A rhodamine B-based “turn-on” fluorescent sensor for detecting Cu²⁺ and sulfur anions in aqueous media

Abstract

A novel selective fluorescent chemosensor L based on rhodamine B derivative has been designed and synthesized to optically detect Cu²⁺ and S²⁻ in aqueous buffer solution. The fluorescence of L was excited by Cu²⁺ with a 1 : 1 binding ratio, and could be used as a “turn-on” type sensor. Otherwise, the emission band would undergo a slight red-shift. Once binding with Cu²⁺, the copper complex can exhibit high selectivity for S²⁻. Upon addition of Cu²⁺, the color of the chemosensor L changes from colorless to pink. Additionally, the resulting pink solution changes to colorless immediately upon the addition of S²⁻. However, no phenomenon change could be observed in the presence of other anions. Conclusively, the color changes suggest that sensor L could serve as a “naked-eye” sensor for Cu²⁺ and S²⁻.

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Introduction

Transition-metal ions are toxic, causing serious environmental and health issues.^1,2 As one of most common transition-metal ions, Cu²⁺ is one of the highly toxic and global widespread pollutants. As the third most abundant transition metal in the human body, Cu²⁺ plays a vital role in many biological processes.^3–5 Some reports showed that excessive levels of Cu²⁺ accumulated could induce severe neurodegenerative diseases, such as Menkes and Wilson's diseases, familial amyotrophic lateral sclerosis, prion disease and Alzheimer's disease.^6–11 Hence, there remains a significant challenge for development of rapid detection and removal of copper from pollutants. Currently, many techniques have been reported for detecting of Cu²⁺ ion, including atomic absorption spectrometry, inductively coupled plasma atomic emission spectroscopy.^12–16 On the other hand, fluorescent chemosensors have attracted considerable attention due to its simplicity, low-cost and sensitivity. In past few years, many chemosensors with “turn-off” signal upon binding of Cu²⁺ have been reported.^17–19 However, it is not as sensitive as fluorescene enhancement response.^20–26 Therefore, it is also necessary to design and synthesize fluorescent probes of “turn-on” type in aqueous systems, especially for the response enhancement.

Recently, the hydrogen sulfide (H₂S) has proved as a novel gasotransmitter, and exhibited important physiological function like nitric oxide (NO), carbon monoxide (CO) in animal systems.^27,28 The sulfur anion is also an important anion in biological and environmental samples. Most environmental sulfides are released by industries which are involved in conversion of sulfur, such as preparation of sulfuric acid and dyes, cosmetic manufacturing, and production of wood pulp. On the other hand, sulfide anions can also be generated from microbial reduction of sulfate by anaerobic bacteria or from the sulfur-containing amino acids in meat protein.²⁹ The protonated forms of sulfur, such as HS⁻ or H₂S are more toxic than sulfide. If mammal drinks the water contaminated with sulfide, it will damage mucous membranes and cause respiratory problem. For example, low concentration of hydrogen sulfide has been proven to produce dizziness, while higher concentration can cause loss of consciousness, permanent damage of brain tissues, or even suffocation.^30,31 However, current major methods for H₂S detection, such as inductively coupled plasma atomic emission spectroscopy and electrochemical determination, titration and ion chromatography, often require complicated sample processing, which would lead to predicament for accurate analysis of this important molecule.^32–36 Therefore, development of a quick and sensitive method for immediate sulfide detection in aqueous media is of high interest.

Recently, rhodamine B-based dyes have been widely used as fluorescence reporting groups due to their excellent spectroscopic properties, such as large molar extinction coefficient, high fluorescence quantum yields, long absorption and emission wavelength.³⁷ The sensing mechanisms of these chemosensors probes are mainly based on the structure changes of spirocyclic towards its open-cycle forms.^38–45 On the grounds of these strategies, we designed a novel rhodamine B moiety of L as a “turn-on” fluorescent probe for Cu²⁺ sensing in aqueous solution (Scheme 1). In the absent of Cu²⁺ ions, the L exists in spirocyclic form, which is colorless with weak fluorescent intensity. Upon addition of Cu²⁺ ion into L, the spirocycle structure opened and the solution showed clear pink color, which also generated a fluorescence enhancement response. Additionally, high selectivity toward Cu²⁺ ion over other metal ions was clearly observed. Meanwhile, we also found L–Cu²⁺ complex can be employed to detect sulfide. By adding sulfide to the solution of L–Cu²⁺ complex, the color of the aqueous change from pink to colorless.

Scheme 1 Synthesis of the fluorescent sensor L.

Experimental

Reagents

All reagents and solvents were purchased from commercial suppliers. All chemicals used in this work were of analytical reagent grade and used without further purification. The solutions of the metal ions were prepared from NaNO₃, Mg(NO₃)₂·6H₂O, Al(NO₃)₃·9H₂O, KNO₃, Ca(NO₃)₂·4H₂O, Cr(NO₃)₃·9H₂O, MnCl₂·4H₂O, Fe(NO₃)₃·9H₂O, FeCl₂·4H₂O, Co(NO₃)₂·6H₂O, Ni(NO₃)₂·6H₂O, Cu(NO₃)₂·3H₂O, Zn(NO₃)₂·6H₂O, AgNO₃, Cd(NO₃)₂·4H₂O, Ba(NO₃)₂, Hg(NO₃)₂·0.5H₂O, Pb(NO₃)₂. The solutions of anions were prepared from NaNO₂, NaNO₃, KCN, KSCN, Na₂SO₄, Na₂SO₃, Na₃PO₄·12H₂O, Na₂HPO₄, KH₂PO₄, NaAc, NaCl, NaBr, KI, Na₂S. Tris–HCl buffer solutions (1 × 10⁻² mol L⁻¹, pH 7.2) and all other solutions were prepared in deionized water.

Instruments

¹H and ¹³C NMR spectra were taken on a Varian Mercury 300 MHz NMR spectrometer in CDCl₃ solutions, with tetramethylsilane (TMS, 0.00 ppm) as an internal standard. Absorption spectra were determined on a Varian UV-Cary100 UV-vis spectrophotometer. Fluorescence spectra measurements were performed on a Hitachi F-4500 fluorescence spectrophotometer. Mass spectra were obtained on a Bruker Daltonics-esquire 6000 mass spectrometer. All pH values were recorded on a PHS-3C digital pH meter (Shanghai, China). All spectra were recorded at room temperature.

Synthesis of compound 2

Compound 2 was synthesized based on the reported procedures with minor changes.⁴⁶ The process is the following steps: p-cresol (33 g, 0.305 mol) were completely dissolved in sodium hydroxide solution (10 mol L⁻¹, 50 ml). Following full development of a gold color, paraformaldehyde (18 g, 0.6 mol) was added to the solution. The resulting solution was heated to 323 K for 1 hour with a magnetic stirrer bar, and then cooled to room temperature. The yellow solid was collected and dissolved again in hot water. The pH value was adjusted to 3 with HCl (2 mol L⁻¹). The yellow precipitate was collected by filtration and dried overnight in a vacuum oven at 323 K, which afforded a creamy yellow solid compound 1. Then, compound 1 (10 g, 0.06 mol) was added to 300 ml of CHCl₃ in a 500 ml round bottom flask, afterwards, activated MnO₂ (21.7 g, 0.25 mol) was added to the solution with mechanically stirring and refluxing overnight, after which the reaction was cooled to room temperature and filtered, washed with CHCl₃ thoroughly until the filtrate became colorless. After the solvent was evaporated under reduced pressure, the crude product was purified by flash column chromatography on silica gel (petroleum ether–ethyl acetate = 2 : 1, v/v), affording compound 2 as yellow solid (6.27 g, yields: 62.89%). ¹HNMR (CDCl₃, 400 MHz): δ 2.22 (s, 3H, Ar–CH₃), 3. 01 (s, H, CH₂–OH), 4.60 (s, 2H, Ar–CH₂), 7.15, (s, 1H, Ar–H), 7.30 (s, 1H, Ar–H), 9.72 (s, 1H, N=C–H), 11.03 (s, 1H, Ar–OH) ppm. ¹³C NMR (CDCl₃, 100 MHz): δ 20.09, 59.96, 119.86, 128.88, 128.97, 132.38, 136.66, 157.00, 196.59 ppm.

Synthesis of rhodamine sensor (compound L)

RBH was synthesized from rhodamine B according to the literature method.⁴⁷

RBH (0.30 g, 0.66 mmol) was dissolved in 15 ml absolute ethanol. Excess compound 2 (0.16 g, 0.99 mmol) was added to the reaction mixture, then the mixture was stirred and reflux under N₂ atmosphere for 6 hour. Then the solvent was removed under reduced pressure, and the residue was purified by recrystallization with ethanol as yellow solid (0.29 g, yields: 72.7%). ¹HNMR (400 MHz, CDCl₃): δ 1.19–1.13 (t, J = 7.0 Hz, 12H, NCH₂CH₃), 2.19 (s, 3H, ben–CH₃), 2.75 (s, 1H, ben–CH₂–OH), 3.37–3.29 (q, J = 7.0 Hz, 8H, NCH₂CH₃), 4.62–4.64 (d, J = 5.5 Hz, 2H, ben–CH₂–OH), 6.25–6.52 (m, 6H, xanthene–H), 6.82–6.79 (d, J = 1.1 Hz, 1H, ben–H), 6.98 (s, 1H, ben–H), 7.11–7.19 (m, 1H, Ar–H), 7.47–7.54 (td, J₁ = 6.5, J₂ = 1.2 Hz, 2H, Ar–H), 7.93–8.04 (m, 1H, Ar–H), 8.91 (s, 1H, N=C–H), 11.17 (s, 1H, ben–OH); ¹³C NMR (101 MHz, CDCl₃): δ 12.60, 20.23, 44.38, 62.09, 66.16, 97.99, 105.13, 108.27, 118.00, 123.37, 124.04, 127.86, 128.09, 128.23, 128.53, 129.37, 130.81, 131.24, 133.56, 149.10, 151.30, 151.83, 153.32, 154.46, 164.33. ESI-MS spectrometry showed a peak with m/z 605.6 [M + H]⁺, calculate for C₃₇H₄₀N₄O₄ = 604.74. Anal. calcd for C₃₇H₄₀N₄O₄ (604.74): C 73.42, H 6.61, N 9.26, O 10.71 found: C 73.21, H 6.65, N 9.06, O 11.08%.

Results and discussion

UV-vis spectroscopic studies of L in presence of Cu²⁺

The spectra responses of chemosensor L in the absence or presence of Cu²⁺ in different pH values were evaluated first. As shown in Fig. 1, the absorption of free L was very weak between pH 4 and 10. When the pH value was lower than 4, it showed increased absorption. It was attributed to the ring opening of rhodamine occurred at acid conditions (pH < 4) for strong protonation (Scheme S1†). However, the absorbance intensity had significant enhancement after the addition of Cu²⁺ ion between pH 4 and 9. It was due to the formation of ring-opened L–Cu²⁺ complex (Scheme 2). Then under basic conditions, the absorbance intensity decreased due to the formation of Cu(OH)₂. The data indicated that L has good absorption response for Cu²⁺ under physiological pH conditions. Therefore, further UV-vis studies were carried out in CH₃CN/Tris–HCl solution (1 : 1 v/v, pH = 7.2).

Fig. 1 UV-vis absorption of free L (2 × 10⁻⁵ mol L⁻¹) and L + 5 equiv. of Cu²⁺ ion in CH₃CN/Tris–HCl solution (1 : 1 v/v) with different pH conditions. The absorption wavelength is 556 nm.

Scheme 2 Proposed binding mode of probe L toward Cu²⁺.

The solution of L in CH₃CN/Tris–HCl solution (1 : 1 v/v, pH = 7.2) is colorless. As shown in Fig. 2, the absorption spectra of L alone (2 × 10⁻⁵ mol L⁻¹) exhibited no band in the region beyond 500 nm, which indicated that L was of the spirolactam form. The addition of Cu²⁺ into the solution immediately resulted in a strong absorption band centered at 556 nm, with an obvious color change from colorless to pink. A slight increase at 555 nm after the addition of Fe²⁺ was also observed at the same concentration. We infer that Fe²⁺ ion have lower binging affinity to L compared with Cu²⁺ in aqueous media. Under the same conditions, no obvious response could be observed upon the addition of other ions. Therefore, L can serve as a “naked-eye” indicator for Cu²⁺ ion. Even in the presence of miscellaneous competitive metal ions, the addition of Cu²⁺ ion still resulted in a large absorption change at 556 nm (Fig. 3). This indicates that the selectivity of L to Cu²⁺ ion is excellent in the presence of other competitive cations in acetonitrile–water medium.

Fig. 2 (a) UV-vis absorption of L (2 × 10⁻⁵ mol L⁻¹) in the presence of different metal ions (1 × 10⁻⁴ mol L⁻¹) in CH₃CN/Tris–HCl solution (1 : 1 v/v, pH = 7.2). (b) Photographs of color changes of 2 × 10⁻⁵ mol L⁻¹ L after the addition of 1 × 10⁻⁴ mol L⁻¹ various metal ions (from left to right: ion-free sensor L, Na⁺, Mg²⁺, Al³⁺, K⁺, Ca²⁺, Cr³⁺, Mn²⁺, Fe²⁺, Fe³⁺, Co²⁺, Ni²⁺, Zn²⁺, Ag²⁺, Cd²⁺, Ba²⁺, Hg²⁺, Pb²⁺, Cu²⁺).

Fig. 3 Absorption spectra of L to various metal ions in CH₃CN/Tris–HCl solution (1 : 1 v/v, pH = 7.2). The black bar represent the spectra of L (2 × 10⁻⁵ mol L⁻¹) obtained with (1 × 10⁻⁴ mol L⁻¹) of metal ions. The red bar represents the spectra that occur upon the subsequent addition of (1 × 10⁻⁴ mol L⁻¹) of Cu²⁺ to the above mentioned solutions. The absorption wavelength is 556 nm.

Titration experiment showed that the absorption spectra of L to Cu²⁺ ion also gradually increased at 401 nm and 556 nm, with higher Cu²⁺ concentration (0–10 × 10⁻⁵ mol L⁻¹) (Fig. 4). The absorbance at 556 nm had an approximate 643-fold enhancement. The Job's plot was conducted to determine the binding stoichiometry of the L–Cu²⁺ complex, wherein the total concentration of L and Cu²⁺ ion is 4 × 10⁻⁵ mol L⁻¹ and the mole fraction of Cu²⁺ ion is in the range from 0 to 1. As shown in Fig. 5, we can observe that the absorbance went through a maximum peak at a molar fraction of about 0.5, indicating a 1 : 1 binding stoichiometry between Cu²⁺ and L. To achieve this stoichiometry, carbonyl O, imino N, and phenol O atoms of L are the most likely binding sites for Cu²⁺. The proposed structure of L–Cu²⁺ is illustrated in Scheme 2.

Fig. 4 UV-vis spectra change of L (2 × 10⁻⁵ mol L⁻¹) upon the addition of increasing amounts of Cu²⁺ ion in CH₃CN/Tris–HCl solution (1 : 1 v/v, pH = 7.2) (from bottom to top: [Cu²⁺] = 0, 2, 4, …, 60, 70, 85, 100 × 10⁻⁶ mol L⁻¹). Inset: absorbance at 556 nm of L as a function of Cu²⁺ concentration.

Fig. 5 Job's plot of L and Cu²⁺ in CH₃CN/Tris–HCl solution (1 : 1 v/v, pH = 7.2). The total concentration of L and Cu²⁺ ions is 4 × 10⁻⁵ mol L⁻¹. The absorbance was collected at 556 nm.

Assuming a 1 : 1 association between L and Cu²⁺, the association constant (K_a) of L–Cu²⁺ was determined using the Benesi–Hildebrand equation as follows.⁴⁸

A and A₀ represent the absorbance of L solution in the presence and absence of Cu²⁺ ion, and A_max is the saturated absorbance of L in the presence of excess amount of Cu²⁺. [Cu²⁺] is the concentration of Cu²⁺ ion added. Plotting of 1/(A − A₀) versus 1/[Cu²⁺] showed a linear relationship (Fig. 6), which indicates that L bound with Cu²⁺ in a 1 : 1 binding stoichiometry, and the association constant K_a is determined from the slope as 6.47 × 10⁴ M⁻¹. The ESI-mass spectra of L–Cu²⁺ also showed a 1 : 1 stoichiometry. The unique peak at m/z = 666.18, (calcd = 666.27) corresponding to [CuL]⁺ was clearly observed when Cu²⁺ was added to L (Fig. S6†).

Fig. 6 Benesi–Hildebrand plot (absorbance at 556 nm) of L using eqn (1), assuming 1 : 1 stoichiometry for association between L and Cu²⁺.

Fluorescence spectroscopic studies of L in presence of Cu²⁺

The effects of pH on the sensor L was evaluated first. Fig. 7 shows that for ion-free sensor L, the fluorescence intensity was strong at pH < 6. It was due to the ring opening of rhodamine for the strong protonation. The fluorescence intensities of ion-free sensor L gradually decreased with the increase of pH value.⁴⁹ In the pH range from 6 to 10, the fluorescence signal of L without Cu²⁺ ion was weak. Upon the addition of Cu²⁺ ion, there was an obvious fluorescence emission at 581 nm in pH from 6 to 9. This result suggested that L could act as a fluorescent probe for Cu²⁺ under physiological conditions (i.e. pH 7.2). The experiments have also been conducted in Tris–HCl solutions, wherein the selectivity of sensor L towards Cu²⁺ is also good. However, the fluorescence intensity in Tris–HCl solutions is not as strong as that in mixed solvents of acetonitrile and Tris–HCl buffer. Therefore, further fluorescent studies were also carried out in CH₃CN/Tris–HCl solution (1 : 1 v/v, pH = 7.2).

Fig. 7 Fluorescence intensity of free L (2 × 10⁻⁵ mol L⁻¹) and L + 5 equiv. of Cu²⁺ ion in CH₃CN/Tris–HCl solution (1 : 1 v/v) with different pH conditions. The excitation wavelength is 530 nm. The emission wavelength is 581 nm.

L in CH₃CN/Tris–HCl solution (1 : 1 v/v, pH = 7.2) showed a very weak fluorescence in the absence of metal ions when excited at 530 nm. However, the addition of Cu²⁺ ion resulted in remarkably enhanced fluorescence intensity. Under the same condition, additions of other metal ions including Na⁺, Mg²⁺, Al³⁺, K⁺, Ca²⁺, Cr³⁺, Mn²⁺, Fe³⁺, Co²⁺, Ni²⁺, Zn²⁺, Ag⁺, Cd²⁺, Ba²⁺, Hg²⁺ and Pb²⁺ did not cause any discernible changes. On the other hand, the Fe²⁺–ligand complex, in contrast to the rest of the ions in study, exhibited lower fluorescence intensity than the free ligand (Fig. 8). Similarly, parallel experiments in the presence of potentially competitive metal ions were also carried out (Fig. 9) and the results suggested that the fluorescent recognition of Cu²⁺ by L was hardly influenced by other co-existing metal ions.

Fig. 8 Fluorescence of L (2 × 10⁻⁵ mol L⁻¹) in the presence of different metal ions (1 × 10⁻⁴ mol L⁻¹) in CH₃CN/Tris–HCl solution (1 : 1 v/v, pH = 7.2). The excitation wavelength is 530 nm.

Fig. 9 Fluorescence spectra of L to various metal ions in CH₃CN/Tris–HCl solution (1 : 1 v/v, pH = 7.2), the black bar represent the spectra of L (2 × 10⁻⁵ mol L⁻¹) obtained with (1 × 10⁻⁴ mol L⁻¹) of metal ions. The red bar represents the spectra that occur upon the subsequent addition of (1 × 10⁻⁴ mol L⁻¹) of Cu²⁺ to the above mentioned solutions. The excitation wavelength is 530 nm. The emission wavelength is 581 nm.

From the fluorescence titration experiments (Fig. 10), upon addition of Cu²⁺ into CH₃CN/Tris–HCl solution (1 : 1 v/v, pH = 7.2) of L, a new emission band centered at 572 nm (the excitation wavelength is 530 nm) was developed. Similar to the absorption response, the fluorescence intensity increased gradually with higher Cu²⁺ concentration. Meanwhile, the emission band underwent a slightly red-shifted from 572 to 581 nm after 2 equiv. of Cu²⁺ were added. The fluorescence intensity at 581 nm had an approximate 7-fold enhancement. The red-shift of the emission peak can be ascribed to the recombination of the orbitals after the formation of ring-opened L–Cu²⁺ complex. The emission intensity of ion-free sensor L was measured 10 times and the standard deviation of blank measurements was determined. The fluorescence intensity of L (2 × 10⁻⁵ mol L⁻¹) at 581 nm was found to increase linearly with the concentration of Cu²⁺ in range of 0–0.8 × 10⁻⁵ mol L⁻¹ (R² = 0.9910) (Fig. S7†). The detection limit was then calculated with the eqn (2):³¹

Detection limit = 3σ/k (2)

where σ is the standard deviation of blank measurement, and k is the slope of the intensity versus Cu²⁺ concentration. The detection limit of Cu²⁺ in CH₃CN/Tris–HCl solution (1 : 1 v/v, pH = 7.2) was measured to be 2.43 × 10⁻⁸ mol L⁻¹.

Fig. 10 Fluorescence emission spectra changes of L (2 × 10⁻⁵ mol L⁻¹) upon the addition of increasing amounts of Cu²⁺ ion in CH₃CN/Tris–HCl solution (1 : 1 v/v, pH = 7.2) (from bottom to top: [Cu²⁺] = 0, 2, 4, …, 40, 50, 60, 70, 80, 90, 100 × 10⁻⁶ mol L⁻¹). Inset: fluorescence titration profile of L at 581 nm in increasing of Cu²⁺ concentration.

Theoretical calculations

In order to further verify the configuration of L–Cu²⁺, we carried out density functional theory (DFT) calculations with the B3LYP exchange functions using Gaussian 09 package, and introduce the LANL2DZ effective core potential (ECP) to represent the core electrons of the Cu atom. The valence electrons are described by LANL2DZ basis set, while all the other atoms are described commonly by 6-31G(d) basis set. All the thermodynamic data are obtained at this computational level. The molecule forms a planar structure, as displayed in Fig. 11. The Cu–N bond length is 1.975 Å, and the distances of Cu–O₁, Cu–O₂, Cu–O₃ are 1.992, 1.912, 1.978 Å, respectively. The optimized configuration shows that Cu²⁺ ions occupy the acylhydrazone coordination centers of L at the same time.

Fig. 11 Calculated energy-minimized structure of L with Cu²⁺.

UV-vis spectroscopic studies of L–Cu²⁺ complex in presence of S²⁻

We have further studied the influence of different anions on UV-vis absorbance of L–Cu²⁺ complex. Upon addition of Na₂S to L (2 × 10⁻⁵ mol L⁻¹) and Cu²⁺ (4 × 10⁻⁵ mol L⁻¹) complex in CH₃CN/Tris–HCl solution (1 : 1 v/v, pH = 7.2), the UV-vis absorption was decreased and the color of the L–Cu²⁺ complex changed from pink to colorless. The optical properties of the L–Cu²⁺ complex were also studied in the presence of different anions such as CN⁻, SCN⁻, SO₃²⁻, SO₄²⁻, PO₄³⁻, HPO₄²⁻, H₂PO₄⁻, Cl⁻, Br⁻, I⁻, NO₂⁻, NO₃⁻, and AcO⁻. It is worth noted that only by adding S²⁻ into the solution of L–Cu²⁺ could cause this change, whereas other anions failed to produce any discernible spectral change (Fig. 12). For further investigation, a solution of L in CH₃CN/Tris–HCl buffer (1 : 1 v/v, pH = 7.2) containing 2 equiv. of Cu²⁺ was titrated by the solution of Na₂S. The UV-vis spectral pattern of the titration experiment was similar but in reverse direction to the titration curve obtained with Cu²⁺ (Fig. 13). Upon the addition of S²⁻ to L–Cu²⁺ complex, black precipitate was formed. This phenomenon showed that Cu²⁺ released from complex L–Cu²⁺ was captured by sulfide anion to produce CuS. The formation of CuS was also ascertained by the XRD measurement. The sensing capability of L–Cu²⁺ complex was further tested in the presence of other anions, which may interfere the estimation of copper and sulfide (Fig. 14). The receptor L–Cu²⁺ complex is well selective in detecting sulfur in the presence of other competitive anions. The mass spectrum of the L–Cu²⁺ system was also studied in the presence of S²⁻. ESI-MS of the above media displayed a molecular peak [L + H⁺] at m/z 605.6 and a molecular-ion peak [L + Na⁺] at m/z 627.5 which confirmed the identity of free L (Fig. S8†).

Fig. 12 UV-vis titration spectra of L (2 × 10⁻⁵ mol L⁻¹) with 4 × 10⁻⁵ mol L⁻¹ of Cu²⁺ upon addition of different anions (4 × 10⁻⁵ mol L⁻¹) in CH₃CN/Tris–HCl solution (1 : 1 v/v, pH = 7.2) (inset, A: L + Cu²⁺. B: L + Cu²⁺+ S²⁻).

Fig. 13 UV-vis titration spectra of L (2 × 10⁻⁵ mol L⁻¹) with 4 × 10⁻⁵ mol L⁻¹ of Cu²⁺ upon addition of sodium sulfide (4 × 10⁻⁵ mol L⁻¹) in CH₃CN/Tris–HCl solution (1 : 1 v/v, pH = 7.2) (from top to bottom: [S²⁻] = 0, 2, 4, …, 40, 50, 60, 70 × 10⁻⁶ mol L⁻¹). Inset: changes in the absorbance at 556 nm with incremental addition of S²⁻.

Fig. 14 UV-vis responses of L–Cu²⁺ to various anions in CH₃CN/Tris–HCl solution (1 : 1 v/v, pH = 7.2). The black bars represent the absorbance responses of L (2 × 10⁻⁵ mol L⁻¹) and Cu²⁺ ion (4 × 10⁻⁵ mol L⁻¹) in the presence of anions (4 × 10⁻⁵ mol L⁻¹) of interest. The red bars represent the change of the absorbance that occurs upon the subsequent addition of S²⁻ (4 × 10⁻⁵ mol L⁻¹) to the above solution. The intensities were recorded at 556 nm. (1) L–Cu²⁺ (2) CN⁻ (3) SCN⁻ (4) SO₃²⁻ (5) SO₄²⁻ (6) PO₄³⁻ (7) HPO₄²⁻ (8) H₂PO₄⁻ (9) Cl⁻ (10) Br⁻ (11) I⁻ (12) NO₂⁻ (13) NO₃⁻ (14) AcO⁻ (15) S²⁻.

Conclusions

In summary, a rhodamine-based fluorimetric probe L was designed and synthesized. Studies showed that L exhibited highly selective binding with Cu²⁺ over other metal ions with a fluorescence turn-on effect. The chemosensor L displayed a one-to-one complex formation with Cu²⁺ ions in a broad pH range. Obvious increases in colorimetric changes were observed upon the addition of Cu²⁺ into the CH₃CN/Tris–HCl solution (1 : 1 v/v, pH = 7.2) of chemosensor L. The complex formed between L and Cu²⁺ is dissociable only in the presence of sulfide anion and the color changed from pink to colorless, which makes the L–Cu²⁺ complex an efficient sensor for sulfide anions.

Acknowledgements

The authors acknowledge financial support from the NSFC (Grants 21301082), the Natural Science Foundation of Gansu (no. 1308RJYA028) and Huayin Ordnance Test Center. We thank Prof. Renqi Wang for giving many suggestions on this work.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra45931d

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