Biosensors based on direct electron transfer of protein - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 536

under appropriate conditions and might be assembled layer by layer by electrostatic
interaction [40-42]. The direct electrochemistry of proteins in LbL fi lms with nano-
particles assembled on electrodes was also studied [43-45]. In these fi lms, inorganic
nanoparticles, with their unique and excellent properties such as large surface area and
good biocompatibility, provided a favorable microenvironment for redox proteins to
transfer electrons directly with underlying electrodes. For example, Lvov [44] assem-
bled LbL fi lms of Mb with MnO 2 or SiO 2 nanoparticles at PG electrodes. A pair of
reversible, symmetric, reduction-oxidation CV peaks of Mb heme Fe(III)/Fe(II) couple
were observed.
17.2.2 Direct electron transfer of proteins
Nowadays, studies of direct electrochemistry of redox proteins at the electrode-
solution interface have held more and more scientists' interest. Those studies are a
convenient and informative means for understanding the kinetics and thermodynamics
of biological redox processes. And they may provide a model for the study of the
mechanism of electron transfer between enzymes in biological systems, and establish
a foundation for fabricating new kinds of biosensors or enzymatic bioreactors.
Historically, several factors have plagued direct electron transfer between electrodes
and proteins [46]. These factors include (i) electroactive prosthetic groups deep within
the protein structure, (ii) adsorptive denaturation of proteins onto electrodes, and (iii)
unfavorable orientations of proteins at electrodes. Remarkable recent progress pro-
vides several strategies for achieving direct electron exchange between electrodes and
proteins. With few exceptions [47], special electrode preparations are required. One
approach employs highly purifi ed protein solution and specially cleaned electrodes
[48]. Another coats electrodes with promoter molecules, which facilitate electron
transfer by blocking adsorptive denaturation and favorably orienting the protein [46].
The fi rst reports on direct electrochemistry of a redox active protein were published
in 1977 by Hill [49] and Kuwana [50]. They independently reported that cytochrome
c (cyt c ) exhibited virtually reversible electrochemistry on gold and tin doped indium
oxide (ITO) electrodes as revealed by cyclic voltammetry, respectively. Unlike using
specifi c promoters to realize direct electrochemistry of protein in the earlier studies,
recently a novel approach that only employed specifi c modifi cations of the electrode
surface without promoters was developed. From then on, achieving reversible, direct
electron transfer between redox proteins and electrodes without using any mediators
and promoters had made great accomplishments.
17.2.2.1 Direct electron transfer of cytochrome c
Cyt c is one of most important and extensively studied electron-transfer proteins, partly
because of its high solubility in water compared with other redox-active proteins.
In vivo , cyt c transfers an electron from complex III to complex IV, membrane-bound
components of the mitochondrial electron-transfer chain. The electrochemical interro-
gation of cyt c has, however, been hindered because the redox-active heme center is