NEAR-IR TWO PHOTON MICROSCOPY IMAGING OF SILICA NANOPARTICLES FUNCTIONALIZED WITH ISOLATED SENSITIZED Yb(III) CENTERS

Bright nano objects emitting in the near infrared with a maximal cross section of 41.4 x 103 GM (Goppert Mayer), were prepared by implanting ca. 180 4,4’–diethylaminostyryl–2,2’–bipyridine (DEAS) Yb(III) complexes on the surface of 12–nm silica nanoparticles. The surface complexes Ln@SiO 2 ], Ln =Y,Yb) were characterized using IR, solid–state NMR, UV–Vis, EXAFS spectroscopies in combination with the preparation and characterization of similar molecular analogues by analytical techniques (IR, solution NMR, UV–Vis, X ray crystallography) as well as DFT calculations. Starting from the partial dehydroxylation of the silica at 700 °C on high vacuum having 0.8 OH.nm–2, the grafting of Ln(N(SiMe 3 ) 2 ) 3 generate ≡SiO–Ln(N(SiMe 3 ) 2 ) 2 , which upon thermal step and coordination of the DEAS chromophore yields (≡SiO) 3 Ln(DEAS). Surface and molecular analogues display similar properties, in terms of DEAS binding constants absorption maxima and luminescence properties (intense emission band assigned to a ligand centered CT fluorescence and life time) in the solid state, consistent with the molecular nature of the surface species. The densely functionalized nanoparticles can be dispersed via ultra-sonication in small ca. 15-20 nm aggregates (1 to 6 elementary particles) that were detected using two–photon microscopy imaging at 720 nm excitation, making them promising nano– objects for bio–imaging.