Three N-stabilized rhodamine-based fluorescent probes for Al³⁺via Al³⁺-promoted hydrolysis of Schiff bases

Abstract

Three probes for Al³⁺ detection by both fluorescence and the naked eye in acetonitrile solution were developed. The mechanism of fluorescence was based on the aluminum complexation with rhodamine and subsequent Al³⁺-promoted hydrolysis of the Schiff base. Theoretical calculations indicated that the aluminum complexes tend to hydrolyze to compound L₆. A simple paper test strip system for the rapid monitoring of Al³⁺ was developed, indicating its convenient use in environmental samples.

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Introduction

As is known to all, aluminum is the third most abundant element in the earth's crust. It widely exists in the environment due to acidic rain and is considered to be toxic in biological activities.^1–3 Regarding toxicological effects of aluminum, its primary targets are different from those of heavy metals.⁴ The widespread use of aluminum in water treatment, as a food additive, and in many industrial activities including the manufacturing of cars and computers often exposes people to this metal.^5,6 Excessive exposure of the human body to Al³⁺ leads to the malfunction of the central nervous system such as Alzheimer's disease and Parkinson's disease.^7,8 The WHO has recommended the average daily human intake of Al³⁺ to be around 3–10 mg and the weekly tolerable dietary intake as 7 mg kg⁻¹ body weight. Therefore, trace level determination of Al³⁺ is highly important.

In the last decade, many methods such as graphite furnace atomic absorption spectrometry, atomic emission spectrometry, inductively coupled plasma atomic emission spectrometry and electrochemical methods have been used for aluminum detection. All of these techniques are generally time-consuming and expensive as well. In contrast, optical detection via fluorescence is an operationally easy technique besides being highly sensitive. Due to great changes in the absorption and/or fluorescence spectra of many organic molecules after coordinating with ions, colorimetric and fluorescence analytical methods are very good and effective ways to detect metal ions.^9–15

The rhodamine moiety has been widely used in the field of chemosensors, especially as a chemodosimeter, given its fluorescence off–on behavior that results from its particular structural properties. Upon metal binding, its structure can undergo a change from the spirolactam to an open ring amide, resulting in a magenta-colored, highly fluorescent compound.¹⁶

While some rhodamine-based chemosensors for Al³⁺ ions have been reported to date,^17–26 dual colorimetric and fluorescent chemosensors for Al³⁺ are still rare and some of them are not efficient enough to be selective toward Al³⁺ or sensed in organic solvents. Therefore, developing sensors which are able to detect Al³⁺ by both fluorescence and the naked eye in aqueous solution are very valuable. In this paper, we report three new N-stabilized rhodamine-based fluorescent probes which exhibited sensitive detection toward Al³⁺via significant fluorescence enhancement in solution, and, at the same time, showed a significant color change from being colorless to red. Similar structures had been designed for compounds L₁, L₂ and L₃. By comparing with each other, we could know that the different electronic distribution among the chemodosimeters' structures had great influence in their recognition toward Al³⁺. Owing to a more stable structure, L₁ showed the best sensing property, so we selected L₁ as a representative example to expatiate in the following discussion. To the best of our knowledge, none of the rhodamine-based probes whose fluorescent mechanism is based on the Al³⁺-promoted hydrolysis of the Schiff base has been reported for detection of Al³⁺ up to now.

Experimental

Apparatus

Fluorescence spectra measurements were performed on a HITACHI F-4500 fluorescence spectrophotometer, and the excitation and emission wavelength band passes were both set to 5.0 nm. Absorption spectra were recorded on a UV-2102 double-beam UV/Vis spectrometer, Perkin Elmer precisely. NMR spectra were recorded on a Bruker DTX-400 spectrometer in CDCl₃, using TMS as an internal standard. Mass spectral determination was carried on a HPLC Q-T of HR-MS.

Materials

All the materials for synthesis were purchased from commercial suppliers and used without further purification. The solutions of metal ions were prepared from their nitrate salts, except for FeCl₃, FeCl₂, CrCl₃ and MnCl₂. The metal ions were prepared as 10.00 mM in water solution.

Syntheses

As shown in Scheme 1, the compounds L_1–3 were prepared by reacting L₄ with an aromatic aldehyde. L₄ was synthesized according to the literature.²⁷

Scheme 1 Synthetic route of target compounds.

L₁ . L₄ (0.5 mmol, 0.24 g) was dissolved in 25 mL ethanol, and then p-nitrobenzaldehyde (1 mmol, 0.15 g) was slowly added. The mixture was stirred and refluxed for 12 h at 80 °C. After distillation in vacuum, the residue was recrystallized with methanol and water to give the final product L₁ in a yield of 78.4%. ¹H NMR (400 MHz, CDCl₃, δ ppm): 8.17 (d, J = 8.0 Hz, 2H), 8.06 (s, 1H), 7.91–7.90 (m, 1H), 7.72 (d, J = 8.4 Hz, 2H), 7.43 (s, 1H), 7.03 (d, J = 4.0 Hz, 1H), 6.44 (d, J = 8.4 Hz, 1H), 6.28 (s, 2H), 6.14–6,12 (m, 1H), 3.52–3.49 (m, 1H), 3.25 (s, 8H), 1.99 (s, 8H); ¹³C NMR (100 MHz, CDCl₃, δ ppm): 168.5, 159.8, 153.9, 153.1, 148.8, 148.7, 141.7, 132.6, 130.9, 128.9, 128.8, 128.1, 123.7, 123.6, 122.8, 108.4, 105.7, 98.0, 65.2, 59.3, 47.6, 41.1, 25.5. HR-MS: C₃₇H₃₅N₅O₄ [M + H]⁺, calcd for 614.2762. Found: 614.2767 (Fig. S1–S3, ESI†).

L₂ . Compound L₂ was prepared using a general procedure which is essentially similar to that used for L₁. Yield of L₂: 69.3%. ¹H NMR (400 MHz, CDCl₃, δ ppm): 8.01 (s, 1H), 7.92–7.90 (m, 1H), 7.58–7.56 (m, 2H), 7.44–7.39 (m, 2H), 7.36–7.31 (m, 3H), 7.05–7.03 (m, 1H), 6.46 (d, J = 8.8 Hz, 2H), 6.29 (s, 2H), 6.16–6.14 (m, 2H), 3.44 (s, 4H), 3.25 (t, J = 3.2 Hz, 8H), 2.01–1.98 (m, 8H); ¹³C NMR (100 MHz, CDCl₃, δ ppm): 168.4, 162.4, 154.0, 153.1, 148.7, 132.4, 130.4, 128.9, 128.4, 128.1, 128.0, 123.7, 122.8, 108.4, 105.8, 98.0, 65.1, 59.0, 47.6, 41.3, 25.5. HR-MS: C₃₇H₃₆N₄O₂ [M + H]⁺, calcd for 569.2911. Found: 569.2910 (Fig. S4–S6, ESI†).

L₃ . The following compound was prepared using a general procedure which is essentially similar to that used for L₁. Yield of L₃: 62.7%. ¹H NMR (400 MHz, CDCl₃, δ ppm): 8.03 (s, 1H), 7.94–7.93 (m, 1H), 7.49 (d, J = 7.6 Hz, 2H), 7.45–7.43 (m, 2H), 7.15 (d, J = 7.6 Hz, 2H), 7.07–7.05 (m, 1H), 6.49 (d, J = 8.8 Hz, 2H), 6.32 (s, 2H), 6.17 (d, J = 8.4 Hz, 2H), 3.77–3.71 (m, 4H), 3.46–3.29 (m, 8H), 2.36 (s, 3H), 2.02 (s, 8H); ¹³C NMR (100 MHz, CDCl₃, δ ppm): 168.3, 162.3, 154.1, 153.1, 148.7, 140.6, 133.7, 132.4, 131.1, 129.1, 128.9, 128.1, 127.9, 123.7, 122.8, 108.4, 105.8, 98.0, 65.1, 59.0, 47.6, 41.3, 25.5, 21.5. HR-MS: C₃₈H₃₈N₄O₂ [M + H]⁺, calcd for 583.3068. Found: 583.3074 (Fig. S7–S9, ESI†).

Results and discussion

The structure of title compounds L₁, L₂ and L₃ were characterized by ¹H NMR, ¹³C NMR and HR-MS spectrometry. The results were in good agreement with the structure shown in Scheme 1. Fluorescence and UV-vis studies were performed using a 10 μM solution of L₁, L₂ and L₃ in a CH₃CN/H₂O (95 : 5, v/v) solution with appropriate amounts of metal ions. Solutions were shaken for 120 min before measuring the absorption and fluorescence in order to make the metal ions chelate with the sensors sufficiently.

Steady-state optical properties

Compared with L₂ + Al³⁺ and L₃ + Al³⁺, L₁ + Al³⁺ had higher fluorescence quantum yield (Fig. S10, ESI†). This means that the strong electron withdrawing group on benzene is beneficial to the chelation of Al³⁺ with sensors. The effect of the reaction media on the binding of L₁ with Al³⁺ was studied, and the results are shown in Fig. S11 (ESI†). It was found that the solvent has a great effect on the coordination reaction. When the coordination reaction was performed in acetonitrile–water solution, high F/F₀ and ΔA values were obtained, indicating that acetonitrile–water media is favorable for fluorescence measurement. Therefore, acetonitrile–water solution was selected for both fluorescence assay and colorimetric assay. As shown in Fig. 1, L₁ exhibited a 190-fold enhancement of fluorescence intensity at a peak wavelength λ_max = 582 nm in the presence of 10 equiv. Al³⁺, so we selected L₁ as the representative when expatiating on the characters of the three compounds in the following discussion.

Fig. 1 F: fluorescence intensity (at 582 nm) of L₁, L₂ and L₃ (10 μM) in CH₃CN/H₂O (95/5, v/v) in the presence of Al³⁺ (100 μM) (λ_ex = 520 nm); F₀: fluorescence intensity (at 582 nm) of L₁, L₂ and L₃ (10 μM) only in CH₃CN/H₂O (95/5, v/v) (λ_ex = 520 nm).

UV-vis spectral responses of L₁

As shown in Fig. 2, the UV-vis spectrum of compound L₁ (10 μM) exhibited only very weak bands over 400 nm. Addition of 10 equiv. Al³⁺ into solution immediately resulted in a significant enhancement of absorbance at about 562 nm simultaneously changing the color to red. Under identical conditions, no obvious response could be observed upon the addition of other ions including Zn²⁺, Mg²⁺, Ca²⁺, Cd²⁺, Cu²⁺, Pb²⁺, Hg²⁺, Ba²⁺, Ni²⁺, Fe²⁺, Mn²⁺, K⁺, Li⁺, Ag⁺, Co²⁺ and Na⁺ except for Fe³⁺ and Cr³⁺, which caused a mild effect compared to Al³⁺. This interesting feature demonstrated that compound L₁ can serve as a selective “naked-eye” chemosensor for Al³⁺.

Fig. 2 Absorbance spectra of L₁ (10 μM) in CH₃CN/H₂O (95 : 5, v/v) solution in the presence of 10 equiv. of various species. Inset: the photos of L₁ with different metal ions in CH₃CN/H₂O (95 : 5, v/v) solution.

To further investigate the interaction of Al³⁺ and L₁, an ultraviolet titration experiment was carried out (Fig. 3). The Al³⁺ binding stoichiometry of L₁ can be determined from titration and the Job plot.²⁸ A plot of [Al³⁺]/{[Al³⁺] + [L₁]} versus the molar fraction of Al³⁺ is provided in Fig. 4. The absorbance reached a maximum when the ratio was 0.5, indicating a 1 : 1 stoichiometry of Al³⁺ to L₁ in the complex.

Fig. 3 Absorbance spectra of L₁ (10 μM) in CH₃CN/H₂O (95 : 5, v/v) upon addition of different amounts of Al³⁺ ions.

Fig. 4 Job's plots of the complexation between L₁ and Al³⁺. The total concentration of L₁ + Al³⁺ was kept constant at 100 μM.

Fluorescence spectral responses of L₁

As shown in Fig. 5, L₁ exhibited a very weak fluorescence in the absence of metal ions. When 10 equiv. Al³⁺ was introduced into a 10 μM solution of L₁ in CH₃CN/H₂O (95 : 5, v/v), a remarkable enhancement of fluorescence spectra was observed. The fluorescence enhancement of Al³⁺ to compound L₁ was as high as 190-fold. Under the same conditions, a mild fluorescence enhancement factor was also detected for Fe³⁺ and Cr³⁺, but Zn²⁺, Mg²⁺, Ca²⁺, Cd²⁺, Cu²⁺, Pb²⁺, Hg²⁺, Ba²⁺, Ni²⁺, Fe²⁺, Mn²⁺, K⁺, Li⁺, Ag⁺, Co²⁺ and Na⁺ showed no obvious changes in the fluorescence intensity and color. From fluorescence spectra of L₂ and L₃, we could know Fe³⁺ and Cr³⁺ had more interference (Fig. 6 and 7). Probe L₁ had better fluorescence properties. It may benefit from the strong electron withdrawing group. Moreover, we also confirmed the competitive experiments that the background metal ions showed very low interference with the detection of Al³⁺ in water solution (Fig. 8). Generally, the detection limit of metal ions is needed for a fluorescent sensor. Under optical conditions, the linear response for the fluorescence intensity response of compound L₁ was between 8 and 18 μM (Fig. 9), and the detection limit of Al³⁺ was measured to be 3.98 μM.

Fig. 5 Fluorescence spectra of L₁ (10 μM) in CH₃CN/H₂O (95 : 5, v/v) in the presence of 10 equiv. of various metal ions (λ_ex = 520 nm, slit = 5 nm).

Fig. 6 Fluorescence spectra of L₂ (10 μM) in CH₃CN/H₂O (95 : 5, v/v) in the presence of 10 equiv. of various metal ions (λ_ex = 520 nm, slit = 5 nm).

Fig. 7 Fluorescence spectra of L₃ (10 μM) in CH₃CN/H₂O (95 : 5, v/v) in the presence of 10 equiv. of various metal ions (λ_ex = 520 nm, slit = 5 nm).

Fig. 8 Fluorescence intensity (at 582 nm) of L₁ upon the addition of 100 μM Al³⁺ in the presence of 100 μM background metal ions in CH₃CN/H₂O (95/5, v/v) (λ_ex = 520 nm, slit = 5 nm).

Fig. 9 Fluorescence intensity (at 582 nm) of compound L₁ (10 μM) as a function of the Al³⁺ concentration in CH₃CN/H₂O (95/5, v/v) solution (λ_ex = 520 nm, slit = 5 nm).

Mechanism

To investigate the Al³⁺ enhancement mechanism, IR spectra of L₁ and L₁ + Al³⁺ were recorded in KBr disks (Fig. S15, ESI†). The peak at 1694 cm⁻¹, which corresponds to the amide carbonyl absorption, disappeared upon the addition of Al³⁺. This supported the notion that the carbonyl group of L₁ is involved in the coordination of metal ions.

In addition, the F⁻-adding experiments were conducted to examine the reversibility of this reaction and the result is shown in Fig. 10. When F⁻ (3 equiv. of Al³⁺) was added to the L₁ + Al³⁺ CH₃CN/H₂O solution, the fluorescence intensity at 582 nm decreased (green line) and further addition of 10 equiv. Al³⁺ could not recover the fluorescence (blue line). To investigate the fluorescence quenching mechanism, the ¹H NMR spectra of the complex of L₁ + Al³⁺ were recorded. As it is seen from Fig. 11, a new single peak was observed at 10.2 ppm when Al³⁺ was added to L₁. Comparing the ¹H NMR spectra of p-nitrobenzaldehyde with ¹H NMR spectra of the complex of L₁ + Al³⁺, it can be confirmed that the new single peak belongs to the aldehyde proton (H_a) of p-nitrobenzaldehyde (Fig. S12, ESI†). The HR-MS spectra of L₁ + Al³⁺ in CH₃CN/H₂O (95/5, v/v) were also recorded (Fig. S13, ESI†). A unique peak at m/z 481.2602 corresponding to [L₆ + H]⁺ was clearly observed when 1 equiv. of Al³⁺ was added to L₁, whereas L₁ without Al³⁺ exhibited peaks only at m/z 614.2 which corresponded to [L₁ + H]⁺ (Fig. S3, ESI†). The form of the L₆–Al³⁺ complex was also confirmed by ESI-MS analysis. As shown in Fig. S14 (ESI†), [M + H]⁺ of L₆ appeared at m/z 481.4. When 1 equiv. of Al³⁺ was added to L₁ in H₂O/CH₃CN, the ESI-MS spectra showed the peak of the L₆–Al³⁺ complex. The signal at m/z 629.4 (calculated value, 629.4) corresponds to [L₆ + Al³⁺ + 2HCOO⁻ + CH₃OH]⁺. Both UV-vis and fluorescence data lead to a significant OFF–ON signal. From the molecular structure and spectral results of L₁, an irreversible fluorescent chemodosimeter for Al³⁺ was constructed as shown in Scheme 2. Firstly, the addition of the Al³⁺ ion induced a ring opening of the spirolactam of rhodamine. Then, L₁–Al was hydrolyzed into p-nitrobenzaldehyde. And it was certified by theoretical calculations (the data are supplied in the ESI†). The DFT calculations were performed using the Gaussian 09 program.²⁹ The structures of L₅, H₂O, L₆ and p-nitrobenzaldehyde were optimized at the B3LYP^30–32/6-31G(d) level in acetonitrile solvent, using the integral equation formalism polarizable continuum model (IEF-PCM).^33,34 The calculated results demonstrate that the free energy of L₆ and p-nitrobenzaldehyde is 14.7 kcal mol⁻¹ lower than those of L₅ and H₂O, indicating that this hydrolysis step is an exothermic process. So in the reaction, L₅ tends to hydrolyze to L₆ (Fig. 12). From Fig. 5, we know that both Cr³⁺ and Fe³⁺ can induce the moderate fluorescence enhancements to L₁. The mechanism of L₁ with Fe³⁺ and Cr³⁺ was thought to be similar to that of L₁ with Al³⁺. ¹H NMR spectra of the complex of L₁ + Fe³⁺ and L₁ + Cr³⁺ were recorded (Fig. S16 and S18, ESI†). The same new single peak was observed at 10.179 ppm and 10.138 ppm, respectively. The ESI mass spectra of L₁ + Fe³⁺ and L₁ + Cr³⁺ in CH₃CN/H₂O (95/5, v/v) were recorded (Fig. S17 and S19, ESI†) and a similar hydrolysis product was found. A kind of Fe³⁺-induced Schiff base hydrolysis mechanism has been reported by Kim and coworkers.³⁵

Fig. 10 Fluorescence intensity (at 582 nm) of L₁ (10 μM) as a function of Al³⁺ concentration in CH₃CN/H₂O (95 : 5, v/v) solutions (1) baseline: 10 μM L₁ only; (2) red line: 10 μM L₁ with 10 equiv. Al³⁺; (3) green line: 10 μM L₁ with 10 equiv. Al³⁺ and then addition of 30 equiv. F⁻; (4) blue line: 10 μM L₁ with 10 equiv. Al³⁺ and 30 equiv. F⁻ then addition of 10 equiv. Al³⁺ (λ_ex = 520 nm, slit = 5 nm).

Fig. 11 ¹H NMR spectra of L₁ + Al³⁺ (a) and L₁ (b) in CDCl₃.

Scheme 2 Possible sensing mechanism of L₁ with Al³⁺.

Fig. 12 Calculated energy-minimized structure of L₅, L₆ and p-nitrobenzaldehyde (gray: C atoms; blue: N atoms; red: O atoms; green: Cl⁻; and brown: Al³⁺).

In order to investigate the influence of different acid concentrations on the spectra of L₁ and find a suitable pH span in which L₁ can selectively detect Al³⁺ efficiently, the acid titration experiments were performed. The addition of Al³⁺ led to the fluorescence enhancement over a wide pH range (1.0–8.0), which is attributed to opening of the rhodamine spirolactam structure (Fig. S20, ESI†). Moreover, a time course of the fluorescence response of L₁ upon addition of Al³⁺ is shown in Fig. S21 (ESI†). The kinetics of fluorescence enhancement at 593 nm by the newly developed fluorescent probe was recorded. It indicated that this reaction between L₁ and Al³⁺ was slow. And, the recognizing event of L₁ with Fe³⁺ could be completed in 80 minutes (Fig. S22, ESI†). The chemodosimeters L₂ and L₃ show similar properties as L₁ (see Fig. S23–S34, ESI†). The detection limits of L₂ and L₃ were measured to be 32 μM (Fig. S27, ESI†) and 49 μM (Fig. S34, ESI†). The three chemodosimeters exhibit irreversible, selective and sensitive recognition toward Al³⁺ over other metal ions. The colorimetric and fluorometric responses of the sensors L₁–L₃ and Al³⁺ can also be conveniently detected by the naked eye. The fluorescence enhancement of Al³⁺ to L₁, L₂ and L₃ is as high as 190, 120 and 60-fold, respectively. These indicate that the differences among the three chemodosimeters' structures have great influence in their recognition toward Al³⁺. The strong electron withdrawing group on the benzene ring results in the Schiff base having a good affinity toward Al³⁺.

Many fluorescent sensors for Al³⁺ detection could only perform in solution, which would limit their applications under special circumstances such as on-site detection in situ. To demonstrate the practical application of our sensor, we prepared the test papers of sensor L₁. It was easily prepared by immersing a filter paper into the solution of L₁ in CH₂Cl₂ (1 mM) and then drying in air. Next, in different Al³⁺ concentration solutions (0, 1.0 × 10⁻⁴ M, 1.0 × 10⁻³ M, and 1.0 × 10⁻² M), these strips were immersed for 5 s and taken out of the solution. As depicted in Fig. 13, the color of the test paper changed from colorless to purple and deepened gradually with the increase of the Al³⁺ concentration. These paper-made test kits may be used as simple tools for detecting Al³⁺ in environmental samples. To validate their practicality in real environmental samples, we employed probe L₁ in a standard addition method^36–38 to determine Al³⁺ concentrations in water samples from Jinshui river (in Zhengzhou, Henan province, China), no fluorescence enhancement was observed. When the water samples were spiked with different Al³⁺ concentrations (25 μM, 50 μM, and 100 μM) and measured using the current methods, Al³⁺ recoveries were about 20% (Table S1, ESI†).

Fig. 13 Photographs of the test kits with L₁ for detecting Al³⁺ in (CH₃CN/H₂O = 95 : 5, v/v) solution with different concentrations: (1) 0; (2) 1.0 × 10⁻⁴ M; (3) 1.0 × 10⁻³ M; (4) 1.0 × 10⁻² M.

Conclusions

In summary, we synthesized three fluorescent chemodosimeters L₁, L₂ and L₃. The colorimetric and fluorescence response to Al³⁺ can be conveniently detected even by the naked eye, which provides a facile method for visual detection of Al³⁺. Theoretical calculations indicated that the electronic distribution of the N atom of C=N is −0.436, −0.46 and −0.464, respectively (log file). Owing to the strong electron withdrawing group on benzene, the Schiff base L₁ became stable, which was favorable for high sensitivity for the detection of Al³⁺. As we know, higher electronegativity of the N atom of C=N of the Schiff base is easier to hydrolyze. Furthermore, the mechanism of fluorescence was found to be the aluminum complexation with rhodamine and subsequent Al³⁺-promoted hydrolysis of the Schiff base. Also, iron and chromium complexation had the same hydrolysis mechanism. The simple and convenient test paper may provide an easy way to detect Al³⁺ in our daily life.

Acknowledgements

This work was financially supported by the National Science Foundation of China (No. 21375113) and the Program for New Century Excellent Talents in University (NCET-11-0950).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4nj01357c

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