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

An analytical device which functions to analyse a sample for the presence of a specific compound is known as sensor. A sensor which utilizes biological material to specifically interact with an analyte is known as biosensor. An analyte refers to the compound which has to be ‘sensed’ or the presence of which has to be determined. The interaction of analyte and biosensor is measured and converted to signals, which are again amplified and displayed. A biosensor thus involves converting a chemical flow of information into electrical signals. The biological materials used in biosensors are mostly enzymes, antibodies, nucleic acids, lectins, a cell as a whole etc.

According to the mode of interaction biosensors are of two types:
Catalytic biosensor: The interaction of biological material in the biosensor and the analyte result in modification of analyte into new chemical molecule. The biological material used is mainly enzymes.
Affinity biosensor: Here, upon interaction, the analyte binds to the biomolecule on the biosensor. These are mainly composed of antibodies, nucleic acids etc.

Essential properties of a biosensor:
(i) Specificity: a biosensor should be specific to the analyte which it interact.
(ii) Durability: it should withstand repeated usage.
(iii) Independent nature: It should not be affected by variations in the environment like temperature, pH etc.
(iv) Stability in results: the results produced by interaction should be corresponding to the concentration of analyte.
(v) Ease of use and transport: it should be small in size so that it can be easily carried and used.

Components and mechanism of a biosensor:
A biosensor mainly consists of two parts
(i) a biological part: this constitutes of enzymes antibodies etc., which mainly interacts with the analyte particles and induce a physical change in these particles.
(ii) a transducer part: which collects information from the biological part, converts, amplifies and display them. In order to form a biosensor, the biological particles are immobilized on the transducer surface which acts as a point of contact between the transducer and analyte.
When a biosensor is used to analyse a sample, the biological part specific to the analyte molecules, interacts specifically and efficiently. This produces a physicochemical change of the transducer surface. This change is picked up by the transducer and gets converted into electric signals. These then undergo amplification, interpretation and finally display of these electric units accounting to the amount of analyte present in the sample.

Types of biosensors:
(i) Calorimetric biosensor: some enzyme- analyte reactions are exothermic and releases heat into the sample. This change in temperature is detected by the transducer. The amount of heat generated is proportional to the analyte concentration present and is processed likewise.
(ii) Potentiometric biosensor: an electric potential is produced as a result of interaction which is detected by the transducer
(iii) Amperometric biosensor: analyte when comes in contact with biological material induces a redox reaction. This results in movement of electrons which is picked up by transducer.
(iv) Optical biosensors: in this, a biosensor reacts with analyte to absorb or release light which is identified by the transducer and interpreted.
(v) Acoustic wave biosensors: biological component of biosensor undergoes a biomass change ascertained by transducer.
The advantages of biosensors include accuracy in results, minute detection capability, ease of use, versatile and continuous monitoring available.

A biosensor has a wide range of applications in different fields.
Medicinal Application: biosensors have been used in various diagnostic procedures to determine various tests.
Industrial application: various manufacturing processes can be monitored by biosensors to provide assistance with regard to increase the quality and quantity of product obtained.
Environmental application: it helps in measuring the toxicity of water bodies, microbial contamination of natural resources helping in developing steps towards a cleaner environment.
Military application: it helps to detect explosives, drugs etc., aiding in defence of the people. Another breakthrough in the field of biosensors was the production of a product called ‘smart skin’. It is a kind of biosensor which detects any chemical or biological attack nearby and warns the person using the same.
Drug development: a biosensor called ‘nano sensors’ has been developed which detects and analyse the binding of proteins to its targets which has proved very useful in drug designing. This also helps to monitor certain side effects caused by some medicines.
Biosensors are potential tool for scientific developments :
For new revolution in science, it will need such tools which will able to sense, analyze and predict the process. One such tool is biosensor. Biosensor are analytical devices which are used for detections of an analyze with the help of physiochemical detector. Biosensor can be any sensitive biological element like tissue, microorganisms,organelles,biomolecules,enzymes etc. Today many biological engineers create biologically sensitive elements. Such biosensor reader are associated with electronic processor that indicates or display the results in user friendly way.
Many of such biosensors are used worldwide; one of the most common out of these is blood glucose biosensor, which uses enzymes glucose oxidase to act on substrate glucose of blood. The reaction uses electrons and reduces the FAD to FADH2. Thus is finally oxidized by electrode. The current is then measured as concentration of glucose.
Also many biosensors are being used to detect lethal gases and thus warn the end user about its leakage. Such biosensors are used in mines and other places where ventilation is the issue. Many microorganism are capable of detecting the harmful chemicals and gases even though they are present in very less amount eg in ppm (part per million). These way biosensors are most important devices when safety is the concern. Other places were these sensors are used are water treatment facilities, trace gas detection and environmental monitoring.
Optical biosensors are one of the types of it which is based on the phenomenon of surface plasmon resonance. Such techniques produce electron waves on the gold surfaces and the specific angle and wavelength of incident light is dependent on the type and surface of gold.