Superstructure Formation and Topological Evolution Achieved by Self

Jan 12, 2016 - While it is still a challenging task, fine-tuning and precise control over ... from the self-assembly of low-molecular-weight building ...
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Superstructure Formation and Topological Evolution Achieved by Self-Organization of a Highly Adaptive Dynamer Pengyao Xing,† Hongzhong Chen,‡ Linyi Bai,‡ Aiyou Hao,*,† and Yanli Zhao*,‡,§ †

Key Laboratory of Colloid and Interface Chemistry of Ministry of Education and School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, People’s Republic of China ‡ Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371, Singapore § School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore S Supporting Information *

ABSTRACT: The adaptive property of supramolecular building blocks facilitates noncovalent synthesis of soft materials. While it is still a challenging task, fine-tuning and precise control over topological nanostructures constructed from the self-assembly of low-molecularweight building blocks are an important research direction to investigate the structure−property relationship. Herein, we report controlled selfassembly evolution of a low-molecular-weight building block bearing cholesterol and naphthalene-dicarboximide moieties, showing ultrasensitivity to solvent polarity. In low-polarity solvents (7.8) favor the formation of P-type helical nanostructures terminated by nanotoroids, having lamellar molecular packing. With a further increase in solvent polarity (up to 9.6), unilamellar and multilamellar vesicles were generated, which could undergo an aggregation-induced fusion process to form branched nanotubes tuned by the concentration. Self-attractive interactions between aggregates were found to be responsible for the formation of superstructures including helix−nanotoroid junctions as well as membrane-fused nanotubes. KEYWORDS: adaptive dynamer, morphological evolution, self-assembly, superstructures, vesicles of protocols in building unusual topologies such as toroids12−14 and the lack of precise modulation of noncovalent interactions to fabricate abundant soft materials from single and simple building blocks.15,16 A secondary self-assembly approach enables the fabrication of superstructures from primary aggregates utilizing interaggregate forces.17,18 The constructions of nonsymmetric micelles15,16 or the nanostructures of a linear heterojunction19 are the reflections of this protocol. One of the reasons for the occurrence of secondary self-assembly is self-attraction, which is assigned to spontaneous contact and merging of individual aggregates.20,21 The presence of noncovalent interactions as well as the similarity and matching of crystalline planes between the surface of aggregates contributes greatly to the self-attraction behavior. The utilization of this phenomenon allows the fabrication of

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he development of self-assembly chemistry provides versatile nanoarchitectures performing as functional materials in a wide range of applications.1 The functions and properties of these self-assembled aggregates are proven to be intimately related to their topological morphologies.2,3 For example, micelles and vesicles (liposomes) have advantages in drug delivery,4 while nanofiber-constituted hydrogels are favorably used as an extracellular matrix (ECM) for cell culture.5 Thus, topological study is of vital importance, and it is meaningful to explore the influence of structural and ambient conditions (stimuli-responsiveness) on the topology of supramolecular nanoarchitectures.6−11 Molecular structure factors, such as H-bonding, π−π stacking, solvophobic interactions, and metal−ligand coordination, and ambient condition factors such as pH, redox, temperature, and light are frequently considered and explored in designing building blocks,6−11 whereby the topological transformation between different dimensions can be controlled flexibly. Despite these facts, several challenges still remain in the area of controllable self-assembly, such as the lack © XXXX American Chemical Society

Received: December 11, 2015 Accepted: January 12, 2016

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between gel fibers as well as vesicle surfaces was evidenced to be a key factor.

superstructures from primary self-assembled nanostructures. For instance, during the incubation and aging process of supramolecular gels, some nanofibers tend to bundle and align together to form thicker fibers or dendritic aggregates.22−25 Another important example is vesicle aggregation mediated by the self-attraction between vesicle surfaces, depending on which process of the membrane fusion of vesicular walls occurs to form giant vesicles or nanochannels. Recently, our group employed a cyanostilbene-appended cholesterol to fabricate a vesicular system that could fuse to generate branched nanotubes controlled by UV light irradition.26 Other researchers have also explored stimuli-responsive vesicle aggregation or fusion.27−31 Most of these examples were fabricated via a stimuli-responsive manner by decorating stimulus-active sites such as special ligands, host−guest units, and H-bonding interactions on the surface of vesicles. On the other hand, spontaneous vesicle aggregation and fusion without external stimuli have been rarely reported so far. We here report a successive topological evolution of selfassembled nanostructures mediated by solvent polarity and concentration. As shown in Scheme 1, the self-assembly of a

RESULTS AND DISCUSSION Supergelation in Low Polarity Solvents. We first explored the self-assembly behavior of CN in solvents with low polarity, and it was found that CN could show an excellent gelation capability in selective solvents. As shown in Table S1 (Supporting Information (SI)), single solvents such as decane or ethyl acetate (EA) and mixed solvents such as dichloromethane (DCM)/hexane (2/8, v/v) or tetrahydrofuran (THF)/hexane (2/8, v/v) could be gelled above the critical gelation concentration (CGC), which was intimately related to the solvent polarities. Notably, CN in decane with an estimated polarity of 0.2 could afford a supergel,22 the CGC of which was lower than 0.1 wt %, suggesting that more than 10 000 decane molecules could be immobilized by a CN molecule. Apart from EA, the mixed solvents also allowed the construction of highsolvent-content gels (CGC 0.16 wt % for DCM/hexane-based gel and 0.20 wt % for THF/hexane-based gel). Due to relatively high polarity of EA (ca. 4.2), although stable gels can be readily generated in a solvation process of CN without sonication, the corresponding CGC has a larger value (0.75 wt %). Nevertheless, the CN-Br precursor (Scheme S1) was not capable of immobilizing these solvents, indicating that the hexyl tail plays a vital role in the gelation. The organogels formed from CN were highly emissive.38,39 Gels made from different solvents displayed slightly different emission colors. The maximum emissive wavelength was versatile in different solvents (Figure S1 in the SI), ascribed to different molecular arrangements and π−π stacking efficacy. That is the reason that decane could afford a yellow emissive gel, while other highly polar solvents gave green-yellowish gels under a UV lamp (365 nm). Concentration-dependent UV−vis absorption studies suggested that J-type π−π stacking arrays between naphthalene dicarboximide moieties formed during the gelation process in decane were ascribed to hyperchromic shift upon the increase of CN concentration (Figure 1a). This hypothesis was further verified by the occurrence of the shoulder peak located at 460 nm in the gels (3 and 10 mM samples). The gels made from EA, DCM/hexane, and THF/hexane also displayed similar shoulder peaks to those shown in Figure S2 (SI). By increasing the concentration or decreasing the solvent polarity, the self-assembly of CN was triggered to form gels. In this process, fluorescent intensities of the systems were quenched gradually through aggregation-caused quenching (ACQ) with red shifts (Figure 1c and Figures S3 and S4 in the SI). With the increase of CN concentration, the emission peak red-shifted from 460 nm to 530 nm due to the aggregation, giving an emission color change from blue to greenish-yellow (inset of Figure 1b). Similar to other physical gels, the obtained gels showed thermal responsiveness, whereby a reversible gel−sol phase transition was induced by heating and cooling. Upon heating a decane-based gel from room temperature to a high temperature (80 °C) in a sealed cuvette, the emission peak from π−π stacked arrays decreased gradually, while the blue-shifted peak intensity increased (Figure 1c and Figure S5 in the SI). This tendency is derived from the dissociation of π−π-stacked aggregates to loosely packed monomers. The thermal induced blue shift was further verified by the bulk emission color changes (inset of Figure 1d). By heating the self-standing gels to a high temperature, they transformed into a free-flowing solution, and the emissive colors changed from green to blue. As shown in

Scheme 1. Schematic representation of the molecular structures of the building blocks as well as the morphological evolution process controlled by solvent polarity and concentration. The five morphologies exhibited from left to right represent twisted gel fibers, partially toroid-terminated helices, separated vesicles, fused vesicles, and branched nanotubes.

building block (CN) containing a naphthalene-1,8-dicarboximide and a cholestrol moiety displays ultrasensitivity to solvent polarity. The self-aggregation of this type of building blocks is effective in selected solvents, as well-demonstrated previously.32−35 Thus, this building block can act as a good model compound to study superstructure formation mediated by the interaggregate forces. With the increase in solvent polarity, gel fibers with M-type handedness, nanotoroid-capped helices with P-type handedness, and vesicles were generated by CN, exhibiting outstanding flexibility and adaptivity in changeable environments. We used “dynamer” to describe this highly adaptive building block according to Lehn’s description.36,37 Vesicles are capable of fusing together into branched nanochannels (or nanotubes) at high concentration ranges. In the formation of nanotoroids and nanochannels, the self-attraction B

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Figure 1. (a) Concentration-dependent UV−vis spectra of CN in decane. (b) Concentration-dependent emission spectra (excited at 430 nm) of CN in decane from 0.005 to 10 mM, insets of which show representative emission colors upon increasing the CN concentration under a UV lamp (365 nm). (c) Temperature-dependent emissive spectra of decane-based gel. Inset shows emissive color change before and after heating. (d) Fluorescent intensity ratio (513/532 nm) changes upon an increase in temperature.

Figure 1d, the fluorescent intensity ratio between 513 and 532 nm exhibited an excellent linear correlation to temperature. This temperature-dependent fluorescent change property indicates a promising potential of CN gels as a fluorescent thermometer. Morphologies of the gels were studied by transmission electron microscopy (TEM), cryogenic transmission electron microscopy (Cryo-TEM), scanning electron microscopy (SEM), and atomic force microscopy (AFM) (Figure 2 and Figures S6 and S7 in the SI). Thin fibers with lengths of several micrometers entangled together to form nanopockets that can trap solvents by capillary forces. The morphologies were almost the same for the four types of gels in different solvents. Cross-section profiles of AFM images indicate that the width of the gel fibers is around 100 nm, and the height ranges from 6 to 12 nm. The high aspect ratio (∼10) between width and height suggests that the fibers are actually nanoribbons. One-dimensional growth of building blocks with molecular chirality may allow the presence of chiral amplification in the formation of superchiral aggregates. In the gel matrix, it was found that, although the chirality of individual fibers was hard to distinguish, some aligned fibers exhibited twisted morphologies (insets of Figure S6b,f in the SI). Apparently, the twisted fibers adopted M-type (left-handed) handedness with a helical pitch of hundreds of nanometers. Active circular dichroism (CD) signals confirmed the formation of aggregates with superchirality (Figure S8 in the SI). Strong positive Cotton effects at 450 and 430 nm as well as a negative Cotton effect at about 345 nm can be found in all gel samples, indicating that solvents with polarities from 0.2 to 4.2 favor the formation of twisted fibers with same handedness (M-type).

Figure 2. Representative (a) TEM, (b) SEM, and (c) AFM images of the CN gels (THF/hexane, 2/8, v/v, 0.5 wt %). Inset of (c) shows the cross-section profile of selected areas. (d) Amplitude AFM image of the gel fibers.

Highly cross-linked fibers could facilitate the formation of ultrastable and robust supramolecular organogels, which were C

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Figure 3. (a) Dynamic oscillatory stress sweep and (b) frequency sweep of the CN gels (0.7 wt %) in THF/hexane (2/8, v/v). (c) Complex modulus (GN) as a function of CN concentration. (d) Average molecular weights between effective cross-links (Me) as a function of CN concentration. (e) G′ and G″ values of gels (0.5 wt %, THF/hexane, 2/8, v/v) in the continuous-stress measurements. The sample was subjected to 30 Pa for 30 s and then back to 1 Pa in the linear regime for 30 s. This process was repeated five times, and the applied frequency was fixed at 1 Hz.

nature endows the gels with a mechanical-strength self-healing property. This feature, also known as thixotropy, is crucial in many applications of supramolecular gels.42 In order to elucidate the self-healing feature of the robust CN gels, the continuousstress measurement was carried out with different amplitude oscillatory forces (Figure 3e). Under a low-amplitude oscillatory force (τ = 1.0 Pa, f = 1 Hz), the gels showed a solid-like behavior, which would be disassociated into a liquid behavior under a highamplitude oscillatory force (τ = 30 Pa, f = 1 Hz). After severe deformation, interestingly, the mechanical strength (G′ and G″) was fully recovered when the amplitude oscillatory force was immediately adjusted to lower values (τ = 1.0 Pa). Furthermore, this rapid recovery was proven to be reversible and reproducible. After five cycles, no apparent mechanical strength loss was observed. After the solvent ratio and concentration of gels were varied, the obtained samples were subjected to TEM observations. It was found that DCM/hexane-based gels exhibited honeycomb-like patterns (Figure S12 in the SI) at the low concentration range. The diameters of the pores can be regulated by changing the CN concentrations and solvent ratios. Spherical pores with a diameter range from 400 to 600 nm (0.3 wt %, 1/9, v/v) were packed into a well-defined hexagonal array on TEM carbon grids. Increasing the concentration (0.5 wt %, 1/9, v/v) would result in disordered meshes constituted by the gel fibers. In addition, a high fraction of DCM (2/8, v/v) favors the formation of similar but less ordered spherical pores with diameters from 100 to 500 nm. When the gels were casted on a silicon wafer, it also displayed the same patterns under SEM observations (Figure S13a−e in the SI). It should be noted that, among four types of gels, only DCM/hexane-based gels could exhibit these patterns, meaning that the utilization of low-boiling-point DCM is crucial in

further investigated by rheological studies. The gels showed a linear viscoelastic region (Figure 3a as well as Figures S9 and S10 in the SI) over a wide applied stress region from 0.01 Pa to yield stress points, after which the gels collapse into the liquid, exhibiting non-Newtonian fluid characteristics.40 According to the dynamic frequency sweep (Figure 3b and Figure S9 in the SI) in the linear viscoelastic region (τ = 1.0 Pa), the gels behaved as a solid. At the applied frequency (f = 0.1−100 Hz), values of the storage modulus (G′) were nearly 1 order of magnitude higher than that of the loss modulus (G″), and both G′ and G″ were invariant with frequency until a certain yield value. Mechanical properties of the gels could be well tailored by controlling the concentration. For example, the stiffness or rigidity that can be reflected by the value of G′/G″ increased from 4 to 7 by varying the concentration from 0.2 to 0.7 wt % (Figure S11 in the SI). The complex plateau modulus (GN) calculated from the equation GN = √G′2 + G″2 exhibited concentration-dependent behavior. From a double logarithmic curve (Figure 3c), a relationship of GN ∝ c1.69 (c stands for concentration) was obtained, the exponent of which was close to the theoretical value of ca. 2.0 for gels,22 indicating the stable and continuous feature of the CN gels. The cross-linking degree of the gel fibers can be indicated by the average molar weights between effective cross-links (Me, calculated from the equation Me = ρRT/GN, where ρ, R, and T represent the concentration of the solution, the molar gas constant, and the absolute temperature, respectively).41 As shown in Figure 3d, the Me value dropped down from ca. 24 000 g/mol to 8000 g/mol after elevating the concentration from 0.2 wt % (CGC) to 0.7 wt %, suggesting that there were more crosslinked joints in the gels under higher concentrations. The significant intertwining between gel fibers and their dynamic D

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Figure 4. (a) FT-IR spectra of different gel samples (I: EA gel, II: decane gel, III: DCM/hexane gel, IV: THF/hexane gel). (b) Powder XRD pattern of the gel sample prepared from THF/hexane. Inset shows hexagonal molecular packing (top view). Black circles represent the π-stacked cores, while red dashed circles represent apolar cholesterol domains. (c) Side view of the π-stacked column that could further grow into twisted gel fibers.

gel samples (Figure S15 in the SI) in the wet state suggested that the H-bonding interactions between the amide groups could be strengthened in the presence of apolar solvents contributed by the solvent−solute interaction. The highly directional property of H-bonding between amides, the π−π stacking interaction between naphthalene dicarboximide moieties, and van der Waals interaction between alkyl chains enables the formation of supergels in low-polarity solvents. The molecular arrangement of CN within gel fibers was investigated by powder X-ray diffraction (XRD) experiments (Figure 4b). Diffraction peaks at 2θ = 1.65°, 2.84°, and 3.21° correspond to the distances of 5.5, 3.1, and 2.75 nm with a correlation of 1:1/√3:1/√4, indicating the formation of a hexagonal columnar structure with a d-space of 5.5 nm. This number is in accordance with the length of double CN molecules with overlapping units.32−35,47 Within this kind of molecular packing, a slight overlap of polar heads induced by π−π stacking may occur. As CN has a polar head and a flexible apolar tail (cholesteryl group), the apolar tail would stack at outside columnar array to contact with apolar solvents in lowpolarity solvents (Figure 4c). Helix, Nanotoroid, and Vesicle Formation in HighPolarity Solvents. The self-assembly studies were then carried out in high-polarity solvents. By adding water into the THF solution of CN to tune the solution polarities, self-assembly was triggered. The bulk solution became turbid until the fraction of water ( f w) was more than 60 vol % at appropriate concentrations, indicating the formation of nanostructures. In contrast to apolar solvents, only weak gels that could be easily deformed by gentle shaking were obtained in highly polar solvents. The low f w (lower than 60 vol %) condition would not generate ordered aggregates, as confirmed by TEM observations. On the other hand, the solution with high f w values contained nanofibers with a zigzag shape (Figure 5a). The formation of superchirality is the main reason for the zigzag morphology according to the enlarged TEM

determining the formation of these superstructures. The patterns resemble the breath figure method using water droplets as templates on various substrates.43,44 In this case, however, no extra moist airflow was needed to generate water droplets on a rapidly cooled solution−air interface. The fast evaporation of DCM in ambient environment led to the rapid cooling, which induced the condensation of water on the air/gel interface. Due to the intrinsic hydrophobicity of CN and immobilized solvents in gels, water droplets would expel the gel fibers and solvents to form spherical pores. This hypothesis was verified by the TEM images of intermediates during the formation of pores (Figure S14 in the SI), where it could be found that flexible gel fibers were repelled to generate loops and walls of pores. After the formation of pores, the initial gel fibers were hard to observe, which may result from interfiber alignment and fusion in the drying process. The rearrangement of water droplets triggered by the thermocapillary flow is the key factor to construct hexagonally packed pores. The theory of minimum energy state has been developed to explain the ordered hexagonal pores.45 The resistance from gels in the low concentration range might be much lower than that under high concentration due to comparatively low fiber density. Thus, it was much easier to obtain the ordered patterns at a low concentration range. If there were no gel fibers (prepared from DCM solution), it was also possible to obtain densely packed pores (Figure S13f in the SI), but the pore size was mainly around 100 nm. Noncovalent forces account for the formation of nanofibers and breath figure arrays. Fourier transform infrared spectroscopy (FT-IR) was employed as a tool to probe the H-bonding information. As shown in Figure 4a, vibration peaks at 3350, 1639, and 1542 cm−1 are assigned to the N−H stretch, CO stretch, and amide-II stretch, respectively. These peaks appeared in all four xerogel samples, suggesting the presence of intermolecular H-bonding between amides.46 FT-IR spectra of E

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Figure 5. (a, b) Representative TEM images of helices and toroid-capped helices prepared in a THF/water mixture ( f w: 70%). (c, d) TEM images of various intermediates of toroid-terminated helices (in a THF/water mixture, f w: 70%). (e, f) AFM images of helices and toroid-capped helices (in a THF/water mixture, f w: 60%), insets of which stand for the cross-section profiles of a helix and a toroid. (g) SEM image of helices and toroidcapped helices (in a THF/water mixture, f w: 70%). (h) TEM image of a sample with 80 vol % water.

and SEM images (Figure S16 in the SI). Apparently, the nanofibers adopt a P-type helicity with a helical pitch of hundreds of nanometers, which can be identified as helices rather than twists observed in the organogels. It is well known that the formation of twist fibers is controlled by the saddle-like (Gaussian) curvature, while the helix superstructure has a cylindrical curvature, and the curvature variation suggests the change of basic molecular arrangements upon increasing the solvent polarity.48 A cross-section profile of the nanofibers (Figure 5e) shows that the width is around 200−300 nm and the height is around 20 nm. The nanofibers are highly self-attractive due to the presence of thick and aligned fibers (Figure 5c,e). Nanofibers were observed when the f w value was between 60 and 80 vol % at a wide concentration range. An important feature of these nanofibers is that a large portion of them were capped by nanotoroids (Figure 5 and Figure S17 in the SI). As an infrequent topology, toroidal structures are hard to prepare due to the lack of guiding principles as compared to other common topologies such as fibers, micelles, and vesicles. Normally, two mechanisms were proposed in explaining the toroidal formation, namely, the induction of curvature and the stacking of macrocycles.12 In our case, it should follow the principle of induction of curvature. This speculated mechanism is possible because the curvature on the zigzag helices was observed, which may be induced by steric repulsion from bulky cholesteryl groups. In polar solvents (THF/water), apolar cholesteryl groups with chiral centers tend to overlap and interdigitate together to enable the chiral amplification and the formation of interfacial curvature. Bates and Wooley utilized amphiphilic block copolymers to construct nanotoroids and proposed a mechanism of cylindrical micelle collapse based on the observation of intermediates.20,21 The mechanism in the present work is similar to theirs. Some intermediate structures were captured to verify the hypothesis, as displayed in Figure 5c,d. Lariats (Figure 5d-1, d-3, d-4, d-7), double helices (Figure 5c, d-5), closed loops (Figure 5d-6), and

tail curl (Figure 5d-2, e) were observed, indicating that the toroids were generated from fibers. Furthermore, both toriods and helical fibers share similar lateral or longitudinal length (Figure 5e,f and Figure S18 in the SI), and thus we could infer their parentage relationship. Buckling of fibers should occur to allow self-contacting, which must be followed by the interfiber fusion process to complete the cyclization. Self-fusion of aggregates is a key factor to the topological transformation assisted by THF that would solvate the interior of aggregates and endow them with a dynamic property. Although most toroids were attached/terminated on the fibers, some of them were found to be independent (Figures S19 and S20 in the SI). Therefore, in addition to the buckling of fibers, end-to-end closure of individual fibers is another possible pathway.20 Unfavorable rim energy of helical fibers tends to be eliminated by closing up to form spherical or ring-like structures. The toroid-terminated helices in polar solvents resemble the helical coiling of plant tendrils such as cucumber, the mechanism of which has been elucidated by Darwin and other scientists.49 The strain caused by selective dehydration was proven to control the mechanical behavior and the toroid formation. Similar to the natural helix−toriod structure, the nanotoroid formation in our case may also be contributed by the compensation to the strain by the curvature at the nanoscale, which is in good agreement with the above-mentioned TEM observations and speculations. By increasing the polarity of solvents (80 vol % water), more helical fibers collapsed, while more independent toroids started to appear (Figure 5h). At the same time, fewer multilamellar vesicles were also observed. Upon increasing the solvent polarity, CN was forced to have more close packing, which in turn gave aggregates with higher interfacial curvature. According to the theory of molecular packing parameter (p = v/la) where v stands for the volume of the hydrophobic chain, l is the length of the hydrophobic chain, and a represents the area of the hydrophilic head groups,50 the increase in solvent polarity would increase the F

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Figure 6. (a) Normalized UV−vis and (b) emission spectra of samples upon increasing the water fraction (in a THF/water mixture, 0.1 mM). The dashed lines in (a) represent the samples with f w from 10 to 50 vol %. (c) Representative normalized CD spectral comparison between a vesicle sample (in THF/water) and a gel sample (in THF/hexane). (d) CD spectra of samples upon increasing the water fraction (in a THF/water mixture, 0.2 mM).

relative solvophobic volume (v), elevating the value of p to favor the formation of vesicles rather than one-dimensional fibers or worm-like micelles. Molecular packing of CN into helices/nanotoroids was also examined by powder XRD experiments (Figure S21 in the SI). Peaks at 3.00°, 5.96°, and 8.86° correspond to distances of 2.92, 1.48, and 0.99 nm with a ratio of 1:1/2:1/3, respectively, indicating the formation of lamellar structure. The d-space of 2.92 nm is much lower than that of gels in low-polarity solvents, meaning effective interdigitation of cholesteryl groups in helix and toroid formation. A H-bonding interaction is still present in the aggregates in high-polarity solvents, verified by the FT-IR spectrum (Figure S22 in the SI). In high-polarity solvents, CN still retained J-type π−π stacking according to obvious red-shifted UV−vis spectra upon the addition of water into THF solutions (Figure 6a and Figure S23 in the SI). Red-shifted emission spectra further confirmed the effective π−π stacking in high-polarity solvents (Figure 6b and Figure S23 in the SI). Indeed, a tiny change (2 vol % water) of solvent polarity could induce a large red shift and quenching of the fluorescence (Figure S23d in the SI). The conclusion is that CN in solution is highly sensitive to solvent polarity, allowing us to tailor its self-assemblies readily. When more than 90 vol % water was present in the system, the solution became stable without any precipitation. The slight opalescence in the obtained solution indicates the presence of colloidal nanoparticles, showing similar shoulder peaks to the organogels in UV−vis and emission spectra (Figure 6a,b and Figure S23 in the SI). The shoulder peaks are contributed by the π−π-stacked CN, while the peak located at 430 nm in the UV−vis spectrum is assigned to

the free CN monomer, suggesting that the aggregation number (Np) decreases in a high-polarity solvent environment (90 vol % water). According to the equation μN = μ∞ + αkT/Np where μN stands for the mean interaction free energy of a molecule in the aggregate state, μ∞ stands for the total energy of a molecule in the aggregate state, and α, T, k, and NP represent the positive constant, the absolute temperature, the Boltzmann constant, and the aggregation number, respectively,51,52 the decrease of the aggregation number (Np) would elevate the mean interaction free energy per molecule (μN). Then, smaller size aggregates such as spherical particles may be generated, since an unfavorable rim energy of the fibers would be eliminated under conditions with a high μN value. As elucidated in morphological discussions, instead of M-type twists, P-type helices were formed in high-polarity solvents. CD experiments were then carried out to verify this superhelicity inversion. As displayed in Figure 6c,d, a mirror spectrum was obtained, which has negative Cotton effects at 430 and 450 nm and a positive Cotton effect at 345 nm, indicating that the spinning orientation of the CN molecule was inversed (Figure S24 in the SI). Solvent can be a factor to tune the superhelicity based on the literature reports,34,53,54 because it is capable of changing the arrangement of building blocks. The interdigitation areas inside bilayers would be changed from naphthalene dicarboximide to cholesteryl groups after the polarity was increased significantly (Figure S24 in the SI). In this rearrangement process, the directionality of H-bonding between amides, which could induce aggregation formation and lead to the emergence of superhelicity, may adopt an opposite manner. A silent CD signal in a sample with 50 vol % water (Figure 6d) G

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Figure 7. TEM images of samples with different concentrations: (a) 0.05 mM, (b) 0.1 mM, and (c, d) 0.15 mM. Red arrows in d-1 and d-2 represent the central channels formed between fused vesicles. SEM images of samples with different concentrations: (e) 0.05 mM, (f) 0.1 mM, and (g, h) 0.2 mM. AFM images of samples with different concentrations: (i, j) 0.03 mM and (k, l) 0.2 mM. Image (l) shows a three-dimensional image of aggregated vesicles. Insets of (i) and (k) are cross-section profiles of vesicle and aggregated vesicles. For all samples, f w was fixed at 90 vol %. Scales of images (i) to (k): 2 × 2 μm, 7 × 7 μm, 5 × 5 μm, and 5 × 5 μm, respectively.

induced by external stimuli or just in a spontaneous manner.29 Normally, artificial vesicle−vesicle aggregation can be controlled by host−guest interaction, metal−ligand coordination, and Hbonding interactions. The flexible and dynamic nature of bilayer membrane is responsible for the vesicle aggregation as compared to other self-assembled structures. Vesicle aggregation could result in several topologies, for example, vesicular clusters from aggregation, bigger vesicles or nanotubes from fusion, and smaller vesicles from fission or membrane contraction. The selfattraction of CN endows it with a high possibility to realize stimulus-free vesicle aggregation to generate nanotubes tuned by the concentration. As summarized in Figure 7, TEM, SEM, and AFM were employed to characterize the aggregation and fusion process of vesicles upon increasing the CN concentrations in a highly polar solvent environment (90 vol % water). When the concentrations were lower than 10−4 M, only separated vesicles (at 5 × 10−5 M) or vesicular clusters (at 10−4 M) were obtained, and almost no fusion process was observed. Further increase of CN concentration resulted in vesicle fusion along with the emergence of figure 8 or peanut-shaped interiors (Figure S26 in the SI). Most of the fused vesicles exhibited a tubular morphology under TEM observations (Figure 7c,d), where the parent vesicle shapes indicated that tubules were derived from the vesicle fusion. On account of the vesicle fusion process, the

indicates no ordered aggregate formation. After helices and nanotoroids were formed under 60 vol % water, strong CD signals appeared and became stronger with the increase in water content. However, a colloidal solution with 90 vol % water showing relatively weak Cotton effects may be due to the absence of one-dimensional aggregates, which is more favorable for chiral amplification. Vesicle-to-Branched Nanotube Transformation via Membrane Fusion. The morphological studies on the colloidal solutions (90 vol % water) were further carried out. Expectedly, vesicles with diameters of 100−300 nm were observed (Figure S25 in the SI) at a low concentration range (10−5 to 10−4 M). On the basis of TEM observations, both unilamellar and multilamellar vesicles were confirmed by distinguishable interiors and exteriors (inset of Figure 7a). The AFM cross-section profile gave a high aspect ratio (larger than 10) of the spherical particles with an obviously collapsed center (Figure 7i), indicating their hollow nature. The lowest height of the vesicles is about 10 nm, which is in good agreement with the thickness of two stacked vesicle membranes.55 Interestingly, the vesicular system exhibits concentrationdependent aggregation behavior. It is well known that bilayer vesicles could interact with each other, whereby some of them show aggregation, fusion, and fission behavior when being H

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Figure 8. (a) Concentration-dependent UV−vis spectra of vesicle samples. (b) Absorption intensity at 550 nm as a function of the CN concentration. (c) Concentration-dependent CD spectra of vesicle samples, where dashed lines represent the samples with a concentration range from 0.02 to 0.09 mM. (d) Concentration-dependent emission spectra of vesicle samples. (e) DLS size distribution of samples upon increasing concentrations. All samples were carried out in a THF/water mixture ( f w = 90 vol %).

scenario, perturbation-induction should be the main route. Upon increasing the CN concentration, the collision and membrane contact (perturbations) induced by inevitable Brownian motion of vesicles are more effective. After the membrane contact, the self-attractive membrane would form a bilayer stalk followed by expanding the fusion pores and completing the mergence to give nanotubes. The aggregation and fusion of vesicles brought changes to the turbidity due to the size increase, and this phenomenon was probed by UV−vis absorption studies (Figure 8a). After effective aggregation and fusion, the monomer peak located at 430 nm was surpassed by the aggregation peak at 450 nm, indicating an elevated aggregation number (Np) to favor the formation of onedimensional or linear aggregates rather than spherical particles. Thereafter, more free CN molecules were involved in the selfassembly during the vesicle fusion process. In addition, absorption peaks were broadened due to the increased turbidity. The inflection point in the absorption at 550 nm as a function of concentration gave a value of about 0.12 mM (Figure 8b), which can be regarded as the critical concentration for the vesicle fusion. The vesicular system also had active CD signals (Figure 8c), suggesting a helical molecular arrangement of CN within the bilayer membrane of the vesicles. After vesicle fusion at a concentration greater than 0.1 mM, however, the CD intensity gradually decreased (dashed lines in Figure 8c), which was in contrast with the increasing tendency before the fusion. Emission spectra (Figure 8d) confirmed the increased aggregation number after the vesicle fusion due to gradual disappearance of the monomer peak at 480 nm and red-shifted main peak with

as-formed tubules showed some specific features in contrast with directly assembled nanotubes. Taking some cases as examples, all of tubules were capped by spherical ends, most of them were linked to each other (branch-like) due to poor directional fusion (although the linear growth seemed to be primary), they were not rigid enough to give straight but flexible nanotubes, and lastly some of the interior areas from the nanotubes were not connected completely. On the basis of these features, it may be more appropriate to name them nanotubes rather than nanocapsules or tubular vesicles. SEM and AFM studies (Figure 7 and Figure S27 in the SI) on the surface of the nanotubes were in good agreement with above discussions. The surface of the nanotubes showed worm-like shapes. Cross-section profiles of AFM images (Figure 7k) suggested that the size of the spherical surface from these nanotubes was almost the same as individual vesicles in the low concentration range. According to the CryoTEM observations (Figure S28 in the SI), the influence of solvent evaporation could be excluded in investigating the vesicle fusion process. This concentration-dependent vesicle fusion process was reversible and dynamic. By diluting the concentrated solution followed by sonication, individual vesicles appeared again. Although some intermediates could be captured, the detailed process of the fusion was hard to probe by electron microscope. On the basis of literature reports and morphological studies indicated above, a mechanism of the fusion process was proposed (Figure S29 in the SI). Protein-assistance and perturbationinduced processes are two main pathways during membrane fusion in biological and artificial systems.56−60 In the present I

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CONCLUSION In summary, we have designed and synthesized a novel building block (CN) that is ultrasensitive to solvent polarity, showing rich self-assembly behavior. Upon increasing the solvent polarity, twisted fiber-constituted superorganogels, nanotoroid-terminated helices, and vesicles have been obtained. Organogel fibers in DCM/hexane are capable of forming breath-figure arrays via a fiber−pore transformation process without wet airflow. At high concentrations of CN in highly polar solvents, vesicles could undergo an aggregation−fusion pathway to generate superstructures of branched nanochannels. The self-attraction, hexagonal/lamellar molecular arrangement, J-type π−π stacking, and molecular chiral centers in cholesteryl group account for the multiple topological phases as well as their transformations. The current study presents a simple protocol of tuning multiple morphologies based on a single entity, which is useful to fabricate advanced assemblies toward a variety of applications.

increasing concentration (0.02 mM to 0.25 mM). Dynamic light scattering (DLS) size distribution studies (Figure 8e) were in good agreement with the morphological evolution from vesicles to nanotubes by fusion. After the vesicle fusion, the mean diameters of aggregations were elevated from ca. 150 nm to more than 1000 nm with significant broadening. Through the studies of UV−vis absorption and fluorescent emission, it can be concluded that, at higher concentration, more CN building blocks are involved in the self-assembly, and the intermolecular interactions such as π−π stacking and H-bonding interactions are strengthened accordingly. Thus, the free energy of the self-assembly is sufficient at high concentration, allowing the CN molecules to generate extended π−π stacking arrays to form nanotubes. At low concentration, the stacked arrays of CN molecules are less compact, favoring the formation of loosely packed vesicles. Li and co-workers have established a theoretical model of concentration-dependent vesicle−nanotube phase transition based on a controllable dipeptide assembly, which can also be applied in the present vesicle−nanotube transition system.61−63 The following equation was derived:

MATERIALS AND METHODS Materials. All chemicals were purchased from Sigma-Aldrich and used without further purifications. All solvents and inorganic agents were commercially available. N-Methyl-4-bromonaphthalene-1,8-dicarboximide was synthesized according to the literature report.38 Charcterizations. 1H NMR spectra were measured on a Bruker-AC 300 spectrometer. 13C NMR spectra were measured on a Bruker BBFO400 spectrometer. High-resolution mass spectrometry (HR-MS) was performed on a Waters Q-tof Premier MS spectrometer. Absorption spectra were recorded on a Shimadzu UV-3600 spectrophotometer. The fluorescence emission spectra were recorded on a Shimadzu RF-5301pc fluorescence spectrophotometer. SEM images were collected from a SEM of a field-emission JSM-6700F (JEOL). DLS size distributions were measured on a Nanobrook 90Plus partical size analyzer. Normal TEM images were collected on a JEM-1400 TEM (JEOL, 100 kV). The Cryo-TEM samples were prepared in liquid ethane at −165 °C and stored in liquid nitrogen until they were transferred to a cryogenic sample holder (Gatan 626) and examined with a JEOL JEM-1400 TEM (120 kV) at about −174 °C. AFM images were recorded under ambient conditions by using a Veeco Nanoscope Multimode III SPM operating in tapping mode. FT-IR spectra were measured by a Fourier transform infrared spectrometer. Powder X-ray diffraction patterns were collected by a Bruker D8 powder X-ray diffractometer at 40 kV and 30 mA using Cu Kα radiation (λ = 1.5418 Å). CD spectra were obtained by a Jasco J810 CD spectrophotometer using a 0.2 cm quartz cell. Rheological properties were measured by a Thermo Haake RS6000 rheometer with cone and plate geometry (35 mm diameter and 0.105 mm cone gap). Synthesis of CN-Br. Cholesteryl chloroformate (0.50 g, 1.1 mmol) in anhydrous THF (20 mL) was dropwise added into a mixed solution of N-methyl-4-bromonaphthalene-1,8-dicarboximide (0.32 g, 1.0 mmol) and triethylamine (0.1 mL) in anhydrous THF (20 mL) at 0 °C (Scheme S1 in the SI). The mixture was stirred at room temperature for 1 h. Then, the mixture was filtered to remove the byproducts. The filtrate was concentrated under reduced pressure, and the residue was recrytallized three times (DCM/hexane, 1:10, v/v) to obtain the crude product, which was purified by column chromatography (ethyl acetate/hexane, 20/1, v/v) to afford the pure compound CN-Br (550 mg, yield 75%). 1H NMR (300 MHz, CDCl3, 298 K): δ = 8.75 (d, 1H), 8.60 (d, 1H), 8.46 (d, 1H), 8.12 (d, 1H), 7.83 (t, 1H), 5.22 (s, 1H), 5.05 (s, 1H), 4.41 (m, 3H), 3.60 (t, 2H), 2.3−0.68 (m, 43H). 13C NMR (100 MHz, CDCl3, 298 K): δ = 163.8, 139.7, 133.3, 132.1, 131.3, 131.0, 130.4, 128.2, 128.0, 122.7, 122.2, 121.8, 74.2, 56.6, 56.1, 49.9, 42.2, 39.9, 39.7, 39.5, 38.3, 36.8, 36.4, 36.1, 35.7, 31.8, 28.2, 27.9, 24.2, 23.8, 22.8, 22.5, 20.9, 19.2, 18.7, 11.8. HRMS (TOF): m/z calcd for C42H56BrN2O4 [M + H]+, 731.3491; found, 731.3423. Synthesis of CN. CN-Br (0.36 g, 0.5 mmol) and hexylamine (500 mg, 5 mM) were added into DMSO (30 mL). The mixture was heated at 100 °C with stirring under N2 protection for 8h. Then, the mixture was cooled to room temperature. After that, the mixture was poured into ice

CTVC = CA e−3γ / CA dκbT

where CTVC stands for the critical nanotube-to-vesicle transition concentration, CA stands for the concentration in aggregate state (a constant value for a given material), γ stands for the tension of the solution/aggregate interface, d stands for the outward growth thickness on the aggregates (molecular size), κb is the Boltzmann constant, and T is the temperature. For CN, in a given ambient environment, CA and d are constant values, and thus CTVC is only determined by the γ value. By increasing the concentration of building blocks such as CN, a critical phase transition concentration can be expected at a certain γ value, giving rise to the topological evolution from vesicles to nanotubes. Another variate in the vesicle to tube transition is the change of curvature, which has a vital impact on the morphology of selfassembled nanostructures, especially in membrane-based aggregates. Normally, a high curvature of the membrane leads to the formation of nanotubular structures, while a low curvature favors forming spherical vesicles.64 Previous studies have demonstrated several methods to change the membrane curvature in order to achieve the vesicle−nanotube transformation, such as tuning the solvent ratio,65 changing the molecular conformation,26 and the addition of chemical additives.64 However, concentration-dependent membrane curvature variation has been seldom reported. Although the curvature of the membrane is mostly determined by the structural parameters such as molecular rigidity, noncovalent interactions could also be involved, as demonstrated by the dipeptide assembly61−63 and the present system. A difference between dilute and concentrated situations of the CN selfassembly is the aggregation number, as discussed above. A high aggregation number would normally give a compact and longrange ordered aggregation mode as compared with that of low aggregation number, leading to the formation of nanostructures with high curvature and extended dimensions. Therefore, upon increasing the concentration, CN tends to grow into onedimensional tubular structures with high curvature rather than zero-dimensional spherical vesicles with low curvature. J

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ACS Nano water, which was then extracted by ethyl acetate three times. The organic layer was dried by MgSO4. After the removel of most solvent, the residue was subjected to column chromatography (DCM/MeOH from 1000/1 to 100/1) to afford the pure compound CN (600 mg, yield 80%). 1H NMR (300 MHz, CDCl3, 298 K): δ = 8.68 (d, 1H), 8.50 (d, 1H), 8.13 (d, 1H), 7.60 (t, 1H), 6.75 (d, 1H), 5.27 (d, 2H), 4.40 (t, 3H), 3.57 (t, 3H), 3.44 (t, 2H), 2.3−0.66 (m, 54H). 13C NMR (100 MHz, CDCl3, 298 K): δ = 165.0, 156.4, 149.8, 139.8, 134.7, 131.1, 129.8, 126.2, 124.4, 122.5, 122.2, 120.0, 109.3, 101.1, 74.3, 56.6, 56.1, 49.9, 43.7, 42.2, 40.9, 39.7, 39.5, 39.3, 38.4, 36.9, 36.4, 36.1, 35.7, 31.8, 31.5, 28.8, 28.2, 27.9, 28.8, 24.2, 23.8, 22.8, 22.6, 22.5, 20.9, 19.2, 18.6, 14.0, 11.8. HRMS (TOF): m/z calcd for C48H70N3O4 [M + H]+, 752.5371; found, 752.5366.

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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/acsnano.5b07800. Synthesis, gelation behavior in different solvents, UV−vis and emission spectra of samples with different concentrations and solvent conditions, rheological data of samples with different concentrations, as well as additional TEM, SEM, and AFM images (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is supported by the National Research Foundation (NRF), Prime Minister’s Office, Singapore, under its Campus for Research Excellence and Technological Enterprise (CREATE) Programme−Singapore Peking University Research Centre for a Sustainable Low-Carbon Future, as well as the NTU-A*Star Silicon Technologies Centre of Excellence under program no. 11235100003. This work was also funded by the China Scholarship Council (CSC). REFERENCES (1) Aida, T.; Meijer, E. W.; Stupp, S. I. Functional Supramolecular Polymers. Science 2012, 335, 813−817. (2) Yu, G.; Jie, K.; Huang, F. Supramolecular Amphiphiles Based on Host−Guest Molecular Recognition Motifs. Chem. Rev. 2015, 115, 7240−7303. (3) Yang, L.; Tan, X.; Wang, Z.; Zhang, X. Supramolecular Polymers: Historical Development, Preparation, Characterization, and Functions. Chem. Rev. 2015, 115, 7196−7239. (4) Xing, P.; Sun, T.; Hao, A. Vesicles from Supramolecular Amphiphiles. RSC Adv. 2013, 3, 24776−24793. (5) Kumar, S. Cellular Mechanotransduction: Stiffness Does Matter. Nat. Mater. 2014, 13, 918−920. (6) Zhang, X.; Wang, C. Supramolecular Amphiphiles. Chem. Soc. Rev. 2011, 40, 94−101. (7) Ma, X.; Tian, H. Stimuli-Responsive Supramolecular Polymers in Aqueous Solution. Acc. Chem. Res. 2014, 47, 1971−1981. (8) Yan, X.; Wang, F.; Zheng, B.; Huang, F. Stimuli-Responsive Supramolecular Polymeric Materials. Chem. Soc. Rev. 2012, 41, 6042− 6065. (9) Theato, P.; Sumerlin, B. S.; O’Reillyc, R. K.; Epps, T. H., III. Stimuli Responsive Materials. Chem. Soc. Rev. 2013, 42, 7055−7056. K

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