Biosensor devices comprise a bio-recognition layer and a physico-transducer. Combined, these form a biotransducer. The bio-recognition layer typically comprises an enzyme or other binding protein such as an antibody. Biorecognition layers, however, may also be formed from oligonucleotide sequences, sub-cellular fragments such as organelles (e.g. mitochondria) and receptor carrying fragments (e.g. cell wall), single whole cells, small numbers of cells on synthetic scaffolds, or thin slices of animal or plant tissues. This biologically derived layer is characteristically in close and intimate contact with a physico-transducer. The usual aim of a biosensor is to produce either discrete or continuous electronic signals that are proportional to a targeted analyte or a related group of analytes. This occurs when biospecific interactions between the targeted analyte and the complementary bio-recognition layer produces a physico-chemical change that is measured by the transducer.

Dr. Guiseppi-Elie
Professor, Chemical and Biomolecular Engineering

In general, transducers take many forms and they dictate the physicochemical parameter that will be measured. Thus, the transducer may be optically-based, measuring such changes as optical absorption, fluorescence, or refractive index. It may be mass-based, measuring the change in mass that accompanies a biologically derived binding reaction. Additionally, it may be thermally based (measuring the change in enthalpy (heat) or impedance based (measuring the change in electrical properties) that accompanies the analyte/bio-recognition layer interaction.

Biosensors generally do not permit the measurement of analytes that are not measurable by some other means. As an example, the monoclonal antibodies that comprise the biorecognition molecules of an immuno-biotransducer could easily be used in a multi-well plate ELISA (enzyme linked immunosorbent assay). In fact, many biosensors are derived from previously performed laboratory-based bio assays. Biosensors do however offer the convenience and facility of distributed measurement, that is, the potential ability to take the assay to the point of concern or care. Properly designed and manufactured, biosensor devices may be conveniently mass-produced. The potential lower cost per test is a compelling advantage. While detection limit and sensitivity are often cited as important impediments to success, these only need be appropriate to the test in question performed in the targeted market.

The most successful biosensors are of course the range of highly competitive blood glucose meters that may be purchased at the local pharmacy. Each has its own proprietary twist on what is essentially an enzyme electrode. In the Boringher-Roche Glucoscan, a freeze-dried layer of the enzyme glucose oxidase is made to react with the glucose found in the patient's whole blood. This enzyme transformation reaction results in the production of hydrogen peroxide. The hydrogen peroxide formed is oxidized directly or through redox mediation, at metallic electrodes. The magnitude of the current is directly proportional to the concentration of hydrogen peroxide produced, which is itself proportional to the rate of conversion of the substrate glucose to gluconolactone (gluconic acid).

Issues in biosensor technology: (i) Time-dependant stability of the bio-recognition layer. (ii) Reproducibility of the activity of the bio-recognition layer. (iii) Vulnerability of the biotransducer to foulants and interferences, (iv) Market sizes for biosensor-based measurement devices. (v) Appropriateness of the recognition/transduction device and form-factor of the integrated system to the targeted market.

Future directions in biosensor technology: (i)Increased range of analytes addressed. (ii) Move towards completely integrated systems possessing sample handling, microfluidics, detection and display (possibly telemetry). (iii) Renewed emphasis on whole cell and tissue biosensors. (iv) Use of embedded systems. (v) Telecommunications capability.

Biosensors have wide potential application in such diverse fields as personal health monitoring, environmental screening and monitoring, bioprocess monitoring, and within the food and beverage industry.