In scanning electrochemical microscopy (SECM) an ultramicroelectrode (UME; commonly r = 5 µm – 25 µm) or nanode (commonly r = 500 nm) is scanned in close proximity (d ≈ r) above a substrate to probe the local electrochemical activity of the substrate surface immersed in an electrolyte solution. The solution contains a redox mediator which can reversibly oxidized and reduced.
UMEs are miniature disk electrodes which are obtained by sealing thin metal wires into pulled glass capillaries. The exposed metal disk is polished to a mirror finish. When referring to the metallic disk of the UME, the term tip is commonly used. Upon reaction of the redox mediator at the tip a hemispherical diffusion field is built up and a steady-state current is measured. Due to these defined conditions of mass transfer, the current at the microelectrode can be expressed in a mathematical formula.
Applying a suitable potential to the UME causes the diffusion-controlled oxidation (or reduction) at the UME and a steady-state faradaic current is measured. A hemispherical diffusion layer forms around the UME tip.
As the UME approaches a surface of a substrate, a change in the measured current can be observed. An electrochemically inactive surface hinders the diffusion to the UME resulting in a smaller current at the UME. This reduced current is called a negative feedback response in SECM.
The current is increased by an electrochemically active surface, because the redox mediator is regenerated at the substrate surface and can diffuse back to the UME. This enhancement of current is called positive feedback response in SECM.
A special strength of SECM is the good mathematical description of microelectrodes which allows the digital simulation of many different experiments. This can help to unravel reaction mechanisms.
The feedback mode (FB) is the most popular mode in SECM. Kinetics of electron transfer reactions are often investigated. A means to extract kinetic parameters in the experiment is by conducting an approach curve with the UME to the substrate surface while measuring the distance-dependent current at the UME and keeping the substrate at open circuit potential (OCP).
By fitting the approach curves kinetic parameters can be extracted and the exact distance of microelectrode to surface can be determined.
Most SECM modes are commonly used for mapping, often called 3D scans. The microelectrode is scanned in x and y direction in a constant height above the surface (usually half a tip diameter above the sample) while the current is recorded. The results are presented in a false color plot where the amplitude of the current is expressed as colored pixel and is a measure for the electrochemical activity of the sample.
This example shows a 3D scan over an interdigitated platinum electrode printed on a glass support. The platinum structures give rise to a positive feedback current which appears orange. The glass shows a negative feedback and the current is close to 0.
As can be seen in the approach curves above, the current is strongly dependent on the tip-sample-distance. On samples with a complex topography where the tip-sample-distance varies during the 3D scan, the resulting color map is a superposition of electrochemical activity and topography. To avoid this, constant-distance modes can be employed, where the distance between tip and sample is adjusted at each point of the map. The constant-distance mode yields two images, one for the topography and one for the pure electrochemical activity. Shear Force Sensing can be used to sense the topography and adjust the distance before recording the current.
Generation Collection Mode
In this mode the UME and substrate of interest can respectively serve as either a generator or collector of redoxactive species, namely (left) sample generation – tip collection (SG-TC) mode, and (right) tip generation – sample collection (SC-TG) mode. In this way, the UME tip current always reflects a close correlation with the sample’s activity.
The direct mode is a special form of the generation collection mode. It can be used for local surface modification. The microelectrode acts as generator of e.g. noble metal ions which are dissolved from its surface by applying a suitable, positive potential. At the substrate, the ions are reduced to the metal and deposited on the surface. In this way, nano- or microstructures (depending on the size of the microelectrode tip).
Redox Competition Mode
This operation mode contains two sequential steps. In step-1 the UME serves as the generator of redox active species; in step-2 both the UME and the substrate of interest are electrochemically controlled to form a competition correlation in the consumption of the newly generated redox active species in step-1. In this way, the tip current reflects the difference in the consumption rate between the UME and substrate surface and hence provides information about the electrochemical activity of the surface in the vicinity of the UME.
Surface Interrogation Mode
In the surface interrogation mode of SECM a titration of surface species is performed in a two-step procedure. The microelectrode is brought into close proximity (≈ 3 µm) to the surface under investigation. It is crucial that a negative feedback response is recorded, and no regeneration of redox mediator takes place. On conducting surfaces, this can be achieved by minimizing the substrate size to that of the microelectrode diameter (2 rT). In the first step, a surface species is generated (this can be done chemically, spontaneous, electrochemically etc.) and the surface is then switched to open circuit potential (OCP). At the microelectrode the potential is switched from OCP to a suitable potential to generate a titrant from a redox mediator which is present in the electrolyte from the beginning. The titrant undergoes a chemical reaction with the surface species and a current transient is recorded. The titration is over and the current drops when all surface species are consumed and the current drops to the negative feedback current. By integration of the current transients, surface species can be quantified.