Ostwald ripening-mediated grafting of metal-organic frameworks on a

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Ostwald ripening-mediated grafting of metal-organic frameworks on a single colloidal nanocrystal to form uniform and controllable MXF Yuan Liu, Yu Yang, Yujia Sun, Jibin Song, Nicholas G. Rudawski, Xiaoyuan Chen, and Weihong Tan J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 15, 2019

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Journal of the American Chemical Society

Ostwald ripening-mediated grafting of metal-organic frameworks on a single colloidal nanocrystal to form uniform and controllable MXF Yuan Liu†, ‡, ¶, Yu Yang†,%, Yujia Sun†, Jibin Song¶, Nicholas G Rudawski §, Xiaoyuan Chen¶ and Weihong Tan*, †, ‡ †Center

for Research at Bio/Nano Interface, Department of Chemistry and Department of Physiology and Functional Genomics, Health Cancer Center, UF Genetics Institute and McKnight Brain Institute, University of Florida, Gainesville, Florida 32611, United States. ‡Molecular Science and Biomedicine Laboratory (MBL), State Key Laboratory of Chemo/Bio-Sensing and Chemometrics, College of Chemistry and Chemical Engineering, College of Biology, Hunan University, Changsha, 410082, China. ¶Laboratory of Molecular Imaging and Nanomedicine, National Institute of Biomedical Imaging and Bioengineering, National Institutes of Health, Bethesda, Maryland 20892, United States; % Institute of Molecular Medicine (IMM), Renji Hospital, Shanghai Jiao Tong University School of Medicine, and College of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, People’ s Republic of China; §College of Engineering Research Service Centers, University of Florida, Gainesville, Florida 32611, United States. ABSTRACT: MXF, a metal-organic framework (MOF) that contains more than two components, such as heterogeneous inorganic nanoparticle@MOF (NP@MOF) with precisely defined structures, are important in applications such as catalysis, energy, and biochemistry. However, the ambiguous growth mechanism of MXF has hindered the exploration of controllable design of nanoparticle level MXF complexes. Here, we report an Ostwald ripening-mediated grafting of MOF on a single multidentate inorganic colloidal nanocrystal via heterogeneous nucleation. The process relies on the carboxylic acid groups anchored on the surface of the colloidal nanocrystal. Ostwald ripening-mediated grafting enabled us to obtain uniform MXF with a wide range of sizes of nanoparticles and a controlled thickness of the MOF layer on the surface of colloidal nanocrystal. A dual FRET-induced singlet oxygen generation with near-infrared light was achieved from an UCNP@ZrMOF hybrid. This generalizable grafting strategy provides insight to the design of MXF nanoparticles for a wide range of applications involving advanced functional materials.

Introduction Heterogeneous nanomaterials are believed to be the most important candidates for catalysis and energy, as well as biochemical detection.1-4 Many efforts have been devoted to preparing alloyed, doped, or hybrid nanomaterials for enhanced or selective catalysis.5-8 Recently, MOFs have brought attention to the benefits of uniform porous crystalline structures with high surface area and have given rise to new hybrid nanomaterials with the ability to adapt their functions and utility.9-14 Typically, an MOF is made of two components, one metal ion and one organic linker. MXF is a metal-organic framework that contains more than two components. One or multi external metal ions or organic linkers replace the parent coordinating units forming new coordination sites in MOF, called MXF. The doping of inorganic components to MOF, forming MXF has noticeably enhanced their physicochemical properties. In light of doping components, metal organic coordination15-16 or ionic exchange17-18 facilitates atomic level MXF. Small clusters (< 3 nm) which can be thermodynamically adsorbed into MOF give cluster level MXF, while nanoparticles that can be encapsulated into MOF contribute to nanoparticle level MXF. Regardless of the doping level, the hybrid MXF may generate new structures and novel properties.1920 As structure begets properties, precise control of the doping level is a significant way to adjust the performance of hybrid materials.2122 There have been some papers mentioning the use of nanoparticleMOF hybrids for catalysis, but no work has discussed the precise

control of MXF complexes because of limited understanding on the preparation of these complexes: only by understanding the growth mechanism can we precisely control the MXF structure. Previous studies on hybrid MXF mainly focused on tentatively mixing inorganic nanoparticles, such as iron oxide, Pt, Cu, Au, or Pd, with MOF precursors.23-27 As a result, randomly agglomerated complexes were formed from the resulting mixtures. Understanding the growth mechanism of MOFs and hybrid MXF can aid the rational design of on-demand heterogeneous MXF and facilitate the construction of multifunctional MXF.28 From the physical adsorption point of view, the insertion of as-prepared small clusters into MOF is determined by the size of the guest cluster. However, the typical small pore size of MOF significantly limits the type and the number of clusters that adapt to MOF. Large nanoparticles cannot even be physically adsorbed into the pores of MOF. From a chemical point of view, MOFs could be constructed with “X”, such as clusters29 or colloidal nanoparticles regardless of the size, that act as building blocks and partially replace metal ions and organic linkers to form covalent bond-based MXF hybrids. Encapsulation of nanoparticles into a zeolitic imidazolate framework (ZIF-8) with help from surfactants to form an MXF has been achieved through the interaction of surfactants with nanoparticles and MOF precursors.30-32 However, the ambiguous interaction between nanoparticle and coordination unit has thus far hindered the exploration of controllable design of novel inorganic nanoparticle-based MXF complexes.

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Figure 1. A) Schematic illustration of grafting ZrMOF on a single nanocrystal. The first step involves anchoring carboxylic acid groups on the surfaces of colloidal nanoparticles. The second step involves mixing the multidentate nanoparticles with ZrMOF precursors to form an MXF core shell structure. B) ZrMOF nanoparticles. C) Iron oxide@ZrMOF nanohybrid. D) Au-20@ZrMOF nanohybrid. E) Au-50@ZrMOF nanohybrid. F) UCNP-sphere@ZrMOF nanohybrid. G) UCNP-hexahedron@ZrMOF nanohybrid. H) Ag@ZrMOF nanohybrid. I) UiO-66@ZrMOF nanohybrid. Scale bar: B, E, F, and I are 100 nm; C, D, G, and H are 50 nm.

Results and Discussion Here, we report a controllable nanoparticle level MXF by an Ostwald ripening-mediated grafting of MOF on a single nanocrystal via chemical coordination between nanoparticles and MOF. This controllable nanoparticle level MXF covers a wide range of sizes from 10 nm to 20 nm, 30 nm, 50 nm, and 100 nm regardless of the shape and materials. Furthermore, a controlled layer of MOF on a single nanoparticle was achieved. As shown in Figure 1, the as-synthesized inorganic colloidal nanocrystals were engineered with carboxylic acid groups (coordination groups) through ligand exchange in tetrahydrofuran and subsequently dispersed in DMF. The grafting of MOF on the surfaces of multidentate colloidal nanoparticles was initiated by heterogeneous nucleation and driven by the chelation between metal ions and organic linkers having multi-coordination groups, such as carboxylic acids in this study. After anchoring carboxylic acid groups on the surface, multidentate inorganic colloidal nanocrystals have probability equal to that of other organic linkers to chelate with Zr ions and form frameworks. Upon introducing carboxylic acid-anchored multidentate nanocrystals to a DMF solution containing PCN-222 (ZrMOF) precursors, including metal ions (ZrCl4) and organic linkers (tetrakis(4-carboxyphenyl) porphyrin (TCPP)), ZrMOF precursors nucleate on the surface of nanocrystals and keep growing until all the precursors are consumed. Inorganic nanocrystals anchored with carboxylic acid groups act as seeds in this system. Metal ions chelate with the carboxylic acid on the surface of nanocrystals first. Small ZrMOF clusters then undergo an Ostwald ripening process whereby small crystals, or sol particles, dissolve and redeposit onto the surfaces of larger sol particles, or, in the present study

nanocrystals, under the driving force of chelation and finally form uniform inorganic NP@MOF core shell structures. This strategy relies on the anchoring of carboxylic acids on the surfaces of inorganic nanocrystals. The carboxylic acid groups serve as nucleation sites to grow MOF. Regardless of the size, shape, and material of inorganic nanocrystals, uniform core shell MXF hybrids have been obtained with this method. Preparation of inorganic nanocrystals and carboxylic acid group anchoring were conducted according to reported methods.33 To maximize the benefit of this grafting strategy, we tested the MOF grafting using different types of inorganic colloidal nanocrystals with different sizes and shapes, including upconversion nanoparticles (UCNPs (spherical and hexagonal shape)), iron oxide, Au (20nm and 50 nm), Ag, and UiO-66 nanoparticles (Figure S1S7).34-38 To graft ZrMOF on the inorganic nanocrystal, all nanoparticles were first modified with carboxylic acid groups. However, it should be noted that different ligand exchange methods were applied in other cases because different materials require different anchoring ligands. For UCNP and iron oxide nanoparticles, 3,4-hydroxycinnamic acid was used as guest ligand to anchor carboxylic acids on their surfaces (Figure S14). For Au and Ag nanoparticles, thiol-modified poly(acrylic acid) (PAA) was capped on their surfaces (Figure S8-S9). In the case of UiO-66 nanoparticles, bis[2-(methacryloyloxy) ethyl] phosphate was first anchored on the surface via coordination between the phosphate group and surface Zr cluster of UiO-66,39 followed by an in situ polymerization of methacrylic acid (Figure S15). As shown in Figure 1 (C-I), we obtained uniform core shell MXF structures with these selected multidentate colloidal nanoparticles, including Iron oxide@ZrMOF, Au-20@ZrMOF, Au-50@ZrMOF, UCNP@ZrMOF (both spherical and hexagonal UCNPs), Ag@ZrMOF, and UiO-66@ZrMOF (Figure S10-S13). Without


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Journal of the American Chemical Society carboxylic acid anchoring, randomly agglomerated complexes were obtained. Thus, to graft ZrMOF on single nanocrystals, carboxylic acid groups must first be anchored on the colloidal nanocrystals. This general grafting strategy demonstrated that the ZrMOF growth on the surfaces of nanoparticles relies on the carboxylic acid groups, rather than the surface ligand or the composition of the inorganic nanoparticle.

Figure 2. Powder X-Ray diffraction and elemental map of UCNP@ZrMOF nanoparticles. A) Powder X-Ray diffraction of ZrMOF and UCNP@ZrMOF. Black triangle indicates ZrMOF diffraction peaks. Blue pentagram indicates UCNP diffraction peaks. Inset is the small angle diffraction comparison between ZrMOF and UCNP@ZrMOF. Consistent peaks from ZrMOF and UCNP@ZrMOF in the small angle diffraction plot demonstrate that the ZrMOF layer on the surface of UCNP has the same crystal structure as that of ZrMOF nanoparticles. B) TEM of UCNP@ZrMOF nanoparticles. C) High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) of UCNP@ZrMOF nanoparticle. D) Zr element map. E) Y element map. F) Yb element map. G) Zr, Y, and Yb composite element map. Scale bar: B is 100 nm; C-G is 20 nm. To investigate the grafting of ZrMOF on a single nanocrystal, UCNPs were selected as a model. In a typical experiment, ZrMOF was prepared by mixing TCPP and ZrCl4 in a mixed solvent of DMF:Ethanol (3:1). The resulting solution was incubated for 5 h at 120 °C. A color change from dark red (at the beginning) to brown (at the end) was observed. To graft ZrMOF on the surfaces of UCNPs, the carboxylic acid anchored multidentate UCNPs were added to a ZrMOF mother solution, followed by observation of a clear colloidal state without agglomeration. Good dispersity of UCNPs in ZrMOF mother solution guarantees obtaining uniform spherical core shell MOF structures. Uniform UCNP@ZrMOF nanoparticles were obtained after incubation at 120 °C. The asprepared UCNP@ZrMOF nanoparticles were characterized by TEM and powder X-Ray diffraction. As shown in Figure 2A, powder X-Ray diffraction showed peaks for both ZrMOF and UCNP, indicating that a crystalline ZrMOF structure had been

grafted on the surface of the UCNP by this chelation-driven chemical coordination. TEM images indicated the presence of a well-dispersed uniform monolayer of UCNP@ZrMOF nanostructure (Figure 2B). Furthermore, elemental mapping (Figure 2C-2G) and EDS line scanning (Figure S16) on a single UCNP@ZrMOF nanoparticle demonstrated that the metal components of this nanoparticle, including Y, Yb, and Zr, were evenly distributed in this core shell structure. Au-20@ZrMOF was also characterized by elemental mapping to confirm this core shell structure (Figure S17). An understanding of the grafting mechanism of ZrMOF on the surface of an inorganic colloidal nanocrystal is critical to design and control the core shell MXF structure. Before proposing the grafting mechanism, we first studied the growth mechanism of ZrMOF nanoparticles. Powder X-ray diffraction in Figure 2A (inset) has confirmed the crystalline structure of ZrMOF. A timedependent TEM characterization was conducted to monitor the formation of ZrMOF. As shown in the Figure 3B, it takes 4 h to obtain uniform and spherical ZrMOF nanoparticles. However, the spontaneous nucleation occurred immediately when the vial containing precursors was placed in a 120 °C oil bath (Figure 3B (1 min and 3 min)). In a saturated environment, clusters were formed from the nuclei under the driving force of rapidly increased temperature and coordination between Zr and carboxylic acid. The clusters were very unstable as a result of high surface energy, and by chance, some of them grew larger than the critical size (Figure S18). Then the small clusters dissociated to molecules, which were then incorporated onto the surfaces of large clusters (marked with yellow circles in Figure 3B (5 min)) that exceeded critical size. The large clusters grew larger until all the small clusters were consumed, essentially because the large particles were more energetically favored than the small clusters. As ZrMOF nanoparticles grew larger, the small clusters gradually decreased in number and finally disappeared. These observations from the time-dependent TEM monitoring indicated a typical Ostwald ripening process.40-42 The TEM pictures confirmed a rapid nucleation of ZrMOF for 3 min, while the growth stage required about 2-4 h.43 A schematic illustration of the Ostwald ripening process is shown in Figure 3A. In summary, burst homogeneous nucleation rapidly occurred after placing the precursors in the oil bath (120 °C), and some of the clusters grew larger than the critical size. As a result of Ostwald ripening, small clusters dissociated into free molecules and further redeposited on the larger particles. In the case of UCNP@ZrMOF synthesis, carboxylic acidanchored multidentate UCNPs were mixed with ZrMOF precursors and then placed into the oil bath (120 °C). It was hypothesized that a heterogeneous nucleation would be involved, as shown in Figure 3C, which can also be explained by Ostwald ripening. Rapid nucleation would produce both small homogeneous ZrMOF clusters and large heterogeneous UCNP-based “clusters”. Because the carboxylic acid groups on the surface of UCNPs can also coordinate with Zr, as other carboxylic acid groups from free TCPPs, small ZrMOF clusters could be formed on the surfaces of UCNPs. The thermodynamically-driven spontaneous process facilitates the producing of multidentate UCNPs-based large heterogeneous clusters, which are more energetically favored than small homogeneous clusters, enabling them to grow larger by grafting a layer of ZrMOF on the surface. As a result of Ostwald ripening, small clusters dissociate to small molecules, which are then deposited on the large core shell clusters until all free clusters have been consumed, and uniform UCNP@ZrMOF core shell nanoparticles are finally obtained.



Figure 3. Ostwald ripening-mediated grafting of ZrMOF on a single nanocrystal. A) Schematic illustration of homogeneous nucleation and growth of ZrMOF. B) Time-dependent TEM monitoring of homogeneous nucleation and growth of ZrMOF. Burst homogeneous nucleation was observed from 1 to 3 min. At 5 min, some small clusters (marked with yellow circles) grew to large clusters. At 10 min, large nanoparticles formed by consuming small clusters. From 30 min to 4 hours, the population of small clusters decreased gradually and finally disappeared as the sizes of spherical large ZrMOF nanoparticles increased. Scale bar: 1min, 3 min, and 5 min are 50 nm; 10 min, 1h, 2h, and 4 h are 100 nm. C) Schematic illustration of heterogeneous nucleation and Ostwald ripening-mediated grafting of ZrMOF on UCNP. D) Time-dependent TEM monitoring of heterogeneous nucleation and Ostwald ripening-mediated grafting of ZrMOF on UCNP. All scale bars are 50 nm. Burst heterogeneous nucleation was observed at 3 min. A ZrMOF layer on UCNP was observed at 5 min. The population of small clusters (marked with yellow circles) decreased gradually and finally disappeared as the thickness of the ZrMOF layer increased.


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Journal of the American Chemical Society To test the Ostwald ripening hypothesis, a time-dependent TEMmonitored growth of UCNP@ZrMOF was conducted. As shown in Figure 3D, many small free clusters (encircled by yellow rings) were observed in addition to UCNPs. At 5 min, a layer of ZrMOF on UCNPs was observed. Because large heterogeneous clusters are more energetically favored, most small clusters were consumed within 30 min, indicating a relatively faster ZrMOF growth in heterogeneous nucleation compared to that of homogeneous nucleation. From 30 min to 4 h, no obvious size change of UCNP@ZrMOF was observed, but small ZrMOF clusters were gone. This core shell UCNP@ZrMOF growth can be divided into two stages. The first stage involves heterogeneous nucleation driven by increased temperature and coordination between Zr cluster and carboxylic acid of UCNP and TCPP. Small homogeneous clusters and large heterogeneous clusters are formed at this stage. The second stage involves Ostwald ripening, as described above, during which the small homogeneous clusters dissociate to molecules and then incorporate onto the surfaces of UCNPs to form UCNP@ZrMOF core shell nanoparticles (Figure S18). Here, the occurrence of heterogeneous nucleation and Ostwald ripening depends on the surface chelating carboxylic acid

groups. The Ostwald ripening-mediated grafting of ZrMOF on the surface of the single nanocrystal, as described above, indicated that the ZrMOF layer originated from clusters through a dissociation and redeposition process after heterogeneous nucleation. The UCNP@ZrMOF nanoparticles stopped increasing in size after all homogeneous clusters had been consumed. We then assumed that the concentration of precursor and UCNP determines the thickness of ZrMOF layer. When the number of UCNPs is constant, the thickness of the ZrMOF layer will increase if the number of precursors increases. To test this assumption, we conducted a controlled experiment in which a constant number of UCNPs, but different numbers of precursors, were mixed to prepare UCNP@ZrMOF nanoparticles. As expected, the thickness of the ZrMOF layer increased from 8 nm to 15 nm and 35 nm when precursors increased by 3-fold and 15-fold, respectively (Figure 4A-4C). To further confirm this hypothesis, Au@ZrMOF was also studied with a constant number of Au nanoparticles, but different Figure 4. Controlled grafting of ZrMOF on UCNP and Au nanoparticles. A-C) UCNP@ZrMOF with ZrMOF layer thickness from 8 nm to 15 nm and 35 nm. D-F) Au-50@ZrMOF with ZrMOF layer thickness from 12 nm to 20 nm and 50 nm. Scale bar: A-E is 50 nm; F is 100 nm. numbers of precursors. As shown in Figure 4D-4E, the thickness of the ZrMOF layer increased from 12 nm to 20 nm and 50 nm when the number of precursors increased by 4-fold and 20-fold, respectively. Thus, when nanoparticle seed is constant, more

precursors will generate more clusters and further contribute to the thickness of the ZrMOF layer. Incorporation of inorganic nanoparticles into MOF typically generates enhanced or selective catalytic activities, as well as collective or extended properties. For example, Au nanoparticles with a thin layer of MOF-5 showed excellent surface-enhanced Raman scattering property for highly sensitive detection of CO2.44 Pt cluster-doped MIL-101 (Fe) showed selective hydrogenation catalytic activity.22 By grafting ZrMOF on the surfaces of UCNPs, we obtained a dual-FRET (fluorescence resonance energy transfer)-induced singlet oxygen generation (Figure 5B) with nearinfrared light irradiation (NIR, 980 nm). UCNPs showed excellent NIR-responsive optical properties and potential applications in deep tissue engineering.45 TCPP, the organic precursor of ZrMOF, is an excellent photosensitizer for singlet oxygen generation.46

Figure 5. Dual-FRET optical property of UCNP@ZrMOF. A) UVVis absorption of ZrMOF (black) and emission of UCNP (red). B) Schematic illustration of dual-FRET of UCNP@ZrMOF. C) Emission of ZrMOF (black) and UCNP@ZrMOF (red). D) Singlet oxygen generation monitoring with SOSG and NIR irradiation (980 nm, 300 mW cm-2). Our as-prepared UCNPs have relatively broad emission in the range of 520-560 nm and 640-670 nm, while the wider absorption Q band range (500-750 nm) of TCPP has two overlaps with the emission of UCNP (Figure 5A). The two overlaps, FRET 1 (640660 nm emission from UCNP to TCPP) and FRET 2 (520-560 nm emission from UCNP to TCPP), enable the ZrMOF shell to generate singlet oxygen upon irradiating UCNP@ZrMOF with 980 nm light. A fluorescence study, as shown in Figure 5C, indicated that ZrMOF itself does not have any NIR (980 nm) responsive property, while UCNP@ZrMOF showed significant decrease in emission because of energy transfer (FRET 1 and FRET 2) from UCNP to ZrMOF. Furthermore, singlet oxygen generation from UCNP@ZrMOF was detected by singlet oxygen sensor green (SOSG) upon irradiating by 980 nm laser (Figure 5D). As irradiation time increased, more and more singlet oxygen was generated, demonstrating the utility of this dual FRET-based singlet oxygen generation from UCNP@ZrMOF. Intracellular singlet oxygen detection by SOSG from Confocal Laser Scanning Microscopy (CLSM) also confirmed this dual-FRET optical property of UCNP@ZrMOF, suggesting potential application in photodynamic cancer therapy (Figure S19). Conclusions In summary, we have demonstrated a fundamental Ostwald ripening-mediated grafting of MOF on single nanocrystal, forming nanoparticle level MXF and applied this process to a library of inorganic colloidal nanoparticles, irrespective of size and shape. A



uniform and controlled thickness of ZrMOF layer of MXF was achieved by controlling the ratio of NP and precursors of ZrMOF. In addition, benefiting from this core shell hybrid MXF structure, the as-prepared UCNP@ZrMOF exhibited excellent dual-FRET NIR-responsive optical properties. This Ostwald ripeningmediated grafting of MOF on single nanocrystal strategy provides the momentum towards the design of inorganic nanoparticle level MXF hybrid structures for applications in catalysis, energy, and biochemistry. ASSOCIATED CONTENT Supporting Information. Detailed TEM, FT-IR, Zeta-potential, NMR, optical absorption spectra, and confocal laser scanning microscopy. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author [email protected] [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors thank K. Williams and K. Abboud at the University of Florida for helpful discussions and suggestions. This work is supported by NSFC grants (NSFC 21521063 and NSFC 21327009), the U.S. National Institutes of Health (GM079359 and CA133086), and the intramural research program of the National Institute of Biomedical Imaging and Bioengineering (NIBIB), National Institutes of Health (NIH). REFERENCES (1) Liu, P.; Zhao, Y.; Qin, R.; Mo, S.; Chen, G.; Gu, L.; Chevrier, D.; Zhang, P.; Guo, Q.; Zang, D.; Wu, B.; Fu, G.; Zheng, N. Photochemical route for synthesizing atomically dispersed palladium catalysts. Science 2016, 352, 797–802. (2) Sun, J.; Lee, H. W.; Pasta, M.; Yuan, H.; Zheng, G.; Sun, Y.; Li, Y.; Cui, Y. A phosphorene-graphene hybrid material as a high-capacity anode for sodium-ion batteries. Nat. Nanotechnol. 2015, 10, 980–985. (3) Wang, F.; Deng, R.; Wang, J.; Wang, Q.; Han, Y.; Zhu, H.; Chen, X.; Liu, X. Tuning upconversion through energy migration in core-shell nanoparticles. Nat. Mater. 2011, 10, 968–973. (4) Sailor, M. J.; Park, J. Hybrid nanoparticles for detection and treatment of cancer. Adv. Mater. 2012, 24, 3779– 3802. (5) Sun, S.; Murray, C.; Weller, D.; Folks, L.; Moser, A. Monodisperse FePt nanoparticles and ferromagnetic. Science. 2000, 287, 3–7. (6) Liang, Y.; Li, Y.; Wang, H.; Zhou, J.; Wang, J.; Regier, T.; Dai, H. Co3O4 nanocrystals on graphene as a synergistic catalyst for oxygen reduction reaction. Nat. Mater. 2011, 10, 780–786. (7) Witham, C. A.; Huang, W.; Tsung, C. K.; Kuhn, J. N.; Somorjai, G. A.; Toste, F. D. Converting homogeneous to heterogeneous in electrophilic catalysis using monodisperse metal nanoparticles. Nat. Chem. 2010, 2, 36–41. (8) Schauermann, S.; Nilius, N.; Shaikhutdinov, S.; Freund, H. Nanoparticles for heterogeneous catalysis: new mechanistic insights. Acc. Chem. Res. 2013, 46, 1673– 1681.

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Ostwald ripening-mediated grafting of metal-organic frameworks on a

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