Biosensors:
- Analytical devices that consists a combination of biological detecting elements like sensor system and a transducer is termed as biosensor.
- Biosensors can be defined as self-sufficient integrated devices that has capacity to provide specific qualitative or semi-quantitative analytical information using a biological recognition element which is in direct-spatial contact with a transductional element.
- In simple words, biosensors are analytical devices that detects changes in biological processes and transform the biological data into electrical signal.
- The main features of biosensors are:
- Stability
- Economical
- Sensitivity
- Reproducibility
Components of biosensor:
- The block diagram of the biosensor consists of three segments namely, sensor, transducer, and electrical circuit.
- i. Sensor or detector: The first segment is the sensor or detector which is a biological component. it is a biochemical receptor. It interacts with the analyte and signal the change in its composition as electrical signal.
- ii. Transducer: The second segment is the transducer and it is a physical component which amplifies the biochemical signal received from detector, alters the resulting signal into electrical and displays in an attainable way.
- iii. Electrical circuit: It is the associated part which consists of Signal Conditioning Unit, a Processor or Micro-controller and a Display Unit.
Principle of Biosensors:
- Biosensors works on the principle of signal transduction and biorecognition of element.
- All the biological materials including-enzyme, antibody, nucleic acid, hormone, organelle or whole cell can be used as sensor or detector in a device. But the desired bio-receptor is usually a specific deactivated enzyme.
- The deactivated enzyme is placed in proximity to the transducer.
- The tested analyte links to the specific enzyme (bio-receptor) and inducing a change in biochemical property of enzyme. The change in in turn gives an electronic response through an electroenzymatic approach.
- Electroenzymatic process is the chemical process of converting the enzymes into corresponding electrical signals with the aid of transducer.
- Now, the outcome from transducer i.e. electrical signal is a direct representation of the biological material (i.e. analyte and enzyme in this case) being measured.
- The electrical signal is usually converted into physical display for its proper analysis and representation.
Working principle of biosensors:
- The union of biological sensitive element and a transducer is responsible to convert the biological material into a corresponding electrical response in form of signal.
- The output of the transducer will be either current or voltage relying on the type of enzyme.
- If the output is voltage, then it is fine. But if the output is current, then this current needs to be converted into equivalent voltage (using an Op-Amp based current to voltage converter) before proceeding further.
- The output voltage signal is generally very low in amplitude and is superimposed on a high frequency noise signal.
- Thus, the signal is amplified (using an Op-Amp based Amplifier) and then it is passed through a Low Pass RC Filter.
- Signal Processing Unit or a Signal Conditioning Unit is accountable for performing the this process of amplifying and filtering the signal .
- The output of the signal processing unit is termed as an analog signal. This output is equivalent to the biological quantity being measured.
- The analog signal can be exhibited directly on an LCD display but usually, this analog signal is passed to a Microcontroller, where the analog signal is converted into digital signal. This is done since it is easy to analyse, process or store a digital signal.
Types:
- On the basis of sensor device as well as the biological material the biosensors are classified as:
- 1. Electrochemical biosensors
- 2. Calorimetric/Thermal detection biosensors
- 3. Optical biosensors
- 4. Piezo-electric biosensors
- 5. Resonant biosensors
1. Electrochemical biosensors:
- Generally, electrochemical biosensor works on the principle that many enzyme catalysis reactions consumes or generates ions or electrons causing some change in electrical properties of the solution which can be detected and used as a measuring parameter.
- For example some biological compounds such as glucose, urea, cholesterol, etc.) are not electroactive, so the combination of reactions by this biosensor produce an electroactive element. This electroactive element results in change of current intensity which is proportional to the concentration of analyte.
- An electrochemical biosensor uses an electrochemical cell with electrodes of different dimension and modifications.
- Three kinds of electrodes are generally used-
- Working electrode
- Reference electrode
- Counter or Auxilary electrode
- It is the working electrode where reaction occurs between electrode substrate and analyte.
Types of electrochemical biosensors
- Electrochemical biosensors are classified into three types:
- Amperometric Biosensors
- Potentiometric Biosensors
- Conductimetric Biosensors
- 1. Amperometric Biosensors
- The Bioelectrochemical reaction in this biosensors generate measurable amount of current which is directly proportional to the substrate concentration.
- The first generation amperometric biosensors use the Clark oxygen electrode which determines the reduction of O2 present in the analyte solution.
- Determination of glucose using glucose oxidase enzyme is a redox reaction which is an example of Amperometric biosensors.
- This first generation biosensors depend on the dissolved O2 to measure the concentration analyte. However, as modification in second generation biosensors, a mediators is being used.
- This mediators transfer the electrons produced by the bioelectrochemical reaction directly to the electrode instead of reducing O2 dissolved in analyte solution.
- Nowadays, the electrodes remove the electrons without the aid of mediators and are coated with electrically conducting organic salts.
- 2. Potentiometric biosensors:
- Potentiometric biosensors use the ion-selective electrodes to convert the biological reaction to electronic response.
- Most commonly used electrodes are pH meter glass electrodes (for cations glass pH electrodes coated with a gas selective membrane for CO2, NH or H2S.) or solid state electrodes.
- Biosensors detects and measures the ions or electrons generated in many reactions, very weak buffer solutions are used in this case.
- Gas sensing electrodes detect and measure the amount of gas produced.
- 3. Conductimetric biosensors:
- These biosensors measure electrical conductance/ resistance of the solution.
- Conductance measurement have comparatively low sensitivity.
- Electrical field is generated by use of sinusoidal (ac) voltage which serves in reducing unwanted effects such as:
- Faradaic processes
- Double layer charging
- Concentration polarization
2. Calorimetric/Thermal detection biosensors:
- Most of the enzyme catalysed reactions are exothermic in nature.
- Calorimetric biosensors measure the change in temperature of analyte solution following enzyme action and interpret it in terms of analyte concentration in the solution.
- The analyte solution is passed through a small packed bed column consisting immobilized enzyme.
- The temperature of the solution is measured just before the entry of the solution into the column, and just as it leaves the column using separate thermistors.
- It is the most usually applicable type of biosensor and can also be used for turbid and colourful solutions.
- There are demerits such as:
- The biggest demerit is to maintain the temperature of the sample stream say + or -0.01°C.
- Low range and sensitivity.
3. Optical biosensors:
- Both catalytic and affinity reactions are measured by this biosensor.
- The products generated during the catalytic reactions cause a change in fluorescence that is measured by the biosensor.
- In other way, biosensors measure the change induced in the intrinsic optical properties of the biosensor surface due to loading on it of di-electric molecules like protein.
- A most advanced biosensor involving luminescence uses luciferase enzyme for detection of bacteria in food or clinical samples.
- In the presence of O2, luciferase takes up the ATP released from the lysis of bacteria to produce light which is detected and measured by biosensor.
4. Piezo-electric biosensors:
- In these biosensors, the surface is coated with antibodies which binds to the complementary antigen present in the sample solution.
- This results in increased mass which decreases their vibrational frequency, this alteration/change is used to determine the amount of antigen present in the sample solution.
4. Resonant biosensors:
- The vibrations of the electron cloud in a molecule is termed as resonant biosensors.
- These plasmons oscillate at a particular frequency characteristic of the material.
- The oscillations in surface plasmons are confined to the surface of the material.
- Generally, gold or silver surfaces are preferred for the SPR based biosensors.
- When electromagnetic radiation falls on the metal surface, at a particular angle of incidence, the frequency of the electromagnetic radiation matches the frequency of vibrations resulting in resonance.
- The resonant angle depends on the refractive index of the medium.
- The refractive index in turn is determined by the local mass density on the metal surface.
- If the surface of the metal film is modified with the antibody/receptor i.e. capture molecule, then specific binding occurs between the capture molecule on addition of the sample and its ligand leading to an alteration in mass and hence change in resonant angle.
- These biosensors are employed to understand the functional aspects of human immune deficiency virus (HIV) both qualitatively and quantitatively.
- The major merits of these biosensors are rapid measurements and relatively high sensitivity,
- The major demerit is that it cannot be used to detect and measure the turbid and coloured solutions. In few cases, ligands may interfere with the binding.