Inorganic Chemistry Communications 74 (2016) 62–65
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Sequential detection of copper(II) and cyanide by a simple colorimetric chemosensor Ji Hye Kang, Seong Youl Lee, Hye Mi Ahn, Cheal Kim ⁎ Department of Fine Chemistry, Bio IT Materials, Seoul National University of Science and Technology, Seoul 139-743, Republic of Korea 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 30 September 2016 Received in revised form 21 October 2016 Accepted 29 October 2016 Available online 01 November 2016 Keywords: Colorimetric chemosensor Sequential detection Copper Cyanide Theoretical calculations
a b s t r a c t A simple colorimetric receptor 1 based on the combination of N-(5-nitro-2-pyridyl)-1,2-ethanediamine and 4(diethylamino)-2-hydroxybenzaldehyde was synthesized for the sequential detection of Cu2+ and CN−. The receptor 1 showed a distinct color change toward Cu2+ from colorless to yellow. The detection limit of 1 for Cu2+ (0.88 μM) was much lower than the World Health Organization guideline (31.5 μM) as the maximum allowable copper concentration in drinking water. In addition, 1-Cu2+ complex could be used to detect cyanide by showing a color change from yellow to colorless, indicating the recovery of 1 from 1-Cu2+. Furthermore, the sensing mechanism of 1 for Cu2+ was supported by theoretical calculations. © 2016 Elsevier B.V. All rights reserved.
Copper ion, as the third most abundant metal ion in human body, plays important roles in variety of fundamental physiological processes [1,2]. As catalyst, copper interacted with enzymes conducts to help a number of body functions such as to transform melanin for pigmentation of the skin and provide energy for biochemical reactions [3]. However, excessive copper accumulation can cause nerve disorder including Alzheimer's, Parkinson's and Wilson's diseases [4–6]. In addition, some copper compounds can cause dermal or eye irritation [7]. Thus, it is absolutely necessary to develop Cu2+ sensors with high selectivity and sensitivity. Cyanide is known as one of the most rapidly acting and powerful poisons. The toxicity results from its propensity to bind to the iron in cytochrome c oxidase, interfering with electron transport and resulting in hypoxia [8,9]. Nevertheless, cyanide is extensively used in many industrial processes such as synthesis of fibers and polymers, gold mining and electroplating, so cyanide is readily exposed
to the environment [10,11]. For these reasons, the recognition and detection of cyanide have also received considerable attention [12]. Herein, we designed and synthesized a novel chemosensor 1 based on the combination of the nitroaniline moiety and diethylaminosalicylaldehyde one, which showed the sequential sensing ability for Cu2+ and CN−. Receptor 1 detected Cu2+ via obvious color change from colorless to yellow, and in situ formed 1-Cu2+ complex showed a highly selective recognition of CN− through a color change from yellow to colorless in aqueous solution. Receptor 1 was synthesized by coupling N-(5-nitro-2-pyridyl)-1,2ethanediamine and 4-diethylaminosalicylaldehyde with 44% yield in ethanol (Scheme 1), and analyzed by 1H NMR and 13C NMR, ESI-mass spectrometry, and elemental analysis. To examine the colorimetric sensing ability of 1, the absorption spectral changes were studied in the presence of 18 different cations such as Na+, K+, Mg2 +, Ca2 +,
Scheme 1. Synthetic procedure of 1. ⁎ Corresponding author. E-mail address:
[email protected] (C. Kim).
http://dx.doi.org/10.1016/j.inoche.2016.10.039 1387-7003/© 2016 Elsevier B.V. All rights reserved.
J.H. Kang et al. / Inorganic Chemistry Communications 74 (2016) 62–65
(a)
63
1.2 1 +Fe
1.0
3+
Absorbance
1 +Fe
2+
0.8 3+
3+
3+
2+
1 , 1 +Al , Ga , In , Zn , 2+ 2+ 2+ 3+ Cd , Cu , Mg , Cr , 2+ 2+ 2+ + + Hg , Co , Ni , Na , K , 2+ 2+ 2+ Ca , Mn , Pb
0.6 0.4
1 +Cu
2+
0.2 0.0 300
400
500
Wavelength (nm)
(b)
Fig. 1. (a) Absorption spectral changes of 1 (20 μM) in the presence of 24 equiv. of various metal ions in bis-tris buffer/DMF (1/1, v/v, 10 mM bis-tris, pH = 7.0). (b) The color changes of 1 (20 μM) in the presence of 24 equiv. of various metal ions. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Cr3 +, Mn2 +, Fe2 +, Fe3 +, Co2 +, Ni2+, Cu2 +, Zn2 +, Cd2 +, Hg2 +, Al3 +, Ga3 +, In3+ and Pb2+ in bis-tris buffer/DMF (1/1, v/v, 10 mM bis-tris, pH = 7.0). As shown in Fig. 1a, 1 showed a particular spectral change to Cu2+ in the visible region, while other metal ions caused either little or no spectral changes in absorption peaks. Consistent with the absorption spectral change, the addition of Cu2+ to 1 showed promptly a color change from colorless to yellow (Fig. 1b), demonstrating that receptor 1 can serve as a potential candidate of “naked-eye” chemosensor for Cu2+ in aqueous solution. The binding property of 1 with Cu2+ was studied by UV–vis titration experiments (Fig. 2). The absorption peak at 385 nm decreased gradually upon the addition of Cu2 + to a solution of 1, while a new absorption peak appeared at 436 nm and reached a maxima at 24 equiv. of Cu2+. Meanwhile, an isosbestic point was clearly observed at 401 nm, demonstrating that only one product was formed
Fig. 3. Positive-ion electrospray ionization mass spectrum of 1 (100 μM) upon addition of Cu2+ (24 equiv.).
between the receptor 1 and Cu2+. In addition, 1 showed a fast reaction with copper ion, as shown in Fig. S1. The 1:1 stoichiometric ratio of the 1-Cu2+ complex was determined by Job plot (Fig. S2) [13]. Moreover, a 1:1 binding mode between 1 and Cu2+ was further confirmed by ESI-mass spectrometry analysis (Fig. 3). The positive-ion mass spectrum demonstrated that a peak at m/z = 419.10 was assignable to 1-H+ + Cu2+ [calcd, 419.10]. Based on UV– vis titration, the binding constant of 1-Cu2 + complex was calculated as 2.4 × 103 M−1 by using non-linear fitting analysis (Fig. S3), which indicates a weak binding between 1 and Cu2+. The detection limit of receptor 1 as a colorimetric sensor for the analysis of Cu2 + ion was found to be 0.88 μM (Fig. S4) by using 3σ/K [14]. This value was much lower than the World Health Organization (WHO) guideline (31.5 μM) in drinking water [15]. To further examine the practical applicability of 1, the affinity of 1 toward other coexistent metal ions such as Na+, K+, Mg2+, Ca2+, Cr3+, Mn2 +, Fe2 +, Fe3 +, Co2 +, Ni2 +, Zn2 +, Cd2 +, Hg2 +, Al3 +, Ga3 +, In3 + and Pb2+ was studied. As shown in Fig. S4, there was no interference except Al3+, Ga3+, In3+ and Cr3+. Although they showed some interference in UV–vis (Fig. S5a), it was still discernable in the color change (Fig. S5b). This result indicates that 1 could be a good colorimetric sensor for Cu2+ over different metal ions in aqueous solution. To investigate the practical applicability, we studied the pH effect on the absorption response of receptor 1 to Cu2+ ions in pH values ranging from 2 to 12 (Fig. S6). 1 showed no color change between pH 2 and 12, while an apparent color change of 1-Cu2+ complex was observed at the pH range of 7–12. These results indicate that Cu2+ could be detected by the naked eye or UV–vis absorption measurements using 1 over the various pH range of 7.0–12.0. In order to check the application validity of the chemosensor 1 to detect Cu2+ in real samples, we constructed a calibration curve (Fig. S7), which exhibited a good linear relationship between the absorbance of 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 6.00a 0.00 6.00a 0.00 6.00a
0.0 6.35 0.0 6.07 0.60 6.34
Drinking water Pond waterb
Fig. 2. Absorption spectral changes of 1 (20 μM) after addition of incremental amounts of Cu2+ in bis-tris buffer/DMF (1/1, v/v, pH = 7.0) at room temperature. Inset: Absorption at 436 nm versus the number of equiv. of Cu2+ added.
Recovery (%)
R.S.D. (n = 3) (%)
105
0.58
101
0.45
96.1
0.14
Conditions: [1] = 20 μmol L−1 in 10 mM bis-tris buffer-DMF solution (1:1, v/v, pH 7.0). a 6.00 μmol L−1 of Cu2+ ions was artificially added. b Pond water samples were collected from a pond in Seoul National University of Science & Technology.
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J.H. Kang et al. / Inorganic Chemistry Communications 74 (2016) 62–65
Fig. 4. The energy-minimized structures of (a) 1 and (b) 1-Cu2+ complex.
Scheme 2. Proposed binding mode of 1-Cu2+ complex.
(a)
1.2
Absorbance
1.0
1-Cu 2+ +N3 -
0.8 1-Cu 2+ , 1-Cu 2+ +OAc -, F -, Cl-, Br -, I-, BzO -, SCN -, NO 2 -
0.6 0.4
1-Cu 2+ +SO 4 22+
1-Cu +CN
0.2
-
1
0.0 300
400
500
Wavelength (nm) (b)
Fig. 5. (a) Absorption spectral changes of 1-Cu2+ complex (20 μM) upon addition of 36 equiv. of different anions in bis-tris buffer/DMF (1/1, v/v, pH = 7.0). (b) The color changes of 1-Cu2+ (20 μM) upon the addition of 36 equiv. of various anions in 10 mM bis-tris buffer/DMF (1/1, v/v, pH = 7.0). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
1 and Cu2 + concentration (5.00–40.00 μM) with a correlation coefficient of R2 = 0.9992 (n = 3). It means that 1 is suitable for quantitative detection of Cu2+. First, tap water samples were applied to the determination of Cu2+. As shown in Table 1, satisfactory recovery and R.S.D. values were obtained. Second, drinking water samples also showed the satisfactory recovery and R.S.D. values. Third, pond water samples were prepared and the results were also summarized in Table 1, which exhibited the satisfactory recovery and R.S.D. values. These results indicated that 1 could be used a suitable chemosensor for the practical detection of Cu2+ in real water samples. To clearly demonstrate the colorimetric sensing mechanisms of 1 toward Cu2+, we performed density functional theory (DFT) calculations with the B3LYP/6-31G (d, p) method basis set using the Gaussian 03 program. The calculated energy-minimized structures of 1 and 1-Cu2+ complex are shown in Fig. 4. The energy-minimized structure of 1 showed a chair structure with the dihedral angle of 1C, 2C, 3N, 4C = 2.363° (Fig. 4a). 1-Cu2 + complex exhibited a curved structure with the dihedral angle of 1C, 2C, 3N, 4C = 23.041°, and Cu2+ was coordinated with 3N, 5N, 6O and 7N of 1 (Fig. 4b). Time-dependent density functional theory (TD-DFT) calculations were also performed with the optimized geometries (S0). In the case of 1, the main molecular orbital (MO) contribution of the 3rd lowest excited state was determined for HOMO → LUMO transition (343.29 nm, Fig. S8). For the 1-Cu2+ complex, the main molecular orbital (MO) contribution of the 6th lowest excited state was also determined for HOMO → LUMO transition (489.14, Fig. S9). The calculated HOMO → LUMO excitation of 1-Cu2+ complex indicated ICT transition from the diethylaniline and Schiff base groups to the nitro pyridine group. Based on the molecular orbitals (MOs), the chelation of Cu2 + with 1 rendered the HOMO-to-LUMO energy gap of 1 decrease (2.878 eV → 2.361 eV(α), 2.878 eV → 2.267 eV(β))
J.H. Kang et al. / Inorganic Chemistry Communications 74 (2016) 62–65
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Scheme 3. Proposed sensing mechanism of CN− by 1-Cu2+ complex.
equiv. of CN− in the presence of other coexistent anions such as OAc−, − − 2− (36 equiv.). There F−, Cl−, Br−, I−, BzO−, N− 3 , SCN , NO2 and SO4 was no interference in the absorbance spectral and color changes. Thus, 1-Cu2 + can be used as a selective colorimetric sensor for CN− detection. In conclusion, we have designed a novel colorimetric chemosensor 1 for sequential detection of Cu2+ and CN−. Sensor 1 exhibited a selective recognition toward Cu2+, which induces a distinct color change from colorless to yellow. The color change was explained by ICT with theoretical calculations. Moreover, 1 could be also used to detect and quantify Cu2+ in water samples. Furthermore, the in situ prepared 1-Cu2+ complex showed the highly selectivity and sensitivity toward CN− by changing color from yellow to colorless.
Acknowledgements Fig. 6. UV–vis spectral changes of 1-Cu2+ complex (20 μM) in response to the addition of increasing amounts of CN− ions in bis-tris buffer/DMF (1/1, v/v, pH = 7.0) at room temperature. Inset: Absorption at 436 nm versus the number of equiv. of CN− added.
(Fig. S10), which is consistent with the red shift in the UV–vis spectrum of 1-Cu2+ complex. Based on UV–vis titration, Job plot, ESI-mass spectroscopy analysis, and theoretical calculations, we proposed the structure of a 1:1 complex of 1 and Cu2+ as shown in Scheme 2. Since we and others have shown that cyanide co-ordinates well to Cu2+ ions to form a very stable complex Cu(CN)X [16–22], the selectivity of 1-Cu2+ complex toward CN− was investigated with various anions in bis-tris buffer/DMF (1/1, v/v, pH = 7.0) (Fig. 5). Upon the addition of CN−, 1-Cu2+ complex showed immediately a specific spectral change and a color change from yellow to colorless, while no change was observed for other anionic species such as OAc−, F−, Cl−, Br−, I−, BzO−, − − 2− to 1NO− 2 and SCN . On the other hand, the addition of N3 and SO4 Cu2+ showed a little spectral change but not color change. The absorption recovery with CN− led us to suggest the release of 1 from the 1-Cu2+ complex through the chelation of CN− with the copper ion (Scheme 3). These results indicated that 1-Cu2+ complex can serve as a potential candidate of “naked-eye” chemosensor for CN− in aqueous solution. The binding properties of 1-Cu2+ with CN− were further studied by UV–vis titration experiments (Fig. 6). On gradual addition of CN− to the solution of 1-Cu2+, the absorption band at 436 nm decreased with a distinct isosbestic point at 399 nm, demonstrating the formation of only one product. With Job plot analysis [13], the binding mode between 1Cu2 + and CN− revealed a 1:1 stoichiometric ratio (Fig. S11), which was further analyzed by ESI-mass spectrometry analysis (Fig. S12). The negative ion mass spectrum showed the formation of the 1-H+ [calcd: 356.17, m/z: 356.33], indicating that the 1 was released from 1Cu2+ complex. Based on Job plot and ESI-mass spectrometry analysis, we propose that 1-Cu2+ complex might undergo the demetallation by CN− as shown in Scheme 3. Based on the UV–vis titration, the association constant between 1-Cu2 + and CN− was calculated as 1.34 × 103 M−1 by using Benesi-Hildebrand equation [23] (Fig. S13). The detection limit for CN− was determined to be 27.21 μM on basis of 3 σ/K [14] (Fig. S14). To check further the preferential selectivity of 1-Cu2+ complex as a − CN -selective receptor, we conducted competition experiments (Fig. S15). For competition tests, the 1-Cu2+ complex was treated with 36
Basic Science Research Program through the National Research Foundation of Korea (NRF) (NRF-2014R1A2A1A11051794 and NRF2015R1A2A2A09001301) is gratefully acknowledged. We thank NanoInorganic Laboratory, Department of Nano & Bio chemistry, Kookmin University to access the Gaussian 03 program packages. Appendix A. Supplementary data Supplementary data to this article can be found online at doi:10. 1016/j.inoche.2016.10.039.
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