page_title: Chemiresistor
section_title: Metal oxide semiconductors
hierarchical_section_title: Chemiresistor / Types of chemiresistor sensors / Sensing materials / Metal oxide semiconductors
caption_attribution_description: English: A molecular model of the structure of graphene
context_page_description: 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.
context_section_description: Metal oxide chemiresistor sensors were first commercialized in 1970 in a carbon monoxide detector that used powdered SnO₂. However, there are many other metal oxides that have chemiresistive properties. Metal oxide sensors are primarily gas sensors, and they can sense both oxidizing and reducing gases. This makes them ideal for use in industrial situations where gases used in manufacturing can pose a risk to worker safety.
  Sensors made from metal oxides require high temperatures to operate because, in order for the resistivity to change, an activation energy must be overcome. Typical metal-oxide gas sensors require temperatures of 200 °C or higher in order to operate.
caption: A graphene monolayer.[13]
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