Spectroscopic insights into the antibacterial and antifungal mechanism of ZnO nanoparticles green-synthesized from Morinda citrifolia leaf extract

Abstract

This study presents a green hydrothermal synthesis process for ZnO nanoparticles using Morinda citrifolia leaf extract at pH conditions (2.5, 5.0, 7.5, and 10.0). The phytochemical constituents of the extract, including alkaloids, flavonoids, phenolics, coumarins, terpenoids, anthocyanins, and glycosides, served as effective reducing and stabilizing agents. Structural and morphological characterization via SEM, XRD, and FTIR confirmed the formation of the hexagonal wurtzite phase (JCPDS 36-1451), consisting of nearly spherical nanoparticles that form dense, agglomerated clusters of ZnO nanoparticles with an average crystallite size of ∼12.61 nm. Importantly, Raman and PL spectra provided insight into the antibacterial and antifungal mechanism, revealing that ZnO nanoparticles synthesized at pH 5.0 had a high concentration of oxygen vacancies (V_O), with visible emission accounting for approximately 80% of the total spectrum. This high defect density correlated with superior antimicrobial efficacy. At a concentration of 0.1 mg mL⁻¹, ZnO nanoparticles exhibited significant inhibition zones against Staphylococcus aureus (17.05 ± 0.25 mm), Escherichia coli (17.62 ± 0.72 mm), and Candida albicans (13.39 ± 1.01 mm). These findings demonstrate the potential of ZnO nanoparticles mediated by Morinda citrifolia as potent antimicrobial agents and highlight the role of surface defects in enhancing their bioactivity.