Electrode materials
play an important role on the electrocatalytic
properties of immobilized biocatalysts. In this regard, achieving
direct electronic communication between the electrode and redox sites
of biocatalysts eliminates the need for additional electron transfer
mediators for biocatalytic applications in fuel cells and other electrochemical
energy devices. In order to increase electrocatalytic currents and
power in fuel cells and metal–air batteries, conductive carbon-nanostructure-modified
large surface area electrodes are quite useful. Among various electrode
materials, freestanding buckypapers made from carbon nanotubes have
gained significance as they do not require a solid support material
and thus facilitate miniaturization. In this article, we present the
effect of buckypaper (BP) thickness on the electrocatalytic properties
of a bilirubin oxidase (BOD) enzyme. In this study, we prepared BPs
of varying thicknesses ranging from 87 μm, the minimum thickness
for suitable handling with a good stability in aqueous experiments,
to 380 μm. BOD was adsorbed overnight onto the BPs, mostly via
hydrophobic and π–π interactions since the nanotubes
used were not chemically functionalized. Furthermore, intercalation
of the BOD molecules onto the nanotubes’ multicylindrical network
is feasible. We determined that the lower range BP thickness (<220
μm) exhibited better sigmoidal shaped electrocatalytic currents
than the higher BP-thickness-based BOD biofilms with larger capacitive
currents. An oxygen reduction current density of up to 3 mA cm–2 is achieved without the use of any redox mediators
or tedious electrode modifications. Using the 87 μm thick BP
as the representative case, we were able to obtain distinguishable
peaks for all Cu sites of BOD and assign their types, T1, T2, and
T3, based on the peak-width at half-maximum in anaerobic cyclic voltammograms.
Our peak assignment is further supported by the appearance of dual
electrocatalytic oxygen reduction waves at a higher scan rate region
(>10 mV s–1) in oxygen-saturated buffer, which
is
identified to be driven by an ∼3.5 times faster electron transfer
rate from the buckypaper to the T2/T3 center than the T1 Cu site.
Findings from this study are significant for designing enzyme electrocatalytic
systems and biosensors in general and fuel cells and aerobic energy
storage devices in particular, where the cathodic oxygen reduction
current is often inadequate.