A biosensor is an analytical device which converts a biological response into an electrical signal and helps in understanding biological samples and their biocomposition, structure, and function. The biosensor is composed of a biological recognition element directly interfaced to a signal transducer which together relates the concentration of an analyte or a group of related analytes to a measurable response.
A biosensor has two distinct types of components
• Biological e.g. enzyme, antibody.
• Physical e.g. transducer, amplifier etc.
Characteristics of Ideal Biosensors
• High selectivity and sensitivity
• Low noise or background signal
• Higher stability up to longer time
• High biocompatibility especially when in direct contact with biological fluids like blood
• Quick response to change in analyte concentration
• Simple calibration
Types of Biosensors
Biosensors can be divided into different categories depending upon biological recognition element and transducer type.
Biological recognition elements
1. Antibodies are the substances which are produced by B-lymphocytes in response to foreign pathogens. Usually larger molecules having size more than 10kDa generates an immune response. Smaller molecules which are called as haptens have to conjugate to some other molecule called as adjuvant to produce an immune response. These antibodies play a very important role in immunoassays. These are usually immobilized on the surface of the transducer by covalent bonding of their amino, carboxyl, aldehyde and sulfhydryl groups. Antibodies are usually immobilized on the surface of the transducer by covalent attachment by conjugation of amino, carboxyl, aldehyde, or sulfhydryl groups. The sensors based on antibodies are called as immunosensors and these sensors usually employ optical or acoustic transducers.
2. Enzyme plays a very important role in every reaction. They act as catalysts and have high selectivity. The enzymes can be used to assay concentration of a variety of analytes in different reactions. These have some drawbacks associated with them in-spite of having so many advantages as they get easily destroyed with a change in factors like ionic strength, temperature, pH, chemical inhibitors etc. Enzymes are immobilized mostly on the surface of electrochemical and fiber- optic transducers by covalent attachment, adsorption, and entrapment.
3. Microbial cells have advantages over enzymes and antibodies as they are much cheaper and more stable than them. Microbial cells undergo metabolic changes and these changes are measured. Micro-organisms are immobilized on transducers and the material mostly used are nylon nets, cellulose nitrate membranes or acetyl cellulose.
Transducers : Electrochemical transducers
Electrochemical transducers include amperometric and potentiometric transducers.
Glucose biosensors It is one of the commercially available examples of amperometric biosensors. In the experiment performed by the scientist Leland C. Clark enzyme glucose oxidase was entrapped in the surface of the platinum electrode and glucose concentration in the surrounding medium is estimated or measured. If glucose is present in surrounding medium; it will react with enzyme glucose oxidase to form gluconic acid and release of two electrons and two protons take place leading to the reduction of glucose oxidase. The glucose reacts with surrounding oxygen and water to form hydrogen peroxide and oxidized glucose oxidase, this glucose oxidase can again react with glucose. Thus higher the concentration of glucose more the oxygen will be consumed and lower concentration of glucose leads to the formation of hydrogen peroxide. This consumption of oxygen and hydrogen peroxide is measured by platinum electrodes and serves as the estimation of glucose concentration.
The potentiometric transducers are based on the principle of measuring potential difference across electrochemical cells with a very low current. In other words, potentiometric biosensors are based on detecting the change in the distribution of charge using ion-selective electrodes, such as pH-meters.
These types of transducers measure the heat so produced by biochemical reaction at the biological sensing element. There are various types of calorimetric transducers such as isothermal calorimeters, heat conduction calorimeters (measures temperature difference reaction vessel and sink) and isoperibol calorimeter.
In these types of transducers, fiber optic probes are used with immobilized fluorescent dyes and enzymes on its tip. These transducers determine changes in light absorption between the reactants and products of a reaction or can measure the light output by a fluorescence process. The fibre optic probes mainly consist of two fibers one connected to a light source and other connected to photodiode which detects the change in optical density. Surface Plasmon Resonance Transducers which can detect even minute change in the refractive index at or near the surface is one of the best examples of optical fibre transducer and this is explained under the topic Surface Plasmon Resonance elaborately.
Applications of Biosensors
• Biosensors can be widely used in the diagnosis of diseases with more precise results. For example, glucose oxidase biosensors are used by diabetic patients for monitoring glucose levels in blood.
• Biosensors can play a very important role in the analysis of food. The biosensors for measuring alcohols, carbohydrates, and acids are commercially available in the market which can be used as tools for quality assurance.
• Immunosensors having antibody as a sensing element can be used in food industry for detecting pathogenic organisms in meat, fish, and poultry.
• Biosensors are widely used in environmental testing by using a flow-through system to monitor wastewater.
• In the field of medicine, biosensors contribute in the manufacturing of pharmaceuticals and replacement organs such as artificial pancreas for diabetic patients.
• In industries, biosensors can be used for quality control and industrial process control.
• Glucose biosensor estimates blood glucose level by sensing the concentration of oxygen.