pubs.acs.org/JACS Communication Assembly of Colloidal Clusters Driven by the Polyhedral Shape of Metal−Organic Framework Particles Yang Liu, Jiemin Wang, Inhar Imaz,* and Daniel Maspoch* Cite This: J. Am. Chem. Soc. 2021, 143, 12943−12947 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: Control of the assembly of colloidal particles into discrete or higher-dimensional architectures is important for the design of myriad materials, including plasmonic sensing systems and photonic crystals. Here, we report a new approach that uses the polyhedral shape of metal−organic-framework (MOF) particles to direct the assembly of colloidal clusters. This approach is based on controlling the attachment of a single spherical polystyrene particle on each face of a polyhedral particle via colloidal fusion synthesis, so that the polyhedral shape defines the final coordination number, which is equal to the number of faces, and geometry of the assembled colloidal cluster. As a proof of concept, we assembled six-coordinated (6-c) octahedral and 8-c cubic clusters using cubic ZIF-8 and octahedral UiO-66 core particles. Moreover, we extended this approach to synthesize a highly coordinated 12-c cuboctahedral cluster from a rhombic dodecahedral ZIF-8 particle. We anticipate that the synthesized colloidal clusters could be further evolved into spherical core−shell MOF@polystyrene particles under conditions that promote a higher fusion degree, thus expanding the methods available for the synthesis of MOF−polymer composites. T he assembly of colloidal molecules or clusters1−5 is a crucially important step in the design of more sophisticated architectures such as patchy particles6−8 and colloidal cluster, or a cubic core particle would template the synthesis of a 6-c octahedral colloidal cluster. In this way, polyhedral particles with a high number of faces could ideally self-assembled colloidal crystals9 (e.g., with diamond or be used to form clusters exhibiting geometries with high pyrochlore structures) and opens novel avenues for the coordination numbers. formation of new photonic crystals,10 micromotors,11 and To implement our colloidal strategy, we selected metal− drug-delivery systems.12 Recently, great progress has been organic framework (MOF) particles as the core polyhedral made in the development of synthetic strategies to form such particles. MOFs are a widely known class of porous crystalline particle clusters.13 These strategies are mainly based on either materials that exhibit very high surface areas and have found a the growth of particles on the surface of a preformed particle, broad variety of applications, including gas storage and via phase-separation phenomena or surface nucleation and separation as well as catalysis and contaminant removal.15,16 growth, or the controlled assembly of presynthesized particles However, in the present work, we targeted MOFs mainly via attractive interactions such as DNA hybridization, electro- because they are an excellent source of crystalline particles static interactions, and/or van der Waals forces.14 These (Figure 1a−c) covering most known polyhedral shapes, thanks synthetic methods typically use spherical particles, such as to the discovery of thousands of MOFs during the last 25 silica, polymers, and inorganic particles. Consequently, they all years.17−19 Moreover, recent advances in MOF synthesis allow must accomplish the difficult task of directing the assembly them to be obtained as colloidal particles with highly and/or growth of particles on the surface of isotropic spherical homogeneous size and shape.20−23 Furthermore, their particle particles via methods such as tuning the size ratio between the size can easily be tuned from ∼50 nm to ∼1 μm.24−27 particles or phase-separation phenomena to control the Taking advantage of these properties, we report herein the coordination number and geometry of the synthesized use of polyhedral MOF particles to direct the assembly of colloidal cluster. Building this type of clusters is therefore spherical polystyrene particles in terms of both number and not an easy task, especially for those involving geometries with position, allowing the synthesis of colloidal clusters through high coordination numbers, in which the assembly or growth mix-and-melt or colloidal fusion synthesis (Figure 1). This of many particles must be controlled. synthesis has previously been reported by Sacanna et al.10,28,29 Herein, we propose a new strategy for controlling colloidal and is based on the use of liquid oil droplets or inorganic assembly, based on replacing the spherical particles with polyhedral ones, whose faces are used to direct both the position and number of assembled satellite polystyrene (PS) Received: May 25, 2021 particles (Figure 1). The polyhedral particles thus act as core Published: August 12, 2021 particles with predetermined “instructions” that direct the coordination number and geometry of the synthesized colloidal clusters. For example, an octahedral core particle would direct the formation of an eight-coordinated (8-c) cubic © 2021 The Authors. Published by American Chemical Society https://doi.org/10.1021/jacs.1c05363 12943 J. Am. Chem. Soc. 2021, 143, 12943−12947 Journal of the American Chemical Society pubs.acs.org/JACS Communication Figure 1. Schematic illustration of the use of polyhedral MOF crystals as core particles to assemble coordination clusters and the way in which their polyhedral shape dictates their coordination number and geometry. Using this strategy, colloidal 8-c cubic, 6-c octahedral, and 12-c cuboctahedral clusters were synthesized using octahedral UiO-66 particles, cubic ZIF-8 particles and rhombic dodecahedral ZIF-8 particles, respectively. Scale bars: 5 μm (parts a, b, c, d), 1 μm (insets in parts a, c), and 500 nm (inset in part b). Figure 2. (a,b) FESEM images of the colloidal cubic clusters synthesized at wPS:wUiO‑66 = 150:1. (c) FESEM image of two cubic clusters positioned in different orientations. (d) FESEM image of two defective colloidal clusters with a coordination number of 7; arrows highlight the missing polystyrene sphere. (e) Statistical distributions of the coordination number for clusters with different wPS:wUiO‑66 ratios. Scale bars: 5 μm (part a) and 2 μm (parts b−d). nanoparticles as core particles onto which oppositely charged (diameter: 1039 ± 30 nm) and a surface charge of colloidal particles are stochastically aggregated. A plasticizer is approximately +45 mV (Figures S1,S2). Commercial aqueous then added to fuse the particles to form stable colloidal colloids of spherical sulfonated polystyrene particles with clusters, patchy particles, or core−shell particles. diameters of 400 nm, 700 nm, and 1 μm as well as a surface To demonstrate the feasibility of our proposed strategy, we charge of approximately −30 mV were used as satellite initially targeted the synthesis of 8-c cubic colloidal clusters particles (Figure 1d). using octahedral UiO-66 core particles (Figure 1a). The In a typical experiment, colloidal clusters were assembled by octahedral UiO-66 particles were first synthesized by heating a adding 100 μL of the polystyrene colloid on top of 100 μL of solution of ZrCl4, terephthalic acid, and acetic acid in DMF at the dispersion of UiO-66 particles. Then, tetrahydrofuran 120 °C for 12 h. The synthesized particles were then collected by centrifugation, cleaned with DMF and methanol, and finally (THF), which acts as the plasticizer, was added to the mixture dispersed in water containing polyvinylpyrrolidone. Field- to give a final THF concentration of 18% (v/v), and the emission scanning electron microscopy (FESEM), X-ray mixture was mixed by hand for 10 s. After this short period, the powder diffraction (XRPD), and zeta-potential measurements assembled colloidal clusters were isolated using density of the resulting colloid revealed the formation of homogeneous gradient centrifugation (10−30 wt % sucrose in water), octahedral UiO-66 particles with an edge size of 735 ± 21 nm washed with water, and finally dispersed in water. 12944 https://doi.org/10.1021/jacs.1c05363 J. Am. Chem. Soc. 2021, 143, 12943−12947 Journal of the American Chemical Society pubs.acs.org/JACS Communication Following this protocol, the assembly of spherical poly- formed by incubating an aqueous solution of Zn(NO3)2·6H2O, styrene particles with various diameters (400 nm, 700 nm, and 2-methylimidazole, and hexadecyltrimethylammonium bro- 1 μm) was first systematically studied to determine the mide (CTAB) at room temperature for 5 h. Rhombic optimum particle size for the attachment of a single dodecahedral ZIF-8 particles were synthesized by incubating polystyrene particle on each triangular facet of the UiO-66 an aqueous solution of zinc acetate and 2-methylimidazole for octahedron. For this set of experiments, the concentrations of 24 h. In both reactions, the particles were collected by polystyrene and the UiO-66 colloid were kept constant at 160 centrifugation, washed with water, and redispersed in the and 2 mg/mL, respectively (i.e., wPS:wUiO‑66 = 80:1). Under presence of CTAB. FESEM and XRPD measurements these conditions, the 700 nm diameter polystyrene spheres confirmed the formation of cubic and rhombic dodecahedral proved to be ideal for individual particle assembly. The use of ZIF-8 particles with edge dimensions of 205 ± 10 nm and 526 spherical polystyrene particles with a diameter of 400 nm ± 27 nm, respectively (Figures S10−S13). Zeta-potential resulted in the attachment of two or more particles on some measurements of either particles showed a surface charge of facets of the octahedra, thus preventing the templating of cubic approximately +40 mV. For the assemblies, we used negatively colloidal clusters by the polyhedral shape of the MOF particle charged polystyrene particles with diameters of 200 and 600 (Figure S3). The 1 μm diameter polystyrene particles did not nm in combination with the cubic and rhombic dodecahedral result in high occupancies of the eight facets of the octahedra, ZIF-8 particles, respectively, to optimize the attachment of a which was attributed to steric hindrance among themselves single polystyrene particle on each face of the ZIF-8 particles. (Figure S4). Both 6-c octahedral (Figure 3a−c) and 12-c cuboctahedral The synthesis of cubic colloidal clusters requires the attachment of a single polystyrene particle on each of the eight facets of the UiO-66 octahedra, i.e., achieving a coordination number of eight. Clusters missing one or more of the eight polystyrene particles can thus be considered defective. To maximize the formation of assemblies with eight polystyrene spheres on each UiO-66 particle, we performed a series of syntheses in which the wPS:wUiO‑66 ratio was systematically varied (Figure 2, S5−S8). Figure 2e shows the statistical distribution of the coordination number of clusters synthesized using wPS:wUiO‑66 ratios of 40:1, 80:1, 150:1, and 300:1. At a ratio of 40:1, the population of cubic colloidal clusters was 35%. However, in this case, most of the clusters were defective with coordination numbers of five, six, and seven, whereby the latter exhibited the highest population Figure 3. Schematic illustrations and corresponding FESEM images (44%). When the ratio was increased to 80:1, cubic clusters of 6-c octahedral (a−c) and 12-c cuboctahedral (d−f) clusters. Scale began to predominate (51%). At this ratio, the formation of bars: 5 μm (part f), 1 μm (parts c−e), and 300 nm (parts a,b). defective clusters with coordination numbers of five and six was low, whereas those with a coordination number of seven represented 38% of the total population. This tendency (Figure 3d−f) colloidal clusters were then assembled following continued at a ratio of 150:1, which provided the optimum a similar process to that used for the assembly of the 8-c cubic conditions for the synthesis of cubic clusters. Under these clusters. It should be noted here that the purification step in conditions, 93% of the population consisted of perfect cubic these assemblies involved multiple sedimentation−redispersion colloidal clusters (61%; Figure 2a−c) or defective cubic processes, as density gradient centrifugation (10−30 wt % clusters missing only one polystyrene sphere (32%; Figure 2d). sucrose in water) caused etching of the ZIF-8 particles (Figure Here, dynamic light scattering measurements further con- S14). firmed the formation of the clusters having a mean diameter of Synthesis by colloidal fusion is a method that provides approximately 1.5 μm (Figure S9), which match those access not only to colloidal clusters but also to patchy and measured by FESEM (edge size: 1.34 ± 0.06 μm; diagonal core−shell particles. The latter type of particles can be size: 1.62 ± 0.08 μm). The use of higher ratios did not synthesized by controlling the amount of plasticizer (in this improve the formation of cubic colloidal clusters, but resulted case: THF) added to the reaction. Indeed, the melting process in similar population percentages. in these reactions is usually controlled by tuning the Having demonstrated the utility of the polyhedral shape of concentration of the plasticizer. As described above, stable the core particles to serve as templates and guide the formation colloidal molecules can be formed when low concentrations of of cubic colloidal clusters, we extended the use of polyhedral the plasticizer are used. However, the addition of higher MOF particles to form colloidal clusters with an octahedral concentrations of plasticizer allows further melting of the geometry (coordination number: six) and with a much higher particles to form core−shell or patchy particles.29 In coordinated cuboctahedral geometry (coordination number: conjunction with the use of MOFs as core particles, this 12). To this end, we selected ZIF-8 particles due to their ability possibility opens the door for the creation of MOF@polymer to be isolated as both cubic and rhombic dodecahedral composites in which one MOF particle is encapsulated in a colloidal particles (Figure 1b,c). According to our strategy, the polymer shell (Figure 4). Accordingly, the above-described attachment of a single polystyrene sphere to the six facets of synthesis of 8-c cubic clusters was repeated with the amount of ZIF-8 cubes and the 12 facets of rhombic dodecahedral ZIF-8 THF increased to 26% or 30% (v/v). FESEM images of the particles should lead to the formation of colloidal octahedra resulting assemblies showed that the cubic clusters evolved and cuboctahedra, respectively. Initially, ZIF-8 cubes were into cellular-type particles when the THF concentration was 12945 https://doi.org/10.1021/jacs.1c05363 J. Am. Chem. Soc. 2021, 143, 12943−12947 Journal of the American Chemical Society ■ pubs.acs.org/JACS Communication ASSOCIATED CONTENT * sı Supporting Information The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.1c05363. Detailed syntheses, FESEM data and images, as well as PXRD diagrams (PDF) ■ AUTHOR INFORMATION Corresponding Authors Inhar Imaz − Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC, and Barcelona Institute of Science and Technology, 08193 Barcelona, Spain; orcid.org/0000-0002-0278-1141; Email: inhar.imaz@ icn2.cat Daniel Maspoch − Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC, and Barcelona Institute of Science and Technology, 08193 Barcelona, Spain; ICREA, Barcelona 08010, Spain; orcid.org/0000-0003-1325- 9161; Email: daniel.maspoch@icn2.cat Authors Yang Liu − Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC, and Barcelona Institute of Science and Technology, 08193 Barcelona, Spain Figure 4. (a) Schematic illustration and corresponding FESEM and dark-field STEM images of the evolution from 8-c cubic colloidal Jiemin Wang − Catalan Institute of Nanoscience and clusters to core−shell particles upon increasing the concentration of Nanotechnology (ICN2), CSIC, and Barcelona Institute of THF. (b) FESEM image of the cellular-type particles synthesized at Science and Technology, 08193 Barcelona, Spain 26% v/v THF. (c−e) FESEM (parts c,d) and dark-field STEM (part Complete contact information is available at: e) images of spherical core−shell UiO-66@polystyrene particles https://pubs.acs.org/10.1021/jacs.1c05363 synthesized at 30% v/v THF. Scale bars: 10 μm (parts b,c), 5 μm (part e), 3 μm (part d), and 1 μm (part a). Notes The authors declare no competing financial interest. increased to 26% v/v (Figure 4a,b). In these particles, the eight polystyrene spheres merged with one another and engulfed the ■ ACKNOWLEDGMENTS This work was supported by the Spanish MINECO (project core UiO-66 particle. This melting process was completed at RTI2018-095622−B-I00), the Catalan AGAUR (project 2017 30% v/v THF, where spherical core−shell particles consisting SGR 238), and by the CERCA Program/Generalitat de of a single UiO-66 particle encapsulated in a polystyrene shell Catalunya. ICN2 is supported by the Severo Ochoa program were formed (Figure 4a,c−e). from the Spanish MINECO (grant SEV-2017-0706). Y.L. In conclusion, we have demonstrated that the attachment of acknowledges the China Scholarship Council for scholarship support. ■ polystyrene particles on each face of a polyhedral metal− organic framework (MOF) particle can be controlled at the single-particle level. This control opens the door to using the REFERENCES polyhedral shape of MOF particles to drive the formation of (1) van Blaaderen, A. 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