Classification of biosensors based on the transducers and the key differences between them.

Biosensors

What is a Biosensor?

Biosensors can be defined as analytical devices which include a combination of biological detecting elements like a sensor system and a transducer. When we compare with any other presently existing diagnostic device, these sensors are advanced in the conditions of selectivity as well as sensitivity. The applications of these Biosensors mainly include checking ecological pollution control, in the agriculture field as well as food industries. The main features of biosensors are stability, cost, sensitivity, and reproducibility.

The short form of the biological sensor is known as a biosensor. In this sensor, a biological element is maybe an enzyme, a nucleic acid otherwise an antibody. The bio-element communicates through the analyte being checked & the biological reply can be changed into an electrical signal using the transducer. Based on the application, biosensors are classified into different types like resonant mirrors, immune, chemical canaries, optrodes, bio-computers, glucometers & biochips.

Biosensors - an overview | ScienceDirect Topics

Classification of biosensors

Conventionally, biosensors consist of a biological recognition element, generally called the bioreceptor, the transducer component, and the electronic system (often combined with the transducer). Biosensors can be classified in terms of the bioreceptor or transducer type used. Bioreceptors are the key tools for the biosensor technology; they are the biological molecular species that exploit the biochemical mechanism for the recognition. Bioreceptors allow the binding of analytes of interest to produce a signal measurable by the transducer . Depending on the bioreceptor type used, biosensors can broadly be classified into four classes: nucleic acid/DNA , enzymes , antibody–antigen , and cells . On the basis of the transducer type being used, biosensors may be designated as optical, thermal, piezoelectric, quartz crystal microbalance (QCM), and electrochemical. Additionally, the electrochemical biosensor can further be categorized as conductometric, amperometric, and potentiometric . There are two broad biosensor categories based on the biorecognition principle: (i) catalytic biosensor, typical of enzyme biosensors and (ii) affinity biosensor, typical of DNA, and antibodies. Therefore, a biosensor with electrochemical transduction method and enzymes as a bioreceptor can be called an enzyme biosensor (based on the bioreceptor) or catalytic biosensor (based on the biorecognition principle). Based on both bioreceptor and transducer, they are also known as enzyme-based electrochemical biosensors. Enzyme biosensors can also be categorized based on specific enzymes used as bioreceptor (glucose biosensor, urea biosensor, cholesterol biosensor, etc.). Other biosensors can also be named as DNA biosensor (DNA as bioreceptor) and immunosensor (antibody as bioreceptor).

Classification based on transducers

Based on the transduction mechanism, biosensors are majorly classified into:
• Optical sensors
• Electrochemical sensors
• Mass-sensitive sensors

Optical sensors

An optical sensor converts light rays into electronic signals. It measures the physical quantity of light and then translates it into a form that is readable by an instrument. An optical sensor is generally part of a larger system that integrates a source of light, a measuring device and the optical sensor.

Working of optical biosensor
These types of biosensors are based on measuring the changes in the intensity of light and convert light signal into an electrical signal that can be recorded in the form of current or potential. Optical biosensors have gained considerable interest for bacterial pathogen detection due to their sensitivity and selectivity. The most commonly used technique of optical detection is surface plasmon resonance (SPR) for pathogen detection

Electrochemical sensors

Based on the reaction of enzymatic catalysis thatconsumes or generates electrons- Redox Enzymes..The object analyte is engaged in the response thathappens on the surface of an active electrode, and thisreaction may source also electron-transfer across thedual layer potential. The current can be calculated at a set potential. Based on the chosen function of a specific electrode,the electrode material and surface modification influences detection ability. Electrochemical sensing requires a reference electrode, a counter or auxiliary electrode and a working electrode (sensing electrode). Reference electrode (Ag/AgCl) kept at a distance from the reaction site to maintain a known and stable potential. The working electrode serves as the transduction element in the biochemical reaction, while the counter electrode establishes a connection to the electrolytic solution so that a current can be applied to the working electrode.

Mass-sensitive sensors

Piezoelectric biosensors are considered as mass based biosensors. They produce an electrical signal when a mechanical force is applied. Most common is the quartz crystal
microbalance (QCM). Specific nucleotides are immobilized on the surface of the crystal and are inserted into a solution containing the target nucleic acid. Upon interaction between the target nuclei and its complementary nucleotides, there is an increase in the mass of the piezoelectric biosensor This decreases the resonant frequency of the crystal Real time, low cost, fast response Lack of sensitivity and selectivity

Calorimetric sensors

There are various types of biological reactions which are connected with the
invention of heat, and this makes the base of thermometric biosensors. These sensors are usually named as thermal biosensors. Thermometric-biosensor is used to
measure or estimate the serum cholesterol. As cholesterol gets oxidized through the enzyme cholesterol oxidase, then the heat will be produced which can be calculated.
Similarly, assessments of glucose, urea, uric acid, and penicillin G can be done with these biosensors.