English

• 0.   Introduction

  As science and technology progress rapidly and large enterprises expand, pollution sources endangering human health and the environment have increased. These pollutants mainly include heavy metal ions, toxic gases, pesticides, and explosives^[1-2]. As a crucial heavy-metal ion in human activities and life, Fe³⁺ plays an essential role in both the environment and the human body^[3-4]. Both excessive and insufficient amounts of Fe³⁺ can cause various health issues, such as weakened immunity, iron deficiency anemia, multiple organ failure, and even esophageal and bladder cancers^[5-6]. Additionally, multiple nitroaromatic compounds (NACs) are widely employed in fields such as blasting, medicine, and chemistry. The 4-nitrophenol (4-NP), a vital organic synthetic raw material, is a significant intermediate in the synthesis of fine chemicals such as pesticides, pharmaceuticals, and dyes^[7-8]. However, this substance also presents certain hazards. It can trigger skin irritation, allergic reactions, and damage the respiratory system^[9-11]. Given the above circumstances, it is urgent to develop an effective method for detecting Fe³⁺ and 4-NP^[12-14].

  The currently reported detection techniques, including metal detection methods, ion migration spectrometry^[15], gas chromatography-mass spectrometry^[16], surface-enhanced Raman spectroscopy^[17], and energy dispersive X-ray diffraction^[18], suffer from time-consuming procedures and high costs^[19-21]. In contrast, the fluorescence-based detection methods have attracted considerable attention owing to their simplicity, high sensitivity, rapid response time, and low cost^[22-23]. Consequently, luminescent materials are important for the detection of 4-NP and Fe^{3+ [24-26]}.

  Coordination polymers (CPs), as an appealing class of highly ordered and crystalline porous functional materials, have developed rapidly in the past two decades due to their novel and diverse structures and unique properties. The application value of CPs has been manifested in a multitude of fields, including but not limited to electrochemistry^[27], homogeneous and heterogeneous catalysis^[28], energy storage^[29], gas separation^[30], and particularly in the domain of luminous sensors^[31-32], where they have demonstrated significant potential and utility.

  Furthermore, the CPs are also constructed by the light-emitting metal centers and carboxylic acid ligands. Cd(Ⅱ) exhibits remarkable advantages in assembling CPs. With a d¹⁰ electron configuration, Cd(Ⅱ) lacks ligand-field non-radiative d-d transitions, facilitating ligand photoluminescence and inducing strong luminescence via electron transitions upon photoexcitation. Moreover, Cd(Ⅱ) with moderate coordination ability, diverse modes, and geometries can stably coordinate with various organic ligands to construct CPs of different structures and topologies, enabling the design of polymers with specific luminescent properties^[33-35]. So far, numerous semirigid multicarboxylate ligands have been chosen to construct novel CPs because of their remarkable advantages. Specifically, semirigid ligands can rotate freely within a certain range, which is conducive to the coordination of ligands with metal ions. Additionally, the carboxyl group can be completely or partially deprotonated to meet the coordination requirements of different metal ions^[36-37]. 5, 5′-[1,1′-biphenyl-4,4′-diylbis(oxy)]diisophthalic acid (H₄L) represents a semirigid tetracarboxylic acid ligand. It features two ether bonds within its main chain and employs a biphenyl ring as a spacer moiety. Notably, the carboxyl groups are situated at the meta-positions of the benzene ring. This particular arrangement gives it a smaller steric volume compared to the ortho-position, enhancing its potential for coordinating with metal ions.

  In this work, a novel CP {[Cd₂(L)(1,4-bimb)_1.5(DMF)₂]·DMF}_n (1) was designed and synthesized through solvothermal reaction, where 1,4-bis(imidazole-1-ylmethyl)-benzene. We comprehensively described the synthesis, crystal structure, and fluorescence sensing characteristics of CP 1. 1 demonstrated high selectivity and low detection limits towards Fe³⁺ and 4-NP. Moreover, the fluorescence quenching mechanism of 1 was explored. Finally, 1 was successfully employed for the determination of the concentrations of Fe³⁺ and 4-NP in water samples collected from the Yanhe River.

  1.   Experimental

  1.1   Reagents and instruments

  All reagents and solvents were commercially available and used directly without further purification. The C, H, and N elemental analyses were conducted with a PerkinElmer PE-2400 elemental analyzer. The crystal data were collected on a Bruker SMART APEX-Ⅱ single-crystal X-ray diffractometer. Powder X-ray diffraction (PXRD) patterns were recorded with a Bruker D8 ADVANCE diffractometer operating at 40 kV and 40 mA using Cu Kα radiation (λ=0.154 18 nm) at a scanning rate of 2 (°)·min^-1 from 5° to 50°. Thermal gravimetric analysis (TGA) was performed with a NETZSCH STA 449F3 thermal gravimetric analyzer in flowing nitrogen at a heating rate of 10 ℃·min^-1. The UV-Vis spectra were measured using a UV-2700 spectrophotometer. Fluorescence experiments were carried out on the Hitachi F-7100 Fluorescence Spectrophotometer. The fluorescence lifetime data were obtained from a FLSP-920 lifetime and steady-state spectrometer.

  1.2   Synthesis of CP 1

  A mixture of Cd (NO₃)₂·4H₂O (0.1 mmol, 0.030 8 g), H₄L (0.05 mmol, 0.025 7 g), 1,4-bimb (0.05 mmol, 0.011 9 g), DMF (3 mL), and HNO₃ (0.2 mL, 6 mol·L^-1) were added into a 10 mL glass bottle and heated at 95 ℃ for 3 d. Colorless transparent massive crystals were obtained with a yield of 86.06% (based on Cd). Anal. Calcd. for C₅₈H₅₆N₉O₁₃Cd₂(%): C, 53.05; H, 4.27; N, 9.60. Found(%): C, 52.83; H, 4.21; N, 9.37.

  1.3   Crystal structure determination

  The crystals of CP 1 with regular shapes, transparent colors, and appropriate sizes were selected for testing. The single crystal data of 1 were collected on the Bruker SMART APEX-Ⅱ diffractometer (Mo Kα radiation, λ=0.071 073 nm). The crystal structure was analyzed by the SHELXS-2018/3 program and refined by the full-matrix least-squares techniques on F². All non-hydrogen atoms were refined anisotropically, and all of the hydrogen atoms were geometrically placed in calculated positions. The crystallographic data of 1 are shown in Table 1. Selected bond lengths and bond angles are listed in Table 2.

  Table 1

  

  Table 1.  Crystal data and structure refinement parameters for CP 1

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  Parameter	1		Parameter	1
  Formula	C58H56Cd2N9O13		γ/(°)	73.168(6)
  Formula weight	1 311.91		V/nm3	3.068 9(19)
  Crystal system	Triclinic		Z	2
  Space group	P1		Dc/(g·cm-3)	1.42
  a/nm	1.021 4(4)		F(000)	1 334
  b/nm	1.327 0(5)		Goodness-of-fit on F2	0.984
  c/nm	2.397 9(8)		R1, wR2[I > 2σ(I)]	0.059 4, 0.156 9
  α/(°)	80.725(7)		R1, wR2 (all data)	0.088 1, 0.170 8
  β/(°)	85.928(6)

  Table 2

  

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

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  Cd1—O1	0.222 9(4)	Cd1—N1D	0.222 3(6)	Cd2—O7B	0.228 2(5)
  Cd1—N6	0.220 3(5)	Cd2—O10A	0.245 7(5)	Cd2—N4	0.224 9(5)
  Cd1—O4C	0.226 4(4)	Cd2—O9	0.225 7(4)	Cd2—O11	0.242 0(8)
  Cd1—O3C	0.255 6(5)	Cd2—O8B	0.246 3(4)
  O1—Cd1—O4C	88.18(17)	N1D—Cd1—O4C	96.6(2)	O7B—Cd2—O10A	83.88(18)
  O1—Cd1—O3C	139.65(17)	O11—Cd2—O10A	177.3(2)	O7B—Cd2—O11	98.7(2)
  N6—Cd1—O1	116.81(18)	O11—Cd2—O8B	91.3(2)	N4—Cd2—O10A	88.59(17)
  N6—Cd1—O4C	132.33(19)	O10A—Cd2—O8B	89.78(15)	N4—Cd2—O11	88.9(2)
  N6—Cd1—N1D	113.5(2)	O9—Cd2—O10A	98.70(15)	N4—Cd2—O8B	93.15(17)
  O4C—Cd1—O3C	53.70(16)	O9—Cd2—O8B	140.85(16)
  N1D—Cd1—O1	104.1(2)	O9—Cd2—O11	82.0(2)
  Symmetry codes: A: 2-x, 1-y, 2-z; B: 1+x, y, z; C: -1+x, y, z; D: -2+x, 1+y, -1+z.

  2.   Results and discussion

  2.1   Crystal structure

  CP 1 belongs to the triclinic system, P1 space group. The asymmetric unit of 1 consists of two independent Cd(Ⅱ) ions, one L^4- ligand, one and a half 1,4-bimb ligands, two coordinated DMF molecules, and one free DMF molecule. As shown in Fig. 1, Cd1 has a five-coordination mode, surrounded by nitrogen atoms (N6, N1D) from two 1,4-bimb ligands and three oxygen atoms (O1, O3C, O4C) from two L^4- ligands. Unlike Cd1, Cd2 has a six-coordination mode. Cd2 is surrounded by one nitrogen atom (N4) from a 1,4-bimb ligand, four oxygen atoms (O7B, O8B, O9, O10A) from three L^4- ligands, and one oxygen atom (O11) from a DMF molecule. The bond lengths of Cd—O and Cd—N are 0.222 9(4)-0.255 6(5) nm and 0.220 3(5)-0.224 9(5) nm, respectively (Table 2). As shown in Fig. 2a, L^4- and 1,4-bimb are alternately connected with Cd(Ⅱ) respectively to form a 2D network, and each network forms a 3D framework structure through 1,4-bimb connection.

  Figure 1

  

  Figure 1.  Coordination environment of Cd(Ⅱ) in CP 1 with 50% ellipsoid probability

  For clarity, all hydrogen atoms are omitted; Symmetry codes: A: 2-x, 1-y, 2-z; B: 1+x, y, z; C: -1+x, y, z; D: -2+x, 1+y, -1+z; E: 2+x, -1+y, 1+z.

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  Figure 2

  

  Figure 2.  (a) Two-dimensional network structure of CP 1; (b) 3D framework of 1

  Symmetry codes: B: 1+x, y, z; C: -1+x, y, z; F: -1-x, 2-y, -z.

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  2.2   PXRD and TGA

  To study the purity of CP 1, PXRD patterns were obtained at room temperature. The experimental results are shown in Fig.S1 (Supporting information). The experimental peak position of 1 was consistent with the simulated peak position, indicating that 1 is a pure phase with high crystal purity.

  To investigate the thermal stability of 1, TGA experiments were carried out. As shown in Fig.S2, the weight loss from room temperature to 176 ℃ was about 16.34% (Calcd. 16.69%), corresponding to the loss of three DMF molecules. When further heated to about 321 ℃, the framework began to decompose. The results of thermal stability analysis were consistent with those of structural analysis.

  2.3   Photoluminescence property of CP 1

  The solid luminescence of CP 1 and H₄L was studied due to the excellent luminescence properties of d¹⁰ metal CPs. As shown in Fig.S3, H₄L had an emission peak at 438 nm when excited at 374 nm, and 1 had an emission peak at 427 nm when excited at 356 nm. The results show that the emission peak of 1 had a partial blue shift, which may be because 1 does not form an effective π-conjugated system, thus reducing the electron delocalization range of the ligand^[38].

  2.4   Cation sensing

  Industrial emissions, agricultural pollution, and domestic pollution have led to the proliferation of cations, posing a huge threat to ecosystems, water resources, and the human body^[39]. We urgently need to find a simple and efficient way to detect cations. Therefore, CP 1 was used as a fluorescence sensor to detect cations. As shown in Fig. 3a and 3b, thirteen cations were selected for fluorescence detection, namely Co²⁺, Ni²⁺, Zn²⁺, Ca²⁺, Sr²⁺, Hg²⁺, Pb²⁺, Nd³⁺, Fe²⁺, Al³⁺, In³⁺, Cu²⁺, and Fe³⁺. The results showed that in the presence of Fe³⁺, the fluorescence of 1 reached the maximum quenching. In the absence of Fe³⁺, the fluorescence intensity of other cations changed little, but when Fe³⁺ was added to other cationic solutions, the fluorescence intensity was significantly quenched, indicating that 1 has excellent selectivity. To deeply analyze and study its quenching condition, quantitative experiments were carried out. As shown in Fig. 3c, in a range of 0-90 μmol·L^-1, the intensity gradually decreased with the gradual increase of Fe³⁺ concentration. To explore the relationship between Fe³⁺ concentration and fluorescence intensity, the relationship between I₀/I-1 and c_{Fe^{3 +}} was plotted according to the Stern-Volmer (SV) equation: I₀/I=1+K_SVc_{Fe^{3 +}} (I₀: the fluorescence intensity of 1 without Fe³⁺, I: the fluorescence intensity of 1 with Fe³⁺, K_SV: quenching constant). As shown in Fig. 3d, there was a good linear relationship between I₀/I-1 and c_{Fe^{3 +}} in the low concentration range. The detection limit was calculated with 3σ/k (σ: standard deviation, k: slope), and the detection limit of 1 for Fe³⁺ was 0.034 μmol·L^-1.

  Figure 3

  

  Figure 3.  (a) Luminescent intensity of CP 1 with different cations; (b) Luminescence intensity of 1 with mixed cations; (c) Emission spectra of 1 with different Fe³⁺ concentrations; (d) SV plot for 1 sensing Fe³⁺ in the low concentration range

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  2.5   NACs sensing

  Because of the high explosive toxicity and chemical toxicity of NACs, they have caused great harm to the environment and human health^[40-41]. Therefore, NACs are vital to human health and national security. In this study, nine different NACs including 3-nitroaniline (3-NT), 2, 4-dinitrophenylhydrazine (DNP), 4-nitrophenol (4-NP), p-nitrobenzoic acid (PNBA), 4-nitrophenylhydrazine (4-NPH), o-nitrobenzaldehyde (ONBA), o-nitroaniline (ONT), trinitrophenol (TNP) and nitrobenzene (NB) were studied. As shown in Fig. 4a, the fluorescence intensity of CP 1 reached the maximum quenching in the presence of 4-NP. The fluorescence intensity of other NACs changed little when there was no 4-NP, but when 4-NP was added to other NACs solutions, the fluorescence intensity was greatly quenched, indicating that 4-NP has an anti-interference property (Fig. 4b). To further analyze and study its quenching condition, quantitative experiments were carried out. In a range of 0-4 μmol·L^-1 of 4-NP, the fluorescence intensity of 1 decreased gradually with the increase of 4-NP concentration. When the concentration of 4-NP reached 15.5 μmol·L^-1, the quenching reached more than eight times (Fig. 4c). The relationship between I₀/I-1 and c_4-NP was plotted according to the Stern-Volmer equation (I₀/I=1+K_SVc_4-NP), showing a good linear relationship in the low concentration range (Fig. 4d). The detection limit was calculated to be 0.031 μmol·L^-1.

  Figure 4

  

  Figure 4.  (a) Luminescent intensity of CP 1 in different NACs; (b) Luminescence intensity of 1 in mixed NACs; (c) Emission spectra of 1 with different 4-NP concentrations; (d) SV plot for 1 sensing 4-NP in the low concentration range

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  2.6   Possible sensing mechanism

  The mechanisms of fluorescence quenching reported are as follows: (1) the collapse and disintegration of the framework, (2) the mechanism of photoinduced electron transfer, (3) the mechanism of energy transfer, (4) the exchange of center metal ions, (5) the mechanism of energy competition and absorption^[42]. To determine the possible fluorescence quenching mechanism, the following experiments were carried out. Firstly, the PXRD pattern of the sample after the fluorescence experiment was tested, and the position of the PXRD peak of the sample after the experiment was roughly the same as that of the simulated PXRD peak (Fig. 5). So, the cause of fluorescence quenching is not the collapse and disintegration of the framework. Secondly, UV-Vis absorption experiments were carried out on the detected substances. The experimental results showed that the absorption spectra of Fe³⁺ and 4-NP overlapped with the excitation spectrum of 1, indicating that the reason for the fluorescence quenching of 1 may be the competition of capabilities and absorption (Fig. 6a and 6b). As is commonly recognized, the fluorescence quenching phenomenon in the detection system can be brought about by two processes, namely static quenching and dynamic quenching. Static quenching results from the ground-state combination of substances, leading to the formation of non-fluorescent complexes. On the other hand, dynamic quenching occurs when the excited fluorophore collides with the quencher. To clarify the quenching mechanism, time-resolved fluorescence measurement experiments were conducted with and without competing substances. Fluorescence lifetime parameters before and after the addition of analytes are listed in Table S1. The calculated average fluorescence lifetime was between 3.10 and 0.04 ns. From this, it can be seen that the mean fluorescence lifetime is significantly affected by the addition of the analytes and that dynamic quenching is responsible for the quenching process.

  Figure 5

  

  Figure 5.  PXRD patterns of CP 1 and the as-synthesized samples immersing in Fe³⁺ or 4-NP solution for 24 h

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  Figure 6

  

  Figure 6.  (a) UV-Vis absorption spectra of the cations and the excitation spectrum of CP 1; (b) UV-Vis absorption spectra of the NACs and the excitation spectrum of 1

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  2.7   Detection of Fe³⁺ and 4-NP in Yanhe River water

  To verify the practicability of CP 1, the recovery experiments of Fe³⁺ and 4-NP in Yanhe River water samples were carried out. As shown in Table 3 and 4, the recoveries of standard addition at different concentrations were 102%-103% and 101%-102%. Relative standard deviation (RSD) values of 1.1%-1.6% and 1.1%-1.8%, respectively, indicate that 1 can be used to detect Fe³⁺ and 4-NP in real samples.

  Table 3

  

  Table 3.  Recovery test of Fe³⁺ spiked in Yanhe water samples

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  c/(μmol·L-1)		RSD*/%	Recovery/%
  Spiked	Detected
  0	—
  3	3.1	1.6	103
  6	6.1	1.1	102
  9	9.3	1.4	103
  *n=3.

  Table 4

  

  Table 4.  Recovery test of 4-NP spiked in Yanhe water samples

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  c/(μmol·L-1)		RSD*/%	Recovery/%
  Spiked	Detected
  0	—
  0.5	0.51	1.8	102
  1.0	1.02	1.1	102
  1.5	1.52	1.1	101
  *n=3.

  3.   Conclusions

  In summary, a novel Cd(Ⅱ) coordination polymer (1) based on a carboxylic acid ligand (H₄L) and a N-donor ligand (1,4-bimb) was successfully synthesized by the solvothermal method. The investigation of the crystallographic structure of 1 indicates that it features a 3D framework. Fluorescence sensing experiments show that 1 has good anti-interference in the detection of Fe³⁺ and 4-NP. The detection limits of Fe³⁺ and 4-NP were 0.034 and 0.031 μmol·L^-1, respectively. In addition, the causes of fluorescence quenching are analyzed in detail. The quenching may be caused by the competition and absorption of energy. Finally, the Fe³⁺ and 4-NP in Yanhe River were detected by using 1.

  
  Supporting information is available at http://www.wjhxxb.cn
  
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  Figure 1  Coordination environment of Cd(Ⅱ) in CP 1 with 50% ellipsoid probability

  For clarity, all hydrogen atoms are omitted; Symmetry codes: A: 2-x, 1-y, 2-z; B: 1+x, y, z; C: -1+x, y, z; D: -2+x, 1+y, -1+z; E: 2+x, -1+y, 1+z.

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  Figure 2  (a) Two-dimensional network structure of CP 1; (b) 3D framework of 1

  Symmetry codes: B: 1+x, y, z; C: -1+x, y, z; F: -1-x, 2-y, -z.

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  Figure 3  (a) Luminescent intensity of CP 1 with different cations; (b) Luminescence intensity of 1 with mixed cations; (c) Emission spectra of 1 with different Fe³⁺ concentrations; (d) SV plot for 1 sensing Fe³⁺ in the low concentration range

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  Figure 4  (a) Luminescent intensity of CP 1 in different NACs; (b) Luminescence intensity of 1 in mixed NACs; (c) Emission spectra of 1 with different 4-NP concentrations; (d) SV plot for 1 sensing 4-NP in the low concentration range

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  Figure 5  PXRD patterns of CP 1 and the as-synthesized samples immersing in Fe³⁺ or 4-NP solution for 24 h

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  Figure 6  (a) UV-Vis absorption spectra of the cations and the excitation spectrum of CP 1; (b) UV-Vis absorption spectra of the NACs and the excitation spectrum of 1

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  Table 1.  Crystal data and structure refinement parameters for CP 1

  Parameter	1		Parameter	1
  Formula	C58H56Cd2N9O13		γ/(°)	73.168(6)
  Formula weight	1 311.91		V/nm3	3.068 9(19)
  Crystal system	Triclinic		Z	2
  Space group	P1		Dc/(g·cm-3)	1.42
  a/nm	1.021 4(4)		F(000)	1 334
  b/nm	1.327 0(5)		Goodness-of-fit on F2	0.984
  c/nm	2.397 9(8)		R1, wR2[I > 2σ(I)]	0.059 4, 0.156 9
  α/(°)	80.725(7)		R1, wR2 (all data)	0.088 1, 0.170 8
  β/(°)	85.928(6)

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

  Cd1—O1	0.222 9(4)	Cd1—N1D	0.222 3(6)	Cd2—O7B	0.228 2(5)
  Cd1—N6	0.220 3(5)	Cd2—O10A	0.245 7(5)	Cd2—N4	0.224 9(5)
  Cd1—O4C	0.226 4(4)	Cd2—O9	0.225 7(4)	Cd2—O11	0.242 0(8)
  Cd1—O3C	0.255 6(5)	Cd2—O8B	0.246 3(4)
  O1—Cd1—O4C	88.18(17)	N1D—Cd1—O4C	96.6(2)	O7B—Cd2—O10A	83.88(18)
  O1—Cd1—O3C	139.65(17)	O11—Cd2—O10A	177.3(2)	O7B—Cd2—O11	98.7(2)
  N6—Cd1—O1	116.81(18)	O11—Cd2—O8B	91.3(2)	N4—Cd2—O10A	88.59(17)
  N6—Cd1—O4C	132.33(19)	O10A—Cd2—O8B	89.78(15)	N4—Cd2—O11	88.9(2)
  N6—Cd1—N1D	113.5(2)	O9—Cd2—O10A	98.70(15)	N4—Cd2—O8B	93.15(17)
  O4C—Cd1—O3C	53.70(16)	O9—Cd2—O8B	140.85(16)
  N1D—Cd1—O1	104.1(2)	O9—Cd2—O11	82.0(2)
  Symmetry codes: A: 2-x, 1-y, 2-z; B: 1+x, y, z; C: -1+x, y, z; D: -2+x, 1+y, -1+z.

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  Table 3.  Recovery test of Fe³⁺ spiked in Yanhe water samples

  c/(μmol·L-1)		RSD*/%	Recovery/%
  Spiked	Detected
  0	—
  3	3.1	1.6	103
  6	6.1	1.1	102
  9	9.3	1.4	103
  *n=3.

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  Table 4.  Recovery test of 4-NP spiked in Yanhe water samples

  c/(μmol·L-1)		RSD*/%	Recovery/%
  Spiked	Detected
  0	—
  0.5	0.51	1.8	102
  1.0	1.02	1.1	102
  1.5	1.52	1.1	101
  *n=3.

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