Buckypaper Thickness Dependent Bilirubin Oxidase Electrochemistry

Abstract

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. We present the effect of buckypaper (BP) thickness on the electrocatalytic properties of bilirubin oxidase (BOD) enzyme. In this study, we prepared BPs of varying thicknesses ranging from 87 micron, the minimum thickness for suitable handling with good stability in aqueous experiments, to 380 micron. BOD was adsorbed overnight onto the BPs. We determined that the lower range BP thickness (< 220 micron) exhibited better sigmoidal shaped electrocatalytic currents than the higher BP thickness based BOD biofilms with larger capacitive currents. Oxygen reduction current density of up to 3 mA cm–2 is achieved without use of any redox mediators or tedious electrode modifications. Using the 87 micron thick BP as the representative case, we were able to obtain distinguishable peaks for all Cu sites of BOD, and assigned their types, T1, T2, and T3, based on peak-width at half-maximum in anaerobic cyclic voltammograms. Our peak assignment is further supported by the appearance of dual electrocatalytic reduction waves at 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 new enzyme electrocatalytic systems and biosensors. In addition, accessing electrochemical and catalytic properties of often-buried inaccessible redox centers of many important metalloenzymes can be achieved by the freestanding BP design of optimal thickness.