Biosensors based on direct electron transfer of protein - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 538

Such functionalized superstructures have been shown to have sensoric and photo-
electrochemical applications [79-81]. These metal colloidal fi lms have also been
used as a basic interface to construct amperometric biosensors to realize the direct
electron transfer between the electrode and redox proteins or enzymes [82-83]. So
far, the immobilization of a redox enzyme on colloidal gold is thought either to help
the protein to assume a favored orientation or to make possible conducting channels
between the prosthetic groups and the electrode surface, and they will both reduce
the effective electron transfer distance, thereby facilitating charge transfer between
electrode and enzyme [76]. Sol-gel technology has been the subject of many stud-
ies, because it provides a unique low temperature methodology to prepare a three-
dimensional network suited for the encapsulation of many different molecules [84-
86]. Wang [16] used (3-mercaptopropyl)-trimethoxysilane (MPTMS) for designing a
three-dimensional interfacial structure of silica gel on a gold electrode via the direct
coupling of sol-gel and self-assembled technology. And quasi-reversible and direct
electrochemistry of cyt c was obtained at a novel electrochemical interface constructed
by self-assembling GNPs onto a three-dimensional silica gel network, without polish-
ing or any modifi cation of the surface. These nanoparticles inhibited the adsorption of
cyt c onto bare electrode and act as a bridge of electron transfer between protein and
electrode.
It is known that CNTs could self-organize with DNA molecules [87-88]. The DNA/
CNTs layer could be used as new electronic materials, which was based on theoreti-
cal prediction [14-89] and experimental confi rmation [89-91]. Since DNA solution
can gelatinize, the mixed DNA/CNTs layer can keep stability on the surface of metal
electrode, and can be used to investigate some electrochemical phenomena or probe
electrochemical properties of some proteins. Furthermore, proteins containing posi-
tive charges could be immobilized on the modifi ed layer due to the massive negative
charges of DNA. In Wang's work [92], multi-walled carbon nanotube (MWNT) was
successfully immobilized on the surface of platinum electrode by mixing with DNA.
Further research indicated that cyt c could be strongly adsorbed on the surface of the
modifi ed electrode, and formed an approximate monolayer. The immobilized MWNTs
could promote the redox of horse heart cyt c , which gave reversible redox peaks with a
formal potential of 81 mV.
17.2.2.2 Direct electron transfer of myoglobin
Myoglobin (Mb) is a kind of heme protein containing a single polypeptide chain with
an iron heme as its prosthetic group. The physiological function of Mb is to store diox-
ygen and increase the diffusion rate of dioxygen in the cell. Although Mb does not
function physiologically as an electron carrier, it undergoes the oxidation and reduction
process in the respiratory system. Thus, its electron-transfer reactions play essential
roles in biological processes. It is an ideal model molecule for the study of electron-
transfer reactions of heme proteins, biosensing, and electrocatalysis.
The electrochemistry of Mb has been achieved by using mercury electrodes [93],
methyl-viologen-modifi ed gold electrodes [94], and ultraclean and hydrophilic indium