Chemical Sensors: A Modular Lecture Series
Recommended background: Electrochemistry, Nernst equation, redox chemistry, sensor dynamic range
1. Introduction
Electrochemical sensors detect chemical species by measuring an electrical signal that reflects the chemical environment. They are broadly classified into:
- Potentiometric sensors – measure the voltage under near-zero current conditions
- Amperometric sensors – measure the current generated by redox reactions at an electrode
Understanding the differences between these sensor types is crucial for designing analytical systems in:
- Clinical diagnostics (blood glucose monitoring)
- Environmental monitoring (heavy metal ions, gases)
- Industrial process control (pH, oxygen levels)
Reference:
2. Potentiometric Sensors
2.1 Principle of Operation
- Measures electrode potential relative to a stable reference electrode
- Operates at essentially zero current to avoid altering analyte activity
- Based on the Nernst equation:
E = E° + (RT / zF) ln(a)
Where:
- E = electrode potential
- E° = standard potential
- a = activity of analyte
- z = charge on the ion
- R = gas constant
- T = temperature
- F = Faraday constant
2.2 Examples of Potentiometric Sensors
- pH electrode: H⁺ detection (covered in Lecture 4)
- Ion-selective electrodes (ISEs): Na⁺, K⁺, Ca²⁺, F⁻
- Gas-sensing electrodes: e.g., NH₃ or CO₂-selective sensors
3. Amperometric Sensors
3.1 Principle of Operation
- Governed by Faraday’s law:
- i = n F A k [X]
Where:
- i = current
- n = number of electrons transferred
- F = Faraday constant
- A = electrode area
- k = rate constant for electron transfer
- [X] = analyte concentration
Amperometric sensors are widely used for oxidisable or reducible species, including gases and small molecules.
3.2 Examples of Amperometric Sensors
- Glucose biosensors: Glucose oxidase catalyses oxidation → H₂O₂ → current at electrode
- Dissolved oxygen sensors: Clark electrode
- Hydrogen peroxide detection in environmental monitoring
Reference: LibreTexts: Amperometric Sensors
4. Linear vs Logarithmic Response
4.1 Potentiometric Sensors
- Logarithmic response: The Nernst equation predicts that the electrode potential scales with log [X]
- Linear range: Small range of concentrations where slope is near-constant (~59 mV/decade for monovalent ions at 25°C)
- Saturation limits: Very high or low [X] → electrode potential deviates from linear Nernstian behaviour
4.2 Amperometric Sensors
- Linear response: Current proportional to analyte concentration over a defined range
- Saturation: At high concentrations, the reaction becomes mass-transport limited → current plateaus
- Sensitivity: Proportional to electrode area, catalyst efficiency, and diffusion rate
A worked example for a glucose Amperometric sensor will be shown below.
5. Worked Example 1: Potentiometric Sensor (K⁺ ISE)
Scenario: Potassium ion-selective electrode at 25°C
- Nernst equation:
- Step 1: Low concentration: [K⁺] = 1 × 10⁻⁴ M
- Step 2: High concentration: [K⁺] = 0.1 M
Observation: The Electrode shows logarithmic potential change across four orders of magnitude. Beyond UDL or below LDL, the potential deviates → saturation.
Reference: IUPAC: Ion-Selective Electrodes
6. Worked Example 2: Amperometric Sensor (Glucose Oxidase)
Scenario: Glucose biosensor with:
Glucose concentration, [X] = 5 mM
Working electrode area, A = 0.1 cm²
Number of electrons transferred, n = 2
Rate constant, k = 0.01 cm/s
Step 1: Faraday’s law
Step 2: Compute current
- Linear response predicted for low [X]
- At higher [X], mass transport limits → current plateaus
Reference: LibreTexts: Amperometric Biosensors
7. Comparison of Potentiometric vs Amperometric Sensors
| Feature | Potentiometric | Amperometric |
| Measured signal | Voltage (potential) | Current |
| Current flow | Minimal | Significant |
| Response type | Logarithmic (Nernstian) | Linear (faradaic current) |
| Sensitivity | Moderate | High |
| Dynamic range | Wide (log scale) | Moderate (linear range limited by mass transport) |
| Examples | pH electrode, ISEs | Glucose biosensor, Clark O₂ electrode |
8. Practical Considerations
- Reference electrode stability – crucial for potentiometric sensors
- Mass transport control – stirring, diffusion layer thickness for Amperometric sensors
- Calibration – two-point or multi-point calibration ensures linearity
- Temperature effects – both sensor types are temperature-dependent
Further reading: RSC Education: Electrochemical Sensors
9. Sensor Arrays and Hybrid Systems
- Combining potentiometric and Amperometric sensors allows simultaneous detection of multiple analytes
- Example: Blood gas analysers measure pH (potentiometric) and oxygen (Amperometric) simultaneously
- Multi-sensor arrays can compensate for cross-sensitivity and extend dynamic range
Reference: Sensors and Actuators B: Chemical
10. Summary
Electrochemical sensors provide robust methods for real-time chemical monitoring:
- Potentiometric sensors: logarithmic voltage response, ideal for ion-selective detection
- Amperometric sensors: linear current response, ideal for redox-active analytes
- Dynamic range: governed by equilibrium constants, site density, and mass transport limitations
- Worked examples illustrate the calculation of expected potentials and currents
- Proper calibration, reference stability, and environmental control are critical for accurate measurements
Electrochemical sensors are versatile and form the foundation for clinical, environmental, and industrial sensing applications.
11. Next Lecture Preview
Lecture 6: Ion-Selective Membranes and Interfaces
- Membrane types: permselective, semipermeable, non-selective
- Donnan potentials and liquid junction effects
- Membrane composition and thickness influence selectivity and response
Further reading:
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