Chemical Sensors: A Modular Lecture Series

Recommended background: Basic electrochemistry, Ohm’s law, adsorption principles, thin-film physics

1. Introduction

Conductimetric sensors and chemiresistors detect chemical species by measuring changes in electrical conductivity or resistance due to analyte adsorption or chemical reactions.

Applications include:

• Gas detection (e.g., NO₂, CO, NH₃)
• Water quality monitoring (ionic species)
• Industrial process control (corrosion monitoring, pollutant detection)

These sensors are highly versatile, with simple readout electronics and fast response times. They are commonly combined with thin films or functional coatings to enhance selectivity.

References:

• LibreTexts: Conductimetric Sensors
• RSC: Chemiresistors for Gas Sensing

2. Principle of Conductimetric Sensing

Based on Ohm’s law:
R = V / I = ρ L / A

Where:

• R = resistance
• V = applied voltage
• I = measured current
• ρ = resistivity of sensor material
• L = length of conductive path
• A = cross-sectional area

Conductivity (σ) is the reciprocal of resistivity:
σ = 1 / ρ

Adsorption of analytes changes ρ or σ, producing a measurable signal.

3. Conductimetric Gas Sensors

• Commonly made from metal oxide semiconductors: SnO₂, ZnO, TiO₂
• Mechanism:

1. Adsorption of oxygen on the sensor surface captures electrons → baseline resistance
2. Exposure to reducing gas (CO, H₂, CH₄) → electrons released back to conduction band → decreases resistance
3. Oxidising gases (NO₂, O₃) → increase resistance

• Response is usually expressed as:

Reference: Sensors and Actuators B: Metal Oxide Gas Sensors

4. Chemiresistors

• A chemiresistor is a resistive sensor functionalised to interact selectively with target analytes.
• Materials include:
  • Conducting polymers (polyaniline, polypyrrole)
  • Carbon nanotubes or graphene composites
• Adsorption of analytes changes carrier density → alters resistance

Advantages:

• Simple design, low cost
• Fast response and recovery
• Compatible with microelectronic integration

5. Adsorption Isotherms and Sensor Response

• Sensor response depends on the surface coverage of adsorbed species.
• Langmuir adsorption isotherm often applies:

θ = (K a_X) / (1 + K a_X)

Where:

• θ = fraction of occupied sites
• K = adsorption equilibrium constant
• a_X = analyte activity (approx. concentration)
• Conductance change is proportional to θ for thin films.

Reference: LibreTexts: Adsorption on Sensor Surfaces

6. Thin-Film Effects

• Film thickness affects sensitivity:
  • Thin films → faster response, higher surface-to-volume ratio
  • Thick films → slower response, diffusion-limited
• Optimal thickness balances signal magnitude and response time.

Example: SnO₂ thin film (100 nm) exposed to CO: resistance decreases within seconds; 1 μm film → response delayed due to diffusion through bulk.

7. Worked Example: NO₂ Detection with SnO₂ Chemiresistor

Sensor baseline resistance in air: R₍air₎ = 10 kΩ

Exposure to 5 ppm NO₂ → measured resistance: R₍gas₎ = 15 kΩ

Step 1: Compute response factor

Observation: 50% increase in resistance due to NO₂ adsorption

Step 2: Approximate surface coverage (Langmuir isotherm, assume K a_X ≪ 1)
θ ≈ K a_X

• If K = 0.1 ppm⁻¹, then θ ≈ 0.1 × 5 = 0.5 → 50% coverage, consistent with the observed resistance change.

Reference: RSC Chemiresistor Gas Sensors Review

8. Selectivity and Interference

• Metal oxide sensors are broadly sensitive and may respond to multiple gases
• Selectivity enhanced via:
  • Catalytic coatings (Pt, Pd)
  • Operating temperature optimisation
  • Sensor arrays (electronic noses)
• Chemiresistors can be functionalised for specific analytes:
  • Polyaniline for NH₃
  • Carbon nanotube composites for VOCs

9. Practical Considerations

1. Calibration: gas concentration vs resistance curve
2. Response and recovery times: dependent on diffusion, adsorption kinetics
3. Environmental conditions: humidity, temperature affect response
4. Long-term stability: metal oxides may sinter at high temperatures; polymer films may degrade

Applications:

• Industrial safety: CO, NO₂ monitoring
• Environmental monitoring: air pollution
• Healthcare: exhaled breath analysis

10. Summary

Conductimetric sensors and chemiresistors demonstrate chemical-to-electrical transduction via resistance changes:

• Metal oxide sensors: redox reactions on the surface alter electron density → resistance changes
• Chemiresistors: functional coatings enhance selectivity for target analytes
• Response: depends on the adsorption isotherms and film thickness
• Worked example: NO₂ detection shows correlation between surface coverage and resistance change
• Fast, low-cost, and integrable with electronics, these sensors are widely used in gas and aqueous-phase sensing

11. Next Lecture Preview

Lecture 10: Sensor Systems and Biological Interfaces

• Integration of sensors into larger analytical systems
• Biological detectors: enzymes, antibodies, living organisms
• Multi-sensor arrays and ultimate selectivity machines

Further reading:

• LibreTexts: Conductimetric and Chemiresistor Sensors
• RSC: Chemiresistors for Gas Sensing

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