Inclusion Complexation of γ-Cyclodextrin and Coumarin Dye inside

Aug 2, 2013 - Inclusion Complexation of γ-Cyclodextrin and Coumarin Dye inside Alumina Nanopores over a Temperature Range of 303–233 K...
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Inclusion Complexation of γ‑Cyclodextrin and Coumarin Dye inside Alumina Nanopores over a Temperature Range of 303−233 K Akira Yamaguchi* and Tetsuya Denda College of Science, Ibaraki University, 2-1-1 Bunkyo, Mito, Ibaraki 310-8512, Japan ABSTRACT: In the present study, we aimed to demonstrate that complexation reactions inside an inorganic water-filled nanopore can be enhanced at below the homogeneous nucleation temperature of bulk water (273 K). Using fluorescence experiments, we examined the inclusion complexation of propylamide coumarin 343 (PAC343) with γ-cyclodextrin (γ-CD) inside cylindrical alumina nanopores over a temperature range of 303−233 K. A PAC343-immobilized porous anodic alumina membrane (PAC343-PAA) with a pore diameter of ca. 70 nm was fabricated, and its fluorescence spectra were measured after introduction of aqueous γ-CD solution into the alumina nanopores. Our results confirmed that the inclusion complexation reaction was gradually enhanced with decreasing temperature, particularly at temperatures between 273 and 233 K. The inclusion complexation reaction was confirmed to be exothermic from van’t Hoff analysis of adsorption constants of γ-CD obtained at temperatures between 288 and 283 K. These results confirm low-temperature (T < 273 K) conditions to be useful for enhancing exothermic complexation reactions that occur inside alumina nanopores filled with water.

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beam.16 On the other hand, even when water freezes inside a confined space, there is a possibility that fluid water will still exist between the frozen water and the inner pore surface.17−21 It can be expected, therefore, that molecules inside inorganic nanopores filled with water can diffuse and interact with functional groups on the inner pore surface even when the temperature is below 273 K; that is, inorganic materials can provide a reaction space under low-temperature (T < 273 K) conditions. Assuming a simple van’t Hoff relationship for an exothermic complexation reaction, the complexation efficiency under lowtemperature (T < 273 K) conditions becomes much greater than that at around 283 K, a common temperature for chemical and bioanalysis. When chromogenic ligands are immobilized within the inorganic nanoporous material, the higher complexation efficiency under low-temperature conditions should prove advantageous for sensitive detection of water-soluble analytes via complexation with the chromogenic ligands. However, the process by which low-temperature conditions enhance the complexation reaction inside the inorganic nanopore has not been elucidated and requires experimental confirmation. In the present study, inclusion complexation between γcyclodextrin (γ-CD) and propylamide coumarin 343 (PAC343) immobilized on the inner pore surfaces of a porous anodic alumina (PAA) membrane (pore diameter = ca. 70 nm) was examined by means of steady-state fluorescence experiments over a temperature range of 303−233 K (Figure 1). The reason

INTRODUCTION Inorganic nanoporous materials are a focus of much attention in the field of chemical and bioanalysis because they provide a stable and well-defined confined nanospace for constructing specific molecular recognition systems.1−8 For sensitive and selective analysis, it is necessary to regulate the complexation property between analytes and ligand molecules inside the pores. When ligand molecules are immobilized on inner pore surfaces of inorganic nanoporous materials, their complexation properties depend on the chemical structure of individual ligand molecules, the self-assembled nanoarchitecture of ligand molecules, and the structure of inorganic frameworks.8 Structural control of ligand assembly and inorganic frameworks is thus an important subject in the field of chemical and bioanalysis using inorganic nanoporous materals.1−8 In the present study, on the other hand, we aimed to demonstrate that a complexation reaction inside an inorganic nanopore filled with water could be enhanced at below the homogeneous nucleation temperature of bulk water (273 K). It has been revealed in the past decade that water inside mesoscopic and micrometer-sized confined space maintains its fluidity at below 273 K.9−16 For water confined inside pores of mesoporous silica, the freezing temperatures, estimated from differential scanning calorimetry (DSC) measurements, have been reported as being much lower than 273 K.9−11 Distortion of the tetrahedral-like hydrogen-bonded structure of water has been proposed for the supercooling of water confined inside the silica mesopores.14 The lowering of freezing temperature has been also observed inside microfluidic channels (channel width = 70−300 μm) made of Pyrex glass15 and in water droplets (diameter = ca. 10 μm) trapped in a focused laser © 2013 American Chemical Society

Received: May 6, 2013 Revised: July 31, 2013 Published: August 2, 2013 17567

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Figure 1. Schematic illustrations for (a) introduction of γ-CD into the alumina pore during immersion of PAC343-PAA in an aqueous γ-CD solution and (b) PAC343-PAA in heptane after introduction of γ-CD aqueous solution into the alumina pore.

for choosing the combination of γ-CD and the coumarin dye is that inclusion complexation can be confirmed by changes in both fluorescence intensity and fluorescence maximum wavelength of the coumarin dye.22 γ-CD also has a sufficiently large cavity to allow insertion of a coumarin moiety in PAC343.22−24 We first examined inclusion complexation inside the alumina pores at temperatures between 303 and 283 K. The thermodynamic parameters for the PAC343/γ-CD complex formation were estimated from van’t Hoff analysis of the adsorption constants of γ-CD. Changes in complexation efficiency over a temperature range of 303−233 K were then qualitatively examined by observing the fluorescence spectra of PAC343-PAA containing an aqueous γ-CD solution within the pores. The results obtained in this study confirmed that the exothermic inclusion complexation reaction could be enhanced by decreasing the temperature, particularly to below 273 K.

Figure 2. Typical SEM top view of PAC343-PAA.

coumarin 343 and propylamine immobilized on the inner pore surfaces.27,28 Immobilization of the propylamine groups was carried out by immersing the PAA membrane in a mixture of 30 mL of dry toluene and 1.5 mL of aminopropyltriethoxysilane (APTES). The mixed solution was refluxed at 130 °C for 12 h in a nitrogen atmosphere. After being rinsed with dry toluene and dry ethanol, the APTES-immobilized PAA membrane was immersed in 30 mL of dichloromethane containing coumarin 343 (10 mg) and EDAC (12 mg). The mixture was stored at 37 °C for 12 h in a nitrogen atmosphere. The PAC343-immobilized PAA membrane (PAC343-PAA) was rinsed with acetone and water until the elution of free coumarin 343 ceased. Measurements. For study of the inclusion complexation at temperatures between 303 and 283 K, the PAC343-PAA was attached to a Teflon holder in a quartz cell (1 cm × 1 cm) filled with a CD solution.13 The fluorescence from the PAC343-PAA was measured by changing the CD concentration in the cell. The fluorescence spectra were measured on a Jasco Model FP6800 spectrofluorophotometer equipped with a thermoelectrically temperature-controlled cell holder (Jasco: model ETC815). To study the inclusion complexation over a temperature range of 303−233 K, the PAC343-PAA was immersed in aqueous γ-CD solution (0, 1, 10, and 50 mM) to introduce aqueous γ-CD solution into the pores of PAC343-PAA. After the membrane was washed and excess water was removed from the membrane surface, the PAC343-PAA was attached to a Teflon holder in a quartz cell filled with heptane.13 Immersion of the PAC343-PAA in heptane prevented the evaporation of water from inside the pores during the fluorescence measurements.13 Fluorescence spectra were measured on a Hitachi

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EXPERIMENTAL Materials and Chemicals. Aluminum plate (99.9%; Nilaco Co., Tokyo, Japan) was used to fabricate a porous anodic alumina (PAA) membrane. Coumarin 343 was purchased from Sigma-Aldrich Japan (Tokyo, Japan). 1-Ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDAC) was purchased from Dojindo Laboratories (Kumamoto, Japan). Other chemicals were purchased from Wako Pure Chemical Industries Ltd. (Osaka, Japan). All chemicals were used as received. MilliQ water was used for all the experiments. Preparation of PAC343-PAA. An Al plate (1 cm × 10 cm × 0.5 mm) was electropolished in a phosphoric acid−sulfuric acid−water−ethylene glycol mixture (66:15:16:3 volume ratio) prior to its anodization. The PAA membrane was fabricated by anodizing the Al plate at a constant potential, as reported in the literature,25 with some modifications: a constant potential of 40 V was applied to the Al plate while immersed in 0.5 M oxalic acid solution at 17 °C. After being anodized for 24 h, the PAA membrane was detached from the Al plate using an electrochemical detachment method26 in 0.5 wt% phosphoric acid solution. The fabricated PAA membrane has a packed array of columnar alumina pores. The pore diameter (ca. 70 nm: Figure 2) and membrane thickness (100 μm) were characterized using a field-emission SEM (Hitachi S-4800). From nitrogen adsorption/desorption isotherm measurements using a Micrometrics ASAP 2020 instrument, the BET surface area of the PAA membrane was estimated to be 12 m2 g−1. The PAA membrane modified with propylamide coumarin 343 (PAC343) was prepared by a coupling reaction of 17568

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greater for β- and γ-CD solutions than for α-CD solutions (Figure 4). It has been reported that inclusion affinities of

model F-4500 spectrofluorophotometer equipped with a cryostat (Unisoku Co. Ltd., CoolSpek USP-230). The fluorescence spectra of PAC343-PAA immersed in ethanol were also measured over a temperature range of 298−233 K to examine the fluorescence properties of PAC343 in a less-polar environment.

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RESULTS AND DISCUSSION Inclusion Complexation at Temperatures between 303 and 283 K. Figure 3a shows fluorescence spectra for

Figure 4. Dependences of F/F0 at 495 nm on the concentrations of αCD (blue open triangles), β-CD (black open squares), and γ-CD (red shaded circles). The red solid line denotes the best fit of data for γ-CD to eq 2.

hydroxycoumarin derivatives, which have molecular sizes similar to PAC343, with α-CD were much smaller than those with β-CD due to the smaller cavity size of α-CD.22,23 It can thus be considered that the inclusion affinities of PAC343 with β- and γ-CD are much greater than that with α-CD, resulting in observation of greater increments of fluorescence intensity for β- and γ-CD solutions. Since the fluorescence spectral change in γ-CD solutions can be ascribed to the inclusion complexation of PAC343 with γCD, the fluorescence intensity (F) for PAC343-PAA in the γCD solutions is described as follows:

F = 1 + (β − 1)x F0

(1)

where F0, β, and x are fluorescence intensities found in the absence of CD, ratio of fluorescence emission efficiencies of PAC343 complexed and uncomplexed with γ-CD, and fraction of PAC343 complexed with γ-CD. In the present study, the relationship between fluorescence intensity ratio (F/F0) and bulk CD concentration (CCD) was analyzed on the basis of the Langmuir adsorption model, which has been often been applied to analyze the inclusion complexation of CD at interfacial regions.29,30 On the basis of the Langmuir adsorption model, the relationship between F/F0 and CCD is described as follows.

Figure 3. (a) Fluorescence spectra observed for PAC343-PAA in aqueous γ-CD solutions. The fluorescence intensities are normalized to fluorescence peak intensity observed in the absence of γ-CD. (b) Dependences of F/F0 (F and F0 are fluorescence intensities in the presence and absence of γ-CD: shaded circles) at 495 nm and λmax (maximum wavelength of fluorescence spectrum: open circles) on the concentration of γ-CD.

PAC343-PAA in γ-CD solutions at 298 K. The spectral shape for PAC343-PAA in the absence of CD is almost the same as that for free PAC343 in a water−dioxane mixture (99.8 mol % of water),28 indicating that the fluorescence spectra shown in Figure 3a are due to fluorescence of PAC343 immobilized on the alumina pore wall. In the presence of γ-CD, the fluorescence of PAC343 rises at higher concentrations of γCD. As shown in Figure 3b, the increased fluorescence intensity is accompanied by a blue-shift of the fluorescence maximum. Both a significant increase in fluorescence intensity and a blueshift of the fluorescence maximum are seen when the γ-CD concentration exceeds around 1 mM. These observed fluorescence responses confirm inclusion complexation of the PAC343 with γ-CD: the fluorescence quantum efficiency of the PAC343 increases with insertion of the coumarin moiety of PAC343 into the hydrophobic γ-CD cavity.22 The inclusion complexation of PAC343 with γ-CD is further supported by the fact that the fluorescence response is dependent on CD type: The increments of fluorescence intensity associated with higher CD concentrations are much

x K C F = 1 + (β − 1) max ad CD F0 1 + K adCCD

(2)

Kad and xmax are the Langmuir adsorption constant and maximum fraction of PAC343 complexed with CD. As shown in Figure 4b, the relationship between F/F0 and CCD can be fitted to eq 2, and thus eq 2 was used to estimate Kad values at different temperatures to discuss inclusion complexation of PAC343 with γ-CD. As shown in Figure 4, significant increases in fluorescence intensity with increasing bulk CD concentration can be seen with β-CD and γ-CD. However, the bulk β-CD concentration was limited to a maximum of 10 mM due to its poor water solubility,24 and this limitation prevented reliable analysis of the adsorption isotherm using eq 2. We therefore focused on the inclusion complexation of PAC343 with γ-CD. Figure 5 shows the temperature dependence of the adsorption constant of γ-CD. It can be seen that the adsorption 17569

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Figure 5. van’t Hoff plot for the adsorption of γ-CD at PAC343-PAA.

Figure 6. Changes in fluorescence spectra for (a) PAC343-PAA with confined water, (b) PAC343-PAA with confined ethanol, and (c) PAC343-PAA with confined aqueous γ-CD solution, by decreasing temperature from 303 to 233 K.

constant is larger at lower temperatures. On the basis of the van’t Hoff equation, the apparent thermodynamic parameters of an adsorption process are related to the Langmuir adsorption constant as follows:31 ΔG◦ ΔH ◦ ΔS◦ = − (3) RT RT R where ΔG°, ΔH°, and ΔS° are Gibbs free energy, standard enthalpy change, and standard entropy change of the adsorption process. As shown in Figure 5, the relationship between ln Kad and 1/T appears to be linear at temperatures between 283 and 298 K. Accordingly, ΔH° and ΔS° values were estimated using the least-squares fitting of eq 3 to the adsorption constant data at temperatures between 283 and 298 K. The adsorption constants and thermodynamic parameters for the adsorption of γ-CD are summarized in Table 1. As can be seen from Table 1, the adsorption of γ-CD based on inclusion complexation is exothermic and enthalpy-driven, with a negative entropic contribution (TΔS°), consistent with inclusion complexation between coumarin dyes and cyclodextrins in bulk solution22,23 and at interfacial regions.32 The estimated thermodynamic parameters imply that, below 283 K, the inclusion affinity between PAC343 and γ-CD increases with falling temperature. Inclusion Complexation over a Temperature Range of 303−233 K. The fluorescence emission efficiency of coumarin dyes is greater at lower temperatures.33,34 We therefore examined changes in fluorescence emission efficiency of PAC343 with temperature by observing the fluorescence spectra of PAC343-PAA with confined water or confined ethanol over a temperature range of 303−233 K. Figure 6 panels a and b show fluorescence spectra for PAC343-PAA with confined water or confined ethanol; the temperature dependencies of fluorescence intensities normalized by their value at 303 K are summarized in Figure 7. In the following discussion, the ratio of fluorescence intensity at temperature T to the intensity at 303 K is defined as the temperature-dependent enhancement factor, F(T)/F(303), of fluorescence intensity. −ln K ad =

Figure 7. Temperature dependencies of enhancement factors F(T)/ F(303) (F(T) and F(303) are fluorescence intensities at temperature T and 303 K) for PAC343-PAA with confined water (black open squares), PAC343-PAA with confined ethanol (black open triangles), PAC343-PAA treated with 50 mM γ-CD solution (blue shaded circles), PAC343-PAA treated with 10 mM γ-CD solution (red shaded circles), and PAC343-PAA treated with 1 mM γ-CD solution (green shaded circles). For all PAC343-PAA samples, F(T)/F(303) values were obtained at the fluorescence peak wavelength found at 303 K. The error bars represent the standard deviation observed for three different PAC343-PAA treated with 10 mM γ-CD solution.

For PAC343-PAA with confined water, the fluorescence intensity tended to rise with falling temperature from 303 to 233 K (Figure 6a), and a maximum enhancement factor of 1.3 was obtained at around 233 K (Figure 7). A maximum enhancement factor of 1.3 was also observed at around 233 K when the pores of the PAC343-PAA were filled with less-polar ethanol rather than water (Figure 7). These results suggest that the maximum enhancement factor is not significantly influenced by polarity around PAC343. It has been reported that the effective polarity of the cyclodextrin cavity is like that of organic solvents such as water−methanol mixture, ethanol, and

Table 1. Adsorption Constants and Thermodynamic Parameters for the Adsorption of γ-CD at PAC343-PAA temp/K

Kad/M−1

−ΔG°a/kcal mol−1

−ΔH°b/kcal mol−1

TΔS°c/kcal mol−1

283 288 293 298 303

28 27 19 14 3.6

1.9 1.9 1.7 1.6 0.78

8.0 8.0 8.0 8.0

−6.1 −6.1 −6.3 −6.5

Values derived from adsorption constants. bValues obtained by the best fit of adsorption constant data at temperature between 298 and 283 K using eq 3. cValues derived from ΔH°−ΔG°. a

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other alcohols.35 It is thus likely that the maximum enhancement factor of PAC343 inside the γ-CD cavity is also 1.3 in the temperature range of 303−233 K. In our study of inclusion complexation over the temperature range of 303−233 K, the PAC343-PAA was immersed in aqueous γ-CD solution (1, 10, and 50 mM) at 298 K to introduce aqueous γ-CD solution into the pores of PAC343PAA. As shown schematically in Figure 1a, it can be considered that a proportion of the γ-CD molecules introduced into the pores form PAC343/γ-CD complex, whereas the remaining γCD molecules do not form the complex and are located in the water phase within the pore and on the pore walls. By assuming that all PAC343 groups can form a 1:1 inclusion complex with γ-CD,36 the adsorption constant value at 298 K (14 M−1) implies that at 298 K, the fractions of PAC343 complexed with γ-CD are 2%, 10%, and 40% during the immersion of PAC343PAA in 1 mM, 10 mM, and 50 mM γ-CD solutions, respectively. Thus, free PAC343 groups, free γ-CD molecules, and the PAC343/γ-CD complex coexist inside the pores when PAC343-PAA is immersed in the γ-CD solutions at 298 K. In the fluorescence measurements, the PAC343-PAA containing aqueous γ-CD solution inside the pores was immersed in heptane (see Figure 1b) and its fluorescence spectra were measured while decreasing the temperature from 303 to 233 K. It should be noted that the amount of γ-CD within the PAC343-PAA is constant during fluorescence measurement due to the poor solubility of γ-CD in heptane. Figure 6c shows typical fluorescence spectra for PAC343PAA treated with 50 mM γ-CD solution. Gradual enhancement of fluorescence intensity with decreasing temperature is seen over the temperature range of 303−233 K, and the fluorescence intensity enhancement is greater than that in the absence of γCD inside the pores (Figure 6a,b). As shown in Figure 7, the enhancement factor exceeds 1.3, which is the maximum enhancement factor due to the change with temperature in fluorescence emission efficiency of PAC343, at below 273 K. These results indicate that the fraction of PAC343 complexed with γ-CD gradually increases with decreasing temperature, particularly at below 273 K: that is, the low-temperature (T < 273 K) condition is capable of accelerating the inclusion complexation reaction. For the PAC343-PAA treated with 50 mM γ-CD solution, the fraction of PAC343 complexed with γCD appears to be less than 40% at 303 K, and free PAC343 groups and γ-CD molecules exist inside the pores, as previously discussed. These free PAC343 groups and γ-CD molecules at 303 K are likely to form inclusion complexes under lowtemperature conditions. As can been seen in Table 1, the inclusion complexation of PAC343 with γ-CD is exothermic and enthalpy-driven, suggesting that the inclusion affinity rises with falling temperature. We believe that the stronger inclusion affinity at lower temperatures is responsible for the acceleration of inclusion complexation reactions under low-temperature conditions. Another plausible reason for the acceleration of inclusion complexation reactions is the decreasing solubility of γ-CD with decreasing temperature: exclusion of γ-CD molecules from the water phase at low temperature might increase the effective number of γ-CD molecules in the vicinity of PAC343 immobilized at the pore walls.18 The water solubility of γ-CD, Csol in mg/mL, has been empirically described as Csol = 9.64 × 106 e−3187/T in the temperature range between 298 and 318 K.37 With this empirical equation, the water solubilities of γ-CD were calculated as 140 mM at 293 K,

63 mM at 273 K, and 11 mM at 233 K. Taking these solubility values into account, we examined the temperature-dependent fluorescence spectra for PAC343-PAA treated with 1 or 10 mM γ-CD solution. As shown in Figure 7, PAC343-PAA treated with 1 or 10 mM γ-CD solution exhibits a higher enhancement factor than 1.3 at below 273 K, indicating the acceleration of the inclusion complexation reaction under the low-temperature conditions. When the PAC343-PAA is immersed in 1 or 10 mM γ-CD solution, the effective concentration of γ-CD in the water phase inside the pores appears to be less than 1 or 10 mM, respectively, which is smaller than the calculated solubility values. It can thus be concluded that the influence of the decreasing solubility of γ-CD is limited. As expected from the thermodynamic parameters, therefore, a strong inclusion affinity under low-temperature conditions is chiefly responsible for accelerating the inclusion complexation reaction. If water inside the alumina pores is completely frozen at the temperature range between 303 and 233 K, the inclusion complexation reaction would not be accelerated at below the freezing temperature because the diffusion of γ-CD is dramatically suppressed in frozen water. In contrast, the results of fluorescence intensity changes with temperature indicated gradual acceleration of the inclusion complex reaction within the measured temperature range. Accordingly, it can be considered that water in the vicinity of the PAC343immobilized alumina pore wall maintains its fluidity at temperatures over 303−233 K. If water confined inside the alumina pores is supercooled as in the microfluidic channel system,15 γ-CD can freely diffuse inside the alumina pores. Even if the freezing of water in the core region of the alumina pore occurs, γ-CD would be able to diffuse at the fluidic water layer between frozen water and organic layer on the alumina surface. In general, the hydrogen-bonded structure of water is distorted at interfacial regions,21 and this distortion is responsible for the presence of fluidic water at the interfacial region at temperatures below the homogeneous nucleation temperature of bulk water.17−21 In water confined inside mesoporous silica, for example, it has been reported that 1−2 monolayers of water do not freeze even if freezing of water in the core region occurs.9 From a molecular dynamic simulation, the thickness of the fluidic water layer on ice has been proposed to be about 1 nm at temperatures below 273 K.20 Since the aminopropyl and PAC343 groups at the alumina pore walls significantly disturb the hydrogen-bonded structure of water,38 the thickness of the fluidic water layer at the PAC343immobilized alumina pore wall would be larger than those proposed for mesoporous silica and ice crystal systems, and would be more than 1 nm. We consider that this thickness of the fluidic water layer is enough for the diffusion of γ-CD regardless of slow diffusivity.

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CONCLUSION In the present study, we examined the inclusion complexation between γ-CD and PAC343 immobilized on the inner pore surfaces of the PAA membrane (pore diameter = ca. 70 nm) over the temperature range of 303−233 K. van’t Hoff analysis of the temperature-dependent adsorption constant of γ-CD at temperatures between 288 and 283 K confirmed that the inclusion complexation of PAC343 with γ-CD was exothermic and enthalpy-driven. On the other hand, when the inclusion complexation was qualitatively examined at 303−233 K in fluorescence experiments, the observed fluorescence responses 17571

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(13) Yamaguchi, A.; Namekawa, M.; Itoh, T.; Teramae, N. Microviscosity of Supercooled Water Confined within AminopropylModified Mesoporous Silica as Studied by Time-Resolved Fluorescence Spectroscopy. Anal. Sci. 2012, 28, 1065−1070. (14) Smirnov, P.; Yamaguchi, T.; Kittaka, S.; Takahara, S.; Kuroda, Y. X-ray Diffraction Study of Water Confined in Mesoporous MCM-41 Materials over a Temperature Range of 223−298 K. J. Phys. Chem. B 2000, 104, 5498−5504. (15) Matsuoka, S.; Hibara, A.; Ueno, M.; Kitamori, T. Supercooled Micro Flows and Application for Asymmetric Synthesis. Lab Chip 2006, 6, 1236−1238. (16) Ishizaka, S.; Wada, T.; Kitamura, N. In Situ Observations of Freezing Processes of Single Micrometer-Sized Aqueous Ammonium Sulfate Droplets in Air. Chem. Phys. Lett. 2011, 506, 117−121. (17) Cho, H.; Shepson, P. B.; Barrie, L. A.; Cowin, J. P.; Zaveri, R. NMR Investigation of the Quasi-Brine Layer in Ice/Brine Mixtures. J. Phys. Chem. B 2002, 106, 11226−11232. (18) Tasaki, Y.; Okada, T. Up to 4 Orders of Magnitude Enhancement of Crown Ether Complexation in an Aqueous Phase Coexistent with Ice. J. Am. Chem. Soc. 2012, 134, 6128−6131. (19) Yoshida, K.; Yamaguchi, T.; Kittaka, S.; Bellissent-Funel, M.-C.; Fouquet, P. Neutron Spin Echo Measurements of Monolayer and Capillary Condensed Water in MCM-41 at Low Temperatures. J. Phys.: Condens. Matter. 2012, 24, 064101. (20) Nada, H.; Furukawa, Y. Anisotropy in Structural Transitions between Basal and Prismatic Faces of Ice Studied by Molecular Dynamics Simulation. Surf. Sci. 2000, 446, 1−16. (21) Huthwelker, T.; Ammann, M.; Peter, T. The Uptake of Acidic Gases on Ice. Chem. Rev. 2006, 106, 1375−1444. (22) Dondon, R.; Fery-Forgues, S. Inclusion Complex of Fluorescent 4-Hydroxycoumarin Derivatives with Native β-Cyclodextrin: Enhanced Stabilization Induced by the Appended Substituent. J. Phys. Chem. B 2001, 105, 10715−10722. (23) Rekharsky, M. V.; Inoue, Y. Complexation Thermodynamics of Cyclodextrins. Chem. Rev. 1998, 98, 1875−1917. (24) Szejtli, J. Introduction and General Overview of Cyclodextrin Chemistry. Chem. Rev. 1998, 98, 1743−1753. (25) Masuda, H.; Satoh, M. Fabrication of Gold Nanodot Array Using Anodic Porous Alumina as an Evaporation Mask. Jpn. J. Appl. Phys. 1996, 35, L126−L129. (26) Furneaux, R. C.; Rigby, W. R.; Davidson, A. P. The Formation of Controlled-Porosity Membranes from Anodically Oxidized Aluminum. Nature 1989, 337, 147−149. (27) Yamashita, T.; Amino, Y.; Yamaguchi, A.; Teramae, N. Solvation Dynamics at the Water/Mica Interface As Studied by Time-Resolved Fluorescence Spectroscopy. Chem. Lett. 2005, 34, 988−989. (28) Yamaguchi, A.; Amino, Y.; Shima, K.; Suzuki, S.; Yamashita, T.; Teramae, N. Local Environments of Couamarin Dyes within Mesostructured Silica-Surfactant Nanocomposites. J. Phys. Chem. B 2006, 110, 3910−3916. (29) Ju, H.; Leech, D. Host−Guest Interaction at a Self-Assembled Monolayer/Solution Interface: An Electrochemical Analysis of the Inclusion of 11-(Ferrocenylcarbonyloxy)undecanethiol by Cyclodextrins. Langmuir 1998, 14, 300−306. (30) Domi, Y.; Yoshinaga, Y.; Shimazu, K. Characterization and Optimization of Mixed Thiol-Derivatized β-Cyclodextrin/Pentanethiol Monolayers with High-Density Guest-Accessible Cavities. Langmuir 2009, 25, 8094−8100. (31) Finette, G. M. S.; Mao, Q.-M.; Hearn, M. T. W. Comparative Studies on the Isothermal Characteristics of Proteins Adsorbed under Batch Equilibrium Conditions to Ion-Exchange, Immobilized Ion Affinity and Dye Affinity Matrices with Different Ionic Strength and Temperature Conditions. J. Chromatogr. A 1997, 763, 71−90. (32) Filippini, G.; Bonal, C.; Malfreyt, P. Why is the Association of Supramolecular Assemblies Different under Homogeneous and Heterogeneous Conditions? Phys. Chem. Chem. Phys. 2012, 14, 10122−10124. (33) Nad, S.; Kumbhakar, M.; Pal, H. Photophysical Properties of Coumarin-152 and Coumarin-481 Dyes: Unusual Behavior in

confirmed that the inclusion complexation reaction could be accelerated under low-temperature (T < 273 K) conditions. The acceleration of the inclusion complexation reaction appears to be due to stronger inclusion affinity at lower temperatures, as predicted by the thermodynamic parameters. The results obtained in this study indicate that low-temperature conditions are potentially useful for enhancing the efficiency of exothermic complexation reactions that take place within inorganic nanopores.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +81-29-228-8389. Fax: +81-29-228-8389. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by Grants-in-Aid for Scientific Research (No. 24350034 and No. 22225003) from the Ministry of Education, Culture, Sports, Science and Technology of Japan.

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ABBREVIATIONS PAA, porous anodic alumina; CD, cyclodextrin; C343, coumarin 343; PAC343, propylamide coumarin 343; PAC343-PAA, PAC343-immobilized PAA membrane.

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REFERENCES

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