A highly selective and instantaneously responsive Schiff base fluorescent sensor for the “turn-off” detection of iron(III), iron(II), and copper(II) ions

Abstract

A new and tri-responsive fluorescent Schiff base probe (DBAB) has been designed and developed for the recognition of iron(III) ions (Fe³⁺), iron(II) ions (Fe²⁺) and copper(II) ions (Cu²⁺) simultaneously. This sensor displays a favorable selectivity for Fe³⁺, Fe²⁺ and Cu²⁺ ions over a range of other common metal cations in DMF solution, leading to prominent fluorescence on–off. The detection limits were 2.17 × 10⁻⁶ M for Fe³⁺, 2.06 × 10⁻⁶ M for Fe²⁺ and 2.48 × 10⁻⁶ M for Cu²⁺ respectively. This tri-responsive fluorescent probe for Fe³⁺, Fe²⁺ and Cu²⁺ was distinguished by further reduction of the ascorbic acid. The recognition mechanism of DBAB toward Fe³⁺, Fe²⁺ and Cu²⁺ has been investigated in detail by Job plot measurements and ¹H NMR.

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1. Introduction

In recent years, the design and synthesis of smart organic molecules used as chemosensors with high selectivity, sensitivity, and instantaneous responsiveness toward metal cations has received considerable attention.¹ Sensing multiple analytes with a single chemosensor is a challenging task.² It involves eliciting different responses from the molecule in the presence of different analytes.

Among biological metal ions, iron ions and copper ions both play crucial roles in many biological and environmental processes.³ Fe³⁺ deficiency and overloading induce serious disorders such as Huntington’s disease.⁴ Cu²⁺ plays a critical role as a catalytic cofactor in a variety of metallo-enzymes, including tyrosinase, superoxide dismutase and cytochrome oxidase. Excessive Cu²⁺ ions will also cause imbalance in cellular processes, resulting in neurodegenerative diseases.⁵ At present, many methods are employed to determine Fe³⁺, Fe²⁺ and Cu²⁺ including inductively coupled plasma mass spectrometry (ICP-MS),⁶ fluorescence,⁷ atomic absorption,⁸ colorimetry,⁹ flow injection and electrochemical methods.¹⁰

Currently, compared with these methods, the fluorescent sensor strategy is proven to be the most convenient and popular approach for metal ion detection due to its superiority over other methods, such as high sensitivity, remarkable selectivity, easy operation, and rapid response. In the past decade, diverse fluorescent chemosensors for transition metal ions have been reported, including Hg²⁺,¹¹ Cu²⁺,¹² Zn²⁺,¹³ Al³⁺,¹⁴ Fe³⁺,¹⁵ Cd²⁺,¹⁶ and Cr³⁺.¹⁷ However, fluorescent chemosensors which can recognize two or more transition metal ions simultaneously and instantaneously remain scarce. Therefore, developing simple sensors which can detect more than one ion in environmental and biological samples has gained increasing attention due to their high efficiency, low-cost, and operational simplicity.¹⁸

Herein we introduce a fluorescent Schiff base ((E)-2-((4-(diethylamino)benzylidene)amino)benzoic acid, DBAB) that exhibits remarkable optical responses when interacting with a metal ion. This sensor displays a favorable selectivity for Fe³⁺, Fe²⁺ and Cu²⁺ ions over a range of other common metal cations in DMF solution, leading to prominent fluorescence on–off. The “turn-off” signals between DBAB and Fe³⁺/Fe²⁺/Cu²⁺ can be distinguished by ascorbic acid (AA) via the reduction ability difference between Fe³⁺/AA, Fe²⁺/AA and Cu²⁺/AA (Scheme 1). In order to investigate the fluorescence quenching mechanism of DBAB toward Fe³⁺, Fe²⁺ and Cu²⁺, Job plot measurements and ¹H NMR were applied to the structure of the chelate of DBAB and Fe³⁺/Fe²⁺/Cu²⁺. The DBAB Schiff base molecule shows great potential to serve as a Fe³⁺, Fe²⁺ and Cu²⁺ sensor.

Scheme 1 DBAB detection pathway for Fe³⁺, Fe²⁺ and Cu²⁺.

2. Results and discussion

2.1 The synthesis of DBAB

The fluorescent probe DBAB was facilely synthesized via 4-(diethylamino)benzaldehyde with 2-aminobenzoic acid in an ethanol and acetic acid mixed solution at 60 °C as shown in Scheme 2, and its structure was then confirmed via¹H NMR, ¹³C NMR, and MS measurements (Fig. S1–S3†).

Scheme 2 Synthesis of DBAB.

2.2 Detection of metal ions

The selectivity of DBAB toward various metal ions, such as Fe³⁺, Fe²⁺, Cu²⁺, Cd²⁺, Cr³⁺, Li²⁺, Mn²⁺, Na⁺, K⁺, Mg²⁺, Zn²⁺, Ba²⁺, Ca²⁺, and Al³⁺, was examined in DMF solvent. As observed by the naked eye, it revealed a very distinctive selectivity towards Fe³⁺, Fe²⁺ and Cu²⁺ ions. When the probe was treated with a series of individual metal ions in DMF, Cu²⁺ showed an obvious colour change from colorless to yellow, Fe³⁺ and Fe²⁺ showed an obvious colour change from colorless to light brown. In contrast, the other metal ions did not show any significant colour change as observed by the naked eye (Fig. 1a). Correspondingly, Fig. 1b shows the photograph of fluorescence change from purple to colorless under UV-light in 1.7 μM DBAB solution with different metal ions. Then, to study the fluorescence properties of DBAB towards metal ions, the emission changes were measured in DMF at room temperature by using only 1.7 uM DBAB with Fe³⁺, Fe²⁺, Cu²⁺ and other metal ions (including Cd²⁺, Cr³⁺, Li²⁺, Mn²⁺, Na⁺, K⁺, Mg²⁺, Zn²⁺, Ba²⁺, Ca²⁺, and Al³⁺). As shown in Fig. 1c, 1.7 μM DBAB solution showed a strong purple emission signal (λ_em = 393 nm) at an excitation wavelength of 341 nm, and the addition of 353 equiv. of Cd²⁺, Cr³⁺, Li²⁺, Mn²⁺, Na⁺, K⁺, Mg²⁺, Zn²⁺, Ba²⁺, Ca²⁺, and Al³⁺, and Ce³⁺ resulted in nearly negligible effects on fluorescence emission. However, when equivalent Fe³⁺, Fe²⁺ and Cu²⁺ were added to the solution of DBAB, the fluorescence intensity showed remarkable quenching (∼98% of Fe³⁺, Fe²⁺ and ∼92% of Cu²⁺) due to the chelation-enhanced fluorescence quenching (CHEQ) effect of paramagnetic Fe³⁺, Fe²⁺ and Cu²⁺.¹⁹

Fig. 1 (a) Colour changes and (b) fluorescence changes under UV light irradiation observed after the addition of metal ions (353 equiv.) to DBAB (1.7 μM) solution in DMF. (c) Fluorescence spectra of DBAB (1.7 μM) in the presence of different metal ions (353 equiv.) in DMF.

To estimate the specificity of DBAB detection for Fe³⁺/Fe²⁺/Cu²⁺, competitive experiments were carried out in the presence of Fe³⁺, Fe²⁺, and Cu²⁺ (353 equiv.) mixed with other competitive metal ions (353 equiv.). Competitive ions were initially added to the detection solution, then Fe³⁺, Fe²⁺, and Cu²⁺ ions were added 10 min later. Changes in fluorescence intensity at 393 nm before and after the addition of Fe³⁺, Fe²⁺, and Cu²⁺ are displayed in Fig. 2. The results showed that other competitive metal ions had no obvious effect on the fluorescence of DBAB besides Fe³⁺, Fe²⁺, and Cu²⁺, which reveals that DBAB is a tri-responsive and highly effective fluorescent probe for the detection of Fe³⁺, Fe²⁺, and Cu²⁺ ions over other common metal ions.

Fig. 2 Selectivity of DBAB (1.7 μM) toward (a) Fe³⁺, (b) Fe²⁺ and (c) Cu²⁺ (353 equiv.) and other competing metal ions (353 equiv.). (1), Fe³⁺ (a)/Fe²⁺ (b)/Cu²⁺ (c); (2), Cd²⁺; (3), Cr³⁺; (4), Li²⁺; (5), Mn²⁺; (6), Na⁺; (7), K⁺; (8), Mg²⁺; (9), Zn²⁺; (10), Ba²⁺; (11), Ca²⁺; (12), Al³⁺.

Then, the time dependent fluorescence intensity of DBAB was measured in the presence of Fe³⁺, Fe²⁺, and Cu²⁺ ions, respectively (Fig. 3). After addition of Fe³⁺/Fe²⁺/Cu²⁺ (353 equiv.), the fluorescence intensity of the DBAB solution at 393 nm quenched instantaneously in 2 seconds owing to the formation of a DBAB complex with Fe³⁺/Fe²⁺/Cu²⁺ metal ions, which indicates that the DBAB Schiff base can be used as an instantaneous fluorescence probe for the detection of Fe³⁺/Fe²⁺/Cu²⁺. This fluorescence quenching phenomenon may be attributed to strong CHEQ of DBAB–Fe³⁺/Fe²⁺/Cu²⁺. The short response time demonstrates that DBAB possesses good responsiveness to Fe³⁺, Fe²⁺, and Cu²⁺, which can be further used for wide practical applications.

Fig. 3 The response time of DBAB–Fe³⁺, DBAB–Fe²⁺ and DBAB–Cu²⁺.

To quantify the quenching effect between DBAB and Fe³⁺, Fe²⁺, and Cu²⁺, the sensing capability of DBAB towards Fe³⁺, Fe²⁺, and Cu²⁺ was further investigated in detail by fluorescence titration analysis. As shown in Fig. 4, as Fe³⁺, Fe²⁺, and Cu²⁺ ions were gradually added to the DBAB solution, the fluorescence intensity (at 393 nm) of the chemosensor gradually decreased and reached a plateau after addition of 1912 equiv. of Fe³⁺, 5882 equiv. of Fe²⁺ and 5294 equiv. of Cu²⁺. There was good linear correlation between the metal ion concentrations and fluorescence intensity in the range 0–0.03 mM (Fig. 4a, d and f), which demonstrated that Fe³⁺, Fe²⁺, and Cu²⁺ could be quantitatively detected by DBAB. The limits of detection (LOD) were calculated as 2.17 × 10⁻⁶ M for Fe³⁺, 2.06 × 10⁻⁶ M for Fe²⁺ and 2.48 × 10⁻⁶ M for Cu²⁺ using the equation LOD = 3 × S_b/S (eqn (S1)†),²⁰ which are far lower than the maximum contaminant levels (MCL) for iron (5.4 M) and copper (20 M) in drinking water based on the EPA guidelines.

Fig. 4 Fluorescence emission spectra of 1.7 μM DBAB in the presence of different concentrations of (a) Fe³⁺, (c) Fe²⁺ and (e) Cu²⁺ in DMF, respectively (λ_ex = 341 nm). The plot of fluorescence intensity of DBAB (1.7 μM) at 393 nm versus concentration of (b) Fe³⁺, (d) Fe²⁺ and (f) Cu²⁺ in DMF, respectively, the inset shows the linear range of the curve.

2.3 Distinguishing Fe³⁺, Fe²⁺ and Cu²⁺ by ascorbic acid reduction

In order to distinguish Fe³⁺, Fe²⁺ and Cu²⁺, we used ascorbic acid (AA) as a reducing agent. It is well known that AA is liable to be oxidized in air.²¹ Fe²⁺ ions were easily obtained via the oxidation–reduction reaction between AA and Fe³⁺ ions, while the reaction of Fe²⁺ and Cu²⁺ with ascorbic acid did not show such a phenomenon. With the addition of the reducing agent AA, Fe³⁺, Fe²⁺ and Cu²⁺ can be well distinguished (Fig. 5). In the experiment, an excess amount of AA (1470 equiv.) was introduced into the testing solution (1.7 μM DBAB and 353 equiv. of Fe³⁺, Fe²⁺, and Cu²⁺); for DBAB–Fe³⁺, the quenched fluorescence increased again gradually, and then reached the maximum fluorescence recovery after 90 min (Fig. 5a and 4e). Contrarily, for DBAB–Fe²⁺, AA had little effect (Fig. 5b). These different phenomena can be attributed to the different reducing abilities of AA toward Fe³⁺ and Fe²⁺. For DBAB–Cu²⁺, the quenched fluorescence increased but only can reach half of the maximum fluorescence recovery value (Fig. 5c). Color changes were also observed under UV light upon the addition of AA, showing that the purple fluorescence remained in the solution containing Fe³⁺, while the solution containing Fe²⁺ remained colorless (Fig. 5f). These results solidify that DBAB can further distinguish Fe³⁺, Fe²⁺ and Cu²⁺ effectively by the reduction method of AA.

Fig. 5 (a) Fluorescence intensity changes of the probe DBAB (1.7 μM) in DMF solution after the addition of 353 equiv. of Fe³⁺ and an excess amount of AA (1470 equiv.). (b) and (c) Under the same conditions, fluorescence intensity changes of the probe DBAB solution after the addition of 353 equiv. of Fe²⁺ and Cu²⁺ with 1470 equiv. of AA. (d) Fluorescence intensity changes of the probe DBAB (1.7 μM) in DMF solution after the addition of 353 equiv. of Fe³⁺ and different amounts of AA. (e) Time dependence of fluorescence intensity of DBAB containing Fe³⁺, Fe³⁺ and Cu²⁺ after the addition of 1470 equiv. of AA. (f) Colour changes under UV light irradiation observed for DBAB solution containing Fe³⁺ (353 equiv.), Fe²⁺ (353 equiv.) and Cu²⁺ (353 equiv.) with the addition of 1470 equiv. of AA after 90 min.

2.4 Mechanism of DBAB sensing of Fe³⁺, Fe²⁺ and Cu²⁺

In order to determine the binding stoichiometry between DBAB and responding metal ions (Fe³⁺, Fe²⁺, and Cu²⁺), Job's method for fluorescence titration was carried out (Fig. 6). The inflection point was fixed at 0.5, indicating a 1 : 1 ratio for the DBAB and Fe³⁺/Fe²⁺/Cu²⁺ complex. Based on the Job's analysis method, the association constants (K_a) were calculated to be 3.82 × 10⁵ M⁻¹ for DBAB–Fe³⁺, 3.61 × 10⁵ M⁻¹ for DBAB–Fe²⁺, and 3.93 × 10⁵ M⁻¹ for DBAB–Cu²⁺ from nonlinear curve fitting of the fluorescence titration data (Fig. 4b, d and f), according to the Benesi–Hildebrand equation (eqn (S2)†).

Fig. 6 Job plot obtained for DBAB and Fe³⁺, Fe²⁺ and Cu²⁺ ions. The total concentration of DBAB and cations was fixed at 20 μM. The fluorescence emission was measured at 393 nm.

2.5 ¹H NMR titration

In order to validate the proposed recognition mechanism, ¹H NMR titration was performed by adding incremental amounts of Fe³⁺, Fe²⁺ and Cu²⁺ ions (in D₂O) to a solution of DBAB in DMSO-d₆. Upon addition of Fe³⁺, Fe²⁺ and Cu²⁺ ions, the proton signals became broad due to the paramagnetic nature of the complexed Fe³⁺, Fe²⁺ and Cu²⁺ ions.

As shown in Fig. 7, the –O=C–OH proton signal of DBAB at 8.67 ppm faded away, the –CH=N– proton signal at 8.04 ppm shifted downfield to 9.63 ppm. At the same time proton signals at 7.75 ppm, 7.34 ppm, and 6.87 ppm shifted upfield to 7.66 ppm, 7.21 ppm, and 6.74 ppm, respectively. These spectra exhibit the binding nature with Fe³⁺ ions in a 1 : 1 stoichiometric ratio, they also indicate that the oxygen in the hydroxyl moiety and the nitrogen in the imine moiety participate in complexation with Fe³⁺ ions. The proton signal shifts of DBAB/Fe²⁺ and DBAB/Cu²⁺ complexes are the same as those of DBAB/Fe²⁺ (Fig. 7b and c).

Fig. 7 ¹H NMR spectra of DBAB in DMSO-d₆ upon titration with 0.2 equiv., 0.6 equiv., 1.0 equiv., and 1.1 equiv. of (a) Fe³⁺, (b) Fe²⁺ and (c) Cu²⁺ ions.

3. Experimental

3.1 Reagents and chemicals

All chemicals for the synthesis of DBAB were ordered from Sigma-Aldrich and used as received. The chlorinated salts (Fe³⁺, Fe²⁺, Cu²⁺, Cd²⁺, Cr³⁺, Li²⁺, Mn²⁺, Na⁺, K⁺, Mg²⁺, Zn²⁺, Ba²⁺, Ca²⁺, and Al³⁺) were purchased from Sinopharm chemical reagent company (Shanghai, China) and used without further purification. Dimethyl formamide (DMF) was purchased from Sigma-Aldrich (chromatographically pure) and used without further purification. All aqueous solutions were prepared with pure water from a Millipore Auto pure WR600A system (Millipore Ltd. USA).

UV-vis spectra were recorded on a Shimadzu UV-2500 spectrophotometer using a 10 mm path length quartz cuvette. ¹H and ¹³C Nuclear Magnetic Resonance (NMR) spectra were recorded on a Varian 600 MHz NMR spectrometer using DMSO-d₆. ¹H NMR titrations were carried out by dissolving DBAB in DMSO-d₆ and the metal salts Fe³⁺, Fe²⁺, and Cu²⁺ in D₂O, respectively, at room temperature and the chemical shifts are reported in values (ppm) relative to TMS. Electrospray ionization mass spectra (ESI) were recorded on a Waters mass spectrometer using a mixed solvent of HPLC methanol and triple distilled water.

3.2 Synthesis and characterization of a fluorescent Schiff base ((E)-2-((4-(diethylamino)benzylidene)amino)benzoic acid, DBAB)

4-(Diethylamino)benzaldehyde (2.00 g, 11.28 mmol) was dissolved in dehydrated ethanol (130 mL) in a 250 mL round-bottom flask. Then 2-aminobenzoic acid (1.55 g, 17.06 mmol) was dissolved in a small amount of N,N-dimethylformamide (DMF). The DMF solution and two drops of acetic acid were added to the flask. The mixture was stirred at 50 °C for 5 h. It was then cooled to room temperature before being filtered. The solid product was washed several times using ethanol and hexane. No further purification was needed. Yield: 53%. ¹H NMR (600 MHz, DMSO-d₆), δ (ppm): δ 8.64 (s, 1H), 8.04–8.02 (d, 1H), 7.72–7.61 (m, 5H), 7.32–7.30 (t, 1H), 7.21–7.18 (t, 1H), 6.82–6.81 (d, 2H), 6.72–6.71 (d, 1H), 6.49–6.46 (t, 1H), 3.45–3.41 (q, 4H), 1.12–1.10 (t, 6H). ESI-MS: m/z: 297.15 ([M + H]⁺ calcd 297.15).

3.3 Chromogenic detection of iron and copper ions

3.3.1 Fluorescence spectroscopic measurement. DBAB was dissolved in DMF for a stock solution of 1.7 mM and diluted to 1.7 μM in DMF for absorbance and fluorescence analysis. The chlorinated salts (Fe³⁺, Fe²⁺, Cu²⁺, Cd²⁺, Cr³⁺, Li²⁺, Mn²⁺, Na⁺, K⁺, Mg²⁺, Zn²⁺, Ba²⁺, Ca²⁺, and Al³⁺) were prepared in deionized water at 60 mM. Test solutions were prepared by placing 2 mL of DBAB solution and 0.02 mL of different chlorinated salt solutions in a test tube, respectively. The resulting solution was mixed well. Fluorescence spectra were recorded at room temperature on a Hitachi spectrophotometer F-7000 at an excitation wavelength of 341 nm, slit 2.5/2.5 nm.

3.3.2 Job plot measurements. DBAB was dissolved in DMF–deionized water solution (1 : 1, v/v) for a stock solution of 1.7 mM. The chlorinated salts (Fe³⁺, Fe²⁺ and Cu²⁺) were prepared in DMF–deionized water solution (1 : 1, v/v) of 6 mM.

For Fe³⁺, 0.142 mL of the DBAB solution were added to 11.40 mL of DMF–deionized water solution (1 : 1, v/v) to obtain a concentration of 20 μM. 0.04 mL of the Fe³⁺ solution were added to 11.96 mL of DMF–deionized water solution (1 : 1, v/v) to obtain a concentration of 20 μM. 2.0, 1.8, 1.6, 1.4, 1.2, 1.0, 0.8, 0.6, 0.4, 0.2 and 0 mL of the DBAB solution were taken and transferred to vials. 0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8 and 2.0 mL of the Fe³⁺ solution were added to each DBAB solution separately. After shaking the vials for a few seconds, the fluorescence spectra were recorded at room temperature at an excitation wavelength of 341 nm, slit 2.5/2.5 nm. The job plots of Fe²⁺ and Cu²⁺ were measured in the same way.

3.3.3 Competitive experiments. DBAB was dissolved in DMF for a stock solution of 1.7 mM and diluted to 1.7 μM in DMF for absorbance and fluorescence analysis. The chlorinated salts (Fe³⁺, Fe²⁺, Cu²⁺, Cd²⁺, Cr³⁺, Li²⁺, Mn²⁺, Na⁺, K⁺, Mg²⁺, Zn²⁺, Ba²⁺, Ca²⁺, and Al³⁺) were prepared in deionized water at 60 mM. 0.02 mL of the different chlorinated salt solutions (Fe³⁺, Cd²⁺, Cr³⁺, Li²⁺, Mn²⁺, Na⁺, K⁺, Mg²⁺, Zn²⁺, Ba²⁺, Ca²⁺, and Al³⁺) were added to 2 mL of DBAB solution, respectively. Then 0.02 mL of the Fe³⁺ solution were taken and added to the solutions prepared above, separately. The resulting solution was mixed well and the fluorescence spectra were recorded at room temperature at an excitation wavelength of 341 nm, slit 2.5/2.5 nm. For Fe²⁺, 0.02 mL of the different chlorinated salt solutions (Fe²⁺, Cd²⁺, Cr³⁺, Li²⁺, Mn²⁺, Na⁺, K⁺, Mg²⁺, Zn²⁺, Ba²⁺, Ca²⁺, and Al³⁺) were added to 2 mL of DBAB solution, respectively. Then 0.02 mL of the Fe²⁺ solution were taken and added to the solutions prepared above, separately. The resulting solution was mixed well and the fluorescence spectra were recorded at room temperature at an excitation wavelength of 341 nm, slit 2.5/2.5 nm. For Cu²⁺, 0.02 mL of the different chlorinated salt solutions (Cu²⁺, Cd²⁺, Cr³⁺, Li²⁺, Mn²⁺, Na⁺, K⁺, Mg²⁺, Zn²⁺, Ba²⁺, Ca²⁺, and Al³⁺) were added to 2 mL of DBAB solution, respectively. Then 0.02 mL of the Cu²⁺ solution were taken and added to the solutions prepared above, separately. The resulting solution was mixed well and the fluorescence spectra were recorded at room temperature at an excitation wavelength of 341 nm, slit 2.5/2.5 nm.

3.3.4 Distinguishing Fe³⁺, Fe²⁺ and Cu²⁺ by ascorbic acid reduction. In the experiment, an excess amount of ascorbic acid (1470 equiv.) was introduced into the testing solution (1.7 μM DBAB and 353 equiv. of Fe³⁺, Fe²⁺, and Cu²⁺) which caused the quenched fluorescence to increase again gradually, and the fluorescence reached the maximum fluorescence recovery and half of the maximum, respectively, after 90 min. Contrarily, ascorbic acid had little effect on DBAB/Fe²⁺ solution. These different phenomena can be ascribed to the different reducing abilities of ascorbic acid toward Fe³⁺, Fe²⁺ and Cu²⁺.

3.3.5 Comparative ¹H NMR analysis of DBAB–Fe³⁺/DBAB–Fe²⁺/DBAB–Cu²⁺. To gain insight into the structure and complexation of Fe³⁺, Fe²⁺, and Cu²⁺ with DBAB, ¹H NMR spectroscopy was performed in DMSO-d₆. 0.2, 0.6, 1.0, and 1.1 equiv. of Fe³⁺, Fe²⁺, and Cu²⁺ in D₂O solution were added to DBAB DMSO-d₆ solution. The ¹H NMR spectra were recorded by scanning 24 times.

4. Conclusions

In summary, a new approach using DBAB as a fluorescence quenching probe has been proposed to determine Fe³⁺, Fe²⁺ and Cu²⁺. This tri-responsive fluorescent probe for Fe³⁺, Fe²⁺ and Cu²⁺ can be distinguished by further reduction with ascorbic acid. The detection limits were 2.17 × 10⁻⁶ M for Fe³⁺, 2.06 × 10⁻⁶ M for Fe²⁺ and 2.48 × 10⁻⁶ M for Cu²⁺, respectively.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 20474005), the General Program of Science and Technology Development Project of Beijing Municipal Education Commission (No. KM201310028007), and Scientific Research Starting Foundation for the Returned Overseas Beijing Scholars (Grant No. 009125403700).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ay02526f

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