Effects of phosphate salts and ultrasonication on the formation and characteristics of phosphate-chitosan microparticles

The physicochemical properties of chitosan-microparticles are related to the crosslinking salts. The effects of sodium-pyrophosphate or sodium-tripolyphosphate and ultrasonication on the formation of phosphate-chitosan microparticles were evaluated. Potentiometric titration curves were used to determine the ionization degree of chitosan by employing the Henderson-Hasselbach equation and milliequivalents. Colloidal formation was validated by turbidity and conductivity at different phosphate-chitosan salt ratios. The ultrasonication effect was characterized by visual appearance, zeta potential, particle size, FTIR, SEM, and EDS analyses. Additionally, the impact of particle size on the cytotoxicity and the encapsulation properties of a curcumin microemulsion was assessed. Chitosan ionization degree was pH dependent and was fully ionized below 4.0. The milliequivalent points were estimated at 0.9 and 1.06 for the two crosslinkers and chitosan, respectively. The maximum turbidity observed during complex formation was 0.5 and 1.0 for the pyrophosphate-chitosan and tripolyphosphate-chitosan samples, respectively, showing a critical conductivity value of 1. FTIR showed the interaction between phosphate salts (PO–) and chitosan amino groups (NH3+) in the region of 1200–1650 cm–1. SEM showed that the particle size decreased with ultrasonication. After ultrasonication, the particle size of the phosphate-chitosan microparticles decreased 17-fold but the zeta potential did not change. The phosphate-chitosan complexes did not present cytotoxic activity at concentrations lower than 1000 µg/mL. The pyrophosphate-chitosan and tripolyphosphate-chitosan microparticles have remarkable loading capacities for curcumin, making them promising carriers for nutraceutical and pharmaceutical applications.