PH-induced Crosslinking of Rice Starch via Schiff Base Formation

Abstract

Rice starch is an abundant, inexpensive, and biocompatible biopolymer with a wide range of potential applications, particularly in the biomedical field. However, the use of rice starch is limited because of its water-solubility. One way of addressing this is to modify the crosslinking properties. If the degree of crosslinking is sufficiently high, the starch becomes insoluble in water, but it remains swellable. In this study, the first synthesis of pH-induced crosslinking of rice starch without the use of any external chemical crosslinking agent was demonstrated. The crosslinked rice starch was prepared by forming a Schiff base reaction (imine) between the two modified starches: oxidized rice starch (ORS) and amino rice starch (ARS). Here, the ORS and ARS were successfully synthesized, with the content of the glucose units being 32.01 and 27.80 mol%, respectively. Imine bonds were shown to be formed from the aldehyde and amine groups. The relationships between the pH and the degree of crosslinking were also investigated. The chemical structures of the synthesized ORS and ARS and their crosslinked products at different pH levels were determined by 1H NMR, 13C NMR, and FTIR Furthermore, the effect of pH on swelling behavior was explored. The relationships between the pH and the chemical structure agreed well with the swelling behavior. The swelling ratio was the highest at pH 5 and decreased with increased pH. Under basic conditions, hydrolysis was initiated, reducing the crosslink density and the efficiency of water uptake. The present study suggests that the pH-induced crosslinked rice starch can be a promising biomaterial for controlled release applications.