Chemistry Reference
In-Depth Information
that are buried deep inside the matrix material. However, silica-based carriers have
widely been used for enzyme immobilization by adsorption, entrapment or cova-
lent attachment due to their inertness and stability under a wide variety of reaction
conditions [
17
].
Several authors [
11
,
18
] have reported data on the deposition of (3-aminopropyl)
triethoxysilane (3-APS or
γ
-APS) on the surface of silica particles under various
conditions. These include solvent, heat, and time, and then exposed to different cur-
ing environments, including air, heat, and ethanol. The authors found that 3-APS
monolayers were formed under very mild reaction and curing conditions (reaction
in dry toluene for 15 min at room temperature, curing in air, or 15 min in 200 °C
oven), whereas thick layers were formed when reaction and curing time was in-
creased. The 3-APS initially adsorbed onto the surface, and curing was found to
be necessary to complete the covalent bond formation between the 3-APS and the
hydroxyl groups present on the silica surface. Deposition of 3-APS from aqueous
solution gave thin layers, due to water molecules being electrostatically bound to
the silica. About 0.4 % APS in solution was the maximum concentration which still
allowed monolayer coverage without physisorption. The monolayer films were de-
posited from dry organic solvent, cured under mild conditions, and incubated in
water before use. However, thick films were deposited from an aqueous medium,
cured in an oven or exposed to an open atmosphere for several days before use.
He et al. [
19
] reported that porcine pancreas lipase (PPL) immobilized on porous
silica beads successfully catalyzed the ring-opening polymerization of ethylene iso-
butyl phosphate (EIBP). The porous silica beads were treated with methanesulfonic
acid and then silanization using 3-APS. The lipase was covalently immobilized onto
these functional porous silica beads by crosslinking with glutaraldehyde. It was
found that the recovered immobilized lipase was more active for the polymerization
of EIBP than for the first use. The number average molecular weight, M
n
was found
to be significantly increased.
In another study, Hwang et al. [
20
] reported that surface-modified silica gels
were used as a carrier material for enzyme immobilization. Hydrophilic and hy-
drophobic silica gels were made by polyethyleneimine coating and silanization,
respectively. Covalently bound lipase was found to be more stable than the lipase
immobilized by physical adsorption. The surface chemistry of carriers was found
to play a significant role on the stability of immobilized lipase, presumably through
molecular interactions with lipase leading to structural effects in the enzyme.
Dragoi and Dumitriu [
21
] have reported that lipase B from
Candida antarc-
tica
(CALB) was physically and chemically immobilized on mesoporous silica
through functionalization and hydrophobization with 3-APS, chlorotrimethylsilane
(CTMS), and propyltrimethoxysilane (PTMS). Physically immobilized CALB on
the hydrophobized supports exhibited higher activities in comparison to covalently
immobilized supports. However, the stability of the enzymatic preparations was im-
proved by covalent immobilization and the biocatalyst exhibited significant activity
after the third cycle of the alcoholysis of ethyl acetate with 1-hexanol and 1-butanol,
respectively. Recently, an alternative approach to enzyme immobilization has been
introduced, which relies on the implementation of mechanisms involved in biosilica
