(MOF) Encaged Pt(II) - ACS Publications - American Chemical Society

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Zeolite-like Metal-organic Framework (MOF) Encaged Pt(II)-porphyrin for Anion-selective Sensing Dilshad Masih, Valeriya Chernikova, Osama Shekhah, Mohamed Eddaoudi, and Omar F. Mohammed ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19282 • Publication Date (Web): 26 Mar 2018 Downloaded from http://pubs.acs.org on March 26, 2018

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ACS Applied Materials & Interfaces

Zeolite-like Metal-organic Framework (MOF) Encaged Pt(II)-porphyrin for Anion-selective Sensing Dilshad Masih,a Valeriya Chernikova,b Osama Shekhah,b Mohamed Eddaoudi,b*and Omar F. Mohammeda* a

King Abdullah University of Science and Technology (KAUST), KAUST Solar Center (KSC), Physical Science and Engineering Division (PSE), Thuwal, 23955-6900, Kingdom of Saudi Arabia b

King Abdullah University of Science and Technology (KAUST), Functional Materials Design, Discovery and Development Research Group (FMD3), Advanced Membranes and Porous Materials Center (AMPMC), Physical Science and Engineering Division (PSE), Thuwal, 239556900, Kingdom of Saudi Arabia ABSTRACT: The selectivity and sensitivity of sensors are of great interest to the materials chemistry community, and a lot of effort is now devoted to improving these characteristics. More specifically, the selective sensing of anions is one of the largest challenges impeding the sensingresearch area due to their similar physical and chemical behaviours. In this work, platinummetallated porphyrin (Pt(II)TMPyP) was successfully encapsulated in a rho-type zeolite-like metal-organic framework (rho-ZMOF) and applied for anion-selective sensing. The sensing activity and selectivity of the MOF-encaged Pt(II)TMPyP for various anions in aqueous and methanolic media were compared to that of the free (non-encapsulated) Pt(II)TMPyP. While the photoinduced triplet-state electron transfer of Pt(II)TMPyP showed a very low detection limit for anions with no selectivity, the Pt(II)TMPyP encapsulated in the rho-ZMOF framework possessed a unique chemical structure to overcome such limitations. This new approach has the potential for use in other complex sensing applications, including biosensors, which require ion selectivity. Keywords: Anion sensing, Fluorescence quenching, Metal-organic framework, Metallated porphyrin, Triplet state

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1. Introduction The quest for selective, luminescent sensing of anions is extremely challenging, due to their peculiar interaction with sensors as a result of their indistinct physical and chemical properties. In particular, halides (F−, Cl−, Br−, and I−), sulfide (S2−), and cyanide (CN−) play essential roles in many biological, chemical, and environmental processes, leading to a negative impact even at very low concentrations.1-8 To date, the various approaches for the detection of anions include liquid chromatographymass spectrometry (LC-MS); liquid chromatography-atomic emission spectrometry (LC-AES); capillary electrophoresis coupled with absorption or fluorescence detection; and electro-chemical techniques.9-14 However, these techniques are rather time-consuming and involve tedious sample preparations. In addition, the performance of these techniques is significantly influenced by the interference of coexisting anions. Recently, different studies have shown great potential for the development of photoluminescence (PL)-based methods for the highly sensitive, rapid, and selective identification of anions.15-23 However, some technical issues, such as the complex preparation of the fluorophores, their low hydrophilicity, and the need for a multi-step assessment, have limited the application of these methods. In a previous study, we demonstrated the application of the free 5,10,15,20-tetra(1-methyl-4pyridino)-porphyrin Pt(II) tetra-chloride (Pt(II)TMPyP)-based PL sensor for the ultra-sensitive, easy, rapid, environmentally friendly, and economical determination of anions, especially for iodide.15 The method is a facile, one-step approach for monitoring iodide anions in an aqueous phase with a pico-molar (pmol) level detection limit. In the presence of iodide anions, the PL quenching of Pt(II)TMPyP was directly observed upon irradiation with visible light. The method


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does not require complex preparatory steps or an additional species such as Hg, which is usually used for the activation of fluorophores, as a fluorescence turn-on signal.22,

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In that study, a

detailed mechanism using cutting-edge ultrafast laser spectroscopy with broad-band capabilities was proposed for PL quenching. However, this method suffered from the absence of selectivity for a certain anion, and required further development in order to achieve selectivity while maintaining this excellent detection limit. Therefore, we targeted a promising solution to overcome such a drawback, which is the immobilization (i.e. encapsulation) of functional moieties (like metalloporphyrins) in solid matrices.24, 25 In this approach, the active molecules are encapsulated inside the cavities of a solid matrix in order to restrict motion and isolate their active sites. Metalloporphyrins have been successfully immobilized in matrices of porous solids (e.g. zeolites, metal-organic frameworks (MOFs), silicates, etc.).26-30 In addition, porphyrins are functionalized with reactive groups serve as building blocks for the construction of porous polymers31,

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or MOFs.33 Porphyrins in

immobilized or embedded form can be used as an active component for different applications, such as sensing and catalysis.34,35 The anticipated benefit of the framework is additional selectivity, due to the restricted movement, controlled size and shape or the accessibility of the pores to the physicochemically driven interaction for certain species. MOFs, among other porous materials, possess a unique structural tunability, which is reflected in their diverse applications.36-38 Although most of these applications require a high material stability, coordination bonding in MOFs often results in their limited stability.39 Turning a disadvantage into an advantage, this feature has been successfully exploited for controlled drug delivery, whereupon the slow pH-sensitive destruction of the MOF, the drug is released.40 Interestingly, recent studies describe the use of instability of the MOF loaded with a fluorescent


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molecule in certain media for selective sensing.41 Finally, both photoinduced electron transfer and molecular rotors in porous materials are common fluorescence sensing mechanisms.15, 42 In this work, we aimed to explore the potential sensing benefits of Pt(II)TMPyP in its encapsulated form. In order to do that our recently reported (indium-imidazole-dicarboxylate)based rho-type zeolite-like metal-organic framework (rho-ZMOF; topological analogous to zeolite RHO)43 was chosen as a host matrix, as it is able to address all the criteria for successful encapsulation of cationic metalloporphyrins. Particularly, the suitable pore size and the anionic nature of the framework allow the simultaneous encapsulation and construction of MOF, while the cationic porphyrin acts as structure-directing agent. Here we present the use of rho-ZMOF as a platform to encapsulate, isolate, and confine the metalloporphyrin (Pt(II)TMPyP), to enhance its selective sensing properties. By doing so, we gain both the limited accessibility of the porphyrin to certain species when incorporated into a MOF, and the porphyrin’s predictable release upon the destruction of the framework in mediadependent anion-selective sensing. 2. Experimental The in situ encapsulation process of the porphyrin is depicted in Fig. 1. Single-metal-ion-based tetrahedral molecular building blocks (MBBs) of InN4(CO2)4 are formed in situ in a solution of indium salt and 4,5-imidazole-dicarboxylate ligand (4,5-H3ImDc). Cationic Pt(II)TMPyP acts as a structure-directing agent, guiding the assembly of the MMBs to construct the rho-ZMOF framework. The anionic framework is electrostatically balanced by the presence of cationic porphyrin and [(CH3)2NH2]+ cations formed from the decomposition of solvent,43 thus, the Pt(II)TMPyP/rho-ZMOF composite is neutral.


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Materials. All reagents were obtained from commercial sources and used without further purification. Pt(II)TMPyP was supplied by Frontier Scientific. Indium nitrate, 4,5-imidazoledicarboxylic acid, alkali metal salts of halides, sulfide, and cyanide were purchased from SigmaAldrich. Deionized water was used in all the aqueous phase experiments. Anhydrous methanol, ethanol (EtOH), and dimethylformamide (DMF) were purchased from Sigma-Aldrich. Pt(II)TMPyP/rho-ZMOF was synthesized following the previously reported procedure with slight modifications:28 Briefly, 200 μmol of 4,5-H3ImDc, 80 μmol of In(NO3)3·xH2O, and 5 μmol of Pt(II)TMPyP were mixed with 3 mL EtOH/DMF mixture (1:1) in a 25 mL scintillation vial. The vial was sealed and sonicated for 20 min, then heated to 85°C for 12 h, and then heated again to 105°C for 48-72 h, before cooling down to room temperature. The phase purity of resultant red crystalline powder was confirmed by powder X-ray diffraction (PXRD) (Fig. S1). It should be noted that the prepared Pt(II)TMPyP/rho-ZMOF was washed and centrifuged in DMF and methanol at least 5 times in each solvent to ensure that no residual porphyrin or other unreacted reagents is left in the final product (Fig. S2). On the other hand, the composition and the size of Pt(II)TMPyP/rho-ZMOF composite was estimated from scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX) and inductively coupled plasma (ICP) analysis (Table S1, Fig. S3-4). The atomic ratio of In/Pt is about 12, as estimated from both SEM-EDX and ICP analysis. Elemental mapping for O, In, and Pt show their homogeneous distribution (Fig. S4). The particle size of the composite is in the range of 0.5–5 micron. The solid state UV-Vis spectra (Fig. S5) further confirm the presence of Pt(II)TMPyP in the rhoZMOF. Finally, the optical image of the suspension of the composite is shown in Fig. S6. PXRD measurements were carried out at room temperature on a PANalyticalX’Pert PRO diffractometer operating at 45 kV, 40 mA for CuKα (λ = 1.5418 Å). The data was recorded with


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a scan speed of 1.0° min-1 and a step size of 0.01° in the 2θ range of 5–40 degree. PXRD pattern of Pt(II)TMPyP/rho-ZMOF was simulated in Material Studio and matched with the experimental data illustrating the presence of Pt-porphyrin inside the framework structure (Fig. S7). Proposed packing of two Pt(II)TMPyP molecules in each cage of the rho-ZMOF, as estimated from the elemental analysis is visualized from the simulations as shown in Fig. S8.

SEM images and EDX analysis were performed on FEI Quanta 600 electron microscope equipped with X-ray mapping with an acceleration voltage of 30 kV. For elemental analysis, ICP and optical emission spectroscopy (OES) experiments were carried out using a Varian 720 ICPOES Spectrometer. Fourier-transform infrared (FT-IR) spectra (4000–600 cm–1) were collected in the solid state on a Nicolet 700 FT-IR spectrometer. Nuclear magnetic resonance (NMR) data of liquid state samples were collected on a 500 MHz Bruker instrument. Steady-state absorption and emission measurements were performed using a rectangular quartz cell with a 1 cm optical path on a Cary5000 UV-visible spectrophotometer (Agilent Technologies) and a Fluoromax-4 spectrofluorometer (Horiba Scientific), respectively. In the steady-state measurements, the concentration of the free (non-encapsulated) Pt(II)TMPyP and its composite was kept constant. The Pt(II)-based free (non-encapsulated) porphyrin was completely soluble in water and methanol. In the case of the composite, a homogeneous suspension was used for recording the steady-state spectra. Small aliquots of dilute anion solution containing porphyrin were successively added into the quartz cell containing the analyst. UV-Vis and photoluminescence spectra were recorded immediately after each addition of the analyte solution. An excitation wavelength of 512 nm was used for the PL experiments. By the end of


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the steady-state measurements, the solid part was separated by filtration. Finally, the obtained product was dried overnight for X-ray and FT-IR measurements.

Figure 1. Synthetic approach of the Pt(II)TMPyP/rho-ZMOF composite: Construction of eight coordinated MBBs, followed by assembly of the MBBs, as directed by Pt(II)TMPyP porphyrin, leading to the incorporation of the porphyrin into the rho-ZMOF.

3. Results and discussion 3.1. Sensing of anions in aqueous phase. The sensing activity and selectivity of the MOFencaged Pt(II)TMPyP for iodide and sulfide ions in aqueous (Fig. 2) and methanolic (Fig. 3) media were compared to that of the free Pt(II)TMPyP. In an earlier study, we reported that the free Pt(II)TMPyP porphyrin has an ultrahigh sensitivity for the detection of iodide anions in the


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aqueous phase (Fig. S10-11).15 A similar level of sensitivity was also observed towards sulfide ions, as indicated by the strong PL quenching shown in Fig. 2a-b. However, the free Pt(II)TMPyP, on the other hand, showed only moderate PL quenching in the case of cyanide and bromide ions (Fig. S11).15 The level of sensitivity varied, but selectivity towards a specific anion was still lacking. Therefore, the free Pt(II)TMPyP demonstrated non-selective PL ‘turn-off’ behaviour towards all the various anions. To overcome its poor sensing selectivity towards certain anions, we encapsulated the Pt(II)TMPyP moiety in the rho-ZMOF (Fig. 1), restraining the Pt(II)TMPyP in the surrounding environment of the framework. Using the ship-in-a-bottle approach,28 composites were obtained by packing the Pt(II)TMPyP moiety in the LTA-type cages of the rho-ZMOF, as was confirmed by the change in both the crystal colour (Fig. S6) and PXRD patterns (Fig. S1), in addition to recorded the photoluminescence spectra (Fig. 2). The size of the ring windows (diameter 9 Å) serves as the gateway for controlling the passage, allowing access to the species but not the elution of the porphyrins. The synthesized composite structure of Pt(II)TMPyP/rho-ZMOF was first tested for the detection of iodide and sulfide ions in the aqueous phase. However, the Pt(II)TMPyP showed insensitivity towards iodide when it was encapsulated inside the rho-ZMOF structure (Fig. 2c), in contrast to the free Pt(II)TMPyP, which was immediately responsive and extremely sensitive to the diluted solution of iodide anions. Also, in the presence of excessive amounts (46 times higher) of the iodide anions, the Pt(II)TMPyP/rho-ZMOF composite showed a negligible change in the PL ‘turn-off’ signal (Fig. 2c). Therefore, it is believed that, after encapsulation of Pt(II)TMPyP in the rho-ZMOF, the surrounding framework and the aqueous phase interactions with iodide anions blocked the photoinduced electron transfer. The charge of cationic porphyrin is noticeably balanced by the electrostatic interaction with the anionic framework. Therefore, even though the window size


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allows iodide anions to enter the pores, the neutral nature of the Pt(II)TMPyP/rho-ZMOF composite inhibits or prevents an anionic framework/I- exchange; thus, the photoinduced reduction does not occur. Interactions of other halide ions, such as fluoride, chloride, and bromide, with the composite Pt(II)TMPyP/rho-ZMOF were also investigated; their behaviour was found to be similar to that of the iodide anions. A trend of sensitivity towards halides was observed for the free (non-encapsulated) Pt(II)TMPyP: iodide > bromide > chloride > fluoride (Fig. S11). However, Pt(II)TMPyP encapsulated in the rho-ZMOF structure became altogether inactive in the presence of halides (Fig. S12). Hence, the photoinduced electron-transfer from the halide to the platinum metal centre, and subsequently to the meso-units, was hampered for the porphyrin fixed inside the MOF structure. Furthermore, this inactivity towards halides reflected on the stability of the Pt(II)TMPyP/rho-ZMOF composite structure in aqueous solutions. It

Figure 2. Steady-state photoluminescence spectra for the interactions of free (nonencapsulated) porphyrin Pt(II)TMPyP in aqueous phase in the presence of a) iodide ions and b) sulfide ions; also for composite Pt(II)TMPyP/rho-ZMOF in the presence of c) iodide ions and d) sulfide ions in the aqueous phase.


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should be noted that if Pt(II)TMPyP molecules are released from their MOF cages, then photoinduced quenching of the PL intensity will result. Besides unchanging PL spectra, the optical analysis also does not show any significant change upon interaction with aqueous-phase halide ions. Interestingly, upon addition of aqueous-phase sulfide ions to the Pt(II)TMPyP/rho-ZMOF composite, a prominent decrease was noticed in the PL intensity of the porphyrin (Fig. 2d). For the same amounts of iodide and sulfide added to the composite, the addition of iodide showed a negligible (~7%) change in the PL intensity, while more than 71% PL quenching was observed with the addition of sulfide. Thus, in the composite form, Pt(II)TMPyP porphyrin demonstrated selective sensing of sulfide ions over halides. The lack of quenching by the encapsulated Pt(II)TMPyP porphyrin for halides, and specifically iodide explains the restriction on photoinduced electron-transfer. In principle, the detection of sulfide (and anions) by Pt(II)TMPyP may follow the same mechanism as verified in our previous study: to cause the PL quenching, the Pt(II)TMPyP was released from the restriction of its LTA cages in the rhoZMOF. Critically, this dissolution of the composite structure and the consequent release of Pt(II)TMPyP porphyrin is the guiding factor in the selective sensing of aqueous-phase sulfide ions. This can be explained by the instability of the rho-ZMOF under basic conditions and the acid-base properties of salt solutions; the aqueous solution of sulfide is more basic than the iodide solution. To check stability, the composite, Pt(II)TMPyP/rho-ZMOF was immersed in aqueous media with pH values ranging from 2 to 10. As depicted in Fig. S9, the PXRD patterns demonstrated the destruction of the ZMOF framework structure under basic conditions. Therefore, upon addition of the Pt(II)TMPyP/rho-ZMOF composite into the aqueous sulfide solution, Pt(II)TMPyP moiety is released by the destruction of the framework, as confirmed by


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PXRD and steady-state infrared spectra (Fig. 4a-b). Even after encapsulation in rho-ZMOF, Pt(II)TMPyP remained similarly effective for detecting anion (sulfide) at extremely low (pmol/nanomol) levels as it was determined in previous study.15 The estimation of detection limit is described in the Section 3 of the Supporting Information. 3.2. Sensing of anions in organic (methanol) phase. The results in the case of free (nonencapsulated) Pt(II)TMPyP have so far shown that, regardless of the solution media, aqueous or methanolic (Fig. S11 & S13), the analyte anion directed the photoinduced triplet-state electron transfer and led to the quenching of the PL intensity. Both the iodide and sulfide ions exhibited fast responses and strong quenching of the PL intensity in the methanol phase, similar to their aqueous phase behaviour (Fig. 3a-b). For the same concentration of iodide and sulfide anions in methanolic solution, the level of PL quenching by free Pt(II)TMPyP was 75% and 60%, respectively. On the other hand, the Pt(II)TMPyP/rho-ZMOF composite was found to be insensitive towards the iodide ions in methanolic solution. For the same concentration of iodide ions that was showing 75% PL quenching of free Pt(II)TMPyP, the PL intensity of the Pt(II)TMPyP/rho-ZMOF composite remained almost unchanged. This inactive behaviour provides further evidence that iodide is not able to perturb the porphyrin unit. In contrast to the negligible PL change observed during the interaction of the composite with iodide ions in methanolic solution, a drastic enhancement of the PL intensity was effected upon interaction with sulfide ions (Fig. 3c-d). In other words, an interesting and important PL ‘turn-on’ signal that extremely enhanced (210%) PL intensity was demonstrated by the interaction of sulfide ions with the Pt(II)TMPyP/rho-ZMOF composite. This enhancement in the PL intensity may result from the restrained movements of the porphyrin by the new chemical environment inside rhoZMOF. In the organic medium, the sulfide ions are perhaps interacting with the structural


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Figure 3. Steady-state photoluminescence spectra for the interactions of free (nonencapsulated) porphyrin Pt(II)TMPyP in methanol phase in the presence of a) iodide ions and b) sulfide ions, also for composite Pt(II)TMPyP/rho-ZMOF in the presence of c) iodide ions and d) sulfide ions in the methanol phase. It should be noted that these data are collected under atmospheric conditions. moieties of the MOF. Thus, a change in the surrounding environment of the encapsulated Pt(II)TMPyP would further hinder the twisting motion of the meso-unit of the porphyrin,44,45 enhancing the radiative process; subsequently, an increase in the PL intensity could be observed. Consequently, this selectivity may come at the expense of the structural regularity of the MOF, as observed from the PXRD (Fig. 4c). From a practical viewpoint, this selective PL ‘turn-on’ signal is highly desirable, since it is least influenced by the presence of any coexisting species. It is worth mentioning that, similar to its insensitivity towards iodide, the Pt(II)TMPyP/rho-ZMOF composite structure was also ineffective for the detection of other halides in methanol phase (Fig. S12). Finally, the estimation of the detection limit of nanomol level for iodide/sulfide is shown in supporting information (see also Fig. S13).


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3.3. Stability of Pt(II)TMPyP/rho-ZMOF composite. In order to understand the behaviour of the Pt(II)TMPyP/rho-ZMOF composite, we have characterized it with powder XRD. The PXRD patterns were recorded to determine the structural changes in the Pt(II)TMPyP/rhoZMOF composite after interactions with the analyte anions in both aqueous and organic solutions. As shown in Fig. 4a, the long-range order of the MOF structure decreased upon interaction with aqueous-phase iodide solution. As a result of the slow and partial dissolution of the MOF structure, a small amount of Pt(II)TMPyP was released into the solution. Subsequently, this released porphyrin reacted with the iodide ions and a PL ‘turn-off’ was observed; this can be attributed to the photoinduced electron transfer process. For comparison, a more deteriorated long-range order of the composite was observed after its interaction with aqueous-phase sulfide ions (Fig. 4a). This reduction in the structural regularity indicates the decomposition/phase change of the framework, which can be explained by the difference in basicity of the sulfide and iodide solutions. These data are in good agreement with the instability of the rho-ZMOF in basic conditions as confirmed by PXRD under different pH conditions (Fig. S9). The framework of the rho-ZMOF is less stable under basic conditions; therefore, the destruction process is accelerated in the presence of sulfide ions, leading to the release of the encapsulated Pt(II)TMPyP moiety which was confirmed by NMR (Fig. S14), and resulting in its availability for the photoinduced electron transfer reaction. This could explain the enhanced quenching of intensity in the PL spectra upon interaction with the sulfide ions. Infrared spectra of the Pt(II)TMPyP/rho-ZMOF were recorded after interactions with the analyte ions in the aqueous and organic phases, to monitor changes in the functional groups and the consequent structural restraint or release of the Pt(II)TMPyP/rho-ZMOF composite. FT-IR


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Figure 4. a) PXRD patterns and b) FT-IR spectra of the Pt(II)TMPyP/rho-ZMOF solid product, obtained after interactions with iodide and sulfide ions in aqueous solutions. c) PXRD patterns and d) FT-IR spectra of the Pt(II)TMPyP/rho-ZMOF solid product, obtained after interactions with iodide and sulfide ions in methanolic solution. It should be noted that the amount of the sulfide or iodide added into the Pt(II)TMPyP/rho-ZMOF composite is extremely small (at the nano-to-micro mole level) therefore, no appreciable changes are seen in the composites after interactions with these ions.


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spectra for the solid product, obtained from the composite structure after interaction with water and anion solutions, are depicted in the Fig. 4b. Essentially all the functional groups, but primarily the organic motif 4,5-imidazole dicarboxylate of the rho-ZMOF and Pt(II)TMPyP moiety were present in the FT-IR spectra. No changes or new IR active peaks were observed in the vibrational spectra for either the aqueous or the aqueous-iodide solution. Hence, the aqueous iodide anions did not merge with the MOF structure, which is consistent with observations from the PXRD analysis. On the other hand, the decrease in IR intensity illustrated a loss-ondissolution for the 4,5-imidazole-dicarboxylate group. A drastic change in the IR stretching modes was observed in the recovered solid product after the interaction with sulfide (Fig. 4b). This decrease in the intensity of the IR peaks indicates the dissolution of the MOF structure. An extensive release of Pt(II)TMPyP at the expense of the MOF structure can explain the enhanced quenching of PL intensity upon interaction with sulfide ions. PXRD patterns of the Pt(II)TMPyP/rho-ZMOF composite after interactions with iodide and sulfide ions in methanolic solutions are shown in Fig. 4c. In comparison with the decreased structural regularity after interaction with an aqueous solution, the long-range order of the composite was better preserved in methanolic solution, and the relative ratio of diffraction peaks did not change much. However, prominent diffraction peaks appeared at different positions for the product obtained from interactions with sulfide ions in the methanolic solution. We believe that upon addition of sulfide, significant modifications in the composite structure have directed these changes to the relative peak intensity. The sulfide ions manifested strong interaction with the composite structure in methanol, and perhaps made the framework structure more rigid. In addition, with the increased structural restraints, the tightly packed porphyrin exhibited an increase in PL intensity. Therefore, these compositional changes in the MOF structure guided the


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selective PL ‘turn-on’ sensing of sulfide ions. It is worth mentioning that there is a small PL spectral shift of the porphyrin when we go from water to methanol solution which could be attributed to the change in the environment around the porphyrin including different polarity, different hydrogen bond strength as well as different degree of solvation.46-47 The FT-IR spectra supported the observations in the crystalline structure analysis (Fig. 4d). All of the IR stretching modes during the interaction of the composite with iodide in methanolic solution was essentially the same (Fig. 4d). On the other hand, FT-IR spectra for the composite revealed drastic changes after interactions with methanolic sulfide solution, supporting the observations in the PXRD pattern (Fig. 4c-b). Perhaps this structural modification of the MOF framework was responsible for constructing restrictions around the packed porphyrin, and thus presented a PL ‘turn-on’ signal. 4. Conclusion The encapsulation of the platinum-metallated porphyrin, Pt(II)TMPyP, was successfully achieved in a rho-ZMOF using the ship-in-a-bottle approach. The sensing activity and selectivity of the Pt(II)TMPyP/rho-ZMOF composite for various anions in aqueous and methanolic solutions were investigated and compared with the free (non-encapsulated) Pt(II)TMPyP. Unlike the negligible PL changes observed during the interaction of the composite with iodide ions in both aqueous and methanolic solutions, drastic changes in the PL intensity were observed upon interaction of the composite with sulfide ions only,. In conclusion, this study provides a strong experimental evidence for anion-selective sensing by Pt(II)TMPyP/rho-ZMOF. Such a unique capability is completely missing in the case of free (non-encapsulated) Pt(II)TMPyP. This new analytical approach can now be applied to other complex systems, including biosensing.


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ASSOCIATED CONTENT Supporting Information. Experimental details of steady-state measurements. The Supporting Information is available free of charge on the ACS Publications website. AUTHOR INFORMATION Corresponding Author *Prof. Omar F. Mohammed; E-mail: [email protected] * Prof. Mohamed Eddaoudi; E-mail: [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT The research reported in this publication was supported by funding from King Abdullah University of Science and Technology (KAUST).


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