Synthesis of small silver nanoparticles under light radiation by fungus Penicillium oxalicum and its application for the catalytic reduction of methylene blue

At present, green and efficient synthetic strategies have been gaining great interest for the synthesis of metal nanoparticles. In this study, the synthesis of extracellular silver nanoparticles (AgNPs) under light radiation was described using the cell filtrate of Penicillium oxalicum 1-208. The pH effect of the cell filtrate on nanosynthesis was investigated by visual observation, ultraviolet-visible absorption spectroscopy, dynamic light scattering and zeta potential. The results showed that the pH of the cell filtrate affected the time of nanosynthesis, and the size, size distribution and stability of the synthesized nanoparticles. The AgNPs synthesized at pH 8.0 and 12.0 were further characterized by X-ray diffraction, selected area electron diffraction, energy-dispersive X-ray spectroscopy and transmission electron microscopy. The synthesized AgNPs were spherical in shape, crystalline in nature and preferentially oriented in (111) plane. Small AgNPs with an average particle size of about 4 nm were successfully synthesized at pH 12.0 and well dispersed in solution without obvious aggregation. Furthermore, the AgNPs synthesized at pH 8.0 were used as catalyst and exhibited excellent catalytic activity for the reduction of methylene blue in the presence of NaBH4 at ambient temperature.