Hybrid Fluorescent Dyes-Silica Nanoparticles: Toward a Possible Molecular Control of the Photophysical Properties

The entrapment of organic fluorophores in silica nanoparticles (NP) allows to obtain hybrid nanomaterials with photoemission brightness and stability much higher than dyes in solution. The photophysical properties of such hybrid systems are strongly dependent on the dispersion of the dyes throughout the silica matrix that is ruled by the molecular events occurring during NPs formation. As a prosecution of a previous work on the assessment, at a molecular level, of the photophysical performances of cyanine-silica NP prepared by the microemulsion method, we investigated the dependence of such performances on the relative affinity of TEOS and dyes molecules towards the water pool within the reverse micelles. The photophysical properties of three cyanine dyes with a different degree of hydrophilicity were systematically compared when dissolved in the microemulsion, during NPs formation and entrapped in the final NP, demonstrating that the photoemission brightness, and then the dispersion of dye molecules within silica, increases as the fluorophore hydrophilicity decreases. It is proposed this dependence results from a decrease of diffusion rate toward the water pool in the reverse micelles with respect to hydrolyzed TEOS monomers, favouring the distribution of dye molecules in outer shells growing around undoped silica nuclei and subparticles.