Bright Luminescent Silica Nanoparticles for Two-Photon Microscopy Imaging via Controlled Formation of 4,4′-Diethylaminostyryl-2,2′-bipyridine Zn(II) Surface Complexes

Silica nanoparticles with a surface functionalized by two-photon absorbing (≡SiO)Zn(OSi(O^tBu)₃)(DEAS-bipy) chromophores are prepared via surface chemistry controlled at the molecular level. This involves the grafting of {Zn[OSi(O^tBu)₃]₂}₂ on the surface silanols of a silica partially dehydroxylated at 700 °C followed by the coordination of DEAS-bipy. The spectroscopic and photophysical properties of the grafted species (≡SiO)Zn(OSi(O^tBu)₃)(DEAS-bipy) compare well with that of the molecular model {DEAS-bipyZn(OAc)₂} with the advantage of allowing a high density of chromophores on a nanometric object (ca. 200 chromophores per silica particle of 12 nm). These particles are luminescent and exhibit a giant two-photon cross-section of about 90 × 10³ GM; such two-photon brightness allows the imaging of a single nanoparticle using two-photon scanning microscopy.