Scanning Gel Electrochemical Microscopy (SGECM) for Topography

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Scanning Gel Electrochemical Microscopy (SGECM) for Topography and Electrochemical Imaging Liang Liu, Mathieu Etienne, and Alain Walcarius Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b01011 • Publication Date (Web): 13 Jul 2018 Downloaded from http://pubs.acs.org on July 15, 2018

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Analytical Chemistry

Scanning Gel Electrochemical Microscopy (SGECM) for Topography and Electrochemical Imaging Liang Liu*, Mathieu Etienne, Alain Walcarius Université de Lorraine, CNRS, Laboratoire de Chimie Physique et Microbiologie pour les Matériaux et l’Environnement (LCPME), UMR 7564, Villers-lès-Nancy 54600, France *Phone: +33-3-72747398; Email: [email protected]

Abstract Scanning electrochemical probe techniques have been widely applied for analyzing the local electrochemical activity of surfaces and interfaces. In this work, we develop a new concept of carrying out local electrochemical measurements by localizing both the electrode and the electrolyte. This is achieved through a gel probe, which is prepared by electrodepositing chitosan-gelatin gel on a microdisk electrode. It is positioned in contact with the sample surface by shear force feedback. The preliminary results indicate that the topography of the sample can be mapped by tapping the probe and recording the coordinates at a given normalized shear force signal, while the local electrochemical activity can be retrieved from local measurements with the probe touching the sample surface. The technique is denoted as scanning gel electrochemical microscopy (SGECM). As compared with existing techniques, it has a major advantage of operating in air with the electrolyte immobilized in gel. This would prevent the spreading and leakage of solution on the sample surface, and may lead to field applications. Keywords: gel, electrodeposition, electrochemical imaging, topography

The analysis of local electrochemical activities of surfaces is highly important for fundamental research as well as applications of electrodes and materials in electrocatalysis, batteries, supercapacitors, corrosion protection, etc. This is commonly achieved by scanning electrochemical probe techniques, notably scanning electrochemical microscopy (SECM) and scanning ion conductance microscopy (SICM) that were both proposed in late 1980s.1-3 SECM is based on measuring electrochemical signals through a micro- or nano-disk electrode, while SICM relies on

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measuring the ion conductance through a micro- or nano- capillary. In both techniques, the sample must be immersed in an electrolyte solution. The probe, namely the micro-/nano- disk electrode or capillary, must be positioned close to the sample surface. This constitutes the basis for analyzing the sample topography (where the name “microscopy” originated from), but also requires accurate control of distance between the probe and the sample. A common practice is to use approach curves to determine the distance, which was problematic due to the influence of the shape of the probe (the ratio between the glass shield and the metal Rg, whether the electrode was recessed, etc.), the roughness and electrochemical activity of the sample, as well as the approach speed.4 Later, the developments involved using non-electrochemical signals (e.g. shear force5-10, atomic force microscopy (AFM)11-14) to independently determine the probe-sample distance for SECM and SICM, which allowed topography of the sample to be analyzed separately from the electrochemical activity. SECM and SICM, as well as their combination with other techniques, have been well developed as powerful methodologies for measuring kinetics of heterogeneous charge transfer on electrodes15-19, analyzing the biological activity of proteins and living cells20-25, evaluating localized corrosion behavior of metals26-30 and patterning/etching various materials15,31-35. The main concept of SECM and SICM is based on localizing the electrode (either solid or liquid) as probe, and the whole sample is immersed in the electrolyte solution. Considering that the scan is relatively slow in order to minimize the convection, the acquisition of a map with decent resolution may take up to tens of minutes and even hours. Within this period, the sample may significantly change, especially when the sample is polarized since potential is applied to the whole sample surface, or when the sample is highly reactive such as magnesium alloys in corrosive media. This would make the mapping results unmeaningful. Besides, immersion of sample in solution also technically makes SECM and SICM difficult to be applied for irregularly-shaped samples. Instead of localizing the electrode, one may also localize the electrolyte for carrying out local electrochemical measurements. This is known as scanning droplet cell (SDC)36,37. It is based on a capillary which holds a droplet at its opening. When the capillary is approached close to the sample, the droplet may touch the surface allowing electrochemical measurements to be carried out only in a confined area. Recently, Unwin et al.38,39 developed scanning electrochemical cell microscopy


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(SECCM) based on a similar concept. It involves a double-barreled capillary with a reference electrode and a counter electrode in each barrier. This provides well-defined potential for the reference electrode, which miniatures the three-electrode system in a compact probe with droplet. Due to matured methods of pulling capillaries, SDC and SECCM may achieve nanometric resolution. The sample is placed in air, and it is only in local contact with electrolyte which would minimize the change of the sample during the measurement. These advantages yield wide applications of SDC and SECCM in evaluating electrocatalysts and local corrosion, etc. Nevertheless, it should be noted that the quantification of SDC and SECCM measurements depends on the spreading of the droplet on the sample, which is strongly affected by the surface properties of the sample such as roughness and hydrophobicity40. In this work, we push forward the development of scanning electrochemical probe techniques by localizing both the electrode and the electrolyte. The concept is based on a gel probe, which consists of a micro-disk electrode covered with an ionically conductive gel in hemispherical shape. The sample is placed in air. When the gel probe is in contact with the sample, electrochemical measurements are carried out for measuring the local electrochemical behavior. The contact is sensed by shear force feedback, which is applicable to both conductive and insulating surfaces. The probe may scan over the surface by tapping, i.e. alternatively approaching and detaching from the sample, for mapping the topography and electrochemical activity of the sample. This technique is denoted as scanning gel electrochemical microscopy (SGECM). As compared with SECM, SGECM offers a major advantage of operating in air, which may allow measuring irregularly-shaped samples. It may also prevent the spreading and leakage of electrolyte on the sample surface, which is a technical advantage over SDC and SECCM. Moreover, it should be noted that SGECM is not just simply replacing the electrolyte solution with a gel as compared with existing techniques. The limited volume of electrolyte would make a major difference in terms of quantification, which is yet to be explored in future.

EXPERIMENTAL SECTION Micro-disk electrodes were prepared with 25 µm diameter Pt wire (Goodfellow, UK) sealed in glass capillaries by epoxy resin. The diameter ratio of the insulating shield to the Pt wire (commonly known



as Rg) was approximately 2. The gel probes were fabricated by electrodepositing chitosan with gelatin on the micro-disk electrodes. The chitosan/gelatin solution was prepared by dissolving 1 wt.% chitosan and 3 wt.% gelatin in a 1:1 (vol. ratio) water/glycerol mixture. The solution was heated to 60 ℃ and pH was adjusted to 5 by HCl to allow chitosan and gelatin fully dissolved. For electrodeposition, the micro-disk electrode was immersed in the solution and connected as working electrode, an Ag/AgCl quasi reference electrode (QRE) was used as reference electrode and a Pt wire was used as counter electrode. The electrodeposition was carried out by applying −1.2 V (vs. Ag/AgCl QRE) for 100 s. After electrodeposition, the probe was either used as is, or soaked in different electrolytes according to the need. The gel could also be removed by mechanical polishing after experiments and the Pt microelectrode could be regenerated for electrodeposition again. The electrochemical behavior of the gel probe was tested in a redox media containing 0.5 mM ferrocenedimethanol, 0.05 M KCl in 1:1 (vol. ratio) glycerol/H2O. Cyclic voltammetry (CV) was measured immediately when the gel probe was immersed in the solution. The CV of the gel probe was also carried out at different immersion times to examine the permeability of the gel. The SGECM experiments were carried out on a shear force SECM setup developed by Sensolytics GmbH (Ruhr-Universität, Bochum, Germany), equipped with a PalmSens bipotentiostat (Palm Instruments BV, Houten, The Netherlands), and modified in our laboratory. Shear force detection was obtained by a set of two piezoelectric plates (Piezomechanik Pickelmann, München, Germany) mechanically attached to the microelectrode on a home-made electrode holder.41 The gel probe was approached to the sample surface in air following the protocol by Etienne et al.42 AC excitation was applied on one plate, and the response signal was recorded from the other one using a lock-in amplifier (model 7270 DSP from Signal Recovery, Oak Ridge, TN, USA). The frequency of the excitation signal was scanned between 150 and 250 kHz, and that corresponding to the maximum amplitude of response was selected for approaching the probe. For measuring the approach/retract curve, both shear force and current responses as a function of z coordinate were recorded by applying a potential between the Pt microelectrode in the gel probe and the sample surface. For mapping, the gel probe was approached to the surface until touching, which was quantified by 0.6% change in the amplitude of the shear force signal. Then the probe was retracted 5 µm away from the surface and kept for 2 s, allowing


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a steady current to be recorded. After measuring one point, the gel probe was withdrawn 70 µm to be detached from the sample before laterally moving to the next measuring point. As a result, the average sampling time for each sampling point (one pixel in the map) is ca. 4-5 s. The mapping was carried out in desired area with 20, 30 or 50 µm lateral scan resolution. The physical resolution of the mapping depends on the size and shape of the gel. It is estimated to be around 50 µm for the gel probes used in this work. Two typical samples were examined for the SGECM experiment. Sample 1 was a steel plate covered by 20 µm-thick Zn-Mg-Al coating and 25 µm-thick polyester paint, which was used as a model sample in a previous work dealing with the combination of shear force regulated SECM with Raman spectrometry42. The coating was scratched with a 1 mm stylus for a depth of ca. 28 µm so that the metal was exposed (as measured by profilometry, Brüker Dektak XT, Supporting Information Figure S-1), and SGECM imaging was carried out in the vicinity of the scratch. This sample mimicked the damage of a corrosion-resistant coating. Sample 2 was made of a conductive copper tape sticking on an AA1050 aluminum alloy plate. The thickness of the tape was ca. 45-50 µm (Supporting information Figure S-2). The whole sample was conductive, yet consisted of two different metals. It mimicked the contact between metals, where galvanic corrosion was likely to occur.

Results and discussion Preparation of the gel probe: The preparation of the gel probe was carried out by electrodepositing chitosan/gelatin on a Pt microdisk electrode. The electrodeposition of chitosan is a well-known process, which is widely used for embedding biological species on electrodes for constructing biosensors43-46. It is driven by local pH change induced by OH- ions generated from electrolysis of water on the cathode. The OH- ions neutralize the charge on the polymer chain of chitosan initiating its precipitation. On micro-disk electrodes, the electrochemically generated OH- ions follow hemispherical diffusion, thus the deposited gel has a hemispherical shape as shown in Figure 1. Similar phenomenon was also observed when electrodepositing silica gel on micro-disk electrodes or nanopore arrays47,48. The shape of the gel



deposit can be tuned by deposition potential and time. The excess deposition on the wall of the microelectrode which usually occurs at more negative potential (e.g. −1.4 V vs. Ag/AgCl QRE, Supporting information Figure S-3) shall be avoided. Gelatin was added to the solution to be codeposited with chitosan, for the purpose of reinforcing the mechanical stability of the gel by increasing the solid content.

Figure 1 Photo of the gel probe prepared by electrodeposition at −1.2 V (vs. Ag/AgCl QRE) for 100 s from 1 wt.% chitosan + 3 wt.% gelatin solution.

The applicability of the electrodeposited gel as electrolyte was examined by cyclic voltammetry (CV). Figure 2 shows the CV of the as-prepared gel probe in ferrocenedimethanol solution. It is seen that the oxidation plateau of the gel probe has lower current as the bare Pt microelectrode. This suggests that ferrocenedimethanol penetrates into the gel, yet the diffusion coefficient of ferrocenedimethanol in the gel is lower than that in the solution. Moreover, the current does not increase by prolonging the immersion time of the gel probe in redox media. This suggests the fast penetration of redox probe in the gel, which reaches equilibrium within a few seconds before starting the measurement. This is useful for the gel probe to soak different species according to the need.


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Figure 2 Cyclic voltammetry of the bare Pt microelectrode (black) and the gel probe (red, green and blue) in 0.5 mM ferrocenedimethanol solution (1:1 vol. glycerol/H2O). Scan rate: 50 mV/s.

Approach and retract curves: In order to carry out SGECM measurements, one needs to approach the gel probe to be in contact with the sample. This was achieved by shear force feedback. Meanwhile, a potential was applied between the probe and the sample, and the current response was recorded. Figure 3 shows the shear force and current signals as a function of z position when approaching the gel probe to a Zn-coated steel surface. The shear force signal is normalized to the amplitude at starting position where the gel probe is far away from the sample. In Figure 3A, It is seen that when the probe is far away from the surface, the shear force signal is stable and the current is 0. At ca. 24 µm, the shear force signal starts to increase, while the current shows a spike. This indicates that the gel touches the conductive sample surface. As the probe is further approached towards the sample, the shear force signal keeps increasing, while the current rapidly decreases. The former is related to the compressing of gel to the surface, and the latter is not trivial to explain. Both the charging of double-layer at electrochemical interfaces (i.e. sample/gel and gel/Pt) and the Faraday current originating from the electrochemical oxidation of the sample may contribute to the measured current response. By plotting the current as a function of time, the time constant of the probe can be estimated to be ca. 100 ms (neglecting the compressing of gel for 2-3 µm). This is significantly longer than a 25 µm Pt electrode in aqueous solutions (around 15 ms), which may be attributed to the lower diffusion coefficient of species in gel and glycerol. Besides, the electrostatic discharge upon contact of gel with the sample might also have an effect. Figure 3A suggests that both



shear force and current signals could sense the touching of gel probe to the sample surface. Nevertheless, the current response is very complicated and obviously not applicable for insulating surfaces such as organic coatings, as shown in the Supporting Information Figure S-4. Therefore, shear force was used as the main indicator for the contact of gel probe with the sample in SGECM.

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Figure 3 Approach (A) and retract (B) curves of the gel probe to Zn coating.

After approaching the gel probe to be in contact with the sample (corresponding to the position with 100.6% shear force amplitude), we further studied the retract behavior of the gel probe. Before retraction, the probe was held in contact with the sample for ca. 5 s with the potential applied. This allowed full charging of the double layer and yielded a steady current. Then the probe was retracted from the surface while recording both the shear force and current signals. It is seen that the shear force signal rapidly decreased to the base line of 100% upon retraction to the same z position of initial touching (ca. 24 µm). However, the current remained almost constant until the gel probe was retracted to ca. 42 µm. This indicates that between 24 and 42 µm, the gel was still attached to the surface, even though the attachment was not reflected in shear force. This was perhaps because the gel was soft containing high content of water. Similar phenomenon was also observed when retracting a liquid droplet in scanning droplet cell techniques49,50. It could be explained by the adhesion (“sticking”) between the sample surface and the gel, which stretched the gel when the probe was pulled away from the sample. When the probe was pulled above 42 µm, the gel was fully detached from the surface so that there was no current response. The approaching-retraction process is illustrated in Figure 4.


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Similar trends were also observed when approaching and retracting the gel probe to aluminum and copper surfaces. (Supporting information Figures S-5 and S-6)

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The approach and retract curves are highly informative. From the shear force signals, the topography of the sample can be revealed based on the position where the gel probe touches the sample surface. In practice, one may approach the gel probe to the position corresponding to a fixed normalized shear force response (e.g. 0.6% change), and record the coordinates for quantitatively determining the topography. Once the gel probe touches the conductive sample surface, high current is observed due to the charging of the double-layer. On one hand, this may indicate the local electronic resistance of the sample, but on the other hand this non-Faraday current shall not be taken into analysis of the local electrochemical activity. Moreover, the stretching of the gel upon retraction may indicate the adhesion between the gel and the sample surface. Further quantitative studies are desired to analyze the results. In this preliminary work, we only use shear force feedback to sense the contact between the gel probe and the sample surface. SGECM imaging of topography and electrochemical activity: The gel probe was used for imaging the topography and electrochemical activity of the sample in SGECM. The parameters were chosen based on approach/retract curves. For each sampling point, the gel probe was approached to the sample until the shear force signal varied by 0.6%. The position was recorded, and then the probe was retracted for 5 µm and kept for 2 s before the current was measured. This allows stable Faraday current to be measured. Figure 5 shows the SGECM images of Sample 1. The topography image clearly shows the scratch of the coating (Figure 5A). The depth of the scratch is ca. 28-30 µm, which is the same as measured by profilometry (Supporting Information Figure S-1). This indicates that the Zn coating is exposed. The



current map shows the area where electrochemical reactions occur. On the organic coating surface, the current response is almost 0, suggesting that the metal is well protected. On the scratch, current in the range of 0.1-0.5 nA is detected (Figure 5B). Since the applied potential on the sample surface is not regulated (i.e. 0.3V vs. Pt), it is difficult to quantitatively interpret the data. Nevertheless, this suggests that electrochemical reactions might occur on the scratch where Zn coating is exposed. As comparison, we conducted the same experiment following the same conditions with a microelectrode without gel (Supporting Information Figure S-7). The topography could still be measured by shear force feedback, but there was no current response all over the sample. The results confirm that gel is essential for simultaneously mapping the topography and current of the sample. We also tested three consecutive scans of the sample using the same gel probe (Supporting information Figure S-8). The results indicated that the gel probes could still be used after placing overnight in air. Nevertheless, the stability of the gel probes needs to be further improved in future work.

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Figure 5 SGECM mapping of Sample 1: (A) Topography; (B) Current mapping at Esurf = 0.3 V vs. EPt with the gel probe retracted for 5 µm after shear force signals varied 0.6%.

It should be noted that the contrast in current mapping for Sample 1 may be attributed to the high contrast in electrical conductivity between the coating and the exposed metal. To further explore the applicability of SGECM in electrochemical imaging, Sample 2 consisting of Cu tape on aluminum substrate was tested. In this sample, the whole surface is highly conductive, therefore the effect of electronic conductivity is eliminated. Before imaging, CV was measured in two-electrode system (with the gel probe connected as working electrode and the sample connected as reference/counter


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electrode) when the gel probe is in contact with the sample surface. Figure 6A compares the CV of the gel probe on copper and aluminum. Two pairs of redox peaks were clearly seen for copper surface, which may be attributed to the two-step redox reactions of Cu/CuCl and CuCl/Cu2+, considering there is Cl- in the gel from the deposition solution. In contrary, only one pair of redox peaks (more separated in potential) was observed after soaking the gel probe in 0.15 mol/L KNO3 solution (Supporting Information Figure S-9). This was probably because the NO3- ions replaced the Cl- in the gel, and the redox of Cu in NO3- is a known one-step two-electron process. When the gel probe is in contact with Al surface, the current is very low in the whole potential range, and there is no significant redox peak. The reason is still unclear. One hypothesis could be that Al is passivated in the potential range. We further studied the effect of probe retraction on the CV behavior of Cu|gel|Pt two-electrode system. It is seen from Figure 6B that the CV curves measured at 32 and 40 µm almost overlap, indicating that the electrochemical behavior is not affected by slightly retracting the gel probe after approaching to be in contact. By further retracting the probe, the current of reduction peaks gradually decreases. This might be due to the stretching of gel as illustrated in Figure 4. The distance between Pt microelectrode and Cu surface increases, which makes the diffusion of Cu2+ more difficult. Besides, the conductivity of the gel, as well as the contact area between the gel and the surface may also decrease due to the stretching. From the CV results, the difference between Cu and Al in electrochemical behavior is clearly seen. Moreover, slight retraction of the gel probe for a few microns would not significantly affect the electrochemical results. This offers some flexibility in positioning the gel probe. In the imaging experiments, the probe was approached to touch the sample as detected by 0.6% shear force signal change, and then it was retracted for 5 µm for carrying our electrochemical measurements. This would avoid accidental electric contact between the Pt and the metallic sample originated from the irregular geometry and roughness of the surface. It should be noted that the quantitative analysis of the CV results is not included in this work, since it is very complicated due to the two-electrode system, limited volume of electrolyte and the close positioning of the two electrodes.



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Figure 6 CV of the gel probe on Sample 2: (A) comparison between copper (a) and aluminum (b); (B) on copper at different distance from the sample. Scan rate: 50 mV/s

Electrochemical imaging was carried out on Sample 2 by applying 0.4V on the sample surface vs. the gel probe and recording the current response. From the shear force feedback, the topography of the sample can be determined. A step of ca. 45-50 µm is clearly seen in Figure 7A. This is in accordance with profilometry results (Supporting Information Figure S-2). Figure 7B shows that the current response on Cu is significantly higher than on Al. Since both Cu and Al are highly conductive, the difference in current reflects the different electrochemical activity rather than the electric conductivity of two surfaces. The results confirm the applicability of SGECM for electrochemical imaging. This system may be further elaborated for studying the intergranular galvanic corrosion in aluminum alloys. Al

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Figure 7 SGECM imaging of copper tape on aluminum alloy: (A) Topography; (B) Current mapping at Esurf = 0.4 V vs. EPt.

Instead of applying a constant potential and measuring the steady state current response between the gel probe and the sample, one may also measure CV at each sampling point. This is more time consuming yet may significantly improve the contrast of imaging. Figure 8 compares the line scans of


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Sample 2, where the peak current at ca. −0.2 V (Esurf vs. EPt) from CV (Figure 8A) and the current response at a fixed potential (Esurf vs. EPt = 0.4 V, Figure 8B) were plotted against the coordinate of the scan axis (x axis). The latter was taken from the mapping in Figure 7. Despite that the area of measurement was not exactly the same, the topography was still reproducible, with a step height of ca. 45-50 µm. Both the peak current in CV and the current measured at a constant potential may differentiate the different metals, but the former shows much higher contrast. Moreover, two peaks with higher current response were observed in Figure 8A, in the ranges of 180-220 and 320-360 µm. This might indicate that these two regions were more reactive (easier to be oxidized and reduced), which was not seen in potentiostatic mapping from Figure 8B. This might suggest that CV was more sensitive to the surface reactivity of the sample and could improve the contrast in electrochemical imaging. Nevertheless, the physical meaning of the results and more quantitative analysis need to be

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Figure 8 (A) Topography and peak current at ca. −0.2 V (Esurf vs. EPt) from CV of Sample 2 in a line scan; (B) Topography and current measured at a constant potential Esurf = 0.4 V vs. EPt for Sample 2 (data extracted from Figure 7).

Conclusions In summary, we developed a novel gel probe for scanning electrochemical probe measurements. The gel probe was based on a microelectrode coated with gel, which can be fabricated by electrodeposition of chitosan with gelatin. The gel probe was approached to the sample surface by shear force feedback, with its vertical position recorded. When the probe was in contact with the surface, electrochemical



measurements can be carried out by measuring the current response while applying potential (either fixed or scanning) between the probe and the sample. By laterally scanning the probe over the sample surface in tapping mode, the topography as well as the electrochemical activity of the sample can be mapped. The technique is denoted as SGECM. Two model samples, i.e. a protective coating on steel with a scratch, and a copper tape on aluminum alloy, were measured. The results confirm that the scratch and the tape were well identified both from the topography and the electrochemical activity maps. Moreover, electrochemical mapping can also be conducted by measuring CV at each sampling point. This enhanced the contrast of mapping by plotting the peak current. As compared with SECM, SGECM is carried out with the sample exposed in air, bringing an advantage of preventing undesired change of the sample due to the immersion in electrolyte during the measurement and thus may be applicable to highly reactive samples such as magnesium. As compared with SDC or SECCM, the gel immobilizes the electrolyte and inhibits the evaporation and leakage. The solid nature of gel allows the contact between the probe and the sample to be sensed by force feedback. The special geometry and limited volume of gel would affect the mass transport and the electrochemical behavior, thus more quantitative interpretation of the data is to be further explored in future.

Supporting Information. Surface profile of the coating scratch in Sample 1 and the copper tape in Sample 2 as measured by profilometry; Photos of the gel probes prepared by electrodeposition at different potentials and a duplicate probe prepared after polishing a gel probe; Approach and retract curves of the gel probe to insulating organic paint, aluminum and copper surfaces; SGECM mapping of Sample 1 using a Pt microelectrode without gel; Consecutive SGECM mapping scans of Sample 1 with the same gel probe; CV of the NO3- soaked gel probe in contact with the copper surface of Sample 2.

Acknowledgement The authors gratefully acknowledge the CNRS-Momentum Project “Electrochemical imaging and guided functionalization of irregularly shaped surfaces at microscale with adjustable resolution” for financial support of the work.


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Scanning Gel Electrochemical Microscopy (SGECM) for Topography

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