Biosensors based on direct electron transfer of protein - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 541

stability of these particles, a kind of gold nanoparticle protected by lipid (DDAB) was
invented [15]. The electron transfer of Hb at electrodes modifi ed with colloidal clay
nanoparticles has also been well studied due to its simplicity, effectiveness, and low
cost. A new electrochemical sensing system for direct electron transfer of heme protein
was developed that relied on the virtues of excellent biocompatibility, nano-dimensions
and semiconductor properties of quantum dots in Lu's work [17]. To demonstrate the
conception, Hb was immobilized in a water-soluble quantum dots (QDs, mercapto-
coated CdSe-ZnS) fi lm on glassy carbon (GC) electrode. From the results of cyclic
voltammetry (CV), a pair of well-defi ned and quasi-reversible peaks for Fe(III)/Fe(II)
redox couple of Hb was obtained, which refl ected direct electron transfer of the heme
protein. Scanning electron microscopy (SEM), fl uorescence (FL), and electrochemi-
cal impedance spectroscopy (EIS) demonstrated that electrostatic attractions existed
between Hb and QDs. These interactions led to the arrangement of Hb in the fi lm in a
favorable orientation for exchanging electrons with the electrode, the inhibition of Hb
adsorption onto bare electrode, and the offering of an electron transfer bridge between
Hb and electrode, which were believed to be responsible for the direct electrochemis-
try of Hb.
Cellulose is one of the naturally occurring biopolymers and widely existent in wood
and other plants. Cellulose in its native form is not soluble in water. It can be rendered
water soluble by chemical reaction of its hydroxyl groups with hydrophilic substituents.
Carboxymethyl cellulose (CMC) is one of the water-soluble cellulose derivatives. CMC
contains a hydrophobic polysaccharide backbone and many hydrophilic carboxyl groups,
and hence shows amphiphilic characteristics. Due to its desirable properties such as non-
toxicity, biocompatibility, biodegradability, high hydrophilicity, and good fi lm forming
ability, CMC has been used in various practical fi elds. CMC has also been used to study
interactions with proteins. For example, Cark and Glatz described the formation of com-
plex of CMC with casein by electrostatic interaction [117]. Lii et al. [118] reported the
formation of CMC-casein complex by covalent bonds with electrosynthesis. These com-
plexes were very stable to pH and ionic strength changes and exhibited very good emul-
sifying properties and increased thermal stability. Thus, protein-CMC fi lms were made
by casting a solution of Hb and CMC on PG electrodes [119]. In pH 7.0 buffers, Hb
incorporated in CMC fi lms gave a pair of well-defi ned and quasi-reversible CV peaks at
about 0.34 V (vs SCE), which was characteristic of heme Fe(III)/Fe(II) redox couples

of the protein. The electrochemical parameters such as apparent standard rate constants
( k s ) of heterogeneous electron transfer and formal potentials ( E 0 ) could be estimated by
square wave voltammetry with non-linear regression analysis. In aqueous solution, stable
CMC fi lms absorbed large amounts of water and formed hydrogel. The more loosening
structure of CMC in its hydrogel form would provide a more suitable microenvironment
for Hb to transfer electrons with underlying PG electrodes.
Recently, a novel method of Hb immobilization was achieved by Lu [120]. The
direct electrochemistry of Hb was successfully achieved by adsorbed Hb onto the sur-
face of a yeast cell through electrostatic attractions on a GC electrode. The bioactivity
of Hb immobilized in yeast cell fi lm was retained, and the catalytic reduction of NO
and H 2 O 2 was estimated.