EPS nanoparticles were prepared by adjusting the pH of the EPS water solution. Briefly, EPS solution (0.75%, w/v) was divided into six groups, and 1.0 M HCl was added dropwise using a microinjector under magnetic stirring. The pH was adjusted to 8, 7, 6, 5, 4 and 3, and the particle size and distribution of each group were determined by a laser scattering instrument (Zetasizer Nano ZS90, Malvern) to analyze the formation of nanoparticles under different pH conditions. In addition, each sample was processed by ultrasound (20K Scientz ®) for 5 min as a control group (the average value was obtained after measurements were performed in triplicate). Next, 100.0 μL of EPS nanoparticle solution was uniformly freeze-dried on the slide. The microstructures of the nanoparticles were observed by SEM and transmission electron microscopy (TEM, HITACHI, HT7700, Japan). SEM was performed according to the above section. Following the Song et al (2020)10 (link) method, nanoparticle-loaded copper grid was negatively stained with 2.0% phosphotungstic acid for 30 min for observation after drying. Particle size and zeta potential of nanoparticles at different pH were determined (Zetasizer Nano ZS90, Malvern, UK) to determine the isoelectric point.20 (link)