Paving Metal-Organic Frameworks with Upconversion Nanoparticles

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Paving Metal-Organic Frameworks with Upconversion Nanoparticles via Self-Assembly Ze Yuan, Lu Zhang, Shaozhou Li, Weina Zhang, Min LU, Yue Pan, Xiaoji Xie, Ling Huang, and Wei Huang J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 23 Oct 2018 Downloaded from http://pubs.acs.org on October 23, 2018

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Journal of the American Chemical Society

Paving Metal-Organic Frameworks with Upconversion Nanoparticles via Self-Assembly Ze Yuan,† Lu Zhang,† Shaozhou Li,‡ Weina Zhang,† Min Lu,† Yue Pan,† Xiaoji Xie,*,† Ling Huang,*,† and Wei Huang*,†,‡,§ †

Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, P.R. China

‡

Key Laboratory for Organic Electronics and Information Displays & Institute of Advanced Materials (IAM), Nanjing University of Posts & Telecommunications, 9 Wenyuan Road, Nanjing 210023, P.R. China § Shaanxi Institute of Flexible Electronics, Northwestern Polytechnical University, Xi'an, China ABSTRACT: The combination of metal-organic frameworks (MOFs) and luminescent nanomaterials with upconversion characteristics could enable the development of new nanomaterials and applications in information security, optical sensing and theranostics. However, currently available methods are not ideally suitable for fabricating composites of MOF and upconversion nanomaterial, and incorporating upconversion nanomaterials with MOFs in a controllable manner remains challenging. Here, we demonstrate an in situ self-assembly route to the nanocomposites in which MOFs are homogeneously paved with upconversion nanoparticles. Without additional assistance, this strategy, mainly under the control of electrostatic interactions, can be used to incorporate different upconversion nanoparticles with diverse MOFs. The as-synthesized composites can be further used to construct composites with unique structures, like MOF@upconversion nanoparticles@MOF sandwiched nanocomposites, and be useful for applications, including luminescence-monitored drug delivery, anti-counterfeiting and photodynamic therapy. These findings should shed light on new avenues to fabricate multifunctional composites of MOF and upconversion nanomaterial for varied applications.

are fabricated by MOFs and quantum dots.20-23 Until recently, lanthanide doped upconversion nanoparticles (UCNPs), which can convert low energy photons, like near infrared light, into high energy photons, such as visible and ultra-violet light, have been integrated with MOFs.24-28 One of the main reasons for the lack of luminescent MOF nanocomposites should be the lack of suitable fabrication methods for luminescent nanomaterials.

INTRODUCTION With the rapid development of nanoscience and nanotechnology, composites constructed by different nanomaterials recently have emerged as promising multifunctional candidates for varied applications, including catalysis, sensing, and therapy.1-3 One distinct characteristic of these multi-nanomaterial fabricated composites is their sophisticated and multifunctional properties, which are greater than the sum of the individuals or their physical mixtures.4 Among diverse analogues, metal-organic framework (MOF) based nanocomposites have received continuous research enthusiasm in the past few years, due to their unique features like tunable structure and well-defined pores.5-10 Indeed, it has been demonstrated that integrating MOFs with other nanomaterials can not only overcome some intrinsic limitations associated with individual material, but also endow MOFs with new functionalities.11-14 For example, when encapsulated within MOFs, Pd nanoparticles can show improved stability, selectivity and activity in catalytic reactions.15 Furthermore, the MOF composites can be transferred to other functional nanomaterials, such as porous metal oxides, metal/metal carbides and porous carbons.16-20

Luminescent MOF nanocomposites currently are mainly fabricated by assembling MOFs around nanomaterials (known as the “bottle around ship” strategy).6,12,21-29 Unfortunately, the typically used approaches still suffer from some limitations. For example, the “bottle around ship” strategy usually requires premodification of luminesecent nanomaterials by capping agents,30 such as polyvinylpyrrolidone, to stabilize the nanomaterials and facilitate the coating of MOFs. Regretfully, the capping agents, which are difficult to be completely removed, may act as luminescence quenchers,31,32 charge/energy transfer barriers,33 and catalytic/absorption hinders,34 and thus are deleterious for further functionalization and applications. Moreover, currently used methods typically are only suitable for a certain MOF and luminescent nanomaterial, and further optimizations are required when the MOF or nanomaterial is changed.6,7,12 New fabrication strategies therefore are highly desired to circumvent the limitations.

Despite the progress and great promise of MOF based nanocomposites, only a few, exhibiting unique luminescent properties that are hard to realize in pure MOFs, have been constructed.7,21 Most of the luminescent MOF nanocomposites 1

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Figure 1. Schematic illustration of the fabrication of upconversion nanoparticles (UCNPs) and metal-organic framework (MOF) nanocomposites. The reaction precursors of MOF and ligand-free UCNPs are mixed directly and the nanocomposites are formed in suit. Yellow shaded steps show the proposed formation mechanisms: (i) MOF nucleation, (ii) attachment of nanoparticles onto MOFs through electrostatic interaction, and (iii) nanocomposite formation. Note that MOF@UCNPs denotes MOF-UCNPs nanocomposites.

Self-assembly of nanomaterials, driven by the interactions including Van der Waals interactions, electrostatic interactions and hydrogen bonds, offers an alternative pathway to construct luminescent MOF nanocomposites.35-40 Herein, we demonstrate a facile strategy for the fabrication of luminescent nanocomposites of MOF and UCNPs without the addition of capping agents. Different from conventional methods, in our strategy, MOFs are in situ paved with UCNPs mainly through self-assembly driven by electrostatic interaction (Figure 1). Particularly, our method allows the fabrication of UCNPs with various MOFs, including zirconium(IV) 2-aminoterephthalic acid MOF (UiO-66-NH2), zirconium(IV) 1,4benzodicarboxylic acid MOF (UiO-66), zirconium(IV) fumaric acid MOF (MOF-801), and zirconium(IV) tetrakis (4carboxyphenyl) porphyrin MOF (PCN-223). Furthermore, the distribution of UCNPs in the composites can be controlled via epitaxial growth of MOF layers, forming MOF@nanoparticles@MOF core-shell-shell sandwiched nanocomposites. Decent performance of the multifunctional MOF-UCNPs nanocomposites have been demonstrated in luminescence-monitored drug delivery, anti-counterfeiting, as well as photodynamic therapy.

kept for 90 min before cooling down to room temperature. The resulting NaYF4:Yb/Er nanoparticles were precipitated by ethanol, collected by centrifugation and washed with ethanol. Secondly, the oleic acid molecules on the surface of NaYF4:Yb/Er nanoparticles were removed by acid treatment. Generally, the obtained NaYF4:Yb/Er nanoparticles (10 mg) were dispersed in ethanol (0.75 mL) by sonication and then mixed with hydrochloric acid (HCl, 0.75 mL, 1 M). The resulting mixture was sonicated for 1 min to remove oleic acid, and the nanoparticles were then collected by centrifugation (16000 rpm, 15 min). The acid treatment was repeated one more time to ensure the complete removal of oleic acid. The ligand-free NaYF4:Yb/Er upconversion nanoparticles were wash with ethanol and redispersed in DMF for further use. Preparation of UiO-66-NH2 and ligand-free nanoparticle nanocomposites. In a typical experiment, a DMF solution (10 mL) containing ZrCl4 (4.375 mM) and 2-aminoterephthalic acid (4.005 mM) was first mixed with acetic acid (1.2 mL), and the resulting mixture was then sonicated for 5 min. Subsequently, the mixture was sonicated for another 5 min after the addition of a DMF dispersion of ligand-free upconversion nanoparticles (0.2 mL, 3 mg). The reaction mixture was then transferred to a vial and kept at 120 oC for 24 h, yielding the nanocomposites. The as-synthesized nanocomposites were collected by centrifugation (6000 rpm, 5 min), washed with DMF twice and methanol once, and finally redispersed in methanol for further use. Other experimental details are provided in the Supporting Information.

EXPERIMENTAL SECTION Preparation of ligand-free NaYF4:Yb/Er upconversion nanoparticles. Ligand-free upconversion nanoparticles were prepared by two steps according to previous reports.41,42 First, oleic acid coated NaYF4:Yb/Er (18/2 mol%) nanoparticles were synthesized by the thermal coprecipitation method.41 Typically, an aqueous solution (2 mL), containing Y(Ac)3 (0.32 mmol), Yb(Ac)3 (0.072 mol), and Er(Ac)3 (0.008 mmol), was mixed with oleic acid (3 mL) and 1-octadecene (7 mL) in a 50 mL two-neck round-bottom flask. The mixture was heated at 150 oC for 60 min to remove water, followed by cooling down to 50 oC. After mixed with a methanol solution (6 mL) containing NH4F (1.6 mmol) and NaOH (1 mmol), the reaction mixture was stirred at 50 oC for 30 min and then heated at 100 oC to remove the low boiling solvent. Subsequently, the mixture was heated to 290 oC under a nitrogen atmosphere and

Characterization. Powder X-ray diffraction analysis was carried out on a Rigaku Smartlab (9 kW) X-ray diffractometer, using Cu Kα radiation (λ = 1.5406 Å). Upconversion luminescence spectra were recorded in a Horiba FluoroLog-3 spectrofluorometer coupled with a 980 nm diode laser. Fourier transform infrared (FTIR) spectra were recorded on a Nicolet (iS10) FTIR spectrometer. Thermogravimetric analysis (TGA) was performed on a TGA2 thermogravimetric analyzer (Mettler Toledo) with a heating rate of 10 oC/min under an N2 flow of 50 mL/min. Scanning electron microscopy (SEM) measurements were performed at both Hitachi S-4800 and JEOL JSM-7800F filed emission scanning electron microscopes. 2


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Figure 2. Characterization of the nanocomposites fabricated by NaYF4:Yb/Er and UiO-66-NH2. (a-b) Typical SEM and TEM images of the obtained UiO-66-NH2 and ligand-free NaYF4:Yb/Er nanocomposites. The inset in (b) is the TEM image of a single nanocomposite. (c) STEM image of a single nanocomposite. (d-e) Corresponding elemental mapping of a single nanocomposite shown in (c). (f) SEM image of the as-synthesized nanocomposites after the treatment of a focused ion beam and the cross sections of the nanocomposites indicate that UCNPs are not encapsulated inside MOF crystals. (g-i) SEM, TEM and STEM images of a randomly chosen nanocomposite after treated with hydrochloric acid. (j) Corresponding elemental mapping of a single nanocomposite shown in (i). Scale bars are 100 nm for (c-e and g-j) and 200 nm for (f). (k) N2 adsorption-desorption isothermals (solid circle: adsorption, empty circle: desorption) and (l) thermogravimetric profiles of UiO-66-NH2 MOF and UiO-66-NH2@NaYF4:Yb/Er nanocomposites, respectively. (m) Room temperature upconversion emission spectrum of UiO-66-NH2@NaYF4:Yb/Er nanocomposites dispersed in DMF under the excitation of a 980 nm diode laser. The insets in (m) are photos of UiO-66-NH2@NaYF4:Yb/Er nanocomposites dispersed in DMF under daylight (left) and excitation of a 980 nm laser (right).

Low resolution transmission electron microscopy (TEM) measurements and TEM tomography analysis were performed at a JEOL JEM-1400 plus transmission electron microscope at an acceleration voltage of 120 kV. High resolution TEM (HRTEM) images, scanning TEM (STEM) images, and energy-dispersive X-ray (EDX) spectra were obtained on a JEOL JEM-2100F transmission electron microscope at an acceleration voltage of 200 kV. Zeta potential measurements were carried out on a NanoPlus-3 analyzer using DMF as the solvent at 25 oC. Microscopy images was obtained on an Olympus BX53 microscope equipped with a 980 nm diode laser and a Nikon DS-Ri2 imaging system.

spectroscopy (Figure S2). After ligand removal, the nanoparticles remained their crystal structure, morphology and upconversion properties, although the size of the nanosparticles was slightly decreased due to the acid etching (Figure S3). We then selected UiO-66-NH2 as the host material to synthesis the luminescent MOF nanocomposites. In contrast to previously reported strategies,7,25-28,30 we here directly mixed the ligand-free NaYF4:Yb/Er UCNPs with MOF precursors and then kept the reaction mixture at 120 oC for 24 h. The resulting UiO-66-NH2 based nanocomposites, as precipitations in the reaction mixture, were then collected. As shown in Figure 2a, scanning electron microscopy (SEM) image reveals that the assynthesized nanocomposites, ~400 nm in size, are octahedron geometry and homogeneously decorated with small nanoparticles on the surface. Power X-ray diffraction (XRD) pattern (Figure S4) confirms that the nanocomposites are composed of UiO-66-NH2 and NaYF4:Yb/Er UCNPs. Transmission electron microcopy (TEM) and scanning TEM (STEM) images (Figure 2b, 2c and S5), together with energydispersive X-ray (EDX) analysis (Figure S6) and elemental mapping of a single nanocomposite (Figure 2d-e and S7), further confirm the efficient integration of UCNPs with UiO66-NH2.

RESULTS AND DISCUSSION Synthesis and Characterization of Nanocomposites. To fabricate the luminescent MOF composites, we first synthesized hexagonal NaYF4 UCNPs, doped with Yb/Er (18/2 mol%), by the thermal coprecipitation method, because of the sophisticated synthesis and their intense upconversion emission.41,42 The resulting nanoparticles capped with oleic acid ligands have a size of ~26 nm with single-crystalline nature (Figure S1), and upon the exaction of 980 nm laser, the nanoparticles can emit characteristic Er3+ emission at ~550 and 650 nm (Figure S1). The surface ligands, oleic acid, were then removed through acid treatment,43 yielding ligand-free UCNPs, as confirmed by Fourier transform infrared (FTIR)

However, a close check on the results of elemental mapping indicates that the distribution of upconversion nanoparticles may be inhomogeneous in the composites, where fewer 3



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N2 absorption-desorption analysis (Figure 2k) reveals that the Brunauer-Emmett-Teller (BET) surface area of UiO-66NH2@NaYF4:Yb/Er nanocomposites (~864 m2/g) is smaller than that of pure UiO-66-NH2 MOF (~1281 m2/g), which should be mainly due to the heavy and nonporous UCNPs. Nevertheless, the isothermal features (Figure 2k) and porous structure (Figure S12) of the nanocomposites are similar to those of pure UiO-66-NH2. Thermal gravimetric analysis (Figure 2l), under nitrogen flow, shows that the nanocomposites begin to decompose at ~400 oC, which is almost identical to pure UiO-66-NH2 MOF. These results indicate that the structure and stability of MOF remains intact after paved with nanoparticles. Furthermore, the nanocomposites exhibit characteristic Er3+ emission (Figure 2m), which is the same as that of pure NaYF4:Yb/Er UCNPs (Figure S1). Taken together, we reason that this binding manner can ensure the stability of the nanocomposites and also preserve the properties of each part of the nanocomposites.

nanoparticles were found in the center of the nanocomposite (Figure 2d and S7). TEM tomography analysis also indicates that few UCNPs are encapsulated inside of MOF crystals (a movie provided in Supporting Information). To identify the positions of UCNPs, the as-synthesized composites were cut by a focused ion beam (FIB) and then characterized by SEM (Figure 2f and S8). Surprisingly, no UCNP was found inside the MOF crystals. These results reveal that UCNPs are only decorated on the surface, instead of the inner part, of UiO-66NH2 crystals, and thus we denoted the nanocomposites as UiO66-NH2@NaYF4:Yb/Er. Although all the UCNPs are on the surface of MOF crystals, they bind firmly enough on MOF surface. For example, the nanocomposites can resist a 30 min sonication without obvious change (Figure S9). In order to further reveal the status of the UCNPs in the nanocomposites, we immerged the nanocomposites into hydrochloric acid solution. Under strong acidic conditions, ligand-free UCNPs can be etched, while UiO66-NH2 crystals can remain intact. As expected, SEM, TEM and STEM images (Figure 2g-j and S10), together with EDX analysis (Figure S11), reveal that UCNPs are etched, leaving nanoscale pits on the surface of UiO-66-NH2 crystals, while MOF crystals maintain their structure and morphology. These results further imply that the assembly of UCNPs on MOF happens during the growth of MOF crystals in solution (please see Figure 3 for detailed discussion) and finally the ligand-free UCNPs are partially buried in the MOF crystals, just like the streets paved with cobblestones. These results also demonstrate that the assembly of UCNPs on MOF happens in solution not on the substrate during drying.

Formation Process and Mechanisms. To reveal the formation process of the UiO-66-NH2@NaYF4:Yb/Er nanocomposites, we first monitored the reaction by extracting reaction mixtures at different time intervals and then characterized the extracted products by SEM (Figure 3 and S13). It should be noted that we here used ~35 nm NaYF4 UCNPs (Figure S14) to ensure easy observation under the scanning electron microscope. At the reaction time of 15 min, only UCNPs were observed (Figure 3a), since no residual was found after treating the intermediate products with hydrochloric acid (Figure 3a inset and S13a). As the reaction progressed, small MOF crystals, ~60 nm, appeared (Figure 3b), which continuously grew into polyhedron shape, and meanwhile UCNPs started to attach onto the MOF surface (Figure 3c). At this stage, the surface of MOF crystals was not

Figure 3. Intermediate stages of the formation of the NaYF4:Yb/Er and UiO-66-NH2 nanocomposites. (a-f) SEM images of the intermediate products collected during the formation of UiO-66-NH2@NaYF4:Yb/Er nanocomposites after reacting for 15, 36, 47, 55, 60 and 180 min, respectively. The insets in (a-f) are corresponding SEM images of intermediate products after treated with hydrochloric acid. Scale bars are 500 nm for (a-f) and 200 nm for all the insets. 4


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paved with nanoparticles as indicated by the smooth surface of MOF crystals after acid treatment (Figure 3c inset). As time prolonging, more UCNPs gradually attached on and paved over the surface of MOF crystals due to the continuous growth of MOF crystals (Figure 3d-f). Finally, the surface of MOF crystals was fully paved with UCNPs (Figure S15). It should also be mentioned that the intermediate and final products observed here (Figure 3 and S15) are not so uniform. This should be due to the disturbances caused by intermediate extraction during the formation of nanocomposites. According to the above results, we deduce that the formation of the nanocomposite should follow three major steps as depicted in Figure 1: (i) MOF nucleation, (ii) UCNP attachment, (iii) nanocomposite formation.

forces, should be the main force for the formation of the nanocomposites. To verify our hypothesis, we separately measured the zeta potentials of both UCNPs and MOF crystals. The obtained zeta potentials show that the ligand-free UCNPs are positively charged while UiO-66-NH2 crystals are negatively charged (Figure 5a), indicating the electrostatic attraction between the two materials. We then measured the zeta potentials of MOF products extracted during the formation of MOF crystals at different time intervals. As shown in Figure 5b, MOF crystals gradually become negatively charged as the reaction proceeds, and the most negatively charged zeta potential was recorded when the reaction continued for ~60 min. Surprisingly, these results are consistent with the observations shown in Figure 3 in which many UCNPs attach on the surface of MOF crystals after the reaction proceeding for ~60 min.

Next, intrigued by the nearly uniform distribution of UCNPs on the surface of MOF crystals, we measured the mean centerto-center interparticle distances according to SEM images. Here, relatively small upconversion nanoparticles, ~16 nm (Figure S16), were used to provide more nanoparticles for the measurement. As shown in Figure 4a and b, the mean interparticle distance is ~27 nm when 3 mg UCNPs are used. Further increase the amount of UCNPs can result in reduced interparticle distance which finally maintains at ~23 nm (Figure 4c-h). Additionally, we did not find obvious aggregation of nanoparticles on the surface of MOF crystals although more nanoparticles were introduced during the synthesis. These observations indicate that the attachment of UCNPs on MOF surface has self-limiting behavior due to energy barriers between nanoparticles, controlling the distribution of nanoparticles.

To gain a clearer view of the electrostatic interaction controlled distribution of UCNPs on MOF surface, we used a simplified model to study the interparticle distance (see details of the model and calculation in Supporting Information). In the model (Figure 5c), the size of UCNPs is considered as the sum of the physical size and excluded size caused by electrostatic repulsion, and the optimum distance between two nanoparticles is assumed to be the distance at which there is no overlap of the electric double layer.44-46 The excluded size can be estimated by the Debye screening length which can be calculated based on Derjaguin, Landau, Verwey and Overbeek (DLVO) theory.46 The calculated interparticle distance on MOF surface is ~25 nm which matches with our experimental observations (Figure 4).

Associating the formation of UiO-66-NH2@NaYF4:Yb/Er nanocomposites with typical self-assembly of nanomaterials,3540 we reason that electrostatic interactions between UCNPs and MOF crystals, together with the always existed Van der Waals

To further prove the generality of our strategy, MOF crystals with different surface charges and structures, including UiO-66, MOF-801, PCN-223, ZIF-8 (zinc-methylimidazolate framework-8), UiO-67 (zirconium(IV) biphenyl-4,4’dicarboxylate MOF), and MIL-101 (iron(III) terephthalate

Figure 4. Interparticle distance between adjacent NaYF4:Yb/Er nanoparticles in the nanocomposites. (a, c, e, and g) SEM images of a single UiO-66-NH2@NaYF4:Yb/Er nanocomposite obtained by adding different amounts of UCNPs, and (b, d, f, and h) corresponding interparticle distance distributions by counting 450 center-to-center distance between two nanoparticles. The amounts of UCNPs used in (a-h) are (a-b) 3, (c-d) 4.5, (e-f) 6, and (g-h) 7.5 mg, respectively. 5



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Figure 5. Investigation of the formation mechanism of the MOF-UCNPs nanocomposites. (a) Zeta potentials of ligand-free NaYF4:Yb/Er UCNPs and UiO-66-NH2 MOF crystals. (b) Zeta potential of the UiO-66-NH2 MOF crystals extracted during the synthesis of MOF crystals at different time intervals. (c) Schematic showing the interparticle distance (D) between two positively charged UCNPs on the surface of MOF crystal. The green sphere represents the UCNP and the grey shades around the green sphere represent the electric double layer of the UCNP. (d) Zeta potentials of UiO-66, MOF-801, PCN-223, MIL-101, ZIF-8, and UiO-67 MOF crystals. (e-j) SEM images of (e) UiO66@NaYF4:Yb/Er, (f) MOF-801@NaYF4:Yb/Er, (g-h) PCN-223@NaYF4:Yb/Er, (i) UiO-66-NH2@LiYF4 and (j) UiO-66-NH2@NaGdF4 nanocomposites, respectively.

growth of a hetero MOF layer can also be realized (Figure 6b, and S28-29). Such new structures can allow us to develop applications on the basis of the nanocomposites, including UCNP based lasing and luminescence monitored drug delivery.47-49 For instance, the UiO-66-NH2@NaYF4:Yb/Er UCNPs@UiO-66-NH2 nanocomposites can be used as drug carrier to load doxorubicin, an anticancer drug, due to the strong interactions between UiO-66-NH2 and doxorubicin molecules (Figure S30). Meanwhile, the absorption of doxorubicin overlaps with the emission of NaYF4:Yb/Er upconversion nanoparticles (Figure S30). By recording the upconversion luminescence of the drug loaded nanocomposites, the process of drug release can be monitored (Figure S31).

MOF) (Figure 5d and S17-18), were used for nanocomposite fabrication. As expected, UCNPs can be evenly decorated on all the negatively charged MOF crystals, such as UiO-66, MOF801 and PCN-223, and the formation of nanocomposites were not affected by the type and morphology of MOF crystals (Figure 5e-h and S19). In stark contrast, for MOF crystals with positively charged surface, including ZIF-8, UiO-67, and MIL101, we only observed the mixture of MOF crystals and UCNPs without the formation of nanocomposites (Figure S20). In addition, we found that the formation of nanocomposites is not dependent on the composition of UCNPs. UCNPs based on different hosts, such as LiYF4, NaYbF4, and NaGdF4, can be used to synthesize the nanocomposites if they are positively charged (Figure 5i-j and S21-24).

One feature of our nanocomposites is that multicolor upconversion emission tuning can be easily obtained by adjusting the amounts of UCNPs with red, green and blue upconversion emission during nanocomposite preparation (Figure 6c and S32). Noteworthily, the uniform distribution of UCNPs on the surface of MOF crystals can ensure the homogeneous upconversion luminescence at microscale or a single particle scale (Figure S33). Compared with commonly used methods for the tuning of upconversion emission color at nanoscale and microscale, our demonstration shown here does not need comprehensive design of the nanomaterials and can

Extension and Application of the Nanocomposites. The successful decoration of UCNPs on MOF offers opportunities to design new nanocomposites with unique structures and multifunctionalities. For example, more complicated MOF@nanoparticles@MOF core-shell-shell sandwiched nanocomposites can be easily fabricated with a controllable MOF out layer by epitaxial growth (Figure 6a, b, and S25-27), which can be hardly achieved by other common methods. By controlling the epitaxial growth of the external MOF layer, the distribution of upconversion nanoparticles inside MOF crystals can be further tuned (Figure S26-27). Furthermore, epitaxial 6


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Figure 6. Tunable properties of the MOF-UCNPs nanocomposites. (a) Schematic illustration of the epitaxial growth of another MOF layer on the MOF@UCNPs nanocomposites. (b) TEM images of the obtained MOF@nanoparticles@MOF core-shell-shell sandwiched nanocomposites. (c) Luminescence photos of the UiO-66-NH2@UCNPs nanocomposites with different upconversion emission, showing multicolour tuning of the nanocomposites. (d) Emission profiles of the “N” pattern recorded under the excitation of 980 nm light (center). The enlarged profiles (left and right) under microscope can be used to create unclonable patterns. The insets under the profiles are the corresponding color analysis (red, green and blue) of the randomly selected lines in the enlarged profiles by software. (e) Schematic presentation of the PCN-223@NaYF4:Yb/Er nanocomposite based singlet oxygen generation and (f) corresponding measured singlet oxygen under the excitation of a 980 nm light. The nanocomposites can generate more singlet oxygen when compared with those of pure NaYF4:Yb/Er UCNPs, PCN-223, and the physical mixture of PCN-223 and NaYF4:Yb/Er. Note that ET, TCPP, and PCN-223/UCNPs denote energy transfer, tetrakis (4-carboxyphenyl) porphyrin, and the physical mixture of PCN-223 and NaYF4:Yb/Er nanoparticles, respectively.

minimize potential interferences among emitting lanthanide ions with different emission.50-53

transfer from UCNPs to the porphyrin molecules in MOF (Figure 6e and f), offering candidates for photodynamic therapy. It should be mentioned that the nanocomposite exhibits a higher efficiency for singlet oxygen generation when compared with those of the single composition and the physical mixture of each part (Figure 6f).

The ability of multicolor upconversion emission tuning allows us to design optical authentication systems for anticounterfeiting.52,54 For example, the nanocomposites with different upconversion luminescence color can be dispersed in solution to form an ink which can be directly stamped or written on a substrate like paper (Figure S34). As a proof-of-concept experiment, we used the ink to write an “N” pattern (Figure 6d), where the pattern exhibited almost indistinguishable upconversion emission by the naked eye under the excitation of 980 nm light. In fact, the two “/” in the pattern were written by two inks with different compositions of the nanocomposites, which can be easily observed under a microscope (Figure 6d). Furthermore, a random line or pattern can be drawn in the micrographs, offering almost unclonable functions for anticounterfeiting if the luminescence color is also taken into account.

CONCLUSION We here have demonstrated a versatile method to pave UCNPs onto the surface of MOF crystal. The simply yet powerful method, mainly driven by electric interactions with self-limiting behavior, can afford nanocomposites with unique structures which can be hardly achieved by other approaches. This capability also enables us to modulate the properties of the MOF-UCNPs nanocomposites for diverse purposes, like information security and theranostics. The strategy, without the requirement of capping agent, should enable the understanding of direct interactions between MOFs and luminescent nanomaterials, and thus may offer new possibilities for exploring unexpected properties of luminescent MOF composites. In addition, we believe that this method, once refined, can be used for the fabrication of nanocomposites

In addition, the combination of MOF and UCNPs can result in nanocomposites with synergistic functionalities. For example, under the excitation of 980 nm light, PCN-223@NaYF4:Yb/Er nanocomposites can generate singlet oxygen due to the energy 7



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framework nanosheets in polymer composite materials for gas separation. Nat. Mater. 2015, 14, 48. (14) Moon, H. R.; Lim, D. W.; Suh, M. P. Fabrication of metal nanoparticles in metal-organic frameworks. Chem. Soc. Rev. 2013, 42, 1807. (15) Zhao, M.; Yuan, K.; Wang, Y.; Li, G.; Guo, J.; Gu, L.; Hu, W.; Zhao, H.; Tang, Z. Metal-organic frameworks as selectivity regulators for hydrogenation reactions. Nature 2016, 539, 76. (16) Li, Z.; Xu, Q. Metal-nanoparticle-catalyzed hydrogen generation from formic acid. Acc. Chem. Res. 2017, 50, 1449. (17) Lim, D. W.; Yoon, J. W.; Ryu, K. Y.; Suh, M. P. Magnesium nanocrystals embedded in a metal-organic framework: hybrid hydrogen storage with synergistic effect on physi- and chemisorption. Angew. Chem., Int. Ed. 2012, 51, 9814. (18) Lee, K.; Lee, J.; Jeoung, S.; Moon, H. Transformation of metalorganic frameworks/coordination polymers into functional nanostructured materials: experimental approaches based on mechanistic insights. Acc. Chem. Res. 2017 50, 2684. (19) Chen. Y.; Zhang, R.; Jiao, L.; Jiang, H. Metal-organic framework-derived porous materials for catalysis. Coord. Chem. Rev. 2018, 362, 1. (20) Ma, T.; Dai, S.; Jaroniec, M.; Qiao, S. Metal–organic framework derived hybrid Co3O4-carbon porous nanowire arrays as reversble oxygen evolution electrodes. J. Am. Chem. Soc. 2014, 136, 13925. (21) Buso, D.; Jasieniak, J.; Lay, M. D. H.; Schiavuta, P.; Scopece, P.; Laird, J.; Amenitsch, H.; Hill, A. J.; Falcaro, P. Highly luminescent metal-organic frameworks through quantum dot doping. Small 2012, 8, 80. (22) Saha, S.; Das, G.; Thote, J.; Banerjee, R. Photocatalytic metalorganic framework from CdS quantum dot incubated luminescent metallohydrogel. J. Am. Chem. Soc. 2014, 136, 14845. (23) Sun, C.; Wang, X.; Zhang, X.; Qin, C.; Li, P.; Su, Z.; Zhu, D.; Shan, G.; Shao, K.; Wu, H.; Li, J. Efficient and tunable white-light emission of metal-organic frameworks by iridium-complex encapsulation. Nat. Commun. 2013, 4, 2717. (24) Li, Y.; Tang, J.; He, L.; Liu, Y.; Liu, Y.; Chen, C.; Tang, Z. Core-shell upconversion nanoparticle@metal-organic framework nano-probes for luminescent/magnetic dual-mode targeted imaging. Adv. Mater. 2015, 27, 4075. (25) Li, Y.; Di, Z.; Gao, J.; Cheng, P.; Di, C.; Zhang, G.; Liu, B.; Shi, X.; Sun, L. D.; Li, L.; Yan, C. H. Heterodimers made of upconversion nanoparticles and metal-organic frameworks. J. Am. Chem. Soc. 2017, 139, 13804. (26) He, L.; Brasino, M.; Mao, C.; Cho, S.; Park, W.; Goodwin, A. P.; Cha, J. N. DNA-Assembled core-satellite upconverting-metalorganic framework nanoparticle superstructures for efficient photodynamic therapy. Small 2017, 13, 1700504. (27) Li, M.; Zheng, Z.; Zheng, Y.; Cui, C.; Li, C.; Li, Z. Controlled growth of metal-organic framework on upconversion nanocrystals for NIR-enhanced photocatalysis. ACS Appl. Mater. Interfaces 2017, 9, 2899. (28) Deng, K.; Hou, Z.; Li, X.; Li, C.; Zhang, Y.; Deng, X.; Cheng, Z.; Lin, J. Aptamer-mediated up-conversion core/MOF shell nanocomposites for targeted drug delivery and cell imaging. Sci. Rep. 2015, 5, 7851. (29) Chen, L.; Chen, H.; Luque, R.; Li, Y. Metal-organic framework encapsulated Pd nanoparticles: towards advanced heterogeneous catalysts. Chem. Sci. 2014, 5, 3708. (30) Lu, G.; Li, S.; Guo, Z.; Farha, O. K.; Hauser, B. G.; Qi, X.; Wang, Y.; Wang, X.; Han, S.; Liu, X.; DuChene, J. S.; Zhang, H.; Zhang, Q.; Chen, X.; Ma, J.; Loo, S. C. J.; Wei, W. D.; Yang, Y.; Hupp, J. T.; Huo, F. Imparting functionality to a metal-organic framework material by controlled nanoparticle encapsulation. Nat. Chem. 2012, 4, 310. (31) Gargas, D. J.; Chan, E. M.; Ostrowski, A. D.; Aloni, S.; Altoe, M. V. P.; Barnard, E. S.; Sanii, B.; Urban, J. J.; Milliron, D. J.; Cohen, B. E.; Schuck, P. J. Engineering bright sub-10-nm upconverting

assembled by MOFs and different nanomaterials, providing a new route to manufacture hybrid materials.

ASSOCIATED CONTENT Supporting Information. This Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Experimental details, supporting figures, and additional characterization; Movie

AUTHOR INFORMATION Corresponding Author *[email protected] (X. Xie) *[email protected] (L. Huang) *[email protected] (W. Huang)

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by grants from the National Natural Science Foundation of China (21507059), Natural Science Foundation of Jiangsu Province (BK20150948), Young Elite Scientists Sponsorship Program by CAST (2017QNRC001), and Qing Lan Project.

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Table of Contents (TOC)

10 ACS Paragon Plus Environment

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Figure 1 180x97mm (300 x 300 DPI)



Figure 2 202x102mm (300 x 300 DPI)


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Figure 3 215x129mm (300 x 300 DPI)



Figure 4 207x97mm (300 x 300 DPI)


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Figure 5 191x117mm (300 x 300 DPI)



Figure 6 207x123mm (300 x 300 DPI)


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TOC 129x130mm (300 x 300 DPI)


Paving Metal-Organic Frameworks with Upconversion Nanoparticles

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