Amphiphilic pillar[5]arenes: influence of chemical structure on self-assembly morphology and application in gas response and λ-DNA condensation

Abstract

In order to investigate the influence of chemical structures on self-assembly morphology, six amphiphilic pillar[5]arenes C1, C2, C3, C4, D, and E with different hydrophilic and hydrophobic groups were synthesized. When they were dissolved in water, they all self-assembled into vesicles first. The amine groups on C1 and C2 can reversibly react with CO₂ in water, so vesicles self-assembled from them could reversibly transform into micelles by bubbling CO₂ and N₂. More interestingly, C1, C2, C3, and C4 with the same N-(2-aminoethyl)acetamide units as the hydrophilic groups all further self-assembled into microtubes, on the other hand, E with acetohydrazide units as the hydrophilic groups further self-assembled into nanosheets in water. Amphiphilic pillar[5]arene D could not further self-assemble into any other morphology in water itself, but the carboxylate groups on D can coordinate with silver ions so it could further self-assemble into dendritic structures in water. Dynamic light scattering, transmission electron microscopy, scanning electron microscopy, UV-Vis spectroscopy, and FT-IR spectroscopy were employed to characterize the self-assembly processes of these six amphiphilic pillar[5]arenes and the resultant self-assemblies. The acidified microtubes, which have ammonium ions on their surfaces, could capture negatively charged DNA through electrostatic interactions in water.

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Introduction

Macrocyclic amphiphiles refer to amphiphiles that are synthesized on the basis of macrocyclic compounds.¹ In view of the rich research on the host–guest chemistry of macrocycles,² when macrocyclic amphiphiles self-assemble into micelles, vesicles, nanotubes, or other nano-structures in water, we can tunably functionalize these nano-structures through the self-selectivity of host–guest interactions to fabricate hierarchical, multidimensional, and environmentally responsive assemblies.³ Great progress has been made in the synthesis and assembly of macrocyclic amphiphiles.⁴ For example, Lee and co-workers reported significant size and structural changes of the aggregates in aqueous solution, from large vesicles to small spherical micelles, with only small variation in the hydrophilic chain length or pH in amphiphilic tetramers based on a calixarene building block.^1a Remita and co-workers reported a novel and promising calix[6]arene-based system that could produce self-assemblies under the control of pH or a metal ion.^4b Leblanc and co-workers reported the Langmuir monolayer and Langmuir–Blodgett film based on an amphiphilic coumaryl crown ether.^4a Conjugation of the natural features of amphiphiles and molecular recognition of macrocyclic hosts will expand applications of macrocyclic amphiphiles in various areas.

As a new class of macrocycles after crown ethers, cyclodextrins, calixarenes, cucurbiturils, and other macrocyclic hosts, pillar[n]arenes consist of hydroquinone units linked by methylene (–CH₂–) bridges at their 2,5-positions and have pillar-like architectures and electron-donating cavities.^5,6 Their syntheses, conformational mobility, derivatization, host–guest complexation, self-assembly in water or organic solvents and applications have been widely explored recently. They have been described as “a new class of macrocycles for supramolecular chemistry”.^6d The first amphiphilic pillar[5]arene was synthesized by our group last year. It can form vesicles in water and further transform into microtubes.^5l Then we used this amphiphilic pillar[5]arene to prepare composite microtubes for green catalysis.^5p Furthermore, we recently synthesized a sugar-functionalized amphiphilic pillar[5]arene. It can be utilized as excellent cell glues to agglutinate E. coli.^5q Therefore, pillararene-based macrocyclic amphiphiles have been demonstrated to be versatile building blocks for the fabrication of functional self-assemblies with various applications. However, how chemical structures of the amphiphilic pillar[5]arenes affect their self-assembly morphologies is still not clear. This is very important to explore since we will be able to do rational design of amphiphilic pillararenes according to what we need if we know the relationship between the chemical structures of amphiphilic pillararenes and the corresponding self-assembly morphologies.

Herein, we reported the syntheses of a series of amphiphilic pillar[5]arenes C1, C2, C3, C4, D, and E, which contain different hydrophobic and hydrophilic groups (Scheme 1). Although C1–C4 contain different lengths of hydrophobic chains, they all form vesicles at first and further self-assemble into microtubes in water at last. The resultant microtubes can be used to capture DNA in water after acidification. Interestingly, vesicles self-assembled from C1 or C2 can reversibly transform into micelles by adding CO₂ and N₂. Amphiphilic pillar[5]arenes D and E with carboxylate and acetohydrazide units as the hydrophilic groups can also form vesicles in water but they can not further self-assemble into microtubes.

Scheme 1 Chemical structures of the amphiphilic pillar[5]arenes C1–C4, D, and E.

Results and discussion

When amphiphilic pillar[5]arenes C1–C4, D, and E were dissolved in water, the water surface tension (γ) as a function of the concentration of pillar[5]arenes (C) was measured to determine their critical aggregate concentration (CAC) in water (Fig. S30†). The junctions of the γ–C plot represent the CAC values.⁷ All the CAC values of the amphiphilic pillar[5]arenes were about 10⁻⁵ M. Tyndall effects were observed for all these amphiphilic pillar[5]arenes when the concentration increased to 1.50 × 10⁻⁴ M because this concentration was higher than all of the CAC values of the amphiphilic pillar[5]arenes, indicating the formation of self-assembled aggregates for all these six amphiphilic pillar[5]arenes at this concentration (Fig. S31†).

The aggregation behaviors of all these amphiphilic pillar[5]arenes in water were then investigated using dynamic light scattering (DLS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The DLS experiments were performed with a 2.00 × 10⁻⁴ M aqueous solution of amphiphilic pillar[5]arenes over a scattering angle range of 90°. C1–C4, D, and E all showed aggregation behavior with a narrow size distribution, indicating well equilibrated structures. The average hydrodynamic diameters of them were observed to be between 150 nm and 220 nm (Fig. 1a–f), which greatly exceeded the corresponding extended molecular lengths of these amphiphilic pillar[5]arenes (Scheme 1, <3 nm), suggesting that these aggregates were vesicular entities rather than simple micelles.

Fig. 1 DLS results: (a) C1; (b) C2; (c) C3; (d) C4; (e) D; (f) E the concentrations of these amphiphilic pillar[5]arenes were 2.00 × 10⁻⁴ M.

Further evidence for the formation of vesicles was provided by SEM experiments (Fig. 2a–f). The resultant micrographs showed spherical aggregates with diameters of ∼200 nm, consistent with the results obtained from the above-mentioned DLS experiments. The TEM images revealed an obvious color contrast between the peripheries and centers of the spheres, characteristic of the projection images of hollow spheres (Fig. 2a–f, insets). Moreover, the wall thicknesses of these vesicles were about 5 nm, which corresponds to the extended length of two amphiphilic pillar[5]arene molecules, suggesting that the vesicles have bilayer walls (Scheme 1 and Fig. S32†).

Fig. 2 SEM and TEM (insets) images: (a) C1; (b) C2; (c) C3; (d) C4; (e) D; (f) E. The concentrations of these amphiphilic pillar[5]arenes were 2.00 × 10⁻⁴ M.

As we all know, CO₂ is an inexpensive, nontoxic, biocompatible, and benign stimulus. It is the properties of some solvents, emulsifiers, polymers, and initiators, which bear CO₂-sensitive moieties, including amidine, guanidine, and amine, that allow for a reversible reaction with CO₂ in water to form cationic adducts.⁸ Due to the amphiphilic pillar[5]arenes C1 and C2 containing primary amine groups, their self-assembly process in water might be reversibly controlled by CO₂. We used C1 as a model molecule, the CO₂-responsive aggregation behavior of C1 was investigated by DLS measurements. As shown in Fig. 3a and b, bubbing CO₂ induced a dramatic decrease in the aggregate diameter from ∼150 nm to ∼15 nm, while the aggregates maintained a narrow size distribution. This unique aggregation behavior seems to be due to a transition from a vesicular structure to a micellar structure. Then the morphological changes of this CO₂-responsive aggregation behavior was confirmed by TEM studies. As shown in Fig. 3c, in the absence of CO₂, vesicles with a diameter ∼150 nm were observed. After bubbing CO₂, micelles with a diameter ∼15 nm were visible in Fig. 3d. The CO₂-switch imparts full reversibility to the process, as demonstrated in Fig. 3e, alternatively bubbing CO₂ and replacing with N₂ switches the diameter of assemblies from ∼15 nm to ∼150 nm, without any alteration of the response over 5 cycles.

Fig. 3 DLS results: (a) C1; (b) after bubbing CO₂ into the solution of C1. TEM images: (c) C1; (d) after bubbing CO₂ into the solution of C1. (e) The average diameter of C1 aggregates in water over repeated cycles of bubbing and removing CO₂. The concentration of C1 in these studies was 2.00 × 10⁻⁴ M.

Amphiphilic pillar[5]arenes C1–C4 have the same N-(2-aminoethyl)acetamide units as the hydrophilic groups, but different lengths of hydrophobic chains. When these four amphiphilic pillar[5]arenes were dissolved in water, UV-Vis spectroscopy was used to investigate the self-assembly process (Table S1†). The characteristic peak at ∼290 nm, which is due to the absorbance of these amphiphilic pillar[5]arenes, decreased because of the decreasing concentration of free pillar[5]arene molecules in solution caused by the formation of assemblies. After 4 weeks, floccules were observed in the solutions of these pillar[5]arenes. The morphologies of the floccules were examined by SEM (Fig. 4) and TEM (Fig. 4, inset) studies. The overview of the floccules provided by SEM demonstrated that amphiphilic pillar[5]arenes C1–C4 all self-assembled into microfibers in water after 4 weeks. TEM images of a single microfiber revealed the diameter of the microfiber was ∼1 µm. The TEM image of the cross section of a single microfiber indicated that these floccules were actually microtubes (Fig. S33†).

Fig. 4 SEM and TEM (insets) images of floccules: (a) C1; (b) C2; (c) C3; (d) C4. The concentrations of these amphiphilic pillar[5]arenes were all 2.00 × 10⁻⁴ M.

To explore the fine structures of these microtubes, TEM investigations were further carried out to study the morphologies of the vertical section (Fig. 5a) and cross section (Fig. 5b) of the microtubes self-assembled from C4. Osmium tetroxide was applied as a staining agent here because it made the high electron density moieties appear darker.⁷ The results showed that these microtubes have an average exterior diameter of ∼1000 nm, an average thickness of ∼100 nm, and an average inner diameter of ∼800 nm. The lamella morphology in Fig. 5c follows the sequence of “black-gray-black”, which corresponds to the alternating lamellae of hydrophilic groups and hydrophobic chains (Fig. 5d).⁹ From the above investigations we can draw a conclusion that amphiphilic pillar[5]arenes which contain N-(2-aminoethyl)acetamide units as the hydrophilic groups can self-assemble into multi-layer microtubes in water.

Fig. 5 TEM images of microtubles self-assembled from C4: (a) vertical-section; (b) cross-section; (c) enlarged image of (b). (d) Schematic of the self-assembly of a C4-based microtube.

Small-angle X-ray diffraction (SAXD) was used to measure the characteristic period of the lamellar structure of the C4-based microtubes. However, no peak was observed in the profile and this failure of SAXD measurement might be attributed to the inhomogeneous lamellar structures.⁹ In addition, Fourier transform IR spectrum of a powder sample of C4 and that of the microtubes self-assembled from C4 showed that the antisymmetric and symmetric vibrations of N–H, and the stretching vibrations of alkyl chains in microtubes all shifted to low wavenumbers (Fig. S34†), indicating that both hydrogen-bonding interactions and van der Waals interactions are enhanced between neighboring alkyl chains from the powder sample to microtubes.^5l,7 What is more, vesicles, fused vesicles and necklace-like structures were observed in the intermediate state, which confirmed that microtubes were transferred from vesicles (Fig. S35†).

In addition, the self-assembly behavior of amphiphilic pillar[5]arene D which has the same hydrophobic chains and different hydrophilic groups with C2 was also investigated in water. There was no floccule in its aqueous solution even after 4 weeks, indicating no further self-assembly for D. From UV-Vis spectra we found that the characteristic absorption peak of D remained the same after 4 weeks (Fig. S36†). This finding strongly supports the view that amphiphilic pillar[5]arene D can not further self-assemble in water.

Considering that the carboxylate groups on D can complex with metal ions, we conducted metallization studies using self-assembled D. Fig. 6 shows SEM images of the 3D dendritic structure of the D⊃silver complex after reduction. Each dendrite consists of a long central backbone with very sharp secondary branches that preferentially grow along two definite directions rather than randomly. Surprisingly, the secondary branches that emerged at 60° in relation to the central backbone have uniform spacing and are parallel to each other. These secondary branches closely resemble needle-like crystals, which are the extreme case of anisotropic growth. The 3D dendritic structures may be induced by diffusion-limited aggregation, which produces complex random dendritic structures.¹⁰ Amphiphilic pillar[5]arene D probably acted as a stabilizer and a shape controller in the diffusion-limited aggregation process.¹¹

Fig. 6 (a) SEM image of the 3D dendritic structures self-assembled from the D⊃silver complex. (b) Enlarged image of (a).

Amphiphilic pillar[5]arene E also has the same hydrophobic chains and different hydrophilic groups with C2. As shown in Fig. S36,† the characteristic absorption peak decreased over time. What is more, floccules were observed in the aqueous solution of E after 4 weeks. The overview of the floccules provided by SEM and TEM demonstrated that different from other amphiphilic pillar[5]arenes, E self-assembled into nanosheets in water (Fig. 7). However, these nanosheets could not further self-assemble into microtubes even after 2 months (Fig. S38†).

Fig. 7 (a) SEM image of nanosheets self-assembled from E. (b) TEM image of nanosheets self-assembled from E.

DNA is a kind of biological macromolecules, it can form the genetic instructions to guide the biological development and life works. DNA capture is one of the most promising prospects in biomedical and bioorganic realms.¹² After the supramolecular microtubes self-assembled from C1, C2, C3, or C4 in the present work were acidized with HCl, they could be used to capture negatively charged DNA through electrostatic interactions of the positively charged primary amine units. A decrease in the characteristic absorbance of λ-DNA from 0.995 to 0.142 after the microtubes assembled from C4 were immersed in an aqueous solution of λ-DNA confirmed that λ-DNA was indeed adsorbed by the acidified microtubes (Fig. S39†). The formation of microtubes/λ-DNA complex was also confirmed by TEM images (Fig. 8). The microtubes form extensive areas of close contact surrounded by λ-DNA.

Fig. 8 TEM images of microtubes self-assembled from C4 (a) before and (b) after capture of λ-DNA.

Conclusions

In summary, amphiphilic pillar[5]arenes C1, C2, C3, C4, D, and E with different hydrophilic and hydrophobic groups were successfully synthesized. Because of the amphiphilic properties, they all self-assembled into vesicles firstly when dissolved in water. With N-(2-aminoethyl)acetamide units as the hydrophilic groups, C1, C2, C3, and C4 all further self-assembled into microtubes. Due to the primary amine groups on the surfaces of such microtubes, they can capture negatively charged DNA through electrostatic interactions in water. More interestingly, the self-assembly structures of C1 and C2 can be reversibly controlled from vesicles to micelles by bubbling CO₂ and N₂ because of their reversible reaction ability with CO₂ in water. Pillar[5]arene E with hydrazide groups as the hydophilic groups further self-assembled into nanosheets. Pillar[5]arene D with carboxylate moieties as the hydrophilic groups could not further self-assemble itself. However, after addition of silver cation into a solution of D in water, the resultant D⊃silver complex can further self-assemble into 3D dendritic structures. These results showed that chemical structures of amphiphilic pillar[5]arenes have a significant influence on their self-assembly morphologies. Such studies can provide useful information for the future design and preparation of advanced functional materials.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21202145 and J1210042) and the China Postdoctoral Science Foundation (2013M541767).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Synthetic procedures, characterizations. See DOI: 10.1039/c3ra46430j

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