English

• 1.   Introduction

  More and more attention has been paid to chemosensor which can be applied to detect heavy metal ions in trace amounts, ^[1-2] since the heavy metal ions play a crucial role during numerous environmental and biological processes.^[3-6] In comparison with electrical approaches, the fluorescence chemosensors are advantageous in terms of their chemical analyses in a real-time manner, excellent selectivity and great sensitivity.^[7-8] Particularly, copper (Cu) plays a vital part in numerous biological processes, like gene expression, enzymatic function, and redox process, thus, it is an important trace element within human body.^[9-11] Nonetheless, the excessively great copper content will induce oxidative stress (OS) as well as neurodegenerative disorders, including Parkinson's disease (PD), Wilson's disease and Alzheimer's disease (AD).^[12-14] Therefore, it is of crucial importance to detect Cu²⁺ ion contents within the environment together with numerous other scientific fields, for the sake of protecting human health.

  Rhodamine, together with its derivatives, serves as the typical fluorescent dye, and it is highly applicable as the fluorescent chromophore, which is ascribed to the excellent photophysical performances, like great visible-light emission wavelengths, high quantum yield, and great extinction coefficients. Great efforts have been made to develop the rhodamine skeleton-containing selective Cu²⁺ fluorescence probes.^[15-19] Among them, a large amount of fluorescent and colorimetric sensors of rhodamine hydrazones towards Cu²⁺ ions have been developed using substituted salicylaldehyde, ^[20] cinnamaldehyde, ^[21] 1, 8-naphthyridine and quinoline-2-aldehyde, ^[22] coumarin-3-aldehyde, ^[23] indole-3- carboxaldehyde, ^[24] furfural, ^[25] formaldehyde.^[26] In addition, the pyrrole derivatives have strong physiological activity and good stability.^[27-28] Therefore, Rhodamine B hydrazide and ethyl 5-formyl-3, 4-dimethyl-pyrrole-2-car- boxylate were used to synthesize ethyl 5-formyl-3, 4-di- methyl-pyrrole-2-carboxylate rhodamine B hydrazone (R1) which could exhibits better performance.

  On the other hand, it has been demonstrated that Schiff bases bearing multi-substituted pyrrole unit can form the stable complexes with Cu²⁺ ions.^[29-30] As a matter of fact, according to our earlier findings, ethyl 5-formyl-3, 4-di- methyl-pyrrole-2-carboxylate-derived rhodamine B hydrazone might be used to be the Cu²⁺ selective sensor in dual-mode via ratiometric displacement as well as rhodamine ring opening approach.^[31] To continue our previous research, ethyl 5-formyl-3, 4-dimethyl-pyrrole-2-carboxy- late rhodamine B hydrazone (R1), together with the possible performances as the fluorescent and colorimetric sensor for Cu²⁺ detection, was reported in this study (Eq. 1). The results suggested that the probe R1 exhibited its function by forming the complex [R1-H+Cu+NO₃]. Firstly, the probe R1 followed by hydrolyzing to a fluorescent rhodamine B in water-containing mediums, which is quite different from the coordination mechanism in our previous work.^[31] In addition, the application of the probe in detecting Cu²⁺ in real water samples has also been demonstrated.

  2.   Results and discussion

  2.1   R1 selectivity towards Cu²⁺ as well as additional metal cations

  The probe R1 selectivity to a variety of metal cations was examined. According to Figure 1, the sensor R1 displayed nearly no absorption at the wavelength of 450~650 nm in the 10 mmol•L^-1 CH₃CN/HEPES (pH=7.40, V/V=5/5) buffer solution, which suggested that R1 existed as the spirocycle-closed form, and such observation is in good accordance with previous report.^[32] When Cu²⁺ was added into 2.5 μmol•L^-1 R1 buffer solution, the mixed solution showed marked absorbance within UV-vis spectra at the wavelength of 560 nm (Figure 1), and fluorescence emission of 585 nm (Figure 2, λ_ex=350 nm). This might be ascribed to the spirolactam ring opening induced by Cu²⁺.^[33-34] Nonetheless, for additional metal cations, including Zn²⁺, Pb²⁺, Na⁺, Mn²⁺, Mg²⁺, K⁺, Hg²⁺, Fe³⁺, Cr³⁺, Co²⁺, Cd²⁺, Ca²⁺, Al³⁺ and Ag⁺, except for Ni²⁺ (absorbance intensity at 560 nm was half of that addition of Cu²⁺), neither fluorescence emission of 585 nm nor absorbance enhancement at 560 nm was detected. It should be noted that Ni²⁺ might form a complex with R1 in spirolactam ring opening form, which is responsible for the UV-vis spectral change, while is non-emissive due to the paramagnetism effect of Ni²⁺.^[35] However, in the case of Cu²⁺, the intermediate complex between Cu²⁺ and R1 is not stable enough, which occurred a hydrolysis process under the tested condition to produce highly emissive rhodamine B (side infra), according to the fluorescence enhancement.

  Figure 1

  

  Figure 1.  UV-vis spectra of 2.5 μmol•L^-1 sensor R1 in buffered CH₃CN/HEPES (V:V=5:5) solution at pH 7.40 with 2 equiv. of metal ions

  Metal ions: Ag⁺, Al³⁺, Ca²⁺, Cd²⁺, Co²⁺, Cr³⁺, Cu²⁺, Fe³⁺, Hg²⁺, K⁺, Mg²⁺, Mn²⁺, Na⁺, Ni²⁺, Pb²⁺, Zn²⁺ and blank. The inset shows the change of the color with the addition of Cu²⁺/Ni²⁺ to R1 in buffered solution

  下载: 全尺寸图片 幻灯片

  Figure 2

  

  Figure 2.  Fluorescence emission spectra of 2.5 μmol•L^-1 sensor R1 in buffered CH₃CN/HEPES (V/V=5/5) solution at pH 7.40 with 2 equiv. of metal ions

  Metal ions: Ag⁺, Al³⁺, Ca²⁺, Cd²⁺, Co²⁺, Cr³⁺, Cu²⁺, Fe³⁺, Hg²⁺, K⁺, Mg²⁺, Mn²⁺, Na⁺, Ni²⁺, Pb²⁺ and Zn²⁺ and blank. The inset shows the fluorescence color of R1 with Cu²⁺

  下载: 全尺寸图片 幻灯片

  Similarly, when Cu²⁺ was added, the 10 mmol•L^-1 CH₃CN/HEPES (pH=7.40, V/V=5/5) buffer solution of 2.5 μmol•L^-1 R1 displayed a marked pink color (Figure 1, inset), besides, the red fluorescence was detected under the 365 nm UV lamp (Figure 2, inset), and the remaining cations (except for Ni²⁺) displayed almost no interference. As expected, R1 was a "naked-eye" chemosensor that showed selectivity to Cu²⁺ ions in the neutral buffer solution.

  2.2   Absorption and fluorescence spectrum titrations of R1 with Cu²⁺

  For obtaining the details regarding the R1 sensor and Cu²⁺ binding manner, fluorescence and absorption spec-trum titration experiments were conducted respectively. As shown in Figure 3, upon the gradual addition of Cu²⁺ to the CH₃CN/HEPES (10 mmol•L^-1, pH=7.40, V/V=5/5) solution of sensor R1 (2.5 μmol•L^-1), the absorbance peaked at 560 nm markedly increased, which obviously indicated spirolactam ring opening induced by Cu²⁺ addition. At the same time, fluorescence intensity at the wavelength of 585 nm showed gradual increasing when Cu²⁺ was added (Figure 4). The quantum yield of CH₃CN/HEPES solution of R1 with 2 equiv. Cu²⁺ was tested to be 0.18 using rhodamine B as standard (Ф_F=0.69 in ethanol). Furthermore, the absorbance at 560 nm and the fluorescence intensity at 585 nm for R1 increased linearly over the Cu²⁺ concentration range of 1.25~6.0 and 1.25~7.5 μmol•L^-1 (Figures 3 and 4, inset), respectively, indicating that the sensor R1 may be used as a candidate for detecting Cu²⁺ in live cell and/or water samples. For R1, the LOD for Cu²⁺ ions was 0.201 μmol•L^-1 based on the equation DL=3S_bi/S (in which S_bi represents blank measurement standard deviation, while S stands for the intensity/sample content slope), which was much lower than the standard value for Cu²⁺ (about 20 μmol•L^-1) in drinking water recommended by the United States Environmental Protection Agency.^[36] In addition, Job's plot experiments were also carried out based on those absorption spectra, which revealed the R1:Cu²⁺ binding of 1:1 stoichiometry. Notably, the optical density reached a peak at the Cu²⁺ molecular fraction of about 0.5. Besides, the R1 and Cu²⁺ binding constant was calculated as 2.79×10⁴ mol•L^-1, which displayed good linearity (R=0.99275), when data were fit based on the Benesi-Hildebrand expression.^[37]

  Figure 3

  

  Figure 3.  Absorption spectra of 2.5 μmol•L^-1 sensor R1 upon the addition of Cu²⁺ (0~25 equiv.) in CH₃CN/HEPES (V/V=5/5) solution at pH 7.40

  The inset shows the absorbance at 560 nm as a function of Cu²⁺ concentration (1.25~6.0 μmol•L^-1)

  下载: 全尺寸图片 幻灯片

  Figure 4

  

  Figure 4.  Fluorescence emission spectra of 2.5 μmol•L^-1 sensor R1 upon the addition of Cu²⁺ (0~25 equiv.) in CH₃CN/HEPES (V/V=5/5) solution at pH 7.40

  The inset shows the fluorescence intensity at 585 nm as a function of Cu²⁺ concentration (1.25~7.50 μmol•L^-1)

  下载: 全尺寸图片 幻灯片

  2.3   Reaction mechanism between R1 and Cu²⁺

  ESI-MS was also carried out for exploring the reaction mechanism between R1 and Cu²⁺. According to our observation, when Cu²⁺ was added to the acetonitrile solution of R1, a peak was detected at m/z 757.73, which was as-signed to intermediate [R1-H+Cu+NO₃] complex. Simultaneously, a peak at m/z 442.84 was assigned to rhodamine B, which may be formed through intermediate hydrolysis, resulting in color and fluorescence changes. In fact, Czarnik et al.^[38] had examined the hydrolysis mechanism. In addition, the emission of fluorescence of sensor R1 (2.5 μmol•L^-1) remained almost unchanged at the excess addition of Na₂EDTA (10 equiv.) into 10 mmol•L^-1 CH₃CN/HEPES (pH=7.40, V/V=5/5) buffer solution, while 2 equiv. of Cu²⁺ was added. Such facts also verified that the mechanism of hydrolysis reaction between R1 and Cu²⁺ based on a hydrolysis process rather than a coordination reaction.

  To further clarify the coordination behavior of the R1-Cu²⁺ complex, energy optimized structures of R1 and R1-Cu²⁺ complex were obtained with the Becke-3-Lee- Yang-Parr (B3LYP) exchange function using the Gaussian 09 package. Figure 5 presents the optimization of molecular geometry at the binding stoichiometry between R1 and Cu²⁺ ions of 1:1. In the free sensor, R1 existed in the form of closed spirocycle, while in the R1-Cu²⁺ complex, it exists in the form of spirocycle opening. Besides, pyrrole N, imine N as well as carbonyl O atoms were also utilized for Cu²⁺ coordination, and the bond length was 0.1990~0.2055 nm. By contrast, rhodamine B hydrazone that bears the identical pyrrole unit (R2) sensors for Cu²⁺ is synthesized according to the coordination mechanism in our previous work.^[30] In the energy-minimized structure for R2-Cu²⁺ complex, Cu²⁺ ions are surrounded by two independent probes, with NO donor set from the carbonyl O and pyrrole N atoms. For Cu—O and Cu—N, their bond lengths are 0.192 and 0.205 nm, respectively, which are slightly shorter than those in the R1-Cu²⁺ complex. Notably, a potent intramolecular N—H…O hydrogen bond exists in the R2-Cu²⁺ complex between the pyrrole ring N atom and the hydrazone O atom (with the D—H…A distance of 0.276 nm), which may enhance the complex stability and thus prevent the hydrolysis process. In this regard, it can be roughly concluded that the rhodamine backbone structure is responsible for the sensor functional mechanism.

  Figure 5

  

  Figure 5.  Optimized structures and HOMO/LUMO of R1 and R1-Cu²⁺ complex by DFT calculation

  H atoms are omitted for clarity

  下载: 全尺寸图片 幻灯片

  For R1 and R1-Cu²⁺ complex, their LUMO and HOMO orbital energies and spatial distributions were also obtained (Figure 5). The results suggested that, for R1, both LUMO and HOMO were related to pyrrole and hydrazone moieties, at the same time, LUMO was on the rhodamine benzene ring. With regard to the R1-Cu²⁺ complex, HOMO was mainly localized near Cu²⁺ ions. Clearly, C—N bond on rhodamine moiety spirocycle was broken for the sake of Cu²⁺ ion binding. In comparison, in R1-Cu²⁺ complex, those electrons within LUMO were localized within the entire xanthene backbone. Typically, for the R1-Cu²⁺ complex, the energy gap of LUMO with HOMO was determined as 1.58 eV, and this figure was remarkably reduced relative to the 3.78 eV of R1. Such findings better verified the R1-Cu²⁺ complex binding pattern, as displayed in Scheme 1.

  Scheme 1

  

  Scheme 1.  Proposed reaction mechanism of R1 and Cu²⁺

  下载: 全尺寸图片 幻灯片

  2.4   Environmental factor impacts and kinetic assay

  The pH has been recognized as a leading factor affecting the rhodamine-based chemosensor response. The spectrum response of R1 (2.5 μmol•L^-1) with or without Cu²⁺ (2 equiv.) in the buffer solutions at different pH values was evaluated at ambient temperature. The fluorescence intensity in R1 as well as R1+Cu²⁺ complex was slightly dependent on pH 6.40~8.00. Specifically, the great difference in the fluorescence intensity of R1 compared with the R1-Cu²⁺ complex suggested that R1 was able to detect Cu²⁺ within such a range of pH. Therefore, the physiological pH 7.40 was chosen as the optimum experimental condition. In addition, the influence of those co-existing metal cations was also investigated. As suggested by our observations, those co-existing metal cations, such as Zn²⁺, Pb²⁺, Ni²⁺, Na⁺, Mn²⁺, Mg²⁺, K⁺, Hg²⁺, Fe³⁺, Cr³⁺, Co²⁺, Cd²⁺, Ca²⁺, Al³⁺ and Ag⁺ ions had almost no interference with Cu²⁺ detection.

  For a chemosensor, the short response time plays a vital role similar to the great selectivity and sensitivity. For real- time monitoring of those target metal ions, we examined the time-fluorescence change response relationship of 2.5 μmol•L^-1 R1 in 10 mmol•L^-1 CH₃CN/HEPES (pH=7.40, V/V=5/5) buffer solution when 2 equiv. Cu²⁺ was added. The R1 fluorescence response towards Cu²⁺ ion occurred quite soon. Such findings suggested the completion of R1-Cu²⁺ interaction with no time delay.

  2.5   Practical applications

  For verifying the sensor practical applicability, Cu²⁺ ions were determined within distilled water as well as the commercial Kangshifu Drinking Water samples according to standard addition approach.^[37] In brief, 1 mL of Cu²⁺ spiked water sample was added into 1 mL of CH₃CN solution containing 1 mmol•L^-1 probe, and then the fluorescence spectra was recorded. The equation to calculate Cu²⁺ contents is shown in Figure 4. As suggested by our results, the recovery obtained was 91.2%~96.8%, which indicted that our sensor had appreciable practicality (Table 1).

  Table 1

  

  Table 1.  Determination of Cu²⁺ concentrations in drinking water samples

  下载: 导出CSV

  Probe	Sample	c/(μmol•L-1)		Recovery/%
  		Spiked	Founda
  R1	Distilled water	0.0	0	—
  		2.5	2.39±0.04	95.6
  		5.0	4.84±0.06	96.8
  	Drinking waterb	0.0	0	—
  		2.5	2.28±0.05	91.2
  		5.0	4.69±0.08	93.8
  a Average value of three determinations. b Kangshifu Drinking water (obtained from the local supermarket): [K+]=25.64~700 μmol•L-1; [Mg2+]=4.17~203.12 μmol•L-1; [Cl-]=281.69~769 μmol•L-1; [SO42-]=4.17~71 μmol•L-1.

  3.   Conclusions

  To sum up, the efficient "turn-on" fluorescent probe based on rodamine is developed to detect the Cu²⁺ ion. According to our results, the fluorescent probe exhibits great sensitivity and selectivity to Cu²⁺, the LOD within the 10 mmol•L^-1 CH₃CN/HEPES (pH 7.40, V/V=5/5) buffer solution is 0.201 μmol•L^-1, and those co-existing metal cations almost have no interference on Cu²⁺ detection. The probe functions via coordination reaction by forming complex [R1-H+Cu+NO₃] and followed by hydrolyzing to a fluorescent rhodamine B. Additionally, the practical application of sensor was also performed in Cu²⁺ spiked water samples.

  4.   Experimental section

  4.1   Materials and methods

  The starting materials and solvents in synthesis were bought from the market, which were utilized directly without further purification. In addition, rhodamine B hydrazide, ^[39] as well as ethyl 5-formyl-3, 4-dimethyl-pyrrole- 2-carboxylate, ^[40] was prepared in accordance with the methods reported in literature. The XT4-100X microscopic melting point machine (made in Beijing, China) was used to determine the sensor melting point. Meanwhile, the Elemental Vario EL analyzer was adopted for elemental analysis. The sensor ¹H NMR spectra were obtained using the Bruker AV400 NMR spectrometer within the DMSO-d₆ solution. Besides, the Bruker V70 FT-IR spectrophotometer was used to determine IR spectra (v=4000~400 cm^-1) according to a KBr pressed disc approach. In the meantime, the Purkinje General TU-1800 spectrophotometer was employed to record UV spectra. The Varian CARY Eclipse spectrophotometer was applied in determining fluorescent spectra, and 5 nm pass width was applied in measuring both excitation and emission spectra. The Bruker Daltonics Esquire 6000 mass spectrometer was utilized to obtain ESI-MS spectra.

  4.2   Preparation of ethyl 5-formyl-3, 4-dimethyl- pyrrole-2-carboxylate rhodamine B hydrazine (R1)

  460 mg (1 mmol) of rhodamine B hydrazide was mixed with 20 mL of ethanol solution supplemented with 195 mg (1 mmol) of ethyl 5-formyl-3, 4-dimethyl-pyrrole-2-car- boxylate. Then, the mixed solution was refluxed for 3 h, followed by cooling until ambient temperature. Later, the isolated solid was subjected to filtering and ethanol washing. Yield 71%. m.p. 124~128 ℃; ¹H NMR (400 MHz, DMSO-d₆) δ: 11.51 (s, 1H, NH), 9.13 (s, 1H, CH=N), 7.81~7.83 (m, 1H, Aryl-H), 7.52~7.57 (m, 2H, Aryl-H), 7.08~7.09 (m, 1H, Aryl-H), 6.26~6.33 (m, 6H, Aryl-H), 4.11~4.16 (q, J=7.5 Hz, 2H, CH₂), 3.24~3.26 (q, J=8.0 Hz, 8H, 4CH₂), 2.03 (s, 3H, CH₃), 1.71 (s, 3H, CH₃), 1.18~1.22 (t, J=8. 0 Hz, 3H, CH₃), 0.99~1.03 (t, J=8.0 Hz, 12H, 4CH₃); ¹³C NMR (100 MHz, DMSO-d₆) δ: 170.4, 161.6, 151.9, 151.9, 149.4, 149.4, 139.5, 139.2, 132.6, 131.3, 129.2, 128.8, 128.8, 128.3, 128.0, 126.9, 123.3, 122.7, 122.7, 114.3, 114.3, 106.5, 97.7, 97.7, 76.3, 60.9, 47.1, 47.1, 47.1, 47.1, 14.1, 12.9, 12.9, 12.9, 12.9, 10.4, 10.00. ESI-MS m/z: 633.84. Anal. calcd for C₃₉H₄₃N₅O₄: C 72.01, H 6.84, N 11.05; found C 72.11, H 6.98, N 10.89.

  4.3   Measurements of general fluorescence and UV-Vis spectra

  The spectral analyses were accomplished in CH₃CN/ HEPES buffer (10 mmol•L^-1, pH=7.40, V/V=5/5) solution at room temperature. The concentration of the sensor R1 for UV-vis and fluorescence measurement was 2.5 μmol•L^-1. The metal ion solutions were prepared using chloride salts or nitrate under CH₃CN. Besides, the fluorescence and UV-vis spectrophotometric titration was carried out within the cuvette (2 mL) through successively adding relevant chemical reagents by the microliter syringe. The solution was sufficiently mixed after adding any aliquot, then the spectra were determined.

  Supporting Information  Job plots of R1 and Cu²⁺ in CH₃CN/HEPES solution; the Benesi-Hildebrand plot of R1-Cu²⁺ complex; ESI-MS spectrum; reversible experiment; the effect of pH; the interference experiment; response time experiment; NMR sectrum of R1. The Supporting Information associated with this article can be found in the online version at http://sioc-journal.cn/.

  
  
• 1. [1]

     Wei, J. H.; Yi, J. W.; Han, M. L.; Li, B.; Liu, S.; Wu, Y. P.; Ma, L. F.; Li, D. S. A. Chem. Asian J. 2019, 14, 3694. doi: 10.1002/asia.201900706

  2. [2]

     Wang, H.; Qin, J.; Huang, C.; Han, Y.; Xu, W.; Hou, H. Dalton Trans. 2016, 45, 12710. doi: 10.1039/C6DT02321E

  3. [3]

     Wu, X.; Guo, Z.; Wu, Y.; Zhu, S.; James, T. D.; Zhu, W. ACS Appl. Mater. Interfaces 2013, 5, 12215. doi: 10.1021/am404491f

  4. [4]

     Guo, Z.; Kim, G. H.; Yoon, J.; Shin, I. Nat. Protoc. 2014, 9, 1245. doi: 10.1038/nprot.2014.086

  5. [5]

     Ma, D. L.; Wong, S. Y.; Kang, T. S.; Ng, H. P.; Han, Q. B.; Leung, C. H. Methods 2019, 168, 3. doi: 10.1016/j.ymeth.2019.02.013

  6. [6]

     Jung, J. H.; Lee, J. H.; Shinkai, S. Chem. Soc. Rev. 2011, 40, 4464. doi: 10.1039/c1cs15051k

  7. [7]

     Patil, P.; Ajey, K. V.; Bhat, M. P.; Sriram, G.; Yu, J.; Jung, H. Y.; Altalhi, T.; Kigga, M.; Kurkuri, M. D. ChemistrySelect 2018, 3, 11593. doi: 10.1002/slct.201802411

  8. [8]

     Suo, F.; Chen, X.; Fang, H. Gong, Q.; Yu, C.; Yang, N. D.; Li, S.; Wu, Q.; Li, L.; Huang, W. Dyes Pigm. 2019, 170, 107639. doi: 10.1016/j.dyepig.2019.107639

  9. [9]

     Ji, Y.; Dai, F.; Zhou, B. Free Radical Biol. Med. 2018, 129, 215. doi: 10.1016/j.freeradbiomed.2018.09.017

  10. [10]

      Singh, N.; Paknikar, K. M.; Rajwade, J. Environ Res. 2019, 175, 367. doi: 10.1016/j.envres.2019.05.034

  11. [11]

      Ogunkunle, C. O.; Jimoh, M. A.; Asogwa, N. T.; Viswanathan, K.; Vishwakarma, V.; Fatoba, P. O. Ecotoxicol. Environ. Saf. 2018, 155, 86. doi: 10.1016/j.ecoenv.2018.02.070

  12. [12]

      Donnelly, P. S.; Xiao, Z.; Wedd, A. G. Curr. Opin. Chem. Biol. 2007, 11, 128. doi: 10.1016/j.cbpa.2007.01.678

  13. [13]

      Squitti, R.; Ghidoni, R.; Simonelli, I.; Ivanova, I. D.; Colabufo, N. A.; Zuin, M.; Benussi, L.; Binetti, G.; Cassetta, E.; Rongioletti, M.; Siotto, M. J. Trace Elem. Med. Biol. 2018, 45, 181. doi: 10.1016/j.jtemb.2017.11.005

  14. [14]

      Gardner, B.; Dieriks, B. V.; Cameron, S.; Mendis, L. H. S.; Turner, C.; Faull, R. L. M.; Curtis, M. A. Sci. Rep. 2017, 7, 10454. doi: 10.1038/s41598-017-10659-6

  15. [15]

      Ren, D.; Liu, Y.; Liu, X.; Li, Z.; Li, H.; Yang, X. F. Sens. Actuators. B 2018, 255, 2321. doi: 10.1016/j.snb.2017.09.048

  16. [16]

      Yoon, J. W.; Chang, M. J.; Hong, S.; Lee, M. H. Tetrahedron Lett. 2017, 58, 3887. doi: 10.1016/j.tetlet.2017.08.071

  17. [17]

      Jiao, Y.; Zhou, L.; He, H.; Yin, J.; Gao, Q.; Wei, J.; Duan, C.; Peng, X. Talanta 2018, 184, 143. doi: 10.1016/j.talanta.2018.01.073

  18. [18]

      Jia, X.; Xiao, Z.; Hui, P.; Liu, C.; Wang, Q.; Qiu, X. He, S.; Zeng, X.; Zhao, L. Dyes Pigm. 2019, 160, 633. doi: 10.1016/j.dyepig.2018.08.060

  19. [19]

      Kang, H.; Fan, C.; Xu, H.; Pu, G.; Liu, S. Tetrahedron 2018, 74, 4390. doi: 10.1016/j.tet.2018.07.002

  20. [20]

      Zhang, J. F.; Zhou, Y.; Yoon, J.; Kim, Y.; Kim, S. J.; Kim, J. S. Org Lett. 2010, 12, 3852. doi: 10.1021/ol101535s

  21. [21]

      Yang, Z.; She, M.; Zhang, J.; Chen, X.; Huang, Y.; Zhu, H.; Liu, P.; Li, J.; Shi Z. Sens. Actuators. B 2013, 176, 482. doi: 10.1016/j.snb.2012.07.035

  22. [22]

      Yu, M.; Yuan, R.; Shi, C.; Zhou, W.; Wei, L.; Li, Z. Dyes Pigm. 2013, 99, 887. doi: 10.1016/j.dyepig.2013.07.030

  23. [23]

      Goswami, S.; Sen, D.; Das, A. K.; Das, N. K.; Aich, K.; Fun, H. K.; Quah, C. K.; Maity, A. K.; Saha, P. Sensors. Actuators. B 2013, 183, 518. doi: 10.1016/j.snb.2013.04.005

  24. [24]

      Kar, C.; Adhikari, M. D.; Ramesh, A.; Das, G. Inorg. Chem. 2013, 52, 743. doi: 10.1021/ic301872q

  25. [25]

      Xiong, K.; Chen, J. G. Catalysis Today. 2018, 339, 289.

  26. [26]

      Xu, Q.; Guo, Z. J. East China Univ. Sci. Technol. 2019, 45, 357.

  27. [27]

      Yi, X.; Li, G.; Huang, L.; Chu, Y.; Liu, Z. Q.; Xia, H.; Zheng, A.; Deng, F. J. Phys. Chem. C. 2017, 121, 3887. doi: 10.1021/acs.jpcc.6b11518

  28. [28]

      Jr, P. A.; Liu, Y.; Palacios, M. A.; Minami, T. Wang, Z.; Nishiyabu, R. Chem.-Eur. J. 2013, 19, 8497. doi: 10.1002/chem.201204188

  29. [29]

      Wang, Y.; Wu, H.; Wu, W. N.; Li, S. J.; Xu, Z. H.; Xu, Z. Q. Fan, Y. C.; Zhao, X. L.; Liu, B. Z. Sens. Actuators. B 2018, 260, 106. doi: 10.1016/j.snb.2017.12.201

  30. [30]

      Moubaraki, R.; Li, B.; Murray, K. S.; Brooker, S. Eur. J. Inorg. Chem. 2009, 19, 2851.

  31. [31]

      Wang, Y.; Chang, H. Q.; Wu, W. N.; Peng, W. B.; Yan, Y. F. He, C. M.; Chen, T. T.; Zhao, X. L.; Xu, Z. Q. Sensors. Actuators B:Chem. 2016, 228, 395. doi: 10.1016/j.snb.2016.01.052

  32. [32]

      Li, M.; Lv, H.; Luo, J. Z.; Miao, J. Y.; Zhao, B. X. Sens. Actuators. B 2013, 188, 1235. doi: 10.1016/j.snb.2013.08.030

  33. [33]

      Ge, F.; Ye, H.; Luo, J. Z.; Wang, S.; Sun, Y. J.; Zhao, B. X.; Miao, J. Y. Sens. Actuators. B 2013, 181, 215. doi: 10.1016/j.snb.2013.01.048

  34. [34]

      Huang, K.; Jiao, X.; Liu, C.; Wang, Q.; Qiu, X.; He, S. Zhao, L.; Zeng, X. Dyes Pigm. 2017, 145, 561. doi: 10.1016/j.dyepig.2017.06.047

  35. [35]

      Artesani, A.; Binet, L.; Tana, F.; Comelli, D.; Nardo, L. D.; Nevin, A.; Touati, N.; Valentini, G.; Gourier, D. Microchem. J. 2019, 151, 104256. doi: 10.1016/j.microc.2019.104256

  36. [36]

      Pressman, J. G.; Richardson, S. D.; Speth, T. F.; Miltner, R. J. Environ. Sci. Technol. 2010, 44, 7184. doi: 10.1021/es9039314

  37. [37]

      Xu, Z.; Wang, H.; Hou, X.; Xu, W.; Xiang, T.; Wu, C. Sens. Actuators. B 2014, 201, 469. doi: 10.1016/j.snb.2014.05.026

  38. [38]

      Dujols, V.; Ford, F.; Czarnik, A. W. J. Am. Chem. Soc. 1997, 119, 7386. doi: 10.1021/ja971221g

  39. [39]

      Kim, H.; Rao, B. A.; Jeon, J. W.; Mallick, S.; Kang, S. M.; Choi, J. S.; Lee, C. S.; Son, Y. A. Sens. Actuators. B 2015, 210, 173. doi: 10.1016/j.snb.2014.12.100

  40. [40]

      Ye, X. P.; Zhu, T. F.; Wu, W. N.; Ma, T. L.; Xu, J.; Zhang, Z. P.; Wang, Y.; Jia, L. Inorg. Chem. Commun. 2014, 47, 60. doi: 10.1016/j.inoche.2014.07.022

• 

  Figure 1  UV-vis spectra of 2.5 μmol•L^-1 sensor R1 in buffered CH₃CN/HEPES (V:V=5:5) solution at pH 7.40 with 2 equiv. of metal ions

  Metal ions: Ag⁺, Al³⁺, Ca²⁺, Cd²⁺, Co²⁺, Cr³⁺, Cu²⁺, Fe³⁺, Hg²⁺, K⁺, Mg²⁺, Mn²⁺, Na⁺, Ni²⁺, Pb²⁺, Zn²⁺ and blank. The inset shows the change of the color with the addition of Cu²⁺/Ni²⁺ to R1 in buffered solution

  下载: 全尺寸图片 幻灯片
  

  Figure 2  Fluorescence emission spectra of 2.5 μmol•L^-1 sensor R1 in buffered CH₃CN/HEPES (V/V=5/5) solution at pH 7.40 with 2 equiv. of metal ions

  Metal ions: Ag⁺, Al³⁺, Ca²⁺, Cd²⁺, Co²⁺, Cr³⁺, Cu²⁺, Fe³⁺, Hg²⁺, K⁺, Mg²⁺, Mn²⁺, Na⁺, Ni²⁺, Pb²⁺ and Zn²⁺ and blank. The inset shows the fluorescence color of R1 with Cu²⁺

  下载: 全尺寸图片 幻灯片
  

  Figure 3  Absorption spectra of 2.5 μmol•L^-1 sensor R1 upon the addition of Cu²⁺ (0~25 equiv.) in CH₃CN/HEPES (V/V=5/5) solution at pH 7.40

  The inset shows the absorbance at 560 nm as a function of Cu²⁺ concentration (1.25~6.0 μmol•L^-1)

  下载: 全尺寸图片 幻灯片
  

  Figure 4  Fluorescence emission spectra of 2.5 μmol•L^-1 sensor R1 upon the addition of Cu²⁺ (0~25 equiv.) in CH₃CN/HEPES (V/V=5/5) solution at pH 7.40

  The inset shows the fluorescence intensity at 585 nm as a function of Cu²⁺ concentration (1.25~7.50 μmol•L^-1)

  下载: 全尺寸图片 幻灯片
  

  Figure 5  Optimized structures and HOMO/LUMO of R1 and R1-Cu²⁺ complex by DFT calculation

  H atoms are omitted for clarity

  下载: 全尺寸图片 幻灯片
  

  Scheme 1  Proposed reaction mechanism of R1 and Cu²⁺

  下载: 全尺寸图片 幻灯片

  Table 1.  Determination of Cu²⁺ concentrations in drinking water samples

  Probe	Sample	c/(μmol•L-1)		Recovery/%
  		Spiked	Founda
  R1	Distilled water	0.0	0	—
  		2.5	2.39±0.04	95.6
  		5.0	4.84±0.06	96.8
  	Drinking waterb	0.0	0	—
  		2.5	2.28±0.05	91.2
  		5.0	4.69±0.08	93.8
  a Average value of three determinations. b Kangshifu Drinking water (obtained from the local supermarket): [K+]=25.64~700 μmol•L-1; [Mg2+]=4.17~203.12 μmol•L-1; [Cl-]=281.69~769 μmol•L-1; [SO42-]=4.17~71 μmol•L-1.

  下载: 导出CSV
•