Langmuir 1997, 13, 3251-3255
Formation of Surface Inclusion Complexes between Cyclodextrins and n-Alkanethiols and Their Self-Assembled Behaviors on Gold Juchao Yan and Shaojun Dong* Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, China Received October 29, 1996. In Final Form: February 24, 1997
Introduction Self-assembled monolayers (SAMs) on gold, almost all based on n-alkanethiols or di-n-alkyl sulfides, are receiving considerable interest in interfacial chemistry1 because they are capable of tailoring surfaces with desired and controlled properties by introducing suitable functional groups into alkyl chains. Undoubtedly, the introduction of functional groups within the assembly affects the packing and ordering of the monolayers, which have been suggested to be a function of intermolecular forces, such as dipole-dipole, hydrogen bonding and van der Waals (VDW), etc. interactions.1f Therefore, detailed studies on the relationship between self-assembled forces and binding phenomena are of the most importance for the design of a supramolecular structure with engineered properties. Cyclodextrins (CDs) are cyclic oligoglycosides of six (RCD), seven (β-CD), or eight (γ-CD) R-D-glucopyranose units linked by R(1f4) bonds, which have a torus shape with all the secondary hydroxyl groups, i.e. O(2)-H and O(3)H, located on the wider end and all the primary hydroxyl groups, i.e. O(6)-H, on the narrower end. The ability of CDs to form inclusion complexes (ICs) with hydrophobic organic molecules has been well established.2 Till now, almost all studies on such ICs are concentrated on aromatic derivatives,3 and very few on aliphatic hydrocarbons,4 probably because of the difficulties of elucidating the complexation behaviors.4b To the best of our knowledge, there are no reports on the synthesis of ICs formed between CDs and n-alkanethiols and no reports on their spontaneous assembly on gold. For the first time, we prepared successfully a new kind of SAM in aqueous solution from the preformed ICs (denoted as CD‚Cn between R-CD, β-CD, and n-alkanethiols (CH3(CH2)n-1SH, n ) 10, 14, 18) on gold electrodes. Our motivation for this work was as follows: first, unlike CH3(CH2)n-1SH, which can assemble only in organic * To whom correspondence should be addressed. Fax: 86-4315685653. Telephone: 86-431-5682801-413. E-mail: dongsj@ sun.ihep.ac.cn. (1) For recent reviews and monographs, see: (a) Zhong, C. J.; Porter, M. D. Anal. Chem. 1995, 67, 709A. (b) Kuhn, H.; Ulman, A. Thin Films; Ulman, A., Ed.; Academic Press: New York, 1995; Vol. 20. (c) Allara, D. L. Biosens. & Bioelectron. 1995, 10, 771. (d) Xu, J.; Li, H. L. J. Colloid Interface Sci. 1995, 176, 138. (e) Mandler, D.; Turyan, I. Electroanalysis 1996, 8 (3), 207. (f) Ulman, A. Chem. Rev. 1996, 96, 1533. (2) For recent reviews, see: (a) Bersier, P. M.; Bersier, J.; Klingert, B. Electroanalysis 1991, 3, 443. (b) Li, S.; Purdy, W. C. Chem. Rev. 1992, 92, 1457. (c) Wenz, G. Angew. Chem., Int. Ed. Engl. 1994, 33, 803. (3) (a) Bender, M. L.; Komiyama, M. Cyclodextrin Chemistry; Springer: New York, 1978. (b) Saenger, W. Angew. Chem., Int. Ed. Engl. 1980, 19, 344. (c) Szejtli, J. Cyclodextrins and Their Inclusion Complexes; Akademiai Kiado: Budapest, 1982. (d) Szejtli, J. Cyclodextrin Technology; Kluwer: Academic Publishers: Dordrech, 1988. (4) (a) Wishnia, A. J. Mol. Biol. 1974, 82, 77. (b) Watanabe, M.; Nakamura, H.; Matsuo, T. Bull. Chem. Soc. Jpn. 1992, 65, 164. (c) Bastos, M.; Briggner, L. E.; Shehatta, I.; Wadsoe, I. J. Chem. Thermodyn. 1990, 22, 1181. (d) Palepu, R.; Reinsborough, V. C. Can. J. Chem. 1989, 67, 1550.
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solvents, such water-soluble ICs can organize spontaneously into quasi-crystalline monolayers in aqueous solution, which can be used as a good model to study biomimetic membranes; second, we wish to know whether the secondary hydroxyl groups located on one CD compared to those located on the neighbor CD stabilize SAMs by forming intermolecular hydrogen bonds; and finally, since CDs have no toxic effects and also they can protect, stabilize, or solubilize the guest molecules, such ICs are of great value to the food, drug, and agricultural industries. In this paper, we report the synthesis of the desired CD‚Cn under mild conditions, the confirmation for the formation of CD‚Cn by 1H-NMR (400 MHz), and the determination for the surface composition of the resulting SAMs by XPS, followed by studies on the blocking effect to oxidation and rereduction of the underlying gold substrates and on the electrochemical behavior of the redox species (both Fe(CN)63- and Ru(NH3)63+) in solution by cyclic voltammetry (CV). Experimental Section Chemicals. CH3(CH2)n-1SH (n ) 10, 14, 18) and R-CD, β-CD were obtained from Sigma or Fluka and used as received without further purification. All other chemicals were analytical reagents. Water used was from a Mill-Q plus ultrapure water system (18.2 MΩ‚cm). Preparation of Gold Electrodes. A gold disk electrode (99.99%, ca. 0.8 mm diameter) sealed in a special kind of soft glass was prepared by manual polishing with graded alumina (Buehler) from 1.0 to 0.05 µm on a soft polishing cloth (Buehler), sonicating in absolute ethanol and then in water, electrocycling between 1.5 and -0.3 V (vs Ag/AgCl (saturated KCl), all potentials in this paper were reported with respect to this reference) in 0.5 M H2SO4 until a stable voltammogram was obtained, and finally washing with copious amounts of water. Its mean geometric area was 0.0056 cm2, obtained from diffusion-controlled experiments with Ru(NH3)63+.5 The roughness factor of the electrode determined by using iodine desorption6 was 2.1, a mean value with a standard derivation of 0.1 (4.7%) given by three individual measurements. Formation of ICs. Typically, CH3(CH2)n-1SH (n ) 10, 14, 18) was added directly to the aqueous solution of CD at room temperature with a molar ratio of 1:2 (in the presence of an excess of CD in order to avoid the formation of precipitation7 and also to ensure that the majority of CH3(CH2)n-1SH presented in solution is bound by CD moieties). The resulting mixture was kept in a water bath (40 ( 0.1 °C) with stirring for at least 48 h and then cooled to room temperature. It was found that CH3(CH2)n-1SH, presented originally in the form of oily layers (n ) 10, 14) or white powder (n ) 18) on the water surface at room temperature, disappeared completely in our control experiments under the same conditions; however, when CH3(CH2)n-1SH (n ) 10, 14, 18) was added directly to pure water, such oily layers or white powder did not disappear because of their very low solubility in pure water, indicating the complexation process really occurred. The resulting solution was used as a loading solution to assemble monolayers. Spontaneous Monolayer Formation. In a typical experiment, a precleaned gold electrode was immediately immersed in the loading solution for 24 h at room temperature. It was removed from solution, vigorously rinsed with absolute ethanol and water, and blown dry with argon (99.999%) prior to use. X-ray Photoelectron Spectroscopy (XPS). XPS was performed on an ESCALAB-MKII spectrometer that focused monochromatic Al KR X-rays onto the sample. A survey spectrum over a binding energy range of 0-1250 eV with an analyzer pass energy of 150 eV and an X-ray spot size of 6 mm was acquired to determine which elements were presented on the outer surface (5) Zhang, L.; Lu, T.; Gokel, G. M.; Kaifer, A. E. Langmuir 1993, 9, 786. (6) Rodriguez, J. F.; Mebrahtu, T.; Soriaga, M. P. J. Electroanal. Chem. 1987, 233, 283. (7) Ramamurthy, V.; Eaton, D. F. Acc. Chem. Res. 1988, 21, 300.
© 1997 American Chemical Society
3252 Langmuir, Vol. 13, No. 12, 1997
Notes
Figure 1. 1H-NMR spectra for (a) CH3(CH2)17SH, (b) β-CD‚C18, (c) β-CD‚C14, (d) β-CD‚C10, and (e) β-CD at room temperature (ca. 25 °C). The compound in part a was dissolved in CDCl3, and the others were dissolved in D2O. [β-CD] ) 8 mM; [Cn] ) 4 mM (n ) 10, 14, 18). of the gold substrate. Elemental compositions were calculated from high-resolution spectra of C 1s, O 1s, and S 2p photoemission lines. The binding energies were referenced to Au 4f7/2 at 83.4 eV. The substrates used were prepared by the resistive evaporation of a 100 nm gold layer onto a precleaned silicon wafer at a base pressure of 1 × 10-7 Torr with a 50 nm layer of chromium deposited on the silicon as an adhesion promoter. Flat terraces of gold on the surface of these substrates were observed by scanning tunneling microscopy (STM). Prior to use, they were treated with freshly prepared “piranha solution” (7:3 concentrated H2SO4 and 30% H2O2. Caution: piranha solution reacts violently with organic compounds and should not be stored in closed containers8) and washed with copious amounts of water. Samples were prepared as described previously for the spontaneous monolayer assembly. 1H-NMR Spectroscopy. The high-resolution 1H-NMR spectra were recorded on a Varion Unity-400 MHz spectrometer at room temperature. Unless specified, samples were dissolved in D2O. Cyclic voltammetry (CV). CV was carried out on a Cypress Systems (Model CS-1090/Model CS-1087 computer-controlled electroanalytical systems) instrument with three electrodes, a Ag/AgCl electrode as reference electrode, a Pt flag as counter electrode, and a SAM-modified gold electrode as working electrode. All solutions were freshly prepared; they were purged with argon for 15 min before measurements.
Results Formation of ICs. For ICs in solution, it was reported that the fitting closeness of the guest molecule within the CD cavity is the decisive factor in the stability of the resultant complex.2 At this point, the hydrocarbon chain with a diameter of 0.5 nm should fit most snugly and easily within the cavity of R-CD, which has the same diameter, and it should also fit within that of β-CD, with a diameter of 0.78 nm. In the case of the inclusion of a guest molecule (whether aromatic9 or aliphatic4b) by CDs in solution, 1H-NMR is known to be an excellent method of determining the existence of ICs. As far as aliphatic guests are concerned, if the polymethylene groups (i.e. (8) Pintchovski, F.; Price, J. B.; Tobin, P. J.; Peavey, J.; Kobold, D. J. Electrochem. Soc. 1979, 126, 1428. (9) Demarco, P. V.; Thakkar, A. L. J. Chem. Soc., Chem. Commun. 1970, 2.
the tails) thread through the CD cavity, some new signals in the aliphatic range of the 1H-NMR spectrum will be observed due to the removal of mirror symmetry upon inclusion in the asymmetric host,10 while the 1H-NMR spectra of CDs themselves do not change by the addition of guests. Figure 1 shows the high-resolution 1H-NMR spectra of (a) C18SH (in CDCl3), (b) β-CD‚C18SH, (c) β-CD‚C14SH, (d) β-CD‚C10SH, and (e) β-CD. It was readily apparent that new signals at δ 1.35-1.40 appeared in the spectra b-d. Simultaneously, no changes were observed for the signals of β-CD, as well as those of the CH3 groups at ca. δ 0.96 and of the CH2-S groups at ca. δ 2.5 (not shown here). Besides, signals assigned to the CH2 groups in the spectra of CH3(CH2)n-1SH (in CDCl3) (singlet, δ 1.28 for n ) 10, δ 1.27 for n ) 14, and δ 1.25 for n ) 18) were split into triplets upon inclusion (δ 1.223 (J ) 7 Hz) for n ) 10, δ 1.224 (J ) 7 Hz) for n ) 14, and δ 1.227 (J ) 7 Hz) for n ) 18). Furthermore, no evidence was obtained for the presence of significant residual nalkanethiols. All of the above indicate that the axial inclusion complexes actually formed. On the basis of the following two factors, we can reasonably assume that CDs gird CH3(CH2)n-1SH (n ) 10, 14, 18) molecules amidships with SH penetrating through their cavities: (1) the 1:1 complex is the most common stoichiometry found when such ICs are formed in aqueous solution,4c and (2) compared with the tails, the surface-active groups (SH) are relatively hydrophilic. Given such ICs in solution, well-ordered and CD-immobilizing SAMs (denoted as MCD‚Cn, MR-CD‚Cn, and Mβ-CD‚Cn, respectively) should be formed by dipping the gold substrate into the solution. In order to confirm this point, as well as to determine the surface composition of the monolayers, we carried out XPS measurements. XPS measurements. A survey scan for Mβ-CD‚Cn (n ) 10, 14, 18) over the binding energy range 0-1250 eV confirmed the presence of the expected carbon (C), oxygen (O), and sulfur (S); however, in our control experiments (10) (a) Saito, H.; Yonemura, H.; Nakamura, H.; Matsuo, T. Chem. Lett. 1990, 535. (b) Wenz, G.; Keller, B. Angew. Chem., Int. Ed. Engl. 1992, 31, 197.
Notes
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Figure 2. High-resolution XPS spectra of the C 1s (a), Au 4f7/2 (b), O 1s (c), and S 2p (d) photoelectrons from SAMs of β-CD‚Cn (n ) 10, 14, 18).
under the same conditions, no C, O, and S were detected for substrates incubated in one aqueous loading solution containing only β-CD, and no O was detected for substrates incubated in another ethanol loading solution containing only CH3(CH2)n-1SH (n ) 18). As shown in Figure 2, the high-resolution spectra recorded for Au 4f, C 1s, O 1s, and S 2p revealed peaks at 83.4, 284.6, 532.6, and 161.9 eV, respectively. All of the binding energies are in good agreement with the reported values.11 Elemental compositions were calculated semiquantitatively from detailed scans of C 1s, O 1s, and S 2p photoemission lines. Table 1 summarizes the surface compositions detected under (11) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. A.; Porter, M. D. Langmuir 1988, 4, 365.
Table 1. Atomic Percentage of Mβ-CD‚Cn Estimated by XPS atom % n ) 10
n ) 14
n ) 18
element
obsd
calcd
obsd
calcd
obsd
calcd
C O S
55.8 43.5 0.7
59.1 39.8 1.1
57.4 42.1 0.5
60.9 38.0 1.1
57.7 41.7 0.6
62.5 36.5 1.0
the experimental conditions used. From this table, we can see that the values observed are in close proximity to those calculated by assuming the formation of 1:1 complexes on the gold surface, indicating that the surface composition of Mβ-CD‚Cn is equal to the composition of β-CD‚Cn in solution. A conclusion can then be drawn that the monolayer is probably formed through the spontaneous
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Figure 3. Cyclic voltammograms of SAM-modified electrodes formed by β-CD‚Cn (a), R-CD‚Cn (b), and Cn (c) in 0.5 M H2SO4 after five repeated cycles. SAMs of a and b were formed in H2O, and SAMs of c in CH3CH2OH; n ) 10, 14, and 18 corresponded to dotted, dashed and solid lines, respectively. Scan rates were 0.1 V/s.
assembly of β-CD‚Cn, rather than through the competitive assembly of β-CD‚Cn and CH3(CH2)n-1SH. That is to say, the loading solution is essentially free of a significant residual of CH3(CH2)n-1SH, which coincides well with the 1 H-NMR observation. Blocking Effect in an Acid Medium Containing No Redox Couples. The voltammogram of bare gold in 0.5 M sulfuric acid exhibits two broad peaks at ca. 1.11.4 V corresponding to the characteristic oxide formation and one spike at ca. 0.9 V corresponding to the oxide reduction. On the SAM-modified electrodes, as shown in Figure 3 (the voltammograms obtained in the gold oxide formation/reduction potential range after five repetitive cycles), these oxidation and rereduction features are suppressed by all of the monolayers to some extent: Mβ-CD‚Cn provides the best barrier to oxidation, followed by MR-CD‚Cn and finally SAMs of CH3(CH2)n-1SH alone (denoted as MCn). To our great surprise, the shorter the chain length, the greater the blocking effect observed for both
Notes
Mβ-CD‚Cn and MR-CD‚Cn. This result is contrary to the wellknown case for MCn.12 In addition, for all of these SAMs, the repeated potential cycling initially from -0.3 to ca. 1.2 V shows the voltammograms whose currents are independent of potential and scale linearly with scan rate, as expected for an impermeable dielectric film. Afterward, a scan to positive potentials is made to determine at what potentials the blocking effect of the monolayer fails. When the positive limit is set at ca. 1.4 V, for instance, there appears the oxidation and rereduction peaks, indicating the gold-sulfur interaction has been partly disrupted.14 After half an hour of successive cycling, these peaks are essentially unchangeable. Only little changes can be observed for MCD‚Cn when the potential window is set from -0.3 to 1.5 V, a potential range at which significant alterations can be observed for MCn (not shown here). In summary, we have the following relative blocking sequence as Mβ-CD‚Cn > MR-CD‚Cn > MCn, which is also the relative order of stability. Blocking Effect in Neutral Media Containing Redox Couples. Figure 4 illustrates CV curves of bare and SAM-modified electrodes in 1 M KNO3 containing 1 mM Fe(CN)63- (solid line) or 1 mM Ru(NH3)63+ (dotted line). As shown in Figure 4a, the CV curves for bare gold show the reduction and reoxidation peaks typical for a diffusion-limited one-electron redox process.13 All of the monolayers inhibit the electron transfer to the electrode in the contacting electrolyte to a certain degree. For Mβ-CD‚C10, the reduction current of Fe(CN)63- or Ru(NH3)63+ at the formal potential E°′ (ca. 0.27 V for the former and ca. -0.17 V for the latter) is ca. 3 orders of magnitude lower than that at the same potential for bare gold. Only when the potential is made more negative does the reduction current increase, which has been previously attributed to the fact that the increased exothermicity of the reaction increases the electron transfer rate.14 The longer the chain length, the larger the reduction current (with respect to the individual charging current). For Mβ-CD‚C18, there appeared a distinct “S-shaped” wave, independent of scan direction, indicating the redox process is at a steady state for each couple. In other words, the reaction on SAM-modified electrodes is kinetically limited. For MR-CD‚Cn, the same inhibiting phenomena were obtained (not shown here). Besides, we also observed the following order of inhibition: Mβ-CD‚Cn > MR-CD‚Cn > MCn. Discussion The barrier property of the monolayer to inhibit both the oxidation of the underlying substrate and the electron transfer to solution-based redox couples is a good measure of the quality of the molecular packing, and this packing is a function of the spacing between the surface-active head-groups and of VDW, hydrogen bonding and dipole interactions between molecules, etc.15 It is generally accepted that the driving force for the tilt of the tails with respect to surface normal is the re-establishment of VDW contact among chains.1f. For example, in the case of CH3(CH2)n-1SH/Au(111), since the head-head spacing (0.499 nm) is greater than the touching VDW distance of the tails (0.424 nm), the tails are forced to tilt ca. 30° from the surface normal to maximize their attractive VDW (12) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (13) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; Wiley: New York, 1980. (14) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682. (15) Ulman, A.; Eilers, J. E.; Tillman, N. Langmuir 1989, 5, 1147.
Notes
Langmuir, Vol. 13, No. 12, 1997 3255
more nearly matching the commensurate spacing of the gold lattice) chemisorbed on gold, a tilt of 16° has been reported by Chidsey,14 who further predicted that “incommensurate packing would allow even denser packing, with no tilt at all”. In our case, on one hand, each adsorbate has a maximum diameter of approximately 1.4-1.6 nm (known from the dimensions of the R-CD and β-CD cavities2a), a spacing about 2 times larger than the headhead spacing. Such a diameter should result in a denser packing for CD rings. On the other hand, the rigid cavity of CDs will define rigorously the interchain distance of the adsorbed ICs. By rough estimate, it should be ca. 1.0 nm, a distance out of VDW works. Thus, the VDW interactions among the tails are missing because only the CD rings are closely packed. A very similar conclusion has been already drawn by Knoll and his co-workers.17 Although we do not know the exact orientation of ICs on the gold surface at present, we can imagine reasonably that all-trans chains require very little or no chain tilt. Apparently, the direct result of chain tilt is the formation of tilt domain boundaries,18 one of the main candidates for defect sites. In other words, the tilted tails produce more easily these boundaries than the tails with very little or no chain tilt. Therefore, it seems that such boundaries in MCD‚Cn are less than those in MCn. Under the assumption that all CD cavities gird initially the tails at the same place, their ends should be located the same distance from the gold surface. The less tilted the tails, the closer the O(2) and O(3) hydroxyl groups among the adjacent CDs. If this is true, the O(2) and O(3) hydroxyl groups of each adjacent glucose residue can form both intra- and intermolecular hydrogen bonds according to Pauling’s hydrogen bond rules,19 which leads to the formation of a network of intermolecular hydrogen bonds. Such a network has already been found in the crystalline ICs of β-CD with fenoprofen.20 The formation of such a kind of network decreases dramatically the free energy of the structure and thus stabilizes the monolayer assembly, which makes MCD‚Cn sturdier than MCn. As is well-known, the more hydrogen bonds formed, the more stable the structure, resulting in Mβ-CD‚Cn being more stable than MR-CD‚Cn. Clearly, the more stable the monolayer formed, the less the tendency to be damaged by repeated potential cycling and, therefore, the stronger the inhibition observed. That is why we have the blocking sequence Mβ-CD‚Cn > MR-CD‚Cn > MCn. To date, we have no good explanations for the relationship between the length of the tails and the blocking effects. Work, such as molecular orientation characterization by ellipsometry and FTIR, wettability characterization by contact angle measurement, and topographic observation by STM, is just in progress.
Figure 4. Cyclic voltammograms of bare gold electrodes (a) in 1 M KNO3 (pH ) 7) containing 1 mM Fe(CN)63- (solid line) and 1 mM Ru(NH3)63+ (dotted line); parts b, c, and d were those of SAM-modified electrodes formed by CD‚C10, CD‚C14, and CD‚C18, respectively. All scan rates were 0.1 V/s.
interactions (1.4-1.8 kcal/mol of CH2).16 For CF3(CF2)7(CH2)2SH (with a chain diameter of 0.56 nm, (16) (a) Salem, L. J. Chem. Phys. 1962, 37, 2100. (b) Hautman, J.; Klein, M. L. J. Chem. Phys. 1989, 91, 4994; 1990, 93 (10), 7483. (c) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (d) Zhang, R.; Gellman, A. J. J. Phys. Chem. 1991, 95, 7433.
Acknowledgment. This work was supposed by the National Natural Science Foundation of China. Supporting Information Available: Cyclic voltammograms of MC18- and Mβ-CD‚C18-modified electrodes (1 page). Ordering information is available on any current masthead page. LA961049E (17) Mittler-Neher, S.; Spinke, J.; Liley, M.; Nelles, G.; Weissei, M.; Back, R.; Wenz, G.; Knoll, W. Biosens. & Bioelectron. 1995, 10, 903. (18) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301. (19) Etter, M. C. Acc. Chem. Res. 1990, 23, 120 and references therein. (20) Hamilton, J. A.; Chen, L. Y. J. Am. Chem. Soc. 1988, 110, 4379.
