Chemical sensors

In recent years, demand for methods to detect chemical species and measure their concentration has grown considerably. This interest is mainly due to environmental, safety, process control or rapid medical diagnostic considerations.

When it comes to detecting chemical species, we need to distinguish between analyzers (analytical instruments) and chemical sensors.

Analyzers are generally relatively complex systems combining mechanical, chemical and electrical components. They are often costly, cumbersome and energy-consuming, making them unsuitable for on-site measurements. What's more, these instruments are often plagued by long response times, either due to the detection technique itself, or to the need for sample handling. This category includes chromatographs and various spectrometers. The main advantages of these instruments are their ability to provide a complete and precise analysis of the gas sample.

Chemical or biochemical sensors, on the other hand, are generally simple systems, consisting of a sensitive layer enabling recognition of the species with which it interacts, and a transducer system transforming the chemical interaction into an electrical signal. Often, the two functions are intimately linked. This dossier will not deal with sensors that use a physical method to determine chemical species, such as those based on infrared absorption or photoacoustic sensors.

The main advantages of chemical sensors are their small size, low energy consumption, low cost and relatively short response times. All these advantages make them ideal for on-site measurements and on-line process control. On the other hand, the analysis of complex mixtures requires the use of a large number of sensors sensitive to one or other species.

There are several families of chemical sensors, distinguished by the type of sensitive layer and the transduction principle. This text describes the main types of sensor.

Various parameters characterize sensor performance. The main ones are :

• Sensitivity: link between sensor signal variations and target species concentration;

• sensitivity limit: minimum detectable concentration of target species;

• selectivity: ability to detect the target species to the exclusion of all others;

• interferents: other species leading to an undesirable sensor response;

• response time: time required to reach 90% of the operating response after contact with the gas to be detected;

• recovery time: time required to return to the basic signal value after evacuation...