging. Results of spin echo measurements on saline colloidal dispersions of the NPs have also shown significant proton relaxivities, indicating that the NPs are interesting MRI contrast agents. Preliminary results on NPs of different sizes show that the relaxivity values increase on decreasing the particle size, as evidenced also for NaGdF4 nanoparticles [4]. Electron spin resonance spectra on the present NPs, measured at room temperature, evidence pronounced magnetization properties. Considering also the scarce toxicity, the present NPs are suitable candidates to be efficiently used as bimodal probes for both in-vitro and in-vivo multimodal optical and magnetic resonance imaging.
[1] G. F. Wang, Q. Peng, Y. D. Li, J. Amer. Chem. Soc., 131 (2009) 14200.
[2] M. Pedroni, F. Piccinelli, T. Passuello, M. Giarola, G. Mariotto, S. Polizzi, M. Bettinelli, and A. Speghini, Nanoscale, 3 (2011) 1456.
[3] N.-N. Dong, M. Pedroni, F. Piccinelli, G. Conti, A. Sbarbati, J. E. Ramírez-Hernández, L. M. Maestro, M. C. Iglesias-de la Cruz, F. Sanz-Rodriguez, A. Juarranz, F. Chen, F. Vetrone, J. A. Capobianco, J. García Solé, M. Bettinelli, D. Jaque, A. Speghini, ACS Nano, 5 (2011) 8665.
[4] N. J. J. Johnson, W. Oakden, G. J. Stanisz, R. Scott Prosser, F. C. J. M. van Veggel, Chem. Mater. 23 (2011) 3714.

Rare earth doped wide bandgap materials have gathered attention in the past decade for various optoelectronic applications. This is mainly due to the fact that the light emission wavelength of rare earths depends only very weakly on the host matrix, and wide bandgap matrices are optically transparent in the visible region. Furthermore, the light emission intensity can be enhanced through thermal annealing treatments. However little is known concerning the different excitation mechanisms. Moreover while the light emission concentration quenching effect is well accepted, there are only few characterizations of this process under different excitation sources.
We present a systematic study of the light emission properties of Tb doped a-SiC:H and c-AlN through PL and CL measurements under different Tb atomic concentrations. The optimal concentration for the highest light emission is found to be different in both experiments. The implications of this effect are evaluated.

An overview of the design and development of highly luminescent Ln(III) complexes (with Ln = Tb, Eu) for applications in biotechnology will be presented. In particular, developments utilizing the 2-hydroxyisophthalamide (IAM) chelate for Tb(III).(1-3)
A focus of our research has been the optimization of these compounds as potential commercial agents for use in Homogeneous Time Resolved Fluorescence (HTRF) technology. These assays have become widespread in pharmacology and biotechnology. By using the long-lived luminescence of a lanthanide energy donor, the sensitivity of this assay format can be vastly improved by using time gated excitation and detection. These compounds are now used in commercial products by three different companies.(4)
(1) S. Petoud, S.M. Cohen, J.-C.G. Bünzli, K.N. Raymond, JACS, 2003, 125, 13324-13325.
(2) A.P.S. Samuel, E.G. Moore, M. Melchior, J. Xu, K.N. Raymond, Inorg. Chem., 2008, 47, 7535-7544.
(3) J. Xu, T.M. Corneillie, E.G. Moore, G.-L. Law, N.G. Butlin, K.N. Raymond, J. Am. Chem. Soc. 2011, 133, 19900-19910.
(4) http://www.lumiphore.com

Theranostic nanomedicines allow site-specific drug delivery and real-time assessment of biodistribution. Because of the high resolution and relative safety of magnetic resonance imaging (MRI), we aim to create a theranostic MRI agent. However, no clinically used MRI agent offers sufficient contrast to assess biodistribution, so our agent must also incorporate gadolinium (Gd) in a way that improves contrast enhancement.
To this end, we designed a hydrogel nanoparticle (nanogel) in which crosslinkers chelate Gd, reducing its tumbling frequency to increase contrast. We synthesized a diethylene triamine pentaacetic acid (DTPA) bis(methacrylamide), to form 80-150 nm nanogels through radical polymerization in inverse emulsion. These nanogels have a high relaxivity (~10 mM-1s-1), and are ~2-3 times brighter than the clinically used Magnevist (at the same Gd concentration) as shown by phantom imaging. In vivo imaging reveals that our nanogel also has a longer circulation time , increasing the time available for accumulation in target sites. Enacapsulating a fluorescein-labeled model protein demonstrated these nanogels are capable of gradually releasing protein drugs.
Transmetallation with Zn2+ induces displacement and release of toxic free Gd. Monitoring the longitudinal relaxation time in the presence of phosphate and Zn2+.revealed that the nanogel is more stable, allowing less dechelation than Magnevist.
Our nanogel&’s high relaxivity and stability support its clinical potential. In the long term, incorporating peptides or aptamers would allow targeted imaging and therapy.

Paramagnetic lanthanide (III)-based complexes have been the focus of the contrast agent development for decades. Recently, nanoparticles (NPs) conjugated with lanthanide have attracted widespread attention as novel biomedical imaging agents. Among many nanomaterials, layered double hydroxide (LDH) stands out for its versatile anion-intercalation capability and inherent presence of water molecules within the structure. The interlayer spaces between positively charged hydroxide layers are occupied by intercalated anions, with water held in place via hydrogen bonding to the hydroxyls. In this study, we report the design of a new type of NP consisting of Gd(DTPA)^2- intercalated into (Mg, Al)-LDH, (GLDH-NP), to be used as a powerful positive contrast agent in MRI.
The synthesis, intercalation, and post heat treatment of GLDH-NP are presented. Crystalline GLDH-NPs of different particle sizes, from a few hundred nanometers to a couple of micrometers, with various Gd(DTPA) loading capacities, can be achieved. X-ray diffraction patterns show that the interlayer spacing expands to 20.55 Å after intercalation. R₁ and R₂ measurements are carried out using inversion recovery and Carr-Purcell-Meiboom-Gill NMR pulse sequences, respectively. At 15.4 MHz frequency, R₁ is ~30 s^-1mM^-1, twice as fast as Gd³⁺ ion, and ~6 times faster than Gd(DTPA).
The relaxivity enhancement of water proton stems from time fluctuation of the dipolar coupling between the electron magnetic moment of the metal ion and the nuclear magnetic moment of the protons in the interlayer water molecules of the LDH NPs. In theoretical descriptions of proton spin relaxation in lanthanide contrast agents several key parameters are stressed: the number of metal-bound water molecules (q), their mean residence time (tau;_M), NP&’s rotational correlation time (tau;_R), and electron relaxation times (T_1,2e). Through a series of experiments, we studied how these parameters may affect the relaxivity enhancement for this particular type of contrast agent. The change in relaxivities due to tau;_R is minimal. Probably, the very long tau;_M is the dominant factor in the relaxivity augmentation. We also optimized the ratio between the inherent water amount and the intercalated Gd(DTPA). Furthermore, we evaluated the effects on the relaxivities due to Larmor frequency changes. From current data on relaxivities at both 15.4 and 300 MHz field strengths, we find that the differences in relaxivities can be substantial. Investigation of this question will continue.
In the near future, we will use this system for in vivo tests; i.e., rat brain MRI scans. A thorough study and understanding of this type of contrast agent will allow us to exploit its potential for applications in biomedicine, as well as to gain new concepts for contrast agent design.