As an inorganic network, sol–gel matrices have been used
for enzyme immobilization through entrapment formed by hydrolytic
polymerization of metal alkoxides or tetraethoxysilane
that was pioneered by Avnir et al., and a wide variety of enzymes
has been used for sol–gel immobilization [19, 25]. It should be
pointed out that silica sol–gelmorphology depends on the drying
methods [26]. Drying by evaporation affords so-called xerogels,in which capillary stress causes the shrinkage of the nano cages
and pores. By adding porous supports, such as Celite, to bind
the enzyme the sol–gel process can lead to enzyme-containing
gels. This “double immobilization” creates materials with higher
thermal stability and activity [27]. Reetz et al. used higher alkyl
groups in a RSi(OMe)3 precursor, and this second generation
sol–gel immobilization lead to high enzyme loadings, high activity,
and recyclable uses [27, 28]. In addition, additives such
as polyethylene glycol, polyvinyl alcohol, and albumin have a
stabilizing effect on sol–gel entrapped enzymes [29].