HETEROGENEOUS NANOSTRUCTURES FOR BIOMEDICAL DIAGNOSTICS - Bio-Inspired and Nanoscale Integrated Computing - page 449

e − e −

Gluconolactone Oxygen

H+

Laccase/
bilirubin oxidase/
microperoxidase11/…

GOx

Glucose Water

Anode electrode Cathode electrode

Membrane

Figure 16.20. Schematics diagram of a biofuel cell where cathode and anode
electrodes' surface are coated with enzymes catalyzing redox reactions.

A widely used driving force to catalyze the oxidation of (beta)-D-glucose to
D-gluconic acid and hydrogen peroxide in the biofuel cells is glucose oxidase
(GOx).

Gox

-D-glucose þ O 2 ! D-glucose-1 ; 5-lactone þ H 2 O 2

X

Flavin adenine dinucleotide (FAD) is a cofactor of GOx, an apoenzyme. As
an electron acceptor, FAD is reduced to FADH 2 and FADH 2 is subsequently
oxidized to FAD by oxygen (O 2 ). This FAD/FADH redox centers are embedded
in the GOx at 13A ˚ beneath the surface. Hence, the direct electron transfer (DET)
rate between active site of GOx and electrode surface is usually slow.
To facilitate the electron transfer between the FAD site and microelectrode,
researchers have engineered the positioning and alignment of electron mediators
between the electrodes and redox sites to improve the output power [42]. One
approach is to functionalize the nanostructure such as carbon nanotubes or gold
particles with the cofactors. The apo-enzymes are positioned proximal to the
cofactor-functionalized nanostructures and the electrodes. This construct provides
a means to physically align the biocatalysts on the conductive surface and to
connect electrically redox enzymes with electrodes (Fig. 16.20) [43].
Chemically inert and highly hydrophobic carbon nanotubes (CNT) are
suitable as direct electron transfer mediators [44]. However, Yan et al. have
demonstrated a miniaturized biofuel using a multiwall carbon nanotube as the
anode and cathode electrodes [45]. The investigators decorate the CNTs with
carboxylic acid functionality as the mediators. The flavoenzymes are covalently
bound to the carboxylic functional group to facilitate the electrical contacts
between the electrodes and the redox enzyme. The immobilization of CNTs to the
bulk electrode has been achieved by covalent binding with a cystamine-mono-
layer-modified electrode (Fig. 16.21) [46].