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A 3D supramolecular network as highly selective and sensitive luminescent sensor for PO43− and Cu2+ ions in aqueous media
Dyes and Pigments 150 (2018) 36–43
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A 3D supramolecular network as highly selective and sensitive luminescent sensor for PO43− and Cu2+ ions in aqueous media
MARK
Yuan Xiea,1, Shigang Ningb,1, Yong Zhangb, Zilong Tangb, Shaowei Zhangb,c,∗, Ruiren Tanga,∗∗ a
School of Chemistry and Chemical Engineering, Central South University, Hunan, 410083, China Key Laboratory of Theoretical Organic Chemistry and Functional Molecule of the Ministry of Education, Hunan Provincial Key Laboratory of Controllable Preparation and Functional Application of Fine Polymers, School of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan, Hunan, 411201, China c Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai University, Tianjin, 300071, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Coordination polymers Crystal structure Luminescent Hydrothermal method Benzotriazole-5-carboxylic acid
A 2D coordination polymer, {[Zn(btca)(py)2]}n (1), (H2btca = benzotriazole-5-carboxylic acid, py = pyridine), has been prepared with high yield by the hydrothermal reaction of H2btca and Zn(NO3)2·6H2O in the presence of py, which has been further characterized by kinds of methods combining elemental analysis, thermogravimetric (TG) analysis, infrared (IR) spectrum, powder X-ray diffraction (PXRD), and X-ray single-crystal diffraction. In 1, each Zn2+ adopts a five-coordinated pentagonal bipyramid geometry with one O atom from one carboxylate group of btca2− ligand and four N atoms from two btca2− ligands and two py molecules. Adjacent Zn2+ are connected by btca2− ligands to generate a 2D layer with the ‘fes’-type topology, which could further form a 3D supramolecular network through hydrogen bonds interactions. Furthermore, the luminescent results reveal that the detection limit of 1 as a luminescent sensor to probe Cu2+ and PO43− could reach 3 and 45 ppm, respectively, suggesting that 1 can sensitively and selectively detect Cu2+ and PO43− in aqueous solution.
1. Introduction The detection and sensing of ions in aqueous solution is of significant importance for environmental science, disease diagnosis and biological process monitoring [1–4]. Compared with various of detection techniques for ions such as electrochemical sensing, inductively coupled plasma atomic emission spectrometry, inductively coupled plasma atomic spectrometry and so on, the luminescent-based sensors have drawn increasing attention because of their simplicity, cost-effectiveness, high sensitivity and fast response [1–4]. Among anions and cations, Cu2+ is one of the most essential ions in biological systems, particularly in the brain, the appropriate concentration of Cu2+ plays an important role in the enzyme activity since the redox-active nature. But, if the concentration is excess in the body, it would bring about gastrointestinal disturbance and even damage to the liver and kidneys such as copper metabolism disordered Wilson's disease [3]. While PO43− as an integral part of nucleotides, plays a key role in energy storage and signal transmission in biological systems, as well as environmental systems [4]. Numerous of luminescent-based sensors for
sensing ions have been designed and prepared in the past decade [1–4], only a few sensors can simultaneously detect two different ions [5], however, to our best knowledge, one sensor simultaneously target Cu2+ and PO43− has not been reported. Moreover, quite a number of sensors are not suitable for detecting Cu2+ or PO43− in a quantitative fashion in aqueous systems, which may be mainly attributed to the poor water stability and selectivity of the sensors. Therefore, it is necessary to exploit highly selective and sensitive luminescent sensors for the straightforward and efficient detection of Cu2+ and PO43− in aqueous systems. Coordination polymers (CPs), as one type of important function materials, which are usually constructed by metal ions/clusters and organic ligands through coordination interactions, have been drawing fascinated interest because of their interesting architectures and potential applications in chemical recognition, magnetism, catalysis, gas storage and separation [6–9]. Metal ions with closed d-shells like d10 (such as Zn2+ and Cd2+) usually produce broad emissions as well as large stokes shifts. On the other hand, the organic ligands with π-acceptor including aromatic rings are suitable for developing luminescent
∗ Corresponding author. Key Laboratory of Theoretical Organic Chemistry and Functional Molecule of the Ministry of Education, Hunan Provincial Key Laboratory of Controllable Preparation and Functional Application of Fine Polymers, School of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan, Hunan, 411201, China. ∗∗ Corresponding author. E-mail addresses: [email protected] (S. Zhang), [email protected] (R. Tang). 1 These authors contribute equally.
https://doi.org/10.1016/j.dyepig.2017.11.008 Received 20 September 2017; Received in revised form 1 November 2017; Accepted 4 November 2017 Available online 05 November 2017 0143-7208/ © 2017 Elsevier Ltd. All rights reserved.
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materials. As a result, metal ions with d10 configuration and organic ligands with π-conjugated systems are often selected to construct luminescent materials [10]. In this contribution, the benzotriazole-5-carboxylic acid (H2btca) ligand was chose as a multi-dentate ligand with π-acceptor to assemble with Zn2+ for the following considerations [11–13]: (i) H2btca can be partially or completely deprotonated to produce Hbtca− and btca2− anions at different pH values, which could form various acid-base-dependent coordination modes and consequently result in abundant structures. Meanwhile, the deprotonated Hbtca− and btca2− can serve as hydrogen-bond donors and/or acceptors, which are in favour of the generation of supramolecular structures. (ii) The special orientation between triazole and carboxyl groups may obtain intriguing porous CPs with exceptional discrete fragments as secondary building units (SBUs), and the multiple coordination sites supply a high likelihood for the construction of multi-dimensional structures. Moreover, it possesses both N and O donors, which can simultaneously link different metal ions to prepare heterometallic CPs according to the theory of hard and soft acids and bases (HSAB) [14]. (iii) The strong π-conjugate system from benzotriazole ring can enhance the stability of the structures. Especially, the d10 configuration Zn2+ and the conjugated π systems are beneficial to develop luminescence materials. Based on the above reasons, we reported a 2D coordination polymer, {[Zn(btca)(py)2]}n (1), (H2btca = benzotriazole-5-carboxylic acid, py = pyridine), which was synthesized with high yield by assembling H2btca and Zn(NO3)2·6H2O in the presence of py under the hydrothermal condition. The as-synthesized compound has been further characterized by many methods combining elemental analysis, thermogravimetric (TG) analysis, infrared (IR) spectrum, powder X-ray diffraction (PXRD), and X-ray single-crystal diffraction. Each Zn2+ in 1 exhibits a five-coordinated pentagonal bipyramid geometry with one O atom from one carboxylate group of btca2− ligand and four N atoms from two btca2− ligands and two py groups. Neighbouring Zn2+ are bridged by btca2− ligands to generate a 2D layer with the ‘fes’-type topology, which could further generate a 3D supramolecular network through hydrogen bonds interactions. Furthermore, the luminescent property of 1 has been studied in detail, and the results suggest that 1 can sensitively and selectively detect Cu2+ and PO43− in aqueous solution.
C17H13N5O2Zn: C 53.07, H 3.41, N 18.20. Found: C 52.98, H 3.52, N 18.12. IR (KBr pellets): 3440 (br), 1627 (s), 1597 (s), 1573 (m), 1486 (m), 1444 (m), 1366 (s), 1277 (m), 1216 (m), 1159 (s), 1070 (w), 1036 (w), 1006 (w), 957 (w), 842 (w), 789 (s), 760 (s), 705 (m), 698 (m), 623 (m), 598 (m) cm−1. Crystal data for 1: C17H13N5O2Zn, Mr = 384.69 g mol−1, monoclinic space group P21/n, a = 9.4366(1) Å, b = 10.2859(1) Å, c = 17.2936(9) Å, β = 93.2219(4)°, V = 1675.93(3) Å3, T = 296(2) K, Z = 4, Dc = 1.525 g⋅m−3, μ = 1.486 mm−1, F(000) = 784, Rint = 0.0256, 2946 reflections, 2621 with I > 2σ(I) for 271 parameters, GOF = 1.035, R1 = 0.0247, wR2 = 0.0569 [I > 2σ(I)] and R1 = 0.0296, wR2 = 0.0598 (all data). 2.3. X-ray crystallography Determination of the unit cell and data collection for 1 was performed on a Bruker Apex-II CCD diffractometer with monochromatic Mo Kα radiation (λ = 0.71073 Å) at 296(2) K. The converted data was integrated and reduced using SAINT with absorption and scaling correction being undertaken with the program SADABS. The structure was solved by the direct method and all non-hydrogen atoms were refined by full-matrix least-square procedure on F2 using the program SHELXL2014 in conjunction with the Olex2 graphical user interface [15]. Anisotropic thermal parameters were assigned to all non-hydrogen atoms. H atoms attached to C atoms are placed in geometrically idealized positions and refined using a riding model, with C-H = 0.93 (aromatic), and all H atoms are assigned Uiso(H) = 1.2Ueq(C) of the parent atoms. One of the py molecules is disordered over two positions [the occupancies refined to 0.50 and 0.50 for C13A/C14A/C15A/C16A/C17A and C13B/C14B/C15B/C16B/C17B, respectively] and the disordered atoms are restrained to have equivalent atomic displacement parameters. The ISOR instruction is used to resolve the ADP errors of these C atoms of disordered py molecules. The distances between adjacent C-N and C-C in disordered py molecules are bundled in the normal ranges by DFIX and DANG instructions. The FLAT instruction is applied to ensure these C and N atoms of disordered py molecules in the same plane. In addition, reflections 216, 043, −122, 146, −402, 145, 241 and −263 are omitted from the refinements due to they having the essentially zero measured intensities and are presumably eclipsed by the beam stop during data collection. CCDC no. 1572560 for 1. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
2. Experimental section 2.1. General methods and materials All reagents were directly purchased and used without further treatment. Elemental analyses (C, H and N) were conducted on a Perkin-Elmer 2400-II CHNS/O analyzer. IR spectra were obtained from a solid sample palletized with KBr pellets on a Nicolet 6700 spectrometer in 4000−400 cm−1. TG analyses were obtained on a Labsys NETZSCH TG 209 Setaram apparatus from 25 to 800 °C with the rate of 10 °C·min−1 under a N2 atmosphere. PXRD spectra were measured on a Bruker D8 Advance instrument with Cu Kα radiation (λ = 1.54056 Å), in the range 2θ = 3–60° with the scan speed of 10° min−1. The luminescent experiments were carried out on a F4500 fluorescence spectrophotometer.
3. Results and discussion 3.1. Structural descriptions Single-crystal X-ray analyses reveal that 1 crystallizes in the monoclinic system with the space group P21/n. The asymmetric skeleton of {[Zn(btca)(py)2]}n (1) consists of one Zn2+ ion, one btca2− ligand, and two coordinated py molecules (Fig. 1a). In 1, each Zn2+ ion displays a five-coordinated pentagonal bipyramid geometry defined by one O atom from one carboxylate group of the btca2− ligand [Zn-O: 1.948(1) Å] and four N atoms from two other btca2− ligands and two py groups [Zn-N: 2.026(2) - 2.304(2) Å], while every btca2− ligand acting as a tridentate ligand connects three Zn2+ ions through one carboxylic O atom and 1,3-site N atoms. Adjacent Zn2+ ions are connected by btca2− ligands to produce a 2D layer (Fig. 1b). It is worth noting that one carboxylic O atom and the 2-site N atom of btca2− ligands don't take part in coordination with Zn2+ ion, which can serve as hydrogen-bond donors and/or acceptors to assist the formation of supramolecular structures. Indeed, the 2D layers are further linked to generate a 3D supramolecular network through intra- and intermolecular C-H⋯O and C-H⋯N hydrogen bonds interactions (Fig. 1c, and the details of hydrogen bonds are provided in Table 1). To better understand the 2D structure of 1, the topological structure
2.2. Synthesis of {[Zn(btca)(py)2]}n (1) A mixture of Zn(NO3)2·6H2O (0.0296 g, 0.10 mmol), H2btca (0.0041 g, 0.025 mmol), py (1.0 mL), and H2O (5.0 mL) was sealed in a 20 mL screw capped vial and placed in an oven at 100 °C for 24 h, and then the mixture was cooled to room temperature. Colorless block crystals were collected by filtering, washed with H2O and dried. The complex is very stable in air at ambient temperature and is almost insoluble in most common solvents such as H2O, CH3OH, CH3CH2OH, CH3CN, CH2Cl2, CHCl3, DMF, DMA, DMSO, THF, acetone, benzene, and n-hexane. Yield: ca. 88% (based on H2btca). Anal. Calcd. (%) for 37
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Fig. 1. a) Ball and stick representation of the asymmetric unit of 1; b) The 2D layer of 1; c) The 3D supramolecular network of 1 built by intra- and intermolecular hydrogen bonds; d) A schematic view of 3-connected net for ‘fes’-type topology in 1.
Table 1 Distances (Å) and angles (°) of hydrogen bonds for 1. D-H
d(D-H)
d(H⋯A)
< DHA
d(D⋯A)
A
C3-H3 C6-H6
0.930 0.930
2.428 2.620
136.75 147.03
3.170 3.438
C8-H8 C11-H11
0.930 0.930
2.507 2.652
139.63 121.05
3.271 3.232
C17B-H17B
0.930
2.634
112.28
3.109
O1 [-x+1, -y+1, -z+1] N2 [-x+1/2, y+1/2, -z +1/2] O2 O2 [-x+1/2, y-1/2, -z+3/ 2] O1
Fig. 2. PXRD patterns of simulated from the X-ray single structure of 1, as-synthesized 1, and 1 samples exposed in air for one week and soaked in water for three days.
Fig. 3. The luminescent spectra of 1 immersed in aqueous solutions containing different metal ions (a); and anions (b).
38
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Fig. 4. The comparisons of the luminescent intensities of 1 immersed in aqueous solutions containing different metal ions (a); and anions (b).
Fig. 5. The concentrations-dependent luminescence spectra of 1 in aqueous solutions: (a) Cu2+ and (b) PO43−.
of 1 could be analyzed by the freely available program TOPOS [16]. Taking each Zn2+ ion as a node and each btca2− ligand as a linker, the 2D layer of 1 can be depicted as a 3-connected network with the Schläfli symbol of {4·82}, which corresponds to the ‘fes’-type topology (Fig. 1d).
ligand coordinates to metal ions. 3.4. TG analysis The thermal stability of 1 was performed on the crystalline sample under N2 atmosphere in the range of 25–800 °C (Fig. S2). The TG curve suggests that the skeleton of 1 can be stable at least 110 °C. As the temperature is raised, the weight of 1 begins to reduce and exhibits a unique weight loss with 41.16% (calcd. 41.07%) between 110 and 300 °C, which can be ascribed to the decomposition of two coordinated py molecules.
3.2. PXRD analyses The high phase purity of bulky sample 1 was confirmed by comparing the PXRD patterns between the simulated one from the singlecrystal data and the experimental result (Fig. 2). The different intensity may be resulted from the variation in preferred orientation of one single crystal and bulky powder. Importantly, when exposing 1 in the open air for one week or immersing 1 in H2O for three days, the PXRD patterns are in accordance with the simulated pattern (Fig. 2), which indicates that the skeleton of 1 can be steady for one week in the open air and three days in H2O.
3.5. Luminescent properties CPs constructed by d10 configuration ions (such as Zn2+ and Cd2+) and π-conjugated systems organic ligands are suitable for the exploitation of luminescent materials [10]. In the case of 1, the btca2− ligand with π-conjugated systems binds with Zn2+ that may be excellent candidates for luminescent materials. Therefore, the solid state luminescence spectrum of 1 was measured at room temperature on powdered sample, which displays two obvious emission peaks at ca. 396 and 415 nm upon excitation at 350 nm (Fig. S3). Whereas the free H2btca ligand shows two similar emission peaks at ca. 350 and 358 nm (λex = 300 nm), which can be attributed to intra-ligand emission states are derived from π → π* transitions [13]. The results indicate that these emission bands of 1 are neither metal-to-ligand charge transfer (MLCT) nor ligand-to-metal charge transfer (LMCT) due to Zn2+ is hard to
3.3. IR spectra The IR spectrum of 1 displays strong absorption bands in the ranges of 1630−1485 and 1445−1365 cm−1 (Fig. S1), which correspond to the asymmetric and symmetric stretching vibrations of C=O and C=N, respectively [12]. No strong absorption bands are observed in the region 1720−1680 cm−1, indicating that the carboxyl groups are completely deprotonated, which is in agreement with the crystal data. In comparison with the spectrum of free H2btca ligand, these bands shift to lower wavenumbers and no bands are observed in the region 1720−1680 cm−1, which can be attributed to the fact that the H2btca 39
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Fig. 6. The concentrations-dependent luminescence spectra of 1 vs. Cu2+ or PO43− in different natural water samples: A: (a for Cu2+, b for PO43−), B: (c for Cu2+, d for PO43−), C: (e for Cu2+, f for PO43−), respectively.
reduce or to oxidize because of their d10 configuration but rather may be mainly assigned to an intra-ligand emission states are derived from π → π* transitions of the H2btca ligand, which is usually observed in previous reports [10,13]. In addition, compared with the free H2btca ligand, the apparent red shift of emission peaks can be observed, which probably resulting from the coordination interactions between btca2− ligands and Zn2+ ions. The excellent luminescent property and water stability of 1 prompt us to exploit its possible application for detecting ions in aqueous systems. The sample of 3 mg of 1 immersed in 1.8 mL aqueous solution to
form a suspension using ultrasound methods, then 0.2 mL aqueous solutions (10−2 mol L−1) containing different cations (Ag+, Al3+, Ca2+, Cd2+, Co2+, Cr3+, Cu2+, Fe2+, Fe3+, Hg2+, K+, Mg2+, Mn2+, Na+, Ni2+, Pb2+, Sr2+, and Zn2+) were added into the above solutions to obtain 2.0 mL 10−3 mol L−1 suspensions containing different cations, respectively. Before the luminescent experiments, all suspensions containing different cations were dispersed uniformly using ultrasound methods for 30 min. Likewise, 2.0 mL 10−3 mol L−1 suspensions containing different anions (Ac−, BF4−, Br−, Cl−, ClO4−, CO32−, Cr2O72−, CrO42−, F−, H2PO4−, HCO3−, HPO42−, I−, IO3−, MoO42−, NO2−, 40
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Fig. 7. The concentrations-dependent luminescence spectra of 1 in aqueous solutions: (a) Fe2+, (b) Co2+, (c) Ni2+, and (d) Ag+.
NO3−, PO43−, S2−, SCN−, SO32−, SO42−, and WO42−) were obtained. The experimental results reveal that different ions have different impacts on the luminescent intensities (Figs. 3 and 4), in other words, CP 1 can be a luminescent sensor for sensitively detecting Cu2+ through luminescent quenching effect, whereas for PO43− through luminescent enhancement effect. In order to evaluate the detection limits of 1 as a luminescent sensor to probe Cu2+ and PO43−, a families of suspensions containing different concentrations of Cu2+ or PO43− were prepared by gradually adding 10−2 mol L−1 aqueous solutions including Cu2+ or PO43− into the suspensions of 1. For Cu2+, the luminescent intensities of 1 gradually decrease with the concentration of Cu2+ ranging from 0 to 36 ppm (Fig. 5a). The decrease of luminescent intensities is still apparently observed even if the concentration of Cu2+ is 3 ppm, suggesting that the detection limit of 1 as a luminescent sensor to probe Cu2+ could reach 3 ppm. In the case of PO43−, the luminescent intensities of 1 gradually increase with increasing the concentration of PO43− from 0 to 190 ppm (Fig. 5b). The increase of luminescent intensities is still apparently observed when the concentration of PO43− is 45 ppm, indicating that the detection limit of 1 as a luminescent sensor to probe PO43− could reach 45 ppm. Moreover, the quenching effect and the concentration of analytes can be quantitatively evaluated using the Stern-Volmer equation [17], (I0/I) = KSV[M] + 1, where I0 and I respectively represent the luminescent intensities before and after adding the analyte, KSV represents the Stern-Volmer constant of the analyte (M−1), and [M] is the molar concentration of the analyte. As depicted in Fig. S4, the KSV values are 2.92 × 104 L mol−1 for Cu2+ and −3.52 × 102 L mol−1 for PO43−, respectively, which are among the
highest values of CPs-based sensors for detecting Cu2+ and PO43− [3,4]. The low detection limits and the high Stern-Volmer constants for Cu2+ and PO43−, indicating that 1 can serve as a highly selective and sensitive luminescent sensor for the detection of Cu2+ and PO43−, especially for Cu2+ in the aqueous system. The luminescent response reason induced by Cu2+ and PO43− might be mainly explained by the donor-acceptor electron transfer effect from ligands to ions [3b]. According to the structure description of 1, one carboxylic O atom and the 2-site N atom of btca2− ligands don't coordinate with Zn2+, which can serve as hydrogen-bond donors and/ or acceptors to construct supramolecular structures. In the case of Cu2+, the weak interactions between Cu2+ and the uncoordinated carboxylic O atom and the 2-site N atom may partly quench the singlet and triplet excited states of the btca2− ligand-to-Zn2+ optically active core. Besides, Zn2+ in 1 might be partly substituted by Cu2+ in the aqueous system, which also results in the luminescent quenching. While PO43− together with the uncoordinated O and N atoms might form hydrogen bonds, thereby enhance the π-conjugated systems. It should be noted that the deep understanding of such quenching and enhancing effects is still unclear, series of work need to be done for exploring the possible detection mechanism and will be discussed in the following work. To demonstrate the potential application in a natural water sample, three natural water samples (named A, B, and C, respectively) were collected from different rivers and pretreated using a syringe filter to remove particulate impurities. Different concentrations of Cu2+ or PO43− were added into the above three water samples containing 3 mg of sample 1, respectively. The results revealed that the luminescent 41
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intensities of 1 gradually decrease with the increase concentration of Cu2+ (Fig. 6a, c, e), whereas the luminescent intensities of 1 gradually increase with increasing the concentration of PO43− (Fig. 6b, d, f), suggesting that 1 can serve as a highly selective and sensitive luminescent sensor for the detection of Cu2+ and PO43− in natural water system. In addition, the luminescent intensities change over the concentrations of several other ions such as Fe2+, Co2+, Ni2+ and Ag+ have also been examined (Fig. 7). The KSV values are 2.25 × 104 L mol−1 for Fe2+, 3.76 × 103 L mol−1 for Co2+, 3.66 × 103 L mol−1 for Ni2+, and 9.6 × 103 L mol−1 for Ag+, respectively.
[4]
4. Conclusions A 2D coordination polymer constructed by H2btca and Zn (NO3)2·6H2O in the presence of py has been prepared with high yield under hydrothermal conditions. Each five-coordinated Zn2+ ion in 1 exhibits a pentagonal bipyramid geometry with one O atom from one carboxylate group of btca2− ligand and four N atoms from two btca2− ligands and two py groups. Adjacent Zn2+ ions are bridged by btca2− ligands to form a 2D sheet with the ‘fes’-type topology, which could further generate a 3D supramolecular architecture through intra- and intermolecular hydrogen bonds interactions. Importantly, the luminescent results demonstrate that 1 could high sensitively and selectively probe Cu2+ and PO43− in aqueous solution.
[5]
[6]
Acknowledgments [7]
This work was supported by the National Natural Science Foundation of China (21601058), the Scientific Research Fund of Hunan Provincial Education Department (16C0628) and Hunan University of Science and Technology (E51677 and E21631). Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx. doi.org/10.1016/j.dyepig.2017.11.008. [8]
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A 3D supramolecular network as highly selective and sensitive luminescent sensor for PO43− and Cu2+ ions in aqueous media
MARK
Yuan Xiea,1, Shigang Ningb,1, Yong Zhangb, Zilong Tangb, Shaowei Zhangb,c,∗, Ruiren Tanga,∗∗ a
School of Chemistry and Chemical Engineering, Central South University, Hunan, 410083, China Key Laboratory of Theoretical Organic Chemistry and Functional Molecule of the Ministry of Education, Hunan Provincial Key Laboratory of Controllable Preparation and Functional Application of Fine Polymers, School of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan, Hunan, 411201, China c Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai University, Tianjin, 300071, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Coordination polymers Crystal structure Luminescent Hydrothermal method Benzotriazole-5-carboxylic acid
A 2D coordination polymer, {[Zn(btca)(py)2]}n (1), (H2btca = benzotriazole-5-carboxylic acid, py = pyridine), has been prepared with high yield by the hydrothermal reaction of H2btca and Zn(NO3)2·6H2O in the presence of py, which has been further characterized by kinds of methods combining elemental analysis, thermogravimetric (TG) analysis, infrared (IR) spectrum, powder X-ray diffraction (PXRD), and X-ray single-crystal diffraction. In 1, each Zn2+ adopts a five-coordinated pentagonal bipyramid geometry with one O atom from one carboxylate group of btca2− ligand and four N atoms from two btca2− ligands and two py molecules. Adjacent Zn2+ are connected by btca2− ligands to generate a 2D layer with the ‘fes’-type topology, which could further form a 3D supramolecular network through hydrogen bonds interactions. Furthermore, the luminescent results reveal that the detection limit of 1 as a luminescent sensor to probe Cu2+ and PO43− could reach 3 and 45 ppm, respectively, suggesting that 1 can sensitively and selectively detect Cu2+ and PO43− in aqueous solution.
1. Introduction The detection and sensing of ions in aqueous solution is of significant importance for environmental science, disease diagnosis and biological process monitoring [1–4]. Compared with various of detection techniques for ions such as electrochemical sensing, inductively coupled plasma atomic emission spectrometry, inductively coupled plasma atomic spectrometry and so on, the luminescent-based sensors have drawn increasing attention because of their simplicity, cost-effectiveness, high sensitivity and fast response [1–4]. Among anions and cations, Cu2+ is one of the most essential ions in biological systems, particularly in the brain, the appropriate concentration of Cu2+ plays an important role in the enzyme activity since the redox-active nature. But, if the concentration is excess in the body, it would bring about gastrointestinal disturbance and even damage to the liver and kidneys such as copper metabolism disordered Wilson's disease [3]. While PO43− as an integral part of nucleotides, plays a key role in energy storage and signal transmission in biological systems, as well as environmental systems [4]. Numerous of luminescent-based sensors for
sensing ions have been designed and prepared in the past decade [1–4], only a few sensors can simultaneously detect two different ions [5], however, to our best knowledge, one sensor simultaneously target Cu2+ and PO43− has not been reported. Moreover, quite a number of sensors are not suitable for detecting Cu2+ or PO43− in a quantitative fashion in aqueous systems, which may be mainly attributed to the poor water stability and selectivity of the sensors. Therefore, it is necessary to exploit highly selective and sensitive luminescent sensors for the straightforward and efficient detection of Cu2+ and PO43− in aqueous systems. Coordination polymers (CPs), as one type of important function materials, which are usually constructed by metal ions/clusters and organic ligands through coordination interactions, have been drawing fascinated interest because of their interesting architectures and potential applications in chemical recognition, magnetism, catalysis, gas storage and separation [6–9]. Metal ions with closed d-shells like d10 (such as Zn2+ and Cd2+) usually produce broad emissions as well as large stokes shifts. On the other hand, the organic ligands with π-acceptor including aromatic rings are suitable for developing luminescent
∗ Corresponding author. Key Laboratory of Theoretical Organic Chemistry and Functional Molecule of the Ministry of Education, Hunan Provincial Key Laboratory of Controllable Preparation and Functional Application of Fine Polymers, School of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan, Hunan, 411201, China. ∗∗ Corresponding author. E-mail addresses: [email protected] (S. Zhang), [email protected] (R. Tang). 1 These authors contribute equally.
https://doi.org/10.1016/j.dyepig.2017.11.008 Received 20 September 2017; Received in revised form 1 November 2017; Accepted 4 November 2017 Available online 05 November 2017 0143-7208/ © 2017 Elsevier Ltd. All rights reserved.
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materials. As a result, metal ions with d10 configuration and organic ligands with π-conjugated systems are often selected to construct luminescent materials [10]. In this contribution, the benzotriazole-5-carboxylic acid (H2btca) ligand was chose as a multi-dentate ligand with π-acceptor to assemble with Zn2+ for the following considerations [11–13]: (i) H2btca can be partially or completely deprotonated to produce Hbtca− and btca2− anions at different pH values, which could form various acid-base-dependent coordination modes and consequently result in abundant structures. Meanwhile, the deprotonated Hbtca− and btca2− can serve as hydrogen-bond donors and/or acceptors, which are in favour of the generation of supramolecular structures. (ii) The special orientation between triazole and carboxyl groups may obtain intriguing porous CPs with exceptional discrete fragments as secondary building units (SBUs), and the multiple coordination sites supply a high likelihood for the construction of multi-dimensional structures. Moreover, it possesses both N and O donors, which can simultaneously link different metal ions to prepare heterometallic CPs according to the theory of hard and soft acids and bases (HSAB) [14]. (iii) The strong π-conjugate system from benzotriazole ring can enhance the stability of the structures. Especially, the d10 configuration Zn2+ and the conjugated π systems are beneficial to develop luminescence materials. Based on the above reasons, we reported a 2D coordination polymer, {[Zn(btca)(py)2]}n (1), (H2btca = benzotriazole-5-carboxylic acid, py = pyridine), which was synthesized with high yield by assembling H2btca and Zn(NO3)2·6H2O in the presence of py under the hydrothermal condition. The as-synthesized compound has been further characterized by many methods combining elemental analysis, thermogravimetric (TG) analysis, infrared (IR) spectrum, powder X-ray diffraction (PXRD), and X-ray single-crystal diffraction. Each Zn2+ in 1 exhibits a five-coordinated pentagonal bipyramid geometry with one O atom from one carboxylate group of btca2− ligand and four N atoms from two btca2− ligands and two py groups. Neighbouring Zn2+ are bridged by btca2− ligands to generate a 2D layer with the ‘fes’-type topology, which could further generate a 3D supramolecular network through hydrogen bonds interactions. Furthermore, the luminescent property of 1 has been studied in detail, and the results suggest that 1 can sensitively and selectively detect Cu2+ and PO43− in aqueous solution.
C17H13N5O2Zn: C 53.07, H 3.41, N 18.20. Found: C 52.98, H 3.52, N 18.12. IR (KBr pellets): 3440 (br), 1627 (s), 1597 (s), 1573 (m), 1486 (m), 1444 (m), 1366 (s), 1277 (m), 1216 (m), 1159 (s), 1070 (w), 1036 (w), 1006 (w), 957 (w), 842 (w), 789 (s), 760 (s), 705 (m), 698 (m), 623 (m), 598 (m) cm−1. Crystal data for 1: C17H13N5O2Zn, Mr = 384.69 g mol−1, monoclinic space group P21/n, a = 9.4366(1) Å, b = 10.2859(1) Å, c = 17.2936(9) Å, β = 93.2219(4)°, V = 1675.93(3) Å3, T = 296(2) K, Z = 4, Dc = 1.525 g⋅m−3, μ = 1.486 mm−1, F(000) = 784, Rint = 0.0256, 2946 reflections, 2621 with I > 2σ(I) for 271 parameters, GOF = 1.035, R1 = 0.0247, wR2 = 0.0569 [I > 2σ(I)] and R1 = 0.0296, wR2 = 0.0598 (all data). 2.3. X-ray crystallography Determination of the unit cell and data collection for 1 was performed on a Bruker Apex-II CCD diffractometer with monochromatic Mo Kα radiation (λ = 0.71073 Å) at 296(2) K. The converted data was integrated and reduced using SAINT with absorption and scaling correction being undertaken with the program SADABS. The structure was solved by the direct method and all non-hydrogen atoms were refined by full-matrix least-square procedure on F2 using the program SHELXL2014 in conjunction with the Olex2 graphical user interface [15]. Anisotropic thermal parameters were assigned to all non-hydrogen atoms. H atoms attached to C atoms are placed in geometrically idealized positions and refined using a riding model, with C-H = 0.93 (aromatic), and all H atoms are assigned Uiso(H) = 1.2Ueq(C) of the parent atoms. One of the py molecules is disordered over two positions [the occupancies refined to 0.50 and 0.50 for C13A/C14A/C15A/C16A/C17A and C13B/C14B/C15B/C16B/C17B, respectively] and the disordered atoms are restrained to have equivalent atomic displacement parameters. The ISOR instruction is used to resolve the ADP errors of these C atoms of disordered py molecules. The distances between adjacent C-N and C-C in disordered py molecules are bundled in the normal ranges by DFIX and DANG instructions. The FLAT instruction is applied to ensure these C and N atoms of disordered py molecules in the same plane. In addition, reflections 216, 043, −122, 146, −402, 145, 241 and −263 are omitted from the refinements due to they having the essentially zero measured intensities and are presumably eclipsed by the beam stop during data collection. CCDC no. 1572560 for 1. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
2. Experimental section 2.1. General methods and materials All reagents were directly purchased and used without further treatment. Elemental analyses (C, H and N) were conducted on a Perkin-Elmer 2400-II CHNS/O analyzer. IR spectra were obtained from a solid sample palletized with KBr pellets on a Nicolet 6700 spectrometer in 4000−400 cm−1. TG analyses were obtained on a Labsys NETZSCH TG 209 Setaram apparatus from 25 to 800 °C with the rate of 10 °C·min−1 under a N2 atmosphere. PXRD spectra were measured on a Bruker D8 Advance instrument with Cu Kα radiation (λ = 1.54056 Å), in the range 2θ = 3–60° with the scan speed of 10° min−1. The luminescent experiments were carried out on a F4500 fluorescence spectrophotometer.
3. Results and discussion 3.1. Structural descriptions Single-crystal X-ray analyses reveal that 1 crystallizes in the monoclinic system with the space group P21/n. The asymmetric skeleton of {[Zn(btca)(py)2]}n (1) consists of one Zn2+ ion, one btca2− ligand, and two coordinated py molecules (Fig. 1a). In 1, each Zn2+ ion displays a five-coordinated pentagonal bipyramid geometry defined by one O atom from one carboxylate group of the btca2− ligand [Zn-O: 1.948(1) Å] and four N atoms from two other btca2− ligands and two py groups [Zn-N: 2.026(2) - 2.304(2) Å], while every btca2− ligand acting as a tridentate ligand connects three Zn2+ ions through one carboxylic O atom and 1,3-site N atoms. Adjacent Zn2+ ions are connected by btca2− ligands to produce a 2D layer (Fig. 1b). It is worth noting that one carboxylic O atom and the 2-site N atom of btca2− ligands don't take part in coordination with Zn2+ ion, which can serve as hydrogen-bond donors and/or acceptors to assist the formation of supramolecular structures. Indeed, the 2D layers are further linked to generate a 3D supramolecular network through intra- and intermolecular C-H⋯O and C-H⋯N hydrogen bonds interactions (Fig. 1c, and the details of hydrogen bonds are provided in Table 1). To better understand the 2D structure of 1, the topological structure
2.2. Synthesis of {[Zn(btca)(py)2]}n (1) A mixture of Zn(NO3)2·6H2O (0.0296 g, 0.10 mmol), H2btca (0.0041 g, 0.025 mmol), py (1.0 mL), and H2O (5.0 mL) was sealed in a 20 mL screw capped vial and placed in an oven at 100 °C for 24 h, and then the mixture was cooled to room temperature. Colorless block crystals were collected by filtering, washed with H2O and dried. The complex is very stable in air at ambient temperature and is almost insoluble in most common solvents such as H2O, CH3OH, CH3CH2OH, CH3CN, CH2Cl2, CHCl3, DMF, DMA, DMSO, THF, acetone, benzene, and n-hexane. Yield: ca. 88% (based on H2btca). Anal. Calcd. (%) for 37
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Fig. 1. a) Ball and stick representation of the asymmetric unit of 1; b) The 2D layer of 1; c) The 3D supramolecular network of 1 built by intra- and intermolecular hydrogen bonds; d) A schematic view of 3-connected net for ‘fes’-type topology in 1.
Table 1 Distances (Å) and angles (°) of hydrogen bonds for 1. D-H
d(D-H)
d(H⋯A)
< DHA
d(D⋯A)
A
C3-H3 C6-H6
0.930 0.930
2.428 2.620
136.75 147.03
3.170 3.438
C8-H8 C11-H11
0.930 0.930
2.507 2.652
139.63 121.05
3.271 3.232
C17B-H17B
0.930
2.634
112.28
3.109
O1 [-x+1, -y+1, -z+1] N2 [-x+1/2, y+1/2, -z +1/2] O2 O2 [-x+1/2, y-1/2, -z+3/ 2] O1
Fig. 2. PXRD patterns of simulated from the X-ray single structure of 1, as-synthesized 1, and 1 samples exposed in air for one week and soaked in water for three days.
Fig. 3. The luminescent spectra of 1 immersed in aqueous solutions containing different metal ions (a); and anions (b).
38
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Fig. 4. The comparisons of the luminescent intensities of 1 immersed in aqueous solutions containing different metal ions (a); and anions (b).
Fig. 5. The concentrations-dependent luminescence spectra of 1 in aqueous solutions: (a) Cu2+ and (b) PO43−.
of 1 could be analyzed by the freely available program TOPOS [16]. Taking each Zn2+ ion as a node and each btca2− ligand as a linker, the 2D layer of 1 can be depicted as a 3-connected network with the Schläfli symbol of {4·82}, which corresponds to the ‘fes’-type topology (Fig. 1d).
ligand coordinates to metal ions. 3.4. TG analysis The thermal stability of 1 was performed on the crystalline sample under N2 atmosphere in the range of 25–800 °C (Fig. S2). The TG curve suggests that the skeleton of 1 can be stable at least 110 °C. As the temperature is raised, the weight of 1 begins to reduce and exhibits a unique weight loss with 41.16% (calcd. 41.07%) between 110 and 300 °C, which can be ascribed to the decomposition of two coordinated py molecules.
3.2. PXRD analyses The high phase purity of bulky sample 1 was confirmed by comparing the PXRD patterns between the simulated one from the singlecrystal data and the experimental result (Fig. 2). The different intensity may be resulted from the variation in preferred orientation of one single crystal and bulky powder. Importantly, when exposing 1 in the open air for one week or immersing 1 in H2O for three days, the PXRD patterns are in accordance with the simulated pattern (Fig. 2), which indicates that the skeleton of 1 can be steady for one week in the open air and three days in H2O.
3.5. Luminescent properties CPs constructed by d10 configuration ions (such as Zn2+ and Cd2+) and π-conjugated systems organic ligands are suitable for the exploitation of luminescent materials [10]. In the case of 1, the btca2− ligand with π-conjugated systems binds with Zn2+ that may be excellent candidates for luminescent materials. Therefore, the solid state luminescence spectrum of 1 was measured at room temperature on powdered sample, which displays two obvious emission peaks at ca. 396 and 415 nm upon excitation at 350 nm (Fig. S3). Whereas the free H2btca ligand shows two similar emission peaks at ca. 350 and 358 nm (λex = 300 nm), which can be attributed to intra-ligand emission states are derived from π → π* transitions [13]. The results indicate that these emission bands of 1 are neither metal-to-ligand charge transfer (MLCT) nor ligand-to-metal charge transfer (LMCT) due to Zn2+ is hard to
3.3. IR spectra The IR spectrum of 1 displays strong absorption bands in the ranges of 1630−1485 and 1445−1365 cm−1 (Fig. S1), which correspond to the asymmetric and symmetric stretching vibrations of C=O and C=N, respectively [12]. No strong absorption bands are observed in the region 1720−1680 cm−1, indicating that the carboxyl groups are completely deprotonated, which is in agreement with the crystal data. In comparison with the spectrum of free H2btca ligand, these bands shift to lower wavenumbers and no bands are observed in the region 1720−1680 cm−1, which can be attributed to the fact that the H2btca 39
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Fig. 6. The concentrations-dependent luminescence spectra of 1 vs. Cu2+ or PO43− in different natural water samples: A: (a for Cu2+, b for PO43−), B: (c for Cu2+, d for PO43−), C: (e for Cu2+, f for PO43−), respectively.
reduce or to oxidize because of their d10 configuration but rather may be mainly assigned to an intra-ligand emission states are derived from π → π* transitions of the H2btca ligand, which is usually observed in previous reports [10,13]. In addition, compared with the free H2btca ligand, the apparent red shift of emission peaks can be observed, which probably resulting from the coordination interactions between btca2− ligands and Zn2+ ions. The excellent luminescent property and water stability of 1 prompt us to exploit its possible application for detecting ions in aqueous systems. The sample of 3 mg of 1 immersed in 1.8 mL aqueous solution to
form a suspension using ultrasound methods, then 0.2 mL aqueous solutions (10−2 mol L−1) containing different cations (Ag+, Al3+, Ca2+, Cd2+, Co2+, Cr3+, Cu2+, Fe2+, Fe3+, Hg2+, K+, Mg2+, Mn2+, Na+, Ni2+, Pb2+, Sr2+, and Zn2+) were added into the above solutions to obtain 2.0 mL 10−3 mol L−1 suspensions containing different cations, respectively. Before the luminescent experiments, all suspensions containing different cations were dispersed uniformly using ultrasound methods for 30 min. Likewise, 2.0 mL 10−3 mol L−1 suspensions containing different anions (Ac−, BF4−, Br−, Cl−, ClO4−, CO32−, Cr2O72−, CrO42−, F−, H2PO4−, HCO3−, HPO42−, I−, IO3−, MoO42−, NO2−, 40
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Fig. 7. The concentrations-dependent luminescence spectra of 1 in aqueous solutions: (a) Fe2+, (b) Co2+, (c) Ni2+, and (d) Ag+.
NO3−, PO43−, S2−, SCN−, SO32−, SO42−, and WO42−) were obtained. The experimental results reveal that different ions have different impacts on the luminescent intensities (Figs. 3 and 4), in other words, CP 1 can be a luminescent sensor for sensitively detecting Cu2+ through luminescent quenching effect, whereas for PO43− through luminescent enhancement effect. In order to evaluate the detection limits of 1 as a luminescent sensor to probe Cu2+ and PO43−, a families of suspensions containing different concentrations of Cu2+ or PO43− were prepared by gradually adding 10−2 mol L−1 aqueous solutions including Cu2+ or PO43− into the suspensions of 1. For Cu2+, the luminescent intensities of 1 gradually decrease with the concentration of Cu2+ ranging from 0 to 36 ppm (Fig. 5a). The decrease of luminescent intensities is still apparently observed even if the concentration of Cu2+ is 3 ppm, suggesting that the detection limit of 1 as a luminescent sensor to probe Cu2+ could reach 3 ppm. In the case of PO43−, the luminescent intensities of 1 gradually increase with increasing the concentration of PO43− from 0 to 190 ppm (Fig. 5b). The increase of luminescent intensities is still apparently observed when the concentration of PO43− is 45 ppm, indicating that the detection limit of 1 as a luminescent sensor to probe PO43− could reach 45 ppm. Moreover, the quenching effect and the concentration of analytes can be quantitatively evaluated using the Stern-Volmer equation [17], (I0/I) = KSV[M] + 1, where I0 and I respectively represent the luminescent intensities before and after adding the analyte, KSV represents the Stern-Volmer constant of the analyte (M−1), and [M] is the molar concentration of the analyte. As depicted in Fig. S4, the KSV values are 2.92 × 104 L mol−1 for Cu2+ and −3.52 × 102 L mol−1 for PO43−, respectively, which are among the
highest values of CPs-based sensors for detecting Cu2+ and PO43− [3,4]. The low detection limits and the high Stern-Volmer constants for Cu2+ and PO43−, indicating that 1 can serve as a highly selective and sensitive luminescent sensor for the detection of Cu2+ and PO43−, especially for Cu2+ in the aqueous system. The luminescent response reason induced by Cu2+ and PO43− might be mainly explained by the donor-acceptor electron transfer effect from ligands to ions [3b]. According to the structure description of 1, one carboxylic O atom and the 2-site N atom of btca2− ligands don't coordinate with Zn2+, which can serve as hydrogen-bond donors and/ or acceptors to construct supramolecular structures. In the case of Cu2+, the weak interactions between Cu2+ and the uncoordinated carboxylic O atom and the 2-site N atom may partly quench the singlet and triplet excited states of the btca2− ligand-to-Zn2+ optically active core. Besides, Zn2+ in 1 might be partly substituted by Cu2+ in the aqueous system, which also results in the luminescent quenching. While PO43− together with the uncoordinated O and N atoms might form hydrogen bonds, thereby enhance the π-conjugated systems. It should be noted that the deep understanding of such quenching and enhancing effects is still unclear, series of work need to be done for exploring the possible detection mechanism and will be discussed in the following work. To demonstrate the potential application in a natural water sample, three natural water samples (named A, B, and C, respectively) were collected from different rivers and pretreated using a syringe filter to remove particulate impurities. Different concentrations of Cu2+ or PO43− were added into the above three water samples containing 3 mg of sample 1, respectively. The results revealed that the luminescent 41
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intensities of 1 gradually decrease with the increase concentration of Cu2+ (Fig. 6a, c, e), whereas the luminescent intensities of 1 gradually increase with increasing the concentration of PO43− (Fig. 6b, d, f), suggesting that 1 can serve as a highly selective and sensitive luminescent sensor for the detection of Cu2+ and PO43− in natural water system. In addition, the luminescent intensities change over the concentrations of several other ions such as Fe2+, Co2+, Ni2+ and Ag+ have also been examined (Fig. 7). The KSV values are 2.25 × 104 L mol−1 for Fe2+, 3.76 × 103 L mol−1 for Co2+, 3.66 × 103 L mol−1 for Ni2+, and 9.6 × 103 L mol−1 for Ag+, respectively.
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4. Conclusions A 2D coordination polymer constructed by H2btca and Zn (NO3)2·6H2O in the presence of py has been prepared with high yield under hydrothermal conditions. Each five-coordinated Zn2+ ion in 1 exhibits a pentagonal bipyramid geometry with one O atom from one carboxylate group of btca2− ligand and four N atoms from two btca2− ligands and two py groups. Adjacent Zn2+ ions are bridged by btca2− ligands to form a 2D sheet with the ‘fes’-type topology, which could further generate a 3D supramolecular architecture through intra- and intermolecular hydrogen bonds interactions. Importantly, the luminescent results demonstrate that 1 could high sensitively and selectively probe Cu2+ and PO43− in aqueous solution.
[5]
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Acknowledgments [7]
This work was supported by the National Natural Science Foundation of China (21601058), the Scientific Research Fund of Hunan Provincial Education Department (16C0628) and Hunan University of Science and Technology (E51677 and E21631). Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx. doi.org/10.1016/j.dyepig.2017.11.008. [8]
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