Using Scanning Ion Conductance Microscopy (SICM SECM) for Simultaneous Electrochemical and Topographical Mapping

Following the creation of scanning tunneling microscopy (STM) [1], a type of scanned probe microscopy (SPM), scientists have looked for ways to utilize these types of techniques to control the spatial positioning of a high resolution probe. The aim is to make high resolution amperometric or voltammetric, conductometric, and topographical imaging of a surface possible, and one that interfaces all at once [2].

More recently, Scanning Ion Conductance Microscopy (SICM) [3], has shown itself as an adaptable non-contact visualization aid and can be implemented for a number of different applications. SICM has been useful in research looking at the surface topography of ion transport via porous materials, both artificial and biological membranes [4, 5], suspended artificial black lipid membranes [9], and the dynamic characteristics of live cells [6, 7, 8].

SICM can also be used in combination with other techniques, such as scanning near-field optical microscopy (SNOM) [10], and patch-clamping [11, 12], which opens it up for use in a range of promising new applications. However, a considerable drawback of using SICM is that it cannot detect specific chemical properties, making it chemically-blind and insensitive to electrochemical characteristics.

The creation of the scanning electrochemical microscopy (SECM), sometimes referred to as the chemical microscope, has led to the ability to collect spatially-resolved electrochemical details. It is used for the investigation of electrochemical properties and the reactivity of a range of materials and interfaces, including electrode surfaces and interfaces [13, 14, 15], membranes [16, 17, 18], and biological systems [19, 20, 21, 22, 23].

Although SECM proved to be a vital tool in lots of different applications, the technique does not allow for dependable probe-sample distance control, and the probe is typically kept at the same height throughout standard SECM scanning. This means that changes in surface topography causes the distance between the probe and the sample to be different. This causes convolution to the recorded faradaic current, and makes the interpretation of the data produced less straightforward [18].

In order to overcome and provide a solution for the problems previously discussed for SICM and SECM, techniques that combine the two types of technology have been created, allowing users to have the best of both worlds. The SICM allows for precise probe-sample distance manipulation, while the SECM part records the faradaic current for electrochemical detail gathering.

In this article, the principle of operation for SICM, SECM and SICM-SECM will be briefly discussed. Next, the probe as well as the sample that are used for SICM-SECM imaging experiments are described. Finally, simultaneous SICM-SECM topography imaging and electrochemical mapping with SmartScan RTM10e using a Park NX12 AFM system is demonstrated.

Principle of Operation

Scanning Ion Conductance Microscopy (SICM)

SICM consists of a probe made from an electrolyte-filled nanopipette in order to raster scan a substrate submerged in a solution of electrolyte, such as a PBS buffer (shown in Figure 1). A current is created between two Ag and AgCl electrodes, one back-inserted in the pipette, which provides a working electrode, and a second is dunked into the solution of electrolyte and acts as a reference electrode by applying potential.

The size of the flow of ions varies depending on the distance between the probe and the substrate (or Dp-s). The flow of ions is reduced as the probe approaches the surface of the sample. This phenomenon occurs because the current is interrupted when between the surface underneath and the end of the probe.

Plotting the flow of ion measurements from experimental data as a function of DP-s, allows the relationship between the current and the probe distance to be calculated. This involves collecting data from each Dp-s and its relative ion current value.

This information on the distance-dependent ion current is valuable for manipulating the positioning of the probe during scanning and allowing it to be fed back to the device. This allows for a topographical map of the sample being investigated to be produced.

SICM schematic illustration. The scanning probe consisted of a working electrode (WE, Ag/AgCl) back-inserted into a nanopipette. The reference electrode (RE) is placed in the bath solution (i.e., PBS buffer). The nanopipette opening is shown in the zoomed-in scanning electron micrograph. As a potential is applied between the WE and RE, a distance-dependent ion current is generated, based on which a piezoelectric positioner is used to maintain a constant probe-substrate distance during scanning. As a result, topographical information of the sample under study can be obtained.

Figure 1. SICM schematic illustration. The scanning probe consisted of a working electrode (WE, Ag/AgCl) back-inserted into a nanopipette. The reference electrode (RE) is placed in the bath solution (i.e., PBS buffer). The nanopipette opening is shown in the zoomed-in scanning electron micrograph. As a potential is applied between the WE and RE, a distance-dependent ion current is generated, based on which a piezoelectric positioner is used to maintain a constant probe-substrate distance during scanning. As a result, topographical information of the sample under study can be obtained.

Scanning Electrochemical Microscopy (SECM)

An electrode, either micro or nano and referred to as the tip, is used in Scanning Electrochemical Microscopy to scan the surface of the sample substrate. There is also a bath solution made up of an electrochemically active species and supporting electrolytes.

The electrochemical active species, when carrying out an experiment using SECM and a top with sufficient potential, will be either reduced or oxidized. Details of the quantitative electrochemical properties of the interfacial region are collected by recording the faradaic current.

There is a choice of multiple different imaging systems that have been designed specifically for SECM methods, such as positive and negative feedback modes and substrate creation or tip collection (SG/TC) mode. This feedback system utilizes the changes in the faradaic current as the tip comes closer to the sample, which can be either positive or negative determined by the conductivity of the surface.

A positive value is produced when redox cycling occurs and results in a raise in the faradaic current. In contrast, a negative value is produced in the presence of an insulative surface which causes diffusion of the redox molecules and a fall in the faradaic current.

Hybrid SICM-SECM

As a solution to the drawbacks of fixed-distance SECM imaging, a hybrid second method has been produced, called SICM-SECM. A variety of pipette-like probes have been made for this new imaging technique. One example is a micropipette with a gold covering and electrophoretic paint that acts as an insulator, designed by Bard and his colleagues [24].

Another attempt included a gold covered pipette again, but underneath there was an insulating layer made up of an atomic layer deposition of aluminum oxide and the use of a focus ion beam (FIB) to uncover the nanopore and gold electrode as created by Hersam and his colleagues [25]. A further option that moves away from using Au is the use of a theta pipette, this device has a single barrel containing electrolyte for SICM and another barrel containing pyrolyzed carbon for SECM. [26]

Techniques that use a SICM-SECM hybrid have the ability to manipulate the probe height and measure electrochemical activity, enhancing the capabilities of topographical and electrochemical imaging.

Experimental

SECM schematic illustration. The electrochemical cell consisted of a working electrode (WE), a counter electrode (CE) and a reference electrode (RE). The bath solution contains supporting electrolyte and an electrochemically active species. During SECM operation, the WE is biased at a sufficient potential at which the redox reaction of the electrochemically active species occurs. By scanning the probe at a constant-height away from the underlying substrate while recording the faradaic current response, the electrochemical activity of the surface can be obtained.

Figure 2. SECM schematic illustration. The electrochemical cell consisted of a working electrode (WE), a counter electrode (CE) and a reference electrode (RE). The bath solution contains supporting electrolyte and an electrochemically active species. During SECM operation, the WE is biased at a sufficient potential at which the redox reaction of the electrochemically active species occurs. By scanning the probe at a constant-height away from the underlying substrate while recording the faradaic current response, the electrochemical activity of the surface can be obtained.

Sample

As can be seen in Figure 3, the standard sample used for SICM-SECM experiment described herein consists of Au bars that are 10 µm in width and 300 nm in height on Pyrex substrate. The pitch width is 20 µm.

SICM-SECM standard sample. The sample consisted of Au bars with a width of 10 µm and a height of 300 nm on Pyrex substrate. The pitch width is 20 µm.

Figure 3. SICM-SECM standard sample. The sample consisted of Au bars with a width of 10 µm and a height of 300 nm on Pyrex substrate. The pitch width is 20 µm.

Probe

The probe used is adopted from a method described in Shi et al. [18]. In brief, nanopipettes obtained via laser pulling were first coated with 10 nm Cr adhesion layer, then by 200 nm Au layer by thermal evaporation. Then, chemical vapor deposition of parylene C was performed such that the Au-coated nanopipettes were entirely covered by parylene C. Finally, a focused ion beam (FIB) technique was employed to expose the nanopore and the Au crescent.

A representative scanning electron micrograph of the SICM-SECM probe can be seen in Figure 4. The diameter of the central pore ranges from 200 – 250 nm. The Au crescent thickness is ~200 nm.

Cyclic Voltammetry

Before SICM-SECM imaging, cyclic voltammetry (CV) in a bulk solution consisted of 100 mM KCl and 10 mM Ru(NH3)63+ was performed. The purpose of the CV measurement is to characterize the Au crescent electrode performance and also to choose the potential at which the Au crescent electrode will be biased at during SICM-SECM imaging experiments.

To perform the CV measurement, a potentiostat (Model 760E, CH instrument, Austin, TX) with a three-electrode electrochemical cell is used. The Au crescent electrode served as the working electrode (WE) with respect to a Ag/AgCl reference electrode (RE) and a Pt counter electrode (CE) in the bulk solution, as indicated in the cartoon illustration seen in Figure 5a.

 a) Cartoon illustration of CV measurement; b) Setup of the CV measurement performed with the probe mounted on the SICM head of a Park NX12 AFM system. In Figure 5b, setup of the CV measurement performed with the probe mounted on the SICM head of a Park NX12 AFM system is shown. The potential window used here is from 0 to -0.5 V, with a step size of 0.1 V/s.

Figure 5. a) Cartoon illustration of CV measurement; b) Setup of the CV measurement performed with the probe mounted on the SICM head of a Park NX12 AFM system.
In Figure 5b, setup of the CV measurement performed with the probe mounted on the SICM head of a Park NX12 AFM system is shown. The potential window used here is from 0 to -0.5 V, with a step size of 0.1 V/s.

a) Cartoon illustration of SICM-SECM imaging; b) Setup of the SICM-SECM measurement performed with the probe mounted on the SICM head of a Park NX12 AFM system.

Figure 6. a) Cartoon illustration of SICM-SECM imaging; b) Setup of the SICM-SECM measurement performed with the probe mounted on the SICM head of a Park NX12 AFM system.

SICM-SECM Imaging

To realize SICM-SECM imaging, a Park NX12 system in combination with an ammeter (Chem Clamp, Dagan Corporation, Minneapolis, MN) is used. A schematic diagram of the SICM-SECM setup is shown in Figure 6a.

Ion current between the Ag/AgCl electrodes inside the pipette (PE: pipette electrode) and another Ag/AgCl pallet electrode in the bath solution (RE: reference electrode) is used as feedback to control probe-substrate distance.

The Au crescent electrode (AuE) is used to acquire electrochemical signal. The potential applied between the PE and RE was 0.1 V, and the potential at the Au E was held at -0.5 V. The ammeter is used for both applying potential to the AuE and measuring the faradaic current at the AuE.

The measured faradaic current is then fed to one of the auxiliary recording channels (AUX IN 2) of the NX12 controller and recorded in real-time using the SmartScan software. As a result, simultaneous topographical imaging (from SICM) and electrochemical activity mapping (from SECM) is accomplished.

Cyclic voltammograms (CVs) taken with the Au crescent electrode in bulk solution containing 100 mM KCl and 5 mM Ru(NH3)63+.

Figure 7. Cyclic voltammograms (CVs) taken with the Au crescent electrode in bulk solution containing 100 mM KCl and 5 mM Ru(NH3)63+.

Results and Discussion

As can be seen in Figure 7, five consecutive CVs taken with the Au crescent electrode in a bulk solution containing 100 mM KCl + 10 mM Ru(NH3)63+ are shown. For Ru(NH3)63+ , starting from ~0.25 V, the reduction reaction of Ru(NH3)63+ to Ru(NH3)62+ occurs, and as the potential is ramped up, at ~-0.5 V, the electrochemical reaction is diffusion-controlled and a steady-state current is achieved. The electrochemical reaction can be seen below:

Ru(NH3)3/6+ + e-↔ Ru(NH3)26+

From the CV data, a steady-state current was reached at -0.5 V, and, therefore, in the following SICM-SECM imaging experiment, the bias applied to the Au electrode will be kept at -0.5 V.

Representative SICM-SECM images. a) SICM topography image; b) SECM faradaic current image.

Figure 8. Representative SICM-SECM images. a) SICM topography image; b) SECM faradaic current image.

In Figure 8, representative SICM-SECM images are seen. In Figure 8a, a topography image of the Au/Pyrex pattern is shown. Figure 8c (top) displays the line profile of the topography image. The measured pitch width is 20.06 µm, which matches the actual pitch width (20 µm). The measured height of the Au bar is 302.03 nm, which agrees with the actual feature height of 300 nm.

In Figure 8b, electrochemical activity map of the same region seen in Figure 8a is shown. The absolute value of the faradaic current over Au is ~4.5 nA, while over Pyrex, the absolute value of the faradaic current is ~3.6 nA. An overall ~981 pA faradaic current difference is observed.

Correlation of the topography and faradaic current images reveals the expected contrast, with enhanced faradaic current over the conductive Au regions, consistent with positive feedback due to redox cycling, and reduced Faradaic current over insulative Pyrex trenches, consistent with negative feedback due to hindered diffusion.

 c) Line profile along the line seen in a) and b). Image size: 50 µm × 25 µm.

Figure 8. c) Line profile along the line seen in a) and b). Image size: 50 µm × 25 µm.

Conclusion

In this article, the use of a Park NX12 AFM is demonstrated in combination with an ammeter for concurrent topography imaging and electrochemical mapping. The SICM-SECM probe used here consisted of a Au crescent electrode (AuE) on the peripheral of a nanopipette.

High resolution probe-substrate distance control was obtained by the ion current feedback from SICM, while simultaneous electrochemical signal collection was attained via the AuE from SECM. As a proof-of-concept experiment, a Au/Pyrex pattern standard sample was imaged with the SICM-SECM technique.

The Au bar and the Pyrex substrate were clearly resolved from the SICM topography image, with the bar height and pitch width closely matching the actual values. In terms of the electrochemical property mapping, higher faradaic current was seen when the probe was scanned over Au bar as a result of redox cycling, while lower faradaic current was observed when the probe was over Pyrex substrate due to hindered diffusion.

The capability of the SICM-SECM technique described here holds promise of many exciting applications in the field of electrochemistry, material science and battery research.

Acknowledgments

Original authors: Wenqing Shi, Cathy Lee, Gerald Pascual, John Paul Pineda, Byong Kim, Keibock Lee, Park Systems Inc., Santa Clara, CA USA.

References and Further Reading

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This information has been sourced, reviewed and adapted from materials provided by Park Systems Inc.

For more information on this source, please visit Park Systems Inc.

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