A Zn(ii) metal–organic framework based on bimetallic paddle wheels as a luminescence indicator for carcinogenic organic pollutants: phthalate esters

Based on the multifunctional ligand 3-(1H-1,2,4-triazol-1-yl)isophthalic acid (H2TIA), a three-dimensional coordination polymer, namely {[Zn(TIA)]·DMA}n (Zn-1) was synthesized solvothermally. Single-crystal X-ray diffraction analyses confirmed that Zn-1 is a 3D framework composed of binuclear Zn2 paddle wheels with one-dimensional channels long the a direction. Further topological analyses revealed that MOF Zn-1 existed as a (3,6)-connected rtl binodal net {4·62}2{42·610·83}. Furthermore, the luminescence explorations indicate that complex Zn-1 is the first MOF for luminescent probing of phthalate esters (carcinogenic organic pollutants) with a high quenching-efficiency constant and low fluorescence-detection limit.


Introduction
Owning to their appealing architectures and potential application value in catalysis, 1 gas storage/separation, 2 luminescence, 3 and so on, 4 metal-organic frameworks (MOFs), especially, those with large cavities or channels have attracted tremendous attention. In particular, a great deal of research has focused on exploring their applications in luminescence sensing, which in turn can be used to detect environmental pollutants selectively and sensitively. Distinguished from conventional carbon-based materials and porous zeolites, the ordered structure, the aperture and shape of the pore, along with the functionalities of MOFs are controllable. 5 For example, the structure of the MOFs can be devised and regulated through reasonably selecting and adjusting of organic linkers and metal salts or clusters. Even so, the initiators characteristics and properties would heavily take impact on the nal structures of coordination polymer. Besides, chemical reaction conditions, such as solvent, pH value, temperature, counter ions and so on, would also create impress inuence on their nal structure. 6 Comparatively speaking, the rational selection of an appropriate organic ligand with a certain spatial conguration and active sites is considered to be the most effective strategy for constructing MOFs with the anticipated topological structures and desirable functions.
Due to the tunable channel size, large surface area and functional sites, MOFs always display amazing interaction properties between the coordination polymers and the analytes, and that makes MOFs the very sensible sensors of much more excellent properties than other luminescent materials. 7 In particular, MOFs which have d 10 or 4f metal centers always exhibit fascinating luminescent properties, they are considered as potential chemosensors, such as selective detectors for anions, 3f,8 heavy metal cations, 9 vapours, 10 toxic organic molecules, 3d,9d,9e,11 and biomolecules. 3e,12 While, an unmet challenge is to design MOFs for quick recognizing persistent toxic organics, especially for phthalate esters (carcinogenic organic pollutants), which are believed having severe impacts on environmental protection and public health.
Phthalate esters (PAEs) are a class of organic chemical reagents, which were already and are being extensively used in commercial and industrial plastics additives. As one of the most produced and consumed chemicals in the world, PAEs undoubtedly exist in atmosphere, 13 water, 14 soil, 15 sediment. 16 And more seriously, their metabolites were reported already been found in human urine and serum. 17 Recent researching results have shown that PAEs also may have potential carcinogenesis effects on human and animals. 18 In addition, studies have also shown a correlation between PAE levels and precocious breast development, long-term exposure to PAE for women can lead to endometriosis and gestational duration. 19 In addition, Gao's research group had also found that PAEs may cause central nervous system depression and obvious renal injury. 20 Therefore, PAEs have posed great potential and obvious threaten to human and wildlife reproduction. Heavy efforts have already been taken to diminish and erase the pollution and ruin from PAEs. Precisely and rapid detection of PAEs, especially of trace concentration and amount, was believed the rst and key step to success.
An effective method was processed for the controllable synthesis of MOFs as phthalate esters sensor, in which, aromatic organic molecules with non-bonded functional ligand sites were selected as the precursors. The aromatic ring of the precursors has uorescent properties, and the nonbonded functional sites will interact with d 10 or 4f metal centers which will enhance uorescence properties. Lately, 1,2,4-triazole and its derivatives with several N-donor atoms, as well as aromatic polyacids with several O-donor atoms are widely used to construct novel MOFs. Corresponding, a great number of coordination polymers with novel architectures, higher stabilities and peculiar functions have been reported successively. 11a,21 In this context, a multi-functional organic linker 3-(1H-1,2,4triazol-1-yl)isophthalic acid (H 2 TIA), which contains long p-p conjugate chain was selected as the MOFs' precursor, for it combines the advantages of the two kinds of functional groups (triazole groups and aromatic carboxylic acid groups). Besides, d 10 metal centers such as Zn 2+ , Ag + , and Cd 2+ , were usually adopted in synthesizing of luminescent MOFs, because these cations can adjust the emission wavelength and intensity of organic linkers. A new MOF {[Zn(TIA)]$DMA} n (Zn-1) with onedimensional channels was successfully obtained from H 2 TIA and Zn(NO 3 ) 2 , which showed as a 3D framework with rtl topology. Fluorescence recognition experiments demonstrate that micro-porous framework of Zn-1 has high efficiency in detecting phthalate esters under ambient conditions through "turn-off" luminescent detection (Scheme 1).

Materials and physical measurement
Most of the starting materials and solvents were commercially available and used without further treatment, except that the solvent dimethyl sulfoxide (DMSO) was treated by 4A molecular sieve and distilled under vacuum. 3-(1H-1,2,4-triazol-1-yl) isophthalic acid (H 2 TIA) was synthesized according to the literature methods. 22 Elemental analyses for C, H, and N were taken on a PerkinElmer elemental analyzer. Infrared spectra were determined on a Bruker TENOR 27 spectrometer in the region of 4000-400 cm À1 . Powder X-ray diffraction patterns (PXRD) were carried out on a D/Max-2500 X-ray diffractometer with Cu-Ka radiation (l ¼ 1.5406 A) at room temperature. UVvis spectra were collected on a UV-2600 spectrophotometer. Thermogravimetric test (TG) was measured on a NETZSCH TG 209 instrument under N 2 atmosphere with the heating rate controlled at 10 C min À1 . Emission/excitation spectra were recorded on F97pro uorescence spectrometer. Gas adsorption isotherms were performed by a volumetric method on a Micromeritics ASAP 2020HD88 surface area and pore analyzer.

Preparation of {[Zn(TIA)]$DMA} n (Zn-1)
H 2 TIA (17.5 mg, 0.075 mmol), Zn(NO 3 ) 2 $4H 2 O (30 mg, 0.1 mmol) and 6 mL CH 3 OH-DMA (2 : 4, in v/v) were added in turn into a 15 mL Teon-lined stainless steel vessel, and then the mixture was heated at 80 C for three days. Aer that, the autoclave was cooled to room temperature at a rate of 1.2 C h À1 . Colorless rectangular crystals Zn-1 (20.54 mg) were obtained (yield 71% based on H 2 TIA). Elemental analysis for
binuclear coordinating units are further connected by carboxylate groups and triazole N atoms of TIA 2À ligands to form a 3D framework (Fig. 1b), in which one-dimensional channels exist along a direction. In Zn-1, the triazolyl ring, the two carboxylate groups in the same TIA 2À ligand are not completely coplanar with the central phenyl ring.
In the 3D framework, if each Zn 2 binuclear unit is treated as a six-connected node and each TIA 2À is regarded as a threeconnected node, Zn-1 can be regarded as a (3,6)-connected rtl binodal net, with the short (Schläi) vertex symbol of {4$6 2 } 2 {4 2 $6 10 $8 3 }, which is shown in Fig. 1c. By using the program of PLATON, the ratio of the accessible porous volume (utilizable) in every unit cell is calculated as 53.5%.

PXRD, thermal stability and sorption properties
As shown in Fig. S2 (ESI †), powder X-ray diffraction patterns for compound Zn-1 were carried out to conrm the phase purity of crystalline materials. The main peak positions of the experiment data coincide with the corresponding simulated ones, suggesting the good phase purity of the bulk crystals Zn-1.
For the purpose of detecting the thermal stability of the framework in Zn-1, thermogravimetric analyses (TGA) was performed at the heating rated of 10 C min À1 under a N 2 atmosphere in the temperature range of 30-800 C. The TGA curve (Fig. S4 †) of Zn-1 displays as a two-step weight loss, the rst step weight loss of 22.93% before 220 C (calcd 22.68%) contributed by the loss of free DMA molecule in the lattice. The second step weight loss attributes to the collapse of the 3D framework at 366 C, which demonstrates the framework is relatively stable.
The permanent porosity of MOF Zn-1 was further detected by the N 2 sorption isotherm, which was carried out at 77 K. As displayed in Fig. S5, † it was a type-I isotherm with a BET surface area of 31.46 m 2 g À1 (Langmuir surface area 47.83 m 2 g À1 ).

Photoluminescent properties of Zn-1 and H 2 TIA in solid state
Organic compounds which contain wide p-p conjugated systems and aromatic rings, along with their relevant metalorganic coordination polymers, have aroused extensive interest of chemists owning to their distinctive luminescent properties and latent applications in uorescent devices, such as lightemitting diodes (LEDs). The architecture of novel coordination polymers by reasonable selection of conjugated organic Fig. 1 (a) The coordination environment of Zn II ions in compound Zn-1. Hydrogen atoms were omitted for clarity. Symmetry operator: A ¼ 1 À x, À0.5 + y, 1.5 À z; B ¼ x, y, 1 + z; C ¼ 1 + x, 1.5 À y, 0.5 + z; D ¼ 2 À x, 1 À y, 1 À z; E ¼ 2 À x, 1 À y, 2 À z. (b) The 3D framework of Zn-1 viewed along a direction. (c) Topology network of the framework Zn-1. connectors and transition metal centers (such as Zn 2+ , Cd 2+ , and Ag + ) may be one of the most efficacious methods to obtain new kinds of luminescent materials, because of the capability to adjust the emission wavelength and intensity of organic linkers. Herein, the photo-luminescent properties of the aromatic H 2 TIA and its relevant complex Zn-1 in the solid state have been investigated.
At ambient temperature, the organic linker H 2 TIA exhibits a broad emission band with the maximum at 350 nm (l ex ¼ 290 nm), which is assigned as p / p* transitions caused by the large conjugated aromatic triazole and phenyl rings (Fig. 2). 23 On comparison, Zn-1 shows an intense uorescence emission band centered at 366 nm (l ex ¼ 290 nm), which is contributed by the intraligand emission. Compared with organic ligand H 2 TIA, the enhancement of uorescence intensity is attributed to that the rigidity of the ligand signicantly increases by metalligand coordination, and the non-radiative decay between intraligand transition states decreases accordingly. For coordination polymers, the emission band based on organic ligand is preferable, since the band gaps of host material can be altered along with exchanged guest molecules diffusing into the channels based on host-guest interaction, thus resulting in different uorescent response on the basis of the change of luminescent intensity or position. 24 Consequently, MOF Zn-1 may be used as a uorescent sensor owning to its structural particularities which contain 1D channels decorated by a number of open coordinating locations. In addition, MOF Zn-1 is insoluble in general organic solvents, therefore its selective sensing ability was investigated for small molecular organic solvents as well as the rmness of the framework and the durability of the micropore.
For the purpose of further investigating the sensing abilities of polymer Zn-1 to PAEs series, the selective sensing tests were conducted by using different amount of PAEs in DMF solution ( Fig. 4-6). Suspensions of Zn-1 in DMF solution were prepared by the same method as above mentioned except by gradually increasing the amount of phthalate esters. It is obvious that the uorescence intensity of DEPAE@Zn-1 weakened dramatically with the increase of diethyl phthalate ester (DEPAE) content and became a half at a low diethyl phthalate ester content of 0.04 vol% (2 mM), and almost completely disappeared at a concentration of 0.4 vol% (20 mM). The quenching percentage of Zn-1 toward diethyl phthalate ester was evaluated to be 99.1%, thus MOF Zn-1 can be seen as a good candidate for highly selectively sensing the DEPAE.
The Stern-Volmer plots of relative luminescent intensity (I 0 / I) versus the concentration of DEPAE were used to quantify the quenching efficiency, which are exhibited in Fig. 4b. The quenching efficiency can be tted through the equation: I 0 /I ¼ 1 + K SV [DEPAE]. I 0 and I are the luminescence intensity of Zn-1 before and aer addition of the analytes DEPAE, [DEPAE] denotes the molar concentration of DEPAE, and K SV is the quenching rate constant. As shown in Fig. 4b, I 0 /I versus DEPAE concentration plots were almost linear under the condition of low concentration, the quenching constant K SV can be determined as 4.67 Â 10 2 M À1 (insets of Fig. 4b). Consequently, it can be concluded that DEPAE can be effectively recognized by Zn-1 in a very low concentration range.
In order to probe the uorescent quenching characteristics of other common PAE series such as DBPAE, DOPAE, a series of experiments were conducted as follows: the samples of Zn-1 (2 mg) was ground and immersed in 5 mL DMF solution and ultrasonicated 20 min to form stable suspensions, and then DBPAE and DOPAE were separately and gradually added to the standard suspensions. The curve of intensity versus the concentration of DBPAE and DOPAE are shown in Fig. 5 and 6, respectively. For DBPAE, when the concentration was 2.1 mM, the uorescence intensity has almost halved, with the quenching efficiency of 43%. At the concentration of 10.5 mM, the luminescence quenching efficiency was as high as 98% (Fig. 5). DOPAE displayed similar property with a quenching efficiency of 47% (at 1.87 mM) and 96% (at 26 mM) (Fig. 6).  What's more, the quenching mechanism of coordination polymer to PAEs was further studied, the UV-absorption spectra of Zn-1 and PAEs in ethanol solution were determined in the range of 200-450 nm. As depicted in Fig. S7, † the excitation peak of Zn-1 overlapped with the absorption band of PAE's, which indicated the competition of absorption of the light source energy between ligand H 2 TIA and PAEs. The energy absorbed by ligand H 2 TIA was transferred to PAEs, which resulted in emission quenching of Zn-1 by means of an excitation energy transfer mechanism.
To measure the quality of a chemosensor, in addition to taking into account the stability, selectivity and sensitivity, the reusability should also be considered as an important factor. Therefore, the tests for the recyclability of Zn-1 to PAEs (DEPAE, DBPAE, DOPAE) were carried out (Fig. 7). The luminescence intensities of Zn-1 could almost restore to their initial level aer @PAE-Zn-1 being washed with DMF solvent for a few times. In addition, powder X-ray diffraction experiments further conrmed that the framework of MOF Zn-1 remained unchanged aer uorescence recycling experiments (Fig. S3 †). Special stability of the framework and recyclability of MOF Zn-1 indicates the potential applicability as a uorescent sensor for PAEs organic pollutants.
According to the reported results, 8c,9e,25 these multifunctional ligands which contain long p-p conjugated systems have a wide application prospect in uorescent probing eld. Such as the 2D layer complexes {[Cd 2 L 2 (H 2 O) 4 ]$H 2 O} n and {[Zn 2 L 2 (H 2 O) 4 ]$ H 2 O} n and 3D polymer [Cd 3 L 3 (DMF) 2 ] n all exhibited highly selective and sensitive chemosensors for Cr VI -anions in aqueous medium. In our previous work, we found MOF [Cd 0.5 (TBC)] n as a bifunctional luminescence sensor for benzaldehyde and Fe 2+ ion. In the near future, we will continue to explore the application of these multifunctional ligands in uorescent probing.

Conclusion
In conclusion, one porous Zn-MOF (Zn-1) was successfully synthesized by using a multifunctional organic linker H 2 TIA as the building block. Zn-1 exists as a 3,6-connected rtl 3D framework with 1D channels, which was composed by binuclear Zn 2 paddle wheels. In addition, the well-dened correlation between the luminescence intensity of Zn-1 and the concentration of phthalate esters provides a reliable relationship that makes MOF Zn-1 as a low-cost and user-friendly luminescence sensor to determine the concentration of different target species. As far as I know, MOF Zn-1 is the rst specialized and efficient luminescence sensor for phthalate esters (carcinogenic organic pollutants). The present study highlights the practical applications of luminescent MOFs as sensors and provides a novel Zn-MOF-based sensor for persistent organic pollutants, which is of great importance for living environments and human health.

Conflicts of interest
There are no conicts to declare.