Biomedical Engineering Reference
In-Depth Information
electrical conductivity, high chemical stability, and extremely remarkable mechanical
strength and modulus [22]. Cai and Chen [14] obtained a stable suspension of CNTs by
dispersing the CNTs in the solution of the surfactant cetyltrimethylammonium bromide
(CTAB). When haemoglobin (Hb) was immobilized onto the surface of CNTs, its direct
electron-transfer rate was greatly enhanced. Cyclic voltammetric (CV) results showed a
pair of well-defi ned redox peaks, which corresponded to the direct electron transfer of
Hb. The electrochemical parameters such as apparent heterogeneous electron-transfer rate
constant ( k s ) and the value of formal potential ( E 0
) were estimated. The experimental
results also demonstrated that the immobilized Hb retained its bioelectrocatalytic activity
to the reduction of H 2 O 2 . Other proteins, such as cyt c , catalyase, xanthine oxidase (XOD)
and GOD, can also be immobilized onto the surface of CNTs, and achieve the direct elec-
trochemistry as reported.
17.2.1.6 Other methods of protein immobilization
Some physical mechanisms that might assure the function of DNA as a molecular wire
have been considered on the basis of recent progress in understanding charge transfer
of biological molecules [23-26]. So, the advent of molecular electronics has stimu-
lated an interest in the possibility of exploiting this molecule in functional mesoscopic
electronic devices [27-28] and considerable interest has focused on the application of
DNA based on its p-stacked base pairs [29-30]. This key structure feature is similar to
the conductive one-dimensional aromatic crystals, which suggests that in the interior
of the double helix, the stack of base pairs can provide a one-dimensional pathway for
charge migration and acts as a “
way” for the effi cient transfer of electrons [31-33].
Using DNA to immobilize proteins is a new embedment method developed recently.
Rusling and Nassar [34] reported that stable fi lms of calf thymus (CT) double stranded
(ds) DNA and proteins on PG electrodes could be obtained. They also achieved direct
electron transfer involving heme protein Fe(III)/Fe(II) couples in DNA/protein fi lms.
In their work, DNA fi lms on PG electrodes also extracted heme proteins from solution.
Mb diffused into pure DNA fi lms much faster than haemoglobin.
Layer-by-layer (LbL) assembly of proteins or enzymes with polyelectrolytes is a
novel general method for protein fi lm fabrication that emerged over the past decade
[35], and establishes a new procedure for studying redox proteins with electrochemical
technology [36]. The principle of the LbL assembly is based on alternate adsorption
of oppositely charged species from their solutions by electrostatic interaction between
them. Compared with cast method, the LbL assembly technology develops a “molecu-
lar architecture” with precise control of the composition, the number of layers, and
the thickness of fi lms at a molecular or nanometer level. Moreover, the LbL method is
simple and suitable to a variety of substrate matrices with different shapes. Recently,
direct electrochemistry of proteins in LbL fi lms assembled with oppositely charged
polyions was studied [37-39]. In general, proteins in these fi lms retained their native
structures and electroactivities, and were used for electrocatalysis. The LbL assem-
bly technique was recently extended to fabricate ultrathin protein fi lms with inorganic
nanoparticles, since proteins and nanoparticles could carry opposite surface charges
π
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