Singlet oxygen generation by hybrid rhodamine B-gold nanostructures in chitosan biomolecule environment: an EPR study

Gold nanoparticles of various shapes were synthesized by reduction of gold salts in the presence of chitosan biomolecules as stabilizing agents. Fluorescence and photosensitizing properties of the rhodamine B dye were studied after its mixing with pure chitosan and chitosan-gold nanoparticle solutions. It was found that gold nanoparticles significantly affect the fluorescence intensity and the singlet oxygen production of the photosensitizer. Metal-enhanced fluorescence and metal-enhanced singlet oxygen generation effects were observed, probably as a direct consequence of the activation of the surface plasmon of the nanoparticles upon irradiation. Photosensitizing activity of the rhodamine B dye was investigated by using electron paramagnetic resonance (EPR) spectroscopy with TEMP as spin-trap molecules. The singlet oxygen generation was followed via changes in the intensity of EPR signal of the radical adduct, TEMPO. It was found that gold nanoparticles facilitate the production of singlet oxygen, while the chitosan molecules influence TEMPO stability and tend reduce the intensity of the EPR signal, especially at prolonged times following the irradiation.