Biogenic synthesis of gold nanoparticles and their application in photocatalytic degradation of toxic dyes

Plants and their extracts play an important role in the green synthesis of nanoparticles mainly because of their environmental benignity. Based on plant extracts number of metal nanoparticles have been synthesized. In our study, we report a green technique for the synthesis of gold nanoparticles using the aqueous extracts of Alpinia nigra leaves and their photocatalytic activities. The antioxidant, antibacterial and antifungal potential of the synthesized nanoparticles were also evaluated. The aqueous extract of the plant is rich in flavonoids with Total Flavonoid Content of 491mgRE/g extract. The presence of flavonoids was further confirmed through analytical High Performance Liquid Chromatography (HPLC) analysis. The A. nigra mediated syntheses of gold nanoparticles (ANL-AuNPs) were characterized by UV-Vis spectrophotometer, Fourier Transform Infrared (FTIR) Spectroscopy, X-ray Diffraction (XRD) and Transmission Electron Microscopy (TEM). The crystalline nature of the ANL-AuNPs was confirmed by the powder XRD analysis. The TEM micrographs showed that the ANL-AuNPs was predominantly spherical in shape and the average particle size was 21.52 nm. The polyphenolics and other functional groups present in the aqueous extract that acted as reducing and capping agent in the synthesis of the Au-NPs were identified via FTIR spectral analysis. These green synthesized nanoparticles exhibited antioxidant activity with IC50 value of 52.16 mu g/ml and showed inhibition in the growth of both gram-positive and gramnegative bacteria. The pathogenic fungus, Candida albicans was also susceptible to these nanoparticles. The ANLAuNPs in the presence of sunlight catalyzed the degradation of the anthropogenic pollutant dyes, Methyl Orange and Rhodamine B with percent degradation of 83.25% and 87.64% respectively. The photodegradation process followed pseudo first order kinetic model. These results confirm that Alpinia nigra is a potential bioresource for the synthesis of Au-NPs with versatile applications.