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 × 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 ([DEAS·Ln@SiO2], Ln = Y, Yb) were characterized using IR, solid-state NMR, UV-vis, and 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 under a high vacuum having 0.8 OH·nm–2, the grafting of Ln­(N­(SiMe3)2)3 generates SiO–Ln­(N­(SiMe3)2)2, which upon thermal step and coordination of the DEAS chromophore yields (SiO)3Ln­(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 lifetime) in the solid state, consistent with the molecular nature of the surface species. The densely functionalized nanoparticles can be dispersed via ultrasonication in small 15–20 nm aggregates (one to six elementary particles) that were detected using two-photon microscopy imaging at 720 nm excitation, making them promising nano-objects for bioimaging.