In common scanning electrochemical microscopy (SECM) experiments a redox mediator is used to probe the electrochemical activity at a surface. The issue of the distance-dependence of the measured current can be circumvented by using Shear Force Sensing in constant-distance scans (in contrast to constant-height scans).
An alternative mode of SECM which does not need a redox mediator to gather information about the electrochemical activity of a surface is alternating current scanning electrochemical microscopy (AC-SECM). In this mode an alternating current with a small amplitude around 40 mV and frequency of around 50 kHz is applied to the microelectrode. Experiments can be conducted in electrolyte solution of low concentration, e.g. 10 mM KCl. When approaching a surface, the measured solution resistivity is changing when getting closer to the surface due to the thinning layer of electrolyte. On insulating sample, a negative-type feedback is recorded. On conducting samples, a positive-type feedback is recorded. As in SECM, the current response is distant-dependent. AC-SECM can also be combined with Shear Force Sensing.
The images in Fig. 1 show the results of an AC-SECM constant-distance scan above a glass sample with deposited gold structures. The topography is recorded by Shear Force Sensing (Fig. 1a). The gold structures are several micrometers in height and seen in green to yellow. The glass substrate is seen in blue. Some protruding particles are distributed on the surface. The material contrast can be seen both in the phase (Fig. 1b) as well as the admittance map (Fig. 1c). The admittance is especially sensitive at the edges between gold and glass.
Fig. 1: AC-SECM in constant-distance mode at a sample with gold structures on a glass substrate; a) topography image via shear force sensing, b) phase image and c) admittance image.
On samples with homogeneous electrochemical activity, such as biological samples in a petri dish, the AC-SECM approach curve shows purely topographical differences and can be used for topography mapping.
The image on the right (Fig. 2) shows the signal of the phase and the admittance during an approach of the microelectrode to a glass surface. They behave quite similar to SECM feedback mode approach curves.
Fig. 2: Approach curves in AC-SECM with the signal of the phase and the admittance.