Abstract

Sneka C., P. Perumal, K. Amuthavalli*

ABSTRACT

Background: The rise of antimicrobial resistance has renewed interest in silver nanoparticles (AgNPs) as broad-spectrum antibacterial agents, and plant-mediated synthesis offers a route that avoids the toxic reducing agents and energy costs of conventional chemical methods. Cordia sebestena L. (Boraginaceae), an ornamental tree used in folk medicine for respiratory complaints, fevers and wounds, is rich in phenolic and flavonoid constituents capable of reducing silver ions, but its use as a bioreductant under microwave irradiation has received little attention. Methods: Dried leaves were defatted with petroleum ether and macerated in ethanol for 48 h. The extract was subjected to qualitative phytochemical screening and used to reduce aqueous silver nitrate, with incubation for 24 h in the dark followed by microwave irradiation at 240 W for 15 min. The resulting AgNPs were characterised by UV–visible spectroscopy, Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM), and their colloidal stability was monitored for 60 days. Antibacterial activity of the extract and the AgNPs was evaluated by disc diffusion at 250, 500 and 1000 µg/mL against Staphylococcus aureus, Bacillus subtilis, Escherichia coli and Klebsiella pneumoniae, with amoxicillin as the reference standard. Results: The extract contained alkaloids, terpenoids, flavonoids, coumarins, steroids, phenolics and tannins, and saponins. AgNP formation was indicated by a yellow-to-brown colour change and a surface plasmon resonance band near 430 nm. FTIR spectra showed bands attributable to C–H, C=C, aromatic, C–N and C–O vibrations, consistent with phytochemical capping. SEM showed polydisperse, spherical, non-agglomerated particles with a mean size of 13 ± 0.63 nm. The colloid showed no sedimentation or change in absorbance over 60 days. Zones of inhibition increased with concentration for both preparations, and at every concentration the AgNPs produced larger zones than the crude extract. At 1000 µg/mL the AgNPs gave zones of 24, 22, 21 and 20 mm against S. aureus, B. subtilis, E. coli and K. pneumoniae respectively, comparable to the standard. Conclusion: Ethanolic leaf extract of C. sebestena is an effective reducing and capping agent for the rapid, microwave-assisted synthesis of small, stable, spherical AgNPs with concentration-dependent antibacterial activity against both Gram-positive and Gram-negative bacteria, and greater activity than the extract alone.