Spherical carbon nanoparticles KetjenBlack (KB) with a high sorption capacity together with conductivity increasing single walled carbon nanotubes (CNTs) were "glued" together by chitosan for the preparation of a composite. A biocathode with bilirubin oxidase (BUD) adsorbed within the composite was characterised and its composition optimised. A renewable biopolymer chitosan present in the composite offered (1) pre-concentration of BUD within the matrix via electrostatic interactions, (2) favourable orientation of BUD for a direct electron transfer (DET) between BUD and the composite, (3) electrochemical visibility of 3 redox sites present in BUD, (4) low charge transfer resistance, (5) high proton conductivity and (6) low overpotential for oxygen reduction. Electrochemical investigation of BUD revealed interesting internal redox communication within the enzyme with some novel insights provided. At least one of tyrosines present in BUD seems to be involved in electron transfer route, as well. The composite used for the biocathode was directly applied for the preparation of a bioanode with fructose dehydrogenase immobilised, working in a DET mode of operation. Finally, integration of the biocathode and the bioanode into a biofuel cell operated in a reagentless and membraneless mode offered a power density of 50 mu W cm(-2) at 300 mV. (C) 2012 Elsevier Ltd. All rights reserved.