Green Synthesis of Cobalt Oxide Nanoparticles from Aqueous Extract of Hunteria umbellata as Antibacterial Agent

Abstract

The study utilized aqueous seed extract of Hunteria umbellata to synthesize cobalt oxide nanoparticles (Co₃O₄ NPs) as a green and sustainable approach. The cobalt nano-oxide was characterized by UV-visible (UV-Vis) and Fourier transform infrared (FTIR) spectrophotometry, X-ray diffraction (XRD), scanning electron microscopy coupled energy dispersive X-ray (SEM-EDX), differential thermal and thermogravimetric analyses (DTA/TGA), Brunauer-Emmett-Theory (BET), dynamic light scattering (DLS) and Zeta potential-conductivity value. The Co3O4 NPs were assayed for antimicrobial potency. The UV-Vis spectrum revealed an absorption maximum at 347 nm consistent with Co3O4 NPs while the FTIR spectrum revealed the formation of Co-O bond at 1043 cm^-1 with bands at 2340- 2653 cm^-1
and 1438 cm^-1 assigned to C-H and C-C stretching frequencies of the glycosides and aromatic system of the alkaloid groups from the plant extract. The XRD data revealed a peak at (311) plane with crystallite size of 38.57 nm, the SEM micrograph showed average diameter of 0.49 µm of irregular to slightly spherical particles with evidence of agglomeration while the EDX revealed the presence of cobalt and oxygen atoms. DTA/TGA data showed endothermic and exothermic peaks consistent with cobalt oxide nanoparticles as the TGA revealed multistep decomposition. BET results present mesoporous material with high surface area while the DLS affirmed the formation of nanoparticles with moderate stability in suspension with evidence of slight agglomeration. Zeta potential of -33.53 mV with conductivity value of 0.633 mS/cm indicate good colloidal stability with the presence of free ions in suspension. The antibacterial potency of the Co₃O₄ NPs was tested against two gram-negative bacteria (Escherichia coli and Salmonella typhi) and one gram-positive bacteria (Staphylococcus aureus). The results of the zones of inhibition showed that the nanoparticles were moderately active against E. coli (14±1.41 mm) at 500 mg/mL only. The nanoparticles were highly and moderately active against S. typhi at both 500 and 250 mg/mL concentrations with mean values of 22.6±1.96 mm and 17.3±1.92 mm respectively. The Co₃O₄ NPs exhibited high and moderate activity against S. aureus at both concentrations of 500 and 250 mg/mL with mean values of 20.3±0.92 mm and 12.3±0.92 mm respectively. In comparison with gentamycin with results ranging between (34±0.05 – 37±0.08) µg/mL for the microbes, Co₃O₄ NPs shows good promise. With optimization and better functionalization, the characterization results suggests that the biosynthesized Co₃O₄ NPs could serve as adsorbents in pollution control and catalysis and as very potent antibacterial agent.