Building Block Symmetry Relegation Induces Mesopore and Abundant Open-Metal Sites in Metal–Organic Frameworks for Cancer Therapy

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE1 Mar 2022Building Block Symmetry Relegation Induces Mesopore and Abundant Open-Metal Sites in Metal–Organic Frameworks for Cancer Therapy Jing Sun†, Xuan Zhang†, Dong Zhang, Ying-Pin Chen, Fei Wang, Lan Li, Tian-Fu Liu, Huanghao Yang, Jibin Song and Rong Cao Jing Sun† State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210 , Xuan Zhang† MOE Key Laboratory for Analytical Science of Food Safety and Biology, College of Chemistry, Fuzhou University, Fuzhou 350116 , Dong Zhang State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 , Ying-Pin Chen NSF's ChemMatCARS, The University of Chicago, Argonne, IL 60439 , Fei Wang State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 University of Chinese Academy of Sciences, Beijing 100049 , Lan Li State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 University of Chinese Academy of Sciences, Beijing 100049 , Tian-Fu Liu State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 University of Chinese Academy of Sciences, Beijing 100049 , Huanghao Yang MOE Key Laboratory for Analytical Science of Food Safety and Biology, College of Chemistry, Fuzhou University, Fuzhou 350116 , Jibin Song *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] MOE Key Laboratory for Analytical Science of Food Safety and Biology, College of Chemistry, Fuzhou University, Fuzhou 350116 and Rong Cao *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210 University of Chinese Academy of Sciences, Beijing 100049 https://doi.org/10.31635/ccschem.021.202000634 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail The judicious choice of metal clusters and organic building blocks leads to a wide variety of structures for metal–organic frameworks (MOFs). In this work, we demonstrated that relegating the symmetry of a building block can also lead to the proliferation of new MOF structures. Herein, a triangle building block was elongated with reduced symmetry for MOF construction, which gave rise to a novel (3,4,6)-connected idp (based on the definition of Reticular Chemistry Structure Resource, http://rcsr.net/) network ( PFC-16) with mesopores and abundant open-metal sites. The framework is composed of the rarely observed tetrakis hexahedral cages, surrounding which are small cages arranged in sodalite topology. The relegated symmetry was required for this novel self-assembly. The obtained MOF with mesopores, a robust backbone, and abundant open-metal sites can incorporate functional species in the structure, which is representatively demonstrated by internalizing the photosensitizer zinc(II) phthalocyanine (ZnPc) and the modifying tumor-targeting molecule folic acid (FA) in PFC-16. The obtained composite [email protected] shows excellent photodynamic therapy (PDT) efficiency for both in vitro and in vivo experiments, representing a promising candidate for cancer therapy. Download figure Download PowerPoint Introduction Metal–organic frameworks (MOFs), self-assembled through metal clusters and organic ligands, are a class of long-range ordered crystalline materials attracting much attention because of their high surface area, diverse structures, and wide applications.1–11 Among them, a great deal of research interest has been focused on Zr-based MOFs since a series of Zr-MOFs were initially reported in 2008.12 Zr-MOFs show not only excellent chemical stability but also highly predictable structure, which offers this class of materials large design space for potential applications. Although a variety of structures have been reported,13–15 there remains a pressing need to explore new Zr-MOFs combining high chemical stability, large pore size, and abundant open-metal sites for specific application demands. Elongation of organic building blocks is a traditional paradigm for synthesis of MOFs with large cavities.16,17 However, this strategy usually causes the obtained MOFs have interpenetrated structures instead of large cavities.18,19 Moreover, attempts of linker extension are very likely to yield the same topologies as the underlying frameworks, which misses out on new structures for MOF library.20–22 Previous studies show that using asymmetric organic ligands can result in highly porous materials, new topologies, and sometimes multiple metal clusters within a framework.23 These findings inspired us to introduce symmetry inequivalence into a multitopic building block for Zr-MOF design, which would bring an opportunity to create new types of structures unprecedented in Zr-MOFs. Moreover, relegated symmetry may induce the absence of connections in certain directions and therefore generate abundant open-metal sites, which would be beneficial to the catalytic performance of bulky MOFs. Taking triangular carboxylate linkers as examples, self-assembly of trimesic acid (H3BTC) with Zr6 clusters results in six-connected MOF-808 ( spn topology; spn = spinel).24 Symmetrically extending H3BTC to H3TATB and H3BTE for Zr-MOF construction gave rise to 6-connected PCN-777 (β-cristobalite topology) and 12-connected MOF-1004 [ sky (based on the definition of Reticular Chemistry Structure Resource) topology], respectively.25,26 If trimesic acid was elongated with a reduced symmetry, would it induce a new structure different from MOF-808, PCN-777, and MOF-1004? This scenario was supported in this work by deliberately designing a building block with reduced symmetry where two of the carboxylic moieties of H3BTC were replaced by ethynylbenzoic acid, and the other one was replaced by the relatively short benzoic acid (named as H3BDBA). Moreover, a methyl group was intentionally introduced on the benzoate group to force the connected carboxylate group perpendicular to the plane of two ethynylbenzoate arms (Figure 1, detail synthetic routine as shown in Supporting Information Scheme S1). With this modification, a triangle planer building block was constructed into a tetrahedral geometry. Self-assembly of H3BDBA and Zr ions generated a (3,4,6)-connected network PFC-16 [PFC = porous materials from FJIRSM (Fujian Institute of Research on the Structure of Matter), Chinese Academy of Science] with an idp network combining both six- and four-connected Zr6 clusters in one structure, which is unprecedented in Zr-MOFs. The entire framework is composed of the sodalite (SOD) cages and rarely observed tetrakis hexahedral cages (Catalan solids), and the twisted direction of three carboxylate groups is the prerequisite for this Catalan solid self-assembly. The obtained structure with mesopore, robust backbone, and abundant open-metal sites allows the incorporation of functional species in the structure, which is representatively demonstrated by internalizing photosensitizer zinc(II) phthalocyanine (ZnPc) and modifying tumor-targeting molecule folic acid (FA) in PFC-16. The obtained composite [email protected] shows excellent photodynamic therapy (PDT) efficiency for both in vitro and in vivo experiments, representing a promising candidate for bio-related applications.27,28 Figure 1 | Conceptual design of organic linker with relegated symmetry. Download figure Download PowerPoint Experimental Procedures Preparation of PFC-16 for single-crystal X-ray diffraction analysis H3BDBA (10 mg, 0.02 mmol) was dissolved in 1 mL of N,N-dimethylformamide (DMF) in a 10-mL uncapped vial, to which ZrCl4 (14 mg, 0.06 mmol) and trifluoroacetic acid (TFA; 100 μL, 1.35 mmol) were added. The mixture was ultrasonicated for 5 min and kept at 120 °C for 3 days. Light yellow cubic crystalline products were attained with 71.9% (12 mg) yield. Preparation of nano-PFC-16 H3BDBA (10 mg, 0.02 mmol) was dissolved in 10 mL DMF in a 20-mL vial, to which ZrCl4 (14 mg, 0.06 mmol) and TFA (10 μL, 0.135 mmol) were added, and then the mixture was kept at 90 °C for 24 h. A light yellow precipitate was attained with 83.9% (14 mg) yield. Preparation of [email protected] ZnPc (3.5 mg, 6 μmol) was added to the reaction mixture of nano-PFC-16, and after the reaction, a blue-green precipitate was attained with 80.7% (16.3 mg) yield. Preparation of [email protected] 10 mg of dried [email protected] powder was suspended in 1 mL DI, 4.4 mg FA was dissolved in 1 mL dimethyl sulfoxide (DMSO). The mixture was kept in a 90 °C oven for 2 days. Then the mixture was centrifuged and washed three times with DMSO/DI (v/v, 1/1) to remove excessive free folic acid. Cell culture MCF-7 (human breast adenocarcinoma cell line) cells were purchased from American Type Culture Collection (ATCC) (Manassas, VA) and cultured at 37 °C containing 5% CO2 in the air with RPMI 1640 (purchased from HyClone, South Logan, UT) supplemented with 1% penicillin-streptomycin solution and 10% fetal bovine serum (FBS; HyClone). In vitro cytotoxicity of [email protected] The cell cytotoxicity was examined by standard CCK-8 assay. In brief, MCF-7 cells were seeded in a 96-well cell-culture plate (104 cells per well, 100 μL) and incubated in a humidified incubator at 37 °C with 5% CO2 for 24 h. Then the cells were treated with [email protected] at concentrations of 0, 10, 20, 40, 60, 100, and 200 μg mL−1 for another 12 h. Each cell well was washed with phosphate-buffered saline (PBS) twice after removing the culture medium. Finally, 100 μL mixture containing 10 μL CCK-8 solution and 90 μL culture medium was added into each well and incubated for 2 h. The OD450 (absorbance at 450 nm) value was measured by microplate reader. In vitro photodynamic performance of [email protected] Briefly, MCF-7 cells were seeded in 96-well plates (104 cells per well) for 24 h. Cells were refreshed with fresh medium containing different concentrations of [email protected] for another 12 h. Next, cells of each concentration group were irradiated with a 660 nm laser (0.8 W cm−2) for 10 min, and then incubated for another 2 h. Ultimately, 10 μL CCK-8 solution and 90 μL culture medium were added to determine the cell viability. Intracellular PDT performance of [email protected] MCF-7 cells were first plated onto confocal dishes (NEST, Scientific USA) for 12 h and incubated with [email protected] for another 12 h. The irradiating experimental group was treated with 660 nm laser for 10 min independently. Then, after washing twice with PBS, 1 μL calcein-AM and 1 μL propidium iodide (PI) were added and the cells were incubated for 30 min to be observed by fluorescence microscope. Intracellular reactive oxygen species generation of [email protected] The cancer cells were seeded in a glass bottom cell culture dish and incubated with [email protected] at 50 μg mL−1 for 24 h. After sequential 10 min of 660 nm irradiation, 10 μM 2′,7′-dichlorofluorescin diacetate (DCFH-DA), which is a reactive oxygen species (ROS) detector, was added into each plate and incubated for another 30 min. After washing with PBS twice, the generated ROS from each group was observed by the Nikon A1 confocal laser scanning microscope (CLSM). In vivo PDT efficacy All the animal experiments abided by the guide for the care and use of laboratory animals (Ministry of Science and Technology of China, 2006) and were approved by the Institutional Animal Care and Use Committee of Fujian Medical University. All the female BALB/c nude mice (6 weeks) were obtained from Shanghai SLAC Laboratory Animal Co., Ltd. The subcutaneously implanted MCF-7 tumor models were constructed by hypodermic inoculation of MCF-7 cells (100 μL, 1 × 106) into the hind limbs of nude mice. When the tumor grew to 40 mm3, all the models were randomly divided into four groups (three mice in each group): PBS group, [email protected] group, laser only group, and [email protected] + laser group. All doses of [email protected] were 20 mg kg−1. A 100 μL dose was injected into each mouse in all groups through tail vein. The laser only and [email protected] + laser groups were irradiated at 660 nm for 15 min. Every 2 days, body weight was measured, and tumor size was calculated as follows: V = L × W2, in which L and W represent the length and width of the tumor. Results and Discussion Crystal structure Slightly yellow cubic crystals of PFC-16 were synthesized by solvothermal reaction of H3BDBA (H3L) and ZrCl4 in DMF at 120 °C for 3 days. Single-crystal X-ray diffraction revealed that PFC-16 crystallizes in space group Im 3 ¯ , and the structure can be formulated as Zr6O4(OH)4(BDBA)2(OH)6 ( Supporting Information Table S1). There are two kinds of Zr-oxo clusters in the structure. One is a disordered cluster connecting with six ethynylbenzoate groups of H3L, and the other one is an ordered cluster connecting with four methylbenzoate groups of H3L, resulting in a (3,4,6)-c net with idp topology (Figure 2 and Supporting Information Figures S1–S3).29 Moreover, the Im 3 ¯ space group is the maximum symmetric embedding of this net, further verifying the conclusion drawn from crystallography analysis.30 Of special note, this is the first observation of Zr-oxo clusters with different connectivity in one MOF structure ( Supporting Information Figure S4). Previous studies show that the Zr6 clusters can exhibit diverse geometries compatible to various organic building blocks. For example, in fcu-a or ftw networks, each Zr6 cluster is fully occupied by 12 carboxylates adopting cuboctahedron geometry (Oh symmetry).12,31 In an eight-connected bcu-a or flu network, the symmetry of Zr6 is downgraded to D4h.24,32 Further reducing the connectivity of the network, for example, in the six-connected β-cristobalite network, the Zr6 cluster acts as an antiprismatic node with D3d symmetry.25 The aforementioned MOFs are all based on highly symmetric organic building blocks. However, with relegating the symmetry of organic linkers, the connectivity and symmetry of Zr6 cluster in PFC-16 can be further reduced to six-connected (octahedron with Oh symmetry) and four-connected (rectangle with C4v symmetry) nodes. The four-connected Zr6 nodes, although having been reported, are still rare in Zr-MOF structure.33–36 In a search for MOFs with idp topology, we found that most reported idp networks are constructed by Zn4O(COO)6 (six-connected node in octahedral geometry) and Cu(COO)4 (four-connected node in rectangular geometry) clusters.29,37 Therefore, this work demonstrates that degrading the symmetry of organic linkers caused downgraded connectivity for Zr6 clusters and therefore gave rise to some topologies which are usually observed in Cu-, Mn-, Zn-MOF, and so forth, presenting an effective strategy for expanding the Zr-MOF library.37–39 Figure 2 | The view of organic building block, Zr6 cluster, and two types of cavities in crystal structure (top). Irregular tiles with SOD topology surrounding the tetrakis hexahedron tile (bottom). Download figure Download PowerPoint Accompanied with the downgraded Zr6 clusters of PFC-16 are the features of novel architecture, abundant open-metal sites, and mesoporosity. The dihedral angles between the central benzene ring and the methylbenzoate fragments in BDBA3− ligand are 90° because of the steric hindrance of the –CH3 group, and therefore the carboxylate group on methylbenzoate is perpendicular to the other two carboxylate groups on the ethynylbenzoate fragments. Further investigation of the detailed structure revealed, 12 BDBA3− ligands connected with 6 four-connected Zr6-SBUs and 8 six-connected Zr6-SBUs generating a tetrakis hexahedron cage (Oh symmetry), which can be considered as a Catalan solid by putting a square pyramid onto each of the faces of a cube ( Supporting Information Figure S2).40–42 All the six-connected Zr6 clusters were located at the eight vertices of the cube, and the four-connected Zr6 clusters were located at the 6 vertices of the square pyramid ( Supporting Information Figure S2). It is known that Catalan solids are a class of polyhedrons composed of irregular polygons. In PFC-16, the symmetry inequivalent BDBA3− ligands served as some of the triangular faces of the pyramid, and we found that this structure was also obtained in other MOF structures based on symmetry inequivalent ligands.29 Moreover, in the tetrakis hexahedron, the pyramids' lateral faces have to be at an angle of strictly <45° to the base plane so that the lateral faces of adjacent pyramids do not lie in the same plane to form a rhombic face. This also required that the building blocks sat on the face of the pyramid must be non equilateral triangles such as H3BDBA. Therefore, besides the appropriate direction of each carboxylate group, the inequivalent extension of the ligand into irregular triangular building blocks is a prerequisite for the construction of this Catalan solid. We can further speculate that this type of ligand has the possibility of generating other Catalan solids unexplored in the MOF field. The tetrakis hexahedron cages exhibit pentagonal windows with an opening of ∼16 Å and an internal diameter of approximately 36 Å ( Supporting Information Figure S1). Such a tetrakis hexahedron was further connected with six adjacent tetrakis hexahedra by a vertex-transitive manner causing a pcu stacking (Figure 2 and Supporting Information Figures S2 and S3), between which are 24 irregular void tiles forming a SOD network with a tetrakis hexahedron embedded inside (Figure 2). This novel architecture gave rise to a highly porous framework with a calculated solvent-accessible surface area of 3510.47 m2 g−1. The experimental powder X-ray diffraction (PXRD) pattern of PFC-16 matched well with the simulated one, indicative of the attainment of the pure phase (Figure 3b). Experimental N2 adsorption isotherm exhibits type IV behavior without hysteresis upon desorption, further supporting the existence of permanent micro- and mesopores in the structure (Figure 3c). The deduced Brunauuer–Emmett–Teller (BET) surface area is 3231 m2 g−1 based on the N2 adsorption isotherms, consistent with the calculated results.43 Despite the high porosity, PFC-16 exhibits good acid and base stability in various pH aqueous solutions (pH 3–12) as indicated by the intact PXRD patterns and N2 uptakes (Figure 3c and Supporting Information Figure S5). A lower synthetic temperature (decreasing from 120 to 90 °C) gave rise to a smaller particle size of approximately 100 nm (denoted as nano-PFC-16) as confirmed by scanning electron microscopy (SEM; Supporting Information Figure S6), suitable for the subsequent in vitro and in vivo experiments. Figure 3 | Incorporation of photosensitizer and targeting molecule in nano-PFC-16. (a) Incorporation of ZnPc and FA into PFC-16 for PDT. (b) PXRD patterns of simulated-PFC-16, bulk-PFC-16, [email protected], and [email protected]. (c) N2 isotherms of PFC-16 treated with different pH aqueous solutions. (d) Digital photographs of nano-PFC-16 (left), [email protected] (middle), and [email protected] (right) suspended in DMF. (e) Solid luminescent spectra of ZnPc, nano-PFC-16, [email protected], and [email protected]. (f) Solid UV spectra of ZnPc, FA, nano-PFC-16, [email protected], and [email protected]. (g) IR spectra of nano-PFC-16, [email protected], FA, and [email protected]. Download figure Download PowerPoint Incorporation of photosensitizer and targeting molecule in nano-PFC-16 The addition of ZnPc or phthalocyanine nickel (NiPc) to the reaction mixtures yields composite material with ZnPc (7.03% mol·mol−1) or NiPc (16.22% mol·mol−1) being successfully encapsulated in nano-PFC-16 (denoted as [email protected] and [email protected], respectively) as proved by inductively coupled plasma (ICP) analysis ( Supporting Information Scheme S2).44 Solid-state fluorescence and UV–vis spectra show the characteristic peaks from the ZnPc component (Figures and the incorporation of ZnPc in nano-PFC-16. The of ZnPc can be to mesopores large for and abundant open-metal sites within the structure or Moreover, the existence of Zr-oxo clusters in the structure allows for some functional through In the carboxylate of FA ( Supporting Information Scheme can with Zr6 clusters to form nano-PFC-16, which is in Zr-MOFs the of we further FA on metal This caused an from to yellow (Figure The incorporation of FA was confirmed by the of [email protected], where new peaks at and were which can be to the and groups of FA, (Figure from the UV–vis (Figure and as shown in Supporting Information Figure can with to the generation of [email protected], nano-PFC-16, and [email protected] was by the of at = UV–vis spectra an excellent generation of [email protected], nano-PFC-16 and [email protected] exhibit much lower or for ( Supporting Information Figures and analysis of PFC-16, [email protected], [email protected], and after are revealed in Supporting Information Figure all of the materials can their frameworks 450 The N2 adsorption of [email protected], and [email protected] been at exhibit type IV behavior without hysteresis upon ( Supporting Information Figure The CO2 adsorption of PFC-16 been and the were calculated based on adsorption isotherms at and by using the ( Supporting Information Figure In vitro photodynamic performance of [email protected] in vitro the generation of [email protected] in aqueous solution was by oxygen The fluorescence of a at nm in the of [email protected] and 660 nm laser (Figure the efficiency was also The solutions with different concentrations of [email protected] were irradiated at 660 and the UV–vis spectra were to determine the of The concentrations of with the of [email protected] Moreover, the efficiency is in a concentration as as 20 μg it is a promising for PDT in vivo (Figure The group without [email protected] not show in concentration the 660 nm laser The of [email protected] was then through the cell after MCF-7 cells with [email protected] in various shown in Figure [email protected] cancer cells to 200 μg the PDT the cancer cells were irradiated with 660 nm laser for 10 min. The cell upon with that of the group without irradiation, the photodynamic of [email protected] (Figure of MCF-7 cells generated by calcein-AM and a much cell in the group treated with [email protected] and laser (Figure that was by photodynamic a which can be to by was as an ROS shown in Figure only MCF-7 cells treated with [email protected] and laser These results indicated that upon the at 660 [email protected] ROS and leads to cell potential for in vivo photodynamic Figure | In vitro photodynamic performance of [email protected]. (a) spectra of solution for generated (b) UV–vis spectra and of aqueous solution treated with various concentrations of [email protected] 20, 40, 60, and 100 μg with the same (c) of MCF-7 cells incubated with various concentration of [email protected] 10, 20, 40, 60, 100, and 200 μg for 12 h. (d) of MCF-7 cells after with [email protected] treated with or without 660 nm laser (e) cells of calcein-AM and 100 (f) confocal laser scanning fluorescence microscope of ROS with the in MCF-7 cells treated with [email protected] at μg mL−1 660 nm laser for 10 min. 100 Download figure Download PowerPoint In vivo PDT efficacy by the in vitro results we constructed groups of mice with of MCF-7 cells to further explore the photodynamic efficacy of [email protected] in in Figure the 40 in measured [email protected] or PBS was injected into four various groups of mice tail ( [email 20 mg 100 μL, and 660 Figure 5 | In vivo PDT (a) In vivo PDT with [email protected] by 660 nm laser MCF-7 mice were randomly divided into four PBS group, [email protected] only group, laser only group, and [email protected] + laser group. (b) tumor and (c) body weight of MCF-7 mice after being treated with different (d) tumor from mice in four (e) treated with after PDT 100 Download figure Download PowerPoint shown in Figure the tumor were other for a of days. of laser only and [email protected] only groups grew at a in the first which was the PBS group without After the PDT on the the of both was different and a tumor the [email protected] + laser group efficacy that with the PBS group. In in body weight was observed for all caused by such a (Figure The photographs of tumor from groups (Figure also revealed the analysis a to the photodynamic performance of in and that the of [email protected] and 660 nm laser where the cancer cells of tumor a

Topics & Concepts