English

• 0.   Introduction

  There has been an increasing demand for the rapid detection of nitroaromatic explosives (NACs) due to their carcinogenicity, mutagenicity, explosiveness, and environmental issues. Several methods including electrochemical methods, spectroscopy, fluorescence response method, and solid-phase micro-extraction-liquid chromatography are commonly employed to detect these explosive chemicals^[1-7]. However, the limit of detection (LOD) of electrochemical sensing is too high to be applicable for trace-level NACs, while solid-phase micro-extraction-liquid chromatography suffers from the issue of weakly volatile explosives. The fluorescence response strategy is extremely appropriate for detecting the trace level of NACs in principle, with rapid response, low cost, non-destructive nature, high sensitivity, and selectivity.

  Coordination polymers (CPs) have garnered significant research attention as functional materials due to their prospective applications in magnetism, gas storage and separation, and sensing^[8-14]. The luminescent CPs offer advantages compared to other sensor materials on account of their high sensitivity, simplicity, and instantaneous response. CP-based probes have been synthesized for the detection of heavy metal ions, small organic molecules, and nitro explosives in wastewater, due to their advantages such as high sensitivity, rapid response, and efficient selectivity. Zinc-based CPs are among the most important constituents in luminescent CPs^{[5, 15-16]}.

  As a part of our research regarding the design of CPs with luminescent properties^[17-18], a unique fluorene-based ligand was selected. A novel zinc CP, namely {[Zn₂(bdc)₂(mfdp)]₂·4DMA·2Me₂NH·3H₂O}_n (1), was synthesized and characterized, where H₂bdc=1, 4-benzenedicarboxylic acid, mfdp=2, 7-bis(4-pyridyl)-9, 9-dimethylfluorene, and DMA=N, N-dimethylacetamide. It exhibited an exceptional sensitivity towards the detection of trinitrophenol (TNP) within a dimethylacetamide (DMA) medium, boasting an exceedingly high quenching constant.

  1.   Experimental

  1.1   Materials and methods

  All reagents, e.g. 2, 4, 6-trinitrophenol(TNP), 1, 4-dinitrobenzene (1, 4-DNB), 1, 2-dinitrobenzene (1, 2-DNB), 1, 3-dinitrobenzene (1, 3-DNB), and 4-nitrotoluene (4-NT), DMA were purchased as analytical grade and used without further purification. Ligand mfdp was prepared according to the literature^[19]. Elemental analysis (C, H, N) was performed on an Elementar Vario EL Ⅲ elemental analyzer. The IR spectrum was recorded on a Bruker Vector 22 spectrophotometer in a range of 4 000-400 cm^-1 using KBr pellets. The luminescence spectra were measured on a Hitachi F-4600 fluorescence spectrometer. Thermogravimetric analysis (TGA) was carried out on a NETZSCH STA 449C unit at a heating rate of 10 ℃·min^-1 under a nitrogen atmosphere.

  1.2   Synthesis of complex 1

  A mixture of Zn(NO₃)₂·6H₂O (0.1 mmol, 29.7 mg), H₂bdc (0.05 mmol, 8.3 mg), mfdp (0.05 mmol, 17.4 mg) in DMA (7.0 mL) was sealed in a 15 mL reactor, heated to 95 ℃ for three days, and then cooled to room temperature. Colorless block crystals were obtained (Yield: 42% based on mfdp). Anal. Calcd. for C₁₀₂H₁₁₂N₁₀O₂₃Zn₄(%): C, 58.13; H, 5.36; N, 6.65. Found(%), C, 58.01; H, 5.25; N, 6.74. IR(KBr, cm^-1): 3 452(m), 3 132(w), 2 964(m), 2 925(m), 1 637(s), 1 612(s), 1 564(m), 1 504(w), 1 463(w), 1 388(s), 1 263(w), 1 224(w), 1 184(w), 1 093(w), 1 070(w), 1 018(m), 889(w), 819(m), 746(s), 630(w), 592(w), 549(m), 455(w).

  1.3   Crystal structure determination

  The X-ray crystallographic data for complex 1 were acquired using a Bruker Smart-1000 CCD diffractometer, which was furnished with a graphite-monochromated Mo Kα radiation source (λ=0.071 073 nm). The data collection was performed in φ and ω scanning modes at 150(2) K. Absorption corrections were applied by SADABS^[20]. The structure was solved by direct method and refined on F² via the full-matrix least-squares techniques^[21-22]. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms of the water molecules were located in a differential Fourier map, and the other hydrogen atoms were generated geometrically. The details of crystal data and refinement for 1 are given in Table 1. Selected bond distances and angles for 1 are listed in Table S1 (Supporting information).

  Table 1

  

  Table 1.  Crystal data and refinement parameters of complex 1

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  Parameter	1		Parameter	1
  Formula	C102H112N10O23Zn4		Dc / (g·cm-3)	1.367
  Formula weight	2 107.5		μ / mm-1	1.001
  Crystal system	Monoclinic		F(000)	4 392
  Space group	C2/c		Unique reflection	10 501
  a / nm	4.410 2(2)		Observed reflection [I > 2σ(I)]	7 383
  b / nm	1.502 99(6)		Number of parameters	645
  c / nm	1.564 81(7)		GOF	1.056
  β / (°)	99.118(2)		Final R indices [I > 2σ(I)]	R1=0.060 7, wR2=0.161 0
  Z	4		R indices (all data)	R1=0.090 0, wR2=0.190 3
  V / nm3	10.241 3(8)

  2.   Results and discussion

  2.1   Crystal structure of complex 1

  Complex 1 crystallizes in the monoclinic C2/c space group. Its asymmetric unit consists of two Zn(Ⅱ) ions, two bdc^2- anions, one mfdp ligand, four DMA molecules, two Me₂NH molecules, and three water molecules. As illustrated in Fig. 1a, each Zn(Ⅱ) ion adopts a slightly distorted square pyramidal geometry, coordinated by four carboxylic oxygen atoms from four different bdc^2- at the basal positions and one N atom from a mfdp molecule at the apical position. The Zn—N distances are 0.202 9(3) and 0.204 3(3) nm, and the Zn—O bonds range between 0.201 7(3) and 0.207 9(3) nm. The bond angles around each Zn(Ⅱ) range from 78.81(8)° to 163.37(12)° (Table S1).

  Figure 1

  

  Figure 1.  Crystal structure of complex 1: (a) coordination environment; (b) simplified 6-connected node; (c) one of the simplified net with the {4¹²·6³} topology; (d) schematic view of the 2-fold interpenetrated network

  Hydrogen atoms, DMA, Me₂NH, and lattice water molecules are omitted for clarity; Symmetry codes: #1: x, 1-y, 1/2+z; #2: x, 1-y, -1/2+z; #3: x, -y, -1/2+z; #4: -1/2+x, 1/2+y, -1+z; #5: -1/2+x, 1/2-y, -1/2+z.

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  Zn1 and Zn2 are linked to four bdc^2- ligands adopting bidentate bridging coordination modes to form dinuclear [Zn₂(bdc)₄] units of square paddlewheels, which are further linked by mfdp to construct a 3D network (Fig. 1a). The void space within the single 3D framework is mainly minimized by the interpenetration of other equivalent frameworks to maintain the stability of the whole structure, leaving only small voids to hold solvent molecules (Fig. 1c).

  From the topological perspective^[23], each [Zn₂(BDC)₂] unit is simplified to a 6-connected node (Fig. 1b), the bdc^2- and mfdp ligands act as linkers, the 3D structure can be rationalized as 2-fold interpenetrating 6-connected uninodal {4¹².6³} topology (Fig. 1c and 1d).

  2.2   TGA of complex 1

  As in Fig 2, it commenced losing weight immediately after the measurement was started. At about 172 ℃, a weight loss of about 6.64% was observed which could be ascribed to the removal of two Me₂NH molecules and three lattice water molecules per formula unit, confirmed by the good agreement between the observed and calculated weight losses (6.83%). A continued weight loss of about 15.04% was observed in a temperature range of 173-392 ℃, which matched with the weight loss caused by the release of four DMA molecules per formula unit (Calcd. 16.51%).

  Figure 2

  

  Figure 2.  TGA curve of complex 1

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  2.3   Luminescence sensing of TNP

  Based on d¹⁰ metal ions and the electron-rich π-conjugated ligands, CPs often demonstrate intriguing luminescence properties, making them useful as chemical sensors^{[15-16, 18, 24]}. The solid-state luminescence spectra of complex 1 and ligand mfdp were measured under the same experimental conditions. As depicted in Fig. 3a, the emission of 1 was observed at 434 nm (λ_ex=284 nm). Such fluorescence behavior may be attributed to the intraligand transition of coordinated mfdp ligands since similar emission at 425 nm (λ_ex=352 nm) was observed for the free mfdp ligand. The red shift of the band compared to the mfdp ligand may originate from the influence of the coordination interactions of the ligand to the metal centers.

  Figure 3

  

  Figure 3.  Luminescence spectra of complex 1 and ligand mfdp in the solid state (a) and in DMA solution (b)

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  The luminescence characteristics of complex 1 suspended in various solvents including DMF, DMA, THF, EtOH, etc., are illustrated in Fig. 4. Notably, the highest emission intensity of 1 was observed in the DMA solution. As shown in Fig. 3b, the emission peak of mfdp in DMA was observed at 382 nm (λ_ex=359 nm), while that of 1 in DMA was found at 385 nm (λ_ex=335 nm). The intense luminescence exhibited by 1 motivates us to explore its potential utility in the detection of NACs. Upon the addition of 100 μL of a 5 mmol·L^-1 TNP solution to the suspension of 1 in DMA (2 mL), the luminescence was quenched by 98.64%. In comparison, this figure was markedly higher than the quenching observed for 1 upon the addition of 100 μL of solutions containing other NACs (74.77%, 38.54%, 35.90%, and 15.63% for 1, 4-DNB, 1, 2-DNB, 1, 3-DNB, and 4-NT, respectively. Fig. 5). The results highlight the potential of 1 as a highly sensitive chemical sensor for detecting TNP.

  Figure 4

  

  Figure 4.  Comparisons of the luminescence intensities of complex 1 in different dispersants

  λ_ex=335 nm.

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  Figure 5

  

  Figure 5.  Comparison of the quenching efficiency of complex 1 for different NACs (5 mmol·L^-1, 100 μL)

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  Luminescence titration experiments were thus performed on a suspension of complex 1 in DMA (2 mL), with the incremental addition of the TNP solution (Fig. 6a). The efficiency of luminescence quenching by TNP can be assessed through a quantitative analysis employing the Stern-Volmer (SV) equation^[24], which is formulated as follows: I₀/I=1+K_SVc_M. Herein, c_M signifies the molar concentration of the analyte, while I₀ and I denote the luminescent peak intensities of the indicator before and after the introduction of the analyte, respectively. The parameter K_SV represents the quenching constant. At low concentrations, the SV curves for nitroaromatic species exhibit a near-linear relationship. This linearity suggests a probable energy transfer process between complex 1 and the analyte, supported by the observation of a blue shift (16 nm) in the emission band following the incremental admixture of TNP solution (Fig. 6a). The quenching constant for complex 1 was determined to be 6.65×10⁴ L·mol^-1 for TNP, based on the linear regression analysis of the curve obtained under low concentration conditions (Fig. 6b). Furthermore, the LOD of the assay, calculated using the formula 3σ/k (where k is the slope and σ is the standard deviation), within the low concentration range of 0-30 μmol·L^-1, was estimated to be approximately 0.164 μmol·L^-1 for TNP (Fig. 7). The substantial magnitude of K_SV coupled with the minute LOD underscores the superior performance of 1 in comparison to previously documented CPs for the detection of TNP, as evidenced by the comparative data presented in Table 2.

  Figure 6

  

  Figure 6.  (a) Emission spectra of complex 1 dispersed in DMA upon the addition of TNP solution (5 mmol·L^-1); (b) SV plot for the detection of TNP by 1 in DMA suspension at the low concentration (0-30 μmol·L^-1)

  λ_ex=335 nm.

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  Figure 7

  

  Figure 7.  Plot of the luminescent peak intensity of complex 1 vs TNP concentration at the low concentration (0-30 μmol·L^-1) for the determination of the LOD of TNP

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  Table 2

  

  Table 2.  Quenching constants of the reported CPs for detecting TNP in DMF, DMA, or H₂O suspension

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  CP	Quenching constant KSV / (L·mol-1)	Solvent	Ref.
  [Zn2(bdc)2(mfdp)]n	6.65×104	DMA	This work
  UiO-68@NH2	5.8×104	H2O	[1]
  {[Cd(adc)(dppc)(H2O)]·2H2O}n	7.6×104	DMF	[18]
  {[Zn2(BIDPS)2(OBA)2]·DMA}n	9.6×103	H2O	[25]
  {[Zn(BIDPT)(PA)]·DMF}n	7.03×103	H2O	[25]
  {[Zn(BIDPS)(PA)(H2O)2]·2H2O}n	8.58×103	H2O	[25]
  {[Zn(2, 5-tdc)(3-abit)]·H2O}n	4.71×104	DMF	[26]
  Cd(INA)(pytpy)(OH)·2H2O	4.3×104	DMF	[24]
  Cd(INA)(pytpy)(OH)·2H2O	3.3×104	H2O	[24]
  (Me2NH2)[Zn2(L)(H2O)]·0.5DMF	1.42×104	DMF	[27]
  [Pr2(TATMA)2·4DMF·4H2O]	1.6×104	DMF	[28]
  [(Zn4O)(DCPB)3]·11DMF·5H2O	3.7×104	DMF	[29]
  [Zn(BDC)(L)]·xG	1.16×104	H2O	[30]
  [Cd(BDC)(L)]·xG	1.35×104	H2O	[30]
  [Zn2(H2L)2(Bpy)2(H2O)3]·H2O	1.36×104	H2O	[31]
  [Zn(L)(HCOO)·H2O]n	2.11×104	H2O	[32]
  [Cd(5-BrIP)(TIB)]n	2.68×104	H2O	[33]
  [Eu3(bpydb)3(HCOO)(μ3-OH)2(H)]	1.5×104	H2O	[34]

  The selectivity of the TNP detection by complex 1 was carried out via an anti-interference experimental protocol, as previously reported^[24]. At the beginning, the emission intensity of 1 was measured. Then, two equal parts of a 10 μL solution of 1, 4-DNB (5 mmol·L^-1) were added to the suspension, causing a negligible reduction in the emission intensity. Nevertheless, a significant drop in the emission intensity was witnessed upon adding an equal amount of a 5 mmol·L^-1 TNP solution (Fig. 8). This tendency remained in the subsequent cycles of analyte addition. Additionally, analogous outcomes were realized when assessing the selectivity of 1 in distinguishing TNP from other NACs. Collectively, these results substantiate the conclusion that 1 demonstrates a pronounced capacity for discriminating against interference when sensing TNP.

  Figure 8

  

  Figure 8.  Competitive luminescence quenching of complex 1 upon addition of other NACs followed by TNP

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  Importantly, complex 1 was recyclable after the luminescence titration experiment. The suspension consisting of 1 and TNP was centrifuged to separate the solids, washed with DMA, and then dried. As depicted in Fig. 9, the reclamation of luminescence intensity was achieved, and the quenching efficiency of 1 remained relatively unchanged across five consecutive cycles. These outcomes collectively underscore the superior performance of complex 1 as a TNP sensor.

  Figure 9

  

  Figure 9.  Recyclability of complex 1 for TNP detection in DMA

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  3.   Conclusions

  In summary, a fluorene-based Zn(Ⅱ) CP (1) was successfully synthesized and characterized. Complex 1 exhibits 2-fold interpenetrating frameworks with {4¹²·6³} topology, constructed by Zn(Ⅱ) ions, mfdp, and bdc^2- ligands. The luminescence studies indicate that complex 1 can detect TNP in DMA solution with high sensitivity through a luminescent quenching effect. The results demonstrate that complex 1 serves as a stable and reusable chemical sensor for the detection of TNP.

  Supporting information is available at http://www.wjhxxb.cn

  
  
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• 

  Figure 1  Crystal structure of complex 1: (a) coordination environment; (b) simplified 6-connected node; (c) one of the simplified net with the {4¹²·6³} topology; (d) schematic view of the 2-fold interpenetrated network

  Hydrogen atoms, DMA, Me₂NH, and lattice water molecules are omitted for clarity; Symmetry codes: #1: x, 1-y, 1/2+z; #2: x, 1-y, -1/2+z; #3: x, -y, -1/2+z; #4: -1/2+x, 1/2+y, -1+z; #5: -1/2+x, 1/2-y, -1/2+z.

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  Figure 2  TGA curve of complex 1

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  Figure 3  Luminescence spectra of complex 1 and ligand mfdp in the solid state (a) and in DMA solution (b)

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  Figure 4  Comparisons of the luminescence intensities of complex 1 in different dispersants

  λ_ex=335 nm.

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  Figure 5  Comparison of the quenching efficiency of complex 1 for different NACs (5 mmol·L^-1, 100 μL)

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  Figure 6  (a) Emission spectra of complex 1 dispersed in DMA upon the addition of TNP solution (5 mmol·L^-1); (b) SV plot for the detection of TNP by 1 in DMA suspension at the low concentration (0-30 μmol·L^-1)

  λ_ex=335 nm.

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  Figure 7  Plot of the luminescent peak intensity of complex 1 vs TNP concentration at the low concentration (0-30 μmol·L^-1) for the determination of the LOD of TNP

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  Figure 8  Competitive luminescence quenching of complex 1 upon addition of other NACs followed by TNP

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  Figure 9  Recyclability of complex 1 for TNP detection in DMA

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  Table 1.  Crystal data and refinement parameters of complex 1

  Parameter	1		Parameter	1
  Formula	C102H112N10O23Zn4		Dc / (g·cm-3)	1.367
  Formula weight	2 107.5		μ / mm-1	1.001
  Crystal system	Monoclinic		F(000)	4 392
  Space group	C2/c		Unique reflection	10 501
  a / nm	4.410 2(2)		Observed reflection [I > 2σ(I)]	7 383
  b / nm	1.502 99(6)		Number of parameters	645
  c / nm	1.564 81(7)		GOF	1.056
  β / (°)	99.118(2)		Final R indices [I > 2σ(I)]	R1=0.060 7, wR2=0.161 0
  Z	4		R indices (all data)	R1=0.090 0, wR2=0.190 3
  V / nm3	10.241 3(8)

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  Table 2.  Quenching constants of the reported CPs for detecting TNP in DMF, DMA, or H₂O suspension

  CP	Quenching constant KSV / (L·mol-1)	Solvent	Ref.
  [Zn2(bdc)2(mfdp)]n	6.65×104	DMA	This work
  UiO-68@NH2	5.8×104	H2O	[1]
  {[Cd(adc)(dppc)(H2O)]·2H2O}n	7.6×104	DMF	[18]
  {[Zn2(BIDPS)2(OBA)2]·DMA}n	9.6×103	H2O	[25]
  {[Zn(BIDPT)(PA)]·DMF}n	7.03×103	H2O	[25]
  {[Zn(BIDPS)(PA)(H2O)2]·2H2O}n	8.58×103	H2O	[25]
  {[Zn(2, 5-tdc)(3-abit)]·H2O}n	4.71×104	DMF	[26]
  Cd(INA)(pytpy)(OH)·2H2O	4.3×104	DMF	[24]
  Cd(INA)(pytpy)(OH)·2H2O	3.3×104	H2O	[24]
  (Me2NH2)[Zn2(L)(H2O)]·0.5DMF	1.42×104	DMF	[27]
  [Pr2(TATMA)2·4DMF·4H2O]	1.6×104	DMF	[28]
  [(Zn4O)(DCPB)3]·11DMF·5H2O	3.7×104	DMF	[29]
  [Zn(BDC)(L)]·xG	1.16×104	H2O	[30]
  [Cd(BDC)(L)]·xG	1.35×104	H2O	[30]
  [Zn2(H2L)2(Bpy)2(H2O)3]·H2O	1.36×104	H2O	[31]
  [Zn(L)(HCOO)·H2O]n	2.11×104	H2O	[32]
  [Cd(5-BrIP)(TIB)]n	2.68×104	H2O	[33]
  [Eu3(bpydb)3(HCOO)(μ3-OH)2(H)]	1.5×104	H2O	[34]

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