SECM is an electrochemical imaging technique that allows to map in-situ the topography of surfaces that are immersed into electrolytes as well as a visualization of spatially-confined variations in the chemical reactivity. Typically, amperometric or potentiometric ultramicroelectrodes with radii, r, in the order of a few µm or less are employed in a SECM as scanning probes. Spatial resolution of SECM is limited by the size of the SECM tip and the minimum current that can be accurately measured. However, SECM has an exceptional chemical selectivity and is therefore an excellent tool for examining local interfacial (electro)chemical properties and reactions. Imaging laterally confined activity of immobilized enzyme microstructures, spotting microscopic superficial phenomena such as localized corrosion or the flux of ions through pores of semi permeable membranes and monitoring the activity of living biological cells are only a few out of an increasing number of applications demonstrating the potential of SECM. Operational principles and technical concept: A Faraday current caused by the electrochemical conversion of a dissolved quasi reversible redoxactive species (the mediator in its reduced or oxidised form) at a suitable constant potential provides the signal, which is exploited in the amperometric feedback mode of SECM for imaging. Generally, hemispherical diffusion of the mediator towards the electroactive disk of the microelectrode is controlling the tip current and a diffusion-limited steady-state value is measured in the bulk of solution (I = I¥, Fig. 1A). However, when the tip is brought very close to a surface, the vicinity and the nature of the sample starts heavily to perturb the tip current. Close to a conductor, an electrochemical recycling of consumed mediator molecules is leading to an increase in the tip current (I > I¥, positive feedback, Fig. 1B). In contrast, proximity to insulating surfaces is physically hindering diffusion of species towards the sensing disk and the tip current decreases (I < I¥, negative feedback, Fig.1C). Positive and negative feedback effects are highly distance-dependent. Approach curves (I/I¥ vs. tip-to-sample separation d) can be used to position the SECM tip at appropriate working distances inside the regime of the electrochemical “near field" (typically a few times the SECM tip radius or less). Image acquisition in the constant-height amperometric feedback mode is achieved by scanning the SECM tip at a user-defined fixed height in the x-y plane above the sample surface and simultaneously recording the tip current as a function of position (Figure 2).
In the generator/collector mode of SECM (Fig. 1D), readily oxidizable or reducible species possibly released from active spots at the sample surface are detected by the SECM tip (Fig. 1D), or vice versa. The tip-generation/substrate-collection mode is the better for gaining knowledge about the kinetics of interfacial redox processes while the substrate-generation/tip-collection mode represents the tool of choice for monitoring enzymatic reaction, corrosion processes, or chemical release from living cells.
Constant-height SECM imaging certainly has limits on heterogeneous surfaces with variations in both, conductivity and topography since current changes associated to distance variations cannot simply be distinguished from ones due to alterations in conductivity. Besides, tip crash is of course at risk on tilted or rough surfaces without a distance control, especially when decreasing the size of the SECM tip for imaging at higher resolution. Constant-distance mode SECM has been established taking advantage of optical and non-optical detection schemes for hydrodynamic shearforces occurring between a liquid/solid interface to SECM tips that vibrate at resonance. The distance control benefits from shearforce-induced dampening of tip vibration as typically obtained in extreme proximity to the surface. The integrated computer-controlled feedback loop of the device continually compares actual measured oscillation amplitudes with a user-defined set point and responds to deviations due to distance variations by repositioning the tip in such a way that a constant level of damping and non-contact scanning at constant distance of a few hundred nanometres is guaranteed. This opened the opportunity to acquire simultaneously the real sample topography along with the localized electrochemical tip response, which actually promotes interpretation of SECM data and effectively prevents tip crash.
In our research group, the development of microelectrochemical methods is mainly directed towards the development of scanning electrochemical microscopes (SECM) aiming on increased resolution, integrating shearforce-based constant distance positioning, allowing for the temperature control and defined temperature variation of the sample, integration of alternating current modes etc. Based on a highly modular software programmed in Visual Basic the developed SECM systems are open for novel experiments. Thus, the obtained SECMs are continuously updated according to the specific needs of the planned experiments. Presently, SECMs with different resolutions, different shearforce based constant-distance positioning modes are mainly applied for the local visualization of immobilized enzyme activity on biosensor surfaces, the high-resolution visualization of corrosion phenomena and catalyst activity, the detection of the release of electroactive transmitter compounds from adherently growing living cells, the detection of DNA hybridization, the visualization of nanopores and nanoelectrode assemblies.