A novel reversible colorimetric chemosensor for the detection of Cu²⁺ based on a water-soluble polymer containing rhodamine receptor pendants

Abstract

A novel reversible colorimetric chemosensor based on a water-soluble polymer containing rhodamine receptor pendants has been developed. The chemosensor exhibited high sensitivity and selectivity to Cu²⁺ ions with a short response time in pure aqueous solution, and the results could be monitored directly by the naked eye. The detection limit was 3.6 × 10⁻⁷ M with UV-vis absorbance experiment. The absorption intensity of P(HEA-co-RhBBA) for Cu²⁺ remained unaffected over a wide pH range of 4–10. In addition, low-cost test strips were fabricated using P(HEA-co-RhBBA) for convenient and efficient detection of Cu²⁺ ions.

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Introduction

As one of the most important transition metal ions, copper ions play a crucial role in the life of organisms. However, unregulated overloading of copper causes toxicity and can result in severe neurodegenerative diseases such as Alzheimer's, Wilson's and Parkinson's diseases.¹ Therefore, highly sensitive and selective detection of Cu²⁺ ions in water is important for human health and environmental protection. During the last couple of decades, considerable attention has been paid to the design and synthesis of Cu²⁺ ion chemosensors, including colorimetric chemosensors,² fluorescent chemosensors³ and electrochemical sensors.⁴ Among them, colorimetric chemosensors based on color changes are more attractive and convenient since they can be monitored directly by the naked eye and avoid using complex instruments.

Since the first report about the detection of Cu²⁺ ions using rhodamine B derivative in 1997,^3a rhodamine derivatives have been considered as ideal candidates for constructing chemosensors due to their excellent spectroscopic properties, and a great deal of small molecule rhodamine derivatives have been synthesized to detect various metal ions.⁵ However, owing to the poor water solubility of the small molecule rhodamine derivatives with spirolactam ring, organic solvents must be used in the process of metal ions detection, and this partially limits their potential applications in the environmental and biological fields. Furthermore, the design of chemosensors for practical application is still a challenge because of the limitations of small molecules in the fabrication of devices. In contrast to small molecular chemosensors, chemosensors based on polymers can be easily employed to fabricate devices.⁶ Recently, Wu et al.⁷ reported a water-soluble conjugated polymer bearing rhodamine 6G moieties, which showed high selectivity and good reversibility toward Fe³⁺ in pure aqueous solution. Covalent incorporation of ion-sensitive receptors into hydrophilic polymeric backbones paves a way to develop novel water-soluble chemosensors, and avoids the second pollution from the use of organic cosolvents in the detection process of metal ions. However, to the best of our knowledge, the reports about rhodamine-based polymers as chemosensors to real-time monitor of Cu²⁺ ions in pure aqueous solution are quite limited.

Herein, we report a novel, reversible water-soluble polymer chemosensor, P(HEA-co-RhBBA), which contains the pendant group of rhodamine B-based dye as the ion-sensitive receptor. Intriguingly, not only does P(HEA-co-RhBBA) exhibit rapid selective response to Cu²⁺ ions in pure aqueous solution with turn-on color, but also the relative metal complex, P(HEA-co-RhBBA)–Cu²⁺, shows turn-off color with high sensitivity to EDTA ions. In order to detect Cu²⁺ ions conveniently, the test strips were prepared using P(HEA-co-RhBBA).

Experimental

Material and methods

Azobisisobutyronitrile (AIBN, Shanghai Chemical Reagent Co., China) was recrystallized from ethanol. Rhodamine B, hydrazine hydrate, 2,4-dihydroxybenzaldehyde, 2-hydroxyethyl acrylate (HEA), nitrate salts of metal ions (Ba²⁺, Co²⁺, Cd²⁺, Cr³⁺, Fe³⁺, Hg²⁺, Mn²⁺, Na⁺, Pb²⁺, Zn²⁺), chloride salt of Fe²⁺ and all other chemicals were purchased from Shanghai Chemical Reagent Co. Deionized water was used throughout the experiment. Rhodamine B hydrazide,⁸ acryloyl chloride,⁹ and benzyl 1H-imidazole-1-carbodithioate (BICDT)¹⁰ were synthesized according to the references, respectively. The normal filter paper was used to prepare the test strip and purchased from Hangzhou Special Paper Industry Co.

The values of number-average molecular weight (M_n) and polydispersity index (PDI) were determined by means of a waters 150C gel permeation chromatograph (GPC) equipped with 10³, 10⁴, and 10⁵ Å waters ultrastyragel columns, using DMF (1.0 mL min⁻¹) as the eluent, and the calibration was carried out with a polystyrene standard. NMR spectra were recorded at room temperature on a Varian Mercury 400 spectrometer (at 400 MHz for ¹H NMR and 100 MHz for ¹³C NMR) using CDCl₃ as the solvent and TMS as the internal standard. High resolution mass spectra (HRMS) were measured on a Thermo Fisher Scientific LTQ Orbitrap XL. UV-vis absorption spectra were measured using a Shimadzu UV-3600 spectrophotometer. Fluorescent spectra were recorded on a Hitachi F-7000 fluorescence spectrophotometer. The pH values were measured on a Mettler-Toledo FE20 pH meter.

Synthesis of compound RhBHB

Under nitrogen, rhodamine B hydrazide (0.46 g, 1 mmol) and 2,4-dihydroxybenzaldehyde (0.276 g, 2 mmol) were combined in ethanol (20 mL) and refluxed for 8 h. After that, the solution was cooled and allowed to stand at room temperature overnight. The precipitate was filtered and washed 3 times with 10 mL cold ethanol. After drying under reduced pressure, RhBHB was obtained as a light pink powder (0.41 g, yield: 72%). ¹H-NMR (400 MHz, CDCl₃), δ (ppm): 1.14 (t, 12H, J = 7 Hz), 3.32 (q, 8H, J = 7.2 Hz), 6.28 (dd, 2H, J = 8.8 Hz, J = 2.4 Hz), 6.31 (dd, 1H, J = 8.8 Hz, J = 2.4 Hz), 6.35 (d, 1H, J = 2.4 Hz), 6.47 (d, 2H, J = 2.4 Hz), 6.50 (d, 2H, J = 8.8 Hz), 6.96 (d, 1H, J = 8.4 Hz), 7.18 (dd, 1H, J = 6.4 Hz, J = 1.6 Hz), 7.51 (m, 2H), 7.97 (dd, 1H, J = 6.8 Hz, J = 1.6 Hz), 9.19 (s, 1H), 11.02 (s, 2H); ¹³C-NMR (100 MHz, CDCl₃), δ (ppm): 12.6, 44.3, 66.7, 98.1, 103.5, 105.6, 107.2, 108.2, 112.3, 123.2, 124.1, 128.1, 128.5, 130.2, 133.1, 133.3, 149.1, 150.6, 153.6, 154.1, 159.0, 160.7, 164.1; HRMS (ESI) calcd for C₃₅H₃₆N₄O₄ [M + H]⁺ (m/z): 577.2770; found 577.2795.

Synthesis of monomer RhBBA

A solution of RhBHB (0.1 g, 0.17 mmol) in dry CH₂Cl₂ (3 mL) was added triethylamine (21.1 mg, 0.21 mmol) at 0 °C, and the mixture was stirred for 5 min. Then, a solution of acryloyl chloride (15.7 mg, 0.17 mmol) in dry CH₂Cl₂ (3 mL) was added dropwise over 15 min. The reaction mixture was allowed to warm to room temperature, and stirred overnight. After the reaction, the mixture was poured into 15 mL CH₂Cl₂ and washed with water (5 × 25 mL). The organic layer was dried over anhydrous Na₂SO₄, filtered and concentrated. The residue was purified with a column chromatography on silica gel (CH₂Cl₂/MeOH = 10/1) and a light pink powder was collected (85.8 mg, yield: 80%). ¹H-NMR (400 MHz, CDCl₃), δ (ppm): 1.16 (t, 12H, J = 7 Hz), 3.33 (q, 8H, J = 7.2 Hz), 6.00 (dd, 1H, J = 10.4 Hz, J = 0.8 Hz), 6.27 (dd, 2H, J = 8.4 Hz, J = 2.4 Hz), 6.30 (m, 1H), 6.47 (d, 2H, J = 2.8 Hz), 6.50 (d, 2H, J = 8.8 Hz), 6.58 (dd, 1H, J = 17.2 Hz, J = 0.8 Hz), 6.59 (dd, 1H, J = 8.8 Hz, J = 2.4 Hz), 6.64 (d, 1H, J = 2.4 Hz), 7.11 (d, 1H, J = 8.4 Hz), 7.19 (dd, 1H, J = 6.4 Hz, J = 1.6 Hz), 7.53 (m, 2H), 7.98 (dd, 1H, J = 6.8 Hz, J = 1.6 Hz), 9.17 (s, 1H), 11.11 (s, 1H); ¹³C-NMR (100 MHz, CDCl₃), δ (ppm): 12.6, 44.3, 45.8, 66.4, 97.9, 105.2, 108.1, 112.4, 116.7, 123.3, 124.1, 127.7, 128.0, 128.5, 129.9, 132.1, 132.6, 133.4, 149.1, 150.7, 151.8, 152.6, 153.5, 159.7, 164.2; HRMS (ESI) calcd for C₃₈H₃₈N₄O₅ [M + H]⁺ (m/z): 631.2876; found 631.2891.

Synthesis of P(HEA-co-RhBBA)

BICDT (15.6 mg, 66.7 μmol), AIBN (2.7 mg, 16.7 μmol), RhBBA (52.0 mg, 82.5 μmol), HEA (0.948 g, 8.16 mmol) and 2-propanol (1 mL) were placed in a glass tube. The glass tube was sealed under vacuum after degassing with three freeze–evacuate–thaw cycles and then placed in an oil bath at 65 °C. After the polymerization was conducted for 2 h, the reaction mixture was cooled to room temperature and precipitated in ethyl acetate for 3 times. The polymer was collected by filtration and dried in a vacuum oven at room temperature for 24 h to give a slightly pink solid (0.72 g, M_n = 11 500, PDI = 1.22).

Results and discussion

Synthesis

As shown in Scheme 1, a novel polymerizable Cu²⁺ receptor monomer, RhBBA, which was also an example of the rhodamine spirolactam family, was firstly synthesized by the reaction of RhBHB with acryloyl chloride. The structures of RhBHB and RhBBA were characterized by ¹H NMR, ¹³C NMR, and HRMS analyses (Fig. S1–S6, ESI†). To overcome the poor water solubility of the small molecule rhodamine derivatives with spirolactam ring, 2-hydroxyethyl acrylate (HEA) as a neutral hydrophilic chain segment was chosen as comonomer to form the water-soluble polymer matrix. And then, well-defined water-soluble polymers P(HEA-co-RhBBA) were facilely synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization (Scheme 1). RhBBA content in the P(HEA-co-RhBBA) chain was determined to be ∼0.9 mol% from the standard working curve in UV-vis absorption spectra. Compared to other small molecule rhodamine derivatives with spirolactam ring, P(HEA-co-RhBBA) exhibits excellent solubility in pure aqueous solution without addition of any organic cosolvent.

Scheme 1 The synthetic routes of RhBBA and P(HEA-co-RhBBA).

Spectroscopic properties

High selectivity is usually required for a chemosensor to accomplish detection successfully. In order to verify the selectivity of the synthesized chemosensor, a variety of metal ions (Na⁺, Ba²⁺, Co²⁺, Cd²⁺, Cr³⁺, Cu²⁺, Fe²⁺, Fe³⁺, Mn²⁺, Zn²⁺, Hg²⁺, and Pb²⁺) were tested. Like most of the spirolactam rhodamine derivatives, the free P(HEA-co-RhBBA) (10 μM RhBBA) remained colorless and did not exhibit apparent absorption above 500 nm in an aqueous solution (Fig. 1), which indicated that the spirolactam form was the predominant species in the pendant group of rhodamine B-based dyes. Upon addition of 5 equiv. of Cu²⁺ ions, a strong absorption band centered at 561 nm was observed and led to the color change of the solution from colorless to purple (Fig. 1a). In contrast, addition of 5 equiv. of other metal ions led to no obvious color change and spectral enhancements (Fig. 1).

Fig. 1 (a) Color changes of P(HEA-co-RhBBA) in aqueous solution (10 μM RhBBA) upon addition of various metal ions; (b) UV-vis absorption spectra of P(HEA-co-RhBBA) in aqueous solution (10 μM RhBBA) upon addition of various metal ions.

To check the applicability of P(HEA-co-RhBBA) as a Cu²⁺ selective colorimetric chemosensor, we carried out competitive experiments in the presence of 5 equiv. of Cu²⁺ ions mixed with Na⁺, Ba²⁺, Co²⁺, Cd²⁺, Cr³⁺, Fe²⁺, Fe³⁺, Mn²⁺, Zn²⁺, Hg²⁺, and Pb²⁺ (50 equiv. each). Results are shown in Fig. 2, and no significant variation in the absorbance is observed by comparison with or without other metal ions. These results demonstrate that the response of the chemosensor to Cu²⁺ ions is unaffected by the presence of other metal ions, even existing in a concentration 10 times higher than that of Cu²⁺ ions. It is well known that most of rhodamine-based chemosensors show the fluorescence enhancement effect upon addition of analytes.³ However, after coordination with Cu²⁺, P(HEA-co-RhBBA) only exhibited the strong absorption and the characteristic color change, and did not generate the strong fluorescence emission of rhodamine moiety (Fig. S7, ESI†). It can be attributed to the quenching property of paramagnetic Cu²⁺. Similar phenomena were also observed in other studies.¹¹ Addition of 5 equiv. of other surveyed metal ions (Na⁺, Ba²⁺, Co²⁺, Cd²⁺, Cr³⁺, Fe²⁺, Fe³⁺, Mn²⁺, Zn²⁺, Hg²⁺, and Pb²⁺, respectively) also did not produce a distinct increase in the emission intensity of P(HEA-co-RhBBA) (Fig. S7, ESI†).

Fig. 2 Absorbance responses of P(HEA-co-RhBBA) (10 μM RhBBA) to various metal ions in aqueous solutions. The black bars represent the aqueous solution mixed with P(HEA-co-RhBBA) (10 μM RhBBA) and 50 equiv. of various other metal ions; the red bars represent the aqueous solution mixed with P(HEA-co-RhBBA) (10 μM RhBBA) and 50 equiv. of various other metal ions and 5 equiv. of Cu²⁺ ions.

UV-vis absorbance titration of P(HEA-co-RhBBA) (10 μM RhBBA) with Cu²⁺ ions was conducted in an aqueous solution. Upon addition of increasing concentrations of Cu²⁺ ions, the color of the aqueous solution changed from colorless to purple gradually (Fig. S8, ESI†), and the absorbance intensity at 561 nm increased obviously (Fig. 3). Until the concentration of Cu²⁺ ions increased to 1.5 equiv., the saturation of the absorbance intensity was achieved (Fig. 3, inset). According to the linear Benesi–Hildebrand expression, the measured absorbance [1/(A − A₀)] at 561 nm varied as a function of 1/[Cu²⁺] in a linear relationship with a correlation coefficient of 0.9927 (Fig. S9†), and the binding constant of P(HEA-co-RhBBA) with the Cu²⁺ ion was calculated to be 2.76 × 10⁵ M⁻¹. The detection limit of P(HEA-co-RhBBA) as a colorimetric chemosensor for the analysis of Cu²⁺ ions was found to be 3.6 × 10⁻⁷ M (Fig. S10, ESI†),¹² which was lower than some of the reported detection limits.^2c–e,5h The U.S. Environmental Protection Agency (EPA) has set the safe limit of Cu²⁺ in drinking water at 20 μM.¹³ Hence, P(HEA-co-RhBBA) can be a powerful tool for the detection of Cu²⁺ ions in drinking water.

Fig. 3 UV-vis absorption spectra of P(HEA-co-RhBBA) in aqueous solution (10 μM RhBBA) upon gradual additions of Cu²⁺ ions. Inset: plot of absorbance intensities of P(HEA-co-RhBBA) in aqueous solution (10 μM RhBBA) depending on the equivalents of Cu²⁺ ions.

Besides high sensitivity and selectivity, a short response time is also an important factor for a colorimetric chemosensor to monitor analytes in practical analysis. And so, the response time of P(HEA-co-RhBBA) to 2 equiv. of Cu²⁺ ions in aqueous solution was investigated. The experimental results showed that the color change of the solution completed within 10 s after addition of Cu²⁺ ions (movie S1, ESI†), which meant the high-speed response of P(HEA-co-RhBBA)-based colorimetric chemosensor to Cu²⁺ ions. Therefore, the colorimetric chemosensor can be used for the real-time monitor of Cu²⁺ ions.

For the efficient monitor of Cu²⁺ ions in environmental samples, the absorption spectral changes of P(HEA-co-RhBBA) should take place over a wide pH range. Thus, the effect of pH on the absorption response of P(HEA-co-RhBBA) in the absence and presence of Cu²⁺ ions was investigated in aqueous solution with the pH range of 2 to 13. As shown in Fig. 4, nearly no obvious change in the absorption band of P(HEA-co-RhBBA) at 561 nm is observed in the pH range from 2 to 13, indicating that the free P(HEA-co-RhBBA) is stable in the wide pH range. Upon the addition of Cu²⁺ ions, absorption intensity at 561 nm increases obviously, meaning the formation of P(HEA-co-RhBBA)–Cu²⁺ complex. The absorption band of the complex at 561 nm remains unaffected in solution with the pH value between 4 and 10. Thus, the uniform activity over such a wide range of pH makes P(HEA-co-RhBBA) suitable for the analysis of environmental samples that would occur well within this extended range of pH, or for use in unbuffered medium.

Fig. 4 Effect of pH on the absorbance intensity of P(HEA-co-RhBBA) in aqueous solutions (10 μM RhBBA) in the absence (a) and presence (b) of Cu²⁺ ions (5 equiv.).

Reversibility studies

In order to achieve the reversibility of the chemosensor, the color and absorbance of P(HEA-co-RhBBA)–Cu²⁺ complex should be successfully recovered through chelating of Cu²⁺ to another competing ligand with stronger binding force. As we know, Cu²⁺ ion can interact with EDTA, which is a hexadentate chelating ligand, to form an octahedral complex. Compared with P(HEA-co-RhBBA), EDTA has stronger binding capability with Cu²⁺, and can capture Cu²⁺ from P(HEA-co-RhBBA)–Cu²⁺ complex easily and transform into a more stable EDTA–Cu²⁺ complex. As shown in Fig. S11 (ESI†), a new absorption peak appears upon addition of Cu²⁺ ions to P(HEA-co-RhBBA) aqueous solution, and the original absorption of P(HEA-co-RhBBA) can recover after addition of EDTA to P(HEA-co-RhBBA)–Cu²⁺ complex solution. Obviously, the absorption of recovered P(HEA-co-RhBBA) can appear again by further adding Cu²⁺ ions, and the absorption of P(HEA-co-RhBBA) can be restored steadily upon repeated addition of EDTA. As can be seen from Fig. 5b, the P(HEA-co-RhBBA)-based colorimetric chemosensor shows excellent stability with no significant absorbance signal decay for several successive cycles, which is important for the practical applications of the chemosensors. A reversible chemosensing example of P(HEA-co-RhBBA) was presented in movie S2 (ESI†), which demonstrated an on–off–on color switching process upon adding Cu²⁺ and EDTA ions alternately.

Fig. 5 (a) Reversible color changes of P(HEA-co-RhBBA) in aqueous solution (10 μM RhBBA) upon alternate addition of Cu²⁺ and EDTA ions; (b) reversible changes in absorbance intensity of P(HEA-co-RhBBA) (10 μM RhBBA) at 561 nm upon alternate addition of Cu²⁺ and EDTA ions.

Proposed mechanism

The proposed binding mechanism of P(HEA-co-RhBBA) with Cu²⁺ is shown in Scheme 2, which is the same as other spirolactam rhodamine derivatives.¹⁴ The solution of P(HEA-co-RhBBA) was colorless and exhibited no apparent absorption above 500 nm because RhBBA existed in the closed spirolactam form. However, upon the addition of Cu²⁺ ions into P(HEA-co-RhBBA) solution, the coordination of Cu²⁺ with carbonyl O, imino N, and phenol O atoms of RhBBA transformed the spirolactam ring into its open-ring form, resulting in color change and strong absorption band appearance. The above-mentioned reversibility experiment can serve as experimental evidence to support this reversible spirolactam ring-opening mechanism.

Scheme 2 Proposed reversible binding mechanism of P(HEA-co-RhBBA) toward Cu²⁺.

To confirm the proposed mechanism, the FT-IR spectra experiments were carried out. As demonstrated in Fig. 6a, the stretching vibration absorption peaks of spirolactam C=O, C=N and phenol C–OH appear at 1693 cm⁻¹, 1515 cm⁻¹ and 1233 cm⁻¹, respectively. When RhBBA coordinates with Cu²⁺, the stretching vibration absorption peaks of spirolactam C=O and C=N disappear, and the stretching vibration absorption peak of phenol C–O shifts to higher wavenumber at 1341 cm⁻¹ (Fig. 6b). It can be concluded that Cu²⁺ coordinates with carbonyl O, imino N, and phenol O atoms of RhBBA. The proposed mechanism can be further confirmed by the Job's plot (Fig. 7). The total concentration of RhBBA and Cu²⁺ ions was kept unchanged at 10 μM, while the concentration of Cu²⁺ ions increased from 0 μM to 10 μM. When the molar fraction of Cu²⁺ ions is 0.5, the absorbance intensity at 561 nm goes through a maximum, which implies that a 1 : 1 stoichiometry is the most possible binding mode of RhBBA and Cu²⁺. More direct evidence for the formation of a 1 : 1 complex was obtained by comparing the HRMS spectra of RhBBA and RhBBA–Cu²⁺ (Fig. S6 and S12, ESI†). The unique peak at m/z = 692.2518 corresponding to [RhBBA + Cu–H]⁺ was clearly observed when 1.5 equiv. of Cu²⁺ was added to RhBBA.

Fig. 6 FT-IR spectra of RhBBA (a) and RhBBA–Cu²⁺ (b).

Fig. 7 Job's plot of P(HEA-co-RhBBA) and Cu²⁺. The total concentration of RhBBA and Cu²⁺ is 10 μM.

Applications

To investigate the practical application of the colorimetric chemosensor, test strips were facilely prepared by immersing the normal filter papers into a methanol solution of P(HEA-co-RhBBA) (10 mg mL⁻¹) and then drying in a vacuum oven. The test strips were utilized to sense Cu²⁺ and other metal ions. As shown in Fig. 8a, when the test strip is dipped in 10⁻⁷ M aqueous solution of Cu²⁺ ions, the color change of the test strip from white to light pink is observed. The color of the test strip changes to purple immediately after being immersed into 10⁻⁴ M Cu²⁺ solution. Furthermore, potentially competitive metal ions have no influence on the detection of Cu²⁺ ions by the test strips (Fig. 8b). The feasibility of the test strips for practical applications was investigated using Cu²⁺ samples (10⁻⁴ M) in tap water and river water (Tuhai river). As can be seen from Fig. S13 (ESI†), the color of the test strips all change from white to purple immediately after being immersed into the different Cu²⁺ solutions. Therefore, the test strips can conveniently detect Cu²⁺ ions in pure aqueous solutions.

Fig. 8 Colorimetric detection of Cu²⁺ ions by test strips: (a) after being immersed into Cu²⁺ aqueous solutions with different concentration (from left to right: 10⁻³ M, 10⁻⁴ M, 10⁻⁵ M, 10⁻⁶ M, 10⁻⁷ M, 0 M.); and (b) after being immersed into various metal ion solutions (10⁻⁴ M) (from left to right: Cd²⁺, Cr³⁺, Fe³⁺, Cu²⁺, Mn²⁺, Hg²⁺, and Pb²⁺).

Conclusions

In summary, we have reported a novel water-soluble polymer P(HEA-co-RhBBA) as the highly selective and sensitive colorimetric chemosensor, which can specifically detect Cu²⁺ ions with a short response time in pure aqueous solution by naked eyes and UV-vis absorption responses. Other competitive ions have nearly no influence on the probing behaviour. The detection limit of P(HEA-co-RhBBA) with Cu²⁺ was 3.6 × 10⁻⁷ M. The absorption intensity of P(HEA-co-RhBBA)–Cu²⁺ complex remained unaffected over a wide pH range of 4–10 and the use of P(HEA-co-RhBBA) to detect Cu²⁺ ions could be applied to analyse environmental samples over the wide pH range. In addition, the response toward Cu²⁺ ions has been established to be reversible by the EDTA-titration experiments. Furthermore, test strips based on P(HEA-co-RhBBA) have been fabricated, which can serve as a practical colorimetric sensor for the detection of Cu²⁺ ions through naked eye inspection with no requirement of other instruments. The present study demonstrates that water-soluble polymer containing pendant ion-sensitive receptors can be a promising platform for the detection of metal ions in pure aqueous solutions.

Acknowledgements

This work was supported by the Scientific Research Award Fund for Excellent Middle-Aged and Young Scientists of Shandong Province (no. 2012CL009, 2011CL011), and the Natural Science Foundation of China (no. 21203085).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Supplementary figures and movies. See DOI: 10.1039/c5ra00745c

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