Unraveling the Self-Assembly of Hetero-Cluster Janus Dumbbells into

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Unraveling the Self-Assembly of Hetero-Cluster Janus Dumbbells into Hybrid Cubosomes with Internal Double Diamond Structure Hong-Kai Liu, Li-Jun Ren, Han Wu, Yong-Li Ma, Sven Richter, Michael Godehardt, Christian Kübel, and Wei Wang J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b08016 • Publication Date (Web): 02 Dec 2018 Downloaded from http://pubs.acs.org on December 2, 2018

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Journal of the American Chemical Society

Unraveling the Self-Assembly of Hetero-Cluster Janus Dumbbells into Hybrid Cubosomes with Internal Double Diamond Structure Hong-Kai Liu,†, ‡ Li-Jun Ren,†, ‡ Han Wu,†, ‡ Yong-Li Ma,†, ‡ Sven Richter,# Michael Godehardt,*,# Christian Kübel,*,§ and Wei Wang*,†,‡ Center for Synthetic Soft Materials, Key Laboratory of Functional Polymer Materials of Ministry of Education and Institute of Polymer Chemistry, College of Chemistry, Nankai University, Tianjin 300071, China †

Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin, 300071, China ‡

Karlsruhe Nano Micro Facility and Institute of Nanotechnology, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, D-76344, Eggenstein-Leopoldshafen, Germany §

Fraunhofer-Institut für Techno- und Wirtschaftsmathematik, Fraunhofer-Platz 1, D67663 Kaiserslautern, Germany #

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ABSTRACT: Cubosomes are bicontinuous cubic-phase particles generated by amphiphile self-assembly with bicontinuous cubic phases, which creates an intricate network of interconnected nanochannels endows these materials with special functions for advanced applications. On the other hand, clusters are an attractive class of molecules that exhibit intriguing functions and properties that differ from those of atoms and nanoparticles. Inspired by lipid self-assembly and attracted to the new functionalities of clusters, we prepared special hetero-cluster Janus dumbbells (HCJDs) composed of dissimilar nanoclusters namely, a polyoxometalate and polyhedral oligomeric silsesquioxane. HCJDs resemble conventional amphiphiles and as such, they self-assemble in solution into faceted hybrid cubosomes via the transformation of vesicles into sponge-like aggregates. Multiple mechanisms that lead to equilibrium, including molecular self-assembly, vesicle accumulation, membrane fusion, inner-structure reorganization, and cubic crystal growth, contributed to the overall process. On the basis of these results, we proposed a strategy for self-assembly — from basic molecular design that goes beyond traditional amphiphiles to the construction of micro-or nanomaterials with hierarchical structures and advanced functions.

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INTRODUCTION Molecular self-assembly, a universal process that occurs in nature, is at the heart of the bottom-up approach for the construction of nanostructures and nano-objects for advanced applications.13 Amphiphiles, such as lipids4 and block copolymers,5 are the best examples of molecules that successfully undergo self-assembly. They spontaneously self-assemble in solution into basic spherical or cylindrical micelles, or tubular bilayers. These aggregates formed by amphiphiles further organize into complex superstructures, of which bicontinuous cubic liquid-crystalline phases are the most interesting owing to the rich variety of their 3D periodic nanochannel structures.4,

615

To date, bicontinuous cubic structures with 𝑃𝑛3𝑚 (Q224), 𝐼𝑚3𝑚

(Q229), and 𝐼𝑎3𝑑 (Q230) space group symmetries have been discovered and intensively studied. The unique network in these structures, which is constructed by two independent nanochannels separated by a bilayer, is believed to be critical in biological membranes16,17 and biophotonic materials,1821 and has been incorporated into synthetic materials, including photonic crystals22 and ion transport materials,23 for special applications. Bicontinuous cubic-phase particles, referred to as cubosomes, have also been constructed through molecular self-assembly in solution.9,2426 The size of a cubosome, ranging from several hundreds of nanometers to micrometers, facilitates the complete characterization of the internal nanostructure.2648 The development of cubosomes is driven by their potential practical applications as, for example, nanocarriers for the controlled release of drugs and flavors4955 and templates for the preparation of porous particles for the controlled drug release and catalyst supports.5662 These applications are enabled by the interpenetrating network of nanochannels that provide confined spaces for compound storage, as well as complex diffusion pathways for compound release. Therefore, cubosomes provide new options for controlled release that differ significantly from those provided by vesicles. Understanding the formation and internal nanostructures of cubosomes has garnered continual interest since their discovery.

Current

challenges

include

constructing

cubosomes

using

new

unconventional lipids and block copolymers, and understanding their formation. Clusters are molecules with well-defined structures.6368 They can be more appropriately referred to as nanoclusters because their size is typically between those of atoms and nanoparticles. The diversity in their structure and composition results in intriguing properties and functions that differ significantly from those of atoms and 3



larger nanocrystals (or bulk solids). Therefore, clusters are ideal candidates for creation of novel materials with advanced functions. Although cluster crystallization is clearly the simplest and most efficient method.69 Assembling hetero-clusters into cluster materials with ordered structures that are different from those formed in crystals remains challenging. Inspired by lipid self-assembly and attracted to nanocluster functionalities, we pioneered the design and subsequent creation of hetero-clusters composed of a pair of dissimilar clusters connected by various organic linkers.7075 These molecules are dumbbell shaped and amphiphiles like lipids owing to the different solubilities of the two linked clusters. Hence, these molecules are hereafter referred to as “hetero-cluster Janus dumbbells” (HCJDs). HCJDs form hard cubosomes with bicontinuous cubic phases in solution via self-assembly. Owing to strong intercluster interactions, the stable and solid nanostructures within cubosomes and intermediate aggregates or particles can be directly characterized using conventional approaches such as transmission electron microscopy (TEM) and high-angle annular dark-field scanning TEM (HAADF-STEM). By monitoring the intermediates, a roadmap leading from molecules to cubosomes was formulated in order to unravel the processes and the corresponding mechanisms involved in cubosome formation. Synthetic Route O Si OSi Si O SiO O O O O OSi O Si O Si O Si

O Cl C

NH2

O Si OSi Si O SiO O O O O Si OSi Si O OSiO

O C Cl

OL

Organic Linkers (OL)

TEA, CH2Cl2, rt

O N H

OL

COOH O

N N

HO

POSS-OL-COOH

POSS-NH2 O Si OSi Si O SiO O O O O OSi O Si O Si O Si

Tris, EEDQ CH3CN, reflux

O N H

OL

H N O

OH OH OH (Bu4N)6H3P2W15V3O62

HO

O

OL

H N O

O

O

OH

DMF, 80 oC O

N H

O

NN

HO

POSS-OL-Tris

O Si OSi Si O SiO O O O O Si OSi Si O OSiO

OH

O O OW O W O O OO O O O OO O O WW OO WW OO WO O VOO O O OOV O O O PO O OOPO O W O O O O O VOO WO O W W O OW O W O O O O O OWO O O OO O W O O N

POSS-OL-POM

OH O

OH

O

O

HO

O HO

H N O

O N H

OH O

6

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Figure 1. Synthetic routes to prepare hybrid molecules from polyoxometalate (POM) and polyhedral oligomeric silsesquioxane (POSS) nanoclusters using various organic linkers.

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RESULTS AND DISCUSSION Synthesis of Hybrid Molecules of Dissimilar Nanoclusters. For this study, we purposefully selected a polyoxometalate (POM) and polyhedral oligomeric silsesquioxane (POSS) (Figure 1) as the two dissimilar nanoclusters. The POM, (Bu4N)6H3(P2W15V3O62) (MW = 5422.2 Da), has an anionic core characterized by a truncated ellipsoid with long and short axes of 1.2 and 1.0 nm, respectively.76 Electrostatic interactions in the complex are mediated by Bu4N+ counterions. The complex dissolves well in polar solvents (i.e., solvophilic) such as acetone (ACT = 21.01), but not in nonpolar solvents (i.e., solvophobic) such as n-decane (DEC = 1.99). On the other hand, 1-aminopropyl-3,5,7,9,11,13,15-heptaisobutyl (T8) POSS (MW = 873.6 Da) cluster possesses a Si8O12 core that has a siloxane group (OSiO) along each edge and is approximately cubic with a diagonal length of 0.5 nm. The cluster is functionalized with octaisobutyl groups and occupies a spherical space with a freely extended diameter of approximately 1.3 nm.76 Isobutyl functionalization results in weak van der Waals interactions between the POSS clusters; hence, they preferentially dissolve in weakly polar or nonpolar solvents. A series of POM-OL-POSS hybrid molecules was generated by covalently linking POM to POSS using various organic linkers (OLs). In this study, the specific reactants

were

T8-POSS,

OLs

with

two

terminal

carboxyl

groups,

tris(hydroxymethy1)aminomethane (Tris), and vanadium-capped POM. The general four-step synthetic route to the hybrid molecules is summarized in Figure 1. Details of the synthesis are provided in Supporting Information and a previous report.70 Briefly, the aminopropyl group of T8-POSS was covalently linked to one of the carboxyl groups of OL to afford POSS-OL-COOH, which then reacted with Tris to produce POSS-OL-Tris. Subsequent reaction with the three vanadium atoms of POM generated the target POM-OL-POSS molecule.70,77 The POM-OL-POSS molecules have a well-defined structure, with a length similar to those of lipids, and atomically precise molecular weight close to those of block copolymers. They also possess a hybrid composition arising from the metallic and nonmetallic elements in the individual POM and POSS clusters. This facilitates direct examination and high-contrast imaging of unstained samples using TEM or HAADF-STEM. The POM-OL-POSS molecules are dumbbell shaped, which is defined by the two spherical or near-spherical POM and POSS clusters linked 5



together by OL “stick”. The clusters play key roles in the formation of ordered structures. The hybrid molecules exhibit conjugated rigidity and flexibility due to the clusters and OL, respectively. Moreover, the unbalanced intercluster interactions and solvophilic/solvophobic effects endow the hetero-clusters with the amphiphilic character. Therefore, they form aggregates with ordered structures through self-assembly in solution. It is noteworthy that strong interaction between the POM clusters results in sufficiently stable self-assembled structures that can be characterized in the dry state or under high vacuum. These synergistic characteristics play a constructive role in defining the self-assembly pathways and manipulating the self-assembled structures.

Figure 2. (A) Clear solution of a hybrid molecule in a 3:2 (v/v) mixture of acetone/n-decane. The lack of Tyndall scattering indicates a molecular solution. (B) Weight fraction fw (blue squares) and calculated  (red circles) of the mixed solvent as a function of time, t. Measurements were made as acetone evaporated from the open vial. (C) Solution after acetone evaporation. The strong Tyndall scattering of the solution (indicated by scattered path of light) at t = 600 min indicates aggregation. The red arrows in A and C highlight the incident laser beam. Self-Assembly Process. It is noteworthy that the evaporation rate of acetone is approximately eighty times faster than that of n-decane (Table S1). Hence, we capitalized on this dynamic condition to facilitate self-assembly from solution. A solution of an HCJD with an azo linker (POM-Azo-POSS, Figure 1) in an acetone/n-decane mixture is a representative example. The 0.2 mg/ml solution in a 3/2 (v/v) acetone/n-decane mixture is clear (Figure 2A). The weight fraction of the mixed solvent (fw) is defined as fw = (w(t))⁄(w(0)), where w(0) and w(t) are the total weights of the mixed solvent at t = 0 and t = t, respectively. Once the vial is opened, fw gradually decreased over a period of 780 min, primarily owing to the evaporation of 6


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acetone at 25 °C. The relationship between fw and t can be divided into three regions: (1) At 0  t  180 min, fw decreases rapidly and linearly from 1.0 to 0.73 at a rate of

1.5  103 min1> (2) Within the intermediate period (i.e., 180  t  550 min) fw decreases gradually but nonlinearly from 0.73 to 0. 37 at rates that vary from 1.5  103 to 3.4  105 min1. (3) At 550  t  780 min, fw also decreases linearly from 0.37 to 0.36 at a rate of 3.4  105 min1. This indicates that the solvent effectively becomes pure n-decane at t > 550 min. The calculated  of the mixed solvent also decreases from 13.51 (t = 0) to 1.991 (t = 780 min) (Figure 2B). A small amount of precipitate at the bottom of the vial is observed at t = 780 min. A strong Tyndall effect exhibited when the vial is shaken (Figure 2C), which is attributed to the formation of truncated octahedral particles (Figures S1A and B) with an average diameter of 690 ± 130 nm (Figure S1C). Acetone evaporation leads to a gradual change in solvent quality, from a good solvent at t = 0 to a selective solvent at t = 780 min. Consequently, the hetero-clusters aggregate into particles under this dynamic condition.

Figure 3. Bright-field TEM images of POM-Azo-POSS demonstrating the size-dependent shapes and inner nanostructures of aggregates or particles growing under the dynamic conditions. (A) Vesicle, (B) Hemifused vesicle, and (C) Particles with possible foam-like structure at t = 270 min. (D) Spherical aggregate with sponge-like nanostructure at t = 300 min. (E) Ordered nanostructure in the center of the spherical aggregate (highlighted by the yellow circle) at t = 350 min. (F) Spherical particle with ordered nanostructure at t = 410 min. (G) Faceted particle with an ordered nanostructure at t = 10 h. 7



Formation and Evolution of Aggregates or Particles. To understand the self-assembly process, the formation and evolution of aggregates and/or particles under dynamic condition was monitored by dynamic light scattering (DLS) and bright-field TEM (BF-TEM). Unfortunately, “in-situ monitoring” by DLS could not be performed owing to aggregate deposition at the bottom of the vial; hence, DLS measurement of each sample was taken only after shaking each vial. These experiments reveal the increases in aggregate size with time, but not the quantitative relationship between two parameters because the size distribution is sometimes bimodal (Figure S2). Therefore, the size-dependent shapes and inner nanostructures of dried self-assembled aggregates or particles were examined by BF-TEM in order to follow their structural evolution. The POM-Azo-POSS sample exhibits weak Tyndall scattering at t = 270 min (fw = 0.60), and the corresponding DLS pattern exhibits two peaks (Figure S2A). Representative BF-TEM images acquired for the same sample (Figure S3) reveal vesicles with a diameter of ~25 nm (Figure 3A), hemifused vesicles (Figure 3B), as well as multishell vesicles and multivesicle aggregates formed through vesicle accumulation (Figure 3C). The flat bilayer between two fused vesicles is particularly noteworthy (Figure 3B). The multivesicular aggregates have diameters of ~50 nm and foam- or sponge-like inner nanostructure. The surface of a typical aggregate (diameter of 230 nm) is fully covered by incomplete hemispherical bubbles, and the inner sponge becomes increasingly dense from the surface to the center (Figure 3D). The formation of a hexagonal closed-packed nanostructure (highlighted by the yellow circle in Figure 3E) is formed in the center of the aggregate (diameter of ~250 nm) indicates the commencement of an ordering process. The ordered nanostructure at the core continues to grow until it fully occupies the interior of the aggregate (Figure 3F). From this point, faceting of the aggregate surface begins, transforming the aggregate into a well-faceted submicron particle (Figure 3G). Thus, the ordered nanostructure controls the mode of further growth of the faceted particle. Similar faceted particles are also observed for the remaining hybrid molecules depicted in Figure 1 (Figure S4), and details of these ordered nanostructures are discussed in the following section. Confirmation of the Double Diamond (DD) Nanostructure. The detailed structure of the faceted POM-Azo-POSS particle was studied by X-ray diffraction (XRD) and small-angle X-ray scattering (SAXS). The XRD pattern in Figure 4A 8


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exhibits a peak at a 2 = 6.46° corresponding to a 1.37 nm d-spacing, which reflects the distance between the POM blocks. These blocks are packed in a disordered manner, as evidenced by the absence of higher-order diffraction peaks. The broad amorphous halo in the 1040° region is typical of the disordered structure of amorphous materials (Figure 4A). The absence of the diffraction peak at 2  8° corresponding to the POSS blocks indicates that the blocks cannot pack themselves independently into their own ordered structure.

Figure 4. (A) XRD profile of faceted POM-Azo-POSS particle showing a well-defined peak at 2 = 6.46° and broad peak in the 1040° region. (B) SAXS profile showing characteristic peaks (red arrows), with corresponding scattering 12

vector q listed in the inset. (C) Linear relationship between 𝑞ℎ𝑘𝑙 and (ℎ2 + 𝑘2 + 𝑙2)

(h, k, and l are the Miller indices of the corresponding planes), indicating the formation of DD nanostructures inside the faceted particle. (D) Rendition of a cubic unit cell of the DD nanostructure, with lattice constant a indicated. The SAXS spectrum of the particles in solution reveals five peaks within q = 0– 1.2 nm1, where q is the scattering vector (Figure 4B). The 2: 3: 4: 6: 8 spacing ratios of the peaks index them to the [110], [111], [200], [211], and [220] reflections, respectively.78,79 The reciprocal spacing 𝑞ℎ𝑘𝑙 of the cubic phase is associated with lattice constant a by the equation 𝑞ℎ𝑘𝑙 = 2𝜋 ℎ2 + 𝑘2 + 𝑙2 𝑎, where h, k, and l are the Miller indices and a is 15.4 nm, as determined from slope of the regression line that 9



passes through the origin (Figure 4C). Hence, we conclude that a DD nanostructure has formed within the faceted particle and as such, the particle is considered a D-surface cubosome. Similarly, the other hybrid molecules shown in Figure 1 form cubosomes with a DD nanostructure (Figure S5 and Table S2). The skeletal drawing of the DD structure of POM-Azo-POSS highlights the channels of the two independent (i.e., not interconnected) diamond networks (blue and red in Figure 4D). Each node sits at the center of a cubic cage, with four nodes on one set of tetrahedral vertices, and four nodes of the other type on the remaining set of cubic-cage tetrahedral vertices. A full unit cell consists of eight such cubic cages, with eight nodes of each type.

Figure 5. (A) Three HAADF-STEM images of POM-Azo-POSS viewed along the [111], [110], and [100] zone axes of the double diamond structure obtained by rotating the crystal by 0°, 35°, and 54°, respectively, around an axis close to (110) (indicated in the first images). (B) Corresponding FFT patterns with the characteristic reflections indexed. (C) Close-up images obtained by unit cell averaging of the original images in A. The bright regions are POM rich, while the dark regions are POSS rich. (D) Model of the double diamond structure viewed in the [111], [110], and [100] orientation. (E) Simulated projections corresponding to the models in D. Electron Microscopy. Z-contrast imaging of the POM distribution in each system were obtained by HAADF-STEM because these images are dominated by the tungsten-containing POM clusters.8083 The tilt angles between the [111] and [110], and [111] and [100] projections for a DD structure are 35.3° and 54.7°, respectively. 10


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Therefore, starting from the [111] orientation, three 2D projection images of a faceted POM-Azo-POSS cubosome were recorded by tilting the sample by 35° and 54° around an (110) axis (almost) (Figure 5A). The corresponding fast Fourier transform (FFT) patterns (Figure 5B) reveal the structural characteristics of these projections in reciprocal space, including symmetry and d-spacing. The observed lattice spacings, namely d110 = 10.2 nm, d111 = 8.6 nm and d002 = 7.3 nm, agree with the DD structure determined by SAXS. The slight difference is due to the shrinkage of the structure induced by the electron beam damage during imaging. This quantitative agreement reveals that the structures in solution are well preserved in the dried samples. Images 1-3 in Figure 5C reflect the characteristic features of the DD structure imaged along the [111], [110], and [100] zone axes, which are confirmed by a simple STEM image simulation. Equation 1 describes the DD channel structure,13,84 although it should be noted that this equation only holds for a 50/50 space division.85 sin 2𝑥 ∙ sin 2𝑦 + sin 2𝑦 ∙ sin 2𝑧 + sin 2𝑥 ∙ sin 2𝑧 + cos 2𝑥 ∙ cos 2𝑦 ∙ cos 2𝑧 = 0 (1) Mathematical models 1-3 in Figure 5D were obtained by sequentially rotating the crystal by 0°, 35.3°, and 54.7° around the (110) axis starting from the [111] orientation. The corresponding simulated surface-rendered structures imaged along the [111], [110], and [100] orientations are shown in Figure 5E. For a simple approximation of the STEM images, a relative scattering strength of 10 was assumed for the POM and POSS phases, and the resulting projected intensities along the three directions are shown in Figure 5E. The simulated projections agree well with those determined experimentally. On the basis of these results, we conclude that the DD structure formed in the faceted cubosome, as well as in the ordered nanostructures (Figures 3E–G), corresponds to that imaged in the [111] orientation.

Figure 6. Definition of the geometrical parameters of the truncated cone of the HCJD molecules. Rb, Rt, and H are the bottom and top radii, and height of the truncated cone. 11



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𝑅POSS and 𝑅POM are the POSS and POM radii. Packing Parameters. For lipids and other surfactants, the molecular shape is a key factor in the definition of their self-assembled morphology.86,87 These molecules typically consist of a hydrophilic head and a hydrophobic tail. The molecular packing parameter, p, is defined as: 𝑝=

𝑣 𝑎𝑙

(2)

where v and l are the volume and extended length of the hydrophobic tail, respectively, and a is the hydrophilic headgroup area. As the HCJD molecule possesses an atomically precise molecular weight, fixed length and truncated cone, p can be estimated semi-quantitatively. v, l, and a are defined in Figure 6. Under the dynamic condition, the POSS cluster is solvophilic to n-decane; thus, it is the head of the HCJD molecule and a is equal to its cross-sectional area 𝑎 = 𝑅2t = 𝑅2POSS

(3)

where 𝑅POSS is the POSS radius and Rt is the top radius of the truncated cone. The POM cluster and amide-containing OLs are solvophobic to n-decane; thus, l corresponds to height of the truncated cone (l = H) and v of the truncated cone is 1 𝑣 = 𝜋𝐻(𝑅2b + 𝑅b𝑅𝑡 + 𝑅2t) (4) 3 where Rb is the bottom of the truncated cone, respectively. The relationship between Rb, H, and the POM radius RPOM is 𝑅b = 𝑅POM𝑡𝑎𝑛𝜃 + 𝑅POM 𝑐𝑜𝑠𝜃

(5)

where 𝜃 = 𝑎𝑟𝑐𝑠𝑖𝑛[(𝑅POM ― 𝑅POSS) (𝐻 ― 𝑅POM + 𝑅POSS)]. The p values of the five hybrid molecules can be obtained by substituting Eqs. 3–5 into Eq. 2. 𝑅POM = 0.685 nm that was determined from the XRD data (Figure 4A) is used in the calculation. Unfortunately, signals related to the POSS size were not observed in the same XRD pattern. Using the single crystal data of octa-isobutyl-POSS,76 an 𝑅POSS of ~0.55 nm is obtained, and the p values (1.32 to 1.35, Table S3) are larger than those determined for the DD cubic phases of the three lipids.88 We believe that 𝑅POSS > 0.55 nm in n-decane because the POSS contains more extended isobutyl groups. 12


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Using 𝑅𝑃𝑂𝑆𝑆 values of 0.575, 0.60, and 0.625 yields the average p values of 1.25, 1.19, and 1.13 (Table S3), respectively, which are within the range reported in ref. 88. The decrease in p with increasing 𝑅POSS indicates that selective solvation changes the molecular shape, that is, acetone evaporation further promotes the evolution of self-assembled structures.

Figure 7. Evolution from HCJDs to faceted cubosomes through different mechanisms. Roadmap from Molecules to Cubosomes. The formation and growth of cubosomes from a lipid/water mixture are undoubtedly the most important issue from the perspectives of both basic and applied research. However, studies on this subject are scarce in comparison with structural studies because it is difficult to directly visualize self-assembled aggregates and monitor the self-assembly process from molecules to cubosomes. Most previous studies clearly revealed the coexistence of cubosomes, larger vesicles, and/or sponge-like nanoparticles.2731,3341,43 Vesicles are either isolated or attached to the surface of cubosomes and gradually decrease in size from the exterior of the cubosome to the center until an ordered bicontinuous cubic structure is formed. A similar structural gradient has also been observed in sponge-like nanoparticles.89 On the other hand, small cubosomes containing only four or five vesicles have also been observed at an early stage.38 These seemingly unrelated results highlight he significant dependence of cubosome formation and growth on experimental conditions. However, membrane fusion through key intermediates, such as stalks, transmonolayer contacts (TMCs) and interlamellar attachments (ILAs),90 is unquestionably recommended mechanism for describing the topological transitions from the curved lamellae to the bicontinuous cubic phases of stable cubosomes.37,39 13



The dynamic conditions used in this work (i.e., decreasing acetone content due to evaporation) resulted in a continuous change from the balanced solvation of both polar POM and non-polar POSS to a final system that favors only POSS. Consequently, the sizes, shapes, and inner nanostructures of the self-assembled aggregates change with changing solvent properties and hence with time. The possible growth mechanisms are summarized in Figure 7, and the proposed four-step process is described below. Step 1. Vesicles with a diameter of ~25 nm are formed through molecular self-assembly (Figure 3A). Based on the sizes of the POM and POSS clusters mentioned above, the hybrid molecules exhibit a molecular shape with 1.0 < p < 1.3; hence, they form a dual bilayer in the vesicles. In this dual bilayer a POM double layer is sandwiched between two POSS layers because the former becomes increasingly more solvophobic under dynamic conditions. More importantly, these vesicles are the basic units for the further construction of the final cubosomes from the early aggregates. Step 2. The vesicles do not grow further but instead accumulate and assemble into larger aggregates. Consequently, the dual bilayer can partially surrender to the curvature of the vesicle. A hemifused vesicle, which is the simplest aggregate, is formed by two deformed vesicles separated by a flat dual bilayer (Figure 3B). Further vesicle accumulation creates multivesicular aggregates composed of several hemifused vesicles (Figure 3C). The irregular dual bilayers between neighboring hemifused vesicles results in a foam-like aggregate structure. Step 3. Further reorganization within larger aggregates causes topological transitions, followed by structural ordering within the aggregate core as the size increases (Figures 3C and 3E). The topological transition may be a change from a foam-like to sponge-like structure via the intermediate states involved in membrane fusion (i.e., stalks, TMCs, and ILAs).90 In this case, the interior of the aggregate forms a bicontinuous sponge structure that is essentially a disordered version of bicontinuous cubic phases.89 Structural ordering from the sponge to the DD structure, from the center to the surface, occurs next. This process continues until the DD structure occupies the entire interior of the spherical aggregate, resulting in the creation of the cubosome (Figure 3F). The two outermost unit cells on the [100] facet of the cubosome still reflect this structural transition from vesicular to DD. A projection of the surface structure is shown in Figure S6. The size of a merged 14


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vesicular structures is 2 times of the size of the DD repeat unit. Moreover, the STEM image shows a fast coherent transition from the surface to the core indicating the easy topological transformation from a merged vesicular to DD structure. Step 4. The cubic DD structure is a key feature of this step because it defines the geometric shape of the cubosome and the growth mechanism. Once facets form on the surfaces of the spherical cubosomes (Figure 3F), further growth occurs on the [100] and [111] facets through crystal nucleation and growth. Consequently, a completely faceted cubosome manifests a truncated octahedral rather than a cubic shape (Figures S1B). This corresponds to the equilibrium shape of cubosomes with Pn3m cubic lattices, as indicated by the minimum surface free energies of the cubosomes determined according to the Wulff construction.91 In contrast to atomic or nanoparticle crystal growth, cubosome growth is associated with molecular self-assembly. At this stage, almost all molecules have formed aggregates or cubosomes and further growth is dominated by Ostwald ripening,92 that is, small aggregates and/or cubosomes dissolve and redeposit onto larger cubosomes. At least two growth modes exist: one is the process described above, in which dissolved molecules first self-assemble into vesicles and then accumulate on the cubosome surface, and the other involves the construction of supramolecular units from the dissolved molecules directly on the cubosome surface. We are unable to clearly distinguish between these growth mechanisms; nevertheless, the formation of a merged vesicular structure on the [100] surface suggests growth through vesicular assembly (Figure S6). Bending Free Energy Analysis of Bilayers. The bending free energy FB, as defined by Helfrich,93 provides further insight on the growth process. The FB of an aggregate is given by 𝐹B =

∮𝑑𝐴[(𝑐

1

+ 𝑐2 ― 2𝑐0)2 + (𝑐1𝑐2)]

(6)

where c1 and c2 are principle curvatures; c0 is the spontaneous curvature and  and  are the mean and Gaussian curvature elastic constants, respectively. c1 = 1/r1 and c2 = 1/r2, where r1 and r2 are the principal radii of the curvature, and the mean and Gaussian curvatures are defined by H = c1 + c2 and K = c1c2, respectively. ∮𝑑𝐴(𝑐1 + 𝑐2 ― 2𝑐0)2 expresses the dependence of FB on aggregate size and is 15



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associated with the mean curvature, while ∮𝑑𝐴(𝑐1𝑐2) determines the topology of the aggregate relevant to the Gaussian curvature. According to the Gauss-Bonnet theorem, ∮𝑑𝐴(𝑐1𝑐2) = 4(1 ― 𝑔), where g is the genus. Hereafter, only the FB values of vesicles and diamond cubic structures are compared. For vesicles, g = 0;94 thus ∮𝑑𝐴[(𝑐1𝑐2)] = 4. On the other hand, g = 3 for D-surface cubic structures;95 thus, ∮𝑑𝐴[(𝑐1𝑐2)] = ―8. The difference between the FB of the two types of aggregates contributes to the spontaneous topological conversion of vesicles to cubic structures. The 𝐹VB of a vesicle depends on the 𝑐1 + 𝑐2 ― 2𝑐0, that is, it changes with vesicle size. 𝐹VB has its lowest value at the spontaneous curvature where c1 + c2 = 2c0; hence, 𝐹VB =

∮𝑑𝐴[(𝑐 𝑐 )] = 4 1 2

(7)

For a cubic phase unit, H = 0 and c1 = c2, hence, 𝐹CP B =

∮𝑑𝐴( ― 2𝑐 ) 0

2

― 8

(8)

The system preferentially forms bicontinuous cubic structures if ∮𝑑𝐴( ― 2𝑐0)2 is smaller than 12. If the system selectively forms vesicles under kinetic control, the topology will eventually convert into cubic structure. Equation 8 also indicates that 𝐹CP B is a parameter that depends only on c0, , and , further indicating that identical unit cells are formed. CONCLUSIONS In summary, we created HCJD molecules by connecting a pair of dissimilar nanoclusters, namely a POM and a POSS using various organic linkers. The molecules self-assembled into diverse aggregates or particles in acetone/n-decane under dynamic condition in which the acetone content gradually decreases owing to evaporation. TEM images revealed that multiple processes are involved in self-assembly and growth. Specifically, vesicles transform to hemifused vesicles, to aggregates with foam- and sponge-like structures, and finally to spherical and faceted particles. SAXS and TEM analyses confirmed existence of the DD lattices within the 16


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faceted particles. The results indicated that the construction of cluster-containing cubosomes involves different growth stages that lead to equilibrium. The mechanisms for these growth stages include molecular self-assembly, vesicle accumulation, membrane fusion, inner-structure reorganization, and cubic-crystal growth. This study introduced a strategy for the formation of bicontinuous cubic liquid crystalline phases of cubosomes while unraveling the processes and corresponding mechanisms involved in the self-assembly of amphiphiles. Supporting Information The Supporting Information is available free of charge on the ACS Publications websites. Detailed synthetic procedure and additional data from experimental characterization and simulations (PDF). AUTHOR INFORMATION Corresponding Authors [email protected] [email protected] [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT We dedicate this work to the 100th anniversary of Nankai University. This work was supported by the National Natural Science Foundation of China (Grant nos.21274069, 21334003, and 21674052). TEM characterization was performed at the Karlsruhe Nano Micro Facility (KNMF). SAXS characterization was performed at the 1W2A SAXS station at the Beijing Synchrotron Radiation Facility (BSRF). The authors gratefully acknowledge the assistance of the beamline scientists at BSRF. REFERENCES (1) Whitesides, G. M.; Mathias, J. P.; Seto, C. T. Science 1991, 254, 1312–1319. (2) Philp, D.; Stoddart, J. F. Angew. Chem. Int. Ed. 1996, 35, 1154–1196. (3) Lehn, J.-M. Science 2002, 295, 2400–2403. (4) Seddon, J. M.; Templer, R. H. Polymorphism of Lipid-Water Systems. In Handbook of Biological Physics; Lipowsky, R., and Sackmann, E., Eds.; Elsevier Science, 1995; Vol. 1, Chapter 3, pp 97−160. (5) Mai, Y.; Eisenberg, A. Chem. Soc. Rev. 2012, 41, 5969–5985. 17



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Graphical Abstract

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Unraveling the Self-Assembly of Hetero-Cluster Janus Dumbbells into

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