Green Synthesis of Gold Nanoparticles from Hardwickia binata Roxb. Bark Extract

Abstract

A simple, single-step and environmentally benign route to gold nanoparticles (AuNPs) is reported in which the aqueous bark extract of Hardwickia binata Roxb.—a drought-adapted leguminous hardwood endemic to the dry deciduous forests of peninsular India—serves simultaneously as the reductant and the capping agent for tetrachloroaurate(III). The bark was selected deliberately as a polyphenol-rich feedstock, and quantitative phytochemical analysis confirmed an exceptionally high total phenolic content of 248.6 ± 6.3 mg gallic acid equivalents per gram and a total flavonoid content of 96.4 ± 4.1 mg quercetin equivalents per gram. Univariate optimisation identified an operating window of 1.0 mM HAuCl₄, 3–4 % (v/v) extract, 80 °C, pH 8 and 30 min, under which a deep ruby-red colloid developed with a sharp, symmetric surface plasmon resonance (SPR) band at approximately 535 nm. The product was characterised by UV–visible spectroscopy, FTIR, X-ray diffraction, SEM, TEM/HRTEM/SAED, dynamic light scattering, zeta-potential measurement and EDS. The nanoparticles were single-crystalline, face-centred-cubic (JCPDS 04-0784), quasi-spherical, with a mean diameter of ≈17–19 nm, a polydispersity index of 0.21 and a zeta potential of −31.6 mV indicative of robust electrosteric stability. FTIR band shifts localised the reduction to phenolic hydroxyl oxidation and the capping to quinone-carbonyl and aromatic π-coordination, providing functional-group-level proof of the mechanism. The AuNPs displayed significant antioxidant (DPPH IC₅₀ ≈ 42 µg mL⁻¹), antibacterial and catalytic dye-degradation activity with good reusability. Statistical analysis (ANOVA, p < 0.05; Pearson r > 0.95 between phenolic content and SPR response) confirmed both the significance of the synthesis parameters and the phenolic-mediated mechanism. The work establishes H. binata bark as a premium green-synthesis feedstock for sustainable nanomanufacturing.