Synthesis temperature and time affect size, distribution and bioactivity of Tetradenia riparia-produced AgNPs

Shakira Shaik, Presley Mokobi and Nisha Singh

Abstract

Leaf extracts of Tetradenia riparia were used to synthesise silver nanoparticles (AgNPs) using 1 mM silver nitrate for 6, 12 and 24 h at room temperature (RT) and at 80 °C. There were no significant differences in nanoparticle (NP) yield between synthesis at RT and at 80 °C, and amongst the different synthesis times (6, 12 and 24 h) (p > 0.05). Fourier transformed infrared spectroscopy analyses confirmed the presence of functional groups. The AgNPs were mostly spherical in shape. The largest AgNPs were observed following 12 h synthesis at RT with an average diameter of 32.2 ± 12.4 nm. Smaller AgNPs (17.5 ± 10.7 nm) were recorded following synthesis for 24 h at 80 °C. The growth of Methicillin-resistant Staphylococcus aureus (MRSA) was inhibited by AgNPs synthesized for 24 h at RT, as well as by those synthesized for 12 h and 24 h at 80 °C with a concentration of 125 μg.ml⁻¹ each, compared to Neomycin, where inhibition was recorded at 250 μg.ml⁻¹. A similar finding was observed for Escherichia coli where the minimum inhibitory concentrations (MICs) of the AgNPs synthesized for 12 h at RT and for 6, 12 and 24 h at 80 °C, were recorded at 125 μg.ml⁻¹ each, compared to Vancomycin where it was 250 μg.ml⁻¹. Nanoparticles synthesized at 80 °C had a significantly lower MIC than Neomycin against MRSA (p = 0.007), and a significantly lower MIC than Vancomycin against E. coli (p = 0.01). It is indicated that anti-bacterial activity may be attributed to the size of NPs.