Sensors

Functional Principle

Suitable sensors are available for measuring physical quantities such as temperature, whereas for measuring chemical quantities (e. g. qualitative or quantitative determination of the chemical composition of substances) it is necessary to convert the chemical properties to a physically measurable signal first.

In addition to determining or observing measuring values, selective detection of an analyte using a bio or chemical sensitive layer plays an important part.

The result is a sensor consisting of a sensitive layer (receptor) and a transducer, combined with an electronic evaluation unit – a setup that is also called a sensor system.

For example, light barriers can detect persons passing through the barrier, they may even indicate whether a person is tall or heavy, but they will not be able to determine their age or the country they come from. Bio and chemical sensors, on the other hand, are able to detect more or less selectively or quantitatively e. g. traces of individual gases in exhaust gasses, smallest volumes of pollutants in water or pharmaceuticals in blood.

Sensitive layers consist e. g. of receptor molecules or polymer layers whose physical or chemical properties are changed specifically through interaction with the analyte.

The transducer converts the effect obtained within the sensitive layer into a measurable signal, using changes in the physical properties of the sensitive layers. These physical properties may be optical properties (e. g. refractive index, reflectivity, adsorptive capacity), electrical or magnetic properties (e. g. conductivity, capacity, potential, magnetization), mechanical properties (e. g. characteristic frequency of membranes) or thermal properties.

Chemical sensors

Chemical sensors are used for measuring exhaust gases (lambda probe) or for ambient air or cooling system monitoring.

Chemical (gas) sensors based on semiconducting metal oxides (SMOX) are currently one of the most investigated groups of gas sensors. They have attracted the attention of many users and scientists interested in gas sensing under atmospheric conditions due to the: low cost and flexibility associated to their production; the simplicity of their use; the large number of detectable gases/possible application fields.

The state of the art sensors are realized by depositing a SMOX poly-crystalline, thick, porous film over a substrate provided with electrodes and a heater. The former are used for the readout of the resistance of the sensing layer, which depends on the composition of the ambient atmosphere. The latter allows for heating at a temperature in the range between 200 to 400 °C, which is needed to speed up the surface reactions and minimize the influence of the humidity. In most cases, minute quantities of noble metal additives (Pt, Pd and Au) are added at the surface in order to tune the selectivity, lower the operation temperature and improve the response time.

In air, at the surface of the SMOX, in the case of a n-type conduction, the ionosorption of oxygen decreases the concentration of the free charge carriers, which are trapped at the surface; this is causing an overall increase of the sensor resistance. In the case in which a reducing gas (e.g. CO) appears in the atmosphere, its reaction with the pre-adsorbed oxygen decreases the negative surface charge with an overall effect of sensor resistance decrease.

The dependence of the sensor’s resistance/conductance on the concentration of the target gas is not linear, the reason being the way in which the charge transfer processes associated to the surface reaction is translated into a change of the concentration of free charge carriers taking part to the conduction in the sensing layer.

For further information on chemosensors please see AG Weimar, AG Gauglitz.

Biosensors

Using the biosensor detection principle, it is generally possible to detect biomolecular interactions, e. g. protein / protein interactions or DNA hybridization, but also to determine the behaviour of cells at interfaces or to detect interactions with pollutants.

Biosensors are therefore used in analytics to detect e. g. harmful substances (pestizides, EDCs etc.) in water and foods.

In clinical diagnostics, biosensors are used for quick and easy determination of disease parameters or e. g. for detection of pharmaceuticals in blood. Meanwhile, they are also used in point-of-care testing.

For more information see AK Gauglitz.

Direct optical detection in bioanalysis: an update;
G. Gauglitz, Anal Bioanal Chem, 398(6), 2363-2372 (2010)
A novel analytical tool for quantification of estrogenicity in river water based on fluorescence labelled estrogen receptor a
A. Le Blanc, C. Albrecht, T. Bonn, P. Fechner, G. Proll, F. Pröll, N. Carlquist, G. Gauglitz, Anal Bioanal Chem, 395(6), 1769-1776 (2009)