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.