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A high selective fluorescent sensor for Cu2+ in solution and test paper strips
Dyes and Pigments 173 (2020) 107914
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A high selective fluorescent sensor for Cu2þ in solution and test paper strips Fu Shi, Shiqiang Cui *, Hongling Liu, Shouzhi Pu ** Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal University, Nanchang, 330013, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Fluorescent sensor Stokes shift ESIPT Copper ion Test paper
A novel and efficient fluorescent sensor with a pyrazine triphenylamine unit was developed. The sensor exhibits large Stokes shift (216 nm) due to the excited-state intramolecular proton transfer (ESIPT) process. After com plexing with Cu2þ, the ESIPT process was blocked and the fluorescence of the sensor was quenched. Job’s plot and ESI-MS results indicated that the stoichiometric ratio between the sensor and Cu2þ was 1:1. The detection limit was determined to be 2.47 � 10 8 mol L 1. The sensor could be used to detect Cu2þ in practical samples with high accuracy. Furthermore, the sensor also could be made into test paper strips for the qualitative and quantitative detection of Cu2þ.
1. Introduction Copper is the third most indispensable abundant trace element in human body, and plays a crucial role in various biological processes, such as electron transfer in cellular respiration, as a cofactor in many enzymes, and as redox regulating components [1–3]. However, abnormal levels of copper ions in the body will leads to many human genetic disorders, such as Menkes [4], Wilson’s diseases [5], Alz heimer’s [6], Parkinson’s [7], Prion [8], and Huntington’s diseases [9], etc. The U.S. Environmental Protection Agency (EPA) has set the maximum tolerable concentration of copper ions in drinking water was 20 μM [10]. Nevertheless, owing to the widespread use of copper in the fields of agriculture, industry, chemistry, medicine, and biotechnology et al., the inevitable problem of pollution and the accompanying po tential toxic effects on human beings continue to pose challenges to the world [11–13]. Therefore, the development of methods of efficient and fast detection of copper ions in biological and environmental samples caused extensive concern around the world. There have many methods were established for detecting Cu2þ in the past few decades, including atomic absorption spectrometry (AAS) [14], surface plasmon resonance spectroscopy (SPR) [15], inductively coupled plasma atomic emission spectrometry (ICP-AES) [16], chro matography [17], voltammetry [18] and so on. They could detect Cu2þ with considerable sensitivity, but these methods require expensive equipment, high operating costs, professional operator, and long anal ysis time. Thus, it is necessary to develop reliable and convenient methods for the detection of Cu2þ. Different from above traditional
analytical methods, fluorescent sensors have attracted many researchers attention due to the advantages of inexpensive, high sensitivity, excel lent selectivity, and high response speed [19–23]. Up to now, various of fluorescent sensors for Cu2þ have been reported with different func tional groups, such as quinoline [24], coumarin [25,26], dansyl amide [27], naphthalimide [28], fluorescein [29], rhodamine [30–33] and so on. However some of the sensors still have poor selectivity, low sensi tivity, and long response time [34,35]. Therefore, it is significant to design and synthesis sensor with high selectivity and high sensitivity for Cu2þ. Pyrazine and its derivatives as food additives have important func tion to the flavor of a variety of roasted or otherwise cooked foods [36]. On the other side, as a heterocyclic compound, pyrazine has preferable affinity for metal ions, especially copper ions [37–39]. Nevertheless, up to present, the chemosensor with pyrazine unit has not been reported. Excited state intramolecular proton transfer (ESIPT) has attracted a lot of attentions [40–44], since it was first reported in 1956 [45]. In recent years, The ESIPT has aroused the development of various applications such as fluorescent chemosensors [43,44], photostabilizers [46], solar collectors [47], laser dyes [48,49], and so on. In the ESIPT mechanism, a fast proton transfer process from enol form (E-E*) to keto form (K–K*) was happened, which could enhance the stability of the electronic excited-state and reduce the possibility of photochemical reactions of excited molecules. For this reason, the ESIPT process also has a larger Stokes shift, which will plays a significant role in reduce the interference from biological matrix and increase the signal-to-noise ratio in biolog ical imaging [41,42].
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (S. Cui), [email protected] (S. Pu). https://doi.org/10.1016/j.dyepig.2019.107914 Received 29 July 2019; Received in revised form 14 September 2019; Accepted 20 September 2019 Available online 21 September 2019 0143-7208/© 2019 Elsevier Ltd. All rights reserved.
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Scheme 1. The synthetic route of 1.
Therefore, in this work a novel fluorescent chemosensor (1) with a hexafluorocyclophentene unit as acceptor and a pyrazine unit as donor was designed and synthesized. It not only exhibited ESIPT character and has larger Stocks shift, but also showed high affinity to Cu2þ with high selectivity, excellent sensitivity, and short response time. The synthetic route of the chemosensor is showed in Scheme 1. The structural char acterization data are presented in Supplementary materials (Figs. S1–S7).
hexane 2.5 mol/L) was dropwise added to the mixture. After stirring for 30 min, compound 3 (3.04 g; 10 mmol) was slowly added to the above solution of mixture, and the solution was stirred for 2 h at 195 K. The reaction mixture was quenched with 10 mL water and extracted with CH2Cl2 (3 � 50 mL), the organic phase was then dried with anhydrous MgSO4, filtrated and evaporated. The crude product was purified by column chromatography on silica gel with petroleum ether/ethyl ace tate (20:1, v/v) as eluent. A light yellow oil was obtained (0.8 g, yield: 17.2%). 1H NMR (400 MHz, DMSO‑d6, TMS), δ (ppm): 7.44 (d, J ¼ 8.8 Hz, 1H), 7.37 (s, 1H), 7.14 (d, J ¼ 8.8 Hz, 1H), 6.81 (s, 1H), 5.94 (s, 1H), 3.86 (t, J ¼ 6.8 Hz, 2H), 3.83 (s, 3H), 3.77 (t, J ¼ 6.8 Hz, 2H), 2.43 (s, 3H), 1.85 (s, 3H). 13C NMR (101 MHz, DMSO‑d6, TMS), δ (ppm): δ 159.3, 138.3, 137.7, 130.6, 127.0, 125.1, 123.3, 118.5, 112.1, 97.4, 64.6, 55.8, 14.6, 13.3. ESI-MS (ESI, m/z): [Mþ] calcd of C21H18F6O3S, 464.1; found, 465.1 [M þ Hþ]þ.
2. Experimental 2.1. General methods All solvents were analytical grade and distilled prior to use. Other reagents were purchased from commercial sources and can be directly used without further purification. 1H NMR and 13C NMR spectra were recorded by a Bruker AV400 (400 MHz) NMR spectrometer using tet ramethylsilane as the internal standard. Mass spectra were performed using an Agilent 1100 ion trap LC/MS MSD system. The metal ion so lutions were prepared in water with metal nitrate salts, except for Fe2þ, Ba2þ and Cuþ (all of their counter anions were chloride ions). EDTA solution was prepared with ethylenediaminetetraacetic acid disodium salt (Na2EDTA) (0.2 mmol) in distilled water (2 mL). Fluorescence spectra were measured with the Hitachi F-4600 fluorescence spectro photometer. UV/vis absorption spectra were recorded on an Agilent 8454 UV/vis spectrophotometer equipped with an MUA-165 UV lamp and MVL-210 visible lamp for photoirradiation. The fluorescence quantum yield was determined with a QYC11347-11 absolute PL quantum yield spectrometer. Melting point was measured by a WRS-1B melting point apparatus.
2.4. Synthesis of compound 5 Compound 4 (0.6 g, 1.3 mmol), p-toluenesulfonic acid (0.02 g) and pyridine (1 mL) were dissolved in mixed solvent of acetone (40 mL) and water (20 mL). Stopping the reaction after refluxing and stirring for 24 h, the mixture was extracted with CH2Cl2 (3 � 20 mL). Then, the organic layer was washed with saturated salt solution, dried over anhydrous Na2SO4, filtrated and evaporated. The crude product was purified by column chromatography on silica gel with petroleum ether/ ethyl acetate (15:1, v/v) as eluent. A yellow oil was obtained with 89.7% yield. 1H NMR (400 MHz, DMSO‑d6, TMS), δ (ppm): 10.28 (s, 1H), 7.73 (s, 1H), 7.56 (d, J ¼ 8.8 Hz, 1H), 7.34 (d, J ¼ 8.8 Hz, 1H), 6.83 (s, 1H), 3.95 (s, 3H), 2.43 (s, 3H), 1.85 (s, 3H). 13C NMR (101 MHz, DMSO‑d6, TMS), δ (ppm): 188.2, 162.9, 138.4, 138.2, 135.9, 127.7, 125.0, 124.4, 122.8, 119.2, 114.2, 56.5, 14.7, 13.5. ESI-MS (ESI, m/z): [Mþ] calcd of C19H14F6O2S, 420.0; found, 419.0 [M Hþ] .
2.2. Synthesis The synthesis of 1 is illustrated in Scheme 1. The synthesis details of the intermediate compounds are showed as follows. Therein compound 3 was obtained according the reported method [50,51].
2.5. Synthesis of compound 6 To a 50 mL of anhydrous CH2Cl2 solution of compound 5 (0.8 g, 1.9 mmol) was added, then, 5 mL of BBr3 (1.0 mol/L) was dropwise added to the above solution under nitrogen atmosphere at 195 K. After stirring for 3 h at 195 K, the reaction mixture was continued to be stirred for 1–2 days at room temperature. The reaction was quenched with 10 mL water and extracted with CH2Cl2 (3 � 20 mL). The organic layer was washed with brine, dried over MgSO4, filtrated and evaporated. The crude product was purified by column chromatography on silica gel using petroleum ether/ethyl acetate (20:1, v/v) as eluent. The final
2.3. Synthesis of compound 4 Compound 2 (2.15 g, 10 mmol), glycol (3 mL), and p-toluenesulfonic acid (0.05 g) were dissolved in 200 mL toluene, the reaction mixture was refluxed for 12 h under the Dean-Stark condition. Then, evaporating the solvent, the residue was directly dissolved in 60 mL of anhydrous THF. Under the conditions of 195 K and argon atmosphere, 4 mL of n-BuLi (in 2
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(s, 2H), 2.36 (s, 3H). 13C NMR (101 MHz, DMSO‑d6, TMS) δ (ppm): 153.6, 147.3, 147.1, 146.1, 139.4, 133.6, 129.7, 129.5, 128.8, 124.0, 123.0, 122.5, 22.6. ESI-MS (ESI, m/z): [Mþ] calcd of C23H20N4, 352.1; found, 353.2, [M þ Hþ]þ. 2.7. Synthesis of compound 1
Fig. 1. Absorption and Fluorescence (λex ¼ 413 nm) (2.0 � 10 5 mol L 1) in THF at room temperature.
spectral
of
Compound 6 (0.08 g, 0.2 mmol) and compound 9 (0.08 g, 0.22 mmol) were added to 5 mL of anhydrous ethanol, after the reaction mixture was stirred for 8 h at room temperature, obtaining few amounts of orange precipitated solid. Then, the mixture was put into a refriger ator overnight. The precipitated orange solid was filtered and washed with 20 mL of anhydrous ethanol to gain the target compound 1 (0.105 g, yield: 71%). M.p.: 463–464 K. 1H NMR (400 MHz, THF-d8 TMS), δ (ppm): 13.61 (s, 1H), 9.55 (s, 1H), 8.58 (s, 1H), 7.86 (s, 1H), 7.62 (d, J ¼ 8.6 Hz, 2H), 7.34 (d, J ¼ 8.4 Hz, 1H), 7.29 (t, J ¼ 8.0 Hz, 4H), 7.14 (d, J ¼ 8.0 Hz, 4H), 7.12 (d, J ¼ 8.8 Hz, 2H), 7.06 (t, J ¼ 7.6 Hz, 2H), 6.97 (d, J ¼ 8.8 Hz, 1H), 6.81 (s, 1H), 2.71 (s, 3H), 2.46 (s, 3H), 1.93 (s, 3H). 13C NMR (101 MHz, THF-d8, TMS) δ (ppm): 165.9, 164.9, 153.6, 151.2, 151.0, 150.8, 149.6, 148.5, 139.5, 135.1, 134.9, 132.8, 131.2, 130.3, 126.2, 125.9, 124.9, 124.4, 122.8, 120.6, 120.1, 119.2, 23.8, 15.0, 14.0. ESI-MS (ESI, m/z): [Mþ] calcd of C41H30F6N4OS, 740.2; found, 741.2, [M þ Hþ]þ.
1
product was obtained as faint yellow solid, yield 32.4%. M. p. ¼ 369–370 K. 1H NMR (400 MHz, DMSO‑d6, TMS), δ (ppm): 11.50 (s, 1H), 10.24 (s, 1H), 7.71 (s, 1H), 7.38 (d, J ¼ 8.8 Hz, 1H), 7.04 (d, J ¼ 8.8 Hz, 1H), 6.81 (s, 1H), 2.42 (s, 3H), 1.84 (s, 3H). 13C NMR (101 MHz, DMSO‑d6, TMS), δ (ppm): 189.5, 162.5, 138.3, 138.1, 135.6, 128.5, 125.0, 122.9, 122.8, 118.6, 118.0, 14.7, 13.4. ESI-MS (ESI, m/z): [Mþ] calcd of C18H12F6O2S, 406.0; found, 405.0 [M Hþ] .
3. Results and discussion 3.1. Absorption and fluorescence spectral of 1
2.6. Synthesis of compound 9
The UV–visible absorption and fluorescence spectra of 1 (2.0 � 10 5 mol L 1) were investigated in THF at room temperature. As shown in Fig. 1, two absorption peaks were observed at 302 nm (ε ¼ 4.03 � 104 L mol 1 cm 1) and 413 nm (ε ¼ 2.30 � 104 L mol 1 cm 1), respectively. The absorption band in the UV region was owing to the π→π* transition [52], while the absorption band in the visible region was ascribed to the charge-transfer transition [53]. At the same time, a broad fluores cence emission band centered at 629 nm was observed when excited at 413 nm. The absolute fluorescence quantum yield was determined to be 0.034, and the Stokes shift was 216 nm. The result is due to the
Compound 9 was prepared by reacting compound 7 (3.18 g; 11 mmol) with compound 8 (1.88 g; 10 mmol) in the presence of Pd (PPh3)4 (250 mg) and Na2CO3 (6.36 g; 60 mmol) in THF (120 mL con taining 10% water) for 12 h at 343 K. Compound 9 was purified by column chromatography on silica gel using petroleum ether/ethyl ace tate (15:1, v/v) as eluent. A yellow solid was obtained with 90% yield. M.p.: 454–455 K. 1H NMR (400 MHz, DMSO‑d6 TMS), δ (ppm): 7.78 (s, 1H), 7.43 (d, J ¼ 8.4 Hz, 2H), 7.30 (t, J ¼ 7.6 Hz, 4H), 7.04 (d, J ¼ 8.0 Hz, 4H), 7.03 (d, J ¼ 5.2 Hz, 2H), 6.99 (d, J ¼ 8.4 Hz, 2H), 6.35
Scheme 2. (A) Fast enol – form (E) to keto – form (K) tautomerization process; (B) the proposed sensing mechanism of 1 with Cu2þ. 3
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Fig. 2. Fluorescence changes of 1 (2.0 � 10 5 mol L 1) induced by the addition of various metal ions (2.0 equiv.) in THF: (A) Fluorescence spectra (λex ¼ 413 nm); (B) Emission intensity at 629 nm; (C) Fluorescence colors of 1 in the presence of different metal ions. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
photoinduced ESIPT tautomerization process resulting in the photo induced electron transfer (PET) was inhibited (Scheme 2) [40,54–56].
under 365 nm light that could be observed by naked eyes, while other metal ions above mentioned could not induce any change in the emis sion intensity and fluorescent color (Fig. 2B and C). These results indi cated that 1 could act as a Cu2þ selective fluorescence chemosensor.
3.2. Selectivity studies
3.3. Fluorescence spectral responses of 1 toward Cu2þ
The fluorescence responses experiments of 1 towards all kinds of metal ions including Cu2þ, Zn2þ, Ba2þ, Mg2þ, Ca2þ, Kþ, Ni2þ, Sr2þ, Co2þ, Mn2þ, Cd2þ, Hg2þ, Pb2þ, Cr3þ, Al3þ, Agþ, Cuþ, Fe2þ and Fe3þ were performed in THF at room temperature. As shown in Fig. 2 A, with the addition of 2.0 equiv. of different metal ions to the solution of 1, only Cu2þ induced a complete fluorescence quenching to the fluorescence of 1. Simultaneously, the fluorescent color change from orange to black
The fluorescence titration experiments were carried out in THF to investigate the interaction between 1 and Cu2þ. As demonstrated in Fig. 3, the fluorescence emission intensity of 1 at 629 nm decreased gradually with the addition of Cu2þ, when the amount of added Cu2þ reached 2.0 equivalents of 1, the emission intensity of 1 was quenched 4
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by ca. 91%, and the emission intensity no longer changed with further titration. Meanwhile, the fluorescence color of 1 changed from orange to dark under 365 nm light. The back titration with 5.0 equiv. EDTA could not recover the fluorescence spectrum of 1, indicating that the interac tion between 1 and Cu2þ was not reversible. The obvious fluorescence quenching may be explained by that the coordination of Cu2þ with the phenolic hydroxyl group of 1 restrained the ESIPT process, as well as the chelation-enhanced fluorescence quenching (CHEQ) effect and para magnetic of Cu2þ (Scheme 2) [57,58]. From the inset of Fig. 3, an excellent linear relationship (R ¼ 0.993) between the addition of Cu2þ and the fluorescence intensity of 1 was displayed in the range of 2.0 � 10 6 - 1.4 � 10 5 mol L 1. According to the reported method [59], the limit of detection (LOD) of 1 for Cu2þ was calculated to be 2.47 � 10 8 mol L 1 (Fig. S8). The result showed that 1 has a higher sensitivity to Cu2þ. To further study the binding mode of 1 with Cu2þ, Job’s plot analysis was carried out. The result showed that when the molar fraction was 0.5, the vertical coordinates value reached the minimum (Fig. 4), indicating the binding stoichiometry between 1 and
Fig. 3. Emission spectral and fluorescent color changes of 1 (2.0 � 10 5 mol L 1) induced by Cu2þ (λex ¼ 413 nm) (inset: the emission in tensity curve of 1 at 629 nm with the addition of Cu2þ). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 4. Job’s plot showing 1:1 complex of 1 (2.0 � 10
5
mol L
Fig. 6. Fluorescence intensity (λex ¼ 413 nm) of 1 (2.0 � 10 5 mol L 1) in THF in the present of competing ions. Black bars represent the addition of 2.0 equiv. of various metal ions to the solution of 1. Red bars represent the addition of Cu2þ (2.0 equiv.) to the above solution. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
1
) with Cu2þ.
Fig. 5. 1H NMR spectra of 1 and 1 - Cu2þ in THF-d8 at room temperature. 5
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Fig. 7. Photograph showing the fluorescent color change of 1 on test paper strips after treating with various metal ions under 365 nm light.
out. In ESI-MS spectra, 1 exhibited two characteristic peaks at m/ z ¼ 741.2 and 763.2 were assigned to [1 þ Hþ]þ (calcd: 741.2) and [1 þ Naþ]þ (calcd: 763.2), respectively (Fig. S7 A). With the addition of Cu2þ, two new peaks located at m/z ¼ 864.0 and 865.0 were observed (Fig. S7 B), due to the formation of [1 Hþ þ Cu2þ þ NO3 ]⋅þ (calcd: 864.1) and [1 þ Cu2þ þ NO3 ]þ (calcd: 865.1), respectively. The result further proved the 1:1 binding mode between 1 and Cu2þ. The 1H NMR titration experiments of 1 were performed in THF-d8. With the addition of Cu2þ, the signal at 13.61 ppm belong to the hydroxyl (a) almost disappeared, indicating the –OH participated in the coordination. The signals of at – N of the Schiff base unit 9.55 ppm and 8.59 ppm belonging to the CH– – (b) and the CH– N of pyrazine (c) ring, respectively, were decreased obviously. The results indicated that the N atoms of Schiff base unit and pyrazine also participated in the formation of coordination bonds. On the basis of the experimental results and the 1:1 stoichiometry, the proposed complexation mode between 1 and Cu2þ was shown in Fig. 5. 3.5. Practical application
Fig. 8. Fluorescence photos of different concentrations Cu2þ( � 10 6 mol L 1) in solution (A) and on test paper strips (B).
In order to verify the possibility of practical application of sensor 1, the competitive experiments were carried out in THF at room temper ature. As shown in Fig. 6, before the addition of Cu2þ, various metal ions were added to the test solution of sensor 1 respectively, the emission intensities of 1 almost have no obvious changes. After adding 2.0 equiv. of Cu2þ to the above mentioned solution respectively, the fluorescence emission intensities were decreased and similar to that of 1 and Cu2þ. The results suggested that 1 could be used for the detection of Cu2þ in practical application with high selectivity and excellent antiinterference character. The test strips experiments were performed. Firstly, compound 1 (3.7 mg, 0.005 mmol) was dissolved in THF (10 mL). Then, the filter papers were immersed into the solution of compound 1 (5.0 � 10 4 mol L 1) for 5 s and dried in vacuum drying oven. As shown in Fig. 7, the test paper strips exhibited orange fluorescence that could be seen by naked eyes under 365 nm light. After treating with different metal ions, only Cu2þ caused a notable fluorescence quenching. The results demonstrated that the sensor 1 could be used as a qualitative test paper strip for a quick detection of Cu2þ. Moreover, the quantitative detections for Cu2þ were also demonstrated with the solution of sensor 1 and the test paper strips. As shown in Fig. 8, when the concentration of Cu2þ increased from 0 to 4.0 � 10 5 mol L 1, the fluorescent color of the solution and the test paper strips gradually changed from orange to black, which could be observed under 365 nm light with naked eyes. The results indicated that sensor 1 could also be made into test paper strips for quantitative detecting Cu2þ. In addition, the applications of sensor 1 for the detection of Cu2þ in practical water samples were studied systematically. The practical water samples were got from Ganjiang River, Qingshan Lake, and tap water of Nanchang City, Jiangxi province. According to the reported methods [61,62], the water samples were filtered with 0.2 μm membrane firstly to remove the large particle impurities. Then different concentrations of Cu2þ solution were spiked to the practical samples. The detection results and recoveries in different samples were summarized in Table 1. The recoveries for Cu2þ are ranged from 97.2% to 103.4%. The results indicated that the sensor 1 could be used to detect Cu2þ in practical water samples with high accuracy.
of
Table 1 Detection of Cu2þ in practical samples. Sample
Cu2þ spiked (μmol L 1)
Cu2þ recovered (μmol L 1)
Recovery (%)
Tap water
1.00 2.00 4.00 1.00 2.00 4.00 1.00 2.00 4.00
1.01 1.99 3.89 0.99 1.97 4.14 0.99 2.02 4.08
101.0 99.5 97.2 99.0 98.5 103.4 99.0 101.0 102.0
Qingshan lake Ganjiang river
Each test was repeated 3 times.
Cu2þ was 1:1. Based on this binding mode and the Benesi-Hildebrand equation [60], the association constant (Ka) of 1 with Cu2þ was calcu lated to be 1.38 � 105 L mol 1 (Fig. S9). 3.4. The complexation mechanism of 1 with Cu2þ In order to explore the complexation mechanism between 1 and Cu2þ, the mass spectrum and NMR titration experiments were carried 6
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4. Conclusion
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In conclusion, a novel fluorescent turn-off sensor with a pyrazine unit for the fast and selective detection of Cu2þ was designed and synthe sized. The sensor exhibited ESIPT process and has large Stokes shift (216 nm) under the inducement of light. The sensor not only could be used to determine Cu2þ in practical water samples accurately, but also could be made into test paper strips for qualitative and quantitative determination of Cu2þ. The results in this study will provide new ideas for the application of novel fluorescence sensor in practical samples in the future. Conflicts of interest There are no conflicts of interest to declare. Acknowledgements The authors are grateful for the financial support from the National Natural Science Foundation of China (41867053), the “5511” science and technology innovation talent project of Jiangxi (2016BCB18015), the key project of Natural Science Foundation of Jiangxi Province (20171ACB20025), the Young Talents Project of Jiangxi Science and Technology Normal University (2015QNBJRC004), the Project of Jiangxi Science and Technology Normal University Advantage Sci-Tech Innovative Team (2015CXTD002). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.dyepig.2019.107914. References [1] Feng S, Gao Q, Gao X, Yin J, Jiao Y. Fluorescent sensor for copper(II) ion based on coumarin derivative and its application in cell imaging. Inorg Chem Commun 2019;102:51–6. [2] Ramdass A, Sathish V, Babu E, Velayudham M, Thanasekaran P, Rajagopal S. Recent developments on optical and electrochemical sensing of copper (II) ion based on transition metal complexes. Coord Chem Rev 2017;343:278–307. [3] Isarankura-Na-Ayudhya C, Tantimongcolwat T, Galla HJ, Prachayasittikul V. Fluorescent protein-based optical biosensor for copper ion quantitation. Biol Trace Elem Res 2010;134(3):352–63. [4] Ziegler BE, Marta RA, Burt MB, McMahon TB. Insight into the gas-phase structure of a copper (II) L-histidine complex, the agent used to treat Menkes disease. Inorg Chem 2014;53(5):2349–51. [5] Lutsenko S. Atp7b / mice as a model for studies of Wilson’s disease. Biochem Soc Trans 2008;36(6):1233–8. [6] Savelieff MG, Lee S, Liu Y, Lim MH. Untangling amyloid-β, tau, and metals in Alzheimer’s disease. ACS Chem Biol 2013;8(5):856–65. [7] Vonk WIM, Kakkar V, Bartuzi P, Jaarsma D, Berger R, Hofker MH, et al. The Copper Metabolism MURR1 domain protein 1 (COMMD1) modulates the aggregation of misfolded protein species in a client-specific manner. PLoS One 2014;9(4). e92408. [8] McDonald AJ, Dibble JP, Evans EGB, Millhauser GL. A new paradigm for enzymatic control of α-cleavage and β-cleavage of the prion protein. J Biol Chem 2014;289(2): 803–13. [9] Xiao G, Fan Q, Wang X, Zhou B. Huntington disease arises from a combinatory toxicity of polyglutamine and copper binding. Proc Natl Acad Sci 2013;110(37): 14995–5000. [10] Jung HS, Kwon PS, Lee JW, Kim J, Hong CS, Kim JW, et al. Coumarin-derived Cu2 þ -selective fluorescence sensor: synthesis, mechanisms, and applications in living cells. J Am Chem Soc 2009;131(5):2008–12. [11] Chandra R, Ghorai A, Patra GK. A simple benzildihydrazone derived colorimetric and fluorescent ‘on-off-on’ sensor for sequential detection of copper (II) and cyanide ions in aqueous solution. Sens Actuators B Chem 2018;255:701–11. [12] Okoye COB, Ugwu JN. Impact of environmental cadmium, lead, copper and zinc on quality of goat meat in Nigeria. Bull Chem Soc Ethiop 2010;24(1):133–8. [13] Lee MR, Correa JA, Seed R. A sediment quality triad assessment of the impact of copper mine tailings disposal on the littoral sedimentary environment in the Atacama region of northern Chile. Mar Pollut Bull 2006;52(11):1389–95. [14] Soylak M, Narin I, Dogan M. Trace enrichment and atomic absorption spectrometric determination of lead, copper, cadmium and nickel in drinking water samples by use of an activated carbon column. Anal Lett 1997;30(15):2801–10. [15] Forzani ES, Zhang H, Chen W, Tao N. Detection of heavy metal ions in drinking water using a high-resolution differential surface plasmon resonance sensor. Environ Sci Technol 2005;39(5):1257–62.
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[46] Paterson MJ, Robb MA, Blancafort L, DeBellis AD. Mechanism of an exceptional class of photostabilizers: a seam of conical intersection parallel to excited state intramolecular proton transfer (ESIPT) in o-hydroxyphenyl-(1, 3, 5)-triazine. J Phys Chem A 2005;109(33):7527–37. [47] Chen DY, Chen CL, Cheng YM, Lai CH, Yu JY, Chen BS, et al. Design and synthesis of trithiophene-bound excited-state intramolecular proton transfer dye: enhancement on the performance of bulk heterojunction solar cells. ACS Appl Mater Interfaces 2010;2(6):1621–9. [48] Mai VTN, Shukla A, Mamada M, Maedera S, Shaw PE, Sobus J, et al. Low amplified spontaneous emission threshold and efficient electroluminescence from a carbazole derivatized excited-state intramolecular proton transfer dye. ACS Photonics 2018;5 (11):4447–55. [49] Wang Y, Hu Y, Wu T, Zhou X, Shao Y. Triggered excited-state intramolecular proton transfer fluorescence for selective triplex DNA recognition. Anal Chem 2015;87(23):11620–4. [50] Pu S, Liu W, Miao W. Photochromism of new unsymmetrical isomeric diarylethenes bearing a methoxyl group. J Phys Org Chem 2009;22(10):954–63. [51] Li ZX, Liao LY, Sun W, Xu CH, Zhang C, Fang CJ, et al. Reconfigurable cascade circuit in a photo-and chemical-switchable fluorescent diarylethene derivative. J Phys Chem C 2008;112(13):5190–6. [52] Ding H, Li B, Pu S, Liu G, Jia D, Zhou Y. A fluorescent sensor based on a diarylethene-rhodamine derivative for sequentially detecting Cu2þ and arginine and its application in keypad lock. Sens Actuators B Chem 2017;247:26–35. [53] Maslak P, Chopra A, Moylan CR, Wortmann R, Lebus S, Rheingold AL, et al. Optical properties of spiroconjugated charge-transfer dyes. J Am Chem Soc 1996;118(6): 1471–81.
[54] Sedgwick AC, Wu L, Han HH, Bull SD, He XP, James TD, et al. Excited-state intramolecular proton-transfer (ESIPT) based fluorescence sensors and imaging agents. Chem Soc Rev 2018;47(23):8842–80. [55] Reja SI, Kumar N, Sachdeva R, Bhalla V, Kumar M. d-PET coupled ESIPT phenomenon for fluorescent turn-on detection of hydrogen sulfide. RSC Adv 2013; 3(39):17770–4. [56] Yue X, Wang Z, Li C, Yang Z. Naphthalene-derived Al3þ-selective fluorescent chemosensor based on PET and ESIPT in aqueous solution. Tetrahedron Lett 2017; 58(48):4532–7. [57] Jeong Y, Yoon J. Recent progress on fluorescent chemosensors for metal ions. Inorg Chim Acta 2012;381:2–14. [58] Hao X, Han S, Zhu J, Hu Y, Chang LY, Pao CW, et al. A bis-benzimidazole PMO ratiometric fluorescence sensor exhibiting AIEE and ESIPT for sensitive detection of Cu2þ. RSC Adv 2019;9(24):13567–75. [59] Wang H, Wang B, Shi Z, Tang X, Dou W, Han Q, et al. A two-photon probe for Al3þ in aqueous solution and its application in bioimaging. Biosens Bioelectron 2015;65: 91–6. [60] Li G, Ma L, Liu G, Fan C, Pu S. A diarylethene-based “on–off–on” fluorescence sensor for the sequential recognition of mercury and cysteine. RSC Adv 2017;7(33): 20591–6. [61] Qu S, Zheng C, Liao G, Fan C, Liu G, Pu S. A fluorescent chemosensor for Sn2þ and Cu2þ based on a carbazole-containing diarylethene. RSC Adv 2017;7(16):9833–9. [62] Wu X, Chen J, Zhao JX. A reversible fluorescent logic gate for sensing mercury and iodide ions based on a molecular beacon. Analyst 2013;138(18):5281–7.
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Dyes and Pigments journal homepage: http://www.elsevier.com/locate/dyepig
A high selective fluorescent sensor for Cu2þ in solution and test paper strips Fu Shi, Shiqiang Cui *, Hongling Liu, Shouzhi Pu ** Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal University, Nanchang, 330013, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Fluorescent sensor Stokes shift ESIPT Copper ion Test paper
A novel and efficient fluorescent sensor with a pyrazine triphenylamine unit was developed. The sensor exhibits large Stokes shift (216 nm) due to the excited-state intramolecular proton transfer (ESIPT) process. After com plexing with Cu2þ, the ESIPT process was blocked and the fluorescence of the sensor was quenched. Job’s plot and ESI-MS results indicated that the stoichiometric ratio between the sensor and Cu2þ was 1:1. The detection limit was determined to be 2.47 � 10 8 mol L 1. The sensor could be used to detect Cu2þ in practical samples with high accuracy. Furthermore, the sensor also could be made into test paper strips for the qualitative and quantitative detection of Cu2þ.
1. Introduction Copper is the third most indispensable abundant trace element in human body, and plays a crucial role in various biological processes, such as electron transfer in cellular respiration, as a cofactor in many enzymes, and as redox regulating components [1–3]. However, abnormal levels of copper ions in the body will leads to many human genetic disorders, such as Menkes [4], Wilson’s diseases [5], Alz heimer’s [6], Parkinson’s [7], Prion [8], and Huntington’s diseases [9], etc. The U.S. Environmental Protection Agency (EPA) has set the maximum tolerable concentration of copper ions in drinking water was 20 μM [10]. Nevertheless, owing to the widespread use of copper in the fields of agriculture, industry, chemistry, medicine, and biotechnology et al., the inevitable problem of pollution and the accompanying po tential toxic effects on human beings continue to pose challenges to the world [11–13]. Therefore, the development of methods of efficient and fast detection of copper ions in biological and environmental samples caused extensive concern around the world. There have many methods were established for detecting Cu2þ in the past few decades, including atomic absorption spectrometry (AAS) [14], surface plasmon resonance spectroscopy (SPR) [15], inductively coupled plasma atomic emission spectrometry (ICP-AES) [16], chro matography [17], voltammetry [18] and so on. They could detect Cu2þ with considerable sensitivity, but these methods require expensive equipment, high operating costs, professional operator, and long anal ysis time. Thus, it is necessary to develop reliable and convenient methods for the detection of Cu2þ. Different from above traditional
analytical methods, fluorescent sensors have attracted many researchers attention due to the advantages of inexpensive, high sensitivity, excel lent selectivity, and high response speed [19–23]. Up to now, various of fluorescent sensors for Cu2þ have been reported with different func tional groups, such as quinoline [24], coumarin [25,26], dansyl amide [27], naphthalimide [28], fluorescein [29], rhodamine [30–33] and so on. However some of the sensors still have poor selectivity, low sensi tivity, and long response time [34,35]. Therefore, it is significant to design and synthesis sensor with high selectivity and high sensitivity for Cu2þ. Pyrazine and its derivatives as food additives have important func tion to the flavor of a variety of roasted or otherwise cooked foods [36]. On the other side, as a heterocyclic compound, pyrazine has preferable affinity for metal ions, especially copper ions [37–39]. Nevertheless, up to present, the chemosensor with pyrazine unit has not been reported. Excited state intramolecular proton transfer (ESIPT) has attracted a lot of attentions [40–44], since it was first reported in 1956 [45]. In recent years, The ESIPT has aroused the development of various applications such as fluorescent chemosensors [43,44], photostabilizers [46], solar collectors [47], laser dyes [48,49], and so on. In the ESIPT mechanism, a fast proton transfer process from enol form (E-E*) to keto form (K–K*) was happened, which could enhance the stability of the electronic excited-state and reduce the possibility of photochemical reactions of excited molecules. For this reason, the ESIPT process also has a larger Stokes shift, which will plays a significant role in reduce the interference from biological matrix and increase the signal-to-noise ratio in biolog ical imaging [41,42].
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (S. Cui), [email protected] (S. Pu). https://doi.org/10.1016/j.dyepig.2019.107914 Received 29 July 2019; Received in revised form 14 September 2019; Accepted 20 September 2019 Available online 21 September 2019 0143-7208/© 2019 Elsevier Ltd. All rights reserved.
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Scheme 1. The synthetic route of 1.
Therefore, in this work a novel fluorescent chemosensor (1) with a hexafluorocyclophentene unit as acceptor and a pyrazine unit as donor was designed and synthesized. It not only exhibited ESIPT character and has larger Stocks shift, but also showed high affinity to Cu2þ with high selectivity, excellent sensitivity, and short response time. The synthetic route of the chemosensor is showed in Scheme 1. The structural char acterization data are presented in Supplementary materials (Figs. S1–S7).
hexane 2.5 mol/L) was dropwise added to the mixture. After stirring for 30 min, compound 3 (3.04 g; 10 mmol) was slowly added to the above solution of mixture, and the solution was stirred for 2 h at 195 K. The reaction mixture was quenched with 10 mL water and extracted with CH2Cl2 (3 � 50 mL), the organic phase was then dried with anhydrous MgSO4, filtrated and evaporated. The crude product was purified by column chromatography on silica gel with petroleum ether/ethyl ace tate (20:1, v/v) as eluent. A light yellow oil was obtained (0.8 g, yield: 17.2%). 1H NMR (400 MHz, DMSO‑d6, TMS), δ (ppm): 7.44 (d, J ¼ 8.8 Hz, 1H), 7.37 (s, 1H), 7.14 (d, J ¼ 8.8 Hz, 1H), 6.81 (s, 1H), 5.94 (s, 1H), 3.86 (t, J ¼ 6.8 Hz, 2H), 3.83 (s, 3H), 3.77 (t, J ¼ 6.8 Hz, 2H), 2.43 (s, 3H), 1.85 (s, 3H). 13C NMR (101 MHz, DMSO‑d6, TMS), δ (ppm): δ 159.3, 138.3, 137.7, 130.6, 127.0, 125.1, 123.3, 118.5, 112.1, 97.4, 64.6, 55.8, 14.6, 13.3. ESI-MS (ESI, m/z): [Mþ] calcd of C21H18F6O3S, 464.1; found, 465.1 [M þ Hþ]þ.
2. Experimental 2.1. General methods All solvents were analytical grade and distilled prior to use. Other reagents were purchased from commercial sources and can be directly used without further purification. 1H NMR and 13C NMR spectra were recorded by a Bruker AV400 (400 MHz) NMR spectrometer using tet ramethylsilane as the internal standard. Mass spectra were performed using an Agilent 1100 ion trap LC/MS MSD system. The metal ion so lutions were prepared in water with metal nitrate salts, except for Fe2þ, Ba2þ and Cuþ (all of their counter anions were chloride ions). EDTA solution was prepared with ethylenediaminetetraacetic acid disodium salt (Na2EDTA) (0.2 mmol) in distilled water (2 mL). Fluorescence spectra were measured with the Hitachi F-4600 fluorescence spectro photometer. UV/vis absorption spectra were recorded on an Agilent 8454 UV/vis spectrophotometer equipped with an MUA-165 UV lamp and MVL-210 visible lamp for photoirradiation. The fluorescence quantum yield was determined with a QYC11347-11 absolute PL quantum yield spectrometer. Melting point was measured by a WRS-1B melting point apparatus.
2.4. Synthesis of compound 5 Compound 4 (0.6 g, 1.3 mmol), p-toluenesulfonic acid (0.02 g) and pyridine (1 mL) were dissolved in mixed solvent of acetone (40 mL) and water (20 mL). Stopping the reaction after refluxing and stirring for 24 h, the mixture was extracted with CH2Cl2 (3 � 20 mL). Then, the organic layer was washed with saturated salt solution, dried over anhydrous Na2SO4, filtrated and evaporated. The crude product was purified by column chromatography on silica gel with petroleum ether/ ethyl acetate (15:1, v/v) as eluent. A yellow oil was obtained with 89.7% yield. 1H NMR (400 MHz, DMSO‑d6, TMS), δ (ppm): 10.28 (s, 1H), 7.73 (s, 1H), 7.56 (d, J ¼ 8.8 Hz, 1H), 7.34 (d, J ¼ 8.8 Hz, 1H), 6.83 (s, 1H), 3.95 (s, 3H), 2.43 (s, 3H), 1.85 (s, 3H). 13C NMR (101 MHz, DMSO‑d6, TMS), δ (ppm): 188.2, 162.9, 138.4, 138.2, 135.9, 127.7, 125.0, 124.4, 122.8, 119.2, 114.2, 56.5, 14.7, 13.5. ESI-MS (ESI, m/z): [Mþ] calcd of C19H14F6O2S, 420.0; found, 419.0 [M Hþ] .
2.2. Synthesis The synthesis of 1 is illustrated in Scheme 1. The synthesis details of the intermediate compounds are showed as follows. Therein compound 3 was obtained according the reported method [50,51].
2.5. Synthesis of compound 6 To a 50 mL of anhydrous CH2Cl2 solution of compound 5 (0.8 g, 1.9 mmol) was added, then, 5 mL of BBr3 (1.0 mol/L) was dropwise added to the above solution under nitrogen atmosphere at 195 K. After stirring for 3 h at 195 K, the reaction mixture was continued to be stirred for 1–2 days at room temperature. The reaction was quenched with 10 mL water and extracted with CH2Cl2 (3 � 20 mL). The organic layer was washed with brine, dried over MgSO4, filtrated and evaporated. The crude product was purified by column chromatography on silica gel using petroleum ether/ethyl acetate (20:1, v/v) as eluent. The final
2.3. Synthesis of compound 4 Compound 2 (2.15 g, 10 mmol), glycol (3 mL), and p-toluenesulfonic acid (0.05 g) were dissolved in 200 mL toluene, the reaction mixture was refluxed for 12 h under the Dean-Stark condition. Then, evaporating the solvent, the residue was directly dissolved in 60 mL of anhydrous THF. Under the conditions of 195 K and argon atmosphere, 4 mL of n-BuLi (in 2
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(s, 2H), 2.36 (s, 3H). 13C NMR (101 MHz, DMSO‑d6, TMS) δ (ppm): 153.6, 147.3, 147.1, 146.1, 139.4, 133.6, 129.7, 129.5, 128.8, 124.0, 123.0, 122.5, 22.6. ESI-MS (ESI, m/z): [Mþ] calcd of C23H20N4, 352.1; found, 353.2, [M þ Hþ]þ. 2.7. Synthesis of compound 1
Fig. 1. Absorption and Fluorescence (λex ¼ 413 nm) (2.0 � 10 5 mol L 1) in THF at room temperature.
spectral
of
Compound 6 (0.08 g, 0.2 mmol) and compound 9 (0.08 g, 0.22 mmol) were added to 5 mL of anhydrous ethanol, after the reaction mixture was stirred for 8 h at room temperature, obtaining few amounts of orange precipitated solid. Then, the mixture was put into a refriger ator overnight. The precipitated orange solid was filtered and washed with 20 mL of anhydrous ethanol to gain the target compound 1 (0.105 g, yield: 71%). M.p.: 463–464 K. 1H NMR (400 MHz, THF-d8 TMS), δ (ppm): 13.61 (s, 1H), 9.55 (s, 1H), 8.58 (s, 1H), 7.86 (s, 1H), 7.62 (d, J ¼ 8.6 Hz, 2H), 7.34 (d, J ¼ 8.4 Hz, 1H), 7.29 (t, J ¼ 8.0 Hz, 4H), 7.14 (d, J ¼ 8.0 Hz, 4H), 7.12 (d, J ¼ 8.8 Hz, 2H), 7.06 (t, J ¼ 7.6 Hz, 2H), 6.97 (d, J ¼ 8.8 Hz, 1H), 6.81 (s, 1H), 2.71 (s, 3H), 2.46 (s, 3H), 1.93 (s, 3H). 13C NMR (101 MHz, THF-d8, TMS) δ (ppm): 165.9, 164.9, 153.6, 151.2, 151.0, 150.8, 149.6, 148.5, 139.5, 135.1, 134.9, 132.8, 131.2, 130.3, 126.2, 125.9, 124.9, 124.4, 122.8, 120.6, 120.1, 119.2, 23.8, 15.0, 14.0. ESI-MS (ESI, m/z): [Mþ] calcd of C41H30F6N4OS, 740.2; found, 741.2, [M þ Hþ]þ.
1
product was obtained as faint yellow solid, yield 32.4%. M. p. ¼ 369–370 K. 1H NMR (400 MHz, DMSO‑d6, TMS), δ (ppm): 11.50 (s, 1H), 10.24 (s, 1H), 7.71 (s, 1H), 7.38 (d, J ¼ 8.8 Hz, 1H), 7.04 (d, J ¼ 8.8 Hz, 1H), 6.81 (s, 1H), 2.42 (s, 3H), 1.84 (s, 3H). 13C NMR (101 MHz, DMSO‑d6, TMS), δ (ppm): 189.5, 162.5, 138.3, 138.1, 135.6, 128.5, 125.0, 122.9, 122.8, 118.6, 118.0, 14.7, 13.4. ESI-MS (ESI, m/z): [Mþ] calcd of C18H12F6O2S, 406.0; found, 405.0 [M Hþ] .
3. Results and discussion 3.1. Absorption and fluorescence spectral of 1
2.6. Synthesis of compound 9
The UV–visible absorption and fluorescence spectra of 1 (2.0 � 10 5 mol L 1) were investigated in THF at room temperature. As shown in Fig. 1, two absorption peaks were observed at 302 nm (ε ¼ 4.03 � 104 L mol 1 cm 1) and 413 nm (ε ¼ 2.30 � 104 L mol 1 cm 1), respectively. The absorption band in the UV region was owing to the π→π* transition [52], while the absorption band in the visible region was ascribed to the charge-transfer transition [53]. At the same time, a broad fluores cence emission band centered at 629 nm was observed when excited at 413 nm. The absolute fluorescence quantum yield was determined to be 0.034, and the Stokes shift was 216 nm. The result is due to the
Compound 9 was prepared by reacting compound 7 (3.18 g; 11 mmol) with compound 8 (1.88 g; 10 mmol) in the presence of Pd (PPh3)4 (250 mg) and Na2CO3 (6.36 g; 60 mmol) in THF (120 mL con taining 10% water) for 12 h at 343 K. Compound 9 was purified by column chromatography on silica gel using petroleum ether/ethyl ace tate (15:1, v/v) as eluent. A yellow solid was obtained with 90% yield. M.p.: 454–455 K. 1H NMR (400 MHz, DMSO‑d6 TMS), δ (ppm): 7.78 (s, 1H), 7.43 (d, J ¼ 8.4 Hz, 2H), 7.30 (t, J ¼ 7.6 Hz, 4H), 7.04 (d, J ¼ 8.0 Hz, 4H), 7.03 (d, J ¼ 5.2 Hz, 2H), 6.99 (d, J ¼ 8.4 Hz, 2H), 6.35
Scheme 2. (A) Fast enol – form (E) to keto – form (K) tautomerization process; (B) the proposed sensing mechanism of 1 with Cu2þ. 3
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Fig. 2. Fluorescence changes of 1 (2.0 � 10 5 mol L 1) induced by the addition of various metal ions (2.0 equiv.) in THF: (A) Fluorescence spectra (λex ¼ 413 nm); (B) Emission intensity at 629 nm; (C) Fluorescence colors of 1 in the presence of different metal ions. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
photoinduced ESIPT tautomerization process resulting in the photo induced electron transfer (PET) was inhibited (Scheme 2) [40,54–56].
under 365 nm light that could be observed by naked eyes, while other metal ions above mentioned could not induce any change in the emis sion intensity and fluorescent color (Fig. 2B and C). These results indi cated that 1 could act as a Cu2þ selective fluorescence chemosensor.
3.2. Selectivity studies
3.3. Fluorescence spectral responses of 1 toward Cu2þ
The fluorescence responses experiments of 1 towards all kinds of metal ions including Cu2þ, Zn2þ, Ba2þ, Mg2þ, Ca2þ, Kþ, Ni2þ, Sr2þ, Co2þ, Mn2þ, Cd2þ, Hg2þ, Pb2þ, Cr3þ, Al3þ, Agþ, Cuþ, Fe2þ and Fe3þ were performed in THF at room temperature. As shown in Fig. 2 A, with the addition of 2.0 equiv. of different metal ions to the solution of 1, only Cu2þ induced a complete fluorescence quenching to the fluorescence of 1. Simultaneously, the fluorescent color change from orange to black
The fluorescence titration experiments were carried out in THF to investigate the interaction between 1 and Cu2þ. As demonstrated in Fig. 3, the fluorescence emission intensity of 1 at 629 nm decreased gradually with the addition of Cu2þ, when the amount of added Cu2þ reached 2.0 equivalents of 1, the emission intensity of 1 was quenched 4
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by ca. 91%, and the emission intensity no longer changed with further titration. Meanwhile, the fluorescence color of 1 changed from orange to dark under 365 nm light. The back titration with 5.0 equiv. EDTA could not recover the fluorescence spectrum of 1, indicating that the interac tion between 1 and Cu2þ was not reversible. The obvious fluorescence quenching may be explained by that the coordination of Cu2þ with the phenolic hydroxyl group of 1 restrained the ESIPT process, as well as the chelation-enhanced fluorescence quenching (CHEQ) effect and para magnetic of Cu2þ (Scheme 2) [57,58]. From the inset of Fig. 3, an excellent linear relationship (R ¼ 0.993) between the addition of Cu2þ and the fluorescence intensity of 1 was displayed in the range of 2.0 � 10 6 - 1.4 � 10 5 mol L 1. According to the reported method [59], the limit of detection (LOD) of 1 for Cu2þ was calculated to be 2.47 � 10 8 mol L 1 (Fig. S8). The result showed that 1 has a higher sensitivity to Cu2þ. To further study the binding mode of 1 with Cu2þ, Job’s plot analysis was carried out. The result showed that when the molar fraction was 0.5, the vertical coordinates value reached the minimum (Fig. 4), indicating the binding stoichiometry between 1 and
Fig. 3. Emission spectral and fluorescent color changes of 1 (2.0 � 10 5 mol L 1) induced by Cu2þ (λex ¼ 413 nm) (inset: the emission in tensity curve of 1 at 629 nm with the addition of Cu2þ). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 4. Job’s plot showing 1:1 complex of 1 (2.0 � 10
5
mol L
Fig. 6. Fluorescence intensity (λex ¼ 413 nm) of 1 (2.0 � 10 5 mol L 1) in THF in the present of competing ions. Black bars represent the addition of 2.0 equiv. of various metal ions to the solution of 1. Red bars represent the addition of Cu2þ (2.0 equiv.) to the above solution. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
1
) with Cu2þ.
Fig. 5. 1H NMR spectra of 1 and 1 - Cu2þ in THF-d8 at room temperature. 5
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Fig. 7. Photograph showing the fluorescent color change of 1 on test paper strips after treating with various metal ions under 365 nm light.
out. In ESI-MS spectra, 1 exhibited two characteristic peaks at m/ z ¼ 741.2 and 763.2 were assigned to [1 þ Hþ]þ (calcd: 741.2) and [1 þ Naþ]þ (calcd: 763.2), respectively (Fig. S7 A). With the addition of Cu2þ, two new peaks located at m/z ¼ 864.0 and 865.0 were observed (Fig. S7 B), due to the formation of [1 Hþ þ Cu2þ þ NO3 ]⋅þ (calcd: 864.1) and [1 þ Cu2þ þ NO3 ]þ (calcd: 865.1), respectively. The result further proved the 1:1 binding mode between 1 and Cu2þ. The 1H NMR titration experiments of 1 were performed in THF-d8. With the addition of Cu2þ, the signal at 13.61 ppm belong to the hydroxyl (a) almost disappeared, indicating the –OH participated in the coordination. The signals of at – N of the Schiff base unit 9.55 ppm and 8.59 ppm belonging to the CH– – (b) and the CH– N of pyrazine (c) ring, respectively, were decreased obviously. The results indicated that the N atoms of Schiff base unit and pyrazine also participated in the formation of coordination bonds. On the basis of the experimental results and the 1:1 stoichiometry, the proposed complexation mode between 1 and Cu2þ was shown in Fig. 5. 3.5. Practical application
Fig. 8. Fluorescence photos of different concentrations Cu2þ( � 10 6 mol L 1) in solution (A) and on test paper strips (B).
In order to verify the possibility of practical application of sensor 1, the competitive experiments were carried out in THF at room temper ature. As shown in Fig. 6, before the addition of Cu2þ, various metal ions were added to the test solution of sensor 1 respectively, the emission intensities of 1 almost have no obvious changes. After adding 2.0 equiv. of Cu2þ to the above mentioned solution respectively, the fluorescence emission intensities were decreased and similar to that of 1 and Cu2þ. The results suggested that 1 could be used for the detection of Cu2þ in practical application with high selectivity and excellent antiinterference character. The test strips experiments were performed. Firstly, compound 1 (3.7 mg, 0.005 mmol) was dissolved in THF (10 mL). Then, the filter papers were immersed into the solution of compound 1 (5.0 � 10 4 mol L 1) for 5 s and dried in vacuum drying oven. As shown in Fig. 7, the test paper strips exhibited orange fluorescence that could be seen by naked eyes under 365 nm light. After treating with different metal ions, only Cu2þ caused a notable fluorescence quenching. The results demonstrated that the sensor 1 could be used as a qualitative test paper strip for a quick detection of Cu2þ. Moreover, the quantitative detections for Cu2þ were also demonstrated with the solution of sensor 1 and the test paper strips. As shown in Fig. 8, when the concentration of Cu2þ increased from 0 to 4.0 � 10 5 mol L 1, the fluorescent color of the solution and the test paper strips gradually changed from orange to black, which could be observed under 365 nm light with naked eyes. The results indicated that sensor 1 could also be made into test paper strips for quantitative detecting Cu2þ. In addition, the applications of sensor 1 for the detection of Cu2þ in practical water samples were studied systematically. The practical water samples were got from Ganjiang River, Qingshan Lake, and tap water of Nanchang City, Jiangxi province. According to the reported methods [61,62], the water samples were filtered with 0.2 μm membrane firstly to remove the large particle impurities. Then different concentrations of Cu2þ solution were spiked to the practical samples. The detection results and recoveries in different samples were summarized in Table 1. The recoveries for Cu2þ are ranged from 97.2% to 103.4%. The results indicated that the sensor 1 could be used to detect Cu2þ in practical water samples with high accuracy.
of
Table 1 Detection of Cu2þ in practical samples. Sample
Cu2þ spiked (μmol L 1)
Cu2þ recovered (μmol L 1)
Recovery (%)
Tap water
1.00 2.00 4.00 1.00 2.00 4.00 1.00 2.00 4.00
1.01 1.99 3.89 0.99 1.97 4.14 0.99 2.02 4.08
101.0 99.5 97.2 99.0 98.5 103.4 99.0 101.0 102.0
Qingshan lake Ganjiang river
Each test was repeated 3 times.
Cu2þ was 1:1. Based on this binding mode and the Benesi-Hildebrand equation [60], the association constant (Ka) of 1 with Cu2þ was calcu lated to be 1.38 � 105 L mol 1 (Fig. S9). 3.4. The complexation mechanism of 1 with Cu2þ In order to explore the complexation mechanism between 1 and Cu2þ, the mass spectrum and NMR titration experiments were carried 6
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4. Conclusion
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In conclusion, a novel fluorescent turn-off sensor with a pyrazine unit for the fast and selective detection of Cu2þ was designed and synthe sized. The sensor exhibited ESIPT process and has large Stokes shift (216 nm) under the inducement of light. The sensor not only could be used to determine Cu2þ in practical water samples accurately, but also could be made into test paper strips for qualitative and quantitative determination of Cu2þ. The results in this study will provide new ideas for the application of novel fluorescence sensor in practical samples in the future. Conflicts of interest There are no conflicts of interest to declare. Acknowledgements The authors are grateful for the financial support from the National Natural Science Foundation of China (41867053), the “5511” science and technology innovation talent project of Jiangxi (2016BCB18015), the key project of Natural Science Foundation of Jiangxi Province (20171ACB20025), the Young Talents Project of Jiangxi Science and Technology Normal University (2015QNBJRC004), the Project of Jiangxi Science and Technology Normal University Advantage Sci-Tech Innovative Team (2015CXTD002). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.dyepig.2019.107914. References [1] Feng S, Gao Q, Gao X, Yin J, Jiao Y. Fluorescent sensor for copper(II) ion based on coumarin derivative and its application in cell imaging. Inorg Chem Commun 2019;102:51–6. [2] Ramdass A, Sathish V, Babu E, Velayudham M, Thanasekaran P, Rajagopal S. Recent developments on optical and electrochemical sensing of copper (II) ion based on transition metal complexes. Coord Chem Rev 2017;343:278–307. [3] Isarankura-Na-Ayudhya C, Tantimongcolwat T, Galla HJ, Prachayasittikul V. Fluorescent protein-based optical biosensor for copper ion quantitation. Biol Trace Elem Res 2010;134(3):352–63. [4] Ziegler BE, Marta RA, Burt MB, McMahon TB. Insight into the gas-phase structure of a copper (II) L-histidine complex, the agent used to treat Menkes disease. Inorg Chem 2014;53(5):2349–51. [5] Lutsenko S. Atp7b / mice as a model for studies of Wilson’s disease. Biochem Soc Trans 2008;36(6):1233–8. [6] Savelieff MG, Lee S, Liu Y, Lim MH. Untangling amyloid-β, tau, and metals in Alzheimer’s disease. ACS Chem Biol 2013;8(5):856–65. [7] Vonk WIM, Kakkar V, Bartuzi P, Jaarsma D, Berger R, Hofker MH, et al. The Copper Metabolism MURR1 domain protein 1 (COMMD1) modulates the aggregation of misfolded protein species in a client-specific manner. PLoS One 2014;9(4). e92408. [8] McDonald AJ, Dibble JP, Evans EGB, Millhauser GL. A new paradigm for enzymatic control of α-cleavage and β-cleavage of the prion protein. J Biol Chem 2014;289(2): 803–13. [9] Xiao G, Fan Q, Wang X, Zhou B. Huntington disease arises from a combinatory toxicity of polyglutamine and copper binding. Proc Natl Acad Sci 2013;110(37): 14995–5000. [10] Jung HS, Kwon PS, Lee JW, Kim J, Hong CS, Kim JW, et al. Coumarin-derived Cu2 þ -selective fluorescence sensor: synthesis, mechanisms, and applications in living cells. J Am Chem Soc 2009;131(5):2008–12. [11] Chandra R, Ghorai A, Patra GK. A simple benzildihydrazone derived colorimetric and fluorescent ‘on-off-on’ sensor for sequential detection of copper (II) and cyanide ions in aqueous solution. Sens Actuators B Chem 2018;255:701–11. [12] Okoye COB, Ugwu JN. Impact of environmental cadmium, lead, copper and zinc on quality of goat meat in Nigeria. Bull Chem Soc Ethiop 2010;24(1):133–8. [13] Lee MR, Correa JA, Seed R. A sediment quality triad assessment of the impact of copper mine tailings disposal on the littoral sedimentary environment in the Atacama region of northern Chile. Mar Pollut Bull 2006;52(11):1389–95. [14] Soylak M, Narin I, Dogan M. Trace enrichment and atomic absorption spectrometric determination of lead, copper, cadmium and nickel in drinking water samples by use of an activated carbon column. Anal Lett 1997;30(15):2801–10. [15] Forzani ES, Zhang H, Chen W, Tao N. Detection of heavy metal ions in drinking water using a high-resolution differential surface plasmon resonance sensor. Environ Sci Technol 2005;39(5):1257–62.
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