Selective Anion Sensing Based on Tetra-amide Calix[6]arene

Characterization of Self-Assembled Monolayers (SAMs) of 1. ... The anion sensing properties of SAMs of 1 were investigated by observing their blocking...
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Langmuir 2006, 22, 10732-10738

Selective Anion Sensing Based on Tetra-amide Calix[6]arene Derivatives in Solution and Immobilized on Gold Surfaces via Self-Assembled Monolayers† Sheng Zhang, Amit Palkar, and Luis Echegoyen* Department of Chemistry, Clemson UniVersity, Clemson, South Carolina 29634 ReceiVed April 27, 2006. In Final Form: June 15, 2006 Two anion receptors, 1 and 2, based on the calix[6]crown-4 architecture were synthesized and characterized by NMR (1H, 13C, COSY), UV-vis, and MALDI-MS. 1H NMR measurements demonstrate that receptors 1 and 2 exhibit the highest binding affinity for fluoride ions compared to other anions including Cl-, Br-, NO3-, HSO4-, H2PO4-, and AcO-. The binding constants of 1 with F- and AcO- are 326 ((32) and 238 ((23) M-1, whereas those of 2 with F- and AcO- are 222 ((25) and 176 ((21) M-1. The fluorescent titration of 2 with various anions such as Cl-, Br-, NO3-, HSO4-, and H2PO4- led to essentially no change in excimer emission and a slight enhancement of monomer emission. In contrast, a dramatic change was observed in the fluorescence spectra upon the addition of Fand AcO- to 2. Self-assembled monolayers (SAMs) of 1 were formed on gold surfaces and characterized by reductive desorption and other techniques. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy were used to monitor anion recognition by the SAM-modified gold electrodes. The gold electrodes modified by SAMs of 1, upon binding with the F- anion, exhibit a dramatic increase in charge-transfer resistance (Rct) values. This is due to the repulsion between the negatively charged electrode surfaces and the negatively charged Fe(CN)63-/4- redox probe in the electrolyte solution. In contrast, smaller increases in Rct values were observed in the cases of other monovalent anions investigated.

Introduction The use of supramolecular chemistry as a tool for cation and anion recognition was first reported in the late 1960s.1,2 This triggered a remarkably rapid progress in supramolecular cation recognition using a broad range of cyclic and acyclic multidentate receptors.3 In contrast, progress in the recognition of anions has been much slower. This is probably due to their higher solvation energy, larger ionic radii, more diffuse nature, wider variety of topologies, and greater pH sensitivity as compared to those of cations.4 Biological detection of anions has proven to be extremely difficult because it takes place mainly in aqueous systems where solvation is strong. However, over the past decade, the design of anion receptors and the study of anion sensing has gained enormous attention and has become an important area in supramolecular chemistry. This interest has been fueled by their important roles in biological processes, food chemistry, medicine, and environmental sciences.5,6 Many structurally sophisticated hosts have been developed, some of which exhibit impressive anion selectivity.7-10 †

Part of the Electrochemistry special issue. * Corresponding author. E-mail: [email protected].

(1) Pedersen, C. J. J. Am. Chem. Soc. 1967, 89, 7017-7036. (2) Shriver, D. F.; Ballas, M. J. J. Am. Chem. Soc. 1968, 90, 2431-2432. (3) (a) de Silva, A. P.; McCaughan, B.; McKinney, B. O. F.; Querol, M. Dalton Trans. 2003, 1902-1913. (b) Rurack, K.; Resch-Genger, U. Chem. Soc. ReV. 2002, 31, 116-127. (c) de Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Huxley, A. J. M.; McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem. ReV. 1997, 97, 1515-1566. (4) Beer, P. D.; Gale, P. A. Angew. Chem., Int. Ed. 2001, 40, 486-516. (5) (a) Schemidtchen, F. P.; Berger, M. Chem. ReV. 1997, 97, 1609-1646. (b) Sessler, J. L.; Davis, J. M. Acc. Chem. ReV. 2001, 34, 989-997. (c) Sancenon, F.; Martinez-Manez, R.; Miranda, M. A.; Segui, M. J.; Soto, J. Angew. Chem., Int. Ed. 2003, 42, 647-650. (6) Bianchi, A., Bowman-James, K., Eds. Garcia-Espana, E. Supramolecular Chemistry of Anions; Wiley-VCH: New York, 1997. (7) Kubik, S.; Reyheller, C.; Stuwe, S. J. Incl. Phenom. 2005, 52, 137-187. (8) (a) Beer, P. D. Acc. Chem. Res. 1998, 31, 71-80. (b) Beer, P. D.; Hayes, E. J. Coord. Chem. ReV. 2003, 240, 167-189. (9) (a) Gale, P. A. Coord. Chem. ReV. 2001, 213, 79-128. (b) Gale, P. A. Coord. Chem. ReV. 2000, 199, 181-223.

Most research is focused on sensing ionic species by molecular recognition in bulk solutions, especially in organic solutions. However, constructing self-assembled monolayers (SAM) of host molecules and allowing molecular recognition on the monolayer itself has been proven to be feasible and more effective.11 A well-ordered and densely packed monolayer provides a preorganized molecular sensing system. Immobilization of receptors onto electrode surfaces facilitates anion sensing in aqueous environments. In addition, it has been found that hydrogen bonding, which plays a very important role in anion recognition, is stronger at the monolayer/solution interface than in bulk solution.12 Furthermore, interfacial complexes are more stable because of the low dielectric constant in the monolayer phase.13 The read-out signal upon anion binding can thus be amplified to improve the sensitivity. Several applications of SAM-modified electrodes for cation recognition have been reported.14-16 Although anion recognition at the monolayer/solution interface is advantageous because of its speed and ease of detection, anion sensing based on SAMs of receptors remains largely unexplored.17 To develop new and simpler approaches, one of our objectives is to assemble easily available anion receptors that do not contain (10) (a) Best, M. D.; Tobey, S. L.; Anslyn, E. V. Coord. Chem. ReV. 2003, 240, 3-15. (b) Bondy, C. R.; Loeb, S. J. Coord. Chem. ReV. 2003, 240, 77-99. (c) Fitzmaurice, R. J.; Kyne, G. M.; Douheret, D.; Kilburn, J. D. J. Chem. Soc., Perkin Trans. 1 2002, 841-864. (11) Gooding, J. J.; Mearns, F.; Yang, W.; Liu, J. Q. Electroanalysis 2003, 15, 81-96. (12) Ariga, K.; Kunitake, T. Acc. Chem. Res. 1998, 31, 371-378. (13) Tamagawa, H.; Sakurai, M.; Inoue, Y.; Ariga, K.; Kunitake, T. J. Phys. Chem. B 1997, 101, 4817-4825. (14) (a) Turyan, I.; Mandler, D. Anal. Chem. 1994, 66, 58-63. (b) Turyan, I.; Mandler, D. Anal. Chem. 1997, 69, 894-897. (15) (a) Yang, W.; Gooding, J. J.; Hibbert, D. B. J. Electroanal. Chem. 2001, 516, 10-16. (b) Yang, W.; Gooding, J. J.; Hibbert, D. B. Analyst. 2001, 126, 1573-1577. (c) Yang, W.; Jaramillo, D.; Gooding, J. J.; Hibbert, D. B.; Zhang, R.; Willett, G. D.; Fisher, K. J. Chem. Commun. 2001, 1982-1983. (16) (a) Liu, H. Y.; Liu, S, G.; Echegoyen, L. Chem. Commun. 1999, 14931494. (b) Fujihara, H.; Nakai, H.; Yoshihara, M.; Maeshima, T. Chem. Commun. 1999, 737-738. (c) Chung, T. D.; Park, J.; Kim, J.; Kim, H.; Choi, M. J.; Kim, J. R.; Chang, S. K.; Kim, H. Anal. Chem. 2001, 73, 3975-3980.

10.1021/la061147s CCC: $33.50 © 2006 American Chemical Society Published on Web 08/01/2006

Anion Sensing Based on Calix[6]arene DeriVatiVes Scheme 1. Structure of Compounds 1 and 2

reporter groups directly on electrode surfaces. Electrochemical impedance spectroscopy can then be used to achieve the signal transduction of electrochemically inactive SAMs in the presence of anions. This has been proven to be an effective technique for detecting protein binding events.18 This approach has also been employed in the detection of redox-inactive metal cations as described originally by Reinhoudt et al.19 Along these lines, we have investigated the selective K+ and other cation binding events using SAMs of oligoethylene glycol and calix[n]arene derivatives.20,21 In these cases, the Ru(NH3)62+/3+ probe can penetrate the SAM and exhibit reasonably reversible redox activity in the absence of surface-bound cations. Once the cations are bound to the monolayer, the surfaces become positively charged and inhibit or block the approach of the positively charged redox probe, thus increasing the charge-transfer resistance. Very recently, we successfully introduced impedance spectroscopy to investigate anion sensing at the interface of SAM-modified electrodes with a cyclotriveratrylene (CTV) receptor in aqueous solutions.22 Here we report the synthesis of two calix[6]crown4-based anion receptors 1 and 2 (Scheme 1), their anion recognition properties, and the use of impedance spectroscopy to study anion sensing by SAMs of 1. Experimental Section General. p-tert-Butylcalix[6]arene, NaCl, NaBr, NaNO3, NaHSO4, NaH2PO4, CH3COONa, NaF, Bu4NCl, Bu4NBr, Bu4NNO3, Bu4NHSO4, Bu4NH2PO4, Bu4NAcO, Bu4NF, and O-(7-azabenzotriazoleyl)-N,N,N′,N′-tetramethyluronium hexafluorophosphate (HATU) were obtained commercially and used as received. p-tert-Butylcalix[6]arene-crown-4 (4) was prepared as described in the literature.23 Gold wire (99.999%) was obtained commercially. NMR spectra were recorded on a Bruker AC 300 or 500 or on a JEOL Eclipse+ 500 spectrometer. UV-vis spectra were recorded on a Shimadzu 2101PC spectrophotometer. Mass spectra were recorded with an (17) (a) Gobi, K. V.; Ohsaka, T. J. Electroanal. Chem. 2001, 485, 61-70. (b) Beer, P. D.; Davis, J. J.; Drillsma-Milgrom, D. A.; Szemes, F. Chem. Commun. 2002, 1716-1717. (c) Beer, P. D.; Cormode, D. P.; Davis, J. J. Chem. Commun. 2004, 414-415. (18) (a) Dijksma, M.; Kamp, B.; Hoogvliet, J. C.; van Bennekom, W. P. Anal. Chem. 2001, 73, 901-907. (b) Ruan, C.; Yang, L.; Li, Y. Anal. Chem. 2002, 75, 4814-4820. (19) Flink, S.; Boukamp, B. A.; Ban den Berg, A.; van Veggel, F. C. J. M.; Reinhoudt, D. N. J. Am. Chem. Soc. 1998, 1201, 4652-4657. (20) (a) Bankyopadhyay, K.; Liu, H. Y.; Liu, S. G.; Echegoyen, L. Chem. Commun. 2000, 141-142. (b) Bankyopadhyay, K.; Liu, S. G.; Liu, H. Y.; Echegoyen, L. Chem.sEur. J. 2000, 6, 4385-4392. (21) (a) Zhang, S.; Echegoyen, L. Tetrahedron Lett. 2003, 44, 9079-9082. (b) Zhang, S.; Song, F.; Echegoyen, L. Eur. J. Org. Chem. 2004, 2936-2943. (22) Zhang, S.; Echegoyen, L. J. Am. Chem. Soc. 2005, 127, 2006-2011. (23) Chen, Y. Y.; Yang, F. F.; Gong, S. L. Tetrahedron Lett. 2000, 41, 4815.

Langmuir, Vol. 22, No. 25, 2006 10733 Omni Flex MALDI-TOF spectrometer. Fluorescence spectroscopy was performed using a custom-built instrument obtained from PTI (Photon Technology International). Elemental analyses were performed using a Carlo Erba EA 1106. Deionized water was prepared with a Nanopure infinity ultrapure water system. Monolayer Preparation. Gold beads were prepared as reported before by heating the gold wire in a natural gas/O2 flame and cleaned by electrolysis in 0.1 M HClO4.21,22 Then the gold beads were washed with copious amounts of water and acetonitrile. Monolayers on gold were prepared by immersing freshly prepared gold beads in 1 mM solutions of compound 1 in CH2Cl2 for 48 h. The monolayer-modified gold beads were thoroughly washed with the appropriate solvent and dried in a stream of argon before characterization and anionsensing experiments. Electrochemical Measurements. All electrochemical measurements were performed with a CHI-660 electrochemical analyzer with a three-electrode configuration, and the solutions were deoxygenated by purging with Ar. All potentials were measured versus a Ag/AgCl electrode; a platinum wire was used as the auxiliary electrode, and a monolayer-modified gold electrode was used as the working electrode. The electrochemical desorption experiments were conducted in a 0.5 M KOH solution purged with Ar for 30 min. Impedance measurements were performed in a 0.1 M NaPF6 solution containing equal concentrations of oxidized and reduced forms of the Fe(CN)63-/4- redox couple. The frequency range used was 1 kHz to 0.1 Hz with an ac amplitude of 5 mV. The formal redox potentials were determined by cyclic voltammetry. Impedance data were analyzed by the program Equivalent Circuit.24 Synthesis of 3. p-tert-Butylcalix[6]arene tetramethylester 4 (0.59 g, 0.43 mmol) was added to 60 mL of ethylenediamine, and the mixture was refluxed under Ar for 4 days. After cooling to room temperature, the resulting mixture was poured into 300 mL of ice water and stored at 4 °C overnight. The precipitate was collected by suction filtration, and recrystallization from hot ethanol afforded 0.54 g of white solid 3 (85%). 1H NMR (300 MHz, CDCl3): δ 7.94 (s, 4 H), 7.34 (s, 4 H), 6.99 (s, 4 H), 6.71 (s, 4 H), 4.56-4.61 (d, J ) 15 Hz, 4 H), 4.39-4.50 (m, 8 H), 4.25-4.30 (d, J ) 15 Hz, 4 H), 2.90-3.57 (m, 36 H), 1.94 (s, broad, 8 H), 1.43 (s, 18 H), 0.96 (s, 36 H); 13C NMR (75 MHz, CDCl3): δ 169.13, 153.93, 150.80, 146.87, 146.58, 133.11, 132.51, 131.63, 128.28, 125.45, 125.10, 72.57, 69.89, 69.50, 42.22, 41.48, 34.34, 34.10, 31.62, 31.37; MS (MALDI): m/z ) 1487 [M+ + H]. Synthesis of 1. Thioctic acid (0.11 g, 0.54 mmol) was dissolved in 5 mL of anhydrous DMF and cooled to 0 °C under argon. Triethylamine (0.12 g, 1.19 mmol) and HATU (0.21 g, 0.55 mmol) were added. The mixture was allowed to stir at room temperature for 10 min and cooled to 0 °C again. Compound 3 (0.10 g, 0.07 mmol) dissolved in 4 mL of anhydrous DMF was added, and the mixture was stirred for another 1 h at 0 °C. The cooling bath was then removed, and the solution was stirred for 48 h at room temperature. The solvent was removed, and the residue was subjected to column chromatography (SiO2, 5% MeOH/CH2Cl2) to give a white solid, 1 (110 mg, 74%). 1H NMR (500 MHz, CDCl3): δ 7.84 (s, 4 H), 7.18 (s, 4 H), 7.00 (s, 4 H), 6.81 (s, 4 H), 6.50 (s, 4 H), 4.34-4.37 (d, J ) 14 Hz, 2 H), 4.27-4.30 (d, J ) 14 Hz, 2 H), 4.19-4.22 (d, J ) 15 Hz, 2 H), 4.09-4.12 (d, J ) 15 Hz, 2 H), 3.23-3.40 (m, 40 H), 2.91-3.03 (m, 12 H), 2.31-2.45 (m, 4 H), 1.95-1.98 (t, J ) 7.5 Hz, 8 H), 1.69-1.75 (m, 4 H), 1.30-1.53 (m, 24 H), 1.25 (s, 18 H), 0.71 (s, 36 H); 13C NMR (125 MHz, CDCl3): δ 173.93, 169.76, 153.99, 150.76, 146.95, 146.55, 133.02, 132.33, 131.71, 128.42, 125.57, 124.99, 72.50, 70.18, 69.46, 56.49, 56.39, 40.28, 39.46, 39.35, 38.49, 36.10, 34.63, 34.35, 34.09, 31.65, 31.14, 28.92, 25.38; MS (MALDI): m/z ) 2241 [M+]; elemental analysis calcd (%) for C120H174O16N8S8: C, 64.31; H, 7.83; found: C, 63.76; H, 8.04. Synthesis of 2. Compound 2 was prepared using the same procedure as the one used for 1 by treating pyrenebutyric acid (0.16 g, 0.55 mmol) and compound 3 (0.10 g, 0.07 mmol) with triethylamine (24) Flink, S.; Van Veggel, F. C. J. M.; Reinhoudt, D. N. J. Phys. Chem. B 1999, 103, 6515-6520.

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Zhang et al. Scheme 2. Synthesis of Compound 1

Scheme 3. Synthesis of Compound 2

(0.12 g, 1.19 mmol) and HATU (0.21 g, 0.55 mmol) in 6 mL of anhydrous DMF and purified by column chromatography (SiO2, 3% MeOH/CH2Cl2) to give a white solid, 2 (135 mg, 78%). 1H NMR (500 MHz, CDCl3): δ 7.94-8.01 (m, 16 H), 7.93 (s, 4 H), 7.777.89 (m, 16 H), 7.52-7.54 (d, J ) 8 Hz, 4 H), 7.17-7.18 (d, J ) 8 Hz, 4 H), 6.91 (s, 4 H), 6.79 (s, 4 H), 6.53 (s, 4 H), 4.33-4.36 (d, J ) 14.5 Hz, 2 H), 4.26-4.29 (d, J ) 14.5 Hz, 2 H), 4.22-4.25 (d, J ) 15 Hz, 2 H), 4.11-4.14 (d, J ) 15 Hz, 2 H), 3.28-3.38 (m, 30 H), 3.00-3.03 (t, J ) 7.5 Hz, 8 H), 2.86 (s, broad, 8 H), 2.022.05 (t, J ) 7.5 Hz, 8 H), 1.88-1.91 (m, 8 H), 1.30 (s, 18 H), 0.75 (s, 36 H); 13C NMR (125 MHz, CDCl3): δ 173.80, 169.71, 154.04, 150.74, 146.87, 146.48, 135.78, 133.03, 132.36, 131.69, 131.29, 130.78, 129.77, 128.57, 128.34, 127.39, 127.25, 127.07, 126.58, 125.76, 125.51, 124.93, 124.82, 124.71, 123.24, 72.52, 70.22, 69.34, 39.49, 39.36, 35.74, 34.29, 34.08, 32.66, 31.59, 31.14, 29.76, 27.26; MS (MALDI): m/z ) 2569 [M+]; elemental analysis calcd (%) for C168H182O16N8: C, 78.54; H, 7.14; found: C, 78.16; H, 7.48.

Results and Discussions Synthesis of Calix[6]crown-4 Derivatives 1 and 2. Compounds 1 and 2 were prepared according to Schemes 2 and 3. Calix[6]crown-4 tetra-amide compound 3 was obtained by reacting calix[6]crown-4 derivative 4 with ethylenediamine.25 Coupling of compound 3 with thioctic acid or 1-pyrenebutyric acid in the presence of HATU and triethylamine afforded compounds 1 and 2 in 74 and 78% yields, respectively.26,27 (25) Veroit, G.; Dutasta, J.-P.; Matouzenko, G.; Collet, A. Tetrahedron 1995, 51, 389-400. (26) Cardon, C. M.; Jannach, S. H.; Huang, H.; Itojima, Y.; Leblanc, R. M.; Gawley, R. E.; Baker, G. A.; Brauns, E. B. HelV. Chim. Acta 2002, 85, 35323558. (27) Carpino, L. A. J. Am. Chem. Soc. 1993, 115, 4397-4398.

Compounds 1 and 2 were characterized by 1H, 13C, COSY NMR, UV-vis, and MALDI mass spectroscopy. The 1H NMR spectra of 1 and 2 show two singlets with an intensity of 2:1 for the tert-butyl groups, doublets for the methylene bridge protons, and three single peaks for the protons on the benzene rings indicative of the 1,4-bridged cone conformation of the compounds. 1H NMR Titration of Compound 1 with Different Anions. The anion binding ability of receptor 1 was investigated initially by the addition of monovalent anions Cl-, Br-, NO3-, HSO4-, H2PO4-, AcO-, and F- as their tetrabutylammonium salts to solutions of 1 in CDCl3. Two different NH signals were observed at 7.00 and 7.84 ppm, which were monitored upon anion binding. The NH signals did not shift upon addition of Br-, NO3-, and HSO4-, indicating that compound 1 did not form complexes with these anions, as shown in Figure 1. Upon addition of F-, significant downfield shifts were observed as a result of the hydrogen bonding interactions. This is not surprising because the small size and high charge density of F- make it a strong hydrogen bond acceptor. Compared to the addition of F-, smaller shifts of the NH signals were found in the cases of Cl-, H2PO4-, and AcO-. Single peaks at 7.03 and 7.84 ppm shift downfield to 7.84 and 8.48 ppm in the presence of 3.4 equiv of F(Supporting Information, S1). The titration of 1 with AcO- results in downfield shifts of the NH signals to 8.05 and 8.68 ppm in the presence of 24 equiv of AcO-. The signals for other protons in compound 1 exhibit essentially no shift upon AcO- or Faddition except for the signals for the methylene bridge protons. Some of the doublets at 4.2-4.6 ppm are overlapped and shifted upon the addition of F-, indicating that the conformation of compound 1 is somehow affected upon anion complexation

Anion Sensing Based on Calix[6]arene DeriVatiVes

Langmuir, Vol. 22, No. 25, 2006 10735

Figure 1. 1H NMR spectra (500 MHz, 298 K) of compound 1 in CDCl3 at room temperature in the absence and presence of 2 equiv of different anions.

(Supporting Information, S1). Obviously, compound 1 binds more strongly with F- than with AcO-. In addition, when AcO- or F- was added, the signals of the amide protons were broadened and the peak intensities decreased and eventually disappeared if a large excess of anions was added. This observation indicates the formation of strong hydrogen bonds between the amide NH and the anions. Binding constants of compound 1 with AcO-and F- are 238 ((23) and 326 ((32) M-1, respectively, calculated using the procedure previously described.28 1H NMR Titration of Compound 2 with Different Anions. Variations of the 1H NMR spectrum of 2 in CDCl3 in the presence of different anions were also followed. The NH signals were not affected upon addition of NO3-, slightly shifted in the presence of Cl-, Br-, H2PO4-, and HSO4-, but changed significantly upon additions of F- and AcO-. Compound 2 was systematically titrated with F- and AcO-. The broad amide NH signal at 7.92 ppm shifts downfield to 8.61 ppm, and that at 6.91 ppm shifts downfield but is overlapped with other signals in the presence of 4.3 equiv of F- (Supporting Information, S2). These NH signals shift downfield to 8.22 and 8.81 ppm upon addition of 17.8 equiv of AcO- (Supporting Information, S3). Again, the NH signals were broadened and the peak intensity decreased with increasing anion concentration. As in titration of 1, some of the doublets for the bridged methylene protons became overlapped upon addition of AcO-and F-, suggesting a conformational change induced by anion complexation. Fitting of the experimental data gave binding constants of 222 ((25) and 176 ((21) M-1 for complexes of 2 with AcO-and F-, respectively (Figure 2). Fluorescence Sensing of Anions by Compound 2. Further insight and quantitative evaluation of the anion binding ability of 2 was derived from fluorescence quenching experiments. The titration experiment was performed in CHCl3 using Cl-, Br-, NO3-, HSO4-, H2PO4-, AcO-, or F- as the tetrabutylammonium salt. With excitation at 350 nm, compound 2 exhibits monomer (28) Hirose, K. J. Inclusion Phenom. Macrocyclic Chem. 2001, 39, 193-209.

Figure 2. Titration results corresponding to the chemical shift of amide protons of 2 when titrated against F-. The solid line indicates the curve obtained by nonlinear fitting, which yields the K value of 222 ((25).

and excimer emission at 380 and 483 nm, respectively. As shown in Figure 3, addition of F- to 2 leads to a drastic change in the fluorescence spectrum, with both monomer and excimer emission decreases. The fluorescence quenching is attributed to the fact that the complexation of F- induces a conformational change of all amide groups, giving rise to quenched excimer emission. It is worth mentioning that the quenching efficiency reached a limiting value when up to 40 equiv of F- was added (Supporting Information, S4). Similarly, quenching was also observed when the acetate anion was added to a solution of 2 (Supporting Information, S5). On the basis of the change in fluorescence intensity upon the stepwise addition of anions, the binding constants of 2 with F- and AcO- were calculated to be 273

10736 Langmuir, Vol. 22, No. 25, 2006

Figure 3. Fluorescence spectra (excited at 346 nm) of compound 2 in CHCl3 at room temperature in the absence and presence of 20 equiv of different anions.

Figure 4. Electrochemical desorption of SAMs of 1 on Au in 0.5 M KOH at scan rate of 0.1 V/s.

((78) and 193 ((63) M-1, respectively, using nonlinear leastsquares curve fitting. In contrast, the fluorescent titration of 2 with Cl-, Br-, NO3-, HSO4-, and H2PO4- results in essentially no change in excimer emission and a slight enhancement of monomer emission. This indicates that even though these anions interact with the amide groups the interaction is not as strong as in the case of F- and AcO-. Characterization of Self-Assembled Monolayers (SAMs) of 1. SAMs of 1 were obtained by dipping the gold beads, prepared as reported previously from gold wire of 99.999% purity, into a CH2Cl2 solution of 1 for 2 days.29 They were then rinsed with copious amounts of CH2Cl2 and dried under a stream of Ar. The SAM-modified gold electrodes were characterized and used for anion sensing. Electrochemical desorption experiments provided evidence for SAM formation. The SAM-modified gold electrodes were immersed into a 0.5 M KOH solution that had been thoroughly purged with high-purity Ar for 30 min. The scans were initiated at a potential of 0 V and swept cathodically to a potential of -1.35 V at a scan rate of 0.1 V/s. The cyclic voltammogram in Figure 4 was recorded for a gold electrode covered with a monolayer of 1 and showed a desorption wave at -1.0 V versus Ag/AgCl, which is due to the reductive (29) (a) Zhang, S.; Echegoyen, L. Org. Lett. 2004, 6, 791. (b) Zhang, S.; Palkar, A.; Fragoso, A.; Prados, P.; de Mendoza, J.; Echegoyen, L. Chem. Mater. 2005, 17, 2063-2068.

Zhang et al.

Figure 5. CVs of Fe(CN)63-/4- at bare gold (solid line) and SAMmodified gold electrodes with 1 in the absence (dashed line) and presence of 60 mM F- at a scan rate of 0.1 V/s.

desorption of the thiolates attached to the surface. The shape and position of the desorption peaks are similar to those reported before for SAMs of thioctic acid.30 By integrating the current under the cathodic wave, an estimated surface coverage of 1.33 × 10-10 mol/cm2 was obtained for SAMs of 1. Anion Sensing Properties of the Monolayer of 1 Using Cyclic Voltammetry (CV). The anion sensing properties of SAMs of 1 were investigated by observing their blocking effect on the cyclic voltammetric response of the Fe(CN)63-/4- redox couple. As compared to the reversible redox behavior observed on a bare gold electrode, SAM-modified gold electrodes with 1 exhibit an obvious attenuation of the redox current (dashed line in Figure 5), suggesting that Fe(CN)63-/4- is partially blocked by the presence of the monolayer. The dotted line in Figure 5 shows the CV response of the Fe(CN)63-/4- redox couple at a SAM-modified electrode in the presence of 60 mM F-. Addition of fluoride to the electrolyte solution results in a dramatic change in the faradaic current. Both cathodic and anodic currents are drastically reduced. This observation demonstrates that compound 1 can bind F- strongly and thus repel the approach of the negatively charged redox probe. The addition of AcOto the electrolyte solution also results in a decrease in the redox currents, but to a smaller extent compared to the addition of F-. However, only very small variations in redox currents were observed upon addition of Cl-, Br-, NO3-, HSO4-, and H2PO4to the electrolyte solution. These results indicate that receptor 1 has a high selectivity for F- over the other anions. As control experiments, the CV responses of the redox probe at a bare gold electrode or an electrode modified with SAMs of octanethiol in the presence of F- are exactly the same as those in the absence of F-. Anion Sensing of the Monolayer of 1 Using Impedance Spectroscopy. The anion recognition properties of SAM-modified gold electrodes were further monitored by impedance spectroscopy because it can give more information about the monolayer/ solution interface. The complex impedance response of the redox probe at the monolayer-modified gold electrodes is a semicircle (Figure 6). A simple Randles equivalent circuit to describe the impedance consists of charge-transfer resistance (Rct) in series with Warburg impedance in parallel with total interfacial capacitance.31 For monolayer-modified gold electrodes, Rct (30) (a) Dong, Y. Z.; Abaci, S.; Shannon, C. Langmuir 2003, 19, 8922-8926. (b) Wang, Y.; Kaifer, A. E. J. Phys. Chem. B 1998, 102, 9922-9927. (31) Casnati, A.; Pochini, A.; Ungaro, R.; Ugozzoli, F.; Arnaud, F.; Fanni, S.; Schwing, M.-J.; Egberink, R. J. M.; de Jong, F.; Reindoudt, D. N. J. Am. Chem. Soc. 1995, 117, 2767.

Anion Sensing Based on Calix[6]arene DeriVatiVes

Figure 6. Impedance response of Fe(CN)63-/4- at the electrode modified by SAMs of 1 in the absence and presence of increasing [F-].

Figure 7. Impedance response of Fe(CN)63-/4- at the electrode modified by SAMs of 1 in the absence and presence of increasing [H2PO4-].

contains both the resistance across the monolayer and the resistance to heterogeneous electron transfer. After fitting the experimental data via the commercially available program Equivalent Circuit, a charge-transfer resistance value of 52.5 kΩ was obtained for SAMs of 1. The charge-transfer resistance is due to the blocking of electron transfer by the monolayer. In contrast, the Nyquist plot obtained on bare gold electrodes shows a simple straight line (Warburg impedance), indicating that the redox reactions are very fast on bare gold surfaces. Figure 6 illustrates the complex impedance response of 1 mM Fe(CN)63-/4- at a SAM-modified gold electrode in the absence and presence of increasing amounts of F-. It can be clearly observed that the charge-transfer resistance (semicircle diameter) increased from 52.5 kΩ in the absence of F- to a limiting value of 156.6 kΩ in the presence of 60 mM of F-. The increase in the Rct value indicates that the complexation of F - at the interface results in the increase in negative surface charge, which repels the approach of Fe(CN)63-/4-. The limiting value reached at [F-] ) 45 mM reveals that surface complexation sites are saturated. The binding affinity of the same monolayer toward other anions is much weaker. As an example, Figure 7 shows the complex impedance response of the redox couple at a monolayer-modified gold electrode in the presence of increasing concentrations of H2PO4-. The Rct value increased to 80.4 kΩ upon addition of 60 mM H2PO4- from 52.5 kΩ in the absence of this anion.

Langmuir, Vol. 22, No. 25, 2006 10737

Figure 8. Impedance response of Fe(CN)63-/4- at the monolayermodified gold electrodes in the absence and presence of different 60 mM anions.

Figure 9. Variation of Rct obtained at the gold electrode modified with the SAM of 1 as a function of [F-].

In Figure 8, impedance responses for a SAM-modified gold electrode in the presence of 60 mM Cl-, Br-, NO3-, HSO4-, H2PO4-, AcO-, and F- are shown. Apparently, impedance responses of the monolayer for different anions are quite different. The addition of 60 mM Cl-, Br-, NO3-, and HSO4- gave rise to small changes in Rct values (∆Rct ) 10-16 kΩ), indicating that these anions are not bound effectively by the receptor immobilized on the gold surfaces. The addition of 60 mM H2PO4and AcO- increased Rct values to 80.4 and 114.8 kΩ, respectively. However, the addition of the same concentration of F- to the electrolyte solution resulted in the largest increase in the Rct value. Figure 13 shows the variation of change in Rct with [F-]. At low concentrations, the plot is approximately linear but exhibits saturation as the concentration increases.

Conclusions Two calix[6]crown-4 derivatives with thioctic ester or pyrene groups have been shown to interact with anions via hydrogen bonding. Both receptors bind F- preferentially over other anions such as Cl-, Br-, NO3-, HSO4-, H2PO4-, and AcO-, as indicated by the 1H NMR study. Results obtained from the 1H NMR study in solutions and from surface binding investigations demonstrate that receptor 1 exhibits a higher affinity for F- than for other anions investigated. These observations were further confirmed by impedance spectroscopy of SAM-modified gold electrodes. Assembling the anion receptor on solid substrates provided an

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effective method for anion sensing in aqueous systems. The use of impedance spectroscopy for anion recognition simplified the synthesis of the anion receptors described here because it eliminated the need to attach reporter groups such as fluorophores or redox-active moieties to the receptor system. This concept could be important for biosensing because biochemical anion binding processes occur in aqueous media. One can expect that using such techniques will eventually facilitate the design of anion receptors.

Zhang et al.

Acknowledgment. Financial support from the National Science Foundation (grant no. CHE-0509989) is greatly appreciated. Supporting Information Available: 1H NMR titration of compounds 1 and 2 with F- and compound 2 with AcO- and fluorescence titration of compound 2 with F- and AcO-. This material is available free of charge via the Internet at http://pubs.acs.org. LA061147S