A highly selective chemosensor for naked-eye sensing of nanomolar Cu(II) in an aqueous medium

Abstract

A novel highly selective and sensitive colorimetric chemosensor L for the detection of Cu²⁺ ion with a fast response time was designed and synthesized. Receptor L detected Cu²⁺ ion by changing its color from colorless to magenta in a semi-aqueous solution. The limit of detection for Cu²⁺ was calculated to be as low as 28 nM. The possible binding mode of compound L with Cu²⁺ ion was studied using the Job's method, HRMS, FTIR spectroscopy and ¹H NMR spectroscopy titration. Importantly, test strips containing L were fabricated as a naked-eye indicator for Cu²⁺ ion in pure water samples.

---

Introduction

The recognition of biologically and chemically important species at ultra-low concentrations has received considerable attention in recent decades due to their important roles in biological and environmental processes, ranging from the diagnosis of life-threatening diseases to analysis of environmental pollutants.¹ Copper ion, an indispensible transition metal ion in the human body, plays various roles in physiological processes and is a key component of a wide range of enzymes such as copper–zinc superoxide dismutase, cytochrome c oxidase, ceruloplasmin, lysyl oxidase, tyrosinase, dopamine b-hydroxylase and peptidylglycine a-amidating monooxygenase.² Aberrations in normal copper levels, both systemic as well as on a tissue or cellular scale, are implicated in a wide range of diseases such as Menkes disease, Wilson's disease, Alzheimer's disease, Parkinson's disease and transmissible spongiform encephalopathy (prion diseases)^2(a),3. On the other hand, Cu²⁺ is a significant environmental pollutant throughout the world due to its widespread use in industry, agriculture, household utensils and water pipes. Under normal conditions, the average concentration of copper in the blood should not exceed 100–150 μg dL⁻¹ (15.7–23.6 μM).⁴ Therefore, it is of increasing importance to develop fast, convenient and reliable methods for the qualitative and quantitative detection of trace amounts of copper ion in light of its biological and environmental implications.

To date, a large amount of study has been reported for the detection of Cu²⁺ ions at trace levels, and a series of conventional analytical methods, such as inductively coupled plasma atomic emission/mass spectroscopy (ICP-AES/ICP-MS),⁵ atomic absorption spectrometry (AAS),⁶ electrochemical methods,⁷ surface plasmon resonance detectors⁸ and quantum-dot-based assays,⁹ have been developed. Although these technologies can detect Cu²⁺ ions selectively with high sensitivity, they require highly sophisticated/expensive instrumentation and time-consuming processes, requiring tedious sample preparation and highly trained operators, which means they cannot be used for real-time detection in their routine application.¹⁰

Because of the inexpensiveness, high sensitivity and simplicity, fluorescence techniques have attracted widespread attention in the recent years, and a rapidly increasing number of metal-responsive fluorescent sensors have been studied.¹¹ However, unfortunately, Cu²⁺ is a notorious fluorescence quencher because of its paramagnetic nature.¹² Therefore, many of the reported Cu²⁺ sensors undergo fluorescence quenching upon binding of Cu²⁺ either by electron or energy transfer mechanisms.¹³ In addition, fluorescence techniques sometimes still require tedious sample preparation procedures and trained operators for bio-imaging research (i.e., preparation of buffer solution with different types and dosage, choice of cell resources and cell culture).

In contrast, unlike the abovementioned techniques, colorimetric methods¹⁴ based on color changes appeared to be the most attractive technique since they can conveniently and easily monitor target ions directly by the naked eye even at the micro/submicromolar levels without any need for expensive and/or sophisticated instrumentation. Upon surveying the literature, many relevant works concerning colorimetric Cu²⁺ probes have been reported,¹⁵ and we noticed that most of the reported Cu²⁺ selective colorimetric sensors have a number of drawbacks (i.e., poor detection limit, long response time and interference from other transition metal ions) (Table 1)^{15(e)–(g),16}. Therefore, exploring more excellent colorimetric chemosensing molecules for the naked-eye detection of Cu²⁺ in an aqueous solution is still in demand. Studies related to this area are of great challenge and continue to be of widespread interest.

Table 1 Comparison of the reported colorimetric chemosensors for naked-eye detection of Cu(II)

Chemosensors	Detection limit (μM)	Water content of solution	Interference	Response time	References
	2.1	40%	None	No data	15e
	3.42	90%	None	Less than 30 s	15f
	1.2	60%	Hg^2+, Fe^3+	No data	15g
Cys-modified AuNR (Cys-AuNR)	0.34	100%	Hg^2+	5 min	16a
	2.7	100%	None	No data	16b
	No data	20%	No data	No data	16c
	2.29	80%	None	No data	16d
	13.6	10%	None	No data	16e
	0.028	50%	Hg^2+	1 min	This study

---

In this study, we successfully synthesized and characterized a simple colorimetric chemosensor L, as depicted in Scheme 1. Intriguingly, L gives a visual color change from colorless to magenta with a fast response time, allowing for the naked-eye detection of Cu²⁺ with high selectivity and sensitivity in a DMSO–water (1 : 1, v/v) solution. The sensing behavior of L towards Cu²⁺ was investigated systematically. Importantly, test strips were prepared as a practical, visible colorimetric detection kit for Cu(II). Using the remarkable colorimetric response of L to Cu(II), a recreational test was performed successfully.

Scheme 1 Synthetic procedure of L.

Experimental

Materials and measurements

All starting materials were purchased from commercial suppliers and used without further purification. Deionized water was used throughout the experiments. NMR spectra were obtained on a Varian 400 MHz NMR spectrometer using CDCl₃ or DMSO-d₆ as the solvent and TMS as the internal standard. ¹H NMR titration experiments were carried out in DMSO-d₆. A UV-1800 UV-Vis spectrophotometer with 1.0 cm quartz cell was used to record the absorbance measurements. HRMS were determined on a LCMS-IT from a Shimadzu TOF (LC30A). FTIR spectra were obtained on a Bruker TENSOR27, Germany. The purity of L–Cu²⁺ complex was determined by elemental analysis performed on a EuroEA Elemental Analyser. TLC analysis was performed on silica gel plates (GF254, model number; 0.20–0.25 mm, thickness) and column chromatography was conducted over a silica gel (100–200, mesh size), both of which were obtained from Qingdao Ocean Chemicals.

General methods

A stock solution of L (0.2 mM) was prepared in DMSO. Metal ion solutions (10 mM) of KCl, Co(NO₃)₂·6H₂O, Ni(NO₃)₂·6H₂O, Zn(NO₃)₂, Fe(NO₃)₃·9H₂O, MgSO₄·7H₂O, Al(ClO₄)₃·9H₂O, Pb(NO₃)₂, Mn(NO₃)₂, HgCl₂, AgNO₃, Ca(ClO₄)₂, NaCl, La(NO₃)₃·6H₂O, Cd(NO₃)₂·4H₂O, FeSO₄·7H₂O, CuCl₂·2H₂O, CuSO₄·5H₂O, Cu(NO₃)₂·3H₂O and Cu(OAc)₂·H₂O and organic or inorganic anion solutions (10 mM) of (CH₃CH₂CH₂CH₂)₄N⁺F⁻, (CH₃CH₂CH₂CH₂)₄N⁺Cl⁻, (CH₃CH₂CH₂CH₂)₄N⁺Br⁻, (CH₃CH₂CH₂CH₂)₄N⁺I⁻, CH₃COONa·3H₂O, NaNO₃, Na₂CO₃, NaHCO₃, Na₂SO₄, NaHSO₄·H₂O, Na₂HPO₄·12H₂O, NaH₂PO₄·2H₂O and Na₂S₂O₃·5H₂O were prepared in deionized water. The solution of L (0.5 mL) was diluted to 10 μM with DMSO and deionized water in a 10 mL volumetric flask and then the ions were added. Spectral data were recorded after a 1 min of incubation period.

Determination of the detection limit

The detection limit was calculated with the following formula¹⁷ based on the absorbance titration:

Detection limit = 3σ/k,

where σ is the standard deviation of blank measurements and k is the slope between the absorption intensity at 562 nm versus Cu²⁺ concentration. The absorbance intensity of the blank L (10 μM) was measured 10 times in a DMSO–water (1 : 1, v/v) solution.

Determination of the association constant

The association constant of L–Cu²⁺ complex was determined from the Benesi–Hildebrand equation:¹⁸
where K denotes the association constant, A₀ is the observed absorption in the absence of cation, A is the observed absorption with added cation, [M] is the concentration of the cation added and a and b are constants. The association constant value K was evaluated graphically by plotting 1/ΔA against 1/[M].

Synthesis of rhodamine B hydrazide (1)

Rhodamine B hydrazide (1) was synthesized as previously reported.¹⁹

Synthesis of 6-hydroxy-4-methylcoumarin. 6 mL of concentrated sulfuric acid was placed in a 50 mL round-bottomed flask in an ice bath followed by the dropwise addition of a solution containing hydroquinone (1.50 g, 13.6 mmol) and excess ethyl acetoacetate (4 mL, 31.6 mmol). The mixture was stirred at 5–10 °C for 12 h and then warmed to room temperature. The resultant dark yellow solution was poured into crushed ice with vigorous stirring. The precipitate was collected by suction filtration, washed several times with cold water, and dried in vacuum to obtain a canary yellow solid (0.89 g, yield: 38%).

Synthesis of 7-formyl-6-hydroxy-4-methylcoumarin (2). 6-Hydroxy-4-methylcoumarin (1.00 g, 5.68 mmol) and hexamethylenetetramine (1.98 g, 14.12 mmol) were placed in a 50 mL schlenk flask, 10 mL of trifluoroacetic acid was added under a dry argon atmosphere and the mixture was kept in an ice bath for 30 min. After warming to room temperature, the mixture was refluxed at 100 °C in an argon protection environment for 14 h. The solvent was evaporated under reduced pressure and the residual solution was poured into 100 mL of cold deionized water with stirring. The precipitate was collected by suction filtration, repeatedly washed with cold water and dried in vacuum to afford a yellow powder. The crude product was further purified through silica gel (100–200, mesh size) column chromatography using 13%–16% ethyl acetate in petroleum ether as the eluent to give 2 as a pale yellow solid (0.28 g, yield: 23%). ¹H NMR (400 MHz, DMSO-d₆) δ: 10.86 (s, 1H), 10.34 (s, 1H), 7.53 (s, 1H), 7.25 (s, 1H), 6.54 (d, J = 1.3 Hz, 1H), 2.40 (d, J = 1.3 Hz, 3H). ¹³C NMR (101 MHz, DMSO-d₆) δ: 190.65, 159.94, 156.66, 152.12, 146.09, 126.15, 124.84, 117.84, 115.52, 112.96, 18.44. HRMS (ESI): calculated for C₁₁H₈O₄ [M − H⁺]⁻ (m/z): 203.0350; found: 203.0358.

Synthesis of probe L. Rhodamine B hydrazide (1, 0.19 g, 0.42 mmol) and 7-formyl-6-hydroxy-4-methylcoumarin (2, 0.08 g, 0.42 mmol) were dissolved in anhydrous methanol (4 mL) and the solution was stirred for 1.5 h under refluxing conditions. After cooling to room temperature, the precipitate was filtered and washed three times with 10 mL cold methanol. The crude product was further purified through silica gel (100–200, mesh size) column chromatography using 10%–15% ethyl acetate in petroleum ether as eluent to give L as a pale yellow powder (0.14 g, yield: 53%). ¹H NMR (400 MHz, CDCl₃) δ: 12.23 (s, 1H), 9.62 (s, 1H), 7.99 (d, J = 7.0 Hz, 1H), 7.56–7.48 (m, 2H), 7.16 (d, t, J = 17.2, 9.1 Hz, 3H), 6.56 (d, J = 8.9 Hz, 2H), 6.48 (d, J = 2.5 Hz, 2H), 6.28 (d, d, J = 8.9, 2.5 Hz, 2H), 6.20 (s, 1H), 3.33 (q, J = 7.1 Hz, 8H), 2.22 (s, 3H), 1.15 (t, J = 7.0 Hz, 12H). ¹³C NMR (101 MHz, CDCl₃) δ: 164.86, 160.16, 156.95, 153.23, 152.20, 151.69, 149.32, 148.17, 148.11, 134.01, 128.72, 128.24, 127.65, 123.93, 123.62, 122.21, 120.82, 118.76, 117.97, 113.05, 108.48, 104.72, 98.31, 66.21, 44.45, 25.39, 12.62. HRMS (ESI): calculated for C₃₉H₃₈N₄O₅ [M + H⁺]⁺ (m/z): 643.2915; found: 643.2921. FTIR (KBr) ν: 3445(–OH), 1727, 1715 (C=O), 1616 (C=N).

Preparation of L–Cu²⁺ complex

CuCl₂·2H₂O (0.08 g, 0.45 mmol) was added to a stirred solution of receptor L (0.19 g, 0.30 mmol) in absolute ethanol. The solution was stirred at 50 °C and the reaction process was monitored by TLC. After the reaction was completed, the solvent was removed under reduced pressure and the solid complex was filtered, washed several times with deionized water and dried in vacuum to obtain a dark purple powder (0.17 g, yield: 76%). Elemental anal.: calculated for C₃₉H₃₉N₄O₆CuCl (%): C 61.74, H 5.18, N 7.38; found: C 61.89, H 5.32, N 6.82. HRMS (ESI): calculated for [L + Cu²⁺ − H⁺]⁺ (m/z): 704.2049, found: 704.2072. FTIR (KBr) ν: 3445 (–OH), 1716, 1699 (C=O), 1590 (C=N).

Results and discussion

Synthesis and structural characteristics of L

Receptor L was synthesized by the nucleophilic addition–condensation reaction of rhodamine B hydrazide (1) and 7-formyl-6-hydroxy-4-methylcoumarin (2) in absolute methanol (Scheme 1). L and the intermediate 2 were characterized by ¹H NMR, ¹³C NMR and HRMS (ESI, Fig. S1–S6†).

Equilibration time

The equilibration time for complexation was evaluated between L and Cu²⁺ ion (Fig. S7†). No obvious absorbance variation of L (10 μM) at 562 nm was observed over a period of 15 min, indicating that the five-membered spirolactam structure of sensor L was stable. After the addition of Cu²⁺ ions, the absorbance intensity at 562 nm increased instantaneously and reached a maximum after 60 s. For further spectral measurements, a reaction time of 1 min was used to ensure that the reaction was completed.

Absorption spectrum of L in the presence of competitive metal ions

To gain an insight into the photochemical properties of L, absorption changes upon addition of various metal ions were performed in a DMSO–H₂O (1 : 1, v/v) solution. As illustrated in Fig. 1, the characteristic absorption peak centered at 562 nm (266-fold absorbance enhancement in comparison with blank L) accompanied with remarkable color response from colorless to magenta was observed upon the addition of Cu²⁺. Among other tested metals, only Fe³⁺ and Fe²⁺ caused a slight color change from colorless to light pink, and a corresponding absorption peak appeared at 562 nm as expected, similar to that observed with Cu²⁺, whereas Na⁺, K⁺, Co²⁺, Ni²⁺, Zn²⁺, Mg²⁺, Al³⁺, Ca²⁺, Pb²⁺, Mn²⁺, Hg²⁺, Ag⁺, La³⁺ and Cd²⁺ exerted either little or no disturbance on the UV-Vis spectra of L. However, these changes induced by Fe³⁺ or Fe²⁺ were distinctly less in magnitude as compared to the changes observed with Cu²⁺ due to its low binding affinity to L, which indicates that composite L can serve as a potential chemosensor for the naked eye detection of Cu²⁺ in an aqueous medium.

Fig. 1 Naked-eye detectable color changes and UV-Vis absorption spectra of L (10 μM, in DMSO) in a DMSO/H₂O (1 : 1, v/v) solution upon addition of various metal ions (10 μM, in H₂O).

UV-Vis titration experiment

To further study the binding properties of L with Cu²⁺, we measured the absorption properties of L (10 μM) upon addition of an increasing concentration of Cu²⁺ (0–10 μM) (Fig. 2). With continuous addition of Cu²⁺ ions, a sharp absorption band centered at 562 nm emerged with increasing intensity, which induced an obvious color change from colorless to magenta. Moreover, three clear isosbestic points at 271 nm, 337 nm and 380 nm were observed, which indicate the formation of only one visible active copper complex. Furthermore, a linear dependence of the absorbance at 562 nm as a function of Cu²⁺ concentration was observed (Fig. S8†). These spectroscopic changes were characteristic of the spirolactam ring of rhodamine as the complexation process was accompanied by ring-opening. For all concentrations of Cu²⁺ ions above 10 μM, the intensity of absorption at 562 nm reached saturation, which suggested the formation of a 1 : 1 complex. The stoichiometry between L and Cu²⁺ was confirmed by utilizing the Job's method (Fig. 3). High resolution mass spectrometry (Fig. S9†) provided further support for the formation of the 1 : 1 complex (m/z calculated for [L + Cu²⁺ − H⁺]⁺ = 704.2049, m/z observed = 704.2072). On the basis of non-linear fitting of the titration curve of a 1 : 1 binding model, the association constant of the Cu²⁺–L complex was determined to be 5.23 × 10⁴ M⁻¹ (Fig. S10†). From the titration experiments, the detection limit for Cu²⁺ was calculated to be ∼28 nM, which was much lower than the WHO limit for Cu²⁺ (31.5 μM) in drinking water.²⁰ This shows that our proposed method based on compound L has the potential to monitor the copper concentrations in water samples.

Fig. 2 Titration curves of L (10 μM, in DMSO) in DMSO/H₂O (1 : 1, v/v) solution upon addition of CuCl₂·2H₂O (0–10 μM, in H₂O). Inset shows the color change of the solution before (left) and after (right) the addition of Cu²⁺.

Fig. 3 Job's plot of L–Cu²⁺ complex in DMSO/H₂O (1 : 1, v/v) solution. The total concentration of L and Cu²⁺ was 10 μM. The absorbance was monitored at 562 nm.

Tolerance of L to Cu²⁺ over other metal ions and anions

One basic requirement of an ion-selective chemosensor is its target selectivity over other competitive substrates. The absorbance response of L was highly selective for Cu²⁺ over biologically and environmentally relevant analytes (Fig. 4). No significant difference in the response of the L–Cu²⁺ system in the absence and presence of the interfering metal ions (5 equivalents) was observed, except for Hg²⁺. To clearly understand the interference of Hg²⁺ on the optical response of L to Cu²⁺, a UV-Vis titration experiment was carried out (Fig. S11†). Upon gradual addition of Hg²⁺ (0–60 μM) to a solution of L–Cu²⁺ complex (10 μM), a slight loss of color was observed. Moreover, the absorption band at 562 nm decreased with distinct isosbestic points at 333 nm, 372 nm and 413 nm, indicating the presence of UV-active species in equilibrium. For all concentrations of Hg²⁺ ions above 50 μM (i.e., 60 μM, 6 equiv.; 100 μM, 10 equiv.), the intensity of absorption at 562 nm almost reached saturation (Fig. S11–S12†). As a result, Hg²⁺ had a negative effect on the response of Cu²⁺, probably because of the strong affinity of Hg²⁺ to L for detaching Cu²⁺ from the L–Cu²⁺ complex^15g, and the existing L–Hg²⁺ complex was colorless, which escapes visual inspection and hence has no spectral response was observed. However, the competitive coordination was removed and generally reached a balance when an excess of Hg²⁺ ions were added into the L–Cu²⁺ system. The effect of different organic and inorganic anions was also investigated under the same conditions. Evidently, the coexistence of these anions did not show any distinct influence on the recognition process of Cu²⁺ by L. To determine the effect of counter anions in the sensing behavior of receptor L, the response of L with Cu²⁺ from different copper salts such as copper chloride, copper sulfate, copper nitrate and copper acetate were investigated, as shown in Fig. S13.† When 10 μM of L encountered 1 equiv. of Cu²⁺, the response of L with Cu²⁺ from copper chloride was almost in accordance with Cu²⁺ from other copper salts. These results clearly demonstrated that receptor L can function as a highly selective/anti-disturbance sensor for Cu²⁺ and could be used for the practical detection of Cu²⁺ in water samples.

Fig. 4 Absorbance intensity change profiles of L (10 μM, in DMSO) in the presence of Cu²⁺ (10 μM, in H₂O) or Cu²⁺ (10 μM, in H₂O) with other metal ions or anions (Mⁿ⁺, Mⁿ⁻, 50 μM, in H₂O) in a DMSO/H₂O (1 : 1, v/v) solution. (1) Blank; (2) Cu²⁺; (3) Cu²⁺ + Co²⁺; (4) Cu²⁺ + Ni²⁺; (5) Cu²⁺ + Zn²⁺; (6) Cu²⁺ + Fe³⁺; Cu²⁺ + CH₃COO⁻; (7) Cu²⁺ + Na⁺; Cu²⁺ + F⁻; (8) Cu²⁺ + Mg²⁺; Cu²⁺ + Cl⁻; (9) Cu²⁺ + Al³⁺; Cu²⁺ + Br⁻; (10) Cu²⁺ + Pb²⁺; Cu²⁺ + I⁻; (11) Cu²⁺ + La³⁺; Cu²⁺ + HPO₄²⁻; (12) Cu²⁺ + Mn²⁺; Cu²⁺ + H₂PO₄⁻; (13) Cu²⁺ + Hg²⁺; Cu²⁺ + S₂O₃²⁻; (14) Cu²⁺ + Ca²⁺; Cu²⁺ + CO₃²⁻; (15) Cu²⁺ + Ag⁺; Cu²⁺ + HCO₃⁻; (16) Cu²⁺ + Cd²⁺; Cu²⁺ + SO₄²⁻; (17) Cu²⁺ + K⁺; Cu²⁺ + HSO₄⁻; (18) Cu²⁺ + Fe²⁺; Cu²⁺ + NO₃⁻. The absorbance was monitored at 562 nm.

Proposed mechanism

To obtain a clear understanding of the structure of the L–Cu²⁺ complex, FTIR measurements were primarily employed (Fig. 5). The characteristic stretching frequencies of L at 1727 cm⁻¹ and 1616 cm⁻¹, corresponding to ν (C=O) (the rhodamine unit) and ν (C=N), respectively, almost disappeared completely in the spectra of the L–Cu²⁺ complex. Moreover, the absorption peak of ν (–OH) at 3445 cm⁻¹ decreased conspicuously. These spectroscopic changes are strong evidence that the spirolactam C=O, C=N and –OH groups participated in Cu²⁺ coordination.

Fig. 5 (a) The whole FTIR spectra of L and L–Cu²⁺ complex. (b) The ∼1700 region of spectra of L and L–Cu²⁺ complex.

A ¹H NMR titration experiment was further carried out in DMSO-d₆. As shown in Fig. S14,† upon the addition of 1 equiv. of Cu²⁺ ions, reduction in the intensity of the hydroxyl group (–OH, δ = 10.43 ppm) accompanied with a slight up field chemical shift (δ = 0.03 ppm) was observed, indicating deprotonation as a result of interaction with Cu²⁺.

To rule out the possibility that the absorbance changes observed were not due to a chemical reaction (i.e., chemodosimeter²¹), the reversible binding of Cu²⁺ and the sensor was established. The reversibility experiment (Fig. S15†) revealed that the 266-fold absorbance enhancement of L caused by the addition of 1 equiv. of Cu²⁺ ions can be removed completely by adding 1.5 equiv. of EDTA–2Na, leading to the reconstitution of the spirolactam ring in the rhodamine moiety and hence the loss of absorbance at 562 nm.

On the basis of the combined spectroscopic information and previously reported literature^15(c),22, a possible Cu²⁺-induced deprotonation mechanism and coordination mode of receptor L with Cu²⁺ was proposed, as shown in Scheme 2.

Scheme 2 Proposed mechanism for the identification of Cu²⁺ triggered by L.

Practical application and recreational test

To investigate the preliminary application of chemosensor L, test strips were facilely prepared by immersing normal filter papers into a saturated DMSO solution of L and then drying in vacuum. These test strips were applied for sensing different Cu²⁺ concentrations, exhibiting colorimetric changes differentiable by the naked eye. As depicted in Fig. 6, the well-marked red color of the test strips intensified from 0 to 1.0 × 10⁻⁵ M, 1.0 × 10⁻⁴ M, 1.0 × 10⁻³ M, 1.0 × 10⁻² M and 1.0 × 10⁻¹ M and show that the discernible concentration of Cu²⁺ can be as low as 1.0 × 10⁻⁴ M. These results indicated that receptor L could serve as a good candidate for conveniently detecting Cu²⁺ in pure water samples.

Fig. 6 Images of the test strips coated with L for colorimetric detecting Cu²⁺ ion in an aqueous solution with different concentrations. Left to right: 0, 1 × 10⁻¹ M, 1 × 10⁻² M, 1 × 10⁻³ M, 1 × 10⁻⁴ M and 1 × 10⁻⁵ M.

Taking advantage of the distinct abovementioned color change, a recreational test was then carried out using an A4 paper (Fig. 7). No obvious color change was observed when “” gaps were filled with Cu²⁺ (1.0 × 10⁻² M). Interestingly, it would turn into magenta immediately after adding the saturated DMSO solution of L dropwise.

Fig. 7 Colorimetric response of Cu²⁺ ion in the absence or presence of L. (Top) none, (middle) Cu²⁺, (bottom) Cu²⁺ + L.

Conclusions

In summary, we developed a new chemosensor L as a colorimetric probe for the sensitive detection of Cu²⁺ ion with a short response time in a semi-aqueous medium. The complex exhibited high selectivity for Cu²⁺ ion over a panel of other metal ions. Importantly, the detection limit (28 nM) of L for Cu²⁺ falls sufficiently below the limit criterion of drinking water (31.5 μM). Due to the colorimetric response of L to Cu(II), test strips containing L were fabricated as a naked-eye indicator for Cu²⁺ ion in pure water samples, a recreational test was also achieved. Based on this simple, rapid and cost-effective method, we believe that receptor L will be an excellent prototype for the development of a novel colorimetric Cu²⁺-chemosensor.

Acknowledgements

The authors would like to thank the National Natural Science Foundation of China (Grant No. 21261008, 21471069, 1601310046) and the International Science and Technology Cooperation Projects of Hainan Province (Grant No. KJHZ2014-05).

Notes and references

1. (a) X. Chen, X. Tian, I. Shin and J. Yoon, Chem. Soc. Rev., 2011, 40, 4783; (b) X. Chen, T. Pradhan, F. Wang, J. S. Kim and J. Yoon, Chem. Rev., 2012, 112, 1910; (c) Z. Liu, W. He and Z. Guo, Chem. Soc. Rev., 2013, 42, 1568; (d) M. E. Moragues, R. Martínez-Máñez and F. Sancenón, Chem. Soc. Rev., 2011, 40, 2593; (e) L. Gao, L. Li, X. Wang, P. Wu, Y. Cao, B. Liang, X. Li, Y. Lin, Y. Lu and X. Guo, Chem. Sci., 2015, 6, 2469.
2. (a) P. Verwilst, K. Sunwoo and J. S. Kim, Chem. Commun., 2015, 51, 5556; (b) S. R. Patil, J. P. Nandre, P. A. Patil, S. K. Sahoo, M. Devi, C. P. Pradeep, F. Yu, L. Chen, C. Redshaw and U. D. Patil, RSC Adv., 2015, 5, 21464; (c) J. Li, Y. Zeng, Q. Hu, X. Yu, J. Guo and Z. Pan, Dalton Trans., 2012, 41, 3623.
3. (a) G. J. Brewer, Curr. Opin. Chem. Biol., 2003, 7, 207; (b) S. Hu, J. Song, F. Zhao, X. Meng and G. Wu, Sens. Actuators, B, 2015, 215, 241.
4. R. B. Jonas, Appl. Environ. Microbiol., 1989, 55, 43.
5. (a) G. P. C. Rao, K. Seshaiah, Y. K. Rao and M. C. Wang, J. Agric. Food Chem., 2006, 54, 2868; (b) T. Kato, S. Nakamur and M. Mirita, Anal. Sci., 1990, 6, 623.
6. I. Karadjova, B. Izgi and S. Gucer, Spectrochim. Acta, Part B, 2002, 57, 581.
7. T. Poursaberi, L. Hajiagha-Babaei, M. Yousefi, S. Rouhani, M. Shamsipur, M. Kargar-Razi, A. Moghimi, H. Aghabozorg and M. R. Ganjali, Electroanalysis, 2001, 13, 1513.
8. S. Hong, T. Kang, J. Moon, S. Oh and J. Yi, Colloids Surf., A, 2007, 292, 264.
9. K. M. Gattas-Asfura and R. M. Leblanc, Chem. Commun., 2003, 2684.
10. (a) F. J. Huo, C. X. Yin, Y. T. Yang, J. Su, J. B. Chao and D. S. Liu, Anal. Chem., 2012, 84, 2219; (b) M. Cui, Q. Liu, Q. Fei, Y. Fei, Y. Liu, H. Shan, G. Feng and Y. F. Huan, Anal. Methods, 2015, 7, 4252.
11. (a) S. Sun, B. Qiao, N. Jiang, J. Wang, S. Zhang and X. Peng, Org. Lett., 2014, 16, 1132; (b) J. Yin, Y. Kwon, D. Kim, D. Lee, G. Kim, Y. Hu, J.-H. Ryu and J. Yoon, J. Am. Chem. Soc., 2014, 136, 5351; (c) S. Khatua and M. Schmittel, Org. Lett., 2013, 15, 4422; (d) T. Anand, G. Sivaraman, P. Anandh, D. Chellappa and S. Govindarajan, Tetrahedron Lett., 2014, 55, 671; (e) J. F. Zhang, Y. Zhou, J. Yoon, Y. Kim, S. J. Kim and J. S. Kim, Org. Lett., 2010, 12, 3852; (f) J. Chan, S. C. Dodani and C. J. Chang, Nat. Chem., 2012, 4, 973; (g) Y. Zhang, Z. Shi, L. Yang, X. Tang, Y. An, Z. Ju and W. Li, Inorg. Chem. Commun., 2014, 39, 86.
12. (a) R. Martínez, A. Espinosa, A. Tárraga and P. Molina, Tetrahedron, 2008, 64, 2184; (b) G. Hennrich, W. Walter, U. Resch-Genger and H. Sonnenschein, Inorg. Chem., 2001, 40, 641; (c) V. Chandrasekhar, P. Bag and M. D. Pandey, Tetrahedron, 2009, 65, 9876.
13. (a) W. Lin, L. Yuan, W. Tan, J. Feng and L. Long, Chem.–Eur. J., 2009, 15, 1030; (b) Z. Jiang, L. Tang, F. Shao, G. Zheng and P. Lu, Sens. Actuators, B, 2008, 134, 414; (c) H. Li, Z. Yang and D. Qin, Inorg. Chem. Commun., 2009, 12, 494; (d) H. S. Jung, P. S. Kwon, J. W. Lee, J. I. Kim, C. S. Hong, J. W. Kim, S. Yan, J. Y. Lee, J. H. Lee, T. Joo and J. S. Kim, J. Am. Chem. Soc., 2009, 131, 2008; (e) S. Kaur and S. Kumar, Tetrahedron Lett., 2004, 45, 5081.
14. (a) J. Shao, Y. Qiao, H. Lin and H. Lin, Spectrochim. Acta, Part A, 2009, 71, 1736; (b) N. Kaur and S. Kumar, Tetrahedron, 2011, 67, 9233; (c) Y. Li, Y. Duan, J. Zheng, J. Li, W. Zhao, S. Yang and R. Yang, Anal. Chem., 2013, 85, 11456; (d) Y. J. Na, G. J. Park, H. Y. Jo, S. A. Lee and C. Kim, New J. Chem., 2014, 38, 5769; (e) J. Zhou, D. Liu, Y. He, X. Kong, Z. Zhang, Y. Ren, Y. Long, R. Huang and L. Zheng, Dalton Trans., 2014, 43, 11579.
15. (a) P. Kaur, S. Kaur and K. Singh, Org. Biomol. Chem., 2012, 10, 1497; (b) M. Wang, K. H. Leung, S. Lin, D. S. H. Chan, D. W. J. Kwong, C. H. Leung and D. L. Ma, Sci. Rep., 2014, 4, 6794; (c) X. Xu, W. L. Daniel, W. Wei and C. A. Mirkin, Small, 2010, 6, 623; (d) P. Kaur, H. Kaur and K. Singh, RSC Adv., 2013, 3, 64; (e) T. G. Jo, Y. J. Na, J. J. Lee, M. M. Lee, S. Y. Lee and C. Kim, New J. Chem., 2015, 39, 2580; (f) Z. Xu, L. Zhang, R. Guo, T. Xiang, C. Wu, Z. Zheng and F. Yang, Sens. Actuators, B, 2011, 156, 546; (g) R. Sheng, P. Wang, Y. Gao, Y. Wu, W. Liu, J. Ma, H. Li and S. Wu, Org. Lett., 2008, 10, 5015.
16. (a) J. Liu, H. Wang and X. Yan, Analyst, 2011, 136, 3904; (b) J. Y. Noh, G. J. Park, Y. J. Na, H. Y. Jo, S. A. Lee and C. Kim, Dalton Trans., 2014, 43, 5652; (c) X. Xie, X. Chen, B. Li and L. Zhang, Dyes Pigm., 2013, 98, 422; (d) H. Kim, Y. J. Na, E. J. Song, K. B. Kim, J. M. Bae and C. Kim, RSC Adv., 2014, 4, 22463; (e) P. Kaur, D. Sareen and K. Singh, Talanta, 2011, 83, 1695.
17. B. K. Datta, D. Thiyagarajan, S. Samanta, A. Ramesh and G. Das, Org. Biomol. Chem., 2014, 12, 4975.
18. M. Zhu, M. Yuan, X. Liu, J. Xu, J. Lv, C. Huang, H. Liu, Y. Li, S. Wang and D. Zhu, Org. Lett., 2008, 10, 1481.
19. J. Zhang, B. Li, L. Zhang and H. Jiang, Chem. Commun., 2012, 48, 4860.
20. WHO, WHO Guidelines Values for Chemicals that are of Health Significance in Drinking Water, Guidelines for Drinking Water Quality, Geneva, 3rd edn, 2008.
21. M. H. Lee, B.-K. Cho, J. Yoon and J. S. Kim, Org. Lett., 2007, 9, 4515.
22. (a) S. Goswamia, D. Sena, A. K. Dasa and N. K. Dasa, Sens. Actuators, B, 2013, 183, 518; (b) W. Y. Liu, H. Y. Li, H. S. Lv, B. X. Zhao and J. Y. Miao, Spectrochim. Acta, Part A, 2012, 95, 658.

---

Footnote

† Electronic supplementary information (ESI) available: ¹H NMR, ¹³C NMR, ESI-MS data of compounds 2 and L, ESI-MS data of L–Cu²⁺ complex, ¹H NMR titration data and additional spectroscopic studies. See DOI: 10.1039/c5ra12407g

---