Combining Atomic Force Microscopy with Scanning Electrochemical Microscopy

Topics Covered

Overview of SECM
Applications of SECM
The Bifunctional AFM-SECM Probe
AFM-SECM Instrumentation
AFM-SECM: Application Examples
Summary
References
Authors
AFM Instrumentation from Keysight Technologies

Overview of SECM

Scanning electrochemical microscopy (SECM) is a powerful scanning probe technique that is suitable for investigating surface reactivity as well as processes at solid/liquid and liquid/liquid interfaces.

Redox reactions and their kinetics involving active species are of fundamental importance in emerging research and application areas ranging from the analysis of biochemical signaling processes (e.g., at live cells and tissues) to relevant questions in materials science(e.g., fuel cell technology, catalysis, sensing, and environmental chemistry).

While electrochemical scanning tunneling microscopy (EC-STM) and electrochemical atomic force microscopy (EC-AFM) are predominantly based on imaging structural/topographical and electronic changes at a biased macroscopic sample surface, SECM advantageously combines fundamental microelectrochemical information via the entire palette of electroanalytical techniques with imaging modalities.

The scanning electrochemical microscope was introduced by Bard and coworkers with the fundamental principle entailing scanning a biased ultramicroelectrode (UME) as an imaging probe across the sample surface, while recordingFaradaic (redox) currents at the UME and, optionally, also at the sample.

In contrast to EC-AFM and EC-STM, SECM is not limited to conductive or biased samples. In addition, due to thoroughly developed theoretical descriptions and models, SECM readily allows the quantification and prediction of experimental data. Since the introduction of SECM, a multitude of imaging modalities have been developed and experimentally demonstrated based on a wide variety of electrochemical analysis techniques, including DC voltammetry, redox competition mode, and alternating current (AC)-SECM imaging.

The so-calledfeedback mode is the most commonly applied imaging mode in SECM. Here, the probe and the sample are immersed in a solution containing a redox active species (e.g., R providing a reversible redox behavior governed by diffusion). If an appropriate potential is applied at the probe, R is oxidized to O, thereby resulting in a steady-state Faradaic current, which is proportional to the radius, r, of the UME and the concentration, c, of the redox species following I = 4nFDcr (n = number of transferred electrons, F = Faraday constant, D = diffusion constant of the redox active species).

If now the biased probe (i.e., the UME) is moved toward a sample surface (Figure 1) at a distance of several electrode radii, the faradaic current measured at the probe is increasingly affected by the sample surface properties. An insulating sample surface or surface feature leads to a diffusion limitation of electroactive species (here, R) towards the electrode, hence resulting in a reduced faradaic current vs. the steady-state current (Figure 1, left). A conductive sample surface or surface feature results in a locally increased concentration of the redox species, as the oxidized species can be regenerated at the sample surface (here by reduction), which leads to an instantaneously increased faradaic current (Figure 1, right) in comparison to the steady-state current.

Figure 1. Feedback mode. Left: Negative feedback effect due to hindered diffusion of the redox active species towards the UME. Right: Positive feedback effect due to the regeneration of the redox mediator at the sample surface.

The generation/collection mode (Figure 2)is based on detecting electroactive species at the UME, which are generated at the substrate surface (substrate generation/tip collection mode, SG/TC, Figure 2, left). This mode has been used for imaging transport phenomena through membranes, investigation of corrosion processes, and imaging of biological samples (e.g., immobilized enzymes). Tip generation/substrate collection mode (TG/SC mode, Figure 2, right) is based on the generation of an electroactive species at the UME, which is detected at a macroscopic electrode surface. This mode is mainly applied to study fast, homogeneous reactions.

Figure 2. Generation collection mode. Left: Substrate generation/tip collection mode. Right: Tip generation/substrate collection mode.

Applications of SECM

SECM has evolved from an expert tool into a flexible scanning probe technique, which is reflected in a steadily increasing number of applications. As SECM is extremely versatile for investigating dynamic surface processes and is not limited to certain sample types or sizes, SECM has been applied in the following ways:

  • Investigation of homogeneous and heterogeneous electron-transfer reactions
  • Imaging of biologically active processes
    • Live-cell monitoring
    • Respiratory activity
    • Analysis of membrane transport
    • Enzyme activity
    • Monitoring release and uptake of signaling molecules
  • Surface modification
    • Redox etching
    • Metal or polymer deposition
    • Patterning of self-assembled monolayers
  • Analysis of thin films (e.g., pinhole detection, conformity, etc.)
  • Screening of catalytic material (e.g., fuel cell catalysts)
  • Corrosion processes
    • Triggering and localized monitoring of corrosion processes
    • Detection of precursor sites (e.g., for pit corrosion)
    • Biocorrosion

The Bifunctional AFM-SECM Probe

A drawback of conventional SECM compared to AFM and STM is its limited resolution, which results from the current-dependent positioning of the UME in close proximity to the sample surface and imaging in a constant-height regime. As the required imaging distance is directly correlated to the UME radius, most SECM experiments are performed using micrometer-sized UMEs for avoiding probe crashes and potential convolution of surface roughness with electrochemical information, which may lead to misinterpretation of the obtained electrochemical data. Although nanoelectrodes are reported in the literature, their usage in conventional SECM experiments for reliable imaging remains challenging.

The EC SmartCart probe offered by Keysight Technologies provides an innovative solution to the aforementioned problem by directly integrating a micro- or nanoelectrode into an AFM probe. This integrated probe (i.e., the AFM- SECM probe) maintains the functionality of both the AFM and SECM techniques via integration of a sub-microelectrode recessed from the end of the AFM tip. Consequently, the electrode is located at a pre-defined distance from the sample surface, determined by the height of the actual AFM tip (Figure 3). Thus, in situ (electro)chemical information on a wide range of homogeneous or heterogeneous electron-transfer processes occurring at surfaces and interfaces can be simultaneously obtained during AFM imaging.

Figure 3. Microfabricated, bifunctional AFM-SECM probe. The probe has a frame-shaped electrode, recessed from the AFM tip, which governs the distance between the electrode and the imaged surface.

AFM-SECM Instrumentation

The combined AFM-SECM system includes a standard Keysight AFM (5500/7500) platform with a built-in bipotentiostat that controls the potential of both the sample (for generation/collection mode) and the tip against the same reference electrode. The bifunctional AFM-SECM tip is mounted to a special SECM nosecone that plugs into a standard AFM scanner (Figure 4).

Figure 4. Combined AFM-SECM system includes an AFM control unit and an SECM unit. The EC SmartCart is pre-mounted on a special nosecone that inserts into a standard AFM scanner from Keysight.

The tip current (i.e., the current flowing through the tip) is measured by a preamplifier built into the SECM nosecone, which is located close to the tip itself in order to minimize electromagnetic noises from the line. The major advantage of this setup is that the tip is pre-mounted onto a cartridge (Figure 4, bottom right photo) and tested in the factory before being delivered. The system user need only plug the cartridge into the nosecone and start the experiment. This eliminates the challenging and time-consuming work normally required to prepare for an SECM experiment (i.e., mounting the tip, making electric contact, insulating the tip, etc.), thus allowing the user to focus on research instead of setup.

AFM-SECM: Application Examples

After a probe is mounted onto the cartridge, the electrochemical performance of the probe will be tested in electrolyte solution (0.1MKCl) containing a standard redox mediator such as 10mM [Ru(NH3)6]3+. A typicalCV of the probe (inset) recorded in the AFM setup is presented in Figure 5.

Figure 5. Combined AFM-SECM measurements based on AFM-tip–integrated electrodes. Bottom: Simultaneously recorded images showing the topography (left) of a pattern deposited from platinum/carbon composite by an ion beam-induced deposition (SEM image, middle) and the electrochemical image recorded in feedback mode SECM (right).

The steady-state current varies with the actual size of the ring electrode. The noise of the measured current is evaluated by measuring the redox current as a function of time at a constant potential. Typical noise level of the measured current is about 10 pA (Figure 5), allowing users to perform low-current experiments.

Results from a simultaneously recorded contact mode AFM and feedback mode SECM experiment on a model sample are presented in Figure 6 to demonstrate the functionality of the combined AFM-SECM system.

Figure 6. Topography (left) and SECM (right) images of an Au/Si sample recorded in 1mM Fe methanol solution/0.1MKCl with a combined AFM-SECM probe biased at 240mV vs. Ag/AgCl. Topography image shows the deposited Au strip on Si substrate; SECM image shows a corresponding larger current on the conductive Au surface.

The model sample contains conductive (gold stripes) and nonconductive (Si wafer) regions coexisting on the surface. This Au/Si sample was imaged in contact mode AFM in a 1mM Fe methanol solution/ 0.1M KCl solution with the AFM-tip– integrated electrode biased at 240mV vs. Ag/AgCl.

The simultaneously recorded topography and current images are shown in Figure 6. The SECM image was obtained in the so-called feedback mode, while the sample was not biased during the experiment. Due to the feedback effect, as explained above, the SECM current is smaller on the insulating Si surface and larger on the conducting Au surface.

Another example of AFM-SECM imaging is presented in Figure 7. The sample is a Pt-coated glass slide with FIB-structured patterns. Images are recorded in AFM contact and SECM feedback mode in 10mM [Ru (NH3)6]3+/0.1MKCl. The SECM image (Figure 7, left) revealed some changes in conductivity that are not clearly visible in the corresponding topography image.

Figure 7. Images recorded in AFM contact and SECM feedback mode in 10mM [Ru(NH3)6] 3+/ 0.1M KCl. The sample is a microstructured, platinum-coated glass slide with a nonconductive star pattern.

The imaging power attainable by combining information on the surface morphology with localized (electro)chemical data can be applied to a wide variety of complex engineering and biological problems, ranging from corrosion science to life sciences. For example, modification of the integrated electrode surface with enzymatic biosensing interfaces results in imaging amperometric nanobiosensors. In addition, boron-doped diamond can be used as electrode material, resulting in a combined probe with exceptional properties in terms of robustness and potential window. The integrated SECM functionality is not limited to amperometric experiments, also imaging potentiometric microsensors (e.g., Ir/IrOx or Sb electrode for laterally resolved pH measurements), or thin film amalgam microelectrodes (Au/Hg or Pt/Hg) for imaging stripping voltammetry (e.g., for heavy metal detection) are envisaged.

Summary

This combined AFM-SECM approach (i.e., utilizing a bifunctional probe) provides topographical and correlated electrochemical information withhigh spatial and temporal resolution, thereby enabling the transformation of scanning probe microscopic techniques into multifunctional devices useful in industrial and academic environments for fundamental or applied interests.

The innovative probe design discussed in this article eliminates certain intrinsic drawbacks in conventional SECM, providing high-resolution topographical information correlated with electro(activity) information of the sample. Keysight’s unique design of an SECM nosecone with pre-mounted probes on exchangeable cartridges allows users to perform SECM measurements without having to deal with the time-consuming process of experimental setup.

References

[1] A. J. Bard, F. R. F. Fan, D. T. Pierce, P. R. Unwin, D. O. Wipf, and F. Zhou, Science (80-), 1991, 254, 68–74.

[2] P. Sun, F. O. Laforge, and M. V Mirkin, Phys. Chem. Chem. Phys., 2007, 9, 802–23.

[3] K. Eckhard, X. Chen, F. Turcu, and W. Schuhmann, Phys. Chem. Chem. Phys., 2006, 8, 5359–5365.

[4] K. Eckhard and W. Schuhmann, Analyst, 2008, 133, 1486–1497.

[5] J. Kwak and A. J. Bard, Anal. Chem., 1989, 61, 1221–1227.

[6] C. Lee, J. Kwak, and F. C. Anson, Anal. Chem., 1991, 65, 1501–1504.

[7] C. Kranz et al., Anal. Chem., 2001, 73, 2491–2500.

[8] C. Kranz et al., Ultramicroscopy, 2004, 100, 127–134.

[9] A. Kueng et al., Angew. Chem., Int. Ed., 2005, 44, 3419–3422.

[10] A. Eifert et al., Electrochem. Commun., 2012, 25, 30–34.

Authors

PD Dr. Christine Kranz, Institute of Analytical and Bioanalytical Chemistry, University of Ulm, Germany

Dr. Shijie Wu, Keysight Technologies, 5301 Stevens Creek Blvd, Santa Clara, CA 95051, USA

AFM Instrumentation from Keysight Technologies

Keysight Technologies offers high-precision, modular AFM solutions for research, industry, and education. Exceptional worldwide support is provided by experienced application scientists and technical service personnel. Keysight’s leading-edge R&D laboratories are dedicated to the timely introduction and optimization of innovative, easy-to-use AFM technologies.

This information has been sourced, reviewed and adapted from materials provided by Keysight Technologies.

For more information on this source, please visit Keysight Technologies.

Ask A Question

Do you have a question you'd like to ask regarding this article?

Leave your feedback
Submit