The effects of ionic strength and pH on antibacterial activity of hybrid biosurfactant-biopolymer nanoparticles

The physicochemical properties of nanoparticles (NPs) directly influence the colloidal stability and bio-nano interface, which makes the study of these factors extremely important for NPs applicability. Here, we report the influence of pH and ionic strength in the size, surface charge, and antimicrobial activity of chitosan/rhamnolipid nanoparticles (C/RL-NPs), synthesized by mixing C and RL in the proportion of 1:1, with the dropwise addition of TPP (sodium tripolyphosphate) in the molar ratio of 1:2.5:1.3, respectively. The obtained NPs were resuspended in different solvents to investigate the ionic strength effect on their stability and antimicrobial activity. By decreasing pH and ionic strength, NPs size was reduced while their surface charge increased. Chitosan nanoparticles (C-NPs) remained stable in the presence of TPP at acidic pH and low ionic strength, indicating that the modulation of physicochemical conditions for the crosslinking between the two ionic components was an efficient approach to enhance C/RL-NPs colloidal stability. Thermogravimetric analysis confirmed the interaction between rhamnolipid and chitosan in C/RL-NPs. The antibacterial activity of C/RL-NPs against Staphylococcus aureus increased significantly, demonstrating the importance of investigating size and surface charge to dictate the bioactivity properties of such hybrid natural-based nanomaterial.