Synthesis and characterization of coffee husk extract (CHE)-capped ZnO nanoparticles and their antimicrobial activity.

Access to safe drinking water is among the main challenges in poor countries due to seriously polluted water by waterborne pathogens. Water treatment technology is also prohibitively expensive, and some of it produces harmful by-products. As a result, phyto-fabricated ZnO nanoparticles (ZnO-NPs) are attracting significant interest for their antibacterial activity and potential applications in water disinfection. This study has focused on optimizing the synthesis parameters of ZnO-NPs using coffee husk extract (CHE) as an efficient reducing and capping agent to improve the size and activities, which had not been investigated before at optimal process parameters, such as temperature, zinc precursor-to-CHE ratio, reaction time, and pH. Furthermore, there has not been a study of indigenous CHE total phenolic content. CHE was obtained from coffee husk via ethanol solid-liquid extraction. The total phenolic content of CHE was determined by the Folin-Ciocalteu colorimetric method using gallic acid as a standard. The contribution of CHE in the formation of ZnO-NPs was first observed by a color change to yellowish-white, which was then confirmed using UV-visible spectrophotometry. The synthesis of ZnO-NPs was optimized at different physico-chemical conditions, and the optimal synthesis parameters were found to be at a precursor (ZnO (Zn(NO₃)₂·6H₂O))-to-CHE ratio of 1:1 (v/v), pH 10.0, a reaction time of 1 h, and reaction temperature of 80 °C. The functional groups of CHE-capped ZnO-NPs were studied using Fourier transform infrared spectroscopy (FTIR), and the results revealed that CHE phytochemicals have successfully capped the NPs. The presence and purity of elemental zinc and oxygen were confirmed by energy dispersive X-ray (EDX) analysis. According to the X-ray diffraction (XRD) analysis, NPs were found to be crystalline with crystal sizes of 9.8 nm. The stability and particle size of ZnO-NPs were analyzed using dynamic light scattering (DLS). A negative zeta potential value of the biosynthesized NPs (− 20.27 mV) demonstrated the stability of the biosynthesized nanomaterials. Furthermore, CHE-capped ZnO-NPs showed potent antimicrobial activity against S. aureus and E. coli but with greater antibacterial effect against S. aureus than against E. coli. This study used a novel green process technique that employed coffee residuals as a source of reducing and capping agents by optimizing synthesis parameters for the green synthesis of ZnO-NPs. The developed process has great practical application potential for waterborne pathogen disinfection because the process is safe, inexpensive, effective, and simple to prepare. [ABSTRACT FROM AUTHOR]