Fig 2: Schematic of a Biosensor
A biosensor is a sensor that is based on the use of biological material for its sensing function. The biocomponent specifically reacts or interacts with the analyte of interest resulting in a detectable chemical or physical change.
A schematic of a biosensor can be seen on the following figure:
The receptor is responsible for the selectivity of the sensor.
The detector is not selective at all. It plays the role of a
transducer.
The fundamental advantage of biosensors over nearly all other sensor devices is their
high selectivity and sensitivity. Depending on the actual design, different
receptor-detector combinations are possible. For example an enzyme receptor can be combined with an oxygen
electrode, a hydrogenperoxide sensor, a thermistor,... See the example of the Glucose sensor
on the next page.
Basically, biosensors can be diveded into to main types:
The following paragraphs will focus on the enzyme and cell based biosensors. For both these
types the Michaelis-Menten model of enzyme kinetics can be applied. The
important role of the enzymes manifests itself in the fact that biosensors were
first called "enzyme electrodes".
Biosensors which employ enzymes as recognition molecules, require that the enzymes are
immobilized by membranes. Depending on the electrode type, there are either two membranes (as
shown on the adjacent figure) or only one membrane to separate the enzyme layer
and the medium. With this additional barriers not only the enzyme
kinetics, but also diffusion effects have to be taken into account.
Other effects that complicate the application of enzyme sensors are inhibition,
temperature and pH dependance. All of which can strongly influence the sensor
signal.
As indicated on the figure, sensors influence the medium due to their
consumption of substrate or substance to be detected. With the exception of
very small fermenters this influence can be neglected.
Fig 5: Biosensor composition
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