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Polyethylene glycol capped ZnO quantum dots as a fluorescent probe for determining copper(II) ion
Sensors and Actuators B 253 (2017) 137–143
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Polyethylene glycol capped ZnO quantum dots as a fluorescent probe for determining copper(II) ion Shuo Geng, Shu Min Lin, Nian Bing Li ∗ , Hong Qun Luo ∗ Key Laboratory of Eco-environments in Three Gorges Reservoirs Region (Ministry of Education), School of Chemistry and Chemical Engineering, Southwest University, Chongqing, 400715, PR China
a r t i c l e
i n f o
Article history: Received 2 January 2017 Received in revised form 14 June 2017 Accepted 17 June 2017 Available online 19 June 2017 Keywords: PEG@ZnO QDs Fluorescent probe Copper(II) ions Determination Aggregation induced quenching
a b s t r a c t As an important heavy metal ion, copper has the negative influence on otherwise healthy individuals, so establishing a valid way for the highly efficient, sensitive, and quantitative determination of Cu2+ ion becomes an emergency in the environmental analysis. In the present work, water-soluble luminescent ZnO quantum dots (QDs) capped by polyethylene glycol (PEG) have been synthesized by a simple solution method. The PEG capped ZnO QDs (PEG@ZnO QDs) showed yellow fluorescence. High-resolution transmission electron microscopy, UV–vis absorption spectroscopy, Fourier transform infrared spectroscopy, and luminescence spectroscopy were applied to elucidate the properties of the PEG@ZnO QDs. In addition, the yellow fluorescence of the PEG@ZnO QDs was quenched when Cu2+ ion was added to the PEG@ZnO QDs solutions. Therefore, a novel fluorescent probe was designed to detect Cu2+ in water solution. The linear relationships were 10–200 nM and 2–10 M, respectively, with the detection limit for Cu2+ at 3.33 nM according to 3/slope (where denotes the standard deviation of the blank measures). The proposed sensor of the PEG@ZnO QDs has also been used in natural water samples to examine the availability of this method. In addition, the quenching mechanism was discussed, which may be attributed to the aggregation induced quenching. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Heavy metal ion has attracted a great attention over the past years because of the negative influence on otherwise healthy individuals [1–4]. As a generally used heavy metal, copper(II) ion can be found in various water samples and is involved in fundamental life process as one of the essential micronutrients [5–7]. It is healthy for people when intake of a small amount of copper(II) ion, while it can be caused a negative effect on the central nervous system, liver, kidneys and gastrointestinal system with an excessive intake of copper(II) ion [2,3,8–15]. The World Health Organization (WHO) has developed drinking-water quality guidelines that copper is perceived as a “chemical of health significance in drinking water” and then German water quality standards declared that copper(II) ion concentration in drinking water should be less than 1 ppm [6,16–18]. Hence, establishing a valid probe for the highly efficient, sensitive, and quantitative detection of Cu2+ ion becomes an emergency in the food testing, biological assay, and environmental analysis.
∗ Corresponding author. E-mail addresses: [email protected] (N.B. Li), [email protected] (H.Q. Luo). http://dx.doi.org/10.1016/j.snb.2017.06.118 0925-4005/© 2017 Elsevier B.V. All rights reserved.
Over the past years, a lot of ways have been established to determine copper(II) ion, for instance, atomic absorption spectrometry (AAS), chemiluminescence, spectrophotometry, visual detection, inductively coupled plasma atomic emission spectroscopy (ICPAES), and electrochemical techniques [5,19–26]. Whereas the above methods have lots of shortcomings, for example, reagent toxicity, costly instruments, complicated operation procedures, and tedious sample preparation. Hence, a sensitive, simple, and inexpensive method for the determination of copper ion needs to be developed. The detection method based on fluorescence has developed very fast among other detection methods in recent years [27–29]. Some fluorescent materials including quantum dots (QDs), carbon dots, and zero-dimensional nanomaterials have been applied diffusely to determination of various metal ions because of the wider absorption spectra, low cost, big Stokes shift, tunable wavelength, and easy preparation [30–32]. Right until today, various types of functionalized QDs have been used to detect various ions such as Ag+ , Cu2+ , Cd2+ , CN− , and Hg2+ [33–38]. Tao et al. demonstrated that the 3-mercaptopropinic acid stabilized CdTe/CdS quantum dots can be applied as a copper(II) ion sensor based on that their near-infrared fluorescence was quenched by the aggregation of the quantum dots caused by the competitive binding between 3-mercaptopropinic
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acid and the copper(II) ion [18]. Liu et al. developed a detection of Cu2+ ion probe according to fluorescence quenching of the graphene oxide (GO) modified Ag-In-Zn-S (AIZS) quantum dots when copper(II) ion was added to the above fluorescence probe with its limit of detection of 0.18 M [16]. Chao and coworkers established the use of 1-(2-thiazolylazo)-2-naphthol coated CdTe QDs as a copper(II) ion sensor [27]. These methods demonstrate that the ion sensors based on QDs are in great potential for the chemical and biological application. Nevertheless, the above quantum dots mostly contain toxic and environmental unfriendly heavy metal elements. In the present work, water-soluble and monodisperse PEG@ZnO QDs were synthesized by a simple sol-gel solution method, which were characterized by fluorescence, high-resolution transmission electron microscope, UV–vis absorption spectroscopy, and Fourier transform infrared spectroscopy. In addition, the fluorescence of PEG@ZnO QDs can be quenched by copper(II) ion. Hence, we developed a novel fluorescent probe for selective and sensitive detection of copper(II) ion in aqueous solution. This proposed method showed that the relative fluorescence intensity had two linearly proportional relationships to the concentration of copper(II) ion within the range 10–200 nM and 2–10 M, with the detection limit down to 3.33 nM. This probe had some advantages, such as simple and fast determination procedure, high sensitivity, wide linear range, good selectivity, nontoxicity, and little equipment investment. And this method offers great promise in copper(II) ion detection in natural water and biological analysis. 2. Experimental section 2.1. Materials All the reagents were used as received without further purification. Ethanol absolute, zinc acetate dihydrate (Zn(CH3 COO)2 ·2H2 O), and potassium hydroxide (KOH) were obtained from Chengdu Kelong Chemical Reagent Co. Ltd. (Chengdu, China). Polyethylene glycol (PEG, average mass 1500) was obtained from Aladdin. Britton-Robinson (BR) buffer (10 mM, pH 5.94–8.05), KH2 PO4 -NaOH buffer (0.1 M), and Na2 HPO4 NaH2 PO4 buffer (0.2 M) were employed. Ultrapure water (18.2 M cm) was used throughout all the experiments. 2.2. Instruments A KQ-400 KDB ultrasonic bath (400 W, Kunshan Ultrasonic Instruments Co., Ltd, China) was applied to dissolve the Zn(CH3 COO)2 ·2H2 O and KOH in ethanol completely. An 85-2 constant temperature magnetic mixer (Zongda Instrument Plant, Jiangsu, China) was applied to mix the above solutions completely. A TGL–16 M high-speed refrigerated centrifuge (Xiangyi, China) was used to collect the precipitates. A Hitachi F-2700 fluorescence spectrophotometer (Hitachi Ltd., Japan) was applied to measure the fluorescence intensity in this work. A Shimadzu UV-2450 spectrophotometer (Suzhou Shimadzu Instrument Co., Ltd., China) was used to measure the ultraviolet-visible absorption spectra with two 1-cm cuvettes. The morphology of the PEG@ZnO quantum dots was studied by a high-resolution transmission electron microscope (HRTEM) from JEM-2100 (JEOL Ltd., Japan). And a software named Nano Measurer was used to measure the size of the PEG@ZnO quantum dots. Fourier transform infrared (FTIR) spectra were recorded by using a Bruker IFS 113 v spectrometer (Bruker, Germany). 2.3. Preparation of PEG@ZnO quantum dots The PEG@ZnO quantum dots (PEG@ZnO QDs) were obtained by a two-step process based on prior works with a little modification
[39,40]. First, 0.1683 g of KOH was dispersed in 10 mL of ethanol and 0.1098 g of Zn(CH3 COO)2 ·2H2 O dispersed in 50 mL of ethanol completely by keeping the above solutions in an ultrasonic bath for an hour at room temperature. Then ZnO quantum dots were synthesized by the dropwise addition of 10 mL of KOH (0.3 mol/L) to the 50 mL of Zn(CH3 COO)2 (0.01 mol/L) followed by continuously stirring for 120 min at room temperature. After that, 5 mL of ethyl acetate was added to the above ZnO quantum dots solutions to obtain the precipitate of ZnO quantum dots, then the precipitate was obtained by centrifugation and purified by washing with ethanol. And the precipitate was dispersed in 20 mL of ethanol for using in next step. In the second step, the PEG-modified ZnO quantum dots were synthesized by the dropwise addition of 4 mL of PEG (0.15 g (0.1 mmol) of PEG was dissolved in 4 mL ethanol) into the above prepared ZnO QDs (4 mL) followed by churning for 60 min at room temperature. Next, the PEG@ZnO QDs were collected by centrifugation and purified by washing with ethanol and ultrapure water. Then the precipitate was dissolved in 10 mL of ultrapure water under ultrasonic and agitation for using in the next procedure and characterization. 2.4. Detection of Cu2+ by PEG@ZnO QDs The Cu2+ detection procedure using PEG@ZnO QDs as the fluorescent probe is described as follows: 5 L of Cu2+ with different levels was added to 400 L of PEG@ZnO QDs solution and then the mixture was diluted to 500 L with ultrapure water. After that, the fluorescence intensity of the above solutions was measured using the fluorescence spectrophotometer with the excitation and emission wavelengths of 351 and 543 nm. The relative fluorescence intensity of PEG@ZnO QDs in the presence (F) and absence (F0 ) of Cu2+ was denoted as F0 –F. And the natural water samples were applied to prove that this method is valid in real sample analysis. 3. Results and discussion 3.1. Characterization of PEG@ZnO QDs Characterization of the PEG@ZnO QDs was explored by using UV–vis absorption, fluorescence, FTIR, and HRTEM. As shown in Fig. 1 (A) (solid line), the absorption maximum was around 351 nm, which shifts to short wavelength compared to the peak at 380 nm of a bulk ZnO, and this can prove that the PEG@ZnO QDs was synthesized in a certain degree. The excitation wavelength and the emission wavelength of the PEG@ZnO QDs were at 351 and 543 nm, respectively (Fig. 1 (B), solid line). And the PEG has no fluorescence emission when excited at 351 nm (Fig. 1 (B)), illustrating that the fluorescence of PEG@ZnO QDs is stemmed from ZnO QDs. A yellow fluorescence and well water-soluble QDs can be seen from the photograph of the PEG @ ZnO QDs (inset of Fig. 1 (B), left and middle). The PEG@ZnO QDs were mono-dispersed well in water and the diameter of the PEG@ZnO QDs was about 6 nm (Fig. 1 (C)). The PEG capped on the ZnO surface was successfully confirmed by the FTIR spectrum. Fig. 1 (D) shows the FTIR spectra of PEG@ZnO QDs, PEG, and ZnO QDs. About pure ZnO QDs, the band around 3396 cm−1 can be assigned to the stretching vibration of OH at ZnO QDs surface, and the peaks at 1504 and 1394 cm−1 corresponded to asymmetric and symmetric C O stretching modes of Zn(CH3 COO)2 . In addition, the band at the region of 474 cm−1 could be assigned to the Zn-O stretching of ZnO QDs. About PEG spectrum, the peaks at 3409 and 1644 cm−1 belong to the stretching vibration and bending vibration of the OH, respectively; the peaks at 2886 and 1468 cm−1 can belong to the symmetrical stretching vibration and bending vibration of CH2 –, respectively; the peaks at 1359, 1344, and 1115 cm−1
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Fig. 1. (A) UV–vis absorption spectra of the PEG-capped ZnO QDs (the synthesized), and PEG-capped ZnO QDs with Cu2+ (10 M); (B) Fluorescence excitation and emission spectra of PEG-capped ZnO QDs, emission spectrum of PEG, and the emission spectrum of PEG-capped ZnO QDs with Cu2+ (10 M). Inset: photograph (left) of the PEG-capped ZnO QDs solution under visible light, photograph (middle) of the PEG-capped ZnO QDs solution excited with UV light at 365 nm and photograph (right) of the PEG-capped ZnO QDs solution with Cu2+ (10 M); (C) TEM image of PEG-capped ZnO QDs; (D) FTIR spectra of ZnO QDs, PEG, and PEG-capped ZnO QDs.
can be assigned to the stretching vibration of C O. After PEG capped the ZnO QDs, stretching vibration and bending vibration of the OH the typical bands of PEG and ZnO QDs are also checked with a little shift, demonstrating that the functional groups of PEG and the ZnO QDs were not destroyed and the PEG was successfully conjugated to the ZnO QDs surface. Both of stretching vibration and bending vibration of the OH of the typical bands of PEG and ZnO QDs are also checked with a little shift, demonstrating that the formation of intermolecular hydrogen bonds by the OH of PEG on the surface of ZnO QDs, thereby confirming the capping of PEG on ZnO QDs [41]. The PEG@ZnO QDs can be stored in a refrigerator at 25 ◦ C for 1 month which made the fluorescence negligible decrease. As shown in Fig. 1B (dash line), when Cu2+ (10 M) was added to the PEG@ZnO QDs, the fluorescence of the PEG@ZnO QDs was quenched quickly (inset of Fig. 1B, right). Therefore, a probe for detection of Cu2+ was developed and designed based on the as prepared PEG@ZnO QDs.
3.2. Optimization of the experimental conditions To achieve the highest quenching efficiency of the PEG@ZnO QDs by Cu2+ ion, we optimized several experimental parameters such as reaction time, buffer solution type, and pH value which affected the quenching efficiency. First, the influence of pH value on the quenching efficiency of the PEG@ZnO QDs/Cu2+ system was tested according to recording the fluorescence intensities of PEG@ZnO QDs in the absence and presence of 10 M Cu2+ in BR buffer at a series of pH values (5.94-7.61). From Fig. 2 (A), we can see that the biggest relative fluorescence intensity is obtained at pH 6.25. Hence, the pH 6.25 can be used as the best pH value in the next experiments. In addition, we examined the influence of different media including BR buffer (10 mM), Na2 HPO4 -NaH2 PO4 buffer (0.2 M), and KH2 PO4 NaOH buffer (0.1 M) on the quenching efficiency. Fig. 2(B) shows
the relative fluorescence intensity of the PEG@ZnO QDs in solution with and without the addition of 10 M Cu2+ in different media indicates that the use of buffer is a disadvantage to the determination of Cu2+ . The relative fluorescence intensity was described as F0 –F. And the quenching efficiency of Cu2+ in water is the best among the four media (Fig. 2(B)). Fig. 2(C) proves that the quenching efficiency of Cu2+ increased indistinctively with the reaction time. Upon addition of 10 M Cu2+ and 10 nM Cu2+ to the PEG@ZnO QDs aqueous solution, both of the fluorescences quenched rapidly, and the fluorescence intensity nearly changed to its minimum value immediately. Hence, the reaction time has a neglectable effect on the detection of Cu2+ ion based on the above results. 3.3. Determination of Cu2+ based on PEG@ZnO QDs According to the above results, we studied the detection limit and linearity of the above method by recording the fluorescence intensity of the PEG@ZnO QDs with different concentrations of Cu2+ . Fig. 3(A) shows that the fluorescence of the PEG@ZnO QDs was quenched increasingly with adding Cu2+ concentration from 0 to 10 M. When adding the Cu2+ concentration to 10 M, the fluorescence of the PEG@ZnO QDs was almost quenched completely. A good linear relationship between the relative fluorescence intensity (F0 –F) of the PEG@ZnO QDs in the presence (F) and absence (F0 ) of Cu2+ and the Cu2+ over the range of 10–200 nM was obtained (Fig. 3(B)), with the equation of regression: F0 –F = 0.1768C + 30.86, (R = 0.996), where C presents the concentration of Cu2+ . The detection limit for Cu2+ was 3.33 nM based on 3/slope. In addition, the (F0 –F) has also a linear relationship to the Cu2+ over the range of 2–10 M (Fig. 3(B)) with the regression equation: F0 –F = 2.875C + 99.58, (R = 0.9907), where C presents the concentration of Cu2+ , suggesting that this method can be used to analyze the higher concentration of Cu2+ . Hence, this proposed method could be applied to determine Cu2+ over a wide range. On the basis of
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Fig. 2. (A) Effect of pH on the fluorescence intensity of PEG-capped ZnO QDs and PEG-capped ZnO QDs with addition of 10 M Cu2+ (BR buffer (10 mM)) (B) Effect of different buffers (pH = 6.25) on the fluorescence intensity of the PEG-capped ZnO QDs in the presence and absence of 10 M Cu2+ (a. BR buffer (10 mM); b. Na2 HPO4 -NaH2 PO4 buffer (0.2 M); c. KH2 PO4 -NaOH buffer (0.1 M).); (C) Effect of reaction time on the fluorescence intensity of PEG-capped ZnO QDs and PEG-capped ZnO QDs with addition of 10 nM and 10 M Cu2+ .
Fig. 3. (A) Fluorescence emission spectra of the PEG@ZnO QDs at various concentrations of Cu2+ (0, 4, 6, 10, 20, 40, 60, 100, 200, 400, and 600 nM, and 1, 2, 4, 6, 8, and 10 M) under excitation at 351 nm. (B) Plot of the relative fluorescence intensity (F0 − F) versus Cu2+ ion. Insets: the linear calibration curve of fluorescence intensity and the concentration of Cu2+ .
Table 1 Comparison of different analytical methods for determination of Cu2+ . Method
Linear range
Limit of detection
Detection time (min)
Reference
DPVa Fluorescence Fluorescence Fluorescence Fluorescence Fluorescence FAASb Chemosensor SWSVc
0.008–7.807 ng/mL 10–1000 nM 50–500 nM – 0–850 M 15–35 M 0.04–1.0 g/mL 0–12 M 0.02–11.10 M, 31.10–111.1 M 0–30 M 23.3 nM–23.3 M 10–200 nM 2–10 M
0.0018 ng/mL 0.2 nM 1.1 nM 5 nM 0.18 M 5.6 M 2.9 ng/mL 2.54 M 9.5 nM
3.5 20 0.2 5 – 1 – – 3
[5] [10] [11] [12] [16] [17] [19] [21] [22]
78 nM 0.5 nM 3.33 nM
– 30 0.1
[26] [27] This work
Fluorescence Fluorescence Fluorescence a b c
Differential pulse anodic stripping voltammetry. Flame atomic absorption spectrometry. Square wave stripping voltammetry.
ten parallel samples determined, the relative standard deviations corresponding to 100 nM and 10 M Cu2+ were 3.81% and 3.18%, which showed that this method had a good repeatability in the determination of Cu2+ . And we compared this method with other proposed methods for Cu2+ detection, including linear range, limit of detection, and detection time (See Table 1).
3.4. Selectivity High selectivity is the key factor in many samples determination. In order to examine whether PEG@ZnO QDs is specific for Cu2+ , the effect of other 11 potential interference ions (30 M for each) on the fluorescence of the PEG@ZnO QDs was measured under the optimum experimental conditions. Fig. 4 reveals that the fluorescence
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Table 2 Results for determining Cu2+ in the tap water sample by this method (n = 3). Sample Tap water
a
Added (nM)
Found (nM)
Recovery (%)
RSD (%)
0 10.0 50.0 150.0 3000
NDa 10.4 49.8 151.2 3020
– 104.00 99.60 100.80 100.67
– 2.17 2.34 1.79 1.73
Not detected.
of PEG@ZnO QDs can be quenched by Cu2+ compared to potential interference ions, whereas the 11 potential interference ions had a neglectable effect on the fluorescence of the PEG@ZnO QDs, demonstrating that using the PEG@ZnO QDs to detect Cu2+ has a good selectivity.
orescence spectrophotometer. There is no Cu2+ detected because of the negligible quenching of the fluorescence. To examine the accuracy of the proposed method, the application of this method was further evaluated with recovery experiments. First of all, real water samples were spiked with different concentrations (10.0, 50.0, 150.0 nM, and 3 M) of the standard Cu2+ solution. Then 400 L of the PEG@ZnO QDs solution mixed with 5 L of the above sample solutions were diluted to 500 L with ultrapure water. Finally, the fluorescence of the above mixture solution was detected and the recoveries of the water samples are shown in Table 2. The above results demonstrated that the PEG@ZnO QDs probe can be used to detect Cu2+ in natural water samples.
3.5. Application of this method for detection of Cu2+ in real water sample
3.6. The possible quenching mechanism of the PEG@ZnO QDs by Cu2+
This method for detection of Cu2+ was tested by analysis of the tap water samples. First, the PEG@ZnO QDs solution (400 L) was mixed with 5 L of water samples, and then ultrapure water was added to the above solution to the volume of 500 L. The fluorescence of the above mixture solution was tested using the flu-
In this work, we investigated the possible quenching mechanism of the PEG@ZnO QDs by Cu2+ . As shown in Fig. 1 (A), compared to the absorption spectrum of the PEG@ZnO QDs, the absorption spectrum of the PEG@ZnO QDs with the addition of 10 M Cu2+ reduces obviously at the absorption band of around 351 nm, which may cause
Fig. 4. Effect of different ions on the fluorescence (FL) intensity of PEG@ZnO QDs.
Fig. 5. TEM images of the PEG@ZnO QDs without the addition of Cu2+ (A) and with the addition of 1 M Cu2+ (B). Stern-Volmer plots for different concentrations of Cu2+ (C) and (D).
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the quenching of PEG@ZnO QDs fluorescence to some extent, and as shown in the inset of Fig. 1(B), there is no QDs precipitation when Cu2+ was added to the QDs solution, so the absorbance decrease may occur even when the aggregated QDs remain dispersed in the solution. The PEG@ZnO QDs dispersed well and did not aggregate without the addition of Cu2+ ion (see Fig. 5(A)), which kept the fluorescence intensity, whereas the PEG@ZnO QDs became aggregation when 1 M Cu2+ was added to the PEG@ZnO QDs solution (see Fig. 5(B)), which made the fluorescence quenching. Therefore, aggregation induced quenching may be another quenching mechanism of the PEG@ZnO QDs by Cu2+ . And the fluorescence of PEG has no influence in this experiment (emission at 380 nm when excited at 300 nm) according to previous report [42]. The reason of aggregation may be due to the complexation of Cu and PEG [43], which can form {Cu2+ (-EO-)4 . (y-1)(H2 O)2 } complexes [44]. Further, the quenching process was discussed based on SternVolmer equation F0 /F = Ksv [Q] + 1, where F0 and F denote the fluorescence of PEG@ZnO QDs in the absence and presence of Cu2+ , Ksv is the Stern-Volmer constant representing the affinity between fluorophore and quencher, and [Q] is the concentration of Cu2+ . As shown in Fig. 5 (C) and Fig. 5 (D), two linear responses of intensity ratio with the concentration of Cu2+ were observed in the range of 10–200 nM and 2–10 M, with Ksv 2.57 × 106 and 3.52 × 105 M−1 , respectively, which shows the strong interaction of PEG@ZnO QDs with Cu2+ . And with increasing concentration of Cu2+ , the relative fluorescence intensity shows downward curvature (Fig. 3 (B)), suggesting a more complex quenching process, which consists of static and dynamic quenching processes, and it is difficult to ascertain the main type of the quenching process [45–48]. 4. Conclusion In this work, a new determination of Cu2+ in water samples based on the fluorescence quenching of PEG@ZnO QDs was proposed. And the fluorescence method shows several advantages such as good selectivity, low expense, simplicity, and nontoxicity. The PEG@ZnO QDs absorption at 351 nm reduced and the PEG@ZnO QDs aggregated with the addition of Cu2+ to the PEG@ZnO QDs solution, resulting in the fluorescence quenching. And the fluorescence quenching mechanism is aggregation induced quenching [49]. The linear relationship was 10–200 nM and 2–10 M, respectively, and the limit of detection for Cu2+ was 3.33 nM. The proposed PEG@ZnO QDs sensor can be applied to the determination Cu2+ in water samples. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Nos. 21675131, 21273174) and the Municipal Science Foundation of Chongqing City (Nos. CSTC2013jjB00002, CSTC-2015jcyjB50001). References [1] G. Aragay, A. Merkoc¸i, Nanomaterials application in electrochemical detection of heavy metals, Electrochim. Acta 84 (2012) 49–61. [2] A.K. Mahapatra, G. Hazra, N.K. Das, S. Goswami, A highly selective triphenylamine-based indolylmethane derivatives as colorimetric and turn-off fluorimetric sensor toward Cu2+ detection by deprotonation of secondary amines, Sens. Actuator B-Chem. 156 (2011) 456–462. [3] Y.J. Song, K.G. Qu, C. Xu, J.S. Ren, X.G. Qu, Visual and quantitative detection of copper ions using magnetic silica nanoparticles clicked on multiwalled carbon nanotubes, Chem. Commun. 46 (2010) 6572–6574. [4] S.L. Yang, D.Q. Zhou, H.Y. Yu, R. Wei, B. Pan, Distribution and speciation of metals (Cu, Zn Cd, and Pb) in agricultural and non-agricultural soils near a stream upriver from the Pearl River, China, Environ. Pollut. 177 (2013) 64–70. [5] B.B. Prasad, S. Fatma, Electrochemical sensing of ultra trace copper(II) by alga-OMNiIIP modified pencil graphite electrode, Sens. Actuator B-Chem. 229 (2016) 655–663.
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Biographies Shuo Geng is an MS candidate in School of Chemistry and Chemical Engineering, Southwest University, China. His major research interest is spectrum analysis. Shu Min Lin is an MS candidate in School of Chemistry and Chemical Engineering, Southwest University, China. Her major research interest is spectrum analysis. Nian Bing Li is a Professor of chemistry in School of Chemistry and Chemical Engineering, Southwest University, China. He received his MS degree in physical chemistry in 1997 and Ph.D. degree in material science in 2000 from Chongqing University. During 2000–2002, he was a Postdoctoral Research Fellow in Fuzhou University, China. Since 2006–2007, he was a Postdoctoral Research Fellow in Korea Advanced Institute of Science and Technology (KAIST), Korea. His research interests are the developments of electrochemical devices such as chemical sensors and biosensors. Hong Qun Luo is a Professor of chemistry in School of Chemistry and Chemical Engineering, Southwest University, China. She received her MS degree in environmental chemistry from Sichuan University in 1991 and Ph.D. degree in analytical chemistry from Southwest China Normal University in 2002. During 2006–2007, she was a Visiting Scholar in Tohoku University, Japan. Her research is focused on molecular spectroscopy and electrochemical sensors.
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Polyethylene glycol capped ZnO quantum dots as a fluorescent probe for determining copper(II) ion Shuo Geng, Shu Min Lin, Nian Bing Li ∗ , Hong Qun Luo ∗ Key Laboratory of Eco-environments in Three Gorges Reservoirs Region (Ministry of Education), School of Chemistry and Chemical Engineering, Southwest University, Chongqing, 400715, PR China
a r t i c l e
i n f o
Article history: Received 2 January 2017 Received in revised form 14 June 2017 Accepted 17 June 2017 Available online 19 June 2017 Keywords: PEG@ZnO QDs Fluorescent probe Copper(II) ions Determination Aggregation induced quenching
a b s t r a c t As an important heavy metal ion, copper has the negative influence on otherwise healthy individuals, so establishing a valid way for the highly efficient, sensitive, and quantitative determination of Cu2+ ion becomes an emergency in the environmental analysis. In the present work, water-soluble luminescent ZnO quantum dots (QDs) capped by polyethylene glycol (PEG) have been synthesized by a simple solution method. The PEG capped ZnO QDs (PEG@ZnO QDs) showed yellow fluorescence. High-resolution transmission electron microscopy, UV–vis absorption spectroscopy, Fourier transform infrared spectroscopy, and luminescence spectroscopy were applied to elucidate the properties of the PEG@ZnO QDs. In addition, the yellow fluorescence of the PEG@ZnO QDs was quenched when Cu2+ ion was added to the PEG@ZnO QDs solutions. Therefore, a novel fluorescent probe was designed to detect Cu2+ in water solution. The linear relationships were 10–200 nM and 2–10 M, respectively, with the detection limit for Cu2+ at 3.33 nM according to 3/slope (where denotes the standard deviation of the blank measures). The proposed sensor of the PEG@ZnO QDs has also been used in natural water samples to examine the availability of this method. In addition, the quenching mechanism was discussed, which may be attributed to the aggregation induced quenching. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Heavy metal ion has attracted a great attention over the past years because of the negative influence on otherwise healthy individuals [1–4]. As a generally used heavy metal, copper(II) ion can be found in various water samples and is involved in fundamental life process as one of the essential micronutrients [5–7]. It is healthy for people when intake of a small amount of copper(II) ion, while it can be caused a negative effect on the central nervous system, liver, kidneys and gastrointestinal system with an excessive intake of copper(II) ion [2,3,8–15]. The World Health Organization (WHO) has developed drinking-water quality guidelines that copper is perceived as a “chemical of health significance in drinking water” and then German water quality standards declared that copper(II) ion concentration in drinking water should be less than 1 ppm [6,16–18]. Hence, establishing a valid probe for the highly efficient, sensitive, and quantitative detection of Cu2+ ion becomes an emergency in the food testing, biological assay, and environmental analysis.
∗ Corresponding author. E-mail addresses: [email protected] (N.B. Li), [email protected] (H.Q. Luo). http://dx.doi.org/10.1016/j.snb.2017.06.118 0925-4005/© 2017 Elsevier B.V. All rights reserved.
Over the past years, a lot of ways have been established to determine copper(II) ion, for instance, atomic absorption spectrometry (AAS), chemiluminescence, spectrophotometry, visual detection, inductively coupled plasma atomic emission spectroscopy (ICPAES), and electrochemical techniques [5,19–26]. Whereas the above methods have lots of shortcomings, for example, reagent toxicity, costly instruments, complicated operation procedures, and tedious sample preparation. Hence, a sensitive, simple, and inexpensive method for the determination of copper ion needs to be developed. The detection method based on fluorescence has developed very fast among other detection methods in recent years [27–29]. Some fluorescent materials including quantum dots (QDs), carbon dots, and zero-dimensional nanomaterials have been applied diffusely to determination of various metal ions because of the wider absorption spectra, low cost, big Stokes shift, tunable wavelength, and easy preparation [30–32]. Right until today, various types of functionalized QDs have been used to detect various ions such as Ag+ , Cu2+ , Cd2+ , CN− , and Hg2+ [33–38]. Tao et al. demonstrated that the 3-mercaptopropinic acid stabilized CdTe/CdS quantum dots can be applied as a copper(II) ion sensor based on that their near-infrared fluorescence was quenched by the aggregation of the quantum dots caused by the competitive binding between 3-mercaptopropinic
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acid and the copper(II) ion [18]. Liu et al. developed a detection of Cu2+ ion probe according to fluorescence quenching of the graphene oxide (GO) modified Ag-In-Zn-S (AIZS) quantum dots when copper(II) ion was added to the above fluorescence probe with its limit of detection of 0.18 M [16]. Chao and coworkers established the use of 1-(2-thiazolylazo)-2-naphthol coated CdTe QDs as a copper(II) ion sensor [27]. These methods demonstrate that the ion sensors based on QDs are in great potential for the chemical and biological application. Nevertheless, the above quantum dots mostly contain toxic and environmental unfriendly heavy metal elements. In the present work, water-soluble and monodisperse PEG@ZnO QDs were synthesized by a simple sol-gel solution method, which were characterized by fluorescence, high-resolution transmission electron microscope, UV–vis absorption spectroscopy, and Fourier transform infrared spectroscopy. In addition, the fluorescence of PEG@ZnO QDs can be quenched by copper(II) ion. Hence, we developed a novel fluorescent probe for selective and sensitive detection of copper(II) ion in aqueous solution. This proposed method showed that the relative fluorescence intensity had two linearly proportional relationships to the concentration of copper(II) ion within the range 10–200 nM and 2–10 M, with the detection limit down to 3.33 nM. This probe had some advantages, such as simple and fast determination procedure, high sensitivity, wide linear range, good selectivity, nontoxicity, and little equipment investment. And this method offers great promise in copper(II) ion detection in natural water and biological analysis. 2. Experimental section 2.1. Materials All the reagents were used as received without further purification. Ethanol absolute, zinc acetate dihydrate (Zn(CH3 COO)2 ·2H2 O), and potassium hydroxide (KOH) were obtained from Chengdu Kelong Chemical Reagent Co. Ltd. (Chengdu, China). Polyethylene glycol (PEG, average mass 1500) was obtained from Aladdin. Britton-Robinson (BR) buffer (10 mM, pH 5.94–8.05), KH2 PO4 -NaOH buffer (0.1 M), and Na2 HPO4 NaH2 PO4 buffer (0.2 M) were employed. Ultrapure water (18.2 M cm) was used throughout all the experiments. 2.2. Instruments A KQ-400 KDB ultrasonic bath (400 W, Kunshan Ultrasonic Instruments Co., Ltd, China) was applied to dissolve the Zn(CH3 COO)2 ·2H2 O and KOH in ethanol completely. An 85-2 constant temperature magnetic mixer (Zongda Instrument Plant, Jiangsu, China) was applied to mix the above solutions completely. A TGL–16 M high-speed refrigerated centrifuge (Xiangyi, China) was used to collect the precipitates. A Hitachi F-2700 fluorescence spectrophotometer (Hitachi Ltd., Japan) was applied to measure the fluorescence intensity in this work. A Shimadzu UV-2450 spectrophotometer (Suzhou Shimadzu Instrument Co., Ltd., China) was used to measure the ultraviolet-visible absorption spectra with two 1-cm cuvettes. The morphology of the PEG@ZnO quantum dots was studied by a high-resolution transmission electron microscope (HRTEM) from JEM-2100 (JEOL Ltd., Japan). And a software named Nano Measurer was used to measure the size of the PEG@ZnO quantum dots. Fourier transform infrared (FTIR) spectra were recorded by using a Bruker IFS 113 v spectrometer (Bruker, Germany). 2.3. Preparation of PEG@ZnO quantum dots The PEG@ZnO quantum dots (PEG@ZnO QDs) were obtained by a two-step process based on prior works with a little modification
[39,40]. First, 0.1683 g of KOH was dispersed in 10 mL of ethanol and 0.1098 g of Zn(CH3 COO)2 ·2H2 O dispersed in 50 mL of ethanol completely by keeping the above solutions in an ultrasonic bath for an hour at room temperature. Then ZnO quantum dots were synthesized by the dropwise addition of 10 mL of KOH (0.3 mol/L) to the 50 mL of Zn(CH3 COO)2 (0.01 mol/L) followed by continuously stirring for 120 min at room temperature. After that, 5 mL of ethyl acetate was added to the above ZnO quantum dots solutions to obtain the precipitate of ZnO quantum dots, then the precipitate was obtained by centrifugation and purified by washing with ethanol. And the precipitate was dispersed in 20 mL of ethanol for using in next step. In the second step, the PEG-modified ZnO quantum dots were synthesized by the dropwise addition of 4 mL of PEG (0.15 g (0.1 mmol) of PEG was dissolved in 4 mL ethanol) into the above prepared ZnO QDs (4 mL) followed by churning for 60 min at room temperature. Next, the PEG@ZnO QDs were collected by centrifugation and purified by washing with ethanol and ultrapure water. Then the precipitate was dissolved in 10 mL of ultrapure water under ultrasonic and agitation for using in the next procedure and characterization. 2.4. Detection of Cu2+ by PEG@ZnO QDs The Cu2+ detection procedure using PEG@ZnO QDs as the fluorescent probe is described as follows: 5 L of Cu2+ with different levels was added to 400 L of PEG@ZnO QDs solution and then the mixture was diluted to 500 L with ultrapure water. After that, the fluorescence intensity of the above solutions was measured using the fluorescence spectrophotometer with the excitation and emission wavelengths of 351 and 543 nm. The relative fluorescence intensity of PEG@ZnO QDs in the presence (F) and absence (F0 ) of Cu2+ was denoted as F0 –F. And the natural water samples were applied to prove that this method is valid in real sample analysis. 3. Results and discussion 3.1. Characterization of PEG@ZnO QDs Characterization of the PEG@ZnO QDs was explored by using UV–vis absorption, fluorescence, FTIR, and HRTEM. As shown in Fig. 1 (A) (solid line), the absorption maximum was around 351 nm, which shifts to short wavelength compared to the peak at 380 nm of a bulk ZnO, and this can prove that the PEG@ZnO QDs was synthesized in a certain degree. The excitation wavelength and the emission wavelength of the PEG@ZnO QDs were at 351 and 543 nm, respectively (Fig. 1 (B), solid line). And the PEG has no fluorescence emission when excited at 351 nm (Fig. 1 (B)), illustrating that the fluorescence of PEG@ZnO QDs is stemmed from ZnO QDs. A yellow fluorescence and well water-soluble QDs can be seen from the photograph of the PEG @ ZnO QDs (inset of Fig. 1 (B), left and middle). The PEG@ZnO QDs were mono-dispersed well in water and the diameter of the PEG@ZnO QDs was about 6 nm (Fig. 1 (C)). The PEG capped on the ZnO surface was successfully confirmed by the FTIR spectrum. Fig. 1 (D) shows the FTIR spectra of PEG@ZnO QDs, PEG, and ZnO QDs. About pure ZnO QDs, the band around 3396 cm−1 can be assigned to the stretching vibration of OH at ZnO QDs surface, and the peaks at 1504 and 1394 cm−1 corresponded to asymmetric and symmetric C O stretching modes of Zn(CH3 COO)2 . In addition, the band at the region of 474 cm−1 could be assigned to the Zn-O stretching of ZnO QDs. About PEG spectrum, the peaks at 3409 and 1644 cm−1 belong to the stretching vibration and bending vibration of the OH, respectively; the peaks at 2886 and 1468 cm−1 can belong to the symmetrical stretching vibration and bending vibration of CH2 –, respectively; the peaks at 1359, 1344, and 1115 cm−1
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Fig. 1. (A) UV–vis absorption spectra of the PEG-capped ZnO QDs (the synthesized), and PEG-capped ZnO QDs with Cu2+ (10 M); (B) Fluorescence excitation and emission spectra of PEG-capped ZnO QDs, emission spectrum of PEG, and the emission spectrum of PEG-capped ZnO QDs with Cu2+ (10 M). Inset: photograph (left) of the PEG-capped ZnO QDs solution under visible light, photograph (middle) of the PEG-capped ZnO QDs solution excited with UV light at 365 nm and photograph (right) of the PEG-capped ZnO QDs solution with Cu2+ (10 M); (C) TEM image of PEG-capped ZnO QDs; (D) FTIR spectra of ZnO QDs, PEG, and PEG-capped ZnO QDs.
can be assigned to the stretching vibration of C O. After PEG capped the ZnO QDs, stretching vibration and bending vibration of the OH the typical bands of PEG and ZnO QDs are also checked with a little shift, demonstrating that the functional groups of PEG and the ZnO QDs were not destroyed and the PEG was successfully conjugated to the ZnO QDs surface. Both of stretching vibration and bending vibration of the OH of the typical bands of PEG and ZnO QDs are also checked with a little shift, demonstrating that the formation of intermolecular hydrogen bonds by the OH of PEG on the surface of ZnO QDs, thereby confirming the capping of PEG on ZnO QDs [41]. The PEG@ZnO QDs can be stored in a refrigerator at 25 ◦ C for 1 month which made the fluorescence negligible decrease. As shown in Fig. 1B (dash line), when Cu2+ (10 M) was added to the PEG@ZnO QDs, the fluorescence of the PEG@ZnO QDs was quenched quickly (inset of Fig. 1B, right). Therefore, a probe for detection of Cu2+ was developed and designed based on the as prepared PEG@ZnO QDs.
3.2. Optimization of the experimental conditions To achieve the highest quenching efficiency of the PEG@ZnO QDs by Cu2+ ion, we optimized several experimental parameters such as reaction time, buffer solution type, and pH value which affected the quenching efficiency. First, the influence of pH value on the quenching efficiency of the PEG@ZnO QDs/Cu2+ system was tested according to recording the fluorescence intensities of PEG@ZnO QDs in the absence and presence of 10 M Cu2+ in BR buffer at a series of pH values (5.94-7.61). From Fig. 2 (A), we can see that the biggest relative fluorescence intensity is obtained at pH 6.25. Hence, the pH 6.25 can be used as the best pH value in the next experiments. In addition, we examined the influence of different media including BR buffer (10 mM), Na2 HPO4 -NaH2 PO4 buffer (0.2 M), and KH2 PO4 NaOH buffer (0.1 M) on the quenching efficiency. Fig. 2(B) shows
the relative fluorescence intensity of the PEG@ZnO QDs in solution with and without the addition of 10 M Cu2+ in different media indicates that the use of buffer is a disadvantage to the determination of Cu2+ . The relative fluorescence intensity was described as F0 –F. And the quenching efficiency of Cu2+ in water is the best among the four media (Fig. 2(B)). Fig. 2(C) proves that the quenching efficiency of Cu2+ increased indistinctively with the reaction time. Upon addition of 10 M Cu2+ and 10 nM Cu2+ to the PEG@ZnO QDs aqueous solution, both of the fluorescences quenched rapidly, and the fluorescence intensity nearly changed to its minimum value immediately. Hence, the reaction time has a neglectable effect on the detection of Cu2+ ion based on the above results. 3.3. Determination of Cu2+ based on PEG@ZnO QDs According to the above results, we studied the detection limit and linearity of the above method by recording the fluorescence intensity of the PEG@ZnO QDs with different concentrations of Cu2+ . Fig. 3(A) shows that the fluorescence of the PEG@ZnO QDs was quenched increasingly with adding Cu2+ concentration from 0 to 10 M. When adding the Cu2+ concentration to 10 M, the fluorescence of the PEG@ZnO QDs was almost quenched completely. A good linear relationship between the relative fluorescence intensity (F0 –F) of the PEG@ZnO QDs in the presence (F) and absence (F0 ) of Cu2+ and the Cu2+ over the range of 10–200 nM was obtained (Fig. 3(B)), with the equation of regression: F0 –F = 0.1768C + 30.86, (R = 0.996), where C presents the concentration of Cu2+ . The detection limit for Cu2+ was 3.33 nM based on 3/slope. In addition, the (F0 –F) has also a linear relationship to the Cu2+ over the range of 2–10 M (Fig. 3(B)) with the regression equation: F0 –F = 2.875C + 99.58, (R = 0.9907), where C presents the concentration of Cu2+ , suggesting that this method can be used to analyze the higher concentration of Cu2+ . Hence, this proposed method could be applied to determine Cu2+ over a wide range. On the basis of
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Fig. 2. (A) Effect of pH on the fluorescence intensity of PEG-capped ZnO QDs and PEG-capped ZnO QDs with addition of 10 M Cu2+ (BR buffer (10 mM)) (B) Effect of different buffers (pH = 6.25) on the fluorescence intensity of the PEG-capped ZnO QDs in the presence and absence of 10 M Cu2+ (a. BR buffer (10 mM); b. Na2 HPO4 -NaH2 PO4 buffer (0.2 M); c. KH2 PO4 -NaOH buffer (0.1 M).); (C) Effect of reaction time on the fluorescence intensity of PEG-capped ZnO QDs and PEG-capped ZnO QDs with addition of 10 nM and 10 M Cu2+ .
Fig. 3. (A) Fluorescence emission spectra of the PEG@ZnO QDs at various concentrations of Cu2+ (0, 4, 6, 10, 20, 40, 60, 100, 200, 400, and 600 nM, and 1, 2, 4, 6, 8, and 10 M) under excitation at 351 nm. (B) Plot of the relative fluorescence intensity (F0 − F) versus Cu2+ ion. Insets: the linear calibration curve of fluorescence intensity and the concentration of Cu2+ .
Table 1 Comparison of different analytical methods for determination of Cu2+ . Method
Linear range
Limit of detection
Detection time (min)
Reference
DPVa Fluorescence Fluorescence Fluorescence Fluorescence Fluorescence FAASb Chemosensor SWSVc
0.008–7.807 ng/mL 10–1000 nM 50–500 nM – 0–850 M 15–35 M 0.04–1.0 g/mL 0–12 M 0.02–11.10 M, 31.10–111.1 M 0–30 M 23.3 nM–23.3 M 10–200 nM 2–10 M
0.0018 ng/mL 0.2 nM 1.1 nM 5 nM 0.18 M 5.6 M 2.9 ng/mL 2.54 M 9.5 nM
3.5 20 0.2 5 – 1 – – 3
[5] [10] [11] [12] [16] [17] [19] [21] [22]
78 nM 0.5 nM 3.33 nM
– 30 0.1
[26] [27] This work
Fluorescence Fluorescence Fluorescence a b c
Differential pulse anodic stripping voltammetry. Flame atomic absorption spectrometry. Square wave stripping voltammetry.
ten parallel samples determined, the relative standard deviations corresponding to 100 nM and 10 M Cu2+ were 3.81% and 3.18%, which showed that this method had a good repeatability in the determination of Cu2+ . And we compared this method with other proposed methods for Cu2+ detection, including linear range, limit of detection, and detection time (See Table 1).
3.4. Selectivity High selectivity is the key factor in many samples determination. In order to examine whether PEG@ZnO QDs is specific for Cu2+ , the effect of other 11 potential interference ions (30 M for each) on the fluorescence of the PEG@ZnO QDs was measured under the optimum experimental conditions. Fig. 4 reveals that the fluorescence
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Table 2 Results for determining Cu2+ in the tap water sample by this method (n = 3). Sample Tap water
a
Added (nM)
Found (nM)
Recovery (%)
RSD (%)
0 10.0 50.0 150.0 3000
NDa 10.4 49.8 151.2 3020
– 104.00 99.60 100.80 100.67
– 2.17 2.34 1.79 1.73
Not detected.
of PEG@ZnO QDs can be quenched by Cu2+ compared to potential interference ions, whereas the 11 potential interference ions had a neglectable effect on the fluorescence of the PEG@ZnO QDs, demonstrating that using the PEG@ZnO QDs to detect Cu2+ has a good selectivity.
orescence spectrophotometer. There is no Cu2+ detected because of the negligible quenching of the fluorescence. To examine the accuracy of the proposed method, the application of this method was further evaluated with recovery experiments. First of all, real water samples were spiked with different concentrations (10.0, 50.0, 150.0 nM, and 3 M) of the standard Cu2+ solution. Then 400 L of the PEG@ZnO QDs solution mixed with 5 L of the above sample solutions were diluted to 500 L with ultrapure water. Finally, the fluorescence of the above mixture solution was detected and the recoveries of the water samples are shown in Table 2. The above results demonstrated that the PEG@ZnO QDs probe can be used to detect Cu2+ in natural water samples.
3.5. Application of this method for detection of Cu2+ in real water sample
3.6. The possible quenching mechanism of the PEG@ZnO QDs by Cu2+
This method for detection of Cu2+ was tested by analysis of the tap water samples. First, the PEG@ZnO QDs solution (400 L) was mixed with 5 L of water samples, and then ultrapure water was added to the above solution to the volume of 500 L. The fluorescence of the above mixture solution was tested using the flu-
In this work, we investigated the possible quenching mechanism of the PEG@ZnO QDs by Cu2+ . As shown in Fig. 1 (A), compared to the absorption spectrum of the PEG@ZnO QDs, the absorption spectrum of the PEG@ZnO QDs with the addition of 10 M Cu2+ reduces obviously at the absorption band of around 351 nm, which may cause
Fig. 4. Effect of different ions on the fluorescence (FL) intensity of PEG@ZnO QDs.
Fig. 5. TEM images of the PEG@ZnO QDs without the addition of Cu2+ (A) and with the addition of 1 M Cu2+ (B). Stern-Volmer plots for different concentrations of Cu2+ (C) and (D).
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the quenching of PEG@ZnO QDs fluorescence to some extent, and as shown in the inset of Fig. 1(B), there is no QDs precipitation when Cu2+ was added to the QDs solution, so the absorbance decrease may occur even when the aggregated QDs remain dispersed in the solution. The PEG@ZnO QDs dispersed well and did not aggregate without the addition of Cu2+ ion (see Fig. 5(A)), which kept the fluorescence intensity, whereas the PEG@ZnO QDs became aggregation when 1 M Cu2+ was added to the PEG@ZnO QDs solution (see Fig. 5(B)), which made the fluorescence quenching. Therefore, aggregation induced quenching may be another quenching mechanism of the PEG@ZnO QDs by Cu2+ . And the fluorescence of PEG has no influence in this experiment (emission at 380 nm when excited at 300 nm) according to previous report [42]. The reason of aggregation may be due to the complexation of Cu and PEG [43], which can form {Cu2+ (-EO-)4 . (y-1)(H2 O)2 } complexes [44]. Further, the quenching process was discussed based on SternVolmer equation F0 /F = Ksv [Q] + 1, where F0 and F denote the fluorescence of PEG@ZnO QDs in the absence and presence of Cu2+ , Ksv is the Stern-Volmer constant representing the affinity between fluorophore and quencher, and [Q] is the concentration of Cu2+ . As shown in Fig. 5 (C) and Fig. 5 (D), two linear responses of intensity ratio with the concentration of Cu2+ were observed in the range of 10–200 nM and 2–10 M, with Ksv 2.57 × 106 and 3.52 × 105 M−1 , respectively, which shows the strong interaction of PEG@ZnO QDs with Cu2+ . And with increasing concentration of Cu2+ , the relative fluorescence intensity shows downward curvature (Fig. 3 (B)), suggesting a more complex quenching process, which consists of static and dynamic quenching processes, and it is difficult to ascertain the main type of the quenching process [45–48]. 4. Conclusion In this work, a new determination of Cu2+ in water samples based on the fluorescence quenching of PEG@ZnO QDs was proposed. And the fluorescence method shows several advantages such as good selectivity, low expense, simplicity, and nontoxicity. The PEG@ZnO QDs absorption at 351 nm reduced and the PEG@ZnO QDs aggregated with the addition of Cu2+ to the PEG@ZnO QDs solution, resulting in the fluorescence quenching. And the fluorescence quenching mechanism is aggregation induced quenching [49]. The linear relationship was 10–200 nM and 2–10 M, respectively, and the limit of detection for Cu2+ was 3.33 nM. The proposed PEG@ZnO QDs sensor can be applied to the determination Cu2+ in water samples. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Nos. 21675131, 21273174) and the Municipal Science Foundation of Chongqing City (Nos. CSTC2013jjB00002, CSTC-2015jcyjB50001). References [1] G. Aragay, A. Merkoc¸i, Nanomaterials application in electrochemical detection of heavy metals, Electrochim. Acta 84 (2012) 49–61. [2] A.K. Mahapatra, G. Hazra, N.K. Das, S. Goswami, A highly selective triphenylamine-based indolylmethane derivatives as colorimetric and turn-off fluorimetric sensor toward Cu2+ detection by deprotonation of secondary amines, Sens. Actuator B-Chem. 156 (2011) 456–462. [3] Y.J. Song, K.G. Qu, C. Xu, J.S. Ren, X.G. Qu, Visual and quantitative detection of copper ions using magnetic silica nanoparticles clicked on multiwalled carbon nanotubes, Chem. Commun. 46 (2010) 6572–6574. [4] S.L. Yang, D.Q. Zhou, H.Y. Yu, R. Wei, B. Pan, Distribution and speciation of metals (Cu, Zn Cd, and Pb) in agricultural and non-agricultural soils near a stream upriver from the Pearl River, China, Environ. Pollut. 177 (2013) 64–70. [5] B.B. Prasad, S. Fatma, Electrochemical sensing of ultra trace copper(II) by alga-OMNiIIP modified pencil graphite electrode, Sens. Actuator B-Chem. 229 (2016) 655–663.
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Biographies Shuo Geng is an MS candidate in School of Chemistry and Chemical Engineering, Southwest University, China. His major research interest is spectrum analysis. Shu Min Lin is an MS candidate in School of Chemistry and Chemical Engineering, Southwest University, China. Her major research interest is spectrum analysis. Nian Bing Li is a Professor of chemistry in School of Chemistry and Chemical Engineering, Southwest University, China. He received his MS degree in physical chemistry in 1997 and Ph.D. degree in material science in 2000 from Chongqing University. During 2000–2002, he was a Postdoctoral Research Fellow in Fuzhou University, China. Since 2006–2007, he was a Postdoctoral Research Fellow in Korea Advanced Institute of Science and Technology (KAIST), Korea. His research interests are the developments of electrochemical devices such as chemical sensors and biosensors. Hong Qun Luo is a Professor of chemistry in School of Chemistry and Chemical Engineering, Southwest University, China. She received her MS degree in environmental chemistry from Sichuan University in 1991 and Ph.D. degree in analytical chemistry from Southwest China Normal University in 2002. During 2006–2007, she was a Visiting Scholar in Tohoku University, Japan. Her research is focused on molecular spectroscopy and electrochemical sensors.