Understanding the Photochemical Response of Zeolitic Imidazolate

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Article

Understanding the Photochemical Response of Zeolitic Imidazolate Framework-8 in the Sight of Framework, Uncoordinated 2-Methylimidazole and ZnO Clusters x

y

Tianyu Du, Hui Jiang, and Xuemei Wang J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 17 May 2017 Downloaded from http://pubs.acs.org on May 18, 2017

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The Journal of Physical Chemistry

Understanding the Photochemical Response of Zeolitic Imidazolate Framework-8

in

the

Sight

of

Framework,

Uncoordinated

2-methylimidazole and ZnxOy Clusters Tianyu Du, Hui Jiang and Xuemei Wang* State Key Laboratory of Bioelectronics (Chien-Shiung Wu Lab), School of Biological Science and Medical Engineering, Southeast University, Nanjing 210096, China *E-mail: [email protected]

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ABSTRACT ZIF-8 particles synthesized by solvothermal and self-assembly strategies were used as fluorescence probes to detect a series of analytes based on its “turn on” or “turn off” fluorescence. Besides the electronic transition, the crucial effect of the evolution of structure as well as the ZnxOy clusters and uncoordinated 2-methylimidazole was investigated and the relevant mechanism of fluorescent response was further explored in this study. It was found that the ZnxOy clusters within the cages of ZIF-8 can increase the fluorescence up to 4 folds. Two types of uncoordinated 2-methylimidazole, resulted from the residue or escape of metal center, also make special contribution to fluorescent response. During the chemical sensing, different substances showed different fluorescence response via one or several of these impacting factors. 1. INTRODUCTION Recently, metal-organic frameworks (MOFs) have received extensive attentions and been widely investigated. They are often compared to zeolites for their large specific surface area and high degree of crystallinity.1,2 Besides these advantages, MOFs also show unique properties such as tunable pore size, controllable shape and flexile structure, which enable them applied in the field of catalysis, separation, energy storage, drug release, and chemical sensing et cetera.3-6 Among these applications, chemical sensing is often based on their “turn on” or “turn off” fluorescence (FL) properties.7-10 The fluorescence of most MOFs always arises from the π electrons in aromatic structure,11 and the immobilization of organic ligands by metal ions also enhances the fluorescence.12,13 For sensing, the large specific surface area and porous structures allow the residence of guest molecules inside MOFs and the interaction between guest and host can impact the fluorescence via either electron transfer or energy transfer. Within transition metal MOFs, Zn-MOFs show potential application in biochemical detection due to its bright fluorescence. As a representative of zeolitic imidazolate framework (ZIF), ZIF-8 has high hydrothermal stability (i.e., with its structure maintained for more than 7 days even in boiling water), large specific surface area and controllable particle size, which enable it further development and modification in various situations.14,15 ZIF-8 can be used as a fluorescence probe to detect ions and small molecules and the fluorescence response of the analyte depends on the electron transfer or “guest to host” effect.2,8 Nevertheless, we also find the significant role of ZnxOy clusters and uncoordinated ligands (2-methylimidazole) for relevant fluorescence response, which was consistent with the previous work on MOF-5.16,17 On the basis of these observations, in this contribution we attempted to understand the ions / molecule sensing of ZIF-8 through the evolution of ZnxOy clusters and uncoordinated ligands. Two types of ZIF-8 probes were synthesized by solvothermal or self-assembly strategy. Four species of analytes, i.e., solvent molecules, cations, anions and small biomolecules, were used to obtain the comprehensive results. It was found that not only the electron transfer, but also the structure transformation, such as degradation of framework or ZnxOy clusters and escapement of Zn2+ from metal centers, will determine the fluorescence of ZIF-8. In addition, by comparing the performance of ZIF-8 probes, we also found different sample has their befitting analytes. 2. EXPERIMENTAL 2.1 Synthesis of ZIF-8

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ZIF-8 were synthesized via solvothermal crystallization and self-assembly methods, as reported in refs 18 and 19. For the solvothermal method, Zn(NO3)2·6H2O of 1.10 g and 2-methylimidazole (HMeIM) of 0.613 g were dissolved in 100 mL N,N’-dimethylformamide (DMF) to form a clear solution. Then, the solution was transferred to a tightly capped Teflon-lined autoclave and heated at 140 oC for 24 h, then cooled down to room temperature. The obtained pale-white powder was centrifuged, washed by using 50 mL DMF for three times and dried at 75 oC. This sample was named DZIF-U. The self-assembly method was conducted in methanol solvent. Similarly to solvothermal method, Zn(NO3)2·6H2O of 1.10 g and HMeIM of 0.613 g were dissolved in 100 mL methanol and stirred for 24 h. The white deposition was collected by centrifuge and washed using 50 mL methanol for three times and then dried at 75 oC for 12 h. The white powder, named MZIF-U, was thus obtained. 2.2 Purification of ZIF-8 The samples DZIF-U and MZIF-U were soaked in methanol for 2 days (with a concentration of 1 g/10 mL) with methanol changed every 12 h. The samples were collected, dried at 75 oC for 12 h and named as DZIF and MZIF. 2.3 Characterization FL spectra were recorded on an RF-5301PC fluoremeter (Shimadzu, Japan). Thermogravimetric (TG) analysis was performed on a TGA/SDTA 851e instrument (Mettler-Toledo, Switzerland) with a ramping rate of 10 K/min in air flow. UV−vis spectra were recorded on Biomate 3S spectrometer (Thermo Scientific, USA). Morphologies and particle size were characterized with an ultra plus SEM (Zeiss, Germany). XRD patterns were recorded on a XD-3 X-ray diffraction (Peking Purkinjie general instrument, China) with Cu Kα of 36 kV voltage and 20 mA current. DRIFTS was collected on Nicolet iZ10 spectrometer (Thermo Scientific, USA). UV-Vis DRS was carried out on a UV-2600 UV-Vis spectrophotometer (Shimadzu, Japan) using BaSO4 as reference. 2.4 Details for FL Tests Generally, the fluorescence spectra of ZIF-8 samples were collected in suspensions with a concentration of 1 g / L. Before tests, the ZIF-8 powder was ultrasonic dispersed for about 10 minutes. The solvent was water, methanol (MeOH), ethanol (EtOH), acetone (Ace), DMF, dimethyl sulfoxide (DMSO) and tetrahydrofuran (THF) in section 3.2, and was DMF in section 3.3. For cations detection, a concentrated DMF stock solution of KNO3, NaNO3, MgSO4, CaCl2, Ce(NO3)3·6H2O, Zn(NO3)2·6H2O, Co(NO3)2·6H2O, NiSO4·6H2O, FeCl3·6H2O, CuCl2 and FeCl3·4H2O (200 mM to 1 M) was diluted into the DMF suspensions of ZIF-8 to reach a series of desired concentrations. The fluorescence of the mixture was checked immediately. For anions detection, the concentrated aqueous solutions of NO3-, SO42-, Ac-, Cl-, CO32-, HCO3-, Brand HxPO4x-3 (x = 0, 1 or 2, K+ as counter ion for Br- and others are all Na+) were added into the suspensions and the FL spectra were collected after 15 minutes. In addition, for HxPO4x-3 detection, the pH (5 to 9) was adjusted by using 6.2 % HCl or 1 M NaOH. Glutathione (GSH), cysteine (Cys), phenylalanine (Phe), glutamic acid (Glu), histidine (His), arginine (Arg) and tyrosine (Tyr) were used in small molecules detection. The FL spectra were collected after 5 minutes. 2.5 Others Degradation of ZIF-8, adsorption of cations onto ZIF-8, checking the ZnxOy clusters are all listed in section 1 of supporting information.

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3. RESULTS AND DISCUSSION 3.1. Characterization of the ZIF-8 and Relevant Impact fFactors for Fluorescence Response Figure 1A shows the XRD patterns of the as-prepared ZIFs. All samples exhibit typical diffraction band of SOD topology structure.14 The particle sizes of DZIF and MZIF are determined by SEM images and DLS (Figures S7 and S8), which are 1.7 μm and 1μm respectively. In the FL spectra of ZIF-8 (Figures 1B and 1C), all samples show a main excitation peak at 398 nm and an emission peak at 460 nm. Furthermore, comparing the FL spectra, there are four points usually indicated, illustrated as follows: Firstly, the FL intensity of DZIF is nearly 4 folds as that of MZIF; Secondly, the impure samples, i.e., MZIF-U and DZIF-U, have stronger fluorescence than their pure counterparts; Thirdly, the impure ZIF-8 also exhibits an enhanced excitation peak at 355 nm; Fourthly, under the excitation of 355 nm, their fluorescent behaviors are different (MZIF-U shows enhanced fluorescence, and MZIF and DZIF-U stay the same while DZIF declined). To make a reasonable explanation, some spectral characterizations were conducted.

Figure 1. (A) XRD patterns of DZIF and MZIF. (B) FL spectra of (a) excitation peak of MZIF-U, (b) emission peak of MZIF-U under 398 nm and (c) 355 nm excitation. (d) excitation peak of MZIF, (e) emission peak of MZIF under 398 nm and (f) 355 nm excitation. (C) FL spectra of (a) excitation peak of DZIF-U, (b) emission peak of DZIF-U under 398 nm and (c) 355 nm excitation. (d) excitation peak of DZIF, (e) emission peak of DZIF under 398 nm and (f) 355 nm excitation. (D) DRIFTS of ZIF-8 samples (diluted by KBr). (E) In situ DRIFTS of DZIF-U sample under air flow at different temperatures. (F) UV-Vis DRS of ZIF-8 samples. Figure 1D shows the DRIFTS of the four types of ZIF-8 structures. Compared to DZIF, DZIF-U exhibits two additional vibration bands at 1090 and 1681 cm-1, and the same result can be also observed between MZIF-U and MZIF samples. This vibration can be attributed to uncoordinated HMeIM, trapped in apertures or cages of ZIF-8,2 which can be further confirmed under a heating condition. With the rising temperatures (in air flow), the above mentioned bands (1000-1100 and 1600-1800 cm-1) lose intensity (Figure 1E). Since ZIF-8 can maintain its structure until the temperature reaches 400 oC, the weight lost below 400 oC should be attributed to the decomposition of extra-framework HMeIM (TG, Figure S9).15

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The metal clusters inside MOF cages are believed to make certain contributions to fluorescence. In this respect, MOF-5 was widely investigated as a represent of Zn MOFs. The MOF-5, with Zn4O13 metal clusters as impurity, shows pronounced different fluorescence from the pure MOFs.17,20 Herein, we use the similar method to investigate the ZIF-8 samples. As shown in Figure 1F, the results of UV-Vis absorption studies indicate that MZIF and HmeIM exhibit the same absorbance at ~280 nm while DZIF shows another broad absorption band at 300-500 nm. This band results from Zn clusters,16,17,20 which located or trapped in the cages of ZIF-8. It was noted that the XRD patterns of DZIF show no diffraction peaks of ZnxOy, implying that the size of Zn clusters is below the detection limit.

Scheme 1. Components of MZIF-U, MZIF, DZIF-U and DZIF samples and their relationship. Based on the above results, a concise relationship is outlined in Scheme 1. ZIF-8 powder may consist of three primary components: the framework, uncoordinated HMeIM and Zn clusters. In the suspension of MZIF-U and DZIF-U, the uncoordinated HMeIM seems to maintain its solid state rather than rapidly dissolving into water, due to the protection of framework (this conclusion is confirmed and discussed in details in section 2 of supporting information). Such a solid-like HMeIM induces the excitation peak at 355 nm (line a in Figures 1B and 1C) and trap energy for ZnxOy clusters.16 This conclusion was supported by the different emission of DZIF and DZIF-U under excitation at 355 and 398 nm (Figure 1C). In chemical sensing, the fluorescence response mostly results from the electron or energy transfer between the analyte and ZIF-8.21-23 Despite this point, the following work attempts to understand the fluorescence sensing behavior of ZIF-8 from another perspective, i.e., the structure, HMeIM and Zn clusters. The investigation of the fluorescence of MZIF-U and DZIF-U provide the initial evidences to illustrate how degradation or corrosion of framework affects the fluorescence. By purifying the MZIF-U and DZIF-U it can yield MZIF and DZIF. Conversely, if the pure ZIF-8 is corroded, it can in turn convert to MZIF-U or DZIF-U. 3.2. Solvent Effects 5



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As Figure 2 shows, solvent molecules affect the fluorescence of both DZIF and MZIF. The fluorescence and structure of ZIF both keep stable during 4 hours (the evolution of structure is indicated by XRD in Figure S10), this phenomenon is attributed to the host-guest interactions between ZIF-8 and solvent molecules.11 Notice that MZIF shows higher fluorescent response than DZIF, it can be deemed the ZnxOy clusters and solvent molecules plays independent roles on fluorescence. If we define the impact of solvent molecules as a linear function: I = kI0 (1) I0 is the FL intensity of MZIF in water, I and k are the corresponding intensity and coefficient in different solvents. Then, for DZIF, it can be written as: I = kI0 + I1 (2) Where I1 is the contribution of ZnxOy clusters, it inactivates the FL sensitivity of DZIF then leads to inferior FL response.

Figure 2. Fluorescence response of (A) DZIF and (B) MZIF after soaking in different solvents for a period. 3.3. Cations ZIF-8 is pH sensitive, in the aqueous suspension, the hydrolysis of cations will release proton and degrades the framework. Thus, the free HMeIM molecules deposit the detected cations. To avoid this interference, the following tests are all conducted in DMF. As shown in Figure 3A, addition of 5 mM metal ions to the suspensions of ZIF-8 induce different FL response, in which Fe2+, Fe3+ and Cu2+ quench the fluorescence rapidly (in seconds) since they can trigger the ligand-metal charge transfer (LMCT) and relax the excitation energy.24-26 We further investigate the relationship between FL intensity and concentration and the results are shown in Figures 3B and 3C. Commonly, FL intensity shows monotonicity to the concentration of metal ions, while an abnormality occurred under the low analyte concentration for MZIF (Figure 3C). By checking the excitation spectra, two different pathways of DZIF and MZIF can be found in this process (Figure 3D). Upon addition of Cu2+ from 5 μM to 5 mM, the excitation intensity for DZIF follows a FL decreasing tendency while that for MZIF shows a slight increase under 5 μM Cu2+. This is due to the hydrolysis of metal ions that corrodes the framework and induces the escape of Zn2+ to form holes (Figure S11). At the same time, the remained framework acts as solid state HMeIM to arouse the excitation at 355 nm, and this process increases the FL intensity in some degree. With the increasing ionic concentration (to 5 mM), most framework degrades and destructs the solid

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HMeIM-like units. It should indicate that the decrease of excitation intensity mainly results from the effect of LMCT and the evolution of framework is the minor factor. Another notable point is that the FL response for metal ions is merely instantaneous. In such a detection process, metal ions will gradually accumulate onto the ZIF-8 particles (Figure S12) and thus decrease the FL intensity after 5 minutes.

Figure 3. (A) Fluorescence response of ZIF-8 for different cations. The relationship between relative fluorescence intensity and analyte concentration for DZIF (B) and MZIF (C). (D) Excitation peaks of DZIF (above) and MZIF (below) and after adding 5 μM or 5 mM Cu2+. 3.4. Anions A series of tests indicate HxPO4x-3 (x = 0, 1 or 2) ions will enhance the FL intensity for both MZIF and DZIF (Figure 4A). As known, phosphate occurs in four forms in aqueous solution (i. e. H3PO4, H2PO4-, HPO42- and PO43-), and their fractions depend on the pH values. The relevant calculated results are shown in Figure 4A. Combining the results in Figures 4B and 4C (FL intensity of M/DZIF at different pH and HxPO4x-3 concentration), it is evident that the H2PO4- ions impact significantly on FL intensity. We notice that the stable fluorescence response cannot be obtained until adding HxPO4x-3 for 15 minutes. While considering the scale of HxPO4x-3 and the pore size of ZIF-8 (i.e., 1.2 nm),15 we believe such a FL response is merely result from a “surface to core” reaction. That is, HxPO4x-3 (or H+) remove Zn2+ from the framework and induce the fluorescence of solid-like HMeIM (with the excitation at 355 nm as depicted in Figures 4D and 4E) then enhance the FL intensity. This process shows a “hook” effect, i.e., the excessive degradation will collapse the framework and even decrease the FL intensity (for the case DZIF under 5 mM HxPO4x-3 and pH =

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5).27,28 By comparing the results of DZIF and MZIF, it can be found that DZIF shows higher resistance to such a “corrosive luminescence”, leading to inferior sensitivity.

Figure 4. (A) Fluorescence response of ZIF-8 for anions. Insert: Mole fractions vs pH for H3PO4. Fluorescence response of DZIF (B) and MZIF (C) for different concentration of HxPO4x-3 (x = 0, 1 or 2) and pH. Excitation peak of DZIF (D) and MZIF (E) under different pH, the concentration of HxPO4x-3 (x = 0, 1 or 2) is 5 mM. A further study was conducted by monitoring the fluorescence in 10 hours. Under a 5 mM HxPO4x-3 and pH = 7, the fluorescence of DZIF keep stable while MZIF loses intensity (Figure 5A). If tracking the release rate of HMeIM in this period, we can find ZIF-8 is continually degraded, especially for MZIF (Figure 5B). MZIF cannot maintain the original framework by resisting such a corrosion process. As a result, the FL intensity decreases. For DZIF, the existence of ZnxOy clusters resists the interference of HxPO4x-3 and the fluorescence is stable in a short period. More information can be obtained by XRD (Figure 5C). After soaking in 5 mM HxPO4x-3 (pH = 7) for 10 hours, trace Zn2+ is detected in solution (Figure S13). The released Zn2+ from MZIF transforms to ZnO. The broad band from 20 to 40 degree in XRD pattern suggests a large number of amorphous ZnO phase. This conclusion is also supported by the corresponding SEM image (Figure S14). While for DZIF, it generally keeps the framework after treatment of phosphate, all the diffraction peaks of ZIF-8 are remained (profile a in Figure 5C), although the treatment product, Zn3(PO4)2·4H2O phase is also detected. If the pH is adjusted to 5, DZIF transforms to Zn2P2O7 while no phase for MZIF can be observed (after 10 hours). Thus it can be seen that the “corrosion fluorescence” is based on the destruction of the framework, i.e., higher fluorescence origins from severer degradation and shorter MOF duration time.

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Figure 5. Monitoring the FL intensity (A) and HMeIM release proportion (B) of MZIF and DZIF under 5 mM HxPO4x-3 (x = 0, 1 or 2) and pH = 7. (C) XRD patterns of (a) DZIF soaked in 5 mM HxPO4x-3 at pH = 7 for 10 hours, (b) DZIF soaked in 5 mM HxPO4x-3 at pH = 5 for 10 hours and (c) MZIF soaked in 5 mM HxPO4x-3 at pH = 7 for 10 hours. 3.5. Small Molecules Several amino acids and GSH were used in this test. In Figure 6A it can found Glu, Cys and GSH quench the fluorescence of ZIF-8. This is attributed to the degradation of ZIF-8 particles. Figures 6B and 6C indicate that it is the only case that DZIF shows higher sensitivity than MZIF. This clue suggests the ZnxOy clusters may play the significant role. The UV-Vis absorption spectra of the degraded DZIF demonstrate that 5 mM Glu, Cys or GSH can dissolve the ZnxOy clusters in DZIF (in the order of Cys > GSH > Glu). As a comparison, several ZnxOy nanostructures were prepared to confirm the dissolving effect of Glu, Cys and GSH (supporting information). Since ZnxOy clusters endow ZIF-8 strong fluorescence (4 folds as pure ZIF-8), sharply decline of FL intensity will occur if they are disrupted. Different form the unwrapped ZnxOy clusters (or particles), the ZnxOy clusters inside the ZIF-8 particles are protected by frameworks, which buff the violent corrosion of Zn clusters. Comparing the fluorescence quenching of DZIF and MZIF, it is evident that Glu (or Cys, GSH) prefer to dissolve the Zn clusters rather than corrode the framework. The excitation spectra (Figure S15) demonstrate that Cys, Glu and GSH degrade DZIF and MZIF without the formation of “uncoordinated HMeIM”. It is believed to be a simple degradation process.

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Figure 6. (A) Fluorescence response of ZIF-8 for biomolecules. The relation between relative fluorescence intensity and analyte concentration for DZIF (B) and MZIF (C). (D) UV-Vis absorption spectra for DMF suspensions of DZIF and after adding 5 mM Cys, GSH or Glu. 4. CONCLUSION In summary, this work has investigated the effect of relevant framework, uncoordinated HMeIM and ZnxOy clusters during chemical responses of ZIF-8 by solvents, cations, anions and small molecules. Different analytes may involve the fluorescence response by following different mechanisms. To make a refined analysis, all the mentioned factors should be taken into consideration. For solvent molecules, host-guest effect makes the fluctuation of fluorescence while the ZnxOy clusters inside the particles inactivate detection sensitivity for DZIF samples. Some metal ions quench the fluorescence by electron transfer while HxPO4x-3 gives rise to a “corrosion induced fluorescence”. Although MZIF shows higher sensitivity than DZIF, its unstable framework limits its application under some conditions. Since GSH, Cys and Glu prefer to degrade the ZnxOy clusters, DZIF shows higher sensitivity than MZIF. ACKNOWLEDGMENT This work is supported by National High Technology Research & Development Program of China (2015AA020502) and the National Natural Science Foundation of China (81325011, 21327902, 21675023 and 21175020). H. Jiang acknowledges support from the Fundamental Research Funds for the Central Universities (2242016K41023). ASSOCIATED CONTENT Supporting Information 10


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The XRD details, detection of ions by UV-Vis spectra, effect of uncoordinated HMeIM on fluorescence, the corrosion effect of Cys, GSH and Glu for ZnO, TG, SEM and DLS of ZIF-8 are available free of charge via the Internet at http://pubs.acs.org. REFERENCES (1)

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(16) Bordiga, S.; Lamberti, C.; Ricchiardi, G.; Regli, L.; Bonino, F.; Damin, A.; Lillerud, K. P.; Bjorgen, M.; Zecchina, A. Electronic and Vibrational Properties of a MOF-5 Metal-Organic Framework: ZnO Quantum Dot Behaviour. Chem. Commun. 2004, 2300-2301. (17) Feng, P. L.; Perry, J. J.; Nikodemski, S.; Jacobs, B. W.; Meek, S. T.; Allendorf, M. D. Assessing the Purity of Metal-Organic Frameworks Using Photoluminescence: MOF-5, ZnO Quantum Dots, and Framework Decomposition. J. Am. Chem. Soc. 2010, 132, 15487-15489. (18) Tran, U. P. N.; Le, K. K. A.; Phan, N. T. S. Expanding Applications of Metal−Organic Frameworks: Zeolite Imidazolate Framework ZIF-8 as an Efficient Heterogeneous Catalyst for the Knoevenagel Reaction. ACS Catal. 2011, 1, 120-127. (19) Venna, S. R.; Jasinski, J. B.; Carreon, M. A. Structural Evolution of Zeolitic Imidazolate Framework-8. J. Am. Chem. Soc. 2010, 132, 18030-10833. (20) Hafizovic, J; Bjørgen, M.; Olsbye, U.; Dietzel, P. D. C.; Bordiga, S.; Prestipino, C.; Lamberti, C.; Lillerud, K. P. The Inconsistency in Adsorption Properties and Powder XRD Data of MOF-5 Is Rationalized by Framework Interpenetration and the Presence of Organic and Inorganic Species in the Nanocavities. J. Am. Chem. Soc. 2007, 129, 3612-3620. (21) Lopez, H. A.; Dhakshinamoorthy, A.; Ferrer, B.; Atienzar, P.; Alvaro, M.; Garcia, H. Photochemical Response of Commercial MOFs: Al2(BDC)3 and Its Use As Active Material in Photovoltaic Devices. J. Phys. Chem. C 2011, 115, 22200-22206. (22) Yang, J. S.; Swager, T. M. Fluorescent Porous Polymer Films as TNT Chemosensors: Electronic and Structural Effects. J. Am. Chem. Soc. 1998, 120, 11864-11873. (23) Sanchez, J. C.; DiPasquale, A. G.; Rheingold, A. L.; Trogler, W. C. Synthesis, Luminescence Properties, and Explosives Sensing with 1,1-Tetraphenylsilole- and 1,1-Silafluorene-vinylene Polymers. Chem. Mater. 2007, 19, 6459-6470. (24) Allendorf, M. D.; Bauer, C. A.; Bhakta, R. K.; Houk, R. J. T. Luminescent Metal-Organic Frameworks. Chem. Soc. Rev. 2009, 38, 1330-1352. (25) Jayaramulu, K.; Narayanan, R. P.; George, S. J.; Maji, T. K. Luminescent Microporous Metal-Organic Framework with Functional Lewis Basic Sites on the Pore Surface: Specific Sensing and Removal of Metal Ions. Inorg. Chem. 2012, 51, 10089-10091. (26) Ma, J.; Huang, X.; Song, X.; Liu, W. Assembly of Framework-Isomeric 4d-4f Heterometallic Metal-Organic Frameworks with Neutral/Anionic Micropores and Guest-Tuned Luminescence Properties. Chem. Eur. J. 2013, 19, 3590-3595. (27) Sun, C. Y.; Qin, C.; Wang, X. L.; Yang, G. S.; Shao, K. Z.; Lan, Y. Q.; Su, Z. M.; Huang, P.; Wang, C. G.; Wang, E. B. Zeolitic Imidazolate Framework-8 as Efficient pH-Sensitive Drug Delivery Vehicle. Dalton trans. 2012, 41, 6906-6909. (28) Li, S.; Wang, K.; Shi, Y.; Cui, Y.; Chen, B.; He, B.; Dai, W.; Zhang, H.; Wang, X.; Zhong, C.; et al. Novel Biological Functions of ZIF-NP as a Delivery Vehicle: High Pulmonary Accumulation, Favorable Biocompatibility, and Improved Therapeutic Outcome. Adv. Funct. Mater. 2016, 26, 2715-2727.

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Understanding the Photochemical Response of Zeolitic Imidazolate

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