Chemiresistive Gas Sensors

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 Chemiresistive Gas Sensors  

Chemiresistive gas sensors are a class of chemical sensors that detect gases by monitoring changes in the electrical resistance of a sensing material when it interacts with gas molecules. Their working principle is simple, cost-effective, and well suited for real-time gas detection, making them widely used in environmental monitoring, industrial safety, healthcare, and domestic applications.

Working Principle

The sensing element is typically a semiconducting material whose electrical resistance changes upon exposure to a target gas. When gas molecules adsorb onto the surface of the sensing material, they either donate or withdraw electrons. This alters the charge carrier concentration in the material, leading to a measurable change in resistance. The magnitude and direction of resistance change depend on the nature of the gas (reducing or oxidizing) and the type of semiconductor (n-type or p-type).

  • Reducing gases (e.g., CO, H₂, NH₃) usually decrease the resistance of n-type materials and increase that of p-type materials.

  • Oxidizing gases (e.g., NO₂, O₃) typically increase the resistance of n-type materials and decrease that of p-type materials.

Sensing Materials

Common materials used in chemiresistive sensors include:

  • Metal oxide semiconductors (MOS): SnO₂, ZnO, TiO₂, WO₃, In₂O₃

  • Conducting polymers: polyaniline, polypyrrole, PEDOT:PSS

  • Carbon-based materials: graphene, carbon nanotubes (CNTs)

  • Hybrid and nanocomposite materials: metal oxide–carbon or polymer composites

Nanostructured materials (nanoparticles, nanowires, nanosheets) are especially favored due to their high surface-to-volume ratio, which enhances gas adsorption and sensitivity.

Device Structure

A typical chemiresistive gas sensor consists of:

  • A sensing layer deposited on an insulating substrate

  • Interdigitated electrodes to measure resistance changes

  • A heater (optional, common in MOS sensors) to operate at elevated temperatures and improve reaction kinetics

Advantages

  • Simple design and easy fabrication

  • Low cost and compact size

  • High sensitivity and fast response

  • Compatibility with miniaturized and portable devices

Limitations

  • Limited selectivity (cross-sensitivity to multiple gases)

  • Drift and aging effects over time

  • Dependence on temperature and humidity

  • Higher power consumption for heated sensors

Applications

Chemiresistive gas sensors are used for detecting toxic and combustible gases (CO, NO₂, H₂, CH₄), air quality monitoring, breath analysis for medical diagnostics, industrial process control, and smart home safety systems.

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