A new ionophore for chemical sensing of F⁻, CN⁻ and Co²⁺ using voltammetric, colorimetric and spectrofluorimetric techniques

Abstract

The TPEI molecule with ether-imine functional groups was synthesized and characterized by spectroscopic techniques. The TPEI receptor was analyzed for its interaction with different cations and anions. TPEI showed instant visual color change on addition of fluoride, cyanide and cobalt ions, which led to a study of the interaction of TPEI with Co²⁺, F⁻ and CN⁻. An obvious color change of TPEI from light yellow to colorless was observed for CN⁻ through a nucleophilic addition mechanism, while F⁻ was detected through a H-bonding mechanism with a distinct color change from light yellow to dark yellow. Co²⁺ was detected by pseudocavity formation of TPEI in the vicinity of cobalt ions causing a color change from yellow to blue. Paper strips of the TPEI receptor were also prepared, which can be used to detect the presence of Co²⁺, F⁻ and CN⁻.

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1. Introduction

The design and synthesis of various efficient artificial molecules for the detection of cations and anions have received much interest in the past decade due to the essential roles that cations and anions play in biology, environment and chemistry.¹ Among various cations, cobalt plays an important role in the metabolism of iron and the synthesis of hemoglobin, and it is also a main component of vitamin B12 and other biological compounds. However, exposure to high levels of cobalt can cause toxicological effects, including heart disease and thyroid enlargement.² Therefore, a highly sensitive and selective analytical method to detect cobalt(II) ions in many scientific fields, including medicine and environmental monitoring, etc. is of great importance to avoid these toxic effects.

Likewise, among various anions, cyanide and fluoride have been intensively researched due to their wide use in our life. Cyanide is one of the most toxic anions, harmful to the environment. It plays an important role in various industrial processes, for example gold mining, synthetic fibers and resins.^3–7 However, cyanide is known to be a damaging anion causing poisoning in biology and environment. It has a propensity to bind to the iron in cytochrome c oxidase, interfering with electron transport and resulting in hypoxia.⁸ Meanwhile, fluoride, being the most electronegative atom, is used for various purposes in biological, medical and chemical processes.⁹ If excess influx of fluoride into living systems occur, it can result in disruption of the immune system, fluorosis, and depression of thyroid activity. As a result, the presence of excess fluoride can cause skeletal fluorosis, bone diseases, mottling of teeth, lesions of the thyroid, liver, and other organs.¹⁰ Thus, there is a strong demand for an efficient sensing method to monitor cyanide and fluoride as well.

Till now, there is hardly any report which can detect cobalt, fluoride as well as cyanide by a single receptor. Single chemosensors for multiple analytes have recently become very popular among the analysts, because of their fast detection time and cost reduction. For example, they include CN⁻/F⁻,^11a CN⁻/OAc⁻,^11b F⁻/OAc⁻,^11c and Cu²⁺/S²⁻.^11d

In view of these requirements, we have developed a novel chemosensor which can recognise cobalt, fluoride and cyanide through different approaches. Devoted to fluoride and cyanide recognition, we have, first, considered the possibility of H-bonding between the phenol moiety (–OH) and fluoride. Secondly, we expected the nucleophilic addition reaction of cyanide by synthesizing a Schiff base (imine). Finally, formation of a cavity was considered to trap metal ions which led to the recognition of cobalt ions. Therefore, we designed a new chemosensor TPEI (Tri Phenyl Ether Imine) and tested its sensing properties toward various cations and anions. As hypothesized, the sensor TPEI showed an effective colorimetric recognition of Co²⁺, F⁻ and CN⁻.

2. Experimental

Reagents and instruments

All reagents were commercially obtained from Sigma-Aldrich and used without further purification. ¹H NMR spectra was recorded using JEOL A1 spectrometer operating at 400 MHz. ¹³C NMR spectra was recorded at 100 MHz. UV-vis studies were carried out on an Analytic Jena spectrophotometer using slit width of 1.0 cm. Fluorescence spectra were determined on a Perkin Elmer LS-55 fluorescence spectrometer. Electrochemical measurements were carried out on Gamry Potentiostat/Galvanostat/ZRA Interface 1000.

All the required solvents used in the spectrophotometric and electrochemical experiments were of HPLC grade (Sd Fine, India). For cation interaction investigation, perchlorate salts (Sigma Aldrich) of the metal were used while for anion interaction studies all the tetrabutylammonium salts were used.

Synthesis

Compound TPEI was synthesized by 3-step procedure by our group itself, as described in ESI.† Synthesized compounds were confirmed for their purity by melting point, TLC, ¹H and ¹³C NMR spectroscopy and mass analysis of final product as detailed in ESI.† Further cation/anion interaction studies were carried out in this report.

3. Results and discussion

Design of a new ionophore

Ionophores undergo host–guest type of interactions with a number of species by forming a cavity in the molecule where the target metal ion interacts with hetero atoms through electrostatic binding while acting as Lewis bases. A new molecule was designed where two nitrogen atoms and two oxygen atoms would form an electronic environment in the pseudocavity conducive to acid–base (in Lewis sense) interaction with the guest species. Another novelty introduced in the design of the molecule is the use of triphenyl ether motif. TPEI is a molecule with triphenylether as a spine of it with naphthol motifs connected through imine linkages on both sides of the terminal phenyl groups of the triphenyl ether backbone. This combination of rigid triphenylether and flexible imine groups are suitable in host–guest chemistry. Linking of the naphthol units through imine linkages ortho to the ether–oxygens has been done to create a pseudo cavity around the ether–oxygens atoms which is suitable to host any guest in the form of transition metals like cobalt. Also, fluoride has been captured by the same molecule through H-bonding with –OH of the naphthol motifs.

Synthesized receptor TPEI was studied for its interaction with different cations and anions. As per our hypothesis, the ionophore showed sharp changes in its optical properties, when allowed to interact with the target species. Experiments revealed that there was sudden change in absorbance in presence of cobalt, fluoride and cyanide ions. From 325 nm to 300 nm for cobalt ions, from 385 nm to 435 nm for fluoride ions and complete loss of absorption bands at 325 nm, 385 nm and 435 nm for cyanide ions. These changes were detectable with naked eye on addition of cobalt/fluoride/cyanide ions in TPEI solution. Based on these visual changes, binding properties of the receptor TPEI were interpreted by using spectrophotometric, spectrofluorimetric and voltammetric techniques.

UV-vis studies

Chromogenic sensing of cobalt, fluoride and cyanide. The absorption response of various cations/anions in CH₃CN : H₂O (95 : 5; v/v) showed visual change of color of solution from yellow to blue in the presence of cobalt, yellow to dark yellow in the presence of fluoride and yellow to colorless in the presence of cyanide, while no such color change was observed in the presence of any other ion (Fig. 1). The UV-vis spectra of TPEI receptor also showed changes in the presence of cobalt, fluoride and cyanide ions consistent with the change of color as seen visually and are shown in Fig. 2a and b. In case of anion detection, these mechanisms could possibly be explained by the hydrogen bonding ability of the heteroatoms and the nucleophilic addition of the anions. The hydrogen bonding ability of the anions is assumed to be in the order of F⁻ > AcO⁻ > H₂PO₄⁻ > N₃⁻, CN⁻, based on their electro-negativity. Therefore, fluoride ion could easily form hydrogen bonds with naphtholic protons of TPEI even in aqueous solution, thus showing a bathochromic shift in UV-vis spectra. In case of interaction of TPEI with CN⁻, the decolorization of TPEI might be due to the nucleophilic addition of CN⁻ onto TPEI, forming the colorless leuconitrile species.¹²

Fig. 1 The color changes of TPEI receptor alone (20 μM) and upon addition of (a) various cations (50 equivalents) and (b) various anions (50 equivalents) in CH₃CN : H₂O (95 : 5; v/v) HEPES buffered solvent system.

Fig. 2 UV-vis spectral changes of TPEI receptor alone (20 μM) and upon addition of (a) various cations (50 equivalents) and (b) various anions (50 equivalents) in CH₃CN : H₂O (95 : 5; v/v) HEPES buffered solvent system.

Detection of cobalt ions. Binding properties of TPEI were studied for cobalt ions by UV-vis calibration experiment (Fig. 3). The absorption band at 325 nm was shifted by 25 nm to appear at 300 nm accompanied with appearance of a new absorption peak at 675 nm, which increased steadily with increasing amount of cobalt ions due to the formation of TPEI–Co²⁺ complex. Calibration plots between concentration of cobalt ions and absorbance showed a linear increase in the range 0–70 μM of cobalt ions with R² = 0.99, as shown in Fig. 3. Job's plot measurements proved that the complexation between cobalt and TPEI was in 1 : 1 ratio (Fig. S1b†). ESI-mass analysis also supported the complexation of TPEI with Co²⁺ as predicted from its m/z value i.e. 660.13 (calcd value m/z 660.14) Fig. S1.† Experiments were also carried out in the presence of a number of other ions like Al³⁺, Cd²⁺, Hg²⁺, Li²⁺, Na⁺, Pb²⁺, Ag⁺, Ca²⁺, Cu²⁺, K⁺, Mg²⁺ and Ni²⁺, as shown in Fig. S2,† no interference was found with any of other metal ions, proving the selectivity of TPEI for cobalt ions only. The binding constant value of TPEI with Co²⁺ was found to be 4.2 × 10⁵ M⁻¹ and the detection limit calculated as 2 μM.

Fig. 3 Absorption spectra change of TPEI in the presence of different concentrations of Co²⁺ at room temperature.

Detection of fluoride ions. In presence of fluoride ions, the band at 385 nm in the absorption spectrum of TPEI disappeared and band at 435 nm increased with addition of fluoride ions with an isosbestic point at 400 nm, suggesting the formation of only one UV-vis active species (Fig. 4). This bathochromic shift of the absorption band from 385 nm to 435 nm led us to propose the change of intramolecular charge transfer (ICT) band through the H-bonding of TPEI with fluoride ions. Job's plot¹³ indicated to a 1 : 1 stoichiometry between TPEI and F⁻ (Fig. S3†). The binding constant (K) for TPEI with F⁻ (2.6 × 10⁵ M⁻¹) was estimated by using Benesi–Hildebrand equation based on the UV-vis calibration.¹⁴ Detection limit of TPEI for F⁻ was determined as 0.15(±0.02) μM from the calibration curve. To explore the ability of TPEI as a colorimetric chemosensor for F⁻, the competition experiments were conducted in the presence of F⁻ mixed with various competing anions. Spectrophotometric titration of TPEI was done against fluoride ion solution in presence of interfering ions taken in different concentrations ranging from 1 to 50 equivalents. At least 5 titrations were done with different equivalents of each interfering ion. When TPEI was treated with 50 equivalents of F⁻ in the presence of the same concentration of other competing anions, there was no interference for the detection of F⁻ in the presence of Cl⁻, Br⁻, I⁻, OAc⁻, H₂PO₄⁻, N₃⁻, SCN⁻, HSO₄⁻ and ClO₄⁻. Only CN⁻ interfered with the detection of F⁻ as shown in Fig. 5. These results indicated that TPEI could be a selective colorimetric sensor for F⁻ in the presence of various competing anions except CN⁻.

Fig. 4 Absorption spectra change of TPEI in the presence of different concentrations of F⁻ at room temperature.

Fig. 5 The color changes of TPEI upon addition of F⁻ and then various interfering anions in CH₃CN : H₂O (95 : 5; v/v) HEPES buffered solvent system.

Detection of cyanide ions. High selectivity of TPEI for cyanide may be due to the deprotonation of TPEI by CN⁻ followed by the subsequent nucleophilic addition of cyanide into the imine moiety of TPEI in a solvent mixture of CH₃CN–H₂O (95 : 5; v/v). Cyanide has weaker hydrogen bonding ability with water molecules than F⁻. Therefore, cyanide with the strongest basic property is able to deprotonate the naphtholic proton of TPEI instead of forming the hydrogen bonding with water molecules. Subsequently, the resulting naphtholate TPEI⁻² might form hydrogen bonding with water molecules. In presence of cyanide salt, the CN⁻ would attack the imine group of TPEI to produce the nucleophilic addition product.^15,16 Importantly, the addition of HCN/NaCN on the imine group, the absorbance at 435 nm (responsible for yellow color of the TPEI solution) goes off giving a clear colorless solution. Solution of TPEI in presence of CN⁻ showed the color change from yellow to colorless against various anions (Fig. 1). In consistency with the color change as seen in Fig. 1, TPEI–CN⁻ showed sharp decrease of absorption band at 325 nm, 385 nm and 435 nm (Fig. 6) which could be only due to nucleophilic addition product ruling out the possibility of ICT process.¹⁷ An isosbestic point was observed at 300 nm which also supported the formation of TPEI + CN⁻–H⁺ adduct. The Job's plot for the binding between TPEI and CN⁻ exhibited a 1 : 2 stoichiometry (Fig. S4†). From the Benesi–Hildebrand equation, the association constant for the TPEI–CN⁻ complexation was found to be 6.0 × 10³ M⁻¹ and detection limit of cyanide ions was found to be 0.1 μM. To check the possible interference of other anions on cyanide complexation with receptor TPEI, competition experiments were performed in the presence of CN⁻ mixed with various anions (Fig. 7). All of these competing anions such as F⁻, Cl⁻, Br⁻, I⁻, OAc⁻, H₂PO₄⁻, N₃⁻, SCN⁻, HSO₄⁻ and ClO₄⁻ did not interfere with naked-eye detection of CN⁻ by TPEI, which means that the receptor TPEI displays a good selectivity for CN⁻ over other competing anions. Therefore, these results clearly showed that the receptor TPEI can act as a specific chromogenic chemosensor for cyanide.

Fig. 6 Absorption spectra change of TPEI in the presence of different concentrations of CN⁻ at room temperature in CH₃CN : H₂O (95 : 5; v/v) HEPES buffered solvent system.

Fig. 7 The color changes of TPEI upon addition of CN⁻ and then various interfering anions in CH₃CN : H₂O (95 : 5; v/v) HEPES buffered solvent system.

Detection of cyanide by fluorescence studies. Fluorescence monitoring studies of TPEI were done with different cations as well as anions to check its emission properties. Receptor TPEI alone displayed no fluorescence emission when excited at 390 nm. With the addition of different cations and anions, no significant change was observed except with cyanide (Fig. 8). On the addition of CN⁻ into TPEI, there was an instant enhancement (52-fold) of the emission intensity at 450 nm. This observation demonstrated that the ICT process in TPEI was inhibited upon the addition of CN⁻, as was also observed in spectrophotometric studies. Therefore, the weak fluorescence intensity of TPEI could be attributable to ICT from the naphthol group to the ether group. The nucleophilic addition of cyanide to the imine group of TPEI prevented ICT resulting in an enhanced fluorescence emission.¹⁸

Fig. 8 Fluorescence spectral response of TPEI (10 μM) upon addition of various anions (50 equivalents) in CH₃CN : H₂O (95 : 5; v/v) HEPES buffered solvent system.

For a detailed study of fluorescent sensing behaviour of TPEI, fluorescence calibration experiments were performed (Fig. 9). When the receptor TPEI (10 μM) was titrated with CN⁻ solution, fluorescence intensity remained almost unchanged upto 0.8 μM of CN⁻ due to insufficient concentration and then increased on addition of CN⁻ from 0.8–20 μM with no further change on more addition of the titrate. Calibration graph can also be looked as a 2-step process taking place for cyanide ion measurement. The two parts of the calibration curve have different slopes but linear in shape. The detection limit for CN⁻ complex was determined as 0.01 μM. According to the World Health Organization (WHO), cyanide concentrations lower than 1.9 μM is acceptable in drinking water.¹⁹ Hence, the detection limit of TPEI for CN⁻ (0.01 μM) is far below the WHO guidelines of drinking water, which means that receptor TPEI could be used as a powerful tool for the detection of cyanide in water samples.

Fig. 9 Fluorescence spectral change of TPEI in the presence of different concentrations of CN⁻ at room temperature in CH₃CN : H₂O (95 : 5; v/v) HEPES buffered solvent system.

To check further the practical applicability of TPEI as a CN⁻ selective fluorescence sensor, competition experiments (Fig. S5†) were carried out. When TPEI was treated with 50 equivalents of CN⁻ in the presence of other anions of the same concentration, there was no fall in fluorescence intensity in presence of F⁻, Cl⁻, Br⁻, I⁻, OAc⁻, H₂PO₄⁻, N₃⁻, SCN⁻, HSO₄⁻ and ClO₄⁻ as interfering ions. These results indicated that receptor TPEI showed an excellent selectivity for cyanide in the presence of other anions, making it very useful in practical applications.

Voltammetric sensing of cobalt/fluoride/cyanide. Electrochemical detection of cobalt/fluoride/cyanide by TPEI receptor was carried out by cyclic and differential pulse voltammetric techniques in CH₃CN : H₂O (95 : 5, v/v) solution containing 0.1 M TBAPF₆ as the supporting electrolyte. Voltammograms for TPEI receptor showed anodic peaks at 0.55 V and 0.78 V and a minor cathodic peak at 0.63 V. Cathodic peak at 0.63 V represented reduction of imine moiety. Anodic peaks at 0.55 V and 0.78 V represented the oxidation of ether group and reoxidation process of reduced imine group, respectively,²⁰ as explained in Scheme 1.

Scheme 1 Electrochemistry of the imine moiety.

Voltammograms for a number of cations/anions were drawn to know the selectivity of TPEI receptor. Cobalt/fluoride/cyanide showed the major shifts in peak potentials on their respective additions in TPEI receptor solution, as shown in Fig. S6.†

Cobalt sensing. Addition of cobalt ion in TPEI receptor caused the diminishing of peak currents at 0.55 V and 0.78 V accompanied with anodic shifts and emergence of a new peak at 0.98 V which rises in peak current abruptly upto 40 μA approximately, as shown in Fig. 10a and b.

Fig. 10 (a) Cyclic and (b) differential pulse voltammograms of TPEI receptor solution (10⁻⁵ M) with incremental addition of Co²⁺ ions (0–12 μM), at pH 7.0 ± 0.1 (10 mM HEPES in CH₃CN : H₂O 95 : 5, v : v), 0.1 M TBAPF₆, scan rate: 50 mV s⁻¹.

Fluoride sensing. In case of fluoride, there was a complete quenching of peaks at 0.55 V and 0.78 V (Fig. 11a and b), due to the H-bonding of fluoride anion with naphtholic protons, which caused the whole molecule to come into resonance. Hence, the lone pairs of electrons on ether group and nitrogen of imine group became unavailable for oxidation at the electrode, as proved spectrophotometrically also.

Fig. 11 (a) Cyclic and (b) differential pulse voltammograms of TPEI receptor solution (10⁻⁵ M) with incremental addition of F⁻ ions (0–10 μM), at pH 7.0 ± 0.1 (10 mM HEPES in CH₃CN : H₂O 95 : 5, v : v), 0.1 M TBAPF₆, scan rate: 50 mV s⁻¹.

Cyanide sensing. Addition of cyanide led to the increase in peak current at 0.68 V first which later on merges with the new peak at 1.0 V till current magnitude 36 μA. Anodic peak at 0.55 V decreased in current only after the merging of peaks at 0.68 V and 1.0 V. Simultaneously, a new peak at 0.2 V is seen which increased its peak current till 12 μA, as shown in Fig. 12a and b. The substantial change in peak potentials and peak current values on the addition of cyanide proved the interaction of TPEI with CN⁻.

Fig. 12 (a) Cyclic and (b) differential pulse voltammograms of TPEI receptor solution (10⁻⁵ M) with incremental addition of CN⁻ ions (0–23 μM), at pH 7.0 ± 0.1 (10 mM HEPES in CH₃CN : H₂O 95 : 5, v : v), 0.1 M TBAPF₆, scan rate: 50 mV s⁻¹.

Sensitivity (LOD and LOQ)

Sensitivity of TPEI was determined from the limit of detection (LOD) and limit of quantification (LOQ) values. The LOD is calculated by the equation LOD = 3(SD/b), where SD is the standard deviation of the intercept and b is the average slope of the regression line. The LOQ is examined by the equation LOQ = 10(SD/b). Peak current (μA) versus concentration (μM) was plotted which resulted in a straight line (Fig. S7–S9†). The experiment was repeated 3 times and the representative curve from the 3 sets of data was used for further calculations of LOD and LOQ. By using 3-sigma method²¹ the limit of detection was found to be 0.03 μM, 0.04 μM, 0.07 μM for cobalt, cyanide and fluoride, respectively. Limit of quantification value comes out to be 0.12 μM, 0.13 μM, 0.25 μM cobalt, cyanide and fluoride, respectively.

Interference investigation. Selectivity of TPEI was confirmed further by interference studies with various interfering cations/anions added in excess to the target ion. Voltammetry was done to study the effect of the presence of other ions on the response of TPEI for Co²⁺ and F⁻/CN⁻ ions. TPEI responded selectively to Co²⁺ ions even in the presence of other interfering metal ions (Table S1†). In case of anions, the interaction of TPEI with cyanide was found selective in the presence of all the competing anions, while in case of fluoride except for cyanide no other ion did interference. Hence, TPEI receptor could be oriented as a cyanide selective chemosensor without any interference of all the other anions as shown in Fig. S10.†

NMR titrations. The detection of anions by receptor TPEI is further supported by ¹H NMR titration. ¹H NMR titrations were carried out in CDCl₃. Tetra butyl ammonium salts of fluoride and cyanide were used as anions sources. Without the addition of anions, receptor TPEI showed signals at δ 9.3 ppm due to imine proton and 15.4 ppm corresponds to OH proton. After the addition of 2 equiv. of F⁻ ions, signal of –OH proton becomes disappeared. This might be due to the formation of hydrogen bonds between the fluoride and the –OH group of receptor TPEI. In addition, the signals correspond to imine and aromatic protons were shifted to upfield slightly with the addition of fluoride ions. But in the case of CN⁻ ions to receptor TPEI, the –OH signal of receptor disappeared and a new peak at 5.5 ppm started appearing. All the aromatic protons were shifted to upfield (Fig. S11†). This result strongly supports the nucleophilic attack of CN⁻ ions toward the carbon atom of an electron deficient imine group of receptor TPEI.

Analytical application (dip strip test)

To check the practical applicability of the proposed sensor, dip strip test was performed in the presence of only ligand and then dipping the strips of ligand with light yellow color into the test solutions of F⁻, CN⁻ and Co²⁺. Ligand strips were prepared by soaking the cut filter paper in a TPEI solution (10⁻³ M) prepared in CH₃CN : H₂O (95 : 5; v/v) and then drying the strips into the air oven maintained at 50 ± 0.1 °C. Small amounts of target ions (F⁻, CN⁻ and Co²⁺) were spotted on the strips to check the visible change in color of these strips for the selective ions. Instantaneous color change from light yellow to dark yellow was observed for F⁻, light yellow to colorless for CN⁻ and light yellow to blue for Co²⁺ ions as shown in Fig. 13. It also explained colorimetric sensing with different ions.

Fig. 13 Dip strip test on TPEI receptor with various analytes.

Conclusions

In the present report, single sensor has been developed which can be used for the detection of multiple analytes (F⁻, CN⁻ and Co²⁺) through different approaches. Colorimetric detection of three analytes make the proposed receptor unique in its use as a chemosensor. Just by dipping the prepared strips of TPEI, we can detect the presence of F⁻, CN⁻ and Co²⁺.

Acknowledgements

We thank BRNS, BARC Mumbai for financial assistance vide research grant no. 2012/37C/5/BRNS/621. The support provided by SAI Labs, Thapar University, Patiala for NMR studies are also highly acknowledged. Authors are also thankful to SAIF Labs, Chandigarh for mass analysis.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra03909j

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