Plant-mediated synthesis of zinc oxide (ZnO) nanoparticles using Alnus nepalensis D. Don for biological applications

An aqueous bark extract of <i>A. nepalensis</i> D. Don was utilized to prepare zinc oxide (ZnO) nanoparticles through a green method, which is more economical, eco-friendly, and effective for exploring several biological applications and toxicity assessments against brine shrimp nauplii. The prepared ZnO nanoparticles were characterized using several characterizing techniques. The surface morphology and the elemental composition of the prepared ZnO NPs was analyzed by field emission scanning electron microscopy (FE-SEM), and energy dispersive X-ray (EDX) analysis. The colour of the solution was changed from reddish-brown to white indicating the formation of ZnO NPs which shows UV-vis absorption at 361 nm. The various functional groups of the organic compounds present in plant extract act as reducing and stabilizing agents in the formation of nanoparticles. The involvement of these functionalities in the formation of nanoparticles is indicated by the shifts and changes in the IR spectra of both the plant extract and the ZnO nanoparticles. The size of the nanoparticles was determined to be 15.31 nm with XRD analysis while the FE-SEM revealed the average grain size of 67.29 nm with irregular shape. The elemental composition of ZnO NPs shows a greater atomic percentage of zinc compared to other elements (C, N, Ni, O, and Ag), with an intense peak of zinc observed at approximately 1 keV. The trace amount of silver is due to the impurities present in the reagent used in the experiment. The antioxidant property of ZnO nanoparticles was evaluated with an IC₅₀ of 53.02 ± 3.43 μg/mL. The ZnO nanoparticles exhibited significant antibacterial activity against <i>Klebsiella pneumoniae</i> and <i>Escherichia coli</i>, with zones of inhibition (ZOI) of 18 mm and 23 mm, respectively as compared to the positive control neomycin of ZOI 28 mm against <i>K. pneumoniae</i>. The potential antibacterial activity of the ZnO NPS was revealed as the MIC and MBC against <i>K. pneumoniae</i> of 0.39 mg/mL and 0.78 mg/mL, respectively. In addition, the prepared ZnO nanoparticles showed toxicity against brine shrimp nauplii of LC₅₀ 16.59 μg/mL. The results of this study impart that plant-assisted synthesized ZnO nanoparticles possess significant antibacterial properties that reduce oxidative stress in human cells, ultimately contributing to cancer prevention.