2. Biosensors:
A biosensor is a self-sufficient tool with a high specificity and sensitivity for quantitative estimation of specific chemical or organism and its toxicity (Purohit et al., 2020). Physiochemical transducing element of biosensor gives quantifiable estimation about the analyte in the form of electric signal. It should be specific, sensitive, portable and independent of physical parameters such as temperature and pH. They can measure the analyte in complex matrix with least sample preparation (Chandra, 2016). It is has both in vivo and in vitro applications (Noh et al., 2012; Baranwal, 2018).
Biosensors are categorized based on different parameters like bioreceptors, transducing elements, signal transduction and biorecognition principles. The transducer is an element that changes one form of energy to another and produces measurable signals. Biosensors are classified according to the type of transducer they use: optical, electrochemical, piezoelectric, and thermal. The basic principle of electrochemical biosensors is to convert biological events into electrons on an electrode, which are then converted into electric signals (Kumar et al., 2019; Turner, 2000).  By measuring current as a function of applied voltage, voltammetric potential determines an analyte. Amperometric biosensors detect the current created when an electroactive biological element is oxidized or reduced. Potentiometric biosensors biorecognition element with a transducer recognizes the deviation in ion concentration.
An optical biosensor’s primary goal is to provide a signal proportional to the concentration of the chemical being analyzed (analyte). The optical biosensor may employ biorecognition components from different biological materials like enzymes, antibodies, antigens, receptors, nucleic acids, entire cells, and tissues. Surface plasmon resonance (SPR), evanescent wave fluorescence, and optical waveguide interferometry detect the interaction of the biorecognition element with the analyte by utilizing the evanescent field near the biosensor surface. There are several variants in the design of optical biosensors, and this study will concentrate on a few that have been chosen for their extensive use and proclivity for detecting physiologically relevant chemicals.
An optical biosensor is an analytical device having a biorecognition sensing element integrated with an optical transducer system. Optical based biosensors are based on the principle of simple light absorption, fluorescence, reflectance, Raman scattering, chemoluminescence, surface Plasmon resonance (SPR), optical resonators, optical waveguides, optical resonators, photonic crystals, and optical fibers (Chen & Wang 2020). Optical biosensors can detect chemical changes in an analyte or organism by estimating the variation in the absorption of light or bioluminescence (Velasco-Garcia, 2009).
Physical factors such as acceleration, tension, and pressure may be converted into an electrical charge using piezoelectric biosensors. This bio element is coupled with piezoelectric components such as quartz crystal coated with gold electrodes. These crystals will vibrate at specific frequency depending on crystal mass as well electrical frequency of crystal and produce electrical signal of a specific frequency directly equivalent to the vibration. Though piezoelectric sensor has disadvantage too, as the surface charged produced due to oscillation, can be neutralized easily by environmental charges and current leakage. These biosensors are sensitive to temperature, as they can lead to crystal deformation and finally an electrical output (Paul & Dertein, 2018).The Cantilever biosensor is the new group of emerging micromechanical biosensors, having microfabricated silicon technology. This biosensor responds to physical (PH and temperature) and chemical changes into a mechanical bending and it can be estimated easily. Thermometric biosensors are the one which measures the temperature change due to heat production or evolution, by analyzing enzymatic reactions.