Ralf Maassen (DTEurope)
Biological sensors are analytical devices that detect chemical or biological reactions using biological components. They are utilized in a wide variety of applications, such as medical diagnostics, environmental monitoring, process monitoring, and control – especially in food and drink – and in the military to screen the battlefield for poison gases and nerve agents.
Biosensors, as they are also known, consist of a biological element or bioreceptor (an enzyme, antibody or nucleic acid) and a transducer. The component behaves like a sensor and reacts with the analyte to generate an electrical, optical, or thermal signal. The transducer converts it to a measurable electrical signal; usually a current or voltage.
Several substances can be used as the bio-element: nucleic acids, proteins (including enzymes or antibodies), plant proteins or lectins, or complex materials such as tissues slices, microorganisms, and organelles.
Biosensors should be small but still an appropriate size for regular use, manufacturable in large numbers at a low cost, and fast, providing a result within a timescale of a process or diagnostic test. Furthermore, they should be economical, self-calibrating, specific, robust, portable, simple, and easy to use.
One of the essential properties of a biosensor is its selectivity - the availability of the bioreceptor to detect the specific analyte in a sample that contains other admixtures and contaminants.
Selectivity is shown, for example, by the interaction of an antigen with an antibody. The antibody is a bioreceptor immobilized on the surface of a transducer. A solution, usually a buffer containing salts as well as the antigen, is exposed to the sensor where only the antibody reacts with the antigen.
Reproducibility is the ability of a biosensor to generate identical responses when experiments are repeated. It is characterized by the precision and accuracy of the transducer and electronics in the biosensor. Reproducible signals deliver high consistency and robustness to the implications made on the response of the biosensor.
Stability is incredibly important; it is the degree of susceptibility to ambient disturbances in and around the biosensor system. Such disruptions could cause drift in the output of the biosensor under measurement, causing errors in the measurement concentration as well as issues with the precision and accuracy of the biosensor.
Stability is a crucial feature in applications where the biosensor might require long incubation steps or continuous monitoring. It can be affected by the affinity of the bioreceptor - the degree to which the analyte binds to the bioreceptor. Those with high affinities foster either strong electrostatic bonding (the electrostatic attraction between ions or molecules) or covalent linkage of analytes that strengthens the stability of the biosensor.
The minimum amount of analyte that can be perceived by a biosensor is the limit of detection or sensitivity. In medical or environmental monitoring applications, biosensors are required to detect analyte concentrations in trace amounts in a sample, and so must be incredibly sensitive.
This property shows the accuracy of a measured response to a straight line and is associated with the resolution of the biosensor and the range of the analyte concentration under test.
The resolution of the biosensor is the smallest change in the concentration of the analyte required to bring a change in response of the biosensor. A high resolution is necessary, as most biosensor applications require analyte detection and measurement of the concentration of analyte over a broad working range.
There are many properties of biosensors that are crucial to their proper functioning; especially as they are used in medical diagnostics and point-of-care monitoring of treatment and disease progression, as well as in food and drink monitoring, forensics, and biomedical research. In addition to being cheap and robust, they must be specific, sensitive, stable, and selective.