Highly selective and sensitive colorimetric chemosensor for detection of Co²⁺ in a near-perfect aqueous solution

Abstract

A new simple and easy-to-make colorimetric chemosensor 1 was designed and synthesized by combination of 4-diethylaminosalicylaldehyde and diethylenetriamine. In a near-perfect aqueous solution, sensor 1 showed a selective color change from colorless to yellow in the presence of Co²⁺ with a 1 : 1 stoichiometric system. To investigate the sensing mechanism of 1 for Co²⁺, UV-vis spectroscopy, ESI-mass and theoretical calculations were conducted. The detection limit (0.65 μM) of 1 for Co²⁺ was comparable to the Federal-State Toxicology and Risk Analysis Committee (FSTRAC) guideline in drinking water (0.68 μM). Practically, 1 functioned as a visible test strip with a silica plate and could be used to quantify Co²⁺ in real water samples. Therefore, probe 1 could be a good alternative method for on-site and real time screening of Co²⁺.

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1. Introduction

Cobalt is a normal earth element present in small amounts in rocks, soil, plants and animals.^1,2 In recent years, chemists were very interested in detecting Co²⁺ among the transition-metal ions because it plays a key role in the metabolism of iron and is also a main component of vitamin B₁₂ (cobalamin).^3–5 Cobalamin is essential for the biological synthesis of molecules such as DNA, persistence of the nervous system, arrangement of red blood cells, and growth and development of children.⁶ Apart from its biological role, exposure to high levels or even long-term at low levels of cobalt can cause toxicological effects, including asthma, decreased cardiac output, cardiac enlargement, heart disease, lung disease, dermatitis and vasodilation.^7–10 Therefore, development of a highly selective and sensitive analytical method capable of sensing trace amounts of the cobalt ion is a great challenge.

Many approaches have been reported for the detection of Co²⁺, which mainly refer to inductively coupled plasma atomic emission spectrometry,¹¹ atomic absorption spectroscopy,¹² electrochemical methods,¹³ nanoparticles^14–17 and fluorescence techniques.^18–23 Unfortunately, these methods have their own limitations such as the requirement of expensive instrumentation, tedious sample preparation procedures, and trained operators. Therefore, there is an increasing demand for a simple, efficient and easily accessible analytical method. In order to meet increasing needs, colorimetric methods have attracted much attention to settle these complicated problems, allowing for naked-eye detection of Co²⁺ with its simplicity, sensitivity, and ease of quantification.^24–28

Schiff bases²⁹ have been broadly studied because of their attractive electronic and photochemical properties, and good chelation property with transition metal ions.^30–33 Diethylaminosalicyl-imine is a well-known chromophore with a hydrophilic character. Therefore, we expected that the combination of a Schiff base and a diethylaminosalicyl-imine moiety would show good chelation for transition metal ions and its metal complexes might be water-soluble.^34–37

Herein, we designed and synthesized a new chemosensor 1, based on the combination of 4-diethylaminosalicylaldehyde and diethylenetriamine, for selective detection of Co²⁺. Receptor 1 showed a distinct color change from colorless to yellow in the presence of Co²⁺ in a near-perfect aqueous solution. Importantly, 1 could detect Co²⁺ down to a concentration of hundreds of nanomolar. Moreover, water sample and test strip experiments showed that 1 could be used as a practical chemosensor.

2. Experimental

2.1. Materials and equipment

All the solvents and reagents (analytical grade and spectroscopic grade) were obtained from Sigma-Aldrich and used as received. ¹H NMR and ¹³C NMR measurements were performed on a Varian 400 MHz and 100 MHz spectrometer, and chemical shifts were 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 a Flash EA 1112 elemental analyzer (thermo) in the Organic Chemistry Research Center of Sogang University, Korea. Absorption spectra were recorded at room temperature using a Perkin Elmer model Lambda 25 UV/vis spectrometer.

2.2. Synthesis of receptor 1

4-Diethylaminosalicylaldehyde (0.39 g, 2.0 mmol) and diethylenetriamine (0.11 mL, 1.0 mmol) were dissolved in absolute ethanol (20 mL). Two drops of HCl were added into the reaction solution, which was stirred for 12 h at room temperature. The solvent was removed under reduced pressure to afford brown oil, which was purified by silica gel chromatography (10 : 1 v/v CH₂Cl₂–CH₃OH). Yield of 1 (liquid): 0.35 g (86%). ¹H NMR (400 MHz, DMSO-d₆, 25 °C): δ = 13.59 (s, 2H), 8.15 (s, 2H), 7.02 (d, J = 8.8 Hz, 2H), 6.14 (dd, J = 8.8 Hz, J = 2.4 Hz, 2H), 5.90 (d, J = 2.4 Hz, 2H), 3.51 (t, J = 6.0 Hz, 4H), 2.79 (t, J = 6.0 Hz, 4H), 1.10 (t, J = 7.2 Hz, 12H), 3.34 (m, 7.2 Hz, 8H); ¹³C NMR (100 MHz, DMSO-d₆, 25 °C): δ = 166.28, 164.13, 151.09, 107.85, 133.13, 107.85, 102.60, 97.53, 56.15, 49.69, 43.74, 12.58. LRMS (ESI): m/z calcd for C₂₆H₄₀N₅O₂ + H⁺: 454.32; found 454.30. Anal. calcd for C₂₆H₃₉N₅O₂ (453.63): C, 68.84; H, 8.67; N, 15.44%. Found: C, 68.56; H, 8.92; N, 15.41%.

2.3. UV-vis titrations

Receptor 1 (4.54 mg, 0.01 mmol) was dissolved in DMSO (1 mL) and 6 μL of receptor 1 (10 mM) was diluted to 2.994 mL with a bis–tris buffer solution (10 mM, pH 7) to make a final concentration of 20 μM. Co(NO₃)₂·6H₂O (5.94 mg, 0.02 mmol) was dissolved in DMSO (1 mL) and 0.3–4.5 μL of this Co²⁺ solution (20 mM) was transferred to each receptor solution (20 μM). After mixing them for a few seconds, UV-vis spectra were taken at room temperature.

2.4. Job plot measurements

Receptor 1 (9.07 mg, 0.02 mmol) was dissolved in DMSO (1 mL). 15.0, 13.5, 12.0, 10.5, 9.0, 7.5, 6.0, 4.5, 3.0, 1.5 and 0 μL of the 1 solution were taken and transferred to vials. Each vial was diluted with the bis–tris buffer solution (10 mM, pH 7) to make a total volume of 2.985 mL. Co(NO₃)₂·6H₂O (5.94 mg, 0.02 mmol) was dissolved in DMSO (1 mL). 0, 1.5, 3.0, 4.5, 6.0, 7.5, 9.0, 10.5, 12.0, 13.5, and 15.0 μL of the cobalt solution was added to each diluted 1 solution. Each vial had a total volume of 3 mL. After reacting them for a few seconds, UV-vis spectra were taken at room temperature.

2.5. Competition experiments

Receptor 1 (4.54 mg, 0.01 mmol) was dissolved in DMSO (1 mL) and 6 μL of this solution (10 mM) was diluted to 2.994 mL with the bis–tris buffer solution (10 mM, pH 7) to make the final concentration of 20 μM. MNO₃ (M = Na, K, 0.02 mmol) or M(NO₃)₂ (M = Mn, Hg, Ni, Cu, Zn, Cd, Mg, Ca, Pb, 0.02 mmol) or M(ClO₄)₂ (M = Fe, 0.02 mmol) or M(NO₃)₃ (M = Fe, Cr, Al, Ga, In, 0.02 mmol) was separately dissolved in DMSO (1 mL). 3.3 μL of each metal solution (20 mM) was taken and added to 3 mL of the solution of receptor 1 (20 μM) to give 1.1 equiv. of metal ions. Co(NO₃)₂·6H₂O (5.94 mg, 0.02 mmol) was dissolved in DMSO (1 mL). Then, 3.3 μL of Co²⁺ solution (20 mM) was added into the mixed solution of each metal ion and 1 to make 1.1 equiv. After mixing them for a few seconds, UV-vis spectra were taken at room temperature.

2.6. pH effect test

A series of buffers with pH values ranging from 2 to 12 were prepared by mixing sodium hydroxide solution and hydrochloric acid in bis–tris buffer. After the solution with a desired pH was achieved, receptor 1 (4.54 mg, 0.01 mmol) was dissolved in DMSO (1 mL), and then 6 μL of the receptor 1 (10 mM) was diluted to 2.994 mL with the bis–tris buffer solution (10 mM) to make the final concentration of 20 μM. Co(NO₃)₂·6H₂O (5.94 mg, 0.02 mmol) was dissolved in DMSO (1 mL). 3.3 μL of the Co²⁺ solution (20 mM) was transferred to each receptor solution (20 μM) prepared above. After reacting them for a few seconds, UV-vis spectra were taken at room temperature.

2.7. Colorimetric test strip

Receptor 1 (4.5 mg, 0.01 mmol) was dissolved in MeCN (100 mL). Receptor 1-test kits were prepared by immersing silica plates into receptor 1 solution (100 μM), and then dried in air. MNO₃ (M = Na, K, Ag, 0.1 μmol) or M(NO₃)₂ (M = Mn, Hg, Ni, Cu, Zn, Cd, Co, Mg, Ca, Pb, 0.1 μmol) or M(ClO₄)₂ (M = Fe, 0.1 μmol) or M(NO₃)₃ (M = Fe, Cr, Al, Ga, In, 0.1 μmol) was separately dissolved in bis–tris buffer (10 mL). The test kits prepared above were added into different metal solutions, and then dried at room temperature.

2.8. Theoretical calculation methods

All DFT/TDDFT calculations based on the hybrid exchange correlation functional B3LYP^38,39 were carried out using Gaussian 03 program.⁴⁰ The 6-31G** basis set^41,42 was used for the main group elements, whereas the Lanl2DZ effective core potential (ECP)^43–45 was employed for Co. For vibrational frequency calculations, there was no imaginary frequency for the optimized geometries of 1 and 1–Co³⁺, suggesting that these geometries represented local minima. For all calculations, the solvent effect of water was considered by using the Cossi and Barone’s CPCM (conductor-like polarizable continuum model).^46,47 To investigate the electronic properties of singlet excited states, time-dependent DFT (TDDFT) was performed in the ground state geometries of 1 and 1–Co³⁺. The 30 singlet–singlet excitations were calculated and analyzed. The GaussSum 2.1 (ref. 48) was used to calculate the contributions of molecular orbitals in electronic transitions.

3. Results and discussion

3.1. Synthesis of receptor 1 and colorimetric detection of Co²⁺

Receptor 1 was obtained by the combination of 4-diethylaminocsalicylaldehyde and diethylenetriamine with 86% yield in absolute ethanol (Scheme 1), and characterized by ¹H NMR and ¹³C NMR, ESI-mass spectroscopy, and elemental analysis.

Scheme 1 Synthesis of 1.

The absorption response of 1 with various nitrate or perchlorate salts of Al³⁺, Ga³⁺, In³⁺, Zn²⁺, Cd²⁺, Cu²⁺, Fe²⁺, Fe³⁺, Mg²⁺, Cr³⁺, Hg²⁺, Ag⁺, Co²⁺, Ni²⁺, Na⁺, K⁺, Ca²⁺, Mn²⁺ and Pb²⁺ was carried out in a bis–tris buffer solution (10 mM, pH 7). Upon the addition of 1.1 equiv. of each metal ion, receptor 1 showed some decrease of the absorption band at 360 nm in the presence of Al³⁺, Ga³⁺, In³⁺, Zn²⁺, Cd²⁺, Cu²⁺, Fe³⁺, Mg²⁺, Cr³⁺, Hg²⁺, Ag⁺, Ni²⁺, Na⁺, K⁺, Ca²⁺, Mn²⁺ and Pb²⁺ (Fig. 1a), while Fe²⁺ showed a large decrease at the band. To further check the sensing behavior of 1 toward Fe²⁺, the time-controlled UV-vis absorption spectral changes of 1 in the presence of Fe²⁺ was monitored for 1 h (Fig. S1†). No spectral change was observed in the visible region. By contrast, the addition of Co²⁺ to 1 showed both a distinct spectral change with a significant bathochromic shift and an instant color change from colorless to yellow (Fig. 1b). To investigate the binding properties between receptor 1 and Co²⁺, UV-vis absorption spectral variation of 1 was monitored through titration with different concentrations of Co²⁺ (Fig. 2). Upon addition of Co²⁺ into 1, the absorption band at 360 nm decreased and a new red-shift band at 420 nm steadily increased up to 1.1 equiv. Meanwhile, clear isosbestic points were observed at 340 nm and 400 nm, suggesting that only one product was produced from the binding of 1 with Co²⁺. The molar extinction coefficient of the new peak in the thousands, 8.2 × 10³ M⁻¹ cm⁻¹ at 420 nm, is too large to be Co-based d–d transitions and thus must be ligand-based transitions. Therefore, this result was considered as the ligand-to-metal charge-transfer (LMCT) mechanism.

Fig. 1 (a) Absorption spectral changes of 1 (20 μM) in the presence of 1.1 equiv. of different metal ions in bis–tris buffer solution (10 mM, pH 7). (b) The color changes of 1 (20 μM) upon addition of various metal ions (1.1 equiv.) in bis–tris buffer solution.

Fig. 2 Absorption spectral changes of 1 (20 μM) in the presence of different concentrations of Co²⁺ ions in bis–tris buffer solution (10 mM, pH 7) at room temperature.

The Job plot analysis⁴⁹ indicated a 1 : 1 stoichiometry for receptor 1 and Co²⁺ (Fig. S2†). To further examine the binding interaction between 1 and Co²⁺, ESI-mass spectrometry analysis was carried out (Fig. 3). The 1–Co³⁺ complex with a 1 : 1 stoichiometry was observed as a major peak, although Co²⁺ was used as the standard metal ion. The positive ion mass spectrum of the ESI-mass indicated that a peak at m/z 510.20 was assignable to 1 + Co³⁺–2H⁺ (calcd 510.23). These results led us to propose that the 1–Co²⁺ complex formed from the reaction of 1 with Co²⁺ might be oxidized to 1–Co³⁺ by oxygen molecules.²⁸ To prove our proposal for the oxidation of Co²⁺ to Co³⁺ in the 1–Co²⁺ complex, we carried out a sensing test of the 1–Co²⁺ complex under degassed conditions (Fig. S3†). No color change was observed for the 1–Co²⁺ complex under degassed conditions. Then, continuous exposure of the 1–Co²⁺ solution to air made the color of the solution change to yellow. These results, again, indicated that Co²⁺ was oxidized to Co³⁺ when binding to receptor 1 under the aerobic conditions (Scheme 2). Furthermore, we demonstrated the proposal of the oxidation of Co²⁺ to Co³⁺ in the 1–Co²⁺ complex by using electron paramagnetic resonance (EPR) spectroscopy. As expected, the Co(NO₃)₂ sample showed the typical high-spin signals at g = 2.00, g = 4.3 and g = 6.6 (Fig. S4†). However, the EPR sample of 1 + Co(NO₃)₂ prepared under aerobic conditions was silent, strongly indicating that Co²⁺ in the 1–Co²⁺ complex was oxidized to Co³⁺.

Fig. 3 Positive-ion electrospray ionization mass spectrum of 1 (100 μM) upon addition of Co(NO₃)₂ (1.0 equiv.).

Scheme 2 Proposed structure of the 1–Co³⁺ complex.

Based on the UV-vis titration data, the binding constant for 1 with Co²⁺ was estimated to be 1.9 × 10⁵ M⁻¹ from the Benesi–Hildebrand equation (Fig. S5†).⁵⁰ The detection limit (3σ/K)⁵¹ of receptor 1 as a colorimetric sensor for the analysis of Co²⁺ was found to be 0.65 μM (Fig. S6†). The value is lower than the Wisconsin drinking water guideline (0.68 μM) for Co²⁺, established by Federal-State Toxicology and Risk Analysis Committee (FSTRAC). In addition, the value of the detection limit is the lowest one among those previously reported for organic chemosensors for sensing of Co²⁺ in an aqueous solution with a high water content (>90%), to the best of our knowledge.^26–28,52

To further study the ability of 1 as a colorimetric sensor for Co²⁺, competition experiments were conducted in the presence of various nitrate or perchlorate salts of Al³⁺, Ga³⁺, In³⁺, Zn²⁺, Cd²⁺, Cu²⁺, Fe²⁺, Fe³⁺, Mg²⁺, Cr³⁺, Hg²⁺, Ag⁺, Ni²⁺, Na⁺, K⁺, Ca²⁺, Mn²⁺ and Pb²⁺. For competition studies, 1 was treated with 1.1 equiv. of Co²⁺ in the presence of the same concentration of other metal ions (Fig. 4). The coexistent metal ions had no influence in the detection of Co²⁺. These results suggested that 1 can be used as an excellent selective sensor for Co²⁺ detection in the presence of competing metal ions.

Fig. 4 (a) Competitive selectivity of 1 (20 μM) toward Co²⁺ (1.1 equiv.) in the presence of other metal ions (1.1 equiv.). (b) Color changes of 1 (20 μM) in the presence of Co²⁺ (1.1 equiv.) and other metal ions (1.1 equiv.).

In order to apply to biological and environmental systems, the pH dependence of 1 in the absence and presence of Co²⁺ was conducted at various pH values (Fig. 5). The increase of absorbance caused by the addition of Co²⁺ ions was observed in a range of pH 7.0–11.0. This result warranted its application under physiological conditions, without any change in the detection of Co²⁺.

Fig. 5 UV-vis absorbance (420 nm) of 1 (20 μM) and the 1–Co²⁺ complex at pH 2–12 in bis–tris buffer solution (10 mM, pH 7) at room temperature.

To clearly understand the sensing mechanism of Co²⁺ with 1, theoretical calculations were performed in parallel to experimental studies. As Job plot and ESI-mass spectrometry analyses showed that 1 reacted with Co²⁺ in a 1 : 1 stoichiometric ratio, all theoretical calculations were performed with the 1 : 1 stoichiometry. The 1–Co³⁺ complex was optimized with a diamagnetic character (S = 0, DFT/uB3LYP/main group atom: 6-31G** and Co: Lanl2DZ/ECP), because the 1–Co²⁺ complex was oxidized to 1–Co³⁺. The significant structural properties of the energy-minimized structures are shown in Fig. 6. The energy minimized structure of 1 showed a bent structure with a dihedral angle of 1N, 2C, 3C, 4C = −58.548° (Fig. 6a). The 1–Co³⁺ complex also exhibited a bent structure with a dihedral angle of 1N, 2C, 3C, 4C = −142.971°, and Co³⁺ was coordinated with 1N, 5N, 6O, 7N and 8O atoms of 1 and the oxygen atom of NO₃⁻ (Fig. 6b). We also investigated the absorption to the singlet excited states of 1 and 1–Co³⁺ species via TDDFT calculations. In the case of 1, the main molecular orbital (MO) contribution of the first lowest excited state was determined for the HOMO → LUMO transition (316.49 nm, Fig. 7 and S7†), which indicated an intramolecular charge transfer (ICT) band from the diethylaniline to the imine group. For the 1–Co³⁺ complex, the third lowest excited state was found to be relevant to the color change (colorless to yellow) showing a predominant LMCT (Fig. 7 and S8†). The LMCT mainly indicated MO changes from the molecular orbitals of the diethylaniline and imine groups to metal-centered orbitals. The chelation of Co³⁺ with 1 induced MLCT transition, resulting in the yellow color of 1 in the presence of Co³⁺.

Fig. 6 The energy-minimized structures of (a) 1 and (b) the 1–Co³⁺ complex.

Fig. 7 Molecular orbital diagrams and excitation energies of 1 and the 1–Co³⁺ complex.

3.2. Practical applicability of receptor 1

To investigate the practical application of receptor 1, we constructed a calibration curve for the determination of Co²⁺ by 1 (Fig. 8). Compound 1 showed a linear range of 0–16 μmol L⁻¹ for Co²⁺ detection in buffered aqueous solution. To confirm the practical ability of receptor 1, drinking and tap water samples were tested (Table 1). Each sample was analyzed with three replicates. As shown in Table 1, a suitable recovery and R.S.D. values of the water samples were obtained. These results suggests that receptor 1 could be useful for the measurement of Co²⁺ in chemical and environmental applications.

Fig. 8 Absorbance (at 420 nm) of 1 as a function of Co²⁺ concentration. [1] = 20 μmol L⁻¹ and [Co²⁺] = 0.00–16.00 μmol L⁻¹ in bis–tris buffer solution (10 mM, pH 7.0).

Table 1 Determination of Co²⁺ in water samples^a

Sample	Co^2+ added (μmol L^−1)	Co^2+ found (μmol L^−1)	Recovery (%)	R.S.D. (n = 3) (%)
a Conditions: [1] = 20 μmol L^−1 in bis–tris buffer solution (10 mM, pH 7.0).b 1.00 μmol L^−1 of Co^2+ ions was artificially added.
Drinking water	0.00	0.00	98.0	1.1
	1.00^b	0.98
Tap water	0.0	0.00	99.0	1.0
	1.00^b	0.99

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For another practical application, colorimetric test strips were prepared by immersing a silica plate in an acetonitrile (MeCN) solution of 1 and then drying in a vacuum. These test kits were used to detect Co²⁺ among various metal ions (Fig. 9). When the test strips coated with 1 were added to different metal-ion solutions (10 μM), an obvious color change was observed only with Co²⁺ in a bis–tris buffer solution. Therefore, the colorimetric test strip would have the potential application to easily and rapidly detect Co²⁺.

Fig. 9 Photographs of the silica plates coated with 1 used for the detection of Co²⁺. (a) Left to right: test strip coated with only receptor 1 (control, 100 μM), test kit coated with only Co²⁺ (control, 10 μM), and receptor-1 test strip immersed in Co²⁺ solution. (b) Receptor-1 test strips (100 μM) immersed in various metal ions (10 μM).

4. Conclusion

We have developed an outstanding colorimetric chemosensor 1, based on the combination of 4-diethylaminosalicylaldehyde and diethylenetriamine. Sensor 1 selectively detected Co²⁺ through a color change from colorless to yellow, which might be attributed to the ligand-to-metal charge-transfer (LMCT) process. The sensing mechanism of Co²⁺ can be successfully explained by UV-vis titration and theoretical calculations. Importantly, the detection limit (0.65 μM) of 1 toward Co²⁺ is the lowest one among those previously reported for Co²⁺ in an aqueous solution with a high water content (>90%) without any interference. 1 could be used to detect and quantify Co²⁺ in real water samples. Moreover, 1 also showed visible colorimetric detection of Co²⁺ with test strips, indicating that 1 could be used as a good guidance for real-time optical application of Co²⁺.

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 NRF-2015R1A2A2A09001301) are gratefully acknowledged. We thank Nano-Inorganic Laboratory, Department of Nano & Bio Chemistry, Kookmin University for access to the Gaussian 03 program packages.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra03364d

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