Green synthesis of gold nanoparticles from macadamia nut shell extracts and their antioxidant, antibacterial and catalytic activities

We extracted phenolic and flavonoid compounds from macadamia nut shells using water-ethanol and water as solvents. We measured the total antioxidant activity (TAA), total phenolic content (TPC), and total flavonoid content (TFC) of each extract using 2,2-diphenyl-1-picrylhydrazyl (DPPH) and Folin-Ciocalteu methods. The ethanol extract had the highest TAA (IC50: 12.026 ± 0.002 ppm), TPC (143.297±0.062 mg GAE g-1 dry weight), and TFC (234.409 ± 0.069 mg QE g-1 dry weight). We used the ethanol extract as a green reducing and stabilizing agent to synthesize gold nanoparticles (AuNPs). The AuNPs had an absorption peak at 567 nm and a size range of 12-15 nm. We tested the antibacterial activity of the AuNPs against Escherichia coli and Staphylococcus aureus and found that they had clear zones of 2.33 ± 0.76 and 6.67 ± 0.29 mm, respectively, compared to penicillin. We also evaluated the catalytic activity of the AuNPs in the reduction of methylene blue and Congo red and found that they had good performance. Therefore, we demonstrated that macadamia nut shells are a promising source of natural antioxidants and antimicrobials, and that AuNPs can be green synthesized and used as nanocatalysts.It can be used as a material to effectively absorb heavy metals.