Biomedical Engineering Reference
In-Depth Information
of tryptophan has been reported at a CNT-modifi ed electrode which indicates a high
catalytic activity toward the oxidation of tryptophan [88]. A novel CNT/(3-mercaptopro
pyl)trimethoxysilane (MPS) bilayer-modifi ed gold electrode has been used to study the
electrochemical behavior of fl uphenazine [89]. With this CNT-modifi ed electrode, the
determination of fl uphenazine in drug samples has been successfully carried out. An
ultrasensitive electrochemical method has been developed for 8-azaguanine in human
urine by using CNT-modifi ed electrodes [90]. Zhu
et al.
have studied the promotion of
CNT-modifi ed electrode to the electrochemical oxidation of theophylline (TP), which
can be applied to the determination of TP in drug [91].
15.3.3 Direct electron transfer of proteins and enzymes on carbon
nanotube electrodes
The active sites in redox proteins and enzymes are buried in a hydrophobic polypeptide
chain, and the redox centers of proteins or enzymes are electrically insulated and inacces-
sible to the electrode surface; thus, the direct electrochemistry of proteins and enzymes
is diffi cult on conventional electrodes such as gold, platinum, and glassy carbon. The
adsorption of macromolecular impurities or protein itself also contributes to a slow elec-
tron transfer. Much effort has been made to promote the electron transfer between pro-
teins or enzymes and the surface of the electrode. CNT-based electrodes have paved the
way for studying the direct electrochemistry of proteins due to their unique electronic
and structural properties. Following the preliminary report regarding the promoted elec-
trochemical response of proteins, namely cytochrome
c
and azurin, at CNT electrodes
[92], Wang
et al.
have displayed a pair of well-defi ned redox peaks for a cytochrome
c
aqueous solution at a GC electrode modifi ed with CNTs. The peak current increases
linearly with the concentration of cytochrome
c
[93]. The direct electrochemistry of
cytochrome
c
adsorbed on the surface of CNTs has also been investigated, and a rea-
gentless biosensor has been constructed for the determination of H
2
O
2
using CNT/Cyt.
c
-modifi ed electrode [94]. Voltammetric studies of myoglobin (Mb) and horseradish
peroxidase (HRP) covalently attached onto the ends of vertically oriented CNT forest
arrays show quasi-reversible redox behavior for Fe(III)/Fe(II) and demonstrate the elec-
trochemically manifested peroxidase activity of Mb and HRP attached to CNTs [48].
CNT-modifi ed electrodes also improved the direct electron transfer of Mb [95-97] and
hemoglobin (Hb) [98-100]. Based on the electrocatalytic activity of the CNT-modifi ed
electrode, reagentless biosensors for H
2
O
2
and NO have been constructed.
Moreover, it has been demonstrated that CNTs promote the direct electrochemis-
try of enzymes. Dong and coworkers have reported the direct electrochemistry of
microperoxidase 11 (MP-11) using CNT-modifi ed GC electrodes [101] and layer-by-
layer self-assembled fi lms of chitosan and CNTs [102]. The immobilized MP-11 has
retained its bioelectrocatalytic activity for the reduction of H
2
O
2
and O
2
, which can
be used in biosensors or biofuel cells. The direct electrochemistry of catalase at the
CNT-modifi ed gold and GC electrodes has also been reported [103-104]. The elec-
tron transfer rate involving the heme Fe(III)/Fe(II) redox couple for catalase on the
CNT-modifi ed electrode is much faster than that on an unmodifi ed electrode or other
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