Characterization and antimicrobial evaluation of biomimetically synthesized silver nanoparticles using aqueous leaf extract of Morinda lucida Benth. (Rubiaceae)

Abstract

Silver nanoparticles (AgNPs) are silver atom masses that are attracting widespread interest due to their diverse applications, particularly their activity as antimicrobial agents. Physico-chemical methods of AgNP synthesis are associated with high costs, high temperatures and toxic byproducts. Thus, the plant mediated pathway represents a better option. The indigenous medicinal plant Morinda lucida was employed in the fabrication of AgNPs. The nanoparticles were characterized using different analytical techniques and also evaluated for their antimicrobial potential. Phytochemical screening of the plant was also carried out. For the synthesis, 10 ml of aqueous M. lucida leaf extract was added to 90 ml of freshly prepared 3 mM silver nitrate (AgNO3) solution in a flask. The mixture was allowed to stand at ambient temperature, in a dark cupboard for 48 hours. Positive AgNP synthesis, indicated by a colour change from red to brown was further validated by UV-vis spectroscopy wherein an absorption peak at 460.51 nm was recorded. The utilitarian aspects of the particles were further characterized using scanning electron microscopy (SEM), fourier transform infrared (FTIR) spectroscopy, dynamic light scattering (DLS), X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDX). The SEM images showed that particles were round to irregular in shape. Amide, amine, alkene and alkynes were the most occurring functional groups from the FTIR spectra. Quantitative phytochemical analysis revealed the presence of flavonoids, terpenoids, phenols, reducing sugars and alkaloids in varying amounts, which play a significant role in the synthesis and stabilization of the AgNPs. The XRD diffractogram of AgNPs showed two peaks at 45.53° and 77.17° that correspond to miller indices of (200) and (311) respectively and an average crystalline size of 62.60 nm obtained using the Debye-Scherrer’s formula. The DLS result indicated a Z–average size of 235.1 nm and a polydispersity index (PDI) of 0.4. EDX analysis showed that elemental silver (Ag) had the highest atomic concentration of 64.50 %. Using the agar well diffusion assay, the nanoparticles exhibited antimicrobial activity against P. aeruginosa (bacteria) and A. flavus (fungi). It can be concluded that M. lucida is capable of synthesizing stable, small-sized AgNPs with antimicrobial potential.