Highly Sensitive Dansyl-Based Chemosensor for Detection of Cu2+ in

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Highly Sensitive Dansyl-Based Chemosensor for Detection of Cu2+ in Aqueous Solution and Zebrafish Ju Byeong Chae,† Dongju Yun,† Hangyul Lee,† Hyojin Lee,‡ Ki-Tae Kim,*,‡ and Cheal Kim*,† †

Department of Fine Chemistry and ‡Department of Environmental Engineering, Seoul National University of Science and Technology, Seoul 01186, Korea

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S Supporting Information *

ABSTRACT: A new dansyl-based chemosensor (2-(4-((5(dimethylamino)naphthalen-1-yl)sulfonyl)piperazin-1-yl)-N(quinolin-8-yl)acetamide) (DC) for detecting Cu2+ was synthesized and characterized. DC showed great selectivity to Cu2+ by a fluorescent “on−off” detection method. Job plot, ESI-mass spectroscopy, and 1H NMR titration suggested a 1 to 1 binding mode between DC and Cu2+. The detection limit was determined to be 43 nM, which is greatly below the WHO guidelines. In addition, DC can be applied to real samples and zebrafish imaging. The fluorescence quenching mechanism was proposed as the enhancement of intramolecular charge transfer with calculations.

1. INTRODUCTION Copper is a well-known conducting material and thus, used in many industrial fields such as electrical equipment, tube, pipe, and other machines.1 In addition, as the third richest transition metal in the body, it is involved in many pivotal functions.2,3 For instance, several enzymes and proteins such as cytochrome c oxidase and metallothionein use copper as a cofactor and depend on it for their activity.4,5 Because of its various roles, deficiency or an overload of copper could cause health problems such as liver cirrhosis and damage to the brain and other organs.6,7 Because human beings could be affected by the amount of copper in the environment, the detection of copper has attracted considerable attention.8,9 Several detection methods for sensing copper ions have been developed so far. For instance, inductively coupled plasma, atomic absorption spectroscopy, and other electrochemical tools have been used. However, they need sample pretreatment and expensive equipment, and the samples could be destroyed during the analysis process.10−12 Moreover, the methods could not apply to living organisms for bioimaging.13,14 On the contrary, chemosensors have received much attention because it could overcome those defects.15−17 It is easy-to-handle, cheap, and sensitive.18,19 Especially, chemosensors using fluorescence could be applied for real-time bioimaging of various analytes. Many chemosensors with a variety of chromophores and fluorophores have been used to detect Cu2+, such as BODIPY,20,21 rhodamine,19,22 coumarin,15,23 benzimidazole,24−26 and naphthalimide.27,28 The dansyl moiety is also used as a fluorophore of chemosensors for sensing various metal ions including Cu2+ because of its high quantum yield.6,29−37 However, its low solubility in water is a huge © 2019 American Chemical Society

obstacle for in vivo imaging. Hence, we envisioned the use of piperazine as a linker to combine the dansyl group and the quinoline moiety because quinoline derivatives show good water solubility with useful chromophore properties.38 That is, the chemosensor having both dansyl and quinoline moieties might show high selectivity to Cu2+ with good solubility in water. Herein, we presented a novel dansyl- and quinoline-based chemosensor (2-(4-((5-(dimethylamino)naphthalen-1-yl)sulfonyl)piperazin-1-yl)-N-(quinolin-8-yl)acetamide) (DC) for detecting Cu2+ by a fluorescence “on−off” mode. DC showed a successful zebrafish imaging with good water solubility. Based on diverse spectroscopic studies and density functional theory (DFT) calculations, we demonstrated the detection mechanism of DC for Cu2+.

2. RESULTS AND DISCUSSION 2.1. Fluorescent Turn-Off Detection of Cu(II) Ions. DC was obtained by the substitution reaction of N,N-dimethyl-5(piperazin-1-ylsulfonyl)naphthalen-1-amine (DP) and 2chloro-N-(quinolin-8-yl)acetamide (CQA) in dichloromethane (DCM, 58%, Scheme 1). It was characterized through 1 H and 13C NMR and ESI-MS (Figures S1 and S2). The stability of DC was tested in bis-tris buffer (pH 7.0; Figure S3), which indicated that its fluorescence intensity at 496 nm was stable enough for 1 h. The fluorescence selectivity test of DC to diverse cations was executed in bis-tris buffer. As shown in Figure 1, DC itself and DC with other metal ions showed Received: April 5, 2019 Accepted: June 14, 2019 Published: July 24, 2019 12537

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Scheme 1. Synthetic Procedure of DC

at 277 and 304 nm (Figure S4), implying that the only a species was given from the reaction of DC and copper ions. The job plot experiment was conducted for studying binding stoichiometry (Figure 3). The largest intensity difference (I0 −

Figure 1. Fluorescent spectra of DC (4 μM) with diverse cations.

strong fluorescence emissions at 496 nm, while the fluorescence was completely quenched with Cu2+ within 15 min. It indicated that DC may be used as a selective probe for detecting Cu(II) ions via a fluorescence “on−off” mode. The quantum yields (Φ) of DC and DC−Cu2+ turned to be 0.033 and 0.0035, respectively, with quinine as a standard fluorophore. For studying the binding properties, fluorescence titration of DC towards Cu2+ ions was conducted (Figure 2). On addition of Cu2+ to DC, the fluorescence emission of 496 nm was persistently decreased up to 16 equiv of Cu2+. UV−vis titration of DC with copper ions showing two defined isosbestic points

Figure 3. Job plot for the binding stoichiometry of DC (4 μM) and Cu2+.

I) between DC and DC−Cu2+ complexes was shown at 0.5 of the mole fractions, indicating that DC and Cu2+ binds with a 1 to 1 ratio. The result of positive ESI-mass analysis also supported the 1 to 1 binding ratio (Figure 4). The peak of m/z 565.30 could be assigned to [DC + Cu2+ − H+]+ (m/z 565.12). Moreover, 1H NMR titration was conducted to further apprehend the binding mechanism of DC−Cu2+

Figure 2. Fluorescent titration of DC (4 μM) with the addition of Cu2+. Inset: Fluorescent emission at 496 nm vs the amount of Cu2+.

Figure 4. Positive ESI-mass spectrum of DC with Cu2+. 12538

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Scheme 2. Proposed Binding Structure of DC−Cu2+

(Figure S5). On the addition of Cu2+ (0.5 equiv), the proton H11 disappeared and the protons H8, H9, and H10 shifted down or upfield. Further addition of Cu2+ ions caused most peaks to be broadened because of its paramagnetic property.53 These results suggested that the three N atoms of DC may coordinate to Cu2+. With the results of Job plot, ESI-MS, and 1H NMR titration, we proposed a possible binding mode of DC−Cu2+ (Scheme 2). On the basis of the 1 to 1 binding ratio, the association constant of DC and Cu2+ calculated was 2.2 × 104 M−1 (Figure S6).54 For practical applications, the competition experiment with other metal ions was conducted (Figure 5). Compared to the

Figure 6. Calibration curve of DC (4 μM) with the addition of Cu2+ (0−4.5 μM).

Table 1. Measurement of Copper(II) Iona samples drinking water tap water artificial pollutionb

Cu(II) added (μM)

Cu(II) found (μM)

0.00 8.00 0.00 8.00 0.00

0.00 8.00 0.00 7.98 0.00

8.00

7.99

recovery (%) 100

RSD (n = 3) (%) 0.55

99.82

0.48

99.89

0.85

Figure 5. Competition test of DC (4 μM) in the presence of Cu and coexisting metal ions (16 equiv).

[DC] = 4 μM in bis-tris buffer (0.01 M, pH = 7.0). bPrepared by using deionized water. 8 μM of Al3+, K+, Pb2+, Na+, and Ca2+ were added, respectively.

DC−Cu2+ complex, other coexisting metal ions did not interfere with fluorescence emission change. Therefore, this result implied that DC could effectively detect copper ions in the presence of several cations. The detecting ability of DC towards copper ions was tested in various pH ranges (Figure S7). DC showed a prominent fluorescence emission in the pH scope from 6 to 9, while the DC−Cu2+ complex showed a complete fluorescence quenching in the same pH range (Figure S7a). It was well matched with the photographs of DC itself and DC with Cu2+ (Figure S7b). This observation demonstrated that DC could be used for sensing copper ions in the pH scope from 6 to 9. In addition, DC was applied to detect copper ions in tap, drinking, and artificially polluted water samples. The calibration curve was used to determine the amount of copper ions in real samples (Figure 6). As displayed in Table 1, plausible recoveries and relative standard deviations (RSD) were obtained from three types of water

samples. The results suggested that DC could be used for quantifying Cu(II) ions in real samples. The detection limit from the calibration curve was 43 nM, which is far lower than the WHO recommendation (31.5 μM).55 To investigate the sensing ability of DC in live organisms, fluorescent imaging for zebrafish was performed (Figure 7). Zebrafish treated with DC (1 μM) for 15 min showed an obvious fluorescence signal (Figure 7a2). On addition of 5 μM of Cu(II) ions, the fluorescence emission in the zebrafish tail started to decrease (Figure 7b2). As the amount of copper ions increased to 20 μM, the fluorescence emissions gradually decreased and become nearly invisible (Figure 7c2,d2). Therefore, these results indicated that organism-permeable probe DC could monitor Cu2+ in living organisms. The mean emission of these images was given with Icy software (Figure S8). The calculated detection limit was 4.37 μM, which is still below the WHO standard. Importantly, this is the first report

a

2+

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major transition of DC was observed at 336.74 nm and its molecular orbital change represented π → π* transition in the quinoline moiety (Figures S9 and S11). The addition of Cu2+ induced the changes of the transition character of DC. The main transition was observed at 316.81 nm and its molecular orbitals showed intramolecular charge transfer (ICT) (Figures S10 and S11). The calculated blue shifted transition (336.74 nm → 316.81 nm) was well matched with the experimental UV−vis spectra. The orbital changes occurred from the dimethylamino group to the sulfonyl one. In addition, the oscillator strength was increased (0.1156 → 0.2081). Therefore, the binding of Cu2+ to DC-enhanced ICT and this nonradiative transition would cause fluorescence quenching. Moreover, the paramagnetic effect of Cu2+ would affect fluorescence quenching. The electron configuration (d9) of Cu2+ made intersystem crossing become faster. Then, the excitation energy was consumed in nonradiative transition and fluorescence was quenched. Based on the Job plot, ESI-MS, and theoretical calculations, we proposed the possible binding form of DC−Cu2+ and the sensing mechanism of copper ions by DC (Scheme 2).

Figure 7. The fluorescence images of living zebrafish treated with 1 μM of DC in the absence and presence of Cu2+. (a1−a3) Zebrafish were incubated with DC (1 μM) for 15 min; (b1−b3) subsequent treatment with Cu2+ (5 μM); (c1−c3) subsequent treatment with Cu2+ (10 μM); (d1−d3) subsequent treatment with Cu2+ (20 μM). Scale bar: 1 mm.

of the detection limit obtained in zebrafish imaging tests with Cu2+ chemosensors (Table S1).56−59 2.2. DFT/Time-Dependent DFT Calculations. The 1:1 stoichiometry for DC−Cu2+ was applied for calculations, based on ESI-mass and Job plot analyses. In Figure 8 energyminimized forms of DC and DC−Cu2+ are displayed. Imaginary frequencies were not observed in both structures, indicating that they were at the local minima. The three N atoms in the quinoline and piperazine moieties of DC coordinated to Cu2+. As Cu2+ bound to DC, the dansyl moiety was more rotated and the dihedral angle was significantly changed (1N, 2N, 3N, and 4N = −37.66°). We calculated the possible transitions of DC and DC−Cu2+ complexes using time-dependent DFT (TD-DFT), which is based on energy-optimized structures (Figures S9−S11). The

3. CONCLUSIONS We presented a unique dansyl- and quinoline-based chemosensor DC for sensing copper ions by a fluorescence “on−off” method. In order to increase solubility in water and selectivity to Cu2+, we combined a dansyl chloride and a quinoline moiety with piperazine as the linker. As we expected, DC displayed great selectivity and sensitivity towards copper ions with a low detection limit (43 nM) in aqueous solution. With the Job plot, ESI-MS, 1H NMR titration, and DFT calculations, we demonstrated the appropriate binding structure and detecting mechanism for Cu2+ by DC. Moreover, DC could be applied to the real water samples and zebrafish imaging. Importantly, we reported, for the first time, the detection limit of Cu2+ in

Figure 8. Energy-optimized forms of (a) DC and (b) DC−Cu2+. 12540

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anesthetized using 0.01% MS222 (ethyl-3-aminobenzoate methanesulfonate) after rinsing with E2 media to discard the surplus sensor DC and Cu2+. Fluorescence images were carried out on a MDG36 (Leica). The Icy software was used to determine the mean intensity of the images. 4.6. Theoretical Calculations. We obtained theoretical calculation results using the Gaussian 09W program.43 Before calculating electronic states of DC and DC−Cu2+ complexes, geometry optimizations were conducted using the DFT method.44,45 The hybrid functional was CAM-B3LYP, and the basis set was 6-31G(d,p). Also, 6-31G(d,p) was applied to all atoms except Cu2+.46,47 The LANL2DZ basis set was employed for applying effective core potential to Cu2+.48−50 In the frequency calculations, imaginary frequencies were not observed, indicating that the structures presented local minima. The solvent effect of water was regarded with CPCM.51,52 Based on energy-optimized structures of DC and DC−Cu2+, the UV−visible transition examination was demonstrated with TD-DFT method with the thirty lowest singlet states.

zebrafish imaging tests. We believed that sensor DC could be used as a competent tool for sensing Cu(II) ions by a fluorescence “on−off” method in aqueous solution and living organisms.

4. EXPERIMENTAL SECTION 4.1. General. Both 1H NMR and 13C NMR were recorded on a Varian spectrometer (400 and 100 MHz). The fluorescence and absorption spectra were gained with PerkinElmer spectrometers (LS45 for fluorescence and Lambda 2S for UV−visible). ESI-MS were recorded by using Waters (Milford, MA, USA) Acquity SQD quadrupole Mass instrument. Chemicals were provided from Sigma-Aldrich. CQA and DP were synthesized according to the literature.39,40 4.2. Synthesis of DC. Thirty milliliters of DCM was chosen to dissolve DP (8.0 mmol, 2.56 g) and CQA (8.0 mmol, 1.76 g). Triethylamine was added to the reaction solution, which was shaken for 4 d at 20 °C. After removing the solvent with an evaporator, the crystallized product was obtained and purified by chromatography (CH3OH/CH2Cl2 = 1:100). The yellow crystalline was obtained (0.59 g; 58%). 1H NMR in DMSO-d6: 11.17 (s, 1H), 8.65 (d, J = 8.4 Hz, 1H), 8.53 (m, 1H), 8.45 (d, J = 8.4 Hz, 1H), 8.34 (m, 1H), 8.21 (m, 1H), 7.76 (m, 2H), 7.63 (m, 2H), 7.53 (t, J = 8.0 Hz, 1H), 7.45 (m, 1H), 7.36 (d, J = 7.6 Hz, 1H), 3.25 (s, 2H), 3.22 (m, 4H), 2.91 (s, 6H), 2.67 (m, 4H). 13C NMR in DMSO-d6: 168.08 (1C), 151.51 (1C), 148.23 (1C), 137.80 (1C), 136.60 (1C), 133.63 (1C), 131.69 (1C), 130.72 (1C), 130.50 (1C), 130.06 (1C), 129.22 (1C), 128.31 (1C), 127.71 (1C), 127.06 (1C), 123.82 (1C), 121.83 (1C), 121.68 (1C), 119.19 (1C), 115.35 (1C), 115.24 (1C), 60.65 (1C), 51.75 (2C), 46.19 (2C), 45.13 (2C). ESI-mass (m/z) for C27H29N5O3S + H+: 504.21; found 504.32. 4.3. General Procedures for Fluorescent Applications. A concentrated solution (0.01 M) of DC was made in dimethylsulfoxide. DC was diluted to bis-tris buffer (pH 7.0, 0.01 M). For concentrated solutions of metal ions, nitrate salts were used, and perchlorate salt for Fe2+. All metal salts (In3+, Mn2+, Ni2+, Fe3+, Ca2+, Zn2+, Ag+, Ga3+, Pb2+, Cd2+, Cu2+, Hg2+, Fe2+, Cr3+, Co2+, Na+, Mg2+, Al3+, K+) were dissolved in DMF for concentrated solutions (0.01 M). 4.4. Quantum Yields of DC and DC−Cu2+. By using quinine (ΦF = 0.54 in 0.1 M of H2SO4) as a standard fluorophore,41 we calculated the quantum yield (Φ). The equation is in this way42

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.9b00970. Table of chemosensor examples for detecting Cu2+ in zebrafish, 1H and 13C NMR spectra of DC, timedependent study of DC and DC−Cu2+ complexes, UV− vis titration of DC with Cu2+, 1H NMR titration of DC with Cu2+, association constant of DC to Cu2+, pH dependent study of DC and DC−Cu2+ complexes, detection limit of DC for detecting Cu2+ in zebrafish imaging, and DFT calculation for DC and DC−Cu2+ complexes (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +82-2-971-6683. Fax: +822-970-9140 (K.-T.K.). *E-mail: [email protected]. Phone: +82-2-972-6640 (C.K.). ORCID

Cheal Kim: 0000-0002-7580-0374 Notes

The authors declare no competing financial interest.

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2

ΦF(X) = ΦF(S)(ASFX /AX FS)(nX /nS)

ACKNOWLEDGMENTS Korea Environment Industry & Technology Institute (2016001970001) and National Research Foundation of Korea (2018R1A2B6001686) are thankfully acknowledged.

where the meaning of each abbreviation is ΦF: fluorescent quantum yield, A: absorbance, F: area of fluorescent emission curve, n: refractive index of solvent, S: standard, and X: unknown. 4.5. Zebrafish Imaging. Living zebrafish embryos were cultured at 28 °C and kept under proper breeding conditions on a 10 h dark/12 h light cycle. For the fluorescent imaging, 6 day-old zebrafish were treated with E2 media (pH 7.1; 5 × 10−4 g/L methylene blue, 7 × 10−4 M sodium carbonate, 0.5 mM potassium chloride, 15 mM sodium chloride, 5 × 10−5 M Na2HPO4, 10−4 M magnesium sulfate, 1.5 × 10−4 M KH2PO4, and 1.0 mM calcium chloride) containing DC (1 μM) for 15 min. In the Cu2+ detection study, living zebrafish embryos were stained with Cu2+ (0, 5, 10, and 20 μM) for 14 min after washing with E2 media. All the fish were terminally

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REFERENCES

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DOI: 10.1021/acsomega.9b00970 ACS Omega 2019, 4, 12537−12543