Enhancing catalytic oxygen reduction via sulfonate and phosphate crosslinkers in redox polyelectrolyte-based O2-glucose membraneless enzymatic biofuel cells

Abstract

The development of enzymatic biofuel cells (EBFCs) that operate under mild conditions and do not require membranes as separators is crucial for advancing low-power device technologies. The use of redox mediators for bacterial laccases represents a promising approach in these systems. In this study, a redox polyelectrolyte based on branched polyethyleneimine modified with an osmium complex (Os(im)PEI) was employed as the mediator. Os(im)PEI aggregates the nanomolar scale in the presence of species containing sulfonate or phosphate groups, depending on their concentration and ionic strength (IS). At 1 mM tripolyphosphate (TPP) under high IS, the polyelectrolyte matrix yielded particles formation (400 nm), while with 50 mM HEPES under low IS resulted in smaller particles (220 nm) along with free polyelectrolyte. These redox particles were self-assembled layer-by-layer in combination with the bacterial laccase SilA to construct a biocathode. The catalytic current density increased consistently up to the third bilayer in both systems, with the highest adsorbed mass observed in the presence of TPP. Notably, the incorporation of both species enhanced the redox polyelectrolyte’s function as an enzyme mediator compared to its free form. This enhancement, linked to increased SilA adsorption, may stem from stronger surface electrostatic interactions and changes in overall wettability—attributable to the polyelectrolyte’s globular conformation—or from specific interactions between phosphate groups and basic residues on laccase. Using these biocathodes along with a bioanode, O₂-glucose membraneless EBFCs were constructed under neutral pH and static conditions, achieving power densities ranging from 25 to 47 µW cm^-2. These values were comparable to, and in some cases exceeded, those reported in previous studies under similar operating conditions.