How are electrochemical sensors, EQCM and spectroelectrochemistry set up?
1 Theory
Electrochemical sensors
Amperometric sensors give a current proportional to the analyte concentration at constant potential (glucose, O₂, H₂O₂); potentiometric sensors (ion-selective electrodes, pH) produce a voltage dependent on the logarithm of concentration with the Nernst slope; voltammetric sensors provide multi-analyte separation with pulsed techniques; impedimetric (bio)sensors measure the change in charge-transfer resistance caused by binding to the surface. Performance metrics: sensitivity (slope), detection limit (3σ/slope), linear range, selectivity (interference), response time, repeatability and stability. Screen-printed electrodes (SPE) are a disposable, low-cost sensor platform.
Electrochemical quartz crystal microbalance (EQCM)
The resonance frequency of a quartz crystal falls with mass added to its surface (Sauerbrey: Δf = −C_f·Δm; ~56 Hz·cm²/µg for a 5 MHz crystal). If the metal film on the crystal is used as working electrode, the mass change accompanying the electrochemical process (plating, polymerisation, intercalation, corrosion) is followed simultaneously with ng/cm² sensitivity; the ratio of charge passed to mass (Faraday) gives the molar mass of the species transported in the reaction. Sauerbrey is valid only for thin, rigid films; on viscoelastic films the damping (resistance, dissipation) must be measured too.
Spectroelectrochemistry
Optical measurement (UV-Vis, NIR, IR, Raman, fluorescence) simultaneous with electrochemical excitation identifies chemically the species produced by charge transfer. Thin-layer cells (short optical path, fast electrolysis) and optically transparent electrodes (ITO, Pt grid, gold mesh) are used; spectra are recorded while the potential is stepped and E°′ and n are determined by Nernst analysis. The electrochromism of conducting polymers, potential titration of redox proteins and in situ Raman of battery electrodes are typical applications. Time synchronisation (triggering) and the light/electrode geometry are critical parts of the set-up design.
2 Application in measurement
In sensor development modified electrodes are characterised with CV, calibration is done with pulsed techniques (DPV/SWV), selectivity and stability are followed with chronoamperometry/EIS. Low-current accuracy (ULC) is decisive on micro/nanoelectrodes.
The quartz crystal microbalance (QCM) converts the change in resonance frequency to mass by the Sauerbrey equation (ng/cm² sensitivity); simultaneously with electrochemistry (EQCM) film deposition, insertion and adsorption are followed. On viscoelastic films dissipation (QCM-D) is required.
Spectroelectrochemistry (UV-Vis/NIR/IR) records the potential-dependent spectral change at the same time; thin-layer cells and spectrometer triggering with EC-Lab® are used.
Key equations
Sauerbrey: Δf = −(2f₀²/(A√(ρq·μq)))·Δm
3 Applicability
Where it applies
- Biosensor and chemical sensor development, electroplating and polymer film growth, intercalation mechanism (EQCM), intermediate identification and mechanism (spectroelectrochemistry).
Limits and caveats
- In EQCM liquid loading, viscosity and roughness affect the frequency; mass interpretation requires calibration and damping measurement.
- In spectroelectrochemistry ohmic drop and slow mass transport in the thin-layer cell distort the voltammogram; no kinetic interpretation should be made.
4 What did the application notes find?
- Sensor AN67 — Shows the comparison of SWV, DPV and NPV in a sensor application and parameter selection; explains that the highest sensitivity at trace concentrations is obtained with SWV.
- Electrochemistry AN13 — Shows the simultaneous monitoring of the mass increase during polypyrrole film growth by potentiostat–QCM coupling; comparison of charge passed with mass through the Faraday relationship.
- Battery AN68 — Shows the study of a LiFePO₄ electrode by in situ EQCM; relating the mass change during cycling to Li insertion/extraction and surface-film formation.
- Electrochemistry AN12 — Shows the coupling of UV-Vis spectroscopy with electrochemistry on a polypyrrole film; explanation of the potential-dependent absorbance changes by polaron/bipolaron formation.
- Spectroelectrochemistry AN52 — Shows the design of the thin-layer spectroelectrochemistry cell for UV-Vis, NIR and IR and the superiority of in situ over ex situ measurement.
5 Which instruments perform this measurement?
References
- article G. Sauerbrey. Verwendung von Schwingquarzen zur Wägung dünner Schichten und zur Mikrowägung. Z. Phys. 155, 206–222, 1959. doi:10.1007/BF01337937
- article D. A. Buttry, M. D. Ward. Measurement of Interfacial Processes at Electrode Surfaces with the Electrochemical Quartz Crystal Microbalance. Chem. Rev. 92, 1355–1379, 1992. doi:10.1021/cr00013a005
- article J. G. Osteryoung, R. A. Osteryoung. Square Wave Voltammetry. Anal. Chem. 57, 101A–110A, 1985. doi:10.1021/ac00279a789