Sol–gel synthesized Fe-doped NiO nanoparticles show tunable size, magnetism, and fluorescence. Enhanced emission of acridine orange, a common cell staining dye, highlights its utility in nanostructured bioimaging and optoelectronic platforms.

Iron-doped nickel oxide (NiO) nanoparticles with composition Fe _x Ni_1−x O (x = 0.00, 0.02, 0.04, 0.06, and 0.08) are synthesized via a facile sol–gel method, and their structural, magnetic, and optical properties are systematically studied. X-ray diffraction study confirms the formation of phase-pure NiO with no secondary phases, indicating successful Fe incorporation and reduced crystallite size with increased doping. Field emission scanning electron microscopy and high-resolution transmission electron microscopy images reveal a predominantly spherical morphology. The average particle size decreases significantly from ≈283 nm to 89 nm with increased Fe-doping. Magnetic measurements indicate a transition from antiferromagnetic to ferromagnetic behavior with higher Fe-doping, highlighting the tunability of magnetic properties. Optical absorption spectra show peaks in the range of 323–345 nm, while fluorescence measurements reveal emission around ≈384 nm, showing violet emissions. A notable decrease in optical bandgap and quenching of fluorescence intensity is observed with increasing Fe content, attributed to the reduction in particle size. Furthermore, the Fe-doped NiO nanoparticles enhance and tune the fluorescence intensity of acridine orange dye, a widely used cell-staining agent, suggesting its potential for biological applications. The combined structural and optical properties make these sol–gel synthesized NiO nanoparticles promising candidates for potential applications in bioimaging, biosensing, and optoelectronics.