Enzyme-Modified Buckypaper for Direct Bioelectrocatalysis

Abstract

Pyrroloquinoline quinone-dependent glucose dehydrogenase (PQQ–GDH) has been employed as a biosensor due to its high activity and lack of sensitivity to oxygen. In addition, direct electron transfer (DET) between the PQQ catalytic center and a conductive support has been demonstrated. This direct bioelectrocatalytic process negates the need for redox mediators and enables basic characterization of the biocatalytic process. Although PQQ–GDH is applied in point-of-care glucose sensors, its substrate range makes it an excellent candidate for biological fuel cell (BFC) anodes. However, the majority of research to date focused on the characterization and optimization of PQQ–GDH as a biosensor, rather than a BFC anode. Development of biocatalytic anodes requires a few considerations including the establishment of an open circuit potential decrease upon the addition of electron donor, a minimal potential drop associated with increased current densities, and sustainable power production. High catalytic activity of the bio-anode is also desirable; therefore, the factors influencing the kinetics of the DET process need to be understood and controlled. Examples of influential factors are the composition and structure of the conductive material and the enzyme tethering method. An electrode material suitable for direct bioelectrocatalysis is carbon nanotube (CNT) paper (i.e., buckypaper), which is a simple, easy-to-fabricate, scalable architecture. DET between multicopper oxidases and buckypaper has been demonstrated;1–3however, electron transfer events taking place between an organic redox molecule and CNTs are not as rigorously characterized. Herein, we demonstrate direct bioelectrocatalysis afforded by PQQ–GDH tethered to a range of CNT materials.4Physical characterization of the buckypaper architectures revealed marked differences attributable to changes in CNT dimensions, including conductivity, packing density, and electrochemically accessible surface area. Buckypaper anodes composed of single-walled CNTs produced higher current densities per geometric area with minimal voltage drops (Fig. 1). In comparison, electrodes made of oxidized CNTs exhibited higher voltage drops. The redox center, PQQ, physically adsorbed to the oxidized CNTs; however, non-oxidized CNTs exhibited redox processes limited by diffusion. In addition, electrode longevity tests on single-walled CNTs revealed that activity was retained for two days but then decreased by 50% on the third day. Lastly, the versatile utility of the PQQ–GDH anode was demonstrated via the oxidation of a range of mono- and disaccharide substrates. Fig. 1: Representative potentiostatic polarization curves of PQQ–GDH-modified SWBP (■), MWBP (●) and MWBP-F (▲). All polarization curves were performed with N2-sparged electrolyte in the presence of 10 mM glucose. Figure from reference 4.