This is the accepted version of the article: Troyano J., Carné-Sánchez A., Avci C., Imaz I., Maspoch D.. Colloidal metal-organic framework particles: The pioneering case of ZIF-8. Chemical Society Reviews, (2019). 48. : 5534 - . 10.1039/c9cs00472f. Available at: https://dx.doi.org/10.1039/c9cs00472f Please do not adjust margins ARTICLE Colloidal Metal-Organic Framework Particles: the Pioneering Case of ZIF-8 Received 00th January 20xx, Javier Troyano,a Arnau Carné-Sánchez,a Civan Avci,a Inhar Imaza and Daniel Maspoch*a,b Accepted 00th January 20xx The production of metal–organic frameworks (MOFs) in the form of colloids have brought a paradigm shift in the design of DOI: 10.1039/x0xx00000x new functional porous materials. Along with their intrinsic interest as porous solids, and contrary to their bulk powders counterparts, colloidal MOF particles can additionally be dispersed, shaped, functionalized, transformed and assembled in a controlled manner, conferring them further properties and applications. In this regard, zeolitic imidazolate framework-8 (ZIF-8) has become a pioneering MOF constituent of colloidal science. Today, the understanding of the role of synthetic parameters, learned after one decade of research, enables the production of monodisperse colloidal ZIF-8 particles with tunable dimensions and morphologies, offering the opportunity to develop new functional materials and composites with novel and promising functionalities.This tutorial review provides a useful guide to prepare ZIF-8 in its colloidal form, covering the published studies on the synthesis of homogeneous ZIF-8 particles with controlled size and shape. In addition, we present the most relevant advances in the development of colloidal ZIF-8 hybrid single-particles, reflecting the great potential and rapid development of this interdisciplinary research field. Finally, we highlight how formulation of ZIF-8 as colloids has led to the emergence of novel physicochemical phenomena that are useful for practical applications. This review aims promoting the development of MOFs as colloids, taking ZIF-8 as a pioneering and successful case that clearly shows the benefits of bridging MOF chemistry and colloidal science. and shape opens doors to materials with new functionalities Introduction and applications. A noteworthy example is the fabrication of metal-organic framework (MOF) particles in the colloidal size The term colloid, derived from the Greek words kolla (glue) and range. eidos (like), was first introduced in 1861 by Thomas Graham, MOFs are crystalline porous compounds based on metal ions or who referred to substances that, when in solution, are unable clusters connected by organic ligands, which can be designed to diffuse across a permeable membrane.1 Graham attributed Field Code Changed into many different architectures and tailored for specific uses this behavior to the relatively larger size of the particles, which (e.g. adsorption, separation, catalysis, etc.) by adjusting their ranges from a few nanometers up to several microns. Thus, the topology, pore size and/or chemical composition. Over the past dispersed particles in a colloid are larger than those in a true decade, growing interest in these materials has led to the solution, yet small enough such that buoyancy and gravity are discovery of useful synthetic tools that, beyond structural and balanced, preventing the suspended particles from sinking or compositional control, enable the production of MOF crystalline floating. Accordingly, colloids occupy an intermediate place particles of uniform sizes and morphologies.3-6 This constitutes Field Code Changed between solutions and suspensions, making them highly a challenging but promising strategy to confer MOFs with amenable to specific applications. For instance, solid particles colloidal properties7 and therefore, with new functionalities for Field Code Changed that form a colloidal system can flow like a liquid and therefore, new applications. can be used in paints, inks, coatings, cosmetics, diagnostics or In the above context, zeolitic imidazolate framework- 8 (ZIF-8) for drug delivery.2 Colloidal particles that are uniform in both Field Code Changed epitomizes the successful combination of MOFs and colloidal size and shape can offer an additional level of functionality. For science. Zeolitic imidazolate frameworks are a subfamily of example, they can spontaneously self-assemble to form well- MOFs that present a crystalline topology like that of zeolites, defined micro- or nano-hierarchical superstructures that exhibit characterized by large cavities connected through small enhanced or even novel properties. Therefore, the windows (Fig. 1). Owing to the pioneering ZIF syntheses by miniaturization of any substance, regardless of its chemical Huang et. al and by Park et. al. in the early 2000s, researchers composition, to form colloidal particles of homogeneous size have expanded the ZIF family, which currently encompasses more than 150 reported structures.8,9 Among them, ZIF-8 [Zn(2- Field Code Changed a. mim)2] (where 2-mim is 2-methylimidazole) has been widely Field Code Changed Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and Barcelona Institute of Science and Technology, Campus UAB, Bellaterra 08193, Barcelona, studied due to its large pore volume and surface area (> 1600 Formatted: Font: Not Italic Spain m2/g), high thermal and chemical stabilities, and ready b. ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Spain synthesis: it can be prepared by simply mixing solutions of Zn(II) Please do not adjust margins Please do not adjust margins ARTICLE Journal Name initial colloids. This combination of excess ligand, methanol as solvent, and room temperature has since formed the basis for most syntheses of nanoscale ZIF-8 crystals. Additional reaction parameters (e.g. metal source, reaction time, molar ratio, reagent concentration and additive) can then be tuned to control the size and homogeneity of the desired colloidal ZIF-8 nanocrystals (Fig. 2). We discuss these parameters below. Schejn et al. studied the influence of the zinc salt used in the synthesis of ZIF-8.11 They observed that more-reactive metal Field Code Changed Fig. 1. a) Representation of the crystal structure of ZIF-8, revealing its cavity. b) precursors led to faster nucleation and consequently, to small Framework structure of ZIF-8, highlighting the [110] and [100] planes. c) Crystal structure of ZIF-8 along the [111] direction. ZIF-8 particles: specifically, Zn(acac)2 gave nanocrystals whose average size was 45 nm; Zn(NO3)2, 141 nm; ZnSO4, 211 nm; and Zn(ClO4)2, 224 nm. Contrariwise, less-reactive salts such as and 2-mim. Although initial research focused on the synthesis Zn(CH3COO)2, ZnCl2, and ZnI2 yielded crystals with sizes varying of ZIF-8 crystals in the micrometer-to-millimeter scale for between 300 nm and 500 nm, and when the poorly reactive structural elucidation, the first synthesis of nanoscale ZIF-8, ZnBr2 was used, the crystals were even larger (ca. 1 μm). reported by Cravillon et al. in 2009, paved the way to colloidal The relative and total concentrations of reagents strongly ZIF-8.10 Synthesis of ZIF-8 has since rapidly evolved to enable influence the size of the colloidal ZIF-8 crystals. For instance, Field Code Changed production of colloidal crystals under a high degree of control Zhang et al. were able to synthesize smaller ZIF-8 particles in over size and morphology. Consequently, colloidal ZIF-8 has methanol by increasing the Zn(NO3)2/2-mim/MeOH molar ratio been explored as a constituent in myriad new materials that from 1:4:500 to 1:16:500 (stirring, room temperature, 1 hour).12 Field Code Changed combine the porosity-derived properties of MOFs with the The authors reported that the average particle size gradually advantages of colloids. Such work has illustrated how, for a decreased from 1 µm to 100 nm; however, an excess of 2-mim given material, greater synthetic control can enable a wider (Zn/2-mim = 1:16) resulted in aggregation of the ZIF-8 range of applications. In this sense, ZIF-8 has not only become a nanocrystals. Furthermore, Yeung et al. recently reported that new constituent of colloidal science but has also expanded the for ZIF-8, increasing the reagent concentration led to a decrease boundaries of crystal engineering. For example, the design of in crystallization rate and consequently, to larger particles.13 For Field Code Changed single-particle composites made of a diverse array of example, by using a Zn(NO3)2/2-mim/MeOH ratio of 1:4:2500, nanoparticles and biomolecules has opened new possibilities in ZIF-8 crystals of 41 ± 19 nm were obtained, whereas larger fields such as photonics, biomedicine and catalysis. However, particles with a broader size distribution (717 ± 267 nm) were full exploitation of the marriage of MOFs and colloidal science obtained using more concentrated solution with a Zn(NO3)2/2- demands further research. mim/MeOH ratio of 1:4:250 (stirring, room temperature, 2 In this review, we cover research on ZIF-8 in colloidal form, hours). They explained this phenomenon by assuming that highlighting important lessons that will apply to future work on there had been a pre-equilibrium of metastable intermediate colloidal ZIFs, MOFs, and molecular crystals in general. Firstly, clusters with different Zn/2-mim ratios. In this scenario, we discuss recent progresses in the synthesis of colloidal ZIF-8. increasing the concentration would inhibit nucleation by Next, we address how the combination of colloidal ZIF-8 crystals favoring over-coordinated species, generating larger ZIF-8 with other materials can yield new hybrid single-particles with particles. new functionalities. Finally, we highlight the new Reaction time is another critical parameter for controlling the physicochemical properties that arise from the controlled size and monodispersity of colloidal ZIF-8 crystals. Venna et al. formulation of colloidal ZIF-8 crystals as well as their influence (2010) studied the change in size of ZIF-8 particles synthesized on the use of colloidal ZIF-8 for practical applications. over time upon mixing solutions of Zn(NO3)2 and 2-mim in MeOH (1:8:705) under stirring at room temperature.17 The Field Code Changed authors observed that after an initial incubation period (t < 10 Colloidal ZIF-8 crystals: size control minutes), a growth stage was reached during which the size of In the synthesis of ZIF-8, as in every solution-phase ZIF-8 increased, evolving from ~50 nm spherical particles (t = 10 crystallization, particle size is determined by the nucleation and minutes) up to 230 nm ± 40 nm (t = 30 minutes). Then, the size growth rates. A higher nucleation rate corresponds to smaller remained constant and more-homogenous faceted crystals particles, as the growth slows very early on, eventually stopping were formed (230 ± 20 nm). At longer reaction times (t = 12 due to a lack of nutrients. Consequently, formation of colloidal hours), larger (500 nm ± 40 nm) crystals were produced, ZIF-8 nanocrystals will only succeed if the synthetic conditions showing a narrower size distribution at 24 hours. In 2018, favor nucleation over growth. In 2009, Cravillon et al. achieved Chocarro-Ruiz et al. proved that using a shorter reaction time the first synthesis of colloidal ZIF-8 (average particle size: ~45 yields smaller ZIF-8 particles.18 They reduced the average size of Field Code Changed nm),10 by simply stirring a mixture of Zn(NO 3)2 and 2-mim in monodisperse ZIF-8 nanocrystals down to 32 ± 5 nm by Field Code Changed methanol (molar ratio Zn(NO3)2/2-mim/MeOH = 1:8:1000) at following a conventional synthetic procedure (Zn(NO3)2/2- room temperature for 1 hour. They found that excess ligand mim/MeOH= 1:8:1000, room temperature) but shortening the was essential for promoting faster nucleation and stabilizing the total reaction time to 7 minutes. After initial homogenization of 2 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins Journal Name ARTICLE Fig. 2. Schematic of size control over ZIF-8 crystals (top). Field-emission scanning electron microscopy (FESEM) images of colloidal ZIF-8 particles (bottom) of 3 µm, 1 µm, 319 nm, 147 nm and 38 nm in size (from left to right).14-16 Field Code Changed the starting solutions, they left the mixture undisturbed. This decrease the number of nucleation points without suppressing Commented [C1]: I think this figure 2 can be reinforced by adding, effect was also observed by Cravillon et al.,14 who obtained a crystal growth. By using both 1-methylimidazole and PVP, they below the SEM line, some arrows with labels “Concentration of narrow size distribution of ZIF-8 crystals only when they did not could synthesize highly monodisperse ZIF-8 crystals of ~5 μm in Reagents”, “Metal to Ligand Ratio”, “Water-Methanol”, “Basicity” “Reactivity of Zn salt” and etc, concordant with their effect on the stir the reagent solutions ; they suggested that the size. size. This could help the people to follow very easily what we say in polydispersity associated with stirring may have resulted from Although most work on the synthesis of colloidal ZIF-8 has been general in this size control section. secondary nucleation caused by turbulence. done in methanol, much effort has been devoted to using water Field Code Changed In the context of MOF synthesis, one of the most common as solvent, both for the inherent advantages of aqueous strategies to achieve well-defined size crystals is to incorporate chemistry (e.g. safety, biological compatibility and low cost) as additives, also known as modulators, that can affect the well as for the possibility that water could enable better control nucleation and growth rates.19,20 Modulators act as capping over the size and homogeneity of nanoscale ZIF-8 crystals. For Field Code Changed ligands that compete with the organic linker, reducing the example, using water as the solvent facilitates the use of Field Code Changed number of nucleation points (thus, increasing the crystal size) surfactants, which have been demonstrated to help control and, conversely, limiting the crystal growth (thus, decreasing colloidal ZIF-8 (vide infra). However, aqueous synthesis of ZIF-8 the crystal size). Given these opposing effects, the net effect of is not trivial, as it can generate by-products or amorphous a modulator depends on its coordination strength to the metal, compounds. Kida et al. reported a study on the influence of its concentration and its basicity. In 2011, Cravillon et al. reaction parameters such as metal/ligand ratio, concentration evaluated the effects of different modulators (formate, 1- and reaction time on the size of ZIF-8 particles synthesized in methylimidazole and n-butylamine) in the synthesis of ZIF-8 in water.22 They observed similar trends as those observed for Field Code Changed methanol,14 using a Zn(NO3)2/2-mim/L/MeOH (L = modulator) methanol. For instance, when they increased the Zn(NO3)2/2- Field Code Changed molar ratio of 1:4:4:1000 and a reaction time of 24 hours at mim ratio from 1:40 to 1:100, the average crystal size decreased room temperature. The authors observed that in the presence from 2 µm to 250 nm and the size distribution narrowed. The of formate or 1-methylimidazole, monodisperse microcrystals authors also investigated the total concentration of reagents (1 µm to 3 µm) were formed (Fig. 2). However, when n- and found that high concentrations (Zn(NO3)2/2-mim/H2O = butylamine was employed, the average size was 18 nm. 1:20:895) were required to form crystalline ZIF-8, but that Moreover, they demonstrated that by varying the compositions higher concentrations led to aggregation of smaller particles. of the reagent solutions with n-butylamine, they could tune the Finally, upon exploring reaction times ranging from 5 minutes nanocrystal size between 9 nm and 65 nm. They attributed this to 24 hours, they observed that ZIF-8 crystals exhibited a similar behavior to the higher basicity of n-butylamine, which can size of ~500 nm. deprotonate the intermediate Zn:2-mim species to accelerate the nucleation rate. Contrariwise, formate and 1- methylimidazole, which are less basic, cannot deprotonate the Colloidal ZIF-8 crystals: shape control intermediate, and instead, compete with 2-mim, thus leading to Morphologically, ZIF-8 crystals are characterized as being a low nucleation rate and consequently, to larger crystals. rhombic dodecahedral (RD), truncated rhombic dodecahedral Furthermore, in 2013, Yanai et al. found that using 1- (TRD) or, to a lesser extent, cubic, as crystal growth begins with methylimidazole as modulator, the crystal size decreases with formation of an all [100]-oriented cube that gradually becomes increasing concentrations greater than 100 mM, due to TRD and finally, RD (Fig. 3).23 The final RD shape of ZIF-8 results Field Code Changed suppression of both nucleation and crystal growth.21 Thus, to from Wulff’s rule, whereby the slowest growing direction (for Field Code Changed produce larger monodisperse ZIF-8 crystals, the authors ZIF-8, [110]) determines the final shape and the most explored the use of a relatively weaker ligand, thermodynamically stable facet orientation. Here, RD refers to polyvinylpyrrolidone (PVP), which ultimately helped to further This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 3 Please do not adjust margins Please do not adjust margins ARTICLE Journal Name a polyhedral shape with twelve rhombus-shaped facets, the concentration up to 0.1 mg/mL afforded cubic crystals. whereas the term truncated is used to indicate polyhedrons Interestingly, the use of analogous surfactants having a similar with cut vertices. For ZIF-8 having SOD topology, the RD shape hydrocarbon tail (e.g. cetyltrimethylammonium chloride (CTAC) comprises twelve equivalent rhomboid (or tetragonal) facets and trimethylstearylammonium chloride (STAC)) enabled a that are all [110]-oriented. Alternatively, the TRD shape is an RD similar level of morphologic control. However, this strategy with square cuts (truncations) in the six [100] vertices. This TRD could not be used for the synthesis of ZIF-8 in methanol, as shape comprises twelve irregular hexagonal facets that are surfactants do not efficiently attach onto the facets of ZIF-8 [110]-oriented and six square facets that are [100]-oriented. particles in methanol. In 2015, Tanaka et al. reported the Commented [C3]: Although there is no clear proof, this is because Finally, the cubic shape is a cube with six facets that are all [100] synthesis of homogeneous cubic ZIF-8 crystals based on stirring of the higher solubility of CTAB in methanol. oriented. Overall, the cubic and TRD shapes of ZIF-8 can be an aqueous solution of Zn(CH3COO)2, 2-mim and CTAC (Zn/2- considered to be the result of “unfinished” crystal-growth, as mim/H2O = 1:60:2220) with a low concentration of CTAC (0.064 nicely demonstrated by Cravillon et al.14 mg/mL).29 In 2018, our group refined the above synthetic Field Code Changed Given the aforementioned crystal-growth process, controlled protocol to control the truncation degree of highly Field Code Changed synthesis of homogeneous RD, TRD or cubic ZIF-8 crystals monodisperse TRD ZIF-8 crystals (polydispersity: < 5 %).15 We Field Code Changed requires selective growth of specific crystalline facets. A left an aqueous solution of Zn(CH3COO)2, 2-mim and CTAB common approach for achieving such control in nanomaterial (Zn/2-mim/CTAB/H2O = 1:20:4:400) undisturbed at room synthesis is the use of surfactants, which can preferentially get temperature for 2 hours. Following this protocol, we could adsorbed onto certain crystalline facets to act as growth modulate the truncation of TRD ZIF-8 crystals by slightly varying inhibitors or capping agents.27 Additionally, surfactants the 2-mim/H2O molar ratio and the amount of CTAB. Overall, Field Code Changed adsorbed onto the surface of nanoparticles also protect them lower 2-mim/H2O molar ratios and higher concentrations of from aggregation, thus facilitating the formation of stable CTAB afforded greater truncation. We also found that TRD ZIF- colloids. In 2011, Pan et al. were the first to demonstrate that 8 particles with lower truncations (close to the RD shape) could surfactants could also be used to control the preferential be prepared without the use of CTAB, if the incubation times growth of specific crystalline facets of MOFs (in this case, ZIF- were kept short. For example, TRD ZIF-8 crystals with a 8).28 They used cetyltrimethylammonium bromide (CTAB) as truncation level of 0.38 were synthesized by simply incubating Field Code Changed capping agent to isolate pure cubic or TRD ZIF-8 crystals. They an aqueous solution of Zn(CH3COO)2 and 2-mim (Zn/2-mim/H2O heated an aqueous solution of Zn(NO3)2, 2-mim (Zn/2-mim/H2O = 1:13:460) for 15 minutes at room temperature. Note that = 1:55:4440) and CTAB at 120 ºC for 6 hours. They found that when we incubated the solution for longer (2 hours), we CTAB molecules preferentially attach onto the energetically- obtained homogeneous RD ZIF-8 crystals, as we had expected. favorable [100] facets, owing to their hydrophobic tail. The importance of the concentration of CTAB in shaping Consequently, when CTAB was used, crystal growth in the [100] colloidal ZIF-8 particles was also reported by Yang et al. in 2018 direction drastically slowed down, enabling the authors to (Fig. 4).30 Briefly, these authors expanded the available Field Code Changed isolate homogeneous cubic or TRD crystals before the RD shape morphologies of ZIF-8 particles to octagonal plates, inter- had been reached. Consistent with their hypothesis, the authors penetration twins and nanorods by further tuning the showed that the use of a relatively low concentration of CTAB concentration of CTAB in a synthesis that comprised heating an Commented [C2]: In fig.3, the 3rd shape (TRD) can be oriented in (0.025 mg/mL) provided TRD ZIF-8 crystals, whereas increasing aqueous solution of Zn(CH3COO)2, 2-mim and CTAB (Zn/2- the same way as the 2nd shape. It is a bit confusing. It’s also not mim/H2O = 1:30:1800) at 120 ºC for 24 hours. The ZIF-8 oriented with the 4th shape. morphology depended on the CTAB concentration: 0.16 wt % gave octagonal plates; 0.24 wt %, interpenetration twins; and 0.35 wt %, anisotropic nanorods. The authors highlighted the 2- mim/H2O molar ratio as another tunable parameter for obtaining well-defined shapes. The fact that surfactants such as CTAB can preferentially attach Field Code Changed onto [100] facets to influence ZIF-8 crystal-growth suggested Commented [C4]: In Fig 4, you kind of want to know how the that if surfactants which preferentially attach onto other facets morphology is changing even though the same surfactant is used. were discovered, they could provide access to other Something to imply that the surfactant concentration is effecting the morphologies. Along these lines, Hong et al. discovered that size can be added. tris(hydroxymetyl)aminomethane (TRIS) preferentially Or, the figure can be re-organized simply as 3 groups of shapes stabilizes the [111] facets. They synthesized ZIF-8 at room obtained by i)CTAB ii)TRIS iii) CTAB+TRIS, instead of having multiple temperature using a similar procedure to one previously arrows labelled “CTAB”, “TRIS” or “CTAB+TRIS” for each shape. described,15 expect except that they replaced CTAB with TRIS, obtaining octahedral, flake-shaped or flower-like ZIF-8 Also, I believe that the cubes and at least one kind of TRD should also be added as the figure looks like a “bank of shapes” you can superstructures depending on the TRIS concentration. Fig. 3. Schematic of the morphologic evolution of ZIF-8 crystals, from cubic to obtain with surfactants. truncated dodecahedra (TRD) to rhombic dodecahedra (RD) (top). FESEM images Remarkably, when they used a combination of CTAB and TRIS, of colloidal ZIF-8 crystals of cubic, TRD or RD morphology (middle and bottom). they obtained hexapod-like and burr puzzle-like ZIF-8 Field Code Changed Scale bars: 250 nm (middle) and 1 µm (bottom). 15,24-26 superstructures (Fig. 4).31 Field Code Changed Field Code Changed 4 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins Journal Name ARTICLE In addition to the aforementioned bottom-up strategies to to a XO solution at pH = 3.5, we obtained tetrahedral ZIF-8 control the shape of ZIF-8 particles are, there are also top-down particles, which we attributed to faster etching along the [100] strategies, whereby the morphology of ZIF-8 particles is altered and [211] directions (the vertices and edges of the RD, post-synthetically (Fig. 5). For example, some unprecedentedly respectively) relative to the [110] direction (the facets of the shaped, hierarchically porous or simply hollow ZIF-8 crystals RD). Remarkably, by decreasing the pH to 2.5, we were able to have been obtained using methods such as wet-chemical slow down the etching along the [100] direction, which in turn etching and sacrificial templating (Fig. 5, top). Such top-down provided hollow ZIF-8 boxes.32 Alternatively, Hu et al. showed Field Code Changed shaping approaches require a mechanism that selectively and that etching of the internal core of ZIF-8 particles to form hollow controllably sculpts the pre-synthesized ZIF-8 crystals. In 2015, ZIF-8 particles could be achieved using tannic and gallic acids.33 Field Code Changed our group proved that anisotropic wet-chemical etching could In their proposed mechanism, the protons generated by the be applied to ZIF-8 particles to re-shape them into cubic, tannic/gallic acid protonated the 2-mim, cause the disassembly tetrahedral or hollow-box-shaped colloidal ZIF-8 crystals, of the framework, and then dissolve the core of the ZIF-8 similarly to what had previously been described for inorganic particles. The authors reasoned that due to the large molecular nanoparticles. Thus, we incubated ZIF-8 particles into an size of these acids, they blocked the pores of the external acidified/basified xylenol orange (XO) solution that surface of the ZIF-8 particles, thereby providing surface Commented [C5]: For zif-8 it is only acidified. simultaneously acts as etchant and chelating agent.26 The XO protection against etching. Field Code Changed solution sequentially protonated the 2-mim ligands, breaking Lee et al. obtained spherical hollow ZIF-8 particles by a the Zn(II)-2-mim coordination bonds and sequestering the template-assisted method using polystyrene spheres as liberated Zn(II)/Co(II) ions to form water soluble metal-XO sacrificial template.36 Firstly, they added carboxylate- Field Code Changed complexes. We found that TRD particles could be etched into terminated polystyrene spheres to a solution of Zn(NO 3)2 and 2- cubic particles, as the etching rate in the [100] direction was mim in methanol. With this incubation, the crystallization of ZIF- faster than in the [110] direction; somehow reverting the crystal 8 occurred on the surface of the polystyrene spheres, yielding Commented [C6]: Can be replaced with ”faster than in any other growth described above. When we exposed RD ZIF-8 particles spherical core-shell polystyrene@ZIF-8 particles. Then, they direction” Fig. 4. Schematic showing the influence of different surfactants on the crystal morphology of colloidal ZIF-8, with the corresponding FESEM images for each of the discovered Field Code Changed morphologies. Scale bar (all micrographs): 1 µm. 30,31 Field Code Changed This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 5 Please do not adjust margins Please do not adjust margins ARTICLE Journal Name treated the polystyrene@ZIF-8 particles with DMF, which ZIF particles from pre-synthesized RD ZIF-8 particles.38,39 Field Code Changed selectively dissolved the polystyrene polymer to afford hollow Another self-sacrificial templating strategy, based on the Field Code Changed spherical ZIF-8 particles. Seeking to increase the complexity of different stability of isostructural ZIF-8 and ZIF-67, has been ZIF-8 particles, Shen et al. recently developed another template developed by Yang et al. and Chen et al. The researchers first strategy involving polystyrene spheres. They synthesized ZIF-8 prepared composite particles containing ZIF-67 and ZIF-8, and crystals (size: 1 µm to 4 µm) with ordered internal macropores then placed the resultant core-shell ZIF-67@ZIF-8 particles in by soaking a solution of ZIF-8 precursors in an ordered hot methanol to sacrifice the less stable ZIF-67 portion, which monolithic superstructure comprising polystyrene spheres. 34 afforded newly shaped pure ZIF-8 particles.40, 35 Field Code Changed During incubation, they added a mixture of ammonia/methanol Field Code Changed to induce in situ crystallization of ZIF-8 inside the Single-particle ZIF-8 composites Field Code Changed superstructure. Interestingly, instead of obtaining the continuous monolithic polystyrene@ZIF-8 superstructure that Combining colloidal ZIF-8 crystals with other materials yields they expected, they observed formation of some polyhedral functional composites that can exhibit new properties. Given Commented [C7]: This was my personal comment in my thesis, crystals with exposed [111], [100] and [110] facets. Finally, they they don’t say something like this in their paper. that the highly stable microporous structure of ZIF-8 can be exposed these crystals to THF to remove the polystyrene easily generated under mild conditions in a controlled manner, template, which yielded hierarchically porous ZIF-8 particles single-particle ZIF-8 composites can be readily prepared. The with ordered macropores (pore size: ~270 nm) (Fig. 5, middle). ability to form ZIF-8 crystals of uniform size and shape offers the Among the various template-assisted methods to shape ZIF-8 possibility to impart the colloidal nature of ZIF-8 to functional particles, several self-sacrificial template methods have been single-particle ZIF-8 composites. The reported methodologies developed (Fig. 5, bottom). For example, Chou et al. produced for constructing single-particle ZIF-8-based hybrid materials can hollow ZIF-8 particles via “double-solvent mediated generally be divided into two categories: encapsulation of guest overgrowth”. In this strategy, pristine ZIF-8 particles (average materials during in situ formation of ZIF-8 colloidal crystals size: 200 nm) produced in water were used as seeds to grow (bottom-up); and post-synthetic hybridization of pre- another layer of ZIF-8 on top of them in methanol, leading to a synthesized colloidal ZIF-8 with functional materials (post- final size of 450 nm.37 They realized that the inner water-loaded synthetic). Field Code Changed ZIF-8 cores became disassociated and dissolved after ageing in methanol, ultimately leaving behind well-shaped hollow ZIF-8 Bottom-up approaches for single-particle ZIF-8 composites particles. They proposed that the pH of the internal water The high degree of control over the synthesis of ZIF-8 crystals environment drops due to deprotonation of the 2-mim in the enables localization of nucleation and subsequent growth external methanol environment during formation of the ZIF-8 around functional guests, creating composites unattainable by shell. Alternatively, Zhang et al. used a similar tannic acid-based other techniques such as guest infiltration, surface adsorption approach to construct yolk-shelled ZIF-8 and bimetallic Zn/Co- or covalent linkage. This in situ (or de novo) encapsulation strategy has emerged as a simple, fast and versatile method whose advantages include high loading efficiency, low leaching, and unlimited cargo size. Interestingly, the possibility of encapsulating large cargo molecules (i.e. larger than the host pores) in a host of controlled size and shape that exhibits exceptional stability and negligible cytotoxicity, has positioned ZIF-8 as among the most attractive porous materials for colloidal bio-composites. The first example of the in situ encapsulation of functional guest into ZIF-8 crystals was reported in 2012 by Lu et al.,41 who developed a strategy based Field Code Changed on functionalization of nanoparticle surfaces with PVP for controlled encapsulation of various inorganic nanoparticles of different sizes, shapes and compositions. They successfully encapsulated Pt, CdTe and Fe3O4 nanoparticles (NPs), lanthanide-doped NaYF4 nanoparticles, Ag cubes, polystyrene spheres, and lanthanide-doped NaYF4 or β-FeOOH rods. Formation of these NP@ZIF-8 composites was based on continuous adsorption of the functionalized nanoparticles onto the surface of the ZIF-8 crystals in formation. This was achieved Fig. 5. Illustration of the top-down strategies to post-synthetically tune the by mixing methanolic solutions of Zn(NO3)2 and 2-mim with a morphology of RD colloidal ZIF-8 crystals (left), and the corresponding FESEM images of the resulting ZIF-8 crystals for each strategy (right). Scale bar (all suspension of the desired PVP-functionalized NPs, also in micrographs): 500 nm.26,34,35 methanol, and then leaving the mixture to react for 24 hours at Field Code Changed room temperature without stirring. The amphiphilic nature of Field Code Changed PVP was crucial for stabilizing the nanoparticles as well as for Field Code Changed 6 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins Journal Name ARTICLE favoring the affinity between the nanoparticles and the ZIF-8 2013, Wang et al. reported encapsulation of highly disperse Pt crystals. Another example that illustrates the importance of nanoparticles into colloidal ZIF-8 crystals to form Pt@ZIF-8 functionalization of nanoparticles to promote surface composites without the use of any capping agent.43 Their two- Field Code Changed nucleation and growth of ZIF-8 is the synthesis of single-particle step method comprised treatment of unprotected Pt NPs with NP@ZIF‐8 composites by Zheng et al. in 2016.42 The authors first 2-mim as ligand under ultrasonication, followed by addition of Field Code Changed synthesized plasmonic nanoparticles of different sizes, shapes zinc nitrate. In this process, the methyl imidazole both stabilizes and compositions (Au nanorods, Au@Ag core–shell nanorods the Pt NPs and acts as the nucleation site. and Au nanostars), stabilized them with quaternary ammonium Apart from inorganic nanoparticles, target organic molecules surfactants (CTAB or CTAC) and finally, induced nucleation and such as dyes, drugs and therapeutic agents can be efficiently growth of ZIF‐8 crystals on the metal surface. Remarkably, they encapsulated inside of the ZIF-8 framework via bottom-up could tune the thickness of the ZIF‐8 shell by varying the approaches. For instance, in 2012, Liédana et al. reported a reaction time or CTAB concentration. Interestingly, although comparative study on the encapsulation efficiency of caffeine in capping agents can be valuable in the synthesis of NP@ZIF‐8 ZIF-8 via in situ and ex situ encapsulation strategies, materials, their presence may affect the performance of the demonstrating that in situ encapsulation was more rapid and embedded NPs, leading some researchers to avoid their use. In efficient, as it yielded caffeine@ZIF-8 composites with higher Field Code Changed Fig. 6. Illustration showing the in situ encapsulation of biomolecules (top) and inorganic nanoparticles (bottom) within the ZIF-8 structure and the corresponding electron microscopy images (TEM or FESEM) for each of the composites. Scale bars (all micrographs): 250 nm.41,42,45 Field Code Changed Field Code Changed This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 7 Please do not adjust margins Please do not adjust margins ARTICLE Journal Name guest loading (ca. 28 wt %) and enabled controlled release of rapid formation of ZIF-8. They determined the optimal conditions the guest.44 The simplicity and efficacy of in situ encapsulation for quantitative (100%) encapsulation of the model carbohydrate Field Code Changed have fostered its use in the synthesis of various colloidal drug- therapeutic CM-dextran and its subsequent controlled release, loaded ZIF-8 materials. For example, 2014, Zhuang et al. triggered by addition of ethylenediaminetetraacetic acid. reported the encapsulation of fluorescein (1 wt %) or the anti- Earlier this year, Feng et al. demonstrated that this protective- cancer drug camptothecin (2 wt %) in ZIF-8, by mixing the coating strategy could be exploited for encapsulation of appropriate ZIF-8 precursors in the presence of these guest antibodies, achieving very high encapsulation efficiencies (> molecules.45 In 2016, Zheng et al. discovered that target 99%) for human immunoglobulin polyclonal antibody, goat anti- Field Code Changed molecules that exhibit coordinating functional groups could BSA IgG polyclonal antibody and the monoclonal antibody form coordination polymers with Zn(II) ions that, upon addition adalimumab.51 The antibody@ZIF composites exhibited Field Code Changed of 2-mim, disassemble.46 Subsequent formation of ZIF-8 enhanced resistance against perturbation environments, such Field Code Changed resulted in micro/mesostructured MOF crystals with loadings of as heating, organic solvents, and mechanical pressure; were up to 20 wt %. amenable to long-term storage (> 3 weeks) under temperature- The possibility to produce colloidal ZIF-8 under strict control and cycling test conditions (4º C ↔ 50 °C) at a fast ramp rate (25 °C in very mild synthetic conditions (i.e. water, room temperature) min−1); and allowed for quick recovery of the antibodies before enables construction of MOF-based protective coatings for use. biomacromolecules via in situ encapsulation. In 2014, Lyu et al. reported the first such example of a biomacromolecule being Post-synthetic single-particle ZIF-8 composites embedded into ZIF-8.47 They first stabilized cytochrome c Field Code Changed protein (Cyt c), mixed it with PVP and then, introduced the PVP- While the aforementioned approaches focused on modified Cyt c into a solution of Zn(II) and 2-mim. Then, the encapsulation of functional guests within colloidal ZIF-8 mixture was incubated at room temperature for 24 hours crystals, guest molecules can also be positioned along the outer without stirring to obtain the desired Cyt c@ZIF-8 composite. surface of such crystals. For example, in 2013, Li et al. reported The authors found that they could encapsulate Cyt c this way the preparation of ZIF-8@Au hybrid composites. They started even without using PVP, suggesting that the strong interaction from colloidal ZIF-8 crystals, creating a single layer of Au NPs between Zn(II) and the amide groups of the protein favors that were uniformly distributed on the crystal surface (Fig. 7, formation of Cyt c@ZIF-8. One year later, the same group left).52 The authors devised two strategies: post-growth Field Code Changed extended their method to the co-embedding of two enzymes attachment and on-site reduction. In the former, they prepared into ZIF-8.48 Thus, they encapsulated glucose oxidase and Au NPs, stabilized them with 3-mercaptopropionic acid (MPA), Field Code Changed horseradish peroxidase within ZIF‐8 in aqueous solution and then added the stabilized NPs to ZIF-8 crystals. In the without adding any capping agent. Interestingly, embedding the second method, they mixed an Au-MPA complex with colloidal two enzymes simultaneously provided a synergetic effect: the ZIF-8 crystals, and then reduced the resultant composites to resulting composite was highly efficient at catalyzing a cascade obtain Au NPs. In both strategies, coordination between the reaction. That same year, inspired by natural biomineralization MPA and the Zn(II) ions of the ZIF-8 crystals enabled localization processes, Liang et al. demonstrated that colloidal ZIF-8 could be employed as protective coatings for various biomacromolecules, including ovalbumin, ribonuclease A, human serum albumin, pyrroloquinoline quinone-dependent glucose dehydrogenase, lipase, hemoglobin, lysozyme, insulin, horseradish peroxidase, trypsin, urease and oligonucleotides.49 Field Code Changed They proposed that the existing interactions between biomacromolecules and the MOF precursors increases the concentration of the building blocks around the biomacromolecules, thereby inducing MOF nucleation. The formation of these composites was achieved by mixing an aqueous solution of Zn(CH3COO)2 with an aqueous solution of 2-mim containing the appropriate biomolecule, and keeping the resulting mixture undisturbed for 12 hours at room temperature. After encapsulation, several of these biomacromolecules maintained their activity, even after having been exposed to extreme environments. Recently, the same group investigated the biomimetic mineralization of ZIF-8 by diverse array of simple sugars and carbohydrates.50 The authors Field Code Changed found that chemical functionalization of carbohydrates (e.g. Fig. 7. Schematic on the versatility of the post-synthetic functionalization of colloidal carboxylation) caused an increase in the concentration of Zn(II)+2 at Field Code Changed ZIF-8 crystals with inorganic nanoparticles (left), metal coatings (centre) or polymers the carbohydrate surface, due to Coulombic attraction between (right), and the corresponding electron microscopy images (bottom row). Scale bars: Field Code Changed the functional groups and the Zn cations, which ultimately favored 20 nm (left) and 500 nm (center and right). 52,54,55 Field Code Changed 8 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins Journal Name ARTICLE of Au NPs on the crystal surface. However, the authors found formation of core–shell ZIF-8@SiO2.56 In 2017, Wu et al. Field Code Changed that on-site reduction minimized aggregation and enlargement prepared superhydrophobic core–shell ZIF-8@SiO2 composites of Au NPs, due to the surface-pinning effect of MPA. One year using a two-step approach.57 First, they coated colloidal ZIF-8 Field Code Changed later, He et al. reported a straightforward method for the nanocrystals with SiO2 by following the aforementioned fabrication of yellow fluorescent ZIF-8@GSH-Au (GSH-Au = method. Next, to enhance the hydrophobicity of the ZIF- glutathione-Au nanoclusters) single-particle composites that 8@SiO2 particles, they modified them with involved directly mixing GSH-Au with the synthesized colloidal hexadecyltrimethoxysilane. ZIF-8 crystals at room temperature in methanol, followed by 12 Incorporation of functional molecules onto the surface of hours of stirring.53 The authors attributed formation of the ZIF- colloidal ZIF-8 crystals has also been explored to construct Field Code Changed 8@GSH-Au composites to coordination interactions between nanostructured composites. In 2018, Zhu et al. reported the the Zn(II) ions and the GSH on the nanocluster surface. In 2016, synthesis of colloidal ZIF-8@DPGG and similar colloids (based our group reported a strategy to fabricate Janus particles by on other MOFs) via direct coordination of the lipid analog 1,2‐ partially coating ZIF-8 crystals (Fig. 7, middle).54 Our method is dipalmitoyl‐sn‐glycero‐3‐galloyl (DPGG) onto the external Field Code Changed based on direct evaporation of metals (Au, Co or Pt) on the surface of ZIF-8 crystals (Fig. 7, right).55 The colloidal stability of Field Code Changed surface of ZIF-8 crystals, previously immobilized onto planar the modified DPGG-ZIF-8 particles in nonapolar solvents surfaces, to yield an asymmetric structure in which the metal- enabled fabrication of ZIF-8 or ZIF-8/polymer monolayers by film thickness can be controlled. We showed that colloidal Janus evaporation‐induced interfacial self‐assembly. Moreover, ZIF-8@Pt particles could behave as motors when immersed in a incorporation of a second functional layer onto the DPGG- solution of sodium cholate and H2O2 in aqueous ethanol. modified ZIF-8 enabled construction of polyhedral core‐shell Colloidal ZIF-8 crystals have been surface-functionalized with structures, hybrid lipid‐modified‐plasmonic vesicles and cargo other than inorganic nanoparticles. For example, the multicomponent supraparticles. outer functionalization of ZIF-8 crystals with SiO2 has been explored for the fabrication of colloidal ZIF-8@SiO2 composites. Thus, in 2014, He et al. coated homogenous rhombic A special case: ZIF@ZIF crystals dodecahedral ZIF-8 nanocrystals with SiO2 by following a sol–gel The bottom-up strategy has also been exploited to obtain new process which involved alkaline-catalyzed hydrolysis and morphologies or complex structures of ZIF@ZIF crystals. Generally, condensation of tetraethyl orthosilicate in ethanol, leading to Fig. 8. (a) Schematic illustrating the post-synthetic growth of alternating layers of ZIF-8 or ZIF-67 on top of ZIF-8 (top) or ZIF-67 (bottom), and the corresponding EM images for characterization of the resulting ZIF-on-ZIF hierarchic structures. Scale bar (all micrographs): 100 nm.61 b) Schematic of the potential of combining top-down shape modification Field Code Changed of ZIF crystals that differ by truncation degree, with subsequent re-growth of the etched structures to yield hierarchic RD ZIF crystals that have cores of different morphologies. The corresponding FESEM images and EDX elemental mapping plots (for the hierarchic structures) are shown below each illustration of the ZIF crystals. Scale bars: 200 nm (top and middle) and 1 µm (bottom).62 Field Code Changed This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 9 Please do not adjust margins Please do not adjust margins ARTICLE Journal Name pre-synthesized crystals are taken as seeds and subjected to combination of deconstructive and constructive strategies to one or more growing steps to yield layered ZIF@ZIF crystals.58-60 obtain sophisticated ZIF-based colloidal nanoparticles. Field Code Changed In 2016, Zhang et al. reported a synthesis of ZIF@ZIF materials in which they controlled the resultant shell thickness (Fig. 8a). First, homogeneous ZIF-8 or ZIF-67 seed crystals were soaked in Outlook a methanolic solution of 2-mim and then, a methanolic solution As we have summarized in this review, the past of Co(II) or Zn(II), respectively, was added. After stirring for 1 decade has witnessed tremendous advances in the hour, the resultant mixtures were aged for 24 hours at room development of colloidal ZIF-8: from the synthesis of highly temperature. The authors systematically studied the effects of monodispersed particles with tunable size and shape, to their the metal/ligand ratio and of the metal source (nitrate or further use as functional materials and composites. After acetate salt) on the shell thickness in ZIF-8 and ZIF-67 particles years of investigation into the respective roles of different (size: 90 nm to 600 nm). They found that, at a fixed reaction reaction parameters in the formation of ZIF-8, the resulting time and 2-mim concentration, lower concentrations of acetate knowledge has led to a veritable toolbox for tailored synthesis salt gave ZIF@ZIF particles with the thinnest shells (ca. 7 nm), of colloidal ZIF-8. Thus, researchers can now prepare complex whereas higher concentrations of nitrate salt yielded the colloidal ZIF-8 particles that exhibit the dimensions, morphology thickest shells (ca. 70 nm).61 Their findings were consistent with and homogeneity required for specific purposes, while pushing Field Code Changed previously reported results on the nucleation and growth rates the limits of crystal engineering to design single-particle of ZIF-8. Later, our group described core-shell ZIF-67@ZIF-8 composites of ZIF-8 with a diverse array of nanomaterials and particles having ZIF-67 cores in cubic, tetrahedral or hollow-box biomolecules. morphologies.62 We first obtained the ZIF-67 cores by selective Various recent examples point to the future of the field (Fig. 9), Field Code Changed chemical etching, and then used the etched ZIF-67 crystals as including formation of self-assembled transparent seeds, optimizing particle growth by varying the Zn(II)/2-mim films/monoliths using ultra-small colloidal ZIF-8 particles for concentrations and reaction time for each core morphology use, for example, in shaping monoliths and in optical (Fig. 8b). Interestingly, all the morphologies — even the hollow- sensing;18,63 self-assembly of colloidal ZIF-8 particles of highly Commented [C8]: In fig 8b On the bottom right, I think the hollow- box — ended up with the TRD or RD shape to form cubic@TRD, box core should be depicted much bigger. monodisperse size and shape into ordered supercrystals for use cubic@RD, tetrahedral@TRD, tetrahedral@RD or hollow- in photonics, sensing and other domains;7,15,64 formation of Field Code Changed box@RD morphologies, as if the etching was reversed. functionalized carbons with interesting electrochemical Field Code Changed Moreover, we obtained multi-shelled particles by repeating the properties using complex colloidal ZIF-8 composites;38,58 and ZIF growth step. This unique example illustrates the Field Code Changed design of highly complex single-particle ZIF-8 composites for use in catalysis and biomedicine.62,_ENREF_6465 These are but a Field Code Changed Field Code Changed Field Code Changed Field Code Changed Field Code Changed Field Code Changed Field Code Changed Field Code Changed Field Code Changed Fig. 9. Diagram of the perspective applications of colloidal ZIF-8 in the different forms discussed in this review. Scale bar (all micrographs): 1 µm.15,18,39,49 Field Code Changed Field Code Changed 10 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins Journal Name ARTICLE few examples of how colloidal particles have expanded the 4 M. Pang, A. J. Cairns, Y. Liu, Y. Belmabkhout, H. C. Zeng and M. properties and applications of ZIF-8. Nonetheless, they Eddaoudi, J. Am. Chem. Soc., 2012, 134, 13176-13179. 5 G. Lu, C. Cui, W. Zhang, Y. Liu and F. Huo, Chem. Asian J., 2013, exemplify how synthesis of MOFs as homogeneous colloids can 8, 69-72. confer materials with new properties. 6 M. Sindoro, A.-Y. Jee and S. Granick, Chem. Commun., 2013, 49, 9576-9578. We believe that recent progress in the synthesis and 7 M. Sindoro, N. Yanai, A.-Y. Jee and S. Granick, Acc. Chem. Res., applications of colloidal ZIF-8 has set the stage for further 2014, 47, 459-469. 8 X.-C. Huang, Y.-Y. Lin, J.-P. Zhang and X.-M. Chen, Angew. development of colloidal MOFs. Indeed, considering the high Chem. Int. Ed., 2006, 45, 1557-1559. versatility of MOFs in terms of structure, composition, 9 K. S. Park, Z. Ni, A. P. Côté, J. Y. Choi, R. Huang, F. J. Uribe- morphology and properties, the possibilities now seem endless. Romo, H. K. Chae, M. O’Keeffe and O. M. Yaghi, Proc. Natl. Thus, recently-observed phenomena such as the locomotion of Acad. Sci. U.S.A, 2006, 103, 10186-10191. Janus colloidal MOFs could be expanded to MOFs with the 10 J. Cravillon, S. Münzer, S.-J. Lohmeier, A. Feldhoff, K. Huber and M. Wiebcke, Chem. Mater., 2009, 21, 1410-1412. required composition, to enable specific applications such as 11 A. Schejn, L. Balan, V. Falk, L. Aranda, G. Medjahdi and R. drug delivery or pollutant removal. Alternatively, fabrication of Schneider, CrystEngComm, 2014, 16, 4493-4500. macroscopic materials (e.g. monoliths or membranes) based on 12 Y. Zhang, Y. Jia, M. Li and L. a. Hou, Sci. Rep., 2018, 8, 9597. close packing of colloidal MOFs for gas sorption and separation 13 H. H.-M. Yeung, A. F. Sapnik, F. Massingberd-Mundy, M. W. applications remains in the very early stages of development. Gaultois, Y. Wu, D. A. X. Fraser, S. Henke, R. Pallach, N. Heidenreich, O. V. Magdysyuk, N. T. Vo and A. L. Goodwin, Therefore, expanding the library of available colloidal MOFs Angew. Chem. Int. Ed., 2019, 58, 566-571. that could be used in the fabrication of macroscopic materials 14 J. Cravillon, R. Nayuk, S. Springer, A. Feldhoff, K. Huber and M. will be critical. Wiebcke, Chem. Mater., 2011, 23, 2130-2141. We are also confident that MOFs could make key contributions 15 C. Avci, I. Imaz, A. Carné-Sánchez, J. A. Pariente, N. Tasios, J. in fundamental colloidal science, by facilitating the discovery of Pérez-Carvajal, M. I. Alonso, A. Blanco, M. Dijkstra, C. López and D. Maspoch, Nat. Chem., 2017, 10, 78. new structures and phenomena. The availability of diverse, 16 E. M. Mahdi and J.-C. Tan, Polymer, 2016, 97, 31-43. highly monodisperse, polyhedral colloidal MOFs of different 17 S. R. Venna, J. B. Jasinski and M. A. Carreon, J. Am. Chem. Soc., morphologies should enable the assembly of colloidal crystals 2010, 132, 18030-18033. with new packing structures. Additionally, the photonic 18 B. Chocarro-Ruiz, J. Pérez-Carvajal, C. Avci, O. Calvo-Lozano, properties of such colloidal crystals could be synergistically M. I. Alonso, D. Maspoch and L. M. Lechuga, J. Mater. Chem. A, 2018, 6, 13171-13177. combined with specific functions of the colloidal MOFs used in 19 S. Hermes, T. Witte, T. Hikov, D. Zacher, S. Bahnmüller, G. their synthesis, such as luminescence, magnetism, chirality and Langstein, K. Huber and R. A. Fischer, J. Am. Chem. Soc., 2007, structural flexibility, to unveil new modular photonic materials 129, 5324-5325. with new properties. For example, the concerted 20 T. Tsuruoka, S. Furukawa, Y. Takashima, K. Yoshida, S. Isoda expansion/shrinking of flexible MOFs embedded in highly- and S. Kitagawa, Angew. Chem. Int. Ed., 2009, 48, 4739-4743. 21 N. Yanai, M. Sindoro, J. Yan and S. Granick, J. Am. Chem. Soc., ordered colloidal crystals could give rise to novel inter-particle 2013, 135, 34-37. cooperativity, which we expect would have a significant impact 22 K. Kida, M. Okita, K. Fujita, S. Tanaka and Y. Miyake, on gas sorption and sensing applications. CrystEngComm, 2013, 15, 1794-1801. 23 J. Cravillon, C. A. Schröder, H. Bux, A. Rothkirch, J. Caro and M. Wiebcke, CrystEngComm, 2012, 14, 492-498. Conflicts of interest 24 X. Xu, Y. Sun, Q. Zhang, S. Wang, L. Zhang, Z. Wu and G. Lu, ChemistrySelect, 2016, 1, 1763-1767. The authors declare no conflict of interest. 25 G. Zhan and H. C. Zeng, Nat. Commun., 2018, 9, 3778. 26 C. Avci, J. Ariñez-Soriano, A. Carné-Sánchez, V. Guillerm, C. Carbonell, I. Imaz and D. Maspoch, Angew. Chem. Int. Ed., 2015, 127, 14625-14629. Acknowledgements 27 W. Niu, S. Zheng, D. Wang, X. Liu, H. Li, S. Han, J. Chen, Z. Tang This work was supported by the ERC under the EU-FP7 (ERC-Co and G. Xu, J. Am. Chem. Soc., 2009, 131, 697-703. 615954), and the CERCA Program/Generalitat de Catalunya. 28 Y. Pan, D. Heryadi, F. Zhou, L. Zhao, G. Lestari, H. Su and Z. Lai, CrystEngComm, 2011, 13, 6937-6940. ICN2 is supported by the Severo Ochoa program from Spanish 29 S. Tanaka, K. Fujita, Y. Miyake, M. Miyamoto, Y. Hasegawa, T. MINECO (Grant No. SEV-2017-0706). A.C.S. thanks the Spanish Makino, S. Van der Perre, J. Cousin Saint Remi, T. Van Assche, MINECO for Juan de la Cierva fellowship (IJCI-2016-29802). G. V. Baron and J. F. M. Denayer, J. Phys. Chem. C, 2015, 119, 28430-28439. 30 F. Yang, H. Mu, C. Wang, L. Xiang, K. X. Yao, L. Liu, Y. Yang, Y. Han, Y. Li and Y. Pan, Chem. Mater., 2018, 30, 3467-3473. 31 G. Zheng, Z. Chen, K. Sentosun, I. Pérez-Juste, S. Bals, L. M. Liz- References Marzán, I. Pastoriza-Santos, J. Pérez-Juste and M. Hong, Nanoscale, 2017, 9, 16645-16651. 1 T. Graham, Philos. Trans. Royal Soc., 1861, 151, 183-224. 32 S. Jeoung, I. T. Ju, J. H. Kim, S. H. Joo and H. R. Moon, J. Mater. 2 D. Myers, in Surfaces, Interfaces, and Colloids: Principles and Chem. A, 2018, 6, 18906-18911. Applications, 2nd Ed., Wiley VCH, New York, 1999. 33 M. Hu, Y. Ju, K. Liang, T. Suma, J. Cui and F. Caruso, Adv. Funct. 3 M. Pang, A. J. Cairns, Y. Liu, Y. Belmabkhout, H. C. Zeng and M. Mater., 2016, 26, 5827-5834. Eddaoudi, J. Am. Chem. Soc., 2013, 135, 10234-10237. This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 11 Please do not adjust margins Please do not adjust margins ARTICLE Journal Name 34 K. Shen, L. Zhang, X. Chen, L. Liu, D. Zhang, Y. Han, J. Chen, J. 62 C. Avci, A. Yazdi, M. Tarrés, E. Bernoud, N. G. Bastús, V. Puntes, Long, R. Luque, Y. Li and B. Chen, Science, 2018, 359, 206-210. I. Imaz, X. Ribas and D. Maspoch, ACS Appl. Mater. Interfaces, 35 H. Chen, K. Shen, J. Chen, X. Chen and Y. Li, J. Mater. Chem. A, 2018, 10, 23952-23960. 2017, 5, 9937-9945. 63 T. Tian, J. Velazquez-Garcia, T. D. Bennett and D. Fairen- 36 H. J. Lee, W. Cho and M. Oh, Chem. Commun., 2012, 48, 221- Jimenez, J. Mater. Chem. A, 2015, 3, 2999-3005. 223. 64 L. Bai, Y. He, J. Zhou, Y. Lim, V. C. Mai, Y. Chen, S. Hou, Y. Zhao, 37 L.-Y. Chou, P. Hu, J. Zhuang, J. V. Morabito, K. C. Ng, Y.-C. Kao, J. Zhang and H. Duan, Adv. Opt. Mater., 0, 1900522. S.-C. Wang, F.-K. Shieh, C.-H. Kuo and C.-K. Tsung, Nanoscale, 65 M. Hoop, C. F. Walde, R. Riccò, F. Mushtaq, A. Terzopoulou, 2015, 7, 19408-19412. X.-Z. Chen, A. J. deMello, C. J. Doonan, P. Falcaro, B. J. Nelson, 38 P. Zhang, B. Y. Guan, L. Yu and X. W. Lou, Angew. Chem. Int. J. Puigmartí-Luis and S. Pané, Appl. Mater. Today., 2018, 11, Ed., 2017, 56, 7141-7145. 13-21. 39 P. Zhang, B. Y. Guan, L. Yu and X. W. Lou, Chem, 2018, 4, 162- 173. 40 J. Yang, F. Zhang, H. Lu, X. Hong, H. Jiang, Y. Wu and Y. Li, Angew. Chem. Int. Ed., 2015, 54, 10889-10893. 41 G. Lu, S. Li, Z. Guo, O. K. Farha, B. G. Hauser, X. Qi, Y. Wang, X. Wang, S. Han, X. Liu, J. S. DuChene, H. Zhang, Q. Zhang, X. Chen, J. Ma, S. C. J. Loo, W. D. Wei, Y. Yang, J. T. Hupp and F. Huo, Nat. Chem., 2012, 4, 310. 42 G. Zheng, S. de Marchi, V. López-Puente, K. Sentosun, L. Polavarapu, I. Pérez-Juste, E. H. Hill, S. Bals, L. M. Liz-Marzán, I. Pastoriza-Santos and J. Pérez-Juste, Small, 2016, 12, 3935- 3943. 43 P. Wang, J. Zhao, X. Li, Y. Yang, Q. Yang and C. Li, Chem. Commun., 2013, 49, 3330-3332. 44 N. Liédana, A. Galve, C. Rubio, C. Téllez and J. Coronas, ACS Appl. Mater. Interfaces, 2012, 4, 5016-5021. 45 J. Zhuang, C.-H. Kuo, L.-Y. Chou, D.-Y. Liu, E. Weerapana and C.-K. Tsung, ACS Nano, 2014, 8, 2812-2819. 46 H. Zheng, Y. Zhang, L. Liu, W. Wan, P. Guo, A. M. Nyström and X. Zou, J. Am. Chem. Soc., 2016, 138, 962-968. 47 F. Lyu, Y. Zhang, R. N. Zare, J. Ge and Z. Liu, Nano Lett., 2014, 14, 5761-5765. 48 X. Wu, J. Ge, C. Yang, M. Hou and Z. Liu, Chem. Commun., 2015, 51, 13408-13411. 49 K. Liang, R. Ricco, C. M. Doherty, M. J. Styles, S. Bell, N. Kirby, S. Mudie, D. Haylock, A. J. Hill, C. J. Doonan and P. Falcaro, Nat. Comm., 2015, 6, 7240. 50 E. Astria, M. Thonhofer, R. Ricco, W. Liang, A. Chemelli, A. Tarzia, K. Alt, C. E. Hagemeyer, J. Rattenberger, H. Schroettner, T. Wrodnigg, H. Amenitsch, D. M. Huang, C. J. Doonan and P. Falcaro, Mater. Horizons, 2019, 6, 969-977. 51 Y. Feng, H. Wang, S. Zhang, Y. Zhao, J. Gao, Y. Zheng, P. Zhao, Z. Zhang, M. J. Zaworotko, P. Cheng, S. Ma and Y. Chen, Adv. Mater., 2019, 31, 1805148. 52 Z. Li and H. C. Zeng, Chem. Mater., 2013, 25, 1761-1768. 53 L. He, L. Li, T. Wang, H. Gao, G. Li, X. Wu, Z. Su and C. Wang, Dalton Trans., 2014, 43, 16981-16985. 54 A. Ayala, C. Carbonell, I. Imaz and D. Maspoch, Chem. Commun., 2016, 52, 5096-5099. 55 W. Zhu, G. Xiang, J. Shang, J. Guo, B. Motevalli, P. Durfee, J. O. Agola, E. N. Coker and C. J. Brinker, Adv. Funct. Mater., 2018, 28, 1705274. 56 L. He, L. Li, L. Zhang, S. Xing, T. Wang, G. Li, X. Wu, Z. Su and C. Wang, CrystEngComm, 2014, 16, 6534-6537. 57 C. Wu, Q. Liu, R. Chen, J. Liu, H. Zhang, R. Li, K. Takahashi, P. Liu and J. Wang, ACS Appl. Mater. Interfaces, 2017, 9, 11106- 11115. 58 J. Tang, R. R. Salunkhe, J. Liu, N. L. Torad, M. Imura, S. Furukawa and Y. Yamauchi, J. Am. Chem. Soc., 2015, 137, 1572-1580. 59 C. Rösler, A. Aijaz, S. Turner, M. Filippousi, A. Shahabi, W. Xia, G. Van Tendeloo, M. Muhler and R. A. Fischer, Chem. Eur. J, 2016, 22, 3304-3311. 60 J. Yang, F. Zhang, H. Lu, X. Hong, H. Jiang, Y. Wu and Y. Li, Angew. Chem. Int. Ed., 2015, 127, 11039-11043. 61 J. Zhang, T. Zhang, K. Xiao, S. Cheng, G. Qian, Y. Wang and Y. Feng, Cryst. Growth Des., 2016, 16, 6494-6498. 12 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins