This is the accepted version of the article: Ayala A., Carbonell C., Imaz I., Maspoch D.. Introducing asymmetric functionality into MOFs: Via the generation of metallic Janus MOF particles. Chemical Communications, (2016). 52. : 5096 - . 10.1039/c6cc01098a. Available at: https://dx.doi.org/10.1039/c6cc01098a Introducing asymmetric functionality to MOFs via generation of metallic Janus MOF particles† Received 00th January 20xx, Accepted 00th January 20xx Abraham Ayala,a Carlos Carbonell,a Inhar Imaz, *a and Daniel Maspoch *ab DOI: 10.1039/x0xx00000x www.rsc.org/ Herein we report a versatile methodology for engineering metallic Janus MOF particles based on desymmetrization at interfaces, whereby each MOF particle is partially coated with a desired metal. We demonstrate that it enables fabrication of homogeneous Janus MOF particles according to the MOF (ZIF-8, UiO-66 or UiO-66-SH), the metal (Au, Co or Pt), the MOF particle size (from the micrometer to the submicrometer regime) and the metal-film thickness (from 5 nm to 50 nm) employed. We anticipate that our strategy could be applied to impart new functionalities to MOFs, including asymmetric functionalization, magnetic-guidance and motorization. Janus particles are a type of multicompartmental, asymmetric particles whose surface has two chemically distinct regions.1-5 Their dual surface composition has recently attracted growing interest Scheme 1. Synthesis of metallic Janus MOF particles via the because it can be exploited to confer a single particle with different desymmetrization at interfaces approach. properties (e.g. anisotropy and directionality). For example, Janus design enables combination of two opposing properties, hydrophobicity and hydrophilicity, into single particles that can be used to build water repellent membranes,6 emulsifiers7 and storage, separation, catalysis, molecular sensing, adsorption for Pickering emulsions.8 In this context, the introduction of porosity heat-pump processes, contaminant removal, contrast agents, and into such particles has become a powerful tool to provide porous drug delivery systems.16-18 Seeking to impart MOFs with new materials with new functionalities (e.g. magnetism,9 mobility,10, 11 functionalities, researchers have recently begun to combine these etc.) and therefore, to enhance their performance in numerous particles with other functional materials (mainly, inorganic applications such as in biomedicine, catalysis, pollutant removal, nanoparticles).19,20 Most of the resulting MOF-based composites are and sensors. For example, Sánchez et al. have demonstrated that isotropic core-shell materials in which the metallic component is hollow mesoporous MCM-41 particles capable of transporting and embedded within the MOF. These composites have shown delivering drugs can be motorized by functionalizing half of their important advantages in some of the afore-mentioned applications, surface with enzymes.11 Also, Pingarrón et al. have shown that but their isotropicity prevents their use in applications that require Janus particles comprising Au and mesoporous silica could be used anisotropic MOF-metal composites. Importantly, Bradshaw, Kuhn et in electrochemical biosensing systems.12 al. have recently demonstrated the synthesis of metallic Janus MOF Among the various porous materials available, metal-organic materials by electrochemically growing ZIF-8 and HKUST-1 crystals frameworks (MOFs) and porous coordination polymers (PCPs) are in selected regions of macroscopic Zn wires and Cu beads.21 Also, attractive choices for making porous Janus particles.13-15 MOFs are Lahann et al. have shown the spatioselective growth of HKUST-1 attainable in various pore sizes and shapes and with extremely large nanocrystals on micrometer polymer spheres.22 However, to date, surface areas, tailored internal surfaces and high flexibility, making no suitable methods for forming submicron-sized metallic Janus them potentially useful for myriad applications, including gas particles comprising single MOF particles have been described. a. Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and The Barcelona Institute of Science and Technology, Campus UAB, Bellaterra, 08193 Barcelona, Spain. b. Institució Catalana de Recerca i Estudis Avançats (ICREA) 08100 Barcelona (Spain) †Electronic Supplementary Information (ESI) available. See DOI: 10.1039/x0xx00000x |1 Herein we report a general strategy to synthesize porous metallic Janus MOF particles, based on direct evaporation of metals on the surface of colloidal MOF crystals that have been pre- immobilized onto planar surfaces. The approach that we used is also known as desymmetrization at interfaces. We have named these Janus particles M@MOF particles, in which M refers to the evaporated metal and MOF, to the selected colloidal MOF. Desymmetrization at interfaces using planar surfaces has already been used to synthesize metal/polystyrene and metal/silica Janus particles.23,24 When a metal is evaporated from above onto particles affixed to a planar surface, it is selectively deposited on the top (exposed) surfaces of the particles. Subsequent detachment of the particles from the surface affords Janus particles whose top surfaces are covered by a thin metallic film and whose bottom surfaces (i.e. the previously unexposed surfaces that had been in contact with the planar surface) remain unaltered. For the work that we report here, we adapted this method to MOF crystals. Initially, in a typical experiment, we synthesized colloidal MOF particles of different sizes by adjusting the conditions of previously reported methods.25-29 The selected MOFs were ZIF-8 (particle sizes = 101 ± 10 nm, 201 ± 9 nm, and 1.3 ± 0.2 µm; SBET = 1272 m2/g, 1123 m2/g and 1272 m2/g, respectively); UiO-66 (particle size = 190 ± 22 nm; SBET = 1332 m2/g) and UiO-66-SH (particle size = 208 ± 54 nm; SBET = 306 m2/g) (Fig. S1-S6, ESI†). Each MOF was then deposited onto Au surfaces that had been pre-functionalized with 4-mercaptobenzoic acid and treated with NaOH to deprotonate the carboxylic groups (Fig. S7, ESI†). For the deposition, a colloidal MOF solution in DMF was spread onto the surface, which was then heated at 60 °C until the DMF had evaporated off. Here, we made a strategic decision to carefully cover the surface with a true monolayer (even though some areas remained uncovered) rather than to fully cover the surface and Fig. 1. (a,b) Representative FESEM images showing a general view risk forming MOF aggregates or multilayers, as any MOF particles of (a) Au@ZIF-8 particles (size = 201 ± 9 nm; Au thickness = 50 nm) underneath the resultant top layer would not be covered in the and (b) Au@UiO-66 particles (size = 190 ± 22 nm; Au thickness = 50 nm). (c) FESEM images of individual Au@ZIF-8 particles (size = 1.3 subsequent next stage. In a third step, the surface-deposited MOF µm ± 0.2 µm), showing different Au thicknesses (from left to right: particles were coated with a metal (M = Au, Co or Pt; metal film 5 nm, 10 nm, 20 nm, 30 nm and 50 nm). (d) FESEM images of thickness = 5 nm to 50 nm) using an E-beam evaporator. Finally, individual Au@ZIF-8 particles (Au thickness = 50 nm), showing the resultant Janus M@MOF particles were detached by different ZIF-8 sizes (from left to right: 101 ± 10 nm, 201 ± 9 nm and ultrasonication of the surfaces for 30 s in MeOH. The particles 1.3 µm ± 0.2 µm). (e) FESEM images of individual Au@UiO-66 were characterized by Scanning Electron Microscopy (SEM), and particles (size = 190 ± 22 nm), showing different Au thicknesses their corresponding porosity capabilities were evaluated. (from left to right: 5 nm, 10 nm, 20 nm, 30 nm and 50 nm). (f) EDX Next, to confirm the generality of our approach, we mapping of a single Au@ZIF-8 particle (size = 201 µm ± 9 nm), systematically studied the formation of diverse Janus Au@MOF showing the distribution of Zn (green) and Au (red). g) Schematic particles that varied by MOF (ZIF-8 and UiO-66), particle size illustration and corresponding confocal image (ZIF-8, blue; FITC- PEG-SH, green), showing the asymmetric functionalization of a (particle sizes = 101 ± 10 nm, 201 ± 9 nm, and 1.3 ± 0.2 µm for ZIF- single Au@ZIF-8 particle (size = 1.3 µm ± 0.2 µm). Scale bars: 500 8 and 190 ± 22 nm for UiO-66) and Au film thickness (5 nm, 10 nm, nm (a-d), 100 nm (e) and 1.5 µm (g). 20 nm, 30 nm or 50 nm). SEM images of all synthesized Janus particles revealed the partial coverage of the external surface of the MOF with an Au thin film (Fig. 1a-e and Fig. S8-S11, ESI†). The asymmetric structure of these particles was further confirmed by experimental X-ray powder diffraction (XRPD) patterns were elemental mapping with energy dispersive X-ray spectrometry consistent (Fig. S13-S16, ESI†), confirming that the ZIF-8 or UiO-66 (EDX) of single particles, which revealed a uniform distribution of in the Janus particles were structurally identical to corresponding Au atoms in approximately half of the particle surface (Fig. 1f and starting MOF particles. As expected, characteristic peaks for Fig. S12, ESI†). Additionally, SEM images confirmed that the Au film crystalline Au were also observed. Additionally, N2 sorption thickness on the Janus particles could be easily controlled with this measurements taken at 77 K up to 1 bar of Au@ZIF-8 (ZIF-8 size = method. This thickness increase was further corroborated by EDX 201 ± 9 nm; Au film thickness = 50 nm) and Au@UiO-66 (Au film analyses, which indicated that the Au:Zn or Au:Zr ratio consistently thickness = 5 nm) particles proved that the sorption capacity of increased with increasing Au film thickness (Fig. S8-S11, ESI†). these particles was equivalent to that of the initial MOF particles One might expect that the many synthetic steps required for (Fig. S17,S18, ESI†). Thus, the SBET values were determined to be the Janus particles would affect the initial crystalline integrity and 456 m2/g for Au@ZIF-8 and 560 m2/g for Au@UiO-66. Given that Au adsorption capabilities of the MOFs; however, we did not observe thin film is non-porous to N2, we attributed the sorption exclusively any losses in these properties. Indeed, for all Janus particles, the to ZIF-8 or UiO-66. Therefore, these SBET values could be expressed simulated (derived from the single crystal structure) and in m2 per grams ZIF-8 (SBET = 1037 m2/g) or UiO-66 (SBET = 982 m2/g), 2| once had we calculated the amount of Au present in each samples (56% w/w for Au@ZIF-8; 43% w/w for Au@UiO-66). The latter step was completed upon selectively dissolving the MOF component using an aqueous HCl solution (pH < 1) for ZIF-8 and an aqueous H2SO4 solution (pH < 1) for UiO-66. From the calculated SBET values, we found that the sorption performance of both MOFs in the Janus particles remains unaltered. As proof-of-concept of the properties that metallic Janus MOF particles can incorporate, we explored asymmetric functionalization of them. To this end, we selectively attached the green fluorophore fluorescein isothiocyanate polyethylene glycol thiol (FITC-PEG-SH) onto the Au thin film. The asymmetric functionalization was performed by incubating Au@ZIF-8 particles in an aqueous solution of FITC-PEG-SH at 4 ᴼC for 24 h. The resulting FITC-PEG-S-Au@ZIF-8 particles were then collected by centrifugation, cleaned twice with methanol, and imaged by confocal laser scanning microscopy (Fig. Fig. 3. (a) Optical microscopy images revealing the movement 1g and Fig. S19, ESI†). The results clearly show two different sequence of a motorized Pt@ZIF-8 particle. Scale bar: 100 µm. (b) fluorescent regions corresponding to ZIF-8 (blue)30 and to FITC-PEG- EDX mapping of a single Au@Co@UiO-66 particle, showing the Zr SH (green) that is selectively attached to the Au surfaces. Our (green), Co (blue) and Au (red) distributions. successful demonstration of asymmetric functionalization of metallic Janus MOF particles suggests that in the future, such particles could be vectorized for biomedical and sensing We next sought to create magnetically guidable porous applications. Furthermore, our approach could be advantageous particles. To this end, we synthesized Janus particles as above, modulating certain properties (e.g. hydrophobicity, hydrophilicity) except that we replaced Au with Co in order to confer the particles of the base MOF without altering its porosity. with magnetism. Figures 2a, S20 and S21 show the resultant Janus Co@ZIF-8, Co@UiO-66 and Co@UiO-66-SH particles (size = ca. 200 nm; Co film thickness = 20 nm). In all cases, EDX and XRPD analyses confirmed the desired asymmetric structure (Fig. 2b,c), the presence of crystalline Co, and that the crystalline integrity of all three MOFs had been fully conserved (Fig. S22-24, ESI†). More importantly, all the Janus particles incorporated the magnetic properties of the Co component, as demonstrated by the fact they could be easily guided by a magnet (Fig. 2d). Magnetic measurements also confirmed the magnetism of the particles, which exhibit a characteristic hysteresis loop with coercive field values of 181 Oe (Co@ZIF-8), 350 Oe (Co@UiO-66) and 23 Oe (Co@UiO-66-SH) at room temperature (Fig. S25, ESI†). To demonstrate that these Janus particles could be employed to adsorb target substances and subsequently magnetically recovered, we tested the performance of Co@UiO-66-SH particles in magnetic solid-phase removal of mercury. To this end, Co@UiO-66-SH particles (2 mg) were dispersed in a solution of 10 ppm Hg2+ in water (4 mL) at room temperature. After 30 min of incubation, the mercury content adsorbed by the Janus particles was determined by ICP-MS. The observed extraction was highly efficient: the particles had sequestered 99% of the mercury from the solution,29 and could then be easily removed using a small magnet (Fig. 2d). Once we had demonstrated that MOF particles could be asymmetrically metallized, we then sought to explore development Fig. 2. (a) Representative FESEM images of Co@UiO-66-SH (Co film of motorized MOFs powered by electrochemistry, using Pt as the thickness = 20 nm). (b) EDX mapping of a single Co@UiO-66-SH particle showing the Zr (green) and Co (red) distribution. (d) metal.14, 31 To this end, we fabricated Janus Pt@ZIF-8 particles (ZIF- Photographs showing the magnetic attraction of Co@UiO-66-SH 8 size = 1.3 ± 0.2 µm, or  8 µm for better visualization under particles dispersed in an aqueous solution of mercury before (left) optical microscopy; Pt film thickness = 60 nm; Fig. S26,27, ESI†), and and during (right) exposure to a magnet. Scale bars: 500 nm (a). then dropped them onto the surface of an aqueous/ethanol solution containing sodium cholate and H2O2. The Pt@ZIF-8 moved, and their motion was recorded using an optical microscopy. As shown in Figure 3a (see also Video S1,2), the 8 µm-in-diameter particles behaved as motors, moving around the surface at a maximum speed of 1.65 mm·s-1 (measured by ImageJ software using 20 frames). Finally, we tested our method for fabrication of polymetallic Janus MOF particles, which we made by sequentially evaporating different metals on the surface of the colloidal MOF crystals. Thus, UiO-66 crystals were first coated with Co (thickness = 20 nm) and |3 then, with Au (thickness = 5 nm). The resulting particles were 19. P. Falcaro, R. Ricco, A. Yazdi, I. Imaz, S. Furukawa, D. Maspoch, confirmed by XRPD and EDX elemental mapping to be polymetallic R. Ameloot, J. D. Evans and C. J. Doonan, Coord. Chem. Rev., Au@Co@UiO-66 particles (Fig. 3b and Fig. S28, ESI†). Interestingly, 2016, 307, 237-254. these Janus particles marry the porosity capabilities of UiO-66, the 20. G. Lu, S. Li, Z. Guo, O. K. Farha, B. G. Hauser, X. Qi, Y. Wang, X. magnetic properties of Co and the easy chemical functionalization Wang, S. Han, X. Liu, J. S. DuChene, H. Zhang, Q. Zhang, X. Chen, of the external Au layer (Fig. S25,S29, ESI†). J. Ma, S. C. J. Loo, W. D. Wei, Y. Yang, J. T. Hupp and F. Huo, Nat. In conclusion, we have described a collection of monometallic Chem., 2012, 4, 310-316. and polymetallic Janus MOF particles fabricated by partially coating 21. S. Yadnum, J. Roche, E. Lebraud, P. Négrier, P. Garrigue, D. colloidal MOF crystals with the desired metal(s). We consider this Bradshaw, C. Warakulwit, J. Limtrakul and A. Kuhn, Angew. approach a novel method for imparting new functionalities to Chem. Int. Ed., 2014, 53, 4001-4005. MOFs, as evidenced by our preparation of asymmetrically 22. T.-H. Park, K. J. Lee, S. Hwang, J. Yoon, C. Woell, J. Lahann, Adv. functionalized MOFs, magnetically guidable MOFs and Mater., 2014, 26, 2883. electrochemically motorized MOFs. Our findings should facilitate 23. J. Choi, Y. Zhao, D. Zhang, S. Chien and Y. H. Lo, Nano Lett., development of MOFs for numerous practical applications, 2003, 3, 995-1000. including drug delivery, theranostics, sensors, pollutant removals 24. S. Ye and R. L. Carroll, ACS Appl. Mater. Interfaces, 2010, 2, 616- and catalysis. 620. This work was supported by the MINECO-Spain through projects 25. J. Cravillon, R. Nayuk, S. Springer, A. Feldhoff, K. Huber and M. PN MAT2012-30994 and EU FP7 ERC-Co 615954. I.I. thanks the Wiebcke, Chem. Mater., 2011, 23, 2130-2141. MINECO for a RyC, and A.A. is grateful to the CONACYT for a 26. L. Chen, Y. Peng, H. Wang, Z. Gu and C. Duan, Chem. Commun., predoctoral grant. ICN2 acknowledges the support of the Spanish 2014, 50, 8651-8654. MINECO through the Severo Ochoa Centers of Excellence Program, 27. J. Cravillon, C. A. Schroder, H. Bux, A. Rothkirch, J. Caro and M. under Grant SEV-2013-0295. Wiebcke, CrystEngComm, 2012, 14, 492-498. 28. G. Lu, C. Cui, W. Zhang, Y. Liu and F. Huo, Chem. 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