A Novel Approach to Eco-Friendly Synthesis of ZnO Nanoparticles and ZnO-Ag Nanocomposites from Butea monosperma: A Comparative Study of Structural, Optical, and Antibacterial Properties

This study reports the synthesis of ZnO nanoparticles (NPs) and ZnO-Ag nanocomposites (NCs) using a simple in situ precipitation method via a green synthesis route. The extract of fresh Butea monosperma (PL) flower, which is highly valued in traditional medicine in South Asia, was used as both a reducing and capping agent, as it is nontoxic, less expensive, and recyclable. The synthesized NPs and NCs were characterized and confirmed by X-ray diffraction analysis (XRD), UV-visible spectroscopy, Fourier transform infrared spectroscopy (FTIR), Field emission scanning electron microscopy (FESEM), Transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDX). From XRD analysis, the crystallite sizes of ZnO NPs and ZnO-Ag NCs were determined to be 31.84 nm and 28.09 nm, respectively. The average particle size was 35.43 nm for ZnO NPs and decreased to 17.70 nm for ZnO-Ag NCs. TEM images confirmed the successful formation and uniform size distribution of ZnO NPs and ZnO-Ag NCs. In addition, the band gap of ZnO NPs was computed to be 2.98 eV, while a slight decrease to 2.91 eV was observed for ZnO-Ag NCs. Moreover, the antimicrobial activity of NPs and NCs was also tested against Gram-positive and Gram-negative microbial pathogens. ZnO–Ag NCs demonstrated enhanced antibacterial activity relative to ZnO NPs against both Gram-positive and Gram-negative bacteria, with increased inhibition zones of 11.5 ± 0.71 (S. aureus) and 19.5 ± 0.71 mm (E. coli) when compared to 10.8 ± 0.35 mm (S. aureus) and 12.0 ± 0.0 mm (E. coli) of the latter for the dose 400 μg/disk. The similar enhancement of antibacterial performance of ZnO–Ag NCs compared to ZnO NPs was also observed against P. aeruginosa and S. flexneri. Furthermore, cytotoxicity was estimated against brine shrimp to confirm the biocompatibility of the synthesized NPs and NCs and to ensure their applicability in real-world implementation.