Effect of calcination temperature on physicochemical and antimicrobial properties of green synthesised ZnO/C/Ca nanocomposites using Calotropis gigantea leaves

G Ambarasan Govindasamy, Rabiatul Basria S. M. N. Mydin, Srimala Sreekantan and Nor Hazliana Harun

Abstract

In the present study, green synthesised ZnO nanoparticles using Calotropis gigantea (C. gigantea) leaf extract which were produced at different calcination temperatures (i.e. 400 °C and 500 °C) were evaluated against skin ulcer pathogens since no study is reviewed before. The results from x-ray powder diffraction and energy-dispersive spectroscopy confirmed the presence of ZnO nanoparticles along with natural compounds from medicinal plant (i.e. Ca and C). The capping and stabilising phytochemical agents of C. gigantea leaves such as phenol and carbonyl were identified through UV–Vis and FTIR. Scanning electron microscopy and transmission electron microscopy revealed that increasing the calcination temperature altered the size and shape of the prepared samples. ZnO nanoparticles which were prepared at calcination temperatures of 400 °C and 500 °C were quasi-spherical and irregular rod in shape. The minimum inhibitory and the minimum bactericidal concentrations of ZnO-400C were 0.625 and 1.25 mg ml⁻¹, whereas those of ZnO-500C were 1.25 and 20 mg ml⁻¹ against Staphylococcus aureus (S. aureus). Remarkably, the ZnO-400C revealed excellent bactericidal activity against skin ulcer pathogens, with MBC ranging from 0.3125 mg ml⁻¹ to 2.5 mg ml⁻¹. The time-kill kinetics assay demonstrated that 1.25 mg ml⁻¹ ZnO-400C captured a significant reduction of ≤2.5 log₁₀ in the S. aureus population after 6 h of incubation. ZnO-400C sample significantly reduced the biofilm formed by S. aureus which might be associated to the presence of Zn²⁺, Ca²⁺ and reactive oxygen species (ROS). A strong interaction and exfoliation between natural calcium and carbon from C. gigantea and ZnO nanoparticles offered new insights to inactivate skin ulcer pathogens synergically.