Properties of ionic liquids on Au surfaces

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Properties of ionic liquids on Au surfaces: non-conventional anion exchange reactions with carbonatew Mathieu Ratel, Mathieu Branca, Julien Breault-Turcot, Sandy Shuo Zhao, Pierre Chaurand, Andreea R. Schmitzer* and Jean-Francois Masson* Received 30th June 2011, Accepted 15th August 2011 DOI: 10.1039/c1cc13914b A simple anion metathesis in diluted aqueous carbonate at room temperature affords 1-(12-mercaptododecyl)-3-methyl-imidazolium carbonate (MDMI-HCO3) from MDMI salts self-assembled on gold films and nanoparticles. The properties of MDMI-SAM differ from MDMI in solution, for which the anion exchange reaction does not proceed. The interest of RT-ILs in bioanalysis was sparked by the observation of improved biocatalytic transformations promoted in an IL environment.1 For example, a fluorescence bioassay using an antibody chip was successfully performed in ILs.2 In order to exploit the benefits of ILs in biochips, while analyzing actual biological samples, there is interest in developing biochips with IL immobilized to the surface in a SAM format. The properties of IL-SAM based on MDMI are still relatively unknown, but the influence of the counter-anion on wettability3 and of the deposition temperature on the conformation4 has been reported in the literature. The length of the alkyl chain may also be influencing the properties of IL-SAM, as solutions of 1-alkyl-3-methylimidazolium of different chain lengths do not organize similarly near mica surfaces.5 IL-SAMs are suited for applications in biosensing,6 which often requires exposure to buffers containing carbonate. However, some important questions need to be addressed to develop these applications and to better characterize the properties of IL-SAM, such as whether it is possible to exchange hydrophilic or hydrophobic counter-anions with HCO3 and to study the effect on surface properties of RT-IL SAM following an exposure to HCO3 . The importance of IL-carbonate as a precursor for the synthesis of various ILs and the persistence of carbonate in water also motivate the investigation of the properties of RT-IL SAM in the presence of carbonate. Indeed, exposing aqueous solution to room air will result in non-negligible concentration of H2CO3 and related anions of approximately 0.01–0.02 mM. From a synthetic point of view, the stable HCO3 IL is an attractive intermediate structure for the Department of Chemistry, Universite´ de Montreál, C.P. 6128 Succ. Centre-Ville, Montreál, QC, Canada. E-mail: [email protected], [email protected]; Fax: +1-514-343-7586; Tel: +1-514-343-7342 w Electronic supplementary information (ESI) available: Experimental details; NMR, XPS, FTIR, and MS spectra. See DOI: 10.1039/ c1cc13914b

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synthesis of a plethora of ILs. The use of any acids stronger than H2CO3 may lead to the formation of the IL with the conjugate base of the acid.7–9 The by-products of the reaction of carbonate IL with acids are water and CO2, with gases being easily removed from the reaction mixture. Hence, this process of IL synthesis with different counter-anions could be halide- and metal-free. Current strategies for the synthesis of imidazolium-2-carboxylates are relatively complex. These include reaction of CO2 with a N-heterocyclic carbene (NHC) obtained from the acidic proton deprotonation at the C2 position of imidazolium10,11 and from the N-alkylation and C-carboxylation of 1-methylimidazole with dimethylcarbonate.12 Simple synthesis of carbonated IL on surfaces would enable these catalytic applications and facilitate the synthesis of IL-modified surfaces with tailored properties. Herein, we report that bicarbonate buffer reacts with MDMI salts in an SAM format, due to an unexpected anion metathesis of RT-IL SAM that relies on the exchange of more or less non-coordinating anions to bicarbonate. RT-ILs exhibit different physico-chemical properties depending on the nature of the anion. This dependence has been exemplified by the water solubility of IL with respect to the nature of the anion, decreasing from Br > BF4 > PF6 > NTf2 .13 For example, Au nanoparticles (NPs) modified with RT-IL bearing a chloride anion are water soluble. When the Cl is exchanged with PF6 , the Au NPs become water insoluble and are transferred to an IL phase.14 The change in solubility of anion-exchanged imidazolium-functionalized material allows the transition between phases of the corresponding material.13 These examples demonstrate the importance of anion exchange in the properties of RT-IL SAM.15 Due to the strong ionic force between the imidazolium ring and the anion,15–21 we initially anticipated that anion metathesis would not occur when mixing a bicarbonate solution and an imidazolium IL with hydrophobic counter-anions such as PF6 or NTf2 . To confirm that metathesis does not occur for IL in solution, we carried out an experiment where an aqueous solution of 1-(undecyl)-3-methylimidazolium NTf2 and a large excess of NaHCO3 were vigorously stirred at room temperature for seven days. As expected, the starting IL was fully recovered, and no trace of carbonate was detected by NMR analysis (ESIw, Fig. S1). To evaluate the influence of carbonate solutions on RT-IL SAM, MDMI-X (Scheme 1) monolayers were formed on a This journal is

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The Royal Society of Chemistry 2011

Scheme 1 Anion exchange reaction of MDMI-X (X: Br, BF4, PF6 and NTf2) with aqueous NaHCO3, where R is 12-mercapto-dodecane immobilized on Au.

smooth Au thin film. To verify the quality of the monolayers, we successfully replicated the contact angle experiment previously reported (Table 1).22 As a first observation, exposing the RT-IL SAMs to saline bicarbonate buffer (20 mM HCO3 at pH 7.4 in 150 mM NaCl, Scheme 1) drastically changed the contact angle to a consistent 431 21 for each starting anion (Table 1). The absence of a change in the contact angle of a 1-dodecanethiol control experiment indicated that the observed change in the contact angle of MDMI-X is a direct consequence of the anion exchange. In order to investigate the anion exchange at the surface of the RT-IL SAM, elemental analysis by XPS provided the chemical composition of the surface. XPS analysis of the monolayer following exposure to saline carbonate buffer did not show the presence of Cl , suggesting that the presence of bicarbonate is the dominant factor for the anion exchange. As fluorine atoms are present in three of the anions investigated, XPS analysis focused on the fluorine 1s orbital (using the Au 4f orbital as an internal standard). The XPS results were inconclusive for MDMI-Br; the bromide peak was observed as a weak shoulder of the Au 4f peak. No residual F was detected in MDMI-X following exposure to bicarbonate buffer (see Fig. S2–S4 and Table S1 provided in ESIw), indicating that the yield of exchange is >99% for BF4 , PF6 and NTf2 . The reversibility of the reaction was measured by exposing each surface to their respective initial salt solution. The yields were 97% for BF4 , 68% for PF6 and 87% for NTf2 . Whilst wettability and XPS provide strong evidence of the change in surface properties and in elemental composition, ATR-mid-IR spectroscopy affords molecular information on the RT-IL SAM. The vibrational bands of BF4 , PF6 and NTf2 were all observed in the mid-IR spectra (Fig. 1, and Fig. S5 and S6 provided in ESIw). The characteristic bands were identified for each counter-anion in the RT-IL SAM format: 1074 cm 1 (BF4 ), 863 cm 1 (PF6 ), 1207 cm 1 ( SO2 of NTf2 ) and 1353 cm 1 ( CF3 of NTf2 ). After exchange with the bicarbonate buffer, the spectra show that the characteristic bands for the counter-anions are greatly reduced Table 1

Properties of MDMI-X RT-IL SAM Contact angle (1)

Counter anion

X

Br BF4 PF6 NTf2

24 38 53 71

HCO3

1 3 2 3

43 42 44 44

1 2 2 1

Exchange yield (%) X to HCO3

HCO3 to X

NA >99 >99 >99

NA 97 68 87

NA: the yield cannot be determined by XPS or vibrational spectroscopy.

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Fig. 1 Mid-IR spectrum of MDMI-NTf2 IL-SAM before (gray) and after (black) HCO3 exchange.

in relative intensity and in some cases the bands are no longer observed. Mid-IR provides compelling evidence that the anions are exchanged at the surface of RT-IL SAM in the presence of saline bicarbonate buffer. However, there is no strong characteristic signature of the final product after exchange. To establish the nature of the RT-IL SAM following anion exchange in bicarbonate buffer, its chemical composition was analyzed by mass spectrometry (MS) employed in combination with MALDI to analyze the chemical content of RTIL-modified surfaces. The MDMI-Br monolayer was characterized by a single peak at m/z 283.8 in positive ionization mode, corresponding to MDMI+ and at m/z 79 and 81 in negative ionization mode, corresponding to the Br ion with the correct isotopic ratio (see Fig. S7 provided in ESIw). After bicarbonate buffer exposure, the characteristic MDMI+ peak at m/z 283.6 is still present in positive ionization mode, in addition to a signal at m/z 326.2. This ion corresponds to MDMI-carboxylate, without precise identification of the position of the carboxylate on the imidazolium ring. While the most acidic proton in position 2 is most likely to react and the synthesis of 2-carboxylate was previously reported, MS analysis cannot elucidate the position of the carboxylation and NMR analysis cannot be performed on the Au surface. In addition, isomerization between 2- and 4-carboxylate has been reported, although requiring high temperature,8 such that the position of carboxylation remains unknown. MS in the negative ionization mode confirms the absence of the Br peaks after anion exchange and the presence of the HCO3 ion at m/z 62.5. The presence of the carbonated and carboxylated MDMI at the surface of the Au film is a consequence of the kinetic equilibrium between MDMI-HCO3 and MDMI-carboxylate. The formation of the carboxylate may proceed by deprotonation of the imidazolium cation by bicarbonate to form the carbene, which reacts with the CO2 thereby formed to afford MDMI-carboxylate. This reaction has also been observed with methylimidazole and dimethylcarbonate.23 Partial anion exchange can occur even in the presence of diluted bicarbonate. MS analysis of MDMI-Br, which has been exposed to water, but not to bicarbonate buffer, shows the characteristic peak of MDMI+ and a trace amount of MDMI-carboxylate, with a peak area ratio heavily favouring MDMI+. Importantly, the peak area in MALDI-MS does not factor probability of ionization that may be different for both molecules, rendering the measurement qualitative. The presence of MDMI-carboxylate is definitely surprising, due to the low Chem. Commun., 2011, 47, 10644–10646

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Fig. 2 SERS spectra of MDMI-X RT-IL SAM immobilized on a 13 nm Au NP before (A) and after (B) HCO3 exchange. X: Br (purple), BF4 (blue), PF6 (black) and NTf2 (red).

concentration of carbonate in water exposed to room air. Henry’s law states that carbonate is present in water at a concentration of 10–20 mM, assuming a typical CO2 concentration of about 0.3–0.6% in a laboratory’s air. By the persistent presence of H2CO3 in water, even when no bicarbonate is intentionally added to water, anion exchange still occurred and led to the formation of MDMI-carboxylate during the hydration of the RT-IL SAM. This result is of utmost importance for longterm storage or exposure of RT-IL SAM to water. Based on these results, MDMI-2-carboxylate was synthesized by deprotonating the imidazolium ring and reacting the carbene with CO2, as previously described.24 MS demonstrated the presence of ions at m/z 283 and 326, characteristic of the carbonate/carboxylate equilibrium. The contact angle of the RT-IL SAM of MDMI-2-carboxylate is 541 31, somewhat different from the 431 observed via the anion exchange route. This is explained by the fact that the RT-IL SAM was intentionally not let to reach equilibrium in water, to demonstrate that the wettability will be different with respect to the proportion of MDMI-HCO3 and MDMI-2-carboxylate. RT-ILs have been reported to stabilize Au NPs.14 It was therefore important to verify if the anion metathesis also occurs on RT-IL SAM functionalized Au NPs. Surface enhanced Raman spectroscopy (SERS) was used to demonstrate that anion exchange also occurs on MDMI-X SAM. For Au NP modified with MDMI-X exposed to bicarbonate, a new vibrational band appeared at 2118 cm 1, assigned to the bicarbonate band (Fig. 2). This vibrational band was absent for the citrate Au NP not exposed to bicarbonate. The assignment was validated by measuring the SERS response of the citrate-capped Au NP exposed to a HCO3 buffer, which confirmed that the 2118 cm 1 band corresponds to HCO3 . Measuring the SERS response of MDMI-2-carboxylate on the Au NP was performed as an additional control. Following a short exposure to water, such that equilibrium is not reached, the Raman spectrum reveals bands at 2118 and 2241 cm 1. These vibrational bands are attributed to the bicarbonate and carboxylate respectively. This result clearly shows that anion exchange was also possible on metal NPs functionalized with MDMI-X, providing a synthetic approach to MDMI-carbonate on Au NPs. This result also raises caution that Au NPs stabilized with RT-IL should be carefully stored in order to maintain their counter-anion. Despite the impossibility to perform counter-anion metathesis for solutions of MDMI-X, anion metathesis to bicarbonate is 10646

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complete for MDMI-X SAM on thin Au films or NPs, regardless of the hydrophobicity or coordination properties of the anion. The exchange at the Au surface may be favoured by the orientation of MDMI-X, where X is located at the surface|solution interface. At this interface, the anion is readily available for the exchange reaction to proceed. In addition, a large excess of carbonate is present in solution (even from CO2 dissolution in water) in comparison with the number of MDMI immobilized to Au, which will thermodynamically favour anion exchange. Considering the practical and environmental benefits, the described study not only offers a new, general route for preparing non-coordinating, non-nucleophilic ILs, including functionalized ILs, but also helps understanding the physicochemical processes that are particular to IL-functionalized metal surfaces.

Notes and references 1 F. van Rantwijk, R. M. Lau and R. A. Sheldon, Trends Biotechnol., 2003, 21, 131–138. 2 S. N. Baker, E. B. Brauns, T. M. McCleskey, A. K. Burrell and G. A. Baker, Chem. Commun., 2006, (27), 2851–2853. 3 B. S. Lee, Y. S. Chi, J. K. Lee, I. S. Choi, C. E. Song, S. K. Namgoong and S. G. Lee, J. Am. Chem. Soc., 2004, 126, 480–481. 4 M. Branca, D. Correia-Ledo, O. R. Bolduc, M. Ratel, A. R. Schmitzer and J.-F. Masson, Phys. Chem. Chem. Phys., 2011, 13, 12015–12023. 5 S. Perkin, L. Crowhurst, H. Niedermeyer, T. Welton, A. M. Smith and N. N. Gosvami, Chem. Commun., 2011, 47, 6572–6574. 6 X. Y. Liu, X. D. Zeng, N. N. Mai, Y. Liu, B. Kong, Y. H. Li, W. Z. Wei and S. L. Luo, Biosens. Bioelectron., 2010, 25, 2675–2679. 7 N. J. Bridges, C. C. Hines, M. Smiglak and R. D. Rogers, Chem.–Eur. J., 2007, 13, 5207–5212. 8 M. Smiglak, J. D. Holbrey, S. T. Griffin, W. M. Reichert, R. P. Swatloski, A. R. Katritzky, H. F. Yang, D. Z. Zhang, K. Kirichenko and R. D. Rogers, Green Chem., 2007, 9, 90–98. 9 I. Tommasi and F. Sorrentino, Tetrahedron Lett., 2005, 46, 2141–2145. 10 I. Tommasi and F. Sorrentino, Tetrahedron Lett., 2006, 47, 6453–6456. 11 H. A. Duong, T. N. Tekavec, A. M. Arif and J. Louie, Chem. Commun., 2004, (1), 112–113. 12 J. D. Holbrey, W. M. Reichert, I. Tkatchenko, E. Bouajila, O. Walter, I. Tommasi and R. D. Rogers, Chem. Commun., 2003, (1), 28–29. 13 M. J. Park, J. K. Lee, B. S. Lee, Y. W. Lee, I. S. Choi and S. G. Lee, Chem. Mater., 2006, 18, 1546–1551. 14 H. Itoh, K. Naka and Y. Chujo, J. Am. Chem. Soc., 2004, 126, 3026–3027. 15 C. Chiappe and D. Pieraccini, J. Phys. Org. Chem., 2005, 18, 275–297. 16 I. Dinares, C. G. de Miguel, A. Ibanez, N. Mesquida and E. Alcalde, Green Chem., 2009, 11, 1507–1510. 17 J. J. Ramsden, Q. Rev. Biophys., 1994, 27, 41–105. 18 B. Liedberg, I. Lundstrom and E. Stenberg, Sens. Actuators, B, 1993, 11, 63–72. 19 G. P. Lopez, M. W. Albers, S. L. Schreiber, R. Carroll, E. Peralta and G. M. Whitesides, J. Am. Chem. Soc., 1993, 115, 5877–5878. 20 C. F. Mandenius, S. Welin, B. Danielsson, I. Lundstrom and K. Mosbach, Anal. Biochem., 1984, 137, 106–114. 21 M. D. Ward and D. A. Buttry, Science, 1990, 249, 1000–1007. 22 D. Teng, J. Ma, Y. Y. Huang, X. Zhang and R. Zheng, in Semiconductor Lasers and Applications III, ed. L. Chen, H. Suzuki, P. T. Rudy and N. Zhu, Spie-Int Soc Optical Engineering, Bellingham, 2008, p. P8240. 23 M. Mrksich and G. M. Whitesides, Annu. Rev. Biophys. Biomol. Struct., 1996, 25, 55–78. 24 H. Zhou, Y. M. Wang, W. Z. Zhang, J. P. Qu and X. B. Lu, Green Chem., 2011, 13, 644–650.

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