Green synthesis of silver nanoparticles using Tropaeolum majus: Phytochemical screening and antibacterial studies

Abstract

Abstract The green biosynthesis of metal nanoparticles of already explored phytomedicines has many advantages such as enhanced biological action, increased bioavailability, etc. In this direction, keeping in view the peculiar medicinal value of Tropaeolum majus L., we synthesized its silver nanoparticles (AgNPs) by adopting eco-friendly and cost-effective protocol by using methanolic and aqueous extract of T. majus. The synthesized AgNPs were characterized by using several techniques including UV spectroscopic analysis, FTIR analysis, and atomic force microscopy. The methanolic/aqueous extracts of T. majus and synthesized AgNPs were assessed for antioxidant potential and antimicrobial effect. The preliminary screening showed that the T. majus extracts have variety of reducing phytochemicals including tannins, terpenoids, flavonoids, and cardiac glycosides. The green synthesis of AgNPs was confirmed by the appearance of sharp peak at 430–450 nm in the UV-Visible spectra. The FTIR spectral analysis of extract and AgNPs exhibited that peaks at 2947.23, 2831.50, 2592.33, 2522.89, and 1,411 cm−1 disappeared in the spectra of FTIR spectra of the AgNPs, indicating carboxyl and hydroxyl groups are mainly accountable for reduction and stabilization of AgNPs. Atomic force microscopic scan of the synthesized AgNPs confirmed its cylindrical shape with size of 25 µm. The extracts and AgNPs were investigated for antioxidant potential by DPPH-free radical essay, which showed that aqueous extract has significant and dose-independent antioxidant activity; however, the synthesized AgNPs showed decline in antioxidant activity. The extracts and synthesized AgNPs were also evaluated for antibacterial activity against Klebsiella pneumonia, Staphylococcus aureus, and Bacillus subtilis. Neither extract nor AgNPs were active against Klebsiella pneumonia. The aqueous and methanolic extract exhibited inhibition against Bacillus subtilis and their synthesized AgNPs were active against Staphylococcus aureus. Our data concluded that the extracts of T. majus have necessary capping and reducing agents which make it capable to develop stable AgNPs. The aqueous extract of T. majus has potential antioxidant effect; however, the AgNPs did not enhance its free radical scavenging effect. The bacterial strains’ susceptibility of the extract and AgNPs was changed from Bacillus subtilis to Staphylococcus aureus, respectively. The biological action of AgNPs is changed in case of antibacterial activity which means that AgNPs might change the specificity of T. majus and likewise other drugs.