Optimization, characterization and antimicrobial activity of silver nanoparticles against plant bacterial pathogens phyto-synthesized by Mentha longifolia

The present study involves the Phyto-synthesis of the colloidal silver nanoparticles (AgNPs) and their applications to biologically control plant bacterial pathogens. The synthesis of AgNPs was monitored by measuring the absorbance and a characteristic surface plasmon resonance (SPR) band was observed at 450 nm. The different reaction conditions such as the temperature, incubation period, the concentration of the silver salt and the pH were optimized using the factorial design of the experiment for the better yield and the synthesis of AgNPs. The microscopic results showed that the AgNPs are anisotropic and nearly spherical and exist in the size range of ∼20–100 nm while the EDX analysis confirmed the presence of the elemental Ag. The x-ray diffraction pattern confirmed that the AgNPs are crystalline. The hydrodynamic diameter of AgNPs has measured in the range of ∼13–35 nm and the average size of a single particle was 15.55 nm. The ability of the AgNPs to biologically control the plant bacterial pathogens was measured in terms of antibacterial activity against gram-negative pathogenic bacterial strains; Pectobacterium carotovorum, Xanthomonas oryzae, Xanthomonas vesicatoria and Ralstonia solanacearum and potential antimicrobial activity were observed between 2–12 μg ml⁻¹. The biocompatibility studies revealed that the AgNPs are highly biocompatible (LD₁₀₀ 208 μg ml⁻¹) against RBCs. These findings endorse the applications of AgNPs to biological control the plant bacterial pathogens and the consumption of the plants treated with NPs is biocompatible for the humans. (paper)