Regulating interaction with surface ligands on Au25 nanoclusters by multivariate metal–organic framework hosts for boosting catalysis

ABSTRACT While atomically precise metal nanoclusters (NCs) with unique structures and reactivity are very promising in catalysis, the spatial resistance caused by the surface ligands and structural instability poses significant challenges. In this work, Au25(Cys)18 NCs are encapsulated in multivariate metal–organic frameworks (MOFs) to afford Au25@M-MOF-74 (M = Zn, Ni, Co, Mg). By the MOF confinement, the Au25 NCs showcase highly enhanced activity and stability in the intramolecular cascade reaction of 2-nitrobenzonitrile. Notably, the interaction between the metal nodes in M-MOF-74 and Au25(Cys)18 is able to suppress the free vibration of the surface ligands on the Au25 NCs and thereby improve the accessibility of Au sites; meanwhile, the stronger interactions lead to higher electron density and core expansion within Au25(Cys)18. As a result, the activity exhibits the trend of Au25@Ni-MOF-74 > Au25@Co-MOF-74 > Au25@Zn-MOF-74 > Au25@Mg-MOF-74, highlighting the crucial roles of microenvironment modulation around the Au25 NCs by interaction between the surface ligands and MOF hosts.


INTRODUCTION
The design and fabrication of structurally precise metal sites are crucial for the development of highperformance heterogeneous catalysts [1 -3 ].Metal nanoclusters (NCs) with atomic-level structural precision have attracted intense attention due to their small size, uniform active sites and unique electronic structures, giving rise to outstanding activity and selectivity in various catalytic reactions [4 -7 ].Unfortunately, the surface of metal NCs is always covered with numerous organic ligands that substantially influence catalysis.On the one hand, these surface ligands are detrimental to the accessibility of metal sites [8 ,9 ]; on the other, they optimize the electronic structure of metal NCs and improve the activity and selectivity of specific reactions [10 -12 ].Therefore, the role of surface ligands is akin to a double-edged sword affecting the catalytic properties [13 ].In this context, it would be of great importance to preserve the strength yet suppress the drawback of surface ligands on metal NCs, so as to promote the catalysis of metal NCs.Inspired by the crucial roles of the surrounding microenvironment around catalytic centers in bio-enzyme catalysis [14 ], through the manipulation of the organic surface ligands on metal NCs, it might be possible to improve the substrate accessibility to and electronic structure of catalytic metal sites.To this end, it would be highly desired to develop porous hosts featuring well-defined structures and tunable interaction with the surface ligands.This would allow the regulation of their configuration and movement, thereby influencing the accessibility and activity of central metal sites.
To achieve the aforementioned goal, metalorganic frameworks (MOFs)-a class of crystalline porous materials with coordinatively unsaturated metal sites (for interacting with surface ligands) and tunable pore sizes (for hosting metal NCs)- would be promising candidates [15 -18 ].MOFs have been demonstrated to be very suitable for incorporating diverse catalytically active species, including single-atom metal sites [19 ,20 ], metal nanoparticles [21 ,22 ], organic molecules [23 ], enzymes [24 ,25 ], etc., into pore spaces for enhanced catalysis.There have been also a couple of reports on the integration of metal NCs with MOFs in catalysis [26 -31 ], where the regulation of surface ligands on metal NCs has not yet been investigated, except for ligand removal in the only study [32 ].Given that the structure and properties of metal NCs are particularly sensitive to the surrounding microenvironment [33 ], it would be reasonable to adopt MOFs to modulate the microenvironment around metal NCs by regulating their surface ligands based on their interaction with MOFs for enhanced catalytic performance [34 ].It is expected that the interactions between surface ligands of metal NCs and MOFs would effectively suppress the free vibration of surface ligands in solution and reduce their hindrance of substrate access to the metal sites [26 ].Moreover, the spatial confinement effect of MOFs would significantly improve the catalytic stability of metal NCs [27 -29 ].However, to our knowledge, it remains unknown how the interaction with porous hosts (e.g.MOFs) regulates the surface ligands on metal NCs for improved catalysis.
In this work, Au 25 (Cys) 18 NCs are encapsulated into representative multivariate MOFs, M-MOF-74 (M = Zn, Ni, Co, Mg), based on coordinated self-assembly and electrostatic interactions to afford Au 25 @M-MOF-74 (Fig. 1 ).Significant differences in the fluorescence properties of Au 25 (Cys) 18 are observed among these materials, which can be attributed to the restricted influence of M-MOF-74 encapsulation on the surface-ligand vibration behavior of Au 25 (Cys) 18 .Accordingly, the encapsulation of Au 25 NCs in M-MOF-74 significantly improves its catalytic efficiency in the intramolecular cascade reaction of 2-nitrophenyl cyanide.Furthermore, X-ray absorption spectroscopy (XAS) results indicate that the electron density of Au 25 (Cys) 18 and the length of the Au-Au bonds within the core of Au 25 (Cys) 18 can be systematically regulated in addition to the interaction strengths between M-MOF-74 and Au 25 (Cys) 18 NCs.Such interaction modulates the microenvironment of Au 25 (Cys) 18 , which improves the Au 25 accessibility and facilitates the electron transfer from the Au 25 NCs to substrates responsible for promoting the catalysis.As a result, the catalytic activity can be further regulated by doping the metal nodes of MOF-74 in the following sequence: Ni > Co > Zn > Mg.Moreover, Au 25 @Ni-MOF-74 exhibits far superior catalytic activity and stability to the control of Au 25 /Ni-MOF-74 that is made by supporting the Au 25 NCs on the outer surface of Ni-MOF-74.As far as we know, this is the first report on regulating interaction by MOFs with surface ligands of metal clusters, resulting in enhanced catalysis.

Synthesis and characterization of the Au 25 @Ni-MOF-74 catalyst
The Au 25 (Cys) 18 NCs were selected due to their relatively stable structure and abundance of carbonyl groups on the surface ( Fig. S1a) [35 ].It was synthesized by the NaOH-mediated NaBH 4 reduction method and the high purity of the synthesized Au 25 (Cys) 18 was confirmed by using UV-vis spectroscopy and high-angle annular darkfield scanning transmission electron microscopy (HAADF-STEM) ( Fig. S2) [35 ].To prevent structural damage to Au 25 (Cys) 18 under hightemperature solution conditions, in situ growth of the MOF-74 outer-shell was developed.The MOF linker, 2,5-dihydroxyterephthalic acid (DHTP) and Au 25 (Cys) 18 were pretreated in aqueous NaOH solution, inducing the deprotonation and subsequently promoting the dissolution of DHTP in water.This facilitates the reaction with metal ions, leading to the growth of MOF at room temperature, and promotes the dispersion and encapsulation of Au 25 (Cys) 18 in MOF-74.Subsequently, the metal ions underwent coordination self-assembly with Au 25 (Cys) 18 and DHTP at room temperature, resulting in the formation of Au 25 @MOF-74 [36 ].Taking advantage of the multivariate feature of MOFs, M-MOF-74 (M = Ni, Co, Mg) with Ni 2 + , Co 2 + or Mg 2 + in the Zn-oxo chains was obtained ( Fig. S1b), ensuring their similar crystallinity and sizes, by mixing these metal acetates together with zinc acetate in the synthesis of Zn-MOF-74.Accordingly, Au 25 @M-MOF-74 (M = Zn, Ni, Co, Mg) were fabricated following a similar synthetic route to Au 25 @MOF-74, which would serve as an ideal platform for regulating the interaction with the surface ligands of Au 25 (Cys) 18 .
In addition, due to the presence of a large number of negatively charged carboxyl groups on the surface, Au 25 (Cys) 18 can be attached to the outer surface of Ni-MOF-74 by electrostatic interaction to yield Au 25 (Cys) 18 /Ni-MOF-74 [30 ,31 ].Powder X-ray diffraction (XRD) patterns indicate that the M-MOF-74 has similar crystallinity and Au 25 (Cys) 18 encapsulation or support has no influence on the structural integrity and crystallinity of the MOFs ( Fig. S3).Nitrogen sorption results demonstrate that Au 25 @M-MOF-74 and Au 25 /Ni-MOF-74 maintain the high porosity and very similar pore size distribution, indicating that the introduction of Au 25 (Cys) 18 does not affect the MOF microporous structure ( Fig. S4).The Brunauer-Emmett-Teller surface area of Au 25 @Ni-MOF-74 is 639 m 2 /g, similar to that of Au 25 @Zn-MOF-74 (604 m 2 /g), Au 25 @Co-MOF-74 (609 m 2 /g) and Au 25 @Mg-MOF-74 (610 m 2 /g), reflecting that different metal nodes have little influence on the surface area.Due to the pore space occupation of MOF-74 by the Au 25 NCs, their surface areas are reasonably lower than Au 25 /MOF-74 (667 m 2 /g).For Au 25 @M-MOF-74, Au 25 (Cys) 18 can be isolated by the removal of MOF-74 using dilute hydrochloric acid.UV-vis spectra for the isolated Au 25 (Cys) 18 NCs, upon removing the MOF, showcase the characteristic bands ( Fig. S5), demonstrating that MOF-74 encapsulation does not influence the integrity of the Au 25 (Cys) 18 structure.
Scanning electron microscopy (SEM) images show that Au 25 @M-MOF-74 and Au 25 /Ni-MOF-74 have similar particle sizes and morphology (Fig.  S12).The Au loading amount in Au 25 @M-MOF-74 is controlled to be ∼2 wt% and the mixed metal molar ratio in M-MOF-74 is also maintained at ∼1 according to inductively coupled plasma atomic emission spectroscopy (ICP-AES) results ( Table S1).

Performance of catalytic reduction of 2-nitrobenzonitrile
Given the unique surface and electronic structure of Au 25 (Cys) 18 NCs, they can serve as electron mediators to initiate catalysis via single electron transfer [37 ,38 ].Therefore, the reduction reaction of 2-nitrobenzonitrile containing electron-transfer processes is adopted, which is an important route for the production of significant pharmaceuticals precursor 2-amniobenzamide [37 ].Control experiments indicate that no product is detected in the absence of catalysts or with the use of the four M-MOF-74 catalysts ( Table S2).When adopting Au 25 (Cys) 18 NCs, the activity and selectivity are 20.6% and 91.0%, respectively, indicating that the Au 25 NCs are able to behave as active species.Unfortunately, they are prone to aggregation, as supported by the UV-vis spectrum and HAADF-STEM observation ( Fig. S13).Moreover, no obvious difference in catalytic activity is observed when the same amount of Au 25 NCs is physically mixed with the four M-MOF-74, respectively, indicating that M-MOF-74 does not affect the catalytic reaction process ( Table S2).
Strikingly, the encapsulation of Au 25 (Cys) 18 NCs in M-MOF-74 significantly improves the activity, in which the conversion and selectivity reach 99.8% and 99.2%, respectively, for Au 25 @Ni-MOF-74 (Fig. 3 a).In comparison, the conversion and selectivity of Au 25 @Co-MOF-74, Au 25 @Zn-MOF-74 and Au 25 @Mg-MOF-74 are decreased, with conversion of 72.7%, 46.2% and 32.1%, and selectivity of 97.1%, 94.3% and 93.5%, respectively (Fig. 3 a).The reaction yield gradually increases along with reaction time, showcasing activity in the order of Au 25 @Ni-MOF-74 > Au 25 @Co-MOF-74 > Au 25 @Zn-MOF-74 > Au 25 @Mg-MOF-74 (Fig. 3 b).The results indicate that the metal doping in M-MOF-74 exerts a pivotal influence on the catalytic efficiency of Au 25 (Cys) 18 .Powder XRD patterns of Au 25 @M-MOF-74 do not exhibit any discernible decrease in the MOF crystallinity after the catalytic reaction ( Fig. S14); the UV-vis spectra of the detached Au 25 NCs upon the MOF removal exhibit no substantial alterations with the as-synthesized Au 25 NCs, effectively demonstrating the structural integrity of both components in Au 25 @M-MOF-74 during the catalytic reaction (Fig. 3 c).As a control, the Au nanoparticles (NPs) with surface ligands of Cys were synthesized; the UV-vis spectrum displays a characteristic surface plasmon resonance peak at 520 nm and the HAADF-STEM image indicates that the Au NPs are ∼2 nm in size ( Fig. S15).Afterward, they were encapsulated into Ni-MOF-74 to obtain Au NPs @Ni-MOF-74 with ret ained Au size s and good MOF crystallinity ( Fig. S16) via the same coordination self-assembly as that of Au 25 @Ni-MOF-74.The conversion and selectivity of Au NPs @Ni-MOF-74 in the reaction are only 15.2% and 89.5%, respectively ( Table S2).This is possibly attributed to the fact that the charge-transfer effect on the surface of Au NPs is too weak to efficiently facilitate the migration of electrons from NaBH 4 to the substrate [37 ].
The incorporation of Au 25 (Cys) 18 into M-MOF-74 has been demonstrated to not only influence the activity as noted above, but also strongly improve the catalytic stability.The catalytic yield of Au 25 @Ni-MOF-74 is maintained at 99% for 1 h in the three cycles; in contrast, the yield of Au 25 /Ni-MOF-74 give s 25% in the first cycle and decreases continuously during the next two cycles ( Fig. S17), possibly due to the unexpected leaching or aggregation of Au 25 (Cys) 18 .HAADF-STEM images show that the size and dispersion of Au 25 (Cys) 18 in Au 25 @Ni-MOF-74 remain unchanged after three reaction cycles due to the confined protection by the MOF, whereas significant agglomeration of Au 25 NCs is observed in Au 25 /Ni-MOF-74 ( Fig. S18).No noticeable change occurs in the Au content in Au 25 @Ni-MOF-74 after the reaction; however, a decrease of ∼10% can be found for Au 25 /Ni-MOF-74 ( Table S1).These results exemplify the much enhanced stability of Au 25 (Cys) 18 by MOF encapsulation.
The stability and recyclability of Au 25 @Ni-MOF-74 have been further demonstrated.The activit y and selectivit y exhibit no significant decrease in the five consecutive c ycles w ith a controlled reaction time of 40 min and ∼80% conversion (Fig. 3 d).Powder XRD patterns, HAADF-STEM images and corresponding EDS elemental mapping analyses suggest that the catalyst microstructure can be retained after five consecutive cycles ( Figs S19 and S20).In addition, the results of the hot filtration experiment for Au 25 @Ni-MOF-74 manifest that no leaching occurs in Au 25 NCs and the process is truly heterogeneous catalysis ( Fig. S21).

Mechanism of the catalytic reaction
Given the significant differences in the activity of Au 25 @M-MOF-74, relevant investigations have been conducted to understand the intrinsic mechanisms involved.The variations in fluorescence emission intensity and wavelength can reflect the extent of motion associated with the vibration and rotation of the ligands on the surface of the metal NCs [39 ,40 ].Under excitation of 420 nm, the photoluminescence (PL) spectrum of Au 25 (Cys) 18 is in the visible region with a maximum value of ∼720 nm (Fig. 4 a) and the PL of Au 25 @M-MOF-74 is substantially enhanced in intensity and undergoes an obvious blue shift compared with that of Au 25 (Cys) 18 .The phenomenon can be ascribed to the coordination interactions between the metaloxo chain in M-MOF-74 and the free carboxyl X-ray photoelectron spectroscopy (XPS) analysis demonstrates a significant increase in the Au electron density of Au 25 (Cys) 18 upon integration with M-MOF-74 (Fig. 4 c).Interestingly, the electron density of Au in Au 25 @M-MOF-74 is in a sequence that is consistent with their activity order, disclosing the fact that the charge-transfer interaction between Au 25 (Cys) 18 and M-MOF-74 benefits the activity.In addition, the change in the Au electron density in Au 25 /Ni-MOF-74 is relatively small, further suggesting that Au 25 (Cys) 18 supported on MOF cannot create a strong interaction with Au 25 (Cys) 18 .
UV-vis spectroscopy and electron spin resonance (ESR) spectroscopy are further adopted to investigate the electron-transfer process in the catalytic reactions.The Au 25 NCs with different charges have unique UV-vis absorption spectra, which can clearly distinguish the charge states of Au 25 NCs [42 ].When 2-nitrobenzonitrile is added as the substrate to the Au 25 (Cys) 18 with a negative charge (abbreviated as Au 25 − ) in the aqueous solution, its UVvis spectrum displays a distinct change from the characteristic spectrum of uncharged Au 25 (Cys) 18 (abbreviated as Au 25 0 ).After NaBH 4 is added, the characteristic absorption peak of Au 25 − reappears ( Fig. S22), indicating the recovery of Au 25 − .Furthermore, ESR experiments indicate that Au 25 @Ni-MOF-74 and the mixture of Ni-MOF-74 with 2nitrobenzonitrile do not result in any signal, whereas the mixture of Au 25 (Cys) 18 or Au 25 @Ni-MOF-74 with 2-nitrobenzonitrile results in a triple peak corresponding to the N radicals ( Fig. S23).These results confirm that Au 25 (Cys) 18 is an electronic mediator that continuously transfers electrons from NaBH 4 to the substrate.In addition, a range of catalytic experiments with substrate analogs have been performed ( Table S3).The functional group of the substrates can be efficiently and completely reduced when the unsaturated group is placed at the ortho position only, suggesting that an intermolecular cascade reaction has occurred.To further investigate the catalytic mechanism, deuterium-labeling experiments are also conducted.When H 2 O is replaced by D 2 O and NaBH 4 is replaced by NaBD 4 in the reaction system, the molecular weight of the products detected by using mass spectrometry increases ( Fig. S24), implying that H 2 O and NaBH 4 are involved in the reaction.As a result, reaction paths can be proposed ( Fig. S25).

The geometric structure and electronic properties of Au 25 (Cys) 18
Given that the X-ray absorption fine structure (XAFS) is sensitive to the structure of metal NCs [43 ,44 ], the Au L 3 -edge XAFS is adopted to investigate the effect of MOF encapsulation on the geometric structure and electronic properties of Au 25 (Cys) 18 .X-ray absorption near edge structure (XANES) spectra display obvious differences between the peak profiles of Au 25 (Cys) 18 and fcc-structured Au foil (Fig. 5 a).Moreover, the peak profile of Au 25 (Cys) 18 almost remains after its integration with M-MOF-74, suggesting that the structure of Au 25 (Cys) 18 is preserved in Au 25 @M-MOF-74, which is in agreement with the above results.The white line peak corresponds to the electronic transition from the core level to the unoccupied 5d valence level and the variation in the intensity of the white line peak can drive the absorption edge to different directions of energy, thus explaining different electronic states [45 ].In comparison with that of the Au foil, the spectrum of Au 25 (Cys) 18 exhibits a more intense white line peak at ∼11 924 eV, which is due to the electron-w ithdraw ing property of the thiol ligand and the reduced 5d electron density of surface Au atoms.On the other hand, the white line intensity of Au 25 @M-MOF-74 is lower than that of Au 25 (Cys) 18, suggesting the greater occupation of the 5d electronic state in Au 25 @M-MOF-74.This is likely due to the MOF's constraining thiol ligand vibrations on the Au 25 (Cys) 18 surface, thereby weakening the electron-w ithdraw ing effect of the thiol lig-and toward the Au atoms.In addition, the intensity order of the white line peaks in Au 25 @M-MOF-74 is further demonstrated by means of differential spectroscopy of XAFS, which is consistent with the results of XPS analysis ( Fig. S26), confirming that the microenvironment modulation caused by the interaction between M-MOF-74 and the thiol ligand regulates the Au 25 electronic structure.
To quantify the local atomic structure of Au 25 (Cys) 18 in MOF-74, the extended X-ray absorption fine structure (EXAFS) region has been analysed.All Au 25 @M-MOF-74 samples exhibit osci l lation patterns that are similar to those of Au 25 (Cys) 18 , whereas the osci l lation intensity increases, suggesting that the surrounding microenvironment modulation driven by the MOF encapsulation can affect the degree of Au 25 (Cys) 18 disorder ( Fig. S27).Fourier transform (FT) is performed in the R-space of the EXAFS spectra for Au 25 (Cys) 18 and Au 25 @M-MOF-74 (Fig. 5 b).For Au 25 (Cys) 18 , three prominent features are shown at 1.9, 2.4 and 2.7 Å; thus, the fittings are performed using three coordination paths.As previously reported [46 ], the first peak represents the S atom directly coordinated to Au, i.e.Au-S on the motif of Au 25 (Cys) 18 , and the other two peaks represent contributions from two Au-Au pathways (Fig. 5 c).By comparing the curve-fitting analysis of Au 25 (Cys) 18 and Au 25 @M-MOF-74 EXAFS results ( Table S4), it is observed that the bond length of Au-S is very similar ( Fig. S28), while Au-Au-1 and Au-Au-2 increase when Au 25 (Cys) 18 NCs are encapsulated in Ni-MOF-74 and Co-MOF-74, suggesting that the Au 25 (Cys) 18 nucleus undergoes expansion in response to the stronger interaction with M-MOF-74 (Fig. 5 d).Notably, the Au-Au bond length sequence in Au 25 @M-MOF-74 is in good agreement with their catalytic activity, suggesting that the catalytic performance of metal NCs can be optimized by manipulating the metal-oxo chains in M-MOF-74 hosts.Furthermore, the Au L 3 -edge X ANES and EX AFS spectra of Au 25 @Ni-MOF-74 after the reaction are similar to those before the reaction ( Fig. S29), further supporting that the structure of the Au 25 NCs in Au 25 @Ni-MOF-74 barely changes after the catalytic reaction, matching the UV-vis spectra above (Fig. 3 c) and demonstrating that MOF encapsulation improves the structural stability of metal NCs.
Based on the above analysis, it can be concluded that the strong coordination interaction between M-MOF-74 and Au 25 (Cys) 18 results in the rigidification of surface ligands and the expansion of the Au nucleus.Given that ligand vibrations on the surface of Au 25 NCs in the reaction solution prevent the substrate from accessing the Au sites, rigidifying the surface ligands and expanding the gold nucleus are expected to increase the available spatial domain for substrate accessibility, thereby boosting activity.Additionally, the different interaction between tunable M-MOF-74 and surface ligands of Au 25 (Cys) 18 effectively regulates the electron transfer from the MOF to Au 25 NCs.Due to the electrophilicity of the nitro and cyano groups in 2-nitrobenzonitrile, metal surfaces with higher electron densities wi l l give stronger interactions with the substrate [47 ,48 ].Overall, optimization of the accessibility and electron density of the Au 25 NCs facilitates electron transfer with the substrate and promotes the conversion [49 ].

CONCLUSION
In summary, atomically precise Au 25 (Cys) 18 NCs have been successfully encapsulated in M-MOF-74 with different metal nodes through coordination self-assembly, yielding Au 25 @M-MOF-74 composites for the intramolecular cascade reaction of 2-nitrobenzonitrile.Strikingly, the activity and stability of Au 25 (Cys) 18 are significantly enhanced upon being incorporated into MOF-74, surpassing those of Au 25 /MOF-74.Remarkably, doping different metal species into the metal-oxo chains in MOF-74 showcases significant activity difference in the order of Au 25 @Ni-MOF-74 > Au 25 @Co-MOF-74 > Au 25 @Zn-MOF-74 > Au 25 @Mg-MOF-74.Both fluorescence and XAFS analyses demonstrate that the engineering of metal-oxo nodes in MOFs gives rise to the rigidification of surface ligands on Au 25 (Cys) 18 and induces expansion at the Au nucleus level, improving the accessibility of the Au sites.Moreover, the stronger coordination interaction between M-MOF-74 and Au 25 (Cys) 18 further increases the electron density of Au 25 NCs, which is favorable to the substrate activation, leading to enhanced activity.This work describes improved single electron transfer and metal site accessibility of metal NCs by regulating the interaction between their surface ligands and multivariate MOF hosts, which opens a new avenue for boosting the catalysis of metal NCs by surface microenvironment modulation.

MERHODS Synthesis of Au 25 @Zn-MOF-74
Typically, 0.2 mL of aqueous solution of Au 25 (Cys) 18 (15 mg/mL) was added to 1 mL of 0.73 M NaOH aqueous solution of DHTP (0.183 mmol) at 25°C.Then, 1 mL of aqueous solution of Zn(CH 3 COO) 2 •2H 2 O (0.732 mmol) was mixed with the solution, sonicated for 1 min and stirred for 6 h at 25°C.The precipitate was collected by centrifugation and washed with H 2 O and MeOH three times.Finally, the precipitate was soaked in MeOH for 48 h and dried under a vacuum at 60°C overnight.
2 a and Fig. S6).From the high-angle annular darkfield-scanning transmission electron microscopy (HAADF-STEM) images, it can be observed that Au 25 (Cys) 18 NCs are uniformly dispersed throughout the Ni-MOF-74 support (Fig. 2 b and Fig. S7a).Direct comparison of the HAADF-STEM and secondary electron STEM (SE-STEM) images acquired at the same location provides direct evidence that Au 25 (Cys) 18 NCs are encapsulated inside Ni-MOF-74 in Au 25 @Ni-MOF-74, as indicated by unobservable Au 25 NCs in the SE-STEM image (Fig. 2 b and c).By comparison, the Au 25 NCs can be clearly observed in both images, showing that the Au 25 NCs are supported on the surface of Ni-MOF-74 in Au 25 /Ni-MOF-74 ( Fig. S8).HAADF-STEM images of Au 25 @Ni-MOF-74 projected at different tilt angles (from + 30°to −30°) show that the Au 25 NCs remain monodispersed in the Ni-MOF-74 ( Fig. S9).The HAADF-STEM images and corresponding energy dispersive X-ray spectroscopy (EDS) mapping analyses reveal that both metals in M-MOF-74 and Au 25 (Cys) 18 are uniformly distributed throughout the MOF particle (Fig. 2 d and e, and Figs S9-