Article pubs.acs.org/Langmuir
Bis‑p‑Sulfonatocalix[4]arene-Based Supramolecular Amphiphiles with an Emergent Lower Critical Solution Temperature Behavior in Aqueous Solution and Hydrogel Xuyang Yao, Xi Wang, Tao Jiang, Xiang Ma,* and He Tian Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science & Technology, Shanghai 200237, P. R. China S Supporting Information *
ABSTRACT: An unexpected lower critical solution temperature (LCST) phenomenon is observed in a bis-psulfonatocalix[4]arene-based supramolecular amphiphile system, and the mechanism of this intriguing phenomenon is studied. The unusual macroscopic thermoresponsive behavior is based on the switch of the system from water-soluble assemblies to insoluble netlike cross-linked nanoparticles under temperature stimulus, which is regulated by multiple weak interactions, including hydrophilic and hydrophobic interactions, π−π stacking, and host−guest recognition. By using the LCST solution as the dispersion medium, a hydrogel with LCST behavior can be fabricated. This work contributes toward better understanding about calixarene-induced aggregation (CIA) and thermoresponsive self-assembled systems. It will also help to enrich the designing of complexed supramolecular amphiphile systems and develop their potential applications in hydrogels.
1. INTRODUCTION Spontaneous assembly of small molecules into functional macromolecules1 or polymers2 facilitated by noncovalent interactions including host−guest recognition,3−13 hydrogen bonds,14,15 metal−ligand interaction,11,16−19 donor−acceptor interaction,20 and van der Waals forces is an intriguing topic in supramolecular chemistry. Smart materials based on supramolecular interactions have been extensively fabricated and applied in various fields, including sensors,21 catalysis,22 biology,23,24 energy,25 and so forth. After great development in the past two decades, supramolecular chemistry are now stepping from constitutional dynamic chemistry to adaptive chemistry.26 Quite recently, the concept of system chemistry27−29 was coined for the field dealing with complex chemical systems exhibiting unpredictable emergent properties. In this field, it is more intriguing to explore the supramolecular systems, especially those far from equilibrium,30,31 since the multicomponent self-assembled systems with elaborate structures and multiple weak interactions often give rise to emergent properties which single component does not possess but appear only as a result of interactions between molecules.32−35 Supramolecular amphiphiles,36−38 which are constructed on the basis of noncovalent interactions or dynamic covalent bonds, play useful roles in fabricating nanomaterials with structural complexity. Although much work has been done in this field, new strategies are still expected to fabricate smart assembly architectures to fulfill the requirements for new applications. Calixarene-induced aggregation (CIA)4,39 has © 2015 American Chemical Society
been proved to be an effective way in fabricating supramolecular macrocyclic amphiphiles40,41 with stimuli-responsiveness and functions, but similar systems based on sulfonatocalix[4]arene (BSC4) or p-sulfonatedcalixarenes (SC4) usually show monotonous thermal response of disassembling at certain high temperatures.42 In the present work, our interest was raised by an unexpected LCST phenomenon during research on supramolecular amphiphiles based on amphiphilic bromonaphthalene derivative 1 as guest molecule and BSC4 as macrocyclic host via CIA (Scheme 1). Lower critical solution temperature (LCST) is a critical temperature below which the components of a mixture are miscible for all compositions. It is a unique temperature for a group of materials previously discovered mainly in the systems of poly(N-isopropylacrylamide) (PNIAM),43−45 oligo(ethylene glycol)s (OEGs),36,46−49 and ionic liquid,50,51 which have potential applications in the medical and biochemical field. Most of these materials are soluble in solution below LCST and turn into turbidity as a result of dehydration with increased aggregation (PNIAM-based and OEGs-based systems) or dissociation with host molecules (ionic liquid-based systems). Modulating the LCST behavior of PNIAM-based, OEGs-based systems with the strategy of supramolecular host−guest interactions has also been discovered and studied.52−55 However, nonpolymeric LCST materials, especially ones Received: September 20, 2015 Published: December 7, 2015 13647
DOI: 10.1021/acs.langmuir.5b04083 Langmuir 2015, 31, 13647−13654
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Scheme 1. Schematic Illustration and Photographs for the Supramolecular Amphiphile System and Its LCST Behaviora
a
A solution of the supramolecular amphiphile system is kept in a cuvette. The left is the sample at a temperature below LCST, while the right is the sample at a temperature above LCST.
standard laboratory methods. The molecular structures were confirmed using 1H NMR spectra, 13C NMR spectra, and highresolution ESI mass spectroscopy. Synthesis of the Compounds. BSC4 was synthesized according to previous research.56 4-((6-Bromonaphthalen-2-yl)oxy)-N,N,N-trimethylbutan1-aminium bromide (1) (Synthetic Routes Shown in Scheme S1). 6-Bromo-2-naphthol (1.0 g, 4.5 mmol), 1,4-dibromobutane (2.0 g, 9.2 mmol), and K2CO3 (1.2 g, 8.7 mmol) were added into acetonitrile (80 mL) and then refluxed overnight. The solution was evaporated under vacuum pressure. 2-Bromo-6-(4bromobutoxy)naphthalene was then obtained by column chromatography as a white powder (1.3 g, 81%). 2-Bromo-6(4-bromobutoxy)naphthalene (500 mg, 1.4 mmol) was dissolved into ethanol (80 mL). Trimethylamine (1 mL, 30% in ethanol) was added into the solution and stirred at 40 °C under argon for 24 h. Excess trimethylamine was removed with ethanol by evaporation under vacuum. Petroleum ether (100 mL) was added into the residue and stirred for 2 h. The solution was filtrated, and the solid was dried in vacuum to obtain 1 as a white powder (320 mg, 55%). 1H NMR (400 MHz, D2O, δ): 7.92 (s, 1H), 7.66 (dd, J = 9.2, 5.2 Hz, 1H), 7.60 (dd, J = 8.8, 4.8 Hz, 1H), 7.48 (d, J = 8.6 Hz, 1H), 7.18 (d, J = 6.2 Hz, 1H), 7.14−7.08 (m, 1H), 4.07 (dd, J = 12.4, 6.0 Hz, 2H), 3.29 (t, J = 7.6 Hz, 1H), 1.88 (m, 2H), 1.77 (m, 2H). 13C NMR (101 MHz, DMSO-d6, δ): 156.74, 132.85, 129.65, 129.35, 129.25, 128.85, 128.61, 119.85, 116.26, 106.81, 66.83, 64.90, 52.11, 25.50, 19.25. HRMS (ESI) m/z: [M − Br−]+ = 336.0963; found, 336.0955. N,N,N-Trimethyl-4-(naphthalen-2-yloxy)butan-1-aminium bromide (2). β-Naphthol (1.0 g, 7.0 mmol), 1,4-dibromobutane (3.0 g, 14 mmol), and K2CO3 (1.9 g, 14 mmol) were added into acetonitrile (80 mL) and then refluxed overnight. The solution was evaporated under vacuum pressure. 2-(4Bromobutoxy) naphthalene was then obtained by column
without functional groups like PNIAM or OEGs, are very rare. As neither guest 1 nor host BSC4 alone has a predictable property of LCST behavior, we believe the LCST behavior is emergent from the supramolecular interactions in the host− guest system. Our curiosity in the unusual LCST property of a supramolecular amphiphile system inspired us to explore the mechanism of this phenomenon, from which we might improve the understanding about CIA and further enrich the designing of complexed supramolecular amphiphile systems with adaptivity and functions.
2. EXPERIMENTAL SECTION Instruments. 1H NMR spectra, 13C NMR spectra, and diffusion-ordered spectroscopy (DOSY) were measured on a Brüker AV-400 spectrometer. 2D Rotating Frame Overhauser Effect Spectroscopy (ROESY) was measured on a Brüker AV500 spectrometer. The electronic spray ionization (ESI) highresolution mass spectra were tested on a HP 5958 mass spectrometer. Dynamic light scattering (DLS) and zeta potentials were measured on MALV RN, ZETA SIZER, model Nano ZS90, 25 °C. Surface tension measurements were taken on a DCAT21, Dataphysics, Germany. AFM images were recorded on a MicroNano D5-A machine. The samples were dropped on a mica plate on a spin-coating machine at a rotating speed of 50 r/min and further dried in vacuum. TEM images were recorded on a JEOL JEM-1400 apparatus. The samples were dropped on a perforated copper grid (400 mesh) covered with a carbon film and dried quickly using a piece of filter paper on the back side. SEM images were recorded on a TESCAN nova III apparatus. The samples were dropped on a mica plate on a spin-coating machine at a rotating speed of 50 r/min and further dried in vacuum. Materials. Unless stated otherwise, all reagents were purchased from Sigma-Aldrich or J&K Chemicals and used without further purification. Solvents were purified according to 13648
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quaternary ammonium moieties like −N+(CH3)3Br− could form stable 1:1 complex with p-sulfonatocalix[4]arene (SC4).56 1D Supramolecular polymer could also be fabricated by employing bis-p-sulfonatocalix[4]arene (BSC4) with homoditopic choline species containing −N+(CH3)3Br−.57 In our study, Job’s plot verified the 1:1 binding ratio between 1 and model host SC4 at relative low concentration of 15 μM (Figure S1). When BSC4 was gradually added to the solution of 1 with the concentration of 5 mM, the clear solution became turbid immediately. This phenomenon was similar to that in our previous research,58 indicating the existence of cross-linked micellar particles connected by BSC4. However, the solution converted back to become clear when the guest/host ratio reached about 2:1 (Figure S2). The quite sharp change of transmittance around the guest/host ratio of 2:1 indicated that the system adopted a different assembly mode rather than a simple dissociation by the increasing charge repulsion brought by BSC4. After a small amount of undesired aggregation was removed as described in Experimental Section, the guest/host ratio was calculated to be about 2:1.2 by 1H NMR in the solution. The interaction between guest molecule 1 and host BSC4 (Figure 1) was validated by 1H NMR. The protons of methyl
chromatography as a white powder (1.7 g, 87%). 2-(4Bromobutoxy) naphthalene (200 mg, 0.72 mmol) was dissolved into ethanol (80 mL). Trimethylamine (1 mL, 30% in ethanol) was added into the solution and stirred at 40 °C under argon for 24 h. Excess trimethylamine was removed with ethanol by evaporation under vacuum. Petroleum ether (100 mL) was added into the residue and stirred for 2 h. The solution was filtrated, and the solid was dried in vacuum to obtain 2 as a white powder (153 mg, 63%).1H NMR (400 MHz, D2O, δ): 7.77 (d, J = 8.8 Hz, 2H), 7.73 (d, J = 8.2 Hz, 1H), 7.42 (t, J = 7.2 Hz, 1H), 7.32 (t, J = 7.6 Hz, 1H), 7.24 (s, 1H), 7.12 (dd, J = 8.8, 2.4 Hz, 1H), 4.10 (t, J = 5.8 Hz, 2H), 3.24 (t, J = 12.0 Hz, 2H), 1.87 (s, 1H), 1.67−1.81 (m, 2H), 1.81−1.93 (m, 2H). 13C NMR (101 MHz, DMSO-d6, δ): 156.31, 134.24, 129.27, 128.46, 127.49, 126.62, 126.38, 123.56, 118.68, 106.74, 66.74, 64.91, 52.13, 25.55, 19.26. HRMS (ESI) m/z: [M-Br−]+ = 258.1858; found, 258.1855. N,N,N-Trimethyl-4-((2-oxo-4a,8a-dihydro-2H-chromen-7yl)oxy)butan-1-aminium bromide (3). 7-Hydroxycoumarin (1.0 g, 6.1 mmol), 1,4-dibromobutane (2.60 g, 12.0 mmol), and K2CO3 (1.70 g, 12.3 mmol) were added into acetonitrile (80 mL) and then refluxed overnight. The solution was evaporated under vacuum pressure. 7-(4-Bromobutoxy)-4a,8adihydro-2H-chromen-2-one was then obtained by column chromatography as a white powder (1.5 g, 82%). 7-(4Bromobutoxy)-4a,8a-dihydro-2H-chromen-2-one (500 mg, 1.67 mmol) was dissolved into ethanol (80 mL). Trimethylamine (1 mL, 30% in ethanol) was added into the solution and stirred at 40 °C under argon for 24 h. Excess trimethylamine was removed with ethanol by evaporation under vacuum. Petroleum ether (100 mL) was added into the residue and stirred for 2 h. The solution was filtrated, and the solid was washed by dichloromethane and dried in vacuum to obtain 3 as a white powder (270 mg, 45%). 1H NMR (400 MHz, D2O, δ): 7.80 (d, J = 9.6 Hz, 1H), 7.42 (d, J = 8.8 Hz, 1H), 6.84 (dd, J = 8.8, 2.4 Hz, 1H), 6.79 (d, J = 2.2 Hz, 1H), 6.16 (d, J = 9.6 Hz, 1H), 4.05 (t, J = 5.8 Hz, 2H), 3.34−3.21 (m, 2H), 3.01 (s, 9H), 1.95−1.82 (m, 2H), 1.82−1.70 (m, 2H). 13C NMR (101 MHz, DMSO-d6, δ): 161.57, 160.27, 155.33, 144.34, 129.51, 112.71, 112.47, 112.37, 101.20, 67.52, 64.82, 52.13, 25.30, 19.10. HRMS (ESI) m/z: calculated [M-Br−]+ = 276.1600; found, 276.1591. Preparation of the LCST Solution. 1 (20.9 mg, 50.1 mmol) and BSC4 (42.3 mg, 25.01 mmol) was dissolved in 10 mL deionized water. The solution was treated with ultrasound to make the solute fully dissolved. After standing at room temperature for 1 h, the solution was filtered to remove a small amount of undesired insoluble aggregation with a 450 nm syringe filter. Preparation of the LCST Polyacrylamide Hydrogels. Acrylamide (85 mg, 1.20 mmol), 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone (IRGACURE 2959) (3 mg, 13.4 mmol), and methylene-bis-acrylamide (MBAAM) (1, 3, and 5 mg, respectively) were dispersed in 1 mL LCST solution in a quartz Schlenk tube. The solution was degassed by bubbling argon for 30 min and then irradiated under a UV lamp (250W, purchased from Philips) at a wavelength of 365 nm for 15 min at 15 °C. Details of the prepared hydrogels were shown in Table S1.
Figure 1. 1H NMR spectra of (a) 1 at the concentration of 5 mM at 25 °C; (b) LCST sample of 1 and BSC4 at 25 °C; (c) LCST sample of 1 and BSC4 at 40 °C; and (d) LCST sample of 1 and BSC4 at 55 °C.
group on hydrophilic −N+(CH3)3Br− shifted upfield from 2.97 to 2.15 as a result of shielding effect of calixarene cavity. Furthermore, the 2D ROESY NMR spectrum of the complex showed strong NOE signals between the protons on −N+(CH3)3Br− and those on the cavity of BSC4, indicating that 1 was captured by BSC4 with quaternary ammonium moieties deeply immersed into the cavity (Figure S3). Although detailed NOE signals of aromatic protons could not be recognized on the 2D ROESY NMR spectrum, the upfield shift and broadness of the aromatic proton signals on the 1D 1 H NMR stood for the stacking of the hydrophobic aromatic
3. RESULTS AND DISCUSSION 3.1. Formation and Characterization of Supramolecular Assemblies. It has been well-studied that some 13649
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Langmuir moiety in the supramolecular assemblies. Diffusion-ordered spectroscopy (DOSY) further validated the formation of supramolecular assemblies. The complexed BSC4 and 1 diffuse together with a main peak of diffusion rate of 1.32 × 10−10 m2 s−1 (Figure S4), which is much slower than that of free 1 (4.90 × 10−10 m2 s−1 at 5.0 mM, Figure S5) and BSC4 (2.14 × 10−10 m2 s−1 at 2.5 mM, Figure S6). It should be clarified that the actual concentrations of 1 and BSC4 in the solution are lower than their original ones. However, the fact has no influence on the judgment that both 1 and BSC4 were found to have faster diffusion rate at lower concentration (Figure S7). TEM was carried out to confirm the formation of nanostructures. When the temperature was below the LCST, BSC4 and 1 self-assembled into platelet-like micelles with good solubility in water (Figure 2a). From an enlarged TEM image,
Scheme 2. Schematic Illustration of the Possible Formation Process of Self-Assembled Nanostructure
multiple interactions including π−π stacking, host−guest interaction, and hydrophobic and hydrophilic interactions. 3.2. LCST Behavior and Mechanism Study. An unexpected LCST phenomenon was observed when the prepared solution of 1 and BSC4 was heated to a temperature of ca. 37 °C (Figure 3a). The cloud point was measured by light transmittance at 750 nm on a temperature-controlled UV−vis spectrophotometer (Figure 3b). From the data, it was revealed that the LCST was more “critical” when the measurement started from the sample kept at room temperature for 2 h than a sample which cooled fresh from LCST. It was reasonable that the complete self-reorganization of nanostructures took longer time at room temperature. In accordance with the TEM pictures of sample which was heated to generate turbidity above LCST, the water-soluble platelet-like micelles turned into cross-linked nanoparticles (Figure 4, panels c and d). As the negative charge of BSC4 on the surface of the nanoparticles was not enough to provide repulsion between nearby ones, these nanoparticles would connect with each other with BSC4 and follow with random collision to engender larger net-like aggregation and further turbidity. It is deduced that the stacking between 1 molecules change adaptively at increased temperature and the host−guest interaction still exists. This assumption was evidenced by 1H NMR. When the temperature was raised, the signals of aromatic protons of 1 trended to split and shift to downfield, indicating a change of stacking (Figure 1, panels b, c, and d). However, the protons of methyl group on −N+(CH3)3Br− went on shifting upfield, suggesting more BSC4 cavities were interacting with 1, which proved that the host−guest interaction still existed and the BSC4 played the role of “connector” above LCST. In addition, the dynamic light scattering (DLS) data (Figure S8) and zeta potential (Figure S15) also accorded with the observation. DLS revealed that the supramolecular micelles were in an average size of 115 nm at 25 °C. When it is above LCST, the average size is beyond the test limit. Zeta potential of the host−guest system is about −69 mV when the temperature is 25 °C, indicating a stable colloid system (Figure S15a). However, it increases rapidly after the temperature reaches the LCST and shows a very poor stability of the whole system with increased temperature (Figure S15, panels b−d). Interestingly, the cloud point of the LCST sample could be regulated by the guest/host ratio. With the increase of the guest/host ratio in a certain range, the cloud point of the samples decreased (Figure 3c). This change was proved to be in accord with the function of BSC4. From the DLS data (Figure S8) and transmission electron microscope (TEM) results (Figure S9, panels a−c), it is clear that the appropriate amount of BSC4 facilitates the existence of the platelet-like
Figure 2. (a) TEM picture and (b) enlarged TEM picture of selfassembled nanostructures. (c) AFM picture of self-assembled nanostructures.
it could be observed that the assemblies grew directionally from rod-like building blocks (Figure 2b). AFM was also carried out to observe the morphology of the micelles, and it was clear that the platelet-like micelles showed a planar morphology with the length of ca. 300 nm, width of ca. 100 nm, and height of ca. 20 nm (Figure 2c). Considering the existence of rod-like building blocks, guest/ host ratio of near 2:1 and negative zeta potential (Figure S15), we deduced that the self-assembly mechanism was similar like the one proposed in previous research of Liu.59 The basic unit containing two guest molecule 1 linked by one BSC4 tends to aggregate at certain concentration with hydrophobic segments of 1 packed together to form rod-like building blocks and further assembled to form the platelet-like micelles (Scheme 2). As BSC4 was proved to have the stacking ability owing to its much more three-dimensional conformation, the aggregation process was facilitated by the stacking ability of BSC4 and 1, even though there was electronic repulsion existing among the building blocks. It is also reasonable that the guest/host ratio was 2:1.2 as some of the cavities of BSC4 are not interacting with 1 on the two tops of the rod. Besides, there is a possibility that some BSC4 are attracted on the surface of the micelles to stabilize them. The whole assembly process was balanced by 13650
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Figure 3. (a) Photographs of the LCST behavior of the sample in a cuvette; (b) temperature-dependence of light transmittance of the LSCT sample ([1]:[BSC4] = 2:1.2); and (c) guest/host ratio dependence of the cloud points of different samples.
guest ratio, indicating that SC4 and 1 performed a different aggregation behavior, which matched with the study in previous research.59 Although BSC4 plays an important role in the formation of the nanostructures, we believe the stacking property of the guest molecule 1 is also important for both the formation of nanoparticles and the LCST behavior. The reason for the disassembly of platelet-like micelles upon heating might be the break of stack between 1 molecules in the assemblies. Thus, the TEM study was carried out to explore the possible self-assembly behavior of 1 itself with the amphiphilic property. From TEM images, it was revealed that 1 would self-assemble to planar micelles at the concentration of 5 mM (Figure 5, panels a and b). The critical micelle concentration (CMC) was estimated to be close to 1 mM by measuring the surface tension (Figure S12), which was also verified by the TEM images (Figure S13). Needle-like crystals were also obtained from the same solution near 10 °C after one month (Figure S14a). Figure 4. (a) TEM picture and (b) enlarged TEM picture of supramolecular assemblies formed by 1 and BSC4 below LCST; (c) TEM picture; and (d) enlarged TEM picture of aggregations formed by 1 and BSC4 above LCST (the white arrows indicate the connected nanoparticles).
micelles and more BSC4 transforms the micelles to nonfunctional spheres with repulsion among each other and makes it difficult for them to form cross-linked aggregation. As a result, the LCST increased with the host/guest ratio. When more BSC4 was added into the system to reach the ratio of guest/ host as 1:1, the system lost the LCST property as a result of the complete change of supramolecular assemblies (Figure S9d).The platelet-like micelles could regenerate below LCST, and the whole process was reversible, benefiting from the supramolecular noncovalent interactions (Figures S10 and S11). To confirm the important role of ditopic BSC4 as “organizer” when it is below LCST and “connector” when it is above LCST, SC4 with only one cavity was used to take the place of BSC4 as host. The solution of 1 at 5 mM formed aggregation immediately after the addition of SC4 and showed no LCST behavior at increased temperature in various host/
Figure 5. TEM pictures of the micelle assemblies of (a and b) ribbonlike assemblies observed in the sample prepared with solution of 1 at 5 × 10−3 M at 25 °C and (c and d) sphere assemblies observed in the sample prepared with solution of 1 at 5 × 10−3 M at 40 °C. 13651
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LCST solution as dispersion medium when it was prepared (see details in Experimental Section). As expected, the transparent hydrogel became turbid when it was heated and converted back to transparent at low temperature (Figure 6). It was found that the cloud point in hydrogel (18 °C) was quite different from that in solution (37 °C), possibly due to the change of the environment for the supramolecular amphiphile system. Moreover, although the hydrogel became turbid quickly upon heating, the time for the hydrogel to become transparent from turbid state was much longer than that in solution. Typically, the process took about 4 h at 4 °C. The prolonged conversion time is presumably attributed to the mobility of the molecules, or assemblies in the cross-linked hydrogel was restricted to some extent. Thus, the reorganization in the molecular level took longer time.
Although it was not suitable for single-crystal X-ray diffraction, SEM pictures verified the existence of ribbon-like assemblies (Figure S14b and S14c). When the solution of 1 at 5 mM was heated, smaller sphere nanoparticles would generate from previous ribbon-like ones, which proved that the stacking of 1 would be changed by increasing the temperature (Figure 5, panels c and d). The formed nanoparticles play an important role as stable building blocks for BSC4 to generate aggregations at temperature above LCST. To confirm the important role of stacking capability of 1, two reference compounds naphthalene derivative 2 and coumarin derivative 3 were synthesized, which also had hydrophobic heads and the same hydrophilic tail like 1. The naphthalene derivative showed general sphere morphology at a concentration of 5 mM (Figure S16) and formed aggregation immediately after the addition of BSC4 with a host/guest of 2:1. The turbid system turned clear when it was heated (Figure S17), which was similar to the previous SC4-based supramolecular assemblies. While for compound 3 at the same condition, no aggregation was formed at the same concentration, possibly because the coumarin core was not easy to stack at this concentration, and it was hard for the system to form aggregation. Considering the result of our research and the previous one on CIA, we believe that the stacking capability of the guest molecule also plays an important role in dominating the structure of the supramolecular assembly in CIA. 3.3. LCST Behavior in Hydrogel. Prepared polyacrylamide hydrogel with LCST behavior is shown in Figure 6.
4. CONCLUSIONS A temperature-responsive supramolecular amphiphile system with an unexpected LCST behavior was discovered, and the mechanism of this intriguing phenomenon was studied thoroughly as well. This emergent property appeared when the self-assembled system with water-soluble assemblies switched to the one with insoluble net-like cross-linked nanoparticles. The stacking change of guest molecule 1 was the primary trigger for the switch under the external thermal stimulus. Moreover, by using the LCST solution as the dispersion medium, a hydrogel with LCST behavior was also obtained. The supramolecular amphiphile system with unique LCST behavior might have potential applications in controlled trapping and release of molecules, sewage treatment, drug delivery, and other related fields. The present study embodied the complexity of a supramolecular self-assembled system and showed not only in the aspect of nanostructures formed by multiple components but also in its emergent structures regulated by noncovalent interactions under environmental stimuli. This work contributes toward improving the understanding of CIA and thermoresponsive self-assembled systems. Also, it will help to enrich the designing of complexed supramolecular amphiphile systems and the development of their applications within the hydrogels. In future research, it might be an attempted approach to regulate or control the behavior of a supramolecular self-assembled system by changing the properties of the gel matrix, such as choosing functional polymeric monomers, tuning the density, and adding functional pendent groups.
Figure 6. Photographs of the prepared polyacrylamide hydrogel with LCST behavior.
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Although much research of supramolecular amphiphiles has been done in aqueous solution, little effort was made to study their properties or behaviors in other applicable matrixes, such as hydrogels. Hydrogels possess a degree of flexibility very similar to natural tissue, due to their significant water content. As introducing stimuli-responsive supramolecular self-assembly systems into hydrogels might bring smart soft materials with adaptivity and promote the potential applications of supramolecular self-assembly systems in related fields, it is interesting to investigate the assembly behavior of the supramolecular amphiphile systems in hydrogel. Considering that the LCST behavior of our system is based on the self-assembly process in the spatial scale from nanometer to micrometer and the reorganization of the assemblies are in molecular scale, we believe that the LCST behavior could also happen in hydrogels. To prove our assumption, a common hydrogel with crosslinked polyacrylamide was exploited as the matrix and with the
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b04083. Job’s plot, 2D ROESY spectrum, 2D DOSY spectrum, DLS, additional pictures of TEM, AFM, SEM, CMC plot, zeta potential, and other pictures, scheme of synthetic routes, additional spectra of compounds, 1H NMR and 13C NMR spectroscopy, and TOF-MS (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Fax: (+86)-21-64252288. Notes
The authors declare no competing financial interest. 13652
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ACKNOWLEDGMENTS This work was financially supported by NNSFC (Grants 21421004, 21476075, and 21272072), the National Basic Research 973 Program, Shanghai Pujiang Program (Grant 13PJD011), and the Fundamental Research Funds for the Central Universities.
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