Structure–optics–biology relationship in sol–gel-synthesized Mg0.97 Ni0.03O nanoparticles

Sol–gel synthesis was used to fabricate dilute Ni-substituted magnesium oxide nanoparticles (Mg₀.₉₇Ni₀.₀₃O) and to interrogate how crystal structure and defect-driven optics relate to antibacterial performance. After gel drying and calcination (550 °C), powder X-ray diffraction indexed all reflections to cubic periclase (space group Fm-3m), with a refined lattice parameter a=4.208 Å—slightly contracted relative to stoichiometric MgO—supporting substitutional Ni²⁺ on Mg²⁺ sites. Line-profile analysis via the Scherrer equation (instrument parameters as standard) gave a mean coherent-domain size of D≈9.53 nm across the (111), (200), (220), (311), and (222) peaks, confirming nanocrystallinity. FTIR spectra showed ν(OH) at ~3416 cm⁻¹ and δ(H₂O) at ~1641 cm⁻¹ (rehydroxylated surfaces), carbonate features at ~1445/1076/860 cm⁻¹ (atmospheric CO₂ uptake on basic sites), and broad lattice M–O bands near 455 and 419 cm⁻¹, consistent with a defect-rich rock-salt lattice. UV–vis–NIR measurements revealed strong UV response and a smooth visible tail; Kubelka–Munk transformed Tauc plots (direct-allowed model) yielded an apparent band gap Eg=4.15 eV, attributed to Ni- and defect-mediated sub-gap transitions rather than the intrinsic MgO edge. Agar disk-diffusion (ADM) assays evidenced inhibition of Escherichia coli and Staphylococcus aureus. Single plates gave zones of 9 mm (E. coli) and 14 mm (S. aureus), corresponding to net inhibitory areas of ~35.3 and ~125.7 mm², respectively. Across triplicate disks per species, however, mean diameters were identical (11.67 ± 2.52 vs 11.67 ± 2.08 mm), and Welch tests on diameter and area were non-significant (p ≥ 0.98), indicating comparable activity within current uncertainty. Overall, dilute Ni substitution contracts the MgO lattice, enhances defect-tail optics (apparent Eg=4.15 eV), and affords modest, diffusion-limited antibacterial effects, establishing a reproducible structure–optics–biology framework for MgO-based antimicrobial nanoceramics