Unconventional Electrochemistry in Micro-/Nanofluidic Systems

Abstract

Electrochemistry is ideally suited to serve as a detection mechanism in miniaturized analysis systems. A significant hurdle can, however, be the implementation of reliable micrometer-scale reference electrodes. In this tutorial review, we introduce the principal challenges and discuss the approaches that have been employed to build suitable references. We then discuss several alternative strategies aimed at eliminating the reference electrode altogether, in particular two-electrode electrochemical cells, bipolar electrodes and chronopotentiometry.

Keywords: bipolar electrode; electrochemistry; floating electrode; potentiometry; reference electrode.

Figure 1

Equivalent circuits for (a) a polarizable and (b) a non-polarizable interface.

Figure 2

(a) Schematic of a conventional electrochemical cell for voltammetric measurement. The cell consists of three electrodes, termed the working (WE), reference (RE), and counter electrode (CE), immersed in the electrolyte solution. A potential, E, is applied to the WE with respect to the RE. If the current through the RE would be high enough to cause a potential shift, a CE is introduced to minimize the current through the RE. At low currents, it is instead possible to operate with a two-electrode configuration and eliminate the CE altogether (highlighted in green), simplifying the detection circuitry. (b) Equivalent circuit diagram of a two-electrode setup. R_s: solution resistance; R_ct: charge-transfer resistance at the WE; C: electrical double layer capacitance at the WE. This circuit treats the RE as ideally non-polarizable.

Figure 3

(a) Reference-less two-electrode system where E is the applied potential between the two WEs. (b) Corresponding equivalent-circuit diagram. R_s: solution resistance; R_ct1,2: (charge transfer) resistance at the WE_1,2.

Figure 4

(a) Schematic diagram of a bipolar electrode (brown) in contact with two separate reservoirs. (b) Alternative concept of a bipolar electrode in which a uniform electric field is applied along a channel filled with electrolytic solution. A band electrode exposed to this solution exhibits bipolarity at its opposing ends (cathodic at left and anodic on right). (c) Equivalent circuit for panel (b). E is the potential applied across the solution, R_s is the resistance of the solution, and R_ct is the charge transfer resistance across the anodic/cathodic ends of the bipolar electrode (BPE).

Figure 5

(a) Schematic diagram of a two-electrode nanogap system in contact with a solution containing reversible redox species. The bottom (unbiased) electrode accumulates charge over time, and the resulting potential shift is used as readout signal. (b) Chronopotentiometric signal versus concentration of redox species (100 µM, 10 µM, and 1 nM Fc(MeOH)₂ in 0.1 M KCl) in response to a triangular potential wave applied to the top electrode (black line).