Biosensors: Components and Characteristics
Today, in an era of continuous revolution, there are alterations in processes- biological or abiological, surrounding, environment, etc. So, due to human intervention in every process of life, there is a need for constant monitoring and analysis of such changes. Since humans are not always available for the same. Hence, we need some kind of device that can make such changes detectable and monitorable. There comes the discovery of sensors to fulfil the same requirement. Basically, sensors are devices that can sense physical phenomena (temperature, pressure, strain, humidity, mass, light, and voltage) and produce output signals. In this article, we going to focus mainly on biosensors. Just like sensors, biosensors measure biological or chemical changes and analyze them.
A biosensor is a measuring or analytical device that detects chemical compounds using a transducer to convert a biological response into an electrical signal. In other words, biosensors can be defined as – self-contained analytical device that incorporates a biologically active material in contact with an appropriate transducer for detecting the concentration or activity of chemical species in any type of sample. The discovery of the first ‘true’ biosensor was in 1956 for oxygen detection. Leland C. Clark, Jr is known as the ‘father of biosensors’ for his invention of the oxygen electrode namely the Clark electrode.
COMPONENTS OF BIOSENSORS
The basic principle of biosensors is signal transduction. However, there are three main parts of a biosensor:
Bioreceptors are biologically derived materials or contain biomimetic components which interact with a specific bio-analyte. Therefore, these can be molecules that specifically recognize the molecule. For example, these may be an enzyme, an antibody, a similar binding molecule, a living cell, or organelles. However, enzymes are the most commonly used bio-receptors in biosensors. Bioreceptors are immobilized in the vicinity of the transducer. So, their interaction with the analyte generates bio-recognition signals. There are two classes of bio-recognition processes- bio-affinity recognition and biocatalytic recognition. Moreover, these signals are in the form of light, pH, heat, charge, mass change, etc. Both bio-affinity and biocatalytic recognitions involve the selective binding of an analyte with the receptor. In bio-affinity recognition, the binding is very strong and the transducer detects the presence of the bound receptor-analyte pair. In biocatalytic recognition, the analytes are chemically altered to form the product molecules.
It is the most important component of biosensors, which transforms the signal resulting from the interaction of the analyte with the bioreceptor into a measurable signal. It converts physical and chemical changes accompanying biorecognition events into electrical signals so that they can further be amplified and detected. The process is called transduction. Transducers are of several types as given below:
1. Electrochemical transducers:
The basic principle for this class of biosensors is that chemical reactions between immobilized bio-receptors and target analyte produce or consume ions or electrons, which affects the measurable electrical properties of the solution. These typically measure tiny small changes in voltage (potentiometry), current (amperometry), and resistance/conductance (conductometry).
2. Optical transducers:
It can employ a number of techniques to detect the presence of a target analyte and is based on well-founded methods. For example, some of the methods include chemiluminescence, fluorescence, light absorbance, phosphorescence, photothermal techniques, surface plasmon resonance (SPR), light polarization and rotation, and total internal reflection.
3. Calorimetric (thermometric) transducers:
It measures the heat of a biochemical reaction. Once the analyte comes in contact with the bio-receptor, the heat of the reaction which is proportional to the analyte concentration is measured.
4. Piezoelectric transducers:
These are mass-sensitive transducers that work on the basis of the coupling of the bio-receptor with a piezoelectric component, usually a quartz-crystal coated with gold electrodes. A crystal oscillates at a certain frequency, which can be modulated by its environment. In addition, when the crystal has a coating of some material, the actual frequency depends on the mass of the crystal and the coating. However, the resonant frequency can be measured with great accuracy hence making it possible to calculate the mass of the analyte adsorbed onto the crystal surface.
3. SIGNAL PROCESSING UNIT
These convert signals into a workable form. Altogether, it contains:
- an amplifier that amplifies the signals
- a processing unit that processes them
- a display to get the output screen
CHARACTERISTICS OF BIOSENSORS
Being the most important characteristic of biosensors, selectivity is defined as the ability of a bioreceptor to detect a specific analyte in a sample containing other admixtures and contaminants.
For example, selectivity is characteristic of the interaction of an antigen with the antibody. Conventionally, antibodies act as bio-receptors that are made to immobilize at the surface of the transducer. After that, usually, a buffer containing salts with the antigen is exposed to the transducer where antibodies interact only with the antigens. To construct a biosensor, the selectivity of the bio-receptor is the main consideration.
Biosensors have the characteristics of reproducibility if they generate identical responses for a similar experimental setup. Additionally, reproducibility is the ability to produce precision and accuracy of the transducer and electronics in a biosensor. If the sensor is precise then it provides alike results every time we measure the sample. And, accuracy is the capacity of sensors to provide a mean value that is close to the true value if we it measure multiple times.
Stability is the ability to resist ambient disturbances in and around the biosensor system. Certainly, these disturbances can cause variations in the output signals of a biosensor under measurement. Further, it causes an error in the measured concentration that affects the precision and accuracy of the biosensor. Certain components of the biosensor like transducers can be temperature-sensitive. Moreover, it may influence the stability of a biosensor. Therefore, appropriate tuning is done to ensure its stable response. It is the most crucial characteristic that can also vary according to the affinity of the analyte to its bio-receptor. In conclusion, it characterizes the change in its baseline or sensitivity over a fixed period of time.
It is the attribute that basically gives the accuracy of the response measured as y=mc. The linearity of the biosensor depicts the resolution of the biosensor and the range of analyte concentrations under analysis.
Biosensors are sensitive to the concentrations of analyte we use. Thus, sensitivity defines the limit of detection of biosensors. In other words, sensitivity is the response of the sensor to per unit change in an analyte concentration. Another characteristic is the range. The range is the concentration range over which the sensitivity of the sensor is good.
APPLICATIONS OF BIOSENSORS
Biosensors propose a very wide range of applications that has the potential to improve the quality of life. Basically, they are useful for environmental monitoring, disease detection, food safety detection, drug discovery, and many more. Eventually, they provide better stability and sensitivity as compared to conventional methods. It can also be used either as an indicator of diseases or in targeted drug delivery. These are cost-effective, sensitive, stable, and disposable biosensors.
Biosensors provide us with real-time monitoring such as in bioreactors. Thus, it is helpful in the determination of various physiological and pharmacological parameters. Biosensors also have applications in nanobiotechnology to deliver drugs and sense changes occurring in the body or in the environment. These have been used to make pregnancy test kits, glucometers, etc.
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