Chinese Chemical Letters  2015, Vol.26 Issue (12): 1485-1489   PDF    
Synthesis of a di(2-picolyl)amino-β-diketone dual-functional ligand that can coordinate to europium(III) for responding to copper(II) and sulfide ions
Yan Liua , Qiu-Ling Shia, Jing-Li Yuanb     
a College of Environmental Science and Engineering, Dalian Maritime University, Dalian 116026, China;
b State Key Laboratory of Fine Chemicals, School of Chemistry, Dalian University of Technology, Dalian 116024, China
Abstract: Lanthanide complex-based luminescent probes/chemosensors have shown great utilities in various biological and environmental assays with time-resolved detection mode to eliminate background noises. In this work, by conjugating di(2-picolyl)amine (DPA) with a tetradentate β-diketone 1,2-bis[4'- (1",1",1",2",2"-pentafluoro-3",5"-pentanedion-5"-yl)benzyl]-4-chlorosulfo-benzene (BPPBCB), a novel dual-functional ligand that can coordinate to Eu3+ for responding to Cu2+ and S2- ions in aqueous media, DPA-BPPBCB, has been designed and synthesized. The β-diketone moiety of DPA-BPPBCB can form a strongly luminescent complex with Eu3+. Upon reaction with Cu2+, accompanied by the formation of heterobimetallic complex Cu2+-DPA-BPPBCB-Eu3+, the Eu3+ luminescence was quenched. While in the presence of S2-, owing to the high affinity of S2- to Cu2+, stable CuS was formed, which resulted in the release of Cu2+ from Cu2+-DPA-BPPBCB-Eu3+, to restore the luminescence of the Eu3+ complex. This unique “on-off-on” luminescence response of the Eu3+ complex enabled Cu2+ and S2- ions in aqueous media to be detected with time-resolved luminescence detection mode.
Key words: Europium complex     Luminescent probe     Copper(II) ion     Sulfide ion     Time-resolved luminescence detection    
1. Introduction

In various organic/inorganic luminescent compounds,luminescent lanthanide complexes have several unique luminescence properties,including long luminescence lifetimes,large Stokes shifts and sharp emission profiles,which allow them to be easily used as luminescent probes/chemosensors for time-resolved luminescence detections of analytes in complicated biological and environmental samples to eliminate short-lived background fluorescence and scattering lights [1, 2, 3, 4]. Among luminescent lanthanide complexes,β-diketonate-Eu3+ complexes showed the greatest utility in luminescence bioassays since they were successfully used in the time-resolved luminescence immunoassay (TR-FIA) technique in 1980s [5, 6]. However,although a number of luminescent β-diketonate-Eu3+ complexes have been developed so far,these complexes were mainly designed for application to biolabeling [7, 8, 9, 10, 11, 12],the functional β-diketonate-Eu3+ complexes that could directly respond to biological or environmental analytes were rarely investigated [13].

As two representative environmental analytes,Cu2+ and S2- ions are present in many environments either as naturally occurring species or as by-products of manufacturing and industrial processes. Copper(II) and S2- ions are known to play important roles in biological systems (Cu2+ is an essential ion for life,and H2S is an endogenous signaling gasotransmitter),but their essential yet toxic nature makes their detection/monitoring be an important work for the environmental safety and human health. To date,a variety of analytical methods for detecting Cu2+ and S2- ions have been established,among them the luminescence assay method using Cu2+/S2--responsive probes is considered to be one of the best choices due to its high sensitivity,selectivity,and experimental simplicity. Based on this opinion,many organic dyebased or transition metal complex-based luminescent probes/ chemosensors for Cu2+ [14, 15, 16, 17, 18, 19, 20] and S2- [20, 21, 22, 23, 24, 25, 26] were successfully developed,and used for the detection of Cu2+ or S2- in biological and environmental samples in recent years. However,because these probes/chemosensors can only be used for the steady-state fluorescence detection,while the co-existence of various components in biological or environmental samples leads to the strong background fluorescence,to interfere the quantitative detections of Cu2+ and S2- ions,the development of luminescent probes/ chemosensors that can avoid the effect of background fluorescence to respond to Cu2+ and S2- ions in complicated samples is still highly desirable.

In this work,by conjugating a Cu2+-binding moiety,di(2- picolyl)amine (DPA),to a tetradentate β-diketone,1,2-bis[40- (10 0,10 0,100,20 0,20 0-pentafluoro-30 0,500-pentanedion-50 0-yl)benzyl]-4- chlorosulfobenzene (BPPBCB),a novel dual-functional ligand, DPA-BPPBCB,was designed and synthesized. In this ligand,the b-diketone moiety can coordinate to Eu3+ to form a strongly luminescent complex DPA-BPPBCB-Eu3+. In the presence of Cu2+, the DPA moiety in the Eu3+ complex binds to Cu2+,which leads to the luminescence quenching of the Eu3+ complex,to give a turn-off luminescence signal. After further reaction with S2-,due to the formation of CuS,Cu2+ is released from the Eu3+ complex,which results in the luminescence of the Eu3+ complex to be restored. Based on this ‘‘on-off-on’’ luminescence response,a simple timeresolved luminescence detection method for Cu2+/S2- ions in aqueous media was developed. Scheme 1 shows the luminescence response mechanism of the Eu3+ complex to Cu2+ and S2-.

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Scheme. 1.Luminescence response mechanism of the Eu3+ complex to Cu2+ and S2-.
2. Experimental 2.1. Materials and physical measurements

The tetradentate β-diketone BPPBCB was synthesized using a previously reported method [12]. Unless otherwise stated,all chemical materials were purchased from commercial sources and used without further purification.

1H NMR and ESI-MS spectra were measured on a Bruker Avance spectrometer and a HP1100LC/MSD MS spectrometer,respectively. Elemental analysis was carried out on a Vario-EL analyser. Absorption spectra were measured on a Perkin-Elmer Lambda 35 UV-vis spectrometer. Time-resolved luminescence spectra were measured on a Perkin-Elmer LS 50B luminescence spectrometer.

2.2. Synthesis of DPA-BPPBCB

A mixture of BPPBCB (748 mg,1.0 mmol),di(2-picolyl)amine (796 mg,4.0 mmol),4-dimethylaminopyridine (24 mg,0.2 mmol) and Et3N (1.5 mL) in 25 mL anhydrous CH2Cl2 was stirred at room temperature for 48 h. After solvent was removed by evaporation, the residue was added into 90 mL of 1.0 mol/L HCl,and the mixture was stirred for 20 min. The precipitate was filtered,washed with water,and then dried. The crude product was recrystallized from ethanol to afford the target compound as pale yellow crystals (135 mg,14.2% yield). 1H NMR (400 MHz,CDCl3): δ 8.40 (d,2H, J = 4.8 Hz),7.87-7.91 (m,4H),7.78-7.84 (m,4H),7.62 (d,2H, J = 8.0 Hz),7.32-7.34 (m,3H),7.20 (t,4H,J = 8.0 Hz),6.61 (s,1H), 6.60 (s,1H),4.78 (s,4H),4.16 (s,2H),4.07 (s,2H). ESI-MS (m/z): 896.1 [M+H]+. Elemental analysis calcd. (%) for C42H31F10N3O6S·HCl·H2O (DPA-BPPBCB·HCl·H2O): C 53.08,H 3.50,N 4.42; found (%): C 53.23,H 3.18,N 4.53.

2.3. Reaction of DPA-BPPBCB with Eu3+

Before the reaction,the stock solutions of 1.0 mmol/L DPA-BPPBCB in ethanol and 1.0 mmol/L EuCl3 in distilled water were prepared,respectively. After 300 μL of 1.0 mmol/L DPA-BPPBCB was mixed with 150 μL of 1.0 mmol/L EuCl3,the solution was diluted to 3.0 mL with 50 mmol/L borate buffer of pH 7.4 containing 0.1% CTAB (cetyltrimethylammonium bromide). The solution was incubated at 50 8C for 1 h,suitably diluted with the borate buffer,and then subjected to the measurements of UV-vis absorption spectra and time-resolved luminescence spectra (delay time,0.2ms; gate time,0.4ms; cycle time,20ms; excitation slit, 10 nm; and emission slit,5 nm).

2.4. Reactions of DPA-BPPBCB-Eu3+ with transition metal ions

The reactions of DPA-BPPBCB-Eu3+ with transition metal ions were carriedout in50mmol/Lboratebuffer ofpH7.4containing 0.1% CTAB. After different metal ions (final concentration: 10 mmol/L) were mixed with the solution of DPA-BPPBCB-Eu3+ (final concentration: 1.0 μmol/L of [(DPA-BPPBCB)2Eu3+]),respectively,the solutions were incubated for 20min at roomtemperature,and then their time-resolved excitation and emission spectra weremeasured on the luminescence spectrometer with the conditions of excitation wavelength,330 nm; delay time,0.2 ms; gate time,0.4ms; cycle time,20ms; excitation slit,5 nm; and emission slit,5 nm.

For reacting DPA-BPPBCB-Eu3+ with different concentrations of Cu2+,various concentrations of Cu2+ (final concentrations: 0.0,2.0, 4.0,6.0,8.0,10,12,14,16,18,20,30,and 40 mmol/L) were mixed with the solution of DPA-BPPBCB-Eu3+ (final concentration: 5.0 μmol/L of [(DPA-BPPBCB)2Eu3+]),respectively. After the solutions were incubated for 15 min at room temperature,their time-resolved emission spectra were measured on the luminescence spectrometer with the conditions of excitation wavelength, 330 nm; delay time,0.4 ms; gate time,0.1 ms; cycle time,20 ms; excitation slit,4 nm; and emission slit,4 nm.

2.5. Reaction of Cu2+-DPA-BPPBCB-Eu3+ with S2-

The reaction of Cu2+-DPA-BPPBCB-Eu3+ with S2- was also carried out in 50 mmol/L borate buffer of pH 7.4 containing 0.1% CTAB. Before the reaction,the stock solution of 1.0 μmol/L NaHS was freshly prepared. After different concentrations of NaHS (final concentrations: 0.0,20,30,40,50,60,70,80,100 μmol/L) were mixed with Cu2+-DPA-BPPBCB-Eu3+ (final concentration: 1.0 μmol/L of [(Cu2+-DPA-BPPBCB)2Eu3+]),respectively,the solutions were incubated for 15 min at room temperature,and then their time-resolved emission spectra were measured on the luminescence spectrometer with the conditions of excitation wavelength,330 nm; delay time,0.4 ms; gate time,0.1 ms; cycle time,20 ms; excitation slit,10 nm; and emission slit,5 nm.

3. Results and discussion 3.1. Complexation of DPA-BPPBCB with Eu3+

To investigate the complexation behavior of DPA-BPPBCB with Eu3+ in aqueous media,the UV-vis absorption spectra of DPA-BPPBCB in the absence and presence of Eu3+ in 50 mmol/L borate buffer of pH 7.4 containing 0.1% CTAB were determined, respectively. As shown in Fig. 1,the ligand DPA-BPPBCB itself showed two absorption peaks at 251 nm and 331 nm (ε331nm = 3.89 × 104 mol-1 L cm-1). Upon reaction with Eu3+,the absorption peak at 251 nm was red-shifted to 262 nm,and that at 331 nm became rather broad with the decrease of the molar extinction coefficient (ε331nm = 3.50 × 104 mol-1 L cm-1). These results suggest that DPA-BPPBCB could coordinate to Eu3+ to form a Eu3+ complex when it was reacted with Eu3+. To confirm this, time-resolved excitation and emission spectra of DPA-BPPBCB in the presence of Eu3+ in 50 mmol/L borate buffer of pH 7.4 containing 0.1% CTAB were recorded. As shown in Fig. 2,the solution of DPA-BPPBCB-Eu3+ exhibited the typical excitation and emission spectra of the β-diketonate-Eu3+ complex with an excitation peak at 330 nm,and a main emission peak at 607 nm with several side emission peaks at 588 nm,649 nm and 697 nm (four peaks from 588 nm to 697 nm are corresponding to the four 5D0→7FJ transations of Eu3+ ion,J = 1,2,3,4). This result strongly demonstrated the formation of the luminescent complex DPA- BPPBCB-Eu3+ in the aqueous solution.

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Fig. 1.UV–vis absorption spectra of DPA–BPPBCB (20 μmol/L) in the absence (black line) and presence (red line) of Eu3+ (10 mmol/L) in 50 mmol/L borate buffer of pH 7.4 containing 0.1% CTAB.

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Fig. 2.Time-resolved excitation (black line, λem = 607 nm) and emission (red line, λex = 330 nm) spectra of DPA–BPPBCB (1.0 μmol/L) in the presence of Eu3+ (0.5 μmol/L) in 50 mmol/L borate buffer of pH 7.4 containing 0.1% CTAB.

To reveal the coordination stoichiometry of DPA-BPPBCB to Eu3+,the Job’s plotting analysis of the reaction between DPA-BPPBCB and Eu3+ was conducted in borate buffer of pH 7.4 containing 0.1% CTAB. As shown in Fig. 3,the reaction product of DPA-BPPBCB with Eu3+ exhibited the maximum luminescence intensity at ~0.67 molecular fraction,which indicates that the coordination reaction of DPA-BPPBCB to Eu3+ has a 2:1 stoichiometry,and the formed Eu3+ complex has a composition of [(DPA-BPPBCB)2Eu3+]. Because the tetradentate b-diketone BPPBCB has been demonstrated to be able to form strongly luminescent complex with Eu3+ [12],it can be considered that the formation of [(DPA-BPPBCB)2Eu3+] is attributed to the coordination of β-diketone moieties to the central Eu3+ ion. Using the solution of [(DPA-BPPBCB)2Eu3+],the lifetime of the Eu3+ emission at 607 nm was determined to be 535 ms,which is similar to the reported luminescence lifetime of the BPPBCB-Eu3+ complex [12]. The long luminescence lifetime indicates that the complex [(DPA-BPPBCB)2Eu3+] can be conveniently used for the time-resolved luminescence measurement with a long delay time,which provides a favorable prerequisite for eliminating the short-lived background fluorescencewhen the complex is used as a probe for detecting analytes in complicated samples.

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Fig. 3.Job’s plot of the reaction between DPA–BPPBCB and Eu3+ in 50 mmol/L borate buffer of pH 7.4 containing 0.1% CTAB (the total concentration of Eu3+ and DPA–BPPBCB was kept at 1.0 μmol/L).
3.2. Luminescence responses of DPA-BPPBCB-Eu3+ toward transition metal ions

Because the DPA moiety in [(DPA-BPPBCB)2Eu3+] can further bind to transition metal ions,to induce the intensity change of the Eu3+ emission. To investigate the luminescence responses of the Eu3+ complex to different metal ions,the time-resolved excitation and emission spectra of [(DPA-BPPBCB)2Eu3+] in the presence of different transition metal ions in 50 mmol/L borate buffer of pH 7.4 containing 0.1% CTAB were measured. As shown in Fig. 4,upon additions of 10 molar equivalent of metal ions,the emission intensity of [(DPA-BPPBCB)2Eu3+] was strongly quenched by Cu2+, while other metal ions including Mn2+,Co2+,Ni2+,Fe3+,Fe2+,Zn2+, Cd2+ and Ag+ showed relatively low luminescence quenching efficiency to the Eu3+ emission. These results indicate that the luminescence response of [(DPA-BPPBCB)2Eu3+] to Cu2+ is more sensitive than that to other transition metal ions in aqueous media.

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Fig. 4.Time-resolved excitation and emission spectra of [(DPA–BPPBCB)2Eu3+] (1.0 μmol/L) in the presence of different metal ions (10 mmol/L) in 50 μmol/L borate buffer of pH 7.4 containing 0.1% CTAB.

To quantitatively evaluate the luminescence response of [(DPA-BPPBCB)2Eu3+] to Cu2+,the time-resolved emission spectra of [(DPA-BPPBCB)2Eu3+] (5.0 mmol/L) in the presence of different concentrations (0-40 mmol/L) of Cu2+ in 50 mmol/L borate buffer of pH 7.4 containing 0.1% CTAB were recorded. As shown in Fig. 5, the emission intensity of [(DPA-BPPBCB)2Eu3+] at 607 nm was gradually decreased with the increase of the Cu2+ concentration. The inserted curve in Fig. 5 showed the variation of the emission intensity as a function of Cu2+ concentration. It is notable that the emission intensity displays remarkable changes in the low concentration range of Cu2+ (0.0-10 mmol/L),which indicates that [(DPA-BPPBCB)2Eu3+] can sensitively respond to Cu2+ in a lower micromolar concentration range through the formation of a dinuclear complex [(Cu2+-DPA-BPPBCB)2Eu3+],and thus to enable the Cu2+ concentration to be quantitatively detected with timeresolved luminescence mode.

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Fig. 5.Time-resolved emission spectra of [(DPA–BPPBCB)2Eu3+] (5.0 μmol/L) in the presence of different concentrations (0.0, 2.0, 4.0, 6.0, 8.0, 10, 12, 14, 16, 18, 20, 30, 40 μmol/L) of Cu2+ ions in 50 mmol/L borate buffer of pH 7.4 containing 0.1% CTAB (the inset shows the emission intensity change at 607 nm as a function of Cu2+ concentration).
3.3. Luminescence response of Cu2+-DPA-BPPBCB-Eu3+ toward S2- anions

Due to the high binding affinity of S2- to Cu2+,the Cu2+-S2- reaction has been demonstrated to be a useful strategy for the design of various luminescent probes/chemosensors for S2-/H2S [21, 22, 25, 26]. In which,the Cu2+-S2- reaction allows the luminophore-conjugated Cu(II) to be removed,to switch on the luminescence of the luminophore. In this work,the dinuclear complex [(Cu2+-DPA-BPPBCB)2Eu3+] is also able to be reversibly transformed to the mononuclear Eu3+ complex [(DPA-BPPBCB)2Eu3+] in the presence of S2-,which enables it possible to restore the luminescence of the Eu3+ complex. Thus,to investigate whether [(Cu2+-DPA-BPPBCB)2Eu3+] can respond to S2- anions,the time-resolved emission spectra of [(Cu2+-DPA- BPPBCB)2Eu3+] (1.0 mmol/L) in the presence of different concentrations of S2- (0.0-100 mmol/L) were recorded in 50 mmol/L borate buffer of pH 7.4 containing 0.1% CTAB. As shown in Fig. 6, the emission intensity of [(Cu2+ [1TD$DIF]-DPA-BPPBCB)2Eu3+] at 607 nm was gradually increased with the increase of the S2- concentration, and the dose-dependent luminescence enhancement showed a good linearity in the S2- concentration range of 0.0-60 mmol/L (the inserted curve in Fig. 6). These results indicate that [(Cu2+-DPA- BPPBCB)2Eu3+] can indeed respond to S2- with the luminescence enhancement,and which allows the S2- concentration to be quantitatively detected with time-resolved luminescence mode.

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Fig. 6.Time-resolved emission spectra of [(Cu2+–DPA–BPPBCB)2Eu3+] (1.0 μmol/L) in the presence of different concentrations (0.0, 20, 30, 40, 50, 60, 70, 80, 100 μmol/L) of S2- anions in 50mmol/L borate buffer ofpH7.4 containing 0.1%CTAB(the inset shows the emission intensity change at 607 nm as a function of S2- concentration).
4. Conclusion

In summary,a novel dual-functional ligand,DPA-BPPBCB,has been designed and synthesized in this work. This ligand can coordinate to Eu3+ to form a strongly luminescent Eu3+ complex DPA-BPPBCB-Eu3+ in aqueous media. In the presence of Cu2+,due to the occurrence of DPA-Cu2+ binding,the luminescence of the Eu3+ complex is weakened. However,the dinuclear complex Cu2+- DPA-BPPBCB-Eu3+ can further react with S2-,to restore the luminescence of the Eu3+ complex. Based on this ‘‘on-off-on’’ luminescence response of the Eu3+ complex to Cu2+ and S2-,a simple time-resolved luminescence method for detecting Cu2+/S2- ions was proposed. The method,with the advantage of timeresolved luminescence detection technique,could be anticipated to be a useful tool for the monitoring of Cu2+ and S2- ions in complicated environmental samples. The further improvement and application of the method are being carried out in the process.

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