Scientists and engineers have developed a variety of sensors for different purposes, and as you can imagine, they all have their own ways of working. After all, a pregnancy test kit is not likely to have the same detection mechanism as a radon detector, right?
All chemical sensors target some sort of analyte, but what happens once the analyte is in the sensor is where the differences emerge. For example, the sensor can bind the analyte (think a lock-and-key type mechanism, but on the molecular level). Or, the sensor may be set up so that the analyte selectively passes through a thin film. Imagine the film being a chemical gatekeeper that only lets the target molecule through and stops everything else from going in. This type of sensor has the positive feature of being continually reusable. A third form of sensor uses up the analyte in a chemical reaction that generates a product that creates the readable signal [source: National Research Council]. These three very broad mechanisms cover the workings of most sensors, but there are still other types.
For example, there are direct-read electrochemical sensors that use the diffusion of charged molecules to look for changes in current, conductivity or potential to see if a target analyte is present. Surface acoustic wave sensors employ acoustic waves sent from one electrode to another across a surface. The sensor is designed so that if the speed of the wave changes or if it loses intensity, it signals the presence of a target molecule bound to the surface. By taking measurements of these changes, the sensor may even be able to detect quantities of the material present [source: National Research Council].
Another cool innovation in chemical sensing technology moves toward detecting inherent properties of different chemical targets instead of using a molecular interaction to drive the detection. Different bonds in molecules each have signature vibration patterns that can be detected in the infrared region of the electromagnetic spectrum. By combining light sources, filters and detectors onto a single chip, scientists at Massachusetts Institute of Technology have been able to detect these molecular fingerprints in order to sense a whole host of molecules, from contaminants in water to electrolytes in the blood of newborn babies [source: Bender].