Functionalization methods for ZnO nanoparticles with citric acid and their effect on the antimicrobial activity

The functionalization of ZnO nanoparticles has proven to be an effective alternative to avoid agglomeration and improve their physic-chemical properties. In this work, we studied the functionalization of ZnO nanoparticles using citric acid (CA) through two methods: in situ functionalization and sonochemical functionalization to compare the effects on the physicochemical and antimicrobial properties of functionalized commercial ZnO nanoparticles with those synthesized and functionalized at the same time. Both methods are considered environmentally friendly (green chemistry) since the use of chemical toxic agents was avoided and the reaction medium was water. The in situ method demonstrated the ability of CA to act as a reducing and stabilizing agent. Fourier transform infrared spectroscopy (FTIR) and RAMAN spectroscopy analyses confirmed the functionality of ZnO nanoparticles, through the formation of the COO- /ZnO complex. The wurtzite-type hexagonal crystalline structure of the ZnO nanoparticles was confirmed by X-ray diffraction (XRD). The thermogravimetric analysis (TGA) analysis showed that in situ functionalization was more favored than sonochemical functionalization since 46.66% and 27.22% of the organic ligand were obtained on the surface of the nanoparticles, respectively. X-ray photoelectron spectroscopy (XPS) results revealed the chemical and electronic state of the zinc which was 2+ and atomic percentages of carbon of 40.64 % and 38.35 %, as well as atomic percentages of oxygen of 38.67 and 36.78 % for ZnO-AC and ZnO-AC-R1 nanoparticles, respectively. The evaluation of the antibacterial efficiency of the nanoparticles functionalized by both methods demonstrated that they possess a superior antimicrobial activity when compared with non-fuctionalized nanoparticles. The ZnO nanoparticles synthesized and functionalized via in situ showed an efficiency of 99.99 % starting concentrations of 200 ppm.