The history of biosensors began with the enzyme electrode, a miniature chemical transducer described by Leland C. Clark in 1962.
Since then, scientists from different areas such as VLSI, Physics, Chemistry, and Material Science, have developed more sophisticated, reliable, and mature biosensing devices for applications in the fields of medicine, agriculture, biotechnology, as well as the military and bioterrorism detection and prevention.
A biosensor is a self-contained integrated device, incorporating a biological recognition element (biochemical receptor) either intimately connected to or integrated within a transducer to produce a digital electronic signal which is proportional to the concentration of a specific chemical or set of chemicals.
Biosensor consists of two main parts: A bioreceptor and a transducer. Bioreceptor is a biological component (tissue, microorganisms, organelles, cell receptors, enzymes, antibodies, nucleic acids, etc.) that determines the target analyte. The difference fraction is a transducer, a physicochemical detector component that changes the detection event into a measurable signal.
Biosensors have several potential applications in clinics, diagnosis, process control, bioreactors, quality control, agriculture and veterinary medicine, bacterial and viral diagnostic, drag production, control of industrial wastewater, mining, military defense industry, etc.
The key benefits of biosensors include the following:
- Rapid and continuous measurement
- High specificity
- Very less usage of reagents required for calibration
- Fast response time
- Ability to measure non-polar molecules that cannot be estimated by other conventional devices.
The function of a biosensor depends on the biochemical specificity of the biologically active material. The choice of the biological material will depend on several factors: specificity, storage, operational and environmental stability. The major application of biosensors so far is in blood glucose sensing because of its abundant market potential. Other major applications of biosensors are listed below:
- Monitoring glucose level in diabetes patients
- Food analysis
- Environmental applications
- Protein engineering and drug discovery applications
- Wastewater treatment.
In this post, we will look at some of the key types of biosensors used as diagnostic tools.
Types of Biosensors
1. Resonant Biosensor
The acoustic wave transducer in this type of biosensor is coupled with an antibody (bio-factor). The analyte molecule (or antigen) gets added to the membrane, the mass of the membrane diversities, resulting in diversification in the mass, subsequently diversities the resonant frequency of the transducer. This frequency change is then measured.
2. Optical biosensors
For this type of biosensor, the output transducer signal measured is light. The biosensor is based on optical diffraction or electrochemiluminescence. Optical transducers are particularly attractive for application to direct (label-free) detection of bacteria. These sensors are accomplished to discover minute conversions in the refractive index or thickness, which happens when cells fasten to receptors immobilized on the transducer surface.
They correlate changes in concentration, mass, or number of molecules to direct changes in light characteristics. Several optical techniques have been reported for the detection of bacterial pathogens, including monomode dielectric waveguides, surface plasmon resonance (SPR), ellipsometry, the resonant mirror, and the interferometer, etc.
- Surface plasmon resonance (SPR) biosensor: This is an evanescent area-based optical sensor applying thin gold film for sensing approaches. The interaction between analyte flowing over immobilized interactant on the gold surface is probed through the detection of reflection minima on photo-detector array sensors. SPR has successfully been applied to the detection of pathogen bacteria utilizing immunoreactions.
- Piezoelectric biosensors: Piezoelectric (PZ) biosensor offers a real-time output, simplicity of use, and cost-effectiveness. The chief idea is based on coating the surface of the PZ sensor with a selectively binding material, for instance, antibodies to bacteria, and then locating it in a solution containing bacteria. The bacteria will bind to the antibodies, and the mass of the crystal will increase while the resonance frequency of oscillation will decrease proportionally.
3. Thermal Biosensors
This type of biosensor exploits one of the fundamental properties of biological reactions: absorption or production of heat, which changes the temperature of the medium in which the reaction takes place. They are combined with temperature sensors by combining immobilized enzyme molecules. When the analyte comes into contact with the enzyme, the enzyme ‘s heat reaction is measured and calibrated against the concentration of the analyte. Common uses of this kind of biosensor include pesticide detection and pathogenic bacteria.
4. Electrochemical Biosensors
Electrochemical biosensors are mainly used to detect hybridized DNA, DNA-binding drugs, glucose concentration, etc. Electrochemical biosensors can be classified based on the calculating electrical guidelines as (i) conductimetric, (ii) amperometric, and (iii) potentiometric. Contrary to optical approaches, electrochemistry gives the analyst to act with turbid samples, and the capital cost of equipment is much lower. On the other hand, electrochemical methods present slightly more limited selectivity and sensitivity than their optical counterparts.
- Conductimetric Biosensors: The measured parameter is a solution’s electric conductivity or resistance. When electrochemical reactions create ions or electrons, the solution’s overall conductivity or resistivity has been altered. This convert is ended and calibrated to an appropriate degree. Conductance measurements have relatively low sensitivity (9).
- Amperometric Biosensors: This is maybe the most common electrochemical discovery approach applied in biosensors. This high sensitivity biosensor can discover electroactive species present in biological test samples. Amperometric biosensors produce a current proportional to the concentration of the substance to be detected. The most common amperometric biosensors use the Clark Oxygen electrode.
- Potentiometric Biosensors: These are the least common of all biosensors, but different strategies may be found; nonetheless, in this category of the sensor, the calculated guideline is oxidation or reduction potential of an electrochemical reaction. The working basis is based on the fact that the current flow occurs due to electrochemical reactions when a voltage is applied to an electrode in a solution. The voltage at which those reactions occur indicates a specific reaction and species.
5. Bioluminescence sensors
Recent advances in bioanalytical sensors have led to the utilization of certain enzymes’ ability to emit photons as a byproduct of their reactions. This phenomenon is known as bioluminescence. The potential applications of bioluminescence for bacterial detection were begun by the development of luciferase reporter phages. The bacterial luminescence lux gene has been broadly exercised as a reporter in an inducible or constitutive mechanism.
In an inducible manner, the reporter lux gene is fused to a promoter regulated by the concentration of a combination of interest. As an effect, the concentration of the compound can be quantitatively assayed by determining the bioluminescence intensity. Bioluminescence systems have been used for the detection of a wide range of microorganisms.
6. Nucleic Acid-based Biosensors
A nucleic acid biosensor is an analytical instrument that integrates an oligonucleotide with a signal transducer. The nucleic acid probe is immobilized on the transducer and behaves as the bio-recognition molecule to detect DNA/RNA fragments.
Nanosensors can be described as sensors based on nanotechnology. The development of nano biosensors is one of the most current advancements in the area of Nanotechnology. The silver and certain other noble metal nanoparticles have many important applications in the field of bio labeling, drug delivery system, filters, and also antimicrobial drugs, sensors.