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).

Time-gated triplet-state optical spectroscopy to decipher organic luminophores embedded in rigid matrices.

Wet-chemically synthesized inorganic materials often exhibit luminescence behavior. We have recently shown that the amorphous yttrium-aluminium-borate (a-YAB) powders obtained by sol-gel and modified Pechini methods exhibit organic impurities, responsible for their intense visible photoluminescence and phosphorescence afterglow. However, the heterogeneity of impurity organic compounds and difficulties in their intact extraction from the solid inorganic host matrix limit the extraction-based chemical analysis for luminophore identification. Here, we propose a photo-physical route based on time-gated triplet-state optical spectroscopy (TGTSS) to construct the electronic structures of the trapped unknown luminophores, which successfully illustrates the luminescence properties of a-YAB powders in more detail and also provides important insights intrinsic to the nature of the luminophores. The experimental results accompanied with TD-DFT calculations of the theoretical electronic structures thus help us to propose the probable luminophore compounds trapped in rigid a-YAB matrices. We anticipate that the present approach will open new opportunities for analyzing similar complex luminescent materials, including carbon dots, graphene oxides, etc., which is vital for their improvement.

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.

Afterglow Luminescence in Wet-Chemically Synthesized Inorganic Materials: Ultra-Long Room Temperature Phosphorescence Instead of Persistent Luminescence.

Wet-chemically synthesized amorphous yttrium-aluminum-borates (a-YAB) exhibit intense visible photoluminescence (PL). Preliminary investigations revealed a correlation of PL with the presence of carbon-related impurities; however, their exact nature is still under investigation. These powders also exhibit afterglow luminescence that lasts for several seconds at room-temperature (RT). A comparison with persistent phosphors and phosphorescent dye revealed that the afterglow in a-YAB is a phosphorescence phenomenon and not the persistence luminescence, which is more common in inorganic solids. The unusual RT phosphorescence in a-YAB could be achieved due to triplet-state stabilization of trapped luminescent organic moieties in glassy matrix. This is indeed an important step forward in understanding the complex luminescence mechanism in such promising wet-chemically synthesized functional materials. Moreover, phosphorescence is detectable for over 10 s at RT, suggesting rigid glassy inorganic matrix is more efficient in preserving phosphorescence at elevated temperatures, opening the path for several attractive applications including time-resolved bioimaging and thermometry.

Evidence of Organic Luminescent Centers in Sol-Gel-Synthesized Yttrium Aluminum Borate Matrix Leading to Bright Visible Emission.

Yttrium aluminum borate (YAB) powders prepared by sol-gel process have been investigated to understand their photoluminescence (PL) mechanism. The amorphous YAB powders exhibit bright visible PL from blue emission for powders calcined at 450 °C to broad white PL for higher calcination temperature. Thanks to 13 C labelling, NMR and EPR studies show that propionic acid initially used to solubilize the yttrium nitrate is decomposed into aromatic molecules confined within the inorganic matrix. DTA-TG-MS analyses show around 2 wt % of carbogenic species. The PL broadening corresponds to the apparition of a new band at 550 nm, associated with the formation of aromatic species. Furthermore, pulsed ENDOR spectroscopy combined with DFT calculations enables us to ascribe EPR spectra to free radicals derived from small (2 to 3 rings) polycyclic aromatic hydrocarbons (PAH). PAH molecules are thus at the origin of the PL as corroborated by slow afterglow decay and thermoluminescence experiments.

NIR-to-NIR Two-Photon Scanning Laser Microscopy Imaging of Single Nanoparticles Doped by Yb(III) Complexes.

The photophysical and nonlinear optical properties of water-soluble chromophore-functionalised tris-dipicolinate complexes [LnL3](3-) (Ln=Yb and Nd) are thoroughly studied, revealing that only the Yb(III) luminescence can be sensitized by a two-photon excitation process. The stability of the complex in water is strongly enhanced by embedding in dispersible organosilicate nanoparticles (NPs). Finally, the spectroscopic properties of [NBu4]3 [YbL3] are studied in solution and in the solid state. The high brightness of the NPs allows imaging them as single objects using a modified two-photon microscopy setup in a NIR-to-NIR configuration.

Y0.9 Er0.1 Al3(BO3)4 thin films prepared by the polymeric precursor method for integrated optics.

This work reports on the optimization of Yo.9 Er0.1 Al3(BO3)4 thin films for integrated optics. The films were deposited on silica and silicon substrates using the spin-coating technique involving solutions previously prepared by the polymeric precursor method. These deposits, 400-800 nm thick, were prepared by a 5-10 multi-layer process and heat treatments at different temperatures from glass transition to crystallization temperature, using heating rates of 2 or 5 degrees C/min. The structural characterizations were performed using grazing incidence X-ray diffraction and Fourier transform infrared spectroscopy (FT-IR). Water and/or hydroxyl contents were also evaluated from FT-IR spectra. Microstructural evolution in term of annealing temperatures was analyzed by high resolution scanning electronic microscopy and atomic force microscopy. Optical transmission spectra were used to determine the refractive index and thickness through the envelope method of the films. Finally, the film guiding and optical properties were studied by m-line spectroscopy. The best film showed a good waveguiding with high light-coupling efficiency close to the theoretical limit.

Photochromism of spiropyran nanocrystals embedded in sol-gel matrices.

Spectroscopic and kinetic properties of a new photochromic medium, consisting of nanocrystals of spyropyran molecules (1,3-dihydro-1,3,3,5',6',pentamethyl-spiro[2H-indole-2,2'-[2H]pyrano [3,2-b]pyridinium] iodide) embedded in an organo-silicate sol-gel film, are presented and compared to microcrystals obtained by slow evaporation of a solvent. High photoconversion efficiencies for both kinds of crystals have been observed. In microcrystals, the photomerocyanine form absorbs at 570 nm with a fading rate of 5 h, in nanocrystals the photomerocyanine form absorbs at 535 nm with a fading rate of 41 h. Therefore, the crystalline structure of nanocrystals is different from the microcrystal one.

In situ diagnostics of the crystalline nature of single organic nanocrystals by nonlinear microscopy.

We elucidate the crystalline nature and the three-dimensional orientation of isolated organic nanocrystals embedded in a sol-gel matrix, using a polarized nonlinear microscopy technique that combines two-photon fluorescence and second harmonic generation. This technique allows the distinction between monocrystalline structures and nanoscale polycrystalline aggregates responsible for incoherent second harmonic signals.