​Phytochemical-Guided Green Synthesis of Structurally Defined ZnO Nanoparticles from Butea monosperma Root Extract and Their Antibacterial Potential

Green synthesis of metal oxide nanoparticles using plant-derived phytochemicals represents a sustainable and surface-engineered approach for developing functional nanomaterials. In the present study, an aqueous root extract of Butea monosperma was employed as a bio reducing and stabilizing agent for the synthesis of zinc oxide nanoparticles (ZnO NPs). Phytochemical profiling using qualitative assays, thin-layer chromatography (TLC), high-performance liquid chromatography (HPLC), and LC–MS confirmed the presence of polyphenols, flavonoids, tannins, and triterpenoids, which are known to facilitate metal ion reduction and nanoparticle stabilization. UV–visible spectroscopy revealed a characteristic absorption edge in the UV region with an optical band gap of 3.69 eV, indicating nanoscale ZnO formation. Powder X-ray diffraction (PXRD) confirmed the formation of crystalline hexagonal wurtzite ZnO (JCPDS No. 36-1451) with an average crystallite size of 12–18 nm. Dynamic light scattering analysis showed a hydrodynamic diameter of 255.8 nm with a polydispersity index of 0.389, while a negative zeta potential (–25 mV) indicated moderate colloidal stability due to phytochemical surface capping. FTIR spectra demonstrated the involvement of hydroxyl and carbonyl functional groups in nanoparticle formation. SEM–EDX analysis confirmed nanoscale morphology and elemental purity (Zn 74.48 wt%, O 25.52 wt%). The synthesized ZnO NPs exhibited concentration-dependent antibacterial activity, with a maximum inhibition zone of 20 mm against Bacillus sp. and 15 mm against Escherichia coli at 1000 µg/mL. These findings establish a direct correlation between root phytochemical composition, nanoparticle physicochemical properties, and antimicrobial performance, demonstrating the potential of B. monosperma -mediated ZnO NPs as phytochemical-engineered antibacterial nanomaterials.