Controlled assembly of metal colloids on dye-doped silica particles to tune the photophysical properties of organic molecules.

The use of plasmonic nanomaterials is a challenging strategy to control radiation and radiation-induced processes at a nanometric scale. The localized surface plasmons of metal nanoparticles have been shown to affect the efficiency of a variety of radiative and non-radiative processes occurring in organic molecules. In this contribution, we present an overview of the results obtained through an original approach based on the hierarchical assembly of plasmonic gold colloids on silica templates, covalently doped with organic dyes. The detailed morphological characterization demonstrates the disposition of gold colloids on silica achieved through the tight control of the synthetic conditions. The studies carried out while gradually increasing the concentration of gold nanoparticles allow the detailed investigation of the effects of the progressive addition of plasmonic particles on the photophysical behaviour of organic molecules. In particular, the fluorescence behaviour of three dyes with different spectral properties, namely fluorescein, rhodamine B and 9-aminoacridine, are investigated in the presence of increasing concentrations of gold nanoparticles. In order to fix the distance between the dye and the gold nanoparticles, the dyes are anchored to silica nanoparticles, and the metal colloids are chemically adsorbed on the silica surface. The steady state and time-resolved data are analysed to evaluate the impact of plasmonic nanoparticles on the radiative and non-radiative processes of the dyes; the data provide evidence that the modulation of the fluorescence intensity (enhancement or quenching) can be achieved by changing the concentration of gold colloids. The plasmonic nanostructures can be employed to favour one deactivation process over the others. For example, we demonstrate that the photoinduced formation of reactive oxygen species (ROS) can be enhanced upon the plasmonic engineering of a photosensitizing agent (Protoporphyrin IX, PpIX). The Vis-excitation of silica-PpIX samples in the presence of gold nanoparticles results in a faster and more efficient photoinduced formation of ROS species either in solution or in a hydrogel. The ROS efficiency data and the fluorescence behaviour of PpIX in the presence of gold colloids suggest that the enhancement of the excitation field occurs through a plasmonic effect. For the application of the assembled hybrid materials, further advantages come from the development of photosensitizer-containing hydrogel films that are able to efficiently produce ROS upon visible excitation. Our preliminary results are herein reported and discussed.