Chemiresistor

A chemiresistor is a material that changes its electrical resistance in response to changes in the nearby chemical environment. Chemiresistors are a class of chemical sensor that rely on the direct chemical interaction between the sensing material and the analyte. The chemical interaction between the sensing material and the analyte can be by covalent bonding, hydrogen bonding, or molecular recognition. Several different materials have chemiresistor properties: metal oxide semiconductors, some conductive polymers, and nanomaterials like graphene, carbon nanotubes and nanoparticles. Typically these materials are used as partially selective sensors in devices like electronic tongues or electronic noses.

A basic chemiresistor consists of a sensing material that bridges the gap between two electrodes or coats a set of interdigitated electrodes. The resistance between the electrodes can be easily measured. The sensing material has an inherent resistance that can be modulated by the presence or absence of the analyte. During exposure, analytes interact with the sensing material. These interactions cause changes in the resistance reading. In some chemiresistors the resistance changes simply indicate the presence of analyte. In other chemiresistors, the resistance changes are proportional to the amount of analyte present. This allows for the amount of analyte present to be measured.

As far back as 1965 there are reports of semiconductor materials exhibiting electrical conductivities that are strongly affected by ambient gases and vapours. However, it was not until 1985 that Wohltjen and Snow coined the phrase chemiresistor. The chemiresistive material they investigated was Copper Phthalocyanine, they demonstrated its resistivity decreased in the presence of ammonia vapour at room temperature.

In recent years chemiresistor technology has been used to develop promising sensors for many applications including conductive polymer sensors for secondhand smoke, carbon nanotube sensors for gaseous ammonia, and metal oxide sensors for hydrogen gas. The ability of chemiresistors to provide accurate real time information about the environment through small devices that require minimal electricity makes them an appealing addition to the internet of things.

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