Comparative study of Neem, Guava, and mixed Neem–Guava extracts in green-synthesized CuO nanoparticles: structural, optical, and antibacterial insights

This study presents a systematic comparative investigation of green-synthesized CuO nanoparticles using distinct phytochemical systems derived from Azadirachta indica , Psidium guajava , and their mixed extracts. Unlike conventional single-plant-mediated green synthesis approaches, this work introduces a dual-extract phytochemical strategy that enables cooperative biomolecular interactions for controlled nucleation, lattice relaxation, and defect-state modulation without the use of external stabilizing agents or chemical additives. X-ray diffraction confirms phase-pure monoclinic CuO formation in all samples. Williamson–Hall analysis reveals that the mixed-extract sample exhibits larger crystallite size and reduced lattice strain, indicating improved crystallographic ordering driven by synergistic phytochemical effects. The apparent discrepancy between crystallite size (XRD) and particle size (SEM) is attributed to agglomeration of multiple crystallites into larger secondary particles. Optical characterization demonstrates systematic defect engineering. The band gap decreases from 2.60 eV to 2.54 eV, accompanied by reduced Urbach energy, confirming suppression of band-tail disorder and improved structural homogeneity. Photoluminescence analysis shows enhanced radiative recombination with controlled deep-level defect contributions, reflecting effective surface passivation and optimized defect distribution. Furthermore, antibacterial studies indicate superior activity, attributed to defect-mediated reactive oxygen species generation and improved nanoparticle–cell interaction. Notably, this study establishes for the first time a dual-extract phytochemical system as an intrinsic defect-engineering platform, demonstrating that cooperative biomolecular interactions can simultaneously regulate crystallinity, lattice strain, and defect-state distribution without external chemical modifiers. It also provides new insight into the role of phytochemical diversity in tuning structure–property relationships and offers a scalable, sustainable, and cost-effective pathway for designing next-generation metal oxide nanomaterials for optoelectronic, photocatalytic, and biomedical applications.