Phytochemically mediated fabrication of nickel oxide nanoparticle/carbon dot composites: structural characterization and enhanced antimicrobial efficacy

Abstract

The escalating prevalence of antimicrobial resistance has created an urgent demand for novel, ecologically responsible antibacterial and antifungal agents. This study reports the phytochemically mediated green synthesis of nickel oxide nanoparticles (NiO NPs) using Croton macrostachyus leaf extract and their subsequent hybridization with carbon dots (C-dots) to produce NiO NPs@C-dot composites with enhanced antimicrobial properties. The NiO NPs were synthesized by combining Croton macrostachyus leaf extract with nickel nitrate hexahydrate, followed by calcination at 450 °C for 4 hours, while C-dots were separately prepared from citric acid and o-phenylenediamine via hydrothermal treatment. Structural and morphological characterization was accomplished through transmission electron microscopy (TEM), field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), UV-visible spectroscopy, and electrochemical impedance spectroscopy (EIS). The average particle sizes of NiO NPs and the NiO NPs@C-dot composite were determined to be 25.34 ± 0.12 nm and 24.95 ± 0.22 nm, respectively. XRD analysis confirmed the face centered cubic crystalline phase of the NiO NPs with diffraction peaks indexed to the (111), (200), (220), (311), and (222) planes. The antimicrobial potential of both bare NiO NPs and the composite was evaluated against Gram positive (Bacillus subtilis and Staphylococcus aureus), Gram negative (Escherichia coli and Pseudomonas aeruginosa) bacterial strains, and the fungal strain Fusarium using the agar well diffusion method. The NiO NPs@C-dot composite exhibited superior antimicrobial activity with zones of inhibition ranging from 22 to 26 mm compared with 21 to 24 mm for bare NiO NPs, attributable to the synergistic enhancement of reactive oxygen species generation and improved surface interactions conferred by the C-dot coating. These findings establish the NiO NPs@C-dot composite as a promising candidate for biomedical antimicrobial applications.