pH-Dependent release of blue dextran from carboxymethyl-β-cyclodextrin hydrogels

Carboxymethyl-β-cyclodextrin (CM-β-CD) hydrogel was prepared by cross-linking β-CD and substituting the hydroxyl groups with carboxymethyl group. Epichlorohydrin (EPI) was used as a cross-linker and the molar ratio of EPI to β-CD was 10:1. The number of carboxymethyl group per β-CD residue was determined by a back-titration method and the value was 0.86. The swelling ratio of CM-β-CD hydrogel increased as the pH of medium increased. The carboxyl group will be ionized with increasing pH and an electrostatic repulsion developed within the hydrogels would be responsible for a higher swelling ratio at a higher pH. The degree of release of blue dextran from the hydrogel also increased as the pH of medium increased. The pH-dependent swelling ratio could account for the pH-sensitive release.