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SECM imaging of micropatterned organic films on carbon surfaces
Electrochemistry Communications 9 (2007) 2387–2392 www.elsevier.com/locate/elecom
SECM imaging of micropatterned organic films on carbon surfaces Dodzi Zigah a, Marie Pellissier a, Fre´de´ric Barrie`re a, Alison J. Downard b, Philippe Hapiot a,* a
b
‘‘Sciences Chimiques de Rennes’’, UMR CNRS-Universite´ de Rennes 1N 6226, Equipe MACSE, Campus de Beaulieu, Baˆt 10C, 35042 Rennes cedex, France MacDiarmid Institute for Advanced Materials and Nanotechnology, Department of Chemistry, University of Canterbury, Private Bag 4800, Christchurch, New Zealand Received 25 June 2007; received in revised form 9 July 2007; accepted 12 July 2007 Available online 18 July 2007
Abstract Micropatterned carbon surfaces were prepared by electrochemical soft-lithography and imaged with scanning electrochemical microscopy. The technique allows the characterization of the surface in terms of resistance to molecules permeating the layer. Two types of electrogenerated organic layers were tested: sub-monolayers of tetraethyleneglycol diamine (TGD) and methylphenyl (MP) layers of variable thickness. TGD surfaces were found to provide little isolation and molecules can freely diffuse inside the layer to reach the surface. MP covered surfaces offer a much better resistance to organic molecules and can be almost insulating in the case of multilayer deposits. The blocking was found to be slightly better in water in agreement with the poor affinity of the aromatic layer with water. 2007 Elsevier B.V. All rights reserved. Keywords: SECM; Surface functionalization; Carbon surface; Electrografting
1. Introduction The modification of carbon surfaces by electrogenerated radicals is an attractive process for attaching a large variety of molecular structures [1–4]. This method is based on the reduction (or oxidation) of a precursor (for example the reduction of an aryldiazonium salt) to generate very reactive radicals in the vicinity of the surface. Molecules are strongly grafted to the surface when compared, for example, with those prepared by alkane-thiol self-assembled monolayers on gold (SAMs). With the diazonium reduction method, it has been shown that the molecules are covalently grafted to the surface which results in strongly attached molecules [3]. However, compared with SAMs, the patterning of carbon surfaces is a more difficult task and very few papers have been devoted to structured functionalizations, especially when more than one constituent is *
Corresponding author. Tel.: +33 2 23 23 59 39; fax: +33 2 23 23 67 32. E-mail address: [email protected] (P. Hapiot).
1388-2481/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2007.07.016
involved in the layer. Recently, the first examples of patterns incorporating two different modifiers and using the electrochemical generation of radicals, were reported for carbon surfaces [5,6]. The simplest methods combine electrochemical radical production with the same soft lithographic techniques that have become very widely utilized for generating micro- and nanoscale patterns of SAMs [6]. However, the continuous generation of radicals generally leads to the formation of multilayers because the radicals can add not only to the surface carbons but also to the already immobilized molecular structure. For aryldiazonium salts, the result is the formation of a sort of polyphenylene structure for which the exact structure depends both on the substituents present on the aromatic ring and on experimental conditions used during the electrochemical grafting [7–9]. The formation of multilayers has also been observed for the oxidation of diamines even if the nature of the structure and mechanism are still open questions [10,11]. In a global scale, i.e. when the electrode is observed by classical cyclic voltammetry, the grafting considerably
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modifies the properties of the surface especially in terms of apparent electron exchange rates (see for example Refs. [11,12] and references therein) and the blocking effect is commonly used to follow the compactness of the layer or the presence of pinholes. In this context, scanning electrochemical microscopy [13–15] is a powerful method for measuring apparent electron transfer rates through modifying layers (see for example Refs. [16–18] and references therein). An advantage of the method is that the sample does not have to be electrically connected and thus investigations of the electrochemical properties of the surface can be easily performed. In this paper, SECM was used first, to image carbon surface modified with two components and secondly, to follow the evolution of the blocking properties with the electrochemical grafting time or as function of the solvent used in the SECM measurements. 2. Experimental part
including the glass insulator over UME radius) [15]. The reference electrode was an Ag/AgCl, aqueous KCl 3 M electrode. UME were characterized by cyclic voltammetry and by typical approach curves recorded on a Pt conductive substrate. Imaging was performed under constant current mode at a tip-substrate distance expressed as L = d/a around 1 (d is the distance between the tip and the substrate and a is the radius of the UME). Fittings were performed using the approximate functions following the Bard–Mirkin formalism [15]. For electron transfer kinetic limitation, approximate equations were used for 0.5 < L < 2, then extended for large L values assuming that the current is equal to iinf at an infinite L. Two redox mediators were used: the hydroxymethylferrocene/hydroxymethylferrocenium couple (FcCH2OH/FcCH2OH+) for experiments in water and DMF, and the ferrocene/ferricenium couple (Fc/Fc+) in DMF, both at a typical concentration of 2 mM. Supporting electrolyte was 0.1 M KCl, in water and 0.1 M NBu4PF6, in DMF.
2.1. Preparation of patterned carbon 3. Results and discussion Pyrolyzed photoresist film (PPF) was used as the carbon substrate [19]. We used the fill-in approach and the experimental procedure for preparation of a MP/TGD pattern, for which details are described in reference [6]. In brief, a small piece of patterned poly(dimethylsiloxane) (PDMS) with overall dimensions of approximately 5 mm · 8 mm was positioned on the center of a piece of PPF. The PDMS incorporated two parallel open-ended microfluidic channels each approximately 22 lm wide and 49 lm deep, with a spacing of 380 lm. A drop of modifier solution was placed at the entrance of the channels and flowed into the channels. After fitting an O-ring (diameter = 9.4 ± 0.2 mm) around the PDMS, the electrochemical cell was assembled and the first modifier (methylbenzenediazonium salt) was electrografted to the PPF surface exposed within the micro-channels and to the surface not covered by PDMS. After the removal of the PDMS mold, the second modifier (tetraethyleneglycol diamine) was grafted to the PPF previously in contact with PDMS. For investigations of the modified layer prepared by cyclic voltammetric reduction of methylbenzenediazonium salt in acetonitrile-0.1 M NBu4PF6 (Fluka electrochemical grade), the concentration of diazonium salt was 5 mM, the scan rate 0.1 V/s and the electrode was cycled between +0.4 to 0.72 V vs. Ag/AgCl. 2.2. SECM experiments SECM measurements were performed using the CHI900B instrument from CH-Instruments equipped with an adjustable stage for tilt correction. The electrochemical cell was the one furnished with the SECM and was used in a typical three-electrode configuration for unbiased experiments. The tip electrode was a 5 lm radius disk Pt ultramicroelectrode (UME, CH-Instruments) with a typical RG = 5–10 (RG is the ratio of the total electrode radius
3.1. Examination of the PPF surface In initial test experiments, SECM was used to locally characterize the carbon substrate (PPF) before modification. The principle of the SECM is based on the electrochemical interactions of a redox species produced at a probe electrode (a UME tip) and the surface under investigation [15]. In the simplest mode used here (feedback mode), the substrate electrode (the modified carbon surface) is not biased and the probe electrode is a metallic (gold or platinum) disk UME. The redox couple used was either Fc/Fc+ or FcCH2OH/FcCH2OH+, which exhibit both fast electron transfer kinetics and are chemically reversible systems. The oxidized species is electrogenerated at the UME tip from a solution containing only the reduced form. After diffusion of the oxidized species to the sample, an electrochemical reaction is possible on a localized spot on the surface where the reduced form of the mediator can be regenerated resulting in an enhancement of the current at the probe electrode. Classically, the process is analyzed by recording the approach curves, i.e. the normalized current It = it/iinf vs. the normalized distance L = d/a, where it 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 at an infinite distance from the substrate, and 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 disk UME. SECM experiments are presented in Fig. 1. which displays the approach curves recorded in water and in DMF on clean PPF surface. The signal shows a strong enhancement of the current as the UME approaches the substrate that is characteristic of a positive feedback showing that electrons are rapidly exchanged between the oxidized form of the
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Fig. 1. Approach curves on unmodified and unbiased PPF electrode. Line is the theoretical behavior for a conductive substrate under diffusion control. RG = 5. Water, (m), DMF (h). Redox mediator: FcCH2OH (in water), Fc (in DMF) at a concentration of 2 mM.
mediator and the PPF sample [15]. Similar approach curves were obtained when the tip UME was positioned at different areas of the sample indicating that the conductivity of the PPF is uniform at the SECM resolution.
Fig. 2. Optical micrograph of water condensation figures formed on the patterned PPF surface.
3.2. Examination of the patterned surface Pattern was produced with sequential grafting of MP in the micro-channels and TGD in the surrounding area following the previously published procedure [6] and as explained in the experimental part. In a previous publication, samples were characterized by scanning electron microscopy before and after assembly of citrate-capped gold nanoparticles. These experiments have confirmed the success of the grafting in each region using these conditions [6]. Two parallel lines of approximately 20 lm wide with a spacing of 380 lm were produced by the reduction of 4methylbenzenediazonium salt which leads to the formation of aryl radicals and thus of the polyphenylene layers. The area between the lines was then grafted with the second modifier (TGD). The grafting conditions result in MP lines of approximately 2.5 nm thickness (multilayer films) and TGD background film of approximately 1 nm average thickness [6]. Obtained patterns were visualized through the formation of water condensation figures at two different scales (Fig. 2) [20]. Water condensation allows the two areas to be observed because they have different wettabilities giving different sizes and distributions of droplets. The drops are smaller on the less hydrophilic surface (the aromatic layer formed with the reduction of the methylbenzenediazonium cation) and show two well defined and parallel lines of the expected width. SECM observations of the sample were first undertaken in DMF. As observed in Fig. 3, one can observe a clear difference of current when the tip is passed over the sample.
Fig. 3. SECM Imaging of PPF modified surface in DMF (a) and water (b). Dark lines correspond to the MP layer and lighter region to the TGD area. Redox mediator: FcCH2OH (in water), Fc (in DMF) at a concentration of 2 mM.
The current was higher over the TGD area than the MP area. It shows that the blocking properties of the layer are higher over the aryl based area. The experiments were performed in water where similar patterns were observed. However, observation in DMF provides better contrast.
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To characterize each region, approach curves were recorded in the two individual modified areas. Curves recorded in the TGD region of the sample are displayed in Fig. 4. Enhancements of the current with the distance, L, were observed when the sample was imaged in water or in DMF corresponding to a positive feedback. The fitting of the data with theoretical curves shows a behavior close to a diffusion controlled positive feedback [15] which indicates a fast electron transfer between the oxidized mediator and the modified carbon. It is also noticeable that the responses recorded before and after the grafting are similar and thus that the grafting has only negligible effect on the apparent electron transfer rate kel, between the oxidized mediator and the carbon surface. More precisely, it indicates that kel, is larger than 0.2 cm s1 in both solvents (considering that the diffusion coefficients for Fc in DMF and FcCH2OH in water are equal to 1.07 · 105 [21] and 7.8 · 106 cm2 s1 [22], respectively), and that no difference can be detected considering the size of our tip UME [15]. Investigation of the phenomena in the aryl modified area would require recording the approach curves on the lines. However, our UME tip has typical dimensions (5 lm radius disk) in the same range as the line patterns (20 lm width) which is sufficient for imaging, but too large for approach curves that could be affected by the vicinity of the TGD layer. For that reason, approach curves were recorded on MP layers that were generated on a PPF carbon surface that was fully exposed to the diazonium salt reduction. MP layers were generated by cycling of the PPF sample in a solution of the diazonium salt. Different SECM examinations of the sample were performed: before the modification, after two modification cycles and after 4 cycles (Fig. 5). The recorded approach curves show the passage from a positive to a negative feedback showing a decrease of the apparent electron transfer rate, kel, when a thicker
Fig. 5. Approach curves on unbiased PPF electrode in water before and after electrografting of MP by different numbers of cycles (see text) (s) before modification, (h) after 2 cycles, (4) after 4 cycles. Lines are (from top): the theoretical behavior for a conductive substrate under diffusion control; j = 5 and j = 0.28 (j = kela/D). Redox mediator: FcCH2OH at a concentration of 2 mM.
film was produced. Two major pathways could be envisaged for the interfacial electron transfer namely: (i) electron tunnelling or electron hopping across the organic layer, (ii) permeation of the electroactive species into the layer, possibly through pinhole defects, followed by a transfer near the electrode surface [16,17]. Under these last conditions, the expected electrochemical response of the system is the same as compared with an unblocked substrate, but with a decrease of the apparent electron transfer rate constant by a ratio k el ¼ k 0el (1 u), where u is the fractional coverage of the electrode by the blocking material [23]. Thus, the kinetics of the transfer can be characterized by measuring the apparent electron transfer rate constant kel. Using the formalism previously introduced by Mirkin and Bard [24,25] when the substrate/solution interfacial electron transfer is the rate determining step, the normalized current IT can be described by the following set of equations: I ins I T ¼ I kS 1 Tc þ I ins ð1Þ T IT 0:78377 þ 0:3315eð1:0672=LÞ I cT ¼ 0:68 þ ð2Þ L 1 1:5358 þ 0:58eð1:14=LÞ þ 0:0908eðL6:3=1:017LÞ I ins T ¼ 0:15 þ L ð3Þ
Fig. 4. Approach curves on unbiased PPF electrode modified by TGD. Line is the theoretical curve calculated for a conductive substrate (RG = 5). Experimental points: water (m), DMF (h). Redox mediator: FcCH2OH (in water), Fc (in DMF) at a concentration of 2 mM.
where I cT ; I kS ; I Ins T are the normalized tip currents for, respectively, the diffusion controlled regeneration, the kinetically limited electron transfer at the substrate interface, and an insulating surface. I kS depends on a single dimensionless parameter j = kela/D where a is the electrode tip radius and D the diffusion coefficient of the mediator. For L > 2 and 0.01 < j < 1000, I kS can be approximated by the following equation:
D. Zigah et al. / Electrochemistry Communications 9 (2007) 2387–2392
I kS ¼
0:78377 0:68 þ 0:3315eð1:0672=LÞ þ Lð1 þ 1=KÞ 1 þ F ðL; KÞ
ð4Þ
ð11þ7:3KÞ and K ¼ jL. with F ðL; KÞ ¼ Kð11040LÞ When considering the results on the TGD surface, the estimated, kel, values appear too large for a transport implicating only mechanism (i) and it is likely that the process involves some permeation of the mediator through the grafted layer. The layer does not create any noticeable blocking effect in water or in DMF, and approach curves suggest that FcCH2OH could freely diffuse inside the organic layer or through the pinholes or defects. This situation can be interesting in devices in which good permeability is required, for example, amperometric sensors or mediated biofuel cells. Indeed, one can provide to the surface some desired chemical properties without impeding the communication between the substrate and a dissolved mediator. Concerning the aryl-modified region, negative feedback is observed both in water and in DMF, corresponding to a decrease of the apparent electron transfer rate, kel, that reflects more difficult access of the mediator to the carbon surface (see Fig. 6). As expected, this effect is increasingly pronounced as successive layers of aryl compounds are generated in agreement with the formation of a multilayer. Even in the most blocking situation examined in this work, the curve indicates that the passage of the current through the layer remains possible. It is also interesting to note that curves recorded in water are slightly below those in DMF (see Fig. 6) corresponding to a decrease of the apparent electron transfer rate by a factor around 2 (kel = 1.2 · 102, 4.4 · 103 cm s1) for the FcCH2OH/FcCH2OH+ mediator, in DMF and in water respectively). However, whereas good fitting to the theoretical curve is observed for data recorded in DMF, in water there is a discrepancy at the smallest approach distances. In this region the experimental data are not well-fitted by the simple theoretical model
Fig. 6. SECM approach curves on unbiased PPF electrode in water (h) and in DMF (4), after diazonium deposition. Lines are the theoretical behavior for j = 0.58, j = 0.28, respectively, from top (j = kela/D). Redox mediator: FcCH2OH at a concentration of 2 mM.
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which assumes a kinetic limitation due to electron transfer at the interface. This finding suggests some complications in the transport mechanism through the organic layer in aqueous solution. 4. Conclusion SECM has been proved to be an interesting tool for observing microscale patterns of modifiers on a carbon substrate. Contrast has been demonstrated between two different organic layers: a TGD layer which offers free passage to the surface for molecules in solution, and an aryl layer through which transport is more difficult. We propose that these differences are related to the permeation mechanism. A slight difference has been found for the behavior of the aryl layer in water and in organic solvent. More detailed experiments are required to understand why the approach curves in water deviate from the simple model, and how this is related to the role of the layer thickness and structure. Acknowledgements We gratefully acknowledge support of this research by the Agence National de la Recherche (Program No ANR-06-BLAN-0296-2) and the France-New Zealand Dumont d’Urville exchange program. D.Z. and M.P. thank Re´gion Bretagne for a studentship. We thank E.S.Q. Tan for assistance with preparation of the patterned samples. References [1] M. Delamar, R. Hitmi, J. Pinson, J.-M. Save´ant, J. Am. Chem. Soc. 114 (1992) 5883. [2] P. Allongue, M. Delamar, B. Desbat, O. Fagebaume, R. Hitmi, J. Pinson, J.-M. Save´ant, J. Am. Chem. Soc. 119 (1997) 201. [3] J. Pinson, F. Podvorica, Chem. Soc. Rev. 34 (2005) 429. [4] A.J. Downard, Electroanalysis 12 (2000) 1085. [5] P.A. Brooksby, A.J. Downard, Langmuir 21 (2005) 1672. [6] A.J. Downard, D.J. Garrett, E.S.Q. Tan, Langmuir 22 (2006) 10739. [7] J.K. Kariuki, M.T. McDermott, Langmuir 17 (2001) 5947. [8] F. Anariba, S.H. Du Vall, R.L. McCreery, Anal. Chem. 75 (2003) 3837. [9] P.R. Marcoux, P. Hapiot, P. Batail, J. Pinson, New. J. Chem. 28 (2004) 302. [10] A.J. Downard, S.L. Jackson, E.S.Q. Tan, Aust. J. Chem. 58 (2005) 275. [11] A.C. Cruickshank, E.Q.S. Tan, P.A. Brooksby, A.J. Downard, Electrochem. Commun. 9 (2007) 1456. [12] M.G. Paulik, P.A. Brooksby, A.D. Abell, A.J. Downard, J. Phys. Chem. C 111 (2007) 7808. [13] F.-R.F. Fan, A.J. Bard, Science 267 (1995) 871. [14] F.-R.F. Fan, J. Kwak, A.J. Bard, J. Am. Chem. Soc. 118 (1996) 9669. [15] A.J. Bard, M.V. Mirkin (Eds.), Scanning Electrochemical Micorscopy, Marcel Dekker Inc., New York, 2001. [16] B. Liu, A.J. Bard, M.V. Mirkin, S.E. Creager, J. Am. Chem. Soc 126 (2004) 1485. [17] J. Ghilane, F. Hauquier, B. Fabre, P. Hapiot, Anal. Chem. 78 (2006) 6019. [18] G. Wittstock, M. Burchardt, S.E. Pust, Y. Shen, C. Zhao, Angew. Chem. – Int. Ed. 46 (2007) 1584. [19] P.A. Brooksby, A.J. Downard, Langmuir 20 (2004) 5038.
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[20] G.P. Lopez, H.A. Biebuyck, C.D. Frisbie, G.M. Whitesides, Science 260 (1993) 647. [21] S.R. Jacob, Q. Hong, B.A. Coles, R.G. Compton, J. Phys. Chem. B 103 (1999) 2963. [22] W. Miao, Z. Ding, A.J. Bard, J. Phys. Chem. 106 (2002) 1392.
[23] C. Amatore, J.-M. Save´ant, D. Tessier, J. Electroanal. Chem. 147 (1983) 39. [24] A.J. Bard, M.V. Mirkin, P.R. Unwin, D.O. Wipf, J. Phys. Chem. 96 (1992) 1861. [25] C. Wei, A.J. Bard, M.V. Mirkin, J. Phys. Chem. 99 (1995) 16033.
SECM imaging of micropatterned organic films on carbon surfaces Dodzi Zigah a, Marie Pellissier a, Fre´de´ric Barrie`re a, Alison J. Downard b, Philippe Hapiot a,* a
b
‘‘Sciences Chimiques de Rennes’’, UMR CNRS-Universite´ de Rennes 1N 6226, Equipe MACSE, Campus de Beaulieu, Baˆt 10C, 35042 Rennes cedex, France MacDiarmid Institute for Advanced Materials and Nanotechnology, Department of Chemistry, University of Canterbury, Private Bag 4800, Christchurch, New Zealand Received 25 June 2007; received in revised form 9 July 2007; accepted 12 July 2007 Available online 18 July 2007
Abstract Micropatterned carbon surfaces were prepared by electrochemical soft-lithography and imaged with scanning electrochemical microscopy. The technique allows the characterization of the surface in terms of resistance to molecules permeating the layer. Two types of electrogenerated organic layers were tested: sub-monolayers of tetraethyleneglycol diamine (TGD) and methylphenyl (MP) layers of variable thickness. TGD surfaces were found to provide little isolation and molecules can freely diffuse inside the layer to reach the surface. MP covered surfaces offer a much better resistance to organic molecules and can be almost insulating in the case of multilayer deposits. The blocking was found to be slightly better in water in agreement with the poor affinity of the aromatic layer with water. 2007 Elsevier B.V. All rights reserved. Keywords: SECM; Surface functionalization; Carbon surface; Electrografting
1. Introduction The modification of carbon surfaces by electrogenerated radicals is an attractive process for attaching a large variety of molecular structures [1–4]. This method is based on the reduction (or oxidation) of a precursor (for example the reduction of an aryldiazonium salt) to generate very reactive radicals in the vicinity of the surface. Molecules are strongly grafted to the surface when compared, for example, with those prepared by alkane-thiol self-assembled monolayers on gold (SAMs). With the diazonium reduction method, it has been shown that the molecules are covalently grafted to the surface which results in strongly attached molecules [3]. However, compared with SAMs, the patterning of carbon surfaces is a more difficult task and very few papers have been devoted to structured functionalizations, especially when more than one constituent is *
Corresponding author. Tel.: +33 2 23 23 59 39; fax: +33 2 23 23 67 32. E-mail address: [email protected] (P. Hapiot).
1388-2481/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2007.07.016
involved in the layer. Recently, the first examples of patterns incorporating two different modifiers and using the electrochemical generation of radicals, were reported for carbon surfaces [5,6]. The simplest methods combine electrochemical radical production with the same soft lithographic techniques that have become very widely utilized for generating micro- and nanoscale patterns of SAMs [6]. However, the continuous generation of radicals generally leads to the formation of multilayers because the radicals can add not only to the surface carbons but also to the already immobilized molecular structure. For aryldiazonium salts, the result is the formation of a sort of polyphenylene structure for which the exact structure depends both on the substituents present on the aromatic ring and on experimental conditions used during the electrochemical grafting [7–9]. The formation of multilayers has also been observed for the oxidation of diamines even if the nature of the structure and mechanism are still open questions [10,11]. In a global scale, i.e. when the electrode is observed by classical cyclic voltammetry, the grafting considerably
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modifies the properties of the surface especially in terms of apparent electron exchange rates (see for example Refs. [11,12] and references therein) and the blocking effect is commonly used to follow the compactness of the layer or the presence of pinholes. In this context, scanning electrochemical microscopy [13–15] is a powerful method for measuring apparent electron transfer rates through modifying layers (see for example Refs. [16–18] and references therein). An advantage of the method is that the sample does not have to be electrically connected and thus investigations of the electrochemical properties of the surface can be easily performed. In this paper, SECM was used first, to image carbon surface modified with two components and secondly, to follow the evolution of the blocking properties with the electrochemical grafting time or as function of the solvent used in the SECM measurements. 2. Experimental part
including the glass insulator over UME radius) [15]. The reference electrode was an Ag/AgCl, aqueous KCl 3 M electrode. UME were characterized by cyclic voltammetry and by typical approach curves recorded on a Pt conductive substrate. Imaging was performed under constant current mode at a tip-substrate distance expressed as L = d/a around 1 (d is the distance between the tip and the substrate and a is the radius of the UME). Fittings were performed using the approximate functions following the Bard–Mirkin formalism [15]. For electron transfer kinetic limitation, approximate equations were used for 0.5 < L < 2, then extended for large L values assuming that the current is equal to iinf at an infinite L. Two redox mediators were used: the hydroxymethylferrocene/hydroxymethylferrocenium couple (FcCH2OH/FcCH2OH+) for experiments in water and DMF, and the ferrocene/ferricenium couple (Fc/Fc+) in DMF, both at a typical concentration of 2 mM. Supporting electrolyte was 0.1 M KCl, in water and 0.1 M NBu4PF6, in DMF.
2.1. Preparation of patterned carbon 3. Results and discussion Pyrolyzed photoresist film (PPF) was used as the carbon substrate [19]. We used the fill-in approach and the experimental procedure for preparation of a MP/TGD pattern, for which details are described in reference [6]. In brief, a small piece of patterned poly(dimethylsiloxane) (PDMS) with overall dimensions of approximately 5 mm · 8 mm was positioned on the center of a piece of PPF. The PDMS incorporated two parallel open-ended microfluidic channels each approximately 22 lm wide and 49 lm deep, with a spacing of 380 lm. A drop of modifier solution was placed at the entrance of the channels and flowed into the channels. After fitting an O-ring (diameter = 9.4 ± 0.2 mm) around the PDMS, the electrochemical cell was assembled and the first modifier (methylbenzenediazonium salt) was electrografted to the PPF surface exposed within the micro-channels and to the surface not covered by PDMS. After the removal of the PDMS mold, the second modifier (tetraethyleneglycol diamine) was grafted to the PPF previously in contact with PDMS. For investigations of the modified layer prepared by cyclic voltammetric reduction of methylbenzenediazonium salt in acetonitrile-0.1 M NBu4PF6 (Fluka electrochemical grade), the concentration of diazonium salt was 5 mM, the scan rate 0.1 V/s and the electrode was cycled between +0.4 to 0.72 V vs. Ag/AgCl. 2.2. SECM experiments SECM measurements were performed using the CHI900B instrument from CH-Instruments equipped with an adjustable stage for tilt correction. The electrochemical cell was the one furnished with the SECM and was used in a typical three-electrode configuration for unbiased experiments. The tip electrode was a 5 lm radius disk Pt ultramicroelectrode (UME, CH-Instruments) with a typical RG = 5–10 (RG is the ratio of the total electrode radius
3.1. Examination of the PPF surface In initial test experiments, SECM was used to locally characterize the carbon substrate (PPF) before modification. The principle of the SECM is based on the electrochemical interactions of a redox species produced at a probe electrode (a UME tip) and the surface under investigation [15]. In the simplest mode used here (feedback mode), the substrate electrode (the modified carbon surface) is not biased and the probe electrode is a metallic (gold or platinum) disk UME. The redox couple used was either Fc/Fc+ or FcCH2OH/FcCH2OH+, which exhibit both fast electron transfer kinetics and are chemically reversible systems. The oxidized species is electrogenerated at the UME tip from a solution containing only the reduced form. After diffusion of the oxidized species to the sample, an electrochemical reaction is possible on a localized spot on the surface where the reduced form of the mediator can be regenerated resulting in an enhancement of the current at the probe electrode. Classically, the process is analyzed by recording the approach curves, i.e. the normalized current It = it/iinf vs. the normalized distance L = d/a, where it 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 at an infinite distance from the substrate, and 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 disk UME. SECM experiments are presented in Fig. 1. which displays the approach curves recorded in water and in DMF on clean PPF surface. The signal shows a strong enhancement of the current as the UME approaches the substrate that is characteristic of a positive feedback showing that electrons are rapidly exchanged between the oxidized form of the
D. Zigah et al. / Electrochemistry Communications 9 (2007) 2387–2392
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Fig. 1. Approach curves on unmodified and unbiased PPF electrode. Line is the theoretical behavior for a conductive substrate under diffusion control. RG = 5. Water, (m), DMF (h). Redox mediator: FcCH2OH (in water), Fc (in DMF) at a concentration of 2 mM.
mediator and the PPF sample [15]. Similar approach curves were obtained when the tip UME was positioned at different areas of the sample indicating that the conductivity of the PPF is uniform at the SECM resolution.
Fig. 2. Optical micrograph of water condensation figures formed on the patterned PPF surface.
3.2. Examination of the patterned surface Pattern was produced with sequential grafting of MP in the micro-channels and TGD in the surrounding area following the previously published procedure [6] and as explained in the experimental part. In a previous publication, samples were characterized by scanning electron microscopy before and after assembly of citrate-capped gold nanoparticles. These experiments have confirmed the success of the grafting in each region using these conditions [6]. Two parallel lines of approximately 20 lm wide with a spacing of 380 lm were produced by the reduction of 4methylbenzenediazonium salt which leads to the formation of aryl radicals and thus of the polyphenylene layers. The area between the lines was then grafted with the second modifier (TGD). The grafting conditions result in MP lines of approximately 2.5 nm thickness (multilayer films) and TGD background film of approximately 1 nm average thickness [6]. Obtained patterns were visualized through the formation of water condensation figures at two different scales (Fig. 2) [20]. Water condensation allows the two areas to be observed because they have different wettabilities giving different sizes and distributions of droplets. The drops are smaller on the less hydrophilic surface (the aromatic layer formed with the reduction of the methylbenzenediazonium cation) and show two well defined and parallel lines of the expected width. SECM observations of the sample were first undertaken in DMF. As observed in Fig. 3, one can observe a clear difference of current when the tip is passed over the sample.
Fig. 3. SECM Imaging of PPF modified surface in DMF (a) and water (b). Dark lines correspond to the MP layer and lighter region to the TGD area. Redox mediator: FcCH2OH (in water), Fc (in DMF) at a concentration of 2 mM.
The current was higher over the TGD area than the MP area. It shows that the blocking properties of the layer are higher over the aryl based area. The experiments were performed in water where similar patterns were observed. However, observation in DMF provides better contrast.
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To characterize each region, approach curves were recorded in the two individual modified areas. Curves recorded in the TGD region of the sample are displayed in Fig. 4. Enhancements of the current with the distance, L, were observed when the sample was imaged in water or in DMF corresponding to a positive feedback. The fitting of the data with theoretical curves shows a behavior close to a diffusion controlled positive feedback [15] which indicates a fast electron transfer between the oxidized mediator and the modified carbon. It is also noticeable that the responses recorded before and after the grafting are similar and thus that the grafting has only negligible effect on the apparent electron transfer rate kel, between the oxidized mediator and the carbon surface. More precisely, it indicates that kel, is larger than 0.2 cm s1 in both solvents (considering that the diffusion coefficients for Fc in DMF and FcCH2OH in water are equal to 1.07 · 105 [21] and 7.8 · 106 cm2 s1 [22], respectively), and that no difference can be detected considering the size of our tip UME [15]. Investigation of the phenomena in the aryl modified area would require recording the approach curves on the lines. However, our UME tip has typical dimensions (5 lm radius disk) in the same range as the line patterns (20 lm width) which is sufficient for imaging, but too large for approach curves that could be affected by the vicinity of the TGD layer. For that reason, approach curves were recorded on MP layers that were generated on a PPF carbon surface that was fully exposed to the diazonium salt reduction. MP layers were generated by cycling of the PPF sample in a solution of the diazonium salt. Different SECM examinations of the sample were performed: before the modification, after two modification cycles and after 4 cycles (Fig. 5). The recorded approach curves show the passage from a positive to a negative feedback showing a decrease of the apparent electron transfer rate, kel, when a thicker
Fig. 5. Approach curves on unbiased PPF electrode in water before and after electrografting of MP by different numbers of cycles (see text) (s) before modification, (h) after 2 cycles, (4) after 4 cycles. Lines are (from top): the theoretical behavior for a conductive substrate under diffusion control; j = 5 and j = 0.28 (j = kela/D). Redox mediator: FcCH2OH at a concentration of 2 mM.
film was produced. Two major pathways could be envisaged for the interfacial electron transfer namely: (i) electron tunnelling or electron hopping across the organic layer, (ii) permeation of the electroactive species into the layer, possibly through pinhole defects, followed by a transfer near the electrode surface [16,17]. Under these last conditions, the expected electrochemical response of the system is the same as compared with an unblocked substrate, but with a decrease of the apparent electron transfer rate constant by a ratio k el ¼ k 0el (1 u), where u is the fractional coverage of the electrode by the blocking material [23]. Thus, the kinetics of the transfer can be characterized by measuring the apparent electron transfer rate constant kel. Using the formalism previously introduced by Mirkin and Bard [24,25] when the substrate/solution interfacial electron transfer is the rate determining step, the normalized current IT can be described by the following set of equations: I ins I T ¼ I kS 1 Tc þ I ins ð1Þ T IT 0:78377 þ 0:3315eð1:0672=LÞ I cT ¼ 0:68 þ ð2Þ L 1 1:5358 þ 0:58eð1:14=LÞ þ 0:0908eðL6:3=1:017LÞ I ins T ¼ 0:15 þ L ð3Þ
Fig. 4. Approach curves on unbiased PPF electrode modified by TGD. Line is the theoretical curve calculated for a conductive substrate (RG = 5). Experimental points: water (m), DMF (h). Redox mediator: FcCH2OH (in water), Fc (in DMF) at a concentration of 2 mM.
where I cT ; I kS ; I Ins T are the normalized tip currents for, respectively, the diffusion controlled regeneration, the kinetically limited electron transfer at the substrate interface, and an insulating surface. I kS depends on a single dimensionless parameter j = kela/D where a is the electrode tip radius and D the diffusion coefficient of the mediator. For L > 2 and 0.01 < j < 1000, I kS can be approximated by the following equation:
D. Zigah et al. / Electrochemistry Communications 9 (2007) 2387–2392
I kS ¼
0:78377 0:68 þ 0:3315eð1:0672=LÞ þ Lð1 þ 1=KÞ 1 þ F ðL; KÞ
ð4Þ
ð11þ7:3KÞ and K ¼ jL. with F ðL; KÞ ¼ Kð11040LÞ When considering the results on the TGD surface, the estimated, kel, values appear too large for a transport implicating only mechanism (i) and it is likely that the process involves some permeation of the mediator through the grafted layer. The layer does not create any noticeable blocking effect in water or in DMF, and approach curves suggest that FcCH2OH could freely diffuse inside the organic layer or through the pinholes or defects. This situation can be interesting in devices in which good permeability is required, for example, amperometric sensors or mediated biofuel cells. Indeed, one can provide to the surface some desired chemical properties without impeding the communication between the substrate and a dissolved mediator. Concerning the aryl-modified region, negative feedback is observed both in water and in DMF, corresponding to a decrease of the apparent electron transfer rate, kel, that reflects more difficult access of the mediator to the carbon surface (see Fig. 6). As expected, this effect is increasingly pronounced as successive layers of aryl compounds are generated in agreement with the formation of a multilayer. Even in the most blocking situation examined in this work, the curve indicates that the passage of the current through the layer remains possible. It is also interesting to note that curves recorded in water are slightly below those in DMF (see Fig. 6) corresponding to a decrease of the apparent electron transfer rate by a factor around 2 (kel = 1.2 · 102, 4.4 · 103 cm s1) for the FcCH2OH/FcCH2OH+ mediator, in DMF and in water respectively). However, whereas good fitting to the theoretical curve is observed for data recorded in DMF, in water there is a discrepancy at the smallest approach distances. In this region the experimental data are not well-fitted by the simple theoretical model
Fig. 6. SECM approach curves on unbiased PPF electrode in water (h) and in DMF (4), after diazonium deposition. Lines are the theoretical behavior for j = 0.58, j = 0.28, respectively, from top (j = kela/D). Redox mediator: FcCH2OH at a concentration of 2 mM.
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which assumes a kinetic limitation due to electron transfer at the interface. This finding suggests some complications in the transport mechanism through the organic layer in aqueous solution. 4. Conclusion SECM has been proved to be an interesting tool for observing microscale patterns of modifiers on a carbon substrate. Contrast has been demonstrated between two different organic layers: a TGD layer which offers free passage to the surface for molecules in solution, and an aryl layer through which transport is more difficult. We propose that these differences are related to the permeation mechanism. A slight difference has been found for the behavior of the aryl layer in water and in organic solvent. More detailed experiments are required to understand why the approach curves in water deviate from the simple model, and how this is related to the role of the layer thickness and structure. Acknowledgements We gratefully acknowledge support of this research by the Agence National de la Recherche (Program No ANR-06-BLAN-0296-2) and the France-New Zealand Dumont d’Urville exchange program. D.Z. and M.P. thank Re´gion Bretagne for a studentship. We thank E.S.Q. Tan for assistance with preparation of the patterned samples. References [1] M. Delamar, R. Hitmi, J. Pinson, J.-M. Save´ant, J. Am. Chem. Soc. 114 (1992) 5883. [2] P. Allongue, M. Delamar, B. Desbat, O. Fagebaume, R. Hitmi, J. Pinson, J.-M. Save´ant, J. Am. Chem. Soc. 119 (1997) 201. [3] J. Pinson, F. Podvorica, Chem. Soc. Rev. 34 (2005) 429. [4] A.J. Downard, Electroanalysis 12 (2000) 1085. [5] P.A. Brooksby, A.J. Downard, Langmuir 21 (2005) 1672. [6] A.J. Downard, D.J. Garrett, E.S.Q. Tan, Langmuir 22 (2006) 10739. [7] J.K. Kariuki, M.T. McDermott, Langmuir 17 (2001) 5947. [8] F. Anariba, S.H. Du Vall, R.L. McCreery, Anal. Chem. 75 (2003) 3837. [9] P.R. Marcoux, P. Hapiot, P. Batail, J. Pinson, New. J. Chem. 28 (2004) 302. [10] A.J. Downard, S.L. Jackson, E.S.Q. Tan, Aust. J. Chem. 58 (2005) 275. [11] A.C. Cruickshank, E.Q.S. Tan, P.A. Brooksby, A.J. Downard, Electrochem. Commun. 9 (2007) 1456. [12] M.G. Paulik, P.A. Brooksby, A.D. Abell, A.J. Downard, J. Phys. Chem. C 111 (2007) 7808. [13] F.-R.F. Fan, A.J. Bard, Science 267 (1995) 871. [14] F.-R.F. Fan, J. Kwak, A.J. Bard, J. Am. Chem. Soc. 118 (1996) 9669. [15] A.J. Bard, M.V. Mirkin (Eds.), Scanning Electrochemical Micorscopy, Marcel Dekker Inc., New York, 2001. [16] B. Liu, A.J. Bard, M.V. Mirkin, S.E. Creager, J. Am. Chem. Soc 126 (2004) 1485. [17] J. Ghilane, F. Hauquier, B. Fabre, P. Hapiot, Anal. Chem. 78 (2006) 6019. [18] G. Wittstock, M. Burchardt, S.E. Pust, Y. Shen, C. Zhao, Angew. Chem. – Int. Ed. 46 (2007) 1584. [19] P.A. Brooksby, A.J. Downard, Langmuir 20 (2004) 5038.
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