A self-assembled triphenylamine-based fluorescent chemosensor for selective detection of Fe³⁺ and Cu²⁺ ions in aqueous solution

Abstract

A novel triphenylamine-based fluorescent sensor tris((4-amino)phenylduryl)amine (m-TAPA) for Fe³⁺/Cu²⁺ ion has been developed. m-TAPA shows high selectivity and sensitivity toward Fe³⁺/Cu²⁺ over alkali and transition metal ions in aqueous solution. The possible mechanism of fluorescence quenching was that Fe³⁺/Cu²⁺ can be captured by the NH₂ groups of m-TAPA to form non-fluorescent complexes, resulting in strong quenching. The detection limits of Fe³⁺ and Cu²⁺ were calculated to be 230 nM and 620 nM, respectively. Furthermore, fluorescent test strips have been prepared for convenient detection of Fe³⁺ and Cu²⁺ ions in environmental water samples, even in drinking water.

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1. Introduction

The detection of metal ions is very important for analytical, environmental and biomedical applications due to their deleterious effects on human health and ecosystems.^1–3 Up to now, numerous methods have been developed for the detection of metal ions, such as atomic absorption spectroscopy,⁴ colorimetric methods,⁵ mass spectrometry,⁶ electrochemical methods^7,8 and fluorescence spectroscopic analysis.⁹ Among these methods, fluorescence detection has attracted the most attention due to its ease of operation, high sensitivity and efficiency. Therefore, the design of fluorescent sensors for metal ions have attracted increasing attentions.

The removal of trace amounts of transition metal ions in all types of water sources is an important factor in monitoring environmental pollution. Furthermore, identifying metal-contaminated sewage and fertilizer is useful in limiting human exposure to such harmful chemicals. Copper is a major trace metal in the environment due to its extensive use in electrical and electronic industry, and poses a serious environmental threat at high levels due to its toxicity.^10–13 The high level of copper causes neurodegenerative diseases such as Alzheimer's, Parkinson's and is also suspected to cause amyloidal precipitation and toxicity.^14–17 According to the U.S. Environmental Protection Agency (EPA), the maximum acceptable level of Cu²⁺ in drinking water is ∼20 μM.¹⁸ Iron (Fe) also is an important limiting trace metal nutrient in natural water, as it limits the growth of phytoplankton and biomass production in rivers and lakes. High quantities (200 μg L⁻¹ to 1000 μg L⁻¹) of Fe³⁺ ion in drinking water, can devastate the central nervous system, kidney, liver, skin, lungs, and bones.^19–21 Thus, there is considerable interest in developing fluorescent sensors for the detection of Cu²⁺ and Fe³⁺ ions have been catching considerable attention in the human health and environmental science.^22,23

A number of fluorescent Cu²⁺ (ref. 24–28) and Fe³⁺ (ref. 29–34) sensors have been reported, and some of them have been successfully applied both in biological and in environmental samples. In addition, single probes for multiple targets have been actively developed because of the advantages such as potential cost and analytical time reduction. For example, they include Cr/Al,³⁵ Cu/Hg,³⁶ Cu/Zn,³⁷ Zn/Cd,³⁸ Ag/Mn,³⁹ Al/Fe³⁺,⁴⁰ Cr/Fe³⁺ (ref. 41) and Zn/Al.⁴² However, single Self-assembled chemosensors for Cu²⁺/Fe³⁺ were reported very rare.^43,44

Herein, m-TAPA was synthesized by the Suzuki–Miyaura coupling reaction⁴⁵ for multiple analytes, which can detect Cu²⁺/Fe³⁺ ions selectively in presence of other metal ions. This is first time we reported the fluorescent sensor m-TAPA detect Cu²⁺/Fe³⁺ ions in aqueous solution at nanomolar level.

2. Experimental

Materials

Tris(4-bromophenyl)amine (TBPA, 98%) was purchased from Energy Chemical Company. 3-Aminophenylboronic acid monohydrate (98%), was purchased from Sukailuchem Company. Terakis(triphenylphosphine)palladium(0) [Pd(PPh₃)₄] (99.8%) were purchased from Aladdin Company. Nitrogen with a purity of 99.99% was provided from commercial source. Other reagents, such as potassium carbonate, acetonitrile, methanol, tetrahydrofuran (THF), ethyl acetate, dichloromethane (DCM) were A.R. grade. Al(NO₃)₃·9H₂O, Mg(NO₃)₂·6H₂O, Zn(CH₃COO)₂, FeCl₂·4H₂O, SnCl₂·2H₂O, Fe₂(SO₄)₃·4H₂O, SbCl₃, K₂(SO₄), Zr(NO₃)₄·5H₂O, Ag(CH₃COO), InCl₃·4H₂O, NiCl₂·6H₂O, La(NO₃)₃·nH₂O, LiBr·H₂O, CuCl₂·2H₂O and CdNO₃·4H₂O were purchased from Aladdin Ltd (Shanghai, China). All the other chemicals were analytical grade and used as received. The aqueous solutions were prepared with twice-distilled water in the whole experiments.

Characterization

¹H NMR, ¹³C NMR were recorded on a Brucker AM 400, 100 MHz spectrometer at 25 °C. Mass spectra were recorded on a HP5989B mass spectrometer. Mass spectra were recorded on a HP5989B mass spectrometer. Fourier transform infrared (FT-IR) spectra were recorded on a DIGIL FTS3000 spectrophotometer using KBr tablets. UV spectra were measured on a TU-1901 spectrophotometer. Fluorescence spectra in solution were measured using a PE LS-55 Luminescence/Fluorescence Spectrophotometer (1%, E_x slide: 4 nm, E_m slide: 6 nm, excitation: 360 nm). The morphology of TMCA was observed by scanning electron microscopy (SEM, ZEISS ULTRA PLUS).

Synthetic

The synthesis of m-TAPA is shown in Scheme 1. Synthesis of m-TAPA: 76% yield as slight yellow solid; R_f = 0.35 (petroleum ether : ethyl acetate = 2 : 1); Mp 250–252 °C; ¹H NMR (400 MHz, CDCl₃): δ = 7.47 [d, 2H, ArH], δ = 7.21–7.19 [d, 3H, J = 6 Hz, ArH], 7.00–6.98 [d, 1H, J = 6 Hz, ArH], 6.90 [s, 1H, ArH], 6.67–6.65 [s, 1H, J = 6 Hz ArH], 3.73 [s, 2H, NH]; ¹³C NMR (DMSO-d₆): 149.07, 140.28, 135.60, 131.36, 129.36, 127.50, 123.99, 114.02, 112.88, 111.75; IR: 696, 781, 1284, 1319, 1485, 1599, 3013, 3358, 3442 cm⁻¹; TOF MS ES⁺: 518.25 (M + 1)⁺; elemental analysis: calcd for C₃₆H₃₀N₄: C 83.37; H 5.83; N 10.80; Found: C 83.20%; H 5.85%; 10.69%.

Scheme 1 Synthetic route to m-TAPA.

Fluorescence measurements Fe³⁺ and Cu²⁺

A fixed concentration of m-TAPA was transferred to a fluorescent curette. The fluorescent intensity of the solution was recorded from 330 to 560 nm with excitation wavelength fixed at 360 nm. After appropriate amount of Fe³⁺/Cu²⁺ ions was titrated, the fluorescent intensity of the solution was again recorded. Similar procedure was performed for other metal ions. For the sake of comparison, the volume of m-TAPA solution was fixed to be 2 mL before the addition of Fe³⁺/Cu²⁺. All measurements were made at room temperature.

Principles of fluorescence quenching

Fluorescence quenching usually originated from collisional or dynamic quenching. Dynamic quenching can be described by the following Stern–Volmer equation.⁴⁶

F₀/F = τ₀/τ = 1 + K_qτ₀[Q] (1)

where F₀ and F are the fluorescence intensities before and after the addition of the quencher, respectively. K_q is the rate constant of dynamic (collisional) quenching; τ₀ is the lifetime of the fluorophore in the absence of the quenchers; τ the lifetime in the presence of quenchers, and [Q] is the quencher concentration in solution.

Another type of quenching (static quenching) occurs as a result of the formation of a non-fluorescent complex between the fluorophore and quencher. For this type of quenching, the decrease of fluorescence intensity has the same form as the Stern–Volmer equation above. However, in eqn (2),

F₀/F = 1 + K_SV[Q] (2)

the K_SV is now the association constant K_S. Since the lifetime of the fluorophore is unperturbed by the static quenching, τ₀/τ = 1, lifetime measurements are a definitive method to distinguish between static and dynamic quenching.⁴⁶

Selectivity and interference measurements

The selectivity of m-TAPA was examined by the interfered metal ions such as Mg²⁺, Zn²⁺, K⁺, Ag⁺, Ni³⁺, La³⁺, Li⁺, Cd⁺, Sn²⁺, In³⁺, Fe²⁺, Zr⁴⁺ and Al³⁺ under the identical conditions. The concentrations of Fe³⁺ (5 equiv.), Cu²⁺ (10 equiv.) and other metal ions were the same concentration. Meanwhile, for studying the interference, the m-TAPA was mixed with Fe³⁺/Cu²⁺ in the absence or presence of the interferent.

3. Result and discussion

The selective detection of environmentally active metal ions is investigated by visual, optical, fluorescence spectroscopy method. The m-TAPA was prepared in 5 × 10⁻⁵ M concentration in CH₃CN and all metal ions were prepared in 5 × 10⁻⁵ M concentration in H₂O. m-TAPA was treated with various metal ions like Mg²⁺, Zn²⁺, K⁺, Ag⁺, Ni³⁺, La³⁺, Li⁺, Cd⁺, Sn²⁺, In³⁺, Fe²⁺, Zr⁴⁺, Al³⁺, Cu²⁺ and Fe³⁺ to study the sensitivity and selectivity towards particular metal ions over other metal ions. For the addition of 200 μL of all metal ions into m-TAPA, the presence of Cu²⁺ ion shows colorimetric turn-off response from colorless to brown for m-TAPA (Fig. 1a), which could be easily distinguished by ‘naked-eye’. The other metal ions like Mg²⁺, Zn²⁺, K⁺, Ag⁺, Ni³⁺, La³⁺, Li⁺, Cd⁺, Sn²⁺, In³⁺, Fe²⁺, Zr⁴⁺, Al³⁺, Sb³⁺ and Fe³⁺ with m-TAPA did not show any color change. Therefore Cu²⁺ ion could be easily identified among all other metal ions under visible light. In order to determine the amount of Fe³⁺ ion required, the color change photographs for Fe³⁺and the other metal ions under illumination with a 365 nm UV lamp as shown in Fig. 1b, it is clear that fluorescence intensity quenching of m-TAPA by Cu²⁺ and Fe³⁺ ion and Fe³⁺ with m-TAPA did not show any color change indicating its effectiveness to detect Fe³⁺ over other metal ions.

Fig. 1 Color (a) and fluorescence (b) changes of m-TAPA (5 × 10⁻⁵ M solution in CH₃CN, 2 mL) after the addition of 200 μL of respective metal ions (5 × 10⁻⁵ M solution in H₂O).

Selectivity is a very important factor to estimate the performance of a new fluorescent sensor. The UV-vis spectra of m-TAPA shows strong absorption band at 340 nm. Fig. 2 shows the absorption spectrum of m-TAPA with metal ions in aqueous medium. With the addition of Fe³⁺/Cu²⁺ into sensor m-TAPA, the intensities of the bands 340 nm (m-TAPA) have been reduced. However, the other metal ions like Mg²⁺, Zn²⁺, K⁺, Ag⁺, Ni³⁺, La³⁺, Li⁺, Cd⁺, Sn²⁺, In³⁺, Fe²⁺, Zr⁴⁺, Al³⁺ and Sb³⁺ did not show any optical changes with m-TAPA. From this, it is clear that m-TAPA can detect Fe³⁺and Cu²⁺ selectively in presence of other metal ions.

Fig. 2 UV-vis spectra of sensor m-TAPA (5 × 10⁻⁵ M, in CH₃CN) upon titration with aqueous solution of metal ions (S = m-TAPA, S + Mg²⁺, S + Zn²⁺, S + K⁺, S + Ag⁺, S + Ni³⁺, S + La³⁺, S + Li⁺, S + Cd⁺, S + Sn²⁺, S + In³⁺, S + Fe²⁺, S + Zr⁴⁺, S + Al³⁺, S + Sb³⁺, S + Cu²⁺ and S + Fe³⁺).

Highly selective detection of Fe³⁺and Cu²⁺ ions over other potentially competing species is a necessity. The fluorescence sensing selectivity of m-TAPA for metal ions was examined. Under the same condition as used above for Fe³⁺and Cu²⁺, we tested the fluorescence responses of m-TAPA toward 16 kinds of metal ions such as Mg²⁺, Zn²⁺, K⁺, Ag⁺, Ni³⁺, La³⁺, Li⁺, Cd⁺, Sn²⁺, In³⁺, Fe²⁺, Zr⁴⁺, Al³⁺, Sb³⁺, Cu²⁺ and Fe³⁺. Metal ions were added to CH₃CN solutions of the m-TAPA (5 × 10⁻⁵ M), and the emission of the m-TAPA was measured immediately after the addition of metal ions. As shown in Fig. 3, among the metal ions studied, a clear fluorescence quenching is observed upon the addition of 200 μL of Cu²⁺ and Fe³⁺ions into m-TAPA. The quenching efficiency of m-TAPA toward Fe³⁺and Cu²⁺ were found to be 99% and 96% respectively (Fig. 4). So, we further investigated the m-TAPA sensing behavior to two kinds of metal ions in aqueous solution.

Fig. 3 Fluorescence spectra of m-TAPA (5 × 10⁻⁵ M, in CH₃CN) upon titration with aqueous solution of cations.

Fig. 4 The relative fluorescence quenching degree of m-TAPA at 425 nm with various metal ions (200 μL).

The fluorescence titration for m-TAPA with Fe³⁺ and Cu²⁺ ions reveals that fluorescence emission intensity rapidly died down upon addition of increasing amounts of Fe³⁺/Cu²⁺ solution at 425 nm (Fig. 5a and b). Furthermore, the emissive property study disclosed that the fluorescence quantum yields of m-TAPA, m-TAPA–Fe³⁺ and m-TAPA–Cu²⁺complexes are 41%, 0.6% and 1.5%, respectively.⁴⁷ The gradual addition of Fe³⁺/Cu²⁺ into m-TAPA, the fluorescence band at 425 nm shifted (red Shift) to ∼462 nm and ∼431 nm, respectively. The result revealed the coordination ability of Fe³⁺ ion with the amine better than Cu²⁺ ion. Interestingly, a new emission band is generated at ∼362 nm which increases with Fe³⁺/Cu²⁺ concentration.⁴⁸ This result also indicate that Fe³⁺/Cu²⁺ complex of m-TAPA is formed. A plot of fluorescence intensity depending on the concentration of Fe³⁺/Cu²⁺ in the range from 0 to 10 equiv. as shown in Fig. 5c and d.

Fig. 5 Fluorescence spectrum of m-TAPA upon titration with aqueous solution of Fe³⁺ (a) and Cu²⁺ (b). A plot of changes of fluorescence intensity upon addition of Fe³⁺ (c) and Cu²⁺ (d) at 425 nm.

As shown in Fig. 6a and b, free m-TAPA shows two absorption bands centered at 275 nm (band A) and 350 nm (band B), which can be assigned respectively as a π–π* transition and an intramolecular charge transfer (ICT) band. Upon Fe³⁺/Cu²⁺ addition (5 equiv.), band A increased gradually and obvious blue shift, while broad band B underwent a decrease and obvious red shift, which can be ascribed to the decrease of electron-donating ability induced by Fe³⁺/Cu²⁺ coordination.⁴⁹ In addition, a new band starts to appear (∼400 nm) at the red side of the absorption spectrum of the free ligand and the absorbances increase with increasing gradually concentration of Fe³⁺ and Cu²⁺ ions, which indicated the different interaction pattern between partners of m-TAPA and Fe³⁺/Cu²⁺.⁵⁰ A plot of absorbance depending on the concentration of Fe³⁺ and Cu²⁺ ions in the range from 0 to 5 equiv. as shown in Fig. 6c and d.

Fig. 6 UV-vis spectrum of m-TAPA (5 × 10⁻⁵ M, in CH₃CN) upon titration with aqueous solution of Fe³⁺ (a) and Cu²⁺ (b). Changes of UV absorbance upon addition of Fe³⁺ (c) and Cu²⁺ (d) at 337.5 nm.

To check further the practical applicability of m-TAPA as Fe³⁺/Cu²⁺ selective fluorescent sensor, the competitive experiments were performed in the presence of various metal ions (Fig. 7 and 8). When m-TAPA was treated with 5 equiv. of Fe³⁺ and 10 equiv. of Cu²⁺ in the presence of the same concentration of other metal ions (Mg²⁺, Zn²⁺, K⁺, Ag⁺, Ni³⁺, La³⁺, Li⁺, Cd⁺, Sn²⁺, In³⁺, Fe²⁺, Zr⁴⁺ and Al³⁺), only Ag⁺ ion inhibited about 55% of the interaction between m-TAPA and Fe³⁺ ion; K⁺ ion inhibited about 70% of the interaction between m-TAPA and Cu²⁺ ion. This result is an added evidence for the high stability of the Fe³⁺/Cu²⁺ ion sensing, even in presence of other metal ions without any interference. Therefore, m-TAPA can be used as a selective fluorescent probe for Fe³⁺ and Cu²⁺ ions in practical environmental application.

Fig. 7 Competitive selectivity of m-TAPA (1) toward Fe³⁺ in the presence of other metal ions (5 equiv.) with an emission of 425 nm.

Fig. 8 Competitive selectivity of m-TAPA (1) toward Cu²⁺ in the presence of other metal ions (10 equiv.) with an emission of 425 nm.

Fig. 9 shows the Stern–Volmer analysis of the quenching experiment (F₀ − F/F versus [Fe³⁺/Cu²⁺]). It is interesting to note the linear nature of the Stern–Volmer Plot over the Fe³⁺ and Cu²⁺ ions concentration range of 0–20 μM. The K_SV are 4.08 × 10⁴ M⁻¹ and 1.76 × 10⁴ M⁻¹ for Fe³⁺/Cu²⁺ ion, respectively. This phenomenon means the charge-transfer nature between m-TAPA and Fe³⁺/Cu²⁺ ion may be a static mechanism.⁵¹ Based on the results, The detection limit was then calculated with the equation: detection limit = 3σ_bi/m, where σ_bi is the standard deviation of blank measurements and m is the slope of the intensity versus sample concentration. The detection limits of Fe³⁺ and Cu²⁺ ions with m-TAPA were 230 nM and 620 nM respectively, which is much lower than the maximum level (200 μg L⁻¹ to 1000 μg L⁻¹) of Fe³⁺ and (∼20 μM) of Cu²⁺ in drinking water permitted.

Fig. 9 Stern–Volmer plots for sensor m-TAPA using Fe³⁺ (a) and Cu²⁺ (b) as quencher at lower concentration.

pH effects on the fluorescence of probe (m-TAPA–Fe³⁺ and m-TAPA–Cu²⁺) was investigated in CH₃CN. pH of the solution adjusted by adding of universal buffer. As shown in Fig. 10, m-TAPA show weak fluorescence in the pH range of 1.0–3.0, because of NH₂ groups of m-TAPA already had protonated. In pH range 4.0–8.0, the probe exhibited very good fluorescence behavior due to the protonation weaken about NH₂ groups of m-TAPA, but the emission intensity decreases at pH > 8.0 that indicates the m-TAPA–Fe³⁺ and m-TAPA–Cu²⁺ complexes were stable formed at high pH values. This result indicates that the sensor m-TAPA could be used for determination of Fe³⁺ and Cu²⁺ ions under common environmental condition.

Fig. 10 Effect of pH on the Fe³⁺ and Cu²⁺ ion sensing ability by the m-TAPA at 425 nm.

The formation of aggregates of m-TAPA is supported by scanning electron microscopy (SEM) images in CH₃CN, which show the presence of very uniform spherical particles about 200 nm (Fig. 11a). However, in the presence of Fe³⁺ and Cu²⁺ ion, the morphologies are changed into ununiformed size (Fig. 11b and c). These results demonstrate the interactions of m-TAPA with Fe³⁺ and Cu²⁺ ions possibly formed strong complexation led the break down of m-TAPA assembly morphology.

Fig. 11 Scanning electron microscopy (SEM) images of aggregates of compounds m-TAPA (a) in CH₃CN; SEM images of [m-TAPA–Fe³⁺] (b) and [m-TAPA–Cu²⁺] (c).

Fig. 12 shows the interaction of m-TAPA with Fe³⁺ and Cu²⁺ ions investigated by ¹H NMR spectroscopic titrations carried out in CD₃CN/D₂O. In the ¹H NMR titration experiments, with the molar ratio of self-assembled m-TAPA (10 mg) and Fe³⁺/Cu²⁺ ion from 1 : 5/1 : 10, the NMR spectra exhibit fast exchange between the Fe³⁺/Cu²⁺ and m-TAPA. We found significant downfield shifts are observed for the peaks corresponding to the signal of protons of NH₂ groups, which can be recognized the presence of a strong charge transfer interaction between the electron-deficient Fe³⁺/Cu²⁺ and the electron-rich m-TAPA, leading to the formation of complex between Fe³⁺/Cu²⁺ and m-TAPA. Thus we inferred the possible binding mode as described in Scheme 2.

Fig. 12 ¹H NMR spectra of m-TAPA and Fe³⁺/Cu²⁺ in CD₃CN/D₂O = 1 : 1.

Scheme 2 Possible binding mode of probe (m-TAPA) with Fe³⁺ and Cu²⁺ ions.

To investigate the convenient application of sensor m-TAPA, test trips were prepared by immersing Thin Layer Chromatography (TLC) into a CH₃CN solution of m-TAPA (0.1 M). The test strips containing m-TAPA was utilized to sense Fe³⁺ and Cu²⁺ ion. As shown in Fig. 13, when Fe³⁺/Cu²⁺ was added on the test trips respectively, the obvious color change was observed under visible light (Fig. 13b and c). The fluorescence quenching were observed when the test strips are dipped into aqueous solutions of Fe³⁺/Cu²⁺ ion under the 365 nm UV lamp illumination (Fig. 13e and f). So, the test strips could conveniently detect Fe³⁺ and Cu²⁺ ions in aqueous solutions.

Fig. 13 Photographs of m-TAPA-coated test strips under visible light (a–c) and 365 nm UV (d–f) illumination. (a and d) Blank. (b and e) After dipping into solutions of Fe³⁺ in CH₃CN. (c and f) After dipping into solutions of Cu²⁺ in CH₃CN.

We also check the effect of various concentrations of Fe³⁺/Cu²⁺ solution on the fluorescent TCL strip of m-TAPA (Fig. 14) by applying small spots of different concentrations of Fe³⁺ and Cu²⁺ (10 μL) on test strips. The visual fluorescence response of Fe³⁺ (a) and Cu²⁺ (b) at different concentrations by contact mode detection on test strips of m-TAPA as shown in Fig. 14. Dark spots of different strengths can be observed, which show the regulation of the quenching behavior of Fe³⁺ and Cu²⁺ (Fig. 14ii–viii), which is also practically applicable by varying the concentration of the two metal ions even up to 5 × 10⁻¹³ M (Fig. 11viii). However, no visible change is observed by applying blank solvent (CH₃CN) over the fluorescent test trips (Fig. 11i).

Fig. 14 Photograph of the fluorescence quenching of TCPA-coated test strips by Fe³⁺ (a) and Cu²⁺ (b) on contact mode (10 μL of Fe³⁺ and Cu²⁺ with a spot area of ∼0.2 cm²) when viewed under 365 nm UV illumination. (i) Blank, (ii) 5 × 10⁻³ M, (iii) 5 × 10⁻⁵ M, (iv) 5 × 10⁻⁷ M, (v) 5 × 10⁻⁹ M, (vi) 5 × 10⁻¹¹ M, (vii) 15 × 10⁻¹² M, (vii) 5 × 10⁻¹³ M.

4. Conclusions

In summary, we have prepared a simple but effective fluorescent sensor, m-TAPA, for Fe³⁺/Cu²⁺. The sensor is highly selective and hardly interfered by other metal ions with the detection limit of Fe³⁺ and Cu²⁺ ions were 230 nM and 620 nM respectively. The K_SV value of Fe³⁺ and Cu²⁺ were calculated as 4.08 × 10⁴ M⁻¹ and 1.76 × 10⁴ M⁻¹ respectively. These results indicate that m-TAPA could meet the selective requirements for environmental application and can be sensitive enough to detect Fe³⁺/Cu²⁺ ion in environmental water samples, even in drinking water. Additionally, m-TAPA-coated TCL strips serve as a convenient, low-cost method for detection of Fe³⁺/Cu²⁺ ion at nanomolar range.

Acknowledgements

This work was supported the National Natural Science Foundation of China (no. 21202133, 21174114, 21361023). We also thank Key Laboratory of Eco-Environment-Related Polymer Materials (Northwest Normal University), the Ministry of Education Scholars Innovation Team (IRT 1177) for financial support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra12025f

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