Chemistry Reference
In-Depth Information
Table 4.3
Examples of a wide variety of lipase immobilisation techniques
Method
Support/Cross-Linking Agent
Ref.
Covalent binding
Glyoxyl agarose
[
7
,
26
,
50
]
Commercial Eupergit support beads
[
7
,
51
,
52
]
Green coconut fibres
[
25
]
Glass coated in polyethylene glycol
[
53
]
Ionic binding
PEI coated agarose
[
41
,
58
]
Physical adsorption
Celite
[
41
,
59
]
Polystyrene resins
[
37
,
42
]
Octyl-agarose
[
16
]
Silica mesoporous nanoparticles
[
17
,
18
,
60
]
Fumed silica
[
19
]
Cross-linking
Glutaraldehyde
[
20
,
64
]
Entrapment
Sol-gel
Tetraethoxysilane (TEOS), tetramethoxysilane
(TMOS),
propyltrimethoxysilane (PTMS)
[
21
,
66
,
71
-
73
]
Agarose and poly (N-isopropylacrylamide)
hydrogels
[
24
]
Siliceous mesocellular foam
[
22
]
Disadvantages of this system include a reliance on a porous network, which can
result in reduced mass transfer, since it is often difficult for large substrate mol-
ecules to gain access to the enzyme due to complex, tortuous networks. This can
greatly restrict the performance of the system [
42
].
The sol-gel route can be used to tune porosities of silica gels and thus allows
mass transfer to be controlled. Sol-gel routes also occur in fairly benign condi-
tions, typically at less than 100 ºC [
70
]. However, sol-gel chemistry when used for
enzyme immobilisation suffers from non-biocompatible precursors, production of
alcohols and gel shrinkage—all causing enzyme deactivation.
In order to render sol-gel synthesis biocompatibility, several modifications to the
process were successfully demonstrated. The formation of alcohol was avoided by
one such modification on the precursor—the synthesis of biocompatible glycerated
silanes or the use of water-soluble silicates.[
68
] The second improvement was re-
alised when stabilising agents such as glycerol, carbohydrates or amino acids were
added to the sol-gel process. A third modification was the introduction of multistep
procedures wherein a pre-formed silica sol would be added to a buffered solution of
enzyme in order to avoid any detrimental effects on the enzyme caused by pH drifts.
Despite these improvements, sol-gel methods still suffer from several issues in-
cluding gelation times of several hours to weeks; the requirement of high silicate
concentrations; tedious multistep protocols; and the requirement of custom-synthe-
sised biocompatible precursors [
69
].
A recent approach involved impregnating a resin support with sol-gel precursor
and enzyme (lipase). After gelation, the resultant immobilised enzyme displayed
