Universal Porphyrinic MetalâOrganic Framework Coating to Various

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A Universal Porphyrinic Metal-Organic Framework Coating to Various Nanostructures for Functional Integration Jin-Yue Zeng, Xiao-Shuang Wang, Mingkang Zhang, Zi-Hao Li, Dan Gong, Pei Pan, Lin Huang, Si-Xue Cheng, Han Cheng, and Xian-Zheng Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14881 • Publication Date (Web): 23 Nov 2017 Downloaded from http://pubs.acs.org on November 27, 2017

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ACS Applied Materials & Interfaces

A Universal Porphyrinic Metal-Organic Framework Coating to Various Nanostructures for Functional Integration Jin-Yue Zeng†, Xiao-Shuang Wang†, Ming-Kang Zhang†, Zi-Hao Li†, Dan Gong†, Pei Pan†, Lin Huang†, Si-Xue Cheng†, Han Cheng,†,* Xian-Zheng Zhang†,* †

Key Laboratory of Biomedical Polymers of Ministry of Education, Institute for Advanced Studies (IAS), Department of Chemistry, Wuhan University, Wuhan 430072, P. R. China KEYWORDS: metal-organic framework, heterogeneous nucleation, polydopamine, graphene oxide, gold nanorod

ABSTRACT: A universal strategy was reported that enables functional group-capped nanostructures with various morphologies and compositions to be coated by porphyrin metal-organic framework (MOF). Based on nanostructure-induced heterogeneous nucleation, the controlled growth of MOF shell on the surface of nanostructures can be realized. It was demonstrated that this modification strategy can realize controlled growth of porphyrin MOF on a series of organic and inorganic nanostructures, such as polydopamine (PDA) nanoparticles, PDA@Pt nanoparticles, graphene oxide (GO) sheets and Au nanorods (AuNR). The as-prepared composites exhibit excellent catalytic and optical properties that originated from the nanostructure as well as the coated porphyrin MOF. We further explored the potential applications of PDA@MOF and PDA@Pt@MOF in nanomedicine and photocatalysis.

INTRODUCTION The controlled growth of metal-organic framework (MOF) on the surface of nanostructure has attracted much attention due to the benefits of versatile chemical and physical properties. The ability to control the structure of incorporated functional materials is critical to construct heterogeneous composites with integrated properties that are unavailable in individual components.1-4 Incorporating nanostructure within MOF to obtain the nanostructure@MOF with improved performance and collective properties has attracted much research attention for various potential applications especially in gas storage, catalysis, chemical sensing and drug delivery.5-10 Recently, several groups have explored that selective nanostructure coating polymers or surfactants can mediate the growth of MOF on the surfaces of metal nanostructures to form [email protected] In general, these composites were prepared by the following three methods: i) both the nanostructure cores and the MOF shells are prepared in a one-pot reaction, ii) the nanostructures are embedded and prepared in the channels or cavities of MOF, and iii) the stabilized nanostructures are coated within the MOF shells.16 The traditional methods based on the growth of MOF on the surfaces of nanostructure are complicated and may lead to the poor porosity and crystallinity of MOF shell.17-19 Moreover, the aggregation of nanostructures and the formation of MOF single crystals are the most common issues in existing MOF-coating strategies, which seriously affect the photophysical and

photochemical properties of the MOF incorporated nanostructures.20,21 Although many studies have been carried out on nanostructure@MOF and their applications, developing an applicable strategy to prepare the well-defined composites with a controllable method is still challenging.22-26 The difficulty in the preparation of nanostructure@MOF is mainly that the lattice mismatch between the nanostructure and MOF would lead to the self-nucleation of MOF in solution. Moreover, it is difficult to control the chemistry reaction at the interface between the nanostructure and the coating layer.27-29 Thus, it is necessary to develop an effective method to realize the controllable growth of MOF on the diverse nanostructures while keeping the dispersity, size, and shape of nanostructures. In addition, the integration of nanostructures in functional MOFs with new physical and chemical properties, such as porphyrin MOF, is rarely reported. Porphyrin MOFs are emerging as promising functional materials for catalysis, sensing, and biomedicine due to the unique biological and chemical properties offered by the porphyrin building blocks.30 Here, we report an extensive coating strategy to incorporate the crystals of a stable functional porphyrin MOF on the surfaces of diverse types of nanostructures (Scheme 1). Porphyrin MOF with well-defined pore structures and extraordinary high surface areas can be applied to stabilize nanostructures with a tunable method.31,32 Our coating strategy involves the modification of nanostructure surface with functional groups and the optimization of the crystallization of

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porphyrin MOF. The nucleation and controllable growth of MOF on the surfaces of the nanostructures can be driven by the coordination interactions between functional groups of nanostructures and zirconium nodes. We have demonstrated that the structural integration strategy can realize the controlled growth of porphyrin MOF on a series of organic and inorganic nanostructures, such as polydopamine (PDA) nanoparticles, graphene oxide (GO) sheets and Au nanorods (AuNR), while keeping the dispersity and shape of nanostructures. The control over the thickness of MOF shell was achieved by adjusting the time of the MOF-formation reaction. The as-prepared composites exhibit excellent catalytic and optical properties that are derived from the incorporated nanostructure as well as the shell of porphyrin MOF.

Scheme 1. Schematic illustration of porphyrinic metal-organic framework coating deposited to various nanostructures.

EXPERIMENTAL SECTION Preparation of Polydopamine. PDA nanoparticles were synthesized in a water-alcohol mixed solvent. The tris-buffer solution (10 mM, 100 mL) was mixed with alcohol. Dopamine (50 mg, 33 mmol) was dissolved in the mixed solvent. The mixture was stirred for more than 48 h. After the reaction finished, the product was centrifuged and washed with DI-water for three times. Preparation of PDA@MOF. PDA nanoparticles (10 mg) were suspended in 12 mL mixture of DMF and alcohol (V/V=5:1) and ZrOCl2·8H2O (30 mg, 0.093 mmol) was added. The mixture was heated to 90 °C for 2 h. Then, Pd-TCPP (10 mg, 0.013 mmol) and benzoic acid (0.28 g, 2.3 mmol) were added and stirred at 90 °C for 4 h. After the reaction finished, the core-shell composites were collected via centrifugation (8000 rpm, 20 min) followed by washing with DMF, 1% triethylamine in ethanol (v/v), and ethanol successively for 3 times. Preparation of PDA@Pt. PDA nanoparticles (10.0 mg) were dispersed in alcohol (100 mL) by sonication. Then,

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H2PtCl6 (120 mM, 1 mL) was added to the PDA nanoparticle suspension and the mixture was stirred at 80 °C for 24 h. After the reaction finished, the mixture was centrifuged and washed with alcohol for 3 times. Preparation of PDA@Pt@MOF. PDA@Pt (10 mg) were suspended in 12 mL mixture of DMF and alcohol (V/V=5:1) and ZrOCl2·8H2O (30 mg, 0.093 mmol) was added. The mixture was heated to 90 °C for 2 h. Thereafter, Pd-TCPP (10 mg, 0.013 mmol) and benzoic acid (0.28 g, 2.3 mmol) were added and stirred at 90 °C for 4 h. After cooling down, the composites were collected via centrifugation (6000 rpm, 20 min) followed by washing with DMF, 1% triethylamine in ethanol (v/v), and ethanol successively for 3 times. Preparation of Gold Nanorods. For preparation of the seed solution, CTAB solution (2.0 mL, 0.20 M) was mixed with HAuCl4 (2.0 mL, 0.5 mM) and then ice-cold NaBH4 (0.24 mL, 0.01 M) was added. The seed solution was vigorously stirred for 2 min and resulted in the formation of a brownish-yellow solution. The growth solution was prepared by mixing with CTAB (0.1 M, 200 mL), AgNO3 (4 mM, 5.6 mL) and HAuCl4 (23 mM, 6.5 mL) in 250 ml flask. Ascorbic acid (0.08M, 1.8 mL) was dropwise added to the mixture, until the solution became colourless. Finally, 1.8 mL of the seed solution was added to the mixture at 30°C. The colour of the mixture gradually changed within 10-15 minutes. The temperature of the AuNR growth was kept at 27-30 ºC for 12 h. Surface Modification of AuNRs. The as-obtained AuNRs were centrifuged at 11000 rpm for 30 min followed by washing with Milli-Q water for 2 times to remove excess CTAB surfactant. After discarding the supernatant, AuNRs were redispersed in 60 mL of Milli-Q water. 50 mg of lipoic acid in 4 mL of ethanol and 10 mg of PEG-SH were added under gently stirring and left to react for 12 h. After that, excess lipoic acid and PEG molecules were removed by repeated centrifugation (11000 rpm, 25min). Preparation of AuNR@MOF. Modified-AuNRs (10 mg) were dispersed in 10 mL of DMF by sonication for 10 min and ZrOCl2·8H2O (30 mg, 0.093 mmol) was added. The mixture was heated to 90 °C for 2 h. Then, TCPP (10 mg, 0.013 mmol) and benzoic acid (0.28 g, 2.3 mmol) were added and stirred at 90 °C for 4 h. After the reaction finished, the composites were collected via centrifugation (10000 rpm, 20 min) followed by washing with DMF, 1% triethylamine in ethanol (v/v), and ethanol successively for 3 times. Preparation of Graphere Oxide. GO sheets were prepared by a modified Hummers’ method.33 The graphite powder (5 g, 416.7 mmol) and NaNO3 (2.5 g, 29.4 mmol) were added to concentrated H2SO4 (18 M, 115 mL) in an ice bath. KMnO4 (15 g, 95 mmol) was gradually introduced with stirring at the temperature lower than 20 °C. Then, the mixture was slowly stirred at 35 °C for 4 h to allow oxidation. Then, 230 mL of deionized water was added to the mixture and stirred at 98 °C for 15 min. The mixture was diluted to 700 mL and stirred for 30 min. The reaction was concluded by adding H2O2 (12 mL, 35 wt %) and stirred at room temperature. The final precipitate was


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washed with deionized water (3*500 mL) and dried at 40 °C for 24 h. Preparation of GO@MOF. GO sheets (10 mg) were suspended in 10 mL of DMF under sonication for 1 h and ZrOCl2·8H2O (30 mg, 0.093 mmol) was added. The mixture was heated to 90 °C for 2 h. Then, TCPP (10 mg, 0.013 mmol) and benzoic acid (0.28 g, 2.3 mmol) were added and stirred at 90 °C for 4 h. After the reaction finished, the composites were collected via centrifugation (8000 rpm, 20 min) followed by washing with DMF, 1% triethylamine in ethanol (v/v), and ethanol successively for 3 times. Generation and Detection of Intracellular ROS. The intracellular ROS were measured using DCFH-DA (the ROS indicator) as the sensor via CLSM. After treating 4T1 cells with PDA@MOF (150 µg mL−1) for 6 h, the medium was removed and DCFH-DA was added (final concentration 1×10−6 M). The cells were further incubated for 30 min. Then, the cells were irradiated with a 630 nm LED (0.22 W cm−2) for 2 min, respectively. All the cells were viewed by CLSM with laser at 488 nm and 543 nm. Photothermal Effect of PDA@MOF. 1 mL of PDA@MOF aqueous solution (100 mg/L, 150 mg/L and 200 mg/L) was added into a 2 mL plastic centrifuge tube. The top of plastic centrifuge tube was fixed and a fiber-coupled continuous semiconductor diode laser (808 nm) was used as the light source. In Vitro MTT Cytotoxicity Assay. The cytotoxicity to 4T1 cells in vitro were investigated by using MTT assay. The cells were seeded into the 96-well plates and incubated in 100 μL of medium at 37 °C for 24 h. Then, the original medium was replaced with another 100 μL of fresh medium containing a particular amount of PDA@MOF. After incubation for 6 h, the medium was replaced by 200 μL of fresh medium. For PDT, the cells were irradiated with a 630 nm LED (0.22 W cm−2) for 2 min. For PTT, the cells were irradiated with 808 nm laser (1.5 W cm−2) for 2 min. For the control groups, the cells were cultured in dark all the time. Then, the cells were cultured for another 24 h and 20 μL of MTT (5 mg mL−1 in PBS) was added and further incubated for 4 h. The supernatants were carefully removed, and 150 μL of DMSO was added into each well. After shaking for several minutes, the optical density (OD) was recorded at 570 nm on a microplate reader (BIO-RAD, Model 550, USA). The relative cell viability was calculated by the following equation: cell viability (%) = OD (sample) × 100/OD (control), where OD (control) was obtained in the absence of samples and OD (sample) was obtained in the presence of samples. Photocatalysis activity of PDA@Pt@MOF. 80 µL ADPA (1 mg/mL) was mixed with 3 mL of PDA@Pt@MOF (40 µg mL−1). The samples were irradiated with 532 nm laser (0.22 W cm-2), and the decrease in absorbance was recorded by UV-vis spectrophotometer.

RESULTS AND DISCUSSION

Figure 1. (A) SEM image of PDA nanoparticles; (B) PDA@MOF nanoparticles. (C) TEM image of one PDA nanoparticle; (D) PDA @MOF. (E) TEM-EDS of PDA @MOF nanoparticles; (F) PXRD of porphyrin MOF and PDA @MOF. In this investigation, we firstly studied the applicability of the modification strategy to prepare porphyrin MOF coated PDA nanoparticles. PDA nanoparticles were prepared via oxidation of dopamine into dopaminequinone followed by intramolecular cyclization reaction, oxidative oligomerization and self-assembly.34 The chelate effect of residual catechol groups and π-π stacking of aromatic groups of PDA were used to offer rapid nucleation, resulting in the formation of the delicate core-shell nanomaterials.35 We confirmed that porphyrin MOF can be coated on the surfaces of PDA using TCPP as organic linkers and zirconium (IV) as metal nodes under solvothermal condition. Both SEM images and TEM images show the size of PDA@MOF increased after porphyrin MOF was coated on the surface of PDA (Figure 1A-D). The SEM image of PDA@MOF shows that hybrid nanoparticles possess an average diameter of 220 ± 16 nm (Figure 1B). To demonstrate the formation of porphyrin MOF shell, the organic linkers of TCPP were marked by Pd (II) through coordination interaction. The TEM-EDS of PDA@MOF indicates the existence of Pd and Zr elements on PDA nanoparticles (Figure 1E). Powder X-ray diffraction (PXRD) patterns of PDA@MOF exhibit the characteristic peaks of porphyrin MOF, confirming the formation of MOF phase on the surface of PDA (Figure 1F). Furthermore, we confirmed



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that the growth of MOF layer on the surface of PDA nanoparticles can be controlled by adjusting the reaction time of MOF-formation (Figure s1).

Figure 2. (A) TEM image of PDA nanoparticles; (B) PDA@Pt nanoparticles; (C) PDA@Pt@MOF; (D) PDA@Pt@MOF-4 h; (E) PDA@Pt@MOF-8 h; (F) one core-shell PDA@Pt@MOF-4 h; (G) one core-shell PDA@Pt@MOF-8 h. On the basis of these results, we speculated that the coordination interactions between functional groups of PDA nanoparticles and zirconium nodes would provide the heterogeneous nucleation basis for the growth of MOF on the surface of nanostructure. To confirm this, the MOF shell was prepared by changing the addition order of organic linkers and metal nodes. We observed that well-defined composites can be formed if PDA nanoparticles were firstly mixed with zirconium metal salt. However, a large number of MOF nanocrystals were

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formed if PDA nanoparticles were firstly mixed with organic ligands, suggesting that the interactions between functional groups of PDA and organic linkers led to lower heterogeneous nucleation rate compared to the self-nucleation of MOF (Figure S2). Encouraged by the successful coating of PDA nanoparticle, the growth process of porphyrin MOF shell was further demonstrated with PDA@Pt nanoparticle. PDA@Pt were prepared by using in-situ method. A large number of redox-active catechol groups remained on the surface of PDA enabled localized reduction of noble metal precursors.36 PDA is able to induce the reduction of Pt precursor, producing uniform Pt nanoparticles with a size of 2-3 nm on the surface of PDA (Figure 2A,B and Figure S3). We found that porphyrin MOF can be readily coated on the surfaces of PDA@Pt nanoparticles. The TEM image of PDA@Pt@MOF clearly shows a MOF shell with a lower contrast surrounding the PDA@Pt nanoparticle, suggesting that the formation of Pt nanoparticles does not affect the growth of MOF on the surface of PDA (Figure 2C). We found that the thickness of MOF shell can be controlled by tuning the formation time of MOF (Figure 2D,E). As shown in Figure 2F, a uniform MOF shell of 10.8 ± 2.1 nm on the surfaces of PDA@Pt is formed when the growth reaction time of MOF is 4 h. The elongated reaction time of 8 h can lead to an increased thickness of 37.8 ± 3.4 nm on the surfaces of PDA@Pt (Figure 2G).

Figure 3. (A) XPS spectra of PDA@Pt and PDA@Pt@MOF. (B) Pt 4f XPS spectra of PDA@Pt and PDA@Pt@MOF. (C) UV–vis spectra of PDA, PDA@Pt and PDA@Pt@MOF. (D) PXRD of porphyrin MOF, PDA@Pt and PDA@Pt@MOF. The formation of MOF shell was further investigated by X-ray photoelectron spectroscopy (XPS), UV-vis spectroscopy and PXRD analysis. XPS reveals that Pt (IV) can be reduced to Pt (0) by the residual catechol groups of PDA and remained in the porphyrin MOF coated nanoparticles (Figure 3A,B). In UV-vis spectrum, the characteristic localized surface plasmon resonance (LSPR) peak of Pt (0) was obseved after Pt (IV) was reduced on


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PDA, and the characteristic peak of Pd-TCPP emerged after the MOF coating (Figure 3C). PXRD for PDA@Pt@MOF demonstrate a good match for the diffraction peaks between the MOF phase of PDA@Pt@MOF and pure MOF (Figure 3D). In the PXRD pattern of PDA@Pt@MOF, two strong diffraction peaks at 39.8 and 46.4 degrees appeared due to the presence of Pt nanoparticles. As evaluated by nitrogen-sorption, the pore size of PDA@Pt@MOF is around 1.8 ± 0.3 nm (Figure S4). We observed that PDA@Pt@MOF possesses smaller gravimetric Brunauer-Emmett-Teller (BET) surface areas of 396 m2 g-1 as compared with the pure MOF due to the introduction of PDA core. Above results indicate that the functional groups on the surfaces of nanostructure played an important role in the formation of MOF shell.

Figure 4. (A) TEM image of Au nanorods; (B) AuNR@MOF-4 h; (C) AuNR@MOF-6h. (D) The structure of one core-shell AuNR@MOF-4 h. (E) The structure of one core-shell AuNR@MOF-6 h. (F) EDX elemental mapping images of one core-shell AuNR@MOF. (G) PXRD of AuNR, porphyrin MOF and AuNR@MOF. (H) PXRD of AuNR@MOF-4 h and AuNR@MOF-6 h. (I) UV–vis spectra of AuNR@MOF-4 h and AuNR@MOF-6 h. To explore the compatibility of our coating strategy, Au nanorods were used for obtaining the core-shell composites. Although the rodlike structure and the surface properties of AuNR are differ from PDA nanoparticles, MOF shell can also be coated on the surfaces of AuNR. CTAB stabilized AuNR was prepared by a seed-mediated growth method.37 TEM image of AuNR shows that the aspect ratio is approximate 3.7 (Figure 4A). The UV-vis spectrum of AuNR shows two plasmon resonances, namely, longitudinal plasmon and transverse plasmon (Figure S5). In organic solvent, noble metal nanoparticles are easily aggregated when the stabilization of surfactant was destroyed.38, 39 In our study, we found Au nanorods were likely aggregated and

deformed under heating conditions (Figure S6). To realize the nanoparticle-induced heterogeneous nucleation, the surface of AuNR was functionalized with lipoic acid. After the functionalization, the well-dispersed surface carboxylated AuNR possesses high physical-chemical stability, which can use as the cores for heterogeneous nucleation for the growth of MOF due to the coordination interactions between carboxyl groups of AuNR and zirconium nodes. AuNR@MOF was fabricated by the solvothermal method using the surface carboxylated Au nanorods as crystal nuclei. The localized MOF nucleation growth on the surface of AuNR can be controlled by the reaction time, leading to the formation of well-defined core-shell composites (Figure 4B,C and Figure S7,8). The TEM image of AuNR@MOF shows that the thickness of MOF shell is about 8.2 ± 2.3 nm after the MOF-formation reaction time at 4 h (Figure 4D). Increasing the reaction time to 6 h gave rise to a thickness of 14.8 ±1.6 nm (Figure 4E). In the UV-vis spectrum, the absorption of porphyrin MOF shell in AuNR@MOF increases with the MOF-formation reaction time (Figure 4I). To prove the formation of MOF shell, a typical core-shell composite of AuNR@MOF was characterized by TEM-EDX elemental mapping. The signals corresponding to the zirconium, carbon and gold can be detected (Figure 4F). PXRD for the core-shell composites demonstrated a good match for the diffraction peaks between the porphyrin MOF phase of AuNR@MOF and pure porphyrin MOF (Figure 4H). Due to the peaks of AuNR at (111) and (200) crystal plane, PXRD for the core-shell composites showed two additional strong diffraction peaks at 38.1 and 44.4 degree compared to pure porphyrinic MOF phase (Figure 4G). Both AFM topography images and 3D topography images indicate the thickness of AuNR@MOF increased after porphyrin MOF was coated on AuNR (Figure S9-12). The porosity of AuNR@MOF was evaluated by nitrogen adsorption–desorption at 77 K. The maximum N2 uptake was up to 321 cm3 g−1 (STP) and the pore size was around 1.6 ± 0.3 nm (Figure S13). These results indicate the formation of porphyrin MOF on the surface of AuNR. To confirm AuNR-induced heterogeneous nucleation, the addition order of organic ligands and zirconium nodes was changed in control experiments. We observed that MOF single nanocrystals were easily formed when AuNR firstly mixed with organic ligands or together with zirconium metal salt (Figure S14). This result indicates that coordination interactions between carboxyl groups of AuNR and zirconium nodes can lead to a higher heterogeneous nucleation rate as compared with the self-nucleation of MOF. To further examine the applicability of the nanostructure-induced heterogeneous nucleation strategy, graphene oxide (GO) sheets with 2D morphologies were used for preparing 2D GO@MOF hybrid structures. GO sheets were prepared by previously developed methods, abundant carboxyl groups were remained on the surface of GO sheets (Figure 5A). The coordination interactions between carboxyl groups of GO sheets and zirconium nodes can offer the heterogeneous nucleation for 2D



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template for the surface coating of porphyrin MOF. The TEM image of GO@MOF obviously shows a MOF shell of higher contrast on the surface of GO sheets, suggesting that the successful coating of porphyrin MOF on GO sheets to form hybrid structures (Figure 5B and Figure S15). The thickness of MOF shell increases with the MOF-formation reaction time, as shown in Figure 5C,D. The growth process of MOF coating was investigated by XPS analysis and UV-vis spectroscopy. XPS for GO@MOF shows the expected characteristic peaks corresponding to graphene oxide and porphyrin MOF. The C 1s band of GO@MOF can be divided into three peaks corresponding to C=C at 284.8 eV, C−O at 286.4 eV and O=C−O at 288.4 eV. The peak ratio of O=C−O to C−O in GO@MOF is obviously enhanced compared to GO sheets, suggesting that the graphene oxide surface is coated with porphyrin MOF (Figure 5E). In the UV-vis spectrum, the characteristic peak of TCPP emerges after the MOF coated GO sheets (Figure 5F). The maximum N2 uptake of GO@MOF is up to 232 cm3 g−1 (STP) and around 1.6 ± 0.4 nm, as determined by nitrogen adsorption–desorption analysis at 77 K (Figure S16). This result suggests that GO@MOF possesses a high surface area and a high porosity.

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Furthermore, the AFM-phase images and AFM-amplitude images also visually show the difference in contrast within the GO sheet and between the GO sheet and porphyrin MOF shell, indicating the structure heterogeneity of GO@MOF (Figure 6B, D and Figure S17, 18). To prove that the coordination interactions between carboxyl groups of GO sheets and zirconium nodes induce the formation of the MOF shell, the reduced GO sheets were mixed with organic ligands or zirconium metal salt for the growth of the MOF shell. We observed that abundant MOF nanocrystals were formed due to the lack of carboxyl groups of reduced GO (Figure S19). These results confirmed that the coordination interactions play an important role in the growth of porphyrin MOF on the surfaces of nanostructures with 2D morphologies.

Figure 6. (A) AFM topography image of GO sheets. (B) AFM-phase image of GO sheets. (C) AFM topography image of GO@MOF. (D) AFM-phase image of GO@MOF.

Figure 5. (A) TEM image of GO sheets. (B) GO@MOF; (C) GO@MOF-4 h. (D) GO@MOF-6 h. (E) C1s XPS spectra of GO, MOF and GO@MOF. (F) UV–vis spectra of GO and GO@MOF. AFM topography images show the monolayer GO sheet possesses a thickness of 0.9 ± 0.1 nm. However, the thickness of GO@MOF can increase to 3.1 ± 0.7 nm after porphyrin MOF coated GO sheet (Figure 6A,C).

This coating strategy is also applicable if the nanostructures are modified with other functional groups that can coordinate with metal nodes. We attributed the successful coating of porphyrin MOF to nanostructure-induced heterogeneous nucleation. Moreover, the surface functional modifications not only stabilize the nanostructures in the reaction, but also offer the nanostructures with an enhanced affinity to the surfaces through coordination interactions between functional groups and Zr-clusters. Thus, the strategy based on nanostructure-induced heterogeneous nucleation is universally applicable to the growth of MOF on the diverse types of nanostructures. We further investigated the potential applications of the nanostructure@MOF composites with complement properties in nanomedicine and photocatalysis. For PDA@MOF, the combination of the photodynamic activty of porphyrin MOF with the photothermal effect of PDA nanoparticles can be used for cancer therapy.40,41 To evaluate the photodynamic activity of PDA@MOF in vitro, DCFH was used as the ROS probe, which can emit green ﬂuorescence in the presence of ROS. In CLSM images, the


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obvious green fluorescence was observed under the excitation of 488 nm when the 4T1 cells were cultured with PDA@MOF and irradiated with 630 nm. The result indicates that PDA@MOF can efficiently generate intracellular ROS under irradiation of 630 nm. In contrast, when the 4T1 cells were cultured with PDA@MOF in dark, the green fluorescence is negligible (Figure 7A-C). In addition, the obvious red fluorescence was observed due to the presence of porphyrin under the excitation of 543 nm when the 4T1 cells were cultured with PDA@MOF (Figure 7A,B). This result suggests excellent cellular uptake of PDA@MOF. To investigate the photothermal ability of PDA@MOF, the temperature trends of PDA@MOF solution were recorded under irradiation with 808 nm laser (Figure 7D and Figure S20). The cancer therapy of PDA@MOF in vitro was investigated by the established MTT assay. As shown in Figure 7E, the cells were incubated with PDA@MOF under the dual light irradiation, the cells viability was only 18%, exhibiting the efficient anticancer performance.

absorbance at 378 nm (Figure 7F). The dramatically different endoperoxide rates for ADPA in various nanoparticles solution (PDA@Pt, MOF, PDA@MOF and PDA@Pt@MOF) indicate that PDA@Pt@MOF exhibits the highest photocatalytic activity and excellent photostability (Figure 7G,H and Figure S21-23). We attribute the excellent photocatalytic performance of PDA@Pt@MOF to the following factors. Firstly, PDA@Pt@MOF has strong photo-harvesting power, and its individual components work synergistically in the composite to activate O2 into 1O2 under visible light irradiation. In addition, the electronic state of Pt nanoparticle, closely associated with the generation of 1O2, can be tuned by the competition between a plasmonic effect and a Schottky junction to achieve enhanced O2 activation.

CONCLUSIONS In summary, we have presented an effective strategy to coat diverse nanostructures, such as PDA nanoparticles, PDA@Pt nanoparticles, Au nanorods and GO nanosheets, with a functional porphyrin MOF material by a controllable method. The general applicability of this strategy involves nanostructure-induced heterogeneous nucleation mechanism, and is capable of controlling the growth of porphyrin MOF shell on the surfaces of different nanostructures while keeping the dispersity and shape of nanostructures. The as-prepared composites exhibit excellent catalytic and optical properties that originated from the nanostructure as well as the coated porphyrin MOF and have potential applications in nanomedicine and photocatalysis. The universal applicability of this strategy opens the access to diverse nanostructure@MOF with the property of the core nanostructures complementing the functionality of the porphyrin MOF shell.

ASSOCIATED CONTENT Figure 7. (A) Confocal images of the cells incubated with PBS (blank) under 630 nm LED irradiation. (B) PDA@MOF in dark. (C) PDA@MOF under 630 nm LED irradiation. A1-C1: red fluorescence of porphyrin. A2-C2: green fluorescence of DCFH-DA. A3-C3: merge image. (D) Temperature changes of PDA@MOF at different concentration irradiated with a constant 808 nm laser power. (E) The cytotoxicity of PDA@MOF. (F) Time-dependent endoperoxide of ADPA caused by 1O2 generated PDA@Pt@MOF. (G) The change of ADPA absorbance at 378 nm as a function of the irradiation time. (H) The change of ADPA absorbance at 378 nm at different cycle times (40 min/cycle time). For PDA@Pt@MOF, the incorporation of Pt nanoparticles and metalloporphyrin can provide a platform for catalysis.42 The photocatalysis activity of PDA@Pt@MOF was investigated by using the endoperoxide reaction of 9,10-anthracenedipropionic acid (ADPA) contact with 1O2. The reaction was monitored by UV-vis spectrophotometer, recording the decrease in

The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. Detailed descriptions of the synthetic preparations, the nitrogen adsorption–desorption for nanostructure@MOF, additional TEM images for nanostructure@MOF, UV-vis spectrum of AuNR, AFM topography and 3D image of AuNR@MOF, AFM-amplitude images of GO and GO@MOF, photothermal images of PDA@MOF and time-dependent endoperoxide of 1 ADPA, as well as ESI and H NMR analysis of TCPP (PDF).

AUTHOR INFORMATION Corresponding Authors * [email protected] (H. C.), [email protected] (X. Z. Z.).

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (51573142, 51533006 and 51233003).



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