Physicochemical characterization and antibacterial activities of silver nanoparticles prepared by amidated low-methoxyl pectin

Pectin-based silver nanoparticles (AgNPs) have been used in the field of antibacterials for food due to their excellent antibacterial properties. Herein, in order to achieve higher antibacterial performance, AgNPs were synthesized using high-methoxyl pectin (HMP) and amidated low-methoxyl pectin (ALMP) as precursors. Initially, ALMP-1, -2, and -4 were obtained by pectin amidation with increasing concentrations of NH₄OH. Later, HMP and ALMPs were used to prepare AgNPs, and their physicochemical property and antibacterial activities were studied. Transmission electron microscopy (TEM) showed that the mean diameters of HMP-Ag and ALMP-4-Ag were 11.9 ± 3.8 and 13.0 ± 5.4 nm, respectively. EDS analysis revealed that ALMP-4-Ag combined with more Ag element than HMP-Ag. X-ray photoelectron spectroscopy (XPS) indicated that ALMP-4-Ag led to a lower ratio of Ag⁰ to Ag⁺ on the surface of AgNPs. Interestingly, ALMP-4-Ag had the strongest antimicrobial effect against Escherichia coli and Staphylococcus aureus, with the lowest inhibitory concentrations (MICs) of up to 33 μg mL^-1, which was 16-fold enhanced compared with HMP-Ag (MICs = 533 μg mL^-1). Finally, ALMP-4-Ag-treated cells revealed higher levels of protein and sugar leakage as well as increased levels of reactive oxygen species (ROS) and malondialdehyde (MDA) than HMP-Ag.