Ultrasonication of chitosan nanoparticle suspension: Influence on particle size

•Ultrafine chitosan nanoparticles were prepared using an ionic gelation method.•The stability of nanoparticles is highly dependent on freeze-drying.•Cryoprotectant utilisation was not effective enough to prevent agglomeration.•A critical ultrasonication time was determined to break down the agglomerates.•Ultrasonication is effective even after 6 months storage of dried nanoparticles. Chitosan nanoparticles have been prepared using an ionic gelation method. The effects of freeze-drying, cryoprotectant (d-trehalose) utilisation and ultrasonication time on the mean particle size, size distribution and stability of nanoparticles were investigated. The size distribution results indicated that freeze-drying caused the particles to agglomerate. Addition of the cryoprotectant and ultrasonication led to a decrease in the particle size. However the sizes of the nanoparticles increased with increasing ultrasonication time. Transmission electron microscopy images revealed that spherical chitosan nanoparticles ranging from 3 to 12nm were obtained by means of freeze-drying in the presence of cryoprotectant and ultrasonication. These results suggest that cryoprotectant utilisation and controlled ultrasonic treatment are effective for imparting nanoparticle stability.