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SCANNING ELECTROCHEMICAL MICROSCOPY (SECM)
Department of Chemistry and Biochemistry
Queens College - City University of New York
Flushing, NY 11367, USA
Scanning electrochemical microscopy (SECM; the same abbreviation is also used
for the device, that is, the microscope) is a technique capable of probing surface
reactivity of materials at microscopic scales. Oftentimes chemical reactions
occur at the interface between two regions, for example, corrosion (metal/air or
metal/sea interface), redox reaction in batteries, photosynthesis in the cell
membrane, dissolution of compounds, etc. SECM enables scientists to investigate
the pathways and speed of such reactions with spatial resolution.
Figure 1 is a scheme of the instrument. Like other scanning probe techniques,
SECM is based on the possibility of precisely positioning a probe close to the
object under investigation (substrate). In the case of SECM, the probe is an ultramicroelectrode, which is an electrode of nanometer to micrometer
dimension (Figure 2). Ultramicroelectrode and substrate are both immersed into an electrolyte
solution containing either an oxidizable (or reducible) chemical species. The
ultramicroelectrode is electrically biased so that a redox current, the tip-current, is
generated. When the ultramicroelectrode is brought near the substrate the tip-current changes
and information about the surface reactivity of the substrate can be extracted.
By scanning the ultramicroelectrode laterally above the substrate one can acquire an image of
its topography and/or its surface reactivity.
| Fig. 2. (A) Schematics of a tip ultramicroelectrode. The exposed metal is the active part of the electrode. (B) Optical micrograph of a tip. The platinum wire (orange) is sealed inside a glass sheath. |
The invention of the scanning tunneling microscope in early 1980s by Binnig and
Rohrer almost coincided with the introduction of ultramicroelectrodes. Since that time, the idea
of scanning electrochemical microscopy was in the air. Several groups employed
small and mobile electrochemical probes to make measurements of concentration of
chemical species in localized spaces, examine and modify electrode surfaces.
However, the SECM technique, as we know it, only became possible after the
introduction of the feedback concept by the group of A. J. Bard at the University of Texas at Austin (1989).
Determination of substrate kinetics
Suppose we immerse a substrate into a solution containing a redox active species
in its reduced form "R". An ultramicroelectrode is also immersed into the solution and is electrically
biased to a potential such that "R" is oxidized into "O" at its tip.
| Fig. 3. Feedback modes: (A) bulk oxidation - no feedback, (B) oxidation near a
perfect conductor - positive feedback, (C) oxidation near a perfect insulator -
negative feedback. (D) Approach curves corresponding to B and C. |
While the tip is far from the substrate (Figure 3A) the faradaic current at the tip
generated by this reaction is constant and limited by the speed at which "R"
diffuses toward the electrode. When the tip is brought to within a few tip radii
of a conductive substrate surface (Figure 3B), the "O" species formed at the tip diffuses to the substrate, where it can be instantly reduced back to "R".
This cycling increases the flux of "R" to the tip and hence creates a "positive
feedback", that is, the tip-current increases. The closer the tip is to the
substrate, the larger the tip-current.
If the substrate is an electrical insulator (for example, glass), the tip-generated
species, "O", cannot react at its surface. In this case "R" is not regenerated by
the substrate and its diffusion toward this tip is hindered. The tip-current
decreases as the tip approaches the substrate ("negative feedback"; Figure 3C).
There is an intermediate situation where regeneration of "R" occurs at a limited rate.
Figure 4 shows a tip-current versus tip-substrate distance curve (also called
approach curve) for that situation. When the tip is approached a few radii from
the substrate, regeneration of "R" is fast enough to "beat" the speeds of diffusion of
species "R" and "O" thus the tip-current increases. As the tip gets closer the
diffusion of species "R" and "O" within the thin electrolyte layer between the tip
and the substrate gets faster, regeneration of "R" now limits the feedback and the
| Fig. 4. SECM approach curve in the case where substrate reaction rate is limited. |
Using the well-developed quantitative theory for each situation, one can
simulate tip-current against distance curves with the help of computers. A fit
between experimental and numerical data will give quantitative information about
the rate of the regeneration reactiom.
When scanning the ultramicroelectrode in a horizontal plane over an insulating substrate the
tip-substrate distance varies. In turn, changes in tip-substrate distance affect
the tip-current (Figure 5A). By recording the tip-current for every tip position a
three-dimensional image of the substrate can be acquired (Figure 5B). Depending on
the nature of the feedback (positive or negative) one can convert the three-dimensional image
into a topographical image of the substrate. The resolution of the image
is determined by the tip radius and the average tip-substrate distance. This is
why the SECM instrument is effectively a microscope.
It is also possible to directly acquire a topographical image of the substrate
if its surface is chemically homogeneous. The trick is to maintain the
tip-current constant, while horizontally scanning the substrate. This is done by
constantly adjusting the tip-substrate distance. For example one can obtain an
SECM image of a human breast cell (considering that its membrane is an
insulator) in the constant current mode. The SECM image in this mode is a direct
representation of the cell shape (Figure 6).
| Fig. 5. Substrate imaging. (A) Tip is maintained at constant height while
scanning horizontally. (B) SECM image of a portion (1 µm × 1 µm) of a human
breast cell membrane acquired with a 47 nm radius ultramicroelectrode. |
| Fig. 6. Substrate imaging (constant current mode). (A) SECM image of a portion
(10 µm × 10 µm) of a human breast cell using a 120 nm radius tip. (B) Optical
micrograph of the same cell showing the SECM image area delimited by a white
The small dimension of the tip allows for the imaging of local reactivity.
If one scans a geometrically flat substrate that is redox active one can obtain
a "chemical image" of the substrate. Figure 7A shows the topography (image
acquired with the atomic force microscope) of a flat polished substrate made of
diamond where 20 mm radius disk-like regions are doped with boron. The
height of the doped region (<1 nm) is negligible compared to the radius of the
SECM tip (2.5 µm) so that for the SECM tip, the surface appears to be flat. In
the corresponding SECM image (Figure 7B) the different colors correspond to
different tip-current intensities. The tip-current changes are attributed to
variations in surface reactivity. The SECM image shows that unlike the undoped
surrounding diamond, the small disk-like region is reactive but its surface
reactivity is not homogenous.
| Fig. 7. (A) 80 µm × 80 µm atomic force microscopy image of a disk-like region of
boron-doped diamond. (B) SECM image over such a region.|
Some applications of SECM
Measurement of rates of reactions at interfaces
The kinetics of reactions at the interface between a solid and a liquid, for example,
metal or semiconductor in solution, can be probed by SECM. The technique
can also be applied to measure kinetics of reactions at the interface between
two immiscible solutions (for example, water/oil). To perform a measurement,
one records the tip-current as the tip is approached toward the interface. The
experimental tip-current versus tip-substrate distance curve is then matched
with computer simulated curves and the value of the speed of reaction occurring
at the interface can be obtained.
SECM is a valuable tool for studies of electrocatalysis. Industry is
looking for new catalytic materials to replace expensive platinum used to
enhance the performance of fuel cells. Scientists used SECM to check the
efficiency of metallic mixtures for their catalytic activities. They
fabricated an array of bimetallic or trimetallic catalyst spots with different
compositions on a glassy carbon substrate (Figure 8.1). SECM images were collected
by scanning the tip over all the spots in an aqueous solution containing oxygen
(Figures. 8.2 and 8.3). The spot giving the highest tip-current was determined to
be the most effective catalyst. SECM enables scientist to quickly scan a large
number of candidates for catalytic activities.
| Fig. 8. Array of metal mixture catalytic spots on a glassy carbon substrate.
(1) Scanning electron micrograph of the array (rotated 90o clockwise). (2) -
(3) SECM images with substrate biased at 0.2 V and 0.75 V respectively against the hydrogen electrode. |
Many enzymes in our body catalyze redox reactions during metabolism. Scientists
are considering using these in fuel cells to produce energy from biofuel,
as part of devices such as glucose sensors to monitor the level of glucose in
blood, etc. Devices based on enzymatic bioactivity are fabricated by
chemically attaching the enzymes to the surface of a substrate and immersing it
in a solution containing the enzyme's reactant (for example, glucose for glucose
oxidase). Enzymes are designed by Nature to work inside living cells so their
efficiency in manmade devices can be altered. SECM is appropriate for the study
of immobilized enzymes because it can measure the efficiency of the enzymes
working in these unusual conditions (Figure 9).
| Fig. 9. Optical micrograph of substrate showing dark-colored immobilized
enzyme pattern (left). SECM image of the enzyme activity (right). |
Corrosion and dissolution
SECM is well-suited for high-resolution studies of metal corrosion in aqueous
environments. The reported studies were mostly focused on
| Fig. 10. Scheme of SECM-induced ionic crystal dissolution. |
- detection of small
pits and pit precursors, that is, electrochemically active sites on which pits
nucleate in the course of an experiment;
- monitoring concentration
profiles of the corrosion participants and products; and
- the use of the tip to induce the pit formation.
Dissolution of ionic crystals is a phenomenon that can be studied by SECM. The
ultramicroelectrode tip can deplete a chemical species by reducing or oxidizing it. If the
initial chemical species is in equilibrium with the crystal then the depletion
of the earlier dissolves the crystal as shown in Figure 10. The high spatial
resolution of SECM enabled scientists to determine that the dissolution of
copper sulfate crystals into water occurred at surface defects.
The SECM can be used to fabricate microstructures on surfaces by deposition of
metals or other solids or by etching the substrate. Figure 11A shows how
one can use the SECM tip to locally reduce and deposit a metal on a substrate. A
very small pattern of silver lines (Figure 11B) can thus be constructed.
| Fig. 11. (A) Scheme of SECM metal deposition, metallic ions are reduced at the
tip and oxidation occurs at the substrate/polymer interface. (B) Scanning
electron micrograph picture of a pattern of silver lines deposited in a Nafion
Some scientists have used SECM to investigate the transport of chemicals through
natural or artificial membranes, and biological tissues. SECM seems
to be particularly suited for such studies because in many cases the transport
happens through micro pores in the material and therefore is spatially
constricted. Elucidating transport mechanisms can help understand the
functionalities of biological tissue such as skin, dentine, or cartilage.
SECM has been used to probe adsorption and desorption of chemical species on
surfaces. It has been applied to metabolism study of single living cells, other biological systems, and charge transfer mechanism in
conductive polymers, efficiency of photoelectrochemical conversion, etc. These applications of SECM and other are further discussed in the
The push for nano
The reduction in size of the tip electrode offers two advantages.
| Fig. 12. Nanoelectrodes polishing station. |
As can be expected, the smaller the electrodes the more fragile they become.
Fabricating SECM tip of nanometer dimension is a very challenging task as these
electrodes need to be polished by grinding the tip with abrasive material.
Figure 12 shows a set up for polishing nanometer size ultramicroelectrodes.
Conclusions and prospects
SECM is a valuable and indispensable tool for the study of surface reactivity.
Scientists are more and more attracted to this method because of its simplicity
of use and the quantitative results it can deliver. The fast expansion of the
SECM field during the last several years has been fueled by the introduction of
new probes, commercially available instrumentation, and new practical
applications. The greatest challenge of SECM will be to routinely do
measurements with nanometer-sized electrodes, as these electrodes are much
harder to produce and use than micron-sized one. The fabrication of
nanometer-sized tips and the development of numerous hybrid techniques have
already greatly enhanced the SECM capacity to solve problems in cell biology,
surface science, and nanotechnology.
- Bibliography for SECM Papers and Closely Related Material, D. O. Wipf, available at www.msstate.edu/dept/Chemistry/dow1/secm/secm_bib.html.
- Physicochemical Applications of Scanning Electrochemical Microscopy, F. O. Laforge, P. Sun, and M. V. Mirkin, in "Advances in Chemical Physics" Vol. 139, pp 177-244, Wiley-Interscience, New York, 2008.
- Scanning Electrochemical Microscopy in the 21st Century, P. Sun, F. O. Laforge, and M. V. Mirkin, "Physical Chemistry Chemical Physics" Vol. 9, pp 802-823, 2007.
- Scanning Electrochemical Microscopy for Direct Imaging of Reaction Rates, G. Wittstock, M. Burchardt, S. E. Pust, Y. Shen, and C. Zhao, "Angewandte Chemie International Edition" Vol. 46, pp 1584-1617, 2007.
- Scanning Electrochemical Microscopy, F.-R. F. Fan, J. Fernandez, B. Liu, and J. Mauzeroll, in "Handbook of Electrochemistry" pp 471-540, C. G. Zoski (editor), Elsevier, Boston, 2007.
- Biological Applications of Scanning Electrochemical Microscopy: Chemical Imaging of Single Living Cells and Beyond, S. Amemiya, J. Guo, H. Xiong, and D. A. Gross, "Analytical and Bioanalytical Chemistry" Vol. 386, pp 1584-1617, 2006.
- Imaging Localized Reactivities of Surfaces by Scanning Electrochemical Microscopy, G. Wittstock, in "Solid-Liquid Interfaces (Topics in Applied Physics, Vol. 85)" pp 335-364, K. Wandelt and S. Thurgate (editors), Springer, New York, 2003.
- Scanning Electrochemical Microscopy, B. R. Horrocks, in "Instrumentation and Electroanalytical Chemistry" pp 444-490, P. R. Unwin (editor), "Encyclopedia of Electrochemistry" A. J. Bard and M. Stratmann (editors), Vol. 3, Wiley-VCH, Weinheim, Germany, 2003.
- Fundamentals of Scanning Electrochemical Microscopy, M. V. Mirkin and B. R. Horrocks, in "Electrochemical Microsystem Technologies (New Trends in Electrochemical Technology, Vol. 2)" pp 66-103, J. W. Schultze, T. Osaka, and M. Datta (editors), Taylor and Francis, New York, 2002.
- Scanning Electrochemical Microscopy, A. J. Bard and M. V. Mirkin (editors), Marcel Dekker, New York, 2001.
- Scanning Electrochemical Microscopy as a Local Probe of Chemical Processes at Liquid Interfaces, A. L. Barker, C. J. Slevin, P. R. Unwin, and J. Zhang, in "Liquid Interfaces in Chemical, Biological, and Pharmaceutical Applications (Surfactant Science Series 95)" pp 283-324, A. G. Volkov (editor), Marcel Dekker, New York, 2001.
- Scanning Electrochemical Microscopy: Beyond the Solid/Liquid Interface, A. L. Barker, M. Gonsalves, J. V. Macpherson, C. J. Slevin, and P. R. Unwin, "Analytica Chimica Acta" Vol. 385, pp 223-240, 1999.
- Scanning Electrochemical Microscopy, A. J. Bard, F.-R. F. Fan, and M. V. Mirkin, in "The Handbook of Surface Imaging and Visualization" pp 667-679, A. T. Hubbard (editor), CRC, Boca Baton, FL 1995.
- Scanning Electrochemical Microscopy, A. J. Bard, F.-R. Fen, and M. Mirkin, in "Physical Electrochemistry: Principles, Methods, and Applications" pp 209-242, I. Rubinstein (editor), Marcel Dekker, New York, 1995.
- Scanning Electrochemical Microscopy, A. J. Bard, F.-R. Fen, and M. Mirkin, in "Electroanalytical Chemistry: a Series of Advances" Vol. 18, pp 244-373, A. J. Bard (editor), Marcel Dekker, New York, 1994.
- Scanning Electrochemical Microscopy. Theory of the Feedback Mode, J. Kwak and A. J. Bard, "Analytical Chemistry" Vol. 61, pp 1221-1227, 1989.
Listings of electrochemistry books, review chapters, proceedings volumes, and full text of some historical publications are also available in the Electrochemistry Science and Technology Information Resource (ESTIR). (http://knowledge.electrochem.org/estir/)
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