Potato Starch-Capped Zinc Oxide Nanoparticles as Bifunctional Photocatalyst and Bactericide

The zinc oxide nanoparticles (ZnO NP) were synthesized from zinc acetate using potato starch as a capping agent through alkaline sol–gel precipitation, without calcination. The produced nanoparticles were characterized using various analytical techniques, including UV–visible absorption spectroscopy (UV-vis), zeta potential analysis, dynamic light scattering (DLS), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM). The ZnO NP exhibited an absorption maximum at 361 nm in the UV-vis spectrum, a z-average value of 138.1 nm and a zeta potential value of −23.8 mV as measured by DLS. The XRD pattern revealed distinctive diffraction peaks corresponding to the hexagonal wurtzite crystal structure characteristic of ZnO. FTIR analysis indicated that NP were capped with hydroxyl functional groups from the starch. The produced NP were quasi spherical, with sizes ranging from 20.7 to 38.1 nm and a mean particle size of 30.6 ± 5.1 nm. The potential application of ZnO NP as a photocatalyst was studied under UV light at 365 nm for the decolourization of tartrazine, a model azo food dye. The effects of varying concentrations of the catalyst (900–4500 µg/mL), tartrazine (2.5–10 µg/mL) and reaction time (30–120 min) on tartrazine removal were monitored using UV-vis at 426 nm. Under optimum conditions of 2700 µg/mL NP, 7.5 µg/mL tartrazine and a reaction time of 90 min, a 92% removal was achieved, with a rate constant (k) of 0.0135 min⁻¹. Additionally, the bactericidal activity of the NP against Escherichia coli and Bacillus subtilis was investigated using the resazurin broth method. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) values against E. coli and B. subtilis were 2700 and 1800 µg/mL and 3600 and 2700 µg/mL, respectively. Thus, the current study highlights the calcination-free, potato starch-capped sol–gel synthesis and the bifunctionality of ZnO NP as a recyclable photocatalyst and a bactericide for the decolourization of dyes and pigments, as well as for the bacterial disinfection of food industry wastewater effluents.