Conductive Polymer Functionalization of Cellulose Derived Hydrogels with Bio-Sensing Applications

Abstract

Two derivatives, carboxymethyl cellulose (CMC) and hydroxyethyl cellulose (HEC), contain functional groups suitable for chemical modification, specifically hydroxyl, carboxyl, or hydroxyethyl substituents. These derivatives are subject to polymerization via etherification to produce alternating polymeric chains (CMC-HEC-CMC) that can then be crosslinked through the addition of a dicarboxylic acid, specifically citric acid. It has been reported that due to the covalent bond formation at three different carboxyl sites, citric acid is an ideal crosslinking, capable of primary (covalent) and secondary (hydrogen bonding) crosslinking of polymer chains. The result is a durable hydrogel scaffold with a high pore density-to-area ratio and increased tensile strength compared to other polysaccharide-based hydrogels (i.e. alginate, pectin, agar) and crosslinking agents. Previous work harnessed a photoinitiated polymerization strategy using Fe (III)−carboxylate complexes to facilitate the integration of polyaniline (PANI) into the cellulose hydrogel matrix. Subsequent doping with hydrochloric acid produced an electrochemically responsive carbon-based polymer network. While an increase in the current output of polysaccharide-based gels was promising, the use of PANI raised environmental concerns. This prompted the shift to perform a survey of conductive polymers (i.e. polythiophene and polypyrrole) with more favorable environmental properties. The ability to functionalize the hydrogel matrix produced a scaffold for a sustainable electrode suited for prolonged submersion with minimal mechanical defects. The successful integration of the conductive polymer was characterized by cyclic voltammetry to quantify the redox properties for the design of an organic electrode. Previous work has shown that polysaccharide base gels can conduct currents on the order of 80 mA at 2 V, increasing as a function of PANI concentration. The hydrogel scaffold can be customized through the addition of pollutant-specific receptors (i.e. IgG antibodies) with sample-specific detection capabilities. By embedding these receptors into the matrix, the electrochemical shifts produced by specific receptor-molecule binding are amplified by the surrounding network. The monitoring of environmentally concerning concentrations of specific target contaminants (i.e. antigens) in a simulated wastewater solution was measured as a function of current. This proposed mechanism shows promise for an aqueous biosensor that can provide immediate and critical analysis.