Optimization of the shearforce signal for scanning electrochemical microscopy and application for kinetic analysis

Electrochimica Acta 88 (2013) 877–884

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Optimization of the shearforce signal for scanning electrochemical microscopy and application for kinetic analysis Mathieu Etienne a,∗ , Sébastien Lhenry a , Renaud Cornut b , Christine Lefrou b a

CNRS and Université de Lorraine, LCPME, UMR7564, 405 rue de Vandœuvre, F-54600 Villers-lès-Nancy, France Laboratoire d’Electrochimie et de Physico-chimie des Matériaux et des Interfaces (LEPMI), UMR5279, CNRS-Grenoble-INP-UdS-UJF, 1130 rue de la piscine, B.P. 75, Domaine Universitaire, 38402 Saint Martin d’Hères Cedex, France b

a r t i c l e

i n f o

Article history: Received 5 July 2012 Received in revised form 2 September 2012 Accepted 21 September 2012 Available online xxx Keywords: SECM Shearforce Apparent geometry Apparent electron transfer kinetic

a b s t r a c t A simple modification of the setup for shearforce control in SECM is proposed. It allows alternately SECM experiment and electrode manipulation and polishing. The good reproducibility of the measurement allows studying some parameters affecting the characteristics of the shearforce signal. A simple protocol is proposed to determine the resonant frequency using the current measurement and to select the optimal frequency for application in SECM. The length of the shearforce approach curve can vary from several hundred of nanometers to several ␮m when operating under hydrodynamic control. As shown in this study, the combination of the shearforce signal with the electrochemical feedback measurement allows the determination of the tip-to-sample distance, the apparent disk geometry of the probe and a larger range of heterogeneous kinetics constants. In principle, the proposed protocol is applicable to any SECM experiment involving a non-electrochemical positioning, e.g. AFM-SECM. © 2012 Published by Elsevier Ltd.

1. Introduction Scanning electrochemical microscopy (SECM) is now a classical analytical method for the characterization of interfaces [1–3]. The method is based on the electrochemical interaction of a microelectrode tip with the interface to be analyzed and is sensitive to the tip-to-sample distance and to the sample reactivity. In the absence of tip-to-sample distance control, the measurements performed above samples with profile in the ␮m scale [4,5] and/or the high resolution SECM experiment with nanoelectrodes can be difficult to interpret, because of the influence of both topography and reactivity [6]. The problem of distance control of the microelectrode versus the analyzed surface has been thoroughly considered in the literature and is still challenging nowadays [5]. After the first report on standing approach curves by the group of Heinze [7], several strategies have been proposed over the years including shearforce detection [8–17], AFM-SECM [18–23], STM-SECM [24], SICM-SECM [25], ACSECM [26–29], intermittent contact by tip position modulation [30] and microfluidic-based SECM probes [31,32]. Shearforce control for SECM was first introduced in the mid nineties with using a laser detection of the shearforce-induced tip vibration dampening [8]. Later, non-optical shearforce control by tuning fork [9,12,13] or piezoelectric plates [11,15] was

∗ Corresponding author. Tel.: +33 383 685250; fax: +33 383 275444. E-mail address: [email protected] (M. Etienne). 0013-4686/$ – see front matter © 2012 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.electacta.2012.09.063

described. The scanning procedure with shearforce control in SECM can involve either the continuous repositioning of the electrode by a feedback loop [6,10,11,15,33] or the standing approach curve in which the electrode is regularly repositioned at the constant distance from the electrode by successive shearforce approach curves [12,13]. The method was applied to the analysis of living cells [34], also in combination with near-field optical microscopy [14], to corrosion analysis at small scale [35] or at a larger scale in a SECM-based automate [16]. Various kinds of electrode have been positioned with shearforce control. A careful look at the shearforce approach curves reported in the literature indicates that depending on the experimental conditions (tip vibration actuator, shearforce detector and electrode tip) these approach curves occurred over distances varying from hundred of nanometers [6,15], consistently with former shearforce implementation for SNOM applications [36,37], to micrometers [8,14,16]. While the electrodes used in SECM can be rather small [38,39], the size of the insulating material remains sometimes much bigger (up to tens of ␮m) [17,40]. At the present state, it is expected that the overall size of the electrode (conducting material and insulator) determines the shearforce interaction with the analyzed substrate, but such influence has never been described systematically in the literature. Precise understanding of the shearforce interactions is all the more challenging as many parameters have a significant influence on the resulting electrode vibration measured by the piezoelectric sensor. The first objective of the work was thus to introduce a modification in the setup for non-optical shearforce control

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Fig. 1. Scheme of the SECM setup equipped with an electrode holder allowing alternately shearforce detection and electrode manipulation for polishing. All the components of the shearforce detection are kept mechanically fixed during all operations.

allowing a reproducible hearforce signal after electrode manipulation and polishing. It was then possible to analyze some parameters affecting the shearforce detection for application in SECM. This non-electrochemical positioning was finally used in a systematic procedure allowing the determination of the tipto-sample distance and the apparent geometric parameters of the probe (conductive and insulating part radii). These are especially relevant for any quantitative interpretation of SECM data obtained with the electrode. In particular, it is shown how, using shearforce measurements, the extraction of an heterogeneous process kinetics parameter becomes possible even in the situations where the signal is generally assumed to be indistinguishable from the situation displaying fast electron transfer kinetics. 2. Experimental Microelectrodes have been prepared with commercial Quartzglass-Pt fiber material (Thomas Recording, Germany, dimensions ± 10%). The 8 ␮m diameter electrode is made of platinum and is inserted in an 85 ␮m quartz fiber (Rg = 10) and the 25 ␮m diameter electrode is made of Platinum/Tungsten and is inserted in a 80 ␮m quartz fiber (Rg = 3). As the fibers are too fragile to be directly introduced in the SECM setup, they were first connected to a copper wire using a conducting epoxy resin (EPO-TEK® ) and inserted in a pulled glass capillary. Araldite® was then used to glue the fibers to the capillary. Smaller electrodes have also been prepared by following a protocol previously described [39]. In this case, a 25 ␮m diameter platinum wire (purity 99.9%; hard, Goodfellow SARL, France) was inserted and pulled in a quartz capillary using Laser Puller (P-2000; Sutter Instrument Company, USA). All electrodes have been shaped by polishing on silicon carbide and aluminium oxide films (Struers, Denmark). The SECM setup is based on the instrument developed by Sensolytics (Ruhr-Universität, Bochum, Germany), equipped with a PalmSens Bipotentiostat (Palm Instruments BV, Houten, The Netherlands). The module allowing alternately SECM experiment and electrode polishing has been designed at the laboratory (LCPME, Nancy, France). All measurements have been performed in 0.1 M KCl solution (99.9%, Wormapur) containing 1 mM ferrocenedimethanol

(98%, Aldrich) or 5 mM K4 Fe(CN)6 (>99.5%, Fluka). The electrochemical setup was composed of the working microelectrode, a gold counter-electrode and an Ag/AgCl pseudo-reference electrode. Electrochemical feedbacks have been obtained on a glassy carbon electrode biased at −0.45 V or on a nonbiased platinum plate (positive feedback), on a flat glass substrate (negative feedback), and on a nonbiased indium-tin oxide (ITO) plate (surface resistance of 8–12 /sq, Delta Technologies, Loveland, CO, USA). 3. Results and discussion 3.1. Setup for successive SECM experiment and electrode polishing Shearforce detection for SECM was done with a set of two piezoelectric plates mechanically attached to the working microelectrode and connected to a lock-in amplifier (Fig. 1, left) [11]. Piezoelectric plate 1 actuates the tip vibration and piezoelectric plate 2 detects the modification of this vibration at close distance from the substrate. The frequency and the amplitude of the sinusoidal potential applied to the piezoelectric actuator can be tuned to optimize the response of the piezoelectric sensor. The resulting signal is analyzed by a lock-in amplifier, whose response is used to detect the shearforce interactions between the vibrating electrode and the sample surface. The exact nature of these shearforce interactions is not clearly defined and depends on the experiment. Capillary, Van der Waals and hydrodynamic forces or direct mechanical contact can be involved [10]. One major drawback of this technology is that the lock-in amplifier allowing shearforce detection is also very sensitive to the exact environment of the electrode. In addition to the effect of liquid level that can be solved by appropriate software control [20], the position of the piezoelectric plates on the electrode body and the position of the electrode on the setup affect the lock-in amplifier response (LIAR) and lead to different resonant frequencies from experiment to experiment. If the electrode needs to be reconditioned by polishing during SECM experiments, as it is often required in electrochemistry, piezoelectric plates have to be unscrewed from the electrode and the electrode has to be removed from the setup. After polishing, the electrode is repositioned on the setup. The new position on the setup can be approximately similar to the previous

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one but not the same. Very problematic is the variation of the relative position of the piezoelectric plates that leads to a significant modification in the LIAR and in the resonant frequencies. Fig. 2A illustrates this problem by comparing the lock-in amplifier response (LIAR) before (curve a) and after electrode polishing (curve b). As one can see, the entire profile of the LIAR was changed during the polishing step. This means that the resonant frequencies have to be determined again and the characteristics of the new shearforce signal (LIAR variations, length of the interaction) will be different from the previous ones, leading to different constantdistance mode operations. It is thus important to introduce more reproducibility in this experiment for further application of the shearforce technology in SECM. In order to introduce some reproducibility in the shearforce detection from experiment to experiment, i.e. similar frequency, actuation potential and signal at the sensor, it would be necessary to have exactly the same setup before and after microelectrode manipulation. This would allow for example electrode polishing, as it is often required in electrochemistry. Fig. 1 presents the modification we introduced in the setup for non-optical shearforce detection in order to reach this goal. An electrode holder has been designed for the manipulation of the electrode from the SECM setup to the polishing machine without modifying the electrode position on the

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setup or the relative position of the two piezoelectric plates. It is important to note that the polishing machine is not introduced in the SECM experiment but remains independent. The electrode can be manipulated and shaped while the sensitive part of the shearforce setup remains mechanically attached to the electrode. The consequence is that the spectra recorded before and after electrode polishing (compare curves a and b in Fig. 2B) are very similar, which indicate that the conditions for shearforce detection could also be similar. In order to further illustrate the practical interest of the device for SECM, successive electrode positioning, electrochemical negative feedback measurement and electrode polishing were performed. The electrode was first prepared by surface polishing and then positioned at the surface of a glass slide by shearforce detection (Fig. 3A, curve a) and the electrochemical measurement was performed (Fig. 3B). The electrode was then removed from the setup, polished, replaced on the setup and a new shearforce positioning and an electrochemical measurement were performed (curves b). Finally the same series of manipulation was done once again (curves c). The good superposition of the measured negative feedback shows that the automatic stop condition obtained by shearforce detection is reproducible after successive manipulation and polishing of the electrode. And this reproducibility gave the opportunity to better explore the shearforce interaction of the electrode with the analyzed surface as described in the next sections.

3.2. Shearforce approach curves for SECM Optimal operation in SECM requires being at close distance from the analyzed sample, in order to be in the feedback interaction, but without coming in contact so as to protect both the microelectrode and the analyzed sample. From a practical point of view, this non-contact interaction can be obtained by stopping the electrode displacement over the z-axis, as soon as the shearforce interaction starts, i.e. when the lock-in amplifier response (LIAR) has varied for more than 3 times the signal-to-noise ratio observed in the setup (about 20 mV, corresponding to about 0.4% of the starting 5 V signal in the conditions we used) [16]. This empirical criterion is of practical interest but does not give the exact information on the distance between the tip and the sample when this electrode approach is stopped. It is first necessary to define properly the conditions of the shearforce detection for application

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Fig. 3. Three successive experiments of (A) electrode positioning by shearforce detection (RLIAR = relative lock-in amplifier response) followed by (B) SECM negative feedback measurement (IR = ratio between tip current and tip current at infinite distance). After experiments (a) and (b), the electrode was removed from the setup, manipulated, polished and replaced on the setup for a new measurement. The dotted line shows the calculated negative feedback approach curve. The experiments were performed on a flat glass substrate in a solution containing 0.1 KCl and 1 mM ferrocenedimethanol with a 4 ␮m radius Pt microelectrode with apparent Rg 8.5. Electrode positioning was done with 344 kHz actuation frequency. Piezoelectric actuation was done at 200 mV peak-to-peak. This actuation was switched off during the electrochemical measurement.

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in SECM measurements. In this work, we have explored how the electrochemical signal measured at the SECM tip can be disturbed by the frequency and the actuation potential used in shearforce detection and how this effect can be used for the determination of some resonant frequencies, a critical step in shearforce based SECM experiment. The basic idea is that hydrodynamic forces are implied in this shearforce interaction [8] and should influence significantly the measured current when mass transport conditions at the electrode tip are disturbed. In a second step (Section 3.3), we will take advantage of a well controlled and reproducible shearforce signal in order to determine precisely the tip-to-sample distance. 3.2.1. Influence of the voltage at the piezoelectric actuator on the shearforce interaction In some conditions, it is possible to observe long shearforce interactions. We have used this situation to study the influence of the actuation potential on the shearforce signal and on the SECM feedback current (Fig. 4). The shearforce signal, characterized here by a length of more than 10 ␮m, has been obtained with an electrode displaying a diameter of 80 ␮m (4 ␮m Pt radius with Rg 10). The amplitude of the sinusoidal signal sent to the piezoelectric actuator was varied from 50 mV to 400 mV. This modification did not affect significantly the shearforce signal as shown by the superposition of the four curves displaying the same feature (Fig. 4A). At the opposite, a significant increase of the current at close tipto-sample distance was observed when the piezoelectric actuator potential was increased, with a maximum intensity at 2 ␮m from the surface (Fig. 4B). The long distance LIAR variations and the current modifications indicate that this signal is clearly controlled by hydrodynamic forces, and not by other forces occurring at shorter distances. A similar effect was observed previously with a microelectrode that vibrated ultrasonically in a lateral direction and was ascribed to deformation of the diffusion layer on the electrode due to motion of the solution [41]. With 50 mV actuation, the electrochemical response was very little affected by the tip vibration and the measured current had almost the characteristic of a negative

feedback. In the following section we have chosen to keep applying high voltage at the piezoelectric actuator and to measure the feedback current with the tip placed at 2 ␮m form the surface, where the impact of the hydrodynamic shearforce interactions seems to be the highest. 3.2.2. Influence of the frequency and tip-substrate distance on SECM response and choice of the optimum resonant frequency Fig. 5A reports a typical LIAR obtained by actuation with a sinusoidal potential wave from 100 to 600 kHz. The response of the system as measured by the piezoelectric sensor and analyzed by a lock-in amplifier is composed of several peaks. As such, it is difficult to choose the most appropriate frequency for the shearforce measurements. For this, the current measured simultaneously at the microelectrode may be helpful. Fig. 5B presents the current measured in a solution containing 1 mM ferrocenedimethanol (FDM) at the same distance from the surface as Fig. 5A, and with the same SECM tip. It shows that for some frequencies (e.g., 150 kHz, 267 kHz and 296 kHz in the situation considered here), the current was significantly affected (up to 10% increase) by the piezoelectric actuation. The current is thus also sensitive to the tip vibration. However, the current variations do not match the LIAR response. As an illustration, only a very small current variation is observed in the region of the spectra where the LIAR response is maximal, i.e. between 210 and 240 kHz (Fig. 5A). This confirms that the peaks observed on the LIAR spectrum are not directly indicative of resonance. The distance also influences the current response. As an illustration, the influence of the excitation frequency on the tip current measured at 2 ␮m (corresponding to d/a = 0.5) from the insulating surface is presented Fig. 5B, curve b. The electrode was in the negative feedback regime characterized by a steady-state current of 0.4 nA. In these conditions, scanning the frequency of the piezoelectric actuation from 100 to 600 kHz induced a sharp increase in the current (up to 100% increase) for some frequencies. Most of the frequencies identified when the tip was at 90 ␮m from the surface were still very sensitive to the tip vibration and some additional frequencies, not or hardly detectable before are very clearly identified at 2 ␮m from the surface. Approaching closer to the surface almost suppresses the effect of the tip vibration on the measured current, probably because of hindered tip-vibration at close tip-tosample distance by shearforce interaction or contact (Fig. 5B and c). To further illustrate the influence of the frequency on the shearforce interaction and to discuss the choice of the resonant frequency for SECM positioning, three different frequencies have been tested for electrode positioning: 190 kHz that did not show any current increase even close to the glass substrate, 267 kHz characterized by a very sharp increase of current, and 526 kHz that was not identified as a resonant frequency when the electrode was far from the surface but showing a significant increase of current when the electrode was positioned at 2 ␮m form the surface. Approach curve at 190 kHz (Fig. 5C) led to a very limited increase in the LIAR, about 50 mV, before reaching a stable value when contacting the glass substrate. The length of this interaction was very short, in the range of 100 nm. It is supposed that this signal is here mainly due to a physical contact between the tip and the glass surface. This contact interaction affects the tip vibration, and this interaction can be observed at most frequency of the spectrum, resonant or non-resonant. However it is not the suitable conditions for a usual non-contact SECM experiment. The interaction occurring at 267 kHz (Fig. 5D) was longer, about 1 ␮m, and led to a larger LIAR change of several hundreds of mV. In these conditions, the vibrating tip interacted with the analyzed surface by shearforce before to come in contact with the sample. Finally, a shearforce interaction was observed at 526 kHz (Fig. 5E), longer than 3 ␮m and the

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Fig. 5. Critical effect of the actuation frequency on the LIAR and the current measured at the microelectrode. (A) LIAR spectrum recorded at 90 ␮m from a glass surface. (B) Current response spectra recorded at (a) 90 ␮m, (b) 2 ␮m and (c) 0 ␮m from the surface. (C–E) Example of LIAR response variation with the height form surface recorded at (C) 190 kHz, (D) 267 kHz and (E) 526 kHz. The experiments were performed on a flat glass substrate in a solution containing 0.1 M KCl and 1 mM ferrocenedimethanol with a 4 ␮m radius Pt microelectrode with apparent Rg 8.5. Piezoelectric actuation was done between 300 and 400 mV peak-to-peak.

LIAR varied for more than 7 V. This was the longest range in which the amplitude changed, in these conditions, with this electrode tip. Note that changing the electrode tip will lead to a comparable behavior, but of course resonances will be observed at other frequencies of the spectrum. To find a resonance at this frequency was not obvious from the LIAR spectrum (Fig. 4A) and from the current spectrum measured first at 90 ␮m from the surface (curve a, Fig. 4B). Only the current spectrum recorded at 2 ␮m form the sample indicated the presence of this resonance frequency. In fact, the current increase is the result of a complex interaction between the probe, the substrate and the solution. It is an interlinked combination of the amplitude and frequency of the vibration, the viscosity of the media, and the diffusion timescale of the mediator. A precise understanding of the phenomena requires complex numerical simulations that are beyond the aim of the present study. As a matter of fact, among the different conditions explored below, the most suitable frequency for further SECM data recording was 267 kHz (Fig. 5D) as it provided a non-contact shearforce interaction over a sufficiently short distance to permit a precise positioning. 3.2.3. Influence of the tip geometry The influence of the tip geometry on the shearforce signal is usually very difficult to characterize because the LIAR depends on the electrode geometry and on the environment of the electrode during the experiment, like the exact position of the piezoelectric plates, the level of liquid or the exact point of fixation of the electrode on the setup. All these parameters can be maintained constant in our setup and this was done for the experiment reported in Fig. 6. The electrode was first polished as shown in scheme (i) and the experiment was done at a resonant frequency chosen between 525 and 550 kHz, i.e. giving rise to several ␮m long shearforce interactions with the native fiber. The accurate choice was done for each experiment on the basis of the maximum current variation measured at 2 ␮m in this region of the frequency spectrum. The length

of the shearforce signal observed with a conically polished tip was rather short, i.e. less than 100 nm long (curve i). The same electrode was then polished to reach the geometry reported in scheme (ii). The diameter of the resulting disk (platinum + quartz) was about 10 ␮m and the corresponding shearforce signal became longer, in the range of 0.5 ␮m (curve ii). A final polishing was done to enlarge the insulation material around the platinum disk to about 35 ␮m diameter. The distance of the shearforce signal variation obtained with this last electrode increased significantly, to be in the range of 1.5 ␮m. This work provides original observations on the relationship between electrode geometry and hydrodynamic shearforce interaction whose length increase with the electrode size. From a practical point of view, the very short interaction length observed with the conical tip is of interest as it allows the positioning of the electrode at a very short tip-to-sample distance, the more interesting region for the application of such electrode geometry in SECM. 3.3. Apparent geometry determination and kinetic measurement The major application of shearforce interactions in SECM concerns the imaging of sample displaying complex topography and electrochemical reactivity, as encountered in corrosion or for living cell analysis. But shearforce control can also be very helpful for precise electrode positioning when performing electrochemical feedback measurement. First, shearforce signal positioning when performing positive and negative feedback measurements can be used to characterize the geometry of the microdisk probe (Fig. 7A) [42]. The reproducible stop criteria based on shearforce detection (Fig. 7B) for the approach allows to have a reference position that does not depend on the reactivity of the substrate: the tip position corresponding to a zero tip–surface distance is still not known but remains the same for all different kinds of substrates. In the interpretation step, the number of unknown is thus decreased, as both curves are translated

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Fig. 6. Influence of (A) the electrode geometry on (B) the shearforce signal. Three different geometries have been tested: (i) conical shape, (ii) disc geometry with small Rg and (iii) disc geometry with bigger Rg. The experiments were performed on a flat glass substrate in a solution containing 0.1 M KCl and 1 mM ferrocenedimethanol with a 4 ␮m radius Pt microelectrode with various shapes (see (A)). The actuation frequency was between 525 and 550 kHz and the piezoelectric actuation were done between 300 and 400 mV peak-to-peak.

by the same value. When adjusting the theoretical curves [43] to experiments, this greatly increases the precision of the extracted parameters, i.e. Rg and radius a, by avoiding “over fitting”, i.e. numerous parameter combinations able to fit the experimental

data. The shearforce based protocol provides then a more reliable evaluation of the geometric parameters of the microdisk tip. The procedure is described in detail in the literature (see [42] and the corresponding supporting informations; an Excel file with the

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Fig. 7. (A) Schematic drawing of the different apparent parameters to be determined during the fitting procedures. (B) Successive shearforce approach curves on (a) glass, (b) Pt and (c) ITO surface. (C) Successive SECM feedback curves recorded after electrode positioning using the shearforce signal on (a) glass, (b) Pt and (c) ITO surfaces. (D) Variations of the dimensionless kinetics parameter  with the tip radius a for different electrodes. All measurements have been performed in a solution containing 0.1 M KCl 4− in the presence of 5 mM Fe(CN)6 . The actuation frequency was between 170 and 270 kHz and the piezoelectric actuation was below 100 mV peak-to-peak.

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preimplemented formula for the fit is available on demand). The procedure was applied to all the probes used in the following. In the present study, another advantage of the shearforce positioning is illustrated: the accurate characterization of a substrate’s reactivity. As well described in the literature [43,44], the high kinetics constant approach curves are very similar to the pure positive feedback approach curves: they only differ by a simple lateral translation, over a distance that is proportional to the inverse of the kinetics constant. This makes that it is generally not possible to extract a kinetics parameter from the curves without the precise knowledge of the reference position. Shearforce control can overcome this, as it is illustrated by the following procedure. First, during the previously described preliminary step, not only the apparent geometric parameters (Rg = 1.8 and a = 11.1 ␮m) could be extracted as detailed in the precedent paragraph, but also the link between the effective tip substrate distance (thus determined electrochemically) and the shearforce based stop criteria (d0 = 1.8 ␮m). Then, an approach curve above an ITO substrate was recorded (curve c of Fig. 7C). It presented a positive like behavior. However, when comparing the curve to pure positive feedback curves, the correspondence between the curves was not obtained: the experimental curve was below the theoretical one, showing limited mediator regeneration rate. In fact, a fit with the kinetics constant as the only adjustable parameter [42] gave a dimensionless kinetics rate constant value of 4. Importantly, without the knowledge of the reference distance, at this step, numerous parameter combinations ( and d0 ) could provide an acceptable fit, making the evaluation of the parameter very imprecise [43]. With the shearforce detection, d0 becomes known parameters, so that the model has only one adjustable parameter left, , and a unique value of  ( = 4) leads to a satisfying correspondence with the experimental results. In order to verify the accuracy of the extracted value, approach curves above the same substrate, but with smaller probes (with a = 2.3 and 0.2 ␮m) were performed and the results are reported in Fig. 7D. The lower feedback behavior that was observed was associated to a smaller kinetics constant . As expected through the  definition ( = ka/D) and already verified in previous studies [38,45], the experimental data show a very satisfying proportionality relationship between  and the radius a, as shown in Fig. 7D, including the high  values. This shows how quantitative shearforce measurements may lead to a reliable evaluation of the kinetics parameter, in a situation where it is normally impossible to discriminate from pure positive feedback. In principle, the proposed protocol is applicable to any SECM experiment involving a non-electrochemical positioning, e.g. AFM-SECM, and becomes particularly relevant when small disk electrodes, i.e., with submicrometric diameter, are manipulated. Simple optical microscopic visualization of the electrode shape does not provide necessarily a very precise evaluation of the exact Rg to be applied in the fitting procedure. As perfect electrode does not exist, the determination of apparent geometry clearly provides a simple approach to improve the accuracy of quantitive SECM experiments. Finally, one must note that soft biological materials have not been considered in this study but shearforce detection could surely be influenced by the mechanical properties of the sample.

4. Conclusion Shearforce control in SECM usually requires dedication and good experimental skill. The simple modification of the setup described here simplifies the experiment, introduces reproducibility and provides new opportunities for a better description the shearforce interaction. The choice of the frequency strongly influences the length of the shearforce approach, from several hundred of nm

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to several ␮m, and the method based on current perturbation at close distance to the substrate permits the determination of these frequencies. Shearforce control can be very helpful in kinetic analysis. The successive approach curves on pure positive and pure negative feedback allows the determination of a reference tip-substrate distance and the geometric parameters of the disk electrode. These parameters are then introduced in a fitting procedure allowing the extraction of the kinetic parameter. For all experiments, the reproducible electrode positioning provided by shearforce detection permits to limit the number of unknown parameters during data analysis, increasing the range of the kinetic constant that it is possible to evaluate in a reliable way. In principle, such procedure is applicable to any SECM experiment involving a non-electrochemical positioning. The determination of apparent geometry provides a simple approach to improve the accuracy of quantitive SECM experiments. Acknowledgements We gratefully acknowledge Krystel Pélissier for help in the early studies on the interest of shearforce-based positioning in the determination of apparent geometric parameters for kinetic analysis. We also acknowledge Jean-Paul Moulin and Gérard Paquot for the construction of the different devices for shearforce control in SECM. References [1] A.J. Bard, M.V. Mirkin (Eds.), Scanning Electrochemical Microscopy, Marcel Dekker Inc., New York, 2001. [2] G. Wittstock, M. Burchardt, S.E. Pust, Y. Shen, C. Zhao, Scanning electrochemical microscopy for direct imaging of reaction rates, Angewandte Chemie International Edition 46 (2007) 1584. [3] P. Sun, F.O. Laforge, M.V. Mirkin, Scanning electrochemical microscopy in the 21st century, Physical Chemistry Chemical Physics 9 (2007) 802. [4] S.E. Pust, W. Maier, G. Wittstock, Investigation of localized catalytic and electrocatalytic processes and corrosion reactions with scanning electrochemical microscopy (SECM), Zeitschrift für Physikalische Chemie 222 (2008) 1463. [5] I. Beaulieu, S. Kuss, J. Mauzeroll, M. Geissler, Biological Scanning electrochemical microscopy and its application to live cell studies, Analytical Chemistry 83 (2011) 1485. [6] M. Etienne, A. Schulte, W. Schuhmann, High resolution constant-distance mode alternating current scanning electrochemical microscopy (AC-SECM), Electrochemistry Communications 6 (2004) 288. [7] K. Borgwarth, D.G. Ebling, J. Heinze, Applications of scanning ultramicroelectrodes for studies on surface conductivity, Electrochimica Acta 40 (1995) 1455. [8] M. Ludwig, C. Kranz, W. Schuhmann, H.E. Gaub, Topography feedback mechanism for the scanning electrochemical microscope based on hydrodynamic forces between tip and sample, Review of Scientific Instruments 66 (1995) 2857. [9] P.I. James, L.F. Garfias-Mesias, P.J. Moyer, W.H. Smyrl, Scanning electrochemical microscopy with simultaneous independent topography, Journal of the Electrochemical Society 145 (1998) L64. [10] A. Hengstenberg, C. Kranz, W. Schuhmann, Facilitated tip-positioning and applications of non-electrode tips in scanning electrochemical microscopy using a shear force based constant-distance mode, Chemistry European Journal 6 (2000) 1547. [11] B. Ballesteros Katemann, A. Schulte, W. Schuhmann, Constant-distance mode scanning electrochemical microscopy (SECM)-Part I: Adaptation of a nonoptical shear-force-based positioning mode for SECM tips, Chemistry European Journal 9 (2003) 2025. [12] Y. Lee, Z. Ding, A.J. Bard, Combined scanning electrochemical/optical microscopy with shear force and current feedback, Analytical Chemistry 74 (2002) 3634. [13] H. Yamada, H. Fukumoto, T. Yokoyama, T. Koike, Immobilized diaphorase surfaces observed by scanning electrochemical microscope with shear force based tip-substrate positioning, Analytical Chemistry 77 (2005) 1785. [14] Y. Takahashi, H. Shiku, T. Murata, T. Yasukawa, T. Matsue, Transfected singlecell imaging by scanning electrochemical optical microscopy with shear force feedback regulation, Analytical Chemistry 81 (2009) 9674. [15] C. Cougnon, K. Bauer-Espindola, D.S. Fabre, J. Mauzeroll, Development of a phase-controlled constant-distance scanning electrochemical microscope, Analytical Chemistry 81 (2009) 3654. [16] M. Etienne, B. Layoussifi, T. Giornelli, D. Jacquet, SECM-based automate equipped with a shearforce detection for the characterization of large and complex samples, Electrochemistry Communications 15 (2012) 70. [17] U.M. Tefashe, G. Wittstock, Quantitative characterization of shear force regulation for scanning electrochemical microscopy, Comptes Rendus Chimie (2012), http://dx.doi.org/10.1016/j.crci.2012.03.011.

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