Biomedical Engineering Reference
In-Depth Information
giant magnetoresistance biosensors—as well as other biosensors based on magnetic
transduction principles—are less common than biosensors based on electrochemi-
cal,s optical, or acoustic detection. We think this is because the magnetic principles
require more sophisticated readout electronics than the methods mentioned above.
On the other hand, magnetic labels are very attractive for use as a separation and
purification tool as there is usually no magnetic background in biological samples.
Depending on the type of magnetic label, they can still be used for detection after-
wards, e.g., using electrochemical or optical transduction principles [
29
,
59
].
2.1.7 Radioactive Transduction Principles
The first immunoassay developed by Yalow and Berson [
62
] in 1959 was a
radioimmunoassay (RIA) for detection of insulin. Radioisotopes were not only the
first labels used, for a long time they also offered one of the most sensitive
detection methods for immunoassays. As it is not possible to differentiate radio-
activity of bound radionuclides from that of free radionuclides, they must be
separated. Therefore, all RIAs follow heterogeneous test formats requiring an
immobilization step on a surface [
63
]. Hence, transduction principles exploiting
radioactivity should almost be predestined for use in biosensor measurements.
However, using radioisotopes is also accompanied by several disadvantages,
particularly regarding technical and safety issues [
63
]. This may be the main
reason why—to the best of our knowledge—radioactivity has not found its way
into transduction principles for biosensors.
2.2 Biorecognition Layers
2.2.1 Requirements
To turn a transducer into a biosensor, it has to be coated with a biorecognition
element. Examples are given in Table
1
. Enzymes [
64
], antibodies [
65
], oligo-
nucleotides [
66
], and, more recently, aptamers [
67
,
68
] are examples of biorec-
ognition elements commonly used for diagnostic applications. Biosensor coatings
consisting of whole cells (microbial biosensors) [
69
,
70
] or of molecularly
imprinted polymers [
71
] and other synthetic binders [
72
] as well as coatings using
lipid bilayers or liposomes for embedding, e.g., membrane proteins [
73
], are
currently only marginally used in this area and therefore will not be covered here.
The immobilization procedures for biorecognition elements depend mainly on
the underlying device material and hence on the chemical environment available
[
74
], but they are largely independent of the transduction mechanism. However,
considering transduction processes near the surface, such as electrochemical
principles based on electron transfer through electric double layers [
33
], evanes-
cent field techniques [
39
], or SAWs [
75
], the thickness of the complete sensing
