Stable Zr(IV)-Based MetalâOrganic Frameworks with Predesigned

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Stable Zr(IV)-Based Metal-Organic Frameworks with Pre-Designed Functionalized Ligands for Highly Selective Detection of Fe(III) Ions in Water Bin Wang, Qi Yang, Chao Guo, Yuxiu Sun, Lin-Hua Xie, and Jian-Rong Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00918 • Publication Date (Web): 01 Mar 2017 Downloaded from http://pubs.acs.org on March 3, 2017

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ACS Applied Materials & Interfaces

Stable Zr(IV)-Based Metal-Organic Frameworks with Pre-Designed

Functionalized

Ligands

for

Highly

Selective Detection of Fe(III) Ions in Water Bin Wang,† Qi Yang,† Chao Guo,‡ Yuxiu Sun,‡ Lin-Hua Xie,*† and Jian-Rong Li† †

Beijing Key Laboratory for Green Catalysis and Separation and Department of

Chemistry and Chemical Engineering, College of Environ-mental and Energy Engineering, Beijing University of Technology, Beijing 100124, P. R. China. ‡

Department of Chemistry, Tianjin Normal University, Tianjin 300387, P. R. China

KEYWORDS. Fe3+ ion detection, fluorescent quenching, ligand design, metal-organic framework, water system.

Abstract: Metal-organic frameworks are a class of attractive materials for fluorescent sensing. Improvement of hydrolytic stability, sensitivity and selectivity of function is the key to advance application of fluorescent MOFs in aqueous media. In this work, two stable MOFs, [Zr6O4(OH)8(H2O)4(L1)2] (BUT-14) and [Zr6O4(OH)8(H2O)4(L2)2] (BUT-15) were designed and synthesized for the detection of metal ions in water. Two

new

ligands

utilized

for

construction

of

the

MOFs,

namely,

5',5'''-bis(4-carboxyphenyl)-[1,1':3',1'':4'',1''':3''',1''''-quinquephenyl]-4,4''''-dicarboxylat e (L1) and 4,4',4'',4'''-(4,4'-(1,4-phenylene)bis(pyridine-6,4,2-triyl))tetrabenzoate (L2), are structurally similar with only difference being that the latter is functionalized by 1 ACS Paragon Plus Environment


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pyridine N atoms. The two MOFs are isostructural with a sqc-a topological framework structure, and highly porous with the Brunauer-Emmett-Teller (BET) surface areas of 3595 and 3590 m2 g–1, respectively. Interestingly, they show intense fluorescence in water, which can be solely quenched by trace amounts of Fe3+ ions. The detection limits towards the Fe3+ ions were calculated to be 212 and 16 ppb, respectively. The efficient fluorescent quenching effect is attributed to the photoinduced electron transfer between Fe3+ ions and the ligands in these MOFs. Moreover, the introduced pyridine N donors in the ligand of BUT-15 additionally donate their lone-pair electrons to the Fe3+ ions, leading to significantly enhanced detection ability. It is also demonstrated that BUT-15 exhibits an uncompromised performance for the detection of Fe3+ ions in a simulated biological system.

Introduction

Metal-organic frameworks (MOFs), formed by the connection of metal centers or clusters and organic linkers through coordination bonding, have gathered immense attention due to not only their intriguing architectures rendered by a variety of bridging ligands and metal ions but also their application potential in gas storage, separation, catalysis, proton conduction, sensing and so on.1-5 Recently, luminescent sensing with MOF materials has been considered as one of the most promising technologies for chemical and biological detection applications because it provides a simple, sensitive, selective, and economical approach for online monitoring of target analytes without the need of complicated sample pretreatments.5-8 So far, an 2 ACS Paragon Plus Environment

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increasing number of luminescence MOFs (LMOFs) have been reported and they have shown great potential as effective luminescence sensors for small molecules, metal cations, anions, protons and DNA.9-12 Particularly, the high porosities of MOFs prompt the pre-concentration of the analytes in their pores, therefore leading to lower detection limits and higher sensing sensitivities.13-15

Metal ions play key roles in life and environment, and the detection of these metal ions is quite important.16-18 Fe3+ ion is one of the most essential elements for either humans or other living organisms on account of their significance in many biochemical processes and biological systems. Both excess and deficiency from the normal permissible limit can induce serious system disorder.19-20 In addition, Fe3+ is a common inorganic pollutant in water. The presence of excess Fe3+ in drinking water could result in a number of problems related to human health. The recommended guideline level of Fe3+ in water is 0.3 ppm.21 Therefore, the selective detection of Fe3+ is a very important subject in biological research as well as water treatment industry. Till now, a few fluorescent MOFs have been found to be efficient for the detection of Fe3+ ions, however, most of them were exploited in organic solvent systems but not in water system because of their poor water stability, which limits their practical application.22-29 Although, several lanthanide MOFs as well as MIL-53(Al) have shown good performance for the detection of Fe3+ ions in water system. However, the detection processes of these MOFs towards Fe3+ ions take relative long time because of the detection mechanisms of them are 3 ACS Paragon Plus Environment


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based on the ion exchange between the framework metal ions and Fe3+ ions.30-34 Thus, there is an urgent need to develop new chemical stable fluorescent MOF sensors for the selective and fast detection of Fe3+ ion in water system. To design a desired MOF sensor for the selective Fe3+ detection in water, chemical stable structure, suitable pore size, appropriate luminophore, and efficient binding sites in it are required. With these considerations in mind, we expected that using large π-conjugated organic carboxylate ligands with Lewis basic sites and high oxidation state metal ions (such as Cr3+, Fe3+, Al3+, and Zr4+) to construct MOFs would be promising. Where, π-conjugated organic ligands can provide fair fluorescence,35 Lewis basic sites can interact with metal ions,36-38 and high oxidation state metals can form strong coordination bonds with carboxylate O donors in the ligands to endow with high stability of resulting MOFs.39-49 Zr(IV)-based MOFs with Lewis basic sites functionalized-conjugated organic carboxylic ligands are thus among the most desired candidates because of the following reasons: (1) Zr(IV)-MOFs are usually based on Zr6 carboxylate clusters, which could lead to their high chemical stability and moderate or large pore sizes; (2) with π-conjugated organic ligands, Zr(IV)-MOFs can perform intense fluorescent emission, (3) Zr4+ ion is hard acid and tends to coordinate with hard basic sites, typically carboxylate O donors, but not soft ones such as pyridine N, and thiophene S donors.40 As a result, these open basic sites can be immobilized on pore surfaces of carboxylate-based Zr(IV)-MOFs to provide binding site for Fe3+. In addition, the non-toxic nature of Zr4+ makes Zr(IV)-MOFs suitable for application in biological systems. 4 ACS Paragon Plus Environment

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In this work, two new π-conjugated organic carboxylic acid ligands, 5',5'''-bis(4-carboxyphenyl)-[1,1':3',1'':4'',1''':3''',1''''-quinque-phenyl]-4,4''''-dicarboxyli c

(H4L1)

acid

and

4,4',4'',4'''-(4,4'-(1,4-phenylene)bis(pyridine-6,4,2-triyl))-tetrabenzoic

acid

(H4L2)

were designed and synthesized, with only difference between them being the existence of functionalized pyridine N donors in the latter. The reactions of these acid ligands with ZrCl4 yielded two isostructural Zr(IV)-MOFs, [Zr6O4(OH)8(H2O)4(L1)2] (BUT-14,

where

BUT

=

Beijing

University

of

Technology)

and

[Zr6O4(OH)8(H2O)4(L2)2] (BUT-15), respectively, with the sqc-a topological framework structure. Both of them represent good chemical stability, high surface areas, moderate pore sizes, as well as intense fluorescent emission. Interestingly, their fluorescence can be selectively and solely quenched by trace amount of Fe3+ ions in water with low detection limits of 212 and 16 ppb, respectively. And, the detection performance was not influenced by the presence of other metal ions.

Results and discussion

Design, synthesis, and characterization

Currently, the Zr(IV)-MOFs constructed from tetratopic carboxylic acid ligands are mainly based on porphyrin type, tetraphenylethylene type, pyrene type, and tetrahedral carboxylate acid ligands, and all these MOFs exhibit excellent stabilities, large surface areas, and good properties in gas adsorption and separation, catalyst, fluorescence sensing, and electrochemistry.40 The Zr(IV)-MOFs constructed form 5 ACS Paragon Plus Environment


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other tetratopic carboxylic acid ligands are thus still worthy to be developed. The two designed tetratopic carboxylic acid ligands, H4L1 and H4L2 share similar structures and the only difference of them is that the latter is functionalized with pyridine groups (Figure 1a, f). Besides, they share relative large conjugated system and can be served as relative strong luminophores. Furthermore, the functionalized pyridine N atoms in H4L2 are soft bases, they tend not to coordinate with hard acids such as Zr4+ ion, thus, the pyridine N atoms can be introduced into pore surface of resulting Zr(IV)-MOFs and can be served as additional binding sites towards specific analytes. Solvothermal reactions of ligands (H4L1 or H4L2) with ZrCl4 in the presence of acetic acid as competing reagent in N,N-dimethylformamide (DMF) yielded

octahedral-shaped

single

crystals

of

[Zr6O4(OH)8(H2O)4(L1)2]

(BUT-14) and [Zr6O4(OH)8(H2O)4(L2)2] (BUT-15), respectively. Their phase purities were checked by PXRD measurement. As shown in Figure 2a and b, the experimental PXRD patterns match well with those simulated from the single-crystal data, indicating the pure phase of them. The thermogravimetric analysis (TGA) curves of BUT-14 and -15 are shown in Figure S2 of the Supporting Information, showing that they are stable up to ca. 420 and 440 oC, respectively, which are comparable to those of other Zr-MOFs.40 In addition, the absence of characteristic bonds near 1710 cm–1 for carboxyl groups of free carboxylic acid ligands in Fourier transform infrared (FT-IR) spectra of BUT-14 and -15 indicates that the acid ligands are deprotonated and their

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carboxylate groups should be coordinated with the metal centers in the MOFs (Figure S1, Supporting Information).

Design, synthesis, and characterization

Single crystal X-ray diffraction revealed that BUT-14 and -15 are isostructural, and the structure of the former is discussed in detail here. BUT-14 crystallizes in tetragonal I41/amd space group. Its framework consists of classical Zr6O4(OH)4 cluster core (Figure 1b) linked by elongated tetratopic ligand. Interestingly, the symmetry of the Zr6O4(OH)4 cluster core in BUT-14 is different from that in PCN-222,50 NU-1000,51 and BUT-1215 (Figure 1g and Figure S10 of the Supporting Information). In the three reported MOFs, six Zr atoms combine 8 μ3-OH/O entities to form an idealized octahedron with the D4h symmetry. While in BUT-14, a highly distorted octahedron with a low symmetry of D2d was observed, which also led to the lower framework symmetry of the MOF (Figure 1b and Figure S10 of the Supporting Information). Similar low symmetric Zr6O4(OH)4 core has also been observed in PCN-22552. Furthermore, 8 of the 12 octahedral edges in the cluster are connected to L1 ligands through the coordination of carboxylate groups and metal centers, while the remaining Zr coordination sites are occupied by 8 terminal –OH/H2O to form a Zr6O4(OH)8(H2O)4(CO2)8 secondary building unit (SBU) (Figure 1b). These SBUs are further linked by L1 ligands to form a 3D framework with two types of channels along either the a or b-axes. The small channels have a quadrangular shape with a size of 9 × 16 Å, whereas the larger ones are of pear-like shape having a 17 × 21 Å size (Figure 7 ACS Paragon Plus Environment


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1c, and Figure S10 of the Supporting Information). From the topological view point, L1 ligands can be seen as 4-connected nodes and Zr6 clusters serve as 8-connected nodes (Figure 1d, e), the 3D structure of BUT-14 can thus be simplified as a 4,8-coordinated bi-nodal net with the point symbol of {44.62}2{48.616.84}, which corresponds to the sqc-a topology (Figure 1h), similar to that in PCN-225.52 The total solvent-accessible volumes in frameworks of BUT-14 and -15 are estimated to be 78.2 and 77.6% of their unit-cell volumes, respectively, as estimated by PLATON53.

It should also be pointed out that the configurations of the ligands in BUT-14 and -15 are slightly different as shown in Figure 1a and f. The only difference of the two ligands is that ring 1 in L1 is a benzene ring and that in L2 is a pyridine ring. The absence of H atoms on the pyridine N atoms leads to a smaller steric hindrance, thus the dihedral angles between ring 1 and 2 in L1 (40.4o) is larger than that (32.6o) in L2. In addition, the aromatic rings in L2 are more coplanar than those in L1. Even with changes of these structural parameters in the ligands, the framework structures of BUT-14 and -15 are totally similar as mentioned above, indicating that the given framework of these MOFs are robust enough to tolerate some changes in ligands.54 BUT-14 and -15 thus provide a good platform for exploring the effect of the functional groups in ligands on the sensing performances as detailed below.


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Figure 1. Reticular construction and the structure of BUT-14 and -15: (a) L1 ligand; (b) Zr6O4(OH)8(H2O)4(COO)8 SBU, highlighting irregular Zr6O4(OH)4 core; (c) framework structure, showing quadrangular and pear-like channels (H atoms are omitted for clarity. color code: Zr, green; C, black; O, red; and N, blue); (d) simplified 4-connected node; (e) 8-connected node; (f) L2 ligand; (g) Zr6O4(OH)8(H2O)4(COO)8 SBU in BUT-12, highlighting idealized Zr6O4(OH)4 core; (h) the sqc-a topology network.

The permanent porosity of BUT-14 and -15 has been confirmed by N2 sorption at 77 K (Figure 2c, d). Saturated N2 uptakes of 970 and 960 cm3 (STP) g-1 are achieved, and evaluated BET surface areas are 3595 and 3590 m2 g-1, respectively (Figure 2c, d). The experimental total pore volumes are 1.50 and 1.49 cm3 g–1 for



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BUT-14 and -15, being in close to the calculated values of 1.51, and 1.50 cm3 g-1, respectively based on PLATON calculation.53 Based on the N2 adsorption data, the pore size distributions were calculated by density functional theory (DFT) method, which gave two types of pores of 9 and 18 Å for both BUT-14 and -15 (Figure 2c, d, insert), being consistent with the crystallographic structures of them.

Figure 2. (a, b) PXRD patterns of BUT-14 and -15; (c, d) N2 adsorption/desorption isotherms at 77 K of BUT-14 and -15 after treated in water, boiling water, and pH = 1 HCl aqueous solution, respectively (inset shows DFT pore size distribution evaluated by using the N2 adsorption data in each case). Chemical stability test

In order to examine the chemical stabilities of BUT-14 and -15, their samples were treated in water, concentrated HCl aqueous solution, and NaOH aqueous 10 ACS Paragon Plus Environment

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solution (pH = 10) at room temperature, as well as in boiling water. After being soaked in these solutions for 48 h, the measured PXRD patterns show retained crystallinity and unchanged structures, implying their excellent stability (Figure 2a, b). Furthermore, the N2 sorption isotherms for the samples after being soaked in water, boiling water, pH = 1 HCl aqueous solution, and pH = 10 NaOH aqueous solution for 48 h were measured and almost the same uptakes after treated in in water, boiling water, pH = 1 HCl aqueous solution were found as that of the pristine samples, which further confirms that the framework stability of the two MOFs under these conditions (Figure 2c, d). For the samples treated in pH = 10 NaOH aqueous solution, however, show decreased N2 uptakes (Figure S9, Supporting Information). It was shown that the saturated N2 uptakes of BUT-14 and -15 are reduced from 921 to 571, and 910 to 518 cm3 (STP) g-1, respectively. In addition, UV-Vis spectrum of the supernatant of BUT-14 and -15 after treated with NaOH aqueous solution were carried out, and relative strong peaks were observed in the range of 304 to 370 nm, indicating the presence of ligand in the supernatants of the two MOFs. These results demonstrate that the two MOFs start to decompose in pH = 10 NaOH aqueous solution, even part of the samples still maintains crystalline. It should be pointed out that up to date there are only limited reported MOFs showing good stability in boiling water and acid-base aqueous solutions with a wide pH range.40, 55-56 The outstanding chemical stabilities of the two MOFs can be attributed to the strong Zr–O coordination bonds.15, 40

Metal ions detection 11 ACS Paragon Plus Environment


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General fluorescent property. In terms of their high porosity, moderate pore sizes, and excellent chemical stability, BUT-14 and -15 were explored for the application in the detection of metal ions in water system based on fluorescent sensing. The solid-state fluorescent properties of BUT-14 and -15, as well as their acid ligands were firstly measured at room temperature. As shown in Figure S11 of the Supporting Information, H4L1 exhibits fluorescent emission at 399 nm upon the excitation at 330 nm, while H4L2 represents emissions at 453 and 548 nm upon the 320 nm excitation. Compared with the free H4L1 and H4L2, BUT-14 and -15 show similar emissions at 397, and 440 and 518 nm, respectively, with the same excitations for their corresponding acid ligands, which indicates that the fluorescence of the two MOFs are mainly attributed to the emission of the ligands.15,36 It should be pointed out that, compared with H4L1 and BUT-14, there are additonal peaks at 548 and 518 nm for H4L2 and BUT-15, respectively, which should be assigned to the n→π* electron transition of pyridine rings. Similar fluorescence spectra have also been observed in pyridyl group contaning polymers.57 Besides, the emissions of H4L1 and H4L2 are weaker than those of corresponding MOFs, probably due to the aggregation-induced quenching effect in solid state.58 Furthermore, the fluorescent properties of BUT-14 and -15 dispersed in different solvents of acetone, CH3CN, DMF, CH2Cl2, dioxane, CH3OH, and water were also checked (Figure S12, Supporting Information). It was found that their emissions are very strong and almost not solvent-dependent. Interestingly, BUT-14 and -15 also show intense fluorescent emissions in water. The highly water stability together with good fluorescent 12 ACS Paragon Plus Environment

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performance of the two MOFs thus prompt us to explore their fluorescent sensing properties in water. Selective Fe3+ ion detection based on fluorescence quenching. To explore the ability of sensing a trace quantity of metal ions, the grounded powder samples of BUT-14 or -15 (10 mg) were immersed in 30 mL deionized water and ultrasonicated over 30 min to form steady turbid suspension (Figure S13, Supporting Information). Then, 1 mL of the as-made MOF suspension was added to a cuvette containing 1 mL of 1 mM aqueous solutions of 14 different metal ions of Fe3+, Fe2+, Cu2+, Cd2+, Ni2+, Cr3+, Mn2+, Ca2+, K+, Hg2+, Na+, Mg2+, Al3+, and Co2+, respectively, to form the metal ion incorporated MOF-Mn+ suspensions for fluorescence studies. The fluorescent intensities of these MOF suspensions were recorded at room temperature and compared (Figure 3b, d and Figure S14 of the Supporting Information). Interestingly, it was found that the fluorescent intensities of them are greatly dependent on the identities of the metal ions. For BUT-14, Ca2+ and Na+ have negligible effect on the fluorescence, Cu2+, Fe2+, Cd2+, Ni2+, Cr3+, Mn2+, K+, Hg2+, Mg2+, Al3+, and Co2+ exhibit moderate (or observed) degree of quenching, but Fe3+ ion represents the most significant quenching effect based on the fluorescent intensity compared with itself. The quenching efficiency of BUT-14 towards Fe3+ was evaluated to be 54%. For BUT-15, Cd2+, Ni2+, Cr3+, Mn2+, Ca2+, K+, Hg2+, Na+, Mg2+, Al3+, and Co2+ show negligible effects on its fluorescence, Fe2+ and Cu2+ exhibits moderate degree of quenching, whereas Fe3+ gives the most significant quenching with the efficiency as high as 98%. Obvious color change from bright yellow to brownish red of the 13 ACS Paragon Plus Environment


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BUT-15 solid samples was also observed after the Fe3+ incorporation (Figure 3d, inset). And, under the irradiation of UV light of 365 nm, the color of BUT-15 suspension changed from bright yellow to dark. Clearly, BUT-15 can highly efficiently sense Fe3+ ions through the fluorescent quenching, which can also be easily recognized by the naked-eye. In addition, further experiments also show that the quenching efficiency of BUT-15 towards Fe3+ ions can almost reach the maximum value within 1 min (Figure S16, Supporting Information). This rapid response is quite appealing and may be attribute to the fact that Fe3+ ions can rapidly diffuse into the channels of BUT-15 and interact with luminophores on the pore surface.

Figure 3. Fluorescent spectra of (a) BUT-14 and (c) BUT-15 in water in the presence of different concentrations of Fe(NO3)3 under excitation at 330 and 320 nm, respectively (inset: SV plots of Fe3+ ions); Fluorescent quenching percentage of (b) 14 ACS Paragon Plus Environment

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BUT-14 and (d) BUT-15 in water by 1 mM different metal ions under excitation at 330 and 320 nm, respectively at room temperature (Inset: photographs of BUT-15 and BUT-15-Fe3+ samples in water under UV light of 365 nm, and photographs of them as solid).

It should be pointed out that usually many metal ions coexist in practical biological and environmental systems. In order to explore the detection selectivity for Fe3+ and examine the influence of other metal ions, the fluorescent quenching of mixed ions on BUT-14 and -15 in water systems were explored. Experimentally, 1 mL of MOF suspension was added to 1 mL of aqueous solution containing all other 13 metal ions rather than Fe3+ (1 mM for each), and then fluorescent intensity of the MOF suspension was measured. It was found that the fluorescences slightly changed by 3% and 2% compared with those of blank suspensions (the mixtures of 1 mL MOF suspension and 1 mL pure water), respectively. The fluorescent intensity changes were unexpectedly low, because, from above experiments, some metal ions, such as Cu2+, exhibits moderate fluorescent quenching for both suspensions of BUT-14 and -15. However, when mixed with other metal ions, these metal ions did not exhibit accumulative fluorescent quenching effect, indicating that the quenching effects for these metal ions are mutually interfered. Then, 1 mL of 1 mM Fe3+ aqueous solution was added to 1 mL above suspension to check the change of fluorescent intensity. Interestingly, the fluorescences of BUT-14 and -15 were dramatically quenched by 54% and 99%, respectively (Figure S17, Supporting Information). This result demonstrates that Fe3+ ions can be solely detected by BUT-14 or -15 through the 15 ACS Paragon Plus Environment


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fluorescent quenching process, and other metal ions have negligible interference. This sole detection for Fe3+ ions is attractive, because utilization of such sensing materials can avoid complicated pretreatment procedures to prevent the interference from other metal ions. For the reported MOF-based fluorescent sensors with functionalized Lewis basic sites used in the detection of Fe3+ ions, other metal ions such as Fe2+, Cu2+, Cr3+, and Co2+ also show good quenching efficiencies, which largely effects their detection abilities towards Fe3+ ion.25,29 In addition, the quenching effects of BUT-14 and -15 as a function of Fe3+ ions concentration in the range of 20-700 μM were also studied (Figure 3a, c). It was found that the fluorescent intensities at 399 nm for BUT-14, and 383 and 536 nm for BUT-15 gradually decrease as the concentration of Fe3+ ions increases. As is well known, the fluorescent quenching efficiency can be quantitatively explained by the Stern-Volmer (SV) equation:59 (I0/I) = 1 + Ksv[M], where I0 and I are the fluorescent intensities of MOF and MOF-Fe3+ suspensions, respectively; [M] represents the molar concentration of Fe3+ ions (μM); and Ksv is the SV constant (M-1). It was found that the SV plot of BUT-14 towards Fe3+ ions is linear at the whole concentration range, but that of BUT-15 is nearly linear at low concentration range, but subsequently deviates from linearity and bends upwards at higher concentrations (Figure 3a and c inset and Figure S15 of the Supporting Information). The Ksv values of BUT-14 and -15 for Fe3+ ions were calculated to be 2.17 × 103 and 1.66 × 104 M–1, respectively, which is comparable to that of other well-designed Fe3+ fluorescent sensors (Table S3, Supporting Information). It is worthy to note that the Ksv of BUT-15 is larger than that 16 ACS Paragon Plus Environment

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of BUT-14, which should be attributed to the effect of N donors in the ligand of BUT-15. Based on the Ksv values and the standard deviations (Sb) from three repeated fluorescent measurements of blank solutions, the detection limits (3Sb/Ksv) of BUT-14 and -15 towards Fe3+ ion in water were calculated to be 3.8 and 0.3 μM (corresponding to 212 and 16 ppb), respectively. The latter is comparable to or better than some previously reported Fe3+ fluorescent sensors (Table S3, Supporting Information). It also should be pointed out that most of reported works in the detection of Fe3+ ion by using MOFs are mainly based on lanthanide-containing luminescent MOFs. And, most of them are exploited in organic solvent systems such as DMF, CH3OH, and DMSO mainly because of poor stability of used MOFs in aqueous environment, which indeed limits their practical application to some extent.22-29 In addition, some lanthanide MOFs and MIL-53(Al) are stable in water and have shown good performance for the detection of Fe3+ ions in water system, however, their detection are based on the ion exchange between the framework metal ions coordinated with ligands and Fe3+ ions in aqueous solutions.30-34 That is to say, during the detection processes, these framework metal ions are replaced by Fe3+ ions. Such an exchange process thus requires a long time thereby reducing efficiency and applicability of the detection. More importantly, these MOF-based sensors cannot be reutilized unless further treatment though ion exchange is performed for the materials. To the best of our knowledge, there are few examples of MOF (including Tb-DSOA, Gd6(L)3(HL)2(H2O)10, Eu-BPDA, Zn5(hfipbb)4(trz)2(H2O), [Cd(L)(BPDC)]·2H2O, 17 ACS Paragon Plus Environment


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and [Cd(L)(SDBA)(H2O)]·0.5H2O) for the detection of Fe3+ ions in water which is not dependent on a cation exchange process.38,60-63 However, the Ksv of Tb-DSOA, Gd6(L)3(HL)2(H2O)10, and Eu-BPDA are 3.54 × 103, 7.98 × 102, and 1.25 × 104 M-1, respectively, smaller than that of BUT-15 (1.66 × 104 M–1). The detection limit of Zn5(hfipbb)4(trz)2(H2O), [Cd(L)(BPDC)]·2H2O, and [Cd(L)(SDBA)(H2O)]·0.5H2O are 11.2, 2.21, and 7.41 ppm, respectively, larger than that of BUT-15 (16 ppb). The Ksv value and detection limit are important indicators of the detection ability for a fluorescent sensor. A good fluorescent sensor for specific analytes owns large Ksv value and small detection limit, thus, the detection ability of BUT-15 towards Fe3+ ions is better than that of the MOFs mentioned above. All above results indicate that BUT-15 is a promising fluorescent sensing material for the selective detection of Fe3+ in water system and has great potential for practical application.

Mechanism of fluorescence quenching. The quenching on fluorescent MOFs by metal ions can mainly be arose by: (1) destroying the MOF structure by the metal ion inbreak; (2) occurring ion exchange between the framework metal centers and the targeted cations; (3) forming strong interaction between incoming metal ions and luminophores in the MOFs.32, 34, 64 Attempting to clarify the mechanism involved in our cases, additional experiments were performed. The PXRD measurements were carried out to study the structures of the original and metal ion incorporated MOF samples of BUT-14-Mn+ and BUT-15-Mn+. As shown in Figure S18 of the Supporting Information, the PXRD patterns of the BUT-14-Mn+ or BUT-15-Mn+ are similar to that of BUT-14 or -15, respectively, ruling out the collapse of the MOF frameworks. 18 ACS Paragon Plus Environment

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To verify whether the fluorescent quenching of the two MOFs is caused by ion exchange, metal elemental analysis of BUT-14-Fe3+ and BUT-15-Fe3+ as well as their parent MOFs was measured by ICP. The results (Table S4, Supporting Information) show that Zr contents in the related materials BUT-14 (18.32%) and BUT-14-Fe3+ (18.13%), and BUT-15 (18.01%) and BUT-15-Fe3+ (17.85%) are close in each case, before and after Fe3+ incorporation, indicating no ion exchange between Fe3+ and Zr4+ during the fluorescent quenching process. Thus, the strong interaction between Fe3+ ions and the luminophores in the two MOFs is proposed as the main reason of the observed fluorescent quenching. Generally, the fluorescences of Zr-MOFs are mainly attributed to the emissions of the ligands, thus, the interaction between Fe3+ ions and the luminophores in the two MOFs is refer to the interaction between Fe3+ ions and the ligands.15, 36, 40

Figure. 4 Reproducibility of the quenching ability of BUT-15 dispersed in water in the presence of 1 mM aqueous solution of Fe3+ ions (The blue bars represent the initial fluorescence intensity and the rose red bars represent the intensity upon addition an aqueous solution of 1 mL 1 mM Fe3+ ions). 19 ACS Paragon Plus Environment


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The interaction between the luminophores in MOF hosts and the analytes is commonly related to two processes: the fluorescence resonance energy transfer (FRET) and photoinduced electron transfer (PET).15,

59, 65, 66

FRET is a distance

dependent interaction between the electronic excited state of a fluorophore and another fluorophore in which the excitation is transferred from a donor molecule to an acceptor molecule without emission of photons. FRET occurs only when the emission spectrum of a fluorophore overlaps with the absorption spectrum of the acceptor. As shown in Figure S19 of the Supporting Information, the absorption band of Fe3+ has the greatest degree of overlapping with the emission spectra of BUT-14 and -15, followed by those of Co2+, Ni2+ and Cr3+. The spectral overlapping is negligible for other ions. It was found that the spectral overlapping is in good agreement with the maximum quenching efficiency observed for Fe3+, but the order of observed quenching efficiency is not fully in accordance with the spectral overlapping of Co2+, Ni2+ and Cr3+. In addition, the area of the spectral overlap area of the two MOFs with Fe3+ is similar to each other, however, the quenching efficiency of BUT-14 toward Fe3+ is far smaller than that of BUT-15. These results indicate that the FRET is not the only mechanism for the fluorescence quenching observed in these systems.

Then, the PET is proposed to be another mechanism herein. PET is a deactivation process involving an internal redox reaction between the excited state of the fluorophore and another species which are able to donate or to accept electrons. Due to the conjugated nature of the ligands in BUT-14 and -15, there exist delocalized electrons on it. When absorb energy form light, these delocalized electrons transferred 20 ACS Paragon Plus Environment

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from ground state to excited state to become excited electrons. However, because of the instability of the excited electrons, they fall back to the ground state of the ligands. During this process, the absorbed energy will release in the form of light, thus yielding fluorescence. When Fe3+ ions were added into this system, the excited electrons transferred to the lowest unoccupied molecular orbital (LUMO) of Fe3+ ions, and then fall back to the ground state of the ligands through a non-radiative path to dissipate the absorbed energy, resulting the fluorescence quenching. A similar mechanism has also been proposed in other reported compounds.25, 65, 67 For BUT-15, except for the delocalized electrons on the ligands, the N atoms in pyridine groups can additionally donates their lone-pair electrons to the Fe3+ ions, thus further enhance the detection ability of BUT-15 towards Fe3+ ions. In addition to the two process mentioned above, the collisional encounter between the luminophores of the two MOFs and the electron-deficient Fe3+ ions is considered as the other mechanism for the observed fluorescent quenching. This dynamic quenching process can be confirmed by the changes of the fluorescent lifetimes, if the fluorescent lifetime of metal ion incorporated MOF samples became shorter than that of the original MOF samples, then, we can say that the dynamic quenching process occurs.25, 60 It was found that the fluorescent lifetime of BUT-14 is shortened from 6.62 to 2.08 ns and that of BUT-15 is from 13.51 to 10.65 ns after the Fe3+ ions incorporation (Figure S20-24, Supporting Information), thus verifying the existence of the dynamic quenching process. There remains a question as to why the strong luminescence of BUT-15-Fe3+ almost completely quenched when the BUT-15 suspension was added 21 ACS Paragon Plus Environment


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to 1 mL 1 mM Fe3+ solution. To elucidate this problem, we carried out the diffuse-reflectance UV-visible spectra study of BUT-15 and BUT-15-Fe3+. It was shown that the intense absorption of the Fe3+ incorporated sample at the excitation wavelength is likely responsible for the observed fluorescent quenching (Figure S25, Supporting Information). The strong absorption at excitation wavelength can originate from the ligand to metal electron transfer of excited electrons of L2 to Fe3+ ions, which then fall back to the ground state of the ligand through a non-radiative path to dissipate the absorbed energy, resulting the significant fluorescence quenching.

Figure. 5 Fluorescent spectra of BUT-15 in simulated physiological buffer aqueous solution in the presence of different concentrations of Fe(NO3)3 under excitation at 320 nm at room temperature.

Cyclic use test. As a sensor, regeneration is an important issue. Therefore, we also investigated the fluorescent properties of regenerated BUT-15, which was simply obtained by washing with acetone (see Experimental Section). It was found that the 22 ACS Paragon Plus Environment

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quenching efficiencies of BUT-15 for Fe3+ ions are basically unchanged up to 6 cycles, demonstrating its good recyclability and stability for the detection applications (Figure 4). Application to the detection of Fe3+ ions in biological system. Encouraged by the observed high performance of BUT-15 in the selective detection of Fe3+ ions in water, its application in a biological system (20 mM HEPES and pH = 7) for the Fe3+ detection was explored (Figure 5).68 It was found that a very low concentration of Fe3+ ions could result in a dramatically decrease of the fluorescent intensity, and it was almost completely quenched when the concentration of Fe3+ increased to 1.6 mM. The PXRD pattern shows that the BUT-15 remains intact after the detection in this simulated biological system (Figure S18, Supporting Information). Compared with the detection in water, it was found that the fluorescent emission of BUT-15 at 532 nm is totally missing in the HEPES buffer solution. This can be explained as that the main

component

in

HEPES

aqueous

2-[4-(2-Hydroxyethyl)-1-piperazinyl]ethanesulfonic

acid,

buffer which

solution

is

contains

–OH

groups. The –OH groups could form hydrogen bonds with the pyridine N donors, which might prevent the contribution of pyridine entities to the fluorescence (at 532 nm) of BUT-15. Anyway, these results demonstrate that BUT-15 can be regarded as a promising fluorescent MOF sensor for the detection of Fe3+ ions, even in a biological system.

Conclusions 23 ACS Paragon Plus Environment


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Two new highly stable fluorescent Zr(IV)-based MOFs, BUT-14 and -15 have been designed, synthesized, and used in the detection of Fe3+ ion in water system. The used ligands have similar structures, with the only difference being that one is functionalized by pyridine N donors. Even with these changes in the ligands, the framework structures of them are isostructural. The two MOFs have moderate pore sizes and represent excellent selective detection ability towards Fe3+ over other metal ions in water system in terms of distinct fluorescent quenching. The detection limits of them for Fe3+ ion are estimated to be 212 and 16 ppb, respectively. The photoinduced electron transfer process between the Fe3+ ions and the ligands of the two MOFs is believed to be the main mechanism of the fluorescent quenching. The introduced pyridine N donors in the ligands of BUT-15 can thus additionally donate their lone-pair electrons to the Fe3+ ions, leading to greatly enhanced detection ability of it. It has also been demonstrated that BUT-15 can be easily recycled and reused with stable fluorescent quenching efficiency. In addition, BUT-15 exhibits good performance for the detection of Fe3+ ions in a simulated biological system. The two MOFs are therefore potentially useful in monitoring/sensing Fe3+ ions in water, as well as in biological systems.

Experimental

General characterizations: All general reagents and solvents (AR grade) were commercially available and used as received. FT-IR data were recorded on an 24 ACS Paragon Plus Environment

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IRAffinity-1 instrument. The powder X-ray diffraction patterns (PXRDs) were recorded on a Rigaku Smartlab3 X-ray Powder Diffractometer equipped with a Cu sealed tube (λ = 1.54178 Å) at room temperature. Simulation of the PXRD patterns was carried out by the single-crystal data and diffraction-crystal module of the Mercury

program

available

free

of

charge

via

internet

at

http://www.ccdc.cam.ac.uk/mercury/. TGA data were obtained on a TGA-50 (SHIMADZU) thermogravimetric analyzer with a heating rate of 5 °C min-1 under air atmosphere. Elemental microanalyses (EA) were performed by Vario Macro cube Elementar. ICP were performed by a PerkinElmer Optima 8000 Optical Emission Spectrometer. Gas adsorption isotherms were measured by the volumetric method using a Micromeritics ASAP2020 surface area and pore analyzer.

The

photoluminescence (PL) spectra were recorded in an F-4600 fluorescence spectrophotometer. The luminescence lifetimes were measured in an FLS980 fluorescence spectrophotometer. The scanning electron microscope (SEM) images were recorded in HITACHI SU3050. UV-vis spectra were obtained with a UV-2600 spectrophotometer in the range of 250-800 nm at room temperature.

Synthesis: The detailed descriptions of the synthesis of the acid ligands, 5',5'''-bis(4-carboxyphenyl)-[1,1':3',1'':4'',1''':3''',1''''-quinque-phenyl]-4,4''''-dicarboxyli c

acid

(H4L1)

and

4,4',4'',4'''-(4,4'-(1,4-phenylene)bis(pyridine-6,4,2-triyl))-tetrabenzoic acid (H4L2) are provided in section 1 of the Supporting Information.



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[Zr6O4(OH)8(H2O)4(L1)2]·S (BUT-14·S) (S represents non-assignable solvent molecules): ZrCl4 (48 mg, 0.20 mmol), H4L1 (43 mg, 0.06 mmol), and acetic acid (8 mL) were ultrasonically dissolved in 10 mL of DMF in a 20 mL Pyrex vial and sealed. The vial was then heated at 120 oC for 48 h in an oven. After cooling to room temperature, the resulting colorless crystals were harvested by filtration and washed with DMF and acetone, and then dried in air (yield 28 mg). Elemental analysis (EA) for BUT-14, C44H24N2O16Zr3 (1108.33): Calcd. C 47.64, H 2.17. Found C 46.92, H 2.05. FT-IR (KBr pellets, cm-1): 3390 (m), 1583(m), 1539(m), 1414(s), 1185(w), 1110(w), 1030(s), 849(w), 781(m), and 644(m). For PXRD pattern of as-synthesized material, see Figure 2a. For FT-IR and TGA spectra, see Figure S1 and S2 of the Supporting Information, respectively. [Zr6O4(OH)8(H2O)4(L2)2]·S (BUT-15·S): ZrCl4 (48 mg, 0.20 mmol), H4L2 (42 mg, 0.06 mmol), and acetic acid (8 mL) were ultrasonically dissolved in 10 mL of DMF in a 20 mL Pyrex vial and sealed. The vial was then heated at 120 oC for 48 h in an oven. After cooling to room temperature, the resulting yellow crystals were harvested by filtration and washed with DMF and acetone, and then dried in air (yield 32 mg). Elemental analysis (EA) for BUT-15, C44H24N2O16Zr3 (1110.31): Calcd. C 47.55, H 2.16, N 2.52. Found C 45.95, H 2.81, N 2.21. FT-IR (KBr pellets, cm–1): 3366(m), 1664(m), 1552(s), 1508(s), 1378(s), 980(w), 830(w), 788(w), and 744(m). For PXRD pattern of as-synthesized material, see Figure 2b. For FT-IR and TGA spectra, see Figure S1 and S2 of the Supporting Information, respectively.


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Single-crystal X-ray diffraction: The crystal data of BUT-14 and -15 were collected on a Rigaku Supernova CCD diffractometer equipped with a graphite-monochromatic enhanced Cu Kα radiation (λ = 1.54184 Å) at 100 K. The datasets were corrected by empirical absorption correction using spherical harmonics, implemented in the SCALE3 ABSPACk69 scaling algorithm. The structure of the two MOFs were solved by direct methods and refined by full-matrix least-squares on F2 with anisotropic displacement using the SHELXTL70 software package. Non-hydrogen atoms were refined with anisotropic displacement parameters during the final cycles. Hydrogen atoms of the ligands were calculated in ideal positions with isotropic displacement parameters. Those in OH groups and coordinated H2O molecules of the Zr(IV)-based clusters were not added, but were calculated into molecular formula of the crystal data. There are large solvent accessible pore volumes in the crystals of the two MOFs, which are occupied by highly disordered solvent molecules. No satisfactory disorder model for these solvent molecules could be assigned, and therefore the SQUEEZE program implemented in PLATON53 was used to remove the electron densities of these disordered species. Thus, all of electron densities from free solvent molecules have been “squeezed” out. The details of structural refinement can be found in Tables S1, and S2 of the Supporting Information, and cif files.

Activation of the as-made MOF samples: As-synthesized sample of BUT-14 or -15 was firstly soaked in fresh DMF for 24 h and then the extract was discarded. Fresh acetone was subsequently added, and the sample was allowed to stay in it for 8 h. This procedure was repeated three times over one day. After decanting the acetone 27 ACS Paragon Plus Environment


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extract, the sample was dried under a dynamic vacuum (< 10-3 Torr) at room temperature for 3 h. Before adsorption measurement, the sample was further activated using the “outgas” function of the adsorption analyzer for 10 h at 100 oC.

Fluorescence measurements: The grounded powder sample of BUT-14 (10 mg) or -15 (10 mg) was immersed in 30 mL of deionized water and ultrasonicated for 30 min to form stable turbid suspension, then 1 mL of it was added to a cuvette containing 1 mL of 1 mM aqueous solution of metal ion Fe3+, Cu2+, Cd2+, Ni2+, Cr3+, Mn2+, Ca2+, K+, Hg2+, Na+, Mg2+, Al3+, or Co2+, respectively, to form the metal ion incorporated MOF-Mn+ suspension for the fluorescence studies. The quenching efficiency of the BUT-14 or -15 fluorescence as a function of Fe3+ ion concentration was evaluated via measuring the fluorescence of BUT-14-Fe3+ or BUT-15-Fe3+ obtained after adding 1 mL of MOF suspension in 1 mL of Fe3+ aqueous solutions with different concentrations (20~700 μM). Time-dependent fluorescence was measured for BUT-15-Fe3+ obtained through immersing the BUT-15 samples in 1 mM Fe3+ aqueous solution for different times: 1 min, 5 min, and 10 min, respectively.

For the interference measurements from other ions, 1 mL of MOF suspension was added into 1 mL of aqueous solution containing 1 mM of 13 metal ions to form the BUT-14-(Mn+)13 or BUT-15-(Mn+)13 system, the fluorescence of which were measured. Then, 1 mL of such solution was added a cuvette containing 1 mL of 1 mM aqueous solution of Fe3+ to form BUT-14-(Mn+)13-Fe3+ or BUT-15-(Mn+)13-Fe3+, respectively, and their fluorescence was measured.


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For the measurement of BUT-15 in biological system, the grounded powder of BUT-15 was immersed in simulated physiological buffer aqueous solution (20 mM HEPES and pH = 7) and ultrasonicated to form a steady turbid suspension. Then, 1 mL of MOF suspension was added to a cuvette containing 1 mL simulated physiological aqueous with different concentrations of Fe3+ ions to form the BUT-5-Fe3+ suspension for fluorescent studies at room temperature.

PXRD measurements: 10 mg finely grounded MOF powders were immersed in 1 mL solution containing 1 mM metal ions for 12 h, then, the MOF powders were centrifuged and used to measure the PXRD patterns of them.

ICP measurements: The finely grounded powder samples of BUT-14 or -15 were introduced into the Fe3+ ions solution for 12 h to obtain the BUT-14-Fe3+ or BUT-15-Fe3+ sample, and the Fe3+ incorporated samples were then filtered and washed with fresh acetone several times to ensured that the physically absorbed Fe3+ ions on the surface of the samples have been washed off. The samples were then dried thoroughly in an oven at 100 oC, and were used for the ICP measurement.

Cyclic tests: The used sample was centrifuged and washed several times with acetone through soaking under stirring at room temperature. After centrifugation, the wet sample was dried to remove the residual solvents. The regenerated sample was used again for the detection of Fe3+ ions.

ASSOCIATED CONTENT



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Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. Synthesis of H4L1 and H4L2, the details of structure refinement, general characterization, additional structural Figures, and metal ions detection (PDF).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was financially supported from the Natural Science Foundation of China (21601008 and 21576006), China Postdoctoral Science Foundation (2015M580027), and the Program for New Century Excellent Talents in University of China (NCET-13-0647).


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