Enzyme Immobilization on Chitosan-Based Supports - Chitosan Based Hydrogels: Functions and Applications - page 377

films and then immobilize enzymes effectively [120]. Also they can be mixed with chitosan
and enzymes to construct biosensors through simple one-step electrodeposition [121].
However, in both of these systems, GNPs need to be prepared previously, which prolongs
the duration of biosensor preparation and makes the procedure a bit complicated. Recently,
the authors investigated a simple method, the coelectrochemical deposition method, for
fabricating a chitosan film containing GNPs. HAuCl 4 solution is mixed with chitosan and
electrochemically reduced to GNPs directly, and the produced GNPs were stabilized by
chitosan and electrochemically deposited onto the GCE under a certain voltage along
with chitosan. The characteristics of the film can be controlled by changing the deposi-
tion conditions, and the whole procedure costs only several minutes. The resulted chito-
san film containing GNPs can be used to construct biosensors through assembling
enzymes (GOx) on the surface of the film, and the immobilized enzymes have good bio-
activity [122].
In recent years, metallic alloy nanoparticles have been of considerable interest in the
field of catalysis and sensors because they often exhibit better catalytic properties than do
their monometallic counterparts. Gold (Au) is relatively less reactive and more electrone-
gative than platinum (Pt), so Au-Pt alloy nanoparticles may have unique effects on cataly-
sis. Many studies of Au-Pt alloy nanoparticles have centered on their optical properties,
selective oxidation, and dehydrogenation catalysts, electrocatalysts, and selective
sensors.
By combining the advantageous features of CNTs and Au-Pt bimetallic nanoparticles, a
novel glucose biosensor has been constructed by integrating CNTs with Au-Pt alloy nano-
particles. In the fabrication course, chitosan is used to disperse CNTs and to immobilize
GOx. The electrodeposition method is applied to form Au-Pt alloy nanoparticles on the
electrode modified with CNTs/chitosan. CNTs functionalized with carboxylic groups are
well dispersed in chitosan solution with good stability owing to the presence of active
amino groups of chitosan, and easily form a uniform film of CNTs/chitosan on a GCE.
Because most of the amino groups of chitosan are protonated when chitosan is positively
charged (pH < p K a 6.3), it can electrostatically bind the negatively charged PtCl 2− and
AuCl 4 − . Therefore, Au-Pt alloy can cause NP size distribution by electrodeposition on the
surface of GCE modified with CNTs/chitosan. The fabrication processes of the modified
electrode are shown in Figure 8.12. For the immobilization of enzymes, GOx cross-link
with GA-activated chitosan on the Au-PtNPs/CNTs/chitosan film. A preliminary study
indicated that Au-PtNPs/CNTs had a better synergistic electrocatalytic effect on the reduc-
tion of hydrogen peroxide than did AuNPs/CNTs or PtNPs/CNTs at a low applied poten-
tial window. In addition, the biosensor was applied for the determination of glucose in
human blood and urine samples, and satisfactory results were obtained [123].

NH +
NH +

AuCl -

NH +

NH +
NH +
NH +
NH +

CNTs+CS

NH +
NH +
NH +

PtCl 2- 1 mM

Electrodeposition
-0.2V. 400s

NH +
NH +

AuCl -
PtCl 2-

AuCl -

PtCl 2-

GCE

NH 2
NH 2

CS

Au-Pt alloy nanoparticle

CNTs

Figure 8.12
Preparation process of the modified electrode. (From Kang, X. H. et al. 2007. Anal Biochem 369: 71-79. With
permission.)