Enzyme Immobilization on Chitosan-Based Supports - Chitosan Based Hydrogels: Functions and Applications - page 343

can be cast or formed into various shapes and sizes including foams and monoliths. Two
kinds of enzyme-chitosan conjugates that can further form a smart gel or immobilize on
the support were introduced in this chapter (see Section 8.4.3).

8.2.4 Preparation Methods of Chitosan-based Supports

Preparation methods of chitosan gel, chitosan modifications and chitosan composites for
enzyme immobilization are shown in Figure 8.3.
The formation of chitosan gel is mainly based on three principles:

1. Change in pH. It is easy to form an insoluble network in solution above pH 6.3
because chitosan containing abundant amino groups with p K a 6.3 is soluble in a
slightly acidic solution. This group includes the solvent evaporation method, the
phase-inversion technique, the electrodeposition method, and the electrospinning
technique.

2. Electrostatic interaction. Chitosan can form a gel by ionotropic gelation with cer-
tain anionic counterions, such as tripolyphosphates (TPPs) or the oppositely
charged surfactant SDS.

3. Cross-linking. Freezing-thawing treatment, as a physical cross-linking, leads to a
gel-like polymer system. Chemical cross-linking methods resulting in gel are
often used by a mixture of an acidic chitosan solution with cross-linking agents.
Mostly, the immobilization of enzymes on such prepared gels does not require
chemical activation, as the cross-linker, normally a bifunctional agent, fulfills two
functions: cross-linking and activation. Thus, the processes by which chitosan
gels are further activated by bifunctional agents are also discussed in this group.

Chitosan-based composites, especially chitosan-inorganic nanocomposites, have been
attracting extensive attention and are widely used in enzyme-membrane reactors and bio-
sensors. The involved inorganic materials include metal and metal oxide nanoparticles,
silica gels, CNTs, clay, and inorganic salt. The above methods can also be applied to fabri-
cate chitosan composites. For instance, chitosan, as a polyelectrolyte, is able to form poly-
electrolyte complexes (PECs). Two different types of complexes are considered here: the

Physically cross-linking
(Freezing-thawing treatment)

Chemical cross-linking
(by GA, EDC, Gly, and so on)

Chitosan composites:
Chitosan-tethered or coated support
Polyelectrolyte complexes
Layer-by-layer self-assembly
Hybrid gel by silylating agents

Chitosan modifications:
Chitosan derivatives
Chitosan grafted copolymer
Chitosan-enzyme conjugate

OH OH

O O

O O

NH 3 NH 2

HO HO

+

Low pH
Polycationic
(Soluble)

High pH
Network-forming
(insoluble)

Solvent evaporation
Phase inversion technique
Electrodeposition
Electrospinning

Ionization gelation method
(by TPP)

Figure 8.3
Preparation methods of chitosan-based supports.