Wikimedia Commons / CSIRO Cooper Simplified schematic of a single gap chemiresistive sensor.

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 sensors that rely on the direct chemical interaction between the sensing material and the analyte. The sensing material and the analyte can interact 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 others, the resistance changes are proportional to the amount of analyte present; this allows for the amount of analyte present to be measured.

Chemiresistors can be made by coating an interdigitated electrode with a thin film or by using a thin film or other sensing material to bridge the single gap between two electrodes. Electrodes are typically made of conductive metals such as gold and chromium which make good ohmic contact with thin films. In both architectures, the chemiresistant sensing material controls the conductance between the two electrodes; however, each device architecture has its own advantages and disadvantages.

Interdigitated electrodes allow for a greater amount of the film's surface area to be in contact with the electrode. This allows for more electrical connections to be made and increases the overall conductivity of the system. Interdigitated electrodes with finger sizes and finger spacing on the order of microns are difficult to manufacture and require the use of photolithography. Larger features are easier to fabricate and can be manufactured using techniques such as thermal evaporation. Both interdigitated electrode and single-gap systems can be arranged in parallel to allow for the detection of multiple analytes by one device.

Source: Wikipedia (All text is available under the terms of the Creative Commons Attribution-ShareAlike License)