Polygonum capitatum -mediated green synthesis of gold nanoparticles and the potential antioxidant and catalytic applications

Gold nanoparticles (AuNPs) have attracted significant attention owing to their tunable sizes, unique surface morphology, and tunable surface chemistry. Herein, we report a green synthesis of AuNPs using an aqueous extract of Polygonum capitatum (syn. Persicaria capitata ), a medicinal herb native to Guizhou, and evaluated their catalytic and antioxidant potential. Sodium triacetoxyborohydride (STAB), a mild reducing agent, was used for the catalytic reduction of azo dyes, representing an approach for this plant-mediated AuNP catalyst. The synthesis parameters were systematically optimized to yield well-dispersed nanoparticles exhibiting high catalytic efficiency. The prepared AuNPs were characterized via ultraviolet–visible spectroscopy, transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy, fourier-transform infrared (FTIR) spectroscopy, dynamic light scattering, and zeta potential analysis. A distinct surface plasmon resonance peak appeared at 532 nm. TEM results revealed an average particle size of 11.9 nm, which closely matched the 11.8 nm estimate derived from X-ray diffraction line-broadening using the Scherrer equation. FTIR spectra verified that water-soluble flavonoids and phenolic biomolecules within the P. capitatum extract functioned as reducing and stabilizing agents during AuNP formation. The synthesized AuNPs efficiently catalyzed the reduction of 4-nitrophenol and azo dyes. For azo dye reduction, STAB was used as a mild alternative to sodium borohydride and maintained a near-neutral pH, preventing the alkaline hydrolysis of dyes. AuNPs also displayed a moderate antioxidant capacity compared with butylated hydroxytoluene. These findings substantiate the viability of a green synthesis of AuNPs via a plant biomass–mediated process and highlight their promising applications as antioxidants and agents for wastewater remediation.