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peroxodisulfate system
Accepted Manuscript Title: Electrochemiluminescence sensor for hexavalent chromium based on the graphene quantum dots/peroxodisulfate system Author: Yingmei Chen Yongqiang Dong Huan Wu Congqiang Chen Yuwu Chi Guonan Chen PII: DOI: Reference:
S0013-4686(14)02276-2 http://dx.doi.org/doi:10.1016/j.electacta.2014.11.068 EA 23740
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
8-8-2014 31-10-2014 11-11-2014
Please cite this article as: Yingmei Chen, Yongqiang Dong, Huan Wu, Congqiang Chen, Yuwu Chi, Guonan Chen, Electrochemiluminescence sensor for hexavalent chromium based on the graphene quantum dots/peroxodisulfate system, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2014.11.068 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Electrochemiluminescence sensor for hexavalent chromium based on the graphene quantum dots/peroxodisulfate system Yingmei Chen,# Yongqiang Dong,# Huan Wu, Congqiang Chen, Yuwu Chi,* Guonan Chen MOE Key Laboratory of Analysis and Detection Technology for Food Safety, Fujian
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Provincial Key Laboratory of Analysis and Detection Technology for Food Safety,
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and Department of Chemistry, Fuzhou University, Fujian 350108, China. *Corresponding author: Tel/Fax: +86 591 22866137 E-mail: [email protected] (Y. Chi).
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# These authors contributed equally to this work
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Abstract
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A electrochemiluminescent (ECL) sensor has been designed to detect hexavalent
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chromium (Cr(VI)) in environmental water samples based on the fact that Cr(VI) can
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collisionally quench the ECL signal of graphene quantum dots/peroxodisulfate (GQD/S2O82-) system. After optimizing some important experimental conditions
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including the concentrations of GQDs and S2O82-, response time, and pH value of
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solution, the ECL sensor has been finally developed. The ECL sensor exhibits a wide
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linear response range (50 nM-60 μM), excellent selectivity, and high sensitivity (detection limit of 20 nM, S/N = 3). Furthermore, the developed sensor has been applied in the detection of Cr(VI) in a spiked river water. The result suggests that the sensor can offer a simple, green, low cost, high selectivity and sensitivity detection of
Cr(VI).
Keywords: Electrochemiluminescence, graphene quantum dots, hexavalent chromium, detection
1. Introduction
Graphene quantum dots (GQDs), graphene nanosheets of less than 100 nm in
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lateral size [1], are emerging luminescent carbon nanomaterials. Due to the quantum
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confinement and edge effects, GQDs exhibit many unique optical properties such as
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photoluminescence (PL), chemiluminescence (CL), and electrochemiluminescence
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(ECL) [2,3]. The PL of GQDs has attracted tremendous attention, and has been
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applied in many fields such as bio-imaging, cell-imaging and sensing [4,5]. Contrarily,
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much less attention has been paid to the ECL of GQDs [6,7]. ECL is a CL that
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triggered by electrochemical methods. The analytical methods based on the ECL
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signal combine the advantages of CL and electrochemical analysis [8], and
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accordingly show many distinct advantages including no optical background, easy
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reaction control, high sensitivity and selectivity, and wide response range [9,10].
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Therefore, the ECL properties of GQDs are apparently very important in terms of
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analytical applications. Actually, GQDs have been applied in the detection of some cations based on their ECL activities [11,12].
Chromium, an important alloying element, is predominantly used in metallurgy, e.g., electroplating, leather tanning, metal smelt stabling and metal finishing industries [13]. In aqueous systems, chromium mainly exists in two oxidation states: trivalent
chromium (Cr(Ⅲ)) and hexavalent chromium (Cr(VI)) [14]. Cr(Ⅲ) is essential and beneficial for humans [15,16], while Cr(VI) is a highly toxic species and a suspected carcinogenic agent [17]. After entered blood circulation through digestive tract, Cr(VI) can damage the structure and function of hepatocytes due to its strong oxidation property, resulting in Cr(VI) hepatotoxicity [18]. Accordingly, it is vitally important to develop an effective method to precisely detect trace concentration Cr(VI). Various
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analysis techniques have been used to determinate Cr(VI), including inductively
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coupled plasma mass spectrometry [19], inductively coupled plasma-optical emission
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spectrometry [20], inductively coupled plasma atomic emission spectrometry [21],
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electrothermal atomic absorption spectrometry [22], graphite furnace atomic
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absorption spectrometry [23], reversed-phase ion-pair chromatography [24], X-ray
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fluorescence spectrometry [25], ECL spectrometry [26], and stripping voltammetric
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methods [27,28]. However, most of these methods suffer from some disadvantages,
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such as complex process, long response time, low sensitivity, and high cost of the
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instruments. Furthermore, many methods can only detect the total quantity of
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chromium, but are not able to distinguish Cr(VI) from Cr(Ⅲ). Therefore, developing a
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simple and fast method to detect Cr(VI) sensitively and selectively is of interesting.
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Herein, Cr(VI) was found to be able to quench sensitively the strong cathodic ECL
signal of GQD/S2O82- system. After optimizing some important experimental conditions, a sensitive and selective sensing system would be developed to detect Cr(VI) in domestic water.
2. Experimental
2.1 Chemicals
Vulcan CX-72 carbon black (Cabot Corporation) were used to prepare the single layer GQDs. K2S2O8 (>99.99%) was obtained from Sigma-Aldrich. All other reagents
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were of analytical grade and used as received. Doubly distilled water was used
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throughout the experiment. Phosphate-buffered saline (PBS) solutions of different pH
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values were prepared by titrating 0.1 M phosphoric acid solution with a concentrated
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sodium hydroxide solution (1 M) to the required pH values. 1 M KNO3 was added
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into PBS solutions through the ECL experiments to improve the conductibility.
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2.2 Preparation of GQDs
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GQDs were synthesized by refluxed Vulcan CX-72 carbon black with concentrated
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nitric acid according a chemical oxidation method that has been described elsewhere
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[29]. Briefly, 2 g of dried CX-72 carbon black was dissolved in 500 mL 6 M HNO3
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and then refluxing for 24 h. After cooling to room temperature, the suspension was centrifuged (2770g) for 10 min. The obtain supernatant was heated at 200℃ to
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evaporate the water and nitric acid to obtain reddish-brown powder.
2.3 Analysis of a real sample
A water sample was collected from Min River (Fujian, China). Suitable aliquots
(500 μL) of this river water were spiked with standard potassium dichromate solutions (10 μL, final concentration, 0−50 μM). The spiked samples were then diluted to 1 mL with PBS (0.1 M, pH 7.0) containing GQDs (final concentration, 0.2 mg mL-1), K2S2O8 (final concentration, 1 M), Ethylenediaminetetraacetic acid disodium salt (EDTA) (final concentration, 0.1 mM) and KNO3 (final concentration, 1 M) then
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analyzed using the developed sensing technique.
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2.4 Instrumentation
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ECL and electrochemical measurements were carried out on an ECL detection system (MPI-E, Remex Electroic Instrument Ltd. Co., Xi’an, China) equipped with a
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home-made ECL cell, which has been described in detail elsewhere [30]. An atomic
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absorption (AAS) spectrophotometer (TAS-986, Beijing Persee Corporation, China)
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was used to detect the Cr(VI) concentration of the Min River water sample.
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3. Results and discussion
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3.1 ECL response of GQD/S2O82- system toward Cr(VI) and the corresponding
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mechanism
The as-prepared GQDs have uniform lateral size of ~10 nm without obvious lattice
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fringe (figures are not shown). The topographic heights range mainly from 0.3 to 0.9 nm, with an average value of ~0.5 nm, suggesting that the obtained GQDs are mainly monolayered (figures are not shown). As shown in Figure 1, the prepared monolayer GQDs exhibit excellent ECL activities, especially in the presence of S2O82-. When the
potential is scanned in the range of +1.3 to -1.5 V, the coreactant system of GQD/S2O82- produces a strong cathodic ECL signal in the potential range of -0.7 to -1.5 V (curve (b) in Figure 3). The strong ECL signal can be sensitively quenched by Cr(VI), i.e. mainly existing as Cr2O72- in neutral aqueous solution (see curve (c) in Figure 3).
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As shown in Figure 2, the ECL intensity of GQD/S2O82- system either in the absence (red and black points) or presence (blue points) of Cr(VI) is obviously
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affected by the concentrations of both GQDs and K2S2O8. When the concentration of
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GQDs is increased from 0.01 to 1 mg mL-1, the ECL intensity increases rapidly and
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linearly with the GQD concentration in the range from 0.01 to 0.2 mg mL-1, but
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increases relatively slowly in the range from 0.2 to 1.0 mg mL-1 (Figure 2a). When the
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concentration of K2S2O8 is increased from 10 to 1000 μM, the ECL intensity increases linearly with the concentration of K2S2O8 in the range from 10 to 200 μM, but is
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relatively insensitive to the fluctuation of the concentration of K2S2O8 higher than 200 μM (Figure 2b). Different from the ECL intensities, the quenching efficiency (I0/I,
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where I0 and I are ECL intensities of the GQD/S2O82- system in the absence and
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presence of Cr(VI), respectively) of Cr(VI) always keeps stable when the
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concentrations of GQDs and S2O82- are increased (see the insets in Figure 2).
The ECL response mentioned above implies an ECL sensor for Cr(VI), the corresponding mechanisms are then worth being discussed. In general, the GQD/S2O82- system shows similar ECL behaviours to those of carbon quantum dot
(CQD)/S2O82- system reported in our previous work [31]. Accordingly, GQD/S2O82system should share a similar ECL mechanism with CQD/S2O82- system and silicon nanocrystal/S2O82- system [32]. In brief, GQDs and S2O82- are electrochemical reduced to produce negatively charged GQDs (GQD • −) and SO4 • − radicals, respectively. Subsequently, the electron-transfer between GQD•− and SO4•− leads to the formation of excited-state GQDs (GQD*), which produces ECL signal when
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coming back to the ground state (see Figure 3). Like PL signal, the ECL signal can be
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quenched through static or dynamic avenues. Static avenues mean the quencher react
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with either the luminophore or the coreactant. Apparently, the quenching efficient
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should be affected by the concentrations of either the luminophore or the coreactant if
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the ECL signal of CQD/S2O82- system was quenched by Cr(VI) through a static
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avenue. Therefore, the ECL signal should be quenched by Cr(VI) through a dynamic
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way. It has been well-reported that heavy atoms can collisionally deactivate the
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excited-state of luminophores, resulting the quenching of PL signal. Then the ECL
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quenching of GQD/S2O82- system by Cr(VI) might undergo the same collisional
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deactivation process. In other words, GQD* is deactivated upon contact with Cr(VI),
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leading to the ECL decrease of GQD/S2O82- system. This hypothesis is further proved by the fact that the relationship between the quenching ratio and the concentration of
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Cr(VI) fits well with Stern-Volmer equation [33], which will be discussed in the following investigation results.
3.2 Optimization of the sensing system
As has been discussed above that the ECL signal of GQD/S2O82- system can be sensitively quenched by Cr(VI), on the basis of which an ECL sensor could be developed to detect Cr(VI). Subsequently, some important experimental conditions including concentrations of GQDs and K2S2O8, response time and pH value were optimized.
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First, the concentrations of GQDs and K2S2O8 were optimized. Although the quenching efficiency of Cr(VI) toward the ECL signal of GQD/S2O82- system is
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nearly independent on the concentrations of GQDs and S2O82-, the relative standard
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deviation (RSD) of the ECL intensity of GQD/S2O82- system decreases when the
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concentrations of GQDs and S2O82- are increased. Correspondingly, the signal/noise
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(S/N) ratio increases obviously as increasing the concentrations of GQDs and S2O82-
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in the range from 0.01 to 0.2 mg mL-1 and from 10 to 200 μM, respectively (see Figure 4). When the concentrations of GQDs and S2O82- are further increased, the S/N
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ratio increases slightly. Therefore, 0.2 mg mL-1 GQDs and 200 μM K2S2O8 were eventually chosen in the detection of Cr(VI) for obtaining the highest accuracy and
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the lowest detection limit.
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The response rate of GQD/S2O82- ECL system to Cr(VI) was subsequently
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investigated. The ECL intensity is quenched by ~50% as soon as 20 μM Cr(VI) is added into the GQD/S2O82- system, and remains stable in the following 1 h (Figure is not shown). This result suggests that the quenching of Cr(VI) towards the ECL of GQD/S2O82- is quite rapid and stable, implying a promising application in fast and
convenient sensing of Cr(VI) without strict time control.
The pH value of solution is another important factor for the sensing system. It affects not only the ECL intensity of GQD/S2O82- system, but also the quenching efficiency of Cr(VI) for the ECL of GQD/S2O82- system (Figure 5). On one hand, the ECL intensity of GQD/S2O82- is relatively high in pH range of 3−8, but is relatively
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low in the pH range of 9-12. The effect of pH value on the ECL intensity of GQD/S2O82- may be related to the surface state of GQDs or the stability of GQD•− and
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SO4•− radicals, and thus is quite difficult to be explained exactly. On the other hand,
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the ECL quenching efficiency has the maximum in the neutral solution (pH=7), and
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decreases when the pH value is either increased or decreased. The effect of pH value
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on the quenching efficiency may be related to the existing forms in the solution of
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both Cr(VI) and GQDs. Cr(VI) exists in solution as either CrO42- or Cr2O72-, depending on the pH value of the solution. In acidic solutions, Cr(VI) exists
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predominately as CrO42-, which may has lower quenching efficiency towards GQD*. Therefore, Cr(VI) can’t efficiently quench the ECL signal of GQD/S2O82- system in
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acidic solutions. In alkaline solutions, the oxygen-containing functional groups on
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GQDs are well deprotonated, leading to the increase of negative charge. It is
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unfavourable for the collision between GQD* and the negatively charged Cr2O72-. As a result, the quenching efficiency decreases as the pH value of solution is increased in the range of pH>7. Accordingly, neutral solution (pH=7) was chosen for the sensing system.
3.3 Specificity of the sensing system
To evaluate the selectivity of the sensing system for Cr(VI), effects of some common ions (MnO4 -, ClO4 -, ClO-, Zn2+, Ca2+, Mg2+, K+, Na+, Al3+, Fe3+, Fe2+, Cu2+, Co2+) and some heavy metal ions (Hg2+, Pb2+, Ag+, Cr3+, Cd2+) on the ECL response of GQD/S2O82- system were also investigated. The counter anions were NO3− or Cl−,
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which were proved to have no obvious effect on the ECL of GQD/S2O82- system. Experimental results indicate that many metal ions including Pb2+, Al3+, Fe3+, Fe2+,
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Ag+, Cr3+, Cd2+, Cu2+ and Co2+ can also quench the ECL signal of GQD/S2O82- system
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(Figure 6, black column). Especially for Cr3+, Cd2+, Cu2+ and Co2+, the quenching
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efficiencies are even higher than that of Cr(VI). Accordingly, a suitable masking
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ligand is necessary to improve the selectively of the sensing system toward Cr(VI).
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EDTA is a well-known masking ligand which can complex most metallic cations. However, EDTA has nearly no effect on Cr(VI) due to that Cr(VI) exists mainly as
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anions (CrO4- and Cr2O72-) in the solution of pH 7. Accordingly, EDTA was added in the sensing system to effectively eliminate the interferences from other metal ions. As
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shown in Figure 6 (red columns), the ECL signal was quenched by only Cr(VI) in the
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presence of 0.1 mM EDTA. Apparently, the present sensing system exhibits excellent
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selectivity toward Cr(VI) ions.
3.4 Sensitivity of the sensing system
Under the optimized conditions discussed above, the ECL response of the sensing system to Cr(VI) was measured. As shown in Figure 7, the ECL intensity of
GQD/S2O82- system is sensitive to Cr(VI), and decreases obviously as increasing the concentration of Cr(VI). The relationship between the quenching ratio (the average value from 10 parallel measurements) and the concentration of Cr(VI) in the range from 50 nM to 60 μM can be described by the well-known Stern-Volmer equation [33]:
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I0/I =0.0496C +1.0291 R2=0.999 (1)
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where C is the concentration of Cr(VI). The detection limit (3 times of the standard
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deviation of 10 parallel blank measurements) was detected to be about 20 nM. Apparently, the dynamic range is quite wide, and is comparable to those of previously
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reported methods [19-28]. The detection limit is not as low as those of many stripping
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voltammetric methods [27,28], but it is comparable to those of most reported
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spectroscopic methods [20-22]. Furthermore, the detection limit is much lower than
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the highest allowed concentration of Cr(VI) in domestic water proposed by
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Environmental Protection Agency (EPA) (less than 0.1 μg mL-1) [34]. In other words,
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this developed sensor for Cr(VI) is sensitive enough in environmental monitoring
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applications. What is more important, the wide response range, the excellent selectivity and the environmental friendliness would be the main advantages.
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Accordingly, the simple, fast and low-cost ECL sensor could be considered to have great potential in environmental analysis applications.
3.5. Application of the sensing system in the detection of a real water sample
On the basis of the above results, a river water sample from Min River (the biggest
river of Fujian, China) was used to evaluate the feasibility of the sensor. The standard addition method was used in estimating the concentration of Cr(VI) in the river water. Experimental results showed that river water sample had no observable effect on the ECL intensity of GQD/S2O82- system, indicated Cr(VI) in the water sample was not detected (below the lowest detectable concentration of the method). The results from an AAS method also indicated the river sample contained no detectable Cr element.
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Accordingly, the river water spiked with 10 μM Cr(VI) was tested using the ECL
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sensor. The recovery was calculated to be about 95%, implying the great potential of
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the ECL sensor in detecting Cr(VI) from real water samples.
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4. Summary
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Cr(VI) was found to be able to quench the ECL signal of GQD/S2O82- system.
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Accordingly, an ECL sensor has been developed to detect Cr(VI). Furthermore, the
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ECL mechanism of GQD/S2O82- system and the quenching mechanism of Cr(VI) on
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the GQD/S2O82- system have been discussed. The developed sensor exhibits several
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advantages including inexpensive, high sensitivity, good selectivity, rapid detection
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and wide linear response range, thus is proposed as an effective alternative to other
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more complex sensing system.
Acknowledgments
This study was financially supported by National Natural Science Foundation of China (21305017, 21375020), Program for New Century Excellent Talents in Chinese University
(NCET-10-0019),
National
Basic
Research
Program
of
China
(2010CB732400), and the Program for Changjiang Scholars and Innovative Research Team in University (No. IRT1116).
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Figure 1. ECL response of GQD/S2O82- coreactant system to Cr2O72- in aqueous solution (concentration of GQDs, 0.2 mg mL-1; concentration of K2S2O8, 200 μM; concentration of Cr2O72-, 20 µM; GQDs potential window, −1.50 to +1.30 V; scan rate, 0.2 V/s; starting potential, 0 V; initial scan direction, positive; solution pH, 7).
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Figure 2. Effects of GQD concentration (a) and K2S2O8 concentration (b) on ECL responses of GQD/S2O82- system in the absence (red and black) and presence (blue) of
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20 μM Cr(VI). Other experimental conditions include 200 μM K2S2O8 in (a); 0.2 mg
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mL-1 GQDs in (b); potential window of −1.50 to +1.30 V; scan rate of 0.2 V/s; and pH
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7. The insets show the ECL quenching radio at different concentration of GQDs (left)
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and K2S2O8 (right).
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Figure 3. ECL reaction mechanism of the GQD/S2O82- system in the presence of
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Cr(VI).
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Figure 4. Effects of GQD concentration (a) and K2S2O8 concentration (b) on the S/N
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of the ECL system toward 20 μM Cr(VI). Other experimental conditions include 200 μM K2S2O8 in (a); 0.2 mg mL-1 GQDs in (b); potential window of −1.50 to +1.30 V; scan rate of 0.2 V/s; and pH 7.
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Figure 5. ECL intensities of GQD/S2O82- system in the absence (blue) and presence
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(red) of 20 μM Cr(VI) in PBS of serial pH values (concentration of GQDs, 0.2 mg
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mL-1 ; concentration of K2S2O8, 200 μM; potential window, −1.50 to +1.30 V; scan
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rate, 0.2 V/s;). The inset shows the ECL quenching radio at different pH.
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Figure 6. Selectivity of the GQD/S2O82- system based sensor for Cr(VI) over other ions in PBS (concentration of GQDs, 0.2 mg mL-1 ; concentration of K2S2O8, 200 μM;
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potential window, −1.50 to +1.30 V; scan rate, 0.2 V/s; pH, 7; concentrations of all
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metal ions and anions were 10 μM; concentration of EDTA, 0.1mM).
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Figure 7. ECL responses of GQD/S2O82- system to the additions of serial concentrations of Cr(VI) in PBS (from top to bottom: 0, 0.02, 0.05, 0.10, 0.20, 0.50,
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1.00, 2.00, 5.00, 10.0, 20.0, 30.0, 40.0, 50.0, 60.0, 70.0, 90.0 μM) (concentration of GQDs, 0.2 mg mL-1 ; concentration of K2S2O8, 200 μM; potential window, −1.50 to
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+1.30 V; scan rate, 0.2 V/s; pH, 7; concentration of EDTA, 0.1 mM). The inset shows
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the linear calibration plot for Cr(VI) detection.
S0013-4686(14)02276-2 http://dx.doi.org/doi:10.1016/j.electacta.2014.11.068 EA 23740
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
8-8-2014 31-10-2014 11-11-2014
Please cite this article as: Yingmei Chen, Yongqiang Dong, Huan Wu, Congqiang Chen, Yuwu Chi, Guonan Chen, Electrochemiluminescence sensor for hexavalent chromium based on the graphene quantum dots/peroxodisulfate system, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2014.11.068 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Electrochemiluminescence sensor for hexavalent chromium based on the graphene quantum dots/peroxodisulfate system Yingmei Chen,# Yongqiang Dong,# Huan Wu, Congqiang Chen, Yuwu Chi,* Guonan Chen MOE Key Laboratory of Analysis and Detection Technology for Food Safety, Fujian
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Provincial Key Laboratory of Analysis and Detection Technology for Food Safety,
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and Department of Chemistry, Fuzhou University, Fujian 350108, China. *Corresponding author: Tel/Fax: +86 591 22866137 E-mail: [email protected] (Y. Chi).
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# These authors contributed equally to this work
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Abstract
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A electrochemiluminescent (ECL) sensor has been designed to detect hexavalent
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chromium (Cr(VI)) in environmental water samples based on the fact that Cr(VI) can
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collisionally quench the ECL signal of graphene quantum dots/peroxodisulfate (GQD/S2O82-) system. After optimizing some important experimental conditions
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including the concentrations of GQDs and S2O82-, response time, and pH value of
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solution, the ECL sensor has been finally developed. The ECL sensor exhibits a wide
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linear response range (50 nM-60 μM), excellent selectivity, and high sensitivity (detection limit of 20 nM, S/N = 3). Furthermore, the developed sensor has been applied in the detection of Cr(VI) in a spiked river water. The result suggests that the sensor can offer a simple, green, low cost, high selectivity and sensitivity detection of
Cr(VI).
Keywords: Electrochemiluminescence, graphene quantum dots, hexavalent chromium, detection
1. Introduction
Graphene quantum dots (GQDs), graphene nanosheets of less than 100 nm in
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lateral size [1], are emerging luminescent carbon nanomaterials. Due to the quantum
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confinement and edge effects, GQDs exhibit many unique optical properties such as
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photoluminescence (PL), chemiluminescence (CL), and electrochemiluminescence
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(ECL) [2,3]. The PL of GQDs has attracted tremendous attention, and has been
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applied in many fields such as bio-imaging, cell-imaging and sensing [4,5]. Contrarily,
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much less attention has been paid to the ECL of GQDs [6,7]. ECL is a CL that
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triggered by electrochemical methods. The analytical methods based on the ECL
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signal combine the advantages of CL and electrochemical analysis [8], and
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accordingly show many distinct advantages including no optical background, easy
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reaction control, high sensitivity and selectivity, and wide response range [9,10].
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Therefore, the ECL properties of GQDs are apparently very important in terms of
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analytical applications. Actually, GQDs have been applied in the detection of some cations based on their ECL activities [11,12].
Chromium, an important alloying element, is predominantly used in metallurgy, e.g., electroplating, leather tanning, metal smelt stabling and metal finishing industries [13]. In aqueous systems, chromium mainly exists in two oxidation states: trivalent
chromium (Cr(Ⅲ)) and hexavalent chromium (Cr(VI)) [14]. Cr(Ⅲ) is essential and beneficial for humans [15,16], while Cr(VI) is a highly toxic species and a suspected carcinogenic agent [17]. After entered blood circulation through digestive tract, Cr(VI) can damage the structure and function of hepatocytes due to its strong oxidation property, resulting in Cr(VI) hepatotoxicity [18]. Accordingly, it is vitally important to develop an effective method to precisely detect trace concentration Cr(VI). Various
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analysis techniques have been used to determinate Cr(VI), including inductively
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coupled plasma mass spectrometry [19], inductively coupled plasma-optical emission
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spectrometry [20], inductively coupled plasma atomic emission spectrometry [21],
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electrothermal atomic absorption spectrometry [22], graphite furnace atomic
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absorption spectrometry [23], reversed-phase ion-pair chromatography [24], X-ray
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fluorescence spectrometry [25], ECL spectrometry [26], and stripping voltammetric
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methods [27,28]. However, most of these methods suffer from some disadvantages,
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such as complex process, long response time, low sensitivity, and high cost of the
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instruments. Furthermore, many methods can only detect the total quantity of
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chromium, but are not able to distinguish Cr(VI) from Cr(Ⅲ). Therefore, developing a
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simple and fast method to detect Cr(VI) sensitively and selectively is of interesting.
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Herein, Cr(VI) was found to be able to quench sensitively the strong cathodic ECL
signal of GQD/S2O82- system. After optimizing some important experimental conditions, a sensitive and selective sensing system would be developed to detect Cr(VI) in domestic water.
2. Experimental
2.1 Chemicals
Vulcan CX-72 carbon black (Cabot Corporation) were used to prepare the single layer GQDs. K2S2O8 (>99.99%) was obtained from Sigma-Aldrich. All other reagents
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were of analytical grade and used as received. Doubly distilled water was used
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throughout the experiment. Phosphate-buffered saline (PBS) solutions of different pH
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values were prepared by titrating 0.1 M phosphoric acid solution with a concentrated
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sodium hydroxide solution (1 M) to the required pH values. 1 M KNO3 was added
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into PBS solutions through the ECL experiments to improve the conductibility.
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2.2 Preparation of GQDs
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GQDs were synthesized by refluxed Vulcan CX-72 carbon black with concentrated
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nitric acid according a chemical oxidation method that has been described elsewhere
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[29]. Briefly, 2 g of dried CX-72 carbon black was dissolved in 500 mL 6 M HNO3
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and then refluxing for 24 h. After cooling to room temperature, the suspension was centrifuged (2770g) for 10 min. The obtain supernatant was heated at 200℃ to
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evaporate the water and nitric acid to obtain reddish-brown powder.
2.3 Analysis of a real sample
A water sample was collected from Min River (Fujian, China). Suitable aliquots
(500 μL) of this river water were spiked with standard potassium dichromate solutions (10 μL, final concentration, 0−50 μM). The spiked samples were then diluted to 1 mL with PBS (0.1 M, pH 7.0) containing GQDs (final concentration, 0.2 mg mL-1), K2S2O8 (final concentration, 1 M), Ethylenediaminetetraacetic acid disodium salt (EDTA) (final concentration, 0.1 mM) and KNO3 (final concentration, 1 M) then
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analyzed using the developed sensing technique.
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2.4 Instrumentation
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ECL and electrochemical measurements were carried out on an ECL detection system (MPI-E, Remex Electroic Instrument Ltd. Co., Xi’an, China) equipped with a
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home-made ECL cell, which has been described in detail elsewhere [30]. An atomic
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absorption (AAS) spectrophotometer (TAS-986, Beijing Persee Corporation, China)
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was used to detect the Cr(VI) concentration of the Min River water sample.
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3. Results and discussion
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3.1 ECL response of GQD/S2O82- system toward Cr(VI) and the corresponding
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mechanism
The as-prepared GQDs have uniform lateral size of ~10 nm without obvious lattice
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fringe (figures are not shown). The topographic heights range mainly from 0.3 to 0.9 nm, with an average value of ~0.5 nm, suggesting that the obtained GQDs are mainly monolayered (figures are not shown). As shown in Figure 1, the prepared monolayer GQDs exhibit excellent ECL activities, especially in the presence of S2O82-. When the
potential is scanned in the range of +1.3 to -1.5 V, the coreactant system of GQD/S2O82- produces a strong cathodic ECL signal in the potential range of -0.7 to -1.5 V (curve (b) in Figure 3). The strong ECL signal can be sensitively quenched by Cr(VI), i.e. mainly existing as Cr2O72- in neutral aqueous solution (see curve (c) in Figure 3).
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As shown in Figure 2, the ECL intensity of GQD/S2O82- system either in the absence (red and black points) or presence (blue points) of Cr(VI) is obviously
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affected by the concentrations of both GQDs and K2S2O8. When the concentration of
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GQDs is increased from 0.01 to 1 mg mL-1, the ECL intensity increases rapidly and
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linearly with the GQD concentration in the range from 0.01 to 0.2 mg mL-1, but
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increases relatively slowly in the range from 0.2 to 1.0 mg mL-1 (Figure 2a). When the
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concentration of K2S2O8 is increased from 10 to 1000 μM, the ECL intensity increases linearly with the concentration of K2S2O8 in the range from 10 to 200 μM, but is
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relatively insensitive to the fluctuation of the concentration of K2S2O8 higher than 200 μM (Figure 2b). Different from the ECL intensities, the quenching efficiency (I0/I,
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where I0 and I are ECL intensities of the GQD/S2O82- system in the absence and
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presence of Cr(VI), respectively) of Cr(VI) always keeps stable when the
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concentrations of GQDs and S2O82- are increased (see the insets in Figure 2).
The ECL response mentioned above implies an ECL sensor for Cr(VI), the corresponding mechanisms are then worth being discussed. In general, the GQD/S2O82- system shows similar ECL behaviours to those of carbon quantum dot
(CQD)/S2O82- system reported in our previous work [31]. Accordingly, GQD/S2O82system should share a similar ECL mechanism with CQD/S2O82- system and silicon nanocrystal/S2O82- system [32]. In brief, GQDs and S2O82- are electrochemical reduced to produce negatively charged GQDs (GQD • −) and SO4 • − radicals, respectively. Subsequently, the electron-transfer between GQD•− and SO4•− leads to the formation of excited-state GQDs (GQD*), which produces ECL signal when
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coming back to the ground state (see Figure 3). Like PL signal, the ECL signal can be
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quenched through static or dynamic avenues. Static avenues mean the quencher react
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with either the luminophore or the coreactant. Apparently, the quenching efficient
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should be affected by the concentrations of either the luminophore or the coreactant if
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the ECL signal of CQD/S2O82- system was quenched by Cr(VI) through a static
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avenue. Therefore, the ECL signal should be quenched by Cr(VI) through a dynamic
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way. It has been well-reported that heavy atoms can collisionally deactivate the
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excited-state of luminophores, resulting the quenching of PL signal. Then the ECL
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quenching of GQD/S2O82- system by Cr(VI) might undergo the same collisional
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deactivation process. In other words, GQD* is deactivated upon contact with Cr(VI),
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leading to the ECL decrease of GQD/S2O82- system. This hypothesis is further proved by the fact that the relationship between the quenching ratio and the concentration of
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Cr(VI) fits well with Stern-Volmer equation [33], which will be discussed in the following investigation results.
3.2 Optimization of the sensing system
As has been discussed above that the ECL signal of GQD/S2O82- system can be sensitively quenched by Cr(VI), on the basis of which an ECL sensor could be developed to detect Cr(VI). Subsequently, some important experimental conditions including concentrations of GQDs and K2S2O8, response time and pH value were optimized.
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First, the concentrations of GQDs and K2S2O8 were optimized. Although the quenching efficiency of Cr(VI) toward the ECL signal of GQD/S2O82- system is
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nearly independent on the concentrations of GQDs and S2O82-, the relative standard
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deviation (RSD) of the ECL intensity of GQD/S2O82- system decreases when the
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concentrations of GQDs and S2O82- are increased. Correspondingly, the signal/noise
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(S/N) ratio increases obviously as increasing the concentrations of GQDs and S2O82-
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in the range from 0.01 to 0.2 mg mL-1 and from 10 to 200 μM, respectively (see Figure 4). When the concentrations of GQDs and S2O82- are further increased, the S/N
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ratio increases slightly. Therefore, 0.2 mg mL-1 GQDs and 200 μM K2S2O8 were eventually chosen in the detection of Cr(VI) for obtaining the highest accuracy and
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the lowest detection limit.
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The response rate of GQD/S2O82- ECL system to Cr(VI) was subsequently
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investigated. The ECL intensity is quenched by ~50% as soon as 20 μM Cr(VI) is added into the GQD/S2O82- system, and remains stable in the following 1 h (Figure is not shown). This result suggests that the quenching of Cr(VI) towards the ECL of GQD/S2O82- is quite rapid and stable, implying a promising application in fast and
convenient sensing of Cr(VI) without strict time control.
The pH value of solution is another important factor for the sensing system. It affects not only the ECL intensity of GQD/S2O82- system, but also the quenching efficiency of Cr(VI) for the ECL of GQD/S2O82- system (Figure 5). On one hand, the ECL intensity of GQD/S2O82- is relatively high in pH range of 3−8, but is relatively
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low in the pH range of 9-12. The effect of pH value on the ECL intensity of GQD/S2O82- may be related to the surface state of GQDs or the stability of GQD•− and
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SO4•− radicals, and thus is quite difficult to be explained exactly. On the other hand,
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the ECL quenching efficiency has the maximum in the neutral solution (pH=7), and
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decreases when the pH value is either increased or decreased. The effect of pH value
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on the quenching efficiency may be related to the existing forms in the solution of
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both Cr(VI) and GQDs. Cr(VI) exists in solution as either CrO42- or Cr2O72-, depending on the pH value of the solution. In acidic solutions, Cr(VI) exists
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predominately as CrO42-, which may has lower quenching efficiency towards GQD*. Therefore, Cr(VI) can’t efficiently quench the ECL signal of GQD/S2O82- system in
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acidic solutions. In alkaline solutions, the oxygen-containing functional groups on
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GQDs are well deprotonated, leading to the increase of negative charge. It is
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unfavourable for the collision between GQD* and the negatively charged Cr2O72-. As a result, the quenching efficiency decreases as the pH value of solution is increased in the range of pH>7. Accordingly, neutral solution (pH=7) was chosen for the sensing system.
3.3 Specificity of the sensing system
To evaluate the selectivity of the sensing system for Cr(VI), effects of some common ions (MnO4 -, ClO4 -, ClO-, Zn2+, Ca2+, Mg2+, K+, Na+, Al3+, Fe3+, Fe2+, Cu2+, Co2+) and some heavy metal ions (Hg2+, Pb2+, Ag+, Cr3+, Cd2+) on the ECL response of GQD/S2O82- system were also investigated. The counter anions were NO3− or Cl−,
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which were proved to have no obvious effect on the ECL of GQD/S2O82- system. Experimental results indicate that many metal ions including Pb2+, Al3+, Fe3+, Fe2+,
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Ag+, Cr3+, Cd2+, Cu2+ and Co2+ can also quench the ECL signal of GQD/S2O82- system
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(Figure 6, black column). Especially for Cr3+, Cd2+, Cu2+ and Co2+, the quenching
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efficiencies are even higher than that of Cr(VI). Accordingly, a suitable masking
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ligand is necessary to improve the selectively of the sensing system toward Cr(VI).
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EDTA is a well-known masking ligand which can complex most metallic cations. However, EDTA has nearly no effect on Cr(VI) due to that Cr(VI) exists mainly as
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anions (CrO4- and Cr2O72-) in the solution of pH 7. Accordingly, EDTA was added in the sensing system to effectively eliminate the interferences from other metal ions. As
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shown in Figure 6 (red columns), the ECL signal was quenched by only Cr(VI) in the
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presence of 0.1 mM EDTA. Apparently, the present sensing system exhibits excellent
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selectivity toward Cr(VI) ions.
3.4 Sensitivity of the sensing system
Under the optimized conditions discussed above, the ECL response of the sensing system to Cr(VI) was measured. As shown in Figure 7, the ECL intensity of
GQD/S2O82- system is sensitive to Cr(VI), and decreases obviously as increasing the concentration of Cr(VI). The relationship between the quenching ratio (the average value from 10 parallel measurements) and the concentration of Cr(VI) in the range from 50 nM to 60 μM can be described by the well-known Stern-Volmer equation [33]:
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I0/I =0.0496C +1.0291 R2=0.999 (1)
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where C is the concentration of Cr(VI). The detection limit (3 times of the standard
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deviation of 10 parallel blank measurements) was detected to be about 20 nM. Apparently, the dynamic range is quite wide, and is comparable to those of previously
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reported methods [19-28]. The detection limit is not as low as those of many stripping
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voltammetric methods [27,28], but it is comparable to those of most reported
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spectroscopic methods [20-22]. Furthermore, the detection limit is much lower than
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the highest allowed concentration of Cr(VI) in domestic water proposed by
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Environmental Protection Agency (EPA) (less than 0.1 μg mL-1) [34]. In other words,
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this developed sensor for Cr(VI) is sensitive enough in environmental monitoring
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applications. What is more important, the wide response range, the excellent selectivity and the environmental friendliness would be the main advantages.
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Accordingly, the simple, fast and low-cost ECL sensor could be considered to have great potential in environmental analysis applications.
3.5. Application of the sensing system in the detection of a real water sample
On the basis of the above results, a river water sample from Min River (the biggest
river of Fujian, China) was used to evaluate the feasibility of the sensor. The standard addition method was used in estimating the concentration of Cr(VI) in the river water. Experimental results showed that river water sample had no observable effect on the ECL intensity of GQD/S2O82- system, indicated Cr(VI) in the water sample was not detected (below the lowest detectable concentration of the method). The results from an AAS method also indicated the river sample contained no detectable Cr element.
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Accordingly, the river water spiked with 10 μM Cr(VI) was tested using the ECL
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sensor. The recovery was calculated to be about 95%, implying the great potential of
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the ECL sensor in detecting Cr(VI) from real water samples.
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4. Summary
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Cr(VI) was found to be able to quench the ECL signal of GQD/S2O82- system.
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Accordingly, an ECL sensor has been developed to detect Cr(VI). Furthermore, the
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ECL mechanism of GQD/S2O82- system and the quenching mechanism of Cr(VI) on
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the GQD/S2O82- system have been discussed. The developed sensor exhibits several
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advantages including inexpensive, high sensitivity, good selectivity, rapid detection
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and wide linear response range, thus is proposed as an effective alternative to other
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more complex sensing system.
Acknowledgments
This study was financially supported by National Natural Science Foundation of China (21305017, 21375020), Program for New Century Excellent Talents in Chinese University
(NCET-10-0019),
National
Basic
Research
Program
of
China
(2010CB732400), and the Program for Changjiang Scholars and Innovative Research Team in University (No. IRT1116).
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Figure 1. ECL response of GQD/S2O82- coreactant system to Cr2O72- in aqueous solution (concentration of GQDs, 0.2 mg mL-1; concentration of K2S2O8, 200 μM; concentration of Cr2O72-, 20 µM; GQDs potential window, −1.50 to +1.30 V; scan rate, 0.2 V/s; starting potential, 0 V; initial scan direction, positive; solution pH, 7).
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Figure 2. Effects of GQD concentration (a) and K2S2O8 concentration (b) on ECL responses of GQD/S2O82- system in the absence (red and black) and presence (blue) of
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20 μM Cr(VI). Other experimental conditions include 200 μM K2S2O8 in (a); 0.2 mg
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mL-1 GQDs in (b); potential window of −1.50 to +1.30 V; scan rate of 0.2 V/s; and pH
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7. The insets show the ECL quenching radio at different concentration of GQDs (left)
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and K2S2O8 (right).
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Figure 3. ECL reaction mechanism of the GQD/S2O82- system in the presence of
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Cr(VI).
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Figure 4. Effects of GQD concentration (a) and K2S2O8 concentration (b) on the S/N
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of the ECL system toward 20 μM Cr(VI). Other experimental conditions include 200 μM K2S2O8 in (a); 0.2 mg mL-1 GQDs in (b); potential window of −1.50 to +1.30 V; scan rate of 0.2 V/s; and pH 7.
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Figure 5. ECL intensities of GQD/S2O82- system in the absence (blue) and presence
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(red) of 20 μM Cr(VI) in PBS of serial pH values (concentration of GQDs, 0.2 mg
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mL-1 ; concentration of K2S2O8, 200 μM; potential window, −1.50 to +1.30 V; scan
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rate, 0.2 V/s;). The inset shows the ECL quenching radio at different pH.
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Figure 6. Selectivity of the GQD/S2O82- system based sensor for Cr(VI) over other ions in PBS (concentration of GQDs, 0.2 mg mL-1 ; concentration of K2S2O8, 200 μM;
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potential window, −1.50 to +1.30 V; scan rate, 0.2 V/s; pH, 7; concentrations of all
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metal ions and anions were 10 μM; concentration of EDTA, 0.1mM).
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Figure 7. ECL responses of GQD/S2O82- system to the additions of serial concentrations of Cr(VI) in PBS (from top to bottom: 0, 0.02, 0.05, 0.10, 0.20, 0.50,
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1.00, 2.00, 5.00, 10.0, 20.0, 30.0, 40.0, 50.0, 60.0, 70.0, 90.0 μM) (concentration of GQDs, 0.2 mg mL-1 ; concentration of K2S2O8, 200 μM; potential window, −1.50 to
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+1.30 V; scan rate, 0.2 V/s; pH, 7; concentration of EDTA, 0.1 mM). The inset shows
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the linear calibration plot for Cr(VI) detection.