Chemically Irreversible Redox Mediator for SECM Kinetics

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Chemically Irreversible Redox Mediator for SECM Kinetics Investigations: Determination of the Absolute Tip−Sample Distance Sebastien Lhenry, Yann R. Leroux, and Philippe Hapiot* Institut des Sciences Chimiques de Rennes, Université de Rennes 1, CNRS, UMR 6226 (Equipe MaCSE), Campus de Beaulieu, 35042 Rennes Cedex, France S Supporting Information *

ABSTRACT: The use of a chemically irreversible redox probe in scanning electrochemical microscopy (SECM) was evaluated for the determination of the absolute tip−substrate distance. This data is required for a quantitative use of the method in the analysis of functional surfaces with an unknown redox response. Associated with the relevant model curves, the electrochemical response allows an easy positioning of the tip versus the substrate that is independent of the nature of the materials under investigation. The irreversible oxidation of polyaromatic compounds was found to be well adapted for such investigations in organic media. Anthracene oxidation in acetonitrile was chosen as a demonstrative example for evaluating the errors and limits of the procedure. Interest in the procedure was exemplified for the local investigations of surfaces modified by redox entities. This permits discrimination between the different processes occurring at the sample surface as the permeability of the probe through the layer or the charge transfer pathways. It was possible to observe small differences with simple kinetic models (irreversible charge transfer) that are related to permeation: charge transport steps through a permeable redox layer.

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canning electrochemical microscopy (SECM)1,2 is a powerful method for probing at a local scale a variety of biological, chemical, and electrochemical processes occurring at interfaces.2 In SECM in feedback mode, the active form of a redox species (redox mediator) is produced at the tip electrode of the SECM that is moved in the vicinity of the surface under analysis. After diffusion of the mediator to the sample, its interactions with the surface are derived from approach curves that figure the variations of the tip current with the tip− substrate distance. In typical approach curves, the normalized current I = i/iinf is plotted versus the normalized distance L = d/a where i is the current at the tip electrode localized at a distance d from the substrate, iinf is the steady-state current when the tip is located at an infinite distance from the substrate (iinf = 4nFDCa, with n the number of electrons transferred per species, F the Faraday constant, D and C the diffusion coefficient and the initial concentration of the mediator, and a the radius of the UME).1,2 This allows the electrochemical examination of a variety of surfaces including those prepared on nonconducting substrates and the quantitative characterization of redox kinetics of the sample.3 In quantitative kinetics analyses, the determination of the absolute position of the tip versus the sample, the dimensionless parameter L, is required. In limiting kinetics cases, namely when the electron transfer kinetics at the electrode surface is fast or on the contrary in the absence of reaction at the surface, it is rigorously possible through an appropriate model to derive the absolute tip− surface distance from the approach curve with an excellent precision.1,2 However, for a generally considered sample that is not totally conductive or not totally insulating, this is not the case as the relation between the electrochemical response and © 2013 American Chemical Society

the distance is not a priori known. It results that errors in L distances lead to inaccurate measurements of rate constants4 or in misinterpretations of the redox phenomena. For example, when the charge transfer between the mediator and the surface is fast, but not infinitely fast, the absolute distance and the rate constants are combined parameters that cannot solely be determined.4a The situation is even more problematic for modified surfaces that could present SECM responses with different slopes and breaking points.5 In this context, several methods have been developed to position the SECM tip without using an electrochemical property, among them shear force detection being the most common one.6,7 Despite these techniques becoming more sophisticated and efficient, determining the absolute tip in SECM is a major difficulty for a quantitative use of the method.8 In this work, we have evaluated the use of specific “chemically irreversible redox mediator” in quantitative investigations of the redox properties. “Chemically irreversible redox mediator” means that the species produced at the tip electrode disappears by a homogeneous chemical reaction before reaching the sample and thus does not interact with the surface. When the chemical decay rate of the electrogenerated species is much faster than the diffusion time of the mediator from the tip electrode to the sample, the feedback loop cannot be achieved and the tip current is solely controlled by the hindered diffusion.1,2 This leads to an SECM response that Received: November 6, 2012 Accepted: January 3, 2013 Published: January 3, 2013 1840

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tends to the “negative feedback” behavior, i.e., when mediator does not react with the surface.9 In such a case, the SECM response does not depend on the nature of the surface under investigations and a single relation exists between the electrochemical approach curve and the distance. In principle, this would allow an absolute determination of the tip−surface distance considering that the geometric parameters of the tip electrode have been previously determined. The conditions for precise applications in kinetic analyses of this strategy were examined and tested with a carbon surface modified by a redox organic layer.10

Scheme 1. Formula of Compounds Used in This Work

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MATERIALS AND METHODS Chemicals. Acetonitrile (ACN, anhydrous, 99.8%, Sigma Aldrich) was used without further purification. We have used the following as redox mediators: anthracene (An, 99%, Alfa Aesar), ferrocene (Fc, 99%, Alfa Aesar), bis(pentamethylcyclopentadienyl)iron (DeFc, 97%, Aldrich), 1,1′-dimethylferrocene (DiFc, 97%, Alfa Aesar), ethynylferrocene (EFc, 97%, Aldrich), tri-p-tolylamine (TPA, 97%, Aldrich). The supporting electrolyte was tetrabutylammonium-hexafuorophosphate (nBu4NPF6, ≥99.0%, Fluka, electrochemical grade). The ferrocenyl modified surface was prepared by electroreduction of 4-(ethynyl)benzenediazonium salt that leads to a polyaryl layer with free ethylnyl moieties. Ferrocenyl groups are then added by “click chemistry” using azidomethylferrocene as previously described.10,11 The carbon substrate was a pyrolyzed photoresist film (PPF) (squares of 15 × 15 mm2) provided by Prof. Alison Downard (University of Canterbury, Christchurch, NZ) and was prepared following previously described methods.12 SECM Experiments. We used a typical three-electrode configuration, with a platinum counter electrode and a quasireference electrode.13 Measurements were performed using a homemade setup similar to that described in ref 14. The SECM setup is equipped with an adjustable stage for the tilt angle correction and is controlled by the SECMx software written by Wittstock, G., et al.14b The applied potential at the microelectrode tip is chosen as being sufficiently positive (or negative) to ensure a fast electron transfer at the tip (diffusion plateau of the mediator). The microelectrode tip was a commercial (IJ Cambria) 5-μm-radius platinum disk with a typical RG around 10 (RG is the ratio of the total electrode radius including the glass insulator over tip radius). The tip electrodes were characterized by cyclic voltammetry using the oxidation of ferrocene as test system and by typical approach curves recorded on conducting and insulating surfaces with the same mediator to determine the tip parameters (a and RG) before a set of experiments. A tip with a large RG was chosen to limit the influence of the parameter in the simulations. The fitting curves were calculated with the MIRA software package14c and provide the dimensionless kinetic constant κ = kela/D where kel is the apparent electron charge transfer at the sample surface. All SECM experiments were performed at room temperature. Fast scan voltammetry was performed using the procedures and equipment described in a previous publication.15

species after charge transfer. Rigorously, the lifetime of the electrogenerated species must be infinitively smaller than the tip-to-sample diffusion time even for the shortest tip−sample distances.1,9 Additionally, all electrogenerated species and the following products must obviously not adsorb or add onto the electrode during the time required to record an approach curve in steady state conditions. It is also suitable that the redox couple presents a very positive oxidation potential (or a very negative potential for investigation of reduction processes) in order to keep the largest available electrochemical windows for the SECM analysis. In organic media, the oxidation of polyaromatic compounds (oxidation of anthracene taken as example in this study) was found as a convenient system for such purpose. The initial aromatic radical cation evolves rapidly to products without a noticeable blocking of the tip electrode.16 To ensure the conditions of irreversibility, high-speed cyclic voltammetry of the oxidation of anthracene in the conditions of the experiments was performed for evaluating the lifetime of the electrogenerated radical cation. Typical cyclic voltammograms recorded with the 5 μm radius disk platinum electrode show that the reversibility appears only for scan rates higher than 40 000 V s−1 (Figure 1). Assuming an ECE mecha-

Figure 1. Voltammetry of anthracene (10−2 mol L−1) ACN + nBu4NPF6 (0.2 mol L−1) on polished platinum tips in acetonitrile with a scan rate of 40 000 V s−1 (black □). Line is smoothing adjacent averaging (red line).

nism,17,18 this corresponds to a decay rate k of the initially produced radical cation around k = 2 × 105 s−1 (see following chemical scheme) that in turn could be transformed to the dimensionless kinetic parameter, K, relevant to steady-state SECM conditions: K = ka2/D.9 K is then on the order of 200 considering a tip electrode with radius a = 5 μm and a diffusion coefficient D of 10−5 cm2 s−1.

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RESULTS AND DISCUSSIONS Required Characteristics for a Chemically Irreversible Mediator. To present a negative feedback whatever the nature of the substrate, the mediator must lead to a highly unstable 1841

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An → An•+ + e−

charge in the same way: both are oxidized or both are reduced. The irreversible mediator must present a sufficiently positive oxidation potential (or negative for a reduction) to limit interferences during the redox analyses with the other mediator. Additionally, it is especially important to minimize the distance drift between the two recordings of the curves. All approach curves were thus obtained in the same set of experiments without disconnecting the tip in a sort of round trip. In a first step, the tip electrode potential is set at the level of the diffusion plateau of the irreversible mediator (oxidation of anthracene). The approach curve with the irreversible mediator is then recorded during the forward movement of the tip. In a second stage, the tip potential is switched back to the potential of the analysis mediator and a new approach curve is immediately recorded during the retrieval of the tip. As test example, we chose the classic SECM investigations of an insulator (glass surface) and of a conducting surface (carbon) using the ferrocene/ferrocenium couple as analysis mediator. Following the above-mentioned procedure, approach curves are recorded with a series of ferrocene/anthracene mixtures. Figure 3 shows the data obtained with a conducting carbon surface. As expected, approach curves recorded with the ferrocene are independent of the anthracene concentration and show a good agreement with the diffusion-controlled behavior that is expected for a conducting surface (Figure 3b). During the irreversible anthracene oxidation (Figure 3a), both anthracene and ferrocene are simultaneously oxidized (redox potential of ferrocene is lower than that of anthracene). To limit the interference of the ferrocene signal to that of anthracene, anthracene concentration should be much larger than that of ferrocene. As seen in Figure 3a, a totally negative feedback is observed and almost unaffected for the highest anthracene/ferrocene ratios. The derived error on the tip position, ΔL, was estimated by comparison between the negative approaches of anthracene with those of ferrocene, the last one being considered as under diffusion control and keeping all other parameters. In these situations, the relation between the tip-current and L is unique; such comparison allows an estimation of the error made by using our procedure. The higher electron stoichiometry for the anthracene oxidation (6e− max for the oxidation of anthracene to the corresponding anthraquinone) versus one electron for the ferrocene oxidation is an advantage in this situation.18 A concentration ratio of anthracene/ferrocene above 10/1 or 20/1 appears reasonably sufficient to get an error ΔL around 0.1−0.2. Such ratio appears as a good compromise between the minimization of the error

k

An•+ → prod

prod → prod•+ + e−

With these K values and using previous theoretical analysis, we could expect that approach curves be very similar to the totally “negative feedback” case for dimensionless distances down to L ≈ 0.2.1,9 Practically, such small distances are rarely exploited in approach curves with tip using a large RG to limit the risk of scratching the sample or of tip breaking. We could thus expect that the oxidation of anthracene will display a behavior close to the totally negative feedback behavior whatever the surface. To a first check, we compared two SECM approach curves using the oxidation of anthracene on an insulating surface (a glass surface) and on a conductive metallic surface (a platinum surface). Both curves were recorded using the same experimental parameters and in the same media (ACN + 0.1 mol L−1 nBu4NPF6). As seen in Figure 2, both curves appear almost identical and present shapes very similar to the one expected for a totally negative feedback.

Figure 2. SECM approach curves with oxidation of anthracene (10−3 mol L−1) in acetonitrile + nBu4NPF6 (0.1 mol L−1) on glass (black □) and platinum (red ○). Line is the theoretical curves for an insulator sample (totally “negative feedback”).

Procedure for Positioning the Tip Electrode. In the following experiments, the solution contains both the irreversible couple for positioning the tip and the “analytical mediator” that is use to probe the redox activity of the sample. To test the feasibility of the procedure for quantitative analyses, we consider the most challenging situation where the two mediators, the reversible and the irreversible ones, transfer their

Figure 3. SECM approach curves on conductive carbon surface using a solution containing a mixture of anthracene + ferrocene + nBu4NPF6 (0.2 mol L−1) in acetonitrile. Potential is set at anthracene oxidation (a) or at the ferrocene oxidation plateaus (b). The concentrations of anthracene and ferrocene and ratio are, respectively, 10−2, 5 × 10−4 mol L−1, 20/1 (black □); 5 × 10−3, 5 × 10−4 mol L−1, 10/1 (red ○); 10−3,10−3, 1/1 mol L−1 (green △). Lines are the fitting curves for an insulator and conductor substrate under diffusion control (see text). 1842

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Figure 4. SECM approach curve on an insulating glass surface (a) and a conducting carbon surface (b) using a solution containing anthracene (10−2 mol L−1) + ferrocene (5 × 10−4 mol L−1) + nBu4NPF6 (0.2 mol L−1) in acetonitrile. Oxidation of anthracene (black □) and of ferrocene (red ○). Lines are the fitting curves for an insulator sample (black line) or a conductor for a diffusion controlled process (red line). Insert is a magnification of the L = 0−1 area showing the difference in the determination of absolute L scale between the ferrocene and anthracene oxidation curves ΔL = 0.06.

on L, the solubility of anthracene in ACN, and the limit of detection for the oxidation of ferrocene. On the basis of a simple addition of the current (see the Supporting Information), the minimum ratio between ferrocene and anthracene oxidation currents is indeed estimated in the same range. We choose to retain a ratio 20/1 in the further studies. Approach curves recorded on the insulating and conducting surface in these conditions are shown in Figure 4. The anthracene oxidation curves are recorded during the forward scan and those of ferrocene oxidation during the retrieval movement. On the insulating substrate, one could observe a good superposition of the two curves with a small offset in L. Individual fits with a completely negative feedback behavior (keeping the same geometric parameters for the tip) lead to a variation of L ≈ 0.1 between each curve. This error is recurrent and remains almost constant on repetitive measurements (of 6 consecutive approaches). This phenomenon could be ascribed to a change in the number of electron stoichiometry in the oxidation of anthracene, which approximates more and more toward one electron exchanged when the electrode is close to the surface. Additionally, the residual convection contributions may be slightly different during the forward and retrieval movements of the tip. To empirically correct from this effect and to improve the quality of the absolute L determination, the RG parameters were let as a free adjustable parameter in the anthracene curve that is treated as a totally negative feedback (insulating case). This empirical adjustment takes into account the different artifacts in the anthracene curve shape and leads to an average error on L around 0.06 ± 0.03 for both insulating and conducting surfaces investigations (see the inset in Figure 4a for the insulating surfaces). Application to the Kinetic Analysis of Modified Surfaces. This method of determination of the absolute L was applied to the SECM investigations of a modified carbon surface supporting immobilized redox entities (ferrocenyl groups). For the solution probe, we used an anthracene/ ferrocene ratio of 20 following the above-presented optimization.10 The principle of the SECM study consists in the interrogation of the surface with a series of redox mediator presenting increasing oxidation potentials. Experiments are performed in unbiased conditions meaning that the sample is not electrically connected (see Table 1). When the standard potential of the probe is much lower than that of the immobilized ferrocene (DeFc), the approach curves reveal the permeation of the probe through and out the deposited layer and charge transport by the carbon substrate (see ref 10 and

Table 1. Kinetic Constants Derived from Fitting Curves on Modified PPF Surfaces DeFc E°mediatora,b (V/SCE) κb,c kelb,c (cm s−1)

DiFc

EFc

TPA

−0.10

0.29

0.59

0.80

0.69 ± 0.01d 0.0252 ± 0.0004e

4.3 ± 0.5d 0.19 ± 0.02

9.3 ± 2.6d 0.44 ± 0.10

∞ ∞

a

The oxidation potential for the redox layer estimated as 0.33 V/SCE. Measured in ACN + 0.2 mol L−1 nBu4NPF6. All E° values are standardized with the E° for the ferrocene/ferrocenium couple taken at 0.405 V/SCE. cTip radius a = 5.23 μm. dErrors are determined considering an absolute offset ΔL = 0.06 ± 0.03. eThe fit deviates from the theoretical behavior predicted by the irreversible electron transfer kinetics model.1c b

references therein for examples of similar investigations). On the contrary, when the mediator displays an oxidation potential higher than that of the layer (EFc and TPA), the approach curves reveal the charge transfer between the oxidized form of the mediator and the immobilized redox entities, allowing a discrimination between the different processes. In such comparative analysis, all curves must be referred to the same absolute distance origin, which is rigorously not possible when approach curves are recorded on distinct experiments unless using another property that is not dependent on the materials under investigation.7,8 An accurate and absolute scaling of the experiments is especially important when the charge transfer rates at the sample are high because all positive feedback curves tend to the same curvature and practically are just shifted on the L scale.4 Additionally, some complicated kinetics lead to irregular curvature, which totally impedes approximate determinations based on the observation of a breaking point.5 It is therefore essential to know the absolute value of L with the maximum of accuracy in such quantitative analysis. Following our procedure, the experiment for each mediator consists in an initial approach to record the positioning curve with the oxidation of anthracene. To avoid any damage to the sample, all approaches were stopped long before touching the surface (when i is about 15% iinf). The tip potential is then switched back to the potential of the analysis mediator, and approach curves are recorded during the retrieval of the tip. After fitting of the anthracene curves as explained above (keeping RG as a free parameter), this determines the absolute L scale for each approach curve. To improve the quality of the L determination, all curves are shifted by a small offset ΔL = +0.06 to take into account the systematic error in our 1843

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Figure 5. SECM approach curves of the ferrocenyl modified carbon surface recorded with different analysis mediators. (a) Principle of the measurement and schematic description of the layer. (b) Experimental approach curves. The mediators are TPA (black), EFc (red), DiFc (green), DeFc (blue) at a 5 × 10−4 mol L−1 concentration in ACN + nBu4NPF6 (0.2 mol L−1). The solution also contains 10−2 mol L−1 anthracene used for the positioning. Lines are the fitting curves for irreversible electron transfer kinetics considering the κ constants reported in Table 1.

main advantage of the procedure is to provide a direct determination of the L scale and not only the reproducible positioning of the tip as is generally the case for other nonelectrochemical methods. The procedure not only permits a precise determination of the rate constants but also, thanks to the better accuracy on the absolute L scale, clearly provides evidence for subtle phenomena as shown in this work and coming from the permeation of the molecule through an organic layer. Oxidation of polyaromatic compounds is well adapted to the SECM investigations in organic media. However, the procedure is easily adaptable to quantitative kinetic determinations by SECM in aqueous media considering other irreversible redox mediators.

experimental conditions (see above). Charge transfer rate constants are then derived for the mediator curves considering the “real” geometric parameters (RG and a) that have been determined in an independent experiment. Figure 5 shows the quality of the obtained fits with the experimental data using such L determination. Values of the charge rate constants kel are summarized in Table 1 and under the form of the dimensionless rate constants κ. Error on the determination essentially arises from the ΔL offset and could be transformed in uncertainties on the κ determination that increases for the highest constants. As found previously, charge transfer constants increase with the E° of the probe.3 For DiFc, EFc, and TPA, experiments fit nicely with the theoretical curves even in the case of the most oxidant mediator (TPA) that is considered as an infinitively fast oxidant of the attached ferrocenyl moieties (diffusion controlled process) and thus validates our procedure. Notice that, for the less oxidant probe, DeFc, that characterizes the permeation of the mediator through the organic layer (it does not oxidize the attached ferrocenyl groups), the experimental data do not follow the behavior expected for the simple irreversible electron transfer kinetics case especially for the lowest distance L. This discrepancy provides evidence for the limitations in the regeneration of the mediator through the pinholes of the organic layer located outside the diffusion cone of the mediator as predicted by Amatore, C., et al.5b Additionally, the relatively large rate constant κ (kel) indicates a considerable permeation of the mediator and thus the presence of a large amount of pinholes in the layer.

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ASSOCIATED CONTENT

* Supporting Information S

Evaluation of the SECM response for different mixtures of a reversible and an irreversible mediator. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by the Agence Nationale de la Recherche, Contract ANR10-BLAN-714 Cavity-zyme(Cu) project. Prof A. J. Downard (University of Canterbury, Christchurch, NZ) is thanked for providing us a sample of PPF substrate.

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CONCLUSION Use of a chemically irreversible redox couple presents interesting and low-cost possibilities for the absolute determination of the tip electrode position that is required in accurate SECM kinetic analysis. Our procedure is based on the irreversible oxidation of a mediator illustrated with the oxidation of anthracene in acetonitrile. This molecule presents a totally negative feedback during an approach curve recorded on any substrate and thus allows a nonequivoque relationship between the tip position and the electrochemical current. The

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