There was a time, when it was thought that biosensors would revolutionize the methods of analysis; that they could sense 'anything, anywhere, any time'! This vision proved to be a bit exaggerated. Nevertheless biosensor were and are still very promising.
So, what is 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:
Fig 2: Schematic of a Biosensor
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:
Fig 3: Main Types of Biosensors
Most biosensor rely on enzymes for the recognition of a certain substance. There are a few exceptions. For example one's which use optical or piezoelectric effects.
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 that 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.
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