Biomedical Engineering Reference
In-Depth Information
electrode surface. Microperoxidase MP-11 with amino group was covalently linked to the
carboxyl groups of free ends of standing SWCNTs. The MP-11 linked to the SWCNT showed
a characteristic cyclic voltammogram at
390 mV vs. Ag/AgCl, indicating the MP-11 being
electrically contacted with the electrode surface. The rate constant of heterogeneous electron
transfer between MP-11 and the SWCNT-modified electrode was similar to that for MP-11
directly attached to cystamine-modified gold electrode. These similar rate constants indicate
the efficiency of the SWCNT as molecular wires. This result allows the understanding of the
redox properties of proteins and also the development of enzyme biosensors without
mediators.
The redox-active site of GOx is also electrically insulated by a protein shell, and thus
cannot be oxidized or reduced at an electrode. However, similar to other redox proteins,
the direct electron transfer of GOx can be performed at CNT-modified electrode. For
example, Guiseppi-Elie et al. [33] reported on the direct electron transfer of GOx sponta-
neously adsorbed onto SWCNTs cast onto the GC surface. A quasi-reversible one-electron
transfer was observed possibly due to the tubular fibrils positioned within a tunneling
distance of the cofactors.
Direct electron transfer of GOx and electrode by means of aligned SWCNT was reported
by Willner's group [34]. An array of SWCNT perpendicularly mounted on a gold electrode
was fabricated by covalently coupling the carboxylated SWCNT to a cystamine monolayer-
functionalized gold-electrode surface. Flavin adenine dinucleotide (FAD) cofactor with
amino groups was covalently linked to the carboxyl groups of free ends of standing
SWCNTs. The FAD units linked to the SWCNT showed a quasi-reversible cyclic voltammo-
gram, indicating the FAD unit being electrically contacted with the electrode surface. Apo-
GOx was then reconstituted on the FAD units coupled to the free ends of the standing
SWCNT. The biocatalytic oxidation of glucose was observed at 0.18 V vs. SCE, and the oxi-
dation current increased as the glucose concentration increased. The electron-transfer rate
showed clear dependence on the length of the SWCNTs. The GOx-CNT with an average
length of 25 nm showed an about 1.5-fold enhanced electrocatalytic current compared to the
GOx-CNT with an average length of 50 nm. Such enzyme reconstitution at the end of CNT
represents an extremely efficient approach for the preparation of mediator-free biosensor.
Luong et al. [35] reported on the promoted electron transfer of GOx at MWCNT-
modified GC electrode. 3-Aminopropylethoxysilane (APTES) was used for solubiliza-
tion of CNTs as well as immobilization of GOx to fabricate a mediator-free glucose
biosensor. The resulting surface acts like bundled ultramicroelectrodes that allow the
access to the active FAD site and facilitate a direct electron transfer to the immobilized
GOx. Cyclic voltammograms for GOx immobilized on the APTES-MWCNT-modified
GC electrode showed nearly symmetrical peaks at
0.45 V vs. Ag/AgCl, which is close
to the redox potential of FAD.
The determination of hydrogen peroxide is of great importance in food, pharmaceutical,
and environmental analysis. The direct electrochemical detection of hydrogen peroxide
requires relatively high overpotentials. Therefore, hydrogen peroxide is often detected
enzymatically at low applied potential by the use of peroxidase. Yu et al. [36] reported on
the direct electrochemistry of horseradish peroxidase (HRP) covalently attached onto the
ends of vertically oriented SWCNT forest arrays. Quasi-reversible Fe(III)/Fe(I) voltamme-
try was observed for the heme group around
0.25 V (vs. SCE). This result indicates that
the individual SWCNT behaves electrically similar to metal, conducting electrons from the
external circuit to the redox sites of the HRP enzymes. The resulting HRP biosensor
exhibited a detection limit of 100 nM.
Yamamoto et al. [37] reported on a CNT-HRP modified electrode for online monitoring
of hydrogen peroxide. MWCNTs were immobilized on the surface of GC electrode by mix-
ing with HRP. The CNT-HRP-modified electrode exhibited good sensitivity by monitoring
