Green synthesis of gold nanoparticles using Tribulus terrestris extract and antibacterial activity against Gram-negative bacteria

Abstract

Aqueous extract of the plant Tribulus terrestris was used to reduce chloroauric acid for synthesizing gold nanoparticles (AuNPs). In this green synthesis, the reaction proceeded to give a red/purple color that was monitored by UV-vis spectrophotometry, where the formed AuNPs had an absorption band with max of 550 nm. In terms of the highest absorbance at 550 nm, reaction conditions were optimized at a temperature of 75°C, at pH 7 and using a reaction time of 4 h. The integrity of the synthesized AuNPs was confirmed and their physical properties were characterized by Fourier-transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM), which also showed evidence that plant metabolites contributed to capping or stabilization of the AuNPs. XRD spectra suggested a particle size of around 40 nm and SEM images revealed spherical and relatively uniform and disperse particles with a size of less than 0.1 m. In a broth microdilution assay, the AuNPs showed inhibitory effects against Gram-negative Escherichia coli, but not against Gram-positive Enterococcus faecalis. The AuNPs showed no hemagglutination activity or cytotoxic activity against human blood cells, which is important for them to be explored as therapeutic antibacterial agents.