Carboxamidoquinoline–coumarin derivative: A ratiometric fluorescent sensor for Cu(II) in a dual fluorophore hybrid

Sensors and Actuators B 218 (2015) 37–41

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Carboxamidoquinoline–coumarin derivative: A ratiometric fluorescent sensor for Cu(II) in a dual fluorophore hybrid Yu Zhang, Xiangfeng Guo ∗ , Xiujuan Tian, Andong Liu, Lihua Jia ∗ College of Heilongjiang Province Key Laboratory of Fine Chemicals, Qiqihar University, Qiqihar 161006, China

a r t i c l e

i n f o

Article history: Received 17 January 2015 Received in revised form 20 April 2015 Accepted 22 April 2015 Available online 2 May 2015 Keywords: Carboxamidoquinoline Coumarin Ratiometric sensor Cu(II)

a b s t r a c t A novel dual fluorophore-based ratiometric Cu2+ sensor CPC was synthesized via integrating carboxamidoquinoline and coumarin fluorophores. CPC showed high selectivity for Cu2+ over other metal ions at pH 7.24 in acetonitrile–water solution. The emission produced by carboxamidoquinoline was obviously quenched upon binding Cu2+ , while the emission produced by coumarin remained almost intact during the whole titration, thus acting as a stable internal standard and validating CPC as a ratiometric sensor. A Job’s plot and the MS analysis implied that there was only the formation of a CPC/Cu2+ complex with 1:1 stoichiometry. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Copper ion is the third most abundant transition metal in the human body and is crucial for the fundamental physiological processes of organisms [1,2]. Meanwhile, as a significant environmental pollutant, excess Cu2+ intake will alter the cellular homeostasis, resulting in neurodegenerative diseases [3–5]. The maximum permissible levels of Cu2+ in drinking water by World Health Organization and the US Environmental Protection Agency are 2.0 ppm (∼30 ␮M) and 1.3 ppm (∼20 ␮M), respectively [6]. Thus, extensive research efforts, especially developing fluorescent sensors has been devoted to the measurement of Cu2+ because the fluorescent technique displays evident advantages in operational simplicity, high sensitivity, and fast response time [7–12]. Generally, fluorescent sensors are classified into intensity-based and ratiometric sensors. The main limitation of the former is that the univariate measurement of the fluorescence intensity is prone to be disturbed in quantitative detection [13,14]. In principle, this problem can be alleviated by using ratiometric sensors, which measure the ratio of fluorescence intensities at two different wavelengths [15]. However, due to the intrinsic paramagnetic nature of Cu2+ [16,17], the design of ratiometric sensors for Cu2+ is more difficult than for other metals [18]. The main strategies of ratiometric

∗ Corresponding author. Tel.: +86 452 2742563; fax: +86 452 2742563. E-mail addresses: [email protected] (X. Guo), [email protected] (L. Jia). http://dx.doi.org/10.1016/j.snb.2015.04.100 0925-4005/© 2015 Elsevier B.V. All rights reserved.

Cu2+ sensors were relied on the intramolecular charge transfer (ICT) [19–22], the fluorescence resonance energy transfer (FRET) [18,23–26], and the monomer–excimer transformation [27–29]. These sensors usually showed the bivariate changes of fluorescent signals, which was evident by the simultaneous decrease and increase in two emission intensities upon binding Cu2+ [30]. Using the univariate changes instead of the bivariate ones in the ratiometric detection would improve the measurement accuracy of ratiometric sensors [31]. And this could be realized via the utilization of a stable internal standard in combination with two different cation-responsive fluorophores of the sensor, which possess two almost independent fluorescent emissions. Recently, Guo et al. developed a dual fluorophore-based ratiometric Cu2+ sensor through this strategy, which could respond to concentrations of Cu2+ at the level of 10−5 M [32]. Carboxamidoquinoline as a fundamental platform was widely applied to construct sensors for Zn2+ or Cu2+ [33–42]. Previously, we reported a series of carboxamidoquinoline-based sensors that showed ratiometric signals for Zn2+ through the ICT process [43,44] or the solubilizing effect of SDS micelles [31]. Herein, two fluorophores, carboxamidoquinoline and coumarin, were conjugated via a piperazine linker as a new sensor CPC (Scheme 1). Since there is a large overlap in excitation spectra of two fluorophores of CPC (280–400 nm for carboxamidoquinoline [44] and 250–380 nm for coumarin [45,46]), the dual independent emissions of two fluorophores would be simultaneously excited by the same excitation wavelength. It was expected that two fluorophores would show different fluorescent signals after binding Cu2+ , offering the ability of CPC as a ratiometric sensor.

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2. Experimental

J = 4.2 Hz, 1H), 8.74 (d, J = 7.2 Hz, 1H), 8.63 (d, J = 8.4 Hz, 1H), 8.55 (d, J = 8.4 Hz, 1H), 8.11 (t, J = 7.5 Hz, 1H), 7.71 (d, J = 8.4 Hz, 1H), 7.60 (m, 2H), 3.36 (COCH2 , s, 2H), 3.05 (CON(CH2 CH2 )2 NH, s, 4H), 2.73 (CON(CH2 CH2 )2 NH, s, 4H) ppm.

2.1. Materials and apparatus Deionized water was used throughout the experiment. All the reagents were obtained from commercial suppliers and were used without further purification. The tris–HCl buffer solution (pH 7.24) was prepared using 10 mM tris, and proper amount of hydrochloric acid under adjustment by a pH meter. The fluorescence quantum yield was determined using that of quinine bisulphate (˚ = 0.55) in 100 mM H2 SO4 as a standard [47]. NMR spectra were measured on a Bruker AV-600 spectrometer with chemical shifts reported as ppm. Mass spectra were measured on a Waters Xevo UPLC/G2-S QTof MS spectrometer. IR spectra were obtained on a Nicolet AVATAR-370 FT-IR instrument, and samples were prepared as KBr pellets. Melting points were determined by an X-6 micro-melting point apparatus and uncorrected. All pH measurements were made with a Sartorius basic pH-meter PB-10. Fluorescence and absorption spectra were recorded on a Hitachi F-4500 fluorescence spectrometer and a Pgeneral TU-1901 UV–vis spectrophotometer. 2.2. Synthesis of CPC 2.2.1. Preparation of 1 2-Chloroacetyl chloride (151 mg, 1.35 mmol) was dissolved in chloroform (5 mL), then added dropwise to a cooled, stirred solution of 2-(pyridin-2-yl)quinolin-8-amine (200 mg, 0.90 mmol) and pyridine (142 mg, 1.80 mmol) in chloroform (10 mL) within 1 h, after stirred 2 h at rt, the mixture was evaporated to afford 1 (95% yield, 255 mg), which was purified by column chromatography on silica gel with chloroform–ethyl acetate (20:1, v/v) as the eluent. Mp: 193.4–194.5 ◦ C. 1 H NMR (CDCl3 , 600 MHz): ı 11.23 (NHCO, s, 1H), 8.75 (m, 2H), 8.68 (m, 2H), 8.31 (d, J = 8.4 Hz, 1H), 7.92 (t, J = 7.6 Hz, 1H), 7.60 (m, 2H), 7.41 (t, J = 5.6 Hz, 1H), 4.40 (COCH2 , s, 2H). 2.2.2. Preparation of 2 Compound 1 (200 mg, 0.68 mmol), piperazine (292 mg, 3.40 mmol), N,N-diisopropylethylamine (176 mg, 1.36 mmol), and potassium iodide (10 mg) were added to acetonitrile (30 mL). The stirred mixture was heated at reflux for 8 h under a nitrogen atmosphere. Then the mixture was cooled to rt and evaporated to afford 2 (76% yield, 179 mg), which was further purified by column chromatography on silica gel with chloroform–methanol (5:1, v/v) as the eluent. Mp: 220.5–221.8 ◦ C. 1 H NMR (DMSO-d6 , 600 MHz): ı 11.26 (NHCO, s, 1H), 8.82 (d, J = 7.8 Hz, 1H), 8.80 (d,

3.1. Selectivity for metal ions sensing To verify the selectivity, the absorptive and fluorescent behavior of CPC was investigated upon addition of various metal ions in acetonitrile–water solution. As seen in Fig. 1a, the absorption spectrum of free CPC showed a maximum centered at 322 nm (ε = 1.7 × 104 M−1 cm−1 ). There were no appreciable changes of the absorption maximum in the presence of Na+ , K+ , Mg2+ , Ca2+ , Ni2+ , Zn2+ , Ag+ , Cd2+ , Hg2+ , or Al3+ , but the absorbance at 322 nm was slightly increased by Co2+ (ε = 1.8 × 104 M−1 cm−1 ). Complexation with Cu2+ led to a considerable red shift of the absorption maximum from 322 to 415 nm (ε = 2.7 × 103 M−1 cm−1 ) due to the coplanar effect between the quinoline ring and the pyridinyl group, which increased the conjugate area and reduced the energy gap between the HOMO–LUMO. When the excitation wavelength was at 290 nm, free CPC (˚0 = 0.062) showed two obvious fluorescent bands centered at 355 and 470 nm (Fig. 1b), which were assigned to the emission of 450

a)

0.34

2+

Co No one, Na+, K+, Mg2+, 2+ 2+ 2+ + Ca , Ni , Zn , Ag , 2+ 2+ 3+ Cd , Hg , Al

0.17

2+

Cu 0.00

3. Results and discussion

Fluorescence Intensity (au)

Absorbance

0.51

2.2.3. Preparation of CPC Coumarin-3-carbonyl chloride (92 mg, 0.44 mmol) and compound 2 (100 mg, 0.29 mmol) were added to chloroform (20 mL). After stirred at rt for 2 h under a nitrogen atmosphere, the mixture was evaporated to afford CPC (79% yield, 119 mg), which was further purified by column chromatography on silica gel with chloroform–methanol (30:1, v/v) as the eluent. Mp: 243.5–244.6 ◦ C. 1 H NMR (CDCl , 600 MHz): ı 11.46 (NHCO, s, 1H), 8.88 (d, J = 9.0 Hz, 3 1H), 8.81 (m, 2H), 8.64 (d, J = 8.4 Hz, 1H), 8.31 (d, J = 8.4 Hz, 1H), 8.09 (t, J = 7.5 Hz, 1H), 7.96 (s, 1H), 7.61 (t, J = 7.8 Hz, 1H), 7.57 (m, 3H), 7.47 (t, J = 5.7 Hz, 1H), 7.38 (d, J = 8.4 Hz, 1H), 7.35 (t, J = 8.1 Hz, 1H), 4.01 (COCH2 , s, 2H), 3.66 (CON(CH2 CH2 )2 N, s, 2H), 3.42 (CON(CH2 CH2 )2 N, s, 2H), 2.86 (CON(CH2 CH2 )2 N, s, 2H), 2.79 (CON(CH2 CH2 )2 N, s, 2H) ppm. 13 C NMR (CDCl3 , 100 MHz): 168.10, 163.64, 157.85, 155.65, 154.33, 154.15, 149.49, 143.68, 138.07, 137.31, 137.03, 134.26, 133.05, 128.60, 128.06, 127.64, 124.99, 124.84, 124.65, 121.85, 121.32, 119.43, 118.21, 116.83, 116.79, 62.84, 53.64, 53.21, 47.09, 41.96 ppm. HRMS m/z (TOF MS ES+ ): calcd for C30 H26 N5 O4 + (M+H+ ) 520.1979, found 520.2164. FT-IR (KBr): max 3309.42, 2921.23, 2827.24, 1728.04, 1683.09, 1634.05, 1519.64, 1470.60, 1237.68, 996.59, 759.59 cm−1 .

300

400 Wavelength (nm)

500

b)

+

+

2+

No one, Na , K , Mg , 2+ 2+ 2+ + Ca ,Ni , Zn , Ag , 2+ 2+ 3+ Cd , Hg , Al 2+

Co

300

150 2+

Cu 0 300

400

500 600 Wavelength (nm)

700

Fig. 1. Absorbance (a) and fluorescence spectra (b) of 10 ␮M CPC in the presence of different metal ions (5 equiv.) at pH 7.24 (10 mM Tris–HCl, CH3 CN–water = 3:2 (v/v), ex = 290 nm).

Y. Zhang et al. / Sensors and Actuators B 218 (2015) 37–41

a)

39

b)

Fig. 2. Absorbance (a) and fluorescence spectra (b) of 10 ␮M CPC in the presence of different concentrations of Cu2+ (0–3 equiv.) at pH 7.24 (10 mM Tris–HCl, CH3 CN–water = 3:2 (v/v), ex = 290 nm). (Inset) Absorbance at 415 nm and ratiometric calibration curve I470 nm /I355 nm as a function of Cu2+ concentration. The red line is the nonlinear fitting curve obtained assuming a 1:1 association between CPC and Cu2+ . (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

coumarin and carboxamidoquinoline, respectively. No appreciable spectral changes of CPC were observed in the presence of different metal ions except for Co2+ and Cu2+ . The fluorescence intensity at 470 nm was slightly quenched by Co2+ (˚Co(II) /˚0 = 0.78). However, the addition of Cu2+ to CPC in the solution led to the obvious fluorescence quenching of the band centered at 470 nm (˚Cu(II) /˚0 = 0.007), whereas the band centered at 355 nm remained invariable compared with that of the solution containing only CPC. This result exhibited that between two fluorophores, only carboxamidoquinoline could be quenching response to Cu2+ through the energy transfer from the photoexcited quinoline ring to its low-lying empty d-orbital [48]. 3.2. Ratiometric sensing of Cu2+ Cu2+

The signal response of CPC toward was recorded in both absorption and emission spectra. Upon the addition of increasing amounts of Cu2+ (0–1 equiv.), three new absorption bands appeared at 300, 352, and 415 nm, accompanied with gradually increasing in intensity. There was a 93 nm red shift of the maximum absorption with two isosbestic points at 361 and 372 nm, respectively (Fig. 2a). The linear increase of the absorbance at 415 nm (A415 nm) with the Cu2+ concentration up to a molar ratio (CPC/Cu2+ ) of 1:1 (linearly dependent coefficient: R2 = 0.9939) and the stable spectra at even higher Cu2+ concentration implied a 1:1 Cu2+ binding stoichiometry of CPC. Meanwhile, as the concentration of Cu2+ was increased (0–1 equiv.), there was a distinct emission quenching of the band centered at 470 nm, and the quenching amplitude reached up to 96%; while the band centered at 355 nm remained almost intact in intensity and position during the whole titration (Fig. 2b). Thus, the emission of 355 nm could act as a stable internal standard, validating CPC as a ratiometric Cu2+ sensor and transforming from the bivariate changes of fluorescent signals to the univariate ones in the ratiometric detection. The emission ratio at 470 and 355 nm (I470 2+ nm /I355 nm ) decreased linearly with the Cu concentration until the 2+ CPC/Cu molar ratio reached 1:1 (linearly dependent coefficient: R2 = 0.9902). Subsequently, the emission spectra of CPC remained stable. This result was consistent with that from absorption spectra, also implying the formation of a complex with 1:1 stoichiometry of CPC and Cu2+ . A Job’s plot, which exhibited a maximum at 0.5 M fraction of Cu2+ , indicated that only a 1:1 complex was formed in acetonitrile–water solution (Fig. 3). According to the titration profiles [49,50], the apparent 1:1 association constants of the CPC/Cu2+ complex was calculated to be 2.2 × 106 M−1 (nonlinearly

Fig. 3. A Job’s plot for CPC (forms 1:1 complexes) at pH 7.24 (10 mM Tris–HCl, CH3 CN–water = 3:2 (v/v)). The total [CPC] + [Cu2+ ] = 10 ␮M.

dependent coefficient: R2 = 0.9971). This data are two orders of magnitude higher than the value reported by Guo [32], also indirectly revealed that the nitrogen atom of the pyridyl group played an important role in chelation to Cu2+ . And the detection limit (LOD) of CPC for Cu2+ was measured to be 4.6 × 10−7 M (3 per slope). Further evidence for the binding mode of CPC with Cu2+ was provided by HRMS. As seen in Scheme 2, the mass spectrum of CPC in the presence of Cu2+ manifested a series of peaks at m/z 581.1336,

(i) N NH2

N O

N

NH Cl

N 1 (ii)

(iii)

N O

NH N

N O N

N O

NH

N

O N

CPC O Scheme 1. The synthetic route of CPC.

NH 2

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Acknowledgments This work was financially supported by the National Natural Science Foundation of China (21176125), the Natural Science Foundation of Heilongjiang Province of China (B201313, B201419), and the Research Foundation of Education Bureau of Heilongjiang Province of China (2012TD012, 1254G066).

Appendix A. Supplementary data

Scheme 2. The proposed binding mechanism of CPC with Cu2+ .

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2015.04.100

References

Fig. 4. Metal ion selectivity profile of 10 ␮M CPC in the presence of Cu2+ (1 equiv.) and additional metal ions (5 equiv.) at pH 7.24 (10 mM Tris–HCl, CH3 CN–water = 3:2 (v/v), ex = 290 nm).

582.1343, 583.1311, 584.1323, and 585.1357, which were assigned as [M−H+ +Cu2+ ]+ , [M+Cu2+ ]2+ , [M+H+ +Cu2+ ]3+ , [M+2H+ +Cu2+ ]4+ , and [M+3H+ +Cu2+ ]5+ , respectively. Moreover, the deprotonation of the 8-carboxamino group induced by Cu2+ was precedent [36,51], and Cu2+ formed four-coordinated complexes [52,53]. The binding of Cu2+ by the nitrogen atoms of the quinoline ring and the pyridinyl group was in good agreement with the large absorption red shift in the ensemble relative to free CPC. However, the repeated efforts for crystallization of the CPC/Cu2+ complex were not successful, and the detailed mechanism warrants further studies. To explore the anti-disturbance of CPC as the Cu2+ -selective sensor, competition experiments were performed in the presence of Cu2+ mixed with background metal ions such as Na+ , K+ , Mg2+ , Ca2+ , Co2+ , Ni2+ , Zn2+ , Ag+ , Cd2+ , Hg2+ , and Al3+ . The solutions containing Cu2+ and various metal ions showed evident fluorescence quenching similar to that of Cu2+ alone, indicating that the Cu2+ -specific response was not disturbed by the other tested metal ions (Fig. 4).

4. Conclusions In summary, a dual fluorophore-based ratiometric Cu2+ sensor CPC was reported through conjugating two different Cu2+ -responsive fluorophores. CPC exhibited high selectivity toward Cu2+ over the other tested metal ions at pH 7.24 in acetonitrile–water solution. The utilization of the stable emission as an internal standard realized the univariate changes in the ratiometric detection. However, the excitation wavelength of CPC is 290 nm and this is a destructive light for many species. Therefore, the development of new ratiometric sensors with visible light excitation by using this strategy is in progress.

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Biographies Yu Zhang received his PhD degree in applied chemistry in 2009 from Dalian University of Technology. He is currently an associate professor at Qiqihar University. His scientific interests mainly focus on developing fluorescent sensors. Xiangfeng Guo received his PhD degree in applied chemistry in 2004 from Dalian University of Technology. He is currently a professor at Qiqihar University. His scientific interests include supramolecular chemistry, surfactant and heterogeneous catalysis. Xiujuan Tian is studying for MS degree in chemical engineering at Qiqihar University. Her scientific interests mainly focus on developing fluorescent sensors. Andong Liu received his BSc degree in chemical engineering and technology in 2012 from Qiqihar University. His scientific interests mainly focus on developing fluorescent sensors. Lihua Jia received her PhD degree in applied chemistry in 2004 from Dalian University of Technology. She is currently a professor at Qiqihar University. Her scientific interests include supramolecular chemistry and heterogeneous catalysis.