Biomedical Engineering Reference
In-Depth Information
is to be used as a non-implantable device, because of the need for a membrane and
casing.
The mediator and enzyme can be immobilized by adsorption onto electrodes, link-
age functionalized to the electrode surface or integration into a polymer layer. The
immobilization of biocatalyst and mediator provides the possibility for membrane-
less biocatalytic fuel cells, because of effective separation of the anode and cathode
reagents. The easiest way to immobilize mediators is through adsorption onto elec-
trode surfaces. Cass et al. [69] deposited solutions of ferrocene derivatives onto pyro-
lytic graphite electrodes and allowed them to air-dry. In this approach, the solubility
of ferrocenes in aqueous solution must be low to aid entrapment within the electrode.
Covalent attachment of GOx (through carbodiimide activation) to the electrode and
covering with polycarbonate membranes provided biocatalytic surfaces for mediated
oxidation of glucose. All the ferrocene derivatives investigated act as rapid oxidants
for the enzyme glucose oxidase [69, 70].
Increased micro- and nanoscopic surface areas can lead to improved signal output
for mediated biocatalytic electrodes. For example, Liu et al. [73] used porous carbon
as a matrix to load glucose oxidase to provide an anode in a biocatalytic fuel cell.
Not surprisingly, given the increased loading of enzyme, this biocatalytic anode dis-
played higher oxidation currents than that observed for GOx on a glassy carbon elec-
trode in the presence of glucose and solution-phase ferrocene monocarboxylic acid as
a mediator. The formal potential of the mediator of
0.34 V vs Ag/AgCl is, however,
remote from the FAD/FADH 2 reduction potential and further refi nement of this system
is focused on selecting more appropriate mediators and the co-immobilization of the
mediator with enzyme.
Co-immobilization of mediator and enzyme may be achieved using a novel recon-
stitution approach. For example, Katz et al. [15] have developed a biocatalytic anode
functionalized by a surface reconstitution of apo-GOx onto FAD that was previously
coupled to a pyrrolo-quinoline quinine (PQQ) relay conjugated to a self-assembled
monolayer of cysteamine on gold. The CV study of this assembly in the presence
of glucose yields an electrocatalytic current for glucose oxidation commencing at
0.12 V vs SCE at pH 7. The chronoamperometric output from this assembly can
achieve current densities of 300 mAcm 2 at
0.2 V vs SCE in 80 mM of glucose [2b].
Further modifi cation, to what the authors term the electrical contacting of the enzyme,
involved the use of alternate conjugation strategies [74], and of nanostructured sur-
faces [75, 76]. Xiao et al. [76] have reconstituted apo-GOX onto FAD-functionalized
gold nanoparticles that are then tethered via a dithiol spacer to a bulk gold electrode.
This gold nanoparticle assembly is an effi cient relay of electrons between the FAD
active site and the bulk electrode. Unfortunately, the biocatalytic oxidation of glucose
using this assembly only commences at potentials greater than
0.4 V vs SCE, with
the overpotential proposed to result from a tunnelling barrier introduced by the dithiol
spacer.
An alternative strategy for co-immobilization of mediator and GOx is based on adsorp-
tion of enzyme, cross-linked, as was described for the laccase-based biocatalytic cathodes
[30, 37-42], to an osmium-based redox polymer fi lm, on carbon electrodes [1-3, 54].
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