Accepted Manuscript Title: A sensitive electrochemical chlorophenols sensor based on nanocomposite of ZnSe quantum dots and cetyltrimethylammonium bromide Author: Jianjun Li Xiao Li Ran Yang Lingbo Qu Peter de B. Harrington PII: DOI: Reference:
S0003-2670(13)01266-X http://dx.doi.org/doi:10.1016/j.aca.2013.09.049 ACA 232861
To appear in:
Analytica Chimica Acta
Received date: Revised date: Accepted date:
26-7-2013 20-9-2013 23-9-2013
Please cite this article as: J. Li, X. Li, R. Yang, L. Qu, P.B. Harrington, A sensitive electrochemical chlorophenols sensor based on nanocomposite of ZnSe quantum dots and cetyltrimethylammonium bromide, Analytica Chimica Acta (2013), http://dx.doi.org/10.1016/j.aca.2013.09.049 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.
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A sensitive electrochemical chlorophenols sensor based on nanocomposite of
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ZnSe quantum dots and cetyltrimethylammonium bromide
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Jianjun Lia , Xiao Lia, Ran Yanga*, Lingbo Qua,b*, Peter de B. Harringtonc
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a
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Zhengzhou 450001, PR China
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b
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Zhengzhou 450001, PR China
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c
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Biochemistry, Clippinger Laboratories, OHIO University, Athens, OH 45701-2979
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School of Chemistry & Chemical Engineering, Henan University of Technology,
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Center for Intelligent Chemical Instrumentation, Department of Chemistry and
USA
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The College of Chemistry and Molecular Engineering, Zhengzhou University,
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Corresponding authour1: Ran Yang E-mail:[email protected] Corresponding authour2:Lingbo Qu , [email protected]
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Abstract: In this work, a very sensitive and simple electrochemical sensor for
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chlorophenols (CPs) based on a nanocomposite of cetyltrimethylammonium bromide
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(CTAB) and ZnSe quantum dots (ZnSe-CTAB) through electrostatic self-assembly
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technology was built for the first time. The composite of ZnSe-CTAB introduced a
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favorable access for the electron transfer and gave superior electrocatalytic activity for
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the oxidation of CPs than ZnSe QDs and CTAB alone. Differential pulse voltammetry
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(DPV) was used for the quantitative determination of the CPs including
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2-chlorophenol (2-CP), 2,4-dichlorophenol (2,4-DCP) and pentachlorophenol(PCP).
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Under the optimum conditions, the peak currents of the CPs were proportional to their
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concentrations in the range from 0.02 to 10.0 µM for 2-CP, 0.006 to 9.0 µM for
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2,4-DCP, and 0.06 to 8.0 for PCP. The detection limits were 0.008 µM for 2-CP,
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0.002 µM for 2,4-DCP, and 0.01 µM for PCP, respectively. The method was
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successfully applied for the determination of CPs in waste water with satisfactory
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recoveries. This ZnSe-CTAB electrode system provides operational access to design
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environment-friendly CPs sensors. Keywords: ZnSe quantum dots; cetyltrimethylammonium bromide; Self-assembly; chlorophenols; electrochemical sensor
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1. Introduction Chlorophenols (CPs) are well-known pollutants of environmental waters and soils
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and are mainly found in the effluent discharges from factories, such as those
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producing paper and pesticides [1]. They are used as general biocides and wood
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preservatives; may also be introduced into the environment as the products of
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metabolic degration of chlorinated pesticides [2]; and result as byproduct of the
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chlorination of drinking water [3]. CPs can cause serious health hazards due to their
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inherent toxicity and relative persistence in the environment. As a consequence, the
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US Environmental Protection Agency (USEP) and European Union (EU) has listed
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2-chlorophenol
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2,4,6-trichlorophenol(2,4,6-TCP), and 2,3,4,5,6-pentachlorophenol (PCP) as priority
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pollutants and regulated the maximum admissible concentration of CPs in drinking
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water at 0.5 ng mL-1 [4].
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2,4-dichlorophenol
(2,4-DCP),
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(2-CP),
Many analytical methods have been established to determine CPs, such as high
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performance liquid chromatography [5], gas chromatography/mass spectrometry [6], spectrophotometry [7], fluorescence [8], enzyme linked immunosorbent assay (ELISA) [9] and electrochemical methods [10–23]. Among them, electrochemical methods are more preferable over the other methods due to the advantages of simple operation,
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fast response, low cost, and small size that affords a portable sensor for on-site
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detection [19]. However, the detection limits of CPs of the reported electrochemical
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methods are too high to meet the regulatory requirements. Furthermore, the electrode
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modified materials in these reports are usually based on bio-enzymes, which are
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difficult to immobilize on the electrode surface, require a chemical mediator such as
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hydrogen peroxide, and can be easily denatured if the pH, ionic strength, and
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temperature are not carefully controlled. Semiconductor quantum dots (QDs) are typically nanocrystals with a size between
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1 and 10 nm that are compounds of cationic groups II–VI mixed with anionic groups
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III–V [24].
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electrochemical sensing systems [25-27] because of their unique properties, such as
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high electron-transfer efficiency and high surface reaction activity. Nevertheless,
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among these reports, most of them are focused on the application of
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cadmium-containing QDs. Some results indicated that any leakage of cadmium from
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cadmium-containing semiconductor nanoparticles would be toxic and even fatal to
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biological organisms [28,29]. Therefore, application of environmentally-friendly QDs
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to replace cadmium-containing QDs in bioanalysis and biosensing is opportune. ZnSe
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QDs are such a type of new nanoparticles with low toxicity and good environmental
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Recently, they have been favorably adopted as potential materials in
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acceptability by replacing cadmium in cadmium-containing QDs with zinc [30]. Peter M. Ndangili et al. [30] assembled ZnSe QDs with cytochrome P450 3A4 enzyme as an electrode material for very sensitive determination of 17β-estradiol. This research demonstrated that ZnSe QDs not only can be employed for electrochemical sensors to
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improve analytical performance, but also have very good biocompatibility to maintain
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enzymatic activity. This study opens up a new challenge and approach to explore the
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electrochemical features of ZnSe QDs for potential utilizations. However, except
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Peter M. Ndangili’s study [30,31], no other papers for exploring ZnSe QDs as
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electrochemical sensors have been published. Cetyltrimethylammonium bromide(CTAB), a cationic surfactant, often used as an
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absorbent for phenols due to the strong hydrophobic interaction between the long
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alkyl chain of CTAB and aromatic ring of phenols [32]. In this case, CTAB can be
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used as electrode modifier for enrichment of phenols on the electrode surface to
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improve sensitivity. Zhou developed an electrochemical sensor for determination of
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nonylphenol with CTAB as the electrode material [33]. Considering the positive
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charge of CTAB which makes it easily assembled with electronegative mercaptan
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carboxylic acid encapsulated QDs, and its strong hydrophobic interaction with
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phenols, in this work, we attempt to integrate ZnSe QDs and CTAB for sensor
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fabrication through the electrostatic self-assembly between them, thereby exploiting
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their synergy for the electrochemical oxidation of CPs. It was found that the
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composite of ZnSe-CTAB has an extraordinary electrocatalytic activity towards CPs.
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Based on the electrocatalytic activities of ZnSe-CTAB, a very sensitive
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electrochemical sensor for the determination of CPs was established. Relative to the reported methods, the established method not only can meet the requirements of international regulatory limits, but also is very simple. This ZnSe-CTAB electrode system represents a new electrochemical platform for designing environment-friendly electrochemical sensors.
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2. Experimental
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2.1. Reagents and apparatus
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2-Chlorophenol
(2-CP,
99.0%),
2,4-dichlorophenol
(2,4-DCP,
99.0%),
pentachlorophenol (PCP, 99.0%), pyrocatechol, hydroquinol, and hydroxyphenol were
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purchased from Sigma–Aldrich. Polyvinylpyrrolidone (PVP), Nafion, chitosan,
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polyvinylalcohol (PVA), CTAB, sodium borohydride (NaHB4) and selenium powder
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were obtained from Sinopharm Chemical Reagent Beijing Co., Ltd. All the other
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chemicals and reagents used in the experiments were of analytical grade and used
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without further purification. Redistilled water was used throughout.
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Fluorescence spectra were acquired on a 970-CRT spectrofluorometer (Shanghai,
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China). The atomic force microscope (AFM) was the Agilent 5500 model (Aijian
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Nanotechnology, USA) in tapping mode. An RST 3000 electrochemical system
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(Suzhou Risetech Instrument Co., Ltd., China) was employed for all voltammetric
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measurements. AC impedance spectroscopy was obtained using CHI660E
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electrochemical workstation (Shanghai Chenhua Co., China). A conventional
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three-electrode system was used, including a bare glassy carbon electrode (GCE)
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(diameter of 4 mm) or PVP/ZnSe-CTAB/GCE as the working electrode, a saturated calomel electrode (SCE) as the reference electrode, and a platinum wire electrode as the auxiliary electrode.
2.2. Preparation of ZnSe-CTAB composite
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ZnSe QDs were synthesized according to the procedure described in the literature
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[34] with some slight modifications. Briefly, 14.8 mg of NaBH4, 7.9 mg of selenium
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powder and 3 mL of ultrapure water were transferred to a small flask in an ice–bath.
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After reacting for 40 min, the NaHSe precursor was obtained. Then, the NaHSe
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precursor was added to 100 mL Zn(Ac)2 solution at a pH of 10.5 in the presence of
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GSH under N2 atmosphere, and the molar ratio of Se2–/Zn2+/GSH was fixed at 1:4:5.
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After heated at 95 ◦C for 9 h, the ZnSe QDs applied in our assay was obtained.
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According to the Se2– concentration, the final concentration of ZnSe QDs was 1.0
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mM.
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ZnSe-CTAB composites were obtained by adding 0.3 mL CTAB (1.0 mM) to 1 mL of the ZnSe solution and sonicating for 30 min to yield a uniform suspension.
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2.3. Preparation of the working electrode
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Before chemical modification, the bare GCE was polished to a mirrorlike finish
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with a 0.05 μm alumina slurry, then washed successively with 1:1 HNO3–H2O (v/v),
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anhydrous alcohol, and double distilled deionized water in an ultrasonic bath and
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dried. The working electrode was prepared as follows: initially, 10μL of the
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ZnSe-CTAB mixture was added onto the freshly prepared GCE surface and dried. Then, 10 μL PVP (0.05%, w/v) solutions were coated on ZnSe-CTAB/GCE and dried under infrared lamp. Lastly, the electrode surface was washed with double distilled water to remove unbound materials from the electrode surface. The obtained electrode is referred to as PVP/ZnSe-CTAB/GCE.
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2.4. Electrochemical measurements A certain volume of CPs stock solution and 5 mL 0.1 M PBS (pH 6.0) was added
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into an electrochemical cell, and then the three–electrode system was installed in it.
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Cyclic voltammetry (CV) was employed between 0.3 and 1.2 V with a scan rate of
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100 mV s-1. Differential pulse voltammetry (DPV) measurements were made from 0.3
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to 1.2 V with the following parameters: increment potential, 15 mV; pulse amplitude,
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0.04 V; pulse width, 0.02 V; pulse period, 0.05 s; sample width, 6 ms; quiet time 3 s.
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AC impedance was performed in 1.0 mM Fe(CN)6 3-/4- (1: 1) solution containing 0.1
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M KCl. The parameters were as follows: frequency range from 0.01 to 105 Hz;
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initiative potential, 0.2 V; amplitude, 0.01 V and quiet time of 2 s.
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2.5 Detection by HPLC
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LC-20A (Shimadzu) HPLC was used for the chromatographic analysis. All
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separations were carried out on a chromosil C18 column(250 mm×4.6 mm, 5 µm).
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The mixture of methanol and 0.1% acetic acid solution (V:V 70:30) was used as the
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mobile phase. The solvent flow rate was 1.0 mL min-1, the injection volume was 20
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µL and the detection wavelength was 290 nm.
3. Results and discussion
3.1. Characterization of ZnSe-CTAB composite by fluorescence and AFM
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Because the fluorescence properties and morphology are important evaluating
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characteristics of QDs, the self-assembly of ZnSe QDs, and CTAB were investigated
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by fluorescence spectroscopy and AFM （Fig.1）. The fluorescence intensity of ZnSe
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quenched gradually with the increase of CTAB (Fig. 1A). The fluorescence quenching
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may result from two causes [35]. One is dynamic quenching which results from the
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collision between the fluorophore and a quencher. The other is static quenching which
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results from the ground-state complex formation between the fluorophore and a
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quencher. To ascertain the cause of the quenching, the Stern–Volmer equation [36]
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was first applied to model the quenching of ZnSe QDs with respect to the CTAB
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concentration (Fig. 1B), it is described as:
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(1)
For which F0 and F are the fluorescence intensities in the absence and presence of the
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quencher Q, Kq is the quenching rate constant of the biomolecule, τ0 is the average
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life-time of molecule without the quencher and its value is 10-8 s. Ksv is the
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Stern–Volmer dynamic quenching constant. Based on the experimental data, Kq was
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calculated as 1.26×1011 L mol-1 S-1, which was a factor of six greater than the
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maximum scatter collision quenching constant of various other quenchers for the
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biopolymer (2.0×1010 L mol-1 S-1) [35]. This result implies that the fluorescence
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quenching arises from the static quenching mechanism: the formation of a non-covalent complex between ZnSe QDs and CTAB [37]. The cationic surfactant CTAB is positively charged, whereas the GSH capped ZnSe
QDs are negatively charged due to the two carboxylate groups of GSH being
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negatively charged at the experimental pH. It is therefore easy for CTAB molecules to
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bind onto the surface of ZnSe QDs through electrostatic interaction. To confirm that
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the non-covalent binding of ZnSe QDs and QDs is driven mainly through the
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electrostatic interaction, the effect of ionic strength on the interaction of ZnSe QDs
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and CTAB was investigated by adding 0.2 M NaCl into the system. With the addition
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of ionic strength, the extent of fluorescence quenching of CTAB to ZnSe QDs
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decreased rapidly (the Kq was about 3.1×1010 L mol-1 S-1)(Fig. 1B) which was in
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accordance with the well known theory that ionic strength has a diminishing effect on
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the electrostatic interaction between two molecules [38]. Furthermore, the
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morphology of ZnSe QDs also showed that the addition of CTAB could cause an
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obvious aggregation for it (Fig. 1C), which was accordance with the self-assembly of
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CdTe QDs and CTAB [37]. So, according to the above studies, we could make a
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conclusion that the self-assembly of ZnSe-CTAB had been accomplished through the
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electrostatic interaction between them.
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3.2. Characterization of PVP/ZnSe-CTAB/GCE by AC impedance
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AC impedance is an efficient tool for studying the interface properties of
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surface-modified electrodes. The electron-transfer resistance (Ret) at the electrode
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surface is equal to the semicircle diameter of the Nyquist plots and can be used to describe the interface properties of the electrode [39]. As can been seen from the Nyquist plots in Fig. 2, the obvious difference of the semicircle portion at higher frequencies among ZnSe/GCE, ZnSe-CTAB/GCE and PVP/ZnSe-CTAB/GCE
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demonstrated that the modification of PVP/ZnSe-CTAB on the surface of GCE had
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been successfully achieved. The almost linear Nyquist plot of CTAB/GCE indicates
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that the CTAB can improve the electron transfer rate greatly. This improvement was
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probably due to the positive charge of CTAB, which promotes the access of
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[Fe(CN)6]3−/4− to the electrode surface. For the ZnSe QDs/GCE, the semicircle portion
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at higher frequencies increased visibly. The reason might be that ZnSe QDs are
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semiconductors and its conductivity was not as good [40]. However, for
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ZnSe-CTAB/GCE, the semicircle portion at higher frequencies remarkably decreased,
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which suggests that the ZnSe-CTAB composite introduced a favorable channel for
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electron transfer.
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3.3. Cyclic voltammetric behavior of 2,4-DCP on PVP/ZnSe-CTAB/GCE
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The CV behavior of 2,4-DCP at a 2.0 μM concentration in PBS at a pH of 7.0 with
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the bare GCE and different modified electrodes was investigated. The typical cyclic
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voltammogram of 2,4-DCP is Fig. 3. At the bare GCE, there were only very weak
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oxidation peaks at 0.75 V. After modification with ZnSe QDs or CTAB, the oxidation
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peak current of 2,4-DCP increased obviously for either modification. This behavior
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indicates good electrocatalytic activity of ZnSe QDs and CTAB towards 2,4-DCP.
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When ZnSe-CTAB was modified on the electrode, the peak current of 2,4-DCP on ZnSe-CTAB/GCE was larger than that of the sum of ZnSe QDs and CTAB, which demonstrated that the ZnSe-CTAB nanocomposite was synergistic for electron transfer. In spite of the high response current of 2,4-DCP on ZnSe-CTAB/GCE, the
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stability of electrode was very poor and the oxidation current gradually decreased
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with successive measurements. The reason may be that the ZnSe-CTAB composite
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was easily dissolved into the reaction solution. To protect against the loss of the
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ZnSe-CTAB composite, a frequently-used electrode fixative, PVP was cast onto the
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surface of ZnSe-CTAB/GCE. It was found that the PVP modified electrode exhibits
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improved and good stability. However, PVP had no obvious electrocatalytic activity
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for the oxidation of 2,4-DCP, it only acted as an electrode fixative for this sensor.
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3.4. Optimization of experimental parameters
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3.4.1 The ratio of CTAB to ZnSe QDs
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According to the synthesis procedure of ZnSe-CTAB, different ZnSe-CTAB
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composites were prepared by adding different amounts of CTAB (1.0 mM) into 1.0
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mL of ZnSe QDs (1.0 mM). A 10 μL aliquot obtained from the ZnSe-CTAB
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composites was used for the fabrication of PVP/ZnSe-CTAB/GCE. The optimum ratio
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CTAB to ZnSe QDs was determined by comparing the electrochemical response of
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2,4-DCP obtained with the PVP/ZnSe-CTAB/GCE. When the ratio of CTAB to ZnSe
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QDs was 1:3, the oxidation current of 2,4-DCP reached a maximum. So, the ratio of
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CTAB to ZnSe QDs for preparing the ZnSe-CTAB composites was selected as 1:3.
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3.4.2 Fixatives
From 3.3, it can be seen that the electrode fixatives had an important role on the
electron transfer from the electrode surface. To obtain the optimum electrochemical
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response, the influence of different fixatives including chitosan, PVA, and Nafion on
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the oxidation of 2,4-DCP was investigated. Although, all of them could provide good
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fixation for the ZnSe-CTAB composite on the electrode surface, they made some
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extent inhibition effect on the oxidation of 2,4-DCP. Relative to other fixatives (Table
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1), only PVP modified electrode could maintain the good response and demonstrated
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stability, so PVP was selected as the best fixative.
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3.4.3 Supporting electrolytes and pH
Supporting electrolyte and pH are important factors affecting the performance of
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ZnSe-CTAB/PVP/GCE to the oxidation of CPs.
The effect of supporting
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electrolytes including phosphate buffer solution (PBS), Britton–Robinson buffer
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solution, acetate buffer solution, citrate buffer solution, and tris-HCl buffer solution
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(each 0.1 M, pH range from 3.0 to 9.0) on the current response can be seen in Fig. 4A.
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The best peak current of 2,4-DCP was obtained with PBS at a pH of 6.0.
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For the PBS solution in the pH range of 3.0 to 9.0, the oxidation peak potential
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shifted negatively with the increased pH. The relationship between the peak potential
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(Epa) and pH can be expressed as: Epa (V) = -0.0572 pH +1.19 (R= -0.9936, SD =
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0.0154). The value of the slope (-57.2 mV/pH) was approximately equal to the
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theoretical Nerstian value of −59 mV/pH, which indicated that the total number of electrons and protons taking part in the oxidation mechanism was the same [41].
3.4.4 Scan rate on the electrochemical response of 2,4-DCP
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The influence of scan rate on oxidation of 2,4-DCP was investigated by the CV
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(Fig. 4B). The anodic peak intensity increased continuously with the increase of scan
284
rate. A good linear relationship between the peak current and the square root of the
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scan rate ( v1/2 ) from 20 to 200 mV/s was obtained. The regression equations was Ipa
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(μA) = 0.43 v1/2 (mV/s) - 0.10 (R = 0.9960, SD = 0.15), demonstrating the oxidation
287
process of 2,4-DCP was diffusion controlled [16]. In addition, the effect of scan rate on the oxidation peak potential was also
289
investigated. According to the Laviron’s theory, the Epa of the totally irreversible
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electrode process could be describe by the Laviron equation,
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Epa = E 0 −
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For which n is electron transfer number and ks is standard rate coefficient. The
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electron transfer coefficient α was calculated based on slopes of Epa with respect to ln
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v. For this work, the Epa was linearly dependent on the ln v with the regression
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equation of Epa = 0.0256 ln v + 0.717 (R=0.9974). Firstly (1-α) n was calculated to be
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0.97 according to the slope of Epa with respect to ln v. Generally, for a totally
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irreversible electrode process, α is assumed to be 0.5 [42]. Then the electron transfer
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number n is obtained to be 2. Therefore, the oxidation process of 2,4-DCP on
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RTk s RT RT ln ln v (2) + (1 − α )nF (1 − α )nF (1 − α )nF
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PVP/ZnSe-CTAB/GCE is a two-electron and two-proton process which is similar to the previous report [16]. According to the oxidation mechanism of CPs [16], the reaction process of 2,4-DCP on this electrode may be depicted in Scheme 1.
3.5. Interferences
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Potential interferences of some compounds in real samples were investigated by
305
analyzing a standard solution of 2.0 µM 2,4-DCP in 0.1 M PBS solution (pH 6.0).
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With exception of Cu2+ and Mn2+, metal ions have negligible effects on the peak
307
current of 2,4-DCP in large concentration (1.0 mM). To eliminate the interference of
Page 14 of 39
Cu2+ and Mn2+, EDTA was added into the detection system. It was found that the
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current of 2,4-DCP remained unchanged when 1.0 mM Cu2+ and Mn2+ were present in
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the 0.1 M PBS after the addition of 5.0 mM EDTA. So the supporting electrolyte of
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0.1 M PBS (pH of 6.0) contained 5.0 mM EDTA in these experiments.
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Considering that the structurally similar phenols maybe have similar
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electrochemical responses and result in the interference for the determination of CPs,
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the electrochemical behaviors of common phenols such as hydroquinol, pyrocatechol,
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hydroxyphenol, 2-chlorophenol, and pentachlorophenol also were investigated at the
316
same conditions (Fig. 5). Hydroquinol and pyrocatechol had no influence on the
317
determination of the CPs due to their oxidation potentials (0.3 V) being far from the
318
CPs’. For hydroxyphenol, although its oxidation potential was very near to the CPs’,
319
its electrochemical signal was much smaller than the CPs’. So, to some extent, the
320
established methods could be applied for selective determination of CPs.
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3.6. Determination of CPs using DPV DPV was used for the determination of CPs with the ZnSe-CTAB/PVP/GCE by
reason of its higher current sensitivity and better resolution than CV. Under the optimum instrumental conditions, the peak currents of the CPs were proportional to
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their concentrations in the range from 0.02 to 10.0 µM for 2-CP, 0.006 to 9.0 µM for
327
2,4-DCP, and 0.06 to 8.0 µM for PCP. The regression equations and CV responses for
328
2-CP, 2,4-DCP and PCP were shown in Fig. 6. The detection limits of 2-CP, 2,4-DCP
329
and PCP were 0.008, 0.002 µM and 0.01 µM respectively.
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By comparison with other CPs electrochemical sensors (Table 2), the proposed
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methods in our work gave higher sensitivities, wider linear ranges, and had a simple
332
fabrication process for the electrode. Furthermore, the use of bio-enzymes as modified
333
materials was avoided which led to more robust and stable detection platform.
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3.7. Repeatability and stability of the modified electrode
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The stability and reproducibility of the ZnSe-CTAB/PVP/GCE were investigated
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by the measurement of the response to the 2 µM 2,4-DCP. The relative standard
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deviation (RSD) of the oxidation peak current by 5 successive measurements was
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1.4%. The fabrication reproducibility was estimated using 5 modified electrodes that
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were prepared under the same conditions, and the RSD was 5.9 %. When the
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electrode was kept at 4 °C for 1 week, the peak currents remained more than 93.5 %
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of their initial values. The above results revealed good stability and reproducibility of
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the ZnSe-CTAB/PVP/GCE due to the strong non-covalent interaction between PVP
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on the electrode surface and the self-assembly of ZnSe QDs with CTAB.
3.8. Practical detection in water samples To investigate the applicability of the proposed method for the determination of
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CPs, local lake water sample (obtained from the Lake of Zhengzhou University) was
349
used for quantitative analysis. The pH of the sample was detected as 6.12, which was
350
similar to the optimum pH, so, no PBS was added to the samples to adjust the pH.
351
Considering the interference of the coexisting metal ions, appropriate EDTA was
Page 16 of 39
added to maintain 5 mM EDTA in the detection system. No obvious DPV
353
electrochemical response was found between 0.8-0.85 V for the pretreated water
354
sample. So, we assumed that the concentration of CPs of the lake was too low to be
355
detected.
ip t
352
To verify the accuracy of the proposed method, the method for determination of
357
2,4-DCP by HPLC also was established (Fig.7). The detection limit of the HPLC
358
method was 0.01 µM (S/N=3). Similarly, no obvious chromatography peak was found
359
for the sample.
an
us
cr
356
Known concentrations of 2,4-DCP were added to the lake water sample and
361
detected by the propose DPV method and HPLC method respectively (Table 3). The
362
recoveries for 2,4-DCP by DPV method and HPLC were ranged from 97.4% to
363
108.0% and 96.6% to 104.0% respectively. Furthermore, the detection results of the
364
two methods showed no obvious difference. These results clearly indicated the
365
applicability and reliability of the proposed method.
367 368 369 370
d
te
Ac ce p
366
M
360
4. Conclusions
In this study, a very sensitive and simple electrochemical sensor for CPs based on
the nanocomposite of CTAB and ZnSe QDs through electrostatic self-assembly was built. Relative to other reported methods, the proposed sensor was more sensitive,
371
simpler, and environment-friendly. This ZnSe-CTAB electrode system represents a
372
new platform for designing environment-friendly electrochemical sensors.
373 374
Acknowledgments
Page 17 of 39
We acknowledge financial support of Zhengzhou University postgraduate scientific
376
research project (research project numbers: 12L10302).
377
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[35] J.R. Lakowicz, G. Weber, Biochem. 12 (1973) 4161-4170. [36] O. Stern, M. Volmer, Physik. Zeitschr. 20 (1919) 183-188. [37] J.R. Lakowicz, Plenum Press, New York, 1983. [38] X.L. Diao, Y.S. Xia, T.L.Zhang, Y. Li, C.Q. Zhu, Anal. Bioanal. Chem. 388
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443
Electrochem. 15 (2011)167-173.
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cr
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Page 21 of 39
Figure Captions:
445
Fig.1 (A) Fluorescence spectra of ZnSe QDs (0.2 mM) with different concentration of
446
CTAB in H2O (a) and 0.2 M NaCl (b). The concentration of CTAB from (1) to (7): 0,
447
0.04, 0.08, 0.12, 0.16, 0.20, 0.24 mM respectively. (B) Stern-Volumer plots of the
448
fluorescence quenching of ZnSe QDs by CTAB in H2O and 0.2 M NaCl. (C) AFM
449
images of ZnSe (a), ZnSe-CTAB (b) respectively.
us
cr
ip t
444
450
Fig.2 The Nyquist plots of bare GCE, CTAB/GCE, ZnSe/GCE, PVP/GCE,
452
ZnSe-CTAB/GCE and PVP/ZnSe-CTAB GCE in 1.0 mM [Fe(CN)6]3−/4− solution
453
containing 0.1 M KCl.
M
an
451
454
Fig.3 Cyclic voltammetric behaviors of 2,4-DCP (2.0 μM) in pH 7.0 PBS at GCE,
456
ZnSe/GCE, CTAB/GCE, PVP/GCE, ZnSe-CTAB/GCE and PVP/ZnSe-CTAB/GCE.
457
Scan rate 100 mV s− 1; Accumulation time: 60 s.
459 460 461 462
te
Ac ce p
458
d
455
Fig.4 (A) The peak current (Ipa) of 2.0 μM 2,4-DCP in the 0.1 M different supporting electrolytes at different pH values at the PVP/ZnSe-CTAB/GCE. (B)The plot shows the linear relationship between Ipa and v1/2, the insert expresses the relationship of Epa with respect to ln v.
463 464
Fig.5 The electrochemical behaviors of common phenols (2.0 μM) at the
465
PVP/ZnSe-CTAB/GCE in the 0.1 M PBS (pH of 6.0).
Page 22 of 39
Fig.6 (A) DPVs of 2-CP at PVP/ZnSe-CTAB/GCE with different concentrations
467
(0.02,0.04, 0.06, 0.08, 0.2, 0.4, 0.6, 0.8, 1.0, 2.0, 4.0, 6.0, 8.0, 10.0 μM), inset is the
468
calibration curve; (B) DPVs of 2,4-DCP at PVP/ZnSe-CTAB/GCE with different
469
concentrations (0.006, 0.01, 0.08, 0.2, 0.4, 0.6, 0.8, 1.0, 2.0, 4.0, 6.0, 8.0, 9.0 μM),
470
inset is the calibration curve; (C) DPVs of PCP at PVP/ZnSe-CTAB/GCE with
471
different concentrations (0.06, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0, 2.0, 4.0, 6.0, 8.0 μM), inset
472
is the calibration curve
us
cr
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466
an
473
Fig.7 Chromatograms of 2,4-DCP with different concentrations(0.05, 0.1, 0.5, 1.0, 2.0, 4.0, 6.0,
475
8.00 μM), inset is the calibration curve.
476
Scheme 1
478
PVP/ZnSe-CTAB/GCE.
480 481 482 483
te
Ac ce p
479
Schematic illustration of oxidation process for 2,4-DCP on
d
477
M
474
484 485 486 487
Page 23 of 39
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488
Ac ce p
te
d
489
490
Page 24 of 39
ip t cr 492
an
Fig. 1
493
M
494
499 500 501
te
498
Ac ce p
497
d
495 496
us
491
Page 25 of 39
ip t cr us an M
502 503
Fig. 2
507 508 509 510 511
te
506
Ac ce p
505
d
504
512 513 514
Page 26 of 39
ip t cr us an M
515 516
519 520 521 522 523
d te
518
Ac ce p
517
Fig. 3
524 525 526 527
Page 27 of 39
ip t cr us an Ac ce p
te
d
M
528
529 530
Fig. 4
Page 28 of 39
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531 532
Fig. 5
536 537 538 539 540
te
535
Ac ce p
534
d
533
541 542 543 544
Page 29 of 39
ip t cr us an Ac ce p
te
d
M
545
546 547
Page 30 of 39
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548 549
Fig.6
553 554 555 556 557
te
552
Ac ce p
551
d
550
558 559 560 561
Page 31 of 39
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562 563
Fig.7
567 568 569 570 571
te
566
Ac ce p
565
d
564
572 573 574 575
Page 32 of 39
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an
Schem. 1
us
576
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578
Page 33 of 39
Table 1 The effects of fixatives for the response current of 2,4-DCP. (n=5) Ea/V
Ipa/μA
RSD/%
Chitosan
0.763
1.139
5.3
PVA
0.773
1.278
8.7
Nafion
0.776
2.748
4.8
PVP
0.780
3.474
579
581
588 589 590
te Ac ce p
587
d
583
586
cr
M
582
585
1.4
an
580
584
ip t
Fixatives
us
578
591 592 593 594
Page 34 of 39
Table 2 Comparison of various electroanalytical methods proposed for detection of CPs. Linear range
Detection limit
analyte
Ref. (μM)
(μM)
HRP/Au NPs/GCE
4-CP
2.5-117.5
0.39
CTAB-MMT/CPE
4-CP
0.05-10
0.02
TiO2/GPE
4-CP
0.05-50
cr
Electrode
ip t
595
AB-DHP/GCE
2-CP
0.2-40
MWNTs-DCP1/GCE
2-CP
0.1-20
HRP/MWNTs/GCE
2,4-DCP
Tyrosinase/MWNTs/GCE
2,4-DCP
Nafion/MWCNT/GCE
20
21
22
0.04
23
1.0–100
0.38
18
2.0–100
0.66
16
2,4-DCP
0.1–100
0.037
15
1–25
12.09
2,4-DCP
1–25
2.70
M
an
0.05
d
us
0.01
10
te
4-CP
17
Lac/PVA/F108/Au NPs/GCE
1–25
9.33
Mb-AG/GCE
2,4-DCP
12.5-208
2.06
13
Graphene/HRP/GCE
2,4-DCP
0.01–13.0
0.005
19
TiO2/DHP/GCE
PCP
0.05-100
0.01
11
MWCNT/EP/GCE
PCP
2-12
0.8
12
2-CP
0.02-10.0
0.008
Ac ce p
2,4,6-TCP
This 2,4-DCP
0.006-9.0
0.002
PCP
0.06-8.0
0.01
work
PVP/ZnSe-CTAB/GCE
596
Page 35 of 39
HRP: horseradish peroxidase; Au NPs: gold nanoparticles ; MMT : montmorillonite; CPE : carbon
598
paste electrode; TiO2: mesoporous TiO2 nanoparticles; GPE: graphite paste electrode; AB :
599
acetylene black; DHP :dihexadecyl hydrogen phosphate; MWNTs:multiwalled carbon nanotubues;
600
DCP1: dicetyl phosphate; Lac: laccase ; PVA: polyvinyl alcohol; F108: polyethylene oxide –
601
polyoxypropylene–polyethylene oxide (PEO–PPO–PEO); DHP:dihexadecylphosphate; EP : epoxy;
602
MB-AG: Myoglobin and agarose
us
cr
ip t
597
603
an
604 605
M
606
610 611 612 613 614
te
609
Ac ce p
608
d
607
615 616 617 618
Page 36 of 39
Table 3
Analytical results of 2,4-DCP in spiked lake water samples by the proposed DPV method and HPLC method （n=5）
Lake water
Found /μM
Recovery/%
Recovery/%
(DPV)
(HPLC)
(DPV)
0.05
0.054±0.010
0.052±0.010
108.0
0.5
0.491±0.021
0.483±0.049
5.0
4.869±0.214
5.120±0.192
Added/μM
621
98.2
97.4
(HPLC) 104.0 96.6
102.4
Ac ce p
te
d
M
an
622
ip t
Sample
Found /μM
cr
620
us
619
Page 37 of 39
ip t cr us
622
A very sensitive and simple electrochemical sensor for chlorophenols (CPs) based on
624
nanocomposite of cetyltrimethylammonium bromide (CTAB) and ZnSe quantum dots
625
(ZnSe-CTAB) through electrostatic self-assembly technology was built for the first
626
time. The nanocomposite of ZnSe-CTAB introduced a favorable access for the
627
electron transfer and showed excellent electrocatalytic activity for the oxidation of
628
CPs.
M
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629
an
623
Page 38 of 39
629 630 631 632
Highlights Nanocomposite based ZnSe QDs and CTAB was prepared and characterized. A novel electrochemical sensor for the determination of CPs was built. The proposed sensor was more sensitive, simple and environment-friendly.
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Page 39 of 39
S0003-2670(13)01266-X http://dx.doi.org/doi:10.1016/j.aca.2013.09.049 ACA 232861
To appear in:
Analytica Chimica Acta
Received date: Revised date: Accepted date:
26-7-2013 20-9-2013 23-9-2013
Please cite this article as: J. Li, X. Li, R. Yang, L. Qu, P.B. Harrington, A sensitive electrochemical chlorophenols sensor based on nanocomposite of ZnSe quantum dots and cetyltrimethylammonium bromide, Analytica Chimica Acta (2013), http://dx.doi.org/10.1016/j.aca.2013.09.049 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.
1
A sensitive electrochemical chlorophenols sensor based on nanocomposite of
2
ZnSe quantum dots and cetyltrimethylammonium bromide
3
Jianjun Lia , Xiao Lia, Ran Yanga*, Lingbo Qua,b*, Peter de B. Harringtonc
4
a
5
Zhengzhou 450001, PR China
6
b
7
Zhengzhou 450001, PR China
8
c
9
Biochemistry, Clippinger Laboratories, OHIO University, Athens, OH 45701-2979
ip t
cr
us
School of Chemistry & Chemical Engineering, Henan University of Technology,
an
Center for Intelligent Chemical Instrumentation, Department of Chemistry and
USA
M
10
The College of Chemistry and Molecular Engineering, Zhengzhou University,
11
te
d
12
14 15 16 17 18
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13
19 20 21
* *
Corresponding authour1: Ran Yang E-mail:[email protected] Corresponding authour2:Lingbo Qu , [email protected]
Page 1 of 39
Abstract: In this work, a very sensitive and simple electrochemical sensor for
23
chlorophenols (CPs) based on a nanocomposite of cetyltrimethylammonium bromide
24
(CTAB) and ZnSe quantum dots (ZnSe-CTAB) through electrostatic self-assembly
25
technology was built for the first time. The composite of ZnSe-CTAB introduced a
26
favorable access for the electron transfer and gave superior electrocatalytic activity for
27
the oxidation of CPs than ZnSe QDs and CTAB alone. Differential pulse voltammetry
28
(DPV) was used for the quantitative determination of the CPs including
29
2-chlorophenol (2-CP), 2,4-dichlorophenol (2,4-DCP) and pentachlorophenol(PCP).
30
Under the optimum conditions, the peak currents of the CPs were proportional to their
31
concentrations in the range from 0.02 to 10.0 µM for 2-CP, 0.006 to 9.0 µM for
32
2,4-DCP, and 0.06 to 8.0 for PCP. The detection limits were 0.008 µM for 2-CP,
33
0.002 µM for 2,4-DCP, and 0.01 µM for PCP, respectively. The method was
34
successfully applied for the determination of CPs in waste water with satisfactory
35
recoveries. This ZnSe-CTAB electrode system provides operational access to design
37 38 39
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environment-friendly CPs sensors. Keywords: ZnSe quantum dots; cetyltrimethylammonium bromide; Self-assembly; chlorophenols; electrochemical sensor
40 41 42 43
Page 2 of 39
44
1. Introduction Chlorophenols (CPs) are well-known pollutants of environmental waters and soils
46
and are mainly found in the effluent discharges from factories, such as those
47
producing paper and pesticides [1]. They are used as general biocides and wood
48
preservatives; may also be introduced into the environment as the products of
49
metabolic degration of chlorinated pesticides [2]; and result as byproduct of the
50
chlorination of drinking water [3]. CPs can cause serious health hazards due to their
51
inherent toxicity and relative persistence in the environment. As a consequence, the
52
US Environmental Protection Agency (USEP) and European Union (EU) has listed
53
2-chlorophenol
54
2,4,6-trichlorophenol(2,4,6-TCP), and 2,3,4,5,6-pentachlorophenol (PCP) as priority
55
pollutants and regulated the maximum admissible concentration of CPs in drinking
56
water at 0.5 ng mL-1 [4].
58 59 60 61
cr
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M
2,4-dichlorophenol
(2,4-DCP),
te
d
(2-CP),
Many analytical methods have been established to determine CPs, such as high
Ac ce p
57
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45
performance liquid chromatography [5], gas chromatography/mass spectrometry [6], spectrophotometry [7], fluorescence [8], enzyme linked immunosorbent assay (ELISA) [9] and electrochemical methods [10–23]. Among them, electrochemical methods are more preferable over the other methods due to the advantages of simple operation,
62
fast response, low cost, and small size that affords a portable sensor for on-site
63
detection [19]. However, the detection limits of CPs of the reported electrochemical
64
methods are too high to meet the regulatory requirements. Furthermore, the electrode
65
modified materials in these reports are usually based on bio-enzymes, which are
Page 3 of 39
66
difficult to immobilize on the electrode surface, require a chemical mediator such as
67
hydrogen peroxide, and can be easily denatured if the pH, ionic strength, and
68
temperature are not carefully controlled. Semiconductor quantum dots (QDs) are typically nanocrystals with a size between
70
1 and 10 nm that are compounds of cationic groups II–VI mixed with anionic groups
71
III–V [24].
72
electrochemical sensing systems [25-27] because of their unique properties, such as
73
high electron-transfer efficiency and high surface reaction activity. Nevertheless,
74
among these reports, most of them are focused on the application of
75
cadmium-containing QDs. Some results indicated that any leakage of cadmium from
76
cadmium-containing semiconductor nanoparticles would be toxic and even fatal to
77
biological organisms [28,29]. Therefore, application of environmentally-friendly QDs
78
to replace cadmium-containing QDs in bioanalysis and biosensing is opportune. ZnSe
79
QDs are such a type of new nanoparticles with low toxicity and good environmental
81 82 83
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Recently, they have been favorably adopted as potential materials in
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acceptability by replacing cadmium in cadmium-containing QDs with zinc [30]. Peter M. Ndangili et al. [30] assembled ZnSe QDs with cytochrome P450 3A4 enzyme as an electrode material for very sensitive determination of 17β-estradiol. This research demonstrated that ZnSe QDs not only can be employed for electrochemical sensors to
84
improve analytical performance, but also have very good biocompatibility to maintain
85
enzymatic activity. This study opens up a new challenge and approach to explore the
86
electrochemical features of ZnSe QDs for potential utilizations. However, except
87
Peter M. Ndangili’s study [30,31], no other papers for exploring ZnSe QDs as
Page 4 of 39
88
electrochemical sensors have been published. Cetyltrimethylammonium bromide(CTAB), a cationic surfactant, often used as an
90
absorbent for phenols due to the strong hydrophobic interaction between the long
91
alkyl chain of CTAB and aromatic ring of phenols [32]. In this case, CTAB can be
92
used as electrode modifier for enrichment of phenols on the electrode surface to
93
improve sensitivity. Zhou developed an electrochemical sensor for determination of
94
nonylphenol with CTAB as the electrode material [33]. Considering the positive
95
charge of CTAB which makes it easily assembled with electronegative mercaptan
96
carboxylic acid encapsulated QDs, and its strong hydrophobic interaction with
97
phenols, in this work, we attempt to integrate ZnSe QDs and CTAB for sensor
98
fabrication through the electrostatic self-assembly between them, thereby exploiting
99
their synergy for the electrochemical oxidation of CPs. It was found that the
100
composite of ZnSe-CTAB has an extraordinary electrocatalytic activity towards CPs.
101
Based on the electrocatalytic activities of ZnSe-CTAB, a very sensitive
103 104 105 106
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89
electrochemical sensor for the determination of CPs was established. Relative to the reported methods, the established method not only can meet the requirements of international regulatory limits, but also is very simple. This ZnSe-CTAB electrode system represents a new electrochemical platform for designing environment-friendly electrochemical sensors.
107 108
2. Experimental
109
2.1. Reagents and apparatus
Page 5 of 39
110
2-Chlorophenol
(2-CP,
99.0%),
2,4-dichlorophenol
(2,4-DCP,
99.0%),
pentachlorophenol (PCP, 99.0%), pyrocatechol, hydroquinol, and hydroxyphenol were
112
purchased from Sigma–Aldrich. Polyvinylpyrrolidone (PVP), Nafion, chitosan,
113
polyvinylalcohol (PVA), CTAB, sodium borohydride (NaHB4) and selenium powder
114
were obtained from Sinopharm Chemical Reagent Beijing Co., Ltd. All the other
115
chemicals and reagents used in the experiments were of analytical grade and used
116
without further purification. Redistilled water was used throughout.
us
cr
ip t
111
Fluorescence spectra were acquired on a 970-CRT spectrofluorometer (Shanghai,
118
China). The atomic force microscope (AFM) was the Agilent 5500 model (Aijian
119
Nanotechnology, USA) in tapping mode. An RST 3000 electrochemical system
120
(Suzhou Risetech Instrument Co., Ltd., China) was employed for all voltammetric
121
measurements. AC impedance spectroscopy was obtained using CHI660E
122
electrochemical workstation (Shanghai Chenhua Co., China). A conventional
123
three-electrode system was used, including a bare glassy carbon electrode (GCE)
125 126 127 128
M
d
te
Ac ce p
124
an
117
(diameter of 4 mm) or PVP/ZnSe-CTAB/GCE as the working electrode, a saturated calomel electrode (SCE) as the reference electrode, and a platinum wire electrode as the auxiliary electrode.
2.2. Preparation of ZnSe-CTAB composite
129
ZnSe QDs were synthesized according to the procedure described in the literature
130
[34] with some slight modifications. Briefly, 14.8 mg of NaBH4, 7.9 mg of selenium
131
powder and 3 mL of ultrapure water were transferred to a small flask in an ice–bath.
Page 6 of 39
After reacting for 40 min, the NaHSe precursor was obtained. Then, the NaHSe
133
precursor was added to 100 mL Zn(Ac)2 solution at a pH of 10.5 in the presence of
134
GSH under N2 atmosphere, and the molar ratio of Se2–/Zn2+/GSH was fixed at 1:4:5.
135
After heated at 95 ◦C for 9 h, the ZnSe QDs applied in our assay was obtained.
136
According to the Se2– concentration, the final concentration of ZnSe QDs was 1.0
137
mM.
cr us
139
ZnSe-CTAB composites were obtained by adding 0.3 mL CTAB (1.0 mM) to 1 mL of the ZnSe solution and sonicating for 30 min to yield a uniform suspension.
140
2.3. Preparation of the working electrode
M
141
an
138
ip t
132
Before chemical modification, the bare GCE was polished to a mirrorlike finish
143
with a 0.05 μm alumina slurry, then washed successively with 1:1 HNO3–H2O (v/v),
144
anhydrous alcohol, and double distilled deionized water in an ultrasonic bath and
145
dried. The working electrode was prepared as follows: initially, 10μL of the
147 148 149 150
te
Ac ce p
146
d
142
ZnSe-CTAB mixture was added onto the freshly prepared GCE surface and dried. Then, 10 μL PVP (0.05%, w/v) solutions were coated on ZnSe-CTAB/GCE and dried under infrared lamp. Lastly, the electrode surface was washed with double distilled water to remove unbound materials from the electrode surface. The obtained electrode is referred to as PVP/ZnSe-CTAB/GCE.
151 152 153
2.4. Electrochemical measurements A certain volume of CPs stock solution and 5 mL 0.1 M PBS (pH 6.0) was added
Page 7 of 39
into an electrochemical cell, and then the three–electrode system was installed in it.
155
Cyclic voltammetry (CV) was employed between 0.3 and 1.2 V with a scan rate of
156
100 mV s-1. Differential pulse voltammetry (DPV) measurements were made from 0.3
157
to 1.2 V with the following parameters: increment potential, 15 mV; pulse amplitude,
158
0.04 V; pulse width, 0.02 V; pulse period, 0.05 s; sample width, 6 ms; quiet time 3 s.
159
AC impedance was performed in 1.0 mM Fe(CN)6 3-/4- (1: 1) solution containing 0.1
160
M KCl. The parameters were as follows: frequency range from 0.01 to 105 Hz;
161
initiative potential, 0.2 V; amplitude, 0.01 V and quiet time of 2 s.
an
us
cr
ip t
154
162
2.5 Detection by HPLC
M
163
LC-20A (Shimadzu) HPLC was used for the chromatographic analysis. All
165
separations were carried out on a chromosil C18 column(250 mm×4.6 mm, 5 µm).
166
The mixture of methanol and 0.1% acetic acid solution (V:V 70:30) was used as the
167
mobile phase. The solvent flow rate was 1.0 mL min-1, the injection volume was 20
169 170 171
te
Ac ce p
168
d
164
µL and the detection wavelength was 290 nm.
3. Results and discussion
3.1. Characterization of ZnSe-CTAB composite by fluorescence and AFM
172
Because the fluorescence properties and morphology are important evaluating
173
characteristics of QDs, the self-assembly of ZnSe QDs, and CTAB were investigated
174
by fluorescence spectroscopy and AFM （Fig.1）. The fluorescence intensity of ZnSe
175
quenched gradually with the increase of CTAB (Fig. 1A). The fluorescence quenching
Page 8 of 39
may result from two causes [35]. One is dynamic quenching which results from the
177
collision between the fluorophore and a quencher. The other is static quenching which
178
results from the ground-state complex formation between the fluorophore and a
179
quencher. To ascertain the cause of the quenching, the Stern–Volmer equation [36]
180
was first applied to model the quenching of ZnSe QDs with respect to the CTAB
181
concentration (Fig. 1B), it is described as:
cr
F0 / F = 1 + Kqτ0[Q] = 1 + Ksv[Q]
us
182
ip t
176
(1)
For which F0 and F are the fluorescence intensities in the absence and presence of the
184
quencher Q, Kq is the quenching rate constant of the biomolecule, τ0 is the average
185
life-time of molecule without the quencher and its value is 10-8 s. Ksv is the
186
Stern–Volmer dynamic quenching constant. Based on the experimental data, Kq was
187
calculated as 1.26×1011 L mol-1 S-1, which was a factor of six greater than the
188
maximum scatter collision quenching constant of various other quenchers for the
189
biopolymer (2.0×1010 L mol-1 S-1) [35]. This result implies that the fluorescence
191 192 193
M
d
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Ac ce p
190
an
183
quenching arises from the static quenching mechanism: the formation of a non-covalent complex between ZnSe QDs and CTAB [37]. The cationic surfactant CTAB is positively charged, whereas the GSH capped ZnSe
QDs are negatively charged due to the two carboxylate groups of GSH being
194
negatively charged at the experimental pH. It is therefore easy for CTAB molecules to
195
bind onto the surface of ZnSe QDs through electrostatic interaction. To confirm that
196
the non-covalent binding of ZnSe QDs and QDs is driven mainly through the
197
electrostatic interaction, the effect of ionic strength on the interaction of ZnSe QDs
Page 9 of 39
and CTAB was investigated by adding 0.2 M NaCl into the system. With the addition
199
of ionic strength, the extent of fluorescence quenching of CTAB to ZnSe QDs
200
decreased rapidly (the Kq was about 3.1×1010 L mol-1 S-1)(Fig. 1B) which was in
201
accordance with the well known theory that ionic strength has a diminishing effect on
202
the electrostatic interaction between two molecules [38]. Furthermore, the
203
morphology of ZnSe QDs also showed that the addition of CTAB could cause an
204
obvious aggregation for it (Fig. 1C), which was accordance with the self-assembly of
205
CdTe QDs and CTAB [37]. So, according to the above studies, we could make a
206
conclusion that the self-assembly of ZnSe-CTAB had been accomplished through the
207
electrostatic interaction between them.
M
208
d
3.2. Characterization of PVP/ZnSe-CTAB/GCE by AC impedance
te
209
an
us
cr
ip t
198
AC impedance is an efficient tool for studying the interface properties of
211
surface-modified electrodes. The electron-transfer resistance (Ret) at the electrode
212 213 214 215
Ac ce p
210
surface is equal to the semicircle diameter of the Nyquist plots and can be used to describe the interface properties of the electrode [39]. As can been seen from the Nyquist plots in Fig. 2, the obvious difference of the semicircle portion at higher frequencies among ZnSe/GCE, ZnSe-CTAB/GCE and PVP/ZnSe-CTAB/GCE
216
demonstrated that the modification of PVP/ZnSe-CTAB on the surface of GCE had
217
been successfully achieved. The almost linear Nyquist plot of CTAB/GCE indicates
218
that the CTAB can improve the electron transfer rate greatly. This improvement was
219
probably due to the positive charge of CTAB, which promotes the access of
Page 10 of 39
[Fe(CN)6]3−/4− to the electrode surface. For the ZnSe QDs/GCE, the semicircle portion
221
at higher frequencies increased visibly. The reason might be that ZnSe QDs are
222
semiconductors and its conductivity was not as good [40]. However, for
223
ZnSe-CTAB/GCE, the semicircle portion at higher frequencies remarkably decreased,
224
which suggests that the ZnSe-CTAB composite introduced a favorable channel for
225
electron transfer.
us
cr
ip t
220
226
3.3. Cyclic voltammetric behavior of 2,4-DCP on PVP/ZnSe-CTAB/GCE
an
227
The CV behavior of 2,4-DCP at a 2.0 μM concentration in PBS at a pH of 7.0 with
229
the bare GCE and different modified electrodes was investigated. The typical cyclic
230
voltammogram of 2,4-DCP is Fig. 3. At the bare GCE, there were only very weak
231
oxidation peaks at 0.75 V. After modification with ZnSe QDs or CTAB, the oxidation
232
peak current of 2,4-DCP increased obviously for either modification. This behavior
233
indicates good electrocatalytic activity of ZnSe QDs and CTAB towards 2,4-DCP.
235 236 237
d
te
Ac ce p
234
M
228
When ZnSe-CTAB was modified on the electrode, the peak current of 2,4-DCP on ZnSe-CTAB/GCE was larger than that of the sum of ZnSe QDs and CTAB, which demonstrated that the ZnSe-CTAB nanocomposite was synergistic for electron transfer. In spite of the high response current of 2,4-DCP on ZnSe-CTAB/GCE, the
238
stability of electrode was very poor and the oxidation current gradually decreased
239
with successive measurements. The reason may be that the ZnSe-CTAB composite
240
was easily dissolved into the reaction solution. To protect against the loss of the
241
ZnSe-CTAB composite, a frequently-used electrode fixative, PVP was cast onto the
Page 11 of 39
surface of ZnSe-CTAB/GCE. It was found that the PVP modified electrode exhibits
243
improved and good stability. However, PVP had no obvious electrocatalytic activity
244
for the oxidation of 2,4-DCP, it only acted as an electrode fixative for this sensor.
ip t
242
245
3.4. Optimization of experimental parameters
247
3.4.1 The ratio of CTAB to ZnSe QDs
us
cr
246
According to the synthesis procedure of ZnSe-CTAB, different ZnSe-CTAB
249
composites were prepared by adding different amounts of CTAB (1.0 mM) into 1.0
250
mL of ZnSe QDs (1.0 mM). A 10 μL aliquot obtained from the ZnSe-CTAB
251
composites was used for the fabrication of PVP/ZnSe-CTAB/GCE. The optimum ratio
252
CTAB to ZnSe QDs was determined by comparing the electrochemical response of
253
2,4-DCP obtained with the PVP/ZnSe-CTAB/GCE. When the ratio of CTAB to ZnSe
254
QDs was 1:3, the oxidation current of 2,4-DCP reached a maximum. So, the ratio of
255
CTAB to ZnSe QDs for preparing the ZnSe-CTAB composites was selected as 1:3.
257 258 259
M
d
te
Ac ce p
256
an
248
3.4.2 Fixatives
From 3.3, it can be seen that the electrode fixatives had an important role on the
electron transfer from the electrode surface. To obtain the optimum electrochemical
260
response, the influence of different fixatives including chitosan, PVA, and Nafion on
261
the oxidation of 2,4-DCP was investigated. Although, all of them could provide good
262
fixation for the ZnSe-CTAB composite on the electrode surface, they made some
263
extent inhibition effect on the oxidation of 2,4-DCP. Relative to other fixatives (Table
Page 12 of 39
264
1), only PVP modified electrode could maintain the good response and demonstrated
265
stability, so PVP was selected as the best fixative.
268
3.4.3 Supporting electrolytes and pH
Supporting electrolyte and pH are important factors affecting the performance of
cr
267
ip t
266
ZnSe-CTAB/PVP/GCE to the oxidation of CPs.
The effect of supporting
270
electrolytes including phosphate buffer solution (PBS), Britton–Robinson buffer
271
solution, acetate buffer solution, citrate buffer solution, and tris-HCl buffer solution
272
(each 0.1 M, pH range from 3.0 to 9.0) on the current response can be seen in Fig. 4A.
273
The best peak current of 2,4-DCP was obtained with PBS at a pH of 6.0.
M
an
us
269
For the PBS solution in the pH range of 3.0 to 9.0, the oxidation peak potential
275
shifted negatively with the increased pH. The relationship between the peak potential
276
(Epa) and pH can be expressed as: Epa (V) = -0.0572 pH +1.19 (R= -0.9936, SD =
277
0.0154). The value of the slope (-57.2 mV/pH) was approximately equal to the
279 280 281
te
Ac ce p
278
d
274
theoretical Nerstian value of −59 mV/pH, which indicated that the total number of electrons and protons taking part in the oxidation mechanism was the same [41].
3.4.4 Scan rate on the electrochemical response of 2,4-DCP
282
The influence of scan rate on oxidation of 2,4-DCP was investigated by the CV
283
(Fig. 4B). The anodic peak intensity increased continuously with the increase of scan
284
rate. A good linear relationship between the peak current and the square root of the
285
scan rate ( v1/2 ) from 20 to 200 mV/s was obtained. The regression equations was Ipa
Page 13 of 39
286
(μA) = 0.43 v1/2 (mV/s) - 0.10 (R = 0.9960, SD = 0.15), demonstrating the oxidation
287
process of 2,4-DCP was diffusion controlled [16]. In addition, the effect of scan rate on the oxidation peak potential was also
289
investigated. According to the Laviron’s theory, the Epa of the totally irreversible
290
electrode process could be describe by the Laviron equation,
291
Epa = E 0 −
292
For which n is electron transfer number and ks is standard rate coefficient. The
293
electron transfer coefficient α was calculated based on slopes of Epa with respect to ln
294
v. For this work, the Epa was linearly dependent on the ln v with the regression
295
equation of Epa = 0.0256 ln v + 0.717 (R=0.9974). Firstly (1-α) n was calculated to be
296
0.97 according to the slope of Epa with respect to ln v. Generally, for a totally
297
irreversible electrode process, α is assumed to be 0.5 [42]. Then the electron transfer
298
number n is obtained to be 2. Therefore, the oxidation process of 2,4-DCP on
300 301 302 303
cr
us
an
M
d
te
Ac ce p
299
RTk s RT RT ln ln v (2) + (1 − α )nF (1 − α )nF (1 − α )nF
ip t
288
PVP/ZnSe-CTAB/GCE is a two-electron and two-proton process which is similar to the previous report [16]. According to the oxidation mechanism of CPs [16], the reaction process of 2,4-DCP on this electrode may be depicted in Scheme 1.
3.5. Interferences
304
Potential interferences of some compounds in real samples were investigated by
305
analyzing a standard solution of 2.0 µM 2,4-DCP in 0.1 M PBS solution (pH 6.0).
306
With exception of Cu2+ and Mn2+, metal ions have negligible effects on the peak
307
current of 2,4-DCP in large concentration (1.0 mM). To eliminate the interference of
Page 14 of 39
Cu2+ and Mn2+, EDTA was added into the detection system. It was found that the
309
current of 2,4-DCP remained unchanged when 1.0 mM Cu2+ and Mn2+ were present in
310
the 0.1 M PBS after the addition of 5.0 mM EDTA. So the supporting electrolyte of
311
0.1 M PBS (pH of 6.0) contained 5.0 mM EDTA in these experiments.
ip t
308
Considering that the structurally similar phenols maybe have similar
313
electrochemical responses and result in the interference for the determination of CPs,
314
the electrochemical behaviors of common phenols such as hydroquinol, pyrocatechol,
315
hydroxyphenol, 2-chlorophenol, and pentachlorophenol also were investigated at the
316
same conditions (Fig. 5). Hydroquinol and pyrocatechol had no influence on the
317
determination of the CPs due to their oxidation potentials (0.3 V) being far from the
318
CPs’. For hydroxyphenol, although its oxidation potential was very near to the CPs’,
319
its electrochemical signal was much smaller than the CPs’. So, to some extent, the
320
established methods could be applied for selective determination of CPs.
322 323 324 325
us
an
M
d
te
Ac ce p
321
cr
312
3.6. Determination of CPs using DPV DPV was used for the determination of CPs with the ZnSe-CTAB/PVP/GCE by
reason of its higher current sensitivity and better resolution than CV. Under the optimum instrumental conditions, the peak currents of the CPs were proportional to
326
their concentrations in the range from 0.02 to 10.0 µM for 2-CP, 0.006 to 9.0 µM for
327
2,4-DCP, and 0.06 to 8.0 µM for PCP. The regression equations and CV responses for
328
2-CP, 2,4-DCP and PCP were shown in Fig. 6. The detection limits of 2-CP, 2,4-DCP
329
and PCP were 0.008, 0.002 µM and 0.01 µM respectively.
Page 15 of 39
By comparison with other CPs electrochemical sensors (Table 2), the proposed
331
methods in our work gave higher sensitivities, wider linear ranges, and had a simple
332
fabrication process for the electrode. Furthermore, the use of bio-enzymes as modified
333
materials was avoided which led to more robust and stable detection platform.
334
3.7. Repeatability and stability of the modified electrode
us
335
cr
ip t
330
The stability and reproducibility of the ZnSe-CTAB/PVP/GCE were investigated
337
by the measurement of the response to the 2 µM 2,4-DCP. The relative standard
338
deviation (RSD) of the oxidation peak current by 5 successive measurements was
339
1.4%. The fabrication reproducibility was estimated using 5 modified electrodes that
340
were prepared under the same conditions, and the RSD was 5.9 %. When the
341
electrode was kept at 4 °C for 1 week, the peak currents remained more than 93.5 %
342
of their initial values. The above results revealed good stability and reproducibility of
343
the ZnSe-CTAB/PVP/GCE due to the strong non-covalent interaction between PVP
345 346 347
M
d
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Ac ce p
344
an
336
on the electrode surface and the self-assembly of ZnSe QDs with CTAB.
3.8. Practical detection in water samples To investigate the applicability of the proposed method for the determination of
348
CPs, local lake water sample (obtained from the Lake of Zhengzhou University) was
349
used for quantitative analysis. The pH of the sample was detected as 6.12, which was
350
similar to the optimum pH, so, no PBS was added to the samples to adjust the pH.
351
Considering the interference of the coexisting metal ions, appropriate EDTA was
Page 16 of 39
added to maintain 5 mM EDTA in the detection system. No obvious DPV
353
electrochemical response was found between 0.8-0.85 V for the pretreated water
354
sample. So, we assumed that the concentration of CPs of the lake was too low to be
355
detected.
ip t
352
To verify the accuracy of the proposed method, the method for determination of
357
2,4-DCP by HPLC also was established (Fig.7). The detection limit of the HPLC
358
method was 0.01 µM (S/N=3). Similarly, no obvious chromatography peak was found
359
for the sample.
an
us
cr
356
Known concentrations of 2,4-DCP were added to the lake water sample and
361
detected by the propose DPV method and HPLC method respectively (Table 3). The
362
recoveries for 2,4-DCP by DPV method and HPLC were ranged from 97.4% to
363
108.0% and 96.6% to 104.0% respectively. Furthermore, the detection results of the
364
two methods showed no obvious difference. These results clearly indicated the
365
applicability and reliability of the proposed method.
367 368 369 370
d
te
Ac ce p
366
M
360
4. Conclusions
In this study, a very sensitive and simple electrochemical sensor for CPs based on
the nanocomposite of CTAB and ZnSe QDs through electrostatic self-assembly was built. Relative to other reported methods, the proposed sensor was more sensitive,
371
simpler, and environment-friendly. This ZnSe-CTAB electrode system represents a
372
new platform for designing environment-friendly electrochemical sensors.
373 374
Acknowledgments
Page 17 of 39
We acknowledge financial support of Zhengzhou University postgraduate scientific
376
research project (research project numbers: 12L10302).
377
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[35] J.R. Lakowicz, G. Weber, Biochem. 12 (1973) 4161-4170. [36] O. Stern, M. Volmer, Physik. Zeitschr. 20 (1919) 183-188. [37] J.R. Lakowicz, Plenum Press, New York, 1983. [38] X.L. Diao, Y.S. Xia, T.L.Zhang, Y. Li, C.Q. Zhu, Anal. Bioanal. Chem. 388
437
(2007) 1191-1197.
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[39] B. Unnikrishnan, V. Mani, S.M. Chen, Sens. Actuator. B. 173 (2012) 274-280.
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[40] Q. Liu, X.B. Lu, J. Li, X. Yao, J.H. Li, Biosens. Bioelectron. 22 (2007)
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3203-3209.
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[41] T. Łuczak, Electrochim. Acta. 53 (2008) 5725-5731.
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[42] H.S. Yin, Y.L. Zhou, L. Cui, X.G. Liu, S.Y. Ai, L.S. Zhu, J Solid State
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Electrochem. 15 (2011)167-173.
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Figure Captions:
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Fig.1 (A) Fluorescence spectra of ZnSe QDs (0.2 mM) with different concentration of
446
CTAB in H2O (a) and 0.2 M NaCl (b). The concentration of CTAB from (1) to (7): 0,
447
0.04, 0.08, 0.12, 0.16, 0.20, 0.24 mM respectively. (B) Stern-Volumer plots of the
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fluorescence quenching of ZnSe QDs by CTAB in H2O and 0.2 M NaCl. (C) AFM
449
images of ZnSe (a), ZnSe-CTAB (b) respectively.
us
cr
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444
450
Fig.2 The Nyquist plots of bare GCE, CTAB/GCE, ZnSe/GCE, PVP/GCE,
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ZnSe-CTAB/GCE and PVP/ZnSe-CTAB GCE in 1.0 mM [Fe(CN)6]3−/4− solution
453
containing 0.1 M KCl.
M
an
451
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Fig.3 Cyclic voltammetric behaviors of 2,4-DCP (2.0 μM) in pH 7.0 PBS at GCE,
456
ZnSe/GCE, CTAB/GCE, PVP/GCE, ZnSe-CTAB/GCE and PVP/ZnSe-CTAB/GCE.
457
Scan rate 100 mV s− 1; Accumulation time: 60 s.
459 460 461 462
te
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458
d
455
Fig.4 (A) The peak current (Ipa) of 2.0 μM 2,4-DCP in the 0.1 M different supporting electrolytes at different pH values at the PVP/ZnSe-CTAB/GCE. (B)The plot shows the linear relationship between Ipa and v1/2, the insert expresses the relationship of Epa with respect to ln v.
463 464
Fig.5 The electrochemical behaviors of common phenols (2.0 μM) at the
465
PVP/ZnSe-CTAB/GCE in the 0.1 M PBS (pH of 6.0).
Page 22 of 39
Fig.6 (A) DPVs of 2-CP at PVP/ZnSe-CTAB/GCE with different concentrations
467
(0.02,0.04, 0.06, 0.08, 0.2, 0.4, 0.6, 0.8, 1.0, 2.0, 4.0, 6.0, 8.0, 10.0 μM), inset is the
468
calibration curve; (B) DPVs of 2,4-DCP at PVP/ZnSe-CTAB/GCE with different
469
concentrations (0.006, 0.01, 0.08, 0.2, 0.4, 0.6, 0.8, 1.0, 2.0, 4.0, 6.0, 8.0, 9.0 μM),
470
inset is the calibration curve; (C) DPVs of PCP at PVP/ZnSe-CTAB/GCE with
471
different concentrations (0.06, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0, 2.0, 4.0, 6.0, 8.0 μM), inset
472
is the calibration curve
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cr
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466
an
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Fig.7 Chromatograms of 2,4-DCP with different concentrations(0.05, 0.1, 0.5, 1.0, 2.0, 4.0, 6.0,
475
8.00 μM), inset is the calibration curve.
476
Scheme 1
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PVP/ZnSe-CTAB/GCE.
480 481 482 483
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Schematic illustration of oxidation process for 2,4-DCP on
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477
M
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484 485 486 487
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490
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an
Fig. 1
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M
494
499 500 501
te
498
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d
495 496
us
491
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502 503
Fig. 2
507 508 509 510 511
te
506
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505
d
504
512 513 514
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515 516
519 520 521 522 523
d te
518
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517
Fig. 3
524 525 526 527
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M
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529 530
Fig. 4
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531 532
Fig. 5
536 537 538 539 540
te
535
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d
533
541 542 543 544
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546 547
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548 549
Fig.6
553 554 555 556 557
te
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d
550
558 559 560 561
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562 563
Fig.7
567 568 569 570 571
te
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d
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572 573 574 575
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Schem. 1
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Page 33 of 39
Table 1 The effects of fixatives for the response current of 2,4-DCP. (n=5) Ea/V
Ipa/μA
RSD/%
Chitosan
0.763
1.139
5.3
PVA
0.773
1.278
8.7
Nafion
0.776
2.748
4.8
PVP
0.780
3.474
579
581
588 589 590
te Ac ce p
587
d
583
586
cr
M
582
585
1.4
an
580
584
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Fixatives
us
578
591 592 593 594
Page 34 of 39
Table 2 Comparison of various electroanalytical methods proposed for detection of CPs. Linear range
Detection limit
analyte
Ref. (μM)
(μM)
HRP/Au NPs/GCE
4-CP
2.5-117.5
0.39
CTAB-MMT/CPE
4-CP
0.05-10
0.02
TiO2/GPE
4-CP
0.05-50
cr
Electrode
ip t
595
AB-DHP/GCE
2-CP
0.2-40
MWNTs-DCP1/GCE
2-CP
0.1-20
HRP/MWNTs/GCE
2,4-DCP
Tyrosinase/MWNTs/GCE
2,4-DCP
Nafion/MWCNT/GCE
20
21
22
0.04
23
1.0–100
0.38
18
2.0–100
0.66
16
2,4-DCP
0.1–100
0.037
15
1–25
12.09
2,4-DCP
1–25
2.70
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an
0.05
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us
0.01
10
te
4-CP
17
Lac/PVA/F108/Au NPs/GCE
1–25
9.33
Mb-AG/GCE
2,4-DCP
12.5-208
2.06
13
Graphene/HRP/GCE
2,4-DCP
0.01–13.0
0.005
19
TiO2/DHP/GCE
PCP
0.05-100
0.01
11
MWCNT/EP/GCE
PCP
2-12
0.8
12
2-CP
0.02-10.0
0.008
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2,4,6-TCP
This 2,4-DCP
0.006-9.0
0.002
PCP
0.06-8.0
0.01
work
PVP/ZnSe-CTAB/GCE
596
Page 35 of 39
HRP: horseradish peroxidase; Au NPs: gold nanoparticles ; MMT : montmorillonite; CPE : carbon
598
paste electrode; TiO2: mesoporous TiO2 nanoparticles; GPE: graphite paste electrode; AB :
599
acetylene black; DHP :dihexadecyl hydrogen phosphate; MWNTs:multiwalled carbon nanotubues;
600
DCP1: dicetyl phosphate; Lac: laccase ; PVA: polyvinyl alcohol; F108: polyethylene oxide –
601
polyoxypropylene–polyethylene oxide (PEO–PPO–PEO); DHP:dihexadecylphosphate; EP : epoxy;
602
MB-AG: Myoglobin and agarose
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603
an
604 605
M
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610 611 612 613 614
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615 616 617 618
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Table 3
Analytical results of 2,4-DCP in spiked lake water samples by the proposed DPV method and HPLC method （n=5）
Lake water
Found /μM
Recovery/%
Recovery/%
(DPV)
(HPLC)
(DPV)
0.05
0.054±0.010
0.052±0.010
108.0
0.5
0.491±0.021
0.483±0.049
5.0
4.869±0.214
5.120±0.192
Added/μM
621
98.2
97.4
(HPLC) 104.0 96.6
102.4
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Sample
Found /μM
cr
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A very sensitive and simple electrochemical sensor for chlorophenols (CPs) based on
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nanocomposite of cetyltrimethylammonium bromide (CTAB) and ZnSe quantum dots
625
(ZnSe-CTAB) through electrostatic self-assembly technology was built for the first
626
time. The nanocomposite of ZnSe-CTAB introduced a favorable access for the
627
electron transfer and showed excellent electrocatalytic activity for the oxidation of
628
CPs.
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Highlights Nanocomposite based ZnSe QDs and CTAB was prepared and characterized. A novel electrochemical sensor for the determination of CPs was built. The proposed sensor was more sensitive, simple and environment-friendly.
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