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A colorimetric chemosensor for the sequential recognition of Mercury (II) and iodide in aqueous media
Inorganic Chemistry Communications 70 (2016) 147–152
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Short Communication
A colorimetric chemosensor for the sequential recognition of Mercury (II) and iodide in aqueous media Seong Youl Lee, Jae Jun Lee, Kwon Hee Bok, Jin Ah Kim, SoYoung Kim, Cheal Kim ⁎ Department of Fine Chemistry, Department of Interdisciplinary Bio IT Materials, Seoul National University of Science and Technology, Seoul 139-743, Korea
a r t i c l e
i n f o
Article history: Received 28 April 2016 Received in revised form 3 June 2016 Accepted 4 June 2016 Available online 07 June 2016 Keywords: Mercury Iodide Colorimetric Sequential recognition Test kit
a b s t r a c t A new highly selective colorimetric chemosensor 1 N′1,N′2-bis((1E,2E)-3-(4-(dimethylamino)phenyl) allylidene)oxalohydrazide was designed and synthesized as a colorimetric sensor for Hg2+. The sensor 1 showed high selectivity toward Hg2+ in a 1:1 stoichiometric system and fast color change from yellow to orange in the presence of Hg2+. The sensing mechanism was explained by theoretical calculations. Moreover, the resulting 1-Hg2+ complex acted as an efficient colorimetric sensor for I−, showing recovery of 1 from 1-Hg2+ complex. Furthermore, 1 and 1-Hg2+ complex could be used as practical, visible colorimetric test kits for Hg2+ and I−, respectively, in an aqueous environment. Therefore, probe 1 could be a good alternative method for onsite and real time screening of Hg2+ and I−. © 2016 Elsevier B.V. All rights reserved.
Mercury is one of the most concerned cations among various heavy and soft cations because of its toxicity [1]. In addition, mercury in aqueous solution could be transformed into lipophilic organo-mercury (CH3HgX), which is absorbed into the food chain and finally accumulates in the human body. The so called ‘Minamata Disease’ is a relentless result of methylmercury [2]. Moreover, mercury ions can cause abnormal problems at human body due to their easy absorption through the skin, respiratory and cell membranes, leading to digestive, cardiac, kidney and DNA damage [3]. Therefore, the detection and control of mercury is important for human health and environmental protection [4]. On the other hand, iodide is a very essential microelement for humans because it plays an important role in biological activities such as brain function, cell growth, neurological activity and thyroid function [5]. Hence the iodide content of breast milk and urine is often required for nutritional, metabolic, and epidemiological studies of thyroid disorder [6]. Therefore, developing chemosensors that can detect iodide is strongly desired [7–18]. Herein, we report the synthesis, characterization, and sensing properties of 1, based on the combination of the hydrazone moiety and the dimethylaniline one, as a selective colorimetric sequential chemosensor for Hg2 + and I−. The receptor 1 could detect Hg2 + by colorimetric response with high selectivity in aqueous media. Subsequently, chemosensing ensemble 1-Hg2+ showed highly selective detection to I− recovering 1 from 1-Hg2+ complex by utilizing the mercury-iodide affinity.
⁎ Corresponding author. E-mail address: [email protected] (C. Kim).
http://dx.doi.org/10.1016/j.inoche.2016.06.004 1387-7003/© 2016 Elsevier B.V. All rights reserved.
Receptor 1 was obtained by the combination of 4dimethylaminocinnamaldehyde and oxalic dihydrazide with 81% yield in ethanol (see Supporting Information and Scheme 1), and characterized by 1H NMR and 13C NMR, ESI-mass spectroscopy, and elemental analysis. The absorption response of 1 with various metal ions were carried out in bis-tris buffer/MeCN (9:1, v/v, pH 7). Upon the addition of 10 equiv of each metal ion, receptor 1 showed almost no change in absorption spectra in the presence of Al3+, Ga3+, In3+, Zn2+, Cd2+, Cu2+, Fe2+, Fe2+, Mg2+, Cr3+, Co2+, Ni2+, Na+, K+, Ca2+, Mn2+ and Pb2+ (Fig. 1a). By contrast, the addition of Hg2+ to 1 showed a distinct spectral change with a significant bathochromic shift and an instant color change from yellow to orange (Fig. 1b). This selectivity toward the soft Hg2+ might be due to the softness through the conjugation system of the whole molecule 1. UV–vis absorption spectral variation of 1 was further monitored through titration with different concentrations of Hg2+ (Fig. S1). Upon addition of Hg2+ into 1, the absorption band at 400 nm decreased and a new red-shift band at 518 nm steadily increased up to 10 equiv. Meanwhile, a clear isosbestic point was observed at 460 nm, suggesting that only one product was produced from the binding of 1 with Hg2 +. The molar extinction coefficient at 518 nm (1.0 × 104 M−1 cm−1) was considered as the ligand-to-metal charge-transfer (LMCT) mechanism [19]. To elucidate the binding mode of the receptor 1 and Hg2+, 1H NMR titrations of 1 were performed by the addition of Hg2+ (Fig. 2). Upon addition of 1.0 equiv of Hg2+, the protons of NH at 12.09 ppm disappeared completely. At the same time, the imine protons at 8.30 ppm shifted slightly upfield. The upfield shift might be due to increase of the electron density of the amide nitrogen by deprotonation of the amide moiety coordinating to Hg2+. On the other hand, the aliphatic protons of NMe2,
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Scheme 1. Synthesis of 1.
and aromatic and ethylenic protons showed no shift. In addition, there was no shift in the position of proton signals on further addition of Hg2 + (N 1.0 equiv), which indicated a 1:1 ratio of 1-Hg2 + complex. These results suggested that Hg2+ might coordinate to the two amide nitrogens (Scheme 2). The Job plot indicated a 1:1 stoichiometric ratio between the Hg2 + and 1 (Fig. S2) [20] Based on the UV-vis titration data, the binding constant for 1 with Hg2 + was estimated to be 6.0 × 103 M−1, which was within the range of those (102 −1010) previously reported for Hg2+ chemosensors (Fig. S3) [21]. The detection limit (3σ/K) of receptor 1 as a colorimetric sensor for the analysis of Hg2+ was found to be 4.6 μM (= 1.9 mg/kg) (Fig. S4), which is lower than the maximum allowable levels of Hg2 + regulated by Health Canada (3 mg/kg) [22,23]. To further study the ability of 1 as a colorimetric sensor for Hg2+, the competition experiments were conducted in the presence of various metal ions such as Al3 +, Ga3 +, In3 +, Zn2 +, Cd2 +, Cu2 +, Fe2 +, Fe2 +, Mg2+, Cr3 +, Hg2 +, Co2+, Ni2 +, Na+, K+, Ca2 +, Mn2+ and Pb2 + (Fig. S5). Most of the coexistent metal ions had no influence for detection of Hg2 + with naked-eye. However, Cu2 + and Co2 + increased about 40% of the absorbance of 1-Hg2 +, and Zn2 +, Cd2 +, Na+, Ca2 +, and Mn2+ did about 25% of it. In order to apply to the biological and environmental systems, the pH dependence of 1 in the absence and presence of Hg2+ was conducted at various pH (Fig. S6). The increase of absorbance caused by addition
Fig. 1. (a) Absorption spectral changes of 1 (20 μM) in the presence of 10 equiv of different metal ions in a mixture of water and MeCN (9:1, v/v). (b) The color changes of 1 (20 μM) upon addition of various metal ions (10 equiv) in a mixture of water and MeCN (9:1, v/v).
of Hg2+ ions was observed in a range of 7.0–12.0. This result warranted its application under physiological conditions, without any change in detection Hg2+. To investigate the practical application of receptor 1, colorimetric paper-made test strips were prepared by immersing filter papers in a DMF solution of 1 (1 mM). When the test kits coated with 1 were added to different metal ion solutions (100 nM), an obvious color change was observed only with Hg2+ (Fig. 3a and b) [24]. In addition, the color change of the test strips can be discernible as low as 5 × 10−9 M (Fig. 3c). To understand the sensing mechanism of 1 toward Hg2+, theoretical calculations were performed with the 1:1 stoichiometry (Hg2+: receptor), based on Job plot and 1H NMR titration. To get the energyminimized structures of 1 and 1-Hg2+ complex, their geometric optimizations were performed by DFT/B3LYP level (S = 0, DFT/B3LYP/main group atom: 6-31G** and Hg: Lanl2DZ/ECP). The significant structural properties of the energy-minimized structures were shown in Fig. 4. The energy minimized structure of 1 showed a planar structure with the dihedral angle of 1O, 2C, 3C, 4O = − 179.815° (Fig. 4a). 1-Hg2 + complex exhibited a tetrahedral structure with the dihedral angle of 1O, 2C, 3C, 4O = − 0.109°, and Hg2 + was coordinated with 5N, 6N atoms of 1 and two oxygen atoms of H2O (Fig. 4b). We further investigated the frontier molecular orbitals of 1 and 1-Hg2 + complex (Fig. S7). In case of 1, the energy gap between HOMO and LUMO was assigned to ICT band which showed the yellow color of 1. On the other hand, the energy gap between HOMO and LUMO for 1-Hg2+ complex was assigned to ligand-to-metal charge-transfer (LMCT). Thus, the chelation of Hg2+ to 1 showed LMCT, which induced the color change from yellow to orange. Based on the 1H NMR titration, Job plot, and theoretical calculations, we proposed the structure of a 1:1 complex of 1 and Hg2+ as shown in Scheme 2. Based on the high stability of HgI2 complex (Kass = 8.3 × 1023), we carried out the selectivity study of 1-Hg2+ complex toward I− in bistris buffer/MeCN (9:1, v/v, pH 7). The addition of I− to 1-Hg2+ complex showed both a significant change of UV-vis spectrum (Fig. 5a) and a color change from orange to yellow (Fig. 5b), while other species such − − − − as CN−, OAc−, F−, Cl−, Br−, H2PO− 4 , N3 , SCN , BzO , and NO2 exhibited almost no change in both the UV–vis spectrum and color of 1 under the same conditions. The binding properties of 1-Hg2+ with I− were further studied by UV–vis titration experiments (Fig. S8). On the gradual addition of I− to a solution of 1-Hg2+, the absorbance at 342 nm continuously increased, whereas the band at 500 nm decreased with an isosbestic point at 425 nm. To get further information for the binding mode of 1-Hg2+ with I−, 1 H NMR titration study was carried out (Fig. S9). As already shown in Fig. 2, upon the addition of 1.0 equiv of Hg2+ to 1, the protons of the NH at 12.09 ppm disappeared completely and the imine protons at 8.30 ppm shifted slightly upfield. Meanwhile, upon the addition of 1.0 equiv of I− to 1-Hg2 + solution, the two NH protons reappeared and the imine protons shifted to the original position. Upon further addition of I− (up to 2.0 equiv), the NH protons showed the complete recovery. In addition, there was no shift in the position of proton signals on further addition of I− (N 2.0 equiv), which indicated a 1:2 ratio of 1-Hg2+ complex and I−. Job plot showed a 1:2 stoichiometric ratio of 1-Hg2+ to I− (Fig. S10). Based on NMR titration and Job plot, we proposed that the 1-Hg2 + complex might undergo the demetallation by two I− to form HgI2 complex (Scheme 2). The binding constant between
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Fig. 2. 1H NMR titration of 1 with Hg2+.
1-Hg2+ and I− was calculated as 1.0 × 109 M−2 (Fig. S11) [25]. Based on the UV–vis titration data, the detection limit for I− was determined to be 0.21 μM on basis of 3σ/K (Fig. S12). Importantly, this is the second lowest one for sensing of I− by the mercury complex-based chemosensors in an aqueous solution with a high water content (N 90%) without any interference, to the best of our knowledge (Table S1). The preferential selectivity of 1-Hg2+ as a colorimetric chemosensor for the detection of I− was studied in the presence of various competing anions. The presence of other background anions showed no change in absorbance (Fig. S13). To investigate the practical application of 1-Hg2+ with I−, test strips were prepared by immersing filter papers into bistris buffer/MeCN (9:1, v/v, pH 7) solution of 1-Hg2+ (0.5 μM) (Fig. 6). The color of the test strip returned to the original yellow color in the
I− solution, while the test strips in the solution of other anions such as − − − and NO− CN−, OAc−, F−, Cl−, Br−, H2PO− 4 , BzO , N3 , SCN 2 did not cause any significant color change. The reversibility of the chemosensor is one of the essential aspects for sensor applications. Therefore, we performed the reversibility of receptor 1 by the alternate addition of Hg2+ and TEAI (Fig. S14). The absorbance was almost reversible for several cycles with the sequentially alternative addition of Hg2+ and I−. In conclusion, we have developed a colorimetric chemosensor 1 for the sequential colorimetric detection of Hg2+ and I−. In the presence of Hg2 +, receptor 1 would form 1-Hg2 + complex, which induces UV–vis absorption band and color changes by the LMCT mechanism. Moreover, the resulting 1-Hg2 + complex was used as a colorimetric chemosensor for I− over other anions.
Scheme 2. Proposed binding mode of 1-Hg2+ complex and sequential recognition of I .
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Fig. 3. Photographs of the filter paper coated with 1 used for the detection of Hg2+. (a) Left to right: test kit coated with only receptor 1 (control, 1 mM), test kit coated with only Hg2+ (control, 0.1 μM), and receptor-1 test kit immersed in Hg2+ solution. (b) Receptor-1 test kits (1 mM) immersed in various metal ions (0.1 μM). (c) Receptor-1 test kits (1 mM) for the detection of Hg2+ with different concentrations.
Fig. 4. The energy-minimized structures of (a) 1 and (b) 1-Hg2+ complex.
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Acknowledgements Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (NRF-2014R1A2A1A11051794 and NRF2015R1A2A2A09001301) are gratefully acknowledged. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.inoche.2016.06.004. References
Fig. 5. (a) Absorption spectral changes of 1-Hg2+ (20 μM) in the presence of 2.0 equiv of different anions. (b) The color changes of 1-Hg2+ (20 μM) upon addition of various anions (2.0 equiv) in bis-tris buffer/MeCN (9:1, v/v, pH 7).
[1] F. Renzoni, E. Zino, Franchi, Environ. Res. 77 (1998) 68–72. [2] P.B. Tchounwou, W.K. Ayensu, N. Ninashvili, D. Sutton, Environ. Toxicol. 18 (2003) 149–175. [3] S. chandraa, K.S. Lokeshb, H. Lang, Sensors Actuators B Chem. 137 (2009) 350–356. [4] S. Goswami, S. Maity, A.C. Maity, A.k. Das, B. Pakhira, K. Khanra, RSC Adv. 5 (2015) 5735–5740. [5] G. Dai, O. Levy, N. Carrasco, Nature 379 (1996) 458–460. [6] G. Aumont, J.-C. Tressol, Analyst 111 (1986) 841–843. [7] A.K. Mahapatra, G. Hazra, J. Roy, J. Lumin. 131 (2011) 1255–1259. [8] P. Aroraa, K. Suyala, N.K. Joshi, Spectrochim. Acta A 94 (2012) 119–125. [9] S. Park, W. Kim, K.M.K. Swamy, J.Y. Jung, G. Kim, Y. Kim, S.J. Kim, J. Yoon, Dyes Pigments 99 (2013) 323–328. [10] S.B. Maity, S. Banerjee, K. Sunwoo, J.S. Kim, P.K. Bharadwaj, Inorg. Chem. 54 (2015) 3929–3936. [11] J.H. Kim, H.J. Kim, S.H. Kim, J.H. Lee, J.H. Do, H.-J. Kim, Tetrahedron Lett. 50 (2009) 5958–5961. [12] F.X. Han, W.D. Patterson, Y. Xia, B.B.M. Sridhar, Y. Su, Water Air Soil Pollut. 170 (2006) 161–171. [13] Y.W. Choi, J.J. Lee, G.R. You, C. Kim, RSC Adv. 5 (2015) 38308–38315. [14] Y. Peng, Y.-M. Dong, M. Dong, Y.-W. Wang, J. Organomet. Chem. 77 (2012) 9072–9080. [15] L. Tang, M. Cai, Sensors Actuators B Chem. 173 (2012) 862–867. [16] L. Tang, P. Zhou, K. Zhong, S. Hou, Sensors Actuators B Chem. 182 (2013) 439–445. [17] L. Tang, M. Cai, P. Zhou, J. Zhao, K. Zhong, S. Hou, Y. bian, RSC Adv. 3 (2013) 16802–16809. [18] L. Tang, X. Dai, M. Cai, J. Zhao, P. Zhou, Z. Huang, Spectrochim. Acta A 122 (2014) 656–660.
Fig. 6. Photographs of the filter paper coated with 1-Hg2+ used for the detection of I−. (a) Left to right: test kit coated with only receptor 1 (control, 1 mM), receptor 1 test kit immersed in Hg2+ (control, 0.5 μM) solution, test kit coated with only I− (control, 0.2 μM), and 1-Hg2+ test kit immersed in I− (control, 0.2 μM) solution. (b) 1-Hg2+ test kits (0.5 μM) immersed in various anions (0.2 μM).
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[19] Y.W. Choi, G.R. You, M. Lee, J. Kim, K.-D. Jung, C. Kim, Inorg. Chem. Commun. 46 (2014) 43–46. [20] P. Job, Ann. Chim. 9 (1928) 113–203. [21] H.A. Benesi, J.H. Hildebrand, J. Am. Chem. Soc. 71 (1949) 2703–2707. [22] Y. Shiraishi, S. Sumiya, T. Hirai, Chem. Commun. 47 (2011) (4953-4905).
[23] X. Cui, L. Zhu, J. Wu, Y. Hou, P. Wang, Z. Wang, M. Yang, Biosens. Bioelectron. 63 (2015) 506–512. [24] Q. Liu, Y.-M.Z. Lin, T.-B. Wei, Sensors Actuators B Chem. 196 (2014) 619–623. [25] G. Grynkiewcz, M. Poenie, R.Y. Tsein, J. Biol. Chem. 260 (1985) 3440–3445.
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Short Communication
A colorimetric chemosensor for the sequential recognition of Mercury (II) and iodide in aqueous media Seong Youl Lee, Jae Jun Lee, Kwon Hee Bok, Jin Ah Kim, SoYoung Kim, Cheal Kim ⁎ Department of Fine Chemistry, Department of Interdisciplinary Bio IT Materials, Seoul National University of Science and Technology, Seoul 139-743, Korea
a r t i c l e
i n f o
Article history: Received 28 April 2016 Received in revised form 3 June 2016 Accepted 4 June 2016 Available online 07 June 2016 Keywords: Mercury Iodide Colorimetric Sequential recognition Test kit
a b s t r a c t A new highly selective colorimetric chemosensor 1 N′1,N′2-bis((1E,2E)-3-(4-(dimethylamino)phenyl) allylidene)oxalohydrazide was designed and synthesized as a colorimetric sensor for Hg2+. The sensor 1 showed high selectivity toward Hg2+ in a 1:1 stoichiometric system and fast color change from yellow to orange in the presence of Hg2+. The sensing mechanism was explained by theoretical calculations. Moreover, the resulting 1-Hg2+ complex acted as an efficient colorimetric sensor for I−, showing recovery of 1 from 1-Hg2+ complex. Furthermore, 1 and 1-Hg2+ complex could be used as practical, visible colorimetric test kits for Hg2+ and I−, respectively, in an aqueous environment. Therefore, probe 1 could be a good alternative method for onsite and real time screening of Hg2+ and I−. © 2016 Elsevier B.V. All rights reserved.
Mercury is one of the most concerned cations among various heavy and soft cations because of its toxicity [1]. In addition, mercury in aqueous solution could be transformed into lipophilic organo-mercury (CH3HgX), which is absorbed into the food chain and finally accumulates in the human body. The so called ‘Minamata Disease’ is a relentless result of methylmercury [2]. Moreover, mercury ions can cause abnormal problems at human body due to their easy absorption through the skin, respiratory and cell membranes, leading to digestive, cardiac, kidney and DNA damage [3]. Therefore, the detection and control of mercury is important for human health and environmental protection [4]. On the other hand, iodide is a very essential microelement for humans because it plays an important role in biological activities such as brain function, cell growth, neurological activity and thyroid function [5]. Hence the iodide content of breast milk and urine is often required for nutritional, metabolic, and epidemiological studies of thyroid disorder [6]. Therefore, developing chemosensors that can detect iodide is strongly desired [7–18]. Herein, we report the synthesis, characterization, and sensing properties of 1, based on the combination of the hydrazone moiety and the dimethylaniline one, as a selective colorimetric sequential chemosensor for Hg2 + and I−. The receptor 1 could detect Hg2 + by colorimetric response with high selectivity in aqueous media. Subsequently, chemosensing ensemble 1-Hg2+ showed highly selective detection to I− recovering 1 from 1-Hg2+ complex by utilizing the mercury-iodide affinity.
⁎ Corresponding author. E-mail address: [email protected] (C. Kim).
http://dx.doi.org/10.1016/j.inoche.2016.06.004 1387-7003/© 2016 Elsevier B.V. All rights reserved.
Receptor 1 was obtained by the combination of 4dimethylaminocinnamaldehyde and oxalic dihydrazide with 81% yield in ethanol (see Supporting Information and Scheme 1), and characterized by 1H NMR and 13C NMR, ESI-mass spectroscopy, and elemental analysis. The absorption response of 1 with various metal ions were carried out in bis-tris buffer/MeCN (9:1, v/v, pH 7). Upon the addition of 10 equiv of each metal ion, receptor 1 showed almost no change in absorption spectra in the presence of Al3+, Ga3+, In3+, Zn2+, Cd2+, Cu2+, Fe2+, Fe2+, Mg2+, Cr3+, Co2+, Ni2+, Na+, K+, Ca2+, Mn2+ and Pb2+ (Fig. 1a). By contrast, the addition of Hg2+ to 1 showed a distinct spectral change with a significant bathochromic shift and an instant color change from yellow to orange (Fig. 1b). This selectivity toward the soft Hg2+ might be due to the softness through the conjugation system of the whole molecule 1. UV–vis absorption spectral variation of 1 was further monitored through titration with different concentrations of Hg2+ (Fig. S1). Upon addition of Hg2+ into 1, the absorption band at 400 nm decreased and a new red-shift band at 518 nm steadily increased up to 10 equiv. Meanwhile, a clear isosbestic point was observed at 460 nm, suggesting that only one product was produced from the binding of 1 with Hg2 +. The molar extinction coefficient at 518 nm (1.0 × 104 M−1 cm−1) was considered as the ligand-to-metal charge-transfer (LMCT) mechanism [19]. To elucidate the binding mode of the receptor 1 and Hg2+, 1H NMR titrations of 1 were performed by the addition of Hg2+ (Fig. 2). Upon addition of 1.0 equiv of Hg2+, the protons of NH at 12.09 ppm disappeared completely. At the same time, the imine protons at 8.30 ppm shifted slightly upfield. The upfield shift might be due to increase of the electron density of the amide nitrogen by deprotonation of the amide moiety coordinating to Hg2+. On the other hand, the aliphatic protons of NMe2,
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Scheme 1. Synthesis of 1.
and aromatic and ethylenic protons showed no shift. In addition, there was no shift in the position of proton signals on further addition of Hg2 + (N 1.0 equiv), which indicated a 1:1 ratio of 1-Hg2 + complex. These results suggested that Hg2+ might coordinate to the two amide nitrogens (Scheme 2). The Job plot indicated a 1:1 stoichiometric ratio between the Hg2 + and 1 (Fig. S2) [20] Based on the UV-vis titration data, the binding constant for 1 with Hg2 + was estimated to be 6.0 × 103 M−1, which was within the range of those (102 −1010) previously reported for Hg2+ chemosensors (Fig. S3) [21]. The detection limit (3σ/K) of receptor 1 as a colorimetric sensor for the analysis of Hg2+ was found to be 4.6 μM (= 1.9 mg/kg) (Fig. S4), which is lower than the maximum allowable levels of Hg2 + regulated by Health Canada (3 mg/kg) [22,23]. To further study the ability of 1 as a colorimetric sensor for Hg2+, the competition experiments were conducted in the presence of various metal ions such as Al3 +, Ga3 +, In3 +, Zn2 +, Cd2 +, Cu2 +, Fe2 +, Fe2 +, Mg2+, Cr3 +, Hg2 +, Co2+, Ni2 +, Na+, K+, Ca2 +, Mn2+ and Pb2 + (Fig. S5). Most of the coexistent metal ions had no influence for detection of Hg2 + with naked-eye. However, Cu2 + and Co2 + increased about 40% of the absorbance of 1-Hg2 +, and Zn2 +, Cd2 +, Na+, Ca2 +, and Mn2+ did about 25% of it. In order to apply to the biological and environmental systems, the pH dependence of 1 in the absence and presence of Hg2+ was conducted at various pH (Fig. S6). The increase of absorbance caused by addition
Fig. 1. (a) Absorption spectral changes of 1 (20 μM) in the presence of 10 equiv of different metal ions in a mixture of water and MeCN (9:1, v/v). (b) The color changes of 1 (20 μM) upon addition of various metal ions (10 equiv) in a mixture of water and MeCN (9:1, v/v).
of Hg2+ ions was observed in a range of 7.0–12.0. This result warranted its application under physiological conditions, without any change in detection Hg2+. To investigate the practical application of receptor 1, colorimetric paper-made test strips were prepared by immersing filter papers in a DMF solution of 1 (1 mM). When the test kits coated with 1 were added to different metal ion solutions (100 nM), an obvious color change was observed only with Hg2+ (Fig. 3a and b) [24]. In addition, the color change of the test strips can be discernible as low as 5 × 10−9 M (Fig. 3c). To understand the sensing mechanism of 1 toward Hg2+, theoretical calculations were performed with the 1:1 stoichiometry (Hg2+: receptor), based on Job plot and 1H NMR titration. To get the energyminimized structures of 1 and 1-Hg2+ complex, their geometric optimizations were performed by DFT/B3LYP level (S = 0, DFT/B3LYP/main group atom: 6-31G** and Hg: Lanl2DZ/ECP). The significant structural properties of the energy-minimized structures were shown in Fig. 4. The energy minimized structure of 1 showed a planar structure with the dihedral angle of 1O, 2C, 3C, 4O = − 179.815° (Fig. 4a). 1-Hg2 + complex exhibited a tetrahedral structure with the dihedral angle of 1O, 2C, 3C, 4O = − 0.109°, and Hg2 + was coordinated with 5N, 6N atoms of 1 and two oxygen atoms of H2O (Fig. 4b). We further investigated the frontier molecular orbitals of 1 and 1-Hg2 + complex (Fig. S7). In case of 1, the energy gap between HOMO and LUMO was assigned to ICT band which showed the yellow color of 1. On the other hand, the energy gap between HOMO and LUMO for 1-Hg2+ complex was assigned to ligand-to-metal charge-transfer (LMCT). Thus, the chelation of Hg2+ to 1 showed LMCT, which induced the color change from yellow to orange. Based on the 1H NMR titration, Job plot, and theoretical calculations, we proposed the structure of a 1:1 complex of 1 and Hg2+ as shown in Scheme 2. Based on the high stability of HgI2 complex (Kass = 8.3 × 1023), we carried out the selectivity study of 1-Hg2+ complex toward I− in bistris buffer/MeCN (9:1, v/v, pH 7). The addition of I− to 1-Hg2+ complex showed both a significant change of UV-vis spectrum (Fig. 5a) and a color change from orange to yellow (Fig. 5b), while other species such − − − − as CN−, OAc−, F−, Cl−, Br−, H2PO− 4 , N3 , SCN , BzO , and NO2 exhibited almost no change in both the UV–vis spectrum and color of 1 under the same conditions. The binding properties of 1-Hg2+ with I− were further studied by UV–vis titration experiments (Fig. S8). On the gradual addition of I− to a solution of 1-Hg2+, the absorbance at 342 nm continuously increased, whereas the band at 500 nm decreased with an isosbestic point at 425 nm. To get further information for the binding mode of 1-Hg2+ with I−, 1 H NMR titration study was carried out (Fig. S9). As already shown in Fig. 2, upon the addition of 1.0 equiv of Hg2+ to 1, the protons of the NH at 12.09 ppm disappeared completely and the imine protons at 8.30 ppm shifted slightly upfield. Meanwhile, upon the addition of 1.0 equiv of I− to 1-Hg2 + solution, the two NH protons reappeared and the imine protons shifted to the original position. Upon further addition of I− (up to 2.0 equiv), the NH protons showed the complete recovery. In addition, there was no shift in the position of proton signals on further addition of I− (N 2.0 equiv), which indicated a 1:2 ratio of 1-Hg2+ complex and I−. Job plot showed a 1:2 stoichiometric ratio of 1-Hg2+ to I− (Fig. S10). Based on NMR titration and Job plot, we proposed that the 1-Hg2 + complex might undergo the demetallation by two I− to form HgI2 complex (Scheme 2). The binding constant between
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Fig. 2. 1H NMR titration of 1 with Hg2+.
1-Hg2+ and I− was calculated as 1.0 × 109 M−2 (Fig. S11) [25]. Based on the UV–vis titration data, the detection limit for I− was determined to be 0.21 μM on basis of 3σ/K (Fig. S12). Importantly, this is the second lowest one for sensing of I− by the mercury complex-based chemosensors in an aqueous solution with a high water content (N 90%) without any interference, to the best of our knowledge (Table S1). The preferential selectivity of 1-Hg2+ as a colorimetric chemosensor for the detection of I− was studied in the presence of various competing anions. The presence of other background anions showed no change in absorbance (Fig. S13). To investigate the practical application of 1-Hg2+ with I−, test strips were prepared by immersing filter papers into bistris buffer/MeCN (9:1, v/v, pH 7) solution of 1-Hg2+ (0.5 μM) (Fig. 6). The color of the test strip returned to the original yellow color in the
I− solution, while the test strips in the solution of other anions such as − − − and NO− CN−, OAc−, F−, Cl−, Br−, H2PO− 4 , BzO , N3 , SCN 2 did not cause any significant color change. The reversibility of the chemosensor is one of the essential aspects for sensor applications. Therefore, we performed the reversibility of receptor 1 by the alternate addition of Hg2+ and TEAI (Fig. S14). The absorbance was almost reversible for several cycles with the sequentially alternative addition of Hg2+ and I−. In conclusion, we have developed a colorimetric chemosensor 1 for the sequential colorimetric detection of Hg2+ and I−. In the presence of Hg2 +, receptor 1 would form 1-Hg2 + complex, which induces UV–vis absorption band and color changes by the LMCT mechanism. Moreover, the resulting 1-Hg2 + complex was used as a colorimetric chemosensor for I− over other anions.
Scheme 2. Proposed binding mode of 1-Hg2+ complex and sequential recognition of I .
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Fig. 3. Photographs of the filter paper coated with 1 used for the detection of Hg2+. (a) Left to right: test kit coated with only receptor 1 (control, 1 mM), test kit coated with only Hg2+ (control, 0.1 μM), and receptor-1 test kit immersed in Hg2+ solution. (b) Receptor-1 test kits (1 mM) immersed in various metal ions (0.1 μM). (c) Receptor-1 test kits (1 mM) for the detection of Hg2+ with different concentrations.
Fig. 4. The energy-minimized structures of (a) 1 and (b) 1-Hg2+ complex.
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Acknowledgements Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (NRF-2014R1A2A1A11051794 and NRF2015R1A2A2A09001301) are gratefully acknowledged. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.inoche.2016.06.004. References
Fig. 5. (a) Absorption spectral changes of 1-Hg2+ (20 μM) in the presence of 2.0 equiv of different anions. (b) The color changes of 1-Hg2+ (20 μM) upon addition of various anions (2.0 equiv) in bis-tris buffer/MeCN (9:1, v/v, pH 7).
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Fig. 6. Photographs of the filter paper coated with 1-Hg2+ used for the detection of I−. (a) Left to right: test kit coated with only receptor 1 (control, 1 mM), receptor 1 test kit immersed in Hg2+ (control, 0.5 μM) solution, test kit coated with only I− (control, 0.2 μM), and 1-Hg2+ test kit immersed in I− (control, 0.2 μM) solution. (b) 1-Hg2+ test kits (0.5 μM) immersed in various anions (0.2 μM).
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