Biosensors based on direct electron transfer of protein - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 537

buried beneath the surface of the protein. This diffi culty has been overcome by the
introduction of modifi ed electrode surfaces, with monolayers able to interact with both
the heme center and the underlying electrode. Since the direct electron transfer of cyt c
was fi rst observed in 1977 [49-50], its direct electrochemistry and voltammetric meas-
urements have been extensively described at various chemically modifi ed electrodes
[51-59]. Direct electron transfer of cyt c can also be achieved by using electrode mate-
rials such as oxides [50]. Otherwise, direct electrochemical studies of cyt c can be
made by using the protein-fi lm voltammetry approach, pioneered by Armstrong and
coworkers, where proteins are adsorbed on rough hydrophilic surfaces such as edge-
plane pyrolytic graphite [60]. These works have led to a good understanding of the
electron-transfer mechanism between cyt c and chemically modifi ed electrodes.
The modifi ers for preparation of those chemically modifi ed electrodes are generally
organic compounds [61]. Compared with organic compounds, inorganic materials are
intrinsically more stable catalysts because of their layered oxide structure. Recently,
a series of inorganic porous materials such as clay [62], montmorillonite [63-64],
porous alumina [65], and sol-gel matrix [66] have been proven to be promising as the
immobilization matrices. They have the advantages of high mechanical, thermal, and
chemical stability, good adsorption and penetrability due to their regular structure and
appreciable surface area. Also, the unique structural and catalytic properties of zeo-
lites for structuring an electrochemical/electron transfer environment and resistance to
biodegradation have also attracted considerable attention [67]. NaY zeolite possesses
a microporous diameter of 0.81 nm [67]. The electrochemical behaviors of redox sub-
stances such as GOD [68-69] and horseradish peroxidase [70] incorporated in NaY
modifi ed matrixes have been studied. Ju [71] used polyvinyl alcohol (PVA) as a sup-
porting medium for NaY immobilization on electrode surface and reported the direct
electrochemistry of cyt c incorporated in NaY-PVA mixed media. An interaction
between cyt c and NaY zeolite particles was observed with UV-Vis spectroscopy and
CV. NaY zeolite particles effectively retained the activity of the immobilized cyt c and
facilitated the electron exchange between the cyt c and the electrode.
It is well known that cyt c adsorbs strongly on Pt, Hg, Au, Ag, and other electrodes,
which results in large changes in its conformation and often in denaturation of the pro-
tein. The denaturation of cyt c will hamper its electrochemistry [48, 72]. Conversely,
direct adsorption of proteins onto the uncoated, nanometer-sized colloidal Au parti-
cles would not denature the proteins, for it has been demonstrated that electrostatically
bound colloidal Au and protein conjugates typically retain biological activity [73-74].
Indeed, Crumbliss and coworkers have shown that several enzymes could maintain
their enzymatic and electrochemical activity when immobilized on gold nanoparticles
(GNPs) [75-76]. Recently, Au nanoparticles were self-assembled onto two-dimensional
and three-dimensional superstructures on a variety of substrates such as glass, alumina,
and so on by using amino/thiolsiloxanes and dithiols/diamines/bipyridinium as cross-
linkers [77]. These GNPs can be strongly bound to the surface through covalent bonds
to the polymer functional groups, such as ß CN, ß NH 2 , or ß SH, and a GNP mon-
olayer can be prepared by self-assembly on the polymer coated substrate [78]. These
nanoparticles can act as tiny conduction centers and facilitate the transfer of electrons.