Elaboration of chitosan nanoparticles: Favorable impact of a mild thermal treatment to obtain finely divided, spherical, and colloidally stable objects

The elaboration of chitosan (CS) nanoparticles via an ionic gelation process using sodium tripolyphosphate (TPP) as cross-linking agent was thoroughly studied in order to develop colloidally stable, spherical CS nanoparticles with a reproducible sub-micrometer size, and a narrow size distribution. To this end, the most relevant parameters involved in the synthesis such as CS chains concentration and molecular weight as well as ionic strength and pH of CS initial solutions were investigated and optimized. The physicochemical characterization of resulting CS nanoparticles was carried out in terms of size, size distribution, and surface charge by quasi-elastic light scattering (QELS), nanoparticle tracking analysis (NTA), and zeta potential measurements. Morphological characterization using classical and cryogenic transmission electron microscopy (TEM and cryo-TEM) was then performed revealing a high number of aggregates mixed to individualized nanoparticles with drop-like and irregular shapes. These limitations were overcome by applying a specific and optimized thermal treatment which appeared to significantly reduce the aggregate number, and to restructure the CS nanoparticles into highly spherical objects. Based on all these findings, reproducible, cationic (zeta potential = +37 ± 5 mV), stable (at least 4 months at 20 °C), spherical, and mainly individualized CS nanoparticles of 100 ± 30 nm (determined from cryo-TEM images) were obtained.