Enzyme Immobilization on Chitosan-Based Supports - Chitosan Based Hydrogels: Functions and Applications - page 375

The presence of ZnO nanoparticles in chitosan results in increased surface area and
enhanced electron transfer kinetics. The low Michaelis-Menten constant ( K m ) value indi-
cates an enhanced affinity of enzyme to the nanobiocomposite [113].
8.5.4.2.2 Chitosan-ZrO 2 Nanocomposite
ZrO 2 has the properties of high hardness, high melting point (2700°C), and small thermal
expansion coefficient. In fact, a porous ZrO 2 particle itself has been used as support for
enzyme immobilization. The surface of porous zirconia has general affinity for the bind-
ing of proteins because the amine and carboxyl groups on the surface of the enzyme act as
ligands to ZrO 2 .
Yang et al. have used a nanoporous ZrO 2 -chitosan composite matrix for fabricating the
glucose biosensor. The surface of nanoporous ZrO 2 was treated with an anionic surfactant
(sodium dodecylbenzene sulfonate) to improve the dispersion of ZrO 2 in chitosan solution.
The results obtained from transmission electron microscopy indicated that a surface-
treated ZrO 2 -chitosan film is porous and highly homogeneous. GOx can be effectively
entrapped in the film with a higher bioactivity compared with that of GOx cross-linked by
glutaraldehyde [114].
It has also been reported that equal weights of chitosan and ZrO 2 powders were mixed
in an acetic acid solution to prepare the composite beads. They were then cross-linked
with GA and stored with and without freeze-drying before use. It was shown that the
activity yield of enzyme immobilized on the dried chitosan-ZrO 2 beads was the highest.
Acid phosphatase immobilized on wet composite beads exhibited the best storage stability
and operation stability even after being reused 50 times [115].
8.5.4.2.3 Chitosan/TiO 2 Nanocomposite
Titanium dioxide (TiO 2 ) can be formed into different morphologies such as nanoparticles,
nanofibers, nanotubes, and nanosheets. TiO 2 nanoparticles, as a semiconductor, showed
excellent electrochemical activity toward H 2 O 2 , ascorbic acid, guanine, l-tyrosine, and
acetaminophen, and provided direct electron transfer ability for GOx, HRP, and Hb. The
catalytic activity was enhanced by taking advantage of the photovoltaic effect of TiO 2 . A
bioactive electrode of uniformly dispersed TiO 2 in chitosan was fabricated on an ITO
substrate for the immobilization of HRP. An enhanced surface porosity and a decrease in
the relative proportion of carbonyl functionality of chitosan in the chitosan-TiO 2 matrix
were observed. The current-voltage characteristic of the chitosan-TiO 2 matrix was
enhanced by a factor of four possibly due to covalent and hydrogen bonding of Ti atoms
with hydroxyl and amino groups of chitosan. The immobilization of HRP on chitosan-
TiO 2 had increased resistance for charge transfer. This is possibly due to the strong bind-
ing of HRP with the chitosan-TiO 2 matrix and controlling the transport of the ions of the
supporting electrolyte [116].
TiO 2 nanotubes are very good biocompatible inorganic material, inexpensive, environ-
mentally benign, and chemically and thermally stable. TiO 2 nanotube arrays have demon-
strated a number of important applications including gas sensing, solar cells, photocatalysts,
tissue engineering, and biosensors. Kafi et al. have reported on a promising H 2 O 2 biosen-
sor based on the coimmobilization of HRP and chitosan onto Au-modified titanium diox-
ide nanotube arrays. These titania nanotube arrays possess large surface area and good
uniformity, providing an excellent matrix for the coimmobilization of HRP and chitosan.
The presence of the Au thin film greatly increases the electrical activity of the formed TiO 2
nanotube arrays. Electrochemical measurements reveal that the immobilized HRP exhib-
its high biological activity and stability. The amperometric response of the developed