Current Response Enhancement According to the Doping Anion’s Nature in Redox Polyelectrolyte─Enzyme Assemblies

High-power density output in enzymatic fuel cells is a key feature to reduce the size of self-powered implantable medical devices. Electron transfer mediated through redox polyelectrolytes allows the transport of electrons from enzymes away from the electrode, improving the current output. It is known that doping ions in polyelectrolytes introduce relevant characteristics in the generation of assemblies regarding mass adsorption and stiffness. In this work, binary 1:1 sodium salts (NaX; X = F–, Cl–, Br–, NO3–, ClO4–) were studied as doping ions of two redox polyelectrolytes (osmium-based branched polyethyleneimine and osmium-based linear polyallylamine) to enhance the adsorption and electron transfer process in glucose oxidase/redox polyelectrolyte assemblies. Cyclic voltammetry, polarization modulation infrared reflection absorption spectroscopy, quartz crystal microbalance with dissipation, and atomic force microscopy were used to understand the growth mechanism of these films and their performance. Ion hydrophobicity plays a key role, bromide being the one that generates the greater absorption and the best electron transfer efficiency for both redox polyelectrolytes. Branched polyethyleneimine doped with bromide was the best combination for the construction of bioanodes. Its application on an O2–glucose enzymatic fuel cell yields a power density output of 2.5 mW cm–2, achieving state-of-the-art performance.