Enzyme Immobilization on Chitosan-Based Supports - Chitosan Based Hydrogels: Functions and Applications - page 367

8.5.3.1 Polysaccharide-Chitosan PECs
A variety of polysaccharide-chitosan PEC gels can be obtained by changing the molecular
properties of the polysaccharide and chitosan polymers, such as molecular weight, degree
of acetylation of chitosan, and carboxylic acid content of polysaccharide, as well as chang-
ing the complexation conditions, such as chitosan solution pH, polymer concentration,
complexation time, and mixing ratio.
8.5.3.1.1 Chitosan-Alginate Microcapsules
Alginate hydrogels, ionically cross-linked in the presence of multivalent cations, contain
carboxylic acid and exhibit negative surface charge, allowing them to be used as negatively
charged templates for PECs. Ehab Taqieddin and Mansoor Amiji described the develop-
ment of a core-shell microcapsule technology for enzyme immobilization. The enzyme is
localized and protected in the core matrix, while the shell can regulate the entry and exit
of the substrate and product, respectively. A model enzyme, β-galactosidase, was immobi-
lized in either calcium alginate or barium alginate core surrounded by a perm-selective
chitosan shell. Cross-linking of chitosan with TPP resulted in the phosphate ions diffusing
into the calcium alginate core and liquefying it. The enzyme loading efficiency was higher
in the barium alginate core (100%) as compared with the calcium alginate core (60%). The
maximum enzymatic rate ( V max ) of calcium alginate-chitosan microcapsules was higher
than that of barium alginate-chitosan microcapsules. However, the enzymatic activities of
calcium alginate or barium alginate core-shell microcapsules were significantly lower
than that of the free enzyme owing to the additional layer necessary for the influx of the
substrate and the efflux of the product [89].
Aranaz et al. used alginate-chitosan polyelectrolyte capsules for coimmobilization of
enzymes to reproduce a multistep enzymatic route for the production of d-amino acids.
Encapsulation of a crude cell extract from Agrobacterium radiobacter containing d-hydantoi-
nase and d-carbamoylase activities into the PECs was accomplished with negligible leak-
age from the formed capsules. The most suitable biocatalysts were prepared using a
chitosan with a medium molecular weight (600 kDa) and a degree of deacetylation of 0.9.
It was indicated that the preparation of the biocatalyst (preparation method and chitosan
characteristics) play a key role in the biocatalyst's properties [90].
8.5.3.1.2 Reversed Chitosan-Alginate Beads
Sankalia et al. explored, using response surface methodology, the main and interaction
effects of some process variables on the preparation of a reversed chitosan-alginate PEC
with entrapped α-amylase for stability improvement [91]. The beads were prepared by
dropping chitosan containing α-amylase into a sodium alginate solution without any salt.
Proper selection of the reaction pH, polymer concentration and hence charge density and
hardening time is important and determines the characteristics of the PECs.
8.5.3.1.3 Chitosan/Alginic Acid Complexation Network
PECs as proton-conducting biopolymer networks have potential use for biosensors. Yapar
et al. reported that cholesterol oxidase was immobilized in the conducting network via
complexation of chitosan with alginic acid. Chitosan was mixed with water containing
cholesterol oxidase, and alginic acid was mixed with 1% glacial acetic acid. Then the two
phases were put together to get the enzyme-entrapped polymer network. The complex
polymer electrolyte with x = 1 ( x is the number of moles of chitosan per mole of -COOH