Linear poly(ethylenimine) (LPEI) was modified by attachment of 3-(tetramethylferrocenyl)propyl groups to ca. 17% of its nitrogen atoms to form a new redox polymer, FcMe(4)-C-3-LPEI, for use as an anodic mediator in glucose/O-2 biofuel cells. Electrochemical properties of this polymer were compared to those of 3(dimethylferrocenyl)propyl-modified LPEI (FcMe(2)-C-3-LPEI). When FcMe(4)-C-3-LPEI was complexed with glucose oxidase (GO(x)) and cross-linked with ethylene glycol diglycidyl ether (EGDGE) to form hydrogels on planar, glassy carbon electrodes, limiting catalytic bioanodic current densities of up to similar to 1.4 mA/cm(2) at 37 degrees C were produced. The use of tetramethylferrocene moieties in place of dimethylferrocene moieties lowered the E-1/2 of the films by ca. 85 mV. FcMe(4)-C-3-LPEI was shown to be the more effective polymer for use in biofuel cells and, when coupled with a stationary O-2 cathode comprised of laccase and crosslinked poly[(vinylpyridine)Os(bipyridyl)(2)Cl2+/3+] (PVP-Os) as a mediator, produced power densities of up to 57 mu W/cm(2) at 37 degrees C. Power density increased to 126 mu W/cm(2) when a rotating biocathode was used. The power densities of biofuel cells made with either FcMe(2)-C-3-LPEI or FcMe(4)-C-3-LPEI were comparable. The FcMe(2)-C-3-LPEI biofuel cells gave somewhat higher maximum currents, but the operating voltage and the stability of biofuel cells constructed with FcMe(4)-C-3-LPEI was higher than that of cells using FcMe(2)-C-3-LPEL These polymers have immediate applications as amperometric glucose sensors as well as in biofuel cell materials and have the potential to be used in a wide range of small implantable electronic devices. (C) 2013 Elsevier Ltd. All rights reserved.