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Bifunctional polydopamine@Fe3O4 core–shell nanoparticles for electrochemical determination of lead(II) and cadmium(II)
Accepted Manuscript Title: Bifunctional polydopamine@Fe3 O4 core-shell nanoparticles for electrochemical determination of Lead (II) and Cadmium (II) Author: Qian Song Maoguo Li Li Huang Qikang Wu Yunyou Zhou Yinling WangTel.: +86 553 3869302; fax: +86 553 3869303
S0003-2670(13)00833-7 http://dx.doi.org/doi:10.1016/j.aca.2013.06.010 ACA 232646
To appear in:
Analytica Chimica Acta
Received date: Revised date: Accepted date:
8-5-2013 2-6-2013 12-6-2013
Please cite this article as: Q. Song, M. Li, L. Huang, Q. Wu, Y. Zhou, Y. Wang, Bifunctional polydopamine@Fe3 O4 core-shell nanoparticles for electrochemical determination of Lead (II) and Cadmium (II), Analytica Chimica Acta (2013), http://dx.doi.org/10.1016/j.aca.2013.06.010 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.
Bifunctional polydopamine@Fe3O4 core-shell nanoparticles for electrochemical
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determination of Lead (II) and Cadmium (II)
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Qian Song, Maoguo Li*, Li Huang, Qikang Wu, Yunyou Zhou*, Yinling Wang
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Anhui Key Laboratory of Chemo/Biosensing; College of Chemistry and Materials Science, Anhui
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Normal University, Wuhu 241000, China
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Corresponding author. Tel.: +86 553 3869302; Fax: +86 553 3869303.
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E-mail address: [email protected] (M. Li); [email protected] (Y. Zhou).
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Highlights The bifunctional nanocomposites were synthesized. A modified magnetic glassy carbon electrode was fabricated. The electrode was used for the selective detection of Pb2+ and Cd2+ ions. The proposed sensor features a wider linear range and higher sensitivity.
Abstract
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The
paper
has
focused
on
the
potential
application
of
the
bifunctional
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polydopamine@Fe3O4 core-shell nanoparticles for development of a simple, stable and highly
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selective electrochemical method for metal ions monitoring in real samples. The electrochemical
13
method is based on electrochemical preconcentration/reduction of metal ions onto a
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polydopamine@Fe3O4 modified magnetic glassy carbon electrode at −1.1 V (versus SCE) in 0.1
15
M pH 5.0 acetate solution containing Pb2+ and Cd2+ during 160 s, followed by subsequent anodic
16
stripping. The proposed method has been demonstrated highly selective and sensitive detection of
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Pb2+ and Cd2+, with the calculated detection limits of 1.4×10−11 M and 9.2×10−11 M. Under the
18
optimized conditions, the square wave anodic stripping voltammetry response of the modified
19
electrode to Pb2+ (or Cd2+) shows a linear concentration range of 5.0 nM to 600 nM (or 20 nM to
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590 nM) with a correlation coefficient of 0.997 (or 0.994). Further, the proposed method has been
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performed to successfully detect Pb2+ and Cd2+ in aqueous effluent.
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Keywords: Polydopamine; Magnetic nanoparticle; Simultaneous determination; Lead ions;
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Cadmium ions
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1. Introduction The declining quality of drinking water due primarily to human activity has become a matter of
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grave concern [1]. The water contaminated with a trace amount of heavy metal ions, such as Pb2+,
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Cd2+, or Hg2+, is colorless and tasteless yet poses dangerous health hazards [2,3], because these
5
ions can be bioaccumulated through the food chain [4]. The U.S. Food and Drug Administration
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(FDA) set guidelines for the maximum amount of lead that can be leached out from ceramics,
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mugs, and flat dish plates to be between 0.5 and 30 μg mL−1 (approximately 2.4−144.8 μM) [5].
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Currently the amount of lead and heavy metal ions in solutions is mainly determined by atomic
9
absorption or emission spectrometry methods, such as atomic adsorption spectroscopy (AAS) and
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inductively coupled plasma (ICP). However, due to the high costs for both the equipment and
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measurement, determining the heavy metal in solutions by using AAS or ICP is impractical in
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certain situations. Therefore, it is of great significance to develop simple and less expensive
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methods for sensitive and selective detection of heavy metal ions.
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Anodic stripping voltammetry (ASV) is a powerful analytical technique for trace metal
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detection [6]. In the past few years, two conventional electrode systems, hanging mercury drop
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electrode and mercury film electrode, were mostly applied in those cases [7]. However, with a
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concern for sustainable development and the eco-design of instruments installed in the natural
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environment, the analytical methods involving toxic compounds must be banned, in particular
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mercury impregnation or films, even if the quantities used are relatively low [8]. Hence,
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mercury-free electrodes have become more attractive. To capture the metal ions in solution for
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improving the sensitivity of sensors, various nanoparticles (NPs) functionalized with species
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bearing terminal groups (e.g., −SH, −COOH, and −NH2), which display high affinity for heavy
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metal ions, have been widely used for sensing of metal ions, as illustrated by the several recent
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review articles [9−11]. Among the different functionalized nanostructures, the magnetic NPs may
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be given preference. Because they can exhibit several features synergistically and deliver more
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than one function simultaneously, such multifunctional magnetic NPs could have unique
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advantages in analytical applications [12].
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Recently, dopamine (DA) self-polymerization was discovered as a powerful approach to
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applying multifunctional coatings onto many surfaces, including noble metals, metal oxides,
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ceramics, and polymers, and served as an adhesion layer to immobilize biological molecules
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[13−15]. The previous reports have been demonstrated that catechol-derivative possesses
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irreversible binding affinity to iron oxide and then forms the multifunctional coatings onto
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magnetic NPs (e.g. polydopamine coated Fe3O4 NPs, denoted as PDA@Fe3O4 NPs), leading to
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ultrastable iron oxide NPs [16−20]. Because PDA@Fe3O4 have functional groups and unique
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magnetic property, the core-shell NPs can be used for separation. Ni et al. [21] described the
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synthesis of PDA@Fe3O4 NPs for Gluconobacter oxydans separation in situ. After optimization,
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21.3 mg (wet cell weight) Gluconobacter oxydans per milligram of nanoparticle was aggregated
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and separated with a magnet. Sahin et al. [22] reported the comparative study of silica-coated
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Fe3O4 NPs and PDA@Fe3O4 NPs for magnetic bio-separation. In most cases, PDA@Fe3O4 NPs
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displayed a significantly higher adsorption capacity for proteins, such as IgG, fibrinogen,
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hemoglobin and myoglobin. However, to the best of our knowledge, to date there has been no
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report on enrichment of heavy metal ions by PDA@Fe3O4 NPs based on its high affinity of amine
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groups for heavy metal ions.
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This contribution describes a simple and versatile electrochemical method using of a
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PDA@Fe3O4 modified magnetic glassy carbon (mGC) electrode to detect Pb2+ and Cd2+ with high
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sensitivity by square wave anodic stripping voltammetry (SWASV). Compared to the reported
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methods for electrochemical sensing toward heavy metal ions based on nanostructures, our
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method indeed possesses the desirable properties of electroanalysis and multifunctional magnetic
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NPs to improve the selectivity, sensitivity and stability. The concept how to get the most out of
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multifunctional materials should be very important for development of sensors with high
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performance.
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2. Experimental
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2.1. Apparatus and reagents
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Dopamine hydrochloride (DA) and 2-amino-2-hydroxymethylpropane-1,3-diol (Tris) were from
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Acros Organics (New Jersey, USA). Ferric chloride hexahydrate (FeCl3·6H2O), sodium acetate
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(NaAc), ethylene glycol (EG), polyethylene glycol 4000 (PEG), were obtained from Aldrich.
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Other reagents were of analytical grade or better quality, which were purchased from J&K
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Chemical Ltd. (Shanghai, China). Stock solution of 10 mM Pb(II) was prepared by dissolving
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Pb(NO3)2 (99.9%, Aldrich) in 4% acetic acid (pH of about 2.3). 0.1 M acetate buffer solutions
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with various pH values were prepared by mixing the stock solutions of HAc and NaAc, and
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adjusted the pH with 0.1 M NaOH or HAc solution. Milli-Q ultrapure water (Millipore, ≥18.2 MΩ
18
cm) was used throughout. Heavy metal compounds, such as Pb(II), Cd(II), and Hg(II), are known
19
to be harmful (toxic to the nervous system) if inhaled and irritating to eyes and skin, so personal
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protective equipment, such as respirators, lab coat, chemical-resistant gloves, and safety goggles,
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should be worn to minimize the chances of exposure during handling. Solution mixing and
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handling were done in a ventilated fume hood, and all chemicals were stored in tightly closed
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containers when not in use. The electrochemical experiments were performed on a CHI660C Electrochemical Analyzer
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(Chenhua Instruments, Shanghai, China). A three-electrode system was employed with an mGC
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electrode (ϕ = 3 mm, Tianjing Incole Union Technology Co., Ltd. www.incole.com ) or modified
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electrode as the working electrode. A saturated calomel electrode (SCE) and a platinum wire
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served as the reference electrode and counter electrode, respectively. All electrochemical
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experiments were performed in solutions deaerated by pure nitrogen at room temperature. The size
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and morphology of the synthesized nanoparticles were measured using a JEOL-2010 transmission
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electron microscope (TEM) operated at 200 kV and an S-4800 field-emission scanning electron
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microscope (Hitachi, Japan) for obtaining the scanning electron microscopy (SEM) images.
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Powder X-ray diffraction (XRD) data were recorded by a Shimadzu XRD−6000 X-ray
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diffractometer (Shimadzu, Japan) based on Cu Kα radiation (λ=0.15406 nm). The 2θ angle of the
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diffractometer was gradated from 5° to 70° at a scan rate of 0.02° s–1. Fourier transform infrared
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(FTIR) spectra were taken on a FTIR-8400S Fourier transform infrared spectrophotometer after 20
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scans within 4000–400 cm–1 at a resolution of 8 cm–1 by measuring the IR absorbance of a KBr
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disk that contented of 1–2 wt% of the sample.
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2.2. Preparation of Fe3O4 NPs
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Fe3O4 NPs were prepared as reported previously [23]. FeCl3·6H2O (1.35 g, 5 mmoL) was
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dissolved in ethylene glycol (40mL) to form a clear solution followed by addition of NaAc (3.6g)
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and polyethylene glycol 4000 (1.0g). The mixture was stirred vigorously for 30 min and then
21
sealed in a teflon-lined stainless-steel autoclave (60 mL, capacity). The autoclave was heated to
22
maintain at 200 ˚C for 12 h and allowed to cool to room temperature. The as-prepared Fe3O4 NPs
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were washed several times with water and ethanol respectively. Finally the products were
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collected with a magnet and then dried at 60 ˚C for 6 h.
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2.3. Synthesis of PDA@Fe3O4 NPs The PDA@Fe3O4 NPs were synthesized according to references with slight modification
5
[16−20]. Briefly, 100 mg dopamine hydrochloride and 50 mg Fe3O4 NPs were added into 10 mL
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deionized water (pH was approximately 6.0) and dispersed by 1 min sonication in water bath. This
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mixture was shaken for 1 h at room temperature. The DA@Fe3O4 NPs were collected with a
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magnet. To obtain the PDA@Fe3O4 NPs, 90 mL of 0.1 M Tris-HCl buffer (pH 8.5) was added into
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the above 10 mL mixture, and then the solution was incubated for 24 h at room temperature under
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shaking. Finally, the products were also collected with a magnet and then dried at 60 ˚C for 6 h.
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2.4. Preparation of modified electrode
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The fabrication of PDA@Fe3O4 NPs modified mGC (PDA@Fe3O4/mGC) electrodes was
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performed as follows: 20 mg PDA@Fe3O4 NPs were suspended by ultrasonic for 10 min in 2 mL
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ethanol to form a suspension. A 10 μL of this suspension was then pipetted onto the surface of a
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freshly pretreated mGC electrode. For comparison, the DA@Fe3O4 modified mGC
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(DA@Fe3O4/mGC) and Fe3O4 modified mGC (Fe3O4/mGC) electrodes were also constructed as
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above procedure.
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2.5. Determination of Pb2+ and Cd2+ by SWASV
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Square wave anodic stripping voltammetry (SWASV) was used for the detection of Pb2+ and
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Cd2+ under optimized conditions [24−27]. Lead and cadmium were deposited at the potential of
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−1.1 V for 160 s by the reduction of Pb(II) and Cd(II) in a 0.1 M pH 5.0 NaAc-HAc. The anodic
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stripping (reoxidation of metal to metal ions) of electrodeposited metal was performed in the
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potential range of −1.2 to 0.2 V at the following optimized parameters: frequency, 15 Hz;
2
amplitude, 25 mV. The simultaneous and selective detection of Pb(II) and Cd(II) has been
3
performed at the same experimental condition. The results, such as the experimental condition,
4
optimization, and calibration curve, were statistical analysis and presented as mean ±SD (standard
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deviation).
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3. Results and discussion
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3.1. Characterization of PDA@Fe3O4 NPs
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Fe3O4 NPs were successfully prepared according to reference [23] as shown in Fig. 1A. Clearly,
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these particles are spherical with a size in rage from 100 nm to 200 nm. It is worth noting that the
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magnetic microspheres are composed of smaller particles and their surface is not smooth, quite
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similar to those reported elsewhere [23,28]. EG is a good reducing agent and has been widely used
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in the polyol process to prepare metal or metal oxide NPs [23]. Herein, EG played both roles as a
13
reducing agent and a solvent during the formation of Fe3O4 NPs. The PEG as the modifier
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stabilized the Fe3O4 in order to circumvent magnetite sedimentation, to ensure well-dispersed
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spherical magnetic particles. The crystalline structure and phase purity were determined by XRD
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as shown in Fig. 1B. The XRD pattern of the sample showed six peaks in the 2θ rage of 20 ~ 70,
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including a high-intensity sharp peak at 2θ = 35.5, corresponding to the (311) plane, and five
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additional weak peaks at 2θ = 30.1, 43.1, 54.4, 57.0 and 62.6, corresponding to the (220),
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(400), (422), (411) and (440) planes, respectively, which match well with the database of a face
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centered cubic (fcc) lattice of Fe3O4 (JCPDS card No. 75-1610).
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Further characterization by FTIR spectroscopy was performed to evaluate surface
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functionalization by DA and PDA coatings. Before ligang exchange by DA, the Fe3O4
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NPs was stabilized by PEG (curve a in Fig. S1). There were only two obvious peaks
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located at 3420 cm–1 (νOH) and 588 cm–1 (due to M–O vibration). Upon ligand
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exchange with dopamine (curve b), the sample displayed the characteristic absorption
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bands of DA at 1615 cm–1 (aromatic ring stretching vibration) and 932 cm–1 (N-H
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wagging vibration), indicating successful exchange of PEG with dopamine. In
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contrast, after the polymerization of DA in weak alkali solution, the absorption band
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at 932 cm–1 has completely disappeared (curve c), suggesting the complete conversion
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of DA coating to PDA coating.
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Figure 1 should be inserted here.
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The previous reports have demonstrated that a room-temperature ligand exchange reaction
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using dopamine could be easily achieved on surface of Fe3O4 NPs [29−31]. In a pH 6.0 solution
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containing of 100 mM DA, the PEG stabilized Fe3O4 NPs can be converted to DA@Fe3O4, as
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shown in Fig. 2 [31].
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Figure 2 should be inserted here.
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When the pH value of solution was adjusted by Tis-buffer (pH 8.5) solution, the PDA film was
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formed on Fe3O4 NPs [32]. Fig. 3 shows typical TEM images of DA@Fe3O4 NPs (3A) and
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PDA@Fe3O4 NPs (3B). As can be seen from the TEM images of as-prepared core-shell NPs, the
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composite particles are composed of a black core of Fe3O4 and a gray shell of DA or PDA. The
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thickness of PDA coating is more than 5 nm. Furthermore, PDA@Fe3O4 NPs can be easily
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attracted within several minutes by placing a mGC electrode on the side of the vessel (Fig. 3C),
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indicating no obvious change in magnetism of the PDA@Fe3O4 NPs.
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Figure 3 should be inserted here.
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3.2. Enhanced electrochemical sensing behavior for Pb2+ at PDA@Fe3O4/mGC electrode by SWV
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To clarify the role of PDA in the nanocomposites, the square wave voltammograms were
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recorded with various electrodes as shown in Fig. 4. In a 0.1 M pH 5.0 NaAc-HAc solution
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containing of 100 nM Pb2+, no obvious peak can be observed for both bare mGC electrode (curve
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a) and Fe3O4/mGC electrode (curve b). Under the same conditions, the well-defined stripping
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peaks at approximately –0.504 V were found for both DA@Fe3O4/mGC and PDA@Fe3O4/mGC
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electrodes (curves c and d), which were ascribed to the reoxidation of Pb (0) to Pb (II). It was
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clear that the current response at the modified electrodes derived from the dopamine coatings on
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the Fe3O4 NPs. Accordingly, dopamine or polydopamine coating has amino group for binding with
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metal ions [33], leading to the enhanced accumulation of Pb2+ from the sample solution. It should
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be noted that the smaller stripping peak observed at DA@Fe3O4/mGC electrode (curve c) is
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probably due to the instability of DA@Fe3O4 composites.
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Figure 4 should be inserted here.
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3.3. Optimum experimental conditions
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The optimum experimental conditions, such as mass effect of the PDA@Fe3O4, supporting electrolytes, pH value, and deposition time, were investigated in detail using Pb(II) as the sample.
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Fig. S2 shows the effect of the mass of the PDA@Fe3O4 NPs on SWASV response in the
19
presence of 50 nM Pb(II) in 0.1 M pH 5.0 NaAc-HAc solution. A well-defined SWASV response
20
can be obtained in different groups (0.05, 0.10, 0.15, and 0.20 mg). The increase of mass more
21
than 0.1 mg resulted in the lower current peaks. This may be mainly attributed to a reduced
22
electrical connectivity between the thicker film of PDA@Fe3O4 NPs and mGC electrode. At the
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same time, decreasing the mass of the modifier beyond 0.10 mg resulted in a lower adsorption of
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PDA@Fe3O4 NPs toward Pb2+ and a sharp decrease in the sensitivity of the response to Pb(II).
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Therefore, 0.10 mg of PDA@ Fe3O4 nanoparticles was chose as modifier. The different supporting electrolytes were also performed in presence of 50 nM Pb(II) using
5
SWASV. The 0.1 M pH 5.0 solutions of phosphate buffer solution, NH4Cl-HCl, and NaAc-HAc
6
were compared to the response toward Pb(II) on the PDA@ Fe3O4, respectively (Fig. S3). No
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obvious signal was obtained in phosphate buffer solution solution, and a very small stripping peak
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was observed in NH4Cl-HCl. In contrast, a well-defined and sharp stripping peak was found in
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NaAc-HAc, quite similar to that reported elsewhere [34].
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The solutions of NaAc-HAc with different pH value (from 2.0 to 7.0) were investigated using
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SWASV in presence of 100 nM Pb(II). The results are illustrated in Fig. S4. As can be seen, the
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stripping peak currents increased with the pH value from 2.0 to 5.0 and decreased from 5.0 to 7.0.
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It was previously reported that the voltammetric signal of metal, such as Pb, was controlled by
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how well the electrode materials can capture Pb and are subsequently collected on the electrode
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surface [35]. Therefore, pH 5.0 was selected for subsequent experiments.
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Different deposition time of 40, 60, 80, 100, 120, 140, 160,180 and 200s were tested using
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SWASV in a 0.1 M pH 5.0 NaAc-HAc solution containing of 100 nM Pb(II). As seen in Fig. S5,
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when the deposition time is less than 160 s, the stripping currents increase linearly with the
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increase of deposition time. It suggests that the electrode surface is saturated by the Pb(II) when
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the deposition time is beyond 160 s. Accordingly, an optimized deposition time of 160 s was used
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throughout.
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3.4. Detection of Pb2+ and Cd2+ under optimum experimental conditions
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The individual measurements of Pb2+ and Cd2+ using the PDA@Fe3O4/mGC electrode were
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performed as shown in Fig. 5. Fig. 5A shows the SWASV responses of the modified electrode for
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Pb2+ at various concentrations, the corresponding calibration curve being inserted in Fig. 5A
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accordingly (inset in Fig. 5A). The concentration of Pb2+ in the rage 5.0 ~ 600 nM is proportional
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to the stripping peak current. The regression equation is y (μA) = 0.378 + 0.120x (nM) with a
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correlation coefficient of 0.997. The sensitivity is 0.235 μA nM−1 and the detection limit is
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calculated to be 1.4×10−11 M. Fig. 5B shows the SWASV responses of the modified electrode for
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Cd2+ at various concentrations. The concentration of Cd2+ in the rage 20 ~ 590 nM is proportional
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to the stripping peak current (inset in Fig. 5B). The regression equation is y (μA) = 0.519 + 0.126x
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(nM) with a correlation coefficient of 0.997. The The sensitivity is 0.196 μA nM−1 and the
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detection limit could be calculated as 9.2×10−11 M at a signal-to-noise ratio of 3.
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Figure 5 should be inserted here.
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Based on the above results, it should be noted that the proposed method has a lower detection
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limit and higher sensitivity toward Pb(II) and Cd(II) than most recently reported electrochemical
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methods for detection of those metal ions. The comparisons for the parameters by use of several
16
modified electrode are listed in Table 1. The excellent sensitivity of PDA@Fe3O4/mGC electrode
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benefits from the interaction between the magnetic NPs and the mGC electrode, which allows the
18
direct electrodeposition of Pb(II) and Cd(II) on the modified electrode.
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Table 1 should be inserted here.
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At the same time, the proposed method was also performed for simultaneous measurement of
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Pb2+ and Cd2+. As shown in Fig. 6, the modified electrode shows stripping peaks at −0.518 V and
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−0.732 V for Pb(II) and Cd(II), respectively. The anodic peak potential difference between Pb(II)
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and Cd(II) is up to more than 200 mV. Thus, their peaks are well-separated; suggesting that
2
simultaneous determination of Pb(II) and Cd(II) can be achieved without interference with each
3
other. Furthermore, as seen (inset), the sensitivities for simultaneous determination of Pb(II) and
4
Cd(II) were comparable to that of individual measurements, implying that simultaneous
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measurement of Pb(II) and Cd(II) is feasible.
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Figure 6 should be inserted here.
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3.4. Selectivity, reproducibility and stability
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The influence of various substances on the determination of 100 nM Pb2+ and Cd2+ was studied.
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The tolerance limit was estimated to less than 5% of the error. It was found that 1000-fold K+, Na+,
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NH4+, Ca2+, Mg2+, Cl–, Br–, NO3– and SO42–, and 500-fold each of Cu2+, Zn2+, Fe2+, Ni2+, Hg2+,
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Al3+, Fe3+, and Ni3+ caused no obvious changes of voltammetric signals for Cd2+ and Pb2+,
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indicating an excellent anti-interference ability of the proposed method.
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The reproducibility of PDA@Fe3O4/mGC electrode was investigated through repetitive
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electrodeposition and stripping of Pb(0). Fig. 7 shows the results after continuous cycling for 30
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times. The peak current of the electrode was found nearly constant with the relative standard
16
deviation (RSD) from 0.3% to 0.7%, indicating that the modified electrode displayed an
17
acceptable reproducibility. To verify the stability of PDA@Fe3O4/mGC electrode, the same
18
electrode had been used for several days, and SWASV for sensing of 100 mM Pb2+ was performed
19
every day under the same experimental condition. The peak position does not change and only
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5.2% decrease in the current for lead stripping peak was noticed after one week.
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Figure 7 should be inserted here.
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3.5. Real sample analysis
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The proposed method was successfully applied to the determination of lead in aqueous effluent
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from Tianneng Battery (Wuhu) Co., LTD. using standard addition method. The accuracy of the
3
method was checked by means of comparison with those data (provided by the environmental
4
protection bureau of Wuhu City) obtained from inductively coupled plasma atomic emission
5
spectrometry (ICP-AES). The assessment by Student’s t-test did not show a statistical significant
6
difference between the methods used (95%), thus confirming the accuracy. The results were listed
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in Table 2.
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Table 2 should be inserted here.
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4. Conclusion
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In this investigation we demonstrate that Fe3O4 NPs can be facilely coated by PDA though a
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room-temperature ligand exchange reaction and subsequent the oxidation of dopamine in weak
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alkali solution by mildly stirring. Combining the advantageous features of high affinity for heavy
13
metal ions provided by PDA and the magnetism of Fe3O4 NPs, a stable PDA@Fe3O4 film was
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constructed on the surface of mGC electrode conveniently and without a further immobilization
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[43−47]. Due to the adsorption toward Pb2+ and Cd2+ of the PDA@Fe3O4 nanoparticles, the
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modified mGC electrode can be used for simultaneous detection of those metal ions. Compared
17
with the reported electrochemical methods for sensing of Pb2+ and Cd2+, this method possesses
18
higher stability, lower cost, better anti-interference ability and higher sensitivity. Thus, the
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PDA@Fe3O4/mGC electrode is expected to develop the heavy metal ion sensors for practical
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applications.
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Acknowledgments
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M. Li is grateful for financial support from the Natural Science Foundation of China (Grant No.
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21075001) and Anhui Provincial Natural Science Foundation (Grant No. 11040606M46). The
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authors gratefully acknowledge Dr. Yan Wei (Wannan Med. Coll., Dept. Chem.) for her help in the
3
paper preparing.
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References
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[1] Z. Feng, S. Zhu, D.R.M. de Godoi, A.C.S. Samia, D. Scherson, Analytical Chemistry 84 (2012)
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Figure captions
2
Fig. 1. (A) SEM image and (B) XRD pattern of the as-prepared Fe3O4 NPs.
3
Fig. 2. Schematic illustration of ligand exchange from oleic to dopamine on Fe3O4 NPs.
4
Fig. 3. TEM images of (A) DA@Fe3O4 NPs, (B) PDA@Fe3O4 NPs and (C) separation of
5
PDA@Fe3O4 NPs by a magnetic glassy carbon (mGC) electrode.
6
Fig. 4. Square wave voltammograms of (a) bare, (b) Fe3O4, (c) DA@Fe3O4 and (d)
7
PDA@Fe3O4-modified mGC electrodes in a 0.1 M pH 5.0 NaAc-HAc solution containing of 100
8
nM Pb2+.
9
Fig. 5. The stripping voltammograms of PDA@Fe3O4/mGC electrode for the individual analysis
10
of (a) Pb2+ over a concentration range of 5.0 to 600 nM and (b) Cd2+ over a concentration range of
11
20 to 590 nM in a 0.1 M pH 5.0 NaAc-HAc solution. The insets show the plot of the SWASV
12
peak current vs. the corresponding metal ions concentration.
13
Fig. 6. The stripping voltammograms of PDA@Fe3O4/mGC electrode for simultaneous
14
measurements of Pb(II) and Cd(II) under optimum conditions. Concentrations: Pb(II), 10 − 700
15
nM; Cd(II), 50 − 600 nM. Inset, the corresponding plots of stripping peaks as a function of
16
concentration.
17
Fig. 7. Selected stripping voltammograms using one PDA@Fe3O4/mGC electrode for repetitive
18
electrodeposition and stripping of Pb(0) in 0.1 M pH 5.0 NaAc-HAc solution containing of 50 nM
19
Pb2+. The top shows the corresponding plot of peak current as a function of the cyclic times.
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Page 19 of 29
1
Table 1
2
Sensitivity, detection limit (DL), and linear range (LR) of different modified electrodes for sensing
3
of Pb(II) and Cd(II) Sensitivity (Pb; Cd) / μA (μg L−1)−1
DL (Pb; Cd) /μg L−1
LR (Pb; Cd) / μg L−1
Reference
ANs/AuDE a
1.0952; −
0.2; −
5.0−60; −
[36]
SbFME b
−; −
3.1; 1.9
[37]
Bi-P-SPCE c
−; −
HGNPs/SPE d
0.014; 0.0073
0.03; 0.34 3; −
ETO/GCE e
−; −
0.0346; −
20−100; 20−100 0.05−30; 1−30 5–100; 5–100 0.1−20; −
Bi/Nafion/DMcT-PANI/MWCNTs/GCE
1.63; 3.44
0.04; 0.01 0.0029; 0.01
0.08−31; 0.02−20 1.0−120; 2.2−64.9
[41]
PDA@Fe3O4/mGCE
M
1.12; 1.74
cr
us
an
f
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Electrodes
a
ANs/AuDE: Au nanowire modified Au-disc electrode.
5
b
SbFME: antimony film microelectrode.
6
c
Bi-P-SPCE: bismuth-coated porous screen-printed carbon electrode.
7
d
HGNPs/SPE: heated graphite nanoparticle-based screen-printed electrode.
8 9 10
e
[39] [40]
This work
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[38]
ETO/GCE: etodolac modified glassy carbon electrode. Bi/Nafion/DMcT-PANI/MWCNTs/GCE: polyaniline, 2,5-dimercapto-1,3,4-thiadiazole, and multiwalled carbon nanotubes modified glassy carbon electrode with Nafion. f
20
Page 20 of 29
1
Table 2
2
Validation of the method for determination of lead in aqueous effluent. Concentration of lead found b (μg L−1) Samples a
1
92.2±3.5
93.1±2.0
2
100.7±2.7
101.0±1.8
3
98.9±1.8
100.1±0.8
0.59 0.19
cr
ICP-AES
3.27
us
Proposed method (μM)
ip t
t calculated
3
a
4
emission spectrometry.
5
b
an
Confidence interval,95%. (n−1)=2; t tabled = 4.303. ICP-AES, inductively coupled plasma atomic
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The results are expressed as mean value ± S.D. based on three replicates (n = 3) determinations.
21
Page 21 of 29
Figure 1
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Fig. 1. (A) SEM image and (B) XRD pattern of the as-prepared Fe3O4 NPs.
pt
4
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Figure 2
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Fig. 2. Schematic illustration of ligand exchange from oleic to dopamine on Fe3O4 NPs.
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2
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Page 23 of 29
Figure 3
us
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1
3
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2
Fig. 3. TEM images of (A) DA@Fe3O4 NPs, (B) PDA@Fe3O4 NPs and (C) separation of
5
PDA@Fe3O4 NPs by a magnetic glassy carbon (mGC) electrode.
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Page 24 of 29
Figure 4
25
a b
bare mGC electrode Fe3O4/mGC electrode
c
DA@Fe3O4/mGC electrode
20
d
PDA@Fe3O4/mGC electrode
ip t
d
15
5
cr
10 c
b a
0 -1.2
-0.8
0.0
0.4
an
2
-0.4 Potential / V
us
Current / A
1
Fig. 4. Square wave voltammograms of (a) bare, (b) Fe3O4, (c) DA@Fe3O4 and (d)
4
PDA@Fe3O4-modified mGC electrodes in a 0.1 M pH 5.0 NaAc-HAc solution containing of 100
5
nM Pb2+
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Page 25 of 29
Figure 5
A
75 y=0.318+0.120x R=0.997
ip t
45 30
cr
Current / A
60
15 0 -0.8
-0.4 0.0 0.4 Potential / V
2
75
y=0.519+0.126x R=0.994
ed
60 45
pt
30 15
Ac ce
Current / A
1.2
M
B
0.8
an
-1.2
us
1
0
-1.2
3
-0.8
-0.4 0.0 0.4 Potential / V
0.8
1.2
4
Fig. 5. The stripping voltammograms of PDA@Fe3O4/mGC electrode for the individual analysis
5
of (a) Pb2+ over a concentration range of 5.0 to 600 nM and (b) Cd2+ over a concentration range of
6
20 to 590 nM in a 0.1 M pH 5.0 NaAc-HAc solution. The insets show the plot of the SWASV
7
peak current vs. the corresponding metal ions concentration.
26
Page 26 of 29
1
Figure 5
Pb(II)
60 Cd(II)
ip t cr
30 15
us
Current / A
45
0
2
-0.4 0.0 Potential / V
0.4
0.8
an
-0.8
Fig. 6. The stripping voltammograms of PDA@Fe3O4/mGC electrode for simultaneous
4
measurements of Pb(II) and Cd(II) under optimum conditions. Concentrations: Pb(II), 10 − 700
5
nM; Cd(II), 50 − 600 nM. Inset, the corresponding plots of stripping peaks as a function of
6
concentration.
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3
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1
Figure 7
16
ip t
8 4
cr
Current / A
12
2
3
6
9
12 15 18 21 24 27 30 33 Stripping Number
an
0
us
0
Fig. 7. Selected stripping voltammograms using one PDA@Fe3O4/mGC electrode for repetitive
4
electrodeposition and stripping of Pb(0) in 0.1 M pH 5.0 NaAc-HAc solution containing of 50 nM
5
Pb2+. The top shows the corresponding plot of peak current as a function of the cyclic times.
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Page 28 of 29
1
Bifunctional polydopamine@Fe3O4 core-shell nanoparticles for
2
electrochemical determination of Lead (II) and Cadmium (II)
3
Qian Song, Maoguo Li*, Li Huang, Qikang Wu, Yunyou Zhou*, and Feng Gao
9
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Page 29 of 29
S0003-2670(13)00833-7 http://dx.doi.org/doi:10.1016/j.aca.2013.06.010 ACA 232646
To appear in:
Analytica Chimica Acta
Received date: Revised date: Accepted date:
8-5-2013 2-6-2013 12-6-2013
Please cite this article as: Q. Song, M. Li, L. Huang, Q. Wu, Y. Zhou, Y. Wang, Bifunctional polydopamine@Fe3 O4 core-shell nanoparticles for electrochemical determination of Lead (II) and Cadmium (II), Analytica Chimica Acta (2013), http://dx.doi.org/10.1016/j.aca.2013.06.010 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.
Bifunctional polydopamine@Fe3O4 core-shell nanoparticles for electrochemical
2
determination of Lead (II) and Cadmium (II)
3
Qian Song, Maoguo Li*, Li Huang, Qikang Wu, Yunyou Zhou*, Yinling Wang
4
Anhui Key Laboratory of Chemo/Biosensing; College of Chemistry and Materials Science, Anhui
5
Normal University, Wuhu 241000, China
6
Corresponding author. Tel.: +86 553 3869302; Fax: +86 553 3869303.
7
E-mail address: [email protected] (M. Li); [email protected] (Y. Zhou).
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Highlights The bifunctional nanocomposites were synthesized. A modified magnetic glassy carbon electrode was fabricated. The electrode was used for the selective detection of Pb2+ and Cd2+ ions. The proposed sensor features a wider linear range and higher sensitivity.
Abstract
10
The
paper
has
focused
on
the
potential
application
of
the
bifunctional
11
polydopamine@Fe3O4 core-shell nanoparticles for development of a simple, stable and highly
12
selective electrochemical method for metal ions monitoring in real samples. The electrochemical
13
method is based on electrochemical preconcentration/reduction of metal ions onto a
14
polydopamine@Fe3O4 modified magnetic glassy carbon electrode at −1.1 V (versus SCE) in 0.1
15
M pH 5.0 acetate solution containing Pb2+ and Cd2+ during 160 s, followed by subsequent anodic
16
stripping. The proposed method has been demonstrated highly selective and sensitive detection of
17
Pb2+ and Cd2+, with the calculated detection limits of 1.4×10−11 M and 9.2×10−11 M. Under the
18
optimized conditions, the square wave anodic stripping voltammetry response of the modified
19
electrode to Pb2+ (or Cd2+) shows a linear concentration range of 5.0 nM to 600 nM (or 20 nM to
20
590 nM) with a correlation coefficient of 0.997 (or 0.994). Further, the proposed method has been
21
performed to successfully detect Pb2+ and Cd2+ in aqueous effluent.
22
Keywords: Polydopamine; Magnetic nanoparticle; Simultaneous determination; Lead ions;
23
Cadmium ions
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Page 2 of 29
1
1. Introduction The declining quality of drinking water due primarily to human activity has become a matter of
3
grave concern [1]. The water contaminated with a trace amount of heavy metal ions, such as Pb2+,
4
Cd2+, or Hg2+, is colorless and tasteless yet poses dangerous health hazards [2,3], because these
5
ions can be bioaccumulated through the food chain [4]. The U.S. Food and Drug Administration
6
(FDA) set guidelines for the maximum amount of lead that can be leached out from ceramics,
7
mugs, and flat dish plates to be between 0.5 and 30 μg mL−1 (approximately 2.4−144.8 μM) [5].
8
Currently the amount of lead and heavy metal ions in solutions is mainly determined by atomic
9
absorption or emission spectrometry methods, such as atomic adsorption spectroscopy (AAS) and
10
inductively coupled plasma (ICP). However, due to the high costs for both the equipment and
11
measurement, determining the heavy metal in solutions by using AAS or ICP is impractical in
12
certain situations. Therefore, it is of great significance to develop simple and less expensive
13
methods for sensitive and selective detection of heavy metal ions.
ed
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2
Anodic stripping voltammetry (ASV) is a powerful analytical technique for trace metal
15
detection [6]. In the past few years, two conventional electrode systems, hanging mercury drop
16
electrode and mercury film electrode, were mostly applied in those cases [7]. However, with a
17
concern for sustainable development and the eco-design of instruments installed in the natural
18
environment, the analytical methods involving toxic compounds must be banned, in particular
19
mercury impregnation or films, even if the quantities used are relatively low [8]. Hence,
20
mercury-free electrodes have become more attractive. To capture the metal ions in solution for
21
improving the sensitivity of sensors, various nanoparticles (NPs) functionalized with species
22
bearing terminal groups (e.g., −SH, −COOH, and −NH2), which display high affinity for heavy
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Page 3 of 29
metal ions, have been widely used for sensing of metal ions, as illustrated by the several recent
2
review articles [9−11]. Among the different functionalized nanostructures, the magnetic NPs may
3
be given preference. Because they can exhibit several features synergistically and deliver more
4
than one function simultaneously, such multifunctional magnetic NPs could have unique
5
advantages in analytical applications [12].
ip t
1
Recently, dopamine (DA) self-polymerization was discovered as a powerful approach to
7
applying multifunctional coatings onto many surfaces, including noble metals, metal oxides,
8
ceramics, and polymers, and served as an adhesion layer to immobilize biological molecules
9
[13−15]. The previous reports have been demonstrated that catechol-derivative possesses
10
irreversible binding affinity to iron oxide and then forms the multifunctional coatings onto
11
magnetic NPs (e.g. polydopamine coated Fe3O4 NPs, denoted as PDA@Fe3O4 NPs), leading to
12
ultrastable iron oxide NPs [16−20]. Because PDA@Fe3O4 have functional groups and unique
13
magnetic property, the core-shell NPs can be used for separation. Ni et al. [21] described the
14
synthesis of PDA@Fe3O4 NPs for Gluconobacter oxydans separation in situ. After optimization,
15
21.3 mg (wet cell weight) Gluconobacter oxydans per milligram of nanoparticle was aggregated
16
and separated with a magnet. Sahin et al. [22] reported the comparative study of silica-coated
17
Fe3O4 NPs and PDA@Fe3O4 NPs for magnetic bio-separation. In most cases, PDA@Fe3O4 NPs
18
displayed a significantly higher adsorption capacity for proteins, such as IgG, fibrinogen,
19
hemoglobin and myoglobin. However, to the best of our knowledge, to date there has been no
20
report on enrichment of heavy metal ions by PDA@Fe3O4 NPs based on its high affinity of amine
21
groups for heavy metal ions.
22
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This contribution describes a simple and versatile electrochemical method using of a
4
Page 4 of 29
PDA@Fe3O4 modified magnetic glassy carbon (mGC) electrode to detect Pb2+ and Cd2+ with high
2
sensitivity by square wave anodic stripping voltammetry (SWASV). Compared to the reported
3
methods for electrochemical sensing toward heavy metal ions based on nanostructures, our
4
method indeed possesses the desirable properties of electroanalysis and multifunctional magnetic
5
NPs to improve the selectivity, sensitivity and stability. The concept how to get the most out of
6
multifunctional materials should be very important for development of sensors with high
7
performance.
8
2. Experimental
9
2.1. Apparatus and reagents
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Dopamine hydrochloride (DA) and 2-amino-2-hydroxymethylpropane-1,3-diol (Tris) were from
11
Acros Organics (New Jersey, USA). Ferric chloride hexahydrate (FeCl3·6H2O), sodium acetate
12
(NaAc), ethylene glycol (EG), polyethylene glycol 4000 (PEG), were obtained from Aldrich.
13
Other reagents were of analytical grade or better quality, which were purchased from J&K
14
Chemical Ltd. (Shanghai, China). Stock solution of 10 mM Pb(II) was prepared by dissolving
15
Pb(NO3)2 (99.9%, Aldrich) in 4% acetic acid (pH of about 2.3). 0.1 M acetate buffer solutions
16
with various pH values were prepared by mixing the stock solutions of HAc and NaAc, and
17
adjusted the pH with 0.1 M NaOH or HAc solution. Milli-Q ultrapure water (Millipore, ≥18.2 MΩ
18
cm) was used throughout. Heavy metal compounds, such as Pb(II), Cd(II), and Hg(II), are known
19
to be harmful (toxic to the nervous system) if inhaled and irritating to eyes and skin, so personal
20
protective equipment, such as respirators, lab coat, chemical-resistant gloves, and safety goggles,
21
should be worn to minimize the chances of exposure during handling. Solution mixing and
22
handling were done in a ventilated fume hood, and all chemicals were stored in tightly closed
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1
containers when not in use. The electrochemical experiments were performed on a CHI660C Electrochemical Analyzer
3
(Chenhua Instruments, Shanghai, China). A three-electrode system was employed with an mGC
4
electrode (ϕ = 3 mm, Tianjing Incole Union Technology Co., Ltd. www.incole.com ) or modified
5
electrode as the working electrode. A saturated calomel electrode (SCE) and a platinum wire
6
served as the reference electrode and counter electrode, respectively. All electrochemical
7
experiments were performed in solutions deaerated by pure nitrogen at room temperature. The size
8
and morphology of the synthesized nanoparticles were measured using a JEOL-2010 transmission
9
electron microscope (TEM) operated at 200 kV and an S-4800 field-emission scanning electron
10
microscope (Hitachi, Japan) for obtaining the scanning electron microscopy (SEM) images.
11
Powder X-ray diffraction (XRD) data were recorded by a Shimadzu XRD−6000 X-ray
12
diffractometer (Shimadzu, Japan) based on Cu Kα radiation (λ=0.15406 nm). The 2θ angle of the
13
diffractometer was gradated from 5° to 70° at a scan rate of 0.02° s–1. Fourier transform infrared
14
(FTIR) spectra were taken on a FTIR-8400S Fourier transform infrared spectrophotometer after 20
15
scans within 4000–400 cm–1 at a resolution of 8 cm–1 by measuring the IR absorbance of a KBr
16
disk that contented of 1–2 wt% of the sample.
17
2.2. Preparation of Fe3O4 NPs
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Fe3O4 NPs were prepared as reported previously [23]. FeCl3·6H2O (1.35 g, 5 mmoL) was
19
dissolved in ethylene glycol (40mL) to form a clear solution followed by addition of NaAc (3.6g)
20
and polyethylene glycol 4000 (1.0g). The mixture was stirred vigorously for 30 min and then
21
sealed in a teflon-lined stainless-steel autoclave (60 mL, capacity). The autoclave was heated to
22
maintain at 200 ˚C for 12 h and allowed to cool to room temperature. The as-prepared Fe3O4 NPs
6
Page 6 of 29
1
were washed several times with water and ethanol respectively. Finally the products were
2
collected with a magnet and then dried at 60 ˚C for 6 h.
3
2.3. Synthesis of PDA@Fe3O4 NPs The PDA@Fe3O4 NPs were synthesized according to references with slight modification
5
[16−20]. Briefly, 100 mg dopamine hydrochloride and 50 mg Fe3O4 NPs were added into 10 mL
6
deionized water (pH was approximately 6.0) and dispersed by 1 min sonication in water bath. This
7
mixture was shaken for 1 h at room temperature. The DA@Fe3O4 NPs were collected with a
8
magnet. To obtain the PDA@Fe3O4 NPs, 90 mL of 0.1 M Tris-HCl buffer (pH 8.5) was added into
9
the above 10 mL mixture, and then the solution was incubated for 24 h at room temperature under
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shaking. Finally, the products were also collected with a magnet and then dried at 60 ˚C for 6 h.
11
2.4. Preparation of modified electrode
M
10
The fabrication of PDA@Fe3O4 NPs modified mGC (PDA@Fe3O4/mGC) electrodes was
13
performed as follows: 20 mg PDA@Fe3O4 NPs were suspended by ultrasonic for 10 min in 2 mL
14
ethanol to form a suspension. A 10 μL of this suspension was then pipetted onto the surface of a
15
freshly pretreated mGC electrode. For comparison, the DA@Fe3O4 modified mGC
16
(DA@Fe3O4/mGC) and Fe3O4 modified mGC (Fe3O4/mGC) electrodes were also constructed as
17
above procedure.
18
2.5. Determination of Pb2+ and Cd2+ by SWASV
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19
Square wave anodic stripping voltammetry (SWASV) was used for the detection of Pb2+ and
20
Cd2+ under optimized conditions [24−27]. Lead and cadmium were deposited at the potential of
21
−1.1 V for 160 s by the reduction of Pb(II) and Cd(II) in a 0.1 M pH 5.0 NaAc-HAc. The anodic
22
stripping (reoxidation of metal to metal ions) of electrodeposited metal was performed in the
7
Page 7 of 29
potential range of −1.2 to 0.2 V at the following optimized parameters: frequency, 15 Hz;
2
amplitude, 25 mV. The simultaneous and selective detection of Pb(II) and Cd(II) has been
3
performed at the same experimental condition. The results, such as the experimental condition,
4
optimization, and calibration curve, were statistical analysis and presented as mean ±SD (standard
5
deviation).
6
3. Results and discussion
7
3.1. Characterization of PDA@Fe3O4 NPs
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Fe3O4 NPs were successfully prepared according to reference [23] as shown in Fig. 1A. Clearly,
9
these particles are spherical with a size in rage from 100 nm to 200 nm. It is worth noting that the
10
magnetic microspheres are composed of smaller particles and their surface is not smooth, quite
11
similar to those reported elsewhere [23,28]. EG is a good reducing agent and has been widely used
12
in the polyol process to prepare metal or metal oxide NPs [23]. Herein, EG played both roles as a
13
reducing agent and a solvent during the formation of Fe3O4 NPs. The PEG as the modifier
14
stabilized the Fe3O4 in order to circumvent magnetite sedimentation, to ensure well-dispersed
15
spherical magnetic particles. The crystalline structure and phase purity were determined by XRD
16
as shown in Fig. 1B. The XRD pattern of the sample showed six peaks in the 2θ rage of 20 ~ 70,
17
including a high-intensity sharp peak at 2θ = 35.5, corresponding to the (311) plane, and five
18
additional weak peaks at 2θ = 30.1, 43.1, 54.4, 57.0 and 62.6, corresponding to the (220),
19
(400), (422), (411) and (440) planes, respectively, which match well with the database of a face
20
centered cubic (fcc) lattice of Fe3O4 (JCPDS card No. 75-1610).
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Further characterization by FTIR spectroscopy was performed to evaluate surface
22
functionalization by DA and PDA coatings. Before ligang exchange by DA, the Fe3O4
8
Page 8 of 29
NPs was stabilized by PEG (curve a in Fig. S1). There were only two obvious peaks
2
located at 3420 cm–1 (νOH) and 588 cm–1 (due to M–O vibration). Upon ligand
3
exchange with dopamine (curve b), the sample displayed the characteristic absorption
4
bands of DA at 1615 cm–1 (aromatic ring stretching vibration) and 932 cm–1 (N-H
5
wagging vibration), indicating successful exchange of PEG with dopamine. In
6
contrast, after the polymerization of DA in weak alkali solution, the absorption band
7
at 932 cm–1 has completely disappeared (curve c), suggesting the complete conversion
8
of DA coating to PDA coating.
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Figure 1 should be inserted here.
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The previous reports have demonstrated that a room-temperature ligand exchange reaction
12
using dopamine could be easily achieved on surface of Fe3O4 NPs [29−31]. In a pH 6.0 solution
13
containing of 100 mM DA, the PEG stabilized Fe3O4 NPs can be converted to DA@Fe3O4, as
14
shown in Fig. 2 [31].
15
Figure 2 should be inserted here.
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16
When the pH value of solution was adjusted by Tis-buffer (pH 8.5) solution, the PDA film was
17
formed on Fe3O4 NPs [32]. Fig. 3 shows typical TEM images of DA@Fe3O4 NPs (3A) and
18
PDA@Fe3O4 NPs (3B). As can be seen from the TEM images of as-prepared core-shell NPs, the
19
composite particles are composed of a black core of Fe3O4 and a gray shell of DA or PDA. The
20
thickness of PDA coating is more than 5 nm. Furthermore, PDA@Fe3O4 NPs can be easily
21
attracted within several minutes by placing a mGC electrode on the side of the vessel (Fig. 3C),
22
indicating no obvious change in magnetism of the PDA@Fe3O4 NPs.
9
Page 9 of 29
Figure 3 should be inserted here.
2
3.2. Enhanced electrochemical sensing behavior for Pb2+ at PDA@Fe3O4/mGC electrode by SWV
3
To clarify the role of PDA in the nanocomposites, the square wave voltammograms were
4
recorded with various electrodes as shown in Fig. 4. In a 0.1 M pH 5.0 NaAc-HAc solution
5
containing of 100 nM Pb2+, no obvious peak can be observed for both bare mGC electrode (curve
6
a) and Fe3O4/mGC electrode (curve b). Under the same conditions, the well-defined stripping
7
peaks at approximately –0.504 V were found for both DA@Fe3O4/mGC and PDA@Fe3O4/mGC
8
electrodes (curves c and d), which were ascribed to the reoxidation of Pb (0) to Pb (II). It was
9
clear that the current response at the modified electrodes derived from the dopamine coatings on
10
the Fe3O4 NPs. Accordingly, dopamine or polydopamine coating has amino group for binding with
11
metal ions [33], leading to the enhanced accumulation of Pb2+ from the sample solution. It should
12
be noted that the smaller stripping peak observed at DA@Fe3O4/mGC electrode (curve c) is
13
probably due to the instability of DA@Fe3O4 composites.
14
Figure 4 should be inserted here.
15
3.3. Optimum experimental conditions
17
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The optimum experimental conditions, such as mass effect of the PDA@Fe3O4, supporting electrolytes, pH value, and deposition time, were investigated in detail using Pb(II) as the sample.
18
Fig. S2 shows the effect of the mass of the PDA@Fe3O4 NPs on SWASV response in the
19
presence of 50 nM Pb(II) in 0.1 M pH 5.0 NaAc-HAc solution. A well-defined SWASV response
20
can be obtained in different groups (0.05, 0.10, 0.15, and 0.20 mg). The increase of mass more
21
than 0.1 mg resulted in the lower current peaks. This may be mainly attributed to a reduced
22
electrical connectivity between the thicker film of PDA@Fe3O4 NPs and mGC electrode. At the
10
Page 10 of 29
1
same time, decreasing the mass of the modifier beyond 0.10 mg resulted in a lower adsorption of
2
PDA@Fe3O4 NPs toward Pb2+ and a sharp decrease in the sensitivity of the response to Pb(II).
3
Therefore, 0.10 mg of PDA@ Fe3O4 nanoparticles was chose as modifier. The different supporting electrolytes were also performed in presence of 50 nM Pb(II) using
5
SWASV. The 0.1 M pH 5.0 solutions of phosphate buffer solution, NH4Cl-HCl, and NaAc-HAc
6
were compared to the response toward Pb(II) on the PDA@ Fe3O4, respectively (Fig. S3). No
7
obvious signal was obtained in phosphate buffer solution solution, and a very small stripping peak
8
was observed in NH4Cl-HCl. In contrast, a well-defined and sharp stripping peak was found in
9
NaAc-HAc, quite similar to that reported elsewhere [34].
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The solutions of NaAc-HAc with different pH value (from 2.0 to 7.0) were investigated using
11
SWASV in presence of 100 nM Pb(II). The results are illustrated in Fig. S4. As can be seen, the
12
stripping peak currents increased with the pH value from 2.0 to 5.0 and decreased from 5.0 to 7.0.
13
It was previously reported that the voltammetric signal of metal, such as Pb, was controlled by
14
how well the electrode materials can capture Pb and are subsequently collected on the electrode
15
surface [35]. Therefore, pH 5.0 was selected for subsequent experiments.
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Different deposition time of 40, 60, 80, 100, 120, 140, 160,180 and 200s were tested using
17
SWASV in a 0.1 M pH 5.0 NaAc-HAc solution containing of 100 nM Pb(II). As seen in Fig. S5,
18
when the deposition time is less than 160 s, the stripping currents increase linearly with the
19
increase of deposition time. It suggests that the electrode surface is saturated by the Pb(II) when
20
the deposition time is beyond 160 s. Accordingly, an optimized deposition time of 160 s was used
21
throughout.
22
3.4. Detection of Pb2+ and Cd2+ under optimum experimental conditions
11
Page 11 of 29
The individual measurements of Pb2+ and Cd2+ using the PDA@Fe3O4/mGC electrode were
2
performed as shown in Fig. 5. Fig. 5A shows the SWASV responses of the modified electrode for
3
Pb2+ at various concentrations, the corresponding calibration curve being inserted in Fig. 5A
4
accordingly (inset in Fig. 5A). The concentration of Pb2+ in the rage 5.0 ~ 600 nM is proportional
5
to the stripping peak current. The regression equation is y (μA) = 0.378 + 0.120x (nM) with a
6
correlation coefficient of 0.997. The sensitivity is 0.235 μA nM−1 and the detection limit is
7
calculated to be 1.4×10−11 M. Fig. 5B shows the SWASV responses of the modified electrode for
8
Cd2+ at various concentrations. The concentration of Cd2+ in the rage 20 ~ 590 nM is proportional
9
to the stripping peak current (inset in Fig. 5B). The regression equation is y (μA) = 0.519 + 0.126x
10
(nM) with a correlation coefficient of 0.997. The The sensitivity is 0.196 μA nM−1 and the
11
detection limit could be calculated as 9.2×10−11 M at a signal-to-noise ratio of 3.
12
Figure 5 should be inserted here.
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Based on the above results, it should be noted that the proposed method has a lower detection
14
limit and higher sensitivity toward Pb(II) and Cd(II) than most recently reported electrochemical
15
methods for detection of those metal ions. The comparisons for the parameters by use of several
16
modified electrode are listed in Table 1. The excellent sensitivity of PDA@Fe3O4/mGC electrode
17
benefits from the interaction between the magnetic NPs and the mGC electrode, which allows the
18
direct electrodeposition of Pb(II) and Cd(II) on the modified electrode.
19
Table 1 should be inserted here.
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20
At the same time, the proposed method was also performed for simultaneous measurement of
21
Pb2+ and Cd2+. As shown in Fig. 6, the modified electrode shows stripping peaks at −0.518 V and
22
−0.732 V for Pb(II) and Cd(II), respectively. The anodic peak potential difference between Pb(II)
12
Page 12 of 29
and Cd(II) is up to more than 200 mV. Thus, their peaks are well-separated; suggesting that
2
simultaneous determination of Pb(II) and Cd(II) can be achieved without interference with each
3
other. Furthermore, as seen (inset), the sensitivities for simultaneous determination of Pb(II) and
4
Cd(II) were comparable to that of individual measurements, implying that simultaneous
5
measurement of Pb(II) and Cd(II) is feasible.
6
Figure 6 should be inserted here.
7
3.4. Selectivity, reproducibility and stability
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The influence of various substances on the determination of 100 nM Pb2+ and Cd2+ was studied.
9
The tolerance limit was estimated to less than 5% of the error. It was found that 1000-fold K+, Na+,
10
NH4+, Ca2+, Mg2+, Cl–, Br–, NO3– and SO42–, and 500-fold each of Cu2+, Zn2+, Fe2+, Ni2+, Hg2+,
11
Al3+, Fe3+, and Ni3+ caused no obvious changes of voltammetric signals for Cd2+ and Pb2+,
12
indicating an excellent anti-interference ability of the proposed method.
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The reproducibility of PDA@Fe3O4/mGC electrode was investigated through repetitive
14
electrodeposition and stripping of Pb(0). Fig. 7 shows the results after continuous cycling for 30
15
times. The peak current of the electrode was found nearly constant with the relative standard
16
deviation (RSD) from 0.3% to 0.7%, indicating that the modified electrode displayed an
17
acceptable reproducibility. To verify the stability of PDA@Fe3O4/mGC electrode, the same
18
electrode had been used for several days, and SWASV for sensing of 100 mM Pb2+ was performed
19
every day under the same experimental condition. The peak position does not change and only
20
5.2% decrease in the current for lead stripping peak was noticed after one week.
21
Figure 7 should be inserted here.
22
3.5. Real sample analysis
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Page 13 of 29
The proposed method was successfully applied to the determination of lead in aqueous effluent
2
from Tianneng Battery (Wuhu) Co., LTD. using standard addition method. The accuracy of the
3
method was checked by means of comparison with those data (provided by the environmental
4
protection bureau of Wuhu City) obtained from inductively coupled plasma atomic emission
5
spectrometry (ICP-AES). The assessment by Student’s t-test did not show a statistical significant
6
difference between the methods used (95%), thus confirming the accuracy. The results were listed
7
in Table 2.
8
Table 2 should be inserted here.
9
4. Conclusion
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In this investigation we demonstrate that Fe3O4 NPs can be facilely coated by PDA though a
11
room-temperature ligand exchange reaction and subsequent the oxidation of dopamine in weak
12
alkali solution by mildly stirring. Combining the advantageous features of high affinity for heavy
13
metal ions provided by PDA and the magnetism of Fe3O4 NPs, a stable PDA@Fe3O4 film was
14
constructed on the surface of mGC electrode conveniently and without a further immobilization
15
[43−47]. Due to the adsorption toward Pb2+ and Cd2+ of the PDA@Fe3O4 nanoparticles, the
16
modified mGC electrode can be used for simultaneous detection of those metal ions. Compared
17
with the reported electrochemical methods for sensing of Pb2+ and Cd2+, this method possesses
18
higher stability, lower cost, better anti-interference ability and higher sensitivity. Thus, the
19
PDA@Fe3O4/mGC electrode is expected to develop the heavy metal ion sensors for practical
20
applications.
21
Acknowledgments
22
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M. Li is grateful for financial support from the Natural Science Foundation of China (Grant No.
14
Page 14 of 29
21075001) and Anhui Provincial Natural Science Foundation (Grant No. 11040606M46). The
2
authors gratefully acknowledge Dr. Yan Wei (Wannan Med. Coll., Dept. Chem.) for her help in the
3
paper preparing.
4
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Figure captions
2
Fig. 1. (A) SEM image and (B) XRD pattern of the as-prepared Fe3O4 NPs.
3
Fig. 2. Schematic illustration of ligand exchange from oleic to dopamine on Fe3O4 NPs.
4
Fig. 3. TEM images of (A) DA@Fe3O4 NPs, (B) PDA@Fe3O4 NPs and (C) separation of
5
PDA@Fe3O4 NPs by a magnetic glassy carbon (mGC) electrode.
6
Fig. 4. Square wave voltammograms of (a) bare, (b) Fe3O4, (c) DA@Fe3O4 and (d)
7
PDA@Fe3O4-modified mGC electrodes in a 0.1 M pH 5.0 NaAc-HAc solution containing of 100
8
nM Pb2+.
9
Fig. 5. The stripping voltammograms of PDA@Fe3O4/mGC electrode for the individual analysis
10
of (a) Pb2+ over a concentration range of 5.0 to 600 nM and (b) Cd2+ over a concentration range of
11
20 to 590 nM in a 0.1 M pH 5.0 NaAc-HAc solution. The insets show the plot of the SWASV
12
peak current vs. the corresponding metal ions concentration.
13
Fig. 6. The stripping voltammograms of PDA@Fe3O4/mGC electrode for simultaneous
14
measurements of Pb(II) and Cd(II) under optimum conditions. Concentrations: Pb(II), 10 − 700
15
nM; Cd(II), 50 − 600 nM. Inset, the corresponding plots of stripping peaks as a function of
16
concentration.
17
Fig. 7. Selected stripping voltammograms using one PDA@Fe3O4/mGC electrode for repetitive
18
electrodeposition and stripping of Pb(0) in 0.1 M pH 5.0 NaAc-HAc solution containing of 50 nM
19
Pb2+. The top shows the corresponding plot of peak current as a function of the cyclic times.
Ac ce
pt
ed
M
an
us
cr
ip t
1
19
Page 19 of 29
1
Table 1
2
Sensitivity, detection limit (DL), and linear range (LR) of different modified electrodes for sensing
3
of Pb(II) and Cd(II) Sensitivity (Pb; Cd) / μA (μg L−1)−1
DL (Pb; Cd) /μg L−1
LR (Pb; Cd) / μg L−1
Reference
ANs/AuDE a
1.0952; −
0.2; −
5.0−60; −
[36]
SbFME b
−; −
3.1; 1.9
[37]
Bi-P-SPCE c
−; −
HGNPs/SPE d
0.014; 0.0073
0.03; 0.34 3; −
ETO/GCE e
−; −
0.0346; −
20−100; 20−100 0.05−30; 1−30 5–100; 5–100 0.1−20; −
Bi/Nafion/DMcT-PANI/MWCNTs/GCE
1.63; 3.44
0.04; 0.01 0.0029; 0.01
0.08−31; 0.02−20 1.0−120; 2.2−64.9
[41]
PDA@Fe3O4/mGCE
M
1.12; 1.74
cr
us
an
f
ip t
Electrodes
a
ANs/AuDE: Au nanowire modified Au-disc electrode.
5
b
SbFME: antimony film microelectrode.
6
c
Bi-P-SPCE: bismuth-coated porous screen-printed carbon electrode.
7
d
HGNPs/SPE: heated graphite nanoparticle-based screen-printed electrode.
8 9 10
e
[39] [40]
This work
Ac ce
pt
ed
4
[38]
ETO/GCE: etodolac modified glassy carbon electrode. Bi/Nafion/DMcT-PANI/MWCNTs/GCE: polyaniline, 2,5-dimercapto-1,3,4-thiadiazole, and multiwalled carbon nanotubes modified glassy carbon electrode with Nafion. f
20
Page 20 of 29
1
Table 2
2
Validation of the method for determination of lead in aqueous effluent. Concentration of lead found b (μg L−1) Samples a
1
92.2±3.5
93.1±2.0
2
100.7±2.7
101.0±1.8
3
98.9±1.8
100.1±0.8
0.59 0.19
cr
ICP-AES
3.27
us
Proposed method (μM)
ip t
t calculated
3
a
4
emission spectrometry.
5
b
an
Confidence interval,95%. (n−1)=2; t tabled = 4.303. ICP-AES, inductively coupled plasma atomic
Ac ce
pt
ed
M
The results are expressed as mean value ± S.D. based on three replicates (n = 3) determinations.
21
Page 21 of 29
Figure 1
cr
ip t
1
Fig. 1. (A) SEM image and (B) XRD pattern of the as-prepared Fe3O4 NPs.
pt
4
Ac ce
3
ed
M
an
us
2
22
Page 22 of 29
Figure 2
ip t
1
pt
ed
M
an
us
Fig. 2. Schematic illustration of ligand exchange from oleic to dopamine on Fe3O4 NPs.
Ac ce
3
cr
2
23
Page 23 of 29
Figure 3
us
cr
ip t
1
3
pt
ed
M
an
2
Fig. 3. TEM images of (A) DA@Fe3O4 NPs, (B) PDA@Fe3O4 NPs and (C) separation of
5
PDA@Fe3O4 NPs by a magnetic glassy carbon (mGC) electrode.
Ac ce
4
24
Page 24 of 29
Figure 4
25
a b
bare mGC electrode Fe3O4/mGC electrode
c
DA@Fe3O4/mGC electrode
20
d
PDA@Fe3O4/mGC electrode
ip t
d
15
5
cr
10 c
b a
0 -1.2
-0.8
0.0
0.4
an
2
-0.4 Potential / V
us
Current / A
1
Fig. 4. Square wave voltammograms of (a) bare, (b) Fe3O4, (c) DA@Fe3O4 and (d)
4
PDA@Fe3O4-modified mGC electrodes in a 0.1 M pH 5.0 NaAc-HAc solution containing of 100
5
nM Pb2+
Ac ce
pt
ed
M
3
25
Page 25 of 29
Figure 5
A
75 y=0.318+0.120x R=0.997
ip t
45 30
cr
Current / A
60
15 0 -0.8
-0.4 0.0 0.4 Potential / V
2
75
y=0.519+0.126x R=0.994
ed
60 45
pt
30 15
Ac ce
Current / A
1.2
M
B
0.8
an
-1.2
us
1
0
-1.2
3
-0.8
-0.4 0.0 0.4 Potential / V
0.8
1.2
4
Fig. 5. The stripping voltammograms of PDA@Fe3O4/mGC electrode for the individual analysis
5
of (a) Pb2+ over a concentration range of 5.0 to 600 nM and (b) Cd2+ over a concentration range of
6
20 to 590 nM in a 0.1 M pH 5.0 NaAc-HAc solution. The insets show the plot of the SWASV
7
peak current vs. the corresponding metal ions concentration.
26
Page 26 of 29
1
Figure 5
Pb(II)
60 Cd(II)
ip t cr
30 15
us
Current / A
45
0
2
-0.4 0.0 Potential / V
0.4
0.8
an
-0.8
Fig. 6. The stripping voltammograms of PDA@Fe3O4/mGC electrode for simultaneous
4
measurements of Pb(II) and Cd(II) under optimum conditions. Concentrations: Pb(II), 10 − 700
5
nM; Cd(II), 50 − 600 nM. Inset, the corresponding plots of stripping peaks as a function of
6
concentration.
Ac ce
pt
ed
M
3
27
Page 27 of 29
1
Figure 7
16
ip t
8 4
cr
Current / A
12
2
3
6
9
12 15 18 21 24 27 30 33 Stripping Number
an
0
us
0
Fig. 7. Selected stripping voltammograms using one PDA@Fe3O4/mGC electrode for repetitive
4
electrodeposition and stripping of Pb(0) in 0.1 M pH 5.0 NaAc-HAc solution containing of 50 nM
5
Pb2+. The top shows the corresponding plot of peak current as a function of the cyclic times.
ed pt Ac ce
6
M
3
28
Page 28 of 29
1
Bifunctional polydopamine@Fe3O4 core-shell nanoparticles for
2
electrochemical determination of Lead (II) and Cadmium (II)
3
Qian Song, Maoguo Li*, Li Huang, Qikang Wu, Yunyou Zhou*, and Feng Gao
9
pt Ac ce
5 6 7 8
ed
M
an
us
cr
ip t
4
29
Page 29 of 29