A simple chemosensor for the dual-channel detection of cyanide in water with high selectivity and sensitivity

Abstract

We have synthesized a simple chemosensor (HY) of acylhydrazones and used it for colorimetric detection of cyanide anions (CN⁻) in DMSO/H₂O (3 : 7, v/v) mixed solvent. The simple chemosensor displays a specificity response for cyanide over other common anions (F⁻, Cl⁻, Br⁻, I⁻, AcO⁻, H₂PO₄⁻, HSO₄⁻, ClO₄⁻, and SCN⁻) in mixed solution. Upon treatment with cyanide, HY displayed a remarkable naked-eye and fluorogenic response, simultaneously with significant changes in its UV-visible and fluorescence spectra. Furthermore, competitive anions did not show any significant changes both in colour and emission intensity, indicating the high selectivity of the sensor to CN⁻. The absorption spectra detection limit of the chemosensor for cyanide was 8.34 × 10⁻⁷ M and the fluorescence spectra detection limit was 2.85 × 10⁻⁸ M. The cyanide test strips based on the chemosensor could serve as a convenient cyanide test kit. Furthermore, the chemosensor was successfully applied to the detection of cyanide in sprouting potatoes.

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Introduction

The concept of devising and synthesizing novel chemosensors for detecting anions has already been a potential subject of interest due to the biological, environmental and practical applications.¹ Among the various anions, cyanide (CN⁻) is one of the most researched anions due to its high lethal effect on the living body and the surroundings.² Nevertheless, cyanide anions (CN⁻) are highly toxic to organisms,³ while CN⁻ is widely used in industrial production, such as in electroplating, gold mining, metallurgy, and the synthesis of fibers, nylon, and resins industries.⁴ Strong binding of CN⁻ to a heme unit of the cytochrome paralyzes cellular respiration and causes serious damages to the central nervous system.⁵ The cyanide standard in industrial wastewater therefore has stringent requirements: for instance, Japan’s Ministry of Environment sets its licensing standards at 38.5 mM.⁶ Precise quantification of CN⁻ in the circumstance specimen is particularly necessary.⁷ The majority of analytical methods such as potentiometry, chromatography, and voltammetry, have been used for CN⁻ detection.⁸ These methods successfully detect very low standards of CN⁻ (<0.1 mM), but demand cumbersome sample preparation or costly analytical instruments.

Therefore, it is highly necessary to develop sensitive, selective, cheap, simple, fluorometric and colorimetric chemosensors for detecting CN⁻.⁹ Optical chemosensors for CN⁻, in which a change in the absorbance and fluorescence spectra is found, have been continually investigated due to their desirable features including simplicity, high selectivity and high sensitivity.¹⁰ The design and synthesis of a colorimetric and fluorometric chemosensor for CN⁻ has attracted great attention because of easy quantification of CN⁻ through absorbance on a UV-vis spectrophotometer and changes in fluorescence in fluorescence spectra. The general methods for cyanide detection are summarized as nucleophilic addition on the carbonyl units,¹¹ electron-deficient alkenes,¹² hydrogen bonding motifs,¹³ cyanide complexation,¹⁴ and so on. But they themselves all have their own drawbacks such as poor solubility in aqueous media, long response time or poor application for naked-eye detection.¹⁵ So due to its operational simplicity, low cost, and rapid implementation, a chromogenic sensor for the detection of cyanide is highly desirable.

In view of the above, and as part of our research interest in ion recognition,¹⁶ in this paper we designed and synthesized a simple fluorescent and colorimetric sensor HY for cyanide anions based on a deprotonated mechanism. As its hydroxy (–OH) and amine (–NH–) groups are effective targets for the deprotonation procedure, cyanide can easily combine with it, inducing a remarkable change in the spectroscopic properties. With the gradual addition of cyanide, the colour of the solution changed from colourless to yellow which offers the possibility for us to detect cyanide with the naked-eye. And the fluorescence colour of the sensor from yellow to colourless similarly can detect cyanide by UV lamps. Other anions such as F⁻, Cl⁻, Br⁻, I⁻, AcO⁻, H₂PO₄⁻, HSO₄⁻, ClO₄⁻, and SCN⁻ could not cause any interference.

Results and discussion

The synthetic procedure of chemosensor HY is shown in Scheme 1. The chemosensor HY has been characterized by ¹H NMR, ¹³C NMR, IR and ESI-MS (Fig. S1–S3†). To detect the selectivity of HY towards CN⁻, we carried out a series of spectral experiments, where 50 equiv. of various anions (F⁻, Cl⁻, Br⁻, I⁻, AcO⁻, H₂PO₄⁻, HSO₄⁻, ClO₄⁻, CN⁻, and SCN⁻) were respectively added to 2.0 × 10⁻⁵ M of HY in mixed solution (DMSO/H₂O, 3 : 7, v/v). When adding 50 equiv. of CN⁻ to the solution of sensor HY, HY responded with dramatic colour changes from colourless to yellow (Fig. 1 inset). As shown in Fig. 1, in the corresponding UV-vis spectrum, a strong and broad absorption band from 378 to 460 nm is observed for the chemosensor HY. However, as shown in Fig. 2, yellow fluorescence with one emission band centered at 512 nm appeared when the solution of chemosensor HY was excited at 378 nm. Upon the addition of CN⁻ solution, the yellow fluorescence emission band decreased remarkably. A variety of other anions, such as F⁻, Cl⁻, Br⁻, I⁻, AcO⁻, H₂PO₄⁻, HSO₄⁻, ClO₄⁻, and SCN⁻, did not cause such a significant change.

Scheme 1 Structure and synthesis of the chemosensor HY.

Fig. 1 UV-vis spectra of HY (c = 2 × 10⁻⁵ M) after addition of 50 equivalents of various anions. Inset: UV-vis colour changes of HY (c = 2 × 10⁻⁵ M) after addition of 50 equivalents of various anions (c = 1 × 10⁻² M).

Fig. 2 Fluorescence spectra (λ_ex = 378 nm) of HY (c = 2 × 10⁻⁵ M) after addition of 50 equivalents of various anions. Inset: fluorescence colour changes of HY (c = 2 × 10⁻⁵ M) after addition of 50 equivalents of various anions.

In order to estimate the specific properties for selective recognition of CN⁻ and the colorimetric changes associated with the chemosensor HY, the chemosensor HY with CN⁻ was studied by UV-vis absorption spectra titration experiments. Along with the increasing concentration of CN⁻, a significant increase of the UV-vis absorbance is observed at 420 nm. Such a red shift led to the solution colour changing from colourless to yellow. On the other hand, clear isosbestic points are observed at 378 nm (Fig. 3). Similarly, the fluorescence spectra of the chemosensor HY change significantly along with the concentration increase of CN⁻. The intensity of the fluorescence band centered at 512 nm of chemosensor HY decreased progressively along with the concentration increase of CN⁻, and it was quenched almost completely after the addition of CN⁻ (Fig. 4). In the meantime, the detection limits of HY for CN⁻ calculated on the basis of 3δ/S (Fig. S4 and S5†) were 8.34 × 10⁻⁷ M for the absorption spectra and 2.85 × 10⁻⁸ M for the fluorescence spectra change respectively, which are both far lower than the WHO guideline of CN⁻ in drinking water (less than 1.9 × 10⁻⁶ M). In addition, the fluorescence quantum yields weakened from 0.64 to 0.007.

Fig. 3 UV-vis spectra of HY (2.0 × 10⁻⁵ M) in DMSO/H₂O (3 : 7, v/v) upon adding an increasing concentration of CN⁻ (0.1 M). Inset: (a) colour change; (b) a plot of absorption at 420 nm versus number of equivalents of CN⁻.

Fig. 4 Fluorescence spectra of HY (2.0 × 10⁻⁵ M) in DMSO/H₂O (3 : 7, v/v) upon adding an increasing concentration of CN⁻ (0.1 M). Inset: a plot of emission at 512 nm versus number of equivalents of CN⁻.

To further exploit the utility of the chemosensor HY as an anion-selective sensor for CN⁻, competitive experiments were carried out in the presence of 50 equiv. of CN⁻ and 50 equiv. of various anions in solution (DMSO/H₂O, 3 : 7, v/v). As shown in Fig. 5, it is noticeable that the miscellaneous competitive anions did not lead to any significant interference. The fluorescence selectivity was examined at an excitation wavelength of 378 nm, as shown in Fig. 6, and none of the competing anions interfered in the detection of CN⁻. This result displays the highly selectivity of the chemosensor HY toward cyanide anions over the other analytes mentioned above.

Fig. 5 UV-vis spectra of sensor HY at 460 nm with addition of 50 equiv. of CN⁻ in the presence of 50 equiv. of other anions in DMSO/H₂O (3 : 7, v/v).

Fig. 6 Fluorescence of sensor HY at 512 nm with addition of 50 equiv. of CN⁻ in the presence of 50 equiv. of other anions in DMSO/H₂O (3 : 7, v/v).

The pH dependence of the chemosensor in the solution system (DMSO/H₂O, 3 : 7, v/v) was checked using its absorption spectrum at 420 nm (Fig. 7) and fluorescence spectroscopy emission at 512 nm (Fig. 8). Cyanide ions were added to the solution of HY at different pH values. The results indicated that HY binding with CN⁻ occurred effectively in the range of pH 5–14.

Fig. 7 Effect of pH on the absorbance spectra (460 nm) of HY + CN⁻ (50 equiv.) from 1 to 14 in DMSO/H₂O (3 : 7, v/v) solution.

Fig. 8 Effect of pH on fluorescence spectra (512 nm) of HY + CN⁻ (50 equiv.) from 1 to 14 in DMSO/H₂O (3 : 7, v/v) solution.

To gain better insight into the sensing mechanism of chemosensor HY to CN⁻, ¹H NMR titration, IR spectroscopy and ESI-MS were carried out. In the ¹H NMR titration spectra (Fig. 9), before the addition of cyanide anions, the ¹H NMR chemical shifts of the O–H_a, O–H_b and –NH_c– protons on HY were at δ 12.75, 12.23 and 11.29 ppm. With the addition CN⁻, the hydroxy protons H_a and H_b, and –NH– proton H_c completely disappeared by the deprotonation. Upon the gradual addition of CN⁻, all the naphthalene ring protons exhibited an upfield shift on different levels, which suggests the increase in the electron density in the naphthalene ring through charge delocalization in the conjugated system. Meanwhile, we also did infrared spectroscopy experiments to prove the mechanism. In the IR spectrum of HY, the stretching vibration absorption peaks of hydroxyl O–H and HC=N appeared at 3409 cm⁻¹, and 1607 cm⁻¹, respectively. However, when HY met with CN⁻, the stretching vibration peaks of hydroxy (O–H) and HC=N absorption miraculously disappeared (Fig. S6†). This is clear proof that HY encountered CN⁻, and the deprotonation effect occurred. Moreover, from the ESI-MS data, we can also see an obvious peak at m/z 422.9846 assignable to [HY − 3H⁺ + 3Na⁺] (m/z = 422) (Fig. S7†). This similarly supports the deprotonation phenomena of HY. These common phenomena could be explained from the proposed sensing mechanism shown in Scheme 2.

Fig. 9 ¹H NMR spectra of HY (0.1 M, DMSO-d₆) and in the presence of varying amounts of CN⁻.

Scheme 2 The sensing mechanism of the chemosensor HY with CN⁻.

Motivated by the favourable CN⁻ recognition properties of the chemosensor HY, we prepared test strips by immersing filter papers into the DMSO/H₂O (3 : 7) solution of the chemosensor HY (1 × 10⁻³ M) and then dried them in air. The test strips could conveniently detect CN⁻, similar to those commonly used for pH measurement. Upon the addition of CN⁻ solution to the test strips, the test strips showed an obvious colour change from colourless to yellow, meanwhile, under the UV lamp, the disappearance of the yellow fluorescence can be clearly observed (Fig. 10). Therefore, the test strips of HY have excellent application value in the detection of CN⁻.

Fig. 10 Left: colour change of test strips upon addition of CN⁻. Right: the detection of test strips upon the addition of CN⁻ under an UV lamp at 365 nm.

It is worth mentioning that we also investigated the practical utilities of the sensing in our daily life. We selected sprouting potatoes to carry out the following experiment. The sprouting potato (187 g) was first mashed before being soaked in water (400 mL) for 5 days until the extract became muddy. The mixture was filtered and the filtrate was washed with 60 mmol L⁻¹ NaOH solution (150 mL) to get the cyanide-containing solution. As shown in Fig. 10, upon the addition of cyanide-containing solution into HY, an obvious colour change from colourless to yellow can be directly observed by the naked-eye (Fig. 11a). The disappearance of yellow fluorescence under the UV lamp confirmed that HY is a promising CN⁻ probe for practical applications (Fig. 11b).

Fig. 11 Fluorescence emission data for the chemosensor HY to detect cyanide in sprouting potatoes. Inset: (a) colour changes observed for HY upon the addition of cyanide-containing solution; (b) colour changes observed for HY upon the addition of cyanide-containing solution under a UV lamp and a photograph of a sprouting potato.

Conclusions

In summary, we have developed a novel CN⁻ chemosensor, which could detect CN⁻ through UV turn-on and fluorescence quenching with specific selectivity and high sensitivity. The investigation of the recognition mechanism indicated that the chemosensor HY recognized CN⁻ by a deprotonation procedure. It is worth noting that the competitive anions did not afford any obvious interference response. The absorption spectra detection limit of HY for CN⁻ was 8.34 × 10⁻⁷ M and the fluorescence spectra detection limit was 2.85 × 10⁻⁸ M, which is far lower than the WHO guideline of CN⁻ in drinking water (less than 1.9 × 10⁻⁶ M). Moreover, test strips based on this chemosensor were fabricated, which could serve as a practical colorimetric and fluorescence test kit to detect CN⁻ with simple and fast measurement and the chemosensor HY is a good way to detect cyanide aqueous extracts of sprouting potatoes.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (NSFC) (no. 21574104; 21161018; 21262032), the Natural Science Foundation of Gansu Province (1308RJZA221) and the Program for Changjiang Scholars and Innovative Research Team in University of Ministry of Education of China (IRT1177).

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Footnote

† Electronic supplementary information (ESI) available: Details of complete experimental procedures. See DOI: 10.1039/c6ra01758d

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