Scanning probe microscopies and the idea of SECM
Scanning electrochemical microscopy (SECM) belongs to the family of scanning probe microscopies (SPM). Aim of these analytical techniques is to measure and visualize properties of surfaces. To achieve this goal, a sharp tip is scanned over a surface, while interactions between this tip and the surface are recorded. Depending on the nature of the tip, a broad variety of interactions and surface properties can be measured. The probably most famous scanning probe technique is the scanning tunneling microscopy (STM), that gives insight into a samples electronic properties. Another widely used member of the family of SPM is the atomic force microscopy (AFM), probing force interactions between tip and sample and eventually imaging surface morphology. The resolution of SPM techniques is limited by the size of the very end of the tip and can reach an atomic level in some of these techniques. To be able to probe interactions between tip and sample surface, the tip has to be within the nearfield regime. The range of the nearfield into space or bulk volume (if measuring in liquid media) depends on the technique, since it is determined by the type of interaction measured.
The basic idea of SECM is the visualization of local electrochemical activity of a sample surface. To achieve this, a microelectrode is scanned over the sample surface in close proximity. The nearfield regime in SECM is about three to five times the diameter of the active area of the used microelectrode. Close to the surface, the microelectrode interacts with the surface depending on the surface properties. Microelectrodes with active electrode radii below 100 µm are chosen as tips because of their unique electrochemical properties. The most important electrochemical property of microelectrodes making them for SECM is the hemispherical diffusion field at a microelectrode assuring fast mass transport towards the microelectrode and a steady-state behaviour. This current as measured at the microelectrode is modulated according to the electrochemical properties of the surface.
The basic modes of SECM
Different modes exist to probe the electrochemical activity of a sample surface by SECM. Of course, all of them operate in liquid environment.
The most easy mode in SECM imaging is the generator / collector mode. An electroactive species is produced or present at one site of the electrochemical cell and collected at the microelectrode forming the SECM tip. When the microelectrode is scanned over the active site, an increase in current can be probed at the microelectrode due to an electrochemical process (oxidation or reduction) occurring at the microelectrode.
A good example for a SECM experiment in generator / collector mode is the imaging of transport phenomena through the pores of a membrane. Oxidizable or reducible species diffuse through the opening of the membrane pores and can be converted by the SECM tip if the latter is poised to a suitable potential. When scanning over a pore, one can detect a rise in current (oxidative or reductive).
A second mode used for SECM imaging is the feedback mode. In this mode, no species of the surface are collected by the SECM tip but an electrochemical reaction at the tip is modulated by the surface properties. In feedback mode, an electrochemically active species has to be present in the measuring solution and the microelectrode forming the SECM tip has to be poised to a potential driving the convertion of the redox species present.
If the microelectrode is now approaching an insulating surface, the diffusion of redox species towards the active area of the microelectrode will be blocked by the surface. In other words, the hemispherical diffusion field established at the microelectrode will be "cut" by the (non-conductive!) surface. The blockage of diffusion towards the microelectrode will lead to a depletion of species available for the conversion and hence the current measured at the microelectrode will drop. This effect is called negative feedback and is observed approaching a insulating surface. The distance from the surface when the current starts to drop down is called the nearfield. The nearfield distance is typically 3-5 times the diameter of the active area of the microelectrode (30-50 µm for a 10 µm microelectrode).
The other effect used in feedback mode is the positive feedback. It is observed when the microelectrode is approaching a conductive surface. Following the Nernst equation, a conductive surface will establish a potential that will be able to drive the opposite reaction to the conversion happening at the microelectrode. If an oxidation process occurs at the microelectrode, the conductive surface's potential will re-reduce the species and hence induce a redox cycling leading to an increase in current at the microelectrode. In other words, the equilibrium that is disturbed by the SECM tip (microelectrode) will be reestablished by the Nernst potential of the surface to re-convert the species converted at the tip and will therefore increase the current at the tip since no depletion of convertible species happens.
The visualization of the electrochemical activity of surfaces using SECM in feedback mode takes advantage of a change in feedback signal, i.e. positive feedback over conductive/electrochemically active areas vs. negative feedback over insulating/non-conductive/electrochemically inactive areas of the surface. Different electrochemical activities will also be distinguished by different amounts of positive or negative feedback.
The starting point for all SECM experiments:
a complete scanning electrochemical microscope, including : a high-quality steppermotor-controlled xyz-positioning system : 25x25x25 mm range at 1/32 µm resolution
a powerful PalmSens bipotentiostat offering all the EC-techniques you need
full system control via an industrial PC (peripherals not included)
This system is directed primarily at scanning rather flat surfaces with electrode diameters typically in the range of 10 to 100 µm.
To increase the imaging quality of the Sensolytics Base-SECM, the option 'High-Res' provides a piezoelectric positioning system.
It offers a 3 nm resolution with a scan range of 100x100x100 µm and allows to include a three-point tilt compensation mechanism.
To satisfy the growing interest in corrosion science, the option 'AC' is equipped to perform alternating current measurements with your Sensolytics SECM.
To precisely separate topographic and electrochemical information, the option 'Shearforce' provides the ability to control the tip-to-sample separation. The tip is held in a constant distance of 50 to 300 nm above the sample surface by means of a sophisticated feedback mechanism.
Imaging the 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.
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