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Original Dual Microelectrode: Writing and Reading a local click reaction with Scanning Electrochemical Microscopy
Electrochimica Acta 201 (2016) 274–278
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Original Dual Microelectrode: Writing and Reading a local click reaction with Scanning Electrochemical Microscopy Vasilica Badetsa , Djoan Duclosa , Damien Quintona , Olivier Fontaineb , Dodzi Zigaha,* a b
Univ. Bordeaux, ISM, CNRS UMR 5255, F-33400 Talence, France Université Montpellier 2 UMR 5253, CC1701, Place Eugène Bataillon, 34095 Montpellier, France
A R T I C L E I N F O
A B S T R A C T
Article history: Received 27 June 2015 Received in revised form 8 October 2015 Accepted 13 October 2015 Available online 26 October 2015
A dual gold-copper microelectrode (diameter 25 mm) was fabricated to be employed in a scanning electrochemical microscopy (SECM) configuration, and was used in feedback mode to both modify (write) and analyze (read) a substrate. This write-and-read procedure was performed on a glassy carbon electrode on which azide groups were introduced by electrochemical reduction of a diazonium salt. The copper part of the microelectrode was used to electrogenerate Cu(I) that catalyzed a “click” reaction between azide moieties on the surface and dissolved ethynylferrocene. This created a pattern of ferrocene moieties on the surface that was analyzed with the gold part of the microelectrode. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Click Chemistry surface patterning SECM diazonium reduction dual microelectrode
1. Introduction “Simultaneously writing and reading electrochemical information using the same microelectrode is an interesting concept in Scanning ElectroChemical Microscopy (SECM). Even if standard lithography method allow to obtain patterns with a high spatial resolution their complexity and high cost led to the proposition of alternative procedures, for example nanofabrication by molding or by printing [1,2]. SECM is a promising technique in this context. In SECM, a microelectrode is moved in 3 dimensions above a substrate, and a controlled electrochemical stimuli triggers the modification of surfaces [3]. For example, a microelectrode placed close to the substrate will be subjected to a potential step to start an electrochemical reaction [4]. The product of this reaction will diffuse toward the substrate to create local patterns with a resolution determined by the size of the diffusion layer [5,6]. Moreover, SECM can be used to visualize the newly created pattern. In the present work, the electrogeneration of Cu(I) was used to locally trigger a click reaction and to pattern ethynylferrocene molecules on a glassy carbon surface covered by azide moieties. The click reaction is a Cu(I) catalyzed Huisgen 1,3-dipolar cycloaddition of alkynes and azide to form 1,2,3-triazole [7]. To simplify existing procedures, we fabricated a dual gold-copper microelectrode. This configuration obliterate the polishing step
* Corresponding author. E-mail address: [email protected] (D. Zigah). http://dx.doi.org/10.1016/j.electacta.2015.10.066 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
between the modification and imaging steps and the requirement of copper in the starting solution. Click chemistry combined with SECM was demonstrated for the first time by Bard et al. [8] for the immobilization of a fluorescent molecule on a glass substrate. For this, a gold microelectrode was used to locally generate Cu(I) species by reduction of an initial solution containing Cu(II). Cu(I) catalyzed the reaction between the fluorescent molecules and the azido-terminated self-assembled monolayer. The same principle was used by Hapiot et al. [9], but in this case, a Pt microelectrode was used to pattern gold macroelectrodes previously modified by azido-terminated self-assembled monolayers. Bedioui et al. [10] used a gold microelectrode and click chemistry to covalently immobilize ferrocene on a glassy carbon surface entirely modified by azido-aryl groups via the diazonium approach. The ferrocene was observed by imaging the surface with SECM using the same gold microelectrode and ferricyanide as redox mediator. One disadvantage of this procedure is the electrodeposition of Cu on the Au microelectrode which demands an additional cleaning step, like anodic striping or mechanical polishing, between the surface writing step and the reading step [10]. Mechanical polishing is not straightforward, firstly the gold electrode is removed, then it is mechanically polished and finally it is repositioned. This procedure is tedious because it requires a perfect repositioning in order to find the spot. In our work we propose a solution to address these limitations. We decided to work without any Cu(II) initially present in the solution and use the oxidation of a copper microelectrode to locally generate Cu(I). But because it is not possible to achieve approach
V. Badets et al. / Electrochimica Acta 201 (2016) 274–278
curves and surfaces imaging with a copper microelectrode, we fabricated a dual microelectrode combining gold and copper wires. The gold part was used to fix the distance between the microelectrode and the substrate and also to obtain an SECM image of the modified surface in feedback mode. Other dual electrodes have been used to study the generation of species on one electrode and collection on the other one, or to detect different species at the two different electrodes [11–13]. To the best of our knowledge it is the first time that a dual Au-Cu microelectrode is used to modify and analyze a substrate. 2. Experimental 2.1. Chemicals All chemicals (4-azidoaniline hydrochloride, sodium nitrite, ethynylferrocene, ascorbic acid, absolute ethanol, hydrochloric acid, potassium ferricyanide, copper sulfate) were purchased form Sigma-Aldrich and used as received. All the solutions were prepared in ultrapure water (resistivity 18.2 MV cm at 25 C).
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diazonium salt was prepared in-situ in the solution by diazotation of the parent aniline [15]. The initial solution contained 1 mM of 4azidoaniline in 0.5 M HCl and sodium nitrite was added to a final concentration of 2 mM. The electroreduction of the 4-phenylazide diazonium was achieved by cycling 6 times the working electrode between 0 and -0.9 V vs Ag/AgCl at 50 mV s1. The characteristic peak corresponding to the diazonium reduction was observed on the first cycle and disappeared on the second cycle. For the click chemistry on the entire carbon surface, a solution of 5 mM ascorbic acid and 10 mM CuSO4 was prepared. Ethynylferrocene, initially prepared as 0.1 mg ml1 in ethanol, was added to a final concentration of 10 mM. The glassy carbon electrode containing the azide group on the surface was immersed in this solution for 60 min [16]. For the local “electroclick” reaction the same ethynylferrocene solution was used but without CuSO4 and ascorbic acid. The microelectrode was positioned close to the substrate (10 mm) above the azido-modified substrate. A potential of 0.5 V was applied to the copper microelectrode to generate Cu(I) ions that locally triggered the click reaction between the ethynylferrocene and the azide moieties.
2.2. Fabrication of dual electrode 2.4. Cycling Voltammetry and SECM experiment A borosilicate theta glass capillary (World Precision Instrument, 1.5 mm O.D. 1.05 mm I.D) was first cleaned in ethanol and then dried in oven at 80 C for 15 min. Copper and gold wires (diameter 25 mm from Goodfellow) were introduced in two different compartments and placed in the middle of the capillary. Then a laser puller (Sutter instruments P2000) was used to seal one part of wires in the capillary by melting the glass. The capillary was then manually broken and the sealed end was polished firstly with a sand paper p4000 and then with 0.3 mm alumina paste to obtain a clean surface (Fig. 1). To avoid misalignment between the two wires the dual microelectrode should be held perfectly at 90 during the polishing step. The electrical connection to the unsealed end of the wires was made with carbon powder to a copper wire (diameter 500 mm). 2.3. Preparation of modified surfaces Substrates made of glassy carbon electrode of 3 mm in diameter were obtained from IJ Cambria Scientific Ltd. The glassy carbon electrodes were polished successively with sand paper and with 0.3 mm alumina paste. The electrodes were thoroughly washed with ultrapure water and put in ultrasonic bath for 5 min to remove the alumina that remained on their surface. Azide functional groups were introduced on glassy carbon electrodes by reduction of 4-phenylazide diazonium salt [14]. The
Fig. 1. a) Optical and b) SEM images of the dual gold-copper microelectrode.
All the electrochemical measurements were performed using a CH instrument (Austin, TX) Model 920C. An Ag/AgCl (KCl 3 M) reference electrode and platinum wire were used in a threeelectrode system. All steady state SECM approach curves were recorded at 1 mm s1 approach rate. The approach curves and the SECM imaging were recorded by applying -0.1 V on the gold microelectrode to generate ferrocyanide under diffusion limited condition. In all the experiments, a potential of 0.5 V was applied on the substrate. 3. Results and Discussion Cyclic voltammetry measurements were first recorded on the gold and copper microelectrode to verify that the dual Au-Cu microelectrode works properly before performing the surface modification and the SECM experiments. Afterwards, the gold microelectrode was characterized by realizing approach curves on insulating and conductive substrates in a ferricyanide solution (Fig. 2). These curves are usually represented in dimensionless form by plotting the microelectrode current (IT) normalized by the current when the microelectrode is far away of the substrate (Iinf) as a function of the distance (d) normalized by the microelectrode radius (a). These results showed that the dual electrode functions as well as a normal microelectrode despite the fact that the gold part is not centered and that the diameter of the entire tool is quite large (300 mm). Therefore, it was important to check whether
Fig. 2. SECM approach curves in K3Fe(CN)6 2 mmol L1 as mediator with NaCl 0.1 mol L1 in aqueous solution. Edual-Au = -0.1 V, Esubstrate = 0.5 V. ( ) On glassy carbon (GC) electrodes, positive feedback. (—) On glass, negative feedback.
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V. Badets et al. / Electrochimica Acta 201 (2016) 274–278
Fig. 3. SECM approach curves in K3Fe(CN)6 2 mmol L1as mediator with NaCl 0.1 mol L1 in aqueous solution. Edual-Au = -0.1 V, Esubstrate = 0.5 V. (—) On azide moieties, negative feedback. ( ) On ferricinium, positive feedback.
classic approach curves could be achieved using this dual microelectrode [17]. As a preliminary experiment, the entire surface of the carbon electrode was modified with ferrocene via classical click chemistry as described in section 2.3. Firstly, the anchoring layer was immobilized by the electroreduction of the insitu generated azidophenyl diazonium leading to the formation of the corresponding phenyl radicals that reacted with the substrate and the already grafted phenyl moieties giving a multilayer deposit. As seen in Fig. 3, when an approach curve is performed on this modified substrate the dimensionless current decreases with the distance between the microelectrode and the substrate. This
corresponds to a negative feedback. This behavior is observed on the phenylazide layer even if the potential applied on the substrate is supposed to be high enough to oxidize the reduced species electrogenerated on the microelectrode. This clearly shows that the anchoring layer have a blocking effect on the mediator. After the click reaction described in section 2.3, ferrocene is immobilized on the surface and a potential of 0.5 V is applied on the substrate. At this potential the ferrocene is oxidized to ferricinium ion which can be reduced by the ferrocyanide mediator generated on the microelectrode. Therefore the current increases when the distance between the microelectrode and the substrate decreases. This situation corresponds to a positive feedback (Fig. 3). The ferricinium ions are reduced back to ferrocene by the ferrocyanide ions. Due to the applied potential (0.5 V) ferrocene is constantly oxidized to ferricinium and hence the steady-state and the feedback loop is established. Observing these two different feedback behaviors is a preliminary but essential step to ensure that it will be possible to distinguish the patterned area from the non-modified one by SECM. Indeed, for imaging the surface the microelectrode is moved above the azide and ferrocene layers in XY direction. The method used to pattern the surface is described on Scheme 1. Firstly, the entire glassy carbon surface is modified by the azide moieties by the electroreduction of the diazonium salts. In a solution containing ferricyanide, the dual Au-Cu microelectrode is approached to the surface using the negative feedback obtained at the Au part of the microelectrode on the azide layer
Scheme 1. Procedure for the localized electroclick reaction with the dual gold-copper microelectrode. a) Approach curve above the azide function, the negative feedback obtained is used to place the microelectrode. b) Constant potential applied on the copper electrode to generate Cu(I) that triggers the click reaction and immobilizes the ferrocene. c) Imaging of the immobilized ferrocene using the gold microelectrode.
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microelectrode making it impossible to observe the spot formation under these conditions. Therefore imaging the surface with the same solution of ethynylferrocene used for the click chemistry is not recommended. In order to have a positive feedback required for a good quality image the potential difference between the mediator and the immobilized species should be high enough. Therefore, the ferricyanide mediator with a formal potential of 0.192 V compared with the formal potential of immobilized ferrocene (0.316 V) was selected. In these conditions, the surface was patterned with a ferrocene spot via localized click chemistry in an ethynylferrocene solution and was successfully imaged by the dual gold-copper microelectrode using a ferricyanide solution. Fig. 4. SECM imaging of the immobilized ferricinium. Esubstrate = 0.5 V. Scan rate 5 mm s1. Edual-Au = -0.1 V in ferricyanide solution at 2 mmol L1 in NaCl mol L1 in aqueous solution.
(Scheme 1a). In a solution containing ethynylferrocene a constant potential is applied to oxidize the copper microelectrode (Scheme 1b). The amount of copper oxidized during this step can be determined using Faraday’s law (Eq. (1)). The main reaction is: Cu ! Cu2þ þ 2e mCu ¼
MðCuÞ Q nF
ð1Þ
Where mCu is the mass of copper oxidized, M(Cu) the molecular weight of copper, Q the electric charge, n the number of electrons and F the Faraday constant. During the 10 min (when the potential was applied) a charge of 2.6 mC corresponding to a mass of copper of 0.86 ng was recorded. Thus, the rate of consumption of copper is 0.086 ng min1. The disc section is 4.9 106 cm2 and the length of the wire that can be oxidized is around 0.5 cm. By consequence the volume of the copper wire is 2.5 106 cm3. The density of copper is 8.96 g cm3. The initial mass of copper is 22.4 mg, thus only 0.004% of metallic copper have been consumed. The part of copper oxidized in the experiment is negligible, this suggests that the dual electrode could be used for a large number of patterning experiments without causing extensive depletion of the copper source. During this procedure a small amount of Cu(I) is generated. In spite of its poor stability in aqueous solution Cu(I) catalyzes the click reaction locally below the copper microelectrode [18]. The small distance between the microelectrode and the substrate (10 mm) facilitates this reaction by minimizing the diffusion of the Cu(I) away from the copper electrode. Finally, the solution used for the click reaction was replaced by the solution of ferricyanide required for the SECM imaging (Scheme 1c). A potential of 0.5 V was applied on the substrate to oxidize the immobilized ferrocene to ferricinium ion which can be reduced by the mediator generated below the gold microelectrode during the scanning electrochemical imaging (Fig. 4). To ensure that the spot observed after scanning the surface was really the immobilized ferrocene, and not a copper deposit the surface had been examined with a stereo microscope which did not reveal any metal deposit. In another control experiment, the ethynylferrocene solution was replaced with ferrocene and the local click chemistry was performed in the same conditions. No spot was observed during the SECM imaging. This blank experiment prove that without the ethynylferrocene there is no spot on the surface. A convenient protocol would have been to mix the ferricyanide mediator with the ethynylferrocene in order to perform patterning and imaging in the same solution. This is not possible due to some side reactions leading to the formation of Prussian blue [19], if ferricyanide is exposed to Cu2+ [20]. Even if Cu2+ is not present initially in the solution this ion is generated below the copper
4. Conclusions A home-made dual gold-copper microelectrode was fabricated and then used to “write” and “read” a substrate by scanning electrochemical microscopy (SECM) technique. This new tool was first characterized with simple approach curves using the gold microelectrode above insulating and conductive surfaces. Using the gold part the dual Au-Cu was approached close to the azidoterminated aryl layer obtained by the reduction of azidophenyl diazonium on a glassy carbon electrode. The copper part of the microelectrode was used to localize an “electroclick” reaction catalyzed by locally generated Cu(I). This allowed the immobilization of ferrocene which was observed by imaging the surface with SECM in feedback mode. This versatile method can be used to deposit locally many different molecules. Acknowledgements The authors thank the PEPS NANO-GLOS and IDEX Bordeaux for the financial support. References [1] B.D. Gates, Q. Xu, M. Stewart, D. Ryan, C.G. Willson, G.M. Whitesides, New Approaches to Nanofabrication: Molding, Printing, and Other Techniques, Chem. Rev. 105 (2005) 1171–1196, doi:http://dx.doi.org/10.1021/cr030076o. [2] H.M. Saavedra, T.J. Mullen, P. Zhang, D.C. Dewey, S.A. Claridge, P.S. Weiss, Hybrid strategies in nanolithography, Rep. Prog. Phys. 73 (2010) 036501, doi: http://dx.doi.org/10.1088/0034-4885/73/3/036501. [3] M. Sheffer, D. Mandler, Scanning Electrochemical Imprinting Microscopy: A Tool for Surface Patterning, J. Electrochem. Soc. 155 (2008) D203–D208, doi: http://dx.doi.org/10.1149/1.2830543. [4] S. Amemiya, A.J. Bard, F.-R.F. Fan, M.V. Mirkin, P.R. Unwin, Scanning Electrochemical Microscopy, Annu. Rev. Anal. Chem. 1 (2008) 95–131, doi: http://dx.doi.org/10.1146/annurev.anchem.1.031207.112938. [5] I. Turyan, U.O. Krasovec, B. Orel, T. Saraidorov, R. Reisfeld, D. Mandler, Writing– Reading–Erasing on Tungsten Oxide Films Using the Scanning Electrochemical Microscope, Adv. Mater. 12 (2000) 330–333, doi:http://dx.doi.org/10.1002/ (SICI)1521-4095(200003)12:5<330::AID-ADMA330>3.0.CO;2-8. [6] S. Meltzer, D. Mandler, Microwriting of Gold Patterns with the Scanning Electrochemical Microscope, J. Electrochem. Soc. 142 (1995) L82–L84, doi: http://dx.doi.org/10.1149/1.2044252. [7] C.W. Tornøe, C. Christensen, M. Meldal, Peptidotriazoles on Solid Phase: [1,2,3]-Triazoles by Regiospecific Copper(I)-Catalyzed 1,3-Dipolar Cycloadditions of Terminal Alkynes to Azides, J. Org. Chem. 67 (2002) 3057–3064, doi:http://dx.doi.org/10.1021/jo011148j. [8] S.-Y. Ku, K.-T. Wong, A.J. Bard, Surface Patterning with Fluorescent Molecules Using Click Chemistry Directed by Scanning Electrochemical Microscopy, J. Am. Chem. Soc. 130 (2008) 2392–2393, doi:http://dx.doi.org/10.1021/ ja078183d. [9] S. Lhenry, Y.R. Leroux, C. Orain, F. Conan, N. Cosquer, N.L. Poul, et al., Locally Induced and Self-Induced Electroclick onto a Self-Assembled Monolayer: Writing and Reading with SECM under Unbiased Conditions, Langmuir 30 (2014) 4501–4508, doi:http://dx.doi.org/10.1021/la405005f. [10] D. Quinton, A. Maringa, S. Griveau, T. Nyokong, F. Bedioui, Surface patterning using scanning electrochemical microscopy to locally trigger a click chemistry reaction, Electrochem. Commun. 31 (2013) 112–115, doi:http://dx.doi.org/ 10.1016/j.elecom.2013.03.021. [11] K. McKelvey, B.P. Nadappuram, P. Actis, Y. Takahashi, Y.E. Korchev, T. Matsue, et al., Fabrication, Characterization, and Functionalization of Dual Carbon
278
[12]
[13]
[14]
[15]
V. Badets et al. / Electrochimica Acta 201 (2016) 274–278 Electrodes as Probes for Scanning Electrochemical Microscopy (SECM), Anal. Chem. 85 (2013) 9–7526, doi:http://dx.doi.org/10.1021/ac401476z. A. Kueng, C. Kranz, B. Mizaikoff, Imaging of ATP membrane transport with dual micro-disk electrodes and scanning electrochemical microscopy, Biosens. Bioelectron. 21 (2005) 346–353, doi:http://dx.doi.org/10.1016/j. bios.2004.10.020. S. Isik, M. Etienne, J. Oni, A. Blöchl, S. Reiter, W. Schuhmann, Dual Microelectrodes for Distance Control and Detection of Nitric Oxide from Endothelial Cells by Means of Scanning Electrochemical Microscope, Anal. Chem. 76 (2004) 6389–6394, doi:http://dx.doi.org/10.1021/ac049182w. P. Allongue, M. Delamar, B. Desbat, O. Fagebaume, R. Hitmi, J. Pinson, et al., Covalent Modification of Carbon Surfaces by Aryl Radicals Generated from the Electrochemical Reduction of Diazonium Salts, J. Am. Chem. Soc. 119 (1997) 201–207, doi:http://dx.doi.org/10.1021/ja963354s. S. Baranton, D. Bélanger, Electrochemical Derivatization of Carbon Surface by Reduction of in Situ Generated Diazonium Cations, J. Phys. Chem. B. 109 (2005) 24401–24410, doi:http://dx.doi.org/10.1021/jp054513+.
[16] D. Evrard, F. Lambert, C. Policar, V. Balland, B. Limoges, Electrochemical Functionalization of Carbon Surfaces by Aromatic Azide or Alkyne Molecules: A Versatile Platform for Click Chemistry, Chem. -Eur. J. 14 (2008) 9286–9291, doi:http://dx.doi.org/10.1002/chem.200801168. [17] R. Cornut, A. Bhasin, S. Lhenry, M. Etienne, C. Lefrou, Accurate and Simplified Consideration of the Probe Geometrical Defaults in Scanning Electrochemical Microscopy: Theoretical and Experimental Investigations, Anal. Chem. 83 (2011) 9669–9675, doi:http://dx.doi.org/10.1021/ac2026018. [18] V.D. Bock, H. Hiemstra, J.H. van Maarseveen, CuI-Catalyzed Alkyne–Azide Click Cycloadditions from a Mechanistic and Synthetic Perspective, Eur. J. Org. Chem. 2006 (2006) 51–68, doi:http://dx.doi.org/10.1002/ejoc.200500483. [19] M.K. Sharma, S.K. Aggarwal, Simultaneous formation of Prussian Blue and copper hexacyanoferrate from a solution of Cu2+ and K3[Fe(CN) 6] in presence of HAuCl4, J. Electroanal. Chem. 705 (2013) 64–67, doi:http://dx.doi.org/ 10.1016/j.jelechem.2013.07.023. [20] M. Kohn, Some reactions of ferrocyanides and ferricyanides, Anal. Chim. Acta. 3 (1949) 297–299, doi:http://dx.doi.org/10.1016/S0003-2670(00)87345-6.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Original Dual Microelectrode: Writing and Reading a local click reaction with Scanning Electrochemical Microscopy Vasilica Badetsa , Djoan Duclosa , Damien Quintona , Olivier Fontaineb , Dodzi Zigaha,* a b
Univ. Bordeaux, ISM, CNRS UMR 5255, F-33400 Talence, France Université Montpellier 2 UMR 5253, CC1701, Place Eugène Bataillon, 34095 Montpellier, France
A R T I C L E I N F O
A B S T R A C T
Article history: Received 27 June 2015 Received in revised form 8 October 2015 Accepted 13 October 2015 Available online 26 October 2015
A dual gold-copper microelectrode (diameter 25 mm) was fabricated to be employed in a scanning electrochemical microscopy (SECM) configuration, and was used in feedback mode to both modify (write) and analyze (read) a substrate. This write-and-read procedure was performed on a glassy carbon electrode on which azide groups were introduced by electrochemical reduction of a diazonium salt. The copper part of the microelectrode was used to electrogenerate Cu(I) that catalyzed a “click” reaction between azide moieties on the surface and dissolved ethynylferrocene. This created a pattern of ferrocene moieties on the surface that was analyzed with the gold part of the microelectrode. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Click Chemistry surface patterning SECM diazonium reduction dual microelectrode
1. Introduction “Simultaneously writing and reading electrochemical information using the same microelectrode is an interesting concept in Scanning ElectroChemical Microscopy (SECM). Even if standard lithography method allow to obtain patterns with a high spatial resolution their complexity and high cost led to the proposition of alternative procedures, for example nanofabrication by molding or by printing [1,2]. SECM is a promising technique in this context. In SECM, a microelectrode is moved in 3 dimensions above a substrate, and a controlled electrochemical stimuli triggers the modification of surfaces [3]. For example, a microelectrode placed close to the substrate will be subjected to a potential step to start an electrochemical reaction [4]. The product of this reaction will diffuse toward the substrate to create local patterns with a resolution determined by the size of the diffusion layer [5,6]. Moreover, SECM can be used to visualize the newly created pattern. In the present work, the electrogeneration of Cu(I) was used to locally trigger a click reaction and to pattern ethynylferrocene molecules on a glassy carbon surface covered by azide moieties. The click reaction is a Cu(I) catalyzed Huisgen 1,3-dipolar cycloaddition of alkynes and azide to form 1,2,3-triazole [7]. To simplify existing procedures, we fabricated a dual gold-copper microelectrode. This configuration obliterate the polishing step
* Corresponding author. E-mail address: [email protected] (D. Zigah). http://dx.doi.org/10.1016/j.electacta.2015.10.066 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
between the modification and imaging steps and the requirement of copper in the starting solution. Click chemistry combined with SECM was demonstrated for the first time by Bard et al. [8] for the immobilization of a fluorescent molecule on a glass substrate. For this, a gold microelectrode was used to locally generate Cu(I) species by reduction of an initial solution containing Cu(II). Cu(I) catalyzed the reaction between the fluorescent molecules and the azido-terminated self-assembled monolayer. The same principle was used by Hapiot et al. [9], but in this case, a Pt microelectrode was used to pattern gold macroelectrodes previously modified by azido-terminated self-assembled monolayers. Bedioui et al. [10] used a gold microelectrode and click chemistry to covalently immobilize ferrocene on a glassy carbon surface entirely modified by azido-aryl groups via the diazonium approach. The ferrocene was observed by imaging the surface with SECM using the same gold microelectrode and ferricyanide as redox mediator. One disadvantage of this procedure is the electrodeposition of Cu on the Au microelectrode which demands an additional cleaning step, like anodic striping or mechanical polishing, between the surface writing step and the reading step [10]. Mechanical polishing is not straightforward, firstly the gold electrode is removed, then it is mechanically polished and finally it is repositioned. This procedure is tedious because it requires a perfect repositioning in order to find the spot. In our work we propose a solution to address these limitations. We decided to work without any Cu(II) initially present in the solution and use the oxidation of a copper microelectrode to locally generate Cu(I). But because it is not possible to achieve approach
V. Badets et al. / Electrochimica Acta 201 (2016) 274–278
curves and surfaces imaging with a copper microelectrode, we fabricated a dual microelectrode combining gold and copper wires. The gold part was used to fix the distance between the microelectrode and the substrate and also to obtain an SECM image of the modified surface in feedback mode. Other dual electrodes have been used to study the generation of species on one electrode and collection on the other one, or to detect different species at the two different electrodes [11–13]. To the best of our knowledge it is the first time that a dual Au-Cu microelectrode is used to modify and analyze a substrate. 2. Experimental 2.1. Chemicals All chemicals (4-azidoaniline hydrochloride, sodium nitrite, ethynylferrocene, ascorbic acid, absolute ethanol, hydrochloric acid, potassium ferricyanide, copper sulfate) were purchased form Sigma-Aldrich and used as received. All the solutions were prepared in ultrapure water (resistivity 18.2 MV cm at 25 C).
275
diazonium salt was prepared in-situ in the solution by diazotation of the parent aniline [15]. The initial solution contained 1 mM of 4azidoaniline in 0.5 M HCl and sodium nitrite was added to a final concentration of 2 mM. The electroreduction of the 4-phenylazide diazonium was achieved by cycling 6 times the working electrode between 0 and -0.9 V vs Ag/AgCl at 50 mV s1. The characteristic peak corresponding to the diazonium reduction was observed on the first cycle and disappeared on the second cycle. For the click chemistry on the entire carbon surface, a solution of 5 mM ascorbic acid and 10 mM CuSO4 was prepared. Ethynylferrocene, initially prepared as 0.1 mg ml1 in ethanol, was added to a final concentration of 10 mM. The glassy carbon electrode containing the azide group on the surface was immersed in this solution for 60 min [16]. For the local “electroclick” reaction the same ethynylferrocene solution was used but without CuSO4 and ascorbic acid. The microelectrode was positioned close to the substrate (10 mm) above the azido-modified substrate. A potential of 0.5 V was applied to the copper microelectrode to generate Cu(I) ions that locally triggered the click reaction between the ethynylferrocene and the azide moieties.
2.2. Fabrication of dual electrode 2.4. Cycling Voltammetry and SECM experiment A borosilicate theta glass capillary (World Precision Instrument, 1.5 mm O.D. 1.05 mm I.D) was first cleaned in ethanol and then dried in oven at 80 C for 15 min. Copper and gold wires (diameter 25 mm from Goodfellow) were introduced in two different compartments and placed in the middle of the capillary. Then a laser puller (Sutter instruments P2000) was used to seal one part of wires in the capillary by melting the glass. The capillary was then manually broken and the sealed end was polished firstly with a sand paper p4000 and then with 0.3 mm alumina paste to obtain a clean surface (Fig. 1). To avoid misalignment between the two wires the dual microelectrode should be held perfectly at 90 during the polishing step. The electrical connection to the unsealed end of the wires was made with carbon powder to a copper wire (diameter 500 mm). 2.3. Preparation of modified surfaces Substrates made of glassy carbon electrode of 3 mm in diameter were obtained from IJ Cambria Scientific Ltd. The glassy carbon electrodes were polished successively with sand paper and with 0.3 mm alumina paste. The electrodes were thoroughly washed with ultrapure water and put in ultrasonic bath for 5 min to remove the alumina that remained on their surface. Azide functional groups were introduced on glassy carbon electrodes by reduction of 4-phenylazide diazonium salt [14]. The
Fig. 1. a) Optical and b) SEM images of the dual gold-copper microelectrode.
All the electrochemical measurements were performed using a CH instrument (Austin, TX) Model 920C. An Ag/AgCl (KCl 3 M) reference electrode and platinum wire were used in a threeelectrode system. All steady state SECM approach curves were recorded at 1 mm s1 approach rate. The approach curves and the SECM imaging were recorded by applying -0.1 V on the gold microelectrode to generate ferrocyanide under diffusion limited condition. In all the experiments, a potential of 0.5 V was applied on the substrate. 3. Results and Discussion Cyclic voltammetry measurements were first recorded on the gold and copper microelectrode to verify that the dual Au-Cu microelectrode works properly before performing the surface modification and the SECM experiments. Afterwards, the gold microelectrode was characterized by realizing approach curves on insulating and conductive substrates in a ferricyanide solution (Fig. 2). These curves are usually represented in dimensionless form by plotting the microelectrode current (IT) normalized by the current when the microelectrode is far away of the substrate (Iinf) as a function of the distance (d) normalized by the microelectrode radius (a). These results showed that the dual electrode functions as well as a normal microelectrode despite the fact that the gold part is not centered and that the diameter of the entire tool is quite large (300 mm). Therefore, it was important to check whether
Fig. 2. SECM approach curves in K3Fe(CN)6 2 mmol L1 as mediator with NaCl 0.1 mol L1 in aqueous solution. Edual-Au = -0.1 V, Esubstrate = 0.5 V. ( ) On glassy carbon (GC) electrodes, positive feedback. (—) On glass, negative feedback.
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Fig. 3. SECM approach curves in K3Fe(CN)6 2 mmol L1as mediator with NaCl 0.1 mol L1 in aqueous solution. Edual-Au = -0.1 V, Esubstrate = 0.5 V. (—) On azide moieties, negative feedback. ( ) On ferricinium, positive feedback.
classic approach curves could be achieved using this dual microelectrode [17]. As a preliminary experiment, the entire surface of the carbon electrode was modified with ferrocene via classical click chemistry as described in section 2.3. Firstly, the anchoring layer was immobilized by the electroreduction of the insitu generated azidophenyl diazonium leading to the formation of the corresponding phenyl radicals that reacted with the substrate and the already grafted phenyl moieties giving a multilayer deposit. As seen in Fig. 3, when an approach curve is performed on this modified substrate the dimensionless current decreases with the distance between the microelectrode and the substrate. This
corresponds to a negative feedback. This behavior is observed on the phenylazide layer even if the potential applied on the substrate is supposed to be high enough to oxidize the reduced species electrogenerated on the microelectrode. This clearly shows that the anchoring layer have a blocking effect on the mediator. After the click reaction described in section 2.3, ferrocene is immobilized on the surface and a potential of 0.5 V is applied on the substrate. At this potential the ferrocene is oxidized to ferricinium ion which can be reduced by the ferrocyanide mediator generated on the microelectrode. Therefore the current increases when the distance between the microelectrode and the substrate decreases. This situation corresponds to a positive feedback (Fig. 3). The ferricinium ions are reduced back to ferrocene by the ferrocyanide ions. Due to the applied potential (0.5 V) ferrocene is constantly oxidized to ferricinium and hence the steady-state and the feedback loop is established. Observing these two different feedback behaviors is a preliminary but essential step to ensure that it will be possible to distinguish the patterned area from the non-modified one by SECM. Indeed, for imaging the surface the microelectrode is moved above the azide and ferrocene layers in XY direction. The method used to pattern the surface is described on Scheme 1. Firstly, the entire glassy carbon surface is modified by the azide moieties by the electroreduction of the diazonium salts. In a solution containing ferricyanide, the dual Au-Cu microelectrode is approached to the surface using the negative feedback obtained at the Au part of the microelectrode on the azide layer
Scheme 1. Procedure for the localized electroclick reaction with the dual gold-copper microelectrode. a) Approach curve above the azide function, the negative feedback obtained is used to place the microelectrode. b) Constant potential applied on the copper electrode to generate Cu(I) that triggers the click reaction and immobilizes the ferrocene. c) Imaging of the immobilized ferrocene using the gold microelectrode.
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microelectrode making it impossible to observe the spot formation under these conditions. Therefore imaging the surface with the same solution of ethynylferrocene used for the click chemistry is not recommended. In order to have a positive feedback required for a good quality image the potential difference between the mediator and the immobilized species should be high enough. Therefore, the ferricyanide mediator with a formal potential of 0.192 V compared with the formal potential of immobilized ferrocene (0.316 V) was selected. In these conditions, the surface was patterned with a ferrocene spot via localized click chemistry in an ethynylferrocene solution and was successfully imaged by the dual gold-copper microelectrode using a ferricyanide solution. Fig. 4. SECM imaging of the immobilized ferricinium. Esubstrate = 0.5 V. Scan rate 5 mm s1. Edual-Au = -0.1 V in ferricyanide solution at 2 mmol L1 in NaCl mol L1 in aqueous solution.
(Scheme 1a). In a solution containing ethynylferrocene a constant potential is applied to oxidize the copper microelectrode (Scheme 1b). The amount of copper oxidized during this step can be determined using Faraday’s law (Eq. (1)). The main reaction is: Cu ! Cu2þ þ 2e mCu ¼
MðCuÞ Q nF
ð1Þ
Where mCu is the mass of copper oxidized, M(Cu) the molecular weight of copper, Q the electric charge, n the number of electrons and F the Faraday constant. During the 10 min (when the potential was applied) a charge of 2.6 mC corresponding to a mass of copper of 0.86 ng was recorded. Thus, the rate of consumption of copper is 0.086 ng min1. The disc section is 4.9 106 cm2 and the length of the wire that can be oxidized is around 0.5 cm. By consequence the volume of the copper wire is 2.5 106 cm3. The density of copper is 8.96 g cm3. The initial mass of copper is 22.4 mg, thus only 0.004% of metallic copper have been consumed. The part of copper oxidized in the experiment is negligible, this suggests that the dual electrode could be used for a large number of patterning experiments without causing extensive depletion of the copper source. During this procedure a small amount of Cu(I) is generated. In spite of its poor stability in aqueous solution Cu(I) catalyzes the click reaction locally below the copper microelectrode [18]. The small distance between the microelectrode and the substrate (10 mm) facilitates this reaction by minimizing the diffusion of the Cu(I) away from the copper electrode. Finally, the solution used for the click reaction was replaced by the solution of ferricyanide required for the SECM imaging (Scheme 1c). A potential of 0.5 V was applied on the substrate to oxidize the immobilized ferrocene to ferricinium ion which can be reduced by the mediator generated below the gold microelectrode during the scanning electrochemical imaging (Fig. 4). To ensure that the spot observed after scanning the surface was really the immobilized ferrocene, and not a copper deposit the surface had been examined with a stereo microscope which did not reveal any metal deposit. In another control experiment, the ethynylferrocene solution was replaced with ferrocene and the local click chemistry was performed in the same conditions. No spot was observed during the SECM imaging. This blank experiment prove that without the ethynylferrocene there is no spot on the surface. A convenient protocol would have been to mix the ferricyanide mediator with the ethynylferrocene in order to perform patterning and imaging in the same solution. This is not possible due to some side reactions leading to the formation of Prussian blue [19], if ferricyanide is exposed to Cu2+ [20]. Even if Cu2+ is not present initially in the solution this ion is generated below the copper
4. Conclusions A home-made dual gold-copper microelectrode was fabricated and then used to “write” and “read” a substrate by scanning electrochemical microscopy (SECM) technique. This new tool was first characterized with simple approach curves using the gold microelectrode above insulating and conductive surfaces. Using the gold part the dual Au-Cu was approached close to the azidoterminated aryl layer obtained by the reduction of azidophenyl diazonium on a glassy carbon electrode. The copper part of the microelectrode was used to localize an “electroclick” reaction catalyzed by locally generated Cu(I). This allowed the immobilization of ferrocene which was observed by imaging the surface with SECM in feedback mode. This versatile method can be used to deposit locally many different molecules. Acknowledgements The authors thank the PEPS NANO-GLOS and IDEX Bordeaux for the financial support. References [1] B.D. Gates, Q. Xu, M. Stewart, D. Ryan, C.G. Willson, G.M. Whitesides, New Approaches to Nanofabrication: Molding, Printing, and Other Techniques, Chem. Rev. 105 (2005) 1171–1196, doi:http://dx.doi.org/10.1021/cr030076o. [2] H.M. Saavedra, T.J. Mullen, P. Zhang, D.C. Dewey, S.A. Claridge, P.S. Weiss, Hybrid strategies in nanolithography, Rep. Prog. Phys. 73 (2010) 036501, doi: http://dx.doi.org/10.1088/0034-4885/73/3/036501. [3] M. Sheffer, D. Mandler, Scanning Electrochemical Imprinting Microscopy: A Tool for Surface Patterning, J. Electrochem. Soc. 155 (2008) D203–D208, doi: http://dx.doi.org/10.1149/1.2830543. [4] S. Amemiya, A.J. Bard, F.-R.F. Fan, M.V. Mirkin, P.R. Unwin, Scanning Electrochemical Microscopy, Annu. Rev. Anal. Chem. 1 (2008) 95–131, doi: http://dx.doi.org/10.1146/annurev.anchem.1.031207.112938. [5] I. Turyan, U.O. Krasovec, B. Orel, T. Saraidorov, R. Reisfeld, D. Mandler, Writing– Reading–Erasing on Tungsten Oxide Films Using the Scanning Electrochemical Microscope, Adv. Mater. 12 (2000) 330–333, doi:http://dx.doi.org/10.1002/ (SICI)1521-4095(200003)12:5<330::AID-ADMA330>3.0.CO;2-8. [6] S. Meltzer, D. Mandler, Microwriting of Gold Patterns with the Scanning Electrochemical Microscope, J. Electrochem. Soc. 142 (1995) L82–L84, doi: http://dx.doi.org/10.1149/1.2044252. [7] C.W. Tornøe, C. Christensen, M. Meldal, Peptidotriazoles on Solid Phase: [1,2,3]-Triazoles by Regiospecific Copper(I)-Catalyzed 1,3-Dipolar Cycloadditions of Terminal Alkynes to Azides, J. Org. Chem. 67 (2002) 3057–3064, doi:http://dx.doi.org/10.1021/jo011148j. [8] S.-Y. Ku, K.-T. Wong, A.J. Bard, Surface Patterning with Fluorescent Molecules Using Click Chemistry Directed by Scanning Electrochemical Microscopy, J. Am. Chem. Soc. 130 (2008) 2392–2393, doi:http://dx.doi.org/10.1021/ ja078183d. [9] S. Lhenry, Y.R. Leroux, C. Orain, F. Conan, N. Cosquer, N.L. Poul, et al., Locally Induced and Self-Induced Electroclick onto a Self-Assembled Monolayer: Writing and Reading with SECM under Unbiased Conditions, Langmuir 30 (2014) 4501–4508, doi:http://dx.doi.org/10.1021/la405005f. [10] D. Quinton, A. Maringa, S. Griveau, T. Nyokong, F. Bedioui, Surface patterning using scanning electrochemical microscopy to locally trigger a click chemistry reaction, Electrochem. Commun. 31 (2013) 112–115, doi:http://dx.doi.org/ 10.1016/j.elecom.2013.03.021. [11] K. McKelvey, B.P. Nadappuram, P. Actis, Y. Takahashi, Y.E. Korchev, T. Matsue, et al., Fabrication, Characterization, and Functionalization of Dual Carbon
278
[12]
[13]
[14]
[15]
V. Badets et al. / Electrochimica Acta 201 (2016) 274–278 Electrodes as Probes for Scanning Electrochemical Microscopy (SECM), Anal. Chem. 85 (2013) 9–7526, doi:http://dx.doi.org/10.1021/ac401476z. A. Kueng, C. Kranz, B. Mizaikoff, Imaging of ATP membrane transport with dual micro-disk electrodes and scanning electrochemical microscopy, Biosens. Bioelectron. 21 (2005) 346–353, doi:http://dx.doi.org/10.1016/j. bios.2004.10.020. S. Isik, M. Etienne, J. Oni, A. Blöchl, S. Reiter, W. Schuhmann, Dual Microelectrodes for Distance Control and Detection of Nitric Oxide from Endothelial Cells by Means of Scanning Electrochemical Microscope, Anal. Chem. 76 (2004) 6389–6394, doi:http://dx.doi.org/10.1021/ac049182w. P. Allongue, M. Delamar, B. Desbat, O. Fagebaume, R. Hitmi, J. Pinson, et al., Covalent Modification of Carbon Surfaces by Aryl Radicals Generated from the Electrochemical Reduction of Diazonium Salts, J. Am. Chem. Soc. 119 (1997) 201–207, doi:http://dx.doi.org/10.1021/ja963354s. S. Baranton, D. Bélanger, Electrochemical Derivatization of Carbon Surface by Reduction of in Situ Generated Diazonium Cations, J. Phys. Chem. B. 109 (2005) 24401–24410, doi:http://dx.doi.org/10.1021/jp054513+.
[16] D. Evrard, F. Lambert, C. Policar, V. Balland, B. Limoges, Electrochemical Functionalization of Carbon Surfaces by Aromatic Azide or Alkyne Molecules: A Versatile Platform for Click Chemistry, Chem. -Eur. J. 14 (2008) 9286–9291, doi:http://dx.doi.org/10.1002/chem.200801168. [17] R. Cornut, A. Bhasin, S. Lhenry, M. Etienne, C. Lefrou, Accurate and Simplified Consideration of the Probe Geometrical Defaults in Scanning Electrochemical Microscopy: Theoretical and Experimental Investigations, Anal. Chem. 83 (2011) 9669–9675, doi:http://dx.doi.org/10.1021/ac2026018. [18] V.D. Bock, H. Hiemstra, J.H. van Maarseveen, CuI-Catalyzed Alkyne–Azide Click Cycloadditions from a Mechanistic and Synthetic Perspective, Eur. J. Org. Chem. 2006 (2006) 51–68, doi:http://dx.doi.org/10.1002/ejoc.200500483. [19] M.K. Sharma, S.K. Aggarwal, Simultaneous formation of Prussian Blue and copper hexacyanoferrate from a solution of Cu2+ and K3[Fe(CN) 6] in presence of HAuCl4, J. Electroanal. Chem. 705 (2013) 64–67, doi:http://dx.doi.org/ 10.1016/j.jelechem.2013.07.023. [20] M. Kohn, Some reactions of ferrocyanides and ferricyanides, Anal. Chim. Acta. 3 (1949) 297–299, doi:http://dx.doi.org/10.1016/S0003-2670(00)87345-6.