Microwave-assisted synthesis of Au nanoparticles using fruit peel waste: antioxidant activity and catalytic reduction of malachite green

Gold nanoparticles (AuNPs) were synthesized using dragon fruit peel extract (DFE) as a reducing agent and stabilizer. Confirmation of AuNP formation included the distinct red coloration of the solution and the appearance of surface plasmon resonance (SPR) peak at approximately 530 nm in the UV–visible spectrum. Synthesis conditions, such as solution pH, HAuCl₄ concentration, and DFE concentration, were varied to observe their influence on the AuNPs. Characterization using TEM revealed well-dispersed, nearly spherical particles with diameters ranging from 6 to 20 nm and a mean diameter of 12 ± 3 nm. XRD and SAED patterns confirmed the face-centered cubic (FCC) crystal structure. FTIR analysis highlighted the role of DFE functionalities in reducing and stabilizing the AuNPs. Zeta potential analysis indicated a negative surface charge on DFE@AuNPs, ensuring colloidal stability. The antioxidant activity of DFE@AuNPs was assessed using the ABTS assay, demonstrating comparable efficacy to ascorbic acid as a standard. Furthermore, the catalytic activity of the nanoparticles was evaluated through the NaBH₄-assisted reduction of malachite green (MG) dye, with over 90% reduction observed within 24 min, following pseudo-first order kinetics with the rate constant of 0.069 ± 0.011 min⁻¹ obtained from exponential fit.