| Summary: | The current study evaluated the biogenic synthesis of nickel oxide nanoparticles (SP-NiONPs) from the root extract of (<i>Salvadora persica</i>) <i>S. persica</i> and their biological properties. The nanoparticles were characterized using spectroscopic and microscopic techniques and then evaluated for their antimicrobial properties against 10 oral pathogens. The ultraviolet-visible (UV–Vis) spectra exhibited a distinctive resonance spectrum at 334 nm for the SP-NiONPs produced from <i>S. persica</i>. The fourier transform infrared (FTIR) analysis revealed the presence of functional groups of biomolecules of <i>S. persica</i> that served as reducing and capping agents of the SP-NiONPs. The scanning electron microscope (SEM) and transmission electron microscopy (TEM) analyses showed that the nanoparticles were spherical-shaped, tightly packed, and ranged in size from 18.20 nm to 45.12 nm. The energy dispersive x-ray (EDX) analysis confirmed 69.9% of the nickel (Ni) content by weight, and the X-ray diffraction (XRD) results showed the face-centered cubic (FCC) crystalline structure of the formed SP-NiONPs. The antioxidant activity of the SP-NiONPs exhibited a dose-dependent profile with an IC50 value of 51.45 ± 0.65 and a 54.13 ± 0.98 DPPH<sup>•</sup> and ABTS<sup>•+</sup> radical scavenging activity, respectively. The SP-NiONPs showed an antibacterial activity against all the test strains; however, <i>E. cloacae</i> was found to be the most sensitive strain, with an inhibition zone of 31 ± 0.50 mm. The SEM image of the <i>E. cloacae</i> cells treated with SP-NiONPs showed irregular shapes and ruptured, destroyed cell membranes. Our findings revealed that SP-NiONPs could be used as excellent antibacterial agents against oral pathogens.
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