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Colorimetric detection of Cu2+ and fluorescent detection of PO43- and S2- by a multifunctional chemosensor Dong Hee Joo, Jin Su Mok, Geon Hwan Bae, Sang Eun Oh, Cheal Kim, and Ji Hye Kang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01115 • Publication Date (Web): 09 Jul 2017 Downloaded from http://pubs.acs.org on July 15, 2017

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Industrial & Engineering Chemistry Research

Colorimetric detection of Cu2+ and fluorescent detection of PO43- and S2- by a multifunctional chemosensor

Dong Hee Joo,a Jin Su Mok,a Geon Hwan Bae,a Sang Eun Oh,a Ji Hye Kang*b and Cheal Kim*ab

a

Nowon Institute of Education for The Gifted at Seoultech, Seoul National University of Science and Technology, Seoul 139-743, Korea

b

Department of Fine Chemistry and Department of Interdisciplinary Bio IT Materials, Seoul National University of Science and Technology, Seoul 139-743, Korea. Fax: +82-2-973-9149; Tel: +82-2-970-6693; E-mail: [email protected] (J.H. Kang), [email protected] (C. Kim).

Abstract A simple multifunctional chemosensor 1 was synthesized by a combination of Naminophthalimide and 8-hydroxyjulolidine-9-carboxaldehyde. The sensor 1 showed a clear color change toward Cu2+ from pale to deep yellow and significant fluorescence enhancements toward PO43- and S2-. The detection limit (0.14 µM) for Cu2+ ion was below the World Health Organization (WHO) guideline for drinking water (31.5 µM). The sensor 1 could be used to quantify copper ion in real water samples. In addition, the sensor 1 could detect phosphate and sulfide with the fluorescent enhancements in aqueous solution. The sensing mechanism of Cu2+ by 1 was proposed to be an intramolecular charge transfer (ICT) and those of PO43- and S2- by 1 were proposed to be a deprotonation process, based on the experimental results and theoretical calculations.

Keywords: colorimetric, fluorescent, copper, phosphate, sulfide, theoretical calculations

1

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1. Introduction In recent decades, the development of multifunctional sensors for the detection of two or more analytes has received considerable attentions due to their simplicity, low cost and efficiency in comparison with the previous single-target responsive chemosensors.1–3 Among diverse approaches for detecting both cations and anions, the colorimetric and fluorogenic analysis methods have received attention owing to their easy operation, instant response time, remote control and multiplicity of measurable parameters.4–8 Thus, the detection of both metal ions and anions by a multifunctional chemosensor via colorimetric and fluorogenic analysis is significantly crucial. Among the various metal ions, copper ion plays important roles as catalytic cofactor in various enzymes such as cytochrome c oxidase, tyrosinase and superoxide dismutase.9–14 However, the abnormal intake of Cu2+ can result in liver or kidney damage, gastrointestinal disturbance, dermal or eye irritation and the nerve disorder such as Alzheimer’s disease, Menke’s disease and Wilson’s disease.15–22 Moreover, copper can be a serious pollution source in environment because it has been used much in industry and agriculture.23–26 Therefore, the development of sensitive probes for Cu2+ is considerably important. Phosphate (Pi) in biological system is indispensable due to its crucial roles such as essential constituents of gene, the pivotal role in energy transduction and bone mineralization.27–31 However, irregular phosphate levels are associated with a number of diseases including calcification of tissues, muscle weakness, impaired leukocyte and irregularity in bone mineralization resulting in rickets or osteomalaciaphosphate.32,33 Nevertheless, the study on Pi is relatively small compared to that on pyrophosphate (PPi). Accordingly, significant interests have been recently paid for developing the chemosensors that could recognize Pi. Sulfide (S2-) is well known as a noxious gas with a rotten egg smell. However, S2- in living system has a significant effect in a variety of physiological processes such as vasodilation, cell growth, anti-oxidation, anti-inflammation, anti-apoptosis, neuromodulation of the brain and inhibition of insulin signaling.34–38 Nonetheless, unusual production of sulfide can cause numerous biochemical problems like Down’s syndrome, Huntington’s and Alzheimer’s diseases, suffocation, liver cirrhosis and hyperglycemia.39–43 For these reasons, substantial 2


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efforts have been devoted to efficiently detect sulfide. Herein, we present on a multifunctional chemosensor 1, obtained by the combination of 8hydroxyjulolidine-9-carboxaldehyde and N-aminophthalimide. The sensor 1 recognized copper ion with an obvious color change from pale to deep yellow and could quantify concentrations of Cu2+ in real water samples. Also, 1 as fluorescent sensor could be used to detect phosphate and sulfide in aqueous solution. Moreover, the sensing mechanisms of Cu2+, PO43-, and S2- were explained by theoretical calculations.

2. Experimental 2.1. Materials and instrumentation All the solvents and reagents (analytical and spectroscopic grade) were purchased from Sigma-Aldrich. Absorption spectra were recorded at room temperature using a Perkin Elmer model Lambda L25 UV/Vis spectrometer. 1H and

13

C NMR spectra were recorded on a

Varian 400 MHz and 100 MHz spectrometer and chemical shifts are recorded in ppm. Electro spray ionization mass spectra (ESI-MS) were collected on a Thermo Finnigan (San Jose, CA, USA) LCQTM Advantage MAX quadrupole ion trap instrument by infusing samples directly into the source using a manual method. Spray voltage was set at 4.2 kV, and the capillary temperature was at 80 oC. Elemental analysis for carbon, nitrogen, and hydrogen was carried out by using a MICRO CUBE elemental analyzer (Germany) in Laboratory Center of Seoul National University of Science and Technology, Korea. 2.2. Synthesis of 1 8-Hydroxyjulolidine-9-carboxaldehyde (0.27 g, 1.2 mmol) and N-aminophthalimide (0.17 g, 1 mmol) were dissolved in 10 mL of 1,4-dioxane. Then, three drops of hydrochloric acid were added into the reaction mixture, which was stirred for 4 h at room temperature until the precipitate appeared. An orange precipitate was collected by filtration, washed several times with cold ethanol and diethyl ether, and dried in vacuum to obtain the pure orange solid. The yield: 76%. 1H NMR (400 MHz, DMF-d7, ppm): δ 11.43 (s, 1H), 9.02 (s, 1H), 7.94 (m, 4H), 6.90 (s, 1H), 3.27 (q, 4H), 2.68 (t, J = 6.0 Hz, 4H), 1.91 (m, 4H); 13C NMR (100 MHz, DMF3



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d7, ppm, 25 °C): δ = 164.48, 163.79, 156.46, 147.17, 134.78, 130.55, 130.19, 123.24, 113.66, 106.44, 105.46, 49.76, 49.28, 26.84, 21.63, 20.73, 20.23. ESI-MS: m/z calcd for C21H19N3O3 + H+, 362.15; found, 362.10. Elemental analysis calcd (%) for C21H19N3O3: C, 69.79; H, 5.30; N, 11.63 %; found: C, 69.22; H, 5.41; N, 11.27%. 2.3. Chromogenic detection for copper ion 2.3.1. UV-vis titration of 1 with Cu2+ 1 (1.8 mg, 0.005 mmol) was dissolved in dimethylformamide (DMF, 1 mL) and 12 µL of this solution (5 mM) was diluted with 2.988 mL of buffer/DMF solution (3/2, v/v, 10 mM bis-tris, pH = 7.0) to make the final concentration of 20 µM. Cu(NO3)2·2.5H2O (23.7 mg, 0.1 mmol) was dissolved in bis-tris buffer (5 mL) and 1.5-13.5 µL of this Cu2+ solution (20 mM) were transferred to the sensor 1 solution (20 µM) prepared above. After mixing them for a few seconds, UV-vis spectra were taken at room temperature. 2.3.2. Job plot measurement 1 (1.8 mg, 0.005 mmol) was dissolved in DMF (1 mL) and 300 µL of 1 (5 mM) was diluted to 29.7 mL with buffer/DMF solution (3/2, v/v, 10 mM bis-tris, pH = 7.0) to make the final concentration of 50 µM. 2.7, 2.4, 2.1, 1.8, 1.5, 1.2, 0.9, 0.6 and 0.3 mL of the 1 solution were taken and transferred to vials. Cu(NO3)2·2.5H2O (0.25 mmol) was dissolved in bis-tris buffer (10 mM, 5 mL) and 30 µL of the Cu2+ solution was diluted to 30.0 mL with buffer/DMF solution (3/2, v/v, 10 mM bis-tris, pH = 7.0) to make the final concentration of 50 µM. 0.3, 0.6, 0.9, 1.2, 1.5, 1.8, 2.1, 2.4 and 2.7 mL of the Cu2+ solution were added to each diluted 1 solution. Each vial had a total volume of 3 mL. After mixing them for a few seconds, UV-vis spectra were taken at room temperature. 2.3.3. Competitive experiment 1 (1.8 mg, 0.005 mmol) was dissolved in DMF (1 mL) and 12 µL of this solution (5 mM) was diluted with 2.988 mL of buffer/DMF solution (3/2, v/v, 10 mM bis-tris, pH = 7.0) to make the final concentration of 20 µM. MNO3 (M = Ag, Na, K, 0.1 mmol) or M(NO3)2 (M = Zn, Cu, Cd, Fe, Hg, Co, Ni, Mg, Ca, Pb, Mn, 0.1 mmol) or M(NO3)3 (M = Al, Ga, In, Fe, Cr, 0.1 mmol) was separately dissolved in 10 mM bis-tris buffer (5 mL). 10.5 µL of each metal4


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ion solution (20 mM) was taken and added to 2.990 mL of the 1 solution (20 µM) to give 3.5 equiv of metal ions. Then, 10.5 µL of Cu2+ solution (20 mM) was added into the mixed solution of each metal ion and 1 to make 3.5 equiv. After mixing them for a few seconds, UV-vis spectra were taken at room temperature. 2.4. Fluorogenic detection for PO43- and S22.4.1. UV-vis titrations 1 (1.8 mg, 0.005 mmol) was dissolved in DMF (1 mL) and 12 µL of this solution (5 mM) was diluted with 2.988 mL of buffer/DMSO solution (7/3, v/v, 10 mM bis-tris, pH = 7.0) to make the final concentration of 20 µM. K3PO4 (or Na2S) (3 mmol) was dissolved in bis-tris buffer (5 mL) and 1.0-23.0 µL of this PO43- (or 1.0-27.0 µL of the S2-) solution (600 mM) were transferred to the sensor 1 solution (20 µM) prepared above. After mixing them for a few seconds, UV-vis spectra were taken at room temperature. 2.4.2. Fluorescence titrations 1 (1.8 mg, 0.005 mmol) was dissolved in DMF (1 mL) and 12 µL of this solution (5 mM) was diluted with 2.988 mL of buffer/DMSO solution (7/3, v/v, 10 mM bis-tris, pH = 7.0) to make the final concentration of 20 µM. K3PO4 (or Na2S) (3 mmol) was dissolved in bis-tris buffer (5 mL) and 1.0-16.0 µL of this PO43- (or 1.0-20.0 µL of the S2-) solution (600 mM) were transferred to the sensor 1 solution (20 µM) prepared above. After mixing them for a few seconds, fluorescence spectra were taken at room temperature. 2.4.3. Job plot measurements 1 (1.8 mg, 0.005 mmol) was dissolved in DMF (1 mL) and 300 µL of 1 (5 mM) was diluted to 29.7 mL with buffer/DMSO solution (7/3, v/v, 10 mM bis-tris, pH = 7.0) to make the final concentration of 50 µM. 2.7, 2.4, 2.1, 1.8, 1.5, 1.2, 0.9, 0.6 and 0.3 mL of the 1 solution were taken and transferred to vials. K3PO4 (or Na2S) (3.0 mmol) was dissolved in bis-tris buffer (10 mM, 5 mL) and 2.5 µL of the PO43- (or S2-) solution was diluted to 30.0 mL with buffer/DMSO solution (7/3, v/v, 10 mM bis-tris, pH = 7.0) to make the final concentration of 50 µM. 0.3, 0.6, 0.9, 1.2, 1.5, 1.8, 2.1, 2.4 and 2.7 mL of the PO43- (or S2-) solution were added to each diluted 1 solution. Each vial had a total volume of 3 mL. After mixing them for 5



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a few seconds, UV-vis spectra were taken at room temperature. 2.4.4. 1H NMR titrations For 1H NMR titration of sensor 1 with PO43-, three NMR tubes of 1 (3.6 mg, 0.01 mmol) dissolved in DMF-d7 were prepared and then three different concentrations (0, 0.01 and 0.02 mmol) of K3PO4 dissolved in D2O were added to each solution of 1. After shaking them for a minute, 1H NMR spectra were taken at room temperature. 2.4.5. Competitive experiments Tetraethylammonium

(TEA)

salts

of

CN-,

F-,

Cl-, Br-

and I-

(3

mmol),

tetrabutylammonium (TBA) salts of OAc-, BzO-, N3- and SCN- (3 mmol), sodium salts of NO2-, S2-, HCO3-, CO32- and SO42- (3 mmol) and potassium salt of PO43- (3 mmol) were separately dissolved in 10 mM bis-tris buffer (5 mL). 18 µL of each anion solution (600 mM) was diluted to 2.955 mL of buffer/DMSO solution (7/3, v/v, 10 mM bis-tris, pH = 7.0). 15 µL of the PO43- (or 18 µL of the S2-) solution (600 mM) was added to solutions prepared above. Then, 12 µL of the sensor 1 solution (5 mM) was added to the mixed solutions to make a total volume of 3 mL. After stirring them for a few seconds, fluorescence spectra were taken at room temperature. 2.5. Theoretical calculations of 1, 1-Cu2+ and 1All theoretical calculations were carried out by the density functional theory (DFT) and time-dependent DFT (TD-DFT) method based on the hybrid exchange correlation functional B3LYP44,45 with the Gaussian 03 program.46 The main atoms were applied by the 6-31G (d, p) basis set47,48 and the copper element was employed by the LANL2DZ effective core potential (ECP).49–51 All geometries were completely minimized in the ground states (S0). By using the Cossi and Barone’s conductor-like polarizable continuum model (CPCM),52,53 we considered the solvent effect of DMF for the calculations of 1 and 1-Cu2+ complex and DMSO for the calculation of 1-. We calculated the lowest 20 singlet-singlet transitions using TD-DFT calculations at the ground state geometries (S0) to examine the transition energies for the minimized structures of 1, 1-Cu2+ complex and 1-. The GaussSum 2.154 was used to calculate the contribution of molecular orbitals (MO) in electronic transitions. 6


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3. Results and discussion Sensor 1 was synthesized by coupling N-aminophthalimide and 8-hydroxyjulolidine-9carboxaldehyde with 76 % yield in 1,4-dioxane (Scheme 1), and analyzed by 1H NMR (Fig. S1) and 13C NMR (Fig. S2), ESI-mass spectrometry and elemental analyses. 3.1. Colorimetric and spectral responses of 1 toward Cu2+ The colorimetric sensing properties of sensor 1 were investigated in the presence of 3.5 equiv of various metal ions such as Al3+, Ga3+, In3+, Zn2+, Cd2+, Cu2+, Fe2+, Fe3+, Mg2+, Cr3+, Hg2+, Ag+, Co2+, Ni2+, Na+, K+, Ca2+, Mn2+ and Pb2+ in buffer/DMF solution (3/2, v/v, 10 mM bis-tris, pH = 7.0). As shown in Fig. 1a, only Cu2+ induced a distinct spectral change, while other metal ions induced little or small spectral changes. Consistent with the UV-vis spectral change, the solution color of 1 in the presence of Cu2+ ion changed from pale yellow to deep yellow (Fig. 1b), demonstrating that sensor 1 could be used as a “naked-eye” chemosensor for Cu2+. Counter anion effect of Cu2+ ion was also studied with various anions such as SO42-, Cl-, and OAc- (Fig. S3). Nearly identical results were observed without respective to the type of counter anions. Additionally, we checked the time dependence of sensor 1 to Cu2+ (Fig. S4). It took about 10 min for the complete formation of 1-Cu2+ complex. To further examine the sensing properties of 1, UV-vis titration of 1 with Cu2+ ion was performed (Fig. 2). Upon the addition of Cu2+ to a solution of 1, the absorption peak at 383 nm gradually decreased, whereas the bands at 280 and 422 nm progressively increased with two clear isosbestic points at 341 and 398 nm. These results suggested that a single species was formed between 1 and copper ion. To investigate the binding mode of 1 and Cu2+ ion, we carried out the Job plot (Fig. S5)55 and ESI-mass spectrometry analysis (Fig. S6), which showed the 1:1 stoichiometric ratio for the formation of 1-Cu2+ complex. The positive-ion mass spectrum exhibited the formation of the 1 + Cu2+ + NaCl + 3·H2O – H+ [m/z: 535.10; calcd, 535.05]. In order to further examine the interaction between 1 and Cu2+, we conducted 1H NMR titrations of 1 with Cu2+, but it was not successful due to a paramagnetic character of 1-Cu2+ complex.

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Based on UV-vis titration data, the binding constant of 1 with Cu2+ was calculated to be 1.6 × 104 M-1 from Li’s equation (Fig. S7).56 The value is within the range of those (103~1012) previously reported for Cu2+ chemosensors. The detection limit for Cu2+ was determined to be 0.14 µM (Fig. S8) via 3σ/K,57 which was about 200 times lower than the World Health Organization (WHO) guideline (31.5 µM) in drinking water.58 Also, a comparison between the present detection limit with the reported ones is presented in Table S1. To check a practical ability of 1 toward Cu2+, sensor 1 was treated with various interfering cations (Na+, K+, Mg2+, Ca2+, Cr3+, Mn2+, Fe2+, Fe3+, Co2+, Ni2+, Zn2+, Cd2+, Hg2+, Ag+, Al3+, Ga3+, In3+ and Pb2+) including 3.5 equiv of Cu2+ in buffer/DMF solution (3/2, v/v, 10 mM bistris, pH = 7.0). As shown in Fig. 3, there was no marked change from diverse metal ions compared with 1-Cu2+. These results indicated that chemosensor 1 could be used as a superior colorimetric sensor for Cu2+ over other relevant species. The pH effect of sensor 1 for detecting Cu2+ was tested in the wide pH range of 2 to 12. As shown in Fig. S9, the absorbance of 1-Cu2+ complex at 422 nm indicated a significant increase between pH 4 and 11. These results warranted that the chemosensor 1 could be used as the detector for Cu2+ ion over a broad pH range of 4-11 by the naked eye or UV-vis absorption measurement. To evaluate the practical use of sensor 1 with Cu2+ in environmental analysis, tap and drinking water samples were prepared and analyzed by using the calibration curve of 1 toward Cu2+ (Fig. S10). As shown in Table 1, the satisfactory recoveries and R.S.D. values were obtained for all samples. This indicated that chemosensor 1 could be suitable and useful for detection of Cu2+ ion with good precision and accuracy in real water samples. To further investigate the colorimetric sensing mechanism of sensor 1 binding to Cu2+, DFT calculations were performed using the Gaussian 03 program. In order to get the energyminimized structures of 1 and 1-Cu2+ complex, their geometric optimizations were carried out. The main atoms were applied by the B3LYP/6-31G (d, p) method basis set and the Cu atom was applied by the LANL2EZ/ECP methods. The energy-optimized structure of 1 indicated a slightly distorted structure with the dihedral angle of 2C, 3N, 6C, 7O = -8.551o (Fig. 4a). 1-Cu2+ complex showed a planer structure with the dihedral angle of 2C, 3N, 6C, 8


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7O = -0.224o (Fig. 4b). For further information on electronic transitions of 1 and 1-Cu2+ complex, TD-DFT calculations were executed at the optimized geometries (S0). The main molecular orbital (MO) contribution of the third lowest excited state of 1 was determined for the HOMO → LUMO+1 transition (392.50 nm, Fig. S11). For 1-Cu2+ complex, the main MO contributions of the sixteenth lowest excited state were determined for the HOMO (α) → LUMO+1 (α) and HOMO (β) → LUMO+2 (β) transitions (409.79 nm, Fig. S12), which manifested intramolecular charge transfer (ICT) transition from the julolidine group to the phthalimide one. As shown in Fig. S13, there was only a red shift (392.50 nm to 409.79 nm) by the chelation of 1 to Cu2+, which is well consistent with the change of UV-vis spectra. Based on Job plot, ESI-mass spectroscopy analysis and theoretical calculations, we proposed the structure of a 1:1 complex between 1 and Cu2+ as shown in Scheme 2.

3.2. Fluorescence spectroscopic studies of 1 toward PO43- and S2The fluorogenic sensing ability of 1 was studied in the presence of 180 equiv of other anionic species such as CN-, OAc-, F-, Cl-, Br-, I-, BzO-, N3-, PO43-, NO2-, SCN- and S2- with an excitation of 426 nm in buffer/DMSO solution (7/3, v/v, 10 mM bis-tris, pH = 7.0). Sensor 1 having no fluorescence intensity at 487 nm exhibited instantly a significant fluorescent enhancement with PO43- and S2- while there were no obvious changes in presence of other anions (Fig. 5). These results suggested that sensor 1 could have a potential ability of fluorescence chemosensor for PO43- and S2-. Importantly, this is the first example that a single chemosensor can detect the three analytes, Cu2+, PO43- and S2-, to the best of our knowledge. To investigate the binding affinity of sensor 1 and PO43-, the fluorescence titration was performed. As shown in Fig. 6, the emission intensity of 1 at 487 nm exhibited a progressive increase up to 150 equiv of PO43-. The photophysical interaction between 1 and PO43- was studied by UV-vis titration experiment (Fig. S14). The UV-vis absorbance gradually decreased at 375 nm and steadily increased at 430 nm with a clear isosbestic point at 320 nm. This red shift led us to propose that the ICT band of 1 might be enhanced by the deprotonation of the hydroxyl proton by phosphate (Scheme 3). For further information on the binding mode between 1 and PO43-, the Job plot analysis 9


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was carried out (Fig. S15), which suggested that sensor 1 responded to PO43- with the ratio of a 1:1 stoichiometry. Moreover, the negative-ion mass spectrum indicated the formation of the 1- + MeOH [m/z: 392.20; calcd, 392.16] (Fig. S16). Furthermore, we performed 1H NMR titrations to study the interaction between 1 and PO43-. Upon the addition of PO43- to 1, the phenolic proton H8 disappeared and the protons H3 and H4 exhibited slightly up-field shifts (Fig. S17). These results supported the proposal that the hydroxyl group of the julolidine moiety might be deprotonated by PO43-. Based on the fluorescence titration data, the association constant was estimated to be 5.70 × 102 M-1 from the Benesi-Hildebrand equation (Fig. S18).59 In order to study the practical efficacy of sensor 1 as a fluorogenic chemosensor for PO43-, the competition experiments were carried out in the presence of PO43- with various anions such as CN-, OAc-, F-, Cl-, Br-, I-, BzO-, N3-, NO2-, SCN-, S2-, HCO3-, CO32- and SO42- in buffer/DMSO solution (7/3, v/v, 10 mM bis-tris, pH = 7.0). As shown in Fig. S19, HCO3- and SO42- inhibited 35 % and 25 %, respectively, whereas there was no interference with other anions. These results indicated that sensor 1 could be used as an excellent fluorescent sensor for detection of PO43-. We investigated the pH dependence of 1 at various pH values for practical application (Fig. S20). There was no fluorescence intensity of 1 at pH range of 2-10, whereas the fluorescent emission of 1 in the presence of PO43- was significantly enhanced at pH range of 7-10. These results indicated that sensor 1 could detect phosphate at pH range of 7-10. Next, fluorescence titration was carried out in order to examine interaction properties of sensor 1 toward S2- (Fig. 7). Upon the addition of S2-, sensor 1 exhibited a gradual emission enhancement up to 180 equiv. To study the binding properties of 1 toward S2-, UV-vis titration experiment was performed (Fig. S21a). If one looks carefully into the UV-vis titration, there was a two-step change (0-110 equiv and 120-270 equiv). In the first step (Fig. S21b), the absorbance at 383 nm considerably decreased and the one at 430 nm increased with clear isosbestic points at 343 and 406 nm. In the second step (Fig. S21c), the absorbance at 377 nm decreased and the one at 430 nm significantly increased with obvious isosbestic points at 320 and 390 nm. The two-step change led us to propose the sensing mechanism of 1 toward S2- as shown in Scheme 4. 10


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In order to understand the binding mode between sensor 1 and S2-, Job plot analysis

55

was

conducted (Fig. S22). Sensor 1 responded to S2- with the ratio of a 1:1 stoichiometry. In addition, the negative-ion mass spectrum displayed the formation of the 1- + MeOH [m/z: 392.19; calcd, 392.16] (Fig. S23). Using the fluorescence titration data, the association constant was determined to be 6.40 × 102 M-1 from the Benesi-Hildebrand equation (Fig. S24).59 To investigate the competitive ability of 1 toward S2- in the presence of various anions, the competition experiments were executed in the solution containing both sulfide and other anions (CN-, OAc-, F-, Cl-, Br-, I-, PO43-, BzO-, N3-, NO2-, SCN-, HCO3-, CO32- and SO42-) in buffer/DMSO solution (7/3, v/v, 10 mM bis-tris, pH = 7.0). As shown in Fig. 8, sensor 1 as detector of sulfide had no or slight interruption by other anions. Therefore, 1 could be utilized as a fluorogenic sensor for detecting sulfide in the presence of diverse anions. These results showed that 1 could be utilized as a fluorogenic sensor for detecting sulfide in the presence of diverse anions. In order to obtain the information on the sensitivity of sensor 1 at wide pH range, we performed pH test (Fig. S25). The fluorescent intensity of 1 exhibited no obvious change at pH range from 2 to 10, while the fluorescent intensity of 1 with S2- showed a significant increase at pH range from 3 to 10. These results suggested that 1 could be used as a fluorescence sensor for S2- in a wide pH range. On the other hand, it is a challenge to distinguish PO43- from S2- by 1. As it has been well known that Cu2+ can form a stable complex with S2-, we added Cu2+ ions to 1-PO43- and 1-S2solutions, respectively (Fig. S26). The mixed solution of 1-PO43- with Cu2+ showed color change from deep yellow to green, and the solution color of 1-S2- with Cu2+ changed from deep yellow to brown. Therefore, when sensor 1 showed a fluorescence enhancement in the presence of an anion, it can be PO43- or S2-. In such a case, the appearance of brown color by the addition of Cu2+ indicates that the anion could be S2-, while it could be PO43- with green color. In order to obtain insight into the sensing mechanisms and optical properties of 1 for detecting phosphate and sulfide, DFT calculations were performed by using Gaussian 03 11



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program. The B3LYP/6-31G (d, p) method basis set was used for all the atoms. The calculated energy-optimized structures of 1 and 1- were shown in Fig. 4. The energyoptimized structure of 1 exhibited a slightly distorted structure with dihedral angles of 165.851o (1O, 2C, 4C, 5C) (Fig. 4a), whereas 1- indicated a twisted structure between the phthalimide and the julolidine groups with a dihedral angle of -107.062o (1O, 2C, 4C, 5C) (Fig. 4c). With TD-DFT calculations, the main transition property of 1 indicated the HOMO → LUMO+1 transition (392.50 nm, Fig. S11) and one of 1- showed the HOMO → LUMO+1 transition (440.63 nm, Fig. S27). These transitions were assigned as the enhanced ICT transition from the julolidine group to the phthalimide one (Fig. S28). There was only bathochromic shift (from 392.50 nm to 440.63 nm) by the deprotonation of 1 by PO43- or S2-, which is well consistent with the change of UV-vis spectra. These results supported the sensing mechanisms of 1 toward PO43- and S2-, which are proposed in Schemes 3 and 4.

4. Conclusion We have developed a selective chemosensor 1 for the colorimetric detection of copper ion and for the fluorescent detection of phosphate and sulfide. Sensor 1 showed the color change from pale to deep yellow in the presence of Cu2+ ion with a 1:1 stoichiometric ratio. The detection limit for Cu2+ was 0.14 µM, which was much lower than the drinking water guideline (31.5 µM) of WHO. In addition, sensor 1 could apply to quantify Cu2+ in real water samples. The colorimetric sensing mechanism of Cu2+ with 1 was proposed to be ICT mechanism by theoretical calculations. Moreover, sensor 1 could detect PO43- and S2- by the significant emission enhancement. Importantly, this is the first example that a single chemosensor can detect the three analytes, Cu2+, PO43- and S2-, to the best of our knowledge. The deprotonation process of 1 by PO43- and S2- was proposed by ESI-mass analysis, 1H NMR titrations and DFT calculations. Therefore, we believe that these results may contribute to the development of a new type of multifunctional chemosensors for multiple analytes including metal ions and anions.

Acknowledgements 12


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Basic Science Research Program through the National Research Foundation of Korea (NRF) (NRF-2016M3D3A1A01913239 for the Korea C1 Gas Refinery R&D Center) is gratefully acknowledged. This subject is also supported by Korea Ministry of Environment (MOE) as "The Chemical Accident Prevention Technology Development Project". We thank Nano-Inorganic Laboratory, Department of Nano & Bio chemistry, Kookmin University to access the Gaussian 03 program packages.

Supplementary Information Supplementary data (1H NMR and 13C NMR spectra, time dependence with copper, Job plot and ESI-mass spectroscopy analyses, association constant, detection limit, pH dependence, calibration curve with copper, theoretical calculations, UV-vis titrations with phosphate and sulfide, 1H NMR titrations with phosphate, competitive selectivity for phosphate and color change of 1 with phosphate and sulfide on addition of copper) associated with this article can be found in the online version, at http:// ~.

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Scheme 1. Synthetic procedure of 1.

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Scheme 2. Proposed binding mode of 1-Cu2+ complex.

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Scheme 3. The proposed sensing mechanism of PO43- by 1.

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Scheme 4. The proposed sensing mechanism of S2- by 1.

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Table 1. Determination of Cu2+ in water samples. Sample

Cu(II) added (µmol L-1)

Cu(II) found (µmol L-1)

Tap water

0.00

0.00

a

Drinking water

0.20

0.19

0.00

0.00

0.20a

0.21

Recovery (%)

R.S.D. (n = 3) (%)

95

0.31

105

0.20

Conditions: [1] = 20 µmol/L in 10 mM bis-tris buffer-DMF solution (3/2, v/v, pH 7.0). a 0.20 µM of Cu2+ ion was artificially added.

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Figure captions Figure 1. (a) Absorption spectra of 1 (20 µM) in the presence of diverse cations (3.5 equiv) in buffer/DMF solution (3/2. v/v, 10 mM bis-tris, pH = 7.0). (b) The color changes of 1 (20 µM) in the presence of diverse cations (3.5 equiv). Figure 2. Absorption spectral changes of 1 (20 µM) upon the addition of different concentrations of Cu2+ at room temperature. Inset: Plot of the absorption at 422 nm versus the number of equiv of Cu2+ added. Figure 3. (a) Competitive selectivity of 1 (20 µM) for Cu2+ (3.5 equiv) in the presence of other metal ions (3.5 equiv) in buffer/DMF solution (3/2. v/v, 10 mM bis-tris, pH = 7.0). (b) The competitive color changes of 1 (20 µM) in the absence and presence of other metal ions (3.5 equiv). Figure 4. Energy-optimized structures of (a) 1, (b) 1-Cu2+ complex and (c) 1-. Figure 5. Fluorescence spectra of 1 (20 µM) in the presence of different anions (180 equiv) in buffer/DMSO solution (7/3, v/v, 10 mM bis-tris, pH = 7.0). Inset: The fluorescence pictures of 1, 1-PO43- and 1-S2- (λex: 426 nm). Figure 6. Fluorescence spectral changes of 1 (20 µM) upon the addition of different concentrations of PO43- at room temperature. Figure 7. Fluorescence spectral changes of 1 (20 µM) upon the addition of different concentrations of S2- at room temperature. Figure 8. Competitive selectivity of 1 (20 µM) toward S2- (180 equiv) in the presence of various anions (180 equiv) in buffer/DMSO solution (7/3. v/v, 10 mM bis-tris, pH = 7.0).

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(a)

0.6

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1 and other metal ions Cu2+

0.4

0.2

Ag+, Fe2+ and Fe3+

0.0 300

400

500

Wavelength (nm) (b)

Fig. 1.

25



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Fig. 2.

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(a)

0.4

0.3

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.2

0.1

0.0

1

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 - - Cu2+ 3+ 3+ 2+ 3+ 2+ 2+ Al3+ Ga In Zn2+ Cd Fe2+ Fe Mg2+ Cr3+ Hg2+ Ag+ Co2+ Ni2+ Na+ K+ Ca2+ Mn Pb

(b)

Fig. 3.

27



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(a)

(b)

(c)

Fig. 4.

28


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Fig. 5.

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Fluorescence Intensity


[PO 3-]

200

4

150

100

50

0 450

500

550

600

Wavelength (nm) Fig. 6.

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[S2-]

200

150

100

50

0 450

500

550

600

Wavelength (nm) Fig. 7.

31


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



200

100

0

1

1 S2-

1 1 1 CN OAc F

1 Cl-

1 Br-

1 I-

1 1 1 1 1 1 1 1 32PO4 BzO N3 SCN NO2 HCO3 CO3 SO42-

Fig. 8.

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Graphical Abstract

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Colorimetric Detection of Cu2+ and Fluorescent ... - ACS Publications

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