Design of Robust Binary Film onto Carbon Surface Using Diazonium

Jul 20, 2011 - Design of Robust Binary Film onto Carbon Surface Using Diazonium Electrochemistry. Yann R. Leroux†, Fei Hui†, Jean-Marc Noël†, Clément ...
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Design of Robust Binary Film onto Carbon Surface Using Diazonium Electrochemistry Yann R. Leroux,† Fei Hui,† Jean-Marc No€el,† Clement Roux,‡ Alison J. Downard,‡ and Philippe Hapiot*,† †

Sciences Chimiques de Rennes (Equipe MaCSE), CNRS, UMR 6226, Universite de Rennes 1, Campus de Beaulieu, Bat 10C, 35042 Rennes Cedex, France ‡ Department of Chemistry, University of Canterbury, MacDiarmid Institute Advanced Materials & Nanotechnology, Private Bag 4800, Christchurch 8140, New Zealand

bS Supporting Information ABSTRACT: The electroreduction of functionalized aryldiazonium salts combined with a protection deprotection method was evaluated for the fabrication of organized mixed layers covalently bound onto carbon substrates. The first modification consists of the grafting of a protected 4-((triisopropylsilyl)ethynyl) benzene layer onto the carbon surface on which the introduction of a second functional group is possible without altering the first grafted functional group. After deprotection, we obtained an ultrathin robust layer presenting high densities of both active ethynylbenzene groups (available for “click” chemistry) and the second functional group. The strategy was successfully demonstrated using azidomethylferrocene to react with ethynyl moieties in the binary film by “click” chemistry, and NO2-phenyl as the second functional group. Two possible modification pathways with different orderings of the various steps were considered to show the influence and importance of the protection deprotection process on the final surface obtained. Using mild conditions for the grafting of the second layer maintains a concentration of active ethynyl groups similar to that obtained for a one-component monolayer while achieving a high surface concentration of the second modifier. Considering the wide range of functional aryldiazonium salts that could be electrodeposited onto carbon surfaces and the versatility and specificity of the “click” chemistry, this approach appears very promising for the preparation of mixed layers in well-controlled conditions without altering the reactivity of either functional group.

’ INTRODUCTION Surface modifications of carbon materials by grafting organic molecules or more generally nanoscale molecular objects are of primary importance because of the wide range of potential applications of such interfaces.1 In this field, formation of an active layer through diazonium redox chemistry is certainly one of the most common methods. It allows the covalent immobilization of many types of functional groups and on a large variety of substrates through the electrogeneration of highly reactive aryl radicals obtained by the simple reduction of a diazonium salt.2 As discussed before, functionalization of carbon surfaces is of fundamental importance due to the numerous applications of such materials (see, for example, refs 1, 2b and references therein). The counterpart drawback is the difficulty of controlling the vertical extension of the produced layer principally because aryl radicals may react on the grafted layer leading to multilayer coatings with irregular morphology. This difficulty also limits the design of more complicated modified carbon surfaces, especially those having more than one chemical constituent or those with spatial arrangements that are of fundamental interest in many applications.3 In such applications, functional groups must be positioned within molecular distances, which require that the groups be mixed in the deposited layer. r 2011 American Chemical Society

Several studies have demonstrated that new properties are obtained with such binary layers prepared from solutions containing two reagents (for example, two thiols on gold surfaces)4 with distinct functional groups (or one functional group diluted with an inactive long alkyl chain).4,5 Concerning carbon substrates, similar strategies have been evaluated for the preparation of multicomponent films by simultaneous electrografting of mixtures of diazonium salts6 or diaryliodonium salts.7,8 However, because of the high and unselective reactivity of aryl radicals, the final composition of the mixed organic layer is hard to predict and depends on the nature of the functional groups. Other approaches are based on twostep procedures. Basically, the electrografting of the first diazonium salt is followed by a second one using a different functionalized aryl diazonium salt.9 These procedures also have some fundamental limitations for preparing mixed layers. First, they require that the electrochemical activity of the surface is not too inhibited or altered after the first electroreduction as is most commonly observed.2 Additionally, the Received: June 15, 2011 Revised: July 19, 2011 Published: July 20, 2011 11222

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Langmuir Scheme 1. Aryldiazonium Salts Used in This Work

second grafting step should preserve the functional properties of the first deposited layer, which is problematic when considering reactive intermediates such as aryl radicals. However, this approach is interesting for submicrometric patterned surfaces and has been successfully demonstrated (see, for example, ref 9). After deposition of the first layer that is initially or subsequently patterned (for example, by AFM scratching technique or nanosphere lithography), the second organic layer is grafted onto the patterned modified surfaces.9 More generally for a better control of the properties and morphology of a layer at the molecular level, an efficient strategy is to build a preoptimized molecular platform that will then be used to chemically attach the functional group of interest. An alternative solution was recently explored by Daasbjerg et al. who successfully used a two-step “formation degradation” procedure to obtain thin thiophenolate or benzaldehyde films.10 Following this idea, we have recently proposed a global strategy to prepare robust active monolayers via the electroreduction of aryldiazonium salt derivatives bearing a silyl protection group (4-((triisopropylsilyl)ethynyl)benzenediazonium tetrafluoroborate or TIPS-Eth-ArN2+).11 After chemical deprotection, a dense and active phenylethynylene monolayer is obtained on the carbon surface that could be used to specifically immobilize functional groups through the well-known “click chemistry” (Cu(I)-catalyzed Huisgen 1,3-dipolar cycloaddition).12 This platform presents a layer of reactive functional groups with a density close to the theoretical (geometric) maximum and a welldefined morphology. Further electrochemical investigations have indicated the formation of nanometric pinholes in the prepared layer.11 In this Article, we want to take advantage of the structure of this platform to establish a different strategy for preparing covalently bonded binary films. Indeed, we expect that, thanks to the protection brought by the silyl groups, it should be possible to introduce a second functional group into the free nanometric pores of the constituted organic layer while maintaining the chemical properties of the first immobilized layer. We will evaluate this procedure with a binary film consisting of electroactive ferrocene moieties attached via “click chemistry” to the ethynylbenzene moieties, and nitrobenzene groups that are directly grafted into the pores of the first layer. These chemical groups are both electroactive and thus could also be used as molecular probes, allowing an easier characterization of the layer in this proof of concept study. Two different possible pathways involving different ordering of the modification steps will be considered to show that the influence and importance of the protection deprotection process depends on the modification pathway.

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’ EXPERIMENTAL SECTION Chemicals and Reagents. Commercially available reagents were used as received without further purification. 4-((Triisopropylsilyl)ethynyl) benzenediazonium tetrafluoroborate (TIPS-Eth-ArN2+, Scheme 1) was synthesized and purified according to already published procedures.11 4-Nitrobenzenediazonium salt (4-NO2 ArN2+, Scheme 1) was synthesized from the corresponding aniline using published procedures.13 Azidomethylferrocene was synthesized according to published procedures.14 Tetrabutylammonium fluoride (TBAF) and ferrocene were purchased from Alfa Aesar. Acetonitrile (99.8% anhydrous) and tetrabutylammonium hexafluorophosphate (NBu4PF6) were from Aldrich. Electrochemical Setup and Procedures. All electrochemical measurements were performed with an Autolab PGSTAT 12 (Metrohm) and a conventional three-electrode system, comprising the modified substrate as working electrode, a platinum foil as the auxiliary electrode, and a SCE electrode (Metrohm) as reference. The glassy carbon electrodes were purchased from CH Instruments, Inc. (TX) as 2-mm-diameter disks. The electrodes were polished successively with 1.0, 0.3, and 0.05 μm alumina slurry made from dry alumina powder and Milli-Q water on microcloth pads (CH Instruments, Inc., TX). The electrodes were thoroughly rinsed with Milli-Q water, acetone, and ethanol. Before derivatization, the electrodes were dried with an argon gas stream. The preparation of pyrolyzed photoresist film (PPF) followed methods described previously.15 The PPF substrates were squares of 15  15 mm2. The total surface coverage of active redox centers (Γ) was derived from the integration of the area under the voltammetric peaks at low scan rates giving the charge passed (Q), according to: Γ = Q/nFA. Here, n is the number of electrons exchanged, F is the Faraday constant, and A is the electroactive surface area of the glassy carbon electrode used in the study. Background current was estimated by extrapolation of the baseline capacitive current under the Faradaic peak current. Atomic Force Microscopy (AFM). All atomic force microscopy (AFM) experiments were carried out with a Pico plus (Molecular Imaging) in tapping mode. A classic NCH acoustic probe was employed for imaging. Experiments were performed to estimate the thickness of the organic layers by the McCreery AFM “scratch” procedure.16 The term “AFM scratching” is used here to describe an intentional damage to a modification layer on a relatively hard substrate. If the applied force is sufficient to disrupt the organic layer but not to damage the substrate, it is possible to “carve out” a rectangular trench in the deposit layer. On modified PPF surface, a 0.4  2 μm2 scratch was made by moving the tip in contact mode with a set-point voltage around 3 V. Note that careful examination confirmed that scratching of the underlying PPF substrate did not occur. Loose debris from the scratch often created AFM tip surface tracking problems, but these could be overcome by gentle air convection near the AFM tip to remove most of the debris. The images shown were recorded in tapping mode after “scratching”, and representative line profiles through each scratch are shown (see the Supporting Information). In all experiments, deposited layers appear as homogeneous with no segregation.

’ RESULTS AND DISCUSSION Electroreduction of 4-Nitrobenzene Diazonium Ion on H-Eth-GC Surface (after TIPS Deprotection). The basic idea

for preparing an active binary layer is the electrodeposition of two different aryldiazonium salts. However, the simultaneous reductions of the two diazoniums using a solution mixture lead to materials for which the composition and structure could be difficult to control due to the high and unselective reactivity of phenyl radicals. The alternative consists of the sequential deposition of the two aryldiazonium salts. As reported before, a carbon 11223

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Scheme 2. Principle of the Different Modification Steps Using H-Eth-GC Surface as Starting Material

Figure 1. Cyclic voltammetry of a 10 3 mol L 1 ferrocene solution in MeCN + 0.1 mol L 1 NBu4PF6 at different modification stages. (1) CV recorded on the initial bare GC electrode, (red 2) on the GC electrode after modification by TIPS-Eth-ArN2+ (TIPS-Eth-GC electrode), (pink 3) on the same modified TIPS-Eth-GC electrode after a 20 min soaking in THF, and (blue 4) on the same TIPS-Eth-GC electrode after deprotection (H-Eth-GC electrode). Scan rate 0.1 V s 1.

surface modified by a dense ethynylbenzene monolayer displays a low blocking character toward electrochemical processes despite a large density of active sites11 and thus appears as a good candidate for making a second electrodeposition. The first considered strategy is summarized in Scheme 2. The starting modified surface (TIPS-Eth-GC) was prepared by electroreduction upon potential cycling of a TIPS-Eth-ArN2+ solution in acetonitrile on a glassy carbon (GC) electrode. Details and procedures were described in ref 11. The blocking properties were investigated in the same solvent (acetonitrile, MeCN) as the one used for the surface modification. This classical technique relies on the variations of the cyclic voltammograms of the ferrocene oxidation (used as a redox probe) recorded after each treatment step. Just after electroreduction of TIPS-Eth-ArN2+ (see Figure 1, curve 2, red), the modified substrate presents the characteristics of a totally blocked electrode, and thus no further electrochemical modification could be envisaged in these experimental conditions. This protected surface TIPS-Eth-GC is then treated with tetrabutylammonium fluoride (TBAF) to remove the TIPS group, leading to a carbon surface modified by a covalently bonded ethynylbenzene monolayer (H-Eth-GC). Noticeably, after deprotection, the voltammogram of the ferrocene oxidation (Figure 1, curve 4, blue) became almost indiscernible from that obtained on the initial glassy carbon electrode (Figure 1, curve 1, black). This behavior agrees well with the behavior expected for a modified surface containing a large number of nanometric pinholes allowing the ferrocene to freely reach the carbon

Figure 2. Cyclic voltammetry of a 10 3 mol L 1 ferrocene solution in MeCN + 0.1 mol L 1 NBu4PF6. (1) On a GC surface modified by TIPSEth-ArN2+ and after deprotection (H-Eth-GC electrode). (blue 2) On the same H-Eth-GC surface after its modification with 4-NO2 ArN2+. Scan rate 0.1 V s 1.

substrate.17 XPS has previously been performed and confirmed that TIPS groups are efficiently removed following this approach.11 Electroreduction of 4-nitrobenzene diazonium ion (4-NO2 ArN2+) was then performed on the H-Eth-GC-modified surface to bind the second layer (see Scheme 2, sample 1) using a classical potential cycling procedure.18 As seen in Figure S6 in the Supporting Information, the reduction peak of 4-NO2 ArN2+ at 0.2 V with no associated oxidation peak is indicative of the loss of N2 and the formation of a 4-nitrophenyl radical followed by covalent binding onto the surface. Subsequent scans (curves 2 5, Figure S6) show a rapid decrease of the current that corresponds to a total inhibition of the electrode. Investigations of this surface using ferrocene as redox probe confirmed the totally inhibited character of the surface (see curve 2 in Figure 2).19 The redox activity of the 4-nitrophenyl moiety was considered for estimating the amount of active NO2 groups deposited on the surface.2,13 The classical procedure consists first of converting the NO2 into NHOH groups in a sulphuric acid solution and then analyzing their electrochemical response. Regarding the charge passed under the oxidation peak (Figure S7 in the Supporting Information), we derived an initial concentration of 3.10 9 mol cm 2 of active NO2 groups, which is typical of the value obtained when a naked carbon surface is modified with 4-nitrobenzene diazonium ion using the same deposition protocol. It is noted that an estimate of the quantity of active NO2 may provide a wrong analysis of the layer thickness as part of the NO2 groups could be silent. In the following experiments, the same global procedure was performed on a pyrolyzed photoresist film (PPF) surface. The PPF 11224

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surface has reactivity similar to that of glassy carbon but is sufficiently flat to allow estimation of organic layer thickness by AFM “scratch” procedure.16 The layer thickness, after TIPS deprotection and grafting of 4-nitrobenzene diazonium ion, was found to be 4.64 ( 0.11 nm, which is similar to the data obtained with a naked PPF surface modified with 4-nitrobenzene diazonium ion under the same experimental conditions (4.65 ( 0.30 nm, see the Supporting Information, Figure S1). Such large thickness clearly corresponds to the formation of a multilayer.2,11 All measurements of the layers thicknesses are gathered in Table 1. Finally, to check the activity and accessibility of ethynylbenzene moieties, we performed an azide alkyne “click” reaction using azidomethylferrocene as a test molecule. All attempts to attach ferrocene moieties were unsuccessful, showing that ethynylbenzene moieties were not available for reaction. This suggests that ethynyl groups were smothered by the nitrophenyl film making them inaccessible and/or that the unsaturated triple bonds had reacted with the electrogenerated nitrophenyl radicals. From this first part, we could conclude that it is possible to bind a second layer using aryldiazonium reduction on H-Eth-GC, but unfortunately the nitrophenyl radicals attack both the aryl group of the grafted layer and the ethynyl groups, which are no longer reactive. Electroreduction of 4-Nitrobenzene Diazonium Ion on TIPS-Eth-GC Surface (Protected Surface). To preserve the reactivity of ethynylbenzene moieties in the final mixed layer, the TIPS protecting group should be present when the second electroreduction is performed. Unfortunately, we observed that the protected TIPS-Eth-GC surfaces display a strong blocking character when examined in the same solvent as used for the electrografting. This is an expected phenomenon because the electrografting of aryldiazonium ion is a self-inhibited electrochemical process that stops when the current cannot pass through the deposited layer.2 However, it is reported that the electrochemical response of redox probes on similar modified electrodes depends on the nature of the probe, but also on the Table 1. Layer Thicknesses of the Deposited Organic Layers on PPF Substrate Shown in AFM Experiments substrate

layer thickness

sample NO2 TIPS-Eth-GC

4.65 ( 0.30 nm 2.06 ( 0.50 nma 0.92 ( 0.17 nmb

a

H-Eth-GC

0.65 ( 0.18 nm a

sample 1

4.64 ( 0.11 nm

sample 2

3.84 ( 0.23 nm

sample 3

2.79 ( 0.16 nm

From ref 11. b After cleaning with THF.

solvent in which the electrode is examined.20 These changes of the blocking properties were ascribed to solvent or ion-induced changes in film permeability that could also lead to the release of loosely attached oligophenylene layers. As a consequence, the response of the ferrocene redox probe in acetonitrile solution at the protected TIPS-Eth-GC electrode was investigated after immersion of the modified TIPS-Eth-GC electrode for 20 min in THF that we found to be a “good solvent” for cleaning this type of layer. After THF treatment, we could clearly observe the ferrocene oxidation (Figure 1, curve 3) that was previously totally inhibited (Figure 1, curve 2). In comparison with the bare glassy carbon electrode (Figure 1, curve 1), the voltammogram obtained after modification and immersion in THF (curve 3) shows a larger peak-to-peak potential separation and a global decrease of the current associated with a higher “S-shape” character, suggesting that the redox probe passes through numerous overlapping diffusion channels.17,21 Such transformation of the electrochemical behavior was found to be irreversible, meaning that the response of the electrode did not change even after long immersion in ACN. AFM scratching measurements made before and after THF treatment show a decrease in the layer thickness from around 2.06 ( 0.50 nm (see ref 11) to 0.92 ( 0.17 nm. The formation of a monolayer shows that the electrogenerated phenyl radicals do not react on the attached layer due to the protective effect of the TIPS. This value is very close to the thickness expected for a monolayer of TIPS-Eth-GC, suggesting that part of the inhibition of the surface observed during the electrografting is due to physisorbed oligomers, which are removed during immersion in THF. Consequently, electroreduction of 4-NO2 ArN2+ ion was performed on the TIPS-Eth-GC electrode after immersion in THF but prior to its deprotection, following the procedure summarized in Scheme 3. The presence of the TIPS prevents the attachment 4-NO2 Ar on the first deposited layer and should direct the second grafting preferentially onto the carbon surface through the pinholes. As seen in Figure 3, it is noticeable that reduction of 4-NO2 ArN2+ occurs at the same potential when experiments are performed on the protected or on the unprotected electrodes, but with lower intensity in the first case (see Figure 3b for a comparison between deposition currents on the different electrodes). Subsequent scans show no electrochemical response, characteristic of the total inhibition of the surface. Analysis of this modified surface (marked as sample 2 in Scheme 3) using ferrocene as the redox probe confirms the blocking character of the binary film even after soaking the film in THF (not shown). The thickness of the binary layer (sample 2) was estimated by AFM scratching as 3.84 ( 0.23 nm. This value is significantly larger than the initial film thickness of TIPS-Eth-GC and corresponds to a growth of around 2.9 nm. However, this increase is

Scheme 3. Principle of the Different Modification Steps Using the TIPS-Eth-GC Surface as the Starting Layer

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Figure 3. (a) Electroreduction of 4-NO2 ArN2+ (conc. = 10 2 mol L 1 in MeCN + 0.1 mol L 1 NBu4PF6) on a TIPS-Eth-GC surface. (1) First cycle, (red 2) second, (green 3) third, (blue 4) fourth, and (pink 5) fifth cycle. (b) Comparison of 4-NO2 ArN2+ electroreduction behaviors on (1) H-Eth-GC and (red 2) TIPS-Eth-GC surfaces.

Figure 4. (Right) 1  1 μm2 topographic picture of TIPS-Eth-GC modified with 4-nitrobenzenediazonium salt and after deprotection with TBAF. rms roughness = 0.41 nm. (Left) For comparison, the original PPF substrate before its modification. rms roughness = 0.34 nm.

Figure 5. Redox analyses of the binary layer (second grafting made under self-inhibition conditions). (a) Electroactivity of NHOH groups in 0.1 mol L 1 sulfuric acid aqueous solution of surfaces marked as samples 2 and 3 in Scheme 3. (red ) Before and ( ) after deprotection treatment with TBAF. (b) Cyclic voltammogram in ethanol +0.1 mol L 1 LiClO4 solution after postfunctionalization of sample 4 with azidomethylferrocene using click chemistry coupling method. Scan rate 0.1 V s 1.

significantly smaller than that obtained with sample 1 (electroreduction of NO2 ArN2+ on the unprotected H-EthGC electrode) or on a naked carbon surface. Similarly, the surface concentration of active nitro groups was calculated as 8.8  10 10 mol cm 2, which is less than one-third the concentration obtained when the 4-nitrophenyl deposition was made on the HEth-GC surface. All of these results agree with the formation of a much thinner 4-nitrophenyl layer when using the TIPS protected layer. TBAF treatment was then performed on this last binary layer leading to the layer marked as sample 3 in Scheme 3. AFM imaging shows that the modification appears as a regular layer with no segregation between the two layers (see Figure 4 or Figures S3 S5 in the Supporting Information). The observed roughness is similar to that of the blank PPF substrate. Figure 5a shows the electrochemical analysis of nitro-groups activity before and after the deprotection. We found that around

one-half of the electroactive nitro groups were lost during the deprotection step. Simultaneously, the film thickness was found to be 2.79 ( 0.16 nm, which corresponds to a film thickness decrease of around 1 nm upon deprotection, in agreement with the film structure marked as sample 3 in Scheme 3.22 The azide alkyne “click” reaction was tested using azidomethylferrocene on this binary layer. In contrast to our first attempts starting from the H-Eth-GC surface, ferrocene moieties were successfully attached onto the ethynylbenzene moieties leading to surface sample 4 in Scheme 3. Figure 5b shows the cyclic voltammogram of the electrode modified with ferrocene units (Ssample 4) examined in blank ethanol solution (+ 0.1 mol L 1 LiCLO4 as supporting electrolyte). A surface concentration Γ = 10 10 mol cm 2 of bound ferrocene groups was derived from these electrochemical measurements. This value is smaller, by a factor of 3 4, than those obtained for a compact monolayer of 11226

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Figure 6. Redox analysis of the binary layer (second grafting under milder conditions). (a) Electroactivity of NHOH groups in sulfuric acid aqueous solution of surfaces (red ) before and ( ) after deprotection treatment with TBAF. (b) Cyclic voltammogram in ethanol (second cycle) + 0.1 mol L 1 LiClO4 solution of sample 4 electrode after postfunctionalization with azidomethylferrocene using click chemistry coupling method. Scan rate 0.1 V s 1.

ferrocene units on carbon surfaces.11 This indicates that only a fraction of the ethynyl groups were available for click chemistry, suggesting that the quality of the film could be improved by using grafting conditions for nitrobenzene that will result in a thinner film. For this purpose, we repeated the process of Scheme 3, but electrografting of the 4-NO2 ArN2+ was performed using only one potential cycle, less negative potentials (between 0.6 and 0 V/SCE), and a 10-time lower concentration of diazonium salt (10 3 mol L 1 instead of 10 2 mol L 1). Analysis of the redox activity (see Figure 6a) shows that surface concentration of active nitro groups (Γ = 4  10 10 mol cm 2) after deprotection is similar to that obtained using more negative potentials and a greater number of potential cycles. This finding confirms that mainly NO2 groups located on the outside layer are redox active.13 However, the quantity of active ethynyl groups detective after labeling with azidoferrocene (Γ = 2.9  10 10 mol cm 2) was now very close to the value obtained after labeling a single monolayer of H-Eth-GC. (See Figure 6b.)

’ CONCLUSION The presented method appears to be an easy and efficient approach to build covalently bonded binary functional layers onto carbon surfaces using electroreduction of aryldiazonium salts and a protection deprotection procedure. Experimental results highlight the importance of retaining the chemical protection of the first modifier during the second grafting step to avoid degradation of the functionality of the first layer by the second layer. Pinholes in the protected surface become electrochemically accessible when the surface is soaked in a second solvent such as THF, allowing the electroreduction of the second aryldiazonium salt. The presence of the protection group also directs the second grafting in the pinholes of the first layer, allowing the design of totally mixed and structured surfaces. Following this global strategy, a mixed layer containing active ethynylbenzene (that was further functionalized with ferrocene moieties) and 4-nitrobenzene moieties was prepared on carbon surfaces as a test example. Using mild conditions for the deposition of the second layer maintains the concentration of active ethynyl groups similar to that obtained for a one-component monolayer. Considering the wide range of functional aryldiazonium salts that could be electrodeposited and the versatility and specificity of the “click” chemistry, this approach appears very promising for the preparation of mixed layers on carbon surfaces in well-controlled conditions without altering the reactivity of either functional group.

’ ASSOCIATED CONTENT

bS

Supporting Information. Scratching AFM experiments details and cyclic voltammetric analysis on the different types of layers. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported in part by the Agence Nationale de la Recherche (ANR-08-BLAN-0161, HI-LIGHT project). ’ REFERENCES (1) (a) See, for example, refs 1b,1c and references therein. (b) Devadoss, A.; Chidsey, C. E. D. J. Am. Chem. Soc. 2007, 129, 5370–5371. (c) Cremer, P. S. J. Am. Chem. Soc. 2011, 133, 167–169. (2) (a) Delamar, M.; Hitmi, R.; Pinson, J.; Saveant, J.-M. J. Am. Chem. Soc. 1992, 114, 5883–5884. (b) Pinson, J.; Podvorica, F. Chem. Soc. Rev. 2005, 34, 429–439. (3) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103–1170. (4) (a) Collman, J. P.; Devaraj, N. K.; Eberspacher, T. P. A.; Chidsey, C. E. D. Langmuir 2006, 22, 2457–2464. (b) Banet, P.; Marcotte, N.; Lerner, D. A.; Brunel, D. Langmuir 2008, 24, 9030–9037. (5) (a) Lee, L. Y. S.; Sutherland, T. C.; Rucareanu, S.; Lennox, R. B. Langmuir 2006, 22, 4438–4444. (b) Azam, M. D.; Fenwick, S. L.; GibbsDavis, J. M. Langmuir 2011, 27, 741–750 and references therein. (6) (a) Liu, G.; Liu, J.; B€ocking, T.; Eggers, P. K.; Gooding, J. J. Chem. Phys. 2005, 319, 136–146. (b) Louault, C.; d’Amours, M.; Belanger, D. ChemPhysChem 2008, 9, 1164–1170. (c) Liu, G.; Paddon-Row, M. N.; Gooding, J. J. Electrochem. Commun. 2007, 9, 2218–2223. (7) Vase, K. H.; Holm, A. H.; Norrman, K.; Pedersen, S. U.; Daasbjerg, K. Langmuir 2007, 23, 3786–3793. (8) (a) Electroreduction of diazonium salts has also been proposed to obtain stable monolayers on gold electrode especially for sensing applications.8b (b) Liu, G.; Gooding, J. J. J. Electroanal. Chem. 2007, 600, 335–344. (9) (a) Brooksby, P. A.; Downard, A. J. Langmuir 2005, 21, 1672–1675. (b) Corgier, B. P.; Belanger, D. Langmuir 2010, 26, 5991–5997. (10) (a) Nielsen, L. T.; Vase, K. H.; Dong, M.; Besenbacher, F.; Pedersen, S. U.; Daasbjerg, K. J. Am. Chem. Soc. 2007, 129, 1888–1889. (b) Malmos, K.; Dong, M.; Pillai, S.; Kingshott, P.; Besenbacher, F.; Pedersen, S. U.; Daasbjerg, K. J. Am. Chem. Soc. 2009, 131, 4928–4936. 11227

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(11) Leroux, Y. R.; Fei, H.; No€el, J.-M.; Roux, C.; Hapiot, P. J. Am. Chem. Soc. 2010, 132, 14039–14041. (12) (a) Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.; Sharpless, K. B. Angew. Chem., Int. Ed. 2002, 41, 2596–2599. (b) Tornoe, C. W.; Christensen, C.; Meldal, M. J. Org. Chem. 2002, 67, 3057–3064. (13) Ceccato, M.; Nielsen, L. T.; Iruthayaraj, J.; Hinge, M.; Pedersen, S. U.; Daasbjerg, K. Langmuir 2010, 26, 10812–10821. (14) Casa-Solvas, J. M.; Vargas-Berenguel, A.; Capitan-Vallvey, L. F.; Santo-Gonzalez, S. Org. Lett. 2004, 6, 3687–3690. (15) Brooksby, P. A.; Downard, A. J. Langmuir 2004, 20, 5038–5045. (16) Anariba, F.; DuVall, S. H.; McCreery, R. L. Anal. Chem. 2003, 75, 3837–3844. (17) (a) Amatore, C.; Saveant, J.-M.; Tessier, D. J. Electroanal. Chem. 1983, 147, 39–51. (b) Streeter, I.; Compton, R. G. Sens. Actuators, B 2008, 130, 620–624 and references therein. (18) Cycling between +0.6 and 0.75 V vs SCE at 50 mV s 1 during five cycles and using a 10 2 mol L 1 concentration of the diazonium salt. (19) For comparison purpose, CVs have been recorded after soaking the modified electrodes in THF for 20 min (see text). (20) (a) Cruickshank, A. C.; Tan, E. S. Q.; Brooksby, P. A.; Downard, A. J. Electrochem. Commun. 2007, 9, 1456–1462. (b) Brooksby, P. A.; Downard, A. J. J. Phys. Chem. B 2005, 109, 8791–8798. (c) No€el, J.-M.; Zigah, D.; Simonet, J.; Hapiot, P. Langmuir 2010, 26, 7638–7643. (21) It is noticeable that the current decrease after the peak is less pronounced. This phenomena designated as a higher “S-shape” character is indicative of more pronounced “steady state” behavior and in our case of spherical diffusion due to pinholes. (See ref 18.) (22) Previously, the deprotection step on TIPS-Eth-GC was found to decrease the deposited organic layer from 2.06 ( 0.50 to 0.65 ( 0.18 nm, leading to an ethynylbenzene (H-Eth-GC) monolayer.11 This decrease may indicate the formation of a physisorbed layer that was deposited during the second grafting but also that part of the second grafting layer was attached on the tri-isopropylsilane group.

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