Biosensors based on immobilization of biomolecules in sol-gel matrices - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 507

hydrolysis and condensation of the alkoxide precursors. Alcohol can have a detri-
mental effect on the activity of the biomolecules. Gill and Ballesteros [16] introduced
poly(glyceryl silicate) (PGS) sol-gel precursors. The PGS can be rapidly hydrolyzed
and gelled in aqueous, buffered milieu without the need for any catalyst, to form sil-
ica hydrogels, which produced transparent, mesoporous, and physically stable silica
xerogels after aging, washing to remove glycerol, and drying. These sol-gel materials
showed good porosity, less shrinkage and high percentage of bioencapsulation, and the
entrapped biomolecules were retained almost complete activity (98%). Later Liu and
Chen [29] reported an alcohol-free aqueous colloidal sol-gel process and encapsulated
cytochrome c , catalase, myoglobin, and hemoglobin with good retained activities. In
another alcohol-free sol-gel approach, sodium silicate was employed as a starting pre-
cursor, in which proteins showed preserved activity [30]. Recently Moraxella spp. cells
engineered to express recombinant organophosphorus hydrolase (OPH) were encapsu-
lated by the sodium silicate method and displayed higher activity retention compared
to those by the traditional alkoxide process [31]. Perhaps in the sodium silicate route
the precursors release a high Na concentration, which must be eliminated through an
acidic cation-exchange resin. Ferrer et al. [32] reported another alcohol-free and sim-
ple aqueous sol-gel method. In this approach the alcohol formed during the hydrolysis
was removed through the rotavapor method. The horseradish peroxidase (HRP) immo-
bilized in this alcohol-free route exhibited completely preserved activity and showed
higher specifi c activity compared with the regular sol-gel method.
Some non-silica sol-gel materials have also been developed to immobilize bioac-
tive molecules for the construction of biosensors and to synthesize new catalysts for
the functional devices. Liu et al. [33] proved that alumina sol-gel was a suitable matrix
to improve the immobilization of tyrosinase for detection of trace phenols. Titania is
another kind of non-silica material easily obtained from the sol-gel process [34, 35].
Luckarift et al. [36] introduced a new method for enzyme immobilization in a bio-
mimetic silica support. In this biosilicifi cation process precipitation was catalyzed by
the R5 peptide, the repeat unit of the silaffi n, which was identifi ed from the diatom
Cylindrotheca fusiformis . During the enzyme immobilization in biosilicifi cation the
reaction mixture consisted of silicic acid (hydrolyzed tetramethyl orthosilicate) and R5
peptide and enzyme. In the process of precipitation the reaction enzyme was entrapped
and nm-sized biosilica-immobilized spheres were formed. Carturan et al. [11] devel-
oped a biosil method for the encapsulation of plant and animal cells.
16.2.3 Advantages and disadvantages
Different sol-gel matrices such as inorganic, organically modifi ed (ormosils), hybrid
sol-gels and interpenetrating polymer networks have been used for encapsulation.
Perhaps each type of sol-gel has its own advantages and disadvantages [1]. Inorganic
sol-gels are good in optical transparency; chemical robustness but brittleness and low
porosity in xerogels are major limitations. Similarly organically modifi ed sol-gels have
good tunable porosity and electrochemical activities, but are relatively fragile and
have limited optical transparency [8, 9]. Hybrid sol-gels can be prepared with fl exible