Quantum confinement effects on the surface enhanced Raman spectra of hybrid systems molecule-TiO₂ nanoparticles

The role of quantum confinement, size, and solvent effects on the surface enhanced Raman spectra of biologically important molecules absorbed on semiconducting titanium dioxide (TiO₂) nanoparticles is investigated using density functional calculations. The results obtained for both the gas phase and solvated systems indicate significant changes in the electronic structure and the Raman spectra of molecules like formic acid and dopamine, when they are adsorbed on small TiO₂ nanoparticles. A number of distinctive features that are determined by the formation of a charge-transfer complex at the nanoparticle-molecule interface can be noted in the Raman spectra. Both the spectra and the electronic properties are strongly size dependent and are also sensitive to the presence of the solvent and the nature of adsorbate interaction. Although these calculations reinforce recent experimental findings on the role of quantum confinement, they also pose new questions about the extension of collective effects and the effect of pH and other environmental variables.