用于痕量监测Zr4+、Cr2O72-、Fe3+、HPO42-和指纹识别的功能化Eu3+/Tb3+配位聚合物荧光探针的构筑

王思璐 张凤凤 张 成 王 潇 唐 龙 岳二林 王记江 侯向阳

(延安大学化学与化工学院，延安 716000)

0 Introduction

According to the national sanitary standard for drinking water, there are clear requirements for water quality[1].The presence of metal ions in water is not appropriate,which will have a serious impact[2].Chemical reagents containing Zr4+, Fe3+, Cr2O72-, and HPO42-have been widely used in industry, agriculture, medicine, and other fields, especially the increase of their content in ecological environments such as in water bodies, which has attracted wide attention[3-5].These metal ions remain in the ecosystem for a long time and can have serious detriment on the human body.For example, improper presence of metal ions can cause pathological changes in the main organs of the human body, including the hematopoietic system, respiratory system, central nervous system, digestive system,immune system, and skeletal system[6-9].Therefore, it is particularly important to identify and monitor these metal ions from water samples to obtain real and detailed information.

Coordination polymers (CPs)are an excellent platform for building multifunctional materials because they can be easily constructed by combining functional organic ligands with metal ion centers.Recently,multifunctional fluorescent sensing CP materials have been widely used in the field of energy and environmental monitoring due to their broad potential[10-12].Bai et al.have synthesized the [Bi(BTC)(H2O)]·H2O (Bi-MOF)compound constructed by 1,3,5-benzene tricarboxylic acid.Based on the luminescent properties of the Bi-MOF, it could be used as a multi-functional test chemical sensor such as Fe3+and Cr2O72-with high selectivity, sensitivity, and uniqueness[13].The CP[Cd(BTBD)2(AIC)]nwas designed and synthesized with a 4-connected sql topology, and it is a rare CPs-based switched fluorescence sensor for detecting Fe3+ions[14].So far,a large number of CP sensors have been reported,most of which are sensed by a luminescence quenching mechanism[15-16].Therefore,in addition to X-ray absorption spectrometry, electrochemical methods, ultraviolet detection, surface-enhanced Raman spectroscopy, and other methods to identify and monitor metal ions in aqueous solutions[17-19], CPs can also be studied as a fluorescence sensor for monitoring metal ions.Hence,it is of great significance to develop fluorescence sensing materials based on CPs to achieve high sensitivity and selectivity detection of Zr4+, Fe3+, Cr2O72-, and HPO42-ions.

Herein, we report two isomorphic new CPs with the formula of {[Eu(PLIA)1.5(H2O)2] ·H2O}n(1) and{[Tb(PLIA)1.5(H2O)2]·H2O}n(2),where H2PLIA=5-((pyridin-4-yl-methyl)oxy)benzene-1,3-dicarboxylic acid.Comprehensive and detailed characterizations for CPs 1 and 2 were performed by IR, powder X-ray diffraction (PXRD), thermogravimetric analysis (TG), and fluorescence techniques.The highly stable CPs 1 and 2 reveal the same structural features and ideal fluorescence properties.The fabricated fluorescence probes have been applied to the highly selective, rapid, good sensitive, strong anti-interference ability and rapid detection of Zr4+, Fe3+, Cr2O72-, and HPO42-ions in sewage.The fluorescence recognition mechanism and fingerprint recognition have been investigated in detail.

1 Experimental

1.1 Materials and methods

All available solvents and starting materials of analytical grade in the experiments were obtained from commercial sources and available without further purification.Fourier transform infrared (FTIR)spectra were recorded with an IRAFFINITY-1S spectrophotometer in a range of 400-4 000 cm-1using KBr disks.TG analyses were performed with a NETZSCH STA 449C thermal analyzer under an N2atmosphere.The PXRD patterns were collected on a Rigaku D/Max-2550VB+/PC diffractometer equipped with CuKαradiation (λ=0.154 06 nm,U=45 kV,I=40 mA, 2θ=5°-60°).Solidstate photoluminescence spectra analyses were performed with an Edinburgh Instrument F920 fluorescence spectrometer at ambient temperature.UV-Vis absorption spectra study was carried out on a Shimadzu UV-2700 spectrophotometer.The luminescent sensing measurements were performed on a Hitachi F-7100 Fluorescence spectrophotometer.

1.2 Syntheses of CPs 1 and 2

A mixture of H2PLIA (24.6 mg, 0.09 mmol) and Eu(NO3)3·6H2O (26.8 mg, 0.06 mmol) were added in 3 mL distilled water and 2 mL NaOH solution ( NaOH:8.0 mg, 0.2 mmol ).After being stirred for 20 min, the resulting mixture was transferred and sealed in a 25 mL small glass bottle, which was heated at 180 ℃for 72 h.And white, block crystals of CP 1 were obtained.Yield: 52% based on H2PLIA.Main FTIR data (KBr pellet, cm-1): 3 413(β), 3 183(w), 3 110(m), 2 603(w),1 751(s), 1 615(s), 1 307(w), 1 071(w), 821(w), 779(s),690(w),627(w),575(w)(Fig.1a).

Fig.1 (a)IR spectra of CPs 1 and 2;(b)TG curves of 1 and 2;(c)PXRD patterns of 1 and 2;(d)Solid-state fluorescence emission and excitation spectra of 1 and 2

A mixture of H2PLIA (24.6 mg, 0.09 mmol) and Tb(NO3)3·6H2O (27.2 mg, 0.06 mmol) were added in 3 mL distilled water and 2 mL NaOH solution (NaOH:8.0 mg, 0.2 mmol).After being stirred for 30 min, the resulting mixture was transferred and sealed in a 25 mL small glass bottle, which was heated at 180 ℃for 72 h.And white, block crystals of CP 2 were obtained.Yield: 67% based on H2PLIA.Main FTIR data (KBr pellet, cm-1): 3 397(β), 3 141(w), 3 095(m), 2 597(w),1 719(s), 1 599(s), 1 281(w), 1 050(w), 810(w), 758(s),669(w),554(w)(Fig.1a).

1.3 Crystal structure determination

The crystals with regular size (0.21 mm×0.16 mm×0.13 mm,CP 1;0.35 mm×0.3 mm×0.2 mm,CP 2),and shape(block)were screened by optical microscope.The diffraction point data of 1 and 2 were collected using a Bruker Smart APEX ⅡCCD diffractometer with graphite monochromatic MoKαradiation (λ=0.071 073 nm) at 296(2) K, usingφ-ωscan[15].A semiempirical absorption correction was applied using the SADABS program.The single crystal structures of 1 and 2 were solved by direct methods using the SHELXS-2014 and refined onF2by the full-matrix least-squares methods using the SHELXL-2014 program package.All nonhydrogen atoms were refined anisotropically by the full-matrix least-squares method.All hydrogen atom coordinates were obtained using theoretical hydrogenation.The main crystal data parameters of 1 and 2 are summarized in Table 1.

Table 1 Crystal and structure refinement data of CPs 1 and 2

CCDC:256713,1;2256714,2.

2 Results and discussion

2.1 Crystal structure of CPs 1 and 2

CPs 1 and 2 are isomorphic.Hence,CP 2 is taken as an example to analyze its structural features.Singlecrystal X-ray diffraction analysis revealed that 2 crystallizes in the orthorhombic crystal system with space group Pnma.As shown in Fig.2a, the asymmetric unit of 2 consists of one Tb3+ion, one half-fully-deprotonated PLIA2-, two coordination water molecules, and one crystal water molecule.Each of the Tb3+ions forms a twisted dodecahedral geometry with eight oxygen atoms, six of which come from the five PLIA2-ligands and the remaining two from the water molecule with which it is coordinated (Fig.2b).The main bond lengths and bond angles of 2 are listed in Table 2, which are within the normal range compared with other CPs reported in the literature[23].The two adjacent Tb3+ions are connected via four carboxylate groups from four PLIA2-exhibiting bidentate bridging mode (μ2-η1∶η1)and forming Tb2(COO)4as the secondary building unit(SBU) with a Tb3+…Tb3+separation distance of 0.423 25(8) nm.The neighboring dinuclear SBUs are connected via the bridged PLIA2-linkers inμ3-kO,O∶kO∶kO modes forming a 1D structure (Fig.2c and 2d).The 1D structure is interconnected through these bridged PLIA2-connectors adoptingμ3-kO,O∶kO∶kO,andμ4-kO∶kO∶kO∶kO modes to form a 2D structure(Fig.2e and 2f).Finally, these 2D structures are connected through ligands to form a 3D network structure(Fig.2g).A detailed structural investigation of 2 shows that there exists hydrogen bonding (O9—H9…N1#5:O9…N1#5 0.293 716 nm; O9—H9…O11: O9…O11 0.236 3(18) nm; O9—H9…O11#4: O9…O11#4 0.286 0(18) nm; O10—H10…O11: O10…O11 0.241 1(18)nm; O10—H10…O11#4: O10…O11#4 0.282 3(18)nm; Symmetry codes: #4:x, 0.5-y,z; #5: 1.5+x, 1-y,0.5+z) andπ…πstacking interactions (Fig.2h, the Cg(1)…Cg(2) and Cg(3)…Cg(4) distance: 0.384 9 nm and 0.380 2 nm, the dihedral angles adjacent toπplane of Cg(1)…Cg(2) and Cg(3)…Cg(4): 78.82° and 81.05°) among the benzene and pyridyl moieties ring present in the spacer PLIA2-linkers.In turn, this 3D network is stabilized by hydrogen bonding andπ…πstacking interactions.

Table 2 Selected bond lengths(nm)and angles(°)for CPs 1 and 2

Fig.2 (a)Coordination environment of Tb3+ion in CP 2 with thermal ellipsoids at 50%level;(b)Distorted dodecahedron geometry of Tb3+ion in 2;(c)Coordination mode of the PLIA2-ligand in 2;(d)1D structure of 2;2D structure of 2 viewed along the(e)c-axis and(f)a-axis,respectively;(g)3D structure of 2;(h)π…π stacking interactions in 2

2.2 Characterization of CPs 1 and 2

As shown in Fig.2b, TG analysis of CPs 1 and 2 showed that a first weight loss of 5.71% (Calcd.5.92%,CP 1) and 5.67% (Calcd.5.85%, CP 2) occurred between 80 and 260 ℃and this could be attributed to the loss of three water molecules.After that, the framework of 1 and 2 exhibited good thermal stability until 290 ℃, followed by the degradation of the framework attributed to the decomposition of the PLIA2-ligand.The residue species of 1 and 2 are Eu2O3and Tb2O3(CP 1: Obsd.27.96%, Calcd.29.73%; CP 2: Obsd.28.16%, Calcd.28.93%), respectively.In Fig.2c, the PXRD patterns of as-synthesized 1 and 2 were consistent with their simulated patterns, indicating that all the prepared materials are in a pure phase state.In addition, the PXRD patterns were obtained after exposing the suspensions of 1 and 2 in water to the aqueous solution of analytes (Fe3+, Zr4+, Cr2O72-, and HPO42-) for 24 h, which were also in good agreement with the synthesized patterns, which is verified that the crystallinity,stability and structural integrity of CPs during the sensing experiment.The solid-state emission spectra of 1 and 2 were investigated.As shown in Fig.2d, the spectra of 1 and 2 showed a strong characteristic emission band at 597 nm (5D0→7F1), 628 nm (5D0→7F2) and 494 nm (5D4→7F6), 549 nm (5D4→7F5), 590 nm(5D4→7F4), 625 nm (5D4→7F3) on being excited at 315 and 328 nm, respectively.In the emission spectra of 1 and 2, the emission band from the PLIA2-ligand was not detected, implying the efficient energy transfer from the PLIA2-ligand to the central Eu3+and Tb3+ions[20].This result suggests that the PLIA2-ligand is suitable for the sensitization of luminescence for Eu3+and Tb3+ions[20].

The above characterization results demonstrated that CPs 1 and 2 possess expected and excellent fluorescence properties crystallinity,stability,and structural integrity.We will investigate the application of 1 and 2 as fluorescent probes in detecting metal ions.

2.3 Fluorescent probe

The experimental methods and processes of relevant fluorescence sensing are based on literature reports[21-22].A series of measurements were performed to investigate the fluorescence sensing properties of CPs 1 and 2 for anions and cations in the aqueous system.The suspensions of 1 and 2 were prepared by dispersing 30 mg of 1 or 2 in 100 mL water with subsequent ultrasonic treatment for 1 h.Then,20 μL of various aqueous solution containing different cationic solutions (5 mmol·L-1M(NO3)xor MClx, Mx+=Ba2+, Ca2+,Ni2+, Nd3+, Pb2+, Cu2+, Tb3+, Hg2+, Zn2+, Dy3+, Er3+, Zr4+,Fe3+, In3+) and anionic solutions (5 mmol·L-1KxA or NaxA, Ax-=H2PO4-, CO32-, CH3COO-, PO43-, Cl-, Br-,SO42-, SO32-, C2O42-, HPO42-, Cr2O72-, HSO3-, NO2-,HCO3-,B4O72-)were added to the suspensions (2 mL)of 1 and 2 and used for measurement by a luminescence spectrometer,respectively.As shown in Fig.3A,the fluorescence intensity of the aqueous solution containing Zr4+, Fe3+, Cr2O72-, and HPO42-were almost quenched,whereas those of other ion solutions barely changed,suggesting that 1 and 2 can serve as a promising luminescence probe for sensing Zr4+, Cr2O72-and Fe3+,HPO42-, respectively.Probe selectivity is an important performance index.Anti-interference experiments were carried out in 20 μL aqueous solutions (containing Zr4+,Fe3+,Cr2O72-,and HPO42-,respectively)and 20 μL of aqueous solutions of other ions to explore the selectivity of 1 and 2 for detecting Zr4+, Cr2O72-and Fe3+,HPO42-, respectively.As shown in Fig.3B, the emission intensities of fluorescent probes 1 and 2 were almost unaffected by the existence of other ions, indicating that 1 and 2 possess an efficient selectivity.

Fig.3 (A)Fluorescence intensities of CPs 1 and 2 dispersed in aqueous solutions upon the addition of different cations and anions;(B)Contrast histogram of fluorescence intensity of 1 and 2 dispersed in water with the addition of different cations and anions(a,c,e,g,respectively)and subsequent addition of identified ions:HPO42-(b),Cr2O72-(d),Fe3+(f),and Zr4+(h),respectively

Fluorescence experiments were performed to examine the detection limit of CPs 1 and 2 for Zr4+,Cr2O72-, Fe3+, and HPO42-.The emission intensities of probes 1 and 2 were measured by adding solutions containing Zr4+,Cr2O72-and Fe3+,HPO42-with different concentrations, respectively.The Stern-Volmer (SV) equation isI0/I=KSVcQ+1, whereI0andIare the luminescence intensities of the sensor without and with adding quenching materials,KSVis the quenching constant (L·mol-1),cQis the molar concentration of the detected target.The equation calculated the limit of detection(LOD): LOD=3σ/KSV, whereσis the standard deviation calculated by the fluorescent intensity of blank probe solution (CP 1, intensity: 1 204, 1 206, 1 207, 1 208,1 209;CP 2,intensity:5 430,5 435,5 444,5 445,5 446).As shown in Fig.4, when the concentrations of Zr4+,Cr2O72-,and Fe3+,HPO42-solution were 500,500 μmol·L-1, and 350, 35 μmol·L-1, the fluorescence quenching rate of 1 (2 mL) and 2 (2 mL) were almost 98.52%,98.52%,and 98.10%,97.14%,respectively.The correlation of the quenching effect (I0/I-1) and the concentration of the identified ions (Zr4+, Cr2O72-, Fe3+, and HPO42-) showed a good linear relation.The quenching constants(KSV)were 2.93×104L·mol-1(CP 1,Zr4+),8.7×103L·mol-1(CP 1, Cr2O72-), 3.9×104L·mol-1(CP 2,Fe3+), 3.5×103L·mol-1(CP 2, HPO42-), respectively,and the LOD values were 0.139 μmol·L-1(1, Zr4+),0.626 μmol·L-1(1, Cr2O72-), 0.430 μmol·L-1(2, Fe3+),1.36 μmol·L-1(2, HPO42-), respectively.These values were close or superior to the values reported in the literature[12-16,23-24], and some values may be reported for the first time.These results confirm that CPs 1 and 2 are ideal fluorescence sensing materials with high selectivity and sensitivity.

Fig.4 Fluorescence intensities(left)and SV equation linear fitting(right)of CPs 1 and 2 in solutions of Zr4+,Fe3+,Cr2O72-,and HPO42-with different concentrations at room temperature

2.4 Fluorescence recognition mechanism and potential fingerprint recognition

The possible fluorescence recognition mechanism of CPs 1 and 2 for Zr4+, Cr2O72-and Fe3+, HPO42-have been investigated.As shown in Fig.5a, in an aqueous solution, the UV-Vis absorption spectra of Zr4+and Cr2O72-overlapped with the excitation spectra of 1, and the UV-Vis absorption spectra of Fe3+and HPO42-overlapped with the excitation spectra of CP 2, resulting in fluorescence quenching.These results indicate that the possible mechanism of fluorescence recognition is due to the competitive absorption of the sensors (CPs 1 and 2)and the recognized ions(Zr4+,Cr2O72-,Fe3+,HPO42-).

Fig.5 (a)Excitation spectra of CPs 1 and 2 and UV-Vis absorption spectra of Zr4+,Cr2O72-,Fe3+,HPO42-in aqueous solution;(b)Fluorescent photographs of the powders of 1 and 2 for fingerprint identification

The powders of CPs 1 and 2 with small particle sizes obtained by grinding treatment can be used to collect fingerprints.The collected fingerprints can be visualized by placing them under an ultraviolet lamp(λ=302 nm).As shown in Fig.5b,the fluorescent fingerprint lines were clear and coherent, and even more fingerprint details could be seen, including forks and cores.These results indicate that the powders of 1 and 2 have high sensitivity to fingerprint recognition and resistance to background interference for the applications of fingerprint identification.

3 Conclusions

In conclusion, two novel isomorphic 3D CP materials based on flexible nitrogen-containing carboxylic acids were prepared, and their basic characterization was carried out by single crystal X-ray diffraction,PXRD, infrared, TG, and fluorescence analysis.CPs 1 and 2 have broad application prospects in the fluorescence detection of Zr4+,Cr2O72-and Fe3+,HPO42-ions in aqueous solution,due to their outstanding chemical stability, thermodynamic stability, and good fluorescence properties.TheKSVand LOD values of 1 and 2 for Zr4+,Cr2O72-and Fe3+,HPO42-ions were close to or better than the values reported in the literature, and some values may be reported for the first time.Interestingly, 1 and 2 have potential applications in fingerprint recognition.