15213757, 2022, 16, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/ange.202117455 by Readcube (Labtiva Inc.), Wiley Online Library on [27/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Angewandte Forschungsartikel Chemie www.angewandte.org Zitierweise: Angew. Chem. Int. Ed. 2022, 61, e202117455 Metal–Organic Frameworks Internationale Ausgabe: doi.org/10.1002/anie.202117455 Deutsche Ausgabe: doi.org/10.1002/ange.202117455 Coloration in Supraparticles Assembled from Polyhedral Metal- Organic Framework Particles Junwei Wang+, Yang Liu+, Gudrun Bleyer, Eric S. A. Goerlitzer, Silvan Englisch, Thomas Przybilla, Chrameh Fru Mbah, Michael Engel, Erdmann Spiecker, Inhar Imaz,* Daniel Maspoch,* and Nicolas Vogel* Introduction Abstract: Supraparticles are spherical colloidal crystals prepared by confined self-assembly processes. A partic- Emerging functional materials for applications in pho- ularly appealing property of these microscale structures tonics, plasmonics, and mechanics are created by assem- is the structural color arising from interference of light bling smaller particles into defined structures.[1] Among with their building blocks. Here, we assemble supra- various assembly strategies, using spherical confinements particles with high structural order that exhibit colora- (e.g. in drying emulsion droplets) to guide the self-assembly tion from uniform, polyhedral metal–organic framework process is garnering increasing interest.[2] This hierarchical (MOF) particles. We analyse the structural coloration as approach entails assembly of a finite number of particles a function of the size of these anisotropic building blocks into a larger supraparticle. The finite number can provide and their internal structure. We attribute the angle- additional effects, such as fluctuations of the thermody- dependent coloration of the MOF supraparticles to the namic stability of defined clusters.[2g] More generally, presence of ordered, onion-like layers at the outermost ordered supraparticles exhibit the collective properties regions. Surprisingly, even though different shapes of present in their corresponding bulk assemblies, while the MOF particles have different propensities to form remaining dispersible, discrete objects that are easy to these onion layers, all supraparticle dispersions show handle and post-process. For supraparticles whose constit- well-visible macroscopic coloration, indicating that local uent particles are of a size in the range of visible light ordering is sufficient to generate interference effects. wavelengths, the regular internal structure causes light diffraction, with constructive and destructive interference occurring for specific ranges of wavelengths that depend on [*] Dr. J. Wang,+ G. Bleyer, Dr. E. S. A. Goerlitzer, Prof. Dr. N. Vogel the particle size, lattice spacing, degree of order, and Institute of Particle Technology, Friedrich-Alexander Universität materials composition (namely the variations in the Erlangen-Nürnberg, 91058 Erlangen (Germany) refractive index), all of which affect the resulting structural E-mail: nicolas.vogel@fau.de color and its angle dependence, thus generating structural Y. Liu,+ Dr. I. Imaz, Prof. Dr. D. Maspoch color. The hue, intensity and saturation of this color Catalan Institute of Nanoscience and Nanotechnology (ICN2), depends on the refractive index, degree of crystallinity and CSIC & The Barcelona Institute of Science and Technology, Bellaterra, 08193 Barcelona (Spain) crystal-type of the supraparticle.[3, 4a] E-mail: inhar.imaz@icn2.cat To date, most supraparticles exhibiting structural color daniel.maspoch@icn2.cat are assembled from spherical primary particles. Significant S. Englisch, Dr. T. Przybilla, Prof. Dr. E. Spiecker efforts have been focused on varying the type of the Institute of Micro- and Nanostructure Research (IMN), primary spherical particles, including polystyrene, silica Center for Nanoanalysis and Electron Microscopy (CENEM), and melanin and other biopolymers.[1c, 4] The extensive use IZNF, Friedrich-Alexander Universität Erlangen-Nürnberg, of spherical primary particles in supraparticles stems from 91058 Erlangen (Germany) their availability and strong tendency to form close-packed, Dr. C. F. Mbah, Prof. Dr. M. Engel ordered structures. At the same time, the type of resulting Institute for Multiscale Simulation, crystal lattices is often limited to close-packings of equal IZNF, Friedrich-Alexander Universität Erlangen-Nürnberg, spheres. In contrast, the variety of ordered structures that 91058 Erlangen (Germany) can be formed by polyhedral particles with potential Prof. Dr. D. Maspoch distinct optical functionalities is massive.[5] Although re- ICREA, Pg. Lluis Companys 23, 08010 Barcelona (Spain) searchers can now exercise precise control over the size [+] These authors contributed equally to this work. and shape of diverse polyhedral particles (mainly metallic © 2022 The Authors. Angewandte Chemie published by Wiley-VCH ones), they continue to face the challenge of assembling GmbH. This is an open access article under the terms of the these particles within droplets to generate supraparticles Creative Commons Attribution Non-Commercial NoDerivs License, which permits use and distribution in any medium, provided the with structural color. This is because the particles are either original work is properly cited, the use is non-commercial and no too small to produce Bragg diffraction color or too heavy modifications or adaptations are made. to remain dispersed within the droplets, so that sedimenta- Angew. Chem. 2022, 134, e202117455 (1 of 8) © 2022 The Authors. Angewandte Chemie published by Wiley-VCH GmbH 15213757, 2022, 16, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/ange.202117455 by Readcube (Labtiva Inc.), Wiley Online Library on [27/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Angewandte Forschungsartikel Chemie tion impedes efficient equilibration.[6] This, in turn, pre- particles were washed with deionized water and O-UiO-66 cludes the use of many functional materials that are with DMF by centrifugation at 9000 rpm in 50 mL Falcon intrinsically polyhedral in the assembly of new multi- tubes. Colloids were finally prepared at concentrations of functional supraparticles. 40 mg mL 1 (C-ZIF-8), 40 mg mL 1 (TRD-ZIF-8), To face this challenge, we envisaged using polyhedral 20 mg mL 1 (RD-ZIF-8) and 6 mg mL 1 (O-UiO-66). Field- metal–organic frameworks (MOFs) as primary particles for emission scanning electron microscopy (FESEM), powder the formation of supraparticles. MOFs are functional porous X-ray diffraction (PXRD), and zeta-potential measurements materials with large surface areas and potential for myriad of the resulting colloids revealed the formation of the applications such as gas storage, catalysis and separation.[7] following homogeneous particles, listed here with their MOFs also exist in the form of crystalline particles that respective particle sizes: C-ZIF-8 (191 � 9 nm; Figure S1), encompass most known polyhedral shapes. Moreover, they TRD-ZIF-8 (181 � 9 nm; 198 � 10 nm; 229 � 9 nm; and 247 � have been recently synthesized with highly homogeneous, 10 nm; Figure S2), RD-ZIF-8 (246 � 12 nm; 267 � 12 nm; and tunable size and shape.[8] Together with their low density 293 � 13 nm; Figure S3) and O-UiO-66 (edge size: 194 � and their amenability to surface functionalization, MOFs 12 nm; 238 � 13 nm; and 247 � 13 nm; Figure S4). The trun- can also be obtained as stable colloids, with particle sizes in cation in TRD-ZIF-8 particles was 0.68. The surface charge the range of � 200 nm to 400 nm, which is in the ideal range of all the ZIF-8 particles was approximately + 30 mV, for to interact with visible light and thus, produce structural the O-UiO-66 particles, ca. + 45 mV. color. Indeed, we have recently demonstrated the use of Having prepared the four types of MOF particles, we such polyhedral MOF particles for three-dimensional pho- then synthesized the corresponding supraparticles. This tonic MOF superstructures.[9, 10] Here, we demonstrate that began with addition of 0.5 wt% non-ionic alcohol ethoxylate similar complex-shaped MOF particles can self-assemble surfactant (Lutensol TO-8, BASF) into separate 1.0 mL within the confinement of an emulsion droplet and form dispersions of the ZIF-8 particles, and of polyvinylpyrroli- spherical, consolidated and ordered photonic supraparticles. done into a dispersion of the UiO-66 particles to improve In the following, we examine in detail how the spherical colloidal stability. Three centrifugation cycles removed confinement affects the resultant self-assembled structure residual reagents, including excess surfactant. Next, the and how this internal structure translates into macroscopi- aqueous MOF particle dispersions were emulsified in cally observable structural color. perfluorinated oil (HFE 7500), either by vigorous shaking The polyhedral MOF particles used in this study are (Figure S5) or by droplet-based microfluidics (Figure 1b, c, cubic zeolitic imidazolate framework-8 (C-ZIF-8), truncated Figure S6). The droplets were stabilized by a perflurosurfac- rhombic dodecahedral TRD-ZIF-8, perfect rhombic dodeca- tant and left in an open glass vial overnight to allow hedral RD-ZIF-8 and octahedral Universitetet i Oslo-66 (O- evaporation and self-organization of the building blocks UiO-66) particles. ZIF-8 is a porous MOF made of ZnII ions inside the droplets. and 2-methylimidazolate (2-Mim) linkers that exhibits a Next, we characterized the supraparticles assembled sodalite-type structure and a large surface area ( � 1200– from each of the four polyhedral MOF particles by FESEM 1500 m2 g 1).[11] UiO-66 is a MOF built up from bridging (Figures 1d). Each supraparticle was spherical and had an [Zr6O4(OH)4] clusters by terephthalic acid linkers, also ordered surface, in which each class of polyhedral MOF showing a large surface area ( � 1200 m2 g 1).[12] We found particle was packed in agreement with the corresponding that the spherical geometry of each supraparticle enforces Monte Carlo simulations of hard polyhedra in spherical ordered, onion-like layered structures of polyhedral MOF confinement (Figures 1e). The RD- and TRD-ZIF-8 par- particles, which, in turn, act as Bragg reflectors and cause ticles formed a triangular unit cell and a rhombic unit cell at interference effects leading to structural coloration. Based the supraparticle surface, respectively, agreeing with their on our results, we discuss the angle-dependent coloration as bulk face-centered cubic packing and rhombohedral a function of the size and shape of the building blocks and packing.[9] The C-ZIF-8 particles formed the expected the internal order of the resultant supraparticles. square unit cell on the spherical supraparticle surface, whereas the O-UiO-66 particles formed a triangular cell. Careful analysis of FESEM images also showed that the flat Results and Discussion faces of the TRD-ZIF-8, C-ZIF-8 and O-UiO-66 particles were exposed to form a smooth, ordered, supraparticle We began with the synthesis of different colloidal polyhedral surface. This was observed both experimentally and in the MOF particles (Figure 1a; Figure S1–S4). C-ZIF-8 particles simulations. We reason that this packing is caused by were synthesized by reacting an aqueous mixture of Zn- entropic force that favors face-to-face configurations and (NO3)2·6 H2O, 2-Mim and cetyltrimethylammonium bromide disfavors vertex-to-face configurations in polyhedral pack- (CTAB) at room temperature for 5 h. Similarly, both TRD- ing, which tends to align the flat faces of particles to the and RD-ZIF-8 particles were prepared by reacting an droplet interface.[13] aqueous solution of ZnAc2·2 H2O, 2-Mim and CTAB (only Spherical confinement distorts crystalline lattices; con- in the case of TRD-ZIF-8) at room temperature for 2 h. In sequently, it distorts the arrangement of MOF particles the case of O-UiO-66, a dimethylformamide (DMF) solution inside the supraparticles.[2f,g, 14, 15] The spherical curvature of ZrCl4, terephthalic acid and acetic acid was heated at forces the primary particle to form a spherical particle 120 °C for 12 h. Afterwards, the three types of ZIF-8 monolayer at the supraparticle surface, which can in turn Angew. Chem. 2022, 134, e202117455 (2 of 8) © 2022 The Authors. Angewandte Chemie published by Wiley-VCH GmbH 15213757, 2022, 16, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/ange.202117455 by Readcube (Labtiva Inc.), Wiley Online Library on [27/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Angewandte Forschungsartikel Chemie Figure 1. Supraparticles from uniform, polyhedral MOF building blocks. a) FESEM of monodisperse rhombic dodecahedron (RD-ZIF-8), truncated rhombic dodecahedron (TRD-ZIF-8), cube (C-ZIF-8) and octahedron (O-UiO-66) particles (left to right). b, c) Monodisperse TRD-ZIF-8 supraparticles prepared by emulsifying the MOF particle dispersion using droplet-based microfluidics. d) FESEM (from left to right) of RD-ZIF-8, TRD-ZIF-8, C-ZIF-8 and O-UiO-66 supraparticles, exhibiting ordered surfaces and characteristic packings. e) Snapshots of Monte Carlo simulations with polyhedra in spherical confinement. cause the particles underneath to form another ordered the image appears homogeneous, indicating the particles are layer. This curvature effect propagates from the surface of less ordered at the core of the supraparticle. Similarly, the supraparticle towards the interior and attenuates as the pronounced onion-like layer structures were observed in curvature increases up to infinity at the core. This suggests TRD-ZIF-8 supraparticles, while RD-ZIF-8 and O-UiO-66 that the supraparticle might exhibit onion-like concentric supraparticles showed only little onion layers (Figure S7). layer structures under the surface,[2c] and that the thickness The impression of the X-ray tomography was corroborated of these onion layers may differ depending on the propen- by SEM-FIB cross sectioning (Figure 2b–e). Onion-like sity of the differently-shaped particles to assemble in the layers were observed for all ZIF-8 polyhedral MOF particles spherical confinement. near the supraparticle surface, conforming to the spherical We confirmed this hypothesis by two independent curvature. Corroborating the X-ray images, the RD-ZIF-8 imaging techniques, namely X-ray tomography and focused and the O-UiO-66 supraparticles only showed lower propen- ion beam (FIB)-assisted cross-sectioning, providing direct sities to form onion-like layers and mostly regions with insight into the inner structure of the different supraparticles short-range order (Figure 2b, e; Figure S8, S11), whereas (Figure 2). Figure 2a shows an X-ray transmission image of TRD- and C-ZIF-8 supraparticles exhibited pronounced a supraparticle assembled from C-ZIF-8 particles, where up onion-like layers penetrating far towards the interior (Fig- to ten concentric layers, forming an ordered, onion-like ure 2c, d; Figure S9, S10). Intriguingly, the TRD-ZIF-8 structure, are evident near the surface. Towards the center, supraparticles exhibited very high crystallinity throughout Angew. Chem. 2022, 134, e202117455 (3 of 8) © 2022 The Authors. Angewandte Chemie published by Wiley-VCH GmbH 15213757, 2022, 16, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/ange.202117455 by Readcube (Labtiva Inc.), Wiley Online Library on [27/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Angewandte Forschungsartikel Chemie Carlo simulations, all four hard polyhedral particles form onion-like layers of similar thickness, indicating that shape itself does not cause the different propensity to form onion- like layers in differently-shaped MOF particles (Figure 1e, Figure S13). However, in a simple bulk crystallization induced by centrifugation, we observed coloration for the TRD- and C-ZIF-8 particles, but not for the RD-ZIF-8 and O-UiO-66 particles (Figure S14), agreeing with the observed trend in the supraparticles. We therefore hypothesize that the effect may be caused by the anisotropic electrostatic repulsions of polyhedral particles. It is likely that there is more screening of charges at sharp edges, i.e. the RD-ZIF-8 particles, making them more prone to aggregate at close distance, compared to C-ZIF-8 with less edges. TRD-ZIF-8 particles on the other hand have rounded and smoothened edges, making such particles even more stable.[16] The ability to form well-ordered MOF supraparticles with defined internal structure enabled us to study their photonic properties as a function of primary particle shape and degree of internal order (Figure 3). To this end, we combined micro-spectroscopy at the individual supraparticle level on a substrate in air with macroscopic angle-dependent spectroscopy of the supraparticle dispersions. First, we assembled four types of supraparticles and chose the primary particle sizes that produce a green reflection color under the microscope (Figure 3, top). As expected from the investigation of internal order (Figure 2), C-ZIF-8 and TRD-ZIF-8 supraparticles showed a pronounced green coloration with a well-resolved central colored dot, which results from constructive interference of light reflected at the spherical-symmetric ordered onion layers (Figure 3b, c). In contrast, RD-ZIF-8 and O-UiO-66 supraparticles showed a less pronounced reflection color (Figure 3a, d), indicative of the lower degree of internal ordering. Noteworthily, the surfaces of all supraparticles exhibited well-ordered arrange- ments of particles (Figure 3a–d, middle), indicating that polydispersity is not the origin of the reduced order of some shapes. Microscopic reflection spectra (Figure 3a–d, bottom) corroborated these observations. TRD- and C-ZIF-8 supra- particles showed narrower and more defined photonic stop bands (full width at half maximum (FWHM) = 66 nm and 56 nm, respectively), while RD-ZIF-8 and O-UiO-66 supra- Figure 2. Internal structure of the supraparticles assembled from particles exhibited broader peaks (FWHM = 84 nm and polyhedral MOFs. a) X-ray transmission image of a C-ZIF-8 supra- 83 nm, respectively). particle, showing its onion-like structure near the outer surface. b)– Next, we used the TRD-ZIF-8 (229 nm) sample to e) Focused ion beam cross-sections of a RD-ZIF-8 (b), TRD-ZIF-8 (c), C-ZIF-8 (d) and O-UiO-66 (e) supraparticles, showing their correspond- exemplarily establish the relationship between supraparticle ing thicknesses of the onion-like layers. size and the reflection color intensity, measured at the peak of the photonic stop band (Figure 3e), using a polydisperse supraparticle sample prepared by simple shaking emulsifica- the interior, albeit in multiple crystalline domains. We tion. A clear increase in color intensity was observed with reason that sufficient equilibration may enable them to increasing supraparticle size, which can be rationalized from evolve towards complete crystallization with global symme- the diminishing effect of frustrated crystallization caused by try, similar to what has been observed in spherical particles the curvature of the spherical confinement. or ultra-small nanocrystals in spherical confinement.[2f,g, 6a] We then established the relationship between primary Indeed, we occasionally found local five-fold patterns in particle dimensions and the structural color of the supra- small supraparticles of TRD-ZIF-8 particles, indicative of an particles. We systematically varied the sizes of the differ- emerging icosahedral symmetry, corroborating results on ently-shaped MOF particles and measured reflection spectra the equilibrium structure of cubic nanoparticles with from the formed MOF supraparticles suspended in the oil truncated edges (Figure S12).[6a] Interestingly, in the Monte- phase in a glass vial (Figure 3f). The reflection peaks of all Angew. Chem. 2022, 134, e202117455 (4 of 8) © 2022 The Authors. Angewandte Chemie published by Wiley-VCH GmbH 15213757, 2022, 16, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/ange.202117455 by Readcube (Labtiva Inc.), Wiley Online Library on [27/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Angewandte Forschungsartikel Chemie Figure 3. Optical properties of MOF supraparticles. a)–d) Top row: FESEM of RD-ZIF-8 (a), TRD-ZIF-8 (b), C-ZIF-8 (c) and O-UiO-66 (d) supraparticles, exhibiting structural coloration from the interaction of the incident light and the supraparticle structure (optical microscopy images in inset). Middle row: magnification of the particle surfaces, showing well-ordered arrangements of the primary particles; bottom row: reflection spectra measured for individual supraparticles. e) Effect of supraparticle size on the intensity of the photonic stop band, exemplarily shown for TRD-ZIF-8 supraparticles. f) Relationship between size and shape of the primary particles on the wavelength of the reflection peak, measured in suspension. g) Angle-dependent reflectance spectra of MOF supraparticles consisting of the same colloidal particle size but different shapes, along with corresponding photographs showing the observable coloration. h) FESEM of spherical and buckled MOF supraparticles, with optical microscopy images showing differences in structural color. The surface of the buckled supraparticle shows the hexagonally-packed particle layer. i) Reflectance spectra of individual spherical and buckled MOF supraparticles. j) Reflectance spectra of spherical and buckled supraparticles measured in suspension. samples shifted linearly with increasing primary particle size, Figure 3a–d, a green reflection color was produced for as the periodicity in the Bragg diffractor scales linearly with TRD-ZIF-8 (197 nm), C-ZIF-8 (191 nm) and O-UiO-66 the particle size. Noteworthily, larger RD-ZIF-8 particles (194 nm) of similar sizes, while RD-ZIF-8 had significantly are required to produce the same reflection peak compared larger dimensions (246 nm). These differences are caused by to other shapes (horizontal box in Figure 3f). As shown in the packing of the differently-shaped particles. Entropy Angew. Chem. 2022, 134, e202117455 (5 of 8) © 2022 The Authors. Angewandte Chemie published by Wiley-VCH GmbH 15213757, 2022, 16, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/ange.202117455 by Readcube (Labtiva Inc.), Wiley Online Library on [27/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Angewandte Forschungsartikel Chemie favors the particles to align their faces within each layer and optical microscopy images (Figure 3h).[3a] Their spectral between layers (Figure S15).[17] For TRD- (and similar for properties, however, showed the presence of a reflection C-shaped) particles, the surface of a close-packed layer is peak (Figure 3i, j). A similar phenomenon was recently “flat”, leaving only small grooves from the truncation at reported for nanocellulose crystals in droplets, where multi- particle edges. As a result, the distance between two stacked layer structures persist through a buckled morphology.[18] It layers is the same as the distance between the two opposite is possible that the MOF particles also form stacked, non- faces of the TRD, which is the particle size by definition concentric layers near the supraparticles surface. However, (note that this does not equal the edge length, Figure S2). due to the larger and more rigid particle nature, only a few However, for RD-ZIF-8 particles, the surface of a close- layers can be reasonably expected under the heavily buckled packed layer has jagged protrusions and indentations (Fig- surface. Additionally, the reported buckled nanocellulose ure S15). When two such layers stack, protrusions and structures exhibit angle-independent color, while the indentations can register, bringing the two layers closer. As buckled MOF supraparticles maintain the same angle- a result, the distance between layers is smaller than the size dependency observed in spherical supraparticles (Fig- of RD-shaped particles. From Figure 3f, we estimate that ure S20). We hypothesize that other mechanisms may RD-shaped particles need to be 1.25 larger in size (than contribute to this coloration. One possibility is an additional TRD-shaped particles) to produce a similar Bragg reflection surface grating effect (Figure S21).[19–21] We noticed that peak. even for buckled supraparticles, the surface consists of We then focused on the macroscopic, angle-dependent hexagonally close-packed particles with sufficient structural coloration of the different MOF supraparticles, which was order (Figure 3h, inset, Figure S22), similar to the surface of measured directly in suspension using a collimated white spherical supraparticles (Figure S23). The inclined angle light source at different angles for illumination (Figure S16). enabled by the polyhedral particles may efficiently diffract Figure 3g shows the spectra of supraparticle suspension part of the incident light, which adds to the weaker Bragg prepared by shaking using similar-sized primary particles of reflection color for the buckled supraparticles. Another different shapes ( � 247 nm; vertical box in Figure 3f). As possible source for the color is the existence of numerous established above, the different packings of the individual tiny crystallites consisting of a few dozen particles randomly shapes lead to different layer spacing and therefore a distributed in the supraparticles, which can be seen in the different photonic stop band at the same illumination angle weak and localized color patches in the optical microscopic (Figure 3f). Note that the reduced refractive index contrast image (Figure 3h). While the exact mechanism and the in suspension efficiently suppress incoherent scattering, individual contributions to the observed structural color of manifested by reduced reflection at lower wavelengths MOF supraparticles requires more detailed studies sup- around 400 nm compared to samples measured in air (Fig- ported by numerical simulations, the existence of macro- ure 3a–d). As the angle between incident light and detector scopic coloration in less ordered structure with non- increases from 30° to 90°, all supraparticle suspensions spherical, buckled morphology is promising for the design of showed a blue shift of the photonic stop band (Figure 3g, structurally colored materials and pigments. Figure S17). A quantitative comparison between all samples revealed nearly identical angle-dependent blue shifts of the stop band, regardless of the primary particle shape (Fig- Conclusion ure S18), suggesting that interference of light reflected at the onion-like layers is the dominating color mechanism. How- In conclusion, we assembled various type of polyhedral ever, it is noteworthy that the RD-ZIF-8 and O-UiO-66 MOF particles into well-defined MOF supraparticles and supraparticles with low propensity to form onion layers and investigated the shape-dependent internal structure. We thus low reflected color intensity under the microscope then correlated this internal order with microscopic and (Figure 3a, d), also showed evident macroscopic coloration. macroscopic structural coloration and established shape- We note that the spectral intensities of the different samples size-coloration relationships. With the growing library of cannot be directly compared, as the mobile supraparticles MOF particle materials and shapes, our study opens a new rapidly accumulate on the top of the suspension due to the avenue to fabricate discrete hierarchical ordered structures density difference, causing a non-uniform concentration that combine intrinsic MOF functionality with additional during measurements. Nevertheless, the macroscopic photo- emerging collective properties. graphs (Figure 3g) of the different dispersions document the visible, angle-dependent coloration of the RD-ZIF-8 and O- UiO-66 supraparticle dispersions with comparably low Acknowledgements order. Interestingly, even buckled MOF supraparticles with We thank C.X. Du and E.G. Teich for discussion of supposedly compromised internal order exhibited observ- polyhedra packing in confinement. This work was supported able structural color (Figure 3h). We fabricated spherical by the Spanish MINECO (project RTI2018-095622-B-I00) and buckled RD-ZIF-8 supraparticles by systematically and the Catalan AGAUR (project 2017 SGR 238). It was removing surfactant via additional centrifugation steps (Fig- also funded by the CERCA Pro-gram/Generalitat de ure S19). The buckled supraparticles did not show the Catalunya. ICN2 is supported by the Severo Ochoa program typical central dot attributed to the onion-like layers in the from the Spanish MINECO (Grant No. SEV-2017-0706). Angew. Chem. 2022, 134, e202117455 (6 of 8) © 2022 The Authors. Angewandte Chemie published by Wiley-VCH GmbH 15213757, 2022, 16, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/ange.202117455 by Readcube (Labtiva Inc.), Wiley Online Library on [27/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Angewandte Forschungsartikel Chemie Y.L. acknowledges the China Scholarship Council for [4] a) J. Wang, U. Sultan, E. S. A. Goerlitzer, C. F. Mbah, M. scholarship support. T.P. and S.E. acknowledges support by Engel, N. Vogel, Adv. Funct. Mater. 2020, 30, 1907730; b) M. DFG within the framework of the research training group Xiao, Z. Y. Hu, Z. Wang, Y. W. Li, A. D. z Tormo, N. L. Thomas, B. X. Wang, N. C. Gianneschi, M. D. Shawkey, A. GRK 1896. This research was supported by the Deutsche Dhinojwala, Sci. Adv. 2017, 3, 170115; c) T. H. Zhao, G. Forschungsgemeinschaft (DFG, German Research Jacucci, X. Chen, D.-P. Song, S. Vignolini, R. M. Parker, Adv. Foundation)—Project-ID 416229255—SFB 1411. N.V. and Mater. 2020, 32, 2002681; d) T. Iwasaki, Y. Tamai, M. 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