Solution-Based Synthesis Routes for the Preparation of Noncentrosymmetric 0-D Oxide Nanocrystals with Perovskite and Nonperovskite Structures.

With the miniaturization of electronic-based devices, the foreseen potential of new optical nanoprobes and the assessment of eventual size and shape effects, elaboration of multifunctional noncentrosymmetric nanocrystals with ferroelectric, pyroelectric, piezoelectric, and nonlinear optical properties are the subject of an increasing research interest. Here, the recent achievements from the solution-based methods (coprecipitation in homogeneous and nanostructured media, sol-gel processes including various chemistries and hydro/solvothermal techniques) to prepare 0-D perovskite and nonperovskite oxides in the 5-500 nm size range are critically reviewed. To cover a representative list of covalent- and ionic-type materials, BaTiO3 and its derivatives, niobate compounds (i.e., K/Na/LiNbO3 ), multiferroic BiFeO3, and crystals of lower symmetry including KTiOPO4 and some iodate compounds such as Fe(IO3 )3 and La(IO3 )3 are systematically in focus. The resulting size, morphology, and aggregation state are discussed in light of the proposed formation mechanisms. Because of a higher complexity related to their chemical composition and crystalline structures, improving the rational design of these multifunctional oxides in terms of finely-tuned compositions, crystalline hosts and structure-property relationships still need in the future a special attention of the research community to the detailed understanding of the reaction pathways and crystallization mechanisms.

Luminescent Yb3+,Er3+-Doped α-La(IO3)3 Nanocrystals for Neuronal Network Bio-Imaging and Nanothermometry.

Dual-light emitting Yb3+,Er3+-codoped α-La(IO3)3 nanocrystals, known to exhibit both second harmonic signal and photoluminescence (PL), are evaluated as optical nanoprobes and thermal sensors using both conventional microscopes and a more sophisticated micro-PL setup. When loaded in cortical and hippocampal neurons for a few hours at a concentration of 0.01 mg/mL, a visible PL signal arising from the nanocrystals can be clearly detected using an epifluorescent conventional microscope, enabling to localize the nanocrystals along the stained neurons and to record PL variation with temperature of 0.5% K-1. No signal of cytotoxicity, associated with the presence of nanocrystals, is observed during the few hours of the experiment. Alternatively, a micro-PL setup can be used to discriminate the different PL lines. From ratiometric PL measurements, a relative thermal sensitivity of 1.2% K-1 was measured.

Autofluorescence-Free In Vivo Imaging Using Polymer-Stabilized Nd3+-Doped YAG Nanocrystals.

Neodymium-doped yttrium aluminum garnet (YAG:Nd3+) has been widely developed during roughly the past 60 years and has been an outstanding fluorescent material. It has been considered as the gold standard among multipurpose solid-state lasers. Yet, the successful downsizing of this system into the nanoregimen has been elusive, so far. Indeed, the synthesis of a garnet structure at the nanoscale, with enough crystalline quality for optical applications, was found to be quite challenging. Here, we present an improved solvothermal synthesis method producing YAG:Nd3+ nanocrystals of remarkably good structural quality. Adequate surface functionalization using asymmetric double-hydrophilic block copolymers, constituted of a metal-binding block and a neutral water-soluble block, provides stabilized YAG:Nd3+ nanocrystals with long-term colloidal stability in aqueous suspensions. These newly stabilized nanoprobes offer spectroscopic quality (long lifetimes, narrow emission lines, and large Stokes shifts) close to that of bulk YAG:Nd3+. The narrow emission lines of YAG:Nd3+ nanocrystals are exploited by differential infrared fluorescence imaging, thus achieving an autofluorescence-free in vivo readout. In addition, nanothermometry measurements, based on the ratiometric fluorescence of the stabilized YAG:Nd3+ nanocrystals, are demonstrated. The progress here reported paves the way for the implementation of this new stabilized YAG:Nd3+ system in the preclinical arena.

Nd3+ doped Gd3Sc2Al3O12 nanoparticles: towards efficient nanoprobes for temperature sensing.

Development of contactless temperature-probing nanoplatforms based on thermosensitive near-infrared (NIR) light-emitting nanoparticles opens up new horizons for biomedical theranostics at a deep tissue level. Here, we report on the crystallinity and relative thermal sensitivity of NIR emitting Nd3+ doped Gd3Sc2Al3O12 (GSAG:Nd3+) nanoparticles synthesized by a solvothermal method. The obtained nanoparticles are well-crystallized, with sizes less than 100 nm, and can be dispersed in water without any additional functionalization. Upon excitation at 806 nm, the nanoparticles exhibit emission in the first and second biological optical transparency windows. The temperature sensing properties were evaluated from the luminescence intensity ratio of the thermally coupled emission lines corresponding to the R1, R2→Z5 transitions between the Stark sublevels of the 4F3/2 and 4I9/2 electronic states of Nd3+ in the physiological temperature range of 20-50 °C. GSAG:Nd3+ nanoparticles exhibit a maximal relative thermal sensitivity of 0.20% °C-1, higher than that of YAG:Nd3+ nanoparticles used as a control, due to the difference in the crystal field of the host matrices. A higher synthesis temperature in the range of 300-400 °C was also provided to improve the crystallinity of the GSAG:Nd3+ nanoparticles which results in a higher relative thermal sensitivity. Our results demonstrate the potential of GSAG:Nd3+ nanoparticles as luminescence nanothermometers and emphasize the interest of the GSAG matrix itself, which with the presence of Gd, could lead to multimodal diagnostic applications in nanothermometry and magnetic resonance imaging (MRI).

Microwave Synthesis and Up-Conversion Properties of SHG-Active α-(La, Er)(IO3)3 Nanocrystals.

Pure α-La(IO3)3 and α-La0.85Er0.15(IO3)3 nanocrystals were synthesized by a microwave-assisted hydrothermal method leading to a reaction yield of 87 ± 4%. Electron microscopy and dynamic light scattering characterizations provide evidence for the formation of nanocrystals with an average size of 45 ± 10 nm for α-La(IO3)3 and 55 ± 10 nm for α-La0.85Er0.15(IO3)3. When dispersed in ethylene glycol, the nanocrystal suspensions exhibit second-harmonic generation under near-infrared excitations at 800 and 980 nm whereas additional photoluminescence by up-conversion is simultaneously observed in the case of α-La0.85Er0.15(IO3)3 nanocrystals. Quantitative assessments of the second-harmonic generation efficiency from second-harmonic scattering experiments at 1064 nm result in relatively high ⟨ d⟩ coefficients measured at 8.2 ± 2.0 and 8.0 ± 2.0 pm V-1 for α-La(IO3)3 and α-La0.85Er0.15(IO3)3, respectively. The relative intensity between second-harmonic generation and photoluminescence is discussed following the excitation wavelength.

A new solvothermal method for the synthesis of size-controlled YAG:Ce single-nanocrystals.

Ce3+-doped Y3Al5O12 (YAG:Ce) nanocrystals were synthesized by a unique solvothermal method, under sub-critical conditions. A home-made autoclave was used, operating in a larger pressure and temperature range than that with conventional commercial equipment and allowing direct in situ photoluminescence (PL) and X-ray absorption characterizations. The study of various synthesis conditions (pressure, temperature, precursor concentration, reaction time) allowed the best reaction conditions to be pinpointed to control YAG:Ce nanocrystal size, as well as crystal quality, and to get efficient optical properties. Without any post thermal treatment, we succeeded in obtaining well-crystallized YAG:Ce nanocrystals (30-200 nm), displaying typical PL properties of YAG:Ce with a maximal emission at 550 nm. The pristine 100 nm-sized YAG:Ce nanoparticles present an internal quantum yield of about 40 ± 5%. In situ X-ray absorption near edge spectroscopy demonstrates the presence of Ce4+ in nanocrystals elaborated at high temperature, resulting from the oxidation of Ce3+ during the crystallization process.

Determination of paramagnetic concentrations inside a diamagnetic matrix using solid-state NMR.

The determination of very low doping levels in solid materials is an important issue for many applications. When considering paramagnetic dopants, the NMR relaxation technique appears to be much more accurate than classical techniques such as Vegard's law resulting from X-ray diffraction (XRD) measurements or chemical analysis that cannot provide information on appropriate dopant spatial distributions. In a recent report, the linear variation of 1/T1, i.e. the nuclear relaxation rate, as a function of Nd3+ content has been used to determine doping levels with a good dispersion homogeneity in the monazite LaPO4 matrix down to 0.1 mol%. We here extend this study to more complex compounds doped with Nd3+, such as YPO4, the solid solution Y0.8Sc0.2PO4, Ba5(PO4)3Cl and a phosphate glass. For all considered compounds except Ba5(PO4)3Cl:Nd, 1/T1 is found to be linearly proportional to the nominal Nd concentration, confirming the ability of the method to investigate the dopant concentration and spatial homogeneity. The results obtained for different compounds open up the discussion on the parameters, such as the orbital overlap and the average P-P distances, influencing the nuclear relaxation rate.

Luminescent nanoparticle trapping with far-field optical fiber-tip tweezers.

We report stable and reproducible trapping of luminescent dielectric YAG:Ce(3+) nanoparticles with sizes down to 60 nm using far-field dual fiber tip optical tweezers. The particles are synthesized by a specific glycothermal route followed by an original protected annealing step, resulting in significantly enhanced photostability. The tweezers properties are analyzed by studying the trapped particles residual Brownian motion using video or reflected signal records. The trapping potential is harmonic in the transverse direction to the fiber axis, but reveals interference fringes in the axial direction. Large trapping stiffness of 35 and 2 pN µm(-1) W(-1) is measured for a fiber tip-to-tip distance of 3 µm and 300 nm and 60 nm particles, respectively. The forces acting on the nanoparticles are discussed within the dipolar approximation (gradient and scattering force contributions) or exact calculations using the Maxwell Stress Tensor formalism. Prospects for trapping even smaller particles are discussed.

A solid-state effect responsible for an organic quintet state at room temperature and ambient pressure.

A stable organic diradicaloid with an intermolecular quintet at room temperature as a polycrystalline solid is studied. The conclusion is supported by the observation of the ΔMs = ±2 forbidden transition, electron spin resonance (ESR) simulations, and density functional theory (DFT) calculations. In addition, the molecule, as the active component of a device, is an outstanding near-infrared photodetector with detectivity over 10(11) cm Hz(1/2) W(-1) at 1200 nm.

Nanoparticulate coatings with efficient up-conversion properties.

Nanoparticulate films with high up-conversion emission (UC) properties were prepared by spray-deposition of nanometer-sized YVO4:Yb,Er particles. The optical properties of YVO4:Yb,Er were optimized upon annealing before the film deposition in order to get the highest possible UC signal in the considered type of system. Thanks to a simple model and some time-resolved spectroscopic investigations, the contribution of the scattering to the UC signal could be separated from the intrinsic properties (crystallinity, surface defects) of the material. The films obtained by this technique present the advantages of having both high UC and good transparency.

NMR and ESR relaxation in Nd- and Gd-doped LaPO4: towards the accurate determination of the doping concentration.

We present an original method based on the (31)P solid-state NMR relaxation to determine low concentrations (<1 at%) of paramagnetic ions in monazite LaPO4 crystals with a high accuracy (∼0.1 at%). NMR experiments under static and MAS (15 kHz) conditions show that the (31)P relaxation time T1 is strongly affected by the presence of paramagnetic ions in the vicinity of the phosphorus nuclei. A linear variation of 1/T1 as a function of Nd(3+) or Gd(3+) concentration is shown in the 0-10 at% range for a homogeneous distribution of the doping ions in the matrix, which is the case when doped LaPO4 is synthesized by a soft chemistry route, i.e. by aqueous co-precipitation followed by thermal annealing. As a proof of concept for the use of this tool to study dopant homogeneity, we show that in the case of a solid-state synthesis at 1350 °C, relaxation measurements show that the homogeneous distribution of the doping ions is ensured when the mixing of the oxide precursors is performed mechanically, but not in the case of manual grinding. The electronic relaxation times of Gd(3+) and Nd(3+) ions are evaluated by ESR measurements under saturation conditions. This allows us to provide a semi-quantitative interpretation of the nuclear (31)P relaxation measurements both in Nd and Gd doped LaPO4. In addition, the comparison between nuclear and electronic relaxation suggests that Nd-Gd codoping may improve the efficiency of the Gd(3+) ion as a relaxing agent in MRI (magnetic resonance imaging) techniques.

How to prepare the brightest luminescent coatings?

We address here the question of studying the parameters affecting the brightness of luminescent nanoparticulate coatings, among which are the absorption rate, the internal quantum yield of the phosphor nanoparticles, and the extraction factor of the emitted light in a solid angle perpendicular to the substrate. Experimental investigations are achieved on spray-deposited YVO4:Eu particles, a system whose synthesis and properties are well documented so that particles of different sizes and microstructure can be considered. This allows a quantitative evaluation of the factors affecting film brightness. Considering a film made from raw colloidal particles, this work shows that its brightness is limited by a factor of 5 due to altered quantum yield of nanoparticles, a factor of 1.75 by dielectric effects and a factor of 2.4 by light extraction issues. This investigation, through providing quantitative evaluations of these different parameters, opens the way toward a possible rational design of inorganic luminescent coatings, with a possible improvement of brightness that could reach a factor of 30 as compared to simple films made directly from colloidal suspensions.

Single KTP nanocrystals as second-harmonic generation biolabels in cortical neurons.

We report an efficient colloidal synthesis of KTiOPO4 (KTP) nanocrystals with excellent crystallinity and the direct observation of optical second-harmonic generation (SHG) from discrete KTP nanocrystals in neurons cultured from mammalian brain cortex. Direct internalization and monitoring of these nanoparticles was successfully achieved without limitations from cytotoxicity, bleaching and blinking emission.

Influence of the internalization pathway on the efficacy of siRNA delivery by cationic fluorescent nanodiamonds in the Ewing sarcoma cell model.

Small interfering RNAs (siRNAs) are powerful tools commonly used for the specific inhibition of gene expression. However, vectorization is required to facilitate cell penetration and to prevent siRNA degradation by nucleases. We have shown that diamond nanocrystals coated with cationic polymer can be used to carry siRNAs into Ewing sarcoma cells, in which they remain traceable over long periods, due to their intrinsic stable fluorescence. We tested two cationic polymers, polyallylamine and polyethylenimine. The release of siRNA, accompanied by Ewing sarcoma EWS-Fli1 oncogene silencing, was observed only with polyethylenimine. We investigated cell penetration and found that the underlying mechanisms accounted for these differences in behavior. Using drugs selectively inhibiting particular pathways and a combination of fluorescence and electronic microscopy, we showed that siRNA gene silencing occurred only if the siRNA:cationic nanodiamond complex followed the macropinocytosis route. These results have potential implications for the design of efficient drug-delivery vectors.

Nanodiamond as a vector for siRNA delivery to Ewing sarcoma cells.

The ability of diamond nanoparticles (nanodiamonds, NDs) to deliver small interfering RNA (siRNA) into Ewing sarcoma cells is investigated with a view to the possibility of in-vivo anticancer nucleic-acid drug delivery. siRNA is adsorbed onto NDs that are coated with cationic polymer. Cell uptake of NDs is demonstrated by taking advantage of the NDs' intrinsic fluorescence from embedded color-center defects. Cell toxicity of these coated NDs is shown to be low. Consistent with the internalization efficacy, a specific inhibition of EWS/Fli-1 gene expression is shown at the mRNA and protein level by the ND-vectorized siRNA in a serum-containing medium.

Photonic crystal patterning of luminescent sol-gel films for light extraction.

Structured luminescent thin films are investigated in the context of improved light extraction of phosphors for solid-state-lighting applications. Thin films composed of a sol-gel titania matrix doped with europium chelates are studied as a model system. These films, patterned with a square photonic lattice by soft nanoimprint lithography, are characterized by angle-resolved fluorescence. Modeling of this simple technique is shown to fit well the experimental data, revealing in great detail the guided modes of the film and their extraction parameters. An eightfold extraction enhancement factor of the film emission is measured. To further improve the extraction efficiency, we investigate the role of an additional low-index mesoporous silica underlayer through its influence on the guided modes of different polarizations and their interactions with the photonic crystal. Results obtained on model systems open the way towards the optimization of light-emitting devices, using a strategy of dielectric microstructure engineering using the sol-gel process.

Electro-optical Pockels scattering from a single nanocrystal.

The electro-optical Pockels response from a single non-centrosymmetric nanocrystal is reported. High sensitivity to the weak electric-field dependent nonlinear scattering is achieved through a dedicated imaging interferometric microscope and the linear dependence of electro-optical signal upon the applied field is checked. Using different incident light polarization states, a priori random spatial orientation of the crystal can be inferred. The electro-optical response from a nanocrystal provides local subwavelength sensor of quasi-static electric fields with potential applications in physics and biology. It also leads to a new sub-wavelength microscopy towards the nanoscale investigation of interesting phenomena such as nanoferroelectricity.

A protected annealing strategy to enhanced light emission and photostability of YAG:Ce nanoparticle-based films.

A significant obstacle in the development of YAG:Ce nanoparticles as light converters in white LEDs and as biological labels is associated with the difficulty of finding preparative conditions that allow simultaneous control of structure, particle size and size distribution, while maintaining the optical properties of bulk samples. Preparation conditions frequently involve high-temperature treatments of precursors (up to 1400 °C), which result in increased particle size and aggregation, and lead to oxidation of Ce(iii) to Ce(iv). We report here a process that we term protected annealing, that allows the thermal treatment of preformed precursor particles at temperatures up to 1000 °C while preserving their small size and state of dispersion. In a first step, pristine nanoparticles are prepared by a glycothermal reaction, leading to a mixture of YAG and boehmite crystalline phases. The preformed nanoparticles are then dispersed in a porous silica. Annealing of the composite material at 1000 °C is followed by dissolution of the amorphous silica by hydrofluoric acid to recover the annealed particles as a colloidal dispersion. This simple process allows completion of YAG crystallization while preserving their small size. The redox state of Ce ions can be controlled through the annealing atmosphere. The obtained particles of YAG:Ce (60 ± 10 nm in size) can be dispersed as nearly transparent aqueous suspensions, with a luminescence quantum yield of 60%. Transparent YAG:Ce nanoparticle-based films of micron thickness can be deposited on glass substrates using aerosol spraying. Films formed from particles prepared by the protected annealing strategy display significantly improved photostability over particles that have not been subject to such annealing.

Plasmon-induced modification of fluorescent thin film emission nearby gold nanoparticle monolayers.

When placed in the vicinity of metal nanoparticles, fluorophore molecules can have their fluorescence intensity enhanced. In order to engineer highly fluorescent thin films, surface plasmon enhancement fluorescence was studied on macroscopic systems composed of gold nanoparticles deposited on a substrate and coated by a dye-containing polymer film. We developed a simple method based on surface silanization to get a good dispersion of up to 100 nm gold nanoparticles on a substrate. While controlling the nanoparticle size and the fluorophore concentration, we measured the fluorescence enhancement factors of systems doped with dyes possessing different quantum yields. We evidenced experimentally that a fluorescence enhancement factor of 4 could be reached for a low-quantum yield dye and that the fluorophore quantum yield affects significantly the enhancement factor. We then discussed how our experimental results agree with previously developed models.

Endohedral metallofullerenes in self-assembled monolayers.

A method has been developed for the attachment of a dithiolane group to endohedral metallofullerenes via a 1,3-dipolar cycloaddition reaction. This sulfur-containing functional group serves as an anchor, enabling efficient immobilisation of endohedral fullerenes on Au(111) surfaces at room temperature, directly from the solution phase. The functionalised fullerenes form disordered monolayers that exhibit no long-range ordering, which is attributed to both the strong bonding of the dithiolane anchor to the surface and to the conformational flexibility of the functional group. Endohedral fullerenes Er(3)N@C(80) and Sc(3)N@C(80) have been used as models for functionalisation and subsequent surface deposition. Their chemical reactivity towards dithiolane functionalisation and their surface behaviour have been compared to that of C(60). The endohedral fullerenes appear to be significantly less reactive towards the functionalisation than C(60), however they bind in a similar manner to a gold surface as their dithiolane terminated C(60) counterparts. The optical activity of Er(3)N@C(80) molecules is preserved after attachment of the functional group. We report a splitting of the endohedral Er(3+) emission lines due to the reduction in symmetry of the functionalised fullerene cage, as compared to the highly symmetrical icosahedral C(80) cage of pristine Er(3)N@C(80).