A polyacrylamide–chitosan semi-interpenetrating self-healing network with embedded Keplerate {Mo132} for pH-controlled release of Eu-fluorescent tags

Abstract

Delivery systems with pH-controlled release of drugs and fluorescent tags meets the challenge of rational design of functional hydrogel structures. Here, we report on a simple and versatile approach to fabrication of biocompatible hydrogels with a pH-dependent release of loaded molecules by harnessing properties of the nontoxic polyoxometalate (POM) Keplerate {Mo₁₃₂}. Due to the template effect of the nanoscale POM, molecular complementarity between photo-polymerized polyacrylamide (PAAm) and low molecular weight chitosan (Chit) is realized through forming a homogeneous, self-healing, semi-interpenetrating polymer network (semi-IPN). We characterize the composition of the semi-IPN with IR and Raman spectroscopy, while also affirming POM's structural integrity within the system. Swelling kinetic experiments suggest that varying both cross-linking frequency via imine bonds and the molecular weight of PAAm (0.67–11.4 MDa) significantly alters the hydrogel's structure and its swelling behavior: mechanical properties and pH-sensitive response. The latter one is governed through electrostatic linkages between Chit and negatively charged {Mo₁₃₂}. The optimized hydrogel formulation – PAAm@Chit@Mo₁₃₂ – was chosen to evaluate parameters needed for the controlled release of the fluorescent tag (Eu^IIIL complex, L= 4-(2-fluorophenyl)-[2,2′-bipyridin] derivative) into a series of aqueous solutions (water, 0.97% NaCl, and PBS) at 25 °C and 37 °C. We found a cumulative release of the loaded Eu^IIIL tag (serving as the model drug) from the PAAm@Chit@Mo₁₃₂ hydrogel to be as high as 75% and assessed how the hydrogel composition and the nature of the chemical environment affect that number. These results make this hydrogel system a promising platform for the rational design of injectable hydrogel-based drug-delivery systems.