Precursor effect on green synthesized <scp>CuO</scp> nanoparticles from Olea europaea for photocatalytic and biomedical applications

Abstract This study introduces a novel, surfactant‐free plant‐assisted thermal decomposition method using Olea europaea leaf powder for green synthesis of CuO nanoparticles, demonstrating precise control of structural and functional properties through simple variation of copper(II) acetate precursor mass (2, 4, and 6 g). XRD confirmed phase‐pure monoclinic CuO with crystallite sizes of 15.5 nm (CuO‐2g) and 28.8–28.9 nm (CuO‐4g and CuO‐6g). UV–Vis Tauc plot analysis yielded direct band gap values of 2.36, 2.24, and 2.10 eV and indirect band gap values of 2.09, 1.81, and 1.63 eV for CuO‐2g, CuO‐4g, and CuO‐6g, respectively, confirming the indirect semiconductor character of monoclinic CuO and a precursor‐dependent optical tunability. SEM revealed porous high‐surface‐area morphology at lower precursor loading versus dense agglomerates at higher loadings. Biological and photocatalytic evaluations identified CuO‐2g as the superior performer, showing the lowest IC₅₀ values in ABTS, DPPH, DPP‐4, α‐amylase, AChE, and BChE assays, strongest antibiofilm activity against Enterococcus faecalis , and &gt;99% photocatalytic degradation of Malachite Green under visible light within 140 min. This enhanced performance is attributed to smaller crystallite size, higher surface reactivity, and greater charge carrier separation efficiency. These findings establish precursor concentration as a critical parameter governing the structure–activity relationship in biosynthesized CuO NPs, positioning Olea europaea ‐mediated synthesis as an efficient, eco‐friendly platform for biomedical and environmental applications.