A biosensor is a device that responds to changes in the biological environment and produces a signal in response that can be read out and interpreted to gives us information about the environment. A biosensor typically consists of three parts. The first part is the biologically sensitive element – this could be anything from enzymes and antibodies to DNA or other bio-active surfaces which are artificially engineered which is responsible for interacting with the analyte species. The second part of the sensor is the detector (or transducer). The
mechanism of detection could be chemical, optical, electrochemical etc. Essential, the detector converts a signal from the interaction of the analyte in the
environment with the biologically sensitive element into some signal that can be easily measured. The third part of the sensor consists of the associated read-out electronics that allow us to access the information about the analyte in a sensible way.
There are primarily two requirements to fulfil when designing a sensor, namely, how sensitively it responds to changes in the environment and how specific the response is to the analyte that one desires to sense. Ideally, for commercial usage, it should also be inexpensive, non-invasive if possible, have a large and appropriate range of detection, be easy to use and reliable in its response over a long period of time.
A special kind of sensor that is designed to be able to detect proteins is called an Immunosensor. An immunosensor is a biosensor where the sensing element is an antibody, where the basis of the detection is the specific affinity and
molecular interaction between an antibody and the protein that the sensor is designed to detect. An antibody is a large Y-shaped protein molecule, also known as an Immunoglobulin that is produced by the immune system in
response to an infection by a pathogen. They typically consist of 2 distinct types of chains of amino acids constituting the Y-Shaped structure, 2 heavy chains and the other 2 being lighter chains. The antibody is designed to be able to recognize and neutralize foreign substances in the body known as antigens. The tip of the
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‘Y’ structure contains a paratope that is specific to an antigen allowing the two molecules to bind to each other using what is known as complementary
determining regions that correspond to the topography of the binding sites and making them unique in their interaction with detected molecule or epitope. This type of binding mechanism has two consequences. Firstly, it tags pathogen molecules with the primary amine sulph-hydryl and carbohydrate groups from the antibody and enables the immune system to recognize and attack them. Secondly it also blocks key sites in the pathogen molecule that are associated with key reproductive or infective processes, thus effectively impairing the pathogen molecules. Most pathogens trigger antibody response in a human immune system. It is believed that there are over 10 billion different antibodies that the human system is capable of synthesizing. Consequently, detection of specific antibodies is often highly indicative of a specific pathogen or its
associated disease and medical diagnostic techniques routinely include tests for specific antibodies. Additionally, many autoimmune diseases can also be
associated with the incorrect immune response where antibodies bind the body’s own epitopes.
Immunosensors employing a variety of different detection mechanisms have been reported (fluorescence, electrochemical, FET etc.) Some of them employ labels such as enzymes or fluorescent molecules that can be optically detected and other methods for label-free detection assays using such as surface plasmon resonance, or direct electrochemistry have also been reported. [16-19]
If a label, such as an enzyme is used, then the technique is referred to as a labelled Assay. These types of sensors are designed to measure the presence and quantity of an antibody and antigen by labelling the molecule with some kind of tag that is luminescent, fluorescent or even radioactive. For instance, the most commonly employed immunosensor assay in clinical diagnosis is called ELISA or Enzyme-Linked Immunosorbent Assay. The time of Assay is used to correlate the event of a specific enzyme labeled antibody with the antigen it is meant to detect and convert it into a optical or electrochemical signal. Although ELISA is extensively used and highly sensitive and reliable, there are some drawbacks
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with this type of assay. For instance, long incubation period, complex apparatus, complicated storage conditions (to ensure the Enzyme does not denature), short shelf life, does not lend itself well to point of care devices etc.
The alternative to labelled immunosensors are referred to as label-free sensors. This category of sensors enables the direct measurement of physico-chemical changes that are triggered at the point of the antibody-antigen complex
formation. This is a useful technique and has some advantages. Firstly, there are several different mechanisms by which the signal can be transduced – typically electronically, optically or electrochemically. Reports also demonstrate
transduction using piezoelectric measurements such as A quartz crystal micro- balance in the detection of Glucose. [20]
The most common method of label-free signal transduction is by using electrochemical measurements such as amperometry, voltammetry,
electrochemical impedance spectroscopy etc. Optical methods include detection of changes in colour, absorption, reflectance as a result of a binding event. Surface plasmon resonance is another technique that is recently being developed and used for the detection of protein molecules. Piezo-electric transducers are also able to recognize a binding event due to an increase in the mass of the antibody complex and finally electrically transduction can be used to detect changes in current, conductance modulations, charge accumulation in response to a binding event that happens near the sensing element.