Effect of Silver Nanoparticles on the Photophysics of Riboflavin: Consequences on the ROS Generation

Metal-enhanced singlet oxygen, O2(1Δg), generation is a desirable effect to augment the amount of O2(1Δg) produced upon irradiation of organic sensitizers. Herein, we employed pectin for stabilizing 9 nm silver nanoparticles and showed that these particles are able to form a ground-sate complex with riboflavin (Rf). Pump–probe transient absorption spectroscopy and laser flash-photolysis experiments proved that the excited state of the riboflavin–Ag complex feeds the triplet state of the sensitizer, which results in an enhanced intersystem crossing process. The higher amounts of riboflavin triplet states formed in the presence of the nanoparticles result in higher yields of singlet oxygen and hydrogen peroxide produced in the irradiated colloidal suspensions. As a result, not only the effect on singlet oxygen but also the effect on superoxide radical ion should contribute to a better performance of Riboflavin as a sensitizer applied to the photodynamic therapy of tumors.