Bio-inspired hierarchical self-assembly of nanotubes into multi-dimensional and multi-scale structures

Abstract

As inspired from nature's strategy to prepare collagen, herein we report a hierarchical solution self-assembly method to prepare multi-dimensional and multi-scale supra-structures from the building blocks of pristine titanate nanotubes (TNTs) around 10 nm. With the help of amylose, the nanotubes was continuously self-assembled into helically wrapped TNTs, highly aligned fibres, large bundles, 2D crystal facets and 3D core–shell hybrid crystals. The amyloses work as the glue molecules to drive and direct the hierarchical self-assembly process extending from microscopic to macroscopic scale. The whole self-assembly process as well as the self-assembly structures were carefully characterized by the combination methods of ¹H NMR, CD, Hr-SEM, AFM, Hr-TEM, SAED pattern and EDX measurements. A hierarchical self-assembly mechanism was also proposed.

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1 Introduction

In recent years, great progress has been made to prepare discrete inorganic or carbon nanostructures such as nanoparticles, nanorods and nanotubes with controlled chemical compositions and morphologies.^1,2 A key challenge for their application is how to assemble them into desired large architectures, which is referred to as a “bottom-up” nanofabrication process.^3–9 Hitherto, engineering techniques such as the use of a pre-programmed template,^7–11lithographic patterning,^12,13chemical vapor deposition (CVD) and micromolding,^14,15 have been invented to achieve this goal. Besides, as a bioinspired and economic route, template-free directed self-assembly of nanosized building blocks into multidimensional and multiscale structures has received more-and-more attention in recent years.^3,4,16–19 As typical examples: Tang and Kotov^20–22 have obtained 1D nanowires, 2D nanosheets and even more complex structures through the self-assembly of quantum dots stabilized by organic molecules; Gang²³ and Mirkin²⁴ have successfully fabricated 3D micrometre-sized crystal structures by the controllable self-assembly of DNA–gold nanoparticles through programmable base-pairing interactions between DNA molecules; and Kumacheva et al.²⁵ have realized solution self-assembly of hydrophilic nanorods tethered with hydrophobic polymer chains at both ends into delicate multidimensional structures.

Besides nanoparticles, nanotubes are a very important 1D nanostructure due to their great potential for application in molecular electronics and chemical/biological sensors.^26,27 Hitherto, there have been many publications on the preparation of highly ordered multidimensional structures based on nanotubes through engineering methods such as CVD or patterning.^28–30 However, reports on the template-free self-assembly of nanotubes, especially solution self-assembly, are greatly limited. Poulin and coworkers³¹ have assembled single-walled carbon nanotubes (SWNTs) into micrometre-sized long ribbons and fibres with excellent flexibility through a flow-induced alignment method. Stoddart³² and Kim³³ have independently reported the side-wall functionalization of SWNTs with helical amylose, and Kim³³ also reported that amylose-wrapped SWNTs can further aggregate into twisted ribbons with a diameter of approximately 30 nm. Mao and coworkers³⁴ have used DNA hybridization to control SWNT aggregation and SWNT–Au nanoparticles heteroaggregation. Dieckmann and coworkers^35,36 have helically coated SWNTs with synthesized peptides, and demonstrated that the peptide-wrapped SWNTs can hierarchically self-assemble into highly orientated fibrous arrays from 100 nm to micrometres in diameter and then to large-scale fibrous films and dendritic structures. In addition, some 2D porous or close-packed nanotube films have also been prepared through interfacial self-assembly.^37–40 It can be summarized that the reported nanotubes self-assemblies are generally limited to 1D fibres or 2D films. The 1D fibres are highly aligned, however, the film structures are less ordered. In addition, although more highly ordered structures with large dimensions, such as 3D nanotube crystals are greatly desired, the solution self-assembly of them is still a big challenge.

The formation of many supramolecular entities in nature, especially those in biological systems, always involves a multi-step, hierarchical self-assembly process.¹⁹ For example, the collagen I molecule is approximately 300 nm in length and 1.5 nm in diameter, and it self-assembles into a triple helix with a periodicity of 67 nm, which in turn associates into higher order structures of fibres and fibre bundles. Inspired from nature, the hierarchical self-assembly has already been accepted as a fascinating and sophisticated way to prepare highly ordered structures with a controllable complexity from the nanobuilding blocks.^41–44

As a mimic to the formation of collagen, herein, we have demonstrated a multilevel hierarchical self-assembly of nanotubes into multidimensional supramolecular structures (Scheme 1). Rigid TNTs, about 10 nm in diameter and several hundred nanometres in length, are chosen as the building blocks. The TNTs were first helically wrapped with amylose on the surface through a primary self-assembly process to form functional TNTs (A/TNTs). Then, by increasing the concentration, the A/TNTs were further aggregated into fibres with the nanotubes aligned along the fibre axis through the secondary self-assembly process. In the tertiary self-assembly process, the fibres were further aggregated into large highly aligned bundles, which nucleated the crystallization of the coated amyloses on the surface to form 2D crystal facets. Finally, as a more high-level self-assembly process, micro-sized shuttle-like hexagonal single crystals with a core–shell structure, in which the highly aligned TNT bundles formed the cores and amylose single crystals formed the shells, were obtained. Although the TNT assemblies are rigid, while collagen is flexible, they are very similar in their manner of hierarchical self-assembly. The amyloses provide hierarchical interactions to drive and direct the whole self-assembly process. To our knowledge, the work represents the first report on template-free solution self-assembly of nanotubes into a 3D highly ordered structure. In addition, we believe such a hybrid self-assembly technology will find applications in constructing functional hybrid nanocomposites with high complexity and order.

Scheme 1 Hierarchical self-assembly process for the formation of 3D nanotube crystals: (a) complex self-assembly to form A/TNTs; (b) lateral (side-by-side) and longitudinal (end-to-end) self-assembly of A/TNTs into fibers through interdigitated (I) or juxtaposed (II) models; (c) self-assembly of A/TNT fibers into bundles; (d) formation of 2D crystal facets on the surface of the bundles, the dotted arrows indicate the possible steps; (e) formation of 3D hexagonal single crystal with or without a pyramidal top.

2 Experimental

2.1 Materials

Titanate nanotubes were prepared according to the previous report.⁴⁴ High amylose corn starch (75%) was purchased from National Starch and Chemical Company. Dimethyl sulfoxide (DMSO) and ethanol were obtained from Shanghai Chemical Reagent Co., and they were previously distilled and kept in the presence of a 4 Å molecular sieve to eliminate any traces of water before use.

2.2 Purification of amylose

3 g amylose starch was dissolved in 100 mL water and heated at 100 °C for 1 h to form a slurry. The slurry was then centrifuged for 15 min to remove the insoluble components, and ethanol was added to the supernatant to precipitate crude amylose. The crude amylose was washed by ethanol several times before finally drying the amylose in a vacuum at 50 °C.

2.3 Preparation of the amylose/titanate nanotubes complex (A/TNTs)

In a typical experiment, 0.2 mg TNTs were sonicated in 5 mL water for 20 min, and the resulting fine suspension of TNTs was mixed with a solution of 20 mg amylose dissolved in DMSO and subsequently sonicated for a few minutes. The weight ratio between TNTs and amylose (R_T/A) is 1%, and the volume percentage of DMSO in the mixed solvent of DMSO/water is kept at 15%. All the experiments were conducted at the room temperature. For purification, the solution of A/TNTs was filtered to eliminate the suspended particles, and then centrifuged for longer than 30 min to separate the A/TNTs. After the supernatant was removed, which was rich in the excess free amylose, the precipitated was washed with sufficient water and centrifuged again. This washing/centrifuged cycle was repeated several times to obtain amylose-coated nanotubes.

2.4 Self-assembly of A/TNT fibres

The same procedure as mentioned above was used to prepare A/TNTs aqueous solution, except more TNTs was added (R_T/A is changed from 1% to 2%). Then the solution was kept for at least 5 days at the room temperature until the formation of sediments. The sediments were collected for the measurements.

2.5 Solvent-evaporation induced self-assembly of A/TNT fibres

The A/TNT aqueous solution was transferred into a Petri dish, and then it was covered with a glass dish. The cover was loosely placed, which ensures evaporation of the solvent inside the culture dish. Then the sample was kept under room temperature and in the air atmosphere, allowing the slow evaporation of water. Generally, ∼5 or more days were needed for drying the samples depending on the solution volume. For SEM measurements, some slides were put into the solution during the evaporation process, and the crystals were deposited on the slide. For TEM measurements, the sample was stopped before becoming totally dry and then some drops of the highly concentrated solution were taken out for the measurement.

2.6 Measurements

TEM images were taken using JEOL 2100F at 200 kV. FE-SEM was carried out on a Sirion 200 under an accelerating voltage of 18.0 kV. AFM was performed on a Vecco AFM NanoScope III in tapping mode. Circular dichroism was measured on a JASCO J-600 spectropolarimeter. ¹H-NMR measurements are performed on a Varian Mercury plus-400 spectrometer, and TMS is used as the internal reference.

3 Results and discussion

Since CNTs have a very long length dimension (on the order of micrometres) and are often flexible, it is very difficult to control intertubular interactions at such a scale, which may inhibit the further self-assembly of CNTs into highly ordered structures. So, in the present work, we selected short and rigid TNTs, about 10 nm in diameter and several hundred nanometres in length, as the building blocks.^44,45 The nanotubes have a monodisperse diameter but a polydisperse length, according to the statistical results (Fig. S1, ESI†). In addition, since polymer crystallization is a very strong driving force to induce the highly ordered packing of molecules, we hoped to introduce some crystallizable polymers into the nanotubes to achieve strong and directed intertubular interactions. Previously, Stoddart³² and Kim³³ have reported the side-wall functionalization of SWCNTs by amyloses. Herein we found TNTs could also be spontaneously wrapped with amyloses by dropwise addition of a TNT aqueous solution into the amylose/dimethyl sulfoxide (DMSO) solution under sonication, even though TNTs are ten times larger than the SWCNTs in diameter. The morphology of the resulting samples was carefully characterized by high resolution (Hr-) SEM, TEM and AFM measurements. The pristine TNTs have a smooth surface and can be dispersed randomly in water according to the Hr-TEM (Fig. 1a) and Hr-SEM (Fig. 1d) images. After side-wall functionalization, the so-called A/TNTs show a rough surface with the amylose chains wrapped in a helical manner (Fig. 1b), and the content of the coated amyloses increases with increasing amylose concentration (Fig. 1c). Hr-SEM (Fig. 1e) and AFM (Fig. 1f) images of the single A/TNT show continuous bead-like protuberances helically distributed along the tube, which provides further evidence to support the formation of a helical amylose superstructure along the nanotubes. Such a helical screw-pitch morphology can also be founded in some inorganic nanomaterials.⁴⁶ The circular dichroism (CD) measurement (Fig. 2) exhibits Cotton effects with crossover points at 208 nm and 212 nm, which supports the idea that the amylose macromolecules form double helices or packed double helices on the TNT surface.⁴⁷ As a control, there is no CD signal for the amylose solution without TNTs. Although, the mechanism for the helical twisting of amylose along the TNTs is still unknown, it can be concluded that the TNTs play an important role in inducing the helical arrangements of amylose, maybe through a process like the helical complex between iodine and amylose.⁴⁸ According to the temperature-variable ¹H NMR measurements (Fig. S2, ESI†), the driving force for the conjugation of TNTs and amyloses is the formation of hydrogen bonds among the amylose molecules and TNTs (Ti–OH or Ti–O groups).

Fig. 1 Morphologies of pristine TNTs and A/TNTs. Hr-TEM images of the pristine TNTs (a) and A/TNTs (b and c); Hr-SEM images of the pristine TNTs (d) and A/TNTs (e); (f) AFM image of A/TNTs.

Fig. 2 Circular dichroism of amylose-wrapped TNTs (A/TNTs) in water.

We found that the increase of the A/TNT content in the solution induced the further self-assembly of A/TNTs into 1D fibres. When more TNTs were added into the amylose solution, for example by increasing the weight ratio between TNTs and amylose (R_T/A) from 1% to 2%, the solution became more turbid (Fig. S3, ESI†). After 5 days a sediment was observed, indicating the aggregation of A/TNTs. The sediments were taken out for Hr-SEM characterization. The result shows that A/TNTs have self-assembled into fibres with a diameter smaller than 500 nm and lengths of several micrometres (Fig. 3a). The magnified SEM images (Fig. 3b and 3c) of a single fibre clearly show the aligned arrangement of A/TNTs along the fibre axis, and each nanotube has a helical arrangement of amylose beads along the length of nanotubes with a screw-pitch estimated to be 14.5 nm. In addition, it seems that A/TNTs are laterally interconnected together in a side-by-side mode via the specific association among the helical amylose beads (Fig. 3c) on the tubes, in the manner of matching the pitch among the helices. Such a lateral association mode can guarantee the parallel packing of nanotubes into the highly aligned structure, which can explain the evolution in the diameter dimension from small TNTs (10 nm) to large fibres (500 nm). In addition, since the fibres (micrometres in length) are much longer than the individual TNTs (300–800 nm in length), there must be a longitudinal self-assembly process for the formation of fibres. Hr-TEM images of the fibres (Fig. 3d and 3e) clearly show the end-to-end longitudinal packing of A/TNTs. As shown, most of the A/TNTs are arranged in an interdigitated manner, while some of them are arranged in a juxtaposed manner (green circles). Such a longitudinal self-assembly process can explain the evolution in length dimension of the fibres.

Fig. 3 Morphologies of A/TNT fibers. Hr-SEM image of A/TNT fibers (a) and the magnified images of one single fiber at the middle (b) and the end (c); Hr-TEM images of the single fibers (d and e). The green circles show the junctions indicating the end-to-end self-assembly of nanotubes.

Now we hope to explore the driving force for the self-assembly of A/TNT fibres. Free amyloses are flexible and water-soluble; however, they become much more rigid and hydrophobic when helically wrapped onto TNTs due to the limited freedom of the helical conformation. Thus, we suppose A/TNTs are spontaneously laterally associated together to form fibres via the enhanced intertubular hydrophobic interactions. In addition, intertubular hydrogen bondings among the amylose chains of each A/TNT may be another important driving force for the stability of the fibre structure. Besides, according to Fig. 3, the match of the pitch among the helical A/TNTs is very important to direct the parallel packing of the tubes into the aligned fibres.

As mentioned in the Introduction section, the aligned nanotube fibre is not a new structure, and it has already been prepared through the template-free self-assembly of carbon nanotubes. So we hope to acquire more complicated supramolecular structures by driving the further self-assembly of A/TNT fibres. For this purpose, a solvent-evaporated method, that has been widely adopted in the self-assembly of 3D colloid crystals,^49,50 was used to induce a further ordering process of A/TNT fibres. Hr-SEM images of the obtained powder-like precipitates show the formation of the 3D hexahedral shuttle-like (Fig. 4a) or hexagonal (Fig. 4b) structures. The Hr-TEM image (Fig. 4c) shows that the shuttle has a core–shell structure: the highly orientated TNT bundle forms the dark core, while the amylase forms the grey shell (white arrows). The sharp diffraction spots in the selected-area electron diffraction (SAED) pattern (Fig. 4d) of the shuttle indicate that it is a single crystal with hexagonal unit cells. The spacing between crystal planes in the unit cell is 0.14 nm according to the SAED pattern, which is far below the diameter of TNTs. In addition, the SAED pattern of the pristine TNTs (Fig. S4, ESI†) indicates they are polycrystalline in nature. Thus, it can be deduced that the single crystal structure is generated from the crystallization of amylose. In other words, the as-prepared shuttles are a kind of hybrid nanotube crystals, in which the aligned TNT bundles construct the cores while the hexagonal amylose single crystals form the shells. By further increasing the growth time through a very slow evaporation process, large shuttle-like (Fig. 4e) or hexagonal (Fig. S5, ESI†) hybrid crystals in the macroscopic scale were obtained, and the aligned TNTs inside the crystal can be discerned from the magnified Hr-SEM image of the crystal end (Fig. S5b, ESI†). These nanotubes crystals are very stable and can keep the morphology and structure under sonication for several hours. Nevertheless, it should be mentioned that the yield of the nanotube crystals is as low as about 5%. Such a low yield is attributed to the wide size distribution of the pristine TNTs as well as the low controllability in the solvent evaporation process. We also noted the self-assembly behavior is temperature-dependent, and we could not obtain big TNT bundles at a higher temperature due to a rapid solvent evaporation rate. Although the yield is not satisfactory, to our knowledge, it is the first report on the 3D nanotube crystals generated through a solution self-assembly process.

Fig. 4 3D nanotube crystals. Hr-SEM image of shuttle-like (a) and hexagonal (b) crystals; (c) Hr-TEM image of the shuttle crystal showing the core–shell structure; (d) Corresponding SAED pattern of the crystal in image (c). B = [0001]; (e) Micro-sized shuttle crystals.

The next question is the self-assembly mechanism. To address this issue, the self-assembly intermediates during the solvent-evaporating process were collected for analysis. Fig. 5 shows some pictures selected from a large number of micrographs and then arranged in what appears to be a reasonable sequence. The nanotube fibres first underwent a secondary aggregation process (Fig. 5a). Some fibres were aggregated randomly (Fig. 5a1), while some of them were packed together along the length direction to form aligned bundles (Fig. 5a2), and large bundles with oriented arrangement of A/TNT fibres were also observed (Fig. 5a3). Subsequently, the highly aligned nanotube bundles further induced the formation of quasi-rhomboid (Fig. 5b) or rectangular (Fig. 5c) crystal facets on the surface, and thus the self-assembly process was extended from 1D to 2D. Fig. 5b1 displays an A/TNT bundle with a quasi-rhomboid surface, which is smooth in the central zone and rough at the peripheral as indicated in the magnified image (Fig. 5b2). With further growth, the rough edges disappeared, and the totally smooth rhomboid crystal facets with (as indicated by the biforked structures, Fig. 5b3) or without defects (Fig. 5b4) were obtained. The result indicates the crystal facets are formed on the surface of nanotube bundles and grow from center to edge. Fig. 5c shows the rod-like A/TNT bundles (Fig. 5c1) with rectangular crystal facets. Energy-dispersive X-ray analysis (EDX) was also measured to determine the chemical composition of the rod samples. Results from the EDX spectra of individual rods show that they contain only titanium, carbon, and oxygen elements (inset of Fig. 5c1), which supports the idea that the rods are made of A/TNTs. The magnified images show that these rods have a tetrahedral (Fig. 5c2) or hexahedral (Fig. 5c3) structure with one or more perfect rectangular crystal facets, and the aligned A/TNTs can still be discerned in the crystal facets in growth (the lateral facets in Fig. 5c2 and 5c3). The rectangular crystal facets on the rod were further characterized by Hr-TEM (Fig. S6, ESI†), and the corresponding SAED pattern is very similar to that of the shuttle-like single crystals (Fig. 4d), indicating the formation of perfect amylose single crystals with hexagonal unit cells in the crystal facets.

Fig. 5 Intermediates in the hierarchical self-assembly: (a) Hr-SEM images of A/TNT bundles self-assembled from small fibers; (b) Hr-SEM images of A/TNT bundles with rhomboid crystal facets; (c) Hr-SEM images of the rod-like crystal intermediates, inset of Fig. 5c1 shows the EDX measurement of the rods; (d) Hr-TEM image (d1) and the corresponding SAED patter (d2) of the A/TNT bundles. The image (d3) shows the large rod-like crystals with biforked ends.

From Fig. 5 we can draw two conclusions. One is that during the solvent-evaporation self-assembly process, the smaller A/TNT fibres will further aggregate into bigger bundles, and the other is that amyloses will further crystallize into 2D crystal facets on the surface of the aligned bundles. We think the former process is a entropy-driven self-assembly process like those in the dense packing of peptide or rosette nanotubes during solvent evaporation.^51,52 In addition, the hydrophobic or H-bonding interactions among the amylose chains of A/TNTs fibres may also contribute to the process. For the latter, the central question is the role of the highly aligned A/TNT bundles in the growth of the 2D single crystal facets. SAED pattern (Fig. 5d2) of the highly orientated A/TNT bundles (Fig. 5d1) indicates the formation of crystal lamellae orientated along the bundle length direction and with a lamella spacing of 0.13–0.15 nm. Such a lamella space is very similar to the crystal plane space (0.14 nm) in the hexagonal unit cell of the 2D crystal facets (Fig. S6, ESI†) and 3D shuttle-like crystals (Fig. 3d). These results indicate that the aligned TNT bundles act as nuclei to induce the orientated arrangement of amylose molecules to form the crystal lamellae, and then perfect single crystals are formed by further integrating the amylose chains into the crystal lamellae through molecular reorganization.⁵³ Thus, it can be concluded that the highly aligned A/TNT bundles act as both orientation templates and nucleating agents for amylose crystallization to create hexagonal single crystals on the surface. Winey,⁵⁴ Kumar⁵⁵ and Bucknall⁵⁶ have also found that the aligned SWNT bundles can nucleate the polymer crystallization.

However, we are still curious as to the growth mechanism of the large macroscopic nanotube crystals shown in Fig. 4e. Naturally, such macroscopic crystals can be generated by the template-crystallization of amyloses on the surface of macroscopic A/TNT bundles. Is there an alternative way? Fig. 5d3 shows some large rod-like intermediates, and each of them is made from small rods according the magnified image of the end. This result indicates that the macrosopic nanotube crystals may be generated through the further aggregation and fusion of small crystals.

Based on the abovementioned results, we can summarize the hierarchical self-assembly mechanism for the formation of 3D nanotube crystals as shown in Scheme 1. The pristine TNTs were spontaneously coated with amylose in a helical manner (step a) to form the hybrid nanotubes of A/TNTs. Then, A/TNTs were aggregated together through lateral and longitudinal self-assembly to construct long and highly orientated nanotube fibres (step b). Subsequently, with the slow evaporation of solvent, the A/TNT fibres were further driven to aggregate together into bundles (step c). In the meantime, the highly aligned A/TNT bundles acted as nuclei to induce the crystallization of amylose on the surface (step d). For the crystallization process, amylose molecules first formed the orientated crystal lamellae on the nanotube bundles, and then the arrangements of amylose molecules became more and more regular through reorganization and refolding along the lamellae; finally perfect 2D crystal facets with hexagonal unit cells were formed. The 2D crystal facets have a rectangular or rhomboid structure depending on the morphology of the A/TNT bundles. The hexahedral bundles transform into perfect 3D hexagonal crystals when six coadjacent rectangular crystal facets are formed one by one (step e). Thus, the finally obtained 3D crystals possess a core–shell structure, with the aligned nanotube bundle consisting of the cores and the amylose single crystal forming the shells, and they are hybrid nanotube/polymer single crystals. In our work, some of the hexagonal crystals have a pyramidal top that is probably generated through the growth of rhomboid crystal facets.

4 Conclusions

In conclusion, we have shown a biomimetic hierarchical solution self-assembly process from the building block of nanotubes into highly ordered multidimensional and multiscale supramolecular structures with a sequence of pristine nanotubes, helical nanotubes, fibres, bundles and crystals, and this represents the first report on the preparation of the 3D nanotube hybrid crystals. During the preparation, several supramolecular techniques including complex self-assembly (formation of A/TNTs), hybrid self-assembly (formation of fibres through end-to-end and side-to-side self-assembly of A/TNTs), evaporation-induced self-assembly and template-nucleated polymer crystallization (formation of 2D crystal facets and 3D crystals) have been combined together. The key points in the work lie in that the highly orientated 1D bundles induces the polymer crystallization on the surface, while the polymer crystallization pushes forward the self-assembly from 1D to 2D and 3D. Although, the final yield is still not satisfactory, we believe that the findings presented here are a demonstration of a practical nanotechnology to realize multi-dimensional and multi-scaled solution self-assembly. Currently we are engaging in extending the self-assembly strategy to realize the hierarchical self-assembly of other nanobuilding-blocks.

Acknowledgements

The authors thank the National Basic Research Program (2009CB930400), the National Natural Science Foundation of China (21074069, 20874060 and 50873058), the Foundation for the Author of National Excellent Doctoral Dissertation of China, the Open Project of State Key Laboratory of Supramolecular Structure and Materials (SKLSSM201118) and the Fok Ying Tung Education Foundation (No. 114029) for financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Characterization of the A/TNTs and TNT crystals. See DOI: 10.1039/c1nr11151e

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