Chemical synthesis versus green synthesis to obtain ZnO powders: Evaluation of the antibacterial capacity of the nanoparticles obtained by the chemical method

In this work, zinc oxide nanoparticles (ZnO-NPs) were synthesized using both a chemical route (polymeric complex method) and a green route (controlled precipitation, using suspensions of garlic, Allium sativum, and chili, Capsicum annuum, as solvents). The obtained solids were characterized using infrared and Raman spectroscopies, X-ray diffraction (XRD) and scanning electron microscopy (SEM). While the solid synthesized by the chemical pathway presented a wurtzite-like crystalline structure of ZnO, those obtained by green synthesis were essentially amorphous. In the IR spectra of the materials synthesized by the two methods, the presence of the characteristic bands of ZnO was evidenced, as well as the active groups of garlic and chili pepper in the IR spectrum of the sample obtained by green synthesis. Something similar happened with the Raman spectra of the synthesized samples. To account for the alterations found in both structure and composition of the obtained solids, a series of reactions were proposed between the zinc precursor with both the active garlic and chili molecules present in the solvent during green synthesis. Considering the potential use of synthesized ZnO as antibacterial material, the action of ZnO-NPs (< 100 nm with a spheroidal morphology) obtained by the chemical route was evaluated on a strain of Escherichia coli in the absence of UV radiation. The results indicated that the synthesized ZnO-NPs, in high and medium concentrations (between 500 and 32 µg/mL), inhibited growth of the bacteria, from ~ 70% to ~ 50% respectively, with a minimum inhibitory concentration, MIC50, of 30.40 µg/mL [Display omitted] •Differences in the physicochemical characteristics of the ZnO-NPs synthesized by chemical and green routes were determined.•Chemical reactions are proposed to explain the formation of ZnO-NPs by a green route at low temperature.•In the absence of UV radiation, the antimicrobial effect of ZnO-NPs obtained by chemical route on E. coli was evaluated.•It is observed that with 500 µg/mL of ZnO-NPs, bacterial growth is inhibited by 70%.•The synthesized ZnO-NPs presented a minimum inhibitory concentration of 30.40 µg/mL.