Biosensors Types and it's Application in Pharmaceutical industries

Introduction

An analytical device used to change biological response into an electrical signal is called a biosensor. The term biosensor refers to sensor devices used for determining the concentration of substances and other biological parameters even where the biological system is not directly involved.
Biosensors use a transducer to couple a biological sensing element with a detector. The first scientifically planned and successfully commercialised biosensors were the electrochemical sensors useful for multiple analytes.

Schematic Diagram Showing Main Components of a Biosensors 

The biosensor contains a biological sensing element (e.g., tissues, microorganisms, organelles, cell receptors, enzymes, antibodies, nucleic acids, natural products, etc.), a material obtained biologically (e.g., recombinant antibodies, engineered proteins, aptamers, etc.) or agents that mimic biological system (e.g., synthetic receptors, biomimetic catalysts, combinatorial ligands, imprinted polymers, etc.) either closely associated to or integrated in a transducer.

Principle

The preferred biological material is generally a specific enzyme that is immobilised using conventional methods (e.g., physical or membrane entrapment, non- covalent or covalent binding) and brought in close contact with the transducer.
The analyte on binding to the biological material forms a bound analyte that produces a measurable electronic response. Sometimes due to the release of heat, gas (oxygen), electrons or hydrogen ions, the analyte converts into a product; and changes associated to this product is transformed by the transducer to electrical signals that are amplified and measured.

Working

The electrical signal coming from the transducer is low and superimposed by high and noisy baseline (could be due to high frequency signal component of random nature, or electrical interference generated in transducer electronic components). A baseline signal derived from a similar transducer without any biocatalyst membrane is called a reference baseline signal.
In signal processing, this reference baseline signal is subtracted from the sample signal; the signal difference obtained is amplified and the unwanted noise signals are electronically filtered (i.e. smoothened). The biosensor response is slow and eases the electrical noise filtration. The analogue signal produced directly is the output; however, it is converted to a digital signal, passed to a microprocessor for processing and manipulating the data to desired units, and then the output is displayed or stored.

Types

A biosensor is of the following different types based on the type of sensor devices and the biological materials:

1. Electrochemical Biosensor : It is a simple device used to measure electronic current, ionic or conductance changes carried by bio-electrodes.
2. Amperometric Biosensor : It determines the movement of electrons or electronic current due to a redox reaction catalysed by enzyme. Usually, a normal contact voltage moves along the electrodes to be analysed. In the enzyme-catalysed reaction, the substrate or product obtained can transfer the electrons with the surface of electrodes to be reduced; hence, an alternate current flow is measurable.
3. Blood Glucose Biosensor : It is employed extensively for diabetic patients. It contains a watch pen-shaped disposable electrode for single use. This electrode has glucose oxide and derivatives of a mediator (Ferrocene). The electrodes are converted using hydrophilic mesh.
4. Potentiometric Biosensor : It measures the changes in the concentration of ionic species with the help of ion-selective electrodes present in it. It generally employs pH electrodes, thus in the release of hydrogen ions a large amount of enzymatic reactions are involved.
5. Conductometric Biosensor : Many reactions occurring in the biological system bring about a change in the ionic species. This change is helpful in measuring the electronic conductivity. Urea biosensor which utilises the immobilised areas is an example of conductometric biosensor.
6. Thermometric Biosensor : Several biological reactions involve production of heat and form the basis of thermometric biosensors. The diagram representing a thermal biosensor consists of a heat insulated box fitted with a heat exchanger.
7. Optical Biosensor : It works on the principle of optical measurements, like fluorescence, absorbance, etc., and is utilised in fibre optics and optoelectronic transducers. Optical biosensor can even be safely used for non-electrical remote sensing of materials. It is involved in enzymes and antibodies in the transducer elements. This biosensor generally does not require any reference sensor, and sampling sensor is used for generating comparative signals.
8. Fibre Optic Lactate Biosensor : It measures the change in oxygen concentration at molecular level by identifying the effects of oxygen in fluorescent dye.
9. Optical Biosensor for Blood Glucose : In diabetic patients, the blood glucose level is important to be monitored. It is based on a simple technique in which paper strips saturated with reagents, like glucose oxide, Horseradish Peroxidase and a chromogen are used. The intensity of the dye colour is measured using a portable reflectance meter. The calorimetric test strips of cellulose layered with suitable enzymes and reagents are also widely used for testing blood and urine parameters.
10. Piezoelectric Biosensor : It is also called acoustic biosensor as its principle relies on sound vibrations. It contains piezoelectric crystals and the characteristic frequencies vibrate with the positively and negatively charged crystals. With the help of electronic devices, certain molecules on the crystal surface can be measured. The response frequencies can be changed by using these crystals with attached inhibitors. For example, the biosensor for cocaine (in the gas phase) works by attaching the cocaine antibodies on crystal surface.

Applications in Pharmaceutical Industries

Biosensors are made up of a biological element and a physiochemical detector used for detecting analytes. These devices have a wide range of applications in fields ranging from clinical to environmental to agricultural and to food industries. Given below are some of the fields in which biosensor technology is used:

1. General healthcare monitoring,
2. Screening of diseases,
3. Clinical analysis and diagnosis of diseases,
4. Veterinary and agricultural applications,
5. Industrial-processing and monitoring, and
6. Environmental pollution control.

 Biosensors can be used for quantitative determination of numerous biologically important substances in body fluids, e.g., glucose, cholesterol, urea. Glucose biosensor is widely used for regular monitoring of blood glucose in diabetic patients.

Also biosensors are used for blood gas monitoring for pH, pCO₂, and pO₂ during critical care and surgical monitoring of patients. Mutagenicity of a few chemicals can be determined by using biosensors. Presence of toxic compounds produced in the body can also be detected.