Shearforce positioning of nanoprobe electrode arrays for scanning electrochemical microscopy experiments

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Electrochimica Acta xxx (2015) xxx–xxx

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Shearforce positioning of nanoprobe electrode arrays for scanning electrochemical microscopy experiments Catherine Adam a , Frédéric Kanoufi b , Neso Sojic a , Mathieu Etienne c,d, * a Groupe Nanosystèmes Analytiques, Institut des Sciences Moléculaires, CNRS UMR 5255, Université de Bordeaux, ENSCBP, 16 avenue Pey-Berland, 33607 Pessac, France b ITODYS CNRS UMR 7086, Université Paris Diderot, 15 rue Jean Antoine de Baïf, 75013 Paris, France c CNRS, LCPME, UMR 7564, 405 rue de Vandoeuvre, F-54600 Villers-lès-Nancy, France d Université de Lorraine, LCPME, UMR 7564, 405 rue de Vandoeuvre, F-54600 Villers-lès-Nancy, France

A R T I C L E I N F O

A B S T R A C T

Article history: Received 24 December 2014 Received in revised form 24 April 2015 Accepted 24 April 2015 Available online xxx

Shearforce detection was applied to the controlled positioning of nanotip arrays at air/solid and liquid/ solid interfaces in scanning electrochemical microscopy (SECM). The arrays were fabricated by wet chemical etching of ordered optical fiber bundles. The shearforce detection have been performed with a non-optical detection setup between 70 and 170 kHz. The hydrodynamic nature of the shearforce interaction led to approach curves with length varying from few mm to more than 80 mm, essentially controlled by the working frequency (resonance frequency), the nature of the medium (air, water, electrophoretic paint solution) and the surface state of the fiber bundle (cleaved or etched). This interface sensitive signal was applied to the positioning of the nanotip array in a 1 mm PDMS film before the electrophoretic deposition of an insulating paint. The resulting nanoprobe electrode array, displaying nanotip electrodes individually insulated, has been characterized by cyclic voltammetry and SECM feedback curves. These results are discussed versus numerical simulations. They demonstrate that, except when in vicinity of a conductive substrate that may reveal the nanotip structure, the electrochemical behavior of the nanotip array is dictated by its micrometer dimension (the fiber bundle). In turn, they confirm the potential of shearforce detection for precise control over the array positioning. ã 2015 Elsevier Ltd. All rights reserved.

Keywords: SECM shearforce nanotip array nanoelectrode array simulation

1. Introduction Recent advances in scanning electrochemical microscopy (SECM) are pushing the method towards new limits: scanning larger surfaces [1], with higher resolution [2], higher aspect ratio samples [3] or coupling the electrochemical data with additional chemical [4] and spectroscopic analysis [5]. For imaging experiments, the electrode-to-sample distance needs to be kept as constant as possible in order to analyze properly the heterogeneous reactivity of the sample. Microelectrode positioning, eventually achieved independently from the electrochemical measurement, has been the focus of considerable research since the early age of SECM [6]. Shearforce detection was one of the first methods successfully applied for non-electrochemical distance control in SECM [7]. Nowadays shearforce sensitive techniques can be implemented in a SECM setup using laser detection [8], tuning fork [9] or a set of

* Corresponding author. E-mail address: [email protected] (M. Etienne).

two piezoelectric plates mechanically attached to the electrode body [10]. Other technologies have also been combined with SECM in order to achieve accurate distance control, for example AFM [11,12], scanning ion conductance microscopy (SICM) [13,14] or soft microfluidic devices [1,15]. In addition, in a more classic setup, the current can be used to position the electrode independently from the reactivity using alternating-current measurement [16] or tip position modulation [17]. Each of these approaches provides some specific advantages. Combination of AFM with SECM is by nature very powerful, taking advantage of micro/nanofabrication techniques to tune the electrode shape and size [18–21]. By comparison, the simplicity of fabrication of SICM probes makes them very successful for laboratory experiment at the interface with living materials [14,22]. Soft microfluidic devices allow today to extend the SECM experiment to large and curved samples and to combine the electrochemical measurement with chemical analysis [1,4,15,23,24]. Moreover, alternating current and tip position modulation permit rapid positioning with SECM setup without the need of complex instrumentation [17,25]. Within this large family of methods, shearforce detection has some peculiarities that

http://dx.doi.org/10.1016/j.electacta.2015.04.140 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.

Please cite this article in press as: C. Adam, et al., Shearforce positioning of nanoprobe electrode arrays for scanning electrochemical microscopy experiments, Electrochim. Acta (2015), http://dx.doi.org/10.1016/j.electacta.2015.04.140

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make it convenient in corrosion analysis [5,26,27], visualization of local electrocatalytic activity [28] and characterization of living material [29–32]. The technology was also combined with SECM for micropatterning [33]. Shearforce detection can be implemented with classic microelectrodes, without the need of micro/nanofabrication, where the only requirement is to have a flexible tip for actuation with a piezoelectric element [8]. Shearforce detection can be obtained with a piezoelectric plate simply attached to the electrode body, a setup that allows a great flexibility [34,35]. The device can be implemented in SECM-based automate [27], is applicable in air [30] and can be combined with confocal Raman microspectrometer [5]. Shearforce interaction can be controlled by hydrodynamic forces or by other forces occurring at shorter distances, i.e. capillary forces, Van der Waals interactions or direct mechanical contact [7,8]. The type of interaction depends on the morphology and physical properties of the sample surface, the geometry and physical properties of the vibrating tip and the viscosity of the electrolyte [8]. The length of the shearforce approach could vary from tens of nanometers to more than 1 mm. The longer interactions were observed with the larger tip diameter [35]. A challenge in SECM is to analyze/modify a large surface area with a high resolution in a reasonable time, typically less than one hour. This can be achieved using template electrode [36] or multiple electrode assembly as recently demonstrated with soft-microelectrode arrays [1]. An alternative approach could be the application of nanoprobe electrode arrays in SECM. First demonstration has been recently presented by Deiss et al. for surface patterning using an ensemble of nanoelectrode tips fabricated from an etched optical fiber array [37]. Optical fiber arrays are well-established tools in analytical and bioanalytical chemistry. Their remarkable characteristics are widely exploited for high-density biochemical sensing and remote imaging, including in situ or in vivo detection [38,39]. Spectroscopic information can be collected through the optical waveguide, and fiber etching associated with a subsequent gold-coating step allows the fabrication of nanotip arrays which show surfaceenhanced Raman scattering spectroscopy properties [40]. Electroless deposition or sputter coating of a conducting layer, typically gold [37,41] or indium tin oxide (ITO) [42] can also convert the nanotip arrays to nanoprobe electrode arrays with further insulation of the electrode tips [43]. These arrays have great potential in the future for spectroelectrochemical experiments, including combined SECM and Raman spectrometry experiments with high spatial resolution. Positioning a nanoelectrode array at close distance to a surface for analysis and/or patterning is not trivial, as a mechanical contact is prohibited due to the relative fragility of the individual nanoelectrodes. The purpose of the present work is to demonstrate that shearforce detection can be used to position with a good accuracy nanotip electrode arrays at the interface between a solid and air or a solution. The technology has been applied here to control the partial insulation of the nanotips in order to achieve an array of nanoelectrodes. Then, Electrochemical approach curves have been recorded on insulating and conducting surfaces and the results have been discussed with regards to numerical simulation. 2. Experimental The nanotip arrays were prepared by an extremely reproducible wet-etch procedure of a coherent optical fiber bundle, as reported previously [42]. The alignment of the fibers in the plan of the electrode is a major requirement for SECM experiment. This problem has been discussed in a previous publication by Deiss et al. [37]. Since that primary work, preparation of electrodes has been

improved in order to align the fibers by cleavage of the bundle before HF etching. In brief, silica imaging fiber bundles with a diameter of 300 mm comprising 6000 individually cladded 3-4 mm diameter optical fibers (Sumitomo Electric Industries, IGN-035/06) were used. The bundle was cleaved with the optical fiber cleaver Vytran LDC 200 and chemically etched to form a regular array of sharp tips. The cleaved side was thus left for 90 min in a buffer of hydrofluoric acid solution (HF) consisting of 40% aqueous NH4F and 48% HF in deionized water in proportions 5/1/1, and was then rinsed with deionized water. (Caution! HF etching solutions are extremely corrosive and safety procedures must be followed accordingly). The nanotip arrays were then sputter-coated with a 100 nm thick gold film to serve as electrodes (Emitech K550X). The diameter of the base of each single nanotip element was 3
0.1 mm. The size and the geometry of the etched tips depend on the etching time, on the etching solution and on the materials used for the core and cladding. In the present report, the height and the curvature radius of the tips were 4
0.2 mm and 0.1 mm, respectively. An average value of (34
2) was estimated for the cone angle. The gold-coated optical fiber bundle was then inserted in a capillary (Clark Electromedical Instruments – GC12OF-10: 1.2 mm O.D.  0.69 I.D.) and the electrical contact was realized with a silver paste (Technifr - EPOTEK-H20S). The capillary was then sealed with an epoxy resin (Radiospare - Quick set epoxy adhesive RS850-940) followed by a heating step of 2 h at 80  C. Nail paint was used to cover the protruding sides of the optical fiber outside of the capillary. The SECM setup is based on an instrument developed by Sensolytics (Bochum, Germany) that has been modified in the laboratory [27]. Electrochemical measurement was performed with a PalmSens bipotentiostat (Palm Instruments BV, Houten, The Netherlands). Shearforce detection was obtained by a set of two piezoelectric plates (Piezomechanik Pickelmann, München, Germany) mechanically attached to the electrode as reported before [10,35]. The signal was analyzed by a DSP lock-in amplifier (model 7280, Signal Recovery). The sensitive frequencies employed in this work were comprised between 70 and 170 kHz and the voltage actuation was comprised between 0.1 and 0.4 V. A thin layer of PDMS was deposited on a glass slide by spincoating a 1:2 solution of PDMS (Momentic Performance Materials – Kit RTV 615) in toluene at 4000 rpm for 30 sec. The thickness of the film was estimated by AFM and was approximately 1 mm. Shearforce detection was used to position the nanotip array in the PDMS layer or at a controlled distance from this surface. The cathodic paint (BASF) was then electrodeposited at 2.8 V for 240 sec. The electrode was carefully rinsed and the deposited paint was cured at 135  C for 30 min. As described, the fabrication procedure requires several steps: cleaving, etching, sputtering, mounting in the capillary, shearforce experiments, positioning in the PDMS and paint deposition. The global success rate to produce the final nanoprobe electrode arrays was around 30% because the tips may be broken or damaged during their manipulation. All electrochemical characterizations were performed in 0.1 M KCl solution (99.9%, Wormapur) containing 1 mM ferrocenedimethanol (98%, Aldrich). The electrochemical setup was composed of a modified optical fiber bundle as working microelectrode, a gold counter-electrode and an Ag/AgCl pseudo-reference electrode. Images of the electrodes were obtained using scanning electron microscopy (SEM, Hitachi FEG S4800). Simulated concentration profiles, voltammetry and approach curves were obtained by finite elements method with the COMSOL 4.3 package. The simulation was performed in 3D. The validity of the 3D solution was obtained from comparison of the simulated current and concentration profiles with a 2D axis symmetry geometry simulation for a plain disk. Briefly, each nanotip and confinement of its nearest diffusion layer was enforced by a 0.5 mm radius and 1 mm in height cylinder.

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These cylinders enforce then a maximum meshing density in the nanotip regions. To maximize the density of the nanotips on the electrode, only a portion of the electrode was simulated, these portions correspond to rotational symmetry of the space group corresponding to the nanotip arrangement periodicity (p/2 and p/6). The bulk condition (wall of the simulated cell) was placed at 600 mm in all directions. The mesh consisted of between 20000 and 50000 elements. The current at the electrode assembly was obtained from integration of the Lagrange multiplier of the concentration. Standard boundary conditions for electrochemical modeling were used [44]. 3. Results and discussion 3.1. Shearforce approach curve with a nanotip array Fig. 1A shows a schematic drawing of the experimental setup used to detect shearforce signal at nanotip array. The optical fiber array has been first etched in diluted HF solution before being covered by a thin gold layer with a sputter coater. The metalized sample was then inserted and glued in a glass capillary with 1.2 mm outer diameter. The periphery of the optical fiber array was finally covered with nail paint before being inserted in the electrode holder allowing shearforce detection [34,35]. Fig. 1B displays a picture of the final electrode assembly. The protruding flexible optical fiber array was about 1 cm long. The two piezoelectric elements allowing actuation (A) and detection (D) of shearforce have been fixed mechanically on the glass capillary with metallic screws as reported before by Schuhmann et al. [10]. The electrode could then be inserted in the setup that was also composed of X,Y,Z positioning elements (stepper motors and PiezoCube1), a potentiostat for electrochemical measurements, a lock-in amplifier for shearforce detection and the computer to control the electrode displacement and the data acquisition. While this setup has been described before, its application to optical fiber positioning had never been reported. Experimental investigations have been first performed with the cleaved optical fiber array before studying the behavior of etched optical fiber array. For sake

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of simplicity, the discussion will be mainly focused on the results related to etched optical fiber array. Fig. 2A reports the typical lock-in amplifier response (LAR) measured between 70 and 170 kHz when the nanotip array was in solution, at about 90 mm from a glass surface. The spectrum was composed of several peaks that were not necessarily related to resonance frequencies, which needed to be determined. In this discussion, only the response of the nanotip array at *72.5 kHz, **82 kHz and ***92 kHz will be considered. The resonance frequencies can be determined by approaching the tip at closer distance to the glass surface and measuring a second spectrum. Fig. 2B reports the absolute value of the difference in LAR measured at 90 mm and at close distance from the surface (DLAR). A sharp peak was observed in this spectrum, at ***92 kHz, corresponding to one resonance frequency of the optical fiber. Other peaks of lower intensities were also observed, e.g. at **82 kHz, while no response was observed at *72.5 kHz. A shearforce approach curve has then been performed by using 92 kHz as actuation frequency (Fig. 2C, curve a). It is worth noting that shearforce detection by piezoelectric plate was not direct here (contrarily to optical shearforce detection [8]) and led to either negative or positive variations of the lock-in amplifier response. The measured lock-in amplifier response is here reported relatively to the initial response at 90 mm from the surface. The LAR was stable when the nanotip array remained far from the solid surface of the glass microscope slide. The approach towards the glass surface led then to a significant variation of the LAR by more than 20 %, the decrease in signal being more significant by being closer to the glass substrate. The length of this approach curve was about 40 mm. We suppose that only hydrodynamic forces can explain such long distance interaction [7,8,35]. For comparison, the same experiment, i.e. using the same frequency, has been performed in air leading to a much shorter approach curve, less than 5 mm long, and also to lower LAR variation upon approaching the glass surface, less than 0.2% (see the inset of Fig. 3C). These results clearly show that the presence of water for the shearforce detection induces specific interactions with the substrate that are not involved in air.

Fig. 1. (A) Schematic drawing of the experimental setup allowing shearforce detection with nanotip array. (B) Picture of the nanotip array holder.

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Fig. 2. (A) Lock-in amplifier response (LAR) spectrum recorded with an etched nanotip array in water at 90 mm from a glass surface. (B) Difference between LAR responses (DLAR) recorded in water at 90 mm and at the vicinity of the glass surface. (C,D,E) Shearforce approach curves towards a glass substrate recorded with an etched nanotip array (a) in water or (b) in air at (C) 92 kHz, (D) 82 kHz or (E) 72.5 kHz. Relative Lock-in amplifier response (RLAR) is obtained by normalization of LAR relatively to the response at 90 mm from the glass sample surface. (F) Shearforce approach curves towards a glass substrate recorded with a cleaved fiber array (a) in water or (b) in air at 119 kHz.

Changing the frequency to **82 kHz (i.e. a frequency displaying a sharp peak in Fig. 2A but only a limited signal in Fig. 2B) also led to a shearforce signal extending over 20 mm (Fig. 2D, curve a). No shearforce signal was observed in air for this frequency (Fig. 2D, curve b). Finally, the experiment performed at *72.5 kHz led to very short distance interactions with the glass substrate, similar in water (Fig. 2E, curve a) and in air (Fig. 2E, curve b). This short distance interaction could involve other forces than hydrodynamic, i.e. capillary forces or Van der Waals interactions, or be the consequence of a mechanical contact [8]. Similar experiments have been performed with a cleaved fiber (i.e. a flat surface without the nanotips). As an illustration, Fig. 2F shows that shearforce approach curves longer than 80 mm could be observed, here at 119 kHz. Similarly to etched fibers, the length of the approach curves was longer in water (Fig. 2F, curve a) than in air (Fig. 2F, curve b) for the same frequency and the chosen frequency had a strong influence on the length of the approach curve (see Figs. S1-3 in the supplementary data).

Optical fiber array can thus be positioned with shearforce detection at close distance to a surface. The length of this shearforce approach curve depends on the frequency used to perform this measurement. The surface state of the array had a critical influence on the length of this approach curve, the etched fiber array leading to shearforce interactions over shorter distances than the cleaved fiber array. The next section will discuss the possible application of this signal for controlled tip insulation. 3.2. Controlled insulation of nanoprobe electrode array Nanoelectrodes can be prepared by controlled insulation of electrode with electrophoretic paint [45,46]. This method was initially considered for the preparation of nanoelectrodes for electrochemical scanning tunneling microscopy (EC-STM) [46] and further applied to insulation of electrode for AFM/SECM experiments [11]. The protocol was then successfully used for the

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Fig. 3. (A) Schematic representation of the experiment performed for controlling the insulation of nanoprobe electrodes. (B) Shearforce approach curve on a 1 mm thick PDMS film deposited by spin-coating on a glass surface. Inset: Shearforce approach on a PDMS monolith.

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insulation of nanoelectrode arrays prepared from etched optical fiber arrays [43]. More recently, Schulte et al. have shown that shearforce detection in SECM could be used as a fabrication tool for needletype carbon-fiber nanoelectrodes [47]. Following this work we would like to evaluate the interest of the shearforce positioning as described in the previous section as a tool for controlled insulation of gold nanotip array with electrophoretic paint. The principle of the experiment is reported in Fig. 3A. A gold-coated nanotip array was positioned with shearforce over or in a PDMS layer of approximately 1 mm thickness. A first shearforce approach towards the PDMS layer was realized in air and the electrode was placed at about 90 mm from this surface. Then the electrophoretic paint was added in the cell. The electrode was repositioned accurately over the PDMS layer just before the paint was deposited electrochemically. The insulation was completed after curing the paint in an oven at 130  C. The objective here was to control the deposition of the electrophoretic paint on the whole fiber electrode except the very end of the tips which were protected by PDMS. The critical step in this experiment was then the shearforce positioning of the extreme end of the fiber in the micrometric PDMS layer. Fig. 3B reports a typical shearforce approach curve over PDMS. The signal of the lock-in amplifier was here increasing while approaching the layer, over a distance of few micrometers. The signal reached then a plateau before increasing again sharply when coming in contact with the glass substrate (see the arrows in Fig. 3B). The characteristics of a shearforce approach curve can be influenced by the viscosity of the medium [7,8]. Here, the etched fiber array penetrated in the PDMS not more than the mm range, i.e. the approximate thickness of the PDMS layer. Following the shearforce approach curve, it was then possible to certify the penetration of the nanotip array into the PDMS layer. For comparison, a shearforce approach curve was recorded in a PDMS monolith. A similar signal was measured and a long plateau was also detected due to the entry of the optical fiber array into the

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PDMS (see inset of Fig. 3B) but, no sharp increase in the shearforce signal was observed under such conditions. This procedure was used to introduce reproducibly the nanotip array in the PDMS before applying 2.8 V for 240 s in electrophoretic paint bath. In these conditions, the quantity of paint deposited was controlled by the small reservoir formed by the volume of solution located between the optical fiber array and the PDMS surface. After paint curing, a significant surface of the nanotip was accessible for electrochemistry (Fig. 4A&B). If the electrode was retracted by 5 (Fig. 4C) or 10 mm (Fig. 4D) from the PDMS, the quantity of paint deposited using the same electrolysis parameters was higher, leading to smaller apparent electrodes. Moreover, a deposition achieved at 20 mm or at higher distances from the PDMS film led to fully insulated electrode tips (see Figs. S4-6 in the supplementary data). 3.3. Electrochemical characterization The electrochemistry of gold-coated nanotip array was characterized before (Fig. 5A) and after (Fig. 5B) insulation by cyclic voltammetry in an aqueous solution containing 1 mM ferrocenedimethanol and 0.1 M KCl. 3.3.1. Non-insulated nanoprobe array Before insulation, the cyclic voltammogram recorded at 0.05 V s1 presented peak current in both oxidation and reduction with intensity in the range of 4 mA (Fig. 5A). It indicates that the current was controlled by linear diffusion. An apparent active surface area of the electrode of the order of 2.6 mm2 may be estimated from the Randles-Sevcik equation. Indeed, not only the surface of the array of nanotips (the terminal etched surface of the fiber) but also the walls of the fiber were coated with gold and exposed to the electrolytic solution. The major contribution to the electrode current without insulation was then from the diffusion towards the non-insulated wall of the fiber, not from the nanotips. As the electrode current response was mainly governed by the

Fig. 4. Scanning electron micrographs of nanoprobe electrode array obtained by depositing electrophoretic paint with the nanotip positioned (A&B) in the PDMS film, (C) 5 mm away and (D) 10 mm away.

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transport of the molecular probes at its wall, it could not be positioned in the near-field of a substrate by SECM (i.e., current control) but only by shearforce modulation. The positioning of such nanoprobe array by SECM required the insulation of the wall, as it was depicted previously [37]. Section 3.2 of the present article already described this insulation protocol. 3.3.2. Array of independent insulated nanoprobes Once insulated, the nanoprobe array could be characterized by its electrochemical signature. The shape and the intensity of the signal were dramatically changed after insulation (Fig. 5B). A sigmoidal stationary signal was observed, that took origin from the hemispherical diffusion at the array of isolated micro/nanoelectrodes. The total intensity was also largely reduced and the current measured at the plateau of the sigmoid was 250 nA. This stationary behavior indicated that the array of nanotips displayed a similar diffusion profile as a microelectrode. Two different situations may explain such behavior depending on whether the diffusion fields developed at nanoprobes overlap or not. The theoretical voltammetric response of macroscopic (millimeter large) arrays of nanoelectrodes has been first described analytically by Amatore [48] and recently revisited to account for arrays submitted to transport processes within micrometric distances [49,50] by finite difference methods computation of the diffusion equation in “unit cells”. Macroscopic arrays of nanoelectrodes present stationary sigmoidal response when the hemispherical diffusion layers developed at each nanotip do not overlap. The nanotips can then be considered as independent. Then, considering each independent tip as a hemisphere, the estimated stationary current for such an array can be calculated

using the following equation [42]: ilim ¼ 2prNnFCD

(1)

Where ilim is the observed stationary current, r is the estimated radius for each tip, N is the number of electrodes in the array, F is the Faraday constant, C the concentration of the redox active species and D = 6 105 cm2 s1 the diffusion coefficient. From the experimentally estimated current equal to 0.25 mA and equation (1), our electrode would correspond to an array of nanotips where each electrode presents a radius around 100 nm. This value is in the same order of magnitude as previously reported, i.e. 300 nm [42]. However, looking more carefully at the different theoretical investigations published, for scan rate of 10 mV s1 as used in Fig. 5B, and for 3.5 mm inter-tip spacing as specified by the optical fiber bundle construction, the situation of diffusionally-independent nanoprobes would be encountered only for tip radius < 50 nm [49]. The SEM images of the isolated array are presented in the Fig. 4A and B. They show the general aspect of the array. One can observe that the base of the tips was coated with a thick insulating film. If we consider just this thick basal layer, the estimated radius of each tip appears much bigger than the one calculated from the electrochemical measurements (about one order of magnitude). However, in a previous work [42], we deposited the electrophoretic paint with a different method and it formed a thinner insulating layer closer to the tip apex. Therefore, we believe that these SEM images do not allow visualizing precisely the radius of each electroactive tip because the border of the insulating paint could not be observed by SEM [47,51]. Another interpretation may be that the nanoprobes are not independent and diffusion layer overlapping is encountered along the whole nanoprobe array.

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3.3.3. Diffusion overlap at nanoprobes array – microelectrode behavior In the presence of a micrometric array of nanotips, a new masstransfer predominance zone is available. Even if nanotips are closely spaced so that their diffusion layers interpenetrate, a stationary hemispherical diffusion layer pertaining to the micrometric dimension of the ensemble of the array can be observed. This has been demonstrated from finite element simulation at arrays of up to 10 nanoelectrodes [52]. It was shown that in this situation a micrometrically large array of nanoelectrodes behaves as a microelectrode. The same situation is also encountered for large arrays of probes when transport can be constrained within micrometric distance through convective phenomena, like in natural convection (or in microfluidic environment) [50]. In these situations where a micrometric characteristic transport length is ruling the transport processes at nanoelectrodes arrays, the voltammetric response is sigmoidal and the current depicts the coverage of the micrometric domain by the nanoelectrodes. It is likely the situation developed at the bundle of nanoprobes investigated here. To better apprehend the transport phenomena at these nanoprobe arrays, one need to recourse to the modeling of the transport properties at such 3D electrodes presenting both a microstructure (the bundle dimension) and a nanostructure (the nanoprobes). Finite elements modeling computing are amenable to standard desktop computer in a reasonable time (< 1 hour for the simulation of a CV or an approach curve) by COMSOL which is used here. The general electrochemical behavior of micrometric arrays of nanotips has been considered. Owing to the large number of nanotips on the experimental system (6000 conical tips over a microdisk of 150 mm radius) simpler approximate situations were preferentially simulated. A full 3D model was used where the nanotips were considered as nanodisks (the experimental situation is rather cones) for minimization of the meshing complexity. These nanodisk electrodes were of radius rt = 0.1, 0.2 or 0.5 mm (the situation observed experimentally was 0.1 mm based on Eq. (1), or

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0.5 mm based on SEM images). They were separated by center-tocenter inter-tip distance ranging from d = 1.15 to 28 mm (the situation observed experimentally was 3.5 mm) on a microelectrode of radius a = 3 to 150 mm (the experimental situation was 150 mm), allowing for modulation of the nanoprobe number, N, on the electrode. For low number of nanoprobes (e.g. N = 19 nanodisks or nanocones), the full cylindrical electrode geometry was considered, however this situation did not fulfill the condition of “infinite array” or rather large array of nanoprobes which would require N > 20 elements [53] at the periphery of the microelectrode. To increase the number of nanoprobes and minimize the number of computed elements, slices (1/4th or 1/12th) of the fiber and solution volume were considered (see Fig. 6A) where, depending on the inter-center tip spacing, up to N = 129 nanodisks were implemented. The results of simulations are presented in Figs. 6 and 7. Fig. 6 illustrates cases close to the experimental situation, i.e., a 150 mm radius disk (fiber bundle) with inter-center tip distance varying between 28, 14 and 7 mm and corresponding to densities of nanotips of N = 750, 1500 and 3000 nanotips. The density of N = 6000 nanotips (inter-center tip distance of 3.5 mm) was obtained for 75 mm radius disk. Examples of simulated cyclic voltammograms at 10 mV s1 (as in Fig. 5B) at these arrays and at the plain microdisks are presented in Fig. 6B and 6C. Even though these figures are drawn for these specific cases, they illustrate the different situations encountered at “infinite arrays” when decreasing the extent of overlapping by decreasing the nanoprobe dimension or increasing the tip-to-tip distance. For all the intercenter tip spacings considered in Fig. 6, there was substantial to significant overlapping of diffusion layers, for 0.5 mm radius nanotips, within the time scale of the cyclic voltammetry. This overlapping of diffusion layer was demonstrated from the concentration profiles of electrochemically consumed redox probe at the peak potential or at the plateau (E = 0.4 V in the simulated cyclic voltammograms) presented in Fig. 6A. The overlapping was such that a quasi-spherical or hemispherical diffusion layer was generated around the microelectrode showing that even very large

Fig. 6. Simulated (A) concentration profiles and (B,C) cyclic voltammetric response of a redox probe electrode transformation at a disk arrays of nanodisks (radius 0.5 mm). (A) Concentration profile at the peak potential or at the plateau for E = 0.4 V, for a 150 mm radius array of nanodisks with inter-center spacing (from left to right) 28, 14, 7 mm; the 3D volumes used for simulations correspond respectively to 1/4th (90 ), 1/4th and 1/12th (60 ) of the array and solution. Cyclic voltammetry at array radius (B) r = 150 mm, (C) r = 75 mm; inter-tip spacing (B) 28, 14, 7 mm, (C) 7, 3.5 mm, scan rate 0.01 V s1. In all simulated concentration profiles, red codes for 1 and blue for 0. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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and to the already limited number of nanotips implemented in the model. However, comparing the cyclic voltammograms (Fig. 6C) at the smaller 75 mm radius array with nanotip density comparable to the experiment, one observed that the stationary current at the array was 93% the value of that expected at the plain disk. It is then anticipated that the 150 mm radius fiber bundle with 6000 nanodisks of radius 0.5 mm would behave as the plain 150 mm radius disk electrode and a current of the order of 0.1 mA would be expected. However this value is still lower than 0.25 mA, the experimentally measured current. It might be inferred to the shape of the nanotips that were cones and not the simulated disks. An array of conical tips would allow for higher electroactive surface area and higher current. As the height, h, of the individual conical nanotips was small compared to the dimension of the diffusion layer developed at the array (of size comparable to the bundle diameter), the expected change in current from full-microdisk to full-microdisk decorated with N conical nanotips should be of the order of the change in the area or roughness [54] between these two limiting situations. Here we have simulated the increase in current for the extreme situation of a 9 mm radius microarray of 19 conical probes of h = 4 mm and rt = 1.5 mm spaced by 3.5 mm fully conductive (except the bundle walls). The current at this microarray was 5.9 nA only 10% higher than the 5.5 nA estimated at the flat microdisk of same radius, ruling out the role of the shape of the nanotip on large change in the limiting current value. 3.3.4. Transition between individual independent nanoprobe array and overlapping array One question arise, can the nanoprobe array sustain higher current by decreasing the inter-tip overlapping and decreasing the nanoprobe dimension? Indeed, using Eq. (1) it was predicted that 100 nm apparent radii nanoprobe would give a current of 250 nA, i.e. higher than the one predicted for steady-state hemi-spherical transport at the plain microelectrode of radius a, imicro given by (2). Fig. 7. Simulated variations of the nanoprobe array current with the nanoprobe radius, for disk or cone nanoprobes. (A) Expected current considering that the nanoprobes are independent and (B) current at the plain equivalent disk microelectrode. The arrows represent the experimental value of the parameter a / Nrt for the optical fiber nanoprobe array considering that the nanoprobe are either 0.5 (SEM image) or 0.1 mm (Eq (1)) radius cones, showing that the experimental configuration corresponds to microelectrode behavior rather than independent array. Line: analytical estimate based on Eq (3). The shift in a/Nrt for cones compared to disks indicates that a nanocone array behaves as an array of nanodisk having twice larger radius (rd,app = 2rc).

arrays of nanoprobes develop diffusion layer dictated by the microstructure of the array. The extent of overlapping of the nanotip diffusion layers can be estimated qualitatively from the average concentration at the array surface in Fig. 6A: the closer to zero this concentration was and the larger was the overlapping (note that the concentration is 1 in the bulk). As expected the larger the inter-center tip spacing was, the lower was the overlapping and the lesser was the redox probe consumption at the array. A nanotip array then works qualitatively as a microelectrode (with hemispherical diffusion profile as seen in Fig. 6A) of micrometric dimension under kinetic control by an apparent slow electrontransfer. For all configurations (Figs. 6B and C), a pseudo or stationary sigmoidal voltammetric response was predicted for all the considered arrays. It was also shown that a plain disk with a radius of 150 mm behaved at 0.01 V s1 as a pseudo microelectrode with quasi-sigmoidal response, as observed experimentally. The simulated stationary current at such plain disk microelectrode of radius a = 150 mm was 0.1 mA smaller but within the order of magnitude of the experimentally measured value. The exact situation of the experiment cannot be simulated due to the complexity in meshing a large number of conical nanotips

imicro ¼ 4nFCDa

(2)

In other words, how would the maximum current sustained by each nanotip be depending on its geometrical configuration within the array? Fig. 7 presents in a log-log representation the evolution of the average current sustained at individual nanoprobe in an array, iind, as a function of the characteristic size of the nanoprobe rt. The average individual nanoprobe current was obtained from the simulated current at the array of N nanoprobes, iarray, through: iind = iarray / N. To take into account different nanoprobe dimensions, Fig. 7 is presented in dimensionless form where the current is normalized by the steady-state current observed at an independent nanoprobe, it (here it = 4FCDrt for a disk or e.g. it = 2pFCDrt for hemispheres or cones as in Eq (1)). The nanoprobe size is also presented in dimensionless form and the parameter a / Nrt is used. This parameter characterizes the extent of coverage of the electrode by the nanoprobes, a / Nrt >1 meaning low coverage (and often poor overlapping of diffusion fields) by the nanoprobes while a / Nrt < 1 rather characterizes high density of nanoprobes. At low coverage, the nanoprobes may be independent and the average nanoprobe current was given by its steady-state value and therefore Eq (1) applies for the expression of the current at the array of such independent nanoprobes. More interestingly, increasing the probe density (or decreasing the inter-tip distance), the average current per nanoprobe decreased with a / Nrt showing the stronger influence of the micrometric support (the microelectrode structure) over the current at low a / Nrt values. In the limited situation of small a / Nrt values the current at the microelectrode was then iarray = 2pFCDa (the factor 2p instead of 4 was used here because

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the insulating layer was negligible and spherical diffusion was observed). It ensued an average individual current iind/it = 2pa/Nrt in agreement with the linear correlation observed at small a/Nrt. In a first approximation, the overall situations could be extrapolated by a unique working curve which considered the two competing source of current: the development of independent nanoprobe diffusion field or of the microelectrode diffusion field yielding,   1 1 1 1 Nrt ¼ þ ¼ 1þb (3) Niind imicro Nit Nit a where b is a geometrical factor comparing the steady-state diffusion regime at the individual nanoprobe and the microelectrode (e.g. for disk nanoprobes and spherical microelectrode as simulated here, b = 4/2p).

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Eq (3) roughly predicted the electrochemical behavior of arrays of nanoelectrodes. It was then expected that whatever the array configuration the current flowing at the array could not be higher than the current flowing at the plain microelectrode of same size and geometry. In conclusion, there was no need to consider smaller size of the nanotips to explain the sigmoidal shape of the voltammogram, the micrometric dimension of the overall array intrinsically ruled such behavior characteristic of microelectrode. Moreover, the high current observed experimentally here could not be attributed to the extended paint coverage of the tips. Two sources may be proposed, to account for the difference between the experimental and simulated currents. The first would be an imperfect insulation of the walls of the bundle increasing the apparent size of the

Fig. 8. Simulated approach curves of insulating (i) and conducting (c) substrates along with the concentration profiles for different array-substrates separation distances d = 60, 20, 2 mm, (d = 400 mm presented in Fig. 6). Micrometer array radius r = 150 mm; inter-center nanodisks spacing (A) 28, (B) 14, (C) 7 mm. In all simulated concentration profiles, red codes for 1 and blue for 0. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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3.4. SECM feedback curves SECM feedback curves have been first measured before insulation of the individual tips, i.e. with electrode reported in Fig. 5A. The electrode was positioned with shearforce detection at the surface of the sample (glass or indium-tin oxide electrode, ITO), before the electrode was retracted at 2 mm s1. At short electrodeto-sample distance the electrode was in the SECM feedback regime, either negative on glass surface (Fig. 5C, curve a), or positive on ITO surface (Fig. 5C, curve b). However the relative current was varying in a limited extend versus the infinite current (measured when the electrode was far from the sample surface). One explanation discussed in the previous section is that before insulation with electrophoretic paint, a large part of the walls of the electrode was exposed to the electrolyte solution, a region that was not sensitive to the SECM feedback. Nanoprobe electrode array prepared by controlled tip insulation has been then used in the SECM experiment. In these conditions, the negative feedback was better defined with a minimum relative current close to 0.2 (Fig. 5D, curve a). The positive feedback was also improved, with a maximum relative current at about 3 (Fig. 5D, curve b). Here again finite element modeling can be used to apprehend the experimentally observed behaviors. 3D modeling of nonsymmetrical SECM problems has been obtained by boundary element method by Sklyar and Wittstock [55]. It is also amenable to finite elements methods using COMSOL environment. The SECM feedback curves of the nanotip arrays were modeled in 3D from the same slices of microdisk arrays of nanotips presented in the cyclic voltammogram section. The approach curves to insulating or conducting substrates were generated for 150 mm radius array of 0.5 mm radius nanodisks spaced by 28, 14 or 7 mm and for a 75 mm disk of 7 and 3.5 mm spacing. They are presented in Fig. 8 along with the illustrations of the redox probe concentration profile in the array/substrate gap for 3 electrode-substrate separation distances (2, 20 and 60 mm). The concentration profiles (Fig. 8AC) clearly show that, even if each nanotip was < 1 mm radius, the array sensed the presence of the substrate for large distance compared to the characteristic nanotip size or distance comparable to that of the micrometric array. The insulating substrate had very similar diffusion hindrance (compare the similar shape of the concentration profiles in Fig. 8Ai, Bi and Ci) without significant influence of the inter-tip spacing. It suggests that the diffusion hindrance mainly relied on the overall micrometric diffusion layer developed over the microdisk. The experimental approach curve for the insulated nanotip array was in reasonable agreement with the simulated ones. Conversely, for a conducting substrate the feedback was increased with higher density of nanotips. Particularly for less dense arrays, a sharp increase in feedback was predicted in the last

4 3

I / I0

microelectrode, the second would be related to the intervention of convective effect that would shorten the characteristic transport layer dimension increasing the current recorded at the microelectrode array. Natural convection is known to interfere at microelectrode larger than 100 mm, mainly leading to the development of mass-transfer limited layer dictated by the characteristic range of natural convection [50]. Vibrations resulting in higher pseudostationary current at > 50 mm large microelectrodes. This effect should not be neglected here due to the size of the fiber bundle. With or without intervention of natural convection, one possible approach to evidence the mode of transport to the nanoprobe array could be provided by SECM approach curves. The next section discusses the behavior of the nanotip electrode array through approaches to insulating and conductive surfaces for the measurement of negative and positive feedback curves, respectively.

2 1 0 0

50

100

150

200

Height /µm Fig. 9. Approach curves: experimental (black symbols), simulated curves (colored lines) for r = 150 mm with from bottom to top inter-center nanodisk spacing 7, 14, 28, 28, 14, 7 mm. Experimental inter-electrode distance was 3.5 mm. Experimental approach curves have been obtained over glass (negative feedback I/I0 < 1) and ITO (positive feedback, I/I0 > 1). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

2-4 mm of the approach curve showing the importance of development of feedback with each nanotip now functioning as an independent nanoprobe (see Fig. 8Ac, Bc and Cc for d = 2 mm). The conductive substrate was then more sensitive to the geometry of the nanotip array (density and size of the nanotip). The influence of tip geometry for 9 mm radius array of 19 disks or cone electrodes is illustrated in Fig. S7 in the supplementary data. Even though for the conical electrodes the height of the cone was not negligible compared to the overall dimension of the bundle, positive or negative feedback were detected for tip-substrate distances lower than 20 mm as for microdisk electrode of similar dimensions. Moreover, a modification in the shape of the nanoprobe did not affect the current by more than 10 % for tip-substrate distances < 2 mm. This situation of large aspect ratio microelectrode was however far from that expected for the 150 mm radius fiber bundle whose situation may then be modeled with reasonable confidence by arrays of disk nanoprobes as in Fig. 9. The fit between the experimental and simulated approach curve for the conductive substrate (Fig. 9, the four upper curves) was worse than for the insulating substrate, but data remained in the range of approach curves for arrays of nanodisks with density between 1500 and 3000 over 150 mm radius disk. This agreement was fair enough, considering the importance of the possible tilt of the array versus the conducting substrate on the aspect of the approach curve. 4. Conclusions We have demonstrated that nanotips array prepared by etching optical fiber bundles could be accurately positioned with shearforce detection at close distance to a liquid/solid or air/solid interfaces. The length of the shearforce approach curve that can vary from few mm to several tens of mm depended on the frequency used to perform the measurement, comprised between 70 and 170 kHz. The surface state of the array had also a critical influence on the length of this approach curve as it was observed that etched fibers led to shorter approach curves than the primary cleaved fibers. The media affected this signal, as demonstrated by comparison of approach curves in air and in water for the same nanotip array and the same frequencies. This surface sensitive signal was applied to control the positioning of the etched fiber array in a 1 mm PDMS film before performing the controlled electrophoretic deposition of an insulating paint. This partial insulation of the individual tips resulted in a nanoprobe electrode array. Electrochemical characterization of the micrometric array of

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nanotips by cyclic voltammetry and the comparison with simulations have shown that the 150 mm radius fiber bundle with 6000 nanodisks should behave at best as a plain 150 mm radius disk electrode. Simulations have demonstrated how the overlapping of diffusion layers developed at nanoprobe may extend toward micrometric regions characteristic of the array or the transport process. Even when nanoprobes were considered as independent, they may overlap to a sufficient extent so that the array behaved as a microelectrode. These simulations delineated the infinite array situation from the array of independent arrays and addressed the case of high density micrometric arrays of nanoprobe which were not described before. The behavior of these dense arrays of nanoprobes in the SECM configuration was then described. SECM feedback experiments demonstrated the importance of controlled insulation of the nanotip array for increasing the feedback interaction with the analyzed surfaces. The good agreement between experimental and simulated data suggested that the diffusion hindrance mainly relied on the overall micrometric diffusion layer developed over the microdisk and not by the size of the individual electrode tips. Conversely the size and the density of the nanotips were revealed by positive feedback during the approach to a conducting substrate as each nanotip acted as an individual nano-antenna of feedback current. Even though it was illustrated with bundle of regularly spaced nanoprobes, the conclusions of this work should apply to any SECM experiment using SECM microelectrode working as an array of nanoprobes such as partially covered/insulated microelectrodes. Both simulations and experiments demonstrated the potential of the shearforce detection for precise control of the nanotip array position over a substrate. The fabricated nanoprobe electrode arrays combine SECM measurement and shearforce positioning. Moreover Raman spectroscopy with these arrays has been reported independently [40]. The ability to merge intimately all three techniques based on different basic principles would offer the opportunity to acquire various and complementary information near an interface such as the local electrochemical reactivity and the Raman signature at the microscopic scale. Furthermore, the shearforce positioning allows now envisaging the use of such nanotip surface for tip-enhanced Raman scattering spectroscopy in an array format. Finally, the capabilities of this future dual electrochemical/optical platform might be exploited to modulate electrochemically the Raman or, more generally, the local optical signal collected remotely. Of course, the application of this “big object” in scanning electrochemical microscopy experiment is not trivial. And the absence of individual control of each nanoelectrode could be seen as a limit for electrochemistry, but not for spectroscopic experiments where each tip is optically independent and individually readable. The outlook of this work is certainly the shearforce positioning of etched fiber displaying the same number of fiber but with a much lower dimension, i.e., in the range of tens of mm. Acknowledgements We gratefully acknowledge Yannick Aubril, for the help in the implementation of the experiment, Jean-Paul Moulin and Gérard Paquot for their technical support, and Patrick Garrigue for electron microscopy imaging. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2015.04.140.

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Please cite this article in press as: C. Adam, et al., Shearforce positioning of nanoprobe electrode arrays for scanning electrochemical microscopy experiments, Electrochim. Acta (2015), http://dx.doi.org/10.1016/j.electacta.2015.04.140