A facile, sensitive, and rapid spectrophotometric method for copper(II) ion detection in aqueous media using polyethyleneimine

2.1 Apparatus

A UV–Vis 2450 spectrophotometer (Shimadzu, Japan) was used to record the UV/Vis absorption spectra in the wavelength range of 200–800 nm. A rapid mixing device (Ronghua Instrument Plant, Jiangsu, China) was used to mix solutions completely. A pHS-3C pH meter (Shanghai Analytical Instrument Factory, Shanghai, China) was used to adjust pH values.

2.2 Reagents

Polyethyleneimine (PEI, Mw 10000 Da; branched; water-free), copper(II) sulfate, sodium chloride, sodium hydroxide, potassium chloride, and potassium phosphate monobasic were purchased from Aladdin. Ltd., Shanghai, China. Glacial acetic acid and other acids were supplied by Chengdu Kelong Chemical Reagent Plant (Sichuan, China). All reagents used were of analytical reagent grade and were prepared using ultra-pure water with a resistivity of 18.2 MΩ cm in this study.

2.3 Standard solutions

For the preparation of the PEI stock solution, 0.9400 g of PEI was accurately weighed, transferred into a 10 mL colorimetric cylinder and dissolved under ultrasonication. The PEI stock solution (94 mg mL⁻¹) was stable for at least a month when stored in the dark at −4 °C. The working standard solution of PEI (0.94 mg mL⁻¹) was prepared by diluting the stock solution with water.

The standard stock solution of copper(II) sulfate (0.1 M) was prepared in water. The concentrations of other metal ions were 0.1 M. The standard working solutions of copper(II) sulfate and other metal ions were prepared by further dilution in water appropriately. The standard working solutions were prepared weekly to avoid any degradation phenomenon. Britton–Robinson (BR) buffer solutions (pH 1.8–11.6) were prepared by mixing 0.2 M NaOH and mixture of 0.04 M H₃PO₄, H₃BO₃, and CH₃COOH in proportion and pH values were adjusted using a pH meter.

2.4 General procedure for copper(II) analysis

A typical Cu²⁺ detection procedure was conducted as follows: Briefly, 100 μL of Britton–Robinson (BR) buffer solution (pH 6.0) was added to a 1.5 mL eppendorf tube with 50 μL of PEI (0.94 mg mL⁻¹) solution. Subsequently, different amounts of Cu²⁺ solutions were added to the eppendorf tube. Then the mixture was diluted to 500 μL with ultra-pure water. The UV–Vis absorption spectrum of the solution in the wavelength range of 200–800 nm was recorded at last. All measurements were performed in triplicate at room temperature.

3.1 Spectral characteristics

PEI is a cationic polymer that has no absorption over the wavelength range from 250 to 800 nm (Fig. 1). When trace amounts of copper(II) ion were added to the colorless PEI solution, the mixture rapidly turned light blue (inset in Fig. 1), and its absorption spectrum exhibited two absorption peaks at 275 and 630 nm, respectively (Fig. 1). The hydrophilic and biocompatible PEI serves as the chelating sites for the Cu²⁺ ion. The occurrence of these absorption bands is due to the formation of the Cu²⁺-PEI complex, since this complexation process is a chromogenic reaction (Rivas et al., 2005). These results clearly indicate that the PEI exhibits a high affinity for copper(II) ion in aqueous solution. In Fig. 1, the absorbance at 630 nm was significantly lower than that at 275 nm, which is in agreement with literature data (Ungaro et al., 2003). In addition, the reagent blanks had a negligible absorbance at both 275 and 630 nm. These results suggested that a higher sensitivity could be achieved at 275 nm, which was selected for the following studies.

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Figure 1 Absorption spectra of PEI (0.094 mg mL⁻¹) in the absence and presence of Cu²⁺ (400 μM) in aqueous solution. The inset displays the color of the PEI solution (9.4 mg mL⁻¹) in the absence (i) and presence (ii) of Cu²⁺ (5 mM).

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3.2 Optimum conditions for polyethyleneimine-copper(II) complexation

3.2.1

3.2.1 Effect of the concentration of polyethyleneimine

The effect of the PEI concentration on the absorbance of polyethyleneimine-copper(II) complexation system was investigated. Different amounts of PEI were added to the solutions containing a fixed amount of copper(II) ion (400 μM), and the absorbance of the solutions was recorded at the absorption peak of 275 nm, as shown in Fig. 2A. It can be seen that the optimum volume of PEI (0.94 mg mL⁻¹) for the system is 50 μL. Copper(II) can form complexes with PEI at a certain stoichiometric ratio. As shown in Fig. 2B, the plots obtained by the molar ratio method indicated that Cu²⁺ can react with PEI to form a complex at about 1:4 Cu/N ratio, which is in agreement with literature data (Kislenko and Oliynyk, 2002; Perrine and Landis, 1967; Villoslada et al., 2005). Herein, the planar structure of Cu²⁺-PEI complex that we deduced is shown in Fig. S1.

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Figure 2 (A) The absorbance of a series of solutions containing a fixed amount of copper(II) ion (400 μM) and different volumes (10, 20, 30, 40, 50, 80, 100, 150, 200 μL) of PEI solution (0.94 mg mL⁻¹). (B) Composition ratio for Cu²⁺-PEI complex in aqueous solution.

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3.3 Copper(II) detection by polyethyleneimine

The linear response range and detection limit of the PEI-based sensing system were measured. Under the optimum experimental conditions, 50 μL of PEI (0.94 mg mL⁻¹) in BR buffer (pH 6.0) at room temperature, we used the probe to detect various concentrations of Cu²⁺ in solution. The absorbance of Cu²⁺-PEI complex was increased with increasing the amount of Cu²⁺ ion (Fig. 3). As shown in the inset of Fig. 3, there is a good linear correlation (R² = 0.9997) between the absorbance and the concentration of Cu²⁺ in the range of 2–400 μM with the following equation:

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Figure 3 Absorbance response of PEI (0.094 mg mL⁻¹) after the addition of Cu²⁺ (a–k: 2, 5, 10, 20, 50, 62.5, 80, 100, 200, 300, 400 μM) under optimum conditions. The inset displays the plot of absorbance of Cu²⁺-PEI complex versus the concentration of Cu²⁺ (2–400 μM).

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The color change that resulted from the addition of Cu²⁺ was discerned by the naked eye at concentrations as low as 0.6 mM (Fig. 4). The extent of color change is linear over the concentration of Cu²⁺ and was shown to be reasonably selective for copper. Overall, these results demonstrate that this method is capable of being a simple, practical and reliable method of quantitatively determining the concentration of copper in water samples.

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Figure 4 The addition of Cu²⁺ (1–10: 0.05, 0.6, 1.2, 2.5, 5, 10, 15, 20, 30, 40 mM) results in a color change discernable by the naked eye at different concentrations. The concentration of PEI was 9.4 mg mL⁻¹.

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3.4 Selectivity

To evaluate the selectivity of the method, we investigated some transition metal cations such as Co²⁺ and Ni²⁺, known to form complexes with PEI, as well as Zn²⁺, Pb²⁺, Mg²⁺, Ca²⁺, Hg²⁺, and Mn²⁺, the competitive ions encountered in environmental and biological analyses. As shown in Fig. S6 A, only Cu²⁺ induced a prominent absorption at the wavelength of 275 nm, whereas the other metal ions, such as Co²⁺, Ni²⁺, Fe³⁺, Pb²⁺, Mg²⁺, Zn²⁺, Hg²⁺, Mn²⁺, and Ca²⁺, can barely affect the absorbance of the system at 275 nm. It should be noted here, in the complexation reactions with amines, Ag⁺ and Cu²⁺ may have similar reactivity. However, Ag⁺ does not inhibit the absorption response of PEI (Fig. S6 A) and the presence of Ag⁺ do not affect the activity of Cu²⁺ either (Fig. S6 B). This might result from that silver-amine complexes have no absorption at the wavelength of 275 nm (Fig. 5). This means that Ag⁺ has nearly no interference with the detection of Cu²⁺. Additionally, absorbance recorded in the presence of copper and other competing ions demonstrated that most of the coexistent ions (Na⁺, K⁺, Mg²⁺, Ca²⁺, Zn²⁺, Hg²⁺, Mn²⁺, Fe³⁺, and Pb²⁺) had a negligible interfering effect on Cu²⁺ sensing by PEI (Fig. S6 B). On the other hand, the relative variation of absorbance does not exceed 6% in the presence of Fe³⁺ and Co²⁺ ions. Therefore, the high selectivity of PEI for Cu²⁺ over some competing metal ions in aqueous media indicates its utility for a wide range of biological and environmental applications.

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Figure 5 Absorption spectra of PEI (9.4 mg mL⁻¹) in the absence and presence of Ag⁺ (4 mM) in aqueous solution.

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3.5 Application

The applicability of this sensing system for detecting Cu²⁺ in a real sample was further evaluated. We applied this new method to detect the concentration of Cu²⁺ in the water sample of ChongDe Lake (the lake in Southwest University, China). The lake water sample was used just by filtration. The concentration of Cu²⁺ ion in the ChongDe Lake water sample detected using this new approach is about 16.34 μM, which is well consistent with that obtained by the AAS method, namely 15.54 μM. The detailed results are listed in Table 1. These results confirmed the validity of this spectrophotometric method for the detection of Cu²⁺ in real samples.