A biosensor is an analytic device which is used for the detection of an analyte (i.e. a chemical substance being measured in an analytical procedure) by using a biological material. For example glucose is the analyte in blood glucose testing.
The analyte combines a biological material with a physicochemical detector component. A biosensor consists of the following three parts.
i. Biological material:
It is a biologically derived material e.g. tissue, microorganisms, organelles, cell receptors, enzymes, antibodies, nucleic acids, etc. It is the sensitive part of the biosensor.
It lies in between both the components and associates them.
It works in a physicochemical way; optical, piezoelectric electrochemical, thermometric or magnetic.
Biosensors incorporate a biological component for highly selective molecular recognition. The blood glucose biosensor is the most widespread example of a commercial biosensor. It uses an enzyme to break down the blood glucose and transfers an electron to an electrode which is then converted into a measure of blood glucose concentration. It has a high demand in the market.
1. Principles of Detection :
The biological material is immobilized onto a permeable membrane which lies on the direct vicinity of a sensor. The substances to be measured pass through the membrane and interact to the immobilized biological material and yield the product.
The product may be electrons, gases, heat or ions. The product is converted into a signal by transducer which is then amplified. The electrical signals are recorded by the recorder. There are different types of biosensors which have been constructed as per needs.
The 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 a source of ions. The reaction of target analyte takes place on the surface of active electrode. The ions produced create a potential which is subtracted from that of the reference electrode. Consequently it gives a signal.
The optical biosensors are based on the phenomenon of electron waves (surface plasmons). This utilises a property of gold and other materials.
Specifically a thin layer of gold can absorb laser light on a high refractive index glass surface and produces an electron waves (surface plasmons) on the surface of gold. This occurs only at a specific angle and wavelength on incident light.
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.
Thermometric and magnetic based biosensors are rare.
2. Applications :
There are many potential applications 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:
1. Glucose monitoring in diabetes patients.
2. Other medical health related targets.
3. Environmental monitoring i.e. the detection of pesticides and river water contaminants.
4. Remote sensing of airborne bacteria e.g. in counter-bioterrorist activities.
5. Detection of pathogens present in edible materials.
6. Determining levels of toxic substances before and after bioremediation.
7. Detection and determination of organophospate.
An electrochemical version of enzyme-modified immunoassay technology (EMIT) has been demonstrated for potential detection of endocrine disruptors in the environment. Design of in vivo biosensors for neurotransmitters to facilitate study of Parkinson’s disease is also in progress. It is also used to detect cancer.
It is hoped that it could enable doctors to diagnose and monitor common types of cancer, and to assess which is the most appropriate therapy. Certainly the device would work by identifying cancer markers proteins or other molecules produced by cancer cells. These vary according to the type of cancer and are distinct from proteins produced by healthy cells.