Immobilization and release of the redox mediator ferrocene monocarboxylic acid from within cross-linked p(HEMA-co-PEGMA-co-HMMA) hydrogels

Cross-linked hydrogels are synthesized from hydroxyethylmethacrylate (HEMA), polyethyleneglycol methacrylate (PEGMA), and N-[tris(hydroxymethyl) methyl]-acrylamide (HMMA) [p(HEMA-co-PEGMA-co-HMMA) hydrogels] containing 1, 3, 5, 7, 9, or 12 M % of the cross-linker tetraethyleneglycol diacrylate (TEGDA) and are loaded during synthesis with the well-known redox mediator, ferrocene monocarboxylic acid (FcCOOH). In the absence of FcCOOH, the M% TEGDA in deionized (DI) water (48%; 1 mol % TEGDA to 32%; 12 mol % TEGDA) scales with the cross-link density in accordance with Flory-Huggins-Rehner theory. The release profiles of FcCOOH from hydrogel slabs (43.0 mM) into 0.1 M HEPES/0.1 M KCl buffer are determined from the oxidation peak current of FcCOOH via cyclic voltammetry (100 mV/s) and are decidedly Fickian with overall diffusion coefficients that range from 2.64 × 10^-8 cm²/s (1 mol % TEGDA) to 4.87 × 10^-9 cm²/s (12 mol % TEGDA) and with n parameters that approximated 0.5 but nonetheless linearly declined from 0.49 (1 mol %) to 0.42 (12 mol %). Diffusion coefficients, like hydration, strongly correlate with the M% TEGDA and hence with the cross-link density or the molecular weight (MW) between cross-links. The temperature dependence of the release profiles measured at 10, 15, 20, 25, 30, 35, 40, and 45°C reveal thermally activated transport with activation energies that are 30 kJ/mol (3 mol %), 36 kJ/mol (5 mol %), 45 kJ/mol (7 mol %), 47 kJ/mol (9 mol %) and 57 kJ/mol (12 mol %). Covalent tethering of the FcCOOH via the UV-polymerizable monomers ferrocene monomethacrylate (Fc-AEMA) and ferrocene polyethylene glycol monomethacrylate (Fc-PEG(3500)-AEMA) to produce pendant redox moieties is shown to eliminate or attenuate release of Fc. While, Fc-AEMA showed no evidence of release (0%) from the hydrogel, its PEG-conjugated equivalent, Fc-PEG(3500)-AEMA, shows release of 16% Fc after 5 days of immersion. These hydrogels will serve as the immobilization matrix for oxidoreductase enzymes of biosensors and the parameters obtained used in the modeling of such systems.