Plant-mediated formation of zinc oxide and silver nanoparticles using Garuga pinnata bark: characterization and bioactivity assessment

Plant-mediated synthesis of nanoparticles has become widely recognized due to its cost-effective, sustainable, and efficient nature. In this study, the bark of Garuga pinnata Roxb., a plant known for its rich phytochemical profile, was utilized to synthesize zinc oxide (ZnO NPs) and silver nanoparticles (AgNPs). The preliminary formation of nanoparticles was confirmed by a sharp absorption peak at 361 nm and 412 nm for ZnO and Ag nanoparticles, respectively. The possible involvement of phytochemicals as stabilizing and reducing agents was indicated by Fourier-Transform Infrared Spectroscopy (FTIR), showing distinct shifts in stretching frequencies of corresponding functional groups. The morphology, size, and elemental composition of the nanoparticles were analyzed using Field Emission Scanning Electron Microscopy (FE-SEM), X-Ray Diffraction (XRD), and Energy Dispersive X-Ray spectroscopy (EDX). XRD analysis confirmed the formation of crystalline ZnO and Ag nanoparticles with hexagonal wurtzite and face-centered cubic structures and crystallite sizes of 12.51 nm and 6.47 nm, respectively. FE-SEM results showed the synthesized nanoparticles were mostly spherical, with relatively uniform size distribution and minimum agglomeration. Moreover, a series of quantitative analyses was performed to evaluate the biological activity of the nanoparticles. The antioxidant activity evaluated by 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay, yielded IC₅₀ values of 92.05 ± 3.67 μg/mL for ZnO NPs and 76.27 ± 1.26 μg/mL for AgNPs. In the antibacterial assay using four bacterial strains, ZnO NPs demonstrated stronger activity with a zone of inhibition (ZOI) of 18 mm and 14 mm against Escherichia coli (ATCC 25,312) and Shigella sonnei (ATCC 25,931), respectively, compared to AgNPs with a ZOI of 8 mm for both strains. α-Amylase inhibition assay revealed slightly stronger antidiabetic activity of AgNPs (IC 50 = 0.755 ± 0.235 mg/mL) compared to ZnO NPs (IC 50 = 0.8083 ± 0.212 mg/mL). Both nanoparticles have shown moderate toxicity toward Artemia salina, with ZnO NPs showing relatively greater toxicity (LC 50 = 553.90 μg/mL) than AgNPs (LC 50 = 672.97 μg/mL). The cytotoxicity evaluated through 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay on cancer cells, A549 and HeLa, and normal epithelial cells showed significant selectivity in their action against HeLa (Selectivity Index (SI) = 4.88 for ZnO NPs and 4.93 for AgNPs. Overall, the synthesized nanoparticles exhibited promising antibacterial, antioxidant, antidiabetic, and cytotoxic activities, highlighting the potential of the green synthesis method in producing nanoparticles with prospective biomedical applications.