Biosensors for Diagnostic Applications - Molecular Diagnostics - page 119

description of the respective measurement setups, including detection limits and
commercialization status of available devices, the reader may refer to the cited
references.
2.1.2 Electrochemical Transduction Principles
Electrochemical biosensors are typically supplied as electrode setups. They are
generally divided into three major categories, depending on the underlying mea-
surable parameter. These are current for amperometric biosensors, potential or
charge accumulation for potentiometric biosensors, and conductive or resistive
properties for conductometric or impedimetric biosensors, where both of the latter
are usually treated as one category [ 28 , 30 ].
Amperometric biosensors are typically used to detect small molecules by means
of an enzyme, e.g., a peroxidase, catalyzing a redox reaction [ 31 ]. The ampero-
metric biosensor for the detection of glucose by means of glucose oxidase was not
only the first biosensor principle introduced, it is still, after several improvements,
the most widespread biosensor [ 28 ]. The principle of potentiometric measurements
is familiar to most from the commonly used pH electrode. Similar to amperometric
biosensors, potentiometric biosensors are best suited to detect small molecules
[ 32 ]. Detectors for potentiometric biosensors include the family of field-effect
transistors, which have been gaining interest in recent years owing to newly
developed nanomaterials, such as carbon or silicon nanotubes and nanowires. They
allow label-free detection, i.e., do not require labeling with enzymes or nanopar-
ticles to get a signal response, which reduces the expense [ 28 ]. Impedimetric
biosensors are the youngest among the electrochemical biosensors and also offer
label-free detection of biomolecules. They allow the investigation of a large
variety of analytes, ranging from small molecules to proteins and even cells [ 33 ].
Ongoing research in the field of electrochemical biosensors is mainly focused on
new materials, e.g., graphene [ 34 ] and diamond [ 35 ], as well as on miniaturization
and parallelization of the devices, e.g., by screen-printed electrodes [ 28 , 36 ].
2.1.3 Optical Transduction Principles
Optical biosensor detectors can generally be divided into labeled and label-free.
The labels are, e.g., fluorophores or nanostructured materials such as nanoparticles,
quantum dots, or carbon nanotubes [ 37 ]. Detectors using labels typically use the
evanescent field outside a waveguide resulting from total reflectance within the
waveguide. This evanescent field can excite, for instance, fluorescent or chemi-
luminescent labels outside the waveguide [ 38 ]. An example of this is the first
optical biosensor which was based on TIRF [ 22 ]. Chemiluminescence can also
be induced electrochemically; it is then called electrochemiluminescence [ 37 ].
With labels, small molecules as well as large molecules, such as proteins, can be
detected.