Paper Title
DI- and Tricarboxylic Acid used for Intramolecular Cross-Linking of the Chitosan Linear Chains for Preparation of Biodegradable Nanoparticles for Biomedical Applications

Abstract
Chitosan, a biopolymer derived from chitin, one of the most abundant biopolymers found in nature, exhibit a large spectrum of application in the biomedical field of science. Chitosan polymer is formed by units of D-glucosamine and N-acetyl-D-glucosamine, randomly distributed. Di- and tricarboxylic acids (malic, tartaric, and citric acid) have been used both for the replacement of acetic acid, traditionally used for the dissolution of chitosan, as well as for intramolecular cross-linking of the chitosan linear chains. Water-soluble carbodiimide was used to promote the condensation reactions between carboxylic groups and the amino groups of chitosan. Changing the ratio between organic acids and chitosan, polycations, polyanions, and polyampholyte nanoparticles were obtained. These nanoparticles, with clear or opalescent appearance, were stable at low and neutral pH and even in mild alkaline environment, although the size of the nanoparticles depends on the pH. Nanoparticles are hydrolyzed at rates similar to chitosan in linear polymer form by lysozyme and chitinase A, indicating that nanoparticles are biodegradable. To test the capabilities of nanoparticles to adsorb and release drugs, adsorption / desorption test experiments were performed with anionic, cationic, or neutral molecules. By varying the organic acid / chitosan ratio, the absorption behavior of the nanoparticles can be controlled to favor the absorption of molecules that had positive or negative charges. Chitosan nanoparticles, in the form of solutions or dispersions in aqueous media, could be useful for different biomedical applications. Keywords - Carbodiimide, Chitosan-Based Nanoparticles, Controlled Adsorption/Desorption, Cross-Linking.