Biomedical Engineering Reference
In-Depth Information
transducer. One example is the detection of TNT by an aptamer within a fiber-optic
biosensor. Because of the selectivity of the aptamer it was possible to discriminate
TNT from other explosives [
7
].
The second is the concept of molecularly imprinted polymers (MIP). Here, a
polymerization is carried out in the presence of the analyte which is also the
template during the imprinting process. Within the polymerization mixture
monomers, so-called functional monomers, are also used which can specifically
interact with the template molecule by covalent or noncovalent means. After
polymerization the template is extracted leaving an artificial binding site in which
the analyte may (re-)bind. First adaptations of the MIP concept to biosensors can
be traced back to the work of Mosbach [
14
]. This concept is also a generic
approach and may be used for a great variety of different analytes. Using the
noncovalent approach it was, for example, possible to obtain a binding polymer
against nitrofurantion, an antibiotic frequently used in farming in former times,
however nowadays prohibited due to toxic side effects. With these polymers it was
possible to detect nitrofurantion directly from bird seed avoiding tedious mass
analytical measurements [
2
]. One prominent example of a covalently imprinted
polymer is the use of boronic acids as functional monomers for the detection of
saccharides such as glucose, fructose, or saccharide derivatives such as fructosyl-
valine [
18
,
20
]. Since the binding event is also not linked to a direct detection in
many cases the transducer chosen for molecularly imprinted polymers is either
based on a mass change measured by quartz crystal microbalance (QCM) or
cantilevers or based on the measurement of the latent heat of the binding using
calorimetry. For a fructosyl-valine imprinted polymer it could be shown that the
thermometric response of the binding event is about forty times higher compared
to a control polymer without imprinted cavities [
18
].
Not only improvements on the recognition site are responsible for better
biosensors. As already mentioned also improvements in the design and production
of transducers may lead to great advancements in how biosensors will perform in
various applications. Miniaturization of transducers is beneficial for cost reduction
as well as user-friendliness. Because of the still ongoing race in miniaturizing
electronics, especially electrochemical- and MEMS-based sensors may be min-
iaturized. A limitation of this trend will be, when problems arise from the small
surface area with small possible surface loadings of enzymes leading to small
signal amplitudes. In this regard different amplification methods have to be applied
to gain signals with a high signal-to-noise ratio. Besides microelectronic devices
also the manufacturing of micromechanical devices finds its way into the research
field of biosensors. To give an example, the fabrication of a microcantilever
enables the measurement of mass changes or changes in viscosity. In a biosensor
for glucose detection Birkholz et al. used resonating microcantilevers to measure
the change in viscosity of a hydrogel in which glucose was bound [
5
].
To summarize, improvements in recognition elements as well as in transducers
are responsible for miniaturization and integration of biosensors and biochips.
With smaller devices and more specific and direct biochemical reactions there is
the potential for many more applications. Because of the specificity of the
