Biosensors for Diagnostic Applications - Molecular Diagnostics - page 118

Table 1 Classification of biosensors
Biosensors by transduction principle Biosensors by biorecognition element

? alternative biosensor term a (if available)

Electrochemical Enzyme ? enzyme sensor

Optical Antibody ? immunosensor

Acoustic or gravimetric Aptamer ? aptasensor

Thermal or calorimetric Oligonucleotide ? DNA sensor, genosensor

Magnetic Cell ? whole (microbial) cell biosensor

Molecularly imprinted polymer

a

Instead of (biorecognition element) biosensor

biosensors are sometimes used to describe the manufacturing method and therefore
may include several types of biosensors, regardless of the transduction or bio-
recognition principle [ 17 ].

2.1 Biosensor Transduction Principles

2.1.1 Overview

According to the categories typically used, the ''enzyme electrode'' introduced by
Clark and Lyons [ 18 ] in 1962 was an amperometric biosensor. This milestone in
biosensor development was followed by other electrochemical biosensors, but it
was not until 10 years later that biosensors based on other transduction principles
were reported. The first gravimetric biosensor was a quartz crystal microbalance
(QCM)-based immunosensor for protein detection; it was reported in 1972 [ 19 ].
This was followed by the first thermal biosensor for enzyme-substrate studies,
called an ''enzyme thermistor'', in 1974 [ 20 ]. The first optical biosensor based on
total internal reflectance fluorescence (TIRF) used fluorescein-labeled antibodies
for detection of small molecules (haptens), and was also reported at the beginning
of the 1970s [ 21 , 22 ]. The first fiber-optic-based biosensor—which was dedicated
to glucose detection just like the first electrochemical biosensor—was reported not
until a decade later [ 23 ]. The first biosensors exploiting magnetism as a trans-
duction principle for a variety of analytes were introduced in the 1990s [ 24 - 26 ].
Biosensor development has been driven by analytical demands, but it was the
technological progress in related fields, such as communication and optics, as well
as in manufacturing, which enabled the state of development of the current devices
[ 27 ]. Electrochemical biosensors, for instance, are now comparatively easy to
miniaturize, which is one of the reasons for their widespread availability [ 28 ]. In
fact, detectors for biosensors used today depend mainly on electrochemical
transduction, followed by optical and acoustic effects [ 4 ]. Thermal transduction is
less frequently used, as are magnetic effects. However, the latter are increasingly
employed as a separation tool in bioanalytical assays [ 29 ]. The following sections
will outline current transduction principles to give the reader an idea of potential
methods which would allow particular diagnostic applications. For a detailed