A new coumarin based chromo-fluorogenic probe for selective recognition of cyanide ions in an aqueous medium

Abstract

The optical behavior of a simple intramolecular charge transfer (ICT) fluorescent probe, a coumarin-nitrobenzene conjugate (CNB), has been described to detect cyanide (CN⁻) selectively in buffered aqueous media. The cyanide addition on the electron-deficient alkene bridge interrupts the π-conjugation in CNB, which obstructs the ICT process and results in a color change from yellow to colorless and a significant fluorescence enhancement at 410 nm. Job's plot analysis revealed a 1 : 1 stoichiometry for the interaction between the probe and cyanide anion with a detection limit of 0.14 μM. The probe was also used for fabrication of test strips that could be used in practical and efficient CN⁻ test kits.

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Introduction

Optical chemosensors for anion sensing have drawn considerable attention due to the important roles of anions in industrial and biological processes. Among anions, cyanide is one of the most lethal poisons known. It can be absorbed through the lungs, gastrointestinal tract, and skin, leading to vomiting, convulsions, loss of consciousness, and eventual death.^1,2 Nevertheless, cyanide is widely used in electroplating, gold mining, mineral extraction, polymer production and other industrial activities.² The “heavy” use of cyanide in these industries, along with its necessary transportation, drastically raises the potential for contamination in the environment and increases the risk of human exposure. According to the World Health Organization (WHO), the permissible level of CN⁻ in drinking water is 1.9 μM.³ Though it is difficult to calculate the exact lethal dosage or exposure limit of cyanide ions in human blood, studies have shown that the poisonous cyanide concentration in the blood of fire victims is ca. 20 μM.⁴ Accordingly, the detection as well as the quantitative determination of cyanide in biological and environmental samples has recently become a necessity.

For fluorescent cyanide sensing, methods have been developed based on versatile mechanisms, including specific chemical reaction,⁵ hydrogen-bonding interaction,⁶ metal complex ensemble displacement,⁷ excited-state intramolecular proton transfer⁸ and supramolecular self-assembly.⁹ However, many of the reported CN⁻ sensors suffer from several limitations such as long responding time, poor application in the “naked-eye” detecting, severely interference from F⁻ and AcO⁻, working only in organic media as well as using tetrabutylammonium cation as a counter cation to exclude the cation effect. Thus, undoubtedly development of an effective and selective cyanide-sensing system in aqueous environment is highly desirable. In this regard, methods based on the nucleophilic addition of cyanide ions on the activated carbon–carbon/heteroatom double bond have attracted much attention due to their high selectivity over other anions. One can plan a particular structural motif to enhance the reactivity of the probe by (i) attachment to electron-withdrawing group (such as –CF₃),¹⁰ (ii) installation of positively charged heteroatom substituent,¹¹ and/or tight association with intramolecular hydrogen bonds¹² and improve the sensitivity accordingly.

In the present work, a simple coumarin-derived fluorescent probe CNB was designed, in which diethylamino and nitrophenylvinyl groups act as electron-donating and electron-withdrawing groups, respectively. Such a molecule possesses an expanded π-conjugation as well as a strong ICT from diethylaminocoumarin to the π-deficient p-nitrophenyl ring. Additionally, strong electron-withdrawing nitro group can enhance the reactivity of the probe to cyanide. Thus it was anticipated that cyanide addition on the electron-deficient alkene bridge is expected to interrupt the π-conjugation, thus the ICT process is blocked to produce a significant spectroscopic response. The probe was fully characterized and applied in the detection of cyanide. Significantly colorimetric and fluorescent responses of the probe toward cyanide were realized in aqueous medium with high selectivity, which allows for the determination of cyanide in realistic samples with good recovery.

Results and discussion

Colorimetric signaling of CN⁻ ions

As shown in Fig. 1, CNB exhibited a strong low energy intramolecular charge transfer (ICT) band at 443 nm. With the addition of increasing amounts of CN⁻, the absorbance at 443 nm deceased gradually while a new absorption band at 349 nm developed, which induced a color change from yellow to colorless (inset in Fig. 2). Notably, an isosbestic point appeared at 387 nm indicating a clear formation of a new species. It is possible that an intramolecular charge transfer takes place in CNB from the coumarin N atom to the nitrophenyl group through the double bond conjugated spacer. The π-conjugation and intramolecular charge transfer were interrupted by a favorable nucleophilic attack of cyanide on the vinyl carbon. Accordingly, absorption band at 349 nm which is primarily attributed to the diethylaminocoumarin moiety was increased,¹⁴ whereas the absorption band at 443 nm due to the ICT interaction gradually disappeared.¹⁵

Fig. 1 UV-vis spectra of CNB (5 μM) in the presence of various concentrations of CN⁻ (0–30 μM) in HEPES buffer (CH₃CN/H₂O = 1 : 1, v/v, pH = 7.4). Inset: plot of absorbance intensity of CNB against concentration for CN⁻. Each spectrum was obtained 2 min after CN⁻ addition.

Fig. 2 UV-vis spectra of CNB (5 μM) in the presence of various anions (50 μM) in HEPES buffer (CH₃CN/H₂O = 1 : 1, v/v, pH = 7.4).

In contrast, the addition of other anions such as F⁻, AcO⁻, Cl⁻, Br⁻, HSO₃⁻, NO₃⁻, HCO₃⁻, HSO₃⁻, SCN⁻, H₂PO₄⁻, ClO₄⁻, NO₂⁻, HS⁻, and SO₃²⁻ did not cause noticeable color change (inset in Fig. 2) manifesting a good chromogenic selectivity of the probe toward CN⁻ ions in aqueous media.

Fluorescence signaling of CN⁻ ions

The emission spectrum of CNB when acquired at 340 nm excitation displayed a very weak emission band at 410 nm due to a strong ICT process in the excited state of CNB, which is in line with the literature reports.^5b,11a,b,12c Upon the addition of CN⁻ a significant enhancement in the emission of diethylaminocoumarin moiety at 410 nm was observed (Fig. 3), indicating that the ICT was restricted due to the conjugation breaking between coumarin and p-nitrophenyl group caused by the nucleophilic attack of CN⁻ at the vinyl group of CNB.¹⁶

Fig. 3 Fluorescence emission spectra of CNB (5 μM) in the presence of various concentrations of CN⁻ (0–30 μM) in HEPES buffer (CH₃CN/H₂O = 1 : 1, v/v, pH = 7.4). Inset shows the fluorescence intensity of CNB as a function of CN⁻ ion concentration. Each spectrum was obtained 2 min after CN⁻ addition.

There is a good linear relationship (R² = 0.9950) between the fluorescence intensity and CN⁻ concentration in the range of 0–30 μM and the addition of a 6-fold CN⁻ results in a 10-fold increase in fluorescence. Indeed, such fluorescence enhancement can be distinguished with the naked eye. As shown in Fig. 4, nonfluorescent color of the solution changed to fluorescent blue. In contrast to CN⁻, no detectable fluorescence responses were induced by other anions such as F⁻, AcO⁻, Cl⁻, Br⁻, HSO₃⁻, NO₃⁻, HCO₃⁻, HSO₃⁻, SCN⁻, H₂PO₄⁻, ClO₄⁻, NO₂⁻, HS⁻ and SO₃²⁻ (Fig. 4a). Clearly, CNB possesses an efficient CN⁻ selective signaling behavior, which can be used for the selective detection of CN⁻ in neutral aqueous media. The higher selectivity of CNB for cyanide over other various anions may come from CNB as an activated electrophile as well as the strong nucleophilicity of cyanide. Further, competitive anions interaction studies also suggest high sensitivity and selectivity of CNB for CN⁻, the emission intensity of CNB + CN⁻ remained unaffected by the addition of other anions in excess (Fig. 4b). This indicated that CNB could be used potentially to quantitatively detect CN⁻ concentration with high selectivity. Following the 3σ IUPAC criteria, the detection limit for CN⁻ was calculated to be 1.4 × 10⁻⁷ mol L⁻¹.¹⁷ This value is much lower than both the WHO cyanide standard in drink water (1.9 μM) and the lethal level of cyanide in fire victims (20 μM), indicating that the probe may be sensitive enough for practical applications.

Fig. 4 (a) Emission spectra of CNB upon addition of different anions in HEPES buffer (CH₃CN/H₂O = 1 : 1, v/v, pH = 7.4). (b) Fluorescence intensities of CNB at 410 nm upon addition of CN⁻ in the presence of interfered anions. C_CNB = 5.0 × 10⁻⁶ mol L⁻¹; C_CN⁻ = 5 × 10⁻⁵ mol L⁻¹; C_anions = 5 × 10⁻⁵ mol L⁻¹. Each spectrum was obtained 2 min after addition of anions.

Sensing mechanism

The spectral changes of CNB upon CN⁻ addition can be attributed to the nucleophilic interaction between CN⁻ and CNB. As the sensing method is based on a reaction mechanism, it should satisfy a 1 : 1 reaction ratio between CNB and the cyanide anion. Job's plot analysis of the fluorescence titration spectra supports the formation of a 1 : 1 adduct CNB–CN⁻ (Fig. 5).

Fig. 5 Job's plot between CNB and cyanide anion in HEPES buffer (CH₃CN/H₂O = 1 : 1, v/v, pH = 7.4). Each spectrum was obtained 2 min after CN⁻ addition.

Now, to confirm the nucleophilic addition by cyanide, we carried out ¹H NMR titrations in DMSO-d₆. As shown in Fig. 6, the ¹H NMR spectrum of CNB exhibits signals at 7.36 and 7.59 ppm corresponding to the vinylic protons (H^c and H^d). Following addition of cyanide ions, these signals progressively decreased and finally disappeared with the addition of 1 equiv. of cyanide anions. Concurrently, new signals appeared at 2.97 and 3.79 ppm. In addition, the aryl ring protons of nitrophenyl rings (H^a and H^b) experience small downfield shifts upon cyanide addition. All these observations are in accordance with the nucleophilic addition of cyanide at the π-carbon of the vinylic linkage. In particular, the chemical shift of H^c shifted to greater extent (Δδ = 4.39 ppm), indicating the cyanide anion attacks the β-position of the nitrophenylvinyl moiety of CNB giving an anionic product. The formation of the CNB–CN adduct was further confirmed by HRMS measurements where a peak at m/z 392.1538 corresponding to [CNB − CN + H]⁺ was clearly observed (Fig. 7).

Fig. 6 The ¹H NMR titration spectra of CNB (10 mM) against KCN in DMSO-d₆.

Fig. 7 The HRMS of product obtained by reaction between CNB and CN⁻.

From the spectroscopic evidence, we propose a plausible mechanism for interaction between CNB and cyanide (Scheme 2). The π-conjugation and the ICT progress of CNB were both inhibited by the nucleophilic addition of cyanide to CNB, resulting in fluorescence and UV-vis spectral changes.

Scheme 1 Synthesis of compound 1 (CNB).

Scheme 2 A plausible mode of cyanide interaction with CNB.

Reaction kinetics

The rate constants were further measured under pseudo-first-order approximation as the cyanide anion was used in a high concentration compared to that of CNB. The reaction between CNB and the cyanide was monitored in aqueous acetonitrile solution at room temperature by recording the fluorescence intensities at 410 nm at various time intervals. A plot of ln[(F_max − F_t)/F_max], where F_max and F_t are fluorescence intensities at time t and end of the reaction, respectively, against time gave a linear fit (Fig. 8a). The pseudo-first-order rate constant of 0.027 s⁻¹ was obtained from the slope of the straight line (Fig. 8b),¹⁸ which strongly supports the high reactivity of the probe.

Fig. 8 (a) Changes in emission spectra of CNB (5.0 × 10⁻⁶ mol L⁻¹) incubated with CN⁻ (5.0 × 10⁻⁵ mol L⁻¹) for 0–140 second. (b) Pseudo-first-order kinetic plot of the reaction of CNB (5.0 × 10⁻⁶ mol L⁻¹) with CN⁻ (5.0 × 10⁻⁵ mol L⁻¹) in HEPES buffer (CH₃CN/H₂O = 1 : 1, v/v, pH = 7.4).

Preliminary analytical application

CNB possesses an excellent selectivity for cyanide anion in the presence of other anions, which makes it very useful in practical applications. To examine its potential applicability, the proposed method was preliminarily applied in the determination of CN⁻ in tap water samples. The water sample was found to be free from cyanide and so the sample was prepared by adding known amounts of cyanide to sample. With a given amount of CN⁻ spiked in water samples, recovery yields were determined using the calibration curve and listed in Table 1.

Table 1 Determination of CN⁻ in water samples

Samples	CN^− added (μM)	CN^− found (μM)	Recovery (%)	RSD^a (%)
a RSD: relative standard deviation, n = 5.
Tap water 1	0	—	—	—
Tap water 2	2.00	1.97	98.5	0.91
Tap water 3	5.00	5.12	102.4	1.02
Tap water 4	10.00	9.93	99.3	0.78
Tap water 5	20.00	19.52	97.6	1.13

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As shown in Table 1, the probe was able to measure the concentrations of spiked CN⁻ with good recovery, suggesting that the method could potentially be used for detecting CN⁻ in real samples.

Motivated by the favourable features of this system in solution, the TLC plates coated with CNB were then utilized to sense CN⁻ in aqueous CH₃CN solution (CH₃CN/H₂O = 1 : 1, v/v). As shown in Fig. 9, obvious color and fluorescence changes were observed with CN⁻ solution. Therefore, such “dip stick” approach could be conveniently used to detect CN⁻ in solutions without resorting to instrumental analysis.

Fig. 9 Color and fluorescence changes visualized on TLC plate: (a) plates immersed into the aqueous CH₃CN solution of CNB; (b) color change of CNB coated plates dipped into the aqueous CH₃CN solution of cyanide (10⁻³ M); (c) TLC plate ‘a’ under UV-illumination at 365 nm; (d) TLC plate ‘b’ under UV-illumination at 365 nm.

Experimental

Materials and instrumentation

4-Diethylaminosalicyladehyde (99%) and diethylmalonate (90%) were purchased from Aladdin Reagent Co. (Shanghai, China). All the other chemical reagents were of analytical grade and used as received without further purification. All the anions were supplied from their corresponding sodium or potassium salts. Stock solution of CNB was prepared in MeOH at 2.0 × 10⁻³ mol L⁻¹. Anion stock solutions (2.0 × 10⁻³ mol L⁻¹) were prepared by dissolving appropriate amounts of sodium or potassium salts in water. Double distilled water was used throughout.

UV-vis absorption spectra were recorded on an Agilent 8453 UV-vis spectrophotometer (Lakewood, NJ). Fluorescence measurements were performed on a Cary Eclipse fluorescence spectrophotometer (Varian). The samples were excited at 340 nm. The excitation and emission slits were both set at 5 nm. ¹H NMR and ¹³C NMR spectra were recorded on a Bruker Avance 300 spectrometer operating at 300 and 75 MHz at 298 K, using tetramethylsilane (TMS) as internal standard. High resolution mass measurements were carried out on a Bruker micrOTOF-Q III mass spectrometer. Melting points were measured on a RD-II digital melting point apparatus and were uncorrected.

Synthesis of CNB

CNB was prepared in three steps as shown in Scheme 1.

The key intermediates 4 and 5 were synthesized according to the reported procedures.¹³ Compound 4 (yield: 81.4%): mp 88–90 °C; ¹H NMR (DMSO-d₆) δ (ppm): 7.82 (d, J = 9.3 Hz, 1H), 7.43 (d, J = 8.7 Hz, 1H), 6.69 (d, J = 10.87 Hz, 1H), 6.52 (s, 1H), 5.99 (d, J = 9 Hz, 1H), 3.43 (q, J = 6.9 Hz, 4H), 1.13 (t, J = 7.2, 6H); ¹³C NMR (CDCl₃) δ (ppm):140.97, 138.50, 126.98, 126.78, 123.54, 122.87, 121.15, 119.67, 118.37, 113.82, 109.19, 38.10, 14.22.

Compound 5 (yield: 61%): mp 169–170 °C; ¹H NMR (CDCl₃) δ (ppm): 10.10 (s, 1H), 8.23 (s, 1H), 7.39 (d, J = 9 Hz, 1H), 6.38 (d, J = 9 Hz, 1H), 6.48 (s, 1H), 3.45 (q, J = 7.2 Hz, 4H), 1.23 (t, J = 6.9 Hz, 6H); ¹³C NMR (CDCl₃) δ (ppm):185.13, 167.42, 161.29, 154.36, 139.46, 119.93, 119.46, 119.07, 108.33, 55.56, 23.10.

CNB. A mixture of 5 (3 g, 12.3 mmol) and 6 (5.82 g, 13.4 mmol) in 30 mL THF was stirred at 0 °C under nitrogen atmosphere, followed by the dropwise addition of a solution of t-BuOK (1.5 g, 13.4 mmol in 10 mL of THF). The mixture was stirred at 0 °C for 3 h and was heated at 70 °C for additional 6 h with stirring. After cooling to room temperature, the resulting mixture was poured into 200 mL of cool water and extracted with CH₂Cl₂ (3 × 100 mL). The collected organic phases were washed with water and dried over anhydrous MgSO₄. The solvent was distilled off and the residue was chromatographed (silica, petroleum : ethyl acetate = 5 : 1) to give a red solid (1.46 g, 48.6% yield). Mp 249–254 °C. ¹H NMR (DMSO-d₆) δ (ppm): 8.19 (d, J = 7.8 Hz, 2H), 8.14 (s, 1H), 7.77 (d, J = 8.1 Hz, 2H), 7.59 (d, J = 6.9 Hz, 1H), 7.46 (d, J = 8.7 Hz, 1H), 7.36 (d, J = 13.2 Hz, 1H), 6.73 (d, J = 9.9 Hz, 1H), 6.55 (1s, 1H), 3.44 (q, J = 11.7 Hz, 4H), 1.12 (t, J = 9, 6H). ¹³C NMR (CDCl₃) δ (ppm): 163.08, 156.78, 151.82, 147.45, 145.30, 145.06, 142.30, 130.28, 129.64, 128.94, 128.48, 127.65, 124.97, 124.76, 116.78, 110.43, 110.17, 109.25, 98.32. HRMS (ESI) calcd for C₂₁H₂₀N₂O₄: 365.1423 (M + H)⁺, found: 365.1428 (M + H)⁺.

General procedure

All spectra were recorded 2 min after the addition of anions in CH₃CN : H₂O (1 : 1, v/v, HEPES buffer, pH 7.4) solution. The titrations were carried out in 10 mm quartz cuvettes at 25 °C. The procedure was as follows: into a CH₃CN : H₂O (1 : 1, v/v, HEPES buffer, pH 7.4) solution, containing 5 μM CNB, a CN⁻ sample was gradually titrated. At the same time, any changes in the fluorescence intensity were monitored using a fluorescence spectrometer (λ_ex = 340 nm, λ_em = 410 nm, slit: 5 nm/5 nm).

Conclusions

In conclusion, we have successfully designed and synthesized a new highly selective probe (CNB) for cyanide in aqueous acetonitrile solution. CNB displayed considerable dual changes in both absorption (blue-shift) and emission (turn-on) bands for CN⁻, which could be observed directly with the naked eye. CNB is highly reactive to CN⁻, with a short responding time and low detection limit of 0.14 μM, which is lower than the maximum permissive level in drinking water according to the World Health Organization (WHO). The binding mechanism was established by NMR as well as HRMS experiments. The spectral changes can be attributed to breaking of the π-conjugation and the ICT by the nucleophilic addition of cyanide on the vinyl carbon of CNB. Moreover, the probe could be developed as a simple “dip stick” system (TLC plates) for the rapid monitoring of CN⁻.

Acknowledgements

This work was financially supported by Shanxi Scholarship Council of China (No. 2011-008), the Natural Science Foundation of Shanxi Province (No. 2013011040-6) and the Project Sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry.

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