Physicochemical and morphological properties of size-controlled chitosan–tripolyphosphate nanoparticles

Chitosan–tripolyphosphate nanoparticles have been extensively studied during the last decade because of their numerous applications. In this study, we describe conditions to optimize chitosan nanoparticles as potential nano-fillers in edible films. The ionic cross-linking between the cationic amino groups on the chitosan (CS) chain and the anionic phosphate groups of sodium tripolyphosphate (TPP) was verified via FTIR. Particle size, polydispersity index (PDI) and surface ζ-potential were controlled by chitosanʹs Mw and concentration, CS:TPP mass ratio, and external conditions such as pH and salinity of the initial chitosan solution. Post-processing methods such as centrifugation and ultra-sonication were used to further control particle size. We show that particle size can be controlled by selecting appropriate conditions. Particles with sizes below 120 nm were produced at different CS:TPP mass ratios depending on the CS concentration. Dilute NaCl was the optimal solution ionic composition that decreased the size by 25% and also resulted in a narrow particle size distribution. We show using UV–vis spectrophotometry that particles of different size, separated by centrifugation had different phosphorus content. Ultra-sonication can be used to reduce the size by 50% but long time caused fragmentation of the nanoparticles. Transmission electron microscopy (TEM) revealed the differences in the morphology of chitosan nanoparticles under various fabrication conditions.