V. Buissette

Aqueous routes to lanthanide-doped oxide nanophosphors

Besides well-known quantum dots, lanthanide-doped oxide nanocrystals form a new promising class of nanophosphors. Lanthanide vanadate and phosphate nanoparticles are prepared as well-dispersed and highly concentrated colloids by a very simple aqueous route, using competition between precipitation and complexation reactions. Under UV excitation, the three basic colors are furnished by YVO4 : Eu (red), LaPO4 : Ce·0.7H2O (blue–violet) and LaPO4 : CeTb·0.7H2O (green). In the case of cerium-doped phosphate nanoparticles, the epitaxial growth of a LaPO4·0.7H2O shell leads to “core–shell” nanocrystals presenting improved stability against cerium oxidation which is detrimental for the luminescence. Nanocrystals exhibiting passivation and chemical or biological functionalization are easily prepared using the silane strategy. The optimized nanophosphors are either dispersed in transparent sol–gel films or used as isolated particles for biological labelling. The luminescence properties of nanoparticles differ from corresponding bulk materials in several ways : (i) spectroscopic changes, as a systematic broadening of the luminescence bands in sub-20 nm nanocrystals, resulting from a structural disorder intrinsic to the small size; (ii) a lower luminescence quantum yield related to the presence of chemical species adsorbed at the surface, especially the OH groups; (iii) a shift towards higher values of the optimum doping concentration, due to the alteration of energy transfers either in the vanadate or cerium sub-networks.

Luminescence properties of YVO4:Ln (Ln = Nd, Yb, and Yb-Er) nanoparticles

Concentrated colloidal solutions of well dispersed YVO4:Ln (Ln ¼ Nd, Yb, and Yb–Er) nanocrystals are synthesized through the precipitation of citrate complexes of rare earth salts with sodium orthovanadate. Luminescence spectra of aqueous suspensions of YVO4:Nd or Yb are very similar to the bulk material, with lower lifetimes of the emitting level. After heating at 600 C, the luminescence properties of silica capped YVO4:Ln nanoparticles are significantly improved. In these conditions, YVO4:Yb, Er nanoparticles exhibit green up-conversion luminescence and 1.55 lm emission of the erbium ion under infrared excitation of the ytterbium ion. 2003 Elsevier Science B.V. All rights reserved.

Functionalized luminescent YVO4: Ln3+ nanoparticles

The synthesis and the emission properties of colloidal YVO4:Ln nanocrystals (8nm in diameter) are investigated, where Ln = Eu, Nd, Yb, Er. The nanoparticles synthesized here are constituted of a (Y,Ln)VO4 crystalline core and stabilized by a lanthanide-citrate complexing shell. Surface derivatization of the particles can be achieved through the controlled growth of a silicate shell using a functionalized silane, with elimination of the citrate shell. The luminescence of the Eu3+ -doped YVO4 colloids is studied in details and compared to the bulk material. Chemical treatments are achieved in order to explain the observed differences. Improvement of the emission quantum yield after the transfer of the colloidal particles into D2O shows that surface OH groups act as efficient quenchers of the Eu3+ emission. The growth of a silica shell around the particles decreases the optimum europium concentration, showing that energy transfers within the nanoparticles are limited by the quenching of the excited states of the vanadate ions. Moreover, site selective excitation spectroscopy seems to prove the coexistence of core sites and surface sites for the europium ions. Finally, colloidal nanoparticles exhibit an emission yield of about 25%, which appears already suitable for some applications.

Formation of hybrid colloids by suspension polycondensation in the presence of hydrophilic block copolymers

Double hydrophilic block copolymers were used to control the growth of inorganic particles and directly prepare hybrid colloidal suspensions. Colloids of metal hydrous oxides were obtained by forced hydrolysis of metal ions in presence of the copolymers. The block copolymers contain a metal-complexing polyelectrolyte block and a stabilizing neutral block. The role of the first block is to ensure a controlled growth of the inorganic phase, while simultaneously, the second block ensures the colloidal stabilization. Phase diagrams presenting the conditions under which precipitation is inhibited are established. The nanoparticles are then characterized in terms of sizes, morphologies and surface charges. The main parameters controlling the size were identified: the copolymer-to-metal ratio and the metal prehydrolysis ratio. The synthesis steps were characterized. First, a key step of induced micellization of the hydrophilic copolymers leads to hybrid core-shell assemblies. The second step consists in mineralization of the micellar core. The suspension polycondensation leads to hairy particles whose morphologies depend on the nature of the metal and on synthesis parameters.

Functionalized luminescent oxide nanoparticles for sodium channel imaging at the single molecule level

Lanthanide-ion doped oxide nanoparticles were functionalized for use as fluorescent biological labels. These nanoparticles are synthesized directly in water which facilitates their functionalization, and are very photostable without emission intermittency. Nanoparticles functionalized with guanidinium groups act as artificial toxins and specifically target sodium channels. They are individually detectable in cardiac myocytes, revealing a heterogeneous distribution of sodium channels. Functionalized oxide nanoparticles appear as a novel tool particularly well adapted to long-term single-molecule tracking.

Highly Luminescent Composite Films from Core-Shell Oxide Nanocrystals

Luminescent nanocrystals can find interesting applications for the elaboration of light emitting transparent materials. The work described here is based on the use of lanthanide doped vanadate (YVO 4 :Eu) and phosphate (La, Ce, Tb)PO 4 -0, 7H 2 O nanoparticles grown through aqueous colloidal synthesis, with average sizes below 10 nm. The well-dispersed colloids are transparent and respectively exhibit red and green luminescence under U.V. excitation with high luminescence yields (20 – 50 %). Improvement of luminescence properties of the nanocrystals is achieved through the elaboration of core/shell nanostructures, obtained after the growth at the surface of an amorphous silica shell or a crystalline lanthanum phosphate shell. Surface derivatization is further achieved through the controlled growth of an organically modified silica coating using a functionalized silane precursor. Concentrated sols are obtained, which are highly luminescent and well-dispersed. They can be spin-coated on various substrates, leading to perfectly transparent and highly luminescent thin films. Multi-layers films and heating treatments are performed, leading to optimized materials.

New Biological Labels Based on Functionalized YVO4:Eu Nanoparticles

Abstract : Lanthanide-ion doped oxide (YVO4:Eu) nanoparticles were synthesized as aqueous colloids and functionalized by a bioactive silane shell to be used as fluorescent biological labels. Nanoparticles functionalized with guanidinium groups were able to act as artificial toxins which specifically target Na+ channels. They were individually detectable in live cardiac myocytes. Functionalized oxide nanoparticles appear as a new interesting tool, especially attractive for long-term single-molecule tracking due to their photo-stability and long luminescence lifetime.