Biosensors based on immobilization of biomolecules in sol-gel matrices - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 504

16.1 INTRODUCTION
In recent years a lot of effort and research have been carried out towards the devel-
opment of biosensors for environmental and biomedical monitoring. A biosensor is an
analytical device composed of a biological sensing element (enzyme, antibody, DNA)
in intimate contact with a physical transducer (optical, mass or electrochemical), which
together relates the concentration of an analyte to a measurable electrical signal [1-3].
The stability of biomolecules and signal transfer to transducer surface are crucial fac-
tors in the stability and sensitivity of biosensors. In aqueous solutions, biomolecules
such as enzymes lose their catalytic activity rather rapidly, because enzymes can suffer
oxidation reactions or their tertiary structure can be destroyed at the air-water interface,
hence making the use of enzymes and reagents both expensive and complex [4]. This
problem can be resolved by immobilization of biomolecules. By attachment to an inert
support material, bioactive molecules may be rendered, retaining catalytic activity and
thereby extending their useful life [5, 6]. In view of the above, a variety of techniques
have been developed to immobilize biomolecules, including adsorption, covalent attach-
ment, and entrapment in various polymers.
In general the immobilization method should have the following characteristics: it
should be simple and fast, inert, biocompatible, have high retention capacity, control-
lable porosity and the fi lm should be stable at different environmental and experimental
conditions such as pH and temperature for the development of biosensors. Sol-gel fi lms
have most of the above-mentioned properties; hence they have been widely employed in
biosensor design in recent years. The sol-gel process can be carried out at low tempera-
ture, be chemically inert, have tunable porosity, optical transparency, mechanical sta-
bility, and negligible swelling behavior [7-11]. Several studies have been carried out to
examine the properties of the porous sol-gel matrix, such as pore size distribution, sur-
face area, pore geometry, morphology, and polarity [12-14]. Using the sol-gel technique
different biorecognition agents such as enzymes [15, 16], antibodies [17], and whole cells
[18, 19] have been successfully immobilized and employed in multifarious applications,
e.g. biosensors [1, 3], solid phase extraction sorbents [20, 21], etc. In recent years sev-
eral reviews on sol-gel technology have appeared in specifi c applied areas [3, 7, 8, 16,
22-24]. The present chapter highlights the advantages, recent developments, biosensing
applications, and future perspectives of sol-gel entrapped biomolecules.
16.2 SOL-GEL
16.2.1 Sol-gel chemistry and matrix characteristics
The sol-gel process involves hydrolysis of alkoxide precursors under acidic or basic
conditions, followed by condensation and polycondensation of the hydroxylated units,
which lead to the formation of porous gel. Typically a low molecular weight metal
alkoxide precursor molecule such as tetramethoxy silane (TMOS) or tetra ethoxysilane
(TEOS) is hydrolyzed fi rst in the presence of water, acid catalyst, and mutual solvent