Cu(II) complex-based fluorescence chemosensor for cyanide in aqueous media

Abstract

A fluorescent quinazoline derivative, 6-(2,4-dihydroxyphenyl)-5,6-dihydrobenzoimidazo[1,2-c]quinazoline (H₂L), was designed and synthesized. Its fluorescence could be quenched by the addition of Cu²⁺ in an aqueous media. The binding constant for Cu²⁺ and H₂L was determined to be 9.56 × 10³ M⁻² in DMSO/H₂O (1 : 1, v/v). The crystal structure of the Cu(II) complex, [Cu(HL¹)(Ac)] (1) (H₂L¹ = 4-{[2-(1H-benzimidazol-2-yl)-phenylimino]-methyl}-benzene-1,3-diol, Ac = acetate), revealed that Schiff base complex 1, H₂L¹, was formed in the response system of H₂L to Cu²⁺ via Cu²⁺-assisted C–N bond breakage of the quinazoline ring in H₂L. Consequently, complex 1 was used as a turn-on fluorescence chemosensor for the direct detection of CN⁻, and showed a high selectivity to CN⁻ over a number of anions in the aqueous media, where CN⁻ replaces HL¹ in 1 to form [Cu(CN)_x]^2−x with HL¹ then released and the fluorescence recovered. The detection limit for CN⁻ was 4.0 × 10⁻⁶ M. The cell images showed that 1 could be used to detect intracellular CN⁻.

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Introduction

The cyanide anion (CN⁻) is widely utilized in electroplating, gold mining, textiles, synthetic fibers, metallurgy, resins and herbicides.^1–5 Cyanide is also present in surface water, cigarette smoke and in the combustion products of synthetic materials.⁶ As is well known, cyanide is an extremely toxic anion due to the formation of a stable complex with cytochrome c oxidase, thereby interfering with the process of cellular respiration and causing hypoxia.⁷ Cyanide can make the body lose consciousness and can even lead to death. According to the US Environmental Protection Agency (EPA), the Maximum Contaminant Level (MCL) of cyanide in drinking water is 0.2 ppm.⁸

Due to its extreme toxicity in physiological systems and its wide usage in many industries, the detection of cyanide is important to address the considerable concerns. There have already been many methods developed for detecting cyanide based on voltammetric,⁹ electrochemical,¹⁰ titrimetric¹¹ and other techniques.^12,13 However, when these techniques are applied, they usually require expensive instruments and consume much time. Therefore, intensive efforts have been directed towards the design and synthesis of chemosensors to detect CN⁻ in biological fields in recent years. Fluorescence spectroscopy already plays an important role in the determination of anions due to its simplicity, high selectivity and low cost, and a variety of fluorescence chemosensors to detect cyanide have been developed, which are based on different reaction mechanisms such as the nucleophilic attack of cyanide on the carbonyl moiety,¹⁴ hydrogen-bonding motifs,¹⁵ electron-deficient alkene addition,¹⁶ and ligand exchange.¹⁷ Many chemosensors for cyanide have drawbacks such as low solubility in an aqueous media,¹⁸ interference from fluoride and acetate,¹⁹ and low selectivity and toxicity.

As is known, Cu²⁺ ion has a high affinity to cyanide and [Cu(CN)_x]^2−x possesses a large association constant. There have been some reports about the sequential detection of Cu²⁺ and CN⁻, wherein CN⁻ is detected indirectly.²⁰ However, this makes the detection of CN⁻ inconvenient. It was found that quinazoline derivatives are fluorophores acting as chemosensors for metal ions,²¹ while many chemosensors for Cu(II) operate on an “on–off” mode.²² Based on the above points, herein, we aim to develop a non-fluorescent copper(II) complex-based chemosensor to probe cyanide directly via an “off–on” mode. To increase the aqueous solubility of the copper(II) complex, a quinazoline derivative with two hydroxyl groups in the meta position was designed. As a result, 6-(2,4-dihydroxyphenyl)-5,6-dihydrobenzoimidazo[1,2-c] quinazoline (H₂L) was synthesized by the condensation of 2-(2-aminophenyl)benzimidazole and 2,4-dihydroxybenzaldehyde via a solvothermal method. Our experiment confirmed that Cu²⁺ could cause the fluorescence quenching of compound H₂L. Furthermore, the association constant of Cu²⁺ with H₂L is much lower than that of Cu²⁺ with CN⁻. Consequently, H₂L was used to react with copper(II) salt, and a copper Schiff base complex was obtained, which was fluorescence responsive to cyanide ions. Herein, this study reports an “on–off” fluorescence chemosensor for Cu(II) and a Cu(II) complex-based “off–on” fluorescence sensor (1) for cyanide in DMSO/H₂O (1 : 1, v/v) with a relatively low cost and non-interference from acetate and fluoride. The sensing mechanisms and the application in cell imaging were investigated.

Experimental section

General information and materials

All the solvents and reagents (analytical grade) were used as received. Elemental analyses were conducted using a Vario EL elemental analyzer. The Fourier transform infrared (FT-IR) analyses was performed on an Avatar 360 Nicolet 380 FT-IR spectrometer using KBr pellets. Powder X-ray diffraction (XRD) analyses were performed on a PANalytical X′ Pert PRO MPD diffractometer for Cu-Kα radiation (λ = 1.5406 Å) at a scan rate of 2° min⁻¹ and a step size of 0.02° in 2θ. The solutions of metal ions were prepared from LiCl, NaCl, KCl, MgCl₂·6H₂O, CrCl₃·6H₂O, Mn(ClO₄)₂·6H₂O, Fe(ClO₄)₂·xH₂O, CaCl₂, Co(ClO₄)₂·6H₂O, Ni(ClO₄)₂·6H₂O, Cu(Ac)₂·H₂O, Hg(ClO₄)₂, Cd(ClO₄)₂·H₂O, Zn(ClO₄)₂·6H₂O, Al(ClO₄)₃·9H₂O, and Pb(ClO₄)₂·3H₂O. UV-vis absorption spectra were obtained by a UV-2450 spectrophotometer and fluorescence spectra were acquired on a Cary Eclipse fluorescence spectrophotometer with a quartz cuvette (path length = 1 cm). ¹H NMR spectra were obtained using a Bruker Avance III 400 MHz spectrometer. Mass spectra (ESI) were obtained on a Micromass Quattro micro API mass spectrometer (Waters, Milford, MA) with an electrospray ionization (ESI) interface.

Synthesis of H₂L

A mixture of 2-(2-aminophenyl)benzimidazole (0.0418 g, 0.2 mmol), 2,4-dihydroxybenzaldehyde (0.0276 g, 0.2 mmol) and 2 mL ethanol in a closed 25 mL Teflon-lined autoclave was heated at 80 °C for 24 h, and then cooled to room temperature naturally. Orange block-shaped crystals were collected and washed with methanol and air-dried in a yield of 31.7% (0.0220 g). ¹H NMR (400 MHz, DMSO-d₆) (Fig. 10a): δ = 9.93 (s, 1H), 9.45 (s, 1H), 7.93 (d, J = 8.0 Hz, 1H), 7.62 (d, J = 8.0 Hz, 1H), 7.22 (t, J = 8.0 Hz, 1H), 7.14 (m, 2H), 7.07 (m, 2H), 6.99 (d, J = 4.0 Hz, 1H), 6.87 (d, J = 4.0 Hz, 1H), 6.79 (t, J = 4.0 Hz, 1H), 6.66 (d, J = 8.0 Hz, 1H), 6.37 (d, 4.0 Hz, 1H), 6.09 (dd, J = 8.0 Hz, 4.0 Hz, 1H) ppm. ¹³C NMR (100 MHz, DMSO-d₆) (Fig. S1†): δ = 159.3, 156.0, 147.9, 144.2, 143.9, 133.2, 132.1, 128.1, 125.1, 122.7, 122.5, 118.8, 118.3, 117.3, 115.1, 111.7, 110.8, 107.2, 102.9, 63.6 ppm. Anal. calcd for C₂₀H₁₅N₃O₂: C, 72.94; N, 12.76; H, 4.591. Found: C, 72.93; N, 12.68; H, 4.693. IR (KBr pellet, cm⁻¹): 3417m, 3374m, 3212m, 1616vs, 1530m, 1492vs, 1407s, 1321w, 1250vs, 846w, 741s. To obtain crystals suitable for single-crystal X-ray diffraction, another 100 μL of DMSO was added with H₂L·DMSO obtained (Scheme S1†).

Synthesis of H₂L¹·HCl

A solution of 2,4-dihydroxybenzaldehyde (0.0552 g, 0.4 mmol) and hydrochloric acid (33 μL, 12 mol L⁻¹) in isopropanol (2 mL) was added into a boiling solution of 2-(2-aminophenyl)benzimidazole (0.0836 g, 0.4 mmol) in isopropanol (5 mL) under stirring conditions and then refluxed for 3 h. Then, the resulting mixture was cooled, filtered, washed with isopropanol and air-dried, and a yellowish-green powder of hydrochloride of the Schiff base was collected in a yield of 25.6% (0.0356 g). Anal. calcd for C₂₀H₁₅N₃O₂·HCl: C, 65.67; N, 11.49; H; 4.41. Found: C, 65.28; N, 11.41; H, 4.36. ¹H NMR (400 MHz, DMSO-d₆) (Fig. 10e): δ = 10.09 (s, 1H), 9.68 (s, 1H), 8.09 (s, 1H), 7.77 (m, 2H), 7.41 (m, 5H), 7.14 (d, J = 8.0 Hz, 2H), 6.90 (d, J = 8.0 Hz, 1H), 6.86 (t, J = 8.0 Hz, 1H), 6.37 (s, 1H), 6.23 (d, J = 8.0 Hz, 1H) ppm. IR (KBr pellet, cm⁻¹): 3358s, 3192s, 1624vs, 1605s, 1578m, 1566m, 1512s, 1491m, 1460s, 1385m, 1314s, 1373m, 1273m, 1227w, 1171s, 1125m, 1086w, 976m, 856m, 806m, 746s.

Synthesis of [Cu(HL¹)(Ac)] (1)

A mixture of 2-(2-aminophenyl)benzimidazole (0.0418 g, 0.2 mmol), 2,4-dihydroxybenzaldehyde (0.0276 g, 0.2 mmol), Cu(Ac)₂·H₂O (0.0200 g, 0.1 mmol) and 6 mL ethanol in a closed 25 mL Teflon-lined autoclave was heated at 80 °C for 72 h, and then cooled to room temperature naturally. Black block-shaped crystals of a Cu(II) complex were collected and washed with methanol, and air-dried to obtain a yield of 37.6% (0.0336 g). Anal. calcd for C₂₂H₁₇CuN₃O₄: C, 58.60; N, 9.32; H; 3.80. Found: C, 58.42; N, 9.31; H, 4.28. IR (KBr pellet, cm⁻¹): 3421m, 3209m, 1618vs, 1570vs, 1535vs, 1493m, 1439vs, 1383s, 1283w, 1206s, 1188s, 752s.

Synthesis of [Cu(HL¹)(Ac)] (1′)

A mixture of H₂L (0.0165 g, 0.05 mmol), Cu(Ac)₂·H₂O (0.0100 g, 0.05 mmol) and 4 mL ethanol in a closed 25 mL Teflon-lined autoclave was heated at 80 °C for 72 h, and then cooled to room temperature naturally. Black block-shaped crystals of a Cu(II) complex were collected and washed with methanol, and air-dried to obtain a yield of 47.5% (0.0126 g). Anal. calcd for C₂₂H₁₇CuN₃O₄: C, 58.60; N, 9.32; H; 3.80. Found: C, 58.24; N, 8.96; H, 4.18. IR (KBr pellet, cm⁻¹): 3422m, 3207m, 1618vs, 1570vs, 1535vs, 1493m, 1439vs, 1383s, 1283w, 1206s, 1188s, 752s.

Structural determination

Single-crystal X-ray diffraction data of compounds H₂L and 1 were collected on a Bruker APEX II CCD diffractometer with graphite monochromated Mo-Kα radiation (λ) at 293 K. The structures were solved by the direct method and refined by full matrix least squares based on F² using the SHELX 97 program.²³ All the non-hydrogen atoms were refined anisotropically. The hydrogen atoms were placed in calculated positions. The structure of H₂L (Fig. S2, Tables S1–S2) was reported (ESI†).²⁴

Methods for cell imaging

A SH-SY5Y cell line was cultured in DMEM (Dulbecco's modified Eagle's medium). The cells were incubated with 20 μM of complex 1 at 37 °C for 16 h. After washing with PBS three times to remove the remaining complex 1, the cells were incubated with 10 μM NaCN for 30 min at room temperature. The incubated cells were washed with PBS and mounted onto a glass slide. The fluorescent images of the mounted cells were obtained using a confocal laser scanning microscope with 458 nm excitation.

Results and discussion

Cation-sensing properties of H₂L

Compound H₂L in DMSO/H₂O (1 : 1, v/v) displayed a strong emission at 432 nm when excited at 358 nm. The sensing properties of H₂L for metal ions were investigated in the presence of Li⁺, Na⁺, K⁺, Ca²⁺, Al³⁺, Cr³⁺, Cd²⁺, Co²⁺, Cu²⁺, Fe²⁺, Hg²⁺, Mg²⁺, Mn²⁺, Ni²⁺, Pb²⁺, and Zn²⁺ in DMSO/H₂O (1 : 1, v/v). Only Cu²⁺ caused significant fluorescence quenching at 432 nm upon excitation at 358 nm (Fig. 1) because of the ligand to metal charge transfer (LMCT).^21e From Fig. 2, we can see that the fluorescence emission was gradually quenched with the increasing amount of Cu²⁺. After the addition of 1.0 equiv. Cu²⁺, there was no change of fluorescence intensity and the intensity reached a minimum. The result showed that the complexation stoichiometry of Cu²⁺ with H₂L was 1 : 1.

Fig. 1 Fluorescence responses of H₂L (10 μM) in DMSO/H₂O (1 : 1, v/v) to 1.0 equiv. Al³⁺, Zn²⁺, Cd²⁺, Co²⁺, Ca²⁺, Cr³⁺, Mn²⁺, Ni²⁺, Fe²⁺, Ca²⁺, Cu²⁺, Pb²⁺, Li⁺, Na⁺, K⁺, and Mg²⁺. λ_ex = 358 nm.

Fig. 2 Changes of fluorescence spectra of 20 μM H₂L upon the addition of Cu²⁺ ions (0, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, 0.45, 0.50, 0.55, 0.60, 0.65, 0.70, 0.75, 0.80, 0.85, 0.90, 0.95, 1.0, 1.2, 1.4, 1.6, 1.8, and 2.0 equiv. Cu²⁺) in DMSO/H₂O (1 : 1, v/v). Inset: fluorescence intensity at 432 nm as a function of [Cu²⁺]/[H₂L].

The binding stoichiometry between H₂L and Cu²⁺ was further confirmed by the Job's plot, wherein changes of the absorbance at 394 nm were observed. The result reveals that the stoichiometry of this complex was 1 : 1 for Cu(II) to H₂L (Fig. 3). According to the fluorescence titration data (Fig. 2), the association constant K of binding H₂L with Cu²⁺ was calculated by the Benesi–Hildebrand expression (Fig. S3a†).²⁵ K was determined to be 9.56 × 10³ M⁻² in DMSO/H₂O (Fig. S3b†).

Fig. 3 Job plot for the determination of the stoichiometry of H₂L and Cu²⁺ in the complex.

Crystal structure of the complex 1

To investigate the species formed in the responsive system, the complex of H₂L with Cu²⁺ was prepared by a direct and one-pot reaction. The results showed that the Schiff base complex was generated instead of the quinazoline complex.

The molecular structure and the atom labeling scheme of compound 1 are shown in Fig. 4. Its crystal data and refinement parameters are tabulated in Table 1. The selected bond lengths and angles are listed in Table S2.† Complex 1 belongs to the monoclinic system, space group P2₁/c. In 1 (Fig. 4), the Cu(II) ion is four-coordinated by one acetate oxygen atom, one phenolato oxygen atom, one imine nitrogen atom, and one benzimidazole nitrogen atom, occupying an ON₂O square plane environment. The crystal structure determination indicated that 1 is mononuclear and consists of the tridentate Schiff base ligand (HL¹) from the quinazoline compound H₂L. In the complex, the deprotonated ligand HL¹ binds with centers by forming two six-membered chelated rings. In addition, the intermolecular hydrogen bonds in 1 can be seen in Fig. S4.† From Fig. S5,† we can see that the PXRD pattern of synthesized 1′ is in good agreement with the one that was simulated from the single-crystal X-ray diffraction data of 1 (Fig. S5a†).

Fig. 4 Molecular structures with an atom labeling scheme for complex 1.

Table 1 Crystal data and structure refinement parameters of compound 1^a

Complexes	1
a R1 = ∑(|Fo| − |Fc|)/|Fo|; wR2 = {∑[(w|Fo^2| − |Fc^2|)^2/∑w(Fo^2)^2]}^1/2.
Formula	C22H17CuN3O4
Fw	450.93
Crystal system	Monoclinic
Space group	P2(1)/c
a (Å)	7.890(5)
b (Å)	16.186(11)
c (Å)	15.193(10)
α (°)	90
β (°)	98.385(10)
γ (°)	90
V (Å^3)	1919(2)
Z	4
Calculated density (g cm^−3)	1.560
F (000)	924
Reflections collected/unique	12 518/4325 [R(int) = 0.0269]
Goodness-of-fit on F^2	1.069
Final R indices [I > 2σ(I)]	R1 = 0.0360, wR2 = 0.1267
R indices (all data)	R1 = 0.0445, wR2 = 0.1338

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Elemental analysis, IR and PXRD confirmed that product 1′ of the bicomponent reaction is the same as product 1 of the one-pot reaction. The structure of [Cu(HL¹)(Ac)] (1) proved the metal-assisted ring-opening of the quinazoline derivative to form the metal Schiff base complex.^21e

In addition, the UV-vis spectra of 1, H₂L + Cu²⁺, and H₂L were investigated (Fig. 5). For complex 1 (Fig. 5a), two bands at 300 and 400 nm were observed. The former may be attributed to a π–π* transition from HL¹ and the latter arises from an HL¹ to Cu(II) charge transfer (LMCT).²⁶ In Fig. 5, the absorption spectrum of the system H₂L + Cu²⁺ is different from that of H₂L, but is the same as that of complex 1 (Fig. 5a–c). This reveals that the fluorescence quench of H₂L results from the formation of 1 in the H₂L + Cu²⁺ system. This can also be confirmed by the Job's plot (Fig. 3), which is in agreement with the coordination ratio in complex 1, and by the ESI-MS result (Fig. S6†), in which the peak at m/z = 391.1 corresponds to [Cu(HL¹)]⁺ (calcd m/z = 391.05).

Fig. 5 The absorption spectra of 1 (a), H₂L + 1.0 equiv. Cu²⁺ (H₂L + Cu²⁺) (b), H₂L (c) in DMSO/H₂O (1 : 1, v/v).

Anion-sensing property of [Cu(HL¹)(Ac)] (1)

The addition of 1 equiv. CN⁻ to H₂L showed no significant change in absorption and fluorescence spectra (Fig. S7†), which indicated that H₂L is not responsive to CN⁻. Considering the weak fluorescence of 1, and the small association constant of H₂L with Cu²⁺, we investigated the sensing property of complex 1 to anions. The UV-vis absorption of the solution of 1 upon the addition of various anions, including F⁻, Cl⁻, Br⁻, I⁻, NO₂⁻, Ac⁻, HCO₃⁻, HSO₃⁻, HSO₄⁻, ClO₂⁻, HPO₄²⁻, H₂PO₄⁻, CN⁻, S²⁻, and P₂O₇⁴⁻, was conducted in DMSO/H₂O (1 : 1, v/v) media. Only CN⁻ caused the absorbance at 300 nm to decrease greatly, and a double signal at 280 nm emerged, while other anions did not cause any obvious changes in the UV-vis spectra (Fig. 6). Furthermore, the addition of CN⁻, S²⁻, and P₂O₇⁴⁻ could lead to a great fluorescence enhancement, as is shown in Fig. 7; while from Fig. 6, we can see that CN⁻ could be differentiated from P₂O₇⁴⁻ and S²⁻ by the absorbance at 400 nm.

Fig. 6 UV-vis spectra of 1 (10 μM) upon the addition of 40 μM anions in DMSO/H₂O (1 : 1, v/v).

Fig. 7 Fluorescence responses of 1 (10 μM) in DMSO/H₂O (1 : 1, v/v) to 40 μM of F⁻, Cl⁻, Br⁻, I⁻, HSO₃⁻, HSO₄⁻, HCO₃⁻, ClO₂⁻, H₂PO₄⁻, HPO₄²⁻, Ac⁻, NO₂⁻, CN⁻, S²⁻, and P₂O₇⁴⁻. λ_ex = 358 nm.

The fluorescence titration of 1 with CN⁻ is shown in Fig. 8. It can be seen from Fig. 8 that the fluorescence intensity increased quickly with the concentration of CN⁻ ranging from 4 to 40 μM. Then, with the increasing amount of CN⁻, the fluorescence intensity remains unchanged. The recovery of fluorescence may result from the formation of [Cu(CN)_x]^2−x and HL¹ was released. In ESI-MS, the peak at m/z 330.4 corresponding to H⁺[H₂L¹] (calcd m/z 330.12) further indicated that the cyanide ion promoted the dissociation of complex 1 to replace the ligand HL¹ because of the formation of stable [Cu(CN)_x]^2−x (Fig. S8†). According to the emission titration data, it was found that the fluorescence of 1 was turned on with a concentration of CN⁻ as low as 4 μM (namely, 0.104 ppm), which shows that complex 1 is a useful chemosensor for the detection of cyanide.

Fig. 8 Changes of fluorescence spectra of 1 (10 μM) upon the addition of CN⁻ ions (0, 4, 8, 12, 16, 20, 24, 28, 32, 36, 40, 44, 50, 55, 60, and 65 μM CN⁻) in DMSO/H₂O (1 : 1, v/v). Inset: fluorescence intensity at 432 nm as a function of [CN⁻].

To gain insight into the sensing mechanism of 1 to CN⁻, UV-vis spectra were obtained. The UV-vis spectra of 1, 1 + 4CN⁻, H₂L + Cu²⁺ + 4CN⁻, and H₂L are shown in Fig. 9. The signal of Fig. 9b is different from those of Fig. 9a and d. This shows that CN⁻ replaced HL¹ from 1 to form [Cu(CN)_x]^2−x, and HL¹ was released. The signals in Fig. 9c and b are the same. Therefore, it can be concluded that the addition of Cu²⁺ to H₂L formed 1 and then HL¹ was replaced when CN⁻ was added to 1, with [Cu(CN)_x]^2−x formed. The signals of Fig. 9b are not exactly the same as those of Fig. 9d and e. Therefore after HL¹ was released, not all of them exist as H₂L¹ in DMSO/H₂O (1 : 1, v/v).

Fig. 9 The absorption spectra of 1 (a), 1 + 4 equiv. CN⁻ (1 + 4 CN⁻) (b), H₂L + 1.0 equiv. Cu²⁺ + 4.0 equiv. CN⁻ (H₂L + Cu²⁺ + 4CN⁻) (c), H₂L (d) and H₂L¹·HCl (e) in DMSO/H₂O (1 : 1, v/v).

To obtain a further understanding of the species formed in the solution, ¹H NMR studies were carried out. There are three single signals at 9.93, 9.45, and 7.14 ppm (Fig. 10a), which may be attributed to H1, H2, and H3, respectively, because the signals of H1, H2, and H3 disappeared when a drop of D₂O was added to H₂L (Fig. 10b). The NMR resonance frequency of the protons can be affected by the paramagnetism and ferromagnetic coupling of Cu²⁺. Thus, the ¹H NMR signals of H₂L with 1.0 equiv. Cu²⁺ almost completely disappeared (Fig. 10c). Upon the addition of 1.0 equiv. Cu²⁺ to H₂L and then 4.0 equiv. CN⁻, the ¹H NMR spectrum (Fig. 10d) resembles that of H₂L (Fig. 10a). However, some of the weak signals of Fig. 10d are in agreement with that of H₂L¹·HCl (Fig. 10e). This also indicated that CN⁻ snatched Cu²⁺ from the Schiff base complex [Cu(HL¹)(Ac)] to form [Cu(CN)_x]^2−x, and HL¹ was replaced, then most of the HL¹ changed into quinazoline derivative H₂L, but a minority of them remain as the Schiff base H₂L¹. This result can be further confirmed by comparing the fluorescence spectra of 1 upon the addition of 4 equiv. CN⁻ with that of H₂L and H₂L¹·HCl (Fig. S9†). The fluorescence spectra of 1 upon the addition of 4 equiv. CN⁻ is much closer to that of H₂L. This indicates that there is a mixture of H₂L and H₂L¹ in DMSO/H₂O (1 : 1, v/v) after HL¹ was released, and H₂L is the main species.

Fig. 10 ¹H NMR spectra in DMSO-d₆: (a) H₂L, (b) a drop of D₂O added to (a), (c) H₂L with 1.0 equiv. Cu²⁺, (d) H₂L with 1.0 equiv. Cu²⁺ and 4.0 equiv. CN⁻, (e) H₂L¹·HCl.

Cell imaging studies

Owning to the encouraging selectivity and sensitivity of the complex 1 toward CN⁻, bioimaging experiments were conducted to prove the ability of 1 to detect CN⁻ in living cells. SH-SY5Y cells were first incubated with 20 μM complex 1 for 16 h, and then treated with 10 μM NaCN for 30 min. As shown in Fig. 11, no fluorescence was observed when the cells were exposed to complex 1, while strong fluorescence was observed by further incubation of the cells with CN⁻. These results demonstrate that complex 1 is cell-permeable and can respond to CN⁻ ions within living cells.

Fig. 11 Live-cell imaging of SH-SY5Y cells treated with complex 1 before (A) and after (B) incubation with NaCN. (a) and (d) represent the bright-field images, (b) and (e) represent the fluorescence images, and (c) and (f) represent the overlay images (λ_ex = 458 nm).

Conclusions

In summary, a quinazoline compound H₂L showed fluorescence quenching towards Cu²⁺ with high selectivity over a number of metal ions and [Cu(HL¹)(Ac)] (1) was formed. As soon as cyanide was added to 1, [Cu(CN)_x]^2−x was formed and fluorescence recovered. This showed that complex 1 could be used for the fluorescence “off–on” recognition of cyanide via a ligand displacement approach. The low detection limit makes H₂L applicable for Cu²⁺, and 1 for CN⁻ in environmental monitoring work and for the trace determination of intracellular CN⁻ ions. The direct detection of cyanide via the Cu(II) complex with organic fluorophore provides a strategy for the design of a new fluorescence chemosensor for anions.

Acknowledgements

This study is supported by the National Natural Science Foundation of China (20971015) and the Fundamental Research Funds for the Central Universities.

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Footnote

† Electronic supplementary information (ESI) available: Information on X-ray crystallographic data in CIF format, Fig. S1–S9, Scheme S1, Tables S1–S2. CCDC 1044059. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra09511e

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