Electrochemistry Open LessonsSustainable Electrochemical Science & Technology, BRIN

Chapters / Chapter 4, Electrochemical sensing / Lesson 4.1

4.1 How electrochemical sensors work

What makes a sensor, the main kinds of electrochemical sensors, why pulse techniques such as DPV and SWV help, and how modified electrodes improve sensitivity and selectivity.

About 25 minutesOpen to everyoneQuiz at the end

What makes a sensor

An electrochemical sensor turns the presence of a chemical, the analyte, into an electrical signal. It has two parts: something that recognises the analyte, and a transducer that turns that recognition into a current, a potential or an impedance. When the recognition element is biological, such as an enzyme, an antibody or an aptamer, the IUPAC calls it a biosensor (Thévenot and co-workers, 1999). Many sensors in our field use no biological part at all, and rely on an electrode material that oxidises or reduces the analyte selectively.

Kinds of electrochemical sensors

TypeWhat is measuredExamples
PotentiometricPotential at zero current, about 59/n mV per decade of activity at 25 °CpH glass electrode, ion-selective electrodes
AmperometricCurrent at a fixed potentialGlucose sensors through H2O2, gas sensors
VoltammetricCurrent while the potential is scanned or pulsedDPV and SWV for dopamine, uric acid, chlorogenic acid, stripping for heavy metals
ImpedimetricChange of impedance, often Rct of a redox probeLabel-free detection of proteins or DNA binding to a surface

Why pulse techniques

In a plain CV, the charging current of the double layer adds a background that hides small Faradaic signals. After a potential step, the charging current dies away quickly, within milliseconds, while the Faradaic current decays slowly with diffusion. Pulse methods use this difference by reading the current at the end of each pulse.

Differential pulse voltammetry waveform and the peak it producesPotentialtime →pulsecurrent read before the pulsecurrent read at the end of the pulsesignal Δi = difference of the twopeak height∝ concentrationE →Δi
Differential pulse voltammetry. A small pulse sits on a slowly rising staircase. The current is read just before and at the end of each pulse, and their difference gives a peak whose height grows with concentration.
  • Differential pulse voltammetry (DPV) gives a peak, not a wave, which makes it easy to read several analytes in one scan. Pulse amplitude, pulse width and step size set the sensitivity and the peak width.
  • Square-wave voltammetry (SWV) uses a square wave on a staircase and can be much faster than DPV.
  • Stripping voltammetry first collects the analyte on the electrode, for example by depositing a metal for minutes, then strips it off in one fast scan. This concentration step is why stripping can reach very low detection limits for heavy metals.

Modified electrodes

A bare electrode often reacts slowly with the analyte, or gives overlapping peaks with other species. A classic example is dopamine, ascorbic acid and uric acid, which oxidise at similar potentials on bare carbon. Modifying the surface can help in three ways:

  • More active area, for example with carbon nanomaterials, MXenes or porous films, which raises the current per geometric area.
  • Electrocatalysis, for example with gold or silver nanoparticles, which speeds up electron transfer, sharpens the peak and moves it to a lower potential. The CV simulator shows how a faster k⁰ gives a taller, narrower peak.
  • Selectivity, from charged films that repel interferents, enzymes, aptamers or molecularly imprinted polymers.

The way the material is made matters. The capping agent left on nanoparticles, for example, can block or help electron transfer, so the surface chemistry should be characterised alongside the electrochemistry.

Modified electrodes can foul, as oxidation products stick to the surface. Test the signal over repeated scans, and say whether the electrode was used once or cleaned between measurements.

Sensors outside the lab

Screen-printed electrodes put the working, reference and counter electrodes on one small strip, and a portable potentiostat, such as the BRINanoStat built in our group, can run DPV or SWV on site. A field measurement still needs the same checks as in the lab: calibration in a matrix close to the sample, a blank, and recovery tests (Lesson 4.2).

Check your understanding

Quiz for lesson 4.1

Pick one answer per question. You see the explanation straight away.

References and further reading

  1. D. R. Thévenot, K. Toth, R. A. Durst, G. S. Wilson, Electrochemical biosensors, recommended definitions and classification, Pure Appl. Chem. 71 (1999) 2333–2348, also in Biosens. Bioelectron. 16 (2001) 121–131. IUPAC page
  2. A. J. Bard, L. R. Faulkner, Electrochemical Methods: Fundamentals and Applications, 2nd ed., Wiley, 2001, chapter 7.
  3. E. P. Randviir, C. E. Banks, Electrochemical impedance spectroscopy, an overview of bioanalytical applications, Anal. Methods 5 (2013) 1098–1115. doi:10.1039/c3ay26476a
  4. N. Elgrishi et al., A practical beginner's guide to cyclic voltammetry, J. Chem. Educ. 95 (2018) 197–206. doi:10.1021/acs.jchemed.7b00361