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AFM Observation of Cation Complexation of Dibenzocrown Ethers Adsorbed on Highly Oriented Pyrolytic Graphite Shinpei Kado, Kaname Yamada, and Keiichi Kimura* Department of Applied Chemistry, Faculty of Systems Engineering, Wakayama University, Sakae-dani, Wakayama 640-8510, Japan Received October 27, 2003. In Final Form: February 3, 2004 The cation complexation behavior of dibenzocrown ethers adsorbed on highly oriented pyrolytic graphite substrates was investigated by means of atomic force microscopy using probe tips modified chemically with ammonium ion by silane coupling. The specific adhesion force based on the intermolecular force between dibenzocrown ether and ammonium ion was observed via force curve measurements in ethanol at the interface between the substrate and tip. The observed specific force decreased in the presence of the alkali metal ion in solution, indicating that the cation in solution interferes with the complexation of the crown ethers adsorbed on the substrate with the ammonium ion immobilized on the tip. The blocking effect of metal ions in solution on the observed force depended on the sizes of both the blocking cation and crown ether ring, suggesting that the surface-adsorbed dibenzocrown ethers possess a selective cation-complexing ability similar to that in their bulk state and that the adhesion force measurements using cation-modified tips allow evaluation of the cation-complexing ability of crown ethers under cation-competitive conditions.
Introduction Molecular recognition phenomena based on noncovalent specific intermolecular forces between synthetic host and guest molecules have been of great interest and have been therefore studied widely from the viewpoint of both pure and applied chemistry.1-4 Cation complexation properties (e.g., stability constants) of many artificial host molecules, such as crown ethers and cryptands, have been already investigated to a great extent in their bulk states by wellestablished analytical methods such as NMR, calorimetry, and potentiometry.5 The specific intermolecular forces in host-guest pairs play important roles in their complexation processes. Thus, understanding molecular recognition phenomena at the molecular level requires direct measurements of the molecular interaction forces, which cannot be carried out by conventional methods. In recent years, atomic force microscopy (AFM), which was originally developed for acquiring the surface topography of insulating materials, has attracted increasing attention as a powerful analytical tool for sensing precisely the interaction force between its probe tip and substrate surfaces thanks to its high force and spatial resolutions and almost no limitation of environment and conditions for the force measurements.6-10 In particular, it has been demonstrated that AFM can be employed to measure directly specific intermolecular forces of ligand-receptor or host-guest molecule pairs, such as biological supramolecules of (strept)avidin-biotin,11,12,36,37 DNA strands13 and its * To whom correspondence should be addressed. E-mail:
[email protected]. (1) Inoue, S.; Gokel, G. W. Cation Binding by Macrocycles; Marcel Dekker: New York, 1990. (2) Dietrich, B.; Viout, P.; Lehn, J.-M. Macrocyclic Chemistry; VCH: Weinheim, 1993. (3) Vo¨gtle, F. Supramolecular Chemistry; Wiley: New York, 1993. (4) Lehn, J.-M. Supramolecular Chemistry; VCH: Weinheim, 1995. (5) Izatt, R. M.; Pawlak, K.; Bradshaw, J. S.; Bruening, R. L. Chem. Rev. 1991, 91, 1721. (6) Takano, H.; Kenseth, J. R.; Wong, S.-S.; O’Brien, J. C.; Porter, M. D. Chem. Rev. 1999, 99, 2845. (7) Heinz, W. F.; Hoh, J. H. Nanotechnology 1999, 17, 143. (8) Samorı`, B. Chem.sEur. J. 2000, 6, 4249. (9) Janshoff, A.; Neitzert, M.; Oberdo¨rfer, Y.; Fuchs, H. Angew. Chem., Int. Ed. 2000, 39, 3212. (10) Hugel, T.; Seitz, M. Macromol. Rapid Commun. 2001, 22, 989.
bases,14 carbohydrates,15 and enzyme-drug16 and also artificial molecule pairs of heavy metal-ligand,17,18 β-cyclodextrin-ferrocene,19,20 charge-transfer complexes,21,22 and crown ether-ammonium ion,23,24 which were immobilized independently on AFM probe tip and substrate surfaces. For the purpose, the probe tip and substrate modified chemically with either host or guest molecule are usually required. In general, the chemical modification with self-assembled monolayers (SAMs) of organic thiol or silane compounds is applied for covalent bonding of target molecules to the probe tip and substrate surfaces except for macromolecules (e.g., proteins and polymers).25-40 Despite recent progress in the methodology by (11) Florin, E.-L.; Moy, V. T.; Gaub, H. E. Science 1994, 264, 415. (12) Lee, G. U.; Kidwell, D. A.; Colton, R. J. Langmuir 1994, 10, 354. (13) Lee, G. U.; Chrisey, L. A.; Colton, R. J. Science 1994, 266, 771. (14) Boland, T.; Ratner, B. D. Proc. Natl. Acad. Sci. U.S.A. 1995, 92, 5297. (15) Tromas, C.; Rojo, J.; de la Fuente, J. M.; Barrientos, A. G.; Garcı´a, R.; Penade´s, S. Angew. Chem., Int. Ed. 2001, 40, 3052. (16) Rigby-Singleton, S. M.; Allen, S.; Davies, M. C.; Roberts, C. J.; Tendler, S. J. B.; Williams, P. M. J. Chem. Soc., Perkin Trans. 2 2002, 1722. (17) Ito, T.; Citterio, D.; Bu¨hlmann, P.; Umezawa, Y. Langmuir 1999, 15, 2788. (18) Conti, M.; Falini, G.; Samorı`, B. Angew. Chem., Int. Ed. 2000, 39, 215. (19) Scho¨nherr, H.; Beulen, M. W. J.; Bu¨gler, J.; Huskens, J.; van Veggel, F. C. J. M.; Reinhoudt, D. N.; Vancso, G. J. J. Am. Chem. Soc. 2000, 122, 4963. (20) Zapotoczny, S.; Auletta, T.; de Jong, M. R.; Scho¨nherr, H.; Huskens, J.; van Veggel, F. C. J. M.; Reinhoudt, D. N.; Vancso, G. J. Langmuir 2002, 18, 6988. (21) Skulason, H.; Frisbie, C. D. J. Am. Chem. Soc. 2002, 124, 15125. (22) Gil, R.; Fiaud, J.-C.; Poulin, J.-C.; Schulz, E. Chem. Commun. 2003, 2234. (23) Kado, S.; Kimura, K. Chem. Lett. 2001, 630. (24) Kado, S.; Kimura, K. J. Am. Chem. Soc. 2003, 125, 4560. (25) Frisbie, C. D.; Rozsnyai, L. F.; Noy, A.; Wrighton, M. S.; Lieber, C. M. Science 1994, 265, 2071. (26) Noy, A.; Frisbie, C. D.; Rozsnyai, L. F.; Wrighton, M. S.; Lieber, C. M. J. Am. Chem. Soc. 1995, 117, 7943. (27) Vezenov, D. V.; Noy, A.; Rozsnyai, L. F.; Lieber, C. M. J. Am. Chem. Soc. 1997, 119, 2006. (28) van der Vegte, E. W.; Hadziioannou, G. J. Phys. Chem. B 1997, 101, 9563. (29) van der Vegte, E. W.; Hadziioannou, G. Langmuir 1997, 13, 4357. (30) Noy, A.; Sanders, C. H.; Vezenov, D. V.; Wong, S. S.; Lieber, C. M. Langmuir 1998, 14, 1508.
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use of AFM combining the modification of tip and substrate, there have been few reports on the direct measurement of specific intermolecular force in artificial compounds.21 The necessity for the covalent bonding onto tip and substrate seems to make it difficult to carry out direct measurements of specific intermolecular forces of various artificial host-guest molecule pairs by means of AFM and to limit the applicability of the methodology, because the synthesis of compounds such as thiol or silane derivatives bearing the proper functional terminal group is essential for the chemical modification. Highly oriented pyrolytic graphite (HOPG) is a wellknown substrate used for observation of organic compounds at high resolution by scanning probe microscopy (SPM),41,42 since HOPG provides an atomically flat surface that is well-defined as well as adequately adsorptive for attachment of objective organic molecules. The observation of organic molecules by scanning tunneling microscopy (STM) is usually conducted in a liquid droplet containing them by immersing a probe tip. Furthermore, electrochemical STM affords a high-resolution image of an organic compound on a substrate even in an electrolyte solution. In these cases, the immobilization of target molecules to the substrate surface is based on simple physical adsorption and is thereby a more experimentally convenient method than that using chemical modification by SAMs described above, although it may be somewhat less stable due to the possible desorption of the target molecules from the substrate during the measurement and then applicable to only limited compounds possessing a powerful adsorption ability. High-resolution STM images of crown ether derivatives adsorbed on substrates have been successfully obtained in air43 as well as in solution.44,45 Recently, Yoshimoto et al. have reported the STM observation of calcium ion complexation of the phthalocyanine derivative bearing four crown ether moieties adsorbed on Au(111) in an electrolyte solution.45 An AFM study on the inclusion complex of host molecules (i.e., cyclodextrins) adsorbed on HOPG and MoS2 substrates was reported by Shigekawa and his colleagues.46 They observed that the complexation of the adsorbed cyclodextrins with their guest molecules caused a significant change in the adhesion forces in air. (31) Wong, S. S.; Joselevich, E.; Woolley, A. T.; Cheung, C. L.; Lieber, C. M. Nature 1998, 394, 52. (32) Han, T.; Williams, J. M.; Beebe, T. P., Jr. Anal. Chim. Acta 1995, 307, 365. (33) Wenzler, L. A.; Moyes, G. L.; Raikar, G. N.; Hansen, R. L.; Harris, J. M.; Beebe, T. P., Jr. Langmuir 1997, 13, 3761. (34) Wenzler, L. A.; Moyes, G. L.; Olson, L. G.; Harris, J. M.; Beebe, T. P., Jr. Anal. Chem. 1997, 69, 2855. (35) Ito, T.; Namba, M.; Bu¨hlmann, P.; Umezawa, Y. Langmuir 1997, 13, 4323. (36) Lo, Y.-S.; Huefner, N. D.; Chan, W. S.; Stevens, F.; Harris, J. M.; Beebe, T. P., Jr. Langmuir 1999, 15, 1373. (37) Lo, Y.-S.; Zhu, Y.-J.; Beebe, T. P., Jr. Langmuir 2001, 17, 3741. (38) Zhang, J.; Kirkham, J.; Robinson, C.; Wallwork, M. L.; Smith, D. A.; Marsh, A.; Wong, M. Anal. Chem. 2000, 72, 1973. (39) Smith, D. A.; Wallwork, M. L.; Zhang, J.; Kirkham, J.; Robinson, C.; Marsh, A.; Wong, M. J. Phys. Chem. B 2000, 104, 8862. (40) Hugel, T.; Holland, N. B.; Cattani, A.; Moroder, L.; Seitz, M.; Gaub, H. E. Science 2002, 296, 1103. (41) Yang, R.; Yang, X. R.; Evans, D. F.; Hendrickson, W. A.; Baker, J. J. Phys. Chem. 1991, 95, 3765. (42) Cyr, D. N.; Venkataraman, B.; Flynn, G. W.; Black, A.; Whitesides, G. M. J. Phys. Chem. 1996, 100, 13747. (43) Samorı´, P.; Engelkamp, H.; de Witte, P.; Rowan, A. E.; Nolte, R. J. M.; Rabe, J. P. Angew. Chem., Int. Ed. 2001, 40, 2348. (44) Wang, D.; Xu, Q.-M.; Wan, L.-J.; Wang, C.; Bai, C.-L. Surf. Sci. 2001, 489, L568. (45) Yoshimoto, S.; Suto, K.; Itaya, K.; Kobayashi, N. Chem. Commun. 2003, 2174. (46) Oyama, S.; Miyake, K.; Yasuda, S.; Takeuchi, O.; Sumaoka, J.; Komiyama, M.; Futaba, D. N.; Morita, R.; Yamashita, M.; Shigekawa, H. Jpn. J. Appl. Phys. 2001, 40, 4419.
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Thus, the immobilization of artificial host molecules on substrates based on physisorption has been utilized for SPM studies on their interfacial complexation, whereas SAMs of thiol derivatives bearing crown47,48 and pseudocrown49,50 ether moieties and their complexing properties have been previously investigated using electrochemical impedance spectroscopy. In the present work, we have investigated the cationcomplexation behavior of dibenzocrown ethers adsorbed on HOPG by measuring the specific adhesion force in ethanol with AFM using probe tips modified chemically with a guest cation, that is, ammonium ion. We also demonstrate that a blocking effect of another cation in solution on the adhesion force can be utilized to elucidate the complexation property of the dibenzocrown ethers immobilized on HOPG. The present results may suggest that a new convenient method without covalent bonding, that is, with simple physical adsorption of the target molecule onto substrates, can be utilized to observe directly the specific interaction between host and guest molecules by means of AFM. Experimental Section Reagents. Dibenzo-18-crown-6 (Aldrich) was used without further purification. Dibenzo-24-crown-8 was prepared according to the established procedure and purified by two-time recrystallization from heptane. The silane-coupling reagent, (3-aminopropyl)triethoxysilane (Tokyo Kasei Kogyo, Tokyo, Japan), was used as received. Other chemicals were of reagent grade and used as received. Preparation of Substrate and Probe Tip. An ethanol solution (50 µL) containing 0.6 mmol dm-3 dibenzo-18-crown-6 (DB18C6) was deposited onto an HOPG substrate (area, 1.44 cm2; Digital Instruments, Santa Barbara, CA) that was freshly cleaved with adhesive tape just prior to use. For dibenzo-24crown-8 (DB24C8), 20 µL of 0.9 mmol dm-3 ethanol solution was dropped on the substrate. The HOPG substrates were then allowed to stand in air for several hours for drying. Commercially available V-shaped Si3N4 cantilevers (100 µm length; nominal spring constant, 0.08 N m-1; Olympus, Tokyo, Japan) were used without further treatment except for the case of their chemical modification mentioned below. AFM probe tips modified chemically with alkylammonium ions were prepared by silane coupling.17,23,35 Tips were pretreated successively with a piranha solution (concentrated H2SO4/28% H2O2, 7/3, v/v) for 30 min and with 1 mol dm-3 aqueous solutions of NaOH, HCl, and NaOH for 5 min. Caution: Piranha solutions react violently with organic compounds and should be handled with great care. The cleaned tips were rinsed thoroughly with deionized water and heated in an oven at about 120 °C for 10 min. The resulting tips were immersed into a toluene solution containing 20 mmol dm-3 (3aminopropyl)triethoxysilane (APTES). After being immersed overnight, the tips were withdrawn from the toluene solution and rinsed successively with toluene, ethanol, and deionized water. Finally, the tips were soaked in 0.1 mol dm-3 HCl aqueous solution for 30 min in order to obtain surface ammonium groups by protonation of the free amino groups immobilized on the tip and then dried in an oven (about 100 °C) for 30 min. Force Curve Measurements. Adhesion force measurements were carried out at room temperature with a scanning probe microscope (SPA300, Seiko Instruments, Tokyo, Japan). The cantilever and HOPG substrate were mounted on the apparatus by using a liquid cell. Force curves were measured 100 times for each tip/substrate combination at a scanning rate of 80 nm s-1. The measurement solutions were prepared using absolute (47) Flink, S.; Boukamp, B. A.; van den Berg, A.; van Veggel, F. C. J. M.; Reinhoudt, D. N. J. Am. Chem. Soc. 1998, 120, 4652. (48) Flink, S.; van Veggel, F. C. J. M.; Reinhoudt, D. N. J. Phys. Chem. B 1999, 103, 6515. (49) Bandyopadhyay, K.; Shu, L.; Liu, H.; Echegoyen, L. Langmuir 2000, 16, 2706. (50) Bandyopadhyay, K.; Liu, S.-G.; Liu, H.; Echegoyen, L. Chem.s Eur. J. 2000, 6, 4385.
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Figure 2. Force curves observed on DB18C6/HOPG using APTES/tip (A) in pure ethanol and (B) in ethanol solution containing 1 mmol dm-3 CF3SO3K.
Observation of the Specific Force between Dibenzocrown Ether and Ammonium Ion. There have been some research works on cation binding of crown ether derivatives at solid/liquid interfaces on SAM-modified substrates. The examples are anchoring a crown ether derivative to an ammonium-modified surface based on their host-guest interaction51 and cation complexation of crown ether47,48 and pseudocrown ether49,50 SAM modified electrodes. Also, specific complexation forces of crown ether with ammonium ion can be measured by using AFM at an interface between probe tip and substrate modified chemically with 18-crown-6 and ammonium groups, respectively.24 In this study, we introduced ammonium ions on the probe tip surface by chemical bonding and crown ethers (i.e., dibenzocrown ethers) on the HOPG substrate by adsorption, as shown schematically in Figure 1, because this experimental setup makes it experimentally easy to replace one crown ether adsorbed on HOPG with another. The dibenzocrown ether derivatives have been selected as a host compound for the adsorption, because they can be expected to show high adsorption ability onto HOPG due to their two benzene rings. Recently, Ohira et al. have reported the immobilization of dibenzo-18-crown-6 ether and its potassium ion complex on Au(111) by a LangmuirBlodgett method and their STM observation in an aqueous solution.52
Figure 2A shows a typical force curve observed on the dibenzo-18-crown-6-immobilized HOPG (DB18C6/HOPG) using the ammonium-modified tip (APTES/tip) in pure ethanol. The choice of ethanol as a measurement solvent was due to the fact that the adhesion force observed without any cation was very small, as will be described later. In the absence of potassium ion (Figure 2A), the force curve exhibited a greater adhesion force than that obtained in the presence of potassium ion (Figure 2B). The histograms of the adhesion forces observed from each set of repeated force measurements are given in Figure 3. Obviously, the presence of potassium ion, which has a high affinity to DB18C6, caused a substantial decrease in the adhesion force observed between DB18C6/ HOPG and APTES/tip. In our previous paper,24 a similar result has been obtained upon addition of potassium ion for the adhesion force between a probe tip and a substrate modified chemically with 18-crown-6 and ammonium ion, respectively. The effect of potassium ion on adhesion forces can be sufficiently explained in terms of the competitive complexation of DB18C6 adsorbed on HOPG with ammonium ions on the tip and potassium ions in the solution. In other words, the potassium ion in solution blocks the complexation of ammonium ions by the crown ether on HOPG. Thus, these results suggest that the adhesion forces observed here can be attributed to the specific interaction based on the complexation of DB18C6 adsorbed on HOPG and ammonium ions bonded to the probe tip. In addition, we carried out a control force measurement using an unmodified (without any cation) tip instead of APTES/tip. Figure 4 summarizes histograms of the adhesion forces observed on the DB18C6/HOPG by using the unmodified tip in the absence and presence of potassium ion. Contrary to the APTES/tip, the presence of a blocking ion, that is, potassium ion, caused no significant decrease in the adhesion force. A comparison of the two histograms in the absence of potassium ion (Figures 3A and 4A) makes it clear that the adhesion force observed with an unmodified tip was smaller than that with APTES/tip. Consequently, these findings confirm that the adhesion forces between DB18C6/HOPG and APTES/ tip can be assigned to the specific interaction based on the
(51) Miura, Y.; Kimura, S.; Imanishi, Y.; Umemura, J. Langmuir 1998, 14, 2761.
(52) Ohira, A.; Sakata, M.; Hirayama, C.; Kunitake, M. Org. Biomol. Chem. 2003, 1, 251.
Figure 1. (A) Schematic representation of the setup that consists of a probe tip modified chemically with guest cation and a crown ether adsorbed substrate for measuring the specific supramolecular interaction by AFM and (B) the chemical structure of dibenzocrown ethers used in the present study. ethanol, deionized water, and alkali metal perchlorates or trifluoromethanesulfonate. The value of adhesion forces was computed as the average and standard deviation of data sets obtained from repetitive force measurements.
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Figure 3. Histograms of adhesion forces obtained from each set of repetitive force measurements (total counts, 100) between DB18C6/HOPG and APTES/tip (A) in pure ethanol and (B) in ethanol solution containing 1 mmol dm-3 CF3SO3K. The force interval is 0.1 nN.
complexation of DB18C6 and the ammonium group. Furthermore, this may indicate that most of the DB18C6 molecules adsorbed on HOPG remained immobilized on the substrate surface during the repetitive force measurements. Solvent Effect on Adhesion Force. The cation complexation of crown ether derivatives with guest cations is influenced significantly by solvent.1,2 In general, cation complexation abilities of crown ethers are stronger in nonpolar solvents than in polar ones such as water. Thus, the effect of solvent on the adhesion force between DB18C6/ HOPG and APTES/tip was examined by changing the ethanol content of the ethanol/water mixture used as the measurement solvent. Figure 5 shows the plot of the adhesion forces as a function of the ethanol content (vol %). The adhesion forces at ethanol contents of less than 80% were much greater than that observed in absolute ethanol. The result conflicts apparently with the general concept of crown ethers that their cation complexation ability decreases in polar solvent. Thus, the greater adhesion forces at ethanol contents of less than 80% cannot be attributed only to the specific complexation force of the crown ethers. It has been reported that the work of the adhesion between a hydrophobic CH3-terminated SAM modified tip and the substrate in the methanol/water mixture increased with increasing water content of the solvent.30 In our force measurements, the hydrophobic nature of the underlying HOPG caused the high surface energy at less than 80% ethanol content of aqueous ethanol
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Figure 4. Histograms of adhesion forces obtained from each set of repeated force measurements between DB18C6/HOPG and an unmodified tip (A) in pure ethanol and (B) in ethanol solution containing 1 mmol dm-3 CF3SO3K. The force interval is 0.1 nN.
Figure 5. Plot of adhesion forces observed between DB18C6/ HOPG and APTES/tip vs ethanol content in ethanol/water mixture solvent. Full circles and squares show the averages of force values obtained from repetitive force measurements with two different tips. The error bars indicate the standard deviations of the adhesion force.
solvents, resulting in the large adhesion force. Therefore, the high water content in the mixture solvent was not suitable for measuring specific force in our system, as Skulason and Frisbie have recently pointed out.21 How-
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The minimum value for the adhesion force was attained with potassium ion, which is most favorable for stable complexation with the 18-crown-6 ring, indicating that potassium ion competes most strongly with ammonium ion on the probe tip among the tested alkali metal ions. The present experimental setup using HOPG has the advantage of easy replacement of a crown ether adsorbed on the substrate with another one. Thus, the crown ether on the substrate was replaced with DB24C8 and then the force measurements were also carried out. Definitely, the cation dependence of the adhesion force for DB24C8/ HOPG, as shown in Figure 6B, was different from that for DB18C6/HOPG (Figure 6A); that is, the smallest adhesion force was observed in the presence of cesium ion. The metal ion dependence of the adhesion force is in good agreement with the cation-binding affinity of the crown ether derivatives having different ring sizes. This means that the blocking extent by metal ions on the adhesion forces for DB18C6 and DB24C8 corresponded to their cation affinity in the bulk state. These results reasonably support that the observed adhesion force would be assigned to the specific complexation force of dibenzocrown ethers and ammonium ion immobilized on HOPG and tip, respectively. Consequently, the cation-dependent blocking may indicate that the force measurements using an AFM probe tip modified with a guest cation can be used to elucidate the interfacial cation-binding behavior of surfaceadsorbed crown ethers on a molecular level. Conclusion
Figure 6. Cation dependence of adhesion force on (A) DB18C6/ HOPG and (B) DB24C8/HOPG substrates using the APTESmodified probe tips in the presence of blocking metal ion in solution. The blocking experiments were carried out in ethanol solutions containing 0.5 mmol dm-3 metal perchlorate. Full circles and squares show averages of force values obtained from repetitive blocking force measurements with two different tips. The error bars indicate the standard deviations of the adhesion force. The data without metal were obtained by separate tips.
ever, the adhesion forces at the content of more than 80% ethanol can be derived only from the complexation force, because the contribution of nonspecific hydrophobic interaction to the observed adhesion forces would be mostly eliminated at such high ethanol contents. This can be supported by the abrupt decrease in the adhesion force around ethanol contents of 80% (Figure 5) as well as by the blocking effect by potassium ion (Figure 3). Metal-Ion-Dependent Blocking. The specific force between crown ether adsorbed on HOPG and ammonium ion on the probe tip decreased substantially based on the competitive complexation of another cation in the measurement solvent, as shown in Figure 3. It is well-known that crown ethers show selectivities on cation complexation.1,2 The extent of decrease in the adhesion force between crown ether immobilized HOPG and APTES/tip is expected to depend on the kind of metal ion in the measurement solvent. Figure 6A demonstrates the cation dependence of the adhesion force obtained by force measurements on DB18C6/HOPG under blocking conditions using alkali metal perchlorates.
We have adopted HOPG, which is used often in STM observation of organic compounds, as the substrate for immobilization of crown ether compounds to observe their supramolecular force with guest cations by means of AFM. The force curve measurements for the specific intermolecular force of dibenzocrown ethers adsorbed on HOPG have been successfully carried out in ethanol by using probe tips modified chemically with ammonium ion. The specific force between the surface-adsorbed crown ethers on the substrate and ammonium ion on the probe tip was suppressed by potassium ion in solution. Furthermore, the blocking extent for the adhesion force depended on the kind of metal ions in solution and the ring size of crown ethers on HOPG substrates. The cation dependence of the adhesion force coincides with the cation selectivity of dibenzocrown ethers on the complexation, indicating that the dibenzocrown ethers on the substrate possess cation-selective complexing ability similar to that in their bulk state. Finally, the present method using an HOPG substrate based on its adsorption ability may provide a new opportunity for the easy design of interfaces between an AFM probe tip and a substrate for measuring directly the specific interaction in molecular pairs such as hostguest complexes. Then, the methodology for immobilization of host molecules in this study may be also applied for guest molecules, if they show an adequate adsorption ability to the HOPG substrate. Acknowledgment. This work was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. LA036012Q
