Correlation between structure, optical band gap, and antibacterial efficiency in sol–gel-synthesized Mg₀.₉₁Fe₀.₉O nanoparticles

Mg₀.₉₁Fe₀.₉O nanoparticles were synthesized via a controlled sol–gel method to investigate their structural, optical, and antibacterial properties, with a focus on the impact of Mg and Fe ion deficiencies. Structural analysis by X-ray diffraction (XRD) confirmed the formation of a single-phase spinel structure with a crystallite size in the nanometer range, consistent with high surface reactivity. Lattice parameters, space group (Fd-3m), and d-spacing values matched standard JCPDS data, confirming structural stability despite nonstoichiometry. Optical characterization using UV–Vis spectroscopy revealed strong absorption in the visible region, with a Tauc plot–derived direct band gap of 3.85 eV, indicating potential photocatalytic antibacterial contributions.\r\nAntibacterial activity against Escherichia coli (Gram-negative) was assessed using two complementary methods: the agar diffusion method (ADM) and the spread plate method (SPM). ADM demonstrated a pronounced inhibition zone of 22.0 ± 0.5 mm, indicating strong bactericidal efficacy. SPM results confirmed substantial antibacterial performance, with viable colony counts reduced to 320, 100, and 25 CFU/mL at dilutions of 10⁻³, 10⁻⁴, and 10⁻⁵, respectively, corresponding to an average 2.73 log reduction and 99.82% bacterial reduction compared to the control. Statistical analysis revealed no significant difference between ADM and SPM outcomes (p > 0.05), although ADM provided more consistent inhibition measurements, while SPM offered precise quantitative verification.\r\nThe integration of structural, optical, and microbiological analyses demonstrates that defect-engineered Mg₀.₉₁Fe₀.₉O nanoparticles (NPs)possess high stability, tunable optical properties, and potent antibacterial activity. These results suggest promising potential for applications in antimicrobial coatings, water purification, and environmental remediation, with future research recommended on Gram-positive bacteria, cytotoxicity, and in vivo biocompatibility.\r\n