Biogenic synthesis of ZnO from Rubia cordifolia root extract: A study on sono-photocatalytic dye degradation and anti-bacterial assay

ZnO is a semiconductor metal oxide with unique antibacterial and photocatalytic properties. Green synthesis of nanoparticle is environmentally friendly procedure, imparting multiple properties to the nanoparticles (NPs). This study reports Rubia cordifolia root extract mediated ZnO NP synthesis. Synthesised NPs were analysed for antibacterial properties and sonication assisted photocatalytic dye degradation experiments were performed. Synthesised ZnO NPs were characterized employing UV-VIS spectroscopy, X-Ray diffraction analysis (XRD), Fourier transform infrared spectroscopy (FT-IR), and scanning electron microscopy (SEM) followed by energy dispersive X-ray spectroscopy (EDX) analysis and was compared with commercial grade ZnO. UV-VIS adsorption peak of NPs was found at 375 nm with a bandgap of 3.2 eV. The NPs reveals to have a crystalline size of 17.54 nm with a hexagonal wurtzite structure and clustered flakes with rose petal morphology were identified through XRD and SEM analysis respectively. The green ZnO possesses good antibacterial properties against both gram positive (Staphyllococcus aureus) and gram negative bacteria (E.coli). Study on the effectiveness of synthesised ZnO for ultrasound sonication-assisted photocatalysis over normal photocatalytic degradation were carried out through response surface methodology with face central composite design. The regression models (R2adj > 90 % and R2pred > 90 %) were confirmed by analysis of variance (ANOVA), and optimization was performed. The green ZnO nanoparticle was capable of degrading alizarine red up to 92.89 % and rhodamine B up to 89.95 % under optimal conditions and were also compared with commercial ZnO NPs. Reusability of the photocatalyst was confirmed by conducting 4 cycles of experiments at optimum condition.