"BiosensorThe field of biosensors has led to an increased interaction between chemists, physicts, biologists,and engineers to develop products which can be used for various applications like monitoringenvironment or human health.A biosensor is a self-contained integrated electronic device capable of providing specificquantitative or semi-quantitative analytical application using a biological recognitionelement which is retained in direct spatial contact with a transducer element. [1]1. Components of BiosensorA biosensor consists of a bio-recognition component (also called the bioreceptor), transducer(electrical,optical or physical), and an amplifier.The bioreceptor uses receptors modelled afterbiological systems to interact with analytes of interest. [2]The basic principles by whichbiosensors operate are same as electronic sensors.Biosensors can beclassified into various types either by type of transducer used (piezoelectric,thermal, optical etc.) or by type of bio recognition element used (enzyme, antibody, DNA,microorganisms).Fig 1: Functioning of Biosensor1.1 DNA BiosensorsDNA sensors are the ones which contain DNA immobilized as bio receptor. Most of the DNAsensors are based on the technique of hybridization which is issued between a known DNAsequence and an unknown counterpart (target or analyte). [3]DNA Biosensor’s function is based on the fact that chemical attractive forces help to stick twostrands of DNA to each other. In such cases, two strands of DNA which match at everynucleotide position lead to immobilization of DNA strand.The immobilized DNA is a part ofbiosensor itself which produces a conformational change which is measured by the transducerand converted to a measurable signal (electrical, optical etc.). This signal is then amplified andmeasured to find out the presence or concentration of analytes. [3] 2. Development of DNA sensors In recent years, there has been considerable practice in the development of DNA sensors, due toits significance in several fields. Due to this, several methods are being investigated to generatethe DNA sensors. Some of the key methods are:2.1 Development of DNA sensors using hydrogelsHydrogels are a type of soft material formed by crosslinking hydrophilic polymers (waterloving).The most important feature of a hydrogel is their ability to absorb a lot of water andswell. Recently DNA has acquired considerable attention in the construction of hydrogels whichcan be used in sensors.[4] DNA is hydrophilic and can easily form hydrogels under certainconditions. Functional DNAs like aptamers or DNAzymes can easily be incorporated intosynthetic polymers to form DNA-hybrid hydrogels which can be used to form DNA sensors. InDNA- hydrogel sensors, a change in environmental conditions or occurrence of an externalstimulus changes the properties of hydrogels (de-hybridization of the DNA helix orconformational change). These changes can be sensed by transducers which convert these to ameasurable signal. The DNA-hydrogel sensors show response to several stimuli such as (i) Temperature: DNA hydrogels take advantage of hybridization of complementary basepairs of DNA thermal dissociation characteristics. An example of this is the DNA-basedhydrogel made from succinimide copolymer. The succinimide ester forms bond with 5'- terminal-amino-modified DNAs and this copolymer remains in solution phase. When thecrosslinker DNA strand is added to this copolymer, it converts the solution to gel. An increase of temperature above the dehybridization temperature causes dehybridization ofDNA strands and hence conversion of gel to sol phase. [5](ii) Ions: Ions such as protons or metal ions have been observed to cause changes in theconformational structure of DNA strands which can be easily detected.(iii) DNA: DNA sequences can also be used as stimuli. For example, DNA can reversiblycontrol the gel-sol transition, induce changes in mechanical properties, or producevolume change.Biomolecules and Photons can also act as stimuli for DNA-hydrogel copolymers and can causethe conversion from gel to sol or vice versa.[6]2.2 Development of DNA sensors using conducting polymersConducting polymers are polyconjugated polymers who have electronic properties similar tometals. However, they still retain the properties of organic polymers. The electronic structure ofconjugated polymers is highly sensitive to conformational changes because of which it is highlysuitable to produce DNA sensors which are helpful in detecting the presence of target analyte.[7]In DNA sensors made from conducting polymers, single-stranded DNA probes are immobilizedon a conducting polymer layer. When the target DNA comes near the bio-receptor, the DNAprobe is hybridized with it (at every nucleotide position). [8] This generates a recognition signalwhich is recorded through an electrode. Since the hybridization takes place on the surface ofconducting polymer, the recognition signal generated passes to the transducer through theconducting polymer. Hence the properties of conducting polymer and orientation of immobilizedDNA probe are important for the sensitivity of DNA sensor. The procedure of DNA probe immobilization should retain theprobe’s affinity forcomplementary target DNA. Ideally, the orientation of probes should be predictable and readilyaccessible to the analyte DNA . [9] Generally, immobilization methods fall into the classes ofelectrochemical entrapment, covalent immobilization or affinity interactions.Fig 2: Functioning of a conducting polymer based DNA sensorOne of the examples of DNA sensors based on conducting polymers is the one prepared frompolythiopene. Water-soluble cationic polythiophene derivatives have, in particular, been widelyused in optical DNA detection schemes based on the principle of a conformational change in thepolymer chains after interaction with ss-DNA and ds-DNAA DNA sensor based on thispolymer has shown good specificity. The sensor responses for centre one-base and end two-basemismatched DNA targets were only 14.3% that for the complementary target. This polymer wasfurther used as a matrix for hydrazine-catalyzed ultrasensitive detection of DNA andproteins.[10] "