Accepted Manuscript Title: Phosphomolybdate-doped-poly(3,4ethylenedioxythiophene) coated gold nanoparticles: Synthesis, characterization and electrocatalytic reduction of bromate Author: Syeda Sara Hassan Yuping Liu Sirajuddin Amber Rehana Solangi Alan M. Bond Jie ZhangTel.: +61 3 99051338; fax: +61 3 99054597. PII: DOI: Reference:
S0003-2670(13)00563-1 http://dx.doi.org/doi:10.1016/j.aca.2013.04.036 ACA 232538
To appear in:
Analytica Chimica Acta
Received date: Revised date: Accepted date:
21-3-2013 17-4-2013 21-4-2013
Please cite this article as: S.S. Hassan, Y. Liu, A.R. Solangi, A.M. Bond, J. Zhang, Phosphomolybdate-doped-poly(3,4-ethylenedioxythiophene) coated gold nanoparticles: Synthesis, characterization and electrocatalytic reduction of bromate, Analytica Chimica Acta (2013), http://dx.doi.org/10.1016/j.aca.2013.04.036 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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Highlights Stable and water soluble phosphomolybdate-doped-PEDOT coated gold nanoparticles were synthesized.
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Electrodes modified with these nanoparticles show well defined voltammetric response and excellent stability in acidic media.
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These nanocomposite catalysts exhibit an excellent catalytic activity towards electroreduction of bromate.
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Phosphomolybdate-doped-poly(3,4-ethylenedioxythiophene) coated
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gold nanoparticles: Synthesis, characterization and electrocatalytic
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reduction of bromate
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Syeda Sara Hassana,b, Yuping Liua, Sirajuddinb, Amber Rehana Solangib, Alan M. Bonda, * and Jie
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Zhanga, **
b
School of Chemistry, Monash University, VIC 3800, Australia
National Center of Excellence in Analytical Chemistry, University of Sindh, Jamshoro 76080, Pakistan
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* Corresponding author. Tel.: +61 3 99051338; fax: +61 3 99054597.
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** Corresponding author. Tel.: +61 3 99056289; fax: +61 3 99054597.
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E-mail addresses: [email protected] (A.M. Bond), [email protected] (J. Zhang).
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Abstract Phosphomolybdate (PMo12)-doped-poly(3,4-ethylenedioxythiophene) (PEDOT) coated
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gold nanoparticles have been synthesized in aqueous solution by reduction of AuCl4- using
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hydroxymethyl EDOT as a reducing agent in the presence of polystyrene sulfonate and
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PMo12. The resulting PMo12-doped-PEDOT stabilized Au nanoparticles are water soluble and
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have been characterized by UV-visible spectroscopy, scanning electron microscopy and
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electrochemistry. Glassy carbon electrodes modified with these Au nanoparticles show
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excellent stability and catalytic activity towards the reduction of bromate in an aqueous
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electrolyte solution containing 10 mM H2SO4 and 0.1 M Na2SO4.
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Keywords
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Polyoxometalate,
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modified electrode, Electrocatalysis, Bromate
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Gold
nanoparticles,
Chemically
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Poly(3,4-ethylenedioxythiophene),
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1. Introduction Chemically
modified
electrodes,
whose
conducting/semiconducting
surfaces
are
functionalized to introduce desirable properties, have received much attention since they were
48
first reported in the 1970s [1-3]. Due to the presence of functional groups on the electrode
49
surface, both the selectivity and sensitivity, the two key analytical performance indicators, of
50
the chemically modified electrodes can be improved significantly, making them ideal
51
candidates for analytical chemistry applications.
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Electrodes modified with polyoxometalates (POMs) have found several key applications [4,
53
5] owing to their attractive electrochemical and catalytic properties [6]. To fabricate this kind
54
of modified lectrodes, numerous strategies have been used for immobilization of POMs onto
55
electrode surfaces, such as adsorption [7, 8], electrochemical deposition [9, 10], layer-by-
56
layer assembly [11-14], entrapment [15, 16], drop casting of insoluble POM salts [17], and
57
incorporation of POMs into conducting polymers as dopant anions [18]. The latter approach
58
offers several advantages, such as (1) high stability, due to strong electrostatic interaction
59
between the highly negatively charged POMs and the positive charged conducting polymer
60
backbone, (2) excellent electrical communication between POMs and electrode surface
61
through conducting polymer, and thus (3) excellent catalytic activity and stability of the
62
resulted modified electrodes. It has been reported that the electrodes modified with Keggin
63
structured
64
poly(3,4ethylenedioxythiophene) (PEDOT) or polypyrrole, exhibits excellent stability and
65
electrocatalytic activity for oxidation of ascorbic acid [19-22], and reduction of bromate
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(BrO3-) [23, 24], Cr(VI) [25] and nitrite [26]. Fernandes et al [27] showed that electrodes
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modified with polyoxotungstates doped-PEDOT, synthesized electrochemically, exhibit high
68
electrochemical stability. Unfortunately, POM doped conducting polymers are usually
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silicomolybdate-doped
conducting
polymers,
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insoluble making it difficult for electrode fabrication with high reproducibility using simple
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strategies, such as drop casting, especially when the loading of the polymer is low.
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Electrochemical polymerization in the presence of a POM may provide better batch to batch
72
reproducibility, but is very time consuming. To overcome these drawbacks, in this paper, we
73
report a new method of preparing water soluble Keggin structured phosphomolybdate
74
(PMo12)-doped-PEDOT (PMo12= H3PMo12O40) using chloroauric acid as an oxidising reagent
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for the oxidative polymerization of hydroxymethyl EDOT monomer in the presence of
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polystyrene sulfonate and PMo12. During oxidative formation of PEDOT, chloroauric acid is
77
reduced to metallic Au nanoparticles. This new composite material is then used for the
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fabrication of modified electrodes for the detection of bromate ion based on the principle of
79
catalytic reduction of the bromate ion in aqueous electrolyte media, by electrogenerated
80
reduced forms of PMo12. The electrocatalytic reduction of bromate was chosen as a model
81
system for this study to demonstrate the advantage of the new materials since (1) bromate,
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regularly used as a food additive, exerts nephrotoxic and ototoxic property to both animals
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and human beings, thus is required frequent monitoring [28-33] and (2) different POMs have
84
been used to modify electrodes employing different modification strategies and shown some
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promising results [34-45].
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2. Experimental
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2.1. Reagents
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The following chemicals were used as received: gold chloride (≥ 99%, HAuCl4), sodium
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polystyrene sulfonate (PSS) and phosphomolybdate (PMo12= H3PMo12O40, ≥ 99.99%) (all
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from Sigma Aldrich); sodium sulphate (≥ 99.0%) and sodium bromate (≥ 99%) (both from
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BDH); sulphuric acid (95-97%, Ajax); acetic acid (AnalaR, BDH); sodium acetate (AnalaR,
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BDH); hydroxymethyl EDOT (99%, ACROS Organics monomer). Water purified with a
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Milli Q system was used throughout the experiments.
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2.2. Synthesis of PMo12-doped-PEDOT stabilized Au nanoparticles and PEDOT
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stabilized Au nanoparticles
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To Synthesize PMo12-doped-PEDOT coated Au nanoparticles (AuNps), 0.5 mg mL-1 PSS,
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1 mM AuCl4- and 0.5 mM PMo12 were dissolved in water. This solution was then
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magnetically stirred for 10 minutes before addition of hydroxymethyl EDOT (final
101
concentration is 2 mM, which is slightly in excess to ensure the complete reduction of AuCl4-
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to metallic Au). The colour of the aqueous solution turned purple upon addition of
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hydroxymethyl EDOT, indicating the formation of AuNps. The PMo12-doped-PEDOT coated
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AuNps were then separated from the reaction media using a Mini Spin Plus Eppendorf
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centrifuge at a rotation rate of 8000 RPM for 10 minutes. After the removal of the
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supernatant, the solid precipitate was dissolved in water (10% of the volume of the original
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solution) and used as the stock solution. PEDOT stabilized Au nanoparticles used for control
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experiments were synthesized in a similar way, but without addition of PMo12.
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2.3. Preparation of modified electrodes
2 μL of the AuNps homogenized colloidal solution (10 times dilution of the stock solution)
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was drop casted onto a 3 mm diameter glassy carbon electrode (GCE) and dried under
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atmospheric conditions. This loading of the PMo12-doped-PEDOT coated AuNps was chosen
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since the resulted modified electrodes produce a highest signal-to-background charging
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current ratio in the presence of analyte BrO3- on the experimental timescale, thus is optimal in
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terms of analytical applications, since under this loading condition, the density of the PMo12
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sites on the electrode surface is just enough to turn the electrode process into a mass transport
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controlled process.
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2.4. Apparatus and Procedures
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2.4.1. Electrochemistry
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Voltammograms were acquired at 22 ± 2 ºC using a BAS 100B electrochemical
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workstation with a conventional three electrode electrochemical cell, consisting of a glassy
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carbon working electrode (GCE, 3 mm diameter), a platinum wire auxiliary electrode, and a
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Ag/AgCl (3 M NaCl) reference electrode. The GCE was polished with 0.3 μm Al2O3 slurry,
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sonicated in water for about 10 seconds, washed with water and then dried with a high purity
127
nitrogen gas immediately before use. Solutions were initially degassed for at least 3 min with
128
high-purity nitrogen for complete removal of O2, and then the electrochemical cell was kept
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under a slightly positive pressure of nitrogen during the experiments.
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2.4.2. Scanning Electron Microscopy (SEM)
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SEM images of the POMs-doped PEDOT coated AuNps films deposited on ITO slides
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were obtained with a JEOL JEM 7001 FEGSEM field-emission SEM instrument using
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accelerating voltages of 15 kV. Energy dispersive X-ray (EDX) spectra of the film were also
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taken with this electron microscope.
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2.4.3. UV-visible Spectroscopy
UV-vis spectra of the PMo12-doped-PEDOT coated AuNps dissolved in water were obtained using a Cary 5000UV-VIS-NIR Spectrophotometer.
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3. Results and Discussion
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3.1. UV-visible spectroscopic and scanning electron microscopic characterization of
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PMo12-doped-PEDOT coated AuNps The UV-vis spectrum of the PMo12-doped-PEDOT coated AuNps (Fig. 1) shows a broad
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absorption band having maximum absorbance at a wavelength of 534 nm. This absorption
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arises from the plasmon resonant band of AuNps [46, 47].
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SEM images were obtained to investigate the morphology and size of the coated AuNps in
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the hybrid materials. Fig. 2a and 2b show representative SEM images of PMo12-doped-
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PEDOT coated AuNps at different magnifications. Core shell structured particles of an
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overall diameter of ~ 100 nm are presented with the bright core being metallic AuNps
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(diameter of ~50 nm) and the grey shell being the PMo12-doped-PEDOT. The EDX spectrum
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(Fig. 2c) confirms the presence of Au (from AuNps) and Mo (from PMo12) as expected.
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3.2. Electrochemistry of the PMo12-doped-PEDOT coated AuNps modified glassy
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carbon electrode in acidic media
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Voltammetric experiments (Fig. 3) were undertaken in aqueous 0.1M H2SO4 solution using
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the modified glassy carbon electrodes. Significantly larger capacitance current compared to
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bare glassy carbon electrode observed in the potential region of > 400 mV is mainly due to
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the presence of highly conductive PEDOT. Three well defined processes (I/I', II/II' and III/III')
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were observed in the potential region of −160 to +500 mV vs Ag/AgCl, implying that PMo12
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remains stable during the synthesis of AuNps. The reversible potential (E0') of these
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processes, taken as the average of the oxidation and reduction peak potentials, are 280 mV
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(I/I'), 142 mV (II/II') and -89 mV (III/III'), respectively. The peak-to-peak separations for all
164
processes are close to zero and the peak currents associated with all processes are
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proportional to the scan rates in the range from 10 mV s-1 to 1 V s-1, suggesting that these
166
processes are reversible and surface confined on the timescale of measurements [48]. Based
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on the ratios of the areas underneath each peak, the three processes were assigned to one-
168
electron, two-electron and one-electron transfer processes as described by equations (1) - (3):
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[PMo12O40]3- (surf) + H+ (soln) + e-
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[H+][PMo12O40]3- (surf) + 2 H+ (soln) + 2 e-
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[H+]3[PMo12O40]3- (surf) + H+ (soln) + e-
[H+][PMo12O40]3-(surf)
(1)
[H+]3[PMo12O40]3-(surf)
(2)
[H+]4[PMo12O40]3-(surf)
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(3)
In principle, these processes also could be assigned to two-electron, four-electron and two-
173
electron transfer processes or any other combination which give a 1:2:1 ratio between the
174
peak areas. However, the processes described by equations (1) - (3) are chemically most
175
likely. Since PMo12 is confined on the electrode surface, transfer of protons from the aqueous
176
phase to the electrode surface is required to achieve charge neutrality.
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During the course of the voltammetric measurements, in this 0.1 M aqueous H2SO4
178
medium, a small decrease of peak current was detected upon cycling of the potential at a scan
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rate of 10 mV s-1 (Fig. 4a), suggesting slow dissolution of reduced PMo12 occurs.
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The modified electrode shows comparable stability in an aqueous solution containing 10
181
mM H2SO4 and 0.1 M Na2SO4. However, all processes in the more acidic solution are shifted
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in the negative potential direction by nearly 60 mV to give the reversible potentials of 231mV
183
(I/I'), 83mV (II/II') and -146 mV (III/III'), as expected on the basis of the Nernst equation
184
since equal number of protons and electrons are involved in these processes (equations (1) -
185
(3)). In contrast, in pH 3.6 acetate buffer medium, although the magnitude of the shift of
186
reversible potentials is consistent with the reaction scheme described in equations (1) - (3),
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the modified electrode shows significant instability. The proton transfer into the film
188
(equations (1) - (3)), required to maintain charge neutrality, is no longer facile due to the low
189
proton concentration. Therefore, charge neutralization is achieved through the dissolution of
190
PMo12 instead. The reversible potential data of all processes obtained in different aqueous
191
electrolyte media are summarized in Table 1 for ease of comparison.
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3.3. Electrocatalytic activity of the PMo12-Doped-PEDOT coated AuNps modified
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electrode towards the reduction of BrO3Several studies have shown that POMs in different redox levels are able of rapidly
196
reducing other species, including BrO3-, at a much lower over potential than in the absence of
197
POMs and hence acting as molecular catalysts [19-27, 34-38, 49]. The catalytic activity of
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the modified electrode towards BrO3- reduction was therefore examined. In aqueous 0.1 M
199
H2SO4 media (Fig. 5), the one-electron reduced form of PMo12 generated from process I is
200
essentially inactive towards BrO3- reduction. In contrast, the second reduced form,
201
[H+]3[PMo12O40]3- (surf), generated from process II induces considerable catalytic activity.
202
The catalytic reduction current increases when the concentration of BrO3- increases from 0.25
203
to 3 mM. Surprisingly, the third reduced from of PMo12, [H+]4[PMo12O40]3- (surf), generated
204
from the process III also shows negligible activity towards BrO3- reduction, evidenced by the
205
observation of the oxidation peak in the reverse scan of potential, even though it’s a stronger
206
reductant compared to the one generated from process II. This is not due to the depletion of
207
BrO3- by [H+]3[PMo12O40]3- (surf) since the catalytic process is clearly kinetically controlled
208
(the catalytic current detected is much smaller than the diffusion-controlled current even if
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BrO3- reduction is limited by a one-electron transfer step in the timescale of measurement).
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The origin of this observation is unknown. No catalytic processes were observed in the
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controlled experiments undertaken using PEDOT stabilized Au nanoparticle modified
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electrodes. Therefore, catalytic current detected is due to PMo12 rather than Au nanoparticles
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or PEDOT.
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In aqueous media containing 10 mM H2SO4 and 0.1 M Na2SO4 (Fig. 6), the first reduced
215
form of PMo12 remains inactive. However, both the second and third reduced forms show
216
catalytic activities in contrast with those observed in the aqueous 0.1 M H2SO4 medium. The
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third reduced form is more active than the second one, judging by the relative sizes of the
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catalytic current. This may be expected since the product of the third process,
219
[H+]4[PMo12O40]3- (surf), is the stronger reducing agent. This result in drastic contrast with
220
the results obtained in 0.1 M H2SO4 media where the third reduced form is essentially
221
inactive (Fig. 5). The mechanism for this observation is unknown. Again, control experiments
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were undertaken using PEDOT stabilized Au nanoparticle modified electrodes to rule out the
223
possibility that the observed catalytic activity is due to either Au nanoparticles or PEDOT.
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The catalytic activity of the modified electrode was not tested in pH 3.6 sodium acetate
225
buffer due to the instability of the modified electrode described above (Fig. 4c).
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Consequently, aqueous medium containing 10 mM H2SO4 and 0.1 M Na2SO4 is the optimal
227
electrolyte for BrO3- detection in terms of both electrode stability and catalytic activity. The
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catalytic reduction currents associated with the third process are plotted as a function of the
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concentration of BrO3- in the inset to Fig. 6. The catalytic current approaches a maximum
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value in the presence of high concentrations of BrO3- as expected, since catalytic process will
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eventually becomes rate limiting step when the concentration of the analyte is sufficiently
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high [50]. The peak current versus [BrO3-] plot shows good linearity when [BrO3-] ≤ 3 mM.
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At higher concentration, this plot deviates from linearity (smaller slope). This could be due to
234
the effect of uncompensated resistance (iRu effect) or the fact that the process may be
235
approaching a kinetic limitation as predicted by the Michaelis-Menten equation [51]. The
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slope (sensitivity) of 68 µA cm-2 mM-1 is comparable to those obtained with similar modified
237
electrodes (Table 2).
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4. Conclusions
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Water soluble and highly stable PMo12-doped-PEDOT coated AuNps have been
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synthesized. The PMo12 anions serve as stabilizer together with PEDOT and PSS for the
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formation of AuNps, and the dopant anion for PEDOT and provide additional stability and
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functionality to this new composite material. This PMo12-doped-PEDOT coated AuNps
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modified glassy carbon electrode shows excellent stability during the course of voltammetric
245
measurements and exhibits high catalytic activity towards the reduction of BrO3- in aqueous
246
electrolyte medium containing 0.01 M H2SO4 and 0.1 M Na2SO4.
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Acknowledgements
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Syeda Sara Hassan gracefully acknowledges the Higher Education Commission of Pakistan
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for the financial support to conduct part of her Ph.D research in Australia in the School of
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Chemistry, Monash University. The authors also thank the Monash Centre for Electron
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Microscopy for assistance in obtaining the SEM images.
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[39] X. Xi, S. Dong, Electrochim. Acta, 40 (1995) 2785-2790. [40] S. Dong, L. Cheng, X. Zhang, Electrochim. Acta, 43 (1997) 563-568. [41] Y. Li, W. Bu, L. Wu, C. Sun, Sensor. Actuat. B Chem., 107 (2005) 921-928. [42] A. Balamurugan, S.M. Chen, Electroanalysis, 19 (2007) 1616-1622. [43] A.Z. Ernst, L. Sun, K. Wiaderek, A. Kolary, S. Zoladek, P.J. Kulesza, J.A. Cox, Electroanalysis, 19 (2007) 2103-2109. [44] M. Zhou, L. Guo, F. Lin, H. Liu, Anal. Chim. Acta, 587 (2007) 124-131. [45] M. Skunik, P.J. Kulesza, Anal. Chim. Acta, 631 (2009) 153-160. [46] Y. Wang, I.A. Weinstock, Chem. Soc. Rev., 41 (2012) 7479-7496. [47] R. Pacios, R. Marcilla, C. Pozo-Gonzalo, J.A. Pomposo, H. Grande, J. Aizpurua, D. Mecerreyes, J. Nanosci. Nanotechnol., 7 (2007) 2938-2941. [48] J. Zhang, S.-X. Guo, A.M. Bond, M.J. Honeychurch, K.B. Oldham, J. Phys. Chem. B, 109 (2005) 8935-8947. [49] S. Dong, W. Jin, J. Electroanal. Chem., 354 (1993) 87-97. [50] A. Rogers, Y. Gibon, in: J. Schwender (Ed.), Plant Metabolic Networks, Springer New York, 2009, p. 71-103. [51] P.W. Atkins, J. De Paula, Atkins' Physical Chemistry, eighth ed., Oxford : Oxford University Press 2006.
us
306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324
an
325
329 330 331 332 333 334 335 336 337
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327
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326
338 339 340 341 342
Page 15 of 25
15 343
Table 1. Reversible potential data for the PMo12 reduction processes in different aqueous electrolyte media. E0' / mV
Electrolyte Medium (I/I')
(II/II')
(III/III')
0.1 M H2SO4
280
142
-89
10 mM H2SO4 + 0.1 M Na2SO4
231
83
-146
sodium acetate buffer pH 3.6
143
22
-241
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344 345 346
Table 2. Linear range and sensitivity of polyoxometalate modified electrodes for BrO3-
an
detection. Electrolyte media
Linear range (μM)
slope (μA cm-2/mM)
Reference
PMo12/ PEDOT/AuNP/GC
0.1 M H2SO4
250-3000
68
(this work)
PMo12/CNTs/PPya
0.5M H2SO4
100-2000
117
[45]
PMo12/CNTs/PEDOT
0.5M H2SO4
100-2000
32
[45]
SiMo12O404-/PEDOT
0.2M H2SO4
30-8000
71
[42]
P2Mo18O626-/OMC/GCb
1 M H2SO4
2.77–4000
87
[44]
Nd(SiMo7W4)213-/PPy
0.1M Na2SO4, pH 1.23
990-31500
27.5
[40]
b.
d
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Electrode composition
CNT = carbon nanotube; PPy = polypyrrole; OMC = ordered mesoporous carbon
Page 16 of 25
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Fig. 1. UV-visible absorbance spectrum and photographic image of PMo12-doped-PEDOT coated AuNps.
Fig. 2. SEM images (a, scale bar is 1 μm) and (b, scale bar is 100 nm) with different magnifications, and EDX spectrum (c) of PMo12-doped PEDOT coated AuNps.
Page 17 of 25
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Fig. 3. (a) Cyclic voltammograms derived from a PMo12-doped-PEDOT coated AuNps modified GCE in 0.1 M H2SO4 electrolyte at scan rates of 10 (shown in red), 50, 100, 200, 500 and 1000 mV s-1 (from inside to outside). (b) Effect of scan rate on the background subtracted reduction peak current for the first reduction process.
Ac ce pt e
Fig. 4. Cyclic voltammograms derived from a PMo12-doped-PEDOT coated AuNps modified GCE in aqueous (a) 0.1 M H2SO4, (b) 0.01 M H2SO4 + 0.1 M Na2SO4 buffer media, and (c) pH 3.6, 0.1 M sodium acetate buffer media, obtained at a scan rate of 10 mV s-1.
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Fig. 5. Cyclic voltammograms derived from a PMo12-doped-PEDOT coated AuNps modified GCE obtained in aqueous 0.1 M H2SO4 solution in the absence (a) and presence of 0.25 mM (b), 0.5 mM (c), 1mM (d), 2 mM (e) and 3 mM (f) of BrO3-. The result obtained from a control experiment undertaken in 0.1 M H2SO4 solution in the presence of 3 mM of BrO3- using a PEDOT stabilized AuNps modified GCE (dashed line) is shown for comparison. Scan rate = 10 mV s-1.
Fig. 6. Cyclic voltammograms derived from PMo12-doped-PEDOT coated AuNps modified glassy carbon electrode in aqueous media containing 10 mM H2SO4 and 0.1 M Na2SO4 in the absence (a) and presence of 0.25 mM (b), 0.5 mM (c), 1 mM (d), 2 mM (e), 3 mM (f), 4 mM (g) and 5 mM (h) of BrO3-. The result obtained from a control experiment undertaken in 10 mM H2SO4 and 0.1 M Na2SO4 solution in the presence of 3 mM of BrO3- using a PEDOT stabilized AuNps modified GCE (dashed line) is shown for comparison. Scan rate = 10 mV s-1.
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S0003-2670(13)00563-1 http://dx.doi.org/doi:10.1016/j.aca.2013.04.036 ACA 232538
To appear in:
Analytica Chimica Acta
Received date: Revised date: Accepted date:
21-3-2013 17-4-2013 21-4-2013
Please cite this article as: S.S. Hassan, Y. Liu, A.R. Solangi, A.M. Bond, J. Zhang, Phosphomolybdate-doped-poly(3,4-ethylenedioxythiophene) coated gold nanoparticles: Synthesis, characterization and electrocatalytic reduction of bromate, Analytica Chimica Acta (2013), http://dx.doi.org/10.1016/j.aca.2013.04.036 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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1
Highlights Stable and water soluble phosphomolybdate-doped-PEDOT coated gold nanoparticles were synthesized.
4 5
Electrodes modified with these nanoparticles show well defined voltammetric response and excellent stability in acidic media.
6 7
These nanocomposite catalysts exhibit an excellent catalytic activity towards electroreduction of bromate.
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2
8
Phosphomolybdate-doped-poly(3,4-ethylenedioxythiophene) coated
9
gold nanoparticles: Synthesis, characterization and electrocatalytic
10
reduction of bromate
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11 12
Syeda Sara Hassana,b, Yuping Liua, Sirajuddinb, Amber Rehana Solangib, Alan M. Bonda, * and Jie
14
Zhanga, **
b
School of Chemistry, Monash University, VIC 3800, Australia
National Center of Excellence in Analytical Chemistry, University of Sindh, Jamshoro 76080, Pakistan
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* Corresponding author. Tel.: +61 3 99051338; fax: +61 3 99054597.
28
** Corresponding author. Tel.: +61 3 99056289; fax: +61 3 99054597.
29 30
E-mail addresses: [email protected] (A.M. Bond), [email protected] (J. Zhang).
31
Page 3 of 25
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31 32
Abstract Phosphomolybdate (PMo12)-doped-poly(3,4-ethylenedioxythiophene) (PEDOT) coated
34
gold nanoparticles have been synthesized in aqueous solution by reduction of AuCl4- using
35
hydroxymethyl EDOT as a reducing agent in the presence of polystyrene sulfonate and
36
PMo12. The resulting PMo12-doped-PEDOT stabilized Au nanoparticles are water soluble and
37
have been characterized by UV-visible spectroscopy, scanning electron microscopy and
38
electrochemistry. Glassy carbon electrodes modified with these Au nanoparticles show
39
excellent stability and catalytic activity towards the reduction of bromate in an aqueous
40
electrolyte solution containing 10 mM H2SO4 and 0.1 M Na2SO4.
41
Keywords
42
Polyoxometalate,
43
modified electrode, Electrocatalysis, Bromate
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Gold
nanoparticles,
Chemically
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Poly(3,4-ethylenedioxythiophene),
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1. Introduction Chemically
modified
electrodes,
whose
conducting/semiconducting
surfaces
are
functionalized to introduce desirable properties, have received much attention since they were
48
first reported in the 1970s [1-3]. Due to the presence of functional groups on the electrode
49
surface, both the selectivity and sensitivity, the two key analytical performance indicators, of
50
the chemically modified electrodes can be improved significantly, making them ideal
51
candidates for analytical chemistry applications.
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47
Electrodes modified with polyoxometalates (POMs) have found several key applications [4,
53
5] owing to their attractive electrochemical and catalytic properties [6]. To fabricate this kind
54
of modified lectrodes, numerous strategies have been used for immobilization of POMs onto
55
electrode surfaces, such as adsorption [7, 8], electrochemical deposition [9, 10], layer-by-
56
layer assembly [11-14], entrapment [15, 16], drop casting of insoluble POM salts [17], and
57
incorporation of POMs into conducting polymers as dopant anions [18]. The latter approach
58
offers several advantages, such as (1) high stability, due to strong electrostatic interaction
59
between the highly negatively charged POMs and the positive charged conducting polymer
60
backbone, (2) excellent electrical communication between POMs and electrode surface
61
through conducting polymer, and thus (3) excellent catalytic activity and stability of the
62
resulted modified electrodes. It has been reported that the electrodes modified with Keggin
63
structured
64
poly(3,4ethylenedioxythiophene) (PEDOT) or polypyrrole, exhibits excellent stability and
65
electrocatalytic activity for oxidation of ascorbic acid [19-22], and reduction of bromate
66
(BrO3-) [23, 24], Cr(VI) [25] and nitrite [26]. Fernandes et al [27] showed that electrodes
67
modified with polyoxotungstates doped-PEDOT, synthesized electrochemically, exhibit high
68
electrochemical stability. Unfortunately, POM doped conducting polymers are usually
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silicomolybdate-doped
conducting
polymers,
Page 5 of 25
5
insoluble making it difficult for electrode fabrication with high reproducibility using simple
70
strategies, such as drop casting, especially when the loading of the polymer is low.
71
Electrochemical polymerization in the presence of a POM may provide better batch to batch
72
reproducibility, but is very time consuming. To overcome these drawbacks, in this paper, we
73
report a new method of preparing water soluble Keggin structured phosphomolybdate
74
(PMo12)-doped-PEDOT (PMo12= H3PMo12O40) using chloroauric acid as an oxidising reagent
75
for the oxidative polymerization of hydroxymethyl EDOT monomer in the presence of
76
polystyrene sulfonate and PMo12. During oxidative formation of PEDOT, chloroauric acid is
77
reduced to metallic Au nanoparticles. This new composite material is then used for the
78
fabrication of modified electrodes for the detection of bromate ion based on the principle of
79
catalytic reduction of the bromate ion in aqueous electrolyte media, by electrogenerated
80
reduced forms of PMo12. The electrocatalytic reduction of bromate was chosen as a model
81
system for this study to demonstrate the advantage of the new materials since (1) bromate,
82
regularly used as a food additive, exerts nephrotoxic and ototoxic property to both animals
83
and human beings, thus is required frequent monitoring [28-33] and (2) different POMs have
84
been used to modify electrodes employing different modification strategies and shown some
85
promising results [34-45].
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2. Experimental
88
2.1. Reagents
89
The following chemicals were used as received: gold chloride (≥ 99%, HAuCl4), sodium
90
polystyrene sulfonate (PSS) and phosphomolybdate (PMo12= H3PMo12O40, ≥ 99.99%) (all
91
from Sigma Aldrich); sodium sulphate (≥ 99.0%) and sodium bromate (≥ 99%) (both from
92
BDH); sulphuric acid (95-97%, Ajax); acetic acid (AnalaR, BDH); sodium acetate (AnalaR,
Page 6 of 25
6 93
BDH); hydroxymethyl EDOT (99%, ACROS Organics monomer). Water purified with a
94
Milli Q system was used throughout the experiments.
95
2.2. Synthesis of PMo12-doped-PEDOT stabilized Au nanoparticles and PEDOT
97
stabilized Au nanoparticles
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To Synthesize PMo12-doped-PEDOT coated Au nanoparticles (AuNps), 0.5 mg mL-1 PSS,
99
1 mM AuCl4- and 0.5 mM PMo12 were dissolved in water. This solution was then
100
magnetically stirred for 10 minutes before addition of hydroxymethyl EDOT (final
101
concentration is 2 mM, which is slightly in excess to ensure the complete reduction of AuCl4-
102
to metallic Au). The colour of the aqueous solution turned purple upon addition of
103
hydroxymethyl EDOT, indicating the formation of AuNps. The PMo12-doped-PEDOT coated
104
AuNps were then separated from the reaction media using a Mini Spin Plus Eppendorf
105
centrifuge at a rotation rate of 8000 RPM for 10 minutes. After the removal of the
106
supernatant, the solid precipitate was dissolved in water (10% of the volume of the original
107
solution) and used as the stock solution. PEDOT stabilized Au nanoparticles used for control
108
experiments were synthesized in a similar way, but without addition of PMo12.
110 111
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2.3. Preparation of modified electrodes
2 μL of the AuNps homogenized colloidal solution (10 times dilution of the stock solution)
112
was drop casted onto a 3 mm diameter glassy carbon electrode (GCE) and dried under
113
atmospheric conditions. This loading of the PMo12-doped-PEDOT coated AuNps was chosen
114
since the resulted modified electrodes produce a highest signal-to-background charging
115
current ratio in the presence of analyte BrO3- on the experimental timescale, thus is optimal in
116
terms of analytical applications, since under this loading condition, the density of the PMo12
Page 7 of 25
7 117
sites on the electrode surface is just enough to turn the electrode process into a mass transport
118
controlled process.
119
2.4. Apparatus and Procedures
121
2.4.1. Electrochemistry
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120
Voltammograms were acquired at 22 ± 2 ºC using a BAS 100B electrochemical
123
workstation with a conventional three electrode electrochemical cell, consisting of a glassy
124
carbon working electrode (GCE, 3 mm diameter), a platinum wire auxiliary electrode, and a
125
Ag/AgCl (3 M NaCl) reference electrode. The GCE was polished with 0.3 μm Al2O3 slurry,
126
sonicated in water for about 10 seconds, washed with water and then dried with a high purity
127
nitrogen gas immediately before use. Solutions were initially degassed for at least 3 min with
128
high-purity nitrogen for complete removal of O2, and then the electrochemical cell was kept
129
under a slightly positive pressure of nitrogen during the experiments.
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2.4.2. Scanning Electron Microscopy (SEM)
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SEM images of the POMs-doped PEDOT coated AuNps films deposited on ITO slides
133
were obtained with a JEOL JEM 7001 FEGSEM field-emission SEM instrument using
134
accelerating voltages of 15 kV. Energy dispersive X-ray (EDX) spectra of the film were also
135
taken with this electron microscope.
136 137 138 139
2.4.3. UV-visible Spectroscopy
UV-vis spectra of the PMo12-doped-PEDOT coated AuNps dissolved in water were obtained using a Cary 5000UV-VIS-NIR Spectrophotometer.
140 141
3. Results and Discussion
Page 8 of 25
8 142
3.1. UV-visible spectroscopic and scanning electron microscopic characterization of
143
PMo12-doped-PEDOT coated AuNps The UV-vis spectrum of the PMo12-doped-PEDOT coated AuNps (Fig. 1) shows a broad
145
absorption band having maximum absorbance at a wavelength of 534 nm. This absorption
146
arises from the plasmon resonant band of AuNps [46, 47].
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144
SEM images were obtained to investigate the morphology and size of the coated AuNps in
148
the hybrid materials. Fig. 2a and 2b show representative SEM images of PMo12-doped-
149
PEDOT coated AuNps at different magnifications. Core shell structured particles of an
150
overall diameter of ~ 100 nm are presented with the bright core being metallic AuNps
151
(diameter of ~50 nm) and the grey shell being the PMo12-doped-PEDOT. The EDX spectrum
152
(Fig. 2c) confirms the presence of Au (from AuNps) and Mo (from PMo12) as expected.
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3.2. Electrochemistry of the PMo12-doped-PEDOT coated AuNps modified glassy
155
carbon electrode in acidic media
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Voltammetric experiments (Fig. 3) were undertaken in aqueous 0.1M H2SO4 solution using
157
the modified glassy carbon electrodes. Significantly larger capacitance current compared to
158
bare glassy carbon electrode observed in the potential region of > 400 mV is mainly due to
159
the presence of highly conductive PEDOT. Three well defined processes (I/I', II/II' and III/III')
160
were observed in the potential region of −160 to +500 mV vs Ag/AgCl, implying that PMo12
161
remains stable during the synthesis of AuNps. The reversible potential (E0') of these
162
processes, taken as the average of the oxidation and reduction peak potentials, are 280 mV
163
(I/I'), 142 mV (II/II') and -89 mV (III/III'), respectively. The peak-to-peak separations for all
164
processes are close to zero and the peak currents associated with all processes are
165
proportional to the scan rates in the range from 10 mV s-1 to 1 V s-1, suggesting that these
166
processes are reversible and surface confined on the timescale of measurements [48]. Based
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Page 9 of 25
9 167
on the ratios of the areas underneath each peak, the three processes were assigned to one-
168
electron, two-electron and one-electron transfer processes as described by equations (1) - (3):
169
[PMo12O40]3- (surf) + H+ (soln) + e-
170
[H+][PMo12O40]3- (surf) + 2 H+ (soln) + 2 e-
171
[H+]3[PMo12O40]3- (surf) + H+ (soln) + e-
[H+][PMo12O40]3-(surf)
(1)
[H+]3[PMo12O40]3-(surf)
(2)
[H+]4[PMo12O40]3-(surf)
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(3)
In principle, these processes also could be assigned to two-electron, four-electron and two-
173
electron transfer processes or any other combination which give a 1:2:1 ratio between the
174
peak areas. However, the processes described by equations (1) - (3) are chemically most
175
likely. Since PMo12 is confined on the electrode surface, transfer of protons from the aqueous
176
phase to the electrode surface is required to achieve charge neutrality.
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During the course of the voltammetric measurements, in this 0.1 M aqueous H2SO4
178
medium, a small decrease of peak current was detected upon cycling of the potential at a scan
179
rate of 10 mV s-1 (Fig. 4a), suggesting slow dissolution of reduced PMo12 occurs.
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The modified electrode shows comparable stability in an aqueous solution containing 10
181
mM H2SO4 and 0.1 M Na2SO4. However, all processes in the more acidic solution are shifted
182
in the negative potential direction by nearly 60 mV to give the reversible potentials of 231mV
183
(I/I'), 83mV (II/II') and -146 mV (III/III'), as expected on the basis of the Nernst equation
184
since equal number of protons and electrons are involved in these processes (equations (1) -
185
(3)). In contrast, in pH 3.6 acetate buffer medium, although the magnitude of the shift of
186
reversible potentials is consistent with the reaction scheme described in equations (1) - (3),
187
the modified electrode shows significant instability. The proton transfer into the film
188
(equations (1) - (3)), required to maintain charge neutrality, is no longer facile due to the low
189
proton concentration. Therefore, charge neutralization is achieved through the dissolution of
190
PMo12 instead. The reversible potential data of all processes obtained in different aqueous
191
electrolyte media are summarized in Table 1 for ease of comparison.
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10 192 193
3.3. Electrocatalytic activity of the PMo12-Doped-PEDOT coated AuNps modified
194
electrode towards the reduction of BrO3Several studies have shown that POMs in different redox levels are able of rapidly
196
reducing other species, including BrO3-, at a much lower over potential than in the absence of
197
POMs and hence acting as molecular catalysts [19-27, 34-38, 49]. The catalytic activity of
198
the modified electrode towards BrO3- reduction was therefore examined. In aqueous 0.1 M
199
H2SO4 media (Fig. 5), the one-electron reduced form of PMo12 generated from process I is
200
essentially inactive towards BrO3- reduction. In contrast, the second reduced form,
201
[H+]3[PMo12O40]3- (surf), generated from process II induces considerable catalytic activity.
202
The catalytic reduction current increases when the concentration of BrO3- increases from 0.25
203
to 3 mM. Surprisingly, the third reduced from of PMo12, [H+]4[PMo12O40]3- (surf), generated
204
from the process III also shows negligible activity towards BrO3- reduction, evidenced by the
205
observation of the oxidation peak in the reverse scan of potential, even though it’s a stronger
206
reductant compared to the one generated from process II. This is not due to the depletion of
207
BrO3- by [H+]3[PMo12O40]3- (surf) since the catalytic process is clearly kinetically controlled
208
(the catalytic current detected is much smaller than the diffusion-controlled current even if
209
BrO3- reduction is limited by a one-electron transfer step in the timescale of measurement).
210
The origin of this observation is unknown. No catalytic processes were observed in the
211
controlled experiments undertaken using PEDOT stabilized Au nanoparticle modified
212
electrodes. Therefore, catalytic current detected is due to PMo12 rather than Au nanoparticles
213
or PEDOT.
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214
In aqueous media containing 10 mM H2SO4 and 0.1 M Na2SO4 (Fig. 6), the first reduced
215
form of PMo12 remains inactive. However, both the second and third reduced forms show
216
catalytic activities in contrast with those observed in the aqueous 0.1 M H2SO4 medium. The
Page 11 of 25
11
third reduced form is more active than the second one, judging by the relative sizes of the
218
catalytic current. This may be expected since the product of the third process,
219
[H+]4[PMo12O40]3- (surf), is the stronger reducing agent. This result in drastic contrast with
220
the results obtained in 0.1 M H2SO4 media where the third reduced form is essentially
221
inactive (Fig. 5). The mechanism for this observation is unknown. Again, control experiments
222
were undertaken using PEDOT stabilized Au nanoparticle modified electrodes to rule out the
223
possibility that the observed catalytic activity is due to either Au nanoparticles or PEDOT.
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The catalytic activity of the modified electrode was not tested in pH 3.6 sodium acetate
225
buffer due to the instability of the modified electrode described above (Fig. 4c).
226
Consequently, aqueous medium containing 10 mM H2SO4 and 0.1 M Na2SO4 is the optimal
227
electrolyte for BrO3- detection in terms of both electrode stability and catalytic activity. The
228
catalytic reduction currents associated with the third process are plotted as a function of the
229
concentration of BrO3- in the inset to Fig. 6. The catalytic current approaches a maximum
230
value in the presence of high concentrations of BrO3- as expected, since catalytic process will
231
eventually becomes rate limiting step when the concentration of the analyte is sufficiently
232
high [50]. The peak current versus [BrO3-] plot shows good linearity when [BrO3-] ≤ 3 mM.
233
At higher concentration, this plot deviates from linearity (smaller slope). This could be due to
234
the effect of uncompensated resistance (iRu effect) or the fact that the process may be
235
approaching a kinetic limitation as predicted by the Michaelis-Menten equation [51]. The
236
slope (sensitivity) of 68 µA cm-2 mM-1 is comparable to those obtained with similar modified
237
electrodes (Table 2).
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238 239
4. Conclusions
240
Water soluble and highly stable PMo12-doped-PEDOT coated AuNps have been
241
synthesized. The PMo12 anions serve as stabilizer together with PEDOT and PSS for the
Page 12 of 25
12
formation of AuNps, and the dopant anion for PEDOT and provide additional stability and
243
functionality to this new composite material. This PMo12-doped-PEDOT coated AuNps
244
modified glassy carbon electrode shows excellent stability during the course of voltammetric
245
measurements and exhibits high catalytic activity towards the reduction of BrO3- in aqueous
246
electrolyte medium containing 0.01 M H2SO4 and 0.1 M Na2SO4.
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247
Acknowledgements
249
Syeda Sara Hassan gracefully acknowledges the Higher Education Commission of Pakistan
250
for the financial support to conduct part of her Ph.D research in Australia in the School of
251
Chemistry, Monash University. The authors also thank the Monash Centre for Electron
252
Microscopy for assistance in obtaining the SEM images.
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Table 1. Reversible potential data for the PMo12 reduction processes in different aqueous electrolyte media. E0' / mV
Electrolyte Medium (I/I')
(II/II')
(III/III')
0.1 M H2SO4
280
142
-89
10 mM H2SO4 + 0.1 M Na2SO4
231
83
-146
sodium acetate buffer pH 3.6
143
22
-241
us
cr
347
ip t
344 345 346
Table 2. Linear range and sensitivity of polyoxometalate modified electrodes for BrO3-
an
detection. Electrolyte media
Linear range (μM)
slope (μA cm-2/mM)
Reference
PMo12/ PEDOT/AuNP/GC
0.1 M H2SO4
250-3000
68
(this work)
PMo12/CNTs/PPya
0.5M H2SO4
100-2000
117
[45]
PMo12/CNTs/PEDOT
0.5M H2SO4
100-2000
32
[45]
SiMo12O404-/PEDOT
0.2M H2SO4
30-8000
71
[42]
P2Mo18O626-/OMC/GCb
1 M H2SO4
2.77–4000
87
[44]
Nd(SiMo7W4)213-/PPy
0.1M Na2SO4, pH 1.23
990-31500
27.5
[40]
b.
d
Ac ce pt e
a.
M
Electrode composition
CNT = carbon nanotube; PPy = polypyrrole; OMC = ordered mesoporous carbon
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Fig. 1. UV-visible absorbance spectrum and photographic image of PMo12-doped-PEDOT coated AuNps.
Fig. 2. SEM images (a, scale bar is 1 μm) and (b, scale bar is 100 nm) with different magnifications, and EDX spectrum (c) of PMo12-doped PEDOT coated AuNps.
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Fig. 3. (a) Cyclic voltammograms derived from a PMo12-doped-PEDOT coated AuNps modified GCE in 0.1 M H2SO4 electrolyte at scan rates of 10 (shown in red), 50, 100, 200, 500 and 1000 mV s-1 (from inside to outside). (b) Effect of scan rate on the background subtracted reduction peak current for the first reduction process.
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Fig. 4. Cyclic voltammograms derived from a PMo12-doped-PEDOT coated AuNps modified GCE in aqueous (a) 0.1 M H2SO4, (b) 0.01 M H2SO4 + 0.1 M Na2SO4 buffer media, and (c) pH 3.6, 0.1 M sodium acetate buffer media, obtained at a scan rate of 10 mV s-1.
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Fig. 5. Cyclic voltammograms derived from a PMo12-doped-PEDOT coated AuNps modified GCE obtained in aqueous 0.1 M H2SO4 solution in the absence (a) and presence of 0.25 mM (b), 0.5 mM (c), 1mM (d), 2 mM (e) and 3 mM (f) of BrO3-. The result obtained from a control experiment undertaken in 0.1 M H2SO4 solution in the presence of 3 mM of BrO3- using a PEDOT stabilized AuNps modified GCE (dashed line) is shown for comparison. Scan rate = 10 mV s-1.
Fig. 6. Cyclic voltammograms derived from PMo12-doped-PEDOT coated AuNps modified glassy carbon electrode in aqueous media containing 10 mM H2SO4 and 0.1 M Na2SO4 in the absence (a) and presence of 0.25 mM (b), 0.5 mM (c), 1 mM (d), 2 mM (e), 3 mM (f), 4 mM (g) and 5 mM (h) of BrO3-. The result obtained from a control experiment undertaken in 10 mM H2SO4 and 0.1 M Na2SO4 solution in the presence of 3 mM of BrO3- using a PEDOT stabilized AuNps modified GCE (dashed line) is shown for comparison. Scan rate = 10 mV s-1.
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