Colorimetric determination of copper(II) ions using gold nanoparticles as a probe

Abstract

Copper is a highly toxic environmental pollutant with bio-accumulative properties. Therefore, it is highly desirable to develop a simple, sensitive, and selective assay for Cu²⁺ recognition. Herein, a colorimetric sensor for Cu²⁺ with high selectivity and sensitivity using gold nanoparticles (AuNPs) is presented. AuNPs were first stabilized by polyvinylpyrrolidone (PVP). 2-Mercaptobenzimidazole (MBI) would cause the aggregation of PVP stabilized AuNPs (PVP-AuNPs) for mercapto ligands self-assembled on the surface of AuNPs. This would lead to a color change from red to purple. In contrast, with the addition of Cu²⁺, MBI would be responsive exclusively towards Cu²⁺. This would block mercapto ligand assembly and AuNPs remained monodispersed, exhibiting a red color. Therefore, taking advantage of this mechanism, a “purple-to-red” colorimetric sensing strategy could be established for Cu²⁺ detection. With this strategy, the concentration of Cu²⁺ can be detected with the naked eye or with ultraviolet-visible spectroscopy, and the detection limits of Cu²⁺ were 5 μM and 0.5 μM, respectively. Additionally, the proposed method shows excellent anti-interference capability against many other metal ions, and real water sample applicability. Taken together, these advantages make this assay simple and robust and therefore promising for on-site water testing.

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1. Introduction

Detecting and recognizing metal ions is an important issue in both environmental monitoring and clinical research. Copper, an important cofactor for many protein enzymes in living organisms, is an essential and significant micronutrient required for multiple functions including bone formation, cellular respiration, and connective tissue development. However, elevated concentrations of Cu²⁺ may lead to adverse effects in organisms, resulting in gastrointestinal distress, liver or kidney damage, and serious neurodegenerative diseases.^1,2 The maximum allowable level of copper in drinking water set by the U.S. Environmental Protection Agency (EPA) is 1.3 ppm (∼20 μM).³ Nevertheless, copper continues to be one of the major components of environmental pollutants due to its widespread application in agriculture and industry. Therefore, it has become increasingly important to establish highly sensitive and effective methods to monitor the Cu²⁺ level in water.

Thus far, various efficient and reproducible methods, such as atomic absorption spectrometry,⁴ inductively coupled plasma mass spectroscopy (ICPMS),⁵ quantum-dot-based assays,⁶ electrochemical sensors,⁷ fluorescence sensors,^8,9 colorimetric detection,^10,11 and surface-enhanced Raman spectroscopy¹² have been developed for the detection of Cu²⁺. Among these developed methods, colorimetric chemosensors, have drawn much attention in recent years for their simplicity and ease of observation using the naked eye without the requiring complicated instrumentation. Colorimetric sensors based on precious metal nanoparticles, especially gold nanoparticles (AuNPs), can offer an attractive approach for simple, rapid, highly sensitive and selective recognition of various analytes including protein,¹³ cancer cells,¹⁴ small molecules and metal ions.^15–22

However, most AuNP-based assays for Cu²⁺ detection rely on the modification of AuNPs with specific binding-ligands, i.e., the concept of target-mediated cross-linking of AuNPs, which is often involved in toxic chemical reagents, time-consuming reactions and tedious purification. For example, Jiang report a method for the detection of Cu²⁺ using azide and alkyne-functionalized AuNPs in aqueous solutions by click chemistry.²³ And the minimum concentration of Cu²⁺ detected by eye is approximately 50 μM. However, this method required a series of complex modified procedures to prepare functionalized AuNPs and overnight incubation to facilitate aggregation, which made this detection method more complex and time-consuming. Kailasa et al. has demonstrated the application of AuNPs decorated with dopamine dithiocarbamate for the colorimetric detection of Cu²⁺ ions in water samples.¹¹ Nevertheless, when the surfaces of AuNPs were decorated with dopamine dithiocarbamate, the AuNPs would be aggregated and the color of bare AuNPs would change from red to purple. Also, low sensitivity (1 mM by eye vision and 14.9 μM by UV-vis spectra) would limit the application of the method. Mirkin's group reported a colorimetric Cu²⁺ sensor based on DNA-functionalized AuNPs.²⁴ The limit of detection (LOD) of the assay is 20 μM. However, this AuNPs-based assay relies on the modification of AuNPs with specifically ligands. In addition, this approach requires alternating temperature in the experimental process, and the LOD is high (20 μM). In order to avoid the elaborate and tedious synthesis of ligand-modified AuNPs, an assay based on anti-aggregation or re-dispersion mechanism of AuNPs has recently been employed as a good alternative way to control AuNPs aggregation. The loss of steric stabilisation for ligand-stabilised AuNPs is generally one of the main approaches to induce colloid aggregation in a non-crosslinking mechanism.^15,25–27

Herein, we report an AuNPs colorimetric probe suitable for the facile, sensitive and selective detection of Cu²⁺ by means of the anti-aggregation of AuNPs with the use of a commonly available small organic molecule 2-mercaptobenzimidazole (MBI). MBI is an important member of the thioureylene compound family, and it has long been known as a corrosion inhibitor for the protection of metals. It effectively protects metals such as copper in various aggressive environments through the formation of a chemisorbed layer on the metals.^28,29 MBI possesses a terminal thiol moiety, thus it can bind onto the surface of AuNPs through the Au–S bond and form the MBI functionalized AuNPs. Also MBI is hydrophobic,³⁰ which would induce the aggregation of AuNPs. It has been reported that Cu²⁺ ions show a strong affinity towards MBI.³¹ Taking advantage of this unique attribute of Cu²⁺, we devised a new, facile and colorimetric sensor to detect Cu²⁺ based on the anti-aggregation of AuNPs, through the competing combination with MBI between Cu²⁺ and AuNPs. PVP-stabilized AuNPs were firstly prepared and MBI was used as aggregation agent. Most of previous methods were based on the fact that AuNPs are induced to aggregate by inter-particle cross-linking in the presence of Cu²⁺, then the color of AuNPs solution changes from red to purple or blue. However, in our method, the detection of Cu²⁺ was realized through interrupting the aggregation of gold nanoparticles induced by MBI, which can be observed by the naked eye according to the color changing from purple to red. However, to our knowledge, there are few reports on the colorimetric detection of Cu²⁺ based on anti-aggregation of AuNPs.^32,33 In addition, it is much simpler and more cost-effective than the other existing methods for Cu²⁺ assay by using a common molecule MBI in this method. In this simple way, 5 μM Cu²⁺ was detected only by the naked eye.

2. Experimental section

Reagents and apparatus

Chloroauric acid (HAuCl₄·4H₂O), CuSO₄, MBI and trisodium citrate were purchased from Sigma-Aldrich. Polyvinylpyrrolidone (PVP), KCl, Fe(NO₃)₂, Fe(NO₃)₃, MgCl₂, HCl, NaOH, and other chemicals were obtained from Shanghai Reagent Co. Ltd. All of these reagents were analytical grade and were used without further purification. Unless otherwise noted, distilled water was used throughout the course of the investigation. The room temperature at which the work was conducted was 25 °C. The working solutions of Cu²⁺ were freshly prepared by dilution from the copper(II) sulfate stock solution (1 M). MBI stock solution (10 mM) in ethanol was freshly prepared and used up within seven days of storage. Both the copper(II) sulfate and DTT stock solutions were stored at 4 °C. The Na₂HPO₄–NaH₂PO₄ buffer solution (200 mM, pH = 6.0) was prepared by dissolving Na₂HPO₄ (0.881 g) and NaH₂PO₄ (2.737 g) in water (100 mL). UV-vis spectra were measured on a Shimadzu UV-2550 UV-vis spectrophotometer operated at a resolution of 0.5 nm. Photographs were taken using a digital camera.

Synthesis of AuNPs

Citrate-capped AuNPs were prepared according to the Frens method.³⁴ In brief, 100 mL aqueous solution of HAuCl₄·4H₂O (0.01%) was rapidly heated to boiling under vigorous stirring in a three-necked flask. After that, 1 mL of trisodium citrate solution (1%) was quickly added, resulting in the change of solution color from pale yellow to deep red. After the color change, the solution was heated for an additional 30 min for complete reduction of the Au(III) ions. The maximum absorption wavelength of the AuNPs, which was measured by UV-visible spectrophotometer, was 530 nm.

Procedure for Cu²⁺ detection

1 mL AuNPs was put into a 1.5 mL centrifugal tube. After that the citrate-AuNPs was collected by centrifugation at 10 000 rpm for 10 min and the supernatant ones were carefully removed up to a residual volume of 50 μL. Distilled water and PVP capping agent were then added to form the PVP-stabilised AuNPs (PVP-AuNPs). The total volume of the solution was 900 μL. While the total PVP concentration in the final PVP-AuNPs suspension was 0.025%. For the detection of Cu²⁺, typically, an aliquot (80 μL) of a solution of MBI in ethanol was placed separately in centrifuge tube, into which 20 μL of Na₂HPO₄–NaH₂PO₄ buffer solution (200 mM, pH 6.0) and different concentrations of Cu²⁺ were added. The total volume of the mixture solution was 100 μL, and the final concentrations of MBI and buffer were 10 μM and 20 mM, respectively. The mixtures were equilibrated at room temperature for 30 min. Afterwards, the solution was sequentially added with 900 μL of PVP-AuNPs. Although the change of color was instant, 5 min was allowed for the full colorimetric response, which was then measured with the naked eye or with UV-vis spectroscopy at room temperature.

3. Results and discussion

General principle for the detection of Cu²⁺

The principle of the Cu²⁺ colorimetric sensor is illustrated in Fig. 1. The as-prepared PVP-AuNPs is stable in the aqueous solution. However, MBI would be anchored onto the surface of the AuNPs through Au–S covalent bond^35–37 and induce the aggregation of AuNPs with a corresponding color change (red to purple or colorless) if no Cu²⁺ is present. Meanwhile, PVP molecules are desorbed from the AuNPs. Interestingly, when MBI, which can induce an observable aggregation of AuNPs, is firstly treated with Cu²⁺ and then mixed with PVP-AuNPs, the color of AuNPs solution changes from purple to original red with increasing concentration of Cu²⁺, which corresponds to PVP-AuNPs changing from aggregation to dispersion state. This anti-aggregation or re-dispersion effect of PVP-AuNPs is due to the higher affinity of Cu²⁺ for MBI. The PVP-AuNPs were first characterized by UV-vis spectroscopy, and then corresponding photographic images were gathered. As shown in Fig. 2, a characteristic surface plasmon resonance (SPR) band of PVP-AuNPs is observed in the spectrum at approximate 530 nm and the solution color is ruby red (inset of Fig. 2). The absorbance band at 530 nm is ascribed to the SPR absorption band of dispersed AuNPs. It is well-known that the NPs solution can be precipitated by adding excess amount of mercapto molecules or organic derivatives on the surfaces of NPs system. In the absence of Cu²⁺, the addition of MBI (10 μM) to the PVP-AuNPs significantly induces a red shift of PVP-AuNPs' maximal absorption band, and the solution color turns purple or colorless owing to the MBI-stimulated aggregation. However, when a MBI solution with a concentration that can induce an observable aggregation of PVP-AuNPs is firstly treated with Cu²⁺ (100 μM) and then mixed with the PVP-AuNPs suspension, the maximal absorption band of PVP-AuNPs goes through a reverse process, and the color changes back to red (Fig. 2). In other words, Cu²⁺ would remarkably prevent MBI from inducing the aggregation of PVP-AuNPs. Because Cu²⁺ could be effectively combined with MBI under the above experimental conditions, leaving little or no chance for MBI to bind on the surface of PVP-AuNPs. If aggregated AuNPs were firstly induced by MBI and then treated with Cu²⁺, the aggregates of the AuNPs cannot be destroyed by Cu²⁺. As can be seen from Fig. S1,† upon addition of Cu²⁺ with concentrations up to 1 mM, there is almost no obvious change of the plasmon band of the aggregated AuNPs. So, our assay should proceed as two individual steps. That is, MBI was firstly reacted with Cu²⁺ and then incubated with PVP-AuNPs. The formed Cu²⁺-MBI complexes could be confirmed by UV-vis absorption spectra. As shown in Fig. S2,† the solution after reaction of MBI with Cu²⁺ gave significant absorption change, which was different from MBI or Cu²⁺. This new absorption is probably characteristic of the complexes of MBI and Cu²⁺. According to the literature, MBI has a rigid structural backbone with a delocalized p-electron cloud and a relatively large [double bond splayed right]S–C–N bond angle, which facilitates the formation of stable complexes of Cu²⁺.^31,38 Thus, the aggregation role of MBI is deactivated by Cu²⁺. Therefore, Cu²⁺ could be readily detected through monitoring the color change of PVP-AuNPs solution by just naked eye. Since the color change of the PVP-AuNPs is directly dependent on the Cu²⁺ concentration, the AuNPs–MBI system can serve as a colorimetric probe for the quantitative detection of Cu²⁺. Furthermore, the affinity of MBI for Cu²⁺ is superior to other metal ions,³¹ so the deactivation of MBI by Cu²⁺ exhibits specificity in coexistence of other metal ions.

Fig. 1 Schematic representation for the Cu²⁺ colorimetric detection method.

Fig. 2 UV-vis spectra of (a) original PVP-AuNPs solution, (b) AuNPs aggregates induced by MBI (10 μM), and (c) the PVP-AuNPs solution containing the mixture of MBI (10 μM) and Cu²⁺ (100 μM); the inset photographic images are the corresponding colorimetric response.

Optimum reaction conditions

As illustrated in Fig. 3a, the as-prepared PVP-AuNPs nanodispersion exhibits wine-red color and shows a strong absorbance band at 530 nm in the UV-vis absorption spectrum. The addition of MBI to the PVP-AuNPs led to an observable aggregation of the AuNPs, and the solution color turned from the original wine red to purple or colorless. We also noticed that the color of PVP-AuNPs remained colorless by the addition of MBI concentration from 10 to 100 μM. In the corresponding UV-vis spectra (Fig. 3a), with the increase of MBI concentration the intensity of the absorption band at 530 nm decreased systematically accompanied with the appearance of a new absorption band at ∼650 nm, which originates from the interparticle coupled plasmon absorbance of the aggregated AuNPs. The absorbances at 650 and 530 nm are related to the amounts of aggregated and dispersed AuNPs, respectively. So, the ratio of the absorbance at 650 nm to that at 530 nm (A₆₅₀/A₅₃₀) was used to express the molar ratio of aggregated to dispersed AuNPs. As shown in Fig. 3b, the abscissa denotes the concentration of MBI, and the ordinate denotes the relative absorption value of the PVP-AuNPs with the addition of MBI. The A₆₅₀/A₅₃₀ of PVP-AuNPs increased with the increase of MBI. When the concentration of MBI is higher than 10 μM, no obvious change could be observed for the value of A₆₅₀/A₅₃₀. This result indicated that the PVP-AuNPs were almost completely aggregated by the addition of 10 μM MBI. Since, improvement in the sensitivity of the Cu²⁺ detection relies on a decrease in the amount of MBI used. Therefore, we selected 10 μM of MBI concentration as the optimal concentration for further experiments.

Fig. 3 (a) UV-vis spectra of PVP-AuNPs containing different concentrations of MBI in buffer. The inset photographic images displays that the aggregation of PVP-AuNPs induced by different concentrations of Cu²⁺. The Cu²⁺ concentrations in μM are listed at the top of the respective solutions. (b) Effect of MBI concentration on the value of A₆₅₀/A₅₃₀ of the PVP-AuNPs-based detection system. Effect of ionic strength on the aggregation of AuNPs (c) in the absence and (d) presence of PVP (0.025%).

Ionic strength of the solution exerts a strong effect on the interaction between citrate coated AuNPs. Fig. S3a† shows the UV-vis spectra of AuNPs in the presence of various Cu²⁺ concentrations. Initially, the as-prepared AuNPs with citrate as the stabilizer appeared red in color. After salt was added, the electrostatic repulsion between negatively charged (citrate) AuNPs was screened at a chosen salt concentration, resulting in the aggregation of AuNPs with a red-to-purple colour change. This aggregation is not selective to Cu²⁺, because other multivalent cations can also lead to such aggregation. Therefore, bare AuNPs could not be used for sensing Cu²⁺ directly. This aggregation could be efficiently inhibited with the prior addition of PVP to a dispersion of AuNPs. PVP is a suitable polymer surfactant employed in our experiment, for it is water-soluble, non-ionic, non-toxic and often used in various medical applications. As a matter of fact, many studies have focused on the synthesis of nanoparticles by using the PVP polymer as the stabilizer or capping agent to protect nanoparticles from coagulation or precipitation. As can be seen from Fig. S3a,† with increasing Cu²⁺ concentration, AuNPs displays an obvious decrease in absorption at 530 nm and an increase at 650 nm. Also the A₆₅₀/A₅₃₀ increases, which indicates the aggregation of AuNPs (Fig. 3c). By contrast, in the absence of MBI, the addition of only 0.025% PVP caused no aggregation of AuNPs with increasing the amount of Cu²⁺ up to 10 mM (Fig. 3d). The results indicated that the PVP-coated AuNPs were significantly more stable than citrate-coated AuNPs, which is likely due to steric repulsion imparted by the large and noncharged polymers. But, at relatively high concentrations of PVP, the adsorption of MBI on the surface of AuNPs decreased, reducing the degree of AuNPs aggregation. Fig. S4† shows the UV-vis absorption spectra of AuNPs stabilized by different concentrations of PVP in the presence of 10 μM MBI. As the concentration of PVP increasing to 0.03%, the AuNPs solution in maximum absorbance at 530 nm would remarkably grow and the absorption band at 650 nm would decrease. The A₆₅₀/A₅₃₀ of AuNPs also decreased with the increase of PVP (Fig. 4a). Obviously, when the concentration of PVP is higher than 0.025%, the A₆₅₀/A₅₃₀ decreased sharply. This result indicated that the AuNPs aggregate induced by MBI was significantly alleviated. Since, the more PVP was added, the more MBI was needed to produce aggregation of AuNPs completely. In view of the sensing effect of our detection system, the PVP concentration is fixed at 0.025% for the subsequent experiments.

Fig. 4 (a) Effect of PVP concentration on the absorbance of AuNPs (900 μL) added with 100 μL of buffer (20 mM) and MBI (10 μM). The concentrations of PVP in the AuNPs were 0, 0.01%, 0.02%, 0.025%, 0.03%, 0.05% and 0.1%, respectively. (b) Effect of pH on the value of A₆₅₀/A₅₃₀ of the PVP-AuNPs-based detection system in the absence (curve 1) and presence (curve 2) of 3 μM Cu²⁺. (c) A₆₅₀/A₅₃₀ values of the PVP-AuNPs solutions after the addition of MBI (10 μM) and Cu²⁺ (10 μM) mixture solutions at different reaction times.

To be a major influence factor on the stability of PVP-AuNPs, the pH of the solution needs to be optimized so that the influence on the stability of the PVP-AuNPs and the background agglomeration will be minimal. The reaction conditions were the same as those of the typical assay except the varied factors that we should explore, and the concentration of MBI was fixed at 10 μM. As can be seen from Fig. 4b, in the pH range of 3.0–8.0, the value of A₆₅₀/A₅₃₀ almost remains unchanged in the absence of Cu²⁺, which illustrates that the pH in this range has little effect on the aggregation of PVP-AuNPs induced by MBI. When the pH levels were lower than 5.0, the high value of A₆₅₀/A₅₃₀ observed in the presence of Cu²⁺ indicated a significant decrease in the stability of PVP-AuNPs against aggregation, which led to poor sensitivity for the detection of Cu²⁺. This shows that chelation between Cu²⁺ and MBI is not favoured at low pH. With the addition of 3 μM Cu²⁺, which could be combined with MBI, the aggregation of AuNPs would be alleviated at the pH range of 6.0–8.0 (Fig. 4b, curve 2). However, when the pH is higher than 8.0, the absorbance ratios (A₆₅₀/A₅₃₀) decreased sharply whether there had Cu²⁺ or not. It was supposed that MBI would be removed from AuNPS surfaces through alkaline cleavages at high pH values. Due to the hydrolysis of Cu²⁺ and formation of some hydroxy complexes of Cu²⁺ under alkaline conditions, Na₂HPO₄–NaH₂PO₄ buffer solution of pH 6.0 was selected to control the acidity of the analytical solution.

The absorbance ratios (A₆₅₀/A₅₃₀) strongly depended on the time of reaction (Fig. 4c). In presence of 10 μM Cu²⁺, the value of A₆₅₀/A₅₃₀ significantly decreased with time until it reached a plateau in ∼30 min. Therefore, to quantitatively analyze the Cu²⁺ concentration, 30 min was chosen as the reaction time for the MBI and Cu²⁺ mixture.

Detection of Cu²⁺

Under the optimized conditions, MBI was reacted with different concentrations of Cu²⁺ for 30 min and then incubated with PVP-AuNPs for a further 5 min. The corresponding colors of the solutions with different concentrations of Cu²⁺ are shown in Fig. 5a. We can observe a gradually blue-to-red color change when the concentration of Cu²⁺ increased from 5 to 20 μM. The detection limit of Cu²⁺ was 5 μM. This result demonstrates that the proposed method can be used for the direct detection of Cu²⁺ with the naked eye.

Fig. 5 (a) UV-vis absorption spectra of PVP-AuNPs in the presence of a series of concentrations of Cu²⁺ ions. From ground to top, the concentrations of Cu²⁺ are 0 μM, 0.5 μM, 1 μM, 3 μM, 5 μM, 7.5 μM, 10 μM, 20 μM, 50 μM, 100 μM, 150 μM, 300 μM and 500 μM, respectively. Naked-eye observation of different concentrations of Cu²⁺ was shown in the inset. The Cu²⁺ concentrations, in μM, are listed at the top of the respective solutions. (b) The plot of absorbance against the concentrations of Cu²⁺. (c) Linear fitting curve of the A₆₅₀/A₅₃₀ value versus the concentrations of Cu²⁺ from 0.5 μM to 10 μM. The scale bars represent the standard deviations of three replicated samples.

To evaluate the minimum concentration of Cu²⁺ that can be detected by this colorimetric method, UV-vis spectroscopy was used to quantitatively determine the concentration of Cu²⁺. As shown in Fig. 5b, with an increase in Cu²⁺ concentration, more MBI would be consumed by Cu²⁺, leading to a decrease in the aggregation of PVP-AuNPs and a corresponding decrease in the ratio of A₆₅₀/A₅₃₀. As shown in Fig. 5c, the calibration curve for the value of A₆₅₀/A₅₃₀ against Cu²⁺ concentration was linear in the range from 0.5 to 10 μM and fit the linear equation y = −0.0433x + 1.0185 (R² = 0.9891). The detection limit of Cu²⁺ was 0.5 μM, which is lower than the safe limit of copper ions in drinking water (1.3 mg L⁻¹, ∼20 μM) set by the United States Environmental Protection Agency (EPA).^8,25

Selectivity of the detection procedure

To evaluate the selectivity of this assay, it was challenged with other environmentally relevant metal ions, including K⁺, Ag⁺, Mg²⁺, Co²⁺, Ca²⁺, Mn²⁺, Ni²⁺, Pb²⁺, Hg²⁺, Zn²⁺, Fe²⁺, Cd²⁺, Ba²⁺, Al³⁺ and Fe³⁺, under typical experimental conditions, one of these metal ions was added to the MBI solution for a 30 min reaction time, and then incubated for a further 5 min in the presence of PVP-AuNPs. As shown in Fig. 6, the figure presents the change in the intensity ratio (y axis) with a series of metal ions (x axis). No noticeable interference for the detection of Cu²⁺ was observed. Apart from the above considerations, it should be mentioned that Au³⁺ would interfere with the assay when its concentration was higher than 1 μM. Here it is worth noting that the employed concentration for all the interfering metal ions was 50 μM. In Fig. 6b we have also presented the absorbance ratio for 0.5 μM Cu²⁺. This was included for the sake of comparison. Clearly, from this figure we observe that when the Cu²⁺ concentration is 0.5 μM, the A₆₅₀/A₅₃₀ value is less than the ratio corresponding to other metal ions at a concentration of 50 μM. Thus, the minimum concentration of Cu²⁺ in a solution that the sensor could detect was 0.5 μM.

Fig. 6 The selectivity of the sensing system for the detection of Cu²⁺. (a) Naked-eye observations of the sensing system towards Cu²⁺ and other metal ions. (b) Diagram of the A₆₅₀/A₅₃₀ value against varied metal ions. The blank is without Cu²⁺; the lowest and second-lowest signals are with 0.5 and 10 μM Cu²⁺, respectively.

Real sample tests

To assess the potential practical application of the assay to measure the Cu²⁺ content, the detection of Cu²⁺ in river water and tap water was carried out. None of them contained Cu²⁺, so they were spiked with three levels of Cu²⁺ (1 μM, 3 μM, and 5 μM) and analyzed. As shown in Table 1, the average recoveries ranged between 96% and 109%, with the relative standard deviations being less than 8% (n = 3). The results demonstrated the potential application of this method for the determination of Cu²⁺ in practical sample analysis.

Table 1 Determination of Cu²⁺ in water samples

Sample	Detected (μM)	Added (μM)	Found (μM)	RSD (%)	Recovery (%)
River water	Undetectable	1	1.09	7.4	109
		3	2.88	5.6	96
		5	5.12	6.3	102
Tap water	Undetectable	1	1.05	5.2	105
		3	3.07	5.6	102
		5	4.96	4.9	99

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4. Conclusion

In conclusion, we have developed a probe for the rapid and selective colorimetric detection of Cu²⁺ based on anti-aggregation of AuNPs induced by MBI and Cu²⁺. The sensitivity of our assay for the detection of Cu²⁺ levels in water is less than the EPA standard limit. The color intensity values were linear with the concentration of Cu²⁺ ranging from 0.5 to 10 μM with a coefficient of 0.9891, and showed good sensitivity with a LOD = 0.5 μM. In comparison to other nanoparticles-based optical methods, our assay for the detection of Cu²⁺ avoided the complex process to modify nanoparticles, and abrogated the need for complicated chemosensors or sophisticated equipment. We validated the practicality of this method through the analysis of river water and tap water samples. It is believed that the present approach holds great potential for the detection of Cu²⁺ in real samples.

Acknowledgements

This research was supported by the Doctor Foundation of Henan Institute of Engineering (D2014016) and the National Natural Science Foundation of China (51302102).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra20381c

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