Solvent based fractional biosynthesis, phytochemical analysis, and biological activity of silver nanoparticles obtained from the extract of Salvia moorcroftiana

Abstract

Multi-drug resistant bacteria sometimes known as "superbugs" developed through overuse and misuse of antibiotics are determined to be sensitive to small concentrations of silver nanoparticles. Various methods and sources are under investigation for the safe and efficient synthesis of silver nanoparticles having effective antibacterial activity even at low concentrations. We used a medicinal plant named Salvia moorcroftiana to extract phytochemicals with antibacterial, antioxidant, and reducing properties. Three types of solvents; from polar to nonpolar, i.e., water, dimethyl sulfoxide (DMSO), and hexane, were used to extract the plant as a whole and as well as in fractions. The biosynthesized silver nanoparticles in all extracts (except hexane-based extract) were spherical, smaller than 20 nm, polydispersed (PDI ranging between 0.2 and 0.5), and stable with repulsive force of action (average zeta value = -18.55±1.17). The tested bacterial strains i.e., Klebsiella pneumoniae, Pseudomonas aeruginosa, Staphylococcus aureus, and Enterococcus faecalis were found to be sensitive to even small concentrations of Ag-NPs, especially P. aeruginosa. The antibacterial effect of these Ag-NPs was associated with their ability to generate reactive oxygen species. DMSO (in fraction) could efficiently extract antibacterial phytochemicals and showed activity against MDR bacteria (inhibition zone = 11-12 mm). Thus, the antibacterial activity of fractionated DMSO extract was comparable to that of Ag-NPs because it contained phytochemicals having solid antibacterial potential. Furthermore, Ag-NPs synthesized from this extract owned superior antibacterial activity. However, whole aqueous extract-based Ag-NPs MIC was least (7-32 μg/mL) as compared to others.

Fig 1. Solvent-based extraction of antimicrobial phytochemicals.

Fig 2

UV-Vis spectra of (a) S. moorcroftiana extracts and (b) silver nanoparticles synthesized from these extracts.

Fig 3. Schematic illustration for Ag-NPs reduction and capping through phytochemicals.

Fig 4

FTIR spectra of a) Aq-extract and Ag-NPs b) Aq*-extract and Ag-NPs c) D-extract and Ag-NPs, D*-extract and Ag-NPs.

Fig 5

Transmission electron microscopy image of a) Aq-Ag-NPs, b) Aq*-Ag-NPs, c) D-Ag-NPs and d) D*-Ag-NPs.

Fig 6. Determination of Zeta potential of silver nanoparticles synthesized via different solvents.

Fig 7. Size distributions of silver nanoparticles synthesized via different extracts.

Fig 8. Comparative analysis of total phenolic content in different extracts and Ag-NPs based on different solvent-based extracts.

Hexane based Ag-NPs synthesis was negligible and could not be analyzed for TPC.

Fig 9. Comparative analysis of total flavonoid content in extracts and silver nanoparticles based on different solvent-based extracts.

Fig 10. Antioxidant activity of plant extracts and silver nanoparticles synthesized from these extracts.

Fig 11. Reactive oxygen quantification in bacterial cultures grown in the presence of different concentrations of different types of Ag-NPs.