English

• 1. INTRODUCTION

  Cu²⁺ ion is a very important transition metal ion and has been widely used in the manufacturing industry. In animals and plants, Cu²⁺ ion also plays a very important physiological role^[1-3]. For example, in the human body, Cu²⁺ ion is an essential trace metal ion, which is the second only to zinc ion and iron ion. Absence or excess of Cu²⁺ ion can cause various diseases, such as osteoporosis, Parkinson's disease, Alzheimer's disease, et al.^[4-7]. Therefore, quantitative detection of Cu²⁺ ion is of great significance. Among many detection methods, the fluorescence sensor has attracted much attention due to its advantages such as good selectivity, high sensitivity, simple operation and real-time online detection. At present, the most researched fluorescent probes are usually divided into rhodamines, fused ring aromatic hydrocarbons, naphthimides, quinolines and borofluorodipyrrole^[8-10]. Some of these probes are applied to the detection of Cu²⁺ ion^[11-14]. However, most of the fluorescent probes have the disadvantages of difficulty in synthesis or poor biocompatibility, which limits their further practical application.

  Owing to the simple synthesis and good coordination ability, Schiff-base derivatives are often designed as fluorescent probes for detecting metal cations^{[15, 16]}. In this work, we have synthesized a novel Schiff-base fluorescent probe L from the 2:1 M condensation of 4-(diethylamino)-2-hydroxybenzaldehyde with ethylenediamine, as shown in Scheme 1. In addition, the possible fluorescent sensors and photochemical properties for certain metal ions have been explored for this Schiff-base L. The designed L exhibited strong yellow fluorescence (excitation: 393 nm, emission: 519 nm), and the fluorescence was quenched clearly by adding Cu²⁺ ion. And L showed high selectivity and sensitivity for Cu²⁺ without interference by many other common metal ions.

  Scheme 1

  

  Scheme 1.  Synthesis of Schiff-base L

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  2. EXPERIMENTAL

  2.1 Chemicals and instruments

  All reagents and solvents for the synthesis and analysis were of analytical grade and used without further purification. Infrared (IR) spectra in the 4000~400 cm^-1 region were recorded on a Niconet AVATAR-360 spectrometer using KBr pellets. ¹H NMR and ¹³C NMR were measured at 400 MHz using a Bruker AV400 spectrometer with CDCl₃ as the solvent and tetramethylsilane (TMS) as the internal standard. The crystallographic data were collected with a Bruker APEX II CCD area detector diffractometer using Mo-Kα radiation (λ = 0.71073 Å).

  2.2 Synthesis of compound L

  A mixture of 4-(diethylamino)-2-hydroxybenzaldehyde (2 mmol), ethylenediamine (1 mmol) and anhydrous ethanol (20 mL) was heated at 80 ℃ and monitored by TLC. After the reaction is over, the reaction mixture was cooled to room temperature and a large amount of solid precipitates appeared. The crude product was filtered from the solvent, and then recrystallized from absolute ethanol to get light yellow crystal of L (0.35 g, yield: 85%). ¹H NMR(CDCl₃, 400MHz) δ (ppm): 13.577 (br, 2H), 7.928 (s, 2H), 6.893 (d, J = 8.703 Hz, 2H), 6.053 (d, J = 8.785 Hz, 2H), 6.006 (s, 2H), 3.668 (s, 4H), 3.276 (q, J = 21.163 Hz, 8H), 1.085 (t, J = 14.080 Hz, 12H). ¹³C NMR (CDCl₃, 100MHz) δ (ppm): 165.034, 163.259, 150.522, 132.026, 107.198, 102.007, 97.077, 58.971, 43.432, 11.682. FT-IR: Selected IR data (KBr, cm^-1): 3423.2, 2971.9, 1614.2, 1452.7, 1284.5, 1036.8, 857.1, 703.3.

  2.3 UV-Vis and fluorescence spectra of L

  The spectral analyses were done in ethanol solution at 25 ℃. The concentration of L was 16 μmol·L^-1. Solutions of Ag⁺, K⁺, Na⁺, Ba²⁺, Ca²⁺, Co²⁺, Cu²⁺, Zn²⁺, Ni²⁺, Pb²⁺, Sn²⁺, Cr³⁺, Al³⁺ and Fe³⁺ ions were prepared with nitrate or hydrochloride salts in water (2 mmol·L^-1). The excitation wavelength was 362 nm and the fluorescence emission spectra were recorded within the scope of 400~600 nm.

  2.4 Single-crystal diffraction studies

  In order to study the binding mode of L and Cu²⁺ ion, the crystal of complex L-Cu²⁺ was prepared. The crystallographic data for L-Cu²⁺ were collected with a Bruker APEX II CCD area detector diffractometer using Mo-Kα radiation (λ = 0.71073 Å). Data collections, cell refinements, data reductions and absorption corrections were performed using multi-scan methods with Bruker software^[17]. The structures were solved by direct methods using SIR2004^[18]. The non-hydrogen atoms were refined anisotropically by full-matrix least-squares method on F² using SHELXL^[19]. Molecular graphics were prepared with the Olex2 program^[20].

  3. RESULTS AND DISCUSSION

  3.1 Synthesis

  As shown in Scheme 1, compound L was obtained by the condensation reaction between tris base with 4-(diethylamino)-2-hydroxybenzaldehyde in ethanol at 80 ℃ with a satisfactory yield. The Schiff-base L is light yellow powder, stable in air and soluble in the most common organic solvents such as MeOH, EtOH, CHCl₃, DMSO and DMF. In the ¹H NMR spectra, the signal for the imine proton in L appears at 7.928 ppm, as a singlet peak. The ¹H NMR spectra of L exhibit -OH_phenolic proton resonances at 13.577 ppm in turn, which suggests that here the -OH_phenolic takes along some positive charges.

  3.2 Spectral characterization

  The UV-Vis spectra of L and L-Cu²⁺ were measured. As shown in Fig. 1, the maximum absorption wavelength of L was red-shifted from 413 to 455 nm, with adding Cu²⁺ ion. To get insight into fluorescence intensity changes with the increase of Cu²⁺ concentration, the fluorescence spectra changes of L towards Cu²⁺ were measured. As shown in Fig. 2, upon the addition of Cu²⁺ to the solution of L, the fluorescence spectra at 519 nm clearly quenched under the excitation of 393 nm. Moreover, there was a good linear relationship between the fluorescence intensity of probe L and the concentration of Cu²⁺ in the range of 0 to 20 μmol·L^-1. According to the reported definition (LOD = 3σ/k), the detection limit of L for Cu²⁺ was found to be 19 nmol·L^-1. The fluorescence spectra results proved that L has a high sensitivity toward Cu²⁺.

  Figure 1

  

  Figure 1.  UV-Vis spectra of L and L-Cu²⁺

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  Figure 2

  

  Figure 2.  Fluorescence spectra of L (16 μmol/L) in the presence of different concentrations of Cu²⁺ (from 0 to 20 μmol·L^-1)

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  The binding constant K of L-Cu²⁺ was calculated according to the Benesi-Hildebrand equation. Depending on the slope, the results obtained were K_L = 7.94 × 10⁴ L·mol^-1, indicating that L had a great binding affinity to Cu²⁺ (Fig. 3). In order to study the binding of L with Cu²⁺, the Job plot was measured by using a total concentration of 16 μmol·L^-1 L and Cu²⁺, and the result indicated that the combination of L and Cu²⁺ is 1:1 stoichiometry (Fig. 4).

  Figure 3

  

  Figure 3.  Benesi-Hildebrand plot from fluorescence titration data of L with Cu²⁺

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  Figure 4

  

  Figure 4.  Job-plot for Cu²⁺ and L showing 1:1 stoichiometry

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  The infrared spectra of L and L-Cu²⁺ were measured. As shown in Fig. 5a, the absorption peak of C=N group in L was shifted from 1614 to 1592 cm^-1 with adding Cu²⁺ ion because the electron cloud density of N atom in C=N group decreased after L complexed with Cu²⁺. And the absorption signal of OH group in L disappeared from 3423 cm^-1, with adding the Cu²⁺ ion, which indicated that OH was deprotonated. By combining the above information, the binding mode of L and Cu²⁺ was speculated, as shown in Fig. 5b.

  Figure 5

  

  Figure 5.  (a) Infrared spectra of L and L-Cu²⁺, (b) Possible binding mode of L-Cu²⁺

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  To further understand the fluorescent property of L with Cu²⁺, the fluorescence response behaviors of L to various metal ions were investigated (Fig. 6). The results showed no obvious changes in the fluorescence intensity after adding other metal ions to L solution, including Ag⁺, K⁺, Na⁺, Ba²⁺, Ca²⁺, Zn²⁺, Ni²⁺, Pb²⁺, Sn²⁺, Cr³⁺, Al³⁺ and Fe³⁺. However, fluorescence intensities declined with adding Co²⁺ possibly because of the heavy metal effect. Moreover, the competitive experiments proved that the presence of metal ions, such as Ag⁺, K⁺, Na⁺, Ba²⁺, Ca²⁺, Co²⁺, Zn²⁺, Ni²⁺, Pb²⁺, Sn²⁺, Cr³⁺, Al³⁺ and Fe³⁺ ions, did not interfere with the quenched fluorescence. The results indicated that L has good sensibility and selectivity toward Cu²⁺.

  Figure 6

  

  Figure 6.  Relative emission of L (16 μmol/L) and its complexation with Cu²⁺ (32 μmol/L) in the presence of different metal ions (32 μmol/L). The response of L on its own is used as a control

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  3.3 Crystal structure

  The crystal of Schiff-base L-Cu²⁺ was cultivated to further confirm the real binding mode of L and Cu²⁺ ion. The molecular structure of L-Cu²⁺ is shown in Fig. 7. It is obvious that L and Cu²⁺ ion are combined according to the ratio of 1:1. Cu and N(1), N(2), O(1), O(2) form four coordinate bonds, with the H atom of the phenol group disappearing.

  Figure 7

  

  Figure 7.  ORTEP representation of L-Cu²⁺

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  4. CONCLUSION

  In summary, one novel Schiff-base probe L was developed as efficient chemical sensor for the highly selective detection of Cu²⁺. The maximum absorption wave length of L was red-shifted from 413 to 455 nm, with adding Cu²⁺ ion. After the addition of Cu²⁺ ion, the fluorescence quenching of L was observed at 519 nm under the excitation of 393 nm, which did not take effect for other metal ions. Probe L is highly selective for Cu²⁺ detection, which may be attributed to the fact that the Jahn-Teller deformation of Cu²⁺ complexes provides excellent thermodynamic stability among all metal cations. The job plot revealed that the binding ratio of probe L to Cu²⁺ ion is 1:1, which could be further confirmed by the crystal structure of the complex L-Cu²⁺. Cu²⁺ ion detection limit of probe L was measured to be 19 nmol·L^-1. Our group job herein could provide certain new insights for exploring highly selective and sensitive Schiff-base fluorescent sensors.

  
  
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• 

  Scheme 1  Synthesis of Schiff-base L

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  Figure 1  UV-Vis spectra of L and L-Cu²⁺

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  Figure 2  Fluorescence spectra of L (16 μmol/L) in the presence of different concentrations of Cu²⁺ (from 0 to 20 μmol·L^-1)

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  Figure 3  Benesi-Hildebrand plot from fluorescence titration data of L with Cu²⁺

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  Figure 4  Job-plot for Cu²⁺ and L showing 1:1 stoichiometry

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  Figure 5  (a) Infrared spectra of L and L-Cu²⁺, (b) Possible binding mode of L-Cu²⁺

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  Figure 6  Relative emission of L (16 μmol/L) and its complexation with Cu²⁺ (32 μmol/L) in the presence of different metal ions (32 μmol/L). The response of L on its own is used as a control

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  Figure 7  ORTEP representation of L-Cu²⁺

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