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Microelectrochemical patterning of gold surfaces using 4-azidobenzenediazonium and scanning electrochemical microscopy
Electrochemistry Communications 13 (2011) 150–153
Contents lists available at ScienceDirect
Electrochemistry Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e l e c o m
Microelectrochemical patterning of gold surfaces using 4-azidobenzenediazonium and scanning electrochemical microscopy Megan Coates a,b,c, Eva Cabet a,b, Sophie Griveau a,b,⁎, Tebello Nyokong c, Fethi Bedioui a,b,⁎ a b c
Unité de Pharmacologie Chimique et Génétique et Imagerie, CNRS n° 8151, École Nationale Supérieure de Chimie de Paris, Chimie ParisTech, Université Paris Descartes, Paris, France INSERM, Unité de Pharmacologie Chimique et Génétique et Imagerie n° 1022, Paris, France Department of Chemistry, Rhodes University, 6140, Grahamstown, South Africa
a r t i c l e
i n f o
Article history: Received 9 November 2010 Received in revised form 25 November 2010 Accepted 29 November 2010 Available online 8 December 2010 Keywords: Diazonium Electrografting Spots Electrochemical scanning microscopy Local derivatization
a b s t r a c t This work describes for the first time the possibility of performing local micro electrochemical grafting of a gold substrate by 4-azidobenzenediazonium by SECM in a single and simple one step without complications from adsorption. The electrografted spots of diazonium were performed by positioning a Pt tip at a given distance above the gold substrate and the SECM was used in a three-electrode configuration (the Pt tip serving as the microanode) in acetonitrile containing 5 mM 4-azidobenzenediazonium and 0.1 M Bu4NBF4 during 10 ms. The dimensions of the derivatized areas of the substrates were finely tuned by using different experimental conditions (tip distance above the substrate, tip diameter, presence or absence of supporting electrolyte). The use of the azido-derivated diazonium molecule and these preliminary results open the gate to important applications and developments devoted to the local micro functionalization of electrodes by thin layers that allow the implementation of the emerging and attractive interfacial click reaction. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Among the methods of surface modification, electrochemical functionalization is one of the simplest and it is a relatively low-cost technique. As a common example of this method, the electrografting of aryldiazonium salts has been shown to efficiently form stable thin organic layers on several kinds of substrates [1–9]. For example, the use of suitable diazonium salts has been successfully applied to the development of biosensors, biochips and immobilization of enzymes [10–12]. But several studies have also now unambiguously shown the occurrence of a spontaneous adsorption of diazonium species on carbon, gold and copper surfaces from acetonitrile and aqueous solutions [13–19], which may however prevent the precise control of further electrochemical grafting. Thus, the use of diazonium salts for the microstructuring of surfaces by ultramicrotechniques such as scanning electrochemical microscopy (SECM) is restrained owing to the spontaneous derivatization of exposed surfaces. To date, only one example was recently reported on direct electrochemical patterning of a diazonium salt via a one-pot sequential reaction for local electrogeneration of the diazonium salt, followed by its electrografting on the substrate [20]. Although this alternative strategy allowed circumventing of the spontaneous adsorption of the diazonium salt on the substrate, the technical protocol needs precise control of the multi step procedure. Indeed, the efficiency of the strategy strongly depends on
the kinetics of the diazotization reaction and the electroreduction of the obtained diazonium salt can also occur at the SECM tip and block the micro grafting process. Besides this approach, one can cite the drawing of patterns of a thin passivating layer on gold electrode surface by electrochemical reduction of an aryliodonium salt at very negative potential values by SECM [21]. The search for a local electrochemical strategy with minimized spontaneous adsorption phenomenon and accurate control of substrate surface led us to combine the use of 4-azidobenzenediazonium tetrafluoroborate and SECM. The use of 4-azidobenzenediazonium tetrafluoroborate offers two major advantages afforded by the azide substituent: (i) its kinetics of adsorption is slower than the commonly used diazonium salts [22,23] and (ii) it offers an emerging attractive and efficient way of further chemical functionalization of the electrografted surface through the well known Sharpless copper(I) catalyzed azide-alkyne cycloaddition reaction [22]. Herein, we perform for the first time the microelectrochemical patterning of gold surfaces by application of SECM in three-electrode configuration using 4-azidobenzenediazonium with a very simple approach. The micro patterned surfaces were characterized by SECM approach curve and SECM imaging in feedback mode. 2. Experimental 2.1. Chemicals
⁎ Corresponding authors. Tel.: +33 153 10 12 98; fax: +33 153 10 12 92. E-mail addresses: [email protected] (S. Griveau), [email protected] (F. Bedioui). 1388-2481/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2010.11.037
All chemicals (reagent grade from Aldrich) were used without further purification. MilliQ water was used to prepare all the aqueous
M. Coates et al. / Electrochemistry Communications 13 (2011) 150–153
151
by optical microscopy (RG = aglass/a with a = tip radius and aglass = glass radius). Approach curves were performed using Pt tip as working electrode and Ag/AgCl wire as a pseudo-reference electrode and counter-electrode.
solutions. 4-azidobenzenediazonium tetrafluoroborate was synthesized from previously reported procedure [24].
2.2. Electrodes and SECM apparatus 3. Results
Polycrystalline gold plates (Goodfellow, France) 1.5 × 1.5 cm and 2 mm thick were used as the substrate in the SECM experiments. Each substrate was manually polished before each experiment with diamond liquid (BioDiamant, Lamplan, France) of 1 μm and ¼ μm. The substrate was then thoroughly rinsed with ultra-pure water (Millipore System). SECM experiments were carried out using Princeton Applied Research equipment (UNISCAN Model 370) and homemade 12.5, 25 and 50 μm Pt microelectrode as the tip. The Pt microelectrodes were made by inserting the Pt wire of desired diameter into tapered glass capillaries, and melting the tapered glass around the wire. The connection was achieved using copper wire, the contact was made with silver glue and the probe was polished to expose the Pt wire. SECM images were obtained by maintaining the tip at a constant z position and scanning in the x–y plane over the desired area (constantheight mode of SECM) and monitoring changes in the steady-state current of Ru(NH3)6Cl3 reduction at −0.45 V vs. Ag/AgCl as the tip travels. RG values of 40 and 20 for the tip of 12.5 μm diameter and 25 μm diameter, respectively, were evaluated before each experiment
First, to perform local electrografting of 4-azidobenzenediazonium by SECM, the Pt tip (12.5 μm diameter) is positioned at a desired close distance from the gold substrate surface using conventional approach curve in feedback mode in aqueous 0.1 M KCl + 5 mM Ru(NH3)6Cl3. The calculated distance between the tip and the gold surface, by comparison with simulated pure positive feedback curve, is ≈1 μm. After rinsing with MilliQ water and then acetonitrile, a solution of acetonitrile containing 5 mM 4-azidobenzenediazonium and 0.1 M Bu4NBF4 is introduced. Preliminary experiments indicated that the electrochemical grafting of 4-azidobenzenediazonium occurs on gold at potentials lower than − 0.4 V [23] as shown in Fig. 1a. The bare gold substrate is then polarized at −0.5 V while the SECM tip acts as a microanode (i.e. as a counter electrode in a three-electrode configuration). The positioning of the microanode (SECM tip) close to the gold surface allows for the confining of the electric and diffusion fields [25] and restricts the reduction of the diazonium moieties to a local
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(d/a) Fig. 1. (a) Schematic electrografting reaction of 4-azidobenzenediazonium (b) SECM line scan with 5 mM Ru(NH3)6Cl3 in 0.1 M KCl aqueous solution above the gold substrate after its local electrografting using a Pt tip of 12.5 μm diameter (duration of the electrografting reaction in acetonitrile containing 5 mM 4-azidobenzenediazonium and 0.1 M Bu4NBF4 = 10 ms) (c) SECM approach curves for a 12.5 μm diameter Pt in 5 mM Ru(NH3)6Cl3 in 0.1 M KCl aqueous solution above the locally electrografted area of the gold substrate (curve 1) and 500 μm away (curve 2). (Etip = − 0.45 V vs. Ag/AgCl; Ilim = 9.8 nA).
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area beneath the tip. After reducing the 4-azidobenzenediazonium during 10 ms, the electrochemical set up is rinsed with acetonitrile and MilliQ water and an aqueous solution of 5 mM Ru(NH3)6Cl3 is introduced and the Pt microelectrode is used again as SECM tip, with no potential being applied to the gold substrate. The SECM tip is not passivated during the electrografting of the gold surface, and so it can be used directly to assess the characteristics of the spotted layer. Fig. 1b shows the SECM line scan of the resulting pattern formed on the gold surface. The current decrease is consistent with a local derivatization and reveals the formation of a ≈ 300 μm wide passivating layer on the substrate. The electrografting is confined in a much larger region than the tip, indicating that, as expected for diazonium moieties, the reaction is very efficient. The area surrounding the micro spotted electrografted zone remains apparently conducting as shown by a comparison of the approach curves performed on the spot
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(Fig. 1c, curve 1) and at a distance of 500 μm from the center of the spot (Fig. 1c, curve 2). The approach curve performed on the spot clearly shows after simulation with theoretical curves that the grafted layer is affecting (slowing) the electron transfer but it is not behaving as pure insulating. Fig. 2a shows the SECM image of the patterned spot by scanning the tip over the gold substrate laterally along X and Y axis within a square region of 1000 μm × 1000 μm centered on the position where the tip was placed for the electrografting. The electrografted spot is clearly visible with a good contrast due to the electron transfer features of Ru(NH3)6Cl3. A similar image was obtained by using the well known surface sensitive K3Fe(CN)6 redox mediator [19], confirming that 4-azidobenzenediazonium was not adsorbed at the gold in our experimental conditions (data not shown). Pattern dimensions can be controlled by the concentration of the diazonium,
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Fig. 2. SECM images of locally electrografted gold substrate with a Pt tip in 5 mM Ru(NH3)6Cl3 in 0.1 M KCl aqueous solution (Etip = − 0.45 V vs. Ag/AgCl; Ilim = 9.8, 19.6 and 48 nA for 12.5, 25 and 50 μm diameter). The tip was scanned at 50 μm/s. The electrografting was obtained by a Pt tip of (a) 12.5 μm diameter positioned 1 μm above the gold surface; (b) 50 μm diameter positioned 3 μm above the gold surface; (c) 50 μm diameter positioned 1.5 μm above the gold surface; (d) 25 μm diameter positioned 1.5 μm above the gold surface and (e) 12.5 μm diameter positioned 1 μm above the gold surface. In all cases the electrografting reaction was performed in acetonitrile containing 5 mM 4-azidobenzenediazonium and 0.1 M Bu4NBF4 during 10 ms except (e) where no supporting electrolyte was added.
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the duration of the electrochemical reduction reaction, the presence or absence of supporting electrolyte and the dimensions of the cone of the electrolytic solution confined between the tip and the substrate [25,26]. In the latter case, the dimensions of the cone were controlled by varying the distance between the tip and the gold substrate and/or the diameter of the tip. The use of a 50 μm diameter tip positioned at ≈3 μm above the gold substrate gives a very diffuse derivatized area (dimension larger than 500 μm, Fig. 2b), while a much smaller electrografted spot is obtained if the same tip is positioned at ≈1.5 μm (Fig. 2c). Reducing the tip diameter to 25 μm results in a smaller electrografted spot as shown in Fig. 2d. Thus, comparison of Fig. 2a–d shows how by moving the tip away from the gold substrate, the electrografting reaction is displaced to vicinal uncovered domains due to lesser radial diffusion restriction of the diazonium to enrich the surface of the substrate and the very fast coupling reaction of the radical electrogenerated at the base of the cone. It is important to note that in all cases the duration of the electrografting reaction was the same (10 ms). Finally, the spotted micro zones were limited to a much smaller area (≈60 μm) by eliminating the supporting electrolyte, thus reducing the charge injected for the electrochemical transformation. This is exemplified in Fig. 2e which has to be compared with Fig. 2a. This approach is thus a very appealing method to produce reduced size spots of electrografted aryldiazoniums. It should also be pointed out that in all cases the SECM images were obtained by using the same Pt tip as the one used for the electrografting of the substrate, without any further polishing.
4. Conclusion We show for the first time the local micro electrochemical grafting of a gold substrate by 4-azidobenzenediazonium by SECM in a simple single-step procedure. The dimensions of the derivatized area can be finely tuned by controlling different experimental conditions. The use of the azido-derivated diazonium molecule opens the gate to important applications and developments devoted to the micro local functionalization of electrodes and the implementation of the emerging interfacial click reaction.
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Acknowledgements This work was supported by the DST/NRF South African Research Chairs Initiative for Professor of Medicinal Chemistry and Nanotechnology as well as Rhodes University and PROTEA project 07F10/SA (FranceSouth Africa). MC thanks PROTEA, Rhodes University Henderson and NRF for funding. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
M. Delamar, R. Hitmi, J. Pinson, J.M. Saveant, J. Am. Chem. Soc. 114 (1992) 5883. A.J. Downard, M.J. Prince, Langmuir 17 (2001) 5581. C. Saby, B. Ortiz, G.Y. Champagne, D. Belanger, Langmuir 13 (1997) 6805. A. Laforgue, T. Addou, D. Bélanger, Langmuir 21 (2005) 6855. J. Lyskawa, D. Bélanger, Chemi. Mater. 18 (2006) 4755. G.Z. Liu, T. Bocking, J.J. Gooding, J. Electroanal. Chem. 600 (2007) 335. M.G. Paulik, P.A. Brooksby, A.D. Abell, A.J. Downard, J. Phys. Chem. C 111 (2007) 7808. A. Adenier, M.C. Bernard, M.M. Chehimi, E. Cabet-Deliry, B. Desbat, O. Fagebaume, J. Pinson, F. Podvorica, J. Am. Chem. Soc. 123 (2001) 4541. A. Chaussé, M.M. Chehimi, N. Karsi, J. Pinson, F. Podvorica, C. Vautrin-Ul, Chem. Mater. 14 (2002) 392. B.P. Corgier, A. Laurent, P. Perriat, L.J. Blum, C.A. Marquette, Angew. Chem. Intern. Ed. 46 (2007) 4108. J.J. Gooding, Electroanalysis 20 (2008) 573. R. Polsky, J.C. Harper, D.R. Wheeler, S.M. Brozik, Electroanalysis 20 (2008) 671. B.L. Hurley, R.L. McCreery, J. Electrochem. Soc. 151 (2004) B252. A. Adenier, E. Cabet-Deliry, A. Chaussé, S. Griveau, F. Mercier, J. Pinson, C. Vautrin-Ul, Chem. Mater. 17 (2005) 491. C. Combellas, M. Delamar, F. Kanoufi, J. Pinson, F.I. Podvorica, Chem. Mater. 17 (2005) 3968. A. Adenier, N. Barre, E. Cabet-Deliry, A. Chausse, S. Griveau, F. Mercier, J. Pinson, C. Vautrin-Ul, Surf. Sci. 600 (2006) 4801. F. Barriere, A.J. Downard, J. Sol. State Electrochem. 12 (2008) 1231. M. Toupin, D. Bélanger, Langmuir 24 (2008) 1910. S. Griveau, S. Aroua, D. Bediwy, R. Cornut, C. Lefrou, F. Bedioui, J. Electroanal. Chem. 647 (2010) 93. C. Cougnon, F. Gohier, D. Bélanger, J. Mauzeroll, Angew. Chem. Int. Ed. 48 (2009) 4006. T. Matrab, C. Combellas, F. Kanoufi, Electrochem. Commun. 10 (2008) 1230. D. Evrard, F. Lambert, C. Policar, V. Balland, B. Limoges, Chem. Eur. J. 14 (2008) 9286. S. Griveau, unpublished results. P. Allongue, M. Delamar, B. Desbat, O. Fagebaume, R. Hitmi, J. Pinson, J.-M. Savéant, J. Am. Chem. Soc. 119 (1997) 201. A.J. Bard, M.V. Mirkin, Scanning Electrochemical Microscopy, Marcel Dekker, New-York, 2001. G. Wittstock, M. Burchardt, S.E. Pust, Y. Shen, C. Zhao, Angew. Chem. Int. Ed. 46 (2007) 1584.
Contents lists available at ScienceDirect
Electrochemistry Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e l e c o m
Microelectrochemical patterning of gold surfaces using 4-azidobenzenediazonium and scanning electrochemical microscopy Megan Coates a,b,c, Eva Cabet a,b, Sophie Griveau a,b,⁎, Tebello Nyokong c, Fethi Bedioui a,b,⁎ a b c
Unité de Pharmacologie Chimique et Génétique et Imagerie, CNRS n° 8151, École Nationale Supérieure de Chimie de Paris, Chimie ParisTech, Université Paris Descartes, Paris, France INSERM, Unité de Pharmacologie Chimique et Génétique et Imagerie n° 1022, Paris, France Department of Chemistry, Rhodes University, 6140, Grahamstown, South Africa
a r t i c l e
i n f o
Article history: Received 9 November 2010 Received in revised form 25 November 2010 Accepted 29 November 2010 Available online 8 December 2010 Keywords: Diazonium Electrografting Spots Electrochemical scanning microscopy Local derivatization
a b s t r a c t This work describes for the first time the possibility of performing local micro electrochemical grafting of a gold substrate by 4-azidobenzenediazonium by SECM in a single and simple one step without complications from adsorption. The electrografted spots of diazonium were performed by positioning a Pt tip at a given distance above the gold substrate and the SECM was used in a three-electrode configuration (the Pt tip serving as the microanode) in acetonitrile containing 5 mM 4-azidobenzenediazonium and 0.1 M Bu4NBF4 during 10 ms. The dimensions of the derivatized areas of the substrates were finely tuned by using different experimental conditions (tip distance above the substrate, tip diameter, presence or absence of supporting electrolyte). The use of the azido-derivated diazonium molecule and these preliminary results open the gate to important applications and developments devoted to the local micro functionalization of electrodes by thin layers that allow the implementation of the emerging and attractive interfacial click reaction. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Among the methods of surface modification, electrochemical functionalization is one of the simplest and it is a relatively low-cost technique. As a common example of this method, the electrografting of aryldiazonium salts has been shown to efficiently form stable thin organic layers on several kinds of substrates [1–9]. For example, the use of suitable diazonium salts has been successfully applied to the development of biosensors, biochips and immobilization of enzymes [10–12]. But several studies have also now unambiguously shown the occurrence of a spontaneous adsorption of diazonium species on carbon, gold and copper surfaces from acetonitrile and aqueous solutions [13–19], which may however prevent the precise control of further electrochemical grafting. Thus, the use of diazonium salts for the microstructuring of surfaces by ultramicrotechniques such as scanning electrochemical microscopy (SECM) is restrained owing to the spontaneous derivatization of exposed surfaces. To date, only one example was recently reported on direct electrochemical patterning of a diazonium salt via a one-pot sequential reaction for local electrogeneration of the diazonium salt, followed by its electrografting on the substrate [20]. Although this alternative strategy allowed circumventing of the spontaneous adsorption of the diazonium salt on the substrate, the technical protocol needs precise control of the multi step procedure. Indeed, the efficiency of the strategy strongly depends on
the kinetics of the diazotization reaction and the electroreduction of the obtained diazonium salt can also occur at the SECM tip and block the micro grafting process. Besides this approach, one can cite the drawing of patterns of a thin passivating layer on gold electrode surface by electrochemical reduction of an aryliodonium salt at very negative potential values by SECM [21]. The search for a local electrochemical strategy with minimized spontaneous adsorption phenomenon and accurate control of substrate surface led us to combine the use of 4-azidobenzenediazonium tetrafluoroborate and SECM. The use of 4-azidobenzenediazonium tetrafluoroborate offers two major advantages afforded by the azide substituent: (i) its kinetics of adsorption is slower than the commonly used diazonium salts [22,23] and (ii) it offers an emerging attractive and efficient way of further chemical functionalization of the electrografted surface through the well known Sharpless copper(I) catalyzed azide-alkyne cycloaddition reaction [22]. Herein, we perform for the first time the microelectrochemical patterning of gold surfaces by application of SECM in three-electrode configuration using 4-azidobenzenediazonium with a very simple approach. The micro patterned surfaces were characterized by SECM approach curve and SECM imaging in feedback mode. 2. Experimental 2.1. Chemicals
⁎ Corresponding authors. Tel.: +33 153 10 12 98; fax: +33 153 10 12 92. E-mail addresses: [email protected] (S. Griveau), [email protected] (F. Bedioui). 1388-2481/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2010.11.037
All chemicals (reagent grade from Aldrich) were used without further purification. MilliQ water was used to prepare all the aqueous
M. Coates et al. / Electrochemistry Communications 13 (2011) 150–153
151
by optical microscopy (RG = aglass/a with a = tip radius and aglass = glass radius). Approach curves were performed using Pt tip as working electrode and Ag/AgCl wire as a pseudo-reference electrode and counter-electrode.
solutions. 4-azidobenzenediazonium tetrafluoroborate was synthesized from previously reported procedure [24].
2.2. Electrodes and SECM apparatus 3. Results
Polycrystalline gold plates (Goodfellow, France) 1.5 × 1.5 cm and 2 mm thick were used as the substrate in the SECM experiments. Each substrate was manually polished before each experiment with diamond liquid (BioDiamant, Lamplan, France) of 1 μm and ¼ μm. The substrate was then thoroughly rinsed with ultra-pure water (Millipore System). SECM experiments were carried out using Princeton Applied Research equipment (UNISCAN Model 370) and homemade 12.5, 25 and 50 μm Pt microelectrode as the tip. The Pt microelectrodes were made by inserting the Pt wire of desired diameter into tapered glass capillaries, and melting the tapered glass around the wire. The connection was achieved using copper wire, the contact was made with silver glue and the probe was polished to expose the Pt wire. SECM images were obtained by maintaining the tip at a constant z position and scanning in the x–y plane over the desired area (constantheight mode of SECM) and monitoring changes in the steady-state current of Ru(NH3)6Cl3 reduction at −0.45 V vs. Ag/AgCl as the tip travels. RG values of 40 and 20 for the tip of 12.5 μm diameter and 25 μm diameter, respectively, were evaluated before each experiment
First, to perform local electrografting of 4-azidobenzenediazonium by SECM, the Pt tip (12.5 μm diameter) is positioned at a desired close distance from the gold substrate surface using conventional approach curve in feedback mode in aqueous 0.1 M KCl + 5 mM Ru(NH3)6Cl3. The calculated distance between the tip and the gold surface, by comparison with simulated pure positive feedback curve, is ≈1 μm. After rinsing with MilliQ water and then acetonitrile, a solution of acetonitrile containing 5 mM 4-azidobenzenediazonium and 0.1 M Bu4NBF4 is introduced. Preliminary experiments indicated that the electrochemical grafting of 4-azidobenzenediazonium occurs on gold at potentials lower than − 0.4 V [23] as shown in Fig. 1a. The bare gold substrate is then polarized at −0.5 V while the SECM tip acts as a microanode (i.e. as a counter electrode in a three-electrode configuration). The positioning of the microanode (SECM tip) close to the gold surface allows for the confining of the electric and diffusion fields [25] and restricts the reduction of the diazonium moieties to a local
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(d/a) Fig. 1. (a) Schematic electrografting reaction of 4-azidobenzenediazonium (b) SECM line scan with 5 mM Ru(NH3)6Cl3 in 0.1 M KCl aqueous solution above the gold substrate after its local electrografting using a Pt tip of 12.5 μm diameter (duration of the electrografting reaction in acetonitrile containing 5 mM 4-azidobenzenediazonium and 0.1 M Bu4NBF4 = 10 ms) (c) SECM approach curves for a 12.5 μm diameter Pt in 5 mM Ru(NH3)6Cl3 in 0.1 M KCl aqueous solution above the locally electrografted area of the gold substrate (curve 1) and 500 μm away (curve 2). (Etip = − 0.45 V vs. Ag/AgCl; Ilim = 9.8 nA).
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area beneath the tip. After reducing the 4-azidobenzenediazonium during 10 ms, the electrochemical set up is rinsed with acetonitrile and MilliQ water and an aqueous solution of 5 mM Ru(NH3)6Cl3 is introduced and the Pt microelectrode is used again as SECM tip, with no potential being applied to the gold substrate. The SECM tip is not passivated during the electrografting of the gold surface, and so it can be used directly to assess the characteristics of the spotted layer. Fig. 1b shows the SECM line scan of the resulting pattern formed on the gold surface. The current decrease is consistent with a local derivatization and reveals the formation of a ≈ 300 μm wide passivating layer on the substrate. The electrografting is confined in a much larger region than the tip, indicating that, as expected for diazonium moieties, the reaction is very efficient. The area surrounding the micro spotted electrografted zone remains apparently conducting as shown by a comparison of the approach curves performed on the spot
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(Fig. 1c, curve 1) and at a distance of 500 μm from the center of the spot (Fig. 1c, curve 2). The approach curve performed on the spot clearly shows after simulation with theoretical curves that the grafted layer is affecting (slowing) the electron transfer but it is not behaving as pure insulating. Fig. 2a shows the SECM image of the patterned spot by scanning the tip over the gold substrate laterally along X and Y axis within a square region of 1000 μm × 1000 μm centered on the position where the tip was placed for the electrografting. The electrografted spot is clearly visible with a good contrast due to the electron transfer features of Ru(NH3)6Cl3. A similar image was obtained by using the well known surface sensitive K3Fe(CN)6 redox mediator [19], confirming that 4-azidobenzenediazonium was not adsorbed at the gold in our experimental conditions (data not shown). Pattern dimensions can be controlled by the concentration of the diazonium,
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Fig. 2. SECM images of locally electrografted gold substrate with a Pt tip in 5 mM Ru(NH3)6Cl3 in 0.1 M KCl aqueous solution (Etip = − 0.45 V vs. Ag/AgCl; Ilim = 9.8, 19.6 and 48 nA for 12.5, 25 and 50 μm diameter). The tip was scanned at 50 μm/s. The electrografting was obtained by a Pt tip of (a) 12.5 μm diameter positioned 1 μm above the gold surface; (b) 50 μm diameter positioned 3 μm above the gold surface; (c) 50 μm diameter positioned 1.5 μm above the gold surface; (d) 25 μm diameter positioned 1.5 μm above the gold surface and (e) 12.5 μm diameter positioned 1 μm above the gold surface. In all cases the electrografting reaction was performed in acetonitrile containing 5 mM 4-azidobenzenediazonium and 0.1 M Bu4NBF4 during 10 ms except (e) where no supporting electrolyte was added.
M. Coates et al. / Electrochemistry Communications 13 (2011) 150–153
the duration of the electrochemical reduction reaction, the presence or absence of supporting electrolyte and the dimensions of the cone of the electrolytic solution confined between the tip and the substrate [25,26]. In the latter case, the dimensions of the cone were controlled by varying the distance between the tip and the gold substrate and/or the diameter of the tip. The use of a 50 μm diameter tip positioned at ≈3 μm above the gold substrate gives a very diffuse derivatized area (dimension larger than 500 μm, Fig. 2b), while a much smaller electrografted spot is obtained if the same tip is positioned at ≈1.5 μm (Fig. 2c). Reducing the tip diameter to 25 μm results in a smaller electrografted spot as shown in Fig. 2d. Thus, comparison of Fig. 2a–d shows how by moving the tip away from the gold substrate, the electrografting reaction is displaced to vicinal uncovered domains due to lesser radial diffusion restriction of the diazonium to enrich the surface of the substrate and the very fast coupling reaction of the radical electrogenerated at the base of the cone. It is important to note that in all cases the duration of the electrografting reaction was the same (10 ms). Finally, the spotted micro zones were limited to a much smaller area (≈60 μm) by eliminating the supporting electrolyte, thus reducing the charge injected for the electrochemical transformation. This is exemplified in Fig. 2e which has to be compared with Fig. 2a. This approach is thus a very appealing method to produce reduced size spots of electrografted aryldiazoniums. It should also be pointed out that in all cases the SECM images were obtained by using the same Pt tip as the one used for the electrografting of the substrate, without any further polishing.
4. Conclusion We show for the first time the local micro electrochemical grafting of a gold substrate by 4-azidobenzenediazonium by SECM in a simple single-step procedure. The dimensions of the derivatized area can be finely tuned by controlling different experimental conditions. The use of the azido-derivated diazonium molecule opens the gate to important applications and developments devoted to the micro local functionalization of electrodes and the implementation of the emerging interfacial click reaction.
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Acknowledgements This work was supported by the DST/NRF South African Research Chairs Initiative for Professor of Medicinal Chemistry and Nanotechnology as well as Rhodes University and PROTEA project 07F10/SA (FranceSouth Africa). MC thanks PROTEA, Rhodes University Henderson and NRF for funding. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
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