A facile, sensitive, and rapid spectrophotometric ...

Arabian Journal of Chemistry (2013) xxx, xxx–xxx

King Saud University

Arabian Journal of Chemistry www.ksu.edu.sa www.sciencedirect.com

ORIGINAL ARTICLE

A facile, sensitive, and rapid spectrophotometric method for copper(II) ion detection in aqueous media using polyethyleneimine Ting Wen, Fei Qu, Nian Bing Li, Hong Qun Luo

*

Key Laboratory of Eco-environments in Three Gorges Reservoir Region (Ministry of Education), School of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, China Received 24 February 2013; accepted 9 June 2013

KEYWORDS Polyethyleneimine; Copper(II); Spectrophotometric method; Absorbance; Cuprammonium complex

Abstract In this work, we developed a facile, sensitive, and rapid spectrophotometric method for copper(II) ion detection in aqueous media using polyethyleneimine. Polyethyleneimine is a cationic polymer that has no absorption in the wavelength range of 250–800 nm. When trace amounts of copper(II) ion was added to the colorless polyethyleneimine solution, copper(II) ion could react with the amino groups of the polyethyleneimine to form a dark blue cuprammonium complex whose absorption spectrum exhibited two absorption peaks at 275 and 630 nm, respectively. The effects of parameters such as polyethyleneimine concentration, pH, temperature, reaction time, and the most suitable medium for the reaction were investigated. A linear relationship (R2 = 0.9997) between absorbance and the concentration of copper(II) ion was found at the maximum absorption peak of 275 nm in the concentration range of 2–400 lM. The detection limit for copper(II) ion was 566 nM. The response of polyethyleneimine toward different metal ions was investigated, and polyethyleneimine displayed a high selectivity for the copper(II) ion among the metal ions examined. This biocompatible and sensitive sensor may find applications in copper(II) ion detection in environmental and biological processes. ª 2013 Production and hosting by Elsevier B.V. on behalf of King Saud University.

1. Introduction

* Corresponding author. Tel./fax: +86 23 68253237. E-mail addresses: [email protected], [email protected] (H.Q. Luo). Peer review under responsibility of King Saud University.

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In recent years, there has been great interest in the development of selective and sensitive probes for various heavy metal ions. Among the heavy metal ions, copper is one of a relatively small group of metallic elements which are essential to human health (Chan et al., 2010; Zong et al., 2011; Kra¨mer, 1998; Viguier and Hulme, 2006). However, unregulated copper can lead to disturbance of the cellular homeostasis, which will cause serious neurodegenerative diseases, such as Menkes disease, Wilson disease, and Alzheimer’s disease (Kim et al.,

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Please cite this article in press as: Wen, T. et al., A facile, sensitive, and rapid spectrophotometric method for copper(II) ion detection in aqueous media using polyethyleneimine. Arabian Journal of Chemistry (2013), http://dx.doi.org/10.1016/j.arabjc.2013.06.013

2 2008). In recent researches, copper has been suspected to damage infant liver (Zhou et al., 2008). Accordingly, the U.S. Environmental Protection Agency (EPA) sets 1.25 ppm (20 lM) as the maximum contamination concentration for Cu2+ in drinking water (EPA, 1991). Copper contamination and its potential toxic effects on human beings continue to be serious problems throughout the world because of the widespread use of Cu2+ in industry. Therefore, many methods to detect copper(II) ion are available, including electrochemistry (Yang et al., 2001, 2003), fluorimetry (Zhang and Ye, 2011; Li et al., 2012), atomic absorption spectrometry (Lin and Huang, 2001; Chan and Huang, 2000), inductively coupled plasma mass spectroscopy (ICPMS) (Wu and Boyle, 1997; Becker et al., 2007), and inductively coupled plasma atomic emission spectrometry (ICP-AES) (Otero-Romani et al., 2005; Liu et al., 2005). These methods are fast, reliable, and accurate for the determination of copper(II) ion in geological, biological, and environmental samples, but they often require expensive equipments or complicated operation. Currently, in order to satisfy the requirements of environmental management and environmental risk assessment, it is necessary and urgent to develop sensitive, fast, reproducible, simple, low-cost, and accurate analytical methods for the determination of copper in environmental and biological samples. Recently, using the spectrophotometric method for the determination of copper and the other heavy metals in water samples is advantageous because it is easily operated and inexpensive. A significant challenge for spectrophotometric assays is to distinguish the color change at extremely low concentrations (for example, nanomolar level) of metal ions with the naked eye. It is well known that polyethyleneimine (PEI), a cationic polymer which has been used for a wide variety of biological applications (Ram et al., 2000; Lei and Segura, 2009; Glodde et al., 2006), can act as an adsorbent for some heavy metal ions via chelation (Rivas et al., 2005; Molinari et al., 2004; Kislenko and Oliynyk, 2002). The chelation ability of Cu2+ to PEI is much stronger than that of Ni2+, Co2+, and other metallic ions. Thus, PEI has a significant selectivity to detect Cu2+. Furthermore, PEI can be associated to oligonucleotides to promote their transfection both in vitro and in vivo. Therefore, PEI can be used for biological and environmental applications. In the present study, we created a facile, direct, and rapid spectrophotometric method for sensing copper(II) ion in aqueous media by using PEI, as shown in Scheme 1. The PEI polymer can effectively and sensitively detect copper(II) ion (down to 566 nM) through forming a dark blue cuprammonium complex. The hydrophilic, low-cost, and biocompatible PEI chain segments serve as the chelating agent for copper(II) ion that displays high selectivity for the Cu2+ ion among other metal ions. The PEI-based sensing system shows many advantages, including rapid detection, high sensitivity, good selectivity, wide linear response range, and low-cost. And this method has been demonstrated to have promising applications for the detection of Cu2+ in environment. 2. Materials and methods 2.1. Apparatus A UV–Vis 2450 spectrophotometer (Shimadzu, Japan) was used to record the UV/Vis absorption spectra in the wave-

T. Wen et al. length 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 MX 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 mL1) was stable for at least a month when stored in the dark at 4 C. The working standard solution of PEI (0.94 mg mL1) 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 lL of Britton–Robinson (BR) buffer solution (pH 6.0) was added to a 1.5 mL eppendorf tube with 50 lL of PEI (0.94 mg mL1) solution. Subsequently, different amounts of Cu2+ solutions were added to the eppendorf tube. Then the mixture was diluted to 500 lL 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. Results and discussion 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


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

Scheme 1 Upon addition of Cu2+, the colorless PEI solution rapidly turns light blue and Cu2+-PEI complex exhibits the maximum absorption peak at 275 nm.

gated. Different amounts of PEI were added to the solutions containing a fixed amount of copper(II) ion (400 lM), 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 mL1) for the system is 50 lL. 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. Figure 1 Absorption spectra of PEI (0.094 mg mL1) in the absence and presence of Cu2+ (400 lM) in aqueous solution. The inset displays the color of the PEI solution (9.4 mg mL1) in the absence (i) and presence (ii) of Cu2+ (5 mM).

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. 3.2. Optimum conditions for polyethyleneimine-copper(II) complexation 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 investi-

3.2.2. The effect of pH In order to achieve the highly sensitive detection of copper(II) by using the PEI, the pH value of solutions was studied and optimized. We tested the absorbance of Cu2+-PEI complex at different pH values (Fig. S2). It was found that the PEI has no absorption in the pH range of 1.8–11.6 at the absorption peak of 275 nm. In the presence of Cu2+, the absorption of Cu2+-PEI complex is quite different over the wide pH range from 1.8 to 11.6. In strongly acidic media (pH < 3.0), the addition of Cu2+ has nearly no effect on the absorption spectrum of the system, which may be attributed to that the amino groups of the PEI are well protonated and are thus unable to chelate Cu2+ to form the cuprammonium complex. In alkaline solutions (pH > 7.0), the absorbance is not satisfied either, which may result from that partial hydrolysis of Cu2+ ion in the alkaline media inhibiting the complex reaction between Cu2+ and the amines of PEI. In contrast, in the weakly acidic media (pH 5.5–6.5), the absorbance has high values, suggesting that these weakly acid media can be chosen for the sensitive detection of Cu2+. Therefore, we chose the pH 6.0 as the optimum value.


4

T. Wen et al. 3.2.5. Effect of reaction time The reaction time of the system was then investigated. As shown in Fig. S5, the absorbance of the Cu2+-PEI chelate reached the maximum value as soon as 400 lM Cu2+ was added to the PEI solution (0.094 mg mL1), and kept stable in the following 1 h observation. This result indicates that the reaction between PEI and Cu2+ is rapid and stable, implying a promising application in fast sensing of Cu2+ without strict time control. 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 lL of PEI (0.94 mg mL1) 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 lM with the following equation: A ¼ 3972:9C þ 0:0152

ð1Þ 2+

Figure 2 (A) The absorbance of a series of solutions containing a fixed amount of copper(II) ion (400 lM) and different volumes (10, 20, 30, 40, 50, 80, 100, 150, 200 lL) of PEI solution (0.94 mg mL1). (B) Composition ratio for Cu2+-PEI complex in aqueous solution.

3.2.3. Effects of buffer solutions When dealing with complex ions, the accuracy of the quantification method depends on the ability of operating without disturbing the complex forming equilibrium (Kuljanin et al., 2002). The complex formation can be affected by the aqueous environment in which the reaction takes place. To find a suitable medium which allows good sensitivity and reproducibility of the response, four different reaction media, such as Na2HPO4–citrate acid, acetate buffer, sodium citrate–citrate acid, and BR buffer, were tested. The results showed (Fig. S3 A) that BR buffer was the best among the buffers, so BR buffer was selected as the proper reaction medium. Subsequently, we investigated the influence of the amount of BR buffer. The result (Fig. S3 B) indicated that the volume of BR buffer in the range of 50–400 lL had nearly no effect on the absorption of the system. In order to reduce costs, we chose 100 lL of pH 6.0 BR buffer for further studies.

where A is the absorbance of Cu -PEI complex and C presents the concentration of Cu2+ ion. We found that the detection limit was 566 nM for Cu2+ ion (at a signal-to-noise ratio of 3). Moreover, this detection limit was satisfactory for the Cu2+ ion detection in drinking water within the US EPA limit (20 lM) (EPA, 1991). The relative standard deviation of 2.6% was obtained for five replicate detections of 100 lM Cu2+ ion, which indicated a good repeatability of the present method. These results showed that the PEI had a very promising application for the detection of Cu2+ ion. 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

3.2.4. Effect of temperature The effect of temperature in the range of 20–50 C on the absorbance of the Cu2+-PEI complex solutions was studied, and the results are shown in Fig. S4. As shown in Fig. S4, the absorbance values of the system are almost the same at different temperatures, illustrating that temperature has little effect on the chelation of Cu2+ ion by PEI. Therefore, the reaction between PEI and Cu2+ ion does not require fine control of temperature.

Figure 3 Absorbance response of PEI (0.094 mg mL1) after the addition of Cu2+ (a–k: 2, 5, 10, 20, 50, 62.5, 80, 100, 200, 300, 400 lM) under optimum conditions. The inset displays the plot of absorbance of Cu2+-PEI complex versus the concentration of Cu2+ (2–400 lM).


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

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 mL1.

ever, 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 silveramine 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. 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 lM, which is well consistent with that obtained by the AAS method, namely 15.54 lM. 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.

Figure 5 Absorption spectra of PEI (9.4 mg mL1) in the absence and presence of Ag+ (4 mM) in aqueous solution.

method of quantitatively determining the concentration of copper in water samples. 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. How-

Table 1

4. Conclusion In conclusion, PEI was found to be an excellent probe for Cu2+ detection. PEI can recognize Cu2+ with very high selectivity, both UV–Vis spectrophotometrically and visually, via a simple coordination action between Cu2+ and PEI. Cu2+ ion can react with the amino groups of the PEI to form a dark blue cuprammonium complex that exhibited two absorption peaks at 275 and 630 nm, respectively. An appropriate selection of the experimental conditions ensured the sensitivity of the analytical method. Under optimized condition, the system displayed a detection limit as low as 566 nM toward Cu2+ and a good selectivity over other metal ions. The method has been successfully applied to the detection of Cu2+ in real samples with satisfactory results. Therefore, the PEI-based system shows many advantages, such as rapid detection, good selectivity, wide linear response range, simple operation, and lowcost. Overall, these results demonstrate that this method is capable of being a simple, practical, and reliable method for the detection of Cu2+ and has a great promise for environmental applications.

Analytical results of Cu2+ in the lake water samples.

Sample

Proposed methoda(lM)

RSD (%)

AAS method (lM)

Relative deviation (%)

1 2 3

16.34 16.40 16.27

2.3 2.5 2.1

15.59 15.55 15.47

4.8 5.4 5.1

a

Mean value of five determinations by the proposed method.


6 Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Nos. 21273174, 20975083), the Municipal Science Foundation of Chongqing City (No. CSTC-2013jjB00002), and the 211 Project of Southwest University (the Third Term).

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