Operational Stability Assays for Bioelectrodes for Biofuel Cells: Effect of Immobilization Matrix on Laccase Biocathode Stability

Abstract

One of the main issues of enzymatic biofuel cells is the need to develop stable bioelectrodes. However, there is no standard method to study bioelectrode stability. In this study, laccase and anthracene modified multiwall carbon nanotubes (anth-MWCNTs) were used in conjunction with different immobilization polymers in order to increase the stability of the biocathodes. A series of stability assays were used to understand the operational stability of the biocathode in a biofuel cell environment. The immobilization matrices used in this study were tetrabutyl ammonium bromide modified Nafion (TBAB modified Nafion), octyl modified linear polyethylenimine (C 8 -LPEI), and vapor deposited tetramethyl orthosilicate (TMOS) gels. The decrease in activity of the galvanostatic and potentiostatic measurements over a 24 hour period of the TBAB modified Nafion were 4.0 ± 0.6% and 4.1 ± 0.4%; C 8 -LPEI were 0.7 ± 0.1% and 9.6 ± 0.5%; and TMOS were 4.1 ± 0.1% and 10 ± 2.0%, respectively. This data show that potentiostatic measurements provide a harsher environment for the enzyme and result in lower stability, as well as showing that the TBAB modified Nafion offers a more stabilizing immobilization strategy compared to the C 8 -LPEI or TMOS matrices. The increasing interest in producing electrical energy from renewable resources has caused the field of biofuel cells to flourish. Enzymatic biofuel cell systems are capable of producing energy from renewable fuels. Basically, enzymatic biofuel cells (EBFC) generate electricity from the oxidation of the fuel at the anode and the reduction of the oxidant at the cathode using biological catalysts (enzymes) instead of traditional metal catalysts. More detailed descriptions of such biofuel cells can be found in reviews that discuss the fundamental concepts of this technology.1,2 Applications of EBFCs range from energy sources for small electronic devices, 3 self-powered sensors 4 and more recently as implantable power sources. [5][6][7] To improve this technology, much research has been focused on engineering and modifying the cathode half of these biofuel cells. [8][9][10][11][12][13][14][15][16][17][18] One such engineered bioelectrode system, previously developed by our group, employs anthracene modified multi-walled carbon nanotubes (anth-MWCNTs) for docking of the oxidoreductase enzyme laccase to the current collector. This biocathode uses anthracene covalently modified to the ends of the multi-walled carbon nanotubes (MWCNTs) in order to favorably orient the laccase enzyme to the ends of the carbon nanotubes, since laccase has a hydrophobic binding site for anthracene and the aromatic structure allows for improved conductivity between the active site and the carbon surface. The steric orientation provides increased catalytic reduction of oxygen to water and favorable distances such that the laccase performs direct electron transfer (DET). 8In biofuel cells, DET occurs through the enzyme's ability to oxidize or reduce a substrate while transferring the necessary electrons to or fro...

O2Reduction in Enzymatic Biofuel Cells

“…They compared galvanostatic and potentiostatic measurements, and different experimental conditions for both techniques. 486 Many mechanisms could account for the relative instability of an enzymatic electrode:…”

“…486 We give here the results obtained in potentiostatic mode, since it has been proved to be harsher than galvanostatic mode. 486 The 3 different electrodes, TBAB Nafion®, C8 LPEI and TMOS, respectively lost 4.1 ± 0.4%, 9.6 ± 0.5% and 10 ± 2% of the initial current densities after 24 hours at Eapplied  0.44 V vs.…”

Miniature Direct Electron Transfer Based Enzymatic Fuel Cell Operating in Human Sweat and Saliva

“…chloride ions, uric and ascorbic acids, cells, and a multitude of different proteins. The chosen type of measurement could also in itself affect the apparent stability, as potentiostatic tests have been shown to cause more rapid degradations than galvanostatic tests [26].…”

Lag Time Spectrophotometric Assay for Studying Transport Limitation in Immobilized Enzymes

“…The influences on activity and lag-time performances for three immobilization techniques were determined, specifically magnetically separable enzyme precipitate coating (Mag-EPC), 9 linear polyethyleneimine (LPEI) hydrogels, 10 and tetrabutylammonium bromide (TBAB)-modified Nafion. 11 The lag time assay allows for the determination of the time it takes substrate to get to the immobilized enzyme, the substrate to react, and the product to diffuse away. Therefore, if the immobilization technique retards transport or requires long transport distances to the enzyme, the lag time will be significantly larger than if there is facile diffusion and a short diffusion length to the enzyme.…”