gold nanoparticle composite

    Fabrication of an ultrasensitive impedimetric electrochemical sensor based on graphene nanosheets/polyethyleneimine/gold nanoparticle composite Azadeh Azadbakht, Amir Reza Abbasi, Zohreh Derikvand, Ziba Karimi PII: DOI: Reference:

S1572-6657(15)30092-8 doi: 10.1016/j.jelechem.2015.08.034 JEAC 2259

To appear in:

Journal of Electroanalytical Chemistry

Received date: Revised date: Accepted date:

6 July 2015 19 August 2015 27 August 2015

Please cite this article as: Azadeh Azadbakht, Amir Reza Abbasi, Zohreh Derikvand, Ziba Karimi, Fabrication of an ultrasensitive impedimetric electrochemical sensor based on graphene nanosheets/polyethyleneimine/gold nanoparticle composite, Journal of Electroanalytical Chemistry (2015), doi: 10.1016/j.jelechem.2015.08.034

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ACCEPTED MANUSCRIPT Fabrication of an ultrasensitive impedimetric electrochemical sensor based

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on graphene nanosheets/polyethyleneimine/gold nanoparticle composite

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Azadeh Azadbakhta*, Amir Reza Abbasi, Zohreh Derikvand, Ziba karimi a

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Departmentof Chemistry, Faculty of Science, Khorramabad Branch, Islamic Azad University , Khorramabad, Iran.

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Abstract

In this study, graphene nanosheets/polyethyleneimine/gold nanoparticle (GNs/PEI/AuNPs)

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composite modified glassy carbon (GC) electrode has been utilized as a platform to immobilize poly acriflavine (PAF). PEI, a cationic polymer, was used both as a non-covalent functionalizing agent for the graphene oxide (GO) nanosheets through electrostatic

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interactions in the aqueous medium and also as a stabilizing agent for the formation of

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AuNPs on PEI wrapped GNs. The prepared GNs/PEI/AuNPs composite exhibits the dispersion of high density AuNPs which were densely decorated on the large surface area of

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the PEI wrapped GNs. The surface structure and composition of the sensor were characterized by scanning electron microscopy (SEM). Electrocatalytic reduction of iodate on the surface of modified electrode was investigated with cyclic voltammetry, electrochemical

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impedance spectroscopy (EIS) and chronoamperometry methods. The cyclic voltammetric results indicated the ability of GNs/PEI/AuNPs /PAF modified GC electrode to catalyze the reduction of iodate.

Keywords: Graphene nanosheets, Polyethyleneimine, Gold nanoparticles, Acriflavine.

*Corresponding author: A. Azadbakht, Tel: +98 6616200399 E-mail:[email protected]

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ACCEPTED MANUSCRIPT 1. Introduction

As a trace element, iodine is essential to animal and plants. It is an essential part of the

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thyroid hormones that play an important role in the growth of cell. Deficiency of iodine

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causes serious delay in neurological development. On the other hand, an excess of iodine or iodide can cause goiter and hypothyroidism as well as hyperthyroidism [1]. One of the most

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effective means against iodine-deficiency is adding iodine-containing nutrientinto table salt (iodised salt). Due to its stability, iodate (as potassium iodate) is usually adopted as the iodine source.However, excess intake of iodine may also lead to thyrotoxicosis, a disease relating to

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excessive amount of thyroid hormones. Therefore, it is of significant importance to detect the amount of iodate in salt and other samples. In order to achieve this goal, various methods,

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have been developed but still a few reports are present concerning the determination of iodate [2,3].

Owing to their unique electrical and mechanical properties as well as large surface area,

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graphene nanosheets (GNs) with two-dimensional (2D) carbonnano structure have emerged

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as a new class of promising materials attractive for potential applications in actuators, solar cells, field-emission devices, field-effect transistors, super capacitors, and batteries [4-6].

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Because of Van der Waals interactions, the GNs tend to form irreversible agglomerates and even restack to form graphite [7]. In order to obtain graphene as individual sheets, attaching some molecules or polymers onto the sheets is an approach to reduce the aggregation [8,9]. Graphene-based composite nanomaterials prepared via molecular level dispersion in

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polymers to improve electronic and thermal conductivity [10]. Recent progress in preparation techniques has made it possible to support metal nanoparticles (NPs) such as Pt and Au on GNs [11-18]. Nowadays, nanocomposites produced by the association of GNs with metallic NPs are expected to have promising potential applications in fields such as chemical sensors, energy storage, catalysis and hydrogen storage [11]. Another important feature of the adhesion of metal NPs to GNs is that the adhesion results in the inhibition of aggregation of the GNs in dry state. The metal NPs also function as a spacer, thus increasing the distance between the GNs, thereby making both faces of graphene accessible [12]. Polyethyleneimine (PEI) is a water-soluble amine-containing cationic PE, has been known to effectively interact with CNTs via physisorption on CNTs' sidewalls [19,20]. More recently, linear PEI has been used both as reducing agent and protecting or stabilizing agent for the preparation of gold NPs [20]. The adsorption of cationic PE on the carboxylated surface of CNTs and the attachment of negatively charged AuNPs to the PE chains through 2

ACCEPTED MANUSCRIPT electrostatic interactions has been reported [21, 22]. Recently, much attention is also focused on the large scale microwave assisted synthesis of metallic NPs, soluble CNTs derivatives, exfoliation of graphite intercalation compounds and large surface area 2-D GNs [23, 24]. A

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simple synthetic route for high density attachment of AuNPs onto the sides of GNs with high

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NPs coverage has been reported and in this work this novel composite used as a platform to immobilize poly acriflavine (PAF) [25].

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Acriflavine, first synthesized in 1912, is a kind of antiseptic agent. It is also an orange or brown dye and a medicine against sleeping sickness. Nevertheless poly acriflavine (PAF) is rarely studied; the monomer is widely used for photosensitizer [26], analytical regents for

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sensing [27], acid–base indicator [28], and luminescence sensors [29] among many others. In the literatures the PAF film was proposed to sense nicotinamide adenine dinucleotide

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(NADH), nitrite and sulfur oxoanions, and that film entrapped Flavin adenine dinucleotide (FAD) to catalyst NAD+[30, 31]. There is a redox reaction around 0.25V vs. Ag/AgCl for poly acriflavine film in acidic solution and the PAF-modified film can lower the oxidation

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potential of NADH from 0.65 to 0.22V vs. Ag/AgCl. This class of polymer films, containing

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with phenyl or amine groups in structure such as polyaniline, has been proposed for use in biosensors [32] and electrochemical devices [33].Generally, acriflavine exists in acidic

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solution as acriflavine hydrochloride and another existing form is acriflavine neutral [34, 35]. In this work, we used a simple and fast microwave assisted chemical reduction method for

the

preparation

(GNs/PEI/AuNPs)

of

graphene

composite.

This

nanosheet/polyethyleneimine/gold synthetic

method

involves

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nanoparticle non-covalent

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functionalization of exfoliated GOs with PEI and a simultaneous chemical reduction of graphene oxides (GOs) and HAuCl4 in a one-step reaction. Moreover, acriflavine was immobilized on the modified GC electrode modified with GNs/PEI/AuNPs through the covalent amide bonds formed by the remaining carboxyl groups on the GNs and the amino groups on the acriflavine and then the modified electrode was utilized as an electrochemical sensor for the high sensitive determination of iodate.

2. Experimental 2.1. Chemicals Polyethylenimine (PEI, branched, Mw 10,000), hydrochloric acid(37%), potassium ferricyanide

(K3Fe(CN)6),

potassium

ferrocyanide

(K4Fe(CN)6.4H2O),

hydrogen

tetrachloroaurate (HAuCl4.4H2O), sodium borohydride (NaBH4), Graphite powder (1–2 μm), potassium iodate and potassium chloride (KCl) were purchased from Merck (Germany) and 3

ACCEPTED MANUSCRIPT Fluka. N-hydroxysuccinimide (NHS), 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC), and 3, 6-diamino-10-methylacri-dinium chloride (acriflavine) were produced by Sigma and they were used as received without further purification. All other

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chemicals were analytical reagent grade and used without further purification. Solutions were

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deaerated by bubbling high purity (99.99%) of nitrogen gas through them prior to the

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experiments. All experiments were carried out at ambient temperature of 25 ±1 ◦C.

2.2. Apparatus

Electrochemical experiments were performed via using a μAutolab III (Eco Chemie,

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Utrecht, Netherlands) potentiostat/galvanostat by NOVA 1.8 software. A conventional three electrode cell was used with an Ag/AgCl electrode (KCl 3 M) as the reference electrode, a Pt

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wire as counter electrode and a modified GC as working electrode. The cell was a one compartment cell with an internal volume of 10 mL. JENWAY pH meter (model 3345) was also applied for pH measurements. To obtain information about the morphology and size of

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the particles, scanning electron microscopy (SEM) was performed using an X-30 Philips

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instrument.

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2.3. The preparation of AuNPs/PEI/GNs composites In this synthesis, graphene oxides were prepared from graphite via modified Hummers method [36]. Five mg of the prepared GOs was dispersed in 9.5 mL of 1 M PEI aqueous solution and 0.5 mL of 25 mM HAuCl4 in a 25 mL round bottom glass flask under

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ultrasonication for 10 min. Then, the mentioned mixture solution was placed in a modified version of the domestic microwave oven with a maximum power of 1100W, equipped with a temperature-control condenser system. Microwave was used with effective power of 200W and total irradiation time of 2 min.

After the completion of 1.30 min of microwave

irradiation, 0.1 mL of 0.05 M NaBH4 was added into reaction mixture in order to be sure about completion reduction of any unreduced site of GOs and HAuCl4 in the PEI solution. The resulting mixture solution was then centrifuged at 4000 rpm for 15 min and washed with pure water for three times. Then, the resultant composite product was dried at room temperature for overnight [25]. Suggested position for Fig.1

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ACCEPTED MANUSCRIPT The driving force behind the non-covalent functionalization of cationic PEI on each sheet of GOs is the electrostatic interaction between the oppositely charged GOs and PEI along with the physisorption process, which is analogous to polymer wrapping process [37,

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38]. The major interaction between the polymer backbone and nano sheet surface is the most

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likely cation–π interactions of PEI and GOs that extends to self-assembly [37]. It is worth mentioning that this approach allows control over the distance between functional groups on

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the GOs surface through variation of the polymer backbone and side chains [39]. This approach allows the dispersion and individualization of GOs in aqueous solution by cation–π interactions with polymer wrapping [39]. More importantly, it has been reported that amines

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possess high affinity for physisorption along CNTs' sidewalls, which is also similar to GNs [38]. Moreover, PEI can form PEI–metal ion complex, which can adsorb easily on the surface of the GOs via electrostatic interaction [40]. PEI has a high density of imino-groups, which

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can serve as primers for the adsorption of anionic AuCl4–. It also acts both as reducing agent and stabilizing agent for AuNPs and GOs [38]. As GOs has been found to contain reactive

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epoxy groups, its exposure to free amine groups of PEI would lead to a ring-opening reaction

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of the reactive three-membered epoxide ring, creating new C–N bonds. The ring- opening reaction of the epoxy group from attack by nucleophiles such as amine groups has been well

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established [39]. Addition of mild amount of NaBH4 to the reaction mixture facilitated reduction of any remaining unreduced GOs sites that evaded chemical functionalization [40]. By combining the multifunction of PEI, the attachment of high density AuNPs on the surface of GNs was achieved. This synthetic method does not need any exhaustive separate reduction

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reaction steps for GOs and gold ions [41].High polarizability of graphene layers causes them to heat rapidly which provides simple and fast routes to the synthesis of this composite [42] and since the metal precursor has large microwave absorption cross sections relative to the solvent, very high effective reaction temperatures can be achieved [42]. This allows the rapid decomposition of the precursors, thus creating highly supersaturated solutions where nucleation and growth can take place to produce the desired gold nanocrystalline product on each PEI wrapped GNs. To investigate the role of GNs in this nanocomposite, AuNPs/PEI was also prepared as described above without GNs.

2.4. Electrode modification To prepare a modified electrode, glassy carbon electrode (GCE) was polished with emery paper followed by alumina (1.0 and 0.05 µm) and then thoroughly washed with twicedistilled water. This electrode was afterwards placed in ethanol container, and then a bath 5

ACCEPTED MANUSCRIPT ultrasonic cleaner was used to remove the adsorbed particles. As-prepared AuNPs/PEI/GNs composite was dispersed in 0.75% nafion (1 mg mL-1) with the aid of ultrasonic agitation. Then, 5 µL of AuNPs/PEI/GNs solution was cast on the

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surface of GC electrode and dried in air to form a film at electrode surface. Afterward, the

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electrode was thoroughly rinsed with water and kept at room temperature for further use. For acriflavine immobilization, the GC/AuNPs/PEI/GNs modified electrode was immersed in a

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0.1 M acetate buffer (pH 5) containing 10 mM EDC for 1 h. The EDC-attached electrode was washed and subsequently incubated in a 3.5μM acriflavine solution for 12 h at room temperature. In this process the acriflavine was immobilized on the GC electrode through the

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covalent amide bonds formed by the remaining carboxyl groups on the GNs and the amino groups on the acriflavine (the electrode denoted GC/AuNPs/PEI/GNs/PAF) as demonstrated

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in Scheme 1. The other used electrodes such as Au/PEI and GC/AuNPs/PEI were fabricated by casting 10 µL of 5% (v/v) PEI and AuNPs/PEI on the surface of bare GC electrode, respectively.

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Suggested position for Scheme 1

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For comparison, the modification of GC/AuNPs/PEI/GNs electrode with acriflavine was also carried out by electrodeposition of acriflavine at the surface of GC/AuNPs/PEI/GNs

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by immersion GC/AuNPs/PEI/GNs electrode in 0.1 M acetate buffer solution containing acriflavine. Polymerization of acriflavine onto GC/AuNPs/PEI/GNs electrode was carried out by the consecutive potential cycling of the working electrode between -0.05 V and +0.8 V (vs. Ag/AgCl) in an acetate buffer solution (pH 4) containing1.5×10-3 M acriflavine

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monomers at scan rate of 50 mVs−1for 50 cycles. The results indicated that there is a considerable enhancement in the charge of voltammogram when the polymerization of acriflavineat the surface of GC/AuNPs/PEI/GNs electrode was occurred compared with the amide bond formation in the first case. This behavior may be attributed to few amount of remaining unreduced GOs sites at the surface of GNs and

saturation of active site of

acriflavine with the amide bond formation.

3. Results and discussion 3.1. Characterization of the modified electrode by SEM Fig. 1 shows the SEM images of the GNs (A) AuNPs/PEI/GNs (B) and GC/AuNPs/PEI/GNs/PAF electrode (C). It could be obviously observed that GNs shows a typical crumpled and flaked structure (Fig. 1A). Fig. 1B shows typical SEM images of AuNPs/PEI/GNs composite. As shown from the Fig. 1B the AuNPs were decorated densely 6

ACCEPTED MANUSCRIPT on the crumpled thin layer of PEI wrapped GNs. As can be seen from Fig.1C, after PAF immobilization, the surface of the AuNPs/PEI/GNs nanocomposite became rougher and the morphology of the electrode surface is obviously changed, which implies that the PAF was

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successfully deposited on the surface of the AuNPs/PEI/GNs electrode.

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3.2. Electrochemical Polymerization of acriflavine

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Suggested position for Fig.1

Fig. 2 shows the consecutive cyclic voltammograms of poly acriflavine film deposition onto GC/AuNPs/PEI/GNs in 0.1 M acetate buffer solution containing 1.5× 10 -3 M

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acriflavine monomers. During the first forward scan, the acriflavine monomer started to oxidize at above +0.53V whereas on reverse scan one new cathodic peak appeared at around -

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0.15 V and its counterpart oxidation peak was appeared at +0.2 V on the subsequent cycle. Moreover, when the number of cycles increased, the peak current corresponding to the new reversible redox couple was found increased. This result indicates that the oxidized product

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of acriflavine leads to the deposition of PAF onto the GC/AuNPs/PEI/GNs surface. In second

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type, electropolymerization of acriflavine onto electrode surface was also performed by holding the potential at +0.8 V for 10 minutes in the buffer solution containing 1.5×10-3 M

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acriflavine [30]. The cyclic voltammogram was characterized by one reversible redox couple, with the formal potential (E°'= Epa + Epc/2) occurring at 0.17 V (vs. Ag/AgCl). The reversible redox couple was attributed to the reduced and oxidized forms of PAF. To ascertain the effect of positive potential on the subsequent generation of reversible

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redox couple, voltammograms were carried out in which positive potential limit was progressively increased. The generation of new wave increased as the positive potential limit increased and reached maximum value at about 1V. Sweeping the potentials to more positive values did not result in the generation of additional material. These results confirm that the oxidation of acriflavine results PAF film deposition onto GC/AuNPs/PEI/GNs [30]. Fig. 2B shows the peak current of the PAF characteristic peak at each cycle, with different concentrations of monomer and AuNPs/PEI/GNs. In Fig. 2B, the polymerized rates increased faster initially and maintained a similar rate (slope) after 15 cycles.No detectable change in current was observed after 30 cycle.The deposit rates of GC/AuNPs/PEI/GNs/PAF were faster than those for the GC/PAF electrodes because the AuNPs/PEI/GNs immobilized on the GC surface not only provided extra specific area for polymerization of PAF, but also increased the conductivity of the film. The increased rate of deposition proved that the PAF 7

ACCEPTED MANUSCRIPT was polymerized on both surfaces of GC and AuNPs/PEI/GNs. Suggested position for Fig.2

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Cyclic voltammograms of PAF onto bare GC (a), GC/PEI (c),GC/AuNPs/PEI(b) and

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GC/AuNPs/PEI/GNs(d) in 0.1 M acetate buffer solution (pH 4) at a scan rate of50 mVs-1

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were carried out. As can be seen in Fig.3, a pair of ill redox peaks was observed when GC and GC/PEI electrodes were used. As was previously reported the anodic and cathodic processes correspond to the PAFox/PAFR redox couple [30]. In order to enhance the surface area of the GC/PEI electrode, improvement of the desired electrode with other compound

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with different properties (AuNPs and AuNPs/GNs) was carried out (GC/AuNPs/PEI and GC/AuNPs/PEI/GNs electrodes). Modification of mentioned electrode by AuNPs and

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AuNPs/GNs enhances the sensor response about 15 and 22 times (curve b and d), respectively. The presence of AuNPs and GNs supplied a larger surface area to allow more deposition of PAF.

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As it is seen from Fig.3, the GC/AuNPs/PEI/GNs electrode improves the reversibility

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of the electrodic process (peaks potential separation was decreased to 65 mV). Also, its peak

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currents increase 1.6 times higher than that obtained with the GC/AuNPs/PEI electrode. Suggested position for Fig.3

EIS is complementary electroanalytical test to analyze the electrochemical double layer on

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the electrodes. In EIS we apply different AC frequencies with specific amplitude superimposed over the equilibrium DC potential and measure the impedance change in the double layer. The resistance in the double layer is named impedance since there are capacitive and inductive components which are frequency dependent resistance terms. We are using Fe(CN)63−/4− system since it is one of the best redox species which could study on an electrode. The redox peaks will give clear indication of the double layer changes on the electrode surfaces. Thus when we are incorporating some compound over an electrode the double layer is changed and this can affect the double layer capacitance and the electron transfer resistance. Thus by monitoring the charge transfer resistance (R ct or Ret) we could understand the charge transfer properties of the double layer. Fe(CN)63−/4− also could act as a mediator for some electrochemical reactions and thereby enhancing the signal since these reaction has a good electron transfer kinetics on almost all the electrodes. 8

ACCEPTED MANUSCRIPT Fig.4 shows the typical Nyquist plots for bare GC, GC/AuNPs/PEI,GC/AuNPs/PEI/GNs and GC/AuNPs/PEI/GNs/PAFelectrodes recorded in 0.1M KCl solution containing 0.5 mM Fe(CN)63−/4−as an electrochemical redox marker. The straight line at low frequency is related

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to the diffusion process known as Warburg element, while the high frequency semicircle is

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related to the electron transfer resistance (Ret), which controls the electron transfer kinetics of the redox probe at the electrode interface. The EIS at a bare GC electrode displays a very

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small semicircle and the charge transfer resistance, which was a characteristic feature of the diffusion controlled electrochemical processes (curve d). After immobilization of AuNPs/PEI, the value of Rct is significantly increased to about 41.3 KΩ (curve a). It indicates

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hindrance to the electron transfer, confirming the successful immobilization of PEI onto GC electrode surface.

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Finally when AuNPs/PEI/GNswere cast at the surface of GC electrode the value of Rct is significantly decreased to about 6.5 KΩ compared with GC/AuNPs/PEI electrodes (curve b). These results may be attributed to the positive charge of a large number of amine

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groups on the AuNPs/PEI/GNs nanocomposite and the negative charge of the [Fe(CN)6]3−/4−

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that causes the electrostatic attraction between [Fe(CN)6]3−/4− redox marker and AuNPs/PEI/GNs nanocomposite with the result of lowering electron transfer kinetics on the

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electrode surface.Therefore, immobilization of AuNPs/PEI/GNs at the surface of GC electrode facilitates the electron transfer of the redox probe on the modified electrode. After modification of GC/AuNPs/PEI/GNs with PAF, the value of Rct is significantly decreased to about 1.37Ω (curve c), confirming the successful immobilization of PAF onto

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GC/AuNPs/PEI/GNselectrode

surface.

These

results

indicate

that

the

GC/AuNPs/PEI/GNs/PAFelectrode could provide good electron conduction pathways between the electrode and electrolyte. Suggested position for Fig.4

3.3. Properties of the nano-structured GC/AuNPs/PEI/GNs/PAFelectrode The recorded cyclic voltammograms of GC/AuNPs/PEI/GNs/PAF film in 0.1M acetate buffer solution (pH 4) at different scan rates were recorded. Anodic and cathodic peak currents vs. the scan rate were plotted and the data revealed that both the anodic and cathodic peak currents are linearly proportional to the scan rate in the range of 10 -100 mVs−1, indicating a surface confined electrode process. The peak-to-peak potential separation is about 82 mV at scan rates below 100 mVs−1, suggesting facile charge transfer kinetics over this range of sweep rates. At higher sweep rates peak separations begin to increase, indicating 9

ACCEPTED MANUSCRIPT the limitation due to charge transfer kinetics [43]. The shifts of peak potentials were proportional to the logarithm of the scan rate for scan rates higher than 100 mV s−1. The cyclic voltammograms of the GC/AuNPs/PEI/GNs/PAF was also found to be

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stable in the pH range between 1 to 7, however, in basic solution the cyclic voltammogram of

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the GC/AuNPs/PEI/GNs/PAF was diminished. These behaviors might be attributed to the

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instability of the polymer film in basic solution. Since the modified electrodes had limited stability in buffers at pH ≥ 8, we investigated the stability and electrochemical properties of prepared modified electrode in buffer solutions in the pH range 1-8 by recording the cyclic

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voltammograms. The results exhibited pH dependent voltammetric peak potentials, i.e., the anodic and cathodic peak potentials of the modified electrode were shifted to a less positive value with increasing pH of the electrolyte solution. In addition, the peak current values and

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peak area also decreased. The formal potential (E°') of the PAF film was evaluated as the mean of the anodic and cathodic peak potentials of the cyclic voltammograms recorded at

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various pH values. The E°' vs. pH plot yields straight line with a slope of 69 mV per unit change in solution pH. It suggests that the overall redox reaction of the polymer film

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comprises a two electron and two-proton process. The possible chemical composition of PAF film redox process was shown analogous to polyaniline [30]. The redox reactions of the PAF

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film with electrons and protons exchange reaction is given in Scheme 2.

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Suggested position for Scheme 2

3.4. Electrocatalytic reduction of iodate at GC/AuNPs/PEI/GNs/PAF electrode Application of the modified electrode for reduction of iodate was evaluated by cyclic voltammetry. Reduction of iodate at GC/AuNPs/PEI/GNs and GC/AuNPs/PEI/GNs/PAF electrodes was investigated in the phosphate buffer solution (PBS) (pH 2). Fig. 5 shows recorded cyclic voltammograms in the absence and presence of 2 mM of iodate at scan rate of 50 mVs-1.As can be seen from Fig.5 (curve b) iodate did not undergo reduction at GC/AuNPs/PEI/GNs electrode in the potential window of -0.5- 1 V in the PBS (pH 2). However, presence of PAF film on GC/AuNPs/PEI/GNs electrode had a catalytic effect for reduction of iodate (curve d).This result reveals that PAF has excellent catalytic activity toward iodate determination. Suggested position for Fig.5

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ACCEPTED MANUSCRIPT To understand the undergoing electrochemical reactions at the different modified electrodes, the EIS experiments are performed in the presence of 4 mM of iodate in 0.1M PBS (pH 2). As can be seen in Fig.6 for GC electrode, a semicircle curve is observed over the

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whole frequency region and the value of Rct is 1.2 KΩ, indicating that the reaction is

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kinetically controlled (curve a). After immobilization of PAF at the surface of gold electrode, the value of Rct is significantly decreased to about 397 Ω (curve b). The results indicate that

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the immobilized PAF decrease the charge transfer kinetics to about one-third of that at the bare GC electrode and it also confirmed that presence of PAF film on GC electrode had a catalytic effect for reduction of iodate. For GC/AuNPs/PEI/PAF (curve c) and

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GC/AuNPs/PEI/GNs/PAF (curve d) the calculated charge transfer resistance are decreased, which proves that the assembly of GNs and AuNPs nanoparticles makes the electron transfer

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easier. The deposition of AuNPs/PEI/GNs on the surface of modified electrode facilitated the electron transfer of the electrochemical probe on the modified electrode.

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Suggested position for Fig.6

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For further investigation of the electrocatalytic properties of different modified electrodes, cyclic voltammograms of these electrodes in the presence of iodate at a wide

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potential range were recorded. The cyclic voltammetric responses of GC bare, GC/PAF and GC/AuNPs/PEI/GNs/PAF electrodes in the absence and presence of 4 mM iodate were recorded. iodate did not undergo reduction at GC electrode and weak catalytic effect was observed when GC/PAF was used. However, presence of PAF film on GC/AuNPs/PEI/GNs

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electrode had excellent catalytic effect for reduction of iodate. The result showed that in the absence of iodate a pair of redox peaks corresponding to the PAFox/PAFR were observed at the surface of the electrodes tested. Upon addition of iodate, an enhancement in the cathodic peak current is observed and the anodic peak current tended to decrease. The reason for this increase is that, along with the anodic potential sweep, iodate oxides PAFR to PAFox, while simultaneous reduction of the regenerated PAFox causes an increase in the cathodic current. For the same reason, the anodic current is smaller in the presence of iodate, indicating that PAFR is consumed during a chemical step. Moreover, the electrocatalytic reduction peak current of iodate at GC/AuNPs/PEI/GNs/PAF electrode was -230 μA, which was 5.5times larger than that at GC/PAF electrode (-42 μA), which is in agreement with EIS data for charge transfer kinetics on the different modified electrodes. These results indicate that presence of AuNPs/PEI/GNs in the modified electrode supplied a larger surface area to allow more deposition of PAF for reduction of iodate. 11

ACCEPTED MANUSCRIPT Cyclic voltammograms of different concentrations of iodate (ranging from 3 to 600mM) at the modified electrode in 0.1M PBS (pH 2) was recorded. The calibration curve based on the cathodic peak current is linear with the iodate concentration in the range of 3-

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600 mM with a correlation coefficient of 0.996 (Fig.7). The response of modified electrode

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was deviated from the linearity for iodate concentration above 600 mM. This behavior may be attributed to saturation of some redox sites on the surface of electrode, which involved in

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the catalytic reaction. As can be seen from Fig.7 by increasing the concentration of iodate, the cathodic peak current of the modified electrode is increased while its anodic peak current decreased, indicating a typical electrocatalytic reduction process (EC´).Therefore, an

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enhancement of peak current for the reduction of iodate indicates strong catalytic activity of GC/AuNPs/PEI/GNs/PAF electrode.

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Suggested position for Fig.7

In order to optimize the electrocatalytic response of the modified electrode, the effect

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of pH on the catalytic behavior of modified electrode was investigated. The cyclic

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voltammograms of the modified electrode in the presence of 2mM of iodate in 0.1M buffer solution a different pH values were recorded. At pH range 1–3, the modified electrode shows

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electrocatalytic activity, but catalytic currents decreased with increasing pH and at pH 4 the catalytic effects of the modified electrode are negligible. The higher peak currents are observed at pH because the reduction reaction of iodate depends strongly on the concentration of proton, as can be seen in Eq. (1): IO3- + 6H+ + 6e-→I− + 3H2O

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(1)

Owing to much higher reduction current obtained at solutions with lower pH values, pH=2 was chosen as the suitable pH value for the detection of iodate. For investigation the electrocatalytic mechanism of the modified electrode toward iodate reduction, cyclic voltammograms of modified electrode in 4 mM iodate at different scan rates were recorded. Fig. 8 illustrates cyclic voltammograms of 4 mM iodate using modified electrode that recorded at potential sweep rates ranging from 10 to 500 mV s-1. The magnification in the low scan rate (10 mVs-1) is shown in the Fig. 8. As can be seen, for a low scan rate a catalytic effect for reduction of iodate was observed and pair of redox peaks corresponding to PAFox/PAFR was completely disappeared. While by increasing the scan rate a redox peaks corresponding PAFox/PAFR

appeared confirming atypical electrocatalytic

reduction process (EC').

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ACCEPTED MANUSCRIPT Suggested position for Fig.8

The peak current for the cathodic reduction of iodate is proportional to the square root

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of the scan rate, suggesting that the process is controlled by diffusion of analyte as expected

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for a catalytic system. It can also be noted that by increasing the sweep rate the peak potential for the catalytic reduction of iodate shifts to more negative values and plot of peak current vs.

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square rate of scan rate deviates from linearity (at v >100mVs-1), suggesting a kinetic limitation in the reaction between the redox sites of the PAF and iodate. Based on the observed results the following catalytic mechanism describes the reaction sequence in the RAF (OX) +2e-+2H+−→RAF(R)

(2) (3)

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3RAF (R) + IO3-→3RAF (OX)+ I− + 3H2O

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reduction of iodate.

The value of catalytic reaction rate constant k can be evaluated by cyclic voltammetry. Andrieux and Saveant [44] developed a theoretical model for a catalytic mechanism. In redox

D

catalysis the catalyst couple is involved in an outer sphere electron transfer reaction with the

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substrate, playing then simply the role of an electron carrier between the electrode and the substrate. Andrieux and Saveant found that for small value of the kinetic parameter an S-

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shaped wave independent of sweep rate is obtained: Ic= (kГFSco)/[1+exp(-ξ)]

(4)

ξ = -(F/RT) (E-Eº) Г

AC

(5)

with its half-wave potential located at E0 of the catalyst,where Ic, k, Γ , F, S and co present the catalytic current,catalytic rate constant, catalyst surface concentration, Faraday constant, electrode surface area, and substrate concentrationin the solution, respectively. For large kinetic parameters (small v, large k and Γ) a peaked-shaped wave proportionalto v1/2 is found with: Ip = 0.49 nFsC0D1/2(Fv/RT)1/2

(6)

The low values of k result in a value of the coefficient lower than 0.496. For low scan rate (10 mVs−1), iodate concentration of 16 mM and a surface coverage of 1.04×10−7 mol cm−2, the average value of this constant was found to be 0.466. Based on approach developed by Andrieux and Saveant, using Fig. 1A in Reference [44], we calculated a mean value of 6.1×102M−1 s−1 for k. 13

ACCEPTED MANUSCRIPT

3.5. Impedimetric detectionof iodate at the modified electrode Under the optimized conditions, the relationship between Rc values and the

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concentrations of iodate was studied. As shown in Fig. 9B, there is a linear relationship

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between Rct and the concentration of iodate over a concentration range from 0.5 nM to 14.0

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nM (R2 0.992). The linear response obtained with the electrochemical sensor indicated that the sensor could be possibly used in the analysis of real samples.

The limit of detection (LOD) of this method was calculated by following IUPAC

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recommendations (3Sb/b, where Sb is the standard deviation (n=8) of the blanks, and b is the slope value of the respective calibration graph). The calibration plot has a correlation

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coefficient of 0.998 and the detection limit of 0.1 nM at signal to noise ratio of 3.

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Suggested position for Fig.9 The detection limit, linear calibration range and applied potential for iodate detection

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were reported in Table 1.This analytical parameter for the proposed modified are comparable or better than the results reported for iodate determination at the surface of recently fabricated

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modified electrodes [45-57].

The EIS response and calibration curve of GC/GNs/PEI, GC/AuNPs/GNs,

AC

GC/PEI/GNs/PAF and GC/AuNPs/PEI/PAF at different concentration ofiodatewere recorded ( Fig.S1 Supporting information).Sinceiodate did not undergo reduction at GC/GNs/PEI and GC/AuNPs/GNs electrodes, no detectable change was observed in EIS response at different concentration

ofiodateat

the

surface

ofGC/GNs/PEI

and

GC/AuNPs/GNs

electrodes.However, presence of PAF film onGC/PEI/GNs//PAF and GC/AuNPs/PEI/PAF reveals decreasing the EIS response by increasing the iodate concentration overa concentrationrangefrom10.0-14.0 nM and 7.0-14.0 nM, respectively.These results indicate that, detection limit, linear calibration range and sensitivity of GC/AuNPs/PEI/GNs/ PAF are better than those obtained for iodatedetermination at the surface of GC/PEI/GNs/PAF and GC/AuNPs/PEI/PAF. 3.6. Chronoamperometric studies The chronoamperometry (CA) method, as well as other electrochemical methods, is 14

ACCEPTED MANUSCRIPT employed for the investigation of electrode processes at chemically modified electrodes. Fig. 10A shows a series of well-defined chronoamperograms for the GC/AuNPs/PEI/GNs/PAF in the absence and presence of different concentrations of iodate at an applied potential of 0.15

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versus Ag|AgCl that was selected from mass transfer controlled region. For an electroactive

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material with diffusion coefficient of D, the corresponding current of the electrochemical

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reaction (under diffusion control) is described by Cottrell’s law [58]: I = nFAD1/2Cπ-1/2t-1/2

(7)

Where D and Co are the diffusion coefficient and bulk concentration, respectively. The

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average value of D obtained from the slopes of I versus t-1/2 plots (Fig. 10B) for different concentrations of iodate is 3.19×10-6 cm2 s-1.Chronoamperometry can be used for the

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evaluation of the catalytic rate constant. At intermediate times, the catalytic current (Icat) is dominated by the rate of electrocatalyzed oxidation of iodate and the rate constant for the chemical reaction between iodate and redox sites of surface-confined is determined according

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D

to the method described in the literature [59]:

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Icat/IL=[γ1/2[π1/2erf(γ1/2) +exp (-γ/γ1/2)]

(8)

Suggested position for Fig.10

Where Icat and IL are the currents of the GC/AuNPs/PEI/GNs/PAF electrode in the presence

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and absence of iodate, and γ = kC0t is the argument of the error function, k is the catalytic rate constant, Co is the bulk concentration of iodate, erf is error function and t is the elapsed time (s). In such cases where γ > 1.5, erf (γ1/2) is almost equal to unity, and the above equation can be reduced to: Icat/IL=γ1/2π1/2=π1/2(k Co t) 1/2

(9)

From the slope of the Icat/IL versus t1/2plot, we can simply calculate the value of kcat for a given concentration of iodate. Some such plots, constructed from the chronoamperograms of GC/AuNPs/PEI/GNs/PAFin the absence and presence of different concentrations of iodate, are shown in Fig. 10C. The average value of kcat was found to7.77 ×102 M-1 s-1. Suggested position for Table 1

15

ACCEPTED MANUSCRIPT 3.7. Stability study The stability of the GC/AuNPs/PEI/GNs/PAF modified electrode and the reversibility of its electrochemical behavior were also investigated. It was found that after storing the

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electrode in ambient condition for one week, the current and potential response of recorded

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cyclic voltammograms remained almost unchanged. We also performed the studies of the long-term storage stability of the proposed modified electrode by measuring current response

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of the after two week and one month. When the electrode was stored ambient condition, it retained 96% of its initial response after two week and 10% decrease of current was observed after one month.

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In addition, the operational stability of the modified electrode was examined by recording of the repetitive cyclic voltammograms in 0.1M acetate buffer solution. The results

observed at

the peak height

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indicated that after 100 repetitive cycles at a scan rate of 50 mV s−1, no detectable change was and potential

separation. The high stability of

GC/AuNPs/PEI/GNs/PAF modified electrode may be related to the mechanical and chemical

D

stability of PAF film which leads to its stabilizing against desorption and avoids it from

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leaching into the solution.

In order to study the reproducibility of the electrode modification, five

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GC/AuNPs/PEI/GNs/PAF modified electrodes were prepared independently and their responses to 5 mM of iodate were measured under identical conditions. The relative standard deviation (RSD) of measuring cathodic peak currents was7.1%, which indicates that the fabrication method exhibits appreciable reproducibility. Similarly, the RSD for 5 successive

sensor.

AC

iodate measurements (5mM) was 2.18%which indicates the acceptable repeatability of the

3.8. Interference study In order to study the selectivity of GC/AuNPs/PEI/GNs/PAF modified electrode, different ions were chosen as interferences. No interference was observed with common cations and anions (100-fold quantities of K+, Na+, Ca+2, Al+3, Cl- and SO4-2 ions) and for 100-fold quantities of glucose (GL), salicylic acid (SA), and tartaric acid (TA) in determination of 5 mM of iodate concentration, the deviation of the determination was within 3.2%.

3.9. Real samples analysis To investigate the applicability of the proposed method to real sample analysis, the 16

ACCEPTED MANUSCRIPT modified electrode was applied to monitoring of iodate in water samples. The standard addition method is used to quantify the iodate concentration in real samples. The reason for using the standard additions method is that the matrix of real sample may contain other

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components that interfere with the analyte signal causing inaccuracy in the determined

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concentration. Table 2 shows the results obtained for iodate content of the real samples. It is obvious that the obtained recovery rates are in the range of 99.5–100.7 %. Therefore, the

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modified proposed sensor can be used for iodate detection in real samples.

4. Conclusion

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We have demonstrated the suitability of GC/AuNPs/PEI/GNs/PAF as an ideal catalyst for low-potential determination of iodate with a high sensitivity. The experimental results

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reported above demonstrate that (i) the PAF can be firmly deposited on the GC/AuNPs/PEI/GNs by amid bonds formation between the carboxyl groups on the graphene and the amino groups on the PAF and electrochemical method; (ii) the GC/AuNPs/PEI/GNs/

D

PAF can catalyze the reduction of iodate at pH 2; (iii) the kinetics of catalytic reaction is

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fast; and (iv) it is stable and has short response time, low detection limit, high sensitivity and low operation potential. It can be used as an impedimetric electrochemical sensor for

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monitoring of iodate. Acknowledgements

The authors gratefully acknowledge the support of this work by the Khorramabad

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Branch, Islamic Azad University for financial support.

17

ACCEPTED MANUSCRIPT References [1]A.G. Gilman, L.S. Goodman, T.W. Rad, F. Murad, The Pharmacological Basis of Therapeutics, 7th edition, Macmillan, New York, 1985.

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[3] M. A. Tabrizi , L. Ebrahimi, J. Electroanal. Chem. 724 (2014) 8-14.

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[34] Y. Li, H. Kuwabara, Y.-K. Gong, Y. Takaki, K. Nakashima, J. Photochem. Photobiol. B: Biol. 70 (2003) 171-177. [35] V. K. Sharma, P. D. Sahare , R. C. Rastogi, S. K. Ghoshal, D.Mohan, Spectrochim. Acta Part A, 59 (2003) 1799-1804. [36] W.S. Hummers, R.E. Offeman, J. Am. Chem. Soc. 80 (1958) 1339. [37] Y.Tian,J.G.Park,Q.Cheng,Z.Liang,C.Zhang,B.Wang,Nanotechnology20 (2009)335601– 335606. [38] X. Hu,T.Wang,X.Qu,S.Dong,J.Phys.Chem.B110(2006)853–857. [39] S. Park,A.D.Dikin,T.S.Nguyen,S.R.Ruoff,J.Phys.Chem.C113(2009) 15801–15804. [40] X. Hu,T.Wang,L.Wang,S.Guo,S.Dong,Langmuir23(2007)6352–6357. [41]

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[45] H. Hung, Y. Li, Y. Chen, L. Wang, Sensor. Actuat. B: Chem. 134 (2008) 780–786. [46] C.R. Tang, Z.h. Su, B.G. Lin, H.W. Huang, Y.L. Zeng, S. Li, H. Huang, Y.J. Wang,

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[53]L. Kosminsky and M. Bertotti, Electroanalysis. 11(1999) 623–626.

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[54] A.H. Liang, Z.L. Jiang, B.M. Zhang, Q.Y. Liu, J. Lan, X. Lu, Anal. Chim. Acta. 530 (2005) 131–134.

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[58] A.J. Bard, L.R. Faulkner, Electrochemical Methods: Fundamentals and Applica-tions, JohnWiley and Sons, New York, 2000 [59] Z. Galus, Fundamentals of Electrochemical Analysis, Horwood, New York, 1976.

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ACCEPTED MANUSCRIPT Figure caption Scheme .1 Overall preparative process of theGC/AuNPs/PEI/GNs/PAF electrode.

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Scheme 2. The redox reactions of PAF film.

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Fig.1 Typical SEM image of GNs (A) AuNPs/PEI/GNs (B) and GC/AuNPs/PEI/GNs/PAF

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electrode (C).

Fig. 2(A)Consecutive cyclic voltammograms of GC/AuNPs/PEI/GNs electrode in 0.1 M acetate buffer solution containing 1.5× 10 -3 M acriflavine at a scan rate of 50 mV s-1.(B) The on

the

peak

currents

of

GC/PAF

(curve

a,

b

and

c)

and

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variation

GC/AuNPs/PEI/GNs/PAF(curve d and e) electrodes as a function of the cycle number during

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electropolymerization in the preset of 0.5 mM (curve a), 1mM (curve b) and 1.5mM (curve c) of acriflavine and 1.5 mM acriflavine and 0.5 mg mL-1 AuNPs/PEI/GNs (curve d) and 1.5

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mM acriflavine and 1 mg mL-1 AuNPs/PEI/GNs composite.

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Fig.3 Cyclic voltammograms of GC/PAF (a), GC/PEI/PAF (c), GC/AuNps/PEI/PAF (b) and GC/AuNPs/PEI/GNs/PAF (d) in 0.1 M acetate buffer solution (pH 5) at a scan rate of 50

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mVs-1.

Fig.4 Nyquist plots for bare GC (curve d), GC/ AuNPs/PEI (curve a), GC/ AuNPs/PEI/GNs (curve b) and GC/AuNPs/PEI/GNs/PAF (curve c) electrodes recorded in 0.1 M KCl solution

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containing 0.5 mM Fe(CN)63−/4− in the frequency range of 10 kHz-0.1 Hz.

Fig.5 (A) CVs of GC/AuNPs/PEI/GNs (a and b) and GC/AuNPs/PEI/GNs/PAF (c and d) in the absence (a and c) and the presence (b and d) of 2 mM of iodate, respectively at a scan rate of 50 mVs-1

Fig.6 Nyquist plots of bare GC electrode (curve a), GC/PAF (curve b), GC/AuNPs/PEI/PAF (curve c) and GC/AuNPs/PEI/GNs/PAF (curve d) in 0.1M PBS (pH 2) containing 4 mM iodate in the frequency range of 10 kHz-0.1 Hz.

21

ACCEPTED MANUSCRIPT Fig.7 (A) CVs of the GC/AuNPs/PEI/GNs/PAF in the presence of different iodate concentration:, 3, 5, 7.5, 10,13,16, 20 ,30, 45, 50, 75, 100,150, 200, 300, 400, 500 and 600 mM respectively in 0.1M PBS (pH 2) at a scan rate of 50 mV s−1. (B) The plot of catalytic

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peak vs. iodateconcentration.

Fig.8 (A) Cyclic voltammograms of the modified electrode in the presence of 4 mM iodate at

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various scan rates: 10, 40, 70, 100, 130, 160,190, 225, 280, 320, 350, 380, 410, 440, 470 and 500 mV s−1 in 0.1M PBS (pH 2).

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Fig. 9 (A) EIS response of the GC/AuNPs/PEI/GNs/PAF in the presence of different iodate concentration: 0.5, 3, 7, 8, 9, 10, 11, 12, 13 and 14 nM respectively in 0.1 M KCl solution (B) the

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containing 0.5 mM Fe(CN)63−/4− in the frequency range of 10 kHz-0.1 Hz. corresponding calibration ploto f Rct versus different concentrations of iodate.

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Fig.10 (A) Chronoamperograms obtained on GC/AuNPs/PEI/GNs/PAF in the presence of

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iodateat various concentrations (B) Plots of I versus t-1/2 and (C) (Icat/IL) versus t1/2 derived

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from the data of the main panel for various concentrations of iodate.

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Scheme 1

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Scheme.2

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Fig.2

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ACCEPTED MANUSCRIPT Table 1: Comparison of the performances toward iodate from different methods. Applied potential

LOD (M)

Linear Range (M)

References

-0.29

1.4× 10-6

5.0×10-6– 5.0×10-4

[45]

-

6.0 × 10-9

CoW11Co/GCE

-0.5

8.0 × 10-7

Molybdenum oxide

-0.20

5.0× 10-7

PW12O403-/CPE

-0.1

3.1 × 10-6

12-MP-CCE

-0.20

LixMoOy/Au

-0.20

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1.0× 10-6 1.0× 10-7

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Quantum dot fluorescence

Nanocluster fluorescence

-

2.8 × 10-9

-0.40

6.0× 10-6

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Molybdenum oxide

-

3 ×10-8

[PEI/PSSCu@AgNPs/PEI/P8W48]5/ITO Ion chromatography with UV

-0.50

4.0 ×10-8

-

2.3 × 10-7

AuNPs/PEI/CNTs-COOH/ORC

+0.15

1.7× 10-7

This work

+0.15

0.1× 10-9

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Resonance scattering spectroscopy

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Analytical method or Electrode materials(Modified electrode) AuNPs/ P-3MTP

NPs: Nanoparticle P-3MTP:poly(3-methylthiophene) composites CPE: carbon paste electrode CCE :carbon ceramic electrode CNTs: carbon nanotube PEI: polyethyleneimine PSS: poly sodium-pstyrenesulfonate ORC: organoruthenium(II) complexes

35

1.0×10-8– 1.0 × 10-5 2.0 × 10-6– 2.8 ×10-4 1.0×10-6– 2.0×10-4 5 × 10-6– 1 × 10-3 5.0×10-6– -6.0×10-3 3.0×10-51.0×10-4 1 ×10-8– 1 ×10-6 1.0×10-51.0×10-3 1×10-7– 2 ×10-6 1.0 × 10-7– 3.0 × 10-4 5.05 ×10-7– 5.05 ×10-5 1.0×10-6– 2.0×10-3 0.50× 10-61.4× 10-7

[46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [57]

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Table 2.Determination of iodate in two water samples (%, ±R.S.D. calculated & based on five

Sample

Added

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measurements).

Found(nM)

Water sample .1

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(nM) 70.0

69.8±0.4

110.6±0.6

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110.0

100.5

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140.0

Water sample.2

70.0

139.9±0.5 99.9

70.4±0.4

110.0

100.5

140.0

109.5±0.3 99.5 141.1±0.4 100.7

36

Recovery(%)

99.7