Scheme 2. Typical Electrochemical Cell Configuration for the Operation of SEPM Techniques.

WE, working electrode; RE, reference electrode; CE, counter electrode; QRCE, quasi reference-counter electrode; colored curved arrows indicate an electron-transfer reaction recorded at the WE 1 (orange arrows) and WE 2 (red arrows). (a) Four-electrode SECM cell, where a disk-shaped micro/nanoelectrode is the SECM tip (WE 2). An electron-transfer reaction at the tip generates the SECM tip current, which is modulated by the underlying surface (WE 1). (b) SICM two-electrode configuration, in which one QRCE is inserted inside the filled nanopipette (SICM tip), while the other QRCE is immersed in the sample solution. A potential is applied between the QRCEs, and the SICM tip current is related to the ion flux through the aperture of the capillary. (c) A sharp EC-STM tip is employed as working electrode (WE 2) in a four-electrode cell configuration. The tunneling current is the set point for the EC-STM, due to the electron-transfer tunneling between the surface (WE 1) and the tip. (d) In the case of SECCM, the electron-transfer reaction occurs directly on the investigated surface (WE 1). A single-barrel or double-barrel pipette is filled with the electrolyte, and a QRCE is inserted in each channel. The SECCM tip droplet confines the electrochemical cell. In the double-barrel configuration, a potential is applied between the two QRCEs to achieve the set point, while the sample potential is applied between the sample and the QRCEs.