A single chemosensor for the detection of dual analytes Cu2+ and S2− in aqueous media

Tetrahedron xxx (2016) 1e9

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A single chemosensor for the detection of dual analytes Cu2þ and S2 in aqueous media Dae Yul Park, Ka Young Ryu *, Jin Ah Kim, So Young Kim, Cheal Kim * Department of Fine Chemistry and Department of Interdisciplinary Bio IT Materials, Seoul National University of Science and Technology, Seoul 139-743, South Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 April 2016 Received in revised form 2 May 2016 Accepted 6 May 2016 Available online xxx

A multifunctional colorimetric chemosensor 1, N-(2-((2,4-dinitrophenyl)amino)phenyl)-2-((2hydroxyethyl)amino)acetamide was synthesized and used to sense Cu2þ and S2 in aqueous solution. The sensor 1 showed highly selective colorimetric responses to Cu2þ and S2 by immediately changing its color from pale yellow to light green and pink, respectively, without any interference from other metal ions and anions. Especially, 1 can detect Cu2þ (10.1 mM) below the guideline (31.5 mM) of WHO. Moreover, the sensor 1 could be used to quantify Cu2þ ion in water samples. The sensing mechanism of Cu2þ by 1 were proposed to be a metal-to-ligand charge-transfer (MLCT) with the experimental results and theoretical calculations, and that of S2 by 1 proposed to be a deprotonation process. Ó 2016 Elsevier Ltd. All rights reserved.

Keywords: Copper Sulfide Colorimetric Naked-eye

1. Introduction The design of artificial synthetic probes that can selectively recognize metal ions, anions and neutral species in chemical, clinical, environmental, and biological samples has gained considerable notice of researcher worldwide.1e4 Copper, the third most plentiful essential trace element found in the human physiology and one of the foremost metals known to humans, is indispensable for carrying out several necessary processes both in the plants and animals.5,6 However, copper in excessive amounts could exhibit toxicity by causing oxidative stress and disorders associated with neurodegenerative diseases including Menkes, Wilson’s, Parkinson’s, Alzheimer’s, and prion diseases.7e16 In recent years, copper has been also suspected to cause kidney and liver damage.17,18 Based on these reports, the World Health Organization (WHO) has set the maximum allowable level of copper in drinking water at 31.5 mM.19 Therefore, the development of copper probes with high sensitivity and selectivity capable of rapidly monitoring Cu2þ has attracted considerable attention and is in high demand. As a member of the reactive sulfur species family, hydrogen sulfide (H2S) has drawn much attention due to its effects on environmental toxins and poisons for centuries.20 It is largely generated in coal and natural gas processing, petroleum industries, biogas

* Corresponding authors. Tel.: þ82 2 970 6693; fax: þ82 2 973 9149 (C.K.); e-mail addresses: [email protected] (K.Y. Ryu), [email protected] (C. Kim).

production, automobile tail gas, and sewage treatment plants.21,22 On the other hand, several studies have shown that H2S participate in many physiological processes, such as angiogenesis, vasodilation, regulation of inflammation, neuromodulation, and apoptosis.23e26 In addition, it has also proved that abnormal H2S production is linked to human diseases such as Alzheimer’s disease, Down’s syndrome, hypertension, and liver cirrhosis.27e30 Therefore, the quantitative detection of H2S is of great significance for both environmental and biological systems.31 Several methods, such as electrochemical, fluorescence techniques, inductively coupled plasma emission spectroscopy, inductively coupled plasma mass spectroscopy and atomic absorption spectroscopy have been used to detect metal ions and anions.32e40 Most of these methods require intricate and expensive procedures, while colorimetric methods can simply and conveniently monitor target ions in the visible range with high sensitivity, low cost, simplicity, and rapidity.41e45 Dinitrobenzene with an electron acceptor part (eNO2 group) has been frequently used in recent years as the chromogenic dye in chemosensors.46e48 In addition, the chromogenic dye with NH2CH2OH moiety is usually water-soluble. Therefore, we designed and synthesized a new receptor 1, based on the combination of the dinitrobenzene and the aminoethanol moieties, which was expected to endow it with a unique photophysical properties and water solubility. Herein, we report on a new receptor 1, which was synthesized by coupling 2-chloro-N-(2-((2,4-dinitrophenyl)amino)phenyl) acetamide with aminoethanol (Scheme 1). The receptor 1 detected

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Scheme 1. Synthetic procedure of 1.

Cu2þ by color change from pale yellow to light green and S2 from pale yellow to pink in aqueous solution. The detection mechanisms were proposed for Cu2þ with metal-to-ligand charge-transfer (MLCT) and for S2 with the deprotonation process. 2. Experimental 2.1. General information All the solvents and reagents (analytical grade and spectroscopic grade) were obtained from SigmaeAldrich and used as received. N1-(2,4-Dinitrophenyl)benzene-1,2-diamine was synthesized according to the literature method.49 1H NMR and 13C NMR spectra were recorded on a Varian 400 and 100 MHz spectrometer. Chemical shifts (d) were reported in ppm, relative to tetramethylsilane Si(CH3)4. Absorption spectra were recorded at room temperature using a Perkin Elmer model Lambda 25 UV/Vis spectrometer. Electrospray ionization mass spectra (ESI-mass) were collected on a Thermo Finnigan (San Jose, CA, USA) LCQTM Advantage MAX quadrupole ion trap instrument. Elemental analysis for carbon, nitrogen, and hydrogen was carried out by using a Vario micro cube elemental analyzer (ELEMENTAR) in laboratory center of Seoul National University of Science and Technology, Korea. 2.2. Synthesis of 2-chloro-N-(2-((2,4-dinitrophenyl)amino) phenyl)acetamide (2) The N1-(2,4-dinitrophenyl)benzene-1,2-diamine (0.27 g, 1 mmol) and 2-chloroacetylchloride (0.16 mL, 2 mmol) were dissolved in 5 mL of tetrahydrofuran. Then, this solution was stirred for 1 h at room temperature. The yellow powder produced was filtered, washed using ethanol and diethyl ether, and air-dried. The yield: 0.28 g (80%, solid) and mp¼100e102  C. IR (KBr): (cm1)¼ 3284(m), 3054(w), 1658(m), 1558(m), 1496(s), 1420(w), 1334(s), 1131(m), 1062(w).y 1H NMR (400 MHz, DMSO-d6) d 9.99 (s, 1H), 9.90 (s, 1H), 8.91 (d, J¼4 Hz, 1H), 8.21 (d, J¼8 Hz, 1H), 7.74 (d, J¼8 Hz, 1H), 7.47 (m, J¼8 Hz, 2H), 7.38 (t, J¼8 Hz, 1H), 6.77 (d, J¼8 Hz, 1H), 4.20 (s, 2H) ppm. 13C NMR (100 MHz, DMSO-d6) d 165.9, 147.4, 136.9, 133.9, 131.8, 130.9, 130.3, 128.5, 128.5, 126.9, 125.6, 123.6, 117.1, 43.3 ppm.z The 13C NMR spectrum of 2 was well consistent with that obtained from theoretical expectation.x 2.3. Synthesis of N-(2-((2,4-dinitrophenyl)amino)phenyl)-2((2-hydroxyethyl)amino)acetamide (1) The 2 (0.35 g, 1 mmol) and ethanolamine (0.09 mL, 1.5 mmol) were dissolved in 5 mL of acetonitrile. Then, triethylamine (0.14 mL, 1 mmol) was added into the reaction mixture, which was stirred for

y We provided physical state, melting point, IR and the yield of compounds 1 and 2 in Sections 2.2 and 2.3. z We provided the J values for multiplets in 1H NMR and corrected the digit of 13C NMR in Sections 2.2 and 2.3. x The 13C NMR spectrum of 2 was satisfied with its the molecular structure. Therefore, we added some comments on it in Section 2.2.

5 d at room temperature. The solvent was removed under reduced pressure to obtain orange oil, which was purified by silica gel column chromatography (9:1 v/v CHCl3/CH3OH). The yield: 0.129 g (34%, oil) and IR (KBr): (cm1)¼3252(m), 3104(w), 2923(m), 1673(m), 1585(s), 1505(s), 1420(m), 1329(s), 1270(m), 1129(m).y 1H NMR (400 MHz, DMSO-d6) d 10.01 (s, 1H), 8.90 (d, J¼4 Hz, 1H), 8.20 (d, J¼8 Hz, 1H), 8.00 (d, J¼8 Hz, 1H), 7.42 (m, J¼8 Hz, 2H), 7.29 (t, J¼8 Hz, 1H), 6.69 (d, J¼8 Hz, 1H), 4.43 (t, J¼8 Hz, 1H), 3.22 (m, J¼8 Hz, 2H), 3.19 (s, 2H), 2.37 (t, J¼8 Hz, 2H). 13C NMR (100 MHz, DMSO-d6) d 171.1, 147.5, 136.9, 134.8, 131.6, 130.5, 129.4, 128.8, 128.6, 125.8, 123.7, 123.5, 117.1, 60.5, 52.8, 52.0 ppm.z 1H NMR spectrum of 1 was well consistent with that obtained from theoretical expectation.{ ESI-MS m/z [MþHþ]þ: calcd, 376.12; found, 376.13. Elemental Anal. Calcd (%) for C16H17N5O6: C, 51.20; H, 4.57; N, 18.66; Found: C, 51.14; H, 4.43; N, 18.27. 2.4. UVevis titration For Cu2þ; receptor 1 (3.75 mg, 0.01 mmol) was dissolved in dimethylsulfoxide (DMSO, 1 mL) and 9 mL of the receptor 1 (10 mM) was diluted to 2.991 mL of bisetris buffer/DMSO (7/3, v/v) to make the final concentration of 30 mM. Cu(NO3)2$2.5H2O (2.4 mg, 0.01 mmol) was dissolved in DMSO (1 mL). 0e81 mL of the Cu(NO3)2 solution (10 mM) was transferred to the receptor 1 solution (30 mM) prepared above. After mixing them for a few seconds, UVevis spectra were taken at room temperature. For S2; receptor 1 (3.75 mg, 0.01 mmol) was dissolved in DMSO (1 mL) and 9 mL of the receptor 1 (10 mM) was diluted to 2.991 mL of bisetris buffer/DMSO (3/2, v/v) to make the final concentration of 30 mM. Sodium sulfide nonahydrate (24.01 mg, 0.1 mmol) was dissolved in bisetris buffer (1 mL). 0e130.5 mL of the S2 solution (100 mM) was transferred to the receptor 1 solution (30 mM) prepared above. After mixing them for a few seconds, UVevis spectra were taken at room temperature. 2.5. Job plot measurement For Cu2þ; receptor 1 (3.75 mg, 0.01 mmol) was dissolved in DMSO (1 mL) and 300 mL of the receptor 1 (10 mM) was diluted to 29.7 mL of bisetris buffer/DMSO (7:3, v/v) to make the final concentration of 100 mM. Cu(NO3)2$2.5H2O (2.4 mg, 0.01 mmol) was dissolved in DMSO (1 mL) and 300 mL of the Cu2þ solution (10 mM) was diluted to 29.7 mL of bisetris buffer/DMSO (7:3, v/v) to make the final concentration of 100 mM. 4.5, 4.0, 3.5, 3.0, 2.5, 2.0, 1.5, 1.0, and 0.5 mL of the 1 solution were taken and transferred to vials. 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, and 4.5 mL of the Cu2þ solution were added to solutions of 1 prepared above. Each vial had a total volume of 5 mL. After shaking the vials for a few seconds, UVevis spectra were taken at room temperature. For S2; receptor 1 (3.75 mg, 0.01 mmol) was dissolved in DMSO (1 mL) and 300 mL of the receptor 1 (10 mM) was diluted to 29.7 mL

{ The 1H NMR spectrum of 1 was satisfied with its the molecular structure. Therefore, we added some comments on it in Section 2.3.

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of bisetris buffer/DMSO (3:2, v/v) to make the final concentration of 100 mM. Sodium sulfide nonahydrate (24.01 mg, 0.1 mmol) was dissolved in bisetris buffer (1 mL) and 30 mL of the S2 solution (100 mM) was diluted to 29.97 mL of bisetris buffer/DMSO (3:2, v/ v) to make the final concentration of 100 mM. 4.5, 4.0, 3.5, 3.0, 2.5, 2.0, 1.5, 1.0, and 0.5 mL of the 1 solution were taken and transferred to vials. 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, and 4.5 mL of the S2 solution were added to solutions of 1 prepared above. Each vial had a total volume of 5 mL. After shaking the vials for a few seconds, UVevis spectra were taken at room temperature. 2.6. Competition with other metal ions or anions For Cu2þ; receptor 1 (3.75 mg, 0.01 mmol) was dissolved in DMSO (1 mL) and 9 mL of this solution (10 mM) was diluted with 2.991 mL of bisetris buffer/DMSO (7:3, v/v) to make the final concentration of 30 mM. MNO3 (M¼Na, K; 0.01 mmol), M(NO3)2 (M¼Mn, Co, Ni, Cu, Zn, Cd, Mg, Ca, Pb; 0.01 mmol), M(ClO4)2 (M¼Fe; 0.01 mmol), and M(NO3)3 (M¼Al, Fe, Cr, Ga, In; 0.01 mmol) was separately dissolved in DMSO (1 mL). 76.5 mL of each metal solution (10 mM) was taken and added into 3 mL of each receptor 1 (30 mM) prepared above to make 8.5 equiv. Then, 76.5 mL of the Cu(NO3)2 solution (10 mM) was added into the mixed solution of each metal ion and receptor 1 to make 8.5 equiv. After mixing them for a few seconds, UVevis spectra were taken at room temperature. For S2; receptor 1 (3.75 mg, 0.01 mmol) was dissolved in DMSO (1 mL) and 9 mL of this solution (10 mM) was diluted with 2.991 mL of bisetris buffer/DMSO (3:2, v/v) to make the final concentration of 30 mM. Tetraethylammonium salts of F, Cl, Br and I, and tetra   butylammonium salts of OAc, PO3 4 , N3 , SCN and BzO (0.1 mmol), 2 and sodium salts of SCN, N 3 , SO4 was separately dissolved in bisetris buffer (1 mL). 126 mL of each anion solution (100 mM) was taken and added into 3 mL of each receptor 1 (30 mM) prepared above to make 140 equiv. Then, 126 mL of the sodium sulfide nonahydrate solution (100 mM) were added into the mixed solution of each anion and receptor 1 to make 140 equiv. After mixing them for a few seconds, UVevis spectra were taken at room temperature. 2.7. pH effect test For Cu2þ; a series of buffers with pH values ranging from 2 to 12 was prepared by mixing sodium hydroxide solution and hydrochloric acid in bisetris buffer. After the solution with a desired pH was achieved, receptor 1 (3.75 mg, 0.01 mmol) was dissolved in DMSO (1 mL), and then 9 mL of the receptor 1 (10 mM) was diluted with 2.991 mL of bisetris buffer/DMSO (7:3, v/v) to make the final concentration of 30 mM. Cu(NO3)2$2.5H2O (2.4 mg, 0.01 mmol) was dissolved in DMSO (1 mL). 76.5 mL of the Cu2þ solution (10 mM) was transferred to each receptor solution (30 mM) prepared above. After mixing them for a few seconds, UVevis spectra were obtained at room temperature. For S2; a series of buffers with pH values ranging from 2 to 12 was prepared by mixing sodium hydroxide solution and hydrochloric acid in bisetris buffer. After the solution with a desired pH was achieved, receptor 1 (3.75 mg, 0.01 mmol) was dissolved in DMSO (1 mL), and then 9 mL of the receptor 1 (10 mM) was diluted with 2.991 mL of bisetris buffer/DMSO (3:2, v/v) to make the final concentration of 30 mM. Sodium sulfide nonahydrate (24.01 mg, 0.1 mmol) was dissolved in bisetris buffer (1 mL). 126 mL of the S2 solution (100 mM) was transferred to each receptor solution (30 mM) prepared above. After mixing them for a few seconds, UVevis spectra were obtained at room temperature. 2.8. Determination of Cu2D in water samples

and 0.60 mL of 50 mmol/L bisetris buffer stock solution to 2.391 mL sample solutions. After well mixed, the solutions were allowed to stand at 25  C for 2 min before the test. 2.9. Theoretical calculation methods All DFT/TDDFT calculations based on the hybrid exchange correlation functional B3LYP50,51 carried out using Gaussian 03 program.52 The 6-31G** basis set53,54 was used for the main group elements, whereas the Lanl2DZ effective core potential (ECP)55e57 was employed for Cu2þ. In vibrational frequency calculations, there was no imaginary frequency for the optimized geometries of 1 and 1-Cu2þ, suggesting that these geometries represented local minima. For all calculations, the solvent effect of water was considered by using the Cossi and Barone’s CPCM (conductor-like polarizable continuum model).58,59 To investigate the electronic properties of singlet excited states, time-dependent DFT (TDDFT) was performed in the ground state geometries of 1 and 1-Cu2þ. The 30 singletesinglet excitations were calculated and analyzed. The GaussSum 2.160 was used to calculate the contributions of molecular orbitals in electronic transitions. 3. Results and discussion The receptor 1 was synthesized by coupling of 2-chloro-N-(2((2,4-dinitrophenyl)amino)phenyl)acetamide (2) with ethanolamine in acetonitrile (Scheme 1). The precursor 2 was obtained by substitution reaction of N1-(2,4-dinitrophenyl)benzene-1,2diamine and 2-chloroacetylchloride in tetrahydrofuran. Compounds 1 and 2 were characterized by 1H NMR, 13C NMR, ESI-mass spectrometry and elemental analysis. 3.1. Sensing properties of 1 toward Cu2D ion We examined the chromogenic sensing ability of 1 in the presence of a various metal ions in a bisetris buffer/DMSO (7:3, v/v, 10 mM, pH 7.0). Only the addition of 8.5 equiv of Cu2þ to 1 caused a significant spectral change at 600 nmjj while other metal ions such as Naþ, Kþ, Mg2þ, Ca2þ, Al3þ, Ga3þ, In3þ, Cr3þ, Mn2þ, Fe2þ, Fe3þ, Co2þ, Ni2þ, Zn2þ Cd2þ, Hg2þ, Agþ, and Pb2þ did induce little or small spectral changes in absorption spectra (Fig. 1a). For example, Fe2þ and Fe3þ showed a small red-shift. Consistent with the change of UVevis spectrum, the receptor 1 showed an instant color change from pale yellow to light green in the presence of the Cu2þ (Fig. 1b). This result indicates that receptor 1 can serve as a potential candidate of a ‘naked-eye’ chemosensor for Cu2þ in aqueous solution. Additionally, we examined the discrimination ability of Cu2þ from Cuþ, but it did not work (Fig. S1).yy The binding properties of 1 with Cu2þ were studied by UVevis titration experiment (Fig. 2). Upon the addition of Cu2þ to a solution of 1, the absorption band at 359 nm significantly decreased and a new broad absorption band in the range of 500e800 nm gradually reached a maximum at 8.5 equiv of Cu2þ, resulting in a color change from pale yellow to light green. Two isosbestic points were observed at 336 nm and 390 nm,zz indicating that only one product was generated from 1 upon binding to Cu2þ. This light green color might be due to metal-to-ligand charge-transfer (MLCT), because the molar extinction coefficient in the thousands, 1.5103 M1 cm1, at 600 nm is too large to be Cu-based ded transitions.61e63

jj

We corrected 450 nme600 nm. We conducted the discrimination ability of Cu2þ and Cuþ. Unfortunately, it did not work. We described these results. zz We revised one isosbestic point to two isosbestic points at 336 nm and 390 nm. yy

UVevis spectral measurements of water samples containing Cu2þ were carried out by adding 9 mL of 1 mM stock solution of 1

3

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Fig. 1. (a) Absorption spectral changes of 1 (30 mM) in the presence of 8.5 equiv of various metal ions in a bisetris buffer/DMSO (7:3, v/v). (b) Color changes of receptor 1 (30 mM) in the presence of 8.5 equiv of various metal ions in a bisetris buffer/DMSO (7:3, v/v).

Fig. 2. UVevis spectra of receptor 1 (30 mM) upon the addition of Cu2þ in a mixture of bisetris buffer/DMSO (7:3, v/v).

The Job plot analysis showed a 1:1 stoichiometry of Cu2þ to 1 (Fig. S2).64 To further examine the binding mode between 1 and Cu2þ, a positive-ion ESI-mass experiment was carried out. The positive-ion mass spectrum of 1 with 1 equiv of Cu2þ showed the formation of the 1-HþþCu2þ complex [m/z 437.04, calcd m/z 437.04] (Fig. 3). The association constant (Ka) for the 1-Cu2þ complexation was determined as 7.8103 M1 through using non-linear fitting analysis (Fig. S3). This value is in the range of (103e1012) those reported for Cu2þ binding chemosensor.65e68 Based on the result of UVevis titration, the detection limit (3s/K)xx of receptor 1 for the analysis of Cu2þ ions was calculated to be 10.1 mM (Fig. S4), which is lower than the WHO guideline (31.5 mM) for Cu2þ in drinking water.19,69

xx

We corrected s to 3s.

Fig. 3. Positive-ion electrospray ionization mass spectrum of 1 (0.1 mM) upon addition of Cu2þ (0.1 mM).

Therefore, receptor 1 could be a good indicator for the detection of copper ions in aqueous solution. The preferential selectivity of 1 as chromogenic chemosensor for the detection of Cu2þ was studied in the presence of various competing metal ions. For the competitive studies, receptor 1 was treated with 8.5 equiv of Cu2þ in the presence of other metal ions (8.5 equiv), as indicated in Fig. 4a. There were no obvious interferences for the detection of Cu2þ in the presence of Cuþ, Naþ, Kþ, Mg2þ, Ca2þ, Ga3þ, Cr3þ, Mn2þ, Fe2þ, Co2þ, Ni2þ, Zn2þ Cd2þ, and Pb2þ, while Al3þ, In3þ, Fe3þ, Cr3þ and Hg2þ inhibited somewhat. Nevertheless, they are still discernible by ‘naked-eye’ (Fig. 4b).{{

{{ As the reviewer 1 suggested, we presented the interference of metal ions with the bar graph (Fig. 4a) and described some interferences with Al3þ, In3þ, Fe3þ, Cr3þ and Hg2þ.

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5

Fig. 4. (a) Competitive selectivity of 1 (30 mM) towards Cu2þ (8.5 equiv) in the presence of other metal ions (8.5 equiv) with a wavelength at 600 nm in a mixture of bisetris buffer/ DMSO (7:3, v/v). (b) Colorimetric competitive experiment of 1 (30 mM) in the presence of Cu2þ (8.5 equiv) and other metal ions (8.5 equiv) in a mixture of bisetris buffer/DMSO (7:3, v/v).

Thus, 1 could be used as an excellent colorimetric sensor for Cu2þ in the presence of most competing metal ions. We investigated the effect of pH on the absorption response of receptor 1 to Cu2þ ion in a series of solutions with pH values

Fig. 5. (a) UVevis spectral changes of 1 (30 mM) after the sequential addition of Cu2þ and EDTA in bisetris buffer/DMSO (7:3, v/v). (b) Color changes of 1 (30 mM) after the sequential addition of Cu2þ and EDTA.

ranging from 2 to 12 (Fig. S5). The color of the 1-Cu2þ complex remained in the light green color between pH 7 and 11. These results indicated that Cu2þ could be clearly detected by the naked eye or UVevis absorption measurements using 1 over the pH range of 7e11. To examine the reversibility of sensor 1 toward Cu2þ in bisetris buffer/DMSO (7:3, v/v), ethylenediaminetetraacetic acid (EDTA, 1 equiv) was added to the (solution of 1-Cu2þ complex) complexed solution of sensor 1 and Cu2þ. As shown in Fig. 5, the absorbance at 450 nm disappeared immediately. Upon addition of Cu2þ in the solution again, the color and absorbance were recovered. This result further supports that the response of 1 in the presence of Cu2þ is not due to decomposition process. The color and absorbance changes were almost reversible even after several cycles with the sequentially alternative addition of Cu2þ and EDTA. These results indicate that receptor 1 could be used as a reversible colorimetric chemosenor in aqueous solution. Additionally, we constructed a calibration curve for the determination of Cu2þ by 1 (Fig. S6). Receptor 1 exhibited a good linear relationship between the absorbance of 1 and Cu2þ concentration (0.00e40.00 mM) with a correlation coefficient of R2¼0.9919 (n¼3), which means that 1 could be suitable for quantitative detection of Cu2þ as well. In order to examine the applicability of the chemosensor 1 in environmental samples, water samples were chosen (Table 1) and showed a suitable recovery and R.S.D. values. These results suggested that the chemosensor 1 was suitable for the determination of Cu2þ concentration. To clearly demonstrate the sensing mechanisms of 1 toward Cu2þ, theoretical calculations were performed.jjjj The energyminimized structure of 1 showed the dihedral angle of 1C, 2C, 3C, 4C¼72.853 (Fig. 6a). 1-Cu2þ complex exhibited the dihedral angle of 1C, 2C, 3C, 4C¼123.409 , and Cu2þ was coordinated with 5N, 6N, 7N, 8O and 9O atoms of 1 (Fig. 6b). We also investigated the absorption to the singlet excited states of 1 and 1-Cu2þ species via

jjjjj

We explained the reason for energy-minimized structure of 1.

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Table 1 Determination of Cu(II) in water samples Sample Water sample

a

Cu(II) added (mmol/L)

Cu(II) found (mmol/L)

Recovery (%)

R.S.D. (n¼3) (%)

10.00c 16.00b,c

9.58 15.34

95.80 95.87

1.20 0.81

light green color. Molecular orbital diagrams and excitation energies of 1 and 1-Cu2þ are shown in Fig S9. Based on UVevis titration, Job plot, ESI-mass spectrometry analysis and theoretical calculations, we propose the binding mode of 1 with Cu2þ, as depicted in Scheme 2.

a Synthesized by deionized water, 10.00 mmol L1 Cu(II), Na(I), K(I), Ca(II), Mg(II), Cd(II), and Pb(II). b 16.00 mmol L1 of Cu2þ ions was artificially added into water sample. Conditions: [1]¼30 mmol L1 in 10 mM bisetris buffer-DMSO solution (7:3, pH 7.0). c As the reviewer 1 pointed out, we revised Table 1 in order to easily understand.

Scheme 2. Proposed structure of 1-Cu2þ complex.

3.2. Sensing properties of 1 toward S2L

Fig. 6. The energy-minimized structures of (a) 1 and (b) 1-Cu2þ complex.

TDDFT calculations. In the case of 1, the main molecular orbital (MO) contribution of the second lowest excited state was determined for the HOMO/LUMOþ1 transition (392.94 nm, Fig. S7), which indicated an intramolecular charge transfer (ICT) band to the dinitrobenzene group. For the 1-Cu2þ complex, the sixth lowest excited state was found to be relevant to the color change (pale yellow to light green) showing predominant MLCT (Fig. S8). The MLCT mainly indicated the MO changes from the metal-centered orbitals to the benzene and dinitrobenzene groups. Therefore, the chelation of Cu2þ with 1 induced MLCT transition, resulting in the

We have also examined the chromogenic sensing ability of 1 in the presence of a various anions in a mixture of bisetris buffer/ DMSO (3:2, v/v, 10 mM, pH 7.0) at room temperature. Upon addition of tetraethylammonium (TEA) salts of CN, F, Cl, Br, and I, and   tetrabutylammonium (TBA) salts of OAc, H2PO 4 , N3 , SCN and  2 2 BzO, and sodium salts of NO , HSO and S , only S induced 2 4 a distinct spectral change while other anions did not induce any spectral changes between 400 nm and 600 nm (Fig 7a). Consistent with the change of UVevis spectrum, the solution color of 1 changed from pale yellow to pink only with sulfide with fast response time (Fig. 7b), indicating that receptor 1 can serve as a ‘naked-eye’ indicator for S2. Interestingly, this is the first example that an organo-chemosensor simultaneously detects both Cu2þ and S2, to the best of our knowledge.70,71,yyy Additionally, S2 ion was added to a solution of 1-Cu2þ at pH 7 (Fig. S10). The color of the solution was changed to brown, indicating that CuS compound generated from the demetalation of 1-Cu2þ by S2.zzz The binding properties of 1 with S2 were studied by UVevis titration experiments (Fig. 8). Upon the treatment with S2 to solution of 1, the absorption band at 360 nm significantly decreased with an induction period, and the broad band in the range of 392e600 nm gradually reached a maximum at 140 equiv of S2. An isosbestic point was observed at 392 nm, indicating that only one product was generated from 1 upon binding to S2. This bathochromic shift of the absorption band led us to propose the transition of intra-molecular charge transfer (ICT) band through the deprotonation of the chemosensor 1 by the sulfide, based on Bhattacharya’s proposal.72e74 The negative charge of 1 generated from the deprotonation of the amine proton of 1 by S2 would lead to the red shift, resulting in the pink color. To further confirm the sensing mechanism between 1 and S2, the interaction between 1 and OH was also investigated (Fig. S11). UVevis spectral change of 1 upon addition of OH was nearly identical to that of 1 obtained from the addition of S2, which indicated the deprotonation between 1 and S2. These results suggest that the strong basicity of sulfide might be the specific interaction of 1 and S2, distinct from other anions.xxx The existence of 11 formed from the deprotonation was further confirmed by ESI-mass spectrometry analysis. The negative-ion mass spectrum showed the formation of the 1-Hþ [m/

yyy As the reviewer 1 pointed out, we added two papers related to the detection of both of Cu(II) and S2 into Refs. 70,71. zzz When S2 ion was added to a solution of 1-Cu2þ at pH 7, the color of the solution was changed to brown. We showed these results in Fig. S10 and described them. xxx We think that the strong basicity of sulfide might be the specific interaction of 1 and S2, distinct from other anions, based on Fig. S11. We added these comments.

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Fig. 7. (a) Absorption spectral changes of 1 (30 mM) in the presence of 140 equiv of various anions in a mixture of bisetris buffer/DMSO (3:2, v/v). (b) Color changes of receptor 1 (30 mM) in the presence of 140 equiv of various anions in a mixture of bisetris buffer/DMSO (3:2, v/v).

Scheme 3. Proposed sensing mechanism of sulfide by 1.

Fig. 8. (a) UVevis spectra of receptor 1 (30 mM) upon the addition of S2 in a mixture of bisetris buffer/DMSO (3:2, v/v).

z 374.60, calcd m/z 374.11] (Fig. S12). The Job plot also indicated a 1:1 stoichiometric ratio between the S2 and 1 (Fig. S13).64 Based on the UVevis titration, Job plot and ESI-mass spectrometry analysis, we proposed the sensing mechanism of 1 for S2 as shown in Scheme 3. In 1H NMR of 1, the protons on the dinitrobenzene ring appeared more down-field than those on the diaminobenzene ring, suggesting that the NH on the dinitrobenzene ring is more acidic than that on the diaminobenzene ring.{{{

{{{ In 1H NMR of 1, the protons on the dinitrobenzene ring appeared more down-field than those on the diaminobenzene ring. These results suggest that the NH on the dinitrobenzene ring might be more acidic than that on the diaminobenzene ring. We added these comments.

Based on the UVevis titration data, the binding constant for 1 with S2 was estimated to be 1.1103 M1 from non-linear fitting analysis (Fig. S14). The detection limit (3s/K) of receptor 1 as a colorimetric sensor for the analysis of S2 was found to be 0.14 mM (Fig S15).69 This value is about 50 times lower than a Secondary Maximum Contaminant Level (7.8 mM) by odor in drinking water, established by the EPA.75 Hence, receptor 1 could be used as a good detector for monitoring S2 in aqueous solution. To explore the ability of 1 as a colorimetric chemosensor for S2, the competition experiments were conducted in the presence of S2 mixed with various competing anions (Fig. 9). When 1 was treated with 140 equiv of S2 in the presence of other anions (140 equiv), there was no interference in the detection of S2 in the 2     presence OAc, I, N 3 , SCN , BzO , SO4 , NO2 and CN . However, there was little interference with the detection of S2 in the presence F, Cl, Br and PO3 4 . Nevertheless, the color change from pale yellow to deep pink was surely observed by ‘naked-eye’. Thus, 1 could be used as a selective colorimetric sensor for S2 in the presence of the competing anions. The effect of pH on the absorption response of receptor 1 to S2 was studied in a series of solutions with pH values ranging from 2 to 12 (Fig. S16). The color of the 1-S2 species remained in the pink color between pH 7 and 11. These results indicated that S2 could be clearly detected by the naked eye or UVevis absorption measurements using 1 over the pH range of 7.0e11.0.

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D.Y. Park et al. / Tetrahedron xxx (2016) 1e9

Fig. 9. (a) Competitive selectivity of 1 (30 mM) towards S2 (140 equiv) in the presence of other anions (140 equiv) in a mixture of bisetris buffer/DMSO (3:2, v/v). (b) Colorimetric competitive experiment of 1 (30 mM) in the presence of S2 (140 equiv) and other anions (140 equiv) in a mixture of bisetris buffer/DMSO (3:2, v/v).

4. Conclusion

Supplementary data

The dinitrobenzene-based chemosensor 1 was developed for the detection of Cu2þ and S2 in aqueous solution. The receptor 1 displayed a highly selective and sensitive colorimetric recognition toward Cu2þ by a color change from pale yellow to light green, and enabled the analysis of Cu2þ ions with a sensitivity limit of 10.1 mM, which was below the WHO acceptable limit (31.5 mM) in drinking water. The chemosensor 1 could be used to quantify Cu2þ ion in water samples. The sensing mechanism of Cu2þ by 1 was proposed to be the MLCT, which was supported by theoretical calculations. Moreover, the receptor 1 also detected S2 selectively, which induced an obvious color change from yellow to pink. Such selectivity resulted from the pushepull effect of the ICT transition by deprotonation between 1 and S2 ion. Interestingly, this is the first example that an organo-chemosensor simultaneously detects both Cu2þ and S2, to the best of our knowledge. On the basis of these results, we believe that receptor 1 will offer an important guidance to the development of single receptors for recognizing both cations and anions.

Supplementary data associated with this article can be found in the online version, at http://dx.doi.org/10.1016/j.tet.2016.05.016.

Acknowledgements Financial support from Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (NRF2014R1A2A1A11051794 and NRF-2015R1A2A2A09001301) are gratefully acknowledged. We thank Nano-Inorganic Laboratory, Department of Nano & Bio chemistry, Kookmin University to access the Gaussian 03 program packages.

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