Tetrahedral, Octahedral, and Triangular Dipyramidal Microgel Clusters

Letter Cite This: ACS Macro Lett. 2018, 7, 80−84

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Tetrahedral, Octahedral, and Triangular Dipyramidal Microgel Clusters with Thermosensitivity Fabricated from Binary Colloidal Crystals Template and Thiol−Ene Reaction Lijuan Yao,† Qian Li,† Ying Guan,*,† X. X. Zhu,‡ and Yongjun Zhang*,† †

Key Laboratory of Functional Polymer Materials, State Key Laboratory of Medicinal Chemical Biology, The Co-Innovation Center of Chemistry and Chemical Engineering of Tianjin, Institute of Polymer Chemistry, College of Chemistry, Nankai University, Tianjin 300071, China ‡ Department of Chemistry, Université de Montréal, C. P. 6128, Succursale Centreville, Montreal, Québec H3C 3J7, Canada S Supporting Information *

ABSTRACT: A template-based strategy to fabricate soft colloidal clusters with distinct symmetries of tetrahedra, octahedra, and triangular dipyramid is described. We use binary microgel colloidal crystals as a template, in which the large microgels with surface thiol groups are arranged into a close-packed lattice and a few small microgels with surface vinyl groups occupy the tetrahedral or octahedral interstitial sites, and then immobilize the structure via in situ thiol−ene reaction under UV irradiation. Both 2D cross sections and reconstructed 3D morphology of these clusters are clearly characterized by confocal laser scanning microscopy. The formation mechanism of microgel clusters is discussed, which is closely related to the microgel soft properties, size ratio, and colloidal crystal packing structure. The resulting clusters inherit the thermosensitivity and defects tolerance of poly(Nisopropylacrylamide) (PNIPAM) microgel, which would facilitate their self-assembly into more complex structures.

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assembly.20−26 Nevertheless, to our knowledge, the synthesis of soft colloidal clusters has barely been studied so far, as microgels are highly deformable and most synthesis strategies initiated for hard sphere colloidal molecules are not suitable for them. The only exception may be one of our recent works in which a kind of thermosensitive cluster containing 13 PNIPAM microgel spheres was successfully synthesized.27 However, preparation of soft colloidal molecules with a wide range of configurations still remains a huge challenge. Herein, we employ another template, that is, binary colloidal crystal template, in which small particles with suitable size could fill in the corresponding octahedral or tetrahedral interstitial voids formed by large particles, to fabricate microgel clusters. For the first time, three new soft colloidal molecules with tetrahedral, octahedral, and triangular dipyramidal configurations, were successfully synthesized from PNIPAM microgels as shown in Scheme 1. They inherit the unique properties of PNIPAM microgels, that is, soft and deformable nature, no gravitational sedimentation in solution, and thermally tunable size. Due to their excellent properties, these new microgel clusters are expected to be attractive colloidal molecules for the study of certain fundamental

olloidal clusters, or colloidal molecules, as termed by van Blaaderen1 in 2003, are clusters of colloidal spheres with shapes resembling the space-filling models of real molecules, which offer several immediate advantages over their spherical counterparts.2,3 They have great potential to act as building blocks to self-assemble into a myriad of complex or lowcoordination architectures, such as diamond, pyrochlore, and other sought-after lattices, which are impossible for spherical colloids, as these open lattices are entropically costly.4,5 In particular, it was suggested that tetrahedral colloidal clusters with directional interactions might promote the formation of a diamond lattice, which possesses unique properties such as full omnidirectional photonic band gaps.6,7 Up to now a large number of colloidal clusters with different configurations and compositions, usually composed of hard spheres, such as polystyrene and silica particles, have been fabricated in a number of ways;8−15 however, reports using them as building blocks to assemble large ordered superstructures, with a few notable exceptions,16−18 remain scarce. One of the most important reasons is that the additional degrees of freedom of anisotropic particles complicate and frustrate the kinetic pathways of self-assembly, and local orientational defects cannot easily heal.16,19 To solve this problem, it occurs to us that soft colloidal molecules, made of poly(N-isopropylacrylamide) (PNIPAM) microgels, may have more potential to assemble into these complex structures, due to their inherited thermally tunable size and defects tolerance, which are key factors for their © XXXX American Chemical Society

Received: November 27, 2017 Accepted: December 21, 2017

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DOI: 10.1021/acsmacrolett.7b00935 ACS Macro Lett. 2018, 7, 80−84

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ACS Macro Letters Scheme 1. Preparation of Microgel Clusters from Binary Colloidal Crystals and Thiol−Ene Reactiona

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(A) Synthesis of PNIPAM microgels with surface thiol and vinyl groups, referring to as SH-microgels and vinyl-microgels. (B) Fabrication of binary colloidal crystals from a binary microgel mixture, containing a majority of SH-microgels that forms closepacked crystals with the minority vinyl-microgels occupying interstitial sites, followed by linking the vinyl-microgel with the surrounding SH-microgels by photo-initiated thiol−ene reaction. Depending on the different interstitial site the small vinyl-microgels occupied, three kinds of clusters with a tetrahedral, octahedral, and triangular dipyramidal configuration are generated.

Figure 1. Characterization of binary colloidal crystals composed of large SH-microgels and small vinyl-microgels. (A, B) Photographs of a thin sample (500 μm) and a thick sample (5 mm) when illuminated with white light from behind, respectively. (C) Diffraction pattern under 532 nm laser excitation. (D, E) Confocal images of hexagonal arrangement of unary (D) and binary (E) colloidal crystals in bright field mode at 20 °C. Red arrows indicate the location of small vinylmicrogel particles in the binary crystal.

questions in condensed-matter physics and be explored to form complex hierarchical assemblies. We start the strategy with the synthesis of monodisperse poly(N-isopropylacrylamide-co-acrylic acid) microgels with a cross-linking density of ∼1.5% by free radical precipitation polymerization.28 Surface thiol and vinyl groups were introduced via EDC coupling of the carboxylic acid groups with cysteamine and allylamine, respectively (Scheme 1A and Figure S1). The resulting microgels with surface thiol and vinyl groups were referred to as SH-microgels and vinyl-microgels, respectively. Their hydrodynamic diameter (Dh) was measured by dynamic light scattering to be ∼1000 and ∼310 nm at 20 °C, respectively. The large SH-microgels were used as the majority “host” particles to self-assemble into close-packed colloidal crystals, with a few small vinyl-microgels added to act as “impurity” particles, which were expected to locate in either tetrahedral or octahedral interstitial sites of the colloidal crystals. The embedded vinyl-microgel particles were then covalently bound with the neighboring SH-microgel particles via photoinitiated thiol−ene click reaction, resulting in microgel clusters with symmetries of tetrahedron, octahedron, and triangular dipyramid, according to the arrangements of colloidal crystals (Scheme 1B). As a template, the highly ordered structure of the colloidal crystals is crucial to the formation of microgel clusters with well-defined structures. Thanks to the soft nature of PNIPAM microgels, microgel binary colloidal crystals were successfully fabricated by simply heating binary microgel dispersions to 37 °C and then allowing them to cool back to room temperature according to our previous study.29 Just like unary colloidal crystal, when illuminated with white light from behind, the binary crystal exhibits iridescent color when assembled in a thin cell (Figure 1A) or a monochromatic diffraction six-arm star with exact 60° angles between each two adjacent arms when assembled in a thick cell (Figure 1B).30,31 The appearance of iridescent color indicates the formation of long-range ordered crystalline structure inside the sample. When the sample was excited with a 532 nm laser beam along

its normal (Figure 1C), a diffraction pattern with six distinct spots set within a diffuse ring was observed, corresponding to diffraction from a 2D colloidal crystals with close-packed hexagonal crystal domain.32,33 Confocal laser scanning microscope (CLSM) was used to observe directly the arrangement of microgel spheres. As shown in Figure 1D,E, the large SHmicrogel spheres all assembled into a highly ordered hexagonal structure both in the absence and in the presence of small vinyl-microgel spheres, suggesting that the introduction of small vinyl-microgel does not influence the crystallization of large SH-microgel. The average center-to-center distance between two adjacent SH-microgel particles is determined to be ∼950 nm, which is slightly smaller than the hydrodynamic diameter (Dh) of the free particles measured in dilute dispersion at the same temperature (∼1000 nm at 20 °C), revealing a slight compression of particles in the crystals. The vinyl-microgel spheres were then covalently linked with the surrounding SH-microgel spheres in situ via a photoinitiated thiol−ene click reaction. This coupling process has distinct advantages over other reactions, including a high reaction yield, tolerance to other functional groups, insensitivity of solvents and oxygen and simple product isolation.34−36 DNA hybridization, which was previously used to link colloidal particles,37 requires the addition of chemicals to turn on the reaction, which may destroy the performed order structure. In contrast the thiol−ene reaction used here can be simply turned on by UV irradiation, which is more convenient and controllable and does not influence the performed structure. After UV irradiation, the dispersion was released from cell and reduced by tris-carboxyethylphosphine (TCEP) before characterization, making sure the clusters are covalently bonded by thiol−ene reaction, not by the self-cross-linking of the thiol groups. After fluorescently labeled with sulfo-Cy3 maleimide, the sample was observed under fluorescence microscopy, revealing that the resulting solution was composed of unreacted large 81

DOI: 10.1021/acsmacrolett.7b00935 ACS Macro Lett. 2018, 7, 80−84

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ACS Macro Letters

difference between them (Figure S4).43 As illustrated in Scheme 2, two types of interstitial sites exist both in fcc and

SH-microgels, microgel clusters, and a few irregular structures (Figure S2A). After separated by density gradient centrifugation,10,38 two distinct bands appeared. The top and bottom bands correspond to single SH-microgels and microgel clusters, respectively (Figure S2B,C). In a control experiment, unary colloidal crystals composed of only large SH-microgels were UV-irradiated in the same way. No clusters were found in the resulting solution, suggesting the importance of introduction of small vinyl-microgel into the colloidal crystal template. The resulting clusters are linked with covalent bonds and therefore highly stable. To study the morphology of these clusters, they were examined by confocal laser scanning microscopy. As expected, tetrahedral and octahedral microgel clusters formed when the small vinyl-microgel particles occupy the tetrahedral and octahedral interstitial voids of colloidal crystals,39,40 respectively. 2D confocal sections and the corresponding reconstructed 3D images of tetrahedral and octahedral clusters are displayed in Figure 2A−C and Figure 2F−H, respectively, as

Scheme 2. Tetrahedral and Octahedral Interstitial Sites Existing in Both a Hexagonal Close Packed (hcp) Structure and a Face Centered Cubic (fcc) Structure

hcp structures; therefore, tetrahedral and octahedral clusters can form in the overall colloidal crystals. For triangular dipyramid clusters shown in Figure 2K, we speculate that this structure originates from two small vinyl-microgels occupying adjacent tetrahedral interstitial sites in ABABAB (hcp) stacking structures simultaneously (Figure 2N,O), which could not happen in ABCABC (fcc) structures. Based on an analysis of 100 purified clusters, this strategy yields tetrahedron, triangular dipyramid and octahedron at a ratio ∼7:2:1 in a single colloidal crystal template. The yield differences are believed to derive from filling mechanism of microgels. In this experiment, we use SH-microgels (Dh ∼ 1000 nm) and vinyl-microgel (Dh ∼ 310 nm), with a size ratio of γS/L = 0.31. According to the geometry analysis of the closepacked fcc lattice of hard spheres, for small sphere to be accommodated into a octahedral site, the γS/L should be between 0.2583 and 0.4142, and it cannot fit into a tetrahedral site only when γS/L ≤ 0.2247.44,45 However, in our present study, most small vinyl-microgel particles occupy the tetrahedral interstitial sites, while only ∼10% of them fill in the octahedral interstitial sites. We speculate that this inconsistent behavior is resulting from the soft property of PNIPAM microgels, which facilitates them to be compressed to a smaller size to fit relatively small tetrahedral voids. Moreover, a closer contact between the small particles and their surrounding large particles is necessary for the occurrence of in situ thiol−ene reaction. To testify our presumption, we added small vinyl-microgel particles with a diameter of 200 nm into the SH-microgels of 1000 nm (γS/L = 0.20). As a result, no clusters can be found, as these small particles may move freely in the tetrahedral or octahedral voids due to Brownian motion and cannot come into close enough contact for the reaction to proceed. Similar theory has been brought by Fischer et al.,46 who coupled functional groups to a longer polyethylene glycol (PEG) chain to permit the click reaction to proceed. For triangular dipyramid clusters, it is conceivable that their formation occurs at a much lower probability than tetrahedral clusters, as the former only exists in hcp structure and requires two small particles filling in adjacent tetrahedral interstitial sites simultaneously. The obtained clusters are expected to inherit the thermosensitivity of PNIPAM microgels, which would be a

Figure 2. Characterization of tetrahedral, octahedral and triangular dipyramidal microgel clusters. (A, F, K) Cross-section confocal images of the tetrahedral, octahedral, and triangular dipyramidal clusters observed from the (111) direction, respectively. (B, G, L) Reconstructed 3D confocal images of the clusters in volume mode. (C, H, M) Reconstructed 3D confocal images of the clusters in depth mode. (D, I, N) Schematic illustrations of the lattice structure from which the clusters are generated. (E, J, O) The inner structure of the clusters.

observed from the (111) directions. Colors marked in Figure 2C,H represent depths of clusters in Z-axis direction, and 3D configurations can be seen more clearly in Movies S1 and S3. Figure 2D,E,I,J give illustrations of the corresponding lattice and inner structure of the clusters. For the tetrahedral clusters, another stack morphology is also presented, as observed from the (100) direction (Figure S3 and Movie S2). As sulfo-Cy3 maleimides specifically labels thiol-groups, the inner vinylmicrogel cannot be identified by confocal microscope. Besides tetrahedral and octahedral clusters, a third kind of microgel cluster with a symmetry of triangular dipyramid was also presented unexpectedly, as shown in Figure 2K and Movie S4. As we all know, the symmetries of the resulting microgel clusters depend on the stacking structure of the colloidal crystals. In accord with previous studies,41,42 our confocal microscopy study indicates that the microgel crystals template is a mixture of ABCABC (face centered cubic, fcc) and ABABAB (hexagonal close packed, hcp) stacking structures in the present study due to the exceedingly narrow energy 82

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ACS Macro Letters highly desirable feature for their future applications. Significant heat-induced deswelling of the three kinds of microgel clusters is shown in Figure S5. The volume phase transition temperature (VPTT) of these clusters is determined to be ∼32 °C in all cases, just as the single SH-microgels. By direct observation under a confocal microscope, we can also see the clusters size at 35 °C are approximately half of that at 20 °C (Figure 3A). Besides, the thermal behavior of the clusters is

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tetrahedral cluster observed from (100) direction. Movie S3: 3D structure of an octahedral cluster. Movie S4: 3D structure of a triangular dipyramidal cluster (ZIP).

AUTHOR INFORMATION

Corresponding Authors

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

X. X. Zhu: 0000-0003-0828-299X Yongjun Zhang: 0000-0002-1079-4137 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank financial support for this work from the National Natural Science Foundation of China (Grants Nos. 21274068, 21374048, and 51625302) and the Tianjin Committee of Science and Technology (16JCZDJC32900).

Figure 3. Thermosensitivity of microgel clusters. (A) Morphology change under a confocal microscope at 20 and 35 °C. (B) Images of microgel clusters solution under different temperatures, showing reversible aggregation and dispersion around the VPTT.

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fully reversible and visible, as shown in Figure 3B. At a temperature below the VPTT, the clusters disperse in water very well, resulting in a homogeneous solution. When heated to 35 °C, which is higher than the VPTT, the particles aggregate and precipitate from the solution due to the enhanced hydrophobic interactions between the collapsed microgel clusters. When cooled back to 20 °C, the precipitate dissolves again and the dispersion becomes homogeneous again. In summary, we present a facile and robust strategy to fabricate several microgel clusters, combining a microgel binary colloidal crystals template with efficient thiol−ene reaction. Microgel clusters with three distinct symmetries of tetrahedron, octahedron, and triangular dipyramid are generated from PNIPAM microgels, depending on the arrangement of colloidal crystals and filling mechanism. The resulting clusters maintain the unique properties of PNIPAM microgels, that is, soft and deformable nature, no gravitational sedimentation in solution, and thermally tunable size. They are expected to be used as “big molecules” to study the phase-transition process in real space using an optical microscope. More importantly, it is expected that the thermally tunable size and volume fraction will make their self-assembly process more easily compared with hard sphere colloidal molecules. As one of their potential applications, the tetrahedral clusters may act as building blocks to assemble into diamond structure with interesting optical properties. To overcome the problem of the rather low refractive index of pure PNIPAM microgel, core−shell microgels with inorganic cores, as T. Hellweg22 suggested, could be used to form a diamond structure with a full omnidirectional photonic bandgap.

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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/acsmacrolett.7b00935. Experimental details and additional characterizations (PDF). Movie S1: 3D structure of a tetrahedral cluster observed from (111) direction. Movie S2: 3D structure of a 83

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