A single colorimetric sensor for multiple targets: the sequential detection of Co²⁺ and cyanide and the selective detection of Cu²⁺ in aqueous solution

Abstract

A new highly selective and multifunctional chemosensor 1 for the detection of Co²⁺, Cu²⁺ and CN⁻, based on 4-diethylaminosalicyl aldehyde and thiophene-2-carbohydrazide moieties, was designed and synthesized. 1 could simultaneously detect both Co²⁺ and Cu²⁺ by changing its color from colorless to yellow in aqueous solution. The binding modes of 1 to Co²⁺ and Cu²⁺ were determined to be a 2 : 1 complexation stoichiometry through job plot and ESI-mass spectrometry analysis. The detection limits (0.19 μM and 0.13 μM) of 1 for Co²⁺ and Cu²⁺ were lower than the DEP guideline (1.7 μM) of Co²⁺ and the WHO guideline (31.5 μM) of Cu²⁺ for drinking water. Importantly, 1 could detect and quantify Co²⁺ and Cu²⁺ in real water samples. Moreover, the resulting Co²⁺-2·1 complex sensed cyanide through naked-eye, showing recovery from Co²⁺-2·1 to 1. The sensing mechanisms of Cu²⁺ by 1 were explained by theoretical calculations.

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1. Introduction

The development of colorimetric chemosensors for detecting metal ions has received considerable interest in chemical, environmental and biological areas.^1–6 Cobalt is an important trace element found in rocks, minerals, soils and seawater, and plays a critical role in biological systems.^7–11 It is well known that Co²⁺ is an important component of vitamin B12 and other biological compounds, and also plays a critical role in the metabolism of iron and the synthesis of hemoglobin.^12–14 However, exposure to high levels of cobalt can lead to toxicological effects, including heart disease, thyroid enlargement, asthma, decreased cardiac output, lung disease, dermatitis and vasodilation.^15–22 For these reasons, a highly selective and sensitive determination of trace amounts of Co²⁺ is necessary in biological and environmental systems.

Copper is the third most abundant metal in the human body.^23–26 It functions as a significant cofactor in the process of superoxide dismutase, cytochrome c oxidase and tyrosinase.^27,28 However, excess intake of Cu²⁺ can trigger adverse health effects such as Alzheimer's, Menke's, Parkinson's and Wilson's diseases.^29–31 The World Health Organization (WHO) has recommended the maximum limit of copper in drinking water at 2 ppm (31.5 μM).^32,33 Therefore, much effort has been devoted to the development of various chemosensors for Cu²⁺ detection.^34–40

Cyanide has received attention, because it is known as one of the most rapidly acting and strong poisons. The toxicity is caused by its propensity to bind to the iron in cytochrome c oxidase, interfering with electron transport and resulting in hypoxia.^41–44 In spite of the toxicity, cyanide has been required in diverse industrial processes such as raw materials for synthetic fibers, resins, herbicides, and the gold-extraction process.^45–48 For these reasons, the development of chemosensors for the recognition and detection of cyanide has also received considerable attention.

Many analytical methods for the detection of Co²⁺, Cu²⁺, and CN⁻ have been developed such as atomic absorption-emission spectrometry, inductively coupled plasma atomic emission spectroscopy and electrochemical methods. However, the methods demand expensive equipment, highly trained operators and complicated pre-treatment for routine monitoring and application.⁴⁹ On the contrary to these analytic methods, colorimetric methods have the several merits such as low cost, high sensitivity, and easy monitoring of the target ions.⁵⁰

Herein, we report on the synthesis and sensing properties of a new thiophene-2-carbohydrazide based chemosensor 1. The chemosensor 1 could simultaneously detect both Co²⁺ and Cu²⁺ ions via color changes from colorless to yellow in aqueous solution. In addition, the in situ formed Co²⁺-2·1 complex exhibited highly selective recognition of CN⁻ through a color change from yellow to colorless in aqueous solution. The sensing mechanisms of 1 toward Co²⁺, Cu²⁺, and CN⁻ were explained by using various analytical methods.

2. Experimental

2.1. Materials and equipment

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 25 UV/vis spectrometer. ¹H and ¹³C NMR spectra were recorded on a Varian 400 MHz and 100 MHz spectrometer and chemical shifts (δ) were recorded in ppm. Electro spray ionization mass spectra (ESI-MS) were collected on a Thermo Finnigan (San Jose, CA, USA) LCQ™ 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 °C. Elemental analysis for carbon, nitrogen, and hydrogen was carried out using a Flash EA 1112 elemental analyzer (thermo) at the Organic Chemistry Research Center of Sogang University, Korea.

2.2. Synthesis of sensor 1

A solution of thiophene-2-carbohydrazide (0.145 g, 1 mmol) was added to quinolone-2-carboxaldehyde (0.19 g, 1.2 mmol) in ethanol (5 mL). The reaction solution was stirred for 12 h at room temperature. A white precipitate formed was filtered, washed several times with methanol and diethyl ether, and dried in vacuum to afford the pure white solid. Yield 0.24 g (85%); ¹H NMR (400 MHz DMSO-d₆, ppm): δ 12.17 (s, 1H), 8.59 (s, 1H), 8.44 (d, J = 8 Hz, 1H), 8.31 (s, 1H), 8.20 (s, 1H), 8.09 (s, 1H), 8.02 (m, J = 24 Hz, 3H), 7.79 (t, J = 16 Hz, 1H), 7.63 (t, J = 12 Hz, 1H), 7.25 (t, J = 8 Hz, 1H); ¹³C NMR (100 MHz, DMSO-d₆, 25 °C): δ = 152.56, 149.86, 142.08, 140.39, 137.69, 135.36, 134.74, 134.11, 133.22, 132.59, 122.72. ESI-MS: m/z [1 + H⁺]⁺ calcd 282.07, found, 282.00.

2.3. UV-vis titration measurements of Co²⁺ and Cu²⁺

Sensor 1 (0.16 mg, 0.001 mmol) was dissolved in dimethyl sulfoxide (DMSO, 1 mL) and 60 μL of this solution (1 mM) was diluted with 2.94 mL of bis–tris buffer/DMSO (95/5, 10 mM bis–tris, pH = 7.0) to make the final concentration of 20 μM. Co(NO₃)₂·6H₂O (1.45 mg, 0.005 mmol) was dissolved in bis–tris buffer (1 mL) and 0.6–2.25 μL of this Co²⁺ solutions (5 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. The same experimental procedures were also carried out for Cu²⁺ ion.

2.4. Job plot measurements of Co²⁺ and Cu²⁺

Sensor 1 (0.16 mg, 0.001 mmol) was dissolved in DMSO (1 mL) and 600 μL of the sensor 1 (1 mM) was diluted to 39.4 mL bis–tris buffer/DMSO (95/5, v/v) solution to make to final concentration of 20 μM. 2.7, 2.4, 2.1, 1.8, 1.2, 0.9, 0.6 and 0.3 mL of the sensor 1 solution were taken and transferred to quartz cells. 40 μL of the Co²⁺ solution (20 mM) was diluted to 39.6 mL buffer/DMSO (95/5, v/v) solution. 0.3, 0.6, 0.9, 1.2, 1.5, 1.8, 2.1, 2.4 and 2.7 mL of the Co²⁺ solution were transferred to each sensor 1 solution. Each cell had a total volume of 3 mL. After mixing them for a few seconds, UV-vis spectra were taken at room temperature. The same experimental procedures were also carried out for Cu²⁺ ion.

2.5. Competition experiments of Co²⁺ and Cu²⁺

Sensor 1 (0.16 mg, 0.001 mmol) was dissolved in DMSO (1 mL) and 60 μL of this solution (1 mM) was diluted with 2.94 mL of bis–tris buffer/DMSO (95/5, v/v) to make the final concentration of 20 μM. MNO₃ (M = Na, K, Ag, 0.02 mmol) or M(NO₃)₂ (M = Mn, Fe, Co, Ni, Cu, Zn, Cd, Hg, Mg, Ca, Pb, 0.02 mmol) or M(NO₃)₃ (M = Fe, Cr, Al, Ga, In, 0.02 mmol) were separately dissolved in bis–tris buffer (1 mL). 2.25 μL of each metal-ion solution (20 mM) was taken and added to 3 mL of the solution of sensor 1 (20 μM) to give 7.5 equiv. Then, 2.25 μL of Co²⁺ solution (20 mM) was added into the mixed solution of each metal ion and 1 to make 7.5 equiv. After mixing them for a few seconds, UV-vis spectra were taken at room temperature. The same experimental procedures were also carried out for Cu²⁺ ion.

2.6. pH effect tests of Co²⁺ and Cu²⁺

A series of solutions with pH values ranging from 2 to 12 were prepared by mixing sodium hydroxide solution and hydrochloric acid. After the solution with a desired pH was achieved, sensor 1 (0.16 mg, 0.001 mmol) was dissolved in DMSO (1 mL), and then 60 μL of the sensor (1 mM) was diluted with 2.94 mL of bis–tris buffer/DMSO (95/5, v/v) to make the final concentration of 20 μM. Co(NO₃)₂·6H₂O (5.82 mg, 0.02 mmol) was dissolved in bis–tris buffer (1 mL). 2.25 μL of the Co²⁺ solution (20 mM) was transferred to each sensor solution (20 μM) prepared above. After mixing them for a few seconds, UV-vis spectra were taken at room temperature. The same experimental procedures were also carried out for Cu²⁺ ion.

2.7. Determination of Co²⁺ and Cu²⁺ in water samples

UV-vis spectral measurements of water samples containing Cu²⁺ were carried by adding 60 μL solution of the sensor 1 (1 mM) and 0.3 mL of 100 mM bis–tris buffer stock solution to 2.64 mL sample solutions. After mixing them for a few seconds, UV-vis spectra were taken at room temperature. The same experimental procedures were also carried out for Cu²⁺ ion.

2.8. Theoretical calculations

All theoretical calculations were performed by using Gaussian 03 program.⁵¹ The molecular geometries were optimized by the Density Functional Theory (DFT) calculations based on the hybrid exchange-correlation functional B3LYP.^52,53 The 6-31G** basis set^54,55 was used for the main group (C, H, O and N) and the Lanl2DZ effective core potential (ECP)^56,57 was considered for Cu²⁺. The vibrational frequency calculations were carried out. There was no imaginary frequency for the optimized geometries of 1 and Cu²⁺-2·1, indicating that these structures were local energy minima. For all calculations, the solvent effect of water was considered by using the Cossi and Barone's CPCM (conductor-like polarizable continuum model).^58,59 The twenty lowest singlet states were calculated by using Time-Dependent Density Functional Theory (TD/DFT). The GaussSum 2.1 (ref. 60) was used to calculate the contributions of molecular orbitals in electronic transition.

2.9. UV-vis titration measurements of Co²⁺-2·1 with CN⁻

Sensor 1 (0.16 mg, 0.001 mmol) was dissolved in DMSO (1 mL) and 60 μL of this solution (1 mM) was diluted with 2.94 mL of bis–tris buffer/DMSO (95/5, 10 mM bis–tris, pH = 7.0) to make the final concentration of 20 μM. 2.25 μL of Co²⁺ solution (20 mM) was transferred to each sensor solution (20 μM) to give 0.75 equiv. Then, tetraethylammonium cyanide (16.56 mg, 0.1 mmol) was dissolved in bis–tris (100 mM, 1 mL) and 1.8–19.8 μL of this CN⁻ solutions (100 mM) were transferred to Co²⁺-2·1 solution (20 μM) to give 33 equiv. After mixing them for a few seconds, UV-vis spectra were taken at room temperature.

2.10. Job plot measurements of Co²⁺-2·1 with CN⁻

Sensor 1 (0.16 mg, 0.001 mmol) was dissolved in DSMO (1 mL) and 600 μL of the sensor 1 (1 mM) were diluted to 39.4 mL bis–tris buffer/DMSO (95/5, v/v) solution to make the final concentration of 20 μM. Co(NO₃)₂·6H₂O (5.94 mg, 0.02 mmol) was dissolved in bis–tris buffer (5 mL) and 40 μL of this Co²⁺ solution (20 mM) was transferred to the sensor solution (20 μM) to make Co²⁺-2·1 complex. 2.7, 2.4, 2.1, 1.8, 1.2, 0.9, 0.6 and 0.3 mL of the Co²⁺-2·1 solution were taken and transferred to quartz cells. Tetraethylammonium cyanide (16.56 mg, 0.1 mmol) was dissolved in bis–tris (100 mM, 1 mL). 8 μL of the cyanide solution (100 mM) was diluted to 39.992 mL bis–tris buffer solution. 0.3, 0.6, 0.9, 1.2, 1.5, 1.8, 2.1, 2.4 and 2.7 mL of the diluted cyanide solution were transferred to each Co²⁺-2·1 solution. Each cell had a total volume of 3 mL. After mixing them for a few seconds, UV-vis spectra were taken at room temperature.

2.11. Competition of Co²⁺-2·1 toward various anions

Sensor 1 (0.16 mg, 0.001 mmol) was dissolved in DMSO (1 mL) and 60 μL of this solution (1 mM) was diluted with 2.94 mL of bis–tris buffer/DMSO (95/5, v/v) to make the final concentration of 20 μM. Co(NO₃)₂·6H₂O (5.94 mg, 0.02 mmol) was dissolved in bis–tris buffer (1 mL). 2.25 μL of this Co²⁺ solution (20 mM) was transferred to the 1 solution (20 μM) to make Co²⁺-2·1 complex. Then, tetraethylammonium salts of F⁻, Cl⁻, Br⁻ and I⁻, and tetrabutylammonium salts of OAc⁻, H₂PO₄⁻, N₃⁻, SCN⁻ and BzO⁻ (0.1 mmol), and sodium salts of S²⁻ (0.1 mmol) were separately dissolved in bis–tris buffer (1 mL). 18 μL of each anion solution (100 mM) was taken and added into each Co²⁺-2·1 complex solution prepared above to make 30 equiv. Then, 18 μL of the tetraethylammonium cyanide solution (100 mM) was added into the mixed solution of each anion and Co²⁺-2·1 complex to make 30 equiv. After mixing them for a few seconds, UV-vis spectra were taken at room temperature.

2.12. pH effect test of Co²⁺-2·1 with CN⁻

A series of solutions with pH values ranging from 2 to 12 were prepared by mixing sodium hydroxide solution and hydrochloric acid. After the solution with a desired pH was achieved, 60 μL of the 1 solution (1 mM) and 2.25 μL of the Co(NO₃)₂ solution (20 mM) were dissolved in bis–tris buffer/DMSO (95/5, v/v, pH 2–12), respectively. Then, 18 μL of tetraethylammonium cyanide solution (100 mM) was transferred to Co²⁺-2·1 complex solution prepared above. After mixing them for a few seconds, UV-vis spectra were taken at room temperature.

3. Results and discussion

Sensor 1 was obtained by the combination of thiophene-2-carbohydrazide and quinolone-2-carboxaldehyde with 85% yield in ethanol (Scheme 1), and characterized by ¹H NMR and ¹³C NMR, ESI-mass spectrometry, and elemental analysis.

Scheme 1 Synthesis of 1.

3.1. Absorption spectroscopic studies of 1 toward Co²⁺ and Cu²⁺

The colorimetric selectivity of sensor 1 toward various metal ions was conducted in bis–tris buffer/DMSO (95/5, v/v, pH = 7) (Fig. 1a). When 0.75 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⁺ were added to 1, only both Co²⁺ and Cu²⁺ showed distinct spectral changes at 400 nm and instant color changes from colorless to yellow, while other metal ions did not show any change in absorbance spectrum and color (Fig. 1b). These results suggested that the sensor 1 could be used as a “naked-eye” chemosensor for Co²⁺ and Cu²⁺ in aqueous media.

Fig. 1 (a) Absorption spectral changes of 1 (20 μM) upon the addition of various metal ions (0.75 equiv.) in bis–tris buffer/DMSO solution (v/v, 95 : 5). (b) The color changes of 1 (20 μM) upon addition of various metal ions (0.75 equiv.) in bis–tris buffer/DMSO solution (v/v, 95 : 5).

First of all, we conducted the UV-vis titration to examine the concentration-dependent signaling of 1 toward Co²⁺ (Fig. 2). On the gradual addition of Co²⁺ to a solution of 1, the absorption band at 325 nm significantly decreased, and a new band at 400 nm gradually reached a maximum at 0.75 equiv. of Co²⁺. A clear isosbestic point was observed at 356 nm, indicating that only one product was generated from 1 upon binding to Co²⁺. The peak at 400 nm with the high molar extinction coefficient, 8.65 × 10³ M⁻¹ cm⁻¹, is too large to be Co-based d–d transitions according to the literatures.^61–63 Therefore, the new peak might be attributed to a ligand-to-metal charge-transfer (LMCT).

Fig. 2 Absorption spectral changes of 1 (20 μM) after addition of increasing amounts of Co²⁺ in bis–tris buffer/DMSO (95/5, v/v) at room temperature. Inset: absorption at 400 nm versus the number of 0.8 equiv. of Co²⁺ added.

The binding mode between 1 and Co²⁺ was determined through job plot analysis (Fig. S1†),⁶⁴ which displayed a 2 : 1 stoichiometry. To further confirm the binding mode of Co²⁺-2·1 complex, ESI-mass spectrometry analysis was carried out (Fig. 3). The positive-ion mass spectrum showed that the peak at m/z = 620.00 was assignable to [2·1 + Co²⁺ − H]⁺ [calcd 620.05]. Based on job plot and ESI-mass spectrometry analysis, we proposed the structure of Co²⁺-2·1 complex as shown in Scheme 2.

Fig. 3 Positive-ion electrospray ionization mass spectrum of 1 (10 μM) upon addition of 1 equiv. of Co²⁺.

Scheme 2 Proposed binding mode of Co²⁺-2·1 complex.

Based on UV-vis titration data, the association constant of 1 for Co²⁺ was calculated to be 1.0 × 10¹⁰ M⁻² by Li's equations (Fig. S2†),⁶⁵ which is within the range of those (10⁴–10²²) reported for Co²⁺ sensing chemosensors.^61,66,67 The limit of detection was calculated to be 0.19 μM on the basis of 3σ/slope (Fig. S3†),⁶⁸ which is lower than the guideline of the New Jersey Ground Water Quality Standards rules (1.7 × 10⁻⁶ M).⁶⁹ Importantly, the detection limit is the lowest one among those previously reported for simultaneous sensing of Co²⁺ and Cu²⁺ in aqueous solution with more than 90% of water, to the best of our knowledge (Table S1†).^70,71 Thus, the sensor 1 could be a powerful tool for the detection of cobalt in groundwater.

To investigate any interference from various competing metal ions in the detection of Co²⁺, the competition experiment was executed in the presence of Co²⁺ mixed with various metal ions (Fig. 4). Significantly, no interference was observed for the detection of Co²⁺ from Al³⁺, Ga³⁺, In³⁺, Zn²⁺, Cd²⁺, Cu²⁺, Fe²⁺, Fe³⁺, Mg²⁺, Cr³⁺, Hg²⁺, Ag⁺, Ni²⁺, Na⁺, K⁺, Ca²⁺, Mn²⁺ and Pb²⁺. These results indicated that 1 could be an excellent colorimetric sensor for Co²⁺ over competing relevant metal ions in aqueous solution.

Fig. 4 (a) Absorption spectral changes of competitive selectivity of 1 (20 μM) toward Co²⁺ (0.75 equiv.) in the presence of other metal ions (0.75 equiv.) in bis–tris buffer/DMSO (95/5, v/v). (b) The color changes of competitive selectivity of 1 (20 μM) toward Co²⁺ (0.75 equiv.) in the presence of other metal ions (0.75 equiv.).

For environmental applications, pH effect on the absorption response of sensor 1 to Co²⁺ ion was studied in a series of solutions with pH values ranging from 2 to 12 (Fig. S4†). The color of the Co²⁺-2·1 complex remained in the yellow region between pH 7.0 and 11.0. These results indicated that the sensor 1 could clearly detect Co²⁺ by the naked eye or UV-vis absorption measurements over a wide pH range of 7.0–11.0. Moreover, the reusability of sensor 1 was examined by adding ethylenediaminetetraacetic acid (EDTA) to the complexed solution of 1 and Co²⁺. The spectral changes were reversible after the alternating successive addition of Co²⁺ and EDTA (Fig. S5†). Therefore, sensor 1 toward Co²⁺ could be recyclable by using a suitable reagent such as EDTA.

For quantitative measurement of Co²⁺ in real samples, we constructed a calibration curve for the determination of Co²⁺ (Fig. S6†). Sensor 1 exhibited a good linear relationship between the absorbance of 1 and Co²⁺ concentration (0–3 μM) with a correlation coefficient of R² = 0.9902 (n = 3). Based on the calibration curve, the chemosensor was applied for the determination of Co²⁺ in drinking and tap water samples. As shown in Table 1, the satisfactory recoveries and R.S.D. values were obtained for the water samples.

Table 1 Determination of Co²⁺ in water samples^a

Sample	Co(II) added (μmol L^−1)	Co(II) found (μmol L^−1)	Recovery (%)	R.S.D (n = 3) (%)
a Condition: [1] = 20 μmol L^−1 in bis–tris buffer/DMSO (95/5, v/v).
Drinking water	0.00	0.00	—	—
	2.00	2.08	104.0	0.66
Tap water	0.00	0.00	—	—
	2.00	1.89	94.5	1.43

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Next, we carried out the UV-vis titration to examine the concentration-dependent signaling of 1 toward Cu²⁺ ion (Fig. 5). Upon the gradual addition of Cu²⁺ to a solution of 1, the absorption band at 325 nm significantly decreased, and a new band at 400 nm gradually increased up to 0.7 equiv. of Cu²⁺. A clear isosbestic point was also observed 357 nm, which means that only one product was generated from 1 upon binding to Cu²⁺.

Fig. 5 Absorption spectral changes of 1 (20 μM) after addition of increasing amounts of Cu²⁺ in bis–tris buffer/DMSO (95/5, v/v) at room temperature. Inset: absorption at 400 nm versus the number of 0.75 equiv. of Cu²⁺ added.

The binding mode between 1 and Cu²⁺ was determined through job plot analysis (Fig. S7†).⁶⁴ It displayed a 2 : 1 complexation stoichiometry between 1 and Cu²⁺. To further understand the binding mode of 1 with Cu²⁺, ESI-mass spectrometry analysis was carried out (Fig. 6). The positive-ion mass spectrum showed that the peak at m/z = 624.00 was assignable to [(2·1 + Cu²⁺ − H)]⁺ [calcd 624.05].

Fig. 6 Positive-ion electrospray ionization mass spectrum of 1 (10 μM) upon addition of 1 equiv. of Cu²⁺.

Through the UV-vis titration data, the association constant of 1 and Cu²⁺ was determined as 7.0 × 10⁹ M⁻¹ on the basis of Li's equations (Fig. S8†).⁶⁵ The value is within those (10³–10¹²) reported for Cu²⁺ sensing chemosensors.^72–77 The detection limit of Cu²⁺ by 1, on the basis of 3σ/slope,⁶⁸ was determined to be 0.13 μM (Fig. S9†). Importantly, the value is much lower than the World Health Organization (WHO) guideline (31.5 μM) for Cu²⁺ in the drinking water.^33,78 Importantly, the detection limit is also the lowest one among those previously reported for simultaneous detection of Co²⁺ and Cu²⁺ by organo-chemosensors in aqueous solution with more than 90% of water, to the best of our knowledge (Table S1†), while a few of inorganic- and nano-materials show better detection limits.^70,71,79,80

To check the practical applicability of sensor 1 as a Cu²⁺ selective sensor, the competition experiment was conducted by the addition of Cu²⁺ ions (0.7 equiv.) to the solution of 1 containing various metal ions (0.7 equiv.) (Fig. S10†). There was no or slight interference in the detection of Cu²⁺ from the metal ions. For example, Mg²⁺, Cr³⁺, Hg²⁺, Na⁺, and K⁺ showed the slight interference, but the color of Cu²⁺-2·1 complex was still discernible. Thus, 1 could be used as a selective colorimetric sensor for Cu²⁺ in the presence of most competing metal ions.

In order to apply to environmental systems, the effect of pH on the absorption response of sensor 1 to Cu²⁺ was examined at pH values ranging from 2 to 12. As shown in Fig. S11,† sensing ability of 1 for Cu²⁺ was maintained between pH 7.0 and 10.0. This result assured of its application under physiological conditions, without any change in the detection of Cu²⁺.

For the reversibility test of sensor 1 toward Cu²⁺, ethylenediaminetraacetic acid (EDTA, 0.7 equiv.) was added to the complexed solution of sensor 1 and Cu²⁺ (Fig. S12†). The absorbance changes were almost reversible after several cycles with the sequentially alternative addition of Cu²⁺ and EDTA. These results showed that sensor 1 toward Cu²⁺ could be recyclable simply through treatment with a proper reagent such as EDTA. Therefore, reversibility and regeneration of sensor 1 could be applicable to the fabrication of chemosensor to detect Cu²⁺.

In order to examine the practical properties of the chemosensor 1 in environmental samples, the chemosensor 1 was applied for the determination of Cu²⁺ in water samples, using the calibration curve of 1 toward Cu²⁺ (Fig. S13†). Drinking and tap water samples were chosen. As shown in Table 2, appropriate recoveries and R.S.D values of the water samples were obtained.

Table 2 Determination of Cu²⁺ in water samples^a

Sample	Cu(II) added (μmol L^−1)	Cu(II) found (μmol L^−1)	Recovery (%)	R.S.D (n = 3) (%)
a Condition: [1] = 20 μmol L^−1 in bis–tris buffer/DMSO (95/5, v/v).
Drinking water	0.00	0	—	—
	2.00	2.10	105.0	4.06
Tap water	0.00	0.00	—	—
	2.00	2.11	105.5	2.74

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3.2. Theoretical calculations for 1 and Cu²⁺-2·1 complex

For a better understanding of the colorimetric sensing mechanism of 1 for Cu, theoretical calculations for 1 and Cu²⁺-2·1 were carried out in parallel to the experimental studies. Based on job plot and ESI-mass analysis, all calculations were conducted with a 2 : 1 binding mode of 1 to Cu²⁺. Under these conditions, geometric optimizations and theoretical calculations of 1 and Cu²⁺-2·1 complex were carried out by applying density functional theory (DFT/B3LYP/main group atom: 6-31G** and Cu:Lanl2DZ/ECP). The energy-minimized structures with the significant structural properties are presented in Fig. 7. Sensor 1 was coordinated to Cu²⁺ via the O atom in the hydroxyl group and the N atoms in the imine and quinolone. The absorptions to the singlet excited states of 1 and Cu²⁺-2·1 complex were investigated using the TD/DFT calculations. Theoretical calculation data of 1 and Cu²⁺-2·1 were well matched with those obtained from their UV-vis absorption spectra. For 1, the main molecular orbital (MO) contribution of the first lowest excited state was determined for HOMO → LUMO transition (346.8 nm), which could be characterized by intramolecular charge transfer (ICT) (Fig. S14†). For Cu²⁺-2·1 complex, the MO contributions of the 14th and 16th lowest excited state were described in Fig. S15,† which could be characterized by ICT and ligand-to-metal charge-transfer (LMCT) transitions. Hence, these results indicated that the chelation of 1 to Cu²⁺ might induce the ICT and slight LMCT, which caused the red-shift (346.8 nm → 432.6 and 435.8 nm) with color change from colorless to yellow. Based on job plot, ESI-mass spectrometry analysis, theoretical calculations and the structures reported to literatures,^47,63 we proposed the structure of Cu²⁺-2·1 complex as shown in Scheme 3.

Fig. 7 The energy-minimized structures of (a) 1 and (b) Cu²⁺-2·1 complex.

Scheme 3 Proposed binding mode of Cu²⁺-2·1 complex.

3.3. Absorption spectroscopic studies of Co²⁺-2·1 complex toward CN⁻

Some metal complexes showed the selectivity toward specific anions in the systems such as Cu–S, Cu–CN, and Hg–I.^40,81,82 Therefore, we also examined the selectivity of Co²⁺-2·1 and Cu²⁺-2·1 complexes toward various anions. When 30 equiv. of various anions such as CN⁻, OAc⁻, F⁻, Cl⁻, Br⁻, I⁻, H₂PO₄⁻, BzO⁻, N₃⁻, SCN⁻, NO₃⁻ and S²⁻ were added to the two complexes in bis–tris buffer/DMSO (95/5, v/v, pH = 7), only CN⁻ showed an instant selectivity to Co²⁺-2·1 complex (Fig. 8). Both UV-vis spectral and color changes from yellow to colorless were observed for CN⁻. These results indicated that Co²⁺-2·1 complex can serve as a “naked-eye” chemosensor for CN⁻ in aqueous solution.

Fig. 8 (a) Absorption spectral changes of the Co²⁺-2·1 complex (20 μM) in the presence of 30 equiv. of different anions in bis–tris buffer/DMSO (95/5, v/v). (b) The color changes of the Co²⁺-2·1 complex (20 μM) upon addition of various anions (30 equiv.) in bis–tris buffer/DMSO (95/5, v/v).

In order to understand the binding properties of Co²⁺-2·1 complex with CN⁻, we carried out the UV-vis titration experiments (Fig. 9). On the treatment with CN⁻ to the solution of Co²⁺-2·1, the absorption band at 400 nm significantly decreased, and the band at 325 nm increased with a distinct isosbestic point at 356 nm, indicating the formation of only one UV-active species. Moreover, the final UV-vis spectrum of Co²⁺-2·1 with CN⁻ was nearly identical to that of 1 (Fig. S16†), indicating the reproduction of 1. These results led us to propose that Co²⁺-2·1 complex might undergo the demetallation by CN⁻ as shown in Scheme 4.

Fig. 9 Absorption spectral changes of Co²⁺-2·1 complex (20 μM) upon addition of CN⁻ (up to 33 equiv.) in bis–tris buffer/DMSO (95/5, v/v). Inset: absorption at 400 nm versus the number of equiv. of Co²⁺ added.

Scheme 4 Proposed sensing mechanism of CN⁻ by Co²⁺-2·1 complex.

The job plot for the binding between Co²⁺-2·1 complex and CN⁻ revealed a 1 : 1 stoichiometry (Fig. S17†). The binding constant between Co²⁺-2·1 and CN⁻ was calculated as 3.0 × 10⁴ M⁻¹ on the basis of the Benesi–Hildebrand equation (Fig. S18†).⁸³ Based on the result of UV-vis titration, the detection limit for CN⁻ was determined to be 24.11 μM on basis of 3 σ/K (Fig. S19†).⁶⁸ Importantly, this is the third example of the cyanide-selective colorimetric chemosensor by using cobalt complex as a sensor in aqueous solution with more than 90% of water, to best of our knowledge (Table S2†).

To check further the practical applicability of Co²⁺-2·1 as a CN⁻-selective sensor, competition experiments were carried out. For the competition test, the Co²⁺-2·1 complex was treated with 30 equiv. of CN⁻ in the presence of various competing anions (30 equiv.) such as OAc⁻, F⁻, Cl⁻, Br⁻, I⁻, H₂PO₄⁻, BzO⁻, N₃⁻, SCN⁻, NO₃⁻ and S²⁻ in bis–tris buffer/DMSO (95/5, v/v, pH = 7) (Fig. 10). The coexistent anions showed no interference in the UV-vis spectral and color changes for the detection of CN⁻. These results suggested that Co²⁺-2·1 could be an excellent chemosensor for the detection of CN⁻ over competing anions through naked eye.

Fig. 10 (a) Absorption spectral changes of competitive selectivity of Co²⁺-2·1 complex (20 μM) toward CN⁻ (30 equiv.) in the presence of other anions (30 equiv.) in bis–tris buffer/DMSO (95/5, v/v). (b) The color changes of competitive selectivity of Co²⁺-2·1 complex (20 μM) toward CN⁻ (30 equiv.) in the presence of other anions (30 equiv.).

In order to investigate pH dependence of Co²⁺-2·1 toward CN⁻, the pH test was carried out in a wide range of pH (Fig. S20†). The optimal range for the colorimetric sensing of CN⁻ by Co²⁺-2·1 was turned out to be between pH 7.0 and pH 11.0. Therefore, CN⁻ could be clearly detected by the naked eye or UV-vis absorption measurements using Co²⁺-2·1 over a range of pH 7.0–11.0.

4. Conclusion

We developed a simple thiophene-2-carbohydrazide-based colorimetric chemosensor 1 for Co²⁺ and Cu²⁺. The sensor 1 selectively detected Co²⁺ and Cu²⁺ through a color change from colorless to yellow. Importantly, 1 could quantify Co²⁺ and Cu²⁺ in real water samples with the satisfactory recoveries and R.S.D values. The binding of 1 with Co²⁺ and Cu²⁺ was reversible with a suitable reagent such as EDTA. In addition, the resulting Co²⁺-2·1 complex can be used as a colorimetric sensor for cyanide with changing its color form yellow to colorless in aqueous solution. The cyanide-sensing mechanism by Co²⁺-2·1 complex was proposed to be a simple demetallation between Co²⁺-2·1 and CN⁻. Therefore, these results may contribute to the development of a novel type of chemosensors for a multifunctional “naked-eye” detection of Co²⁺ and Cu²⁺ and for the sequential recognition of Co²⁺ and CN⁻ by a colorimetric method in aqueous solution.

Acknowledgements

Financial support from Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (NRF-2014R1A2A1A11051794 and NRF-2015R1A2A2A09001301) are gratefully acknowledged. This subject is also supported by Korea Ministry of Environment (MOE) as “The Chemical Accident Prevention Technology Development Project”.

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Footnote

† Electronic supplementary information (ESI) available: Supplementary data (experimental procedures and additional experimental data) associated with this article can be found. See DOI: 10.1039/c7ra01580a

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