Detection of multiple analytes (CN− and F−) based on a simple pyrazine-derived chemosensor in aqueous solution: Experimental and theoretical approaches

Sensors and Actuators B 207 (2015) 123–132

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Detection of multiple analytes (CN− and F− ) based on a simple pyrazine-derived chemosensor in aqueous solution: Experimental and theoretical approaches Jae Jun Lee, Gyeong Jin Park, Ye Won Choi, Ga Rim You, Yong Sung Kim, Sun Young Lee, Cheal Kim ∗ Department of Fine Chemistry and Department of Interdisciplinary Bio IT Materials, Seoul National University of Science and Technology, Seoul 139-743, Republic of Korea

a r t i c l e

i n f o

Article history: Received 13 August 2014 Received in revised form 14 October 2014 Accepted 18 October 2014 Available online 28 October 2014 Keywords: Cyanide Fluoride Colorimetric Multiple analytes DFT calculations

a b s t r a c t A new colorimetric receptor 1 for the detection of CN− and F− has been simply developed. Receptor 1 showed selectively colorimetric responses to CN− in a near-perfect aqueous solution and F− in 10% aqueous solution, respectively. An obvious color change of 1 from yellow to colorless was observed for CN− through a nucleophilic addition mechanism, while F− was detected through deprotonating mechanism with a distinct color change from yellow to orange. The binding modes of receptor 1 with two analytes (CN− and F− ) were proposed to be 1:1, based on Job plot, 1 H NMR titration, and ESI-mass spectrometry analysis. Moreover, the sensing mechanisms for F− and CN− were theoretically supported by DFT and TD-DFT calculation. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Development of chemical sensors for anions has received considerable interest, due to their important roles in biological, industrial and environmental processes [1]. Among the various anions, cyanide and fluoride have been intensively researched due to their wide use in our life. The wide use of cyanide as raw materials for synthetic fiber, resins, herbicides, and the gold-extraction process is inevitable, and also many industries produce nearly 140,000 tons of cyanide per year worldwide [2]. However, cyanide is known to be damaging anion causing poison in biology and environment. It has propensity to bind to the iron in cytochrome c oxidase, interfering with electron transport and resulting in hypoxia [3]. Meanwhile, fluoride, being the most electro-negative atom, is used for various purposes in biological, medical and chemical processes [4]. For example, fluoride prevents dental problems and osteoporosis [5,6]. Nevertheless, the presence of excess fluoride can cause skeletal fluorosis, bone diseases, mottling of teeth, lesions of the thyroid, liver and other organs [7]. Thus, there is a strong demand for an efficient sensing method to monitor cyanide and fluoride.

∗ Corresponding author. Tel.: +82 2 970 6693; fax: +82 2 973 9149. E-mail address: [email protected] (C. Kim). http://dx.doi.org/10.1016/j.snb.2014.10.081 0925-4005/© 2014 Elsevier B.V. All rights reserved.

While various approaches such as fluorescence techniques and electrochemical methods have been developed for the detection of cyanide and fluoride, the most attractive approach focuses on novel colorimetric cyanide and fluoride sensors, which allow nakedeye detection of the color change without resorting to the use of expensive instruments [8]. Therefore, colorimetric sensors that are capable of recognizing cyanide and fluoride in aqueous environment have to be developed. Over the past decades, many efforts have been devoted to design various chemosensors to detect cyanide and fluoride, including the formation of anion complexes with transition metals [9], boron center [10], ESIPT mechanism [11], hydrogen-bonding interactions [12], deprotonation [13], luminescence life time measurement [14], excimer–monomer switch [15] and nucleophilic addition reactions [16]. Especially, nucleophilic addition reactions and deprotonation mechanism have been intensively studied in the anion sensing area. Nevertheless, the development of chemosensors using both addition reactions and deprotonation process is still regarded as a challenge [17]. To overcome the challenge, therefore, we developed a new colorimetric sensor, which has both an imine moiety acting as a nucleophilic acceptor and an OH functional group acting as a deprotonation donor. Herein, we report a new pyrazine-based chemosensor 1, which was synthesized in one step by condensation reaction of aminopyrazine and 2-hydroxy-1-naphthalehyde (Scheme 1). Chemosensor

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Scheme 1. Synthetic procedure of chemosensor 1.

1 detected cyanide by color change from yellow to colorless in a near-perfect aqueous solution and fluoride by color change from yellow to orange in aqueous solution. The detection mechanisms were proposed for F− with the deprotonation process and for CN− with the nucleophilic addition reaction, which were supported by the DFT/DT-DFT calculation method. 2. Results and discussion The receptor 1 was obtained by coupling of aminopyrazine and 2-hydroxyl-1-naphthaldehyde with 70% yield in ethanol (Scheme 1) and analyzed by 1 H NMR, 13 C NMR, ESI-mass spectrometry and elemental analysis. 2.1. Colorimetric cyanide and fluoride sensing The absorption response of 1 toward various anions (TEA salts: F− , Cl− , Br− , I− , CN− , and TBA salts: OAc− , H2 PO4 − , N3 − , SCN− , BzO− ) in an aprotic solvent DMSO showed large changes with more basic anions OAc− , F− , H2 PO4 − and BzO− , while less basic anions Cl− , Br− , I− , and SCN− showed slight absorption changes and N3 − showed intermediate change (Fig. S1). Consistent with the change of UV–vis spectra, F− , OAc− , H2 PO4 − and BzO− caused the color of 1 to change from yellow to orange, whereas Cl− , Br− , I− and SCN− showed no color change. Intermediate basic anion N3 − showed a slight change in color (Fig. S1). Also, similar results were observed in MeCN (Fig. S2). On the other hand, the solution of 1 in a protic solvent MeOH did not show any color change in the presence of various anions, except for CN− . CN− caused the color of 1 to disappear regardless of solvent systems (DMSO, MeCN, and MeOH, see Figs. S1 and S2). For a practical application, we increased gradually the amount of water in DMSO. To our surprise, only F− and CN− showed color changes from yellow to orange and to colorless, respectively, in a mixture of DMSO/bis-tris (9/1, v/v), while other anions had no change in color (Fig. 1 (a)). The changes of UV–vis absorption of 1 were also observed with only F− and CN− (Fig. 1 (b)), consistent with the change of color. These phenomena could be possibly explained by the hydrogen bonding ability and the nucleophilic addition of the anions. The hydrogen bonding ability of the anions is assumed to be in order of F− > AcO− > H2 PO4 − > N3 − ∼ CN− , based on their electro-negativity. Thus, fluoride ion could form strong hydrogen bonding with 1 even in aqueous solution, which eventually renders the phenolic proton of 1 deprotonate, thus showing a bathochromic shift in UV–vis spectra. In case of 1 with CN− , the decolorization of 1 might be expected by nucleophilic addition process of CN− toward 1, forming the colorless leuconitrile species. First of all, the binding property of 1 with F− was studied by UV–vis titration experiment (Fig. 2). Upon addition of F− into 1, the absorption band at 434 nm decreased and a new absorption band at 468 nm steadily increased. An isosbestic point was observed at 337 nm, suggesting the formation of only one UV–vis active species. This bathochromic shift of the absorption band led us to propose the change of intramolecular charge transfer (ICT) band through the deprotonation of 1 by fluoride. The negative charge of

Fig. 1. (a) The color changes of 1 (15 ␮М) upon addition of various anions (150 equiv) in DMSO/bis-tris buffer (9/1, v/v, containing 10 mM bis-tris, pH = 7.0). (b) Absorption spectra changes of 1 (15 ␮М) in the presence of 150 equiv of different anions.

Fig. 2. Absorption spectra changes of 1 (15 ␮M) in the presence of different concentrations of F− (from 0 to 160 equiv) at room temperature. Inset: Absorbance at 471 nm versus the number of equiv of F− added.

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Fig. 3. Job plot for the binding of 1 with F− . Absorbance at 400 nm was plotted as a function of the molar ratio of [F− ]/([1] + [F− ]). The total concentration of F− with receptor 1 was 4 × 10−5 M.

1−1 generated from the deprotonation of the naphtholic proton of 1 by F− would lead to the red shift, resulting in the orange color [18]. To further confirm the sensing mechanism between 1 and F− , the interaction between 1 and OH− was also investigated (Fig. S3). UV–vis spectral change of 1 upon addition of OH− was nearly identical to that of 1 obtained from the addition of F− , which indicated the deprotonation between 1 and F− . The formation of 1−1 generated from the deprotonation was further confirmed by ESI-mass spectrometry analysis. The negative-ion mass spectrum showed the formation of the 1-H+ [calcd: 248.083, m/z: 248.467] (Fig. S4). The Job plot [19] referred to a 1:1 stoichiometry between 1 and F− (Fig. 3). Based on the UV-vis titration, Job plot and ESI-mass analysis, we proposed the sensing mechanism of 1 for F− as shown in Scheme 2. The binding constant (K) for 1 with F− was estimated by using Benesi–Hildebrand equation based on the UV–vis titration [20]. The K value was found to be 1.9 × 103 M−1 (Fig. S5). The detection limit of 1 for F− was determined to be 210 ␮M, based on the 3␴/slope (Fig. S6) [21]. To explore the ability of 1 as a colorimetric chemosensor for F− , the competition experiments were conducted in the presence of F− mixed with various competing anions (Fig. 4). When 1 was treated with 150 equiv of F− in the presence of the same concentration of other anions, there was no interference for the detection of F− in the presence of Cl− , Br− , I− , OAc− , H2 PO4 − , N3 − , SCN− , BzO− , NO2 − , HSO4 − and SH− . Only CN− interfered with the detection of F− . These results indicate that 1 could be a selective colorimetric sensor for F− in the presence of various competing anions.

Fig. 4. (a) Absorption spectra changes of 1 (15 ␮M) upon addition of fluoride (150 equiv) in the absence and presence of 150 equiv of various anions in DMSO/bistris buffer (9/1, v/v, containing 10 mM bis-tris, pH = 7.0). (b) The color changes of 1 (15 ␮М) upon addition of fluoride (150 equiv) in the absence and presence of 150 equiv of various anions in DMSO/bis-tris buffer (9/1, v/v, containing 10 mM bis-tris, pH = 7.0).

To further investigate the interaction behavior between chemosensor 1 and F− , 1 H NMR titration of 1 was investigated in the absence and presence of F− in DMSO-d6 (Fig. 5). Upon addition of 0.5 equiv of F− to the chemosensor 1, the proton signal of -OH at 14.9 ppm almost disappeared, indicating that the fluoride would deprotonate the naphtholic proton. Most of aromatic protons were shifted to upfield, which indicates that the negative charge originated from deprotonation of 1 by F− might be delocalized through the whole receptor molecule. On excess addition of F− (20 equiv) to 1 solution, a new peak at 16.1 ppm appeared, indicating the formation of [HF2 − ] species. Next, we carried out the sensing test of 1 to various anions in buffer solution. Importantly, only CN− showed the color change from yellow to colorless against various anions (Fig. 6). In consistency with the color change, only 1-CN− showed a complete decrease of absorption band at 447 nm. These results suggest that CN− might attack the imine group of 1 to produce the nucleophilic addition product, resulting in colorless (Scheme 3) [22].

Scheme 2. The proposed sensing mechanism of 1 for F− .

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Fig. 5.

1

H NMR titration of 1 with F− in DMSO-d6 .

Fig. 6. (a) The color changes of 1 (20 ␮М) upon addition of various anions (35 equiv) in bis-tris buffer/DMSO (99.99/0.01, v/v, containing 10 mM bis-tris, pH = 7.0). (b) Absorption spectra changes of 1 (20 ␮М) in the presence of 35 equiv of different anions.

In order to further investigate the binding property of 1 with CN− , UV–vis titration was carried out (Fig. 7). The absorption spectrum of 1 showed a broad band in a range of 410–490 nm, which might be attributed to the transition of ICT band. On treatment with CN− to solution of 1, the absorption band at 447 nm was gradually attenuated and reached minimum at 35 equiv of CN− (Fig. 7a). A noticeable decrease of the absorption band at 447 nm suggests that the transition of ICT might be interrupted from the nucleophilic addition reaction of CN− to 1 [23]. If one looks carefully into the UV–vis titration in Fig. 7a, there is the two-step change of the absorption bands. The first step up to 10 equiv of CN− shows that the absorption band at 447 nm gradually decreased with two isosbestic points at 341 and 499 nm (Fig. 7b). At this process, the solution color of 1 disappeared completely. The second step up to 12 equiv of CN− shows that the absorption peak at 447 nm also decreased, and one isosbestic point was observed at 348 nm (Fig. 7c). Based on these UV–vis processes, we propose that the first step is the nucleophilic addition process and the second step is an intra-molecular hydrogen bonding process (Scheme 3). The Job plot [19] analysis showed 1:1 stoichiometry for the 1-CN− (Fig. S7), which was further confirmed by ESI-mass spectrometry analysis. The negative-ion mass spectrum showed the formation of the 1-CN− complex [calcd: 275.094, m/z: 275.067 for 1-CN− and calcd: 543.382, m/z: 543.133 for 1-CN− + CN− +TBA+ ] (Fig. S8). Based on the UV–vis titration, Job plot, and ESI-mass spectrometry analysis, we propose the structure of 1:1 stoichiometry of 1 with CN− as shown in Scheme 3. The binding constant (K) for the nucleophilic addition step of 1 with CN− was estimated to be 2.1 × 105 M−1 (0–10 equiv) by using the Benesi–Hildebrand equation [20] based on UV–vis titration (Fig. S9). The value is within the range of those (1.0–1.0 × 105 ) reported for CN− chemosensor [24]. The detection limit (DL) of 1 for CN− was determined to be 19.09 ␮M (0–10 equiv), based on the 3␴/slope

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Scheme 3. The proposed sensing mechanism of 1 for CN− .

(Fig. S10) [21]. To further explore the ability of 1 as colorimetric sensor for CN− , the competition experiments were conducted in presence of CN− with other competing anions such as F− , Cl− , Br− , I− , OAc− , H2 PO4 − , N3 − , SCN− , BzO− , NO2 − , HSO4 − and SH− (Fig. 8). When 1 was treated with 35 equiv of CN− in presence of the same concentration of other anions, all these competing anions showed no obvious interference with naked-eye detection of CN− by 1. Also, KCN instead of TBACN was used to examine a counter-cation effect. As shown in Fig. S11, there was no effect for the replacement of TBACN with KCN. For biological application, the pH dependences of 1 in the absence and presence of CN− were examined at various pH. The decrease of absorbance caused by adding CN− was observed between 4 and 9 (Fig. S12), which warrants its application under physiological conditions with detection of CN− by 1. In order to further examine the proposed nucleophilic addition of CN− toward chemosensor 1, 1 H NMR titrations were performed (Fig. 9). Upon addition of 3.0 equiv of CN− , the H8 protons of imine group at 10.0 ppm gradually disappeared and a new H8 proton at 6.8 ppm started to appear. This result strongly suggests that the nucleophilic addition of CN− occur at the carbon atom of imine group of 1 [22,25(a)]. Most of aromatic protons were shifted to upfield, which suggest that the negative charge developed from the nucleophilic addition of CN− toward 1 might be delocalized through the whole receptor molecule. Furthermore, 13 C NMR spectra of 1 in the absence and presence of 3.0 equiv CN− were taken (Fig. S13). The C15 carbon of the imine group at 155.1 ppm was shifted to upfield (141.5 ppm) and a new peak corresponding to CN carbon (C16 ) appeared at 119.6 ppm. Most of aromatic carbons were shifted to upfield. These results also support that 1 would detect CN− by the nucleophilic addition. 3. Theoretical calculations for sensing mechanism of the two analytes F− and CN− 3.1. Optimized structures In parallel to the experimental study, to further get understanding on the electronic structures of 1, 1-F− and 1-CN− , we

optimized energy-minimized structures of chemosensor 1, 1-F− and 1-CN− at DFT/B3LYP/6-31G** level. Their energy-minimized structures are shown in Fig. S14 and the various physical properties are compared between 1 and the two adducts in Table S1. The comparison of the dipole moments of 1 and 1-F− showed the change from 2.6904 D to 7.0216 D, indicating the deprotonation of the naphtholic proton (entry 5 of columns 3 and 4). For 1 and 1-CN− , the relationship between orbital hybridization and conjugation was compared. 1 has a sp2 hybridized imine group (bond angle = 119.5◦ (C11, C17, H18)) and planar conformation (dihedral angle = 180.0◦ (C11 , C17 , N19 , C22)), whereas 1-CN− showed a sp3 hybridized carbon bond (bond angle = 110.4◦ (N10, C11, C31)) and tilted conformation (dihedral angle = −149.4◦ (C13 , C11 , N10 , C2 )). This structural difference causes a significant difference in conjugation between 1 and 1-CN− . Therefore, it is expected that no ICT was observed in the 1-CN− adduct [25]. 3.2. Theoretical sensing mechanism To gain an insight into colorimetric sensing mechanism for the two adducts (1-F− and 1-CN− ), time-dependent density functional theory (TD-DFT) calculations were performed at the optimized geometries (S0 ). The main transition property of 1 showed that the HOMO to LUMO (405 nm, 99% contribution) and HOMO-1 to LUMO (355 nm, 92% contribution) transitions predominantly contribute to the electronic configurations (Table S2). Especially, the HOMO-LUMO transition of 1 assigned the ICT band ( → *), which is consistent with the experimental absorption wavelength of 1 in MeCN (406 nm, ε = 4.37 × 103 L mol−1 cm−1 ) (Fig. S2 (a)). The TDDFT calculations for 1-F− indicated the obvious red shift of ICT band (433 nm, HOMO → LUMO, Table S3). Thus, deprotonating process of 1 caused the decrease of HOMO to LUMO energy gap, which substantially results in the bathochromic shift. In case of the 1CN− adduct, the TD-DFT calculations indicated the major oscillator strength at 378 nm (HOMO→LUMO + 1), which was assigned as the  → * transition of naphthol group (Table S4). In contrast, the absorption band at 447 nm (HOMO → LUMO) indicated the minor oscillator strength (Table S4). Therefore, the sensing mechanism of

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Fig. 8. (a) Absorption spectra changes of 1 (20 ␮M) upon addition of cyanide (35 equiv) in the absence and presence of 35 equiv of various anions in bis-tris buffer/DMSO (99.99/0.01, v/v, containing 10 mM bis-tris, pH = 7.0). (b) The color changes of 1 (20 ␮М) upon addition of cyanide (35 equiv) in the absence and presence of 35 equiv of various anions.

F− and CN− through different signaling channels. Chemosensor 1 showed a highly selective colorimetric response to fluoride based on a deprotonation process and to cyanide based on a nucleophilic addition. The detection of both F− and CN− by 1 was found to be free of interference from any other anions in aqueous solution. Moreover, DFT and TD-DFT studies supported the experimental data and the proposed sensing mechanisms. Thus, this sensor exhibits a new method to simultaneously assay F− and CN− by two different detection modes. 5. Experimental 5.1. Reagents and instruments

Fig. 7. (a) Absorption spectra changes of 1 (20 ␮M) in the presence of different concentrations of CN− (from 0 to 44 equiv) at room temperature. (b) Absorption spectra changes of 1 (20 ␮M), extracted from the range of 0–10 equiv of CN− in (a). (c) Absorption spectra changes of 1 (20 ␮M), extracted from the range of 12–44 equiv of CN− in (a).

CN− could be explained by blocking of HOMO to LUMO transition by nucleophilic addition of CN− at imine carbon, resulting in the formation of the colorless leuconitrile species. 4. Conclusion We have developed an outstanding single chemosensor 1, based on a naphtholic Schiff base bearing an aminopyrazine group, for

All reagents were commercially obtained from Sigma–Aldrich (St. Louis, Mo, USA) and used without further purification. Anhydrous ethanol was prepared by the simple distillation from MgSO4 . The 1 H NMR measurements were performed on a Varian 400 MHz spectrometer and the chemical shifts are recorded in ppm. Electrospray ionization mass spectra (ESI-MS) 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 Flash EA 1112 Elemental analyzer (thermo) in Organic Chemistry Research Center of Sogang University, Korea. Absorption spectra were recorded at 25 ◦ C using the Perkin Elmer model Lambda 25 UV/vis spectrometer. 5.2. Synthesis of 1 Aminopyrazine (95.1 mg, 1.2 mmol) in ethanol (10 mL) was added to a solution containing 2-hydroxy-1-naphthalehyde (206.6 mg, 1 mmol) in ethanol (10 mL). The reaction mixture was stirred for 3 h and then, three drops of HCl were added into the

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Fig. 9.

1

129

H NMR titration of 1 with CN− in DMSO-d6 /CD3 OD (4:3, v/v).

reaction solution. After 24 h, the yellow precipitate appeared. The precipitate was filtered and washed with ether (10 mL × 2) and ethanol (10 mL) (Scheme 1). The yield was 70% (211.2 mg). 1 H NMR (DMSO-d6 , 400 MHz) ␦ 14.94 (d, 1H, J = 4 Hz), 9.98 (d, 1H, J = 5 Hz), 8.97 (s, 1H), 8.62–5.57 (m, 2H), 8.39 (d, 1H, J = 8 Hz), 8.01 (d, 1H, J = 9 Hz), 7.83 (d, 1H, J = 8 Hz), 7.61 (t, 1H, J = 7 Hz), 7.41 (t, 1H, J = 7 Hz), 7.02 (d, 1H, J = 9 Hz); 13 C NMR (400 MHz DMSO-d6 , ppm): 172.43, 155.10, 151.04, 143.07, 142.05, 138.12, 138.66, 133.04, 129.18, 128.68, 126.90, 124.10, 122.23, 120.08, 108.97. HRMS (ESI): m/z calcd for C15 H10 N3 O: 248.082 ([M-H+ ]): found, 248.267. Anal. calcd for C15 H11 N3 O (249.273): C, 72.28; H, 4.45; N, 16.86. Found: C, 72.03; H, 4.78; N, 16.58. 5.3. UV–vis measurements of receptor 1 with CN− and F− For F− ; Receptor 1 (0.37 mg, 0.0015 mmol) was dissolved in DMSO (0.5 mL) and 15 ␮L of the receptor 1 (3 mM) were diluted to 2.985 mL DMSO/bis-tris buffer (v/v = 9:1) to make the final concentration of 15 ␮M. Tetraethylammonium fluoride (TEAF) (78.4 mg, 0.3 mmol) was dissolved in DMSO (1 mL). 1.5–160 ␮L of the F− solution (300 mM) were transferred to each receptor solution (15 ␮M) prepared above. After mixing them for a few seconds, UV–vis absorption spectra were taken at room temperature. For CN− ; Receptor 1 (0.37 mg, 0.0015 mmol) was dissolved in DMSO (0.5 mL) and 20 ␮L of the receptor 1 (3 mM) were diluted to 2.980 mL bis-tris buffer to make the final concentration of 20 ␮M. Tetrabutylammonium cyanide (TBACN) (80.5 mg, 0.3 mmol) was dissolved in bis-tris buffer (1 mL). 2–100 ␮L of the CN− solution (300 mM) were transferred to each receptor solution (20 ␮M) prepared above. After mixing them for a few seconds, UV–vis absorption spectra were taken at room temperature.

5.4. Job plot measurement For F− ; Receptor 1 (12.5 mg, 0.05 mmol) was dissolved in DMSO (5 mL). 12, 10.8, 9.6, 8.4, 7.2, 6.0, 4.8, 3.6, 2.4, 1.2 and 0 ␮L of the receptor 1 solution were taken and transferred to vials. Each vial was diluted with DMSO/bis-tris buffer (v/v = 9:1) to make a total volume of 2.988 mL. TEAF (13.1 mg, 0.05 mmol) was dissolved in DMSO (5 mL). 0, 1.2, 2.4, 3.6, 4.8, 6.0, 7.2, 8.4, 9.6, 10.8, and 12 ␮L of the TEAF solution were added to each diluted receptor 1 solution. Each vial had a total volume of 3 mL. After shaking the vials for a few minutes, UV–vis spectra were taken at room temperature. For CN− ; Receptor 1 (12.5 mg, 0.05 mmol) was dissolved in DMSO (5 mL). 12, 10.8, 9.6, 8.4, 7.2, 6.0, 4.8, 3.6, 2.4, 1.2 and 0 ␮L of receptor 1 solution were taken and transferred to vials. Each vial was diluted with bis-tris buffer to make a total volume of 2.988 mL. TBACN (13.4 mg, 0.05 mmol) was dissolved in bis-tris buffer (5 mL). 0, 1.2, 2.4, 3.6, 4.8, 6.0, 7.2, 8.4, 9.6, 10.8, and 12 ␮L of the TBACN solution were added to each diluted receptor 1 solution. Each vial had a total volume of 3 mL. After shaking the vials for a few minutes, UV–vis spectra were taken at room temperature. 5.5. Competition with other anions For F− ; Receptor 1 (0.37 mg, 0.0015 mmol) was dissolved in DMSO (0.5 mL) and 15 ␮L of the receptor 1 (3 mM) were diluted to 2.985 mL DMSO/bis-tris buffer (v/v = 9:1) to make the final concentration of 15 ␮M. Tetraethylammonium salts of Cl− , Br− , and I− , and tetrabuthylammonium salts of CN− , OAc− H2 PO4 − , N3 − , SCN− and BzO− , and Na salts of NO2 − , HSO4 − , and SH− (0.1 mmol) were dissolved in DMSO (1 mL). 27 ␮L of each anion solution (100 mM) were taken and added to 2.973 mL of the solution of receptor 1 (15 ␮M) to give 150 equiv of anions. TEAF (26.2 mg, 0.1 mmol) was dissolved

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in DMSO (1 mL). Then, 21 ␮L of TEAF solution (100 mM) were added into the mixed solution of each anion and 1 to make 150 equiv. After mixing them for a few seconds, UV–vis spectra were taken at room temperature. For CN− ; Receptor 1 (0.37 mg, 0.0015 mmol) was dissolved in DMSO (0.5 mL) and 20 ␮L of the receptor 1 (3 mM) were diluted to 2.980 mL bis-tris buffer to make the final concentration of 20 ␮M. Tetraethylammonium salts of F− , Cl− , Br− , and I− , and tetrabuthylammonium salts of OAc− H2 PO4 − , N3 − , SCN− , BzO− and Na salts of NO2 − , HSO4 − , and SH− (0.1 mmol) were dissolved in bis-tris buffer (1 mL). 21 ␮L of each anion solution (100 mM) were taken and added to 2.979 mL of the solution of receptor 1 (20 ␮M) to give 35 equiv of anions. TBACN (26.8 mg, 0.1 mmol) was dissolved in bistris buffer (1 mL). Then, 21 ␮L of the TBACN solution (100 mM) were added into the mixed solution of each anion and 1 to make 35 equiv. After mixing them for a few seconds, UV–vis spectra were taken at room temperature. 5.6.

1H

NMR titration

For 1 H NMR titrations of receptor 1 with F− , four NMR tubes of receptor 1 (2.5 mg, 0.01 mmol) dissolved in DMSO-d6 were prepared and then four different concentrations (0, 0.005, 0.01 and 0.02 mmol) of TEAF dissolved in DMSO-d6 were added to each solution of receptor 1. After shaking them for a minute, 1 H NMR spectra were taken at room temperature. For 1 H NMR titrations of receptor 1 with CN− , five NMR tubes of receptor 1 (2.5 mg, 0.01 mmol) dissolved in DMSO-d6 /CD3 OD (v/v = 4:3) were prepared and then five different concentrations (0, 0.005, 0.01, 0.02 and 0.03 mmol) of TBACN dissolved in CD3 OD were added to each solution of receptor 1. After shaking them for a minute, 1 H NMR spectra were taken at room temperature. 5.7.

13 C

NMR spectra of 1 and 1-CN−

Two NMR tubes of receptor 1 (12.5 mg, 0.05 mmol) dissolved in DMSO-d6 /CD3 OD (v/v = 4:3) were prepared and then two different concentrations (0 and 0.15 mmol) of TBACN dissolved in DMSO-d6 were added to each solution of receptor 1. After shaking them for a minute, 13 C NMR spectra were taken at 40 ◦ C for 3 h. 5.8. Calculation methods All theoretical calculations were performed by DFT/TD-DFT method with the hybrid exchange-correlation functional B3LYP [26] applying the 6-31 G** [27] basis set without any symmetry restrictions in the gas phase. The energy-minimized structure of 1 was obtained in various geometric forms. On the basis of the optimized ground (S0 ) structure of 1, the optimized structures of 1-CN− and 1-F− were also obtained. In order to investigate the transition energies for the optimized structures of 1, 1-CN− and 1-F− , we calculated the lowest 20 singlet–singlet transition using their ground state geometry (S0 ) with TD-DFT (B3LYP) method. The GaussSum 2.1 was used to calculate the contribution of molecular orbital in electronic transitions [28]. All the calculations were performed with Gaussian 03 suite [29]. Acknowledgements Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012001725 and 2012008875) are gratefully acknowledged. We thank Nano-Inorganic Laboratory, Department of Nano & Bio chemistry, Kookmin University to access the Gaussian 03 program packages.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2014.10.081.

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Biographies Jae Jun Lee earned the BS degree in 2014 at Seoul National University of Science and Technology. He is currently a Master Student at Seoul National University of Science and Technology. His scientific interest includes, molecular modeling, chemical sensors, and synthesis of catalyst. Gyeong Jin Park earned the BS degree in 2013 at Seoul National University of Science and Technology. She is currently a Master Student at Seoul National University of Science and Technology. Her scientific interest includes chemical sensors, synthesis of catalyst and inorganic medicine. Ye Won Choi received the BS degree in 2013 at Seoul National University of Science and Technology. She is currently a Master Student at Seoul National University of

Science and Technology. Her scientific interest includes chemical sensors, MOF, DNA cleavage by the transition metal complexes, and inorganic medicine. Ga Rim You received the BS degree in 2013 at Seoul National University of Science and Technology. He is currently a Master Student at Seoul National University of Science and Technology. His research interest includes chemical sensors, synthesis of catalyst and Metal-organic framework. Yong Sung Kim earned the BS degree in 2014 at Seoul National University of Science and Technology. He is currently a Master Student at Seoul National University of Science and Technology. His scientific interest includes chemical sensors, synthesis of catalyst and inorganic medicine. Sun Young Lee earned the BS degree in 2014 at Seoul National University of Science and Technology. She is currently a Master Student at Seoul National University of Science and Technology. Her scientific interest includes reactivity study of the transition metal complexes and inorganic medicine. Cheal Kim is currently a professor at Seoul National University of Science and Technology. He received the PhD degree in 1993 at University of California, San Diego in USA. He is now interested in development of chemical sensors, reactivity study of the transition metal complexes, DNA cleavage by their metal complexes and MOF.