Antimicrobial and antioxidant activity of Silver, Gold and Silver-Gold Alloy Nanoparticles phytosynthesized using extract of Opuntia ficus-indica

Aqueous extract of Opuntia ficus-indica (OF) was employed to reduce salt solutions of silver nitrate, gold chloride and silver nitrate/gold chloride to OFAgNPs, OFAuNPs, and OFAg-AuNPs respectively. Characterization of the particles was carried out using different standard methods, and their antimicrobial and antioxidant activities were also evaluated. The UV-vis spectroscopy showed silver, gold and silver-gold alloys nanoparticles with surface plasmon resonance at 462, 545 and 539 nm, respectively. The significant FTIR peaks: OFAgNPs (3300 and 1635 cm^-1), OFAuNPs (3288 and 1635 cm^-1) and OFAg-AuNPs (3307 and 1637 cm^-1) for synthesized nanoparticles pointed to protein as both capping and stabilizing agent. Generally, particles were spherical with size range of 27-38 nm, 11-28 nm and 15-54 nm for silver, gold and silver-gold alloys nanoparticles respectively. OFAgNPs gave highest antagonistic effects against tested bacteria (35-82%), followed by OFAg-AuNPs (12-75%) and AuNPs (11-48%). The fungal inhibition of 44-70% for OFAgNPs, 42-79% for OFAuNPs and 52-92% for OFAg-AuNPs were obtained. DPPH scavenging activities were obtained as follows: OFAgNPs (52.25-57.50%), OFAuNPs (37.07-59.07%) and OFAg-AuNPs (37.07-63.00%) at tested concentrations of 20-100 μg/ml, which were dose-dependent in comparison with standard BHA (42.57-91.27%) and ascorbic acid (52.13-84.20%). The bleaching inhibition assay of ABTS showed activities of 41.33-61.83% (OFAgNPs), 27.37-50.60% (OFAuNPs) and 32.83-44.33% (OFAg-AuNPs). The synthesized nanoparticles demonstrated an excellent antioxidant and antimicrobial activity. The current study, to the best of our knowledge, is the first time to use Opuntia ficus-indica to synthesize silver, gold and silver-gold alloys nanoparticles.