Biomedical Engineering Reference
In-Depth Information
π
-cation radical. H 2 O 2 fi rst oxidizes CatFe(III) to form Compound I and H 2 O, and then
reduces Compound I to CatFe(III) and produces O 2 . As a well-known effi cient catalyst,
cat acts either as a reductant or as an oxidant in the reactions, and returns to its resting
state after one catalytic cycle, while H 2 O 2 undergoes dismutation to produce H 2 O and
O 2 . Although the heme groups are the electroactive center of cat, it is usually diffi -
cult to observe the direct electron exchange of cat in solution with electrodes, probably
because the hemes are buried deeply inside the polypeptide chains of relatively large
cat molecules. Films modifi ed on solid electrodes provided an approach to realize its
direct electrochemistry. A recent report by Kong [151] showed that in DDAB fi lms
cast on PG electrodes, cat gave a pair of direct CV peaks in blank buffers. Rusling
and coworkers [152] used dimyristoylphosphatidylcholine (DMPC) as a fi lm-forming
material to incorporate catalase. The Cat-DMPC fi lms cast on PG electrodes showed a
pair of well-defi ned, quasi-reversible peaks, characteristic of heme Fe(III)/Fe(II) redox
couples of cat. Both DDAB and DMPC are water-insoluble, double-chain surfactants,
and can form multibilayer fi lms from their organic solution or aqueous dispersion. It is
these biomembrane-like surfactant fi lms that provided a suitable microenvironment for
cat to transfer electrons with PG electrodes.
Polyacrylamide (PAM) is a kind of widely used polymer. PAM has a long
hydrophobic hydrocarbon backbone with hydrophilic amide groups, showing the
amphiphilic property. The electrochemistry of heme proteins in PAM polymer environ-
ment has been studied. Murray and coworkers has reported “solid-state” voltammetry
of cyt c in PAM gel solvent [153]. In the previous study, it was found that PAM could
form stable fi lms on PG surface and absorb considerable amounts of water in aqueous
solution. Heme proteins such as Hb [154] and Mb [155] in the PAM hydrogel fi lms-
modifi ed PG electrodes demonstrated a reversible CV response for the heme Fe(III)/
Fe(II) redox couple. As Lu reported [156], PAM hydrogel fi lms could also provide a
favorable microenvironment for incorporated cat, and cat in PAM fi lms gave a direct
electrochemical response at the electrode surface. Cat-PAM fi lm electrode showed a
pair of well-defi ned and nearly reversible cyclic voltammetry peaks for Cat Fe(III)/
Fe(II) redox couples at approximately
0.46 V (vs SCE) in pH 7.0 buffers. This sug-
gested that the electron transfer between cat and PG electrode was greatly facilitated
in the microenvironment of PAM fi lms. The apparent heterogeneous electron-transfer
rate constant ( k s ) and formal potential ( E 0
) were estimated by fi tting square wave vol-
tammograms with non-linear regression analysis. The formal potential of Cat Fe(III)/
Fe(II) couples in PAM fi lms have a linear relationship with pH between pH 4.0 and 9.0
with a slope of
56 mV pH 1 , suggesting that one proton is coupled with single-elec-
tron transfer for each heme group of cat in the electrode reaction. UV-Vis absorption
spectroscopy demonstrated that cat retained a nearly native conformation in PAM fi lms
at medium pH. The embedded cat in PAM fi lms showed the electrocatalytic activity
toward O 2 and H 2 O 2 .
Several studies have shown that the direct electron transfer between cat and graph-
ite or carbon soot electrodes in deoxygenated solutions was sluggish, with large peak
separations [157-159]. Based on the consideration that CNTs might be the best can-
didate among the various carbonaceous substrates for promoting the electron transfer
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