Near-IR Two Photon Microscopy Imaging of Silica Nanoparticles
Functionalized with Isolated Sensitized Yb(III) Centers
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Abstract
Bright nano-objects
emitting in the near-infrared with a maximal
cross section of 41.4 × 10<sup>3</sup> 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@SiO<sub>2</sub>], 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<sup>–2</sup>, the grafting of
Ln(N(SiMe<sub>3</sub>)<sub>2</sub>)<sub>3</sub> generates SiO–Ln(N(SiMe<sub>3</sub>)<sub>2</sub>)<sub>2</sub>, which upon thermal step and coordination
of the DEAS chromophore yields (SiO)<sub>3</sub>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 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
