Biosensor

A biosensor is a device for the detection of an analyte that combines a biological component with a physicochemical detector component.

It consists of 3 parts:

• the sensitive biological element (biological material (eg. tissue, microorganisms, organelles, cell receptors, enzymes, antibodies, nucleic acids etc), a biologically derived material or biomimic) The sensitive elements can be created by biological engineering.
• the transducer in between (associates both components)
• the detector element (works in a physicochemical way; optical, piezoelectric electrochemical, thermometric, or magnetic.)

The most widespread example of a commercial biosensor is the blood glucose biosensor, which uses an enzyme to break blood glucose down. In doing so it transfers an electron to an electrode and this is converted into a measure of blood glucose concentration. The high market demand for such sensors has fueled development of associated sensor technologies.

Recently, arrays of many different detector molecules have been applied in so called electronic nose devices, where the pattern of response from the detectors is used to fingerprint a substance.

A canary in a cage, as used by miners to warn of gas could be considered a biosensor. Many of today's biosensor applications are similar, in that they use organisms which respond to toxic substances at a much lower level than us to warn us of their presence. Such devices can be used both in environmental monitoring and in water treatment facilities.

Principles of Detection

Piezoelectric biosensors and optical biosensors based on the phenomenon of surface plasmon resonance are both evanescent wave techniques. This utilises a property shown of gold and other materials; specifically that a thin layer of gold on a high refractive index glass surface can absorb laser light, producing electron waves (surface plasmons) on the gold surface. This occurs only at a specific angle and wavelength of incident light and is highly dependent on the surface of the gold, such that binding of a target analyte to a receptor on the gold surface produces a measurable signal.

Other optical biosensors are mainly based on changes in absorbance or fluorescence of an appropriate indicator compound.

Piezoelectric sensors utilise crystals which undergo a phase transformation when an electrical current is applied to them. An alternating current (A.C.) produces a standing wave in the crystal at a characteristic frequency. This frequency is highly dependent on the surface properties of the crystal, such that if a crystal is coated with a biological recognition element the binding of a (large) target analyte to a receptor will produce a change in the resonant frequency, which gives a binding signal.

Electrochemical biosensors are normally based on enzymatic catalysis of a reaction that produces ions. The sensor substrate contains three electrodes, a reference electrode, an active electrode and a sink electrode. A counter electrode may also be present as an ion source. The target analyte is involved in the reaction that takes place on the active electrode surface, and the ions produced create a potential which is subtracted from that of the reference electrode to give a signal.

Thermometric and magnetic based biosensors are rare.

Applications

There are many potential application of biosensors of various types. The main requirements for a biosensor approach to be valuable in terms of research and commercial applications are the identification of a target molecule, availability of a suitable biological recognition element, and the potential for disposable portable detection systems to be preferred to sensitive laboratory-based techniques in some situations. Some examples are given below:

• Glucose monitoring in diabetes patients <-- historical market driver
• Other medical health related targets
• Environmental applications e.g. the detection of Pesticides and river water contaminants
• Remote sensing of airborne bacteria e.g. in counter-bioterrorist activities
• Detection of pathogens
• Determining levels of toxic substances before and after bioremediation
• Detection and determining of organophospate

External links

• Biosensor research at Diepenbeek Belgium.
• Physiological Sensing Facility at Purdue University
• Label-free biosensor system and sensor chips
• Paisly University