Bioactive ZnO Nanoparticles: Biosynthesis, Characterization and Potential Antimicrobial Applications

Abstract

In recent years, biosynthesized zinc oxide nanoparticles (ZnONPs) have gained tremendous attention because of their safe and non-toxic nature and distinctive biomedical applications. A diverse range of microbes (bacteria, fungi and yeast) and various parts (leaf, root, fruit, flower, peel, stem, etc.) of plants have been exploited for the facile, rapid, cost-effective and non-toxic synthesis of ZnONPs. Plant extracts, microbial biomass or culture supernatant contain various biomolecules including enzymes, amino acids, proteins, vitamins, alkaloids, flavonoids, etc., which serve as reducing, capping and stabilizing agents during the biosynthesis of ZnONPs. The biosynthesized ZnONPs are generally characterized using UV-VIS spectroscopy, TEM, SEM, EDX, XRD, FTIR, etc. Antibiotic resistance is a serious problem for global public health. Due to mutation, shifting environmental circumstances and excessive drug use, the number of multidrug-resistant pathogenic microbes is continuously rising. To solve this issue, novel, safe and effective antimicrobial agents are needed urgently. Biosynthesized ZnONPs could be novel and effective antimicrobial agents because of their safe and non-toxic nature and powerful antimicrobial characteristics. It is proven that biosynthesized ZnONPs have strong antimicrobial activity against various pathogenic microorganisms including multidrug-resistant bacteria. The possible antimicrobial mechanisms of ZnONPs are the generation of reactive oxygen species, physical interactions, disruption of the cell walls and cell membranes, damage to DNA, enzyme inactivation, protein denaturation, ribosomal destabilization and mitochondrial dysfunction. In this review, the biosynthesis of ZnONPs using microbes and plants and their characterization have been reviewed comprehensively. Also, the antimicrobial applications and mechanisms of biosynthesized ZnONPs against various pathogenic microorganisms have been highlighted.

Keywords: ZnONPs; antimicrobial applications; antimicrobial mechanisms; biosynthesis; characterization.

Figure 1

Schematic illustration of biosynthesis and potential antimicrobial applications of bioactive ZnONPs.

Figure 2

(a) Reduction of Zn²⁺ to ZnONPs by (i) cell-free supernatant and (ii) cell biomass of L. plantarum TA4. (b) UV-Vis spectrum of (i) ZnONPs-CFS and (ii) ZnONPs-CB. This figure has been reprinted with permission from Ref. [13], copyright 2020, Nature Portfolio.

Figure 3

(a) Transmission electron microscope (TEM) scale bar: 100 nm; (b) particle size distribution histogram; (c) scanning electron microscopy (SEM) scale bar: 1 μm; (d) the EDX spectra of biosynthesized ZnONPs using leaf extract of Salvia officinalis. This figure has been reprinted with permission from Ref. [62], copyright 2021, MDPI.

Figure 4

FTIR spectra of (a) aqueous leaf extract of S. officinalis, (b) ZnONPs dried at 80 °C, and (c) ZnONPs calcinated at 400 °C. This figure has been reprinted with permission from Ref. [62], copyright 2021, MDPI.

Figure 5

Zones of inhibition around discs impregnated with 1% DMSO (A), 2 µg/mL amphotericin B (B), ½ MIC of ZnONPs (C), MIC of ZnONPs (D), and MFC of ZnONPs (E) against different Candida albicans isolates. This figure has been reprinted with permission from Ref. [62], copyright 2021, MDPI.

Figure 6

Scanning electron micrographs (SEM) of C. albicans SC5314: (A) represents untreated control cells, whereas (B,C) represent the cells exposed to MIC and MFC of biosynthesized ZnONPs, respectively. This figure has been reprinted with permission from Ref. [62], copyright 2021, MDPI.

Figure 7

SEM (a) and TEM (b) images of rice bacterial pathogen B. glumae and B. gladioli cells after 8 h treatment with (50 µg mL⁻¹) and without (control) biogenic ZnONPs. This figure has been reprinted with permission from Ref. [42], copyright 2021, MDPI.