Chem Soc Rev This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online REVIEW ARTICLE View Journal | View Issue Self-assembly of colloidal metal–organic framework (MOF) particles Cite this: Chem. Soc. Rev., 2023, Open Access Article. Published on 17 March 2023. Downloaded on 11/2/2023 2:44:12 PM. 52, 2528 Javier Fonseca,ab Lingxin Meng,ab Inhar Imaz*ab and Daniel Maspoch *abc Self-assembly of colloidal particles into ordered superstructures enables the development of novel advanced materials for diverse applications such as photonics, electronics, sensing, energy conversion, energy storage, diagnosis, drug or gene delivery, and catalysis. Recently, polyhedral metal–organic frame- work (MOF) particles have been proposed as promising colloidal particles to form ordered superstruc- Received 23rd November 2022 tures, based on their colloidal stability, size-tunability, rich polyhedral shapes, porosity and DOI: 10.1039/d2cs00858k multifunctionality. In this review, we present a comprehensive overview of strategies for the self-assembly of colloidal MOF particles into ordered superstructures of different dimensionalities, highlighting some of rsc.li/chem-soc-rev their properties and applications, and sharing thoughts on the self-assembly of MOF particles. 1. Introduction superstrutures.1 Natural self-assembly is ubiquitous and under- Self-assembly is the process by which individual building lies the formation of many essential biological structures such blocks, ranging from (bio)molecules to macroscopic as DNA, proteins, and lipid vesicles.2–5 Beyond (bio)molecules, objects, spontaneously organise themselves into ordered self-assembly of colloidal micro- or nanoscale particles into ordered superstructures has also attracted much attention due to the possibility to generate advanced materials with proper- a Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and The ties that could enable diverse applications such as photonics, Barcelona Institute of Science and Technology, Campus UAB, Bellaterra, 08193, electronics, sensing, energy conversion, energy storage, diag- Barcelona, Spain. E-mail: inhar.imaz@icn2.cat, daniel.maspoch@icn2.cat b Departament de Quı́mica, Facultat de Ciències, Universitat Autònoma de nosis, drug or gene delivery, and catalysis.6–8 Traditionally, Barcelona, 08193 Bellaterra, Spain most colloidal ordered superstructures have been assembled c ICREA, Pg. Lluıs Companys 23, 08010, Barcelona, Spain from spherical or near-spherical particles of silica,9,10 metallic Javier Fonseca obtained his Lingxin Meng received his MSc bachelor’s degree in Chemical degree in Chemical Engineering Engineering from Salamanca from the Northeastern University University (Salamanca, Spain) in (Shenyang, China) in 2021. He is 2015 and his master’s degree in currently a PhD student in Prof. Material Science from Rovira i Maspoch’s group at the Virgili University (Tarragona, Autonomous University of Spain) in 2016. He received his Barcelona and the Catalan PhD degree in Chemical Institute of Nanoscience and Engineering from Northeastern Nanotechnology (Barcelona, Spain) University (Boston, USA) in 2020 thanks to a CSC scholarship. His under the supervision of Prof. research interests focus on Javier Fonseca Sunho Choi. He joined Prof. Chin Lingxin Meng controlling the size and group at University of Vienna morphology of MOF nanocrystals, (Vienna, Austria) as a postdoctoral researcher in 2022 and is currently their colloidal self-assembly at the mesoscale, their substantial a postdoctoral researcher in Prof. Maspoch’s group at the Catalan applications (such as photonics) at the macroscale, and MOF- Institute of Nanoscience and Nanotechnology (Barcelona, Spain). His composites. research focuses on the design and preparation of MOF-based superstructures and MOF composites. 2528 | Chem. Soc. Rev., 2023, 52, 2528–2543 This journal is © The Royal Society of Chemistry 2023 View Article Online Review Article Chem Soc Rev quantum dots (e.g. CdSe11,12 and PbS13), metal nanoparticles shapes. Consequently, their use should enable the generation (e.g. gold nanoparticles14), and polymers (e.g. polystyrene15,16 of superstructures with multiple, different packings, including and poly-(methyl methacrylate) (PMMA)).17,18 These particles in novel ones. turn generate superstructures with face-centred cubic (fcc),  MOFs can be obtained as colloidal particles of very homo- This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. hexagonal close-packed (hcp), or random hexagonal close- geneous size and shape.39–41 Furthermore, their particle size packed (rhcp) lattices. can easily be tuned from B50 nm to B1 mm, thus providing Progress in the controlled self-assembly of colloidal particles access to photonic crystals. has recently led to the use of non-spherical, polyhedral particles  Given that MOFs are porous, their self-assembly can yield based on crystalline inorganic nanoparticles (e.g. gold,19 ordered porous superstructures, thereby making them suitable silver,20 CdS,21 PbSe22 and MnO23),24–27 which can adopt many for diverse applications (e.g. sensors).  MOFs can exhibit various other properties (e.g. conductiv- Open Access Article. Published on 17 March 2023. Downloaded on 11/2/2023 2:44:12 PM. polyhedral shapes such as cubic, tetrahedral, octahedral, rod- like, and rhombic dodecahedral morphologies. Compared to ity, chirality, magnetism, optics, etc.) that can lead to multi- spherical colloidal particles, these polyhedral particles can functional ordered superstructures. widely expand the gamut of possible superstructures, thus  Polyhedral MOF particles can exhibit anisotropic shapes, providing access to new packing geometries.28–34 For example, the self-assembly of which can enable the controlled orienta- Mirkin et al. assembled ordered three-dimensional (3D) super- tion of their pores. structures exhibiting fcc, cubic, and body-centred cubic (bcc)  Some MOF structures are flexible and will confer their lattices using gold nanoparticles with rhombic dodecahedral, flexibility to ordered superstructures. truncated cubic and octahedral shapes, respectively.35 Simi-  MOFs can be post-synthetically modified, and so can their larly, Yang et al. showed the use of cubic, octahedral, and corresponding ordered superstructures. truncated octahedral silver particles to assemble 3D super-  MOFs can be calcinated to form inorganic and carbon- structures with cubic, space-filling Kelvin, and Minkowski- based materials, a transformation that can be used to create packing structures, respectively.36 superstructures of these derived materials. Among the existing types of polyhedral particles, crystalline Several excellent reviews have been published on the self- metal–organic framework (MOF) particles have very recently assembly of colloidal particles, covering topics such as the been postulated as new polyhedral building units to construct principles that govern self-assembly,42–44 the types of colloidal ordered superstructures.37,38 As colloidal building units, MOF particles that can self-assemble,45–47 and the characterisation, particles offer several advantages, which are outlined below. properties and applications of the resultant self-assembled These characteristics make polyhedral MOF particles highly superstructures.48–50 We encourage interested readers to con- attractive for self-assembly into ordered superstructures with sult these references. Here, we examine the self-assembly of new properties and applications that otherwise would be inac- MOF particles into ordered superstructures of different dimension- cessible from spherical colloidal particles or other polyhedral alities (Table 1). We aimed to explore the recent approaches that building units. have been exploited for creating the first ordered MOF superstruc-  Polyhedral MOF particles can be an excellent source of tures, including solvent evaporation, depletion-assisted assembly, crystalline particles that encompass most known polyhedral electric field-assisted assembly, DNA-assisted assembly, anisotropic Inhar Imaz received his PhD in Daniel Maspoch is an ICREA Materials Science from the Research Professor at the Institut Université Bordeaux I in 2005, Català de Nanociència i where he studied the formation of Nanotecnologia (ICN2). He heterometallic metal–organic archi- received his BS degree at the tectures from tetrahedral building Universitat de Girona and his PhD blocks. He joined the CIN-2 (ICN- degree at the Universitat Autònoma CSIC) centre in 2005 as a de Barcelona & Institut de Ciència Postdoctoral fellow. At present, he de Materials de Barcelona. He is senior researcher at the Catalan worked as a postdoctoral fellow at Institute of Nanoscience and Northwestern University. His Nanotechnology (ICN2). His research interests include reticular Inhar Imaz research interests are focused on Daniel Maspoch materials (MOFs, COFs and MOPs) controlling the supramolecular and delivery systems. assembly of molecules, metal ions and nanoscale building blocks at the macro and nanoscale for the design of novel metal–organic frame- works and functional supramolecular architectures with interesting properties and applications in gas storage, catalysis. This journal is © The Royal Society of Chemistry 2023 Chem. Soc. Rev., 2023, 52, 2528–2543 | 2529 View Article Online Chem Soc Rev Review Article Table 1 Superstructures assembled from colloidal MOF particles Reagent to screen the interactions between MOF This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Self-assembled superstructure MOF particle particles Assembly technique Ref. 0D microsphere made of onion- C-ZIF-8 (191  9 nm) Lutensol TO-8/ Drying of emulsion droplets 59 like layers perfluoro-surfactant 0D microsphere made of onion- RD-ZIF-8 (246  12, 267  12, and 293  Lutensol TO-8/ Drying of emulsion droplets 59 like layers 13 nm) perfluoro-surfactant 0D microsphere made of onion- TRD-ZIF-8 (181  9, 198  10, 229  9, and Lutensol TO-8/ Drying of emulsion droplets 59 like layers 247  10 nm) perfluoro-surfactant 0D microsphere made of onion- O-UiO-66 (194  12, 238  13, and 247  PVP/perfluoro- Drying of emulsion droplets 59 Open Access Article. Published on 17 March 2023. Downloaded on 11/2/2023 2:44:12 PM. like layers 13 nm) surfactant 0D pseudospherical photonic TRD-ZIF-8 (184, 206, 242, and 275 nm) CTAB/PVA Spray-drying of emulsion droplets 60 supraparticle 0D pseudospherical photonic TRD-ZIF-67 (193, and 205 nm) CTAB Spray-drying of emulsion droplets 60 supraparticle 0D pseudospherical photonic TRD-ZIF-8 (275 nm)/Au nanoparticle CTAB Spray-drying of emulsion droplets 60 supraparticle (51 nm) 0D hollow microsphere C-Fe-soc-MOF (310  10 nm) Tween-85 Drying of emulsion droplets 61 8-c cubic cluster O-UiO-66 (735  21 nm) PVP Colloidal fusion synthesis 66 6-c octahedral cluster C-ZIF-8 (205  10 nm) CTAB Colloidal fusion synthesis 66 12-c cuboctahedral cluster RD-ZIF-8 (526  27 nm) CTAB Colloidal fusion synthesis 66 1D chains RD-ZIF-8 (5.3 mm) PVP External electric field 71 1D chains UiO-66-on-MIL-96 Truncated hexagonal — Anisotropic pattern 75 bipyramidal MIL-96 (mm-size) 1D chains (on a smooth substrate) RD-ZIF-8 (0.9  0.3 mm) CTAC Depletion interaction in capillary 76 1D chains (on a smooth substrate) TRD-ZIF-8 (1.2  0.2 mm) CTAC Depletion interaction in capillary 76 1D chains (on a smooth substrate) MIL-88A (1.6  0.1 mm) SDS Depletion interaction in capillary 76 Anisotropic quasi-1D stripe-like O-UiO-66 (1.0  0.1 mm) CTAC Depletion interaction in capillary 76 superstructure (on a rough substrate) 2D hexagonal lattices RD-ZIF-8 (830 nm) — Solvent evaporation 38 2D snowflake-like network (on a MIL-88A (1.6  0.1 mm) SDS Depletion interaction in capillary 76 rough substrate) 2D hexagonal lattices (on a O-UiO-66 (1.0  0.1 mm) CTAC Depletion interaction in capillary 76 smooth substrate) 2D square lattices (on a smooth Truncated hexagonal bipyramidal MIL-96 CTAC Depletion interaction in capillary 76 substrate) degree of truncation = 0.24 (1.0  0.1 mm) 2D centered rectangular lattices Truncated hexagonal bipyramidal MIL-96 CTAC Depletion interaction in capillary 76 (on a smooth substrate) degree of truncation = 0.52 (0.8  0.1 mm) 2D hexagonal lattices TC-In-soc-MOF (1.60  0.04 mm) PVP Solvent evaporation 79 2D simple cubic lattices C-Ga-soc-MOF (360  20 nm) PVP Solvent evaporation 79 2D hexagonal lattices TRD-ZIF-8 (174  15 nm) CTAB/PMMA Air–liquid interface 80 2D hexagonal lattices O-UiO-66 (409  29 nm) PVP/SDS Air–liquid interface 81 2D films HKUST-1 — LB 82 2D films Al12O(OH)18(H2O)3(Al2(OH)4)(BTC)6 — LB 82 2D films Cu2(BDC)2(bipy) — LB 82 2D hexagonal lattices O-UiO-66 (80 nm) PBnMA Air–liquid interface 84 2D hexagonal lattices O-UiO-66 (80 nm) PMA Air–liquid interface 84 2D hexagonal lattices O-UiO-66 (120 nm) PMA Air–liquid interface 84 2D hexagonal lattices RD-ZIF-8 (590, and 745 nm) PVP LB 89 2D simple cubic lattices TC-ZIF-8 (600 nm) PVP LB 89 2D hexagonal lattices TRD-ZIF-8 (500 nm) PVP LB 89 2D hexagonal lattices PCN-222 nanorods (38  8 nm  159  — Anisotropic pattern (oligonucleo- 90 25 nm) tides) [self-complementary DNA sticky ends] 2D tetragonal lattices PCN-222 nanorods (38  8 nm  159  — Anisotropic pattern (oligonucleo- 90 25 nm) tides) [complementary DNA sticky ends] 2D quasi-ordered superstructures TRD-ZIF-8 (182  11 nm) CTAB Ice-templating strategy 92 2D quasi-ordered superstructures C-ZIF-8 (150  12 nm) — Ice-templating strategy 92 2D quasi-ordered superstructures O-UiO-66 (from 300 to 800 nm) — Ice-templating strategy 92 2D quasi-ordered superstructures MIL-88B(Fe)-NH2 hexagonal nanorods — Ice-templating strategy 92 (300 nm) 3D rhombohedral packing TRD-ZIF-8 (TRD-ZIF-8) degree of trunca- CTAB Solvent evaporation 37 tion 40.66 (210  10 nm) 3D fcc packing TRD-ZIF-8 (TRD-ZIF-8) degree of trunca- CTAB Solvent evaporation 37 tion o0.66 (263  13 nm) 3D hexagonal packing O-UiO-66 (340  30 nm) CTAB Solvent evaporation 37 3D superstructures NU-1000 (length-to-diameter TMPTA External electric field 70 ratio = 3.4  0.2) 2530 | Chem. Soc. Rev., 2023, 52, 2528–2543 This journal is © The Royal Society of Chemistry 2023 View Article Online Review Article Chem Soc Rev Table 1 (continued) Reagent to screen the interactions between MOF This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Self-assembled superstructure MOF particle particles Assembly technique Ref. 3D superstructures MIL-68(In) (length-to-diameter ratio = 6.8 TMPTA External electric field 70  0.9) (length-to-diameter ratio = 1.2  0.2) 3D superstructures MIL-53-NH2(Al) (length-to-diameter ratio = TMPTA External electric field 70 10  5) Quasi-3D superstructures (on a RD-ZIF-8 (0.9  0.3 mm) CTAC Depletion interaction in capillary 76 rough substrate) Open Access Article. Published on 17 March 2023. Downloaded on 11/2/2023 2:44:12 PM. 3D hexagonal packing O-UiO-66 (409  29 nm) PVP Solvent evaporation 81 3D fcc packing O-UiO-66 (86  10 nm) — Anisotropic pattern (oligonucleo- 90 tides) [self-complementary DNA sticky ends] 3D bcc packing O-UiO-66 (86  10 nm) — Anisotropic pattern (oligonucleo- 90 tides) [complementary DNA sticky ends] 3D CsCl-type lattices Spherical UiO-66 (37  8 nm)/spherical Au — Anisotropic pattern (oligonucleo- 90 nanoparticle (40 nm) tides) [complementary DNA sticky ends] 3D CsCl-type lattices Spherical UiO-66 (37  8 nm)/spherical Au — Anisotropic pattern (oligonucleo- 90 nanoparticle (20 nm) tides) [complementary DNA sticky ends] Notation: cubic (C); rhombic dodecahedral (RD); truncated rhombic dodecahedral (TRD); octahedral (O); cetyltrimethylammonium bromide (CTAB); cetyltrimethylammonium chloride (CTAC), polyvinyl alcohol (PVA); polyvinylpyrrolidone (PVP); poly-(methyl methacrylate) (PMMA); poly- (benzyl methacrylate) (PBnMA); poly-(methyl methacrylate) (PMA); sodium dodecyl sulphate (SDS); Langmuir–Blodgett (LB); truncated cubic (TC); face centred-cubic (fcc); body centred-cubic (bcc); trimethylolpropane triacrylate (TMPTA). pattern-assisted assembly, ice-templated self-assembly, and air– uniform MOF particles. Another influential factor in the pre- liquid interface assembly. To help the reader navigate this review, paration of colloidal MOF particles with uniform size is the we have organised each assembled superstructure here by dimen- modulators used, as they can modify the nucleation and the sionality. We conclude by providing our thoughts on MOF particle growth rates.53 On the other hand, the shape of the MOF self-assembly, potential applications of prepared superstructures, particles is defined by the most thermodynamically stable facet and current challenges in the field. orientation. In this regard, the use of surfactants is a common approach to control the morphology of MOF particles, as they selectively adhere to certain specific crystalline facets, thereby 2. From uniform colloidal MOF inhibiting growth at those facets. The shape of MOF particles can also be controlled post-synthetically by top-down strategies particles to ordered superstructures such as wet-chemical etching.54 Before discussing the approaches to create ordered MOF super- Once uniform colloidal MOF particles have been prepared, their structures, we first present an overview of the fundamentals of colloidal stability must be ensured to enable their self-assembly into the self-assembly of colloidal MOF particles. We begin by superstructures. A colloidal dispersion of uniform MOF particles is discussing the preparation of uniform colloidal MOF particles, stable when the interparticle interaction is repulsive. Thus, MOF- then explain the forces that govern these colloidal dispersions, based colloidal solutions become destabilised when interparticle and close by exploring the interparticle forces that influence the interactions change from repulsive to attractive, which results in an self-assembly (Fig. 1). unbalanced and uncontrolled MOF flocculation. Typically, the van The preparation of colloidal MOF particles of uniform size der Waals attractive interaction among MOF particles favours their and morphology forms the basis of experiments on MOF-based flocculation. There are two different mechanisms to screen van der self-assembly. Accordingly, researchers are constantly seeking Waals interactions and stabilise colloidal MOFs: steric stabilisation, to expand the library of uniform colloidal MOF particles by tethering molecular chains to the MOF;55,56 and electrostatic accessible for experimental work.51,52 The importance of syn- stabilisation, by adjusting the surface charge of the MOF.57 In the thetic parameters (e.g. precursors, modulators, reaction time, first mechanism, the osmotic and elastic repulsion among the and reaction temperature) on the preparation of colloidal MOF ligand coronas that tether the MOF particles cause the steric particles has been revealed over the past two decades.39,41 For stabilisation of the particles. In the second mechanism, surface- example, nucleation and growth rates have been found to charged MOF particles are neutralised by oppositely charged coun- define the size of the resultant MOF particles. Thus, the terions, which promotes the formation of an electrical double-layer. formation of colloidal MOF particles of uniform size is usually The overlapping of the electrical clouds of MOF particles generates successful when nucleation is favoured over growth. Generally, an osmotic repulsion among them, thus screening their van der an excess of ligand promotes nucleation, leading to small, Waals interaction. This journal is © The Royal Society of Chemistry 2023 Chem. Soc. Rev., 2023, 52, 2528–2543 | 2531 View Article Online Chem Soc Rev Review Article the unknown contribution of each term, accurately reporting on the interactions among MOF particles in self-assembly is extraordinary difficult. In fact, the interactions among MOF particles in the self- assembly have not yet been explored in any study. However, since This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. self-assembly occurs under near-equilibrium conditions,58 in which the interparticle attractions remain comparable to the thermal energy (kBT; where kB is the Boltzmann constant, and T is the temperature) of the system, we consider that understanding and modelling the interparticle interaction will contribute to the success of the self-assembly. Open Access Article. Published on 17 March 2023. Downloaded on 11/2/2023 2:44:12 PM. 3. Self-assembly of metal–organic framework particles into zero- dimensional ordered superstructures Self-assembly of colloidal particles into zero-dimensional (0D) ordered superstructures usually occurs in confined nano- or microscale spaces, typically in nano- or microdroplets. These spaces restrict the freedom of movement of the particles, which could favour their self-organisation and ultimately self- assembly. Two techniques are used to induce self-assembly of 0D MOF superstructures within droplets: solvent evaporation at the surface of emulsion droplets; and spray-drying.59,60 In 2013, Eddaoudi et al. pioneered the formation of 0D Fig. 1 Schematic of the self-assembly of MOF particles, including the superstructures via drying of emulsion droplets. These super- synthesis of uniform MOF particles, the stabilisation of colloidal MOF structures were hollow microspheres comprising a monolayer particles and finally, the self-assembly of MOF particles into ordered shell of cubic Fe-soc-MOF (C-Fe-soc-MOF) particles (Fig. 2(a)– superstructures. SEM images of colloidal RD-ZIF-8 particles sized (d)).61 Interestingly, the authors named these superstructures (a) 2 mm or (b) 1 mm, and (c) colloidal TRD-ZIF-8 particles sized 500 nm. SEM images of (d) a 0D superstructure of TRD-ZIF-8 particles, (e) 1D colloidosomes, and such structures are also known as supra- superstructures of MOF-based patchy particles, and (g) a 3D superstruc- balls or supraparticles.62,63 They prepared monodisperse C-Fe- ture of TRD-ZIF-8 particles. (f) TEM image of a 2D superstructure of O- soc-MOF particles in the presence of tert-butylamine, which UiO-66 particles. Adapted with permission from ref. 51, copyright 2018 acted as structure-directing agent, and of polyoxyethylene (20) American Chemical Society; ref. 52, copyright 2011 American Chemical sorbitan trioleate (also known as tween-85), which acted as Society; ref. 59, copyright 2022 John Wiley and Sons; ref. 75, copyright 2021 John Wiley and Sons; ref. 90, copyright 2020 Springer Nature; and emulsifier. This ultimately led to spontaneous formation of ref. 37, copyright 2017 Springer Nature. hollow superstructures in two steps: firstly, emulsified droplets were formed, and subsequently, the monodispersed C-Fe-soc- MOF cubes were synthesised and spontaneously assembled at Having highlighted the forces that govern the preparation of the interface of the droplets. Self-assembly occurred as those MOF-based colloidal dispersions, we would like to underscore droplets evaporated. The dimensions of the emulsion droplet that these and other interparticle forces must be considered in and consequently, of the final superstructure, were found to the self-assembly of MOF colloidal particles. Notably, the depend on the amount of emulsifier. Thus, the diameter of the physicochemical properties of MOF particles (e.g. size, shape, hollow colloidosomes was inversely proportional to the amount surface charge, high porosity, low density, etc.) and the physi- of tween-85. The smaller hollow colloidosomes were more rigid, cochemical properties of the molecular chains that tether the a phenomenon that the authors attributed to the stabilisation MOFs (e.g. length, charge, hydrophobicity, density, molecular that was achieved when more emulsifier had been used.61 weight, etc.) strongly influence the interactions among MOF More recently, cubic ZIF-8 (C-ZIF-8), rhombic dodecahedral particles during self-assembly. For example, charged MOF ZIF-8 (RD-ZIF-8), truncated rhombic dodecahedral ZIF-8 (TRD- particles exhibit electrostatic interactions. Other forces may ZIF-8), and octahedral UiO-66 (O-UiO-66) particles have also also be present, depending on the self-assembly methodology been self-assembled via drying of emulsion droplets.59 These employed (e.g. solvent evaporation, depletion-assisted assem- MOF particles self-assembled into spherical, onion-like ordered bly, electric field-assisted assembly, DNA-assisted assembly, superstructures that exhibit structural colour. Unlike the study anisotropic pattern-assisted assembly, ice-templated self- discussed above, in which hollow colloidosomes were reported assembly, and air–liquid interface assembly). For example, to have formed during the synthesis of MOF particles, the when evaporation is used, gravity may be a plausible driving force formation of these colloidal 0D superstructures occurred for large MOF particles. Given the abundance of interactions and from the controlled assembly of pre-synthesised MOF particles. 2532 | Chem. Soc. Rev., 2023, 52, 2528–2543 This journal is © The Royal Society of Chemistry 2023 View Article Online Review Article Chem Soc Rev This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Open Access Article. Published on 17 March 2023. Downloaded on 11/2/2023 2:44:12 PM. Fig. 2 (a)–(c) SEM and (d) TEM images of hollow colloidosomes assembled from C-Fe-soc-MOF particles (edge length of B310  10 nm). The estimated hollow diameter was from 3 to 5 mm. Adapted with permission from ref. 61. Copyright 2013 American Chemical Society. (e) FESEM images of spherical, onion-like ordered superstructures made of RD-ZIF-8, TRD-ZIF-8, C-ZIF-8, or O-UiO-66 particles. (f) Monte Carlo simulations of hard polyhedra in spherical confinement. These simulations matched the experimental packings. Adapted with permission from ref. 59. Copyright 2022 John Wiley and Sons. (g) Schematic of the preparation of photonic supraparticles by self-assembly of MOF particles via spray-drying. (h) SEM image of a ZIF-8 photonic supraparticle. Scale bar: 1 mm. (i) SEM image of a ZIF-8/PVA photonic supraparticle. Scale bar: 2 mm. Adapted with permission from ref. 60. Copyright 2021 John Wiley and Sons. Initially, stable colloidal solutions of ZIF-8 and UiO-66 particles as the smaller MOF particles, favoured the formation of curved were prepared using Lutensol TO-8 and polyvinylpyrrolidone photonic superstructures. Interestingly, spray-drying dries the gen- (PVP), respectively. These colloidal dispersions were then emulsi- erated droplets almost instantly,64,65 which tends to induce buckling fied in perfluorinated oil using ultrasound or microfluidics, and or crumpling effects.60 the resulting droplets were stabilised with a perfluoro-surfactant. Beyond spherical 0D superstructures, another type of 0D Onion-like layered superstructures were finally obtained by drying superstructures are colloidal clusters (also known as colloidal the emulsion droplets. The different MOF particles packed differ- molecules), in which a small number of colloidal particles are ently on the surface of these superstructures (Fig. 2(e) and (f)). arranged in organised geometric systems resembling a ‘‘cluster’’ Thus, C-ZIF-8 particles formed a square unit-cell; RD-ZIF-8 and O- or a ‘‘molecule’’. Recently, the polyhedral shape of colloidal UiO-66 particles, a triangular unit-cell; and TRD-ZIF-8 particles, a MOF particles has also been used to direct the assembly of rhombic unit-cell. The planar faces of the MOF particles were colloidal clusters. For this assembly, MOF particles act as core suggested to align along the droplet interface, thus forming the particles to direct the assembly of spherical polystyrene particles outer layer of the superstructure. Similarly, the MOF particles (Fig. 3).66 This approach is based on attaching a single spherical underneath formed another ordered layer following the curvature polystyrene particle to each face of a polyhedral particle via of the outer layer and so on. Therefore, the superstructures were colloidal-fusion synthesis.67 Accordingly, the faces of the MOF proposed to exhibit an onion-like order under the surface, the particle define the final coordination number and geometry of thickness of which depended on the tendency of the different MOF the assembled colloidal cluster. Using this approach, octahe- particles to self-assemble when confined in the spherical droplet.59 dral, cubic and cuboctahedral clusters were assembled using C- The spray-drying technique, which is based on atomised droplet ZIF-8, O-UiO-66 and RD-ZIF-8 core particles, onto which 6, 8 and drying,64 has been reported to induce self-assembly of MOF parti- 12 polystyrene spherical particles were attached, respectively. cles. Instead of forming perfect spherical superstructures,59,61 spray- drying has led to self-assembly of colloidal MOF particles into pseudospherical photonic superstructures (Fig. 2(g)–(i)).60 Specifi- 4. Self-assembly of metal–organic cally, ZIF-8, ZIF-67 and ZIF-8/Au nanoparticles composite photonic framework particles into one- superstructures were prepared by spray-drying emulsion droplets. dimensional ordered superstructures The pseudo-sphericity was caused by the preference of faceted polyhedral MOF particles to self-assemble into faceted superstruc- The assembly of one-dimensional (1D) MOF particle super- tures with the densest possible packing. The larger droplets, as well structures (also known as chains) requires that MOF particles This journal is © The Royal Society of Chemistry 2023 Chem. Soc. Rev., 2023, 52, 2528–2543 | 2533 View Article Online Chem Soc Rev Review Article referred to as anisotropic pattern-assembly, is limited by the difficulty of preparing Janus MOF particles with surfaces that contain the necessary information to guide the assembly in a controlled manner. Importantly, anisotropic pattern-assembly This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. encodes assembly information locally, through short–ranged interactions, unlike other approaches, in which constraints are used to direct the assembly of MOF particles. Wang et al. have recently developed ordered superstructures with different dimensionalities, by destabilising the colloidal solution through the depletion interaction within a capillary.76 Open Access Article. Published on 17 March 2023. Downloaded on 11/2/2023 2:44:12 PM. The depletion interaction fosters the short–range attractive force among colloids.77,78 Specifically, this attractive force arises among colloidal particles when they are diluted in solution together with small, non-interacting co-solutes called Fig. 3 Schematic of the colloidal-fusion strategy, and corresponding SEM depletants (e.g., small nanoparticles, micelles, polymers, salts, images of (a) an 8-c cubic cluster, (b) a 6-c octahedral cluster, and (c) a 12- c cuboctahedral cluster. Scale bars: (a) 2 mm; (b) 300 nm; and (c) 1 mm. etc.).76 An effective osmotic pressure appears when two parti- Adapted with permission from ref. 66. Copyright 2021 American Chemical cles approach each other at a separation distance shorter than Society. the size of the depletants. This osmotic pressure causes the particles to assemble. Furthermore, self-assembly increases the free volume for the depletants to move, thus increasing be correctly aligned. One way to promote alignment is through the entropy of the system. In the study by Wang and colleagues, the use of electric and magnetic fields.68–70 Specifically, electric cetyltrimethylammonium chloride (CTAC) and sodium dodecyl fields have been shown to induce self-assembly of RD-ZIF-8 sulphate (SDS) facilitated the self-assembly of different MOF particles into 1D superstructures: for instance, in polarising the particles (RD and TRD-ZIF-8 particles, hexagonal rods of MIL- electrostatic double layer on the surface of ZIF-8 particles.71 88A, O-UiO-66 particles, and truncated hexagonal bipyramidal Dipole–dipole interaction between electrostatic double layers of MIL-96 particles) in an aqueous environment inside a capillary. ZIF-8 particles led to surface-to-surface contact of the Researchers have suggested that these surfactants induce a particles,71–74 which resulted in the formation of 1D chains depletion interaction among the MOF particles. Furthermore, along the electric-field direction (Fig. 4(a) and (b)). When ZIF-8 they found that different parameters (the size and shape of the particles self-assembled in a low frequency (1 kHz) electric MOF particles, the interactions among MOF particles and the field, in which the dominant attraction mechanism is electro- substrate, and the degree of truncation of MOF particles) can static double-layer polarisation, the particle chains remained alter the dimension and organisation of the resulting MOF stable even after the electric field had been turned off. However, superstructures. Specifically, while RD-ZIF-8 (0.9 mm) particles when assembled in a high frequency (1 MHz) electric field, were found to self-assemble into quasi-3D fcc superstructures where dielectric polarisation of the crystal particle itself dom- (Fig. 6(e)), RD-ZIF-8 (2.6 mm) and TRD-ZIF-8 (1.2 mm) particles inates, the corresponding 1D superstructures were not rearranged into 1D chains (Fig. 4(h) and (i)). These results stable.72,73 Moreover, the chains also fell apart when the facets evidenced the influence of the size and shape of the MOF of ZIF-8 particles exhibited curvature.71 particles on the dimension of the final superstructure. The Another way of controlling the assembly of 1D superstruc- interaction among the MOF particles and the substrate tures is based on the use of Janus MOF particles, whose two (whether smooth or rough) was also reported to affect the opposing surface sides differ in composition. If there is an dimension of the final superstructure. Smooth substrates led attractive interaction between the two sites, then this can be to self-assembly of MIL-88A hexagonal rods into 1D chains exploited for controlled assembly of the particles into 1D (Fig. 4(j)), whereas rough substrates led to rearrangement of superstructures. Such is the case of the formation of 1D particle MOF particles into two-dimensional (2D) snowflake-like super- chains using MOF-based patchy particles (also called UiO-66- structures. Similarly, a smooth substrate led to rearrangement on-MIL-96 particles).75 These particles were synthesised by of O-UiO-66 particles into 2D hexagonal lattices, but a rough selectively growing UiO-66 on the (0002) facets of truncated substrate favoured self-assembly of a quasi-1D stripe-like hexagonal bipyramidal MIL-96 particles. This heteroepitaxial superstructure.76 growth was favoured by the close match of the (0002) planes of MIL-96 and the (111) planes of UiO-66. Then, 3-(trimethoxysi- lyl)propyl methacrylate oligomers (o-TPM) were deposited as a 5. Self-assembly of metal–organic thin liquid layer on the UiO-66 patches, whereas o-TPM deposi- framework particles into two- tion on the MIL-96 surface was completely suppressed. Conse- dimensional ordered superstructures quently, o-TPM acted as a glue between UiO-66 patches of two particles, thus maximising their contact and leading to the Initial efforts to self-assemble 2D MOF particle superstructures formation of 1D particle chains (Fig. 4(c)–(g)).75 This strategy, were focused on controlling the evaporation of a colloidal 2534 | Chem. Soc. Rev., 2023, 52, 2528–2543 This journal is © The Royal Society of Chemistry 2023 View Article Online Review Article Chem Soc Rev This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Open Access Article. Published on 17 March 2023. Downloaded on 11/2/2023 2:44:12 PM. Fig. 4 Confocal microscopy of (a), (c) and (d) 1D chains of RD-ZIF-8 particles in the direction of the electric field (1 MHz). (b) Cross-sections perpendicular to the electric field. Adapted with permission from ref. 71. Copyright 2013 American Chemical Society. (e) Schematic of the self-assembly of MOF-based patchy particles. (f and g) SEM images of chains made of self-assembled, MOF-based patchy particles. Scale bars: (f) 10 mm; (g) 2 mm. Adapted with permission from ref. 75. Copyright 2021 John Wiley and Sons. (h) Confocal microscopy of 1D chains of TRD-ZIF-8 particles. Inset is the zoomed-in image of a chain. (i) SEM image of chains made of self-assembled TRD-ZIF-8 particles. (j) Optical image of chains of MIL-88A particles formed on a smooth substrate. Inset is the zoomed-in image of the chains. Scale bars: (h) 5 mm; (h, inset) 2 mm; (i) 10 mm; (j) 5 mm; (j, insets) 1 mm. Adapted with permission from ref. 76. Copyright 2022 Springer Nature. solution containing the MOF particles. Using this approach, the Other approaches have recently been developed to improve Granick group and the Eddaoudi group simultaneously reported the 2D-ordering of MOF particles. Among the most important the feasibility of ordering MOF particles into 2D superstructures of these methods is air–liquid interface assembly (Fig. 5(a)), (usually called layers or films). Granick et al. showed that 2D which entails first spreading a colloidal solution of MOF hexagonal lattices could be assembled by slow evaporation of a particles over an air–liquid interface, and then modifying the colloidal solution of RD-ZIF-8 particles in N,N-dimethylformamide interface to drive the assembly and packing of the particles. (DMF).38 In ethylene glycol and using sodium nitrate to screen Methods to alter the interface to favour the contact, assembly electrostatic repulsions between particles, colloidal RD-ZIF-8 particles and/or packing of MOF particles include evaporation, increas- were found to assemble into linear, triangular, and U-shaped trimers, ing the surface tension of the liquid,80,81 and compressing the linear, rhombic, and square tetramers, and fcc-lattice portions.38 MOF particles floating on the liquid subphase.82 The process of Similarly, Eddaoudi et al. demonstrated that the evaporation of spreading particles over an air–liquid interface, and then colloidal solutions of cubic particles of MIII-based MOFs with compressing them into a highly condensed state, is known as square-octahedral (soc) topology (In-soc-MOF and Ga-soc-MOF) con- the Langmuir–Blodgett (LB) process (Fig. 5(d)).83 Examples of taining surfactants (e.g. PVP) led to the formation of 2D hcp and such work include the formation of 2D ordered superstructures square close-packed layers.79 Importantly, the evaporation assembly via self-assembly of monodisperse TRD-ZIF-8 particles on an is mainly based on entropic crystallisation. Since the evaporation of air–liquid interface, reported by Cohen et al.80 Core–shell ZIF-8 the solvent is entropically unfavourable, the MOF particles pursue particles (PMMA@ZIF-8) were made by growing a layer of an ordered phase to maximise entropy and therefore, to minimise PMMA on the ZIF-8 particle. A colloidal toluene solution of free energy. Further evaporation of the solvent ultimately fixes the these PMMA@ZIF-8 particles was then spread on water. After- MOF particles into ordered superstructures. wards, evaporation of toluene induced the 2D assembly of This journal is © The Royal Society of Chemistry 2023 Chem. Soc. Rev., 2023, 52, 2528–2543 | 2535 View Article Online Chem Soc Rev Review Article This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Open Access Article. Published on 17 March 2023. Downloaded on 11/2/2023 2:44:12 PM. Fig. 5 (a) Schematic of air–liquid interface assembly. (b) SEM image of a 2D monolayer of O-UiO-66 crystals transferred to a silicon platform. (c) Cross sectional SEM image of one, two, and three monolayers of O-UiO-66 crystals. Scale bar: (b) 1 mm. Adapted with permission from ref. 81. Copyright 2013 John Wiley and Sons. (d) Schematic of Langmuir–Blodgett assembly. SEM images and optical microscopic images (inset) of (e)–(g) Al12O(OH)18(- H2O)3(Al2(OH)4)(BTC)6 crystals assembled by the LB process on (e) gold substrate, (f) silicon wafer and (g) glass slide. Adapted with permission from ref. 82. Copyright Royal Society of Chemistry 2012. (h) Schematic of the assembly of colloidal MOF particles via complementary oligonucleotides. (i) Bright field (BF) cryo-STEM image of bcc lattices formed by the assembly of O-UiO-66 crystals with self-complementary DNA sticky ends. Scale bar: (i) 100 nm. Adapted with permission from ref. 90. Copyright 2020 Springer Nature. PMMA@ZIF-8 particles at the air–liquid interface. The resulting length and hydrophilicity of the graft as well as on the size of 2D particle films were free-standing, due to fusion of PMMA@- the O-UiO-66. Thus, order was lost when the grafts became ZIF-8 shells. Monodispersed O-UiO-66 particles have also been longer or strongly hydrophilic. Regarding particle size, the self-assembled into 2D ordered superstructures on an air– larger particles favoured the formation of ordered, 2D hexagonal liquid interface (Fig. 5(b) and (c)).81 First, PVP-modified UiO- lattices.84 Kitagawa, Furukawa, and co-workers assembled mono- 66 particles were spread over water. Subsequently, the surface dispersed MOF [HKUST-1, Al12O(OH)18(H2O)3(Al2(OH)4)(BTC)6, tension of water was modified by adding an aqueous solution and Cu2(BDC)2(bipy)] particles into 2D ordered superstructures of SDS, leading the UiO-66 particles to consolidate into a 2D by the LB process (Fig. 5(e)–(g)).82 A colloidal suspension of such hexagonal monolayer. The presence of disorder within this MOF particles in dry methanol was spread over water giving rise to monolayer was ascribed to the rapid packing of UiO-66 particles a temporary bilayer of alcohol and water. MOF particles were then after the addition of SDS.81 Very recently, Cohen et al. investi- assembled, by compression along the air–liquid interface, into 2D gated the effects of the molecular chains attached to O-UiO-66 ordered superstructures floating on the water subphase.82 The particles on the formation of free-standing self-assembled UiO- assembly of MOF particles was suggested to be induced by 66 monolayers.84 Firstly, they grafted O-UiO-66 particles of interparticle attractive forces and capillary forces on the water three different sizes (80 nm, 120 nm, and 250 nm) with sub-phase.85–88 The LB process has also been applied to assemble different polymers (poly-[methyl acrylate] [PMA], PMMA, and PVP-modified RD, slightly truncated cubic (TC), and TRD-ZIF-8 poly-[benzyl methacrylate] [PBnMA]) at different molecular particles into 2D monolayers. Within the monolayers, the RD weights. Subsequently, the grafted particles self-assembled at particles were oriented along the [110] axis, and the TC ZIF-8 the air–water interface. When O-UiO-66 particles were grafted particles, along the [100] axis. Contrariwise, the TRD-ZIF-8 particles with polymers of high molecular weight and density brush, did not exhibit any unique orientation within the monolayers.89 certain mechanical properties (toughness and flexibility) of the Another approach to assemble colloidal MOF particles into monolayers improved. Since greater particle size generates 2D and 3D superstructures (Fig. 5(h) and (i)) is by harnessing larger spaces among the particles, it also negatively correlates the interactions between complementary oligonucleotides.90,91 with the mechanical properties of the monolayers. The degree DNA-modified colloidal MOF particles self-assemble in a very con- of order within the monolayers was found to depend on the trolled manner, as hydrogen bonding between DNA base pairs is a 2536 | Chem. Soc. Rev., 2023, 52, 2528–2543 This journal is © The Royal Society of Chemistry 2023 View Article Online Review Article Chem Soc Rev highly specific interaction.92,93 Using this chemistry, 2D close- particles dispersed in water slowly settled by gravity in a glass packed hexagonal lattices have been assembled from PCN-222 capillary. After drying, UiO-66 particles were arranged into a nanorods functionalised with self-complementary DNA sticky ends hexagonal matrix to form compact oriented layers, which in (50 -GCGC). The use of two sets PCN-222 nanorods functionalised turn were stacked on top of each other to generate 3D-ordered This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. with complementary DNA sticky ends (50 -AAGGAA and 50 -TTCCTT) superstructures.81 In 2018, our group demonstrated that cetyl- drove the formation of 2D tetragonal lattices. Similarly, DNA- trimethylammonium bromide (CTAB)-coated MOF colloidal functionalised O-UiO-66 particles containing self-complementary particles can also self-assemble by water evaporation into DNA linkers self-assembled into 3D fcc superstructures. By exten- different 3D lattices (Fig. 6(a)–(d)).37 Using this strategy, TRD- sion, two sets of O-UiO-66 particles functionalised with complemen- ZIF-8 particles with a truncation (t) value greater than 0.66 were tary DNA sticky ends self-assembled into 3D bcc superstructures.90 assembled into the densest rhombohedral packing, whereas Open Access Article. Published on 17 March 2023. Downloaded on 11/2/2023 2:44:12 PM. In another, very recent example, TRD-ZIF-8, C-ZIF-8, O-UiO-66 those TRD-ZIF-8 particles with a lower t (o0.66) were organised and hexagonal nanorods of MIL-88B(Fe)-NH2 were self-assembled into a fcc crystal. UiO-66 particles adopted a hexagonal lattice into 2D quasi-ordered superstructures using an ice-templating closely related to the Minkowski lattice. Remarkably, the use of strategy.94 Aqueous suspensions of MOF particles were initially this approach enabled the 3D assembly of MOF particles of frozen in liquid N2 and, during this freezing stage, the formed different sizes, opening the door to create 3D photonic MOF polycrystalline ice pushed the MOF particles into the channels super-crystals when particles sized ca. 200 nm (range: 178  between adjacent ice crystals. Then, self-assembly of MOF particles 8 nm to 227  10 nm) were used. Since ZIF-8 and UiO-66 was induced by freeze-drying. Van der Waals forces held the MOF particles that had been synthesised without CTAB did not particles together after sublimation of the ice templates. Using this exhibit ordered packing, it was suggested that this ionic sur- strategy, mono- and bilayer 2D quasi-ordered superstructures could factant balanced the van der Waals attractions between the be created depending on the initial concentration of the MOF MOF particles. Therefore, ionic surfactants have been postu- colloids.94 Remarkably, these layers could be calcinated without lated to be critical to stabilising the suspension and to prevent- losing their initial 2D shape, to form mono- and bilayers of 2D ing irreversible colloidal aggregation.35,36 Thus, using this hollow carbon particles that exhibited a high oxygen-reduction drying method, different (111)-, (100)-, and (110)-oriented fcc catalytic activity in alkaline media. Given the assemblies obtained superstructures were recently assembled from TRD-ZIF-8 par- at the air–liquid interfaces,81 emulsion droplets,59 capillaries,76 and ticles (t = 0.63) by adjusting the amount of surfactant CTAB in ice-templates,94 one can conclude that these approaches performed the colloidal solution.95 The formation of less-dense packings by confining MOF particles to pre-engineered interfaces provide a was favoured by increasing the amount of surfactant. This was high level of control over self-assembly. Importantly, these meth- attributed to the ability of the surfactant to increase repulsive odologies allow for free movement of MOF particles within the interactions during self-assembly and to decrease surface ten- interface, which in turn serves to correct any defects, thereby leading sion. Interestingly, the photonic properties of superstructures to the formation of energetically favourable ordered superstructures. also were shown to depend on their growth orientation.95,96 The dimensionality of the interface usually conditions the dimen- The importance of surfactants in MOF self-assembly has sionality of the superstructures. Thus, while MOF particles generally been further demonstrated by Wang et al,76 who found that assemble into 2D superstructures when air–liquid interfaces are surfactants could induce the attractive depletion force between used, 0D superstructures are formed when emulsion droplets are MOF particles. Specifically, they observed the attractive deple- used. Interface-assisted MOF self-assembly often needs to be com- tion force as effective osmotic pressure, as two MOF particles in bined with other self-assembly approaches, such as those based on a solution containing surfactants moved closer to each other. evaporation or depletion. In these cases, both the driving force and This osmotic pressure ultimately caused the MOF particles to the interface define self-assembly. assemble. Recently, electric field followed by photopolymeriza- As mentioned in the previous section, 2D superstructures tion of trimethylolpropane triacrylate (TMPTA) also allowed the can also be prepared by destabilising the colloidal solution assembly and orientation of NU-1000, MIL-68(In) and MIL-53- through the depletion interaction inside a capillary.76 In this NH2(Al) rod-like particles into 3D MOF superstructures study, the final superstructures were found to be influenced by (Fig. 6(f)).70 In this study, Chin et al. studied the importance the degree of truncation of the MOF particles: hexagonal of the aspect ratio of these rod-like MOF particles for their self- bipyramidal MIL-96 particles (t = 0.24) rearranged into a 2D assembly, achieving well-ordered 3D MOF superstructures square lattice, whereas hexagonal bipyramidal MIL-96 particles using rods with a length-to-diameter ratio ranging from 10 (t = 0.52) self-assembled into centred rectangular lattices.76 to 1.2. 6. Self-assembly of metal–organic 7. Outlook framework particles into three- dimensional ordered superstructures Colloidal particles spontaneously arrange into ordered super- structures via self-assembly. These particles can be viewed as Self-assembly of MOF particles into 3D ordered superstructures building units for superstructures, just as atoms are for mole- was first achieved by sedimentation.81 PVP-modified O-UiO-66 cules. Today, MOFs can be considered to be a new type of This journal is © The Royal Society of Chemistry 2023 Chem. Soc. Rev., 2023, 52, 2528–2543 | 2537 View Article Online Chem Soc Rev Review Article This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Open Access Article. Published on 17 March 2023. Downloaded on 11/2/2023 2:44:12 PM. Fig. 6 (a)–(d) FE-SEM images of a 3D ordered rhombohedral superstructure made of TRD-ZIF-8 crystals (t = 0.69). Scale bars: (a) 200 mm; (a, insets) 5 mm; (b) 10 mm; (b, inset) 1 mm; (c) 2 mm; (d) 500 nm. Adapted with permission from ref. 37. Copyright 2017 Springer Nature. (e) Quasi-3D superstructures of RD-ZIF-8 particles formed on a rough substrate. (100)-, (111)-, and (110)-oriented colloidal crystals. Scale bars: (e) 5 mm for microscope images, and 2 mm for SEM images. Adapted with permission from ref. 76. Copyright 2022 Springer Nature. (f) SEM image of a 3D superstructure of MIL-68(In) particles made in a vertical E-field. Adapted with permission from ref. 70. Copyright 2022 Elsevier. colloidal polyhedral particles for the assembly of superstruc- entropic crystallisation caused by evaporation can lead to tures (Table 1). This is mainly due to the recent synthetic flocculation of MOFs. To circumvent this limitation, research- advances in making them monodisperse in size and shape ers could exploit depletion forces and/or external fields for and in conferring them with high colloidal stability. Accord- evaporation-based assembly. Well-established self-assembly ingly, we expect that further developments in the synthesis of techniques for other colloidal particles could also be extrapo- monodispersed colloidal MOF particles, encompassing new lated to colloidal MOFs. For example, the assembly of MOF MOFs and polyhedral shapes, will expand the gamut of acces- particles grafted with hydrocarbon-based polymers could be sible superstructures, thus opening the door to new packings induced by slowly increasing the polarity of the colloidal and perhaps to more complex and even counterintuitive solution.58 Thinking one step further, one could imagine assemblies. sophisticated monodisperse MOF particles with intrinsic However, accessing new superstructures will require not assembly information that spontaneously drive the formation only greater efforts in the synthesis of MOF particles, but also of superstructures, or the co-assembly of different polyhedral the improvement of their functionalisation (e.g. with oligonu- MOF particle to form multicomponent superstructures. cleotides) and the development (or adaptation) of new strate- Yaghi, Wuttke, and co-workers have proposed a new strategy gies to induce their self-assembly. For example, inefficient to self-assemble colloidal MOF particles into ordered 2538 | Chem. Soc. Rev., 2023, 52, 2528–2543 This journal is © The Royal Society of Chemistry 2023 View Article Online Review Article Chem Soc Rev superstructures.97,98 They theorised that the concepts and knowl- potential. MOF-based superstructures have been used primarily edge of reticular chemistry could be extended to structures within for photonics-related applications such as chemical sensing, the macroscopic regime, referring to this concept as ‘‘augmented thermal imaging, and anti-counterfeiting measures (Fig. 7). reticular chemistry’’. Therefore, they envisage that MOFs and MOF particles can self-assemble into photonic crystals that This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. COFs could serve as building units that would link together to reflect light at specific wavelengths and directions. The photo- form superstructures, analogously to how discrete molecular nic bandgap of MOF-based superstructures depends on the size building blocks form MOFs and COFs. In doing so, superstruc- and orientation of the assembled MOF particles, the lattice tures with complex functionality could thus be designed with the spacing, the degree of internal order of the structures, and the precision of molecular chemistry. However, augmented reticular adsorption of guest molecules.59 Photonic MOF-based super- chemistry has yet to be reported experimentally.97,98 Continued structures have been reported to detect the presence of C1–C8 Open Access Article. Published on 17 March 2023. Downloaded on 11/2/2023 2:44:12 PM. advances in synthetic techniques to control the interactions of alcohols (methanol, ethanol, isopropanol, n-butanol, etc.), and MOF particles, as well as the interactions between MOF particles of C5–C8 linear alkanes and alkenes.89 The adsorption of those and their environment, will surely yield more complex species shifts the spectral position of the photonic bandgap of superstructures. the structures, thus modifying their structural colours. Photo- Regarding the characterisation of MOF-based superstruc- nic superstructures have also been applied to local thermal and tures, the recent use of in situ imaging techniques, including photothermal sensing. Specifically, MOF-based photonic supra- confocal microscopy and in situ atomic force microscopy (AFM), particles have been found to act as temperature probes in the enables real-time visualisation of self-assembly processes and presence of alcohol vapour.60 The application range of photonic consequently, exploration of the kinetics of self-assembly. MOF-based superstructures has recently been extended to Similarly, in situ small-angle X-ray scattering (SAXS) provides anticounterfeiting. The information can be encrypted by play- real-time information about the spatial arrangement of MOF ing with the reflection signals of different superstructures.99 particles. Therefore, conventional techniques such as scanning Interestingly, birefringent 2D superstructures have been electron microscopy (SEM), transmission electron microscopy reported. These include 2D superstructures made of dye- (TEM), scanning transmission electron microscopy (STEM), adsorbed MOF particles of coordinated orientation, AFM, and SAXS all complement in situ techniques well. which exhibit anisotropic fluorescence, which may eventually Although the applications of superstructures made from expand the development of advanced materials for sensing, colloidal MOF particles remain in their infancy, they hold great optics, or photonics.76 MOF-based superstructures have also Fig. 7 Schematic illustration of photonics-related applications of MOF-based superstructures. (a) Photographs of 3D ordered superstructures made of TRD ZIF-8 particles of different sizes (178  8 nm (blue); 193  8 nm (green); 210  10 nm (yellow); and 227  10 nm (red)). Adapted with permission from ref. 37. Copyright 2017 Springer Nature. (b) Reflectance spectra of monolayers made of RD ZIF-8 as a function of exposure to 1-heptane vapour (up) and after exposure to simulated gasoline vapours with varying octane numbers (bottom). Adapted with permission from ref. 89. Copyright 2021 The Royal Society of Chemistry. (c) Information encryption by combining MOF-based superstructures with different lattices and thus photonic properties. Adapted with permission from ref. 99. Copyright 2021 John Wiley and Sons. (d) Hyperspectral and optical microscopy image of laser-irradiated ZIF-8 photonic balls (up) and its corresponding 2D (middle) and 3D (bottom) thermal map. Adapted with permission from ref. 60. Copyright 2021 John Wiley and Sons. This journal is © The Royal Society of Chemistry 2023 Chem. Soc. Rev., 2023, 52, 2528–2543 | 2539 View Article Online Chem Soc Rev Review Article been harnessed for photocatalysis.90 Specifically, 2D hexagonal post-synthetic transformations of these assemblies have not yet lattices made of PCN-222 nanorods have shown photocatalytic been reported, although similar chemistry has been done on activity towards the selective partial oxidation of 2-chloroethyl homologous superstructures prepared by self-assembly of non- ethyl sulphide to the non-toxic compound 2-chloroethyl ethyl MOF building blocks.58 Regardless, we are confident that the field This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. sulfoxide. These superstructures exhibited higher catalytic con- of MOF self-assembly will continue to garner attention from the version efficiency compared to bulk PCN-222 crystals, which research community and is poised for growth and exciting could be attributed to the higher photosensitisation efficiency advances. associated with the superstructures.90 Moreover, self- assembled MOF particles may benefit the countless applica- tions for which MOFs offer promise, such as adsorption, Abbreviations separation, catalysis, and drug delivery.100–109 Open Access Article. Published on 17 March 2023. Downloaded on 11/2/2023 2:44:12 PM. PMMA Poly-(methyl methacrylate) The practical exploitation of MOF-based superstructures is fcc Face-centred cubic limited by their chemical stability and susceptibility to mechan- hcp Hexagonal close-packed ical stress. The stability of MOF-based superstructures has not rhcp Random hexagonal close-packed yet been extensively studied; therefore, the current data set is 3D Three-dimensional insufficient. However, the low mechanical stabilities of these bcc Body-centred cubic superstructures is undeniable. Importantly, the molecular MOF Metal–organic framework chains, which tether MOF particles and stabilise sterics, estab- 0D Zero-dimensional lish relatively weak interactions among neighbouring MOF C-Fe-soc-MOF particles. Logically, these weak interactions have a detrimental Cubic Fe-soc-MOF effect on the mechanical performance of MOF-based super- C-ZIF-8 Cubic ZIF-8 structures. Accordingly, a self-assembly strategy based on aug- RD-ZIF-8 Rhombic dodecahedral ZIF-8 mented reticular chemistry would ensure the stability of the TRD-ZIF-8 superstructures, since this theoretical approach is based on the Truncated rhombic dodecahedral ZIF-8 strong covalent bonding of the frameworks.97,98 As Cohen et al. O-UiO-66 revealed, the stability of superstructures can also be improved Octahedral UiO-66 by grafting the particles with a polymer of high molecular PVP Polyvinylpyrrolidone weight and density brush.84 A similar approach was also used 1D One-dimensional by Chin et al., who photopolymerized TMPTA to fix the oriented o-TPM 3-(Trimethoxysilyl)propyl methacrylate MOF particles embedded within the photopolymer. However, to CTAC Cetyltrimethylammonium chloride date, this approach could suffer from a trade-off between increasing 2D Two-dimensional mechanical stability and decreasing superstructure-order or inher- DMF N,N-Dimethylformamide ent porosity of the embedded MOF particles. Thus, further studies LB Langmuir–Blodgett on the mechanical behaviour of MOF-based superstructures— SDS Sodium dodecyl sulphate including measurements of hardness, elastic modulus, shear mod- PMA Poly-(methyl acrylate) ulus, and yield strength—are needed to comprehensively under- PBnMA Poly-(benzyl methacrylate) stand the variables that influence their stability. TC-ZIF-8 Truncated cubic ZIF-8 The field of MOF self-assembly presents many opportunities CTAB Cetyltrimethylammonium bromide and challenges. For example, controlling the self-assembly of t Truncation MOF particles remains a very challenging task. We believe that SEM Scanning electron microscopy computational modelling may help with controlling the self- TEM Scanning electron microscopy assembly process of MOF particles, guiding experimental synthesis STEM Scanning transmission electron microscopy efforts, and ultimately, designing new superstructures.110 How- AFM Atomic force microscopy ever, we recognise that this has not yet been reported in the SAXS Small-angle X-ray scattering literature. Therefore, we expect that theoretical predictions will TMPTA Trimethylolpropane triacrylate accelerate advances in MOF particle self-assembly. Future work should aim to expand the set of MOF-based superstructures. Indeed, MOF particle self-assembly is restrained to highly sym- Conflicts of interest metric structures such as microspheres, linear chains, periodic lattices, and periodic 3D structures.37,59,71,79 The ability to design There are no conflicts to declare. superstructures with dynamic behaviour that could rival those found in natural materials (e.g. muscle, wood, or cytoskeletons) Acknowledgements perhaps remains a far-off goal. Research could also focus on expanding MOF-based superstructures through post-synthetic This work was supported by grant ref. PID2021-123265NB-I00 transformations, including calcination processes, to obtain super- funded by MCIN/AEI/10.13039/501100011033/and by ‘‘ERDF A structures with otherwise inaccessible properties.94 Furthermore, way of making Europe’’. It was also funded by the Catalan 2540 | Chem. Soc. 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