Optimization of biosynthesis zinc oxide nanoparticles: Desirability-function based response surface methodology, physicochemical characteristics, and its antioxidant properties

Abstract

The optimization of zinc oxide nanoparticles (ZnO NPs) with maximum yield and smaller particle size synthesized using cell-free supernatant (CFS) of Lactobacillus plantarum TA4 were investigated using a desirability-function based response surface methodology (RSM). A central composite rotatable design (CCRD) with five levels and three factors namely zinc concentration (200-500 mM), pH (6-10), and CFS volume ratio (20-50%) were employed to study the response variables. A total of 20 experimental runs were performed and desirability-function showed that the optimal conditions for the maximum yield and minimum particle size were 352.4 mM of Zn concentration, pH 9, and 25 % of CFS volume ratio. At these optimal conditions, the predicted yield and size of the optimized ZnO NPs were 2.47 g and 75.8 nm, respectively. The validation test showed 2.41 g of yield and an average size of 80.5 nm. The UV-Vis spectroscopy showed the characteristic surface plasmon resonance band (SPR) at an absorption peak of 360 nm, confirming the formation of optimized ZnO NPs. Dynamic light scattering (DLS) demonstrated the small hydrodynamic size and low polydispersity index (PDI) of ZnO NPs at 85.9 nm and 0.243, respectively. A high-resolution transmission electron microscope (HRTEM) analysis illustrated spherical and oval-shaped ZnO NPs with an average particle size of 29.7 nm. Furthermore, Fourier-transform infrared (FTIR) analysis showed that biological compounds (proteins, enzymes, and carbohydrates) from CFS were involved in the reduction and capping of ZnO NPs. Raman spectroscopy and thermogravimetric analysis (TGA) demonstrated the crystallinity and thermal stability of ZnO NPs. Furthermore, the total antioxidant capacity (TAC) and free radical scavenging activity (RSA) of ZnO NPs demonstrated promising antioxidant properties when compared to their bulkier counterparts. Overall, this work paves the way for a cleaner and environmentally friendly production of ZnO NPs for industrial use.