Biosynthesis of Silver Nanoparticles from the Mangrove Rhizophora mucronata: Its Characterization and Antibacterial Potential

The green synthesis of nano-sized particles with specific functions is of great significance in the present bio-nanotechnology. In this study, the biosynthesis of silver nanoparticles (AgNPs) from the aqueous extract of stem, root and leaf of Rhizophora mucronata, one of the two native mangrove plants in southern Iran, and its activity against some bacterial pathogens are reported for the first time. A green procedure for the synthesis of AgNPs using the mangrove extract as reducing agent was used. Synthesized AgNPs were investigated using UV-visible spectrophotometry, transmission electron microscopy (TEM), field emission scanning electron microscopy, X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS) and Fourier transform infrared (FTIR) spectroscopy. Antibacterial effect of the biosynthesized AgNPs was investigated by agar well diffusion assay against pathogenic bacteria. The leaf extract yielded the highest production of biosynthesized AgNPs among the plant parts. UV-visible spectrophotometry at a wavelength of 420 nm confirmed the synthesis of AgNPs. The results from the XRD analysis approved the formation of crystalline AgNPs with crystallinity percentage of 88.54. As shown by TEM evaluation, AgNPs had the same spherical morphology. The size of the biosynthesized AgNPs was between 1 and 80 nm with average size of 32.44 nm. According to the size distribution histogram, nanoparticles in the size range of 20-25 nm were more than other size groups. SEM images identified silver nanoparticles ranging in size from 10 to 19 nm. Results of EDS showed the existence of carbon, silver, oxygen and chlorine in the nanoparticles synthesized from the leaf extract of R. mucronata. Silver was the major constituent element, 73.5%. FTIR analysis distinguished different functional groups such as aromatic loops, alcohol, phenol group, alkanes and alkyl halides in the biosynthesis process. The biosynthesized AgNPs had potential antibacterial activity against the bacterial pathogens. Out of the selected bacterial strains, the Gram-positive bacteria of Bacillus cereus with the maximum inhibition zone and the Gram-negative bacteria of Pseudomonas aeruginosa with the minimum inhibition zone were the most sensitive and resistant bacteria, respectively. The mangrove Rhizophora mucronata extract was found to be a suitable reducing agent for biosynthesis of AgNPs with approved antibacterial activity.