Inclusion Complex of α-Cyclodextrin with Poly(ethylene glycol) Brush

Sep 2, 2016 - LLC. 6-Mercapto-1-hexanol was obtained from Tokyo Chemical Industry Co., Ltd. D2O was purchased from Wako Pure Chemical Industries, Ltd...
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Inclusion Complex of α‑Cyclodextrin with Poly(ethylene glycol) Brush Shoko Takahashi,† Norifumi L. Yamada,‡ Kohzo Ito,† and Hideaki Yokoyama*,† †

Graduate School of Frontier Sciences, The University of Tokyo, Chiba 277-8561, Japan High Energy Accelerator Research Organization, Ibaraki 319-1108, Japan

‡

S Supporting Information *

ABSTRACT: The inclusion complex (IC) of α-cyclodextrin (α-CD) with poly(ethylene glycol) (PEG) brush was investigated by neutron reflectometry and grazing-incidence wide-angle X-ray scattering. When the PEG brush was exposed to 10% aqueous α-CD solution, an IC consisting of randomly oriented α-CD polycrystals appeared. On the other hand, when the PEG brush was exposed to 5% aqueous α-CD solution, a uniform 10-nm-thick IC layer with α-CD stacked perpendicular to the substrate was formed. A 10-nm-thick IC was also found in the diluted PEG brush, even when exposed to the 10% α-CD solution. The characteristic 10-nm-thick layer is related to the folded crystalline structure of α-CD on the PEG brush.

1. INTRODUCTION

2. EXPERIMENTAL SECTION

Since the first report of inclusion complex (IC) formation between α-cyclodextrin (α-CD) and poly(ethylene glycol) (PEG),1 much attention has been paid to the IC between PEG and α-CD. The IC formation is believed to be strongly enhanced by hydrogen bonding between neighboring cyclodextrins on the same polymer axis as well as by hydrophobic interactions between the inner cavity of the cyclodextrin and PEG.1−4 However, the IC formation between a PEG chain and α-CDs in solution induces the crystallization of α-CDs1,5 and therefore precipitation or gelation of the solution;6−8 thus, detailed analysis of the complex formation by conventional analytical techniques in solution such as light scattering, smallangle X-ray scattering, and small-angle neutron scattering, has been an extremely difficult. With the aim to reveal the IC formation between PEG and αCD, tethering polymer main-chains onto a solid surface to prevent aggregation between ICs has been attempted. A minimum concentration of the α-CD solution for the IC formation with the PEG brush was found,9 and the PEG brush was reported to start extending upon IC formation.10 However, the detailed structure of the IC formed has not been revealed. In this report, the structure of the IC of α-CDs with PEG brushes tethered on a flat substrate was investigated by in situ time-resolved neutron reflectometry (NR). High neutron flux available at the Japan Proton Accelerator Research Complex (JPARC)11 enabled time-resolved observation of IC formation. In addition, the crystal structure of the IC brush on the substrate after drying was analyzed by grazing-incidence wideangle X-ray scattering measurements (GIWAXS).

2.1. Materials. α-CD was purchased from Nihon Shokuhin Kako Co. Ltd. Poly(ethylene glycol) methyl ether thiol (mPEG SH) with a molecular weight of 5500 (determined by GPC) was purchased from Shearwater. Poly(ethylene glycol) dithiol (PEG SH) with a molecular weight of 12 000 (determined by GPC) was purchased from SigmaAldrich Co. LLC. 6-Mercapto-1-hexanol was obtained from Tokyo Chemical Industry Co., Ltd. D2O was purchased from Wako Pure Chemical Industries, Ltd., and Sigma-Aldrich Co. LLC. All reagents were used without further purification. 2.2. Sample Preparation. PEG Brush. A 10 nm Au layer on a 3 nm Cr adhesive layer was fabricated on a quartz wafer. mPEG SH and 6-mercapto-1-hexanol (MH) were dissolved in D2O. The Cr/Audeposited wafer was immersed in the solution for 1 h to plant the PEG brush. Three types of PEG brushes with various densities were prepared from the mixed solution of mPEG SH and MH, as shown in Table 1. The total concentration of mPEG SH and MH is 100 μmol/

© XXXX American Chemical Society

Table 1. Feed Ratio of mPEG SH and MH Used To Prepare PEG Brushes and Concentrations of α-CD Solution Used To Prepare the IC sample PEG PEG PEG PEG

brush brush brush brush

1 2 3 4

feed ratioa

α-CD concn [w/v, %]b

100:0 100:0 75:25 50:50

5 10 10 10

a

mPEG SH [μmol/L]/MH [μmol/L]. b1% is defined as 10 mg α-CD dissolved in 1 mL D2O. Received: June 9, 2016 Revised: August 26, 2016

A

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Macromolecules L. For IC formation, the PEG brushes on the wafers were washed with D2O and then exposed to either 5% or 10% α-CD solutions in D2O. Looped PEG Brush. A 10 nm gold (Au) layer on a 5 nm chrome (Cr) adhesive layer was fabricated on a quartz wafer by resistance heating-type vacuum vapor deposition. The Cr/Au-deposited wafer was immersed in the 100 μmol/L PEG SH aqueous solution for 1 h, to plant the PEG loop. Because PEG SH has thiol functional groups on both ends of the PEG chain, the attached PEG brushes are expected to form loop structures. The looped PEG brush on the wafer was washed with D2O and then exposed to a 10% α-CD solution in D2O. Sample Preparation for GIWAXS Measurement. The PEG brush was prepared on a silicon wafer by using the same method used for NR sample preparation. A mPEG SH aqueous solution (100 μmol/L) was used to fabricate the PEG brush wafer, which was then immersed into the α-CD aqueous solution for IC formation. After 24 h, the wafer was carefully removed from the α-CD solution, and the solution remaining on the wafer was flushed by complexed air. Two types of samples were prepared: one is obtained in a 5% α-CD solution in D2O and the other in a 10% α-CD solution. 2.3. Neutron Reflectivity (NR) Measurement. In-situ NR experiments were conducted with Soft Interface Analyzer (SOFIA) at the J-PARC, Ibaraki, Japan. The specular neutron reflectivity of the interface of the polymer brush on the substrate with pure D2O and with the α-CD solution was measured. After the polymer brush was exposed to the α-CD solution, time-resolved neutron reflectometry measurement was conducted by using remote-controlled stopped-flow cell. The depth profiles of SLD were computed by fitting the reflectivity curves using the Motofit package.12 The reflectivity curves were fitted with a multilayer model consisting of a quartz substrate, a Cr layer, a Au layer, a D2O-swollen brush layer or IC layer, and an ambient solution. The SLDs of PEG and α-CD were assumed to be 0.63 and 3.78 × 10−4/nm2, respectively. In order to calculate its SLD, α-CD was regarded as C36H42D18O30 because, in deuterium oxide solution, its OH groups are replaced by OD groups. The NR measurements of the Cr/Au coated substrate, the PEG brush on the substrate and IC layer on the substrate were conducted sequentially so that the SLD profile of the substrate is fixed parameters when the brush profiles are to be fit. 2.4. Grazing-Incidence Wide-Angle X-ray Scattering Measurement (GIWAXS). GIWAXS measurements in air were conducted at the BL-6A beamline of Photon Factory (High energy accelerator research organization, Ibaraki, Japan) by using a Pilatus detector. The wavelength of the X-ray beam was 0.15 nm. The camera length was calibrated with a silver behenate standard.

Figure 1. Time-resolved NR profiles of PEG brush 1 with 5% α-CD solution (a) and PEG brush 2 with 10% α-CD solution (b).

scale accessible for the NR experiment. The only 2-fold difference in the concentration induced a much larger difference in the IC formation time. This significant difference is attributed to the kinetics of the IC formation. If the IC formation is simply governed by the hydrophobic interactions between PEG and the inner cavity of CD, the probability of the PEG chain end finding the CD is proportional to the concentration of CD. Thus, an order of difference in time scale cannot be explained solely by the hydrophobic interactions. We further discuss about the origin of IC formation later in this paper, but we focus only on the equilibrium structure of IC in this section. Figure 2a shows the experimental and the simulated reflectivity profiles of PEG brushes 1 and 2, different brush samples prepared by the same procedure at D2O interface and at α-CD solution interface. The scattering length density (SLD) profiles used to simulate the experimental results in Figure 2a are shown in Figure 2b. The SLD profiles of substrates with Cr

3. RESULTS AND DISCUSSION 3.1. Inclusion Complex Formation of PEG Brush and α-CD. We prepared the quartz substrates coated with Cr and Au and measured the scattering length density (SLD) profiles using NR as described in the Experimental Section. The substrates were subsequently immersed in the D2O solution of PEG with thiol functional group(s) to grow PEG brush and their interfacial structures were evaluated using NR. Furthermore, the PEG grafted quartz substrates were immersed in αCD solution in D2O and monitored the growth of ICs as a function of time as shown in Figure 1. Figure 1 shows the time-resolved NR profiles of PEG brushes in 5% (a) and 10% (b) α-CD solutions. The NR profile of PEG brush in 5% α-CD solution exhibits a drastic change around 300−500 s after the exposure to the solution. In contrast, the similarly prepared PEG brush in 10% α-CD solution shows no time dependence in the reflectivity profile. It should be noted that the reflectivity of 10% is different from the reflectivity of the PEG brush; therefore, it is suggested that IC formation takes more than 200−300 s in the 5% solution, but less than 10 s in the 10% solution, which is the shortest time

Figure 2. NR curves of PEG brushes 1 and 2 in D2O, and PEG brush 1 in 5% α-CD solution, PEG brush 2 in 10% α-CD solution (a) and their corresponding SLD profiles (b). The reflectivities are shifted for clarity. B

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Macromolecules and Au layers were determined by the measurement of the substrate before attaching PEG brush, so the profiles of substrate is fixed in the analysis of PEG brush and IC complex. The reflectivity profiles of the two PEG brushes agree each other within experimental precision; hence, the properties of the brush layers are equivalent. The brush densities for brushes 1 and 2, calculated from the SLD profiles, are 0.18 and 0.14 chains/nm2, respectively. The method to determine brush densities are shown in Supporting Information. Note that the brush densities are not too high to prevent IC formation. PEG brushes were then immersed in 5% and 10% solutions of α-CD in D2O. The ICs formed in 5% and 10% α-CD solutions exhibit very different NR profiles as shown in Figure 2a. The difference in the width of the appearing fringes directly indicates the thickness of the IC layer, even without simulation. The thickness of the IC layers formed in 10% α-CD solutions is much larger than that of the IC layers formed in 5% α-CD solution. The detailed layer structures are simulated by the multilayer model and shown in Figure 2b. In the SLD profile of the 5% α-CD solution, a single layer of IC with a thickness of 10 nm clearly appeared. In contrast, the layer with broader interface appeared in the 10% α-CD solution. Filling ratios of the ICs in 5% and 10% solutions calculated from the SLD profiles are 59% and 96%, respectively. Filling ratio is the value defined as 100% when α-CD: PEG monomer = 1:2 and the PEG chain is fully covered by α-CD.13 The large values of filling ratio are simply indicating the PEG chains are highly covered with α-CDs. Note that NR is very sensitive to the appearance of layer and its thickness but is not so sensitive to the amount of PEG brush and α-CD included. It has been clearly shown that some kind of aggregation occurs between PEG brush and α-CD, but it has not directly proved that the aggregation is the inclusion complex formation of PEG brush and α-CD. In the following section, we show the evidence of inclusion complex formation of PEG brush and αCD. In general, contrast variation technique by NR with H2O and D2O mixtures should provide more precise structure. Although both H2O and D2O form hydrogen bonds, the strengths of the hydrogen bonds are slightly different and may bring quantitative differences in the complex structure and reaction rate. The contrast variation brings additional uncertainty and thus cannot be employed for the system where hydrogen bonds have significant influence on the structure as shown later. 3.2. Inclusion Complex Formation of Looped PEG Brush and α-CD. We monitored the IC formation on the PEG brushes using NR, however, similar aggregation may occur when α-CDs are simply adsorbed onto the PEG brush without being threaded. In order to show that the aggregation is not caused by adsorption but by IC formation by threading, we also conducted the same interfacial IC formation experiment with looped PEG and α-CD solution, where the looped PEG was fabricated by attaching α, ω-functionalized PEG on to the substrate. Figure 3a shows the experimental and the simulated reflectivity profiles of the PEG loop in D2O and in the α-CD solution. The SLD profiles used to simulate the experimental results in Figure 3a are shown in Figure 3b. The reflectivity profiles of the PEG loop in D2O and in α-CD solution are almost indistinguishable. The SLD profiles show a slight difference near the interface between the PEG brush and solution; however, this difference is related only to the SLD value of solution since the SLD of the solution of α-CD in D2O is smaller than that of pure D2O. This result indicates that the

Figure 3. NR curves at the interface of the PEG loop with D2O and with 10% α-CD solution (a). SLD depth profiles of PEG loop (b). The reflectivities are shifted for clarity.

α-CDs in solution do not deposit on the PEG loop layer. Although it was reported that the loop formation of α,ωfunctionalized polymer onto a substrate may not be perfect and depending on the molecular weight,14 no adsorption of α-CD with the α,ω-functionalized PEG brush is evident. This fact clearly indicates that the reflectivity changes observed in the PEG brushes in the previous section is the strong evidence of the IC formation through threading. 3.3. Crystalline Structure of Inclusion Complex of PEG Brush and α-CD. NR suggested the existence of relatively concentrated IC layer of PEG and α-CD. Because of the ability of hydrogen bonding between the hydroxyl groups of α-CD, the ICs are known to form crystals in solution. Therefore, the brush of IC may also form crystals. To investigate the small scale structure in the IC layer, the ICs on the substrates were quickly dried because the IC complex formation is reversible, and then examined by grazing incidence wide-angle X-ray scattering (GIWAXS). The GIWAXS images of the IC of PEG brushes in 5% and 10% α-CD solutions are shown in parts a and b of Figure 4, respectively. The 2D scattering pattern of the IC prepared in the 5% solution shows the diffraction spots on the qy and qz axes. The q values of these intense spots are in good agreement with those expected from the channel-type crystal structure of α-CD (a = b = 1.365 nm, c = 1.64 nm, γ = 120°), found in the IC of α-CD with low molecular weight PEG.5 This indicates that α-CDs in the 5% α-CD solution stack perpendicular to the surface. On the other hand, the GIWAXS image of the IC prepared in the 10% α-CD solution shows isotropic ring patterns in addition to the weak diffraction spots identical to the image from 5% solution. Although α-CDs in the 10% solution also form a channel type crystal structure, the C

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3.4. Inclusion Complex of Low Density PEG Brush and α-CD. In order to understand the origin of the 10-nm-thick IC formed in the 5% α-CD solution, the IC formation was investigated between the PEG brush with lower brush density and α-CD. Figure 6a shows the NR profiles and the simulated

Figure 4. GIWAXS images of ICs of PEG brush and α-CD in 5% αCD D2O solution (a) and in 10% α-CD D2O solution (b). The GIWAXS images were collected after the IC complex was removed from the solution and dried.

structure has a lower degree of orientation than those in 5% solution. Schematic images of the IC of PEG brush and α-CD are shown in Figure 5. From the GIWAXS experiments, the c-axis

Figure 6. NR curves of PEG brushes 3 and 4 (PEG brushes diluted with MH) in D2O and in α-CD 10% solution (a). Corresponding SLD depth profiles of PEG brushes 3 and 4 (b). The reflectivities are shifted for clarity.

Figure 5. Schematic images of ICs formed in 5% (a) and in 10% α-CD solution (b).

reflectivity of the PEG brushes 3 and 4 with reduced brush density as listed in in Table 1. Brush density was reduced using 6-mercapto-1-hexanol (MH). MH competes with mPEG SH in the reaction with the gold surface and, hence, reduces the PEG brush density. The SLD profiles used to simulate the reflectivity are shown in Figure 6b. Thin layer of SLD value of zero next to Au layer represent the monolayer of MH. The PEG brushes were successfully diluted with MH, and two PEG brushes with reduced densities (0.10 and 0.067 chains nm−2) were prepared. Those PEG brushes were in contact with α-CD solution for inclusion complex formation to be evaluated by NR. The IC layers with 10 nm thickness were also observed for the low density PEG brushes in 10% α-CD solution. Filling ratios of the ICs of PEG brushes of 0.10 and 0.067 chains nm−2 calculated from the SLD profiles are 66% and 77%, respectively. It is clearly shown in Figure 6 that the same 10-nm-thick layer exists irrespective of the brush density. The layers of lower density of α-CD (higher value of SLD) are found near the gold surface (the distance of 12−15 nm from quartz substrate) in both brushes. This can be the less dense α-CD zone near the substrate as illustrated in Figure 5a. If such folded crystals formed to cover the surface uniformly, the thickness of the crystal would be thinner for the lower density brushes. The

of CD crystal is perpendicular to the surface. PEG chains are confined in the narrow channel of CD crystal so they must be highly stretched in the direction of c-axis. Because the thickness of the dense IC layer prepared in 5% α-CD solution is 10 nm, which is a quarter of the length of the fully extended PEG chain (44 nm), the PEG chains in the IC are folded, as shown in Figure 5a. The density of the IC column in 5% α-CD solution (0.72 chains/nm2 in case of 4 times folding) estimated by the in situ NR measurement in D2O is similar to that of a hexagonally packed IC column (0.62 chains/nm2), calculated from the lattice constant estimated by GIWAXS in air.5 Thus, in the 5% α-CD solution, the substrate is almost completely covered by the folded crystal of PEG and α-CD in D2O as well as in air after drying. In the 10% α-CD solution, a less-defined layer was also observed by NR in D2O; moreover, the α-CD crystal in the IC observed by GIWAXS in air after drying showed a low degree of orientation. This structure is quite different from the folded stacked IC found in the 5% α-CD solution and possibly contains randomly crystallized α-CDs in D2O, as shown in Figure 5b. This remarkable difference could be explained by the extremely fast IC formation in 10% α-CD solution as shown in Figure 1 which end up with polycrystalline of α-CD, whereas in 5% α-CD solution, the IC develops relatively slowly with high crystallinity and orientation. D

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structure likely. The characteristic thickness of 10 nm corresponds to the PEG molecules with a molecular weight of 1,000, where rapid inclusion for extended crystal structure in solution has been reported.1 It could be that 10 nm is the persistent length of PEG threading α-CDs and begins to fold when the length of the inclusion complex exceeds 10 nm even in PEG and α-CD solutions. Further study is necessary to reveal the origin of the phenomena.

same folded crystal thickness of 10 nm irrespective of brush density is the characteristics of IC of α-CD and PEG. Figure 7 summarizes the IC structures depending on the brush density and the α-CD concentration. The 10-nm-thick

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01238. Calculation of brush density and filling ratio (PDF)

Figure 7. Summary of brush density and α-CD concentration dependence of IC structure.

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AUTHOR INFORMATION

Corresponding Author

folded IC appears irrespective of brush density and concentration of α-CD except for the high density and 10% α-CD, where IC formation was extremely rapid. The sparse folded 10-nm-thick IC was uniquely found in the PEG brush with lower density. Therefore, the characteristic 10-nm-thick layer is not the result of covering surface with limited amount of ICs but is rather the characteristic IC structure of PEG and α-CD. Evidently, the characteristic 10-nm-thick crystal structure was also found in a mixture of PEG and α-CD solution.8 On the other hand, less-defined thick randomly crystallized α-CDs appeared only when both brush density and α-CD concentration are high, where crystallization is rapid and intermolecular crystallization is likely. These findings raised the question about the origin of the 10-nm-thick IC universally observed in those polymer brushes. The molecular weight of PEG with a contour length of 10 nm is about 1000, which is the molecular weight known for very rapid IC formation and precipitation in α-CD solution in H2O.1 Moreover, the ICs crystallize into a well-defined columnar structure. These findings suggest that, up to 10 nm, α-CDs form extended crystals with PEG, and then they begin to fold and form folded crystals. It can be concluded that the inclusion of PEG and αCD are controlled by the crystallization of α-CDs on the PEG chains rather than hydrophobic interaction of inner cavity of αCD and PEG.

*(H.Y.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI Grant Number 15H03862, and S.T. expresses thanks for financial support by a Grant-in-Aid for Scientific Research (No. 24·6653) from the Japan Society for the Promotion of Science (JSPS). This work was partially supported by ImPACT Program of Council for Science, Technology and Innovation (Cabinet Office, Government of Japan). The neutron reflectometry experiments were performed using SOFIA at J-PARC with the support from the S-type (2009S08, 2014S08) and general use (2015A0253) research project of KEK. This work was performed under the approval of the Photon Factory, KEK (Proposal No. 2015G716, 2015G073).

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REFERENCES

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4. CONCLUSIONS The IC formation of α-CDs with PEG was investigated by NR and GIWAXS measurements. ICs with densely packed column structures oriented perpendicular to the substrate with characteristic thickness of 10 nm were formed irrespective of the brush density, except in the case of high brush density and high concentration (10%) of α-CD. The fact that 10-nm-thick folded crystal appears even for the low density brush, where each PEG chain is well separated, suggests that the IC single molecule begins to fold as the number of α-CD per chain increases. Therefore, the characteristic thickness of 10 nm appears due to the characteristics of a PEG chain threading αCDs. A randomly oriented thick polycrystalline layer was formed when concentration of α-CD and PEG brush density are high. High PEG density makes the intermolecular interaction dominant and high concentration of α-CD speeds up the inclusion complex formation as time-resolved NR measurement revealed. Both effects make polycrystalline E

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