Calcium Ion−Calixquinone Complexes Adsorbed on a Silver Electrode

Oct 26, 2009 - According to the RAIRS results, the Ca2+ ions in the complexes lie in vicinity to the quinone moieties. Monte Carlo simulation provides...
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J. Phys. Chem. C 2009, 113, 19981–19985

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Calcium Ion-Calixquinone Complexes Adsorbed on a Silver Electrode Sun Kil Kang,†,§ One-Sun Lee,†,⊥ Suk-Kyu Chang,‡ Doo Soo Chung,† Hasuck Kim,† and Taek Dong Chung*,† Department of Chemistry, Seoul National UniVersity, Seoul 151-747, Korea, and Department of Chemistry, Chung-Ang UniVersity, Seoul 156-756, Korea ReceiVed: July 16, 2009; ReVised Manuscript ReceiVed: October 1, 2009

The structures and redox-switching behavior of Ca2+/calix[4]quinone complexes on a Ag surface were investigated by reflection-absorption infrared spectroscopy (RAIRS), Monte Carlo simulation, and voltammetry. RAIRS and Monte Carlo simulation show that the carboxylate groups of calix[4]arene-triacidmonoquinone (CTAQ) and calix[4]arene-diacid-diquinone (CDAQ) are involved in not only binding with the Ca2+ ions but also spontaneous adsorption of their complexes onto the Ag surface. According to the RAIRS results, the Ca2+ ions in the complexes lie in vicinity to the quinone moieties. Monte Carlo simulation provides more detailed information about the probable conformations of the Ca2+/calix[4]quinone complexes adsorbed on the Ag surface. Voltammetric behavior is consistent with the RAIRS and Monte Carlo simulation results and shows the effect of Ca2+ ions to the redox processes of the CTAQ- or CDAQ-modified Ag electrodes. I. Introduction Concerning the development of improved electrochemical sensors based on self-assembled monolayers, huge attention has been paid to the functionalization of metal surfaces.1-4 A variety of self-assembled synthetic receptors were immobilized onto the electrode surface to create functional transducers equipped with selective recognition ability.1,5 Calixarene derivatives have been regarded as promising supramolecules for this purpose and, thereby, intensively investigated.6-11 The first example of the self-assembled monolayers of calixarenes for the redox-dependent ionic recognition was the calixquinone-modified Au electrode that took advantage of the redox property of the quinone moiety in the annular structure.12 In this work, thiol groups were incorporated in the calixquinone for anchoring through spontaneous chemisorption onto the Au surface. It was followed by the report on voltammetric behavior showing the interaction of free Ca2+ ions in an aqueous solution with the calixquinones that were spontaneously adsorbed on a Ag surface.13 This study employed two calixquinones, which were calix[4]arene-triacidmonoquinone (CTAQ) and calix[4]arene-diacid-diquinone (CDAQ) containing no thiolate group but carboxylate groups. The lower rims of CTAQ and CDAQ in Figure 1 have three and two carboxylic acids, respectively.14 These two calixquinones are water-soluble, adsorptive onto Ag surfaces, and likely to associate with free Ca2+ ions selectively. Therefore, CTAQ and CDAQ are very fascinating electroactive receptors for advanced voltammetric solid-state Ca2+ ion sensors and also informative molecules that can provide valuable insight into how to form a functional monolayer on a solid electrode and how to utilize such systems to recognize a specific metal ion. However, there have been few reports on the structural understanding of * To whom correspondence should be addressed. E-mail: tdchung@ snu.ac.kr. Phone: +82-2-880-4362. Fax: +82-2-887-4354. † Seoul National University. ‡ Chung-Ang University. § Current address: LG Electronics, Korea. ⊥ Current address: Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208-3113.

Figure 1. Molecular structures of calix[4]arene-triacid-monoquinone (CTAQ) and the calix[4]arene-diacid-diquinone (CDAQ).

the Ca2+ ion recognition by the adsorbed CTAQ and CDAQ monolayers on the Ag surface. Because the carboxylate groups are predicted to make the calixquinones anchor onto a Ag surface and also to assist association with Ca2+ ions, the conformation of the complexes on the Ag surface is the important knowledge to design better receptors and modified electrode systems for future advances toward selective and sensitive miniaturized voltammetric ion sensors. In this work, the most probable model is proposed and discussed for the calixquinone-ion complex adsorbed on the Ag surface. The molecular interaction of Ca2+ ions with CTAQ or CDAQ on the Ag electrode surface is examined by reflection-absorption infrared spectroscopy (RAIRS). In addition to the RAIRS, the Monte Carlo (MC) conformation search method allows us to investigate the conformational change of

10.1021/jp9067262 CCC: $40.75  2009 American Chemical Society Published on Web 10/26/2009

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CTAQ and CDAQ. On the basis of the results of RAIRS and MC studies, we suggest a plausible model for the Ca2+/ calixquinone complexes on the Ag electrode surface. The validity of the proposed model is confirmed by the electrochemical characterization of the Ca2+/calixquinone complexes for the fascinating applications of voltammetric Ca2+ ion sensors. 2. Experiments 2.1. Reagents. Water-soluble and redox-active CTAQ and CDAQ were synthesized by following the procedure in the literature.12,13 The reagents for the synthesis of CTAQ and CDAQ, including ethyl bromoacetate, p-tert-butylcalix[4]arene, tert-butyl bromoacetate, trifluoroacetic acid, thallium trinitrate (Aldrich), and p-H-calix[4]arene (Janssen), were purchased and used without further purification. Column chromatography was performed with silica gel 60 (230-400 mesh, ASTM, Merck). Anthraquinone-1,5-disulfonic acid disodium salt was obtained from Tokyo Chemical Industry (TCI, Japan). Absolute ethanol (99.9%) and calcium nitrate were purchased from Hayman and Junsei Chemicals (Tokyo, Japan), respectively. 2.2. Electrochemistry. Electrochemical experiments were performed with a Windows-driven electrochemical analyzer (BAS100B/W, Bioanalytical Systems, West Lafayette, IN) using positive feedback routines to compensate for resistance. The Ag electrodes were prepared by evaporating silver at 10-5-10-6 Torr in a thermal resistive evaporator on the batches of previously sonicated 2.5 × 3.7 cm2 glass slides. In aqueous media, an Ag|AgCl (in KCl 3 M) reference electrode was used for voltammetric experiments, which were carried out in 0.05 M 4-(2-hydroxyethyl)-piperazine-1-ethanesulfonic acid (HEPES) buffer at pH 7.4. The electrochemistry of CTAQ is not only affected by pH but also sensitive to the presence of metal ions.12-14 Therefore, it was necessary to use a buffer solution containing no metal ion to examine the effect of Ca2+ ions. In this study, HEPES buffer solution containing no metal ion was employed and its pH was adjusted to 7.4 by adding tetraethylammonium hydroxide (TEAOH) to maintain the solution pH close to the physiological condition. All experiments were conducted in nitrogen atmosphere at room temperature. The surface roughness factor of the Ag substrate was determined by the underpotential deposition (UPD) of Pb2+ ions.15 The surface area of the Ag substrate was calculated to be 1.28 cm2 from the charge passed during the deposition that was estimated from the area under a linear sweep wave in the potential region of UPD. This result revealed ca. 1.24 of the surface roughness factor, which is largely consistent with the value of 1.1 measured by the images of atomic force microscopy. 2.3. Reflection-Absorption Infrared Spectroscopy (RAIRS). To record the RAIR spectra of CTAQ and CDAQ adsorbed on a Ag substrate, a specular reflection attachment (Harrick VRA, Harrick Scientific Co., NY) was used in conjunction with a Harrick wire grid polarizer. The incident angle for the p-polarized light was at 80°. Each spectrum was obtained by averaging 2048 interferograms at 4 cm-1 resolution. The polarization modulation method was employed for all spectra of RAIRS. The Ag substrates for RAIRS measurements were the same as those prepared for electrochemical experiments. The free CTAQ- and CDAQ-modified films were formed by immersing a Ag substrate into the 10-4 M CTAQ and CDAQ in ethanol for more than 12 h, respectively. The films of Ca2+/ calixquinone complexes on the Ag surface were prepared by immersing a Ag substrate into CTAQ or CDAQ solution containing 10 mM Ca(NO3)2 overnight. 2.4. Monte Carlo (MC) Simulation. The AMBER* parameters implemented in the Macromodel 6.016 were used for the

Figure 2. (a) Cyclic voltammograms of (1) CTAQ- and (2) CDAQmodified Ag electrodes. The CTAQ film was prepared by immersing a Ag electrode into 0.1 mM CTAQ in EtOH solution for 12 h and drying for subsequent experiments in aqueous media. The voltammograms were obtained in a HEPES buffer at pH 7.4. The scan rate is 100 mV s-1. (b) Cyclic voltammograms of the anthraquinone-1,5disulfonic acid at 1 bare Ag and 2 CTAQ-film modified Ag in a HEPES buffer at pH 7.4.

MC conformational search and molecular mechanics calculations of CTAQ and CDAQ to obtain the minimum energy structures. The dielectric constant of 25.3 is used for ethanol solvent,17 and the number of structures tried in the conformational search was 1,000. Energy minimization based on the conjugate gradient algorithm was then performed on every sampled structure to a gradient norm of less than 0.001 kJ mol-1 Å-1. 3. Results and Discussion 3.1. Electrochemistry of the Calixquinones Adsorbed on Ag. The electrochemical properties of CTAQ and CDAQ adsorbed on a Ag electrode were examined. At first, the calixquinones were deposited onto the Ag surface by just immersing an Ag electrode into the 0.1 mM ethanol solution. The Ag electrode was then dried and carefully rinsed before voltammetry in the aqueous HEPES solution. Figure 2a shows the cyclic voltammograms of the adsorbed CTAQ and CDAQ on the Ag electrodes in an aqueous HEPES solution. The CTAQ- and CDAQ-modified Ag electrodes produce apparently irreversible voltammetric behavior of two electron transfer followed by two proton transfer for each quinone moiety, which is understood on the basis of the electrochemistry of quinone-

Ca2+-Calixquinone Complexes Adsorbed on a Ag Electrode derivatized compounds.18 Reportedly, a very fast protonation process follows electrochemical reduction of the quinone moieties in the well-buffered medium at a neutral pH so that the observed voltammetric behavior appears to be irreversible.19 The area under the reduction wave in the cyclic voltammogram gives the charge passed during the reduction. The number densities of CTAQ and CDAQ adsorbed on the surface are 1.2 × 10-10 and 8.5 × 10-11 mol cm-2, assuming the reduction processes of two and four electrons, respectively. Quinone in the annular system of a calix[4]arene can rotate with low energy barrier at room temperature and thus is not supposed to significantly contribute to the adsorption of CTAQ and CDAQ onto the Ag surface. On the other hand, carboxylic lariats can play the role of an anchor, immobilizing the CDAQ, as shown by RAIRS and MC simulation (vide infra). The lower surface density of CDAQ than that of CTAQ is supposedly attributed to quinone moieties rather than carboxylate groups of CDAQ. Figure 2b offers another evidence of the adsorbed CTAQ on the Ag surface. When anthraquinone-1,5-disulfonic acid disodium salt is dissolved in the bulk solution and cyclic voltammograms are obtained with a bare Ag electrode, the cathodic and anodic waves in curve 1 are observed at around -0.5 and -0.3 V, respectively. However, the same experiment using a CTAQ-modified Ag electrode gives clearly different voltammogram 2, in which the cathodic wave appears at around -0.2 V and the anodic one disappears. Curve 2 in Figure 2b is almost the same as curve 1 in Figure 2a. It should be noted that no electrochemical reduction of anthraquinone-1,5-disulfonic acid disodium salt is observed and the presence of anthraquinone1,5-disulfonic acid disodium salt in the solution has negligible influence on the voltammetric reduction of the CTAQ film around -0.2 V. This means that the reduction of the quinone moiety of the adsorbed CTAQ is responsible for the cathodic wave at around -0.2 V in curve 2 of Figure 2b and the adsorbed CTAQ molecules do not act as electron mediators. Instead, the CTAQ film substantially interferes in the electron transfer from the electrode to anthraquinone-1,5-disulfonic acid disodium salt in the solution, which is reasonably explained by the fast proton transfer and apparently irreversible reduction of the quinone moiety of CTAQ. The CDAQ film behaves similarly but less severely blocks the electron transfer to the redox species in the solution, presumably due to the lower surface density of the CDAQ film. 3.2. Spectroscopy of the Complexes on the Ag Surface. Leyton et al. used RAIRS to look into the interaction of esterfunctionalized calixarene as a host molecule with Ag for potential applications of sensory devices.20 They also demonstrated that the IR technique is useful to examine the structural changes due to the adsorption and the complexation with the analyte.21 In this study, RAIRS was employed to investigate the Ca2+/calixquinone complexes on the Ag surface. The solid curve in Figure 3a is the RAIR spectrum obtained from a CTAQ film on the Ag surface. The stretching band due to -COOH appears at 1740 cm-1, and the symmetric stretching mode of -COO- is observed at 1414 cm-1. This means that both -COOH and -COO- coexist in the CTAQ film on the Ag surface. Because CTAQ needs at least one carboxylic acid group as an anchor, one or two carboxylic acid groups make the CTAQ immobilized on the Ag surface. In the case of CDAQ, the ν(-COOH) band disappears in Figure 3b (solid line), whereas the strong ν(-COO-) band is observed at 1408 cm-1. This indicates that carboxylic acid groups of a CDAQ on the Ag surface are fully deprotonated and thus both of them are available for adsorption and complexation.

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Figure 3. RAIRS spectra of (a) CTAQ and (b) CDAQ adsorbed on the Ag surface in the absence (solid line) and presence (dotted line) of 10 mM Ca(NO3)2 in 0.1 mM CTAQ ethanol solution.

Comparison between the spectra of the free calixquinones and of the Ca2+/calixquinone complexes on the Ag surfaces provides more information about which moieties of the CTAQ and CDAQ interact with Ca2+ ions and the Ag substrate. The dotted curves in Figure 3a,b exhibit that the absorption peaks due to the ν(-COO-) mode shift to higher frequencies by 9 and 15 cm-1 for CTAQ and CDAQ, respectively. This indicates that the deprotonated -COO- group is closely involved in trapping a Ca2+ ion. In addition to the -COO- group, the intensity of the band corresponding to the ν(CdO) mode of the quinone at 1645 and 1647 cm-1 undergo a drastic decrease. Furthermore, the bands due to the ν(CdC) band of quinone in the region of 1560-1620 cm-1 change in frequency as well as intensity. These results unequivocally indicate that Ca2+ ions in the complexes with CTAQ and CDAQ are present significantly close to the quinone and the -COO- group, suggesting the cooperative roles of the two moieties in capturing the Ca2+ ions. On the other hand, from RAIR spectra of CDAQ, it is clear that both carboxylic acid groups of CDAQ exist in the deprotonated form. In the case of CTAQ, however, the number of deprotonated COOH groups is not evident because a peak corresponding to the protonated COOH group is observed as well in the spectrum. CTAQ has three carboxylic acid groups, and RAIR spectra indicate that at least one deprotonated COOH group is required for anchoring on the Ag surface. To predict the number of deprotonated COOH groups in the anchored

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CTAQ structure, a few conditions should be considered, such as the charge neutrality of the system, coordination with Ca2+ ions, and anchoring onto the silver surface. Taking account of these conditions, the form of two deprotonated COOH groups is expected to be more probable than that of one deprotonated COOH. Nonetheless, it should be noted that there is no experimental evidence to rule out the presence of one deprotonated COOH form. In this study, simulations were performed for both CDAQ and CTAQ with two deprotonated COOH groups, which are expected to be the most probable forms. 3.3. Monte Carlo Simulation. Simulation based on classical force field has been employed to elucidate the complex structure and the relative orientation between the calixquinones and Ca2+ ions in solution.22-25 The structures of CTAQ and CDAQ can be closely examined by MC conformation search simulation, and thereby, the model of adsorption can be proposed. For the protonation of the carboxylic acid groups of CTAQ2-, there are two possible structures: either the carboxylic acid group in the middle of the three or one of two carboxylic acid groups adjacent to the quinone can be protonated. Both of the possible structures complexed with a Ca2+ ion are scrutinized by MC simulation in this study. Another issue of the conformation of calixarene is its isomers due to the rotation of the aromatic or quinone group. Many previous experimental and theoretical investigations showed that calixarene derivatives normally have many isomers due to the conversion of its ring structure.23-28 According to our previous reports, however, CTAQ and CDAQ maintain a cone conformer in solution.12,14 The NMR spectra of CTAQ and CDAQ show the features of the cone-shaped calixarene. In the NMR spectrum of CTAQ, the magnitude of the split of chemical shifts (∆δ) due to the methylene group between the moieties (ArCH2Ar) is 1.12 ppm and that between the aryl and the quinone moieties (ArCH2Q) is 0.67 ppm. It indicates that a cone conformation with rapid quinone rotation is predominant on the time scale of NMR measurements. Compared with that of CTAQ, CDAQ shows a simple NMR spectrum, in which one pair of doublets at 3.29 and 3.93 ppm, corresponding to the methylene protons (QCH2Ar) between the aryl and the quinone moieties, is a clear evidence of cone-shaped calixarenes. Therefore, the structure of CTAQ2- or CDAQ2was retained to be a cone shape with a harmonic constraint during the MC conformation search simulation. The global minimum structures of the cone-shaped calixarene and calcium ion obtained are shown in Figure 4. The top and side views of the CTAQ2- and calcium ion complex when the middle carboxylic acid of CTAQ2- is protonated (Ta1) are shown in Figure 4a. The position of the calcium ion is slightly moved from the center of CTAQ2- to the quinone group. Figure 4b shows the complex structure of CTAQ2- and a calcium ion when the carboxylic acid group next to the quinone is protonated (Tb1). The position of the calcium ion is slightly skewed to the carboxylic acid groups because of the electrostatic interaction. The complex structure of cone-shaped CDAQ2- and a calcium ion (D1) is shown in Figure 4c. The calcium ion is placed in the middle of CDAQ2-. The driving force of the adsorption of the carboxylic acid group on the metal surface is the formation of a salt between the anionic carboxylate and a surface metal cation.29 Therefore, we propose how the complexes of the calixquinones and calcium ion are adsorbed on the silver surface, as shown in Figure 4d-f. The deprotonated carboxylate groups act as anchors for adsorption of the calixquinones in the form of cone-shaped conformers. A calcium ion is captured inside the cavity of the calixquinones, and its position is significantly affected by the electrostatic interactions.

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Figure 4. Global minimum structures of the calixarene and calcium ion complex obtained from MC simulations. The structure of calixarene is fixed to the cone conformer during the simulations. (a) Top and side views of the CTAQ2- and calcium ion complex when the middle carboxylic acid of CTAQ2- is protonated (Ta1). (b) Top and side views of the CTAQ2- and calcium ion complex when one of the carboxylic acids next to the quinone group is protonated (Tb1). (c) Top and side views of the CDAQ2- and calcium ion complex (D1). The adsorption models of Ta1, Tb1 and D1 on silver surface are shown in (d)-(f).

3.4. Electrochemistry of Ca2+/Calix[4]quinone Complexes on Ag. Park et al. reported that two quinone moieties in a calix[4]arene undergo electrochemical redox processes as a result of selective interaction with alkaline earth metal ions, such as Sr2+, Ca2+, and Ba2+ ions.12,13 In that work, a new reduction wave, corresponding to the complex of the reduced quinone and Ca2+ ion, appeared at less negative potential than that of the free calixquinone. Figure 5a,b shows the cyclic voltammograms that were obtained in an aqueous HEPES solution in the absence of CTAQ and Ca2+ ions using the Ag electrodes, on which calixquinones and Ca2+ ion complexes were previously adsorbed. Voltammetric features reveal a few important aspects of the Ca2+/calix[4]quinone complexes on the Ag surface. First, the reduction peaks of both Ca2+/CTAQ and Ca2+/CDAQ appear at a little less negative potential than -0.1 V, which is significantly more positive than those of free adsorbed CTAQ and CDAQ. This is a typical voltammetric behavior of redox-active ionophores in the presence of cationic guests.12,18,30,31 Hence, CTAQ and CDAQ act as redoxactive ionophores that exhibit the electrochemical processes sensitively affected by the presence of Ca2+ ions. Second, the oxidation peaks of the Ca2+ ion complexes on the Ag surfaces emerge around 0 V in Figure 5, whereas no oxidation was observed in both Figure 2a and curve 2 in Figure 2b. This indicates that a Ca2+ ion bound to CTAQ or CDAQ prevents subsequent protonation normally following electron transfer.12,14 Additionally, the oxidation waves in Figure 5 are close to the reduction waves. This means that the observed voltammetric behavior comes from the surface-confined reversible electron transfer. Linear dependence of the scan rate on the reduction peak current confirms the adsorption behavior (data not shown). Third, repetitive potential cycling produces reproducible and constant redox behavior, indicating the presence of the stable Ca2+/CTAQ and Ca2+/CDAQ complexes on the Ag surface without significant loss of Ca2+ ions or calixquinones. The current should have continuously decreased if Ca2+ ions on the surface was released into the solution. The electrochemical observations are consistent with the results of RAIRS and MC simulation. The complexes of CTAQ and CDAQ with Ca2+ ions are spontaneously adsorbed on the Ag surface and the films of Ca2+/CTAQ and Ca2+/CDAQ complexes are sufficiently stable before and after repetitive redox processes in aqueous and ethanol solutions.

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J. Phys. Chem. C, Vol. 113, No. 46, 2009 19985 0(2009)), a grant from the Fundamental R&D Program for Core Technology of Materials, the Ministry of Knowledge Economy in Korea [09MC3310, Programmable Bio-CMOS Field Effect Transistors], and the Technology Development Program for Agriculture and Forestry, Ministry for Agriculture, Forestry and Fisheries, Republic of Korea (109146-03-1-SB030) (TDC). S.K.K. and O.-S.L. thank the Ministry of Education of Korea for the Brain Korea 21 fellowship. This work was partly supported by the National Research Foundation of Korea Grant funded by the Korean Government (20090063005) (H.K.). References and Notes

Figure 5. Cyclic voltammograms of (a) CTAQ-modified and (b) CDAQ-modified Ag electrodes in a HEPES solution. CTAQ and CDAQ films were deposited in EtOH containing 0.1 mM CTAQ (or CDAQ) + 10 mM Ca(NO3)2.

4. Conclusion RAIRS and MC simulation were performed to study the interaction between Ca2+ ions and calix[4]quinones, CTAQ and CDAQ, adsorbed on the Ag surface. CTAQ and CDAQ are attached to the Ag surface via a carboxylate group and associate with a Ca2+ ion by another one. The trapped Ca2+ ion also interacts with the quinone moieties, which offer the redoxswitching properties of calix[4]quinones. RAIRS and MC simulation unequivocally tell how the calix[4]quinones of CTAQ and CDAQ are adsorbed on the Ag electrode surfaces and how the Ca2+ ions are trapped in the complexes. The electrochemical observations agree with the model that is proposed by the results of RAIRS and MC simulation. Moreover, the electrochemical reactions on the Ag electrodes have a negligible effect on the stability of the Ca2+/CTAQ and Ca2+/CDAQ complexes, which is a practical, important advantage for applications. Consequently, the adsorptive and ion-selective behavior of redox active calix[4]quinones on the Ag surfaces provides insight into the redox-dependent complexes and suggests further progress toward more elaborate Ca2+ ion sensors based on voltammetry or surface-enhanced Raman scattering. Acknowledgment. This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MEST) (No. R11-2007-012-02002-

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