Buckypaper–Bilirubin Oxidase Biointerface for Electrocatalytic Applications: Buckypaper Thickness

Abstract

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.

Efficiency of Site‐Specific Clicked Laccase–Carbon Nanotubes Biocathodes towards O2 Reduction

“…This distance as well as the presence of insulating medium between the enzyme active site and the electrode have to be minimized. This challenge requires thin‐layer modification of the electrode, the use of nanostructured material and a favorable orientation of the enzyme . Since the seminal work of Shleev and Gorton, numerous articles have described DET reactions of MCOs on electrodes.…”

“…This challenge requires thin-layer modificationo ft he electrode, the use of nanostructured material and af avorable orientation of the enzyme. [3][4][5][6][7][8] Since the seminalw ork of Shleev and Gorton, [9][10][11] numerousa rticles have described DET reactions of MCOs on electrodes. All together they cover important variations of experimental conditions includinge nzymet ype (laccase,B OD, CueO), electrode type (stationary,o rr otating-diske lectrode), electrode material (gold, graphite), immobilization method (physisorption, supramolecular,c ovalent), operating conditions (enzyme purity,p H, T).…”

Pyrenyl Carbon Nanotubes for Covalent Bilirubin Oxidase Biocathode Design: Should the Nanotubes be Carboxylated?

“…The site‐directed immobilization of BOD is another promising strategy to favorably tune the BOD activity and additionally obtain mechanistic insights into catalytic intermediates and pathways under various experimental conditions (e. g., pH, Cl − concentration) . Thus, designs of new electron‐transfer mediators, electrostatic adsorption favored nanomaterial designs, molecular linkers for covalent attachment strategy, electrode materials, and site‐directed mutagenesis of BOD to efficiently connect the enzyme Cu sites with electrodes have been actively investigated .…”

Molecular Release Associated with Interfacial pH Change Stimulated by a Small Electrical Potential Applied

“…While the molecular release stimulated by local pH change upon O 2 reduction has been studied by us before, the important advantage of the present system is the low potential required due to the bioelectrocatalysis provided by BOx. It should be noted that the effective bioelectrocatalysis was obtained with the buckypaper electrode, which is particularly well known for providing direct (non‐mediated) electron transfer with enzymes, particularly for BOx . The experimentally demonstrated dye release represents only a convenient model, while in the future applications biochemically important molecules, e. g., drugs, can be delivered according to the same approach.…”

“…It should be noted that the effective bioelectrocatalysis was obtained with the buckypaper electrode, which is particularly well known for providing direct (non-mediated) electron transfer with enzymes, [23] particularly for BOx. [28] The experimentally demonstrated dye release represents only a convenient model, while in the future applications biochemically important molecules, e. g., drugs, can be delivered according to the same approach. The designed system can operate as a general platform for the signalactivated (bio)molecular release processes.…”