Biomedical Engineering Reference
In-Depth Information
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Figure 1.3
Entrapment approach to attach a protein. Biomolecule is bound to a
monomer prior to polymerization.
leakage and/or denaturation. An additional consideration is the necessity for
the target to diffuse into the polymer matrix in order for the biochemical
interaction to take place. This process may result in a slow kinetic response in
terms of transduction. Despite these problems the strategy has been employed
widely in surface plasmon resonance (see later).
An analogous procedure to the one described above is the use of sol-gel
technology, which not only serves to immobilize a biomolecule on a surface but
also to potentially stabilize it with regard to its tertiary structure.
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This tech-
nology has been exploited for many years with respect to applications in surface
coating, production of nanomaterials and optomechanical structures.
A solution of a monomer such as an alkoxide (the sol) is induced to polymerize
into a biphasic configuration (the gel) which incorporates both liquid and solid.
A typical monomer among many is tetraethylorthosilicate [TEOS, Si(OC
2
H
5
)
4
],
which is readily hydrolyzed by water to produce a siloxane bond-based
polymeric structure with a gel consistency. When it comes to placement of the
biomolecule, usually a protein, into the gel the term encapsulation is often used
rather than entrapment. A typical approach is shown in Figure 1.3. Particularly
interesting in terms of this technology is the potential of the process to stabilize
the tertiary structure and conformation of the biomolecule, although it remains
to be seen if this result applies to a large number of protein molecules.
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1.3.3 Covalent binding
With regard to the two-dimensional attachment of biomolecules to transducer
surfaces in a planar format it is certainly the case that attachment via covalent
bonds has been by far the most used approach. A very wide variety of
chemistries have been employed with modest success in terms of the criteria
outlined above.
21,22
Many functional groups, whether directly present on the
device substrate or obtained by modification, have been utilized to form a
probe partial monolayer. Examples of these are given in Table 1.1. Such groups
are also available on biomolecules to instigate the surface link and examples of
these for proteins are presented in Table 1.2. Oligonucleotides are very often
attached to the surface through functionalized (-OH, -NH
2,
-COOH, -SH,
etc.) short alkyl chains bound to the 3
0
or 5
0
terminus of the nucleic acid chain.
Single-strand DNA and RNA can be attached via a similar strategy, but the
