Colloids and Surfaces A: Physicochem. Eng. Aspects 465 (2015) 137–146 Contents lists available at ScienceDirect Colloids and Surfaces A: Physicochemical and Engineering Aspects journa l h om epage: www.elsevier.com/locate/colsurfa Physicochemical and morphological properties of size-controlled chitosan–tripolyphosphate nanoparticles John Antoniou a , Fei Liu a , Hamid Majeed a , Jing Qi a , Wallace Yokoyama b , Fang Zhong a,∗ a Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Food Science and Technology, Jiangnan University, Wuxi 214122, PR China b Processed Foods Research Unit, Western Regional Research Center, ARS, USDA, Albany, CA 94710, United States h i g h l i g h t s • Chitosan–tripolyphosphate nanopar- ticles were prepared through ionic- crosslinking. • Control of particle size by selecting appropriate conditions. • Influence of initial pH and salinity of chitosan solution on nanoparticles formation. • Separation of aggregates from nanoparticles by centrifugation. • Ultra-sonication reduced particle size but caused fragmentation on their structure. g r a p h i c a l a b s t r a c t a r t i c l e i n f o Article history: Received 17 June 2014 Received in revised form 9 October 2014 Accepted 23 October 2014 Available online 30 October 2014 Keywords: Chitosan nanoparticles Ionic crosslinking Ionic strength Particle size Morphology Ultra-sonication a b s t r a c t 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 -potential were controlled by chitosan’s Mw and concentration, CS:TPP mass ratio, and external conditions such as pH and salinity of the initial chi- tosan 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 parti- cles 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. © 2014 Elsevier B.V. All rights reserved. ∗ Corresponding author. Tel.: +86 510 85328307. E-mail address: fzhong@jiangnan.edu.cn (F. Zhong). 1. Introduction Chitosan (CS) is obtained by the deacetylation of the naturally occurring polysaccharide chitin, one of the most abundant biopoly- mers in nature. It is composed of -(1-4)-linked d-glucosamine and N-acetyl-d-glucosamine units [1]. The degree of deacetylation (DD) http://dx.doi.org/10.1016/j.colsurfa.2014.10.040 0927-7757/© 2014 Elsevier B.V. All rights reserved.