Green Synthesis and Potential Antibacterial Applications of Bioactive Silver Nanoparticles: A Review

Abstract

Green synthesis of silver nanoparticles (AgNPs) using biological resources is the most facile, economical, rapid, and environmentally friendly method that mitigates the drawbacks of chemical and physical methods. Various biological resources such as plants and their different parts, bacteria, fungi, algae, etc. could be utilized for the green synthesis of bioactive AgNPs. In recent years, several green approaches for non-toxic, rapid, and facile synthesis of AgNPs using biological resources have been reported. Plant extract contains various biomolecules, including flavonoids, terpenoids, alkaloids, phenolic compounds, and vitamins that act as reducing and capping agents during the biosynthesis process. Similarly, microorganisms produce different primary and secondary metabolites that play a crucial role as reducing and capping agents during synthesis. Biosynthesized AgNPs have gained significant attention from the researchers because of their potential applications in different fields of biomedical science. The widest application of AgNPs is their bactericidal activity. Due to the emergence of multidrug-resistant microorganisms, researchers are exploring the therapeutic abilities of AgNPs as potential antibacterial agents. Already, various reports have suggested that biosynthesized AgNPs have exhibited significant antibacterial action against numerous human pathogens. Because of their small size and large surface area, AgNPs have the ability to easily penetrate bacterial cell walls, damage cell membranes, produce reactive oxygen species, and interfere with DNA replication as well as protein synthesis, and result in cell death. This paper provides an overview of the green, facile, and rapid synthesis of AgNPs using biological resources and antibacterial use of biosynthesized AgNPs, highlighting their antibacterial mechanisms.

Keywords: antibacterial application; antibacterial mechanisms; green synthesis; silver nanoparticles.

Figure 1

Different methods of nanoparticle synthesis.

Figure 2

Schematic illustration of green synthesis and characterizations of AgNPs.

Figure 3

Optimization of parameters for stable, monodispersed, rapid and high-yield of AgNPs.

Figure 4

R2A broth with AgNO₃ as control (A); biosynthesized AgNPs (B); UV–vis spectra (C); FE-TEM images (D,E); SAED pattern (F); EDX spectrum (G); X-ray diffraction pattern (H); and FT-IR spectra of biosynthesized AgNPs (I). This figure has been reprinted with permission from Ref. [2], copyright 2020, Informa UK Limited.

Figure 5

Possible antibacterial mechanisms of AgNPs. Disruption of cell wall and cell membrane, damage of ATP molecules due to the production of reactive oxygen species, DNA inactivation, protein denaturation and ribosome degradation.

Figure 6

FE–SEM images of normal P. aeruginosa cells (A); 1 × MBC AgNPs treated P. aeruginosa cells (B); normal S. aureus cells (C); 1 × MBC AgNPs treated S. aureus cells (D). This figure has been reprinted with permission from Ref. [6], copyright 2020, MDPI.