Biosensors are nowadays used in biomedical diagnosis as well as in a wide range of other areas such as point-of-care monitoring of treatment and disease progression, environmental monitoring, food control, drug discovery, forensics and biomedical research. A wide range of techniques can be used for the development of biosensors. Their coupling with high-affinity biomolecules allows the sensitive and selective detection of a range of analytes. One type of such sensor is electronic biosensor which uses “Bioreceptor, Transducer and Display” to electronically transmit signals.
This is the part of a biosensor that processes the transduced signal and prepares it for display. It consists of complex electronic circuitry that performs signal conditioning such as amplification and conversion of signals from analogue into the digital form. The processed signals are then quantified by the display unit of the biosensor.
Types of Electronic biosensors
- Potentiometric: Potentiometric sensor is a type of chemical sensor that may be used to determine the analytical concentration of some components of the analyte gas or solution. These sensors measure the electrical potential of an electrode when no current is present.
- Amperometric: The principle of amperometric sensor is based on measuring current generated by enzymatic or bio affinity reaction at the electrode surface, at a constant working potential with respect to the reference electrode.
- Cantilever-based sensor: Cantilever–basedsensors are extremely versatile, they can be operated in air, vacuum and liquid environment, they can transduce a number of different signals, such as magnetic, stress, electric, thermal, chemical, mass, and flow, into a mechanical deflection detected with sub-Angstrom resolution.
Important Characteristics of Electronic Sensors:
- Selectivity: Selectivity is perhaps the most important feature of a biosensor. Selectivity is the ability of a bioreceptor to detect a specific analyte in a sample containing other admixtures and contaminants. The best example of selectivity is depicted by the interaction of an antigen with the antibody. Classically, antibodies act as bioreceptors and are immobilised on the surface of the transducer. A solution (usually a buffer containing salts) containing the antigen is then exposed to the transducer where antibodies interact only with the antigens. To construct a biosensor, selectivity is the main consideration when choosing bioreceptors.
- Sensitivity: The minimum amount of analyte that can be detected by a biosensor defines its limit of detection (LOD) or sensitivity. In a number of medical and environmental monitoring applications, a biosensor is required to detect analyte concentration of as low as ng/ml or even fg/ml to confirm the presence of traces of analytes in a sample. For instance, a prostate-specific antigen (PSA) concentration of 4 ng/ml in blood is associated with prostate cancer for which doctors suggest biopsy tests. Hence, sensitivity is considered to be an important property of a biosensor.
- Reproducibility: Reproducibility is the ability of the biosensor to generate identical responses for a duplicated experimental set-up. The reproducibility is characterised by the precision and accuracy of the transducer and electronics in a biosensor. Precision is the ability of the sensor to provide alike results every time a sample is measured and accuracy indicates the sensor’s capacity to provide a mean value close to the true value when a sample is measured more than once. Reproducible signals provide high reliability and robustness to the inference made on the response of a biosensor.
Challenges Faced in Field of Electronic Biosensors
Although biosensors employ fundamental sciences, it can hardly be rationalised as ‘curiosity-driven’ research. On the other hand, research in industry obeys the trend of ‘follow the money’ to some extent. Given the success of commercial glucose sensors, biosensor research is, of course, very lucrative for the industry’s long-term sustainability. However, it takes quite a long time to produce a commercially viable device from a proof of concept demonstrated in academia. This also involves a number of risks that industries are reluctant to face.
As a result, there are unaddressed mandatory issues concerning the production of a commercial biosensor, such as:
- Identification of the market that is interested in a biosensor for a specific analyte of interest.
- Clear-cut advantages over existing methods for analyses of that analyte.
- Testing the performance of the biosensor both in use and after storage. Response of a biosensor after 6 months of storage is the absolute minimum for any practical commercial application.
- Stability, costs and ease of manufacturing each component of the biosensor.
- Hazards and ethics associated with the use of the developed biosensor.
Although, there have been challenges in implementing biosensors but the nature of biosensors is ubiquitous which outweighs its disadvantages to a great extent.
The rapid development in the field of biosensors over the past decades, both at the research and product development level, is mainly due to: (i) developments in miniaturisation and microfabrication technologies; (ii) the use of novel bio-recognition molecules; (iii) novel nanomaterials and nanostructured devices; and (iv) better interaction between life scientists and engineering/physical scientists.