Electrochemistry of Electroactive Species Loaded inside Polymeric Hydrogels. Effect of Viscoelastic Properties on Mass Transport and Heterogeneous Charge Transfer
Authors: Martinez Maria Victoria , Coneo Rodriguez Rusbel , Rivarola Claudia R , Bruno Mariano M , Barbero Cesar Alfredo 
Published: February 2020

The loading and mass transfer of electroactive species inside polymeric hydrogels is of interest in drug delivery, analytical chemistry and water remediation. The electrochemical properties of the species could be affected by the viscoelastic properties of the gel, local ionic force and pH, as well as interactions (e.g. hydrophobic) between the ions and the polymer chains. Usually, a thin film of the hydrogel is deposited on the electrode, loaded with electroactive species and electrochemically investigated. However, the mass transfer is controlled by finite diffusion and depends of an unknown, the thickness of the layer in the experimental conditions. On the other hand, we show here that a simple set-up, consisting of a disk electrode pressed on the soft hydrogel, allows performing electrochemistry of electroactve species (redox complexes, arsenite, nitrite) loaded inside the hydrogel matrix. The redox complexes are present as dilute solutions and the hydrogel dimensions fulfill the semi-infinite diffusion boundary conditions. Therefore, the data analysis can be performed using the theoretical framework developed for electrochemical measurements in liquid solutions. The physicochemical properties of the bulk hydrogels are evaluated by measuring the swelling kinetics, in the same conditions of the electrochemical measurements. Redox cations (e.g. tris(phenanthroline)iron(II) (TPFeII)) or anions (e.g. Fe(CN)6
-3, (FeCN) are loaded into anionic, neutral and cationic hydrogels showing no Donnan exclusion effects. The cyclic voltammetry of the loaded species show a quasireversible electron transfer mechanism (Eqre). The electrochemical parameters (diffusion coefficient and charge transfer constant) inside hydrogels are measured using chronoamperometry and digital simulation of the cyclic voltammetry. The Stokes-Einstein equation is used to calculate the effective viscosity of the hydrogel matrixes using the diffusion coefficients of redox complexes determined inside the hydrogels and in aqueous solutions (of known viscosities). The calculated viscosities correlate, with a negative slope, with the swelling rate constant of the hydrogel matrix. The heterogeneous charge transfer constants correlate with the calculated viscosities, revealing the effect of solvent dynamics on the charge transfer, according to Marcus theory for strongly adiabatic electron transfer. In that way, it is shown that the electrochemical measurements are able to monitor the local solvation properties of the hydrogel matrix. The method can be applied in electrochemical sensing of environmental contaminants. In the case of nitrite, the charge transfer is slow on glassy carbon (GC) electrodes. Therefore a redox catalyst (TPFeII) and nitrite are simultaneously absorbed in an anionic hydrogel and the redox electrocatalysis occurs inside the hydrogel matrix. On the other hand, the sensing of arsenite is made using Co oxide nanoparticles adsorbed on the GC electrode with arsenite loaded inside the hydrogel.