Green synthesis of gold nanoparticles using Kaempferia parviflora rhizome extract and their characterization and application as an antimicrobial, antioxidant and catalytic degradation agent

•BG-AuNPs was synthesized by aqueous extract of black ginger (K. parviflora) rhizome.•BG-AuNPs were spherical, smooth surface without aggregation and about 20–60 nm size.•BG-AuNPs showed an excellent antimicrobial and high antioxidant ability.•BG-AuNPs act as a good catalyst in the methyl orange degradation with NaBH4.•BG-AuNPs can be used in various biomedical and environmental applications.