Photocatalytic redox on the surface of colloidal silver nanoparticles revealed by second harmonic generation and two-photon luminescence.

The redox of silver on the surface of Ag nanoparticles (AgNPs) has received extensive attention because of its significant impact on the biological, physical and chemical properties of AgNPs and their applications. Here we demonstrate that the surface redox reaction of AgNPs in colloids may be investigated by the second harmonic generation (SHG) and two-photon luminescence (TPL) emission from the AgNPs. It was revealed that the oxidation of silver on the surface of AgNPs was accelerated upon femtosecond laser excitation, accompanied by a decrease in the SHG and TPL emissions from the AgNPs. The photon-induced reduction of oxidized silver on AgNPs and the formation of surface defects were also revealed by the changes in the SHG and TPL emissions. Size and morphology changes have not been detected by dynamic light scattering and TEM measurements. The changes in the UV-vis extinction spectra were also very weak compared with previous reports. However, the occurrence of redox reactions on the Ag surface upon femtosecond laser irradiation has been confirmed by multiple control experiments. This work demonstrates that SHG and TPL can sensitively probe the subtle structural change on the surface of AgNPs.