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Electrochemical

Department of Chemistry, Faculty of Sciences

79161-93145, Iran. E-mail: s.dianat@hormo

† Electronic supplementary informa10.1039/d1ra00845e

Cite this: RSC Adv., 2021, 11, 8993

Received 31st January 2021Accepted 22nd February 2021

DOI: 10.1039/d1ra00845e

rsc.li/rsc-advances

© 2021 The Author(s). Published by

investigation and amperometrydetermination iodate based on ionic liquid/polyoxotungstate/P-doped electrochemicallyreduced graphene oxide multi-componentnanocomposite modified glassy carbon electrode†

Minoo Sharifi, Somayeh Dianat * and Amin Hosseinian

A novel modified glassy carbon electrode (GCE) was successfully fabricated with a tetra-component

nanocomposite consisting of (1,10-(1,4-butanediyl)dipyridinium) ionic liquid (bdpy), SiW11O39Ni(H2O)

(SiW11Ni) Keggin-type polyoxometalate (POM), and phosphorus-doped electrochemically reduced

graphene oxide (P-ERGO) by electrodeposition technique. The (bdpy)SiW11Ni/GO hybrid nanocomposite

was synthesized by a one-pot hydrothermal method and characterized by UV-vis absorption, Fourier

transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD) analysis, thermogravimetric-differential

thermal analysis (TGA/DTA), and transmission electron microscopy (TEM). The morphology,

electrochemical performance, and electrocatalysis activity of the nanocomposite modified glassy carbon

electrode ((bdpy)SiW11Ni/P-ERGO/GCE) were analyzed by field emission scanning electron microscopy

(FE-SEM) coupled with energy-dispersive X-ray spectroscopy (EDS), cyclic voltammetry (CV), square

wave voltammetry (SWV), and amperometry, respectively. Under the optimum experimental conditions,

the as-prepared sensor showed high sensitivity of 28.1 mA mM�1 and good selectivity for iodate (IO3�)

reduction, enabling the detection of IO3� within a linear range of 10–1600 mmol L�1 (R2 ¼ 0.9999) with

a limit of detection (LOD) of 0.47 nmol L�1 (S/N ¼ 3). The proposed electrochemical sensor exhibited

good reproducibility, and repeatability, high stability, and excellent anti-interference ability, as well as

analytical performance in mineral water, tap water, and commercial edible iodized salt which might

provide a capable platform for the determination of IO3�.

1. Introduction

Polyoxometalates (POMs) are a family of anionic nanoclustersconsisting of early transition metals and oxygen atoms to forma closed 3-dimensional framework.1,2 POMs, especially Keggin-type POMs, have different applications in the elds of catal-ysis,3–6 medicine,7–9 and molecular materials10,11 due to theirexcellent redox character, unique molecular structure, elec-tronic versatility, and easy preparation. One of the attractiveproperties of POMs is that the anionic cluster can undergoa fast, stepwise, reversible, and multi-electron transfer reactionwhile keeping the structural integrity.2,12–14 The multiple redoxproperties make them attractive candidates in surface modi-cation, electroanalysis, and electrocatalysis.2,13–15

Different methods have been developed to achieve chemi-cally modied electrodes (CMEs) with POMs such as

, University of Hormozgan, Bandar Abbas

zgan.ac.ir; Tel: +98 76 33670121

tion (ESI) available. See DOI:

the Royal Society of Chemistry

electrodeposition at a very negative potential,16,17 adsorption,18

entrapment into conducting,19,20 or non-conducting polymersmatrixes,21,22 layer-by-layer self-assembly,23,24 Langmuir–Blodg-ett (LB) technique,25,26 preparation of self-assembled monolayerand multilayer thin-lms,27 and bulk modication of carboncomposites and carbon paste matrices.28 However, the electro-catalytic activity of POMs-modied electrodes is limited due totheir low stability in aqueous solutions15,27,29 and low specicsurface area.30

To achieve the enhanced stability of POMs, many studieshave been focused on the design and construction of modifyingand assemble POMs with suitable organic matrices, which isone of the most exciting elds in the materials chemistry.27,31–33

Recently, ionic liquids (ILs) have concerned more attention,34–36

due to owning highlight properties such as good aqueoussolubility, high electrical conductivity, non-volatility, non-ammability, low toxicity, high thermal and electrochemicalstabilities, and large electrochemical potential window.37

Furthermore, immobilization or dispersion of POMs ondifferent supporting carbon materials has attracted signicantattention due to their small background currents, wide

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potential windows, low-cost, excellent chemical stability, anda strong affinity for POMs.38–43

Graphene was discovered in 2004 as a new carbon nano-structure by Novoselov and Geim.44 Graphene is a two-dimensional (2D) crystal composed of a monolayer of sp2

hybridized carbon atoms arranged in a hexagonal packingstructure.45 Graphene is the thinnest and strongest materialknown to exist in the world.46 Graphene has been widely studiedin the nanotechnology and nanoscience applications due toexclusive properties such as good chemical stability, greatspecic surface area (SSA, about 2600 m2 g�1), high electronmobility, and good thermal conductivity.13 Graphene is a propercandidate for a POM support material to solve both high solu-bility and low surface area of POMs.47 Moreover, the interestingproperties of POMs, particularly as catalysis,48,49 electro-catalysis,50,51 and capacitance52,53 can be improved by the largeSSA and good conductivity of graphene. So far, many studieshave been reported on POM/graphene nanocomposites, inwhich graphene oxide (GO) is applied as support and POMs areimmobilized onto the reduced GO (RGO) through variousreduction methods such as UV-photoreduction,54–57 and chem-ical reduction with the help of hazardous substances (e.g.,hydrazine).58 However, these methods generally obtainedpowder POM/RGO nanocomposites that need to be depositedon the electrode surface using the drop-casting technique. Thispreparation method has intrinsic limitations such as lowstability and lack of control of the lm thickness.59,60 The self-assembly technique is the best method for the construction ofthe stable and uniform modied surfaces. In some research,POM/RGO nanocomposite is prepared through the layer-by-layer electrostatic assembly of POM/GO followed by the UV-photoreduction step.24,61 Lately, the electrochemical reductionof GO to RGO has attracted great attention due to its green andsimple preparation method.59,60,62,63 Chen et al. deposited RGOon the surface of a glassy carbon electrode (GCE) by cyclic vol-tammetric (CV) reduction of a GO colloidal solution, whichpresented the high-resolution ability to the catechol andhydroquinone isomers.60 Weihua Guo et al. reported the prep-aration and electrocatalytic activity of Keggin-type hetero-polymolybdate H4SiMo12O40 and chitosan-electrochemicallyRGO multilayer composite lms on indium tin oxide electrode(ITO) for determination of S2O8

2� with excellent sensitivity anda low detection limit.59 Mai et al. reported the preparation ofRGO/copper composite from a surfactant-free colloidal solutioncomprised of [CuIIEDTA]2� complexes and GO sheets by elec-trochemical reduction.62 Recently, many studies have conrmedthat doping of heteroatoms can well modify the electronicstructure and enhance the electrochemical properties of RGO.For instance, boron, nitrogen, phosphorus, and sulfur atomsdoped into graphene can improve the electrical conductivityand electrocatalytic activity of RGO, which can be applied ashigh-performance supercapacitor electrode,64,65 lithium-ionbatteries,66–69 fuel cells,70,71 and dye-sensitized solar cells.72

Although the electrochemical reduction method has manyadvantages in preparing of the P-doped graphene monolayerlm modied electrode, few studies have been reported con-cerning the modied electrode with POM/P-ERGO and its

8994 | RSC Adv., 2021, 11, 8993–9007

derivatives prepared by this method. Using the electrochemicalreduction method for the fabrication of the POM/P-ERGOmodied electrode still has challenges.

In this study, a new organic–inorganic nanocomposite (1,10-(1,4-butanediyl)dipyridinium)3 [SiW11O39Ni(H2O)]/GO has beenprocessed on GCE through electrochemical reduction method((bdpy)SiW11Ni/P-ERGO/GCE). For this purpose, (bdpy)SiW11Ni/GO nanohybrid compound was deposited on a GCE by one-stepdrop-casting, followed by the electrochemical reduction of GOthrough chronoamperometry at a constant potential of �1.5 V(vs. Ag/AgCl), for 60 s, in a phosphoric acid solution(0.1 mol L�1) containing bdpy (1.0 mmol L�1). (bdpy)SiW11Nican interact with P-ERGO via Pd+ and p-electrons of bdpywithout affecting the electronic structure of POM. Conse-quently, the presence of bdpy provides an additional advantagefor increasing the loading of POM and improving the stability ofnanocomposite lm due to the strong electrostatic attractionbetween [SiW11Ni]

6� and positively charged bdpy. Then, elec-trochemical and electrocatalytic behaviors, selectivity, repeat-ability, reproducibility of the proposed sensor for iodate (IO3

�)determination, and also the recovery in the mineral water, tapwater, and a commercial edible iodized salt were investigated.

2. Experiment2.1. Materials and reagents

The K6[SiW11O39Ni(H2O)]$nH2O (SiW11Ni) was synthesizedaccording to the previous literature.73 Tungstosilicic acidhydrate (H4SiW12O40$nH2O, HSiW), potassium acetate (CH3-COOK), nickel(II) nitrate hexahydrate (Ni(NO3)2$6H2O), 1,10-(1,4-butanediyl)dipyridinium dibromide (bdpy) ionic liquid, glacialacetic acid (CH3COOH), phosphoric acid (H3PO4), sulfuric acid(H2SO4), graphite powder (<20 mm), ethanol (C2H5OH), meth-anol (CH3OH), hydrogen peroxide (H2O2), potassium perman-ganate (KMnO4), potassium hexacyanoferrate(III) (K3[Fe(CN)6]),and potassium iodate (KIO3) with analytical grade werecommercially purchased from Sigma or Merck companies. Allsolutions were prepared with deionized water (18 MU cm (25�C), Milli Q, Millipore Inc.).

2.2. Apparatus

Attenuated total reectance-Fourier transform infrared (ATR-FTIR) spectra were acquired on a Spectrum Two spectrometer-PerkinElmer in the 2400–400 cm�1 wavenumber range. UV-visabsorption spectra of the SiW11Ni, bdpy and (bdpy)SiW11Niwere recorded using a Unico SQ-4802 double beam UV-visspectrophotometer in a 1 cm matched quartz cell. The crystal-line structures of the SiW11Ni, bdpy, (bdpy)SiW11Ni, GO, and(bdpy)SiW11Ni/GO were investigated by an X-ray diffractiontechnique (XRD, Panalytical X'pert PRO, The Netherlands)using Cu Ka radiation. The thermal features of bdpy, SiW11Ni,and (bdpy)SiW11Ni/GO were evaluated usingthermogravimetric/differential thermal analysis (TGA/DTA,Rheometric Scientic STA 1500) under air ow in the temper-ature range of 25–800 �C at a heating rate of 10 �C min�1.Transmission electron microscope (TEM, EM10C-100 kV series,

© 2021 The Author(s). Published by the Royal Society of Chemistry

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Zeiss Co., Germany) was used to study and compare themorphology of the (bdpy)SiW11Ni/GO with GO. The morphologyand elemental analysis of the bare GCE and (bdpy)SiW11Ni/P-RGO/GCE were analyzed by eld emission scanning electronmicroscopy (FE-SEM, SIGMA VP, Zeiss Co., Germany), coupledwith energy-dispersive X-ray spectrometry (EDS) and EDS-mapping.

2.3. Electrochemical measurements

The electrochemical measurements were carried out using anAutolab potentiostat (P/GSTAT 302N, Metrohm Autolab Eco-Chemie Utrecht, The Netherlands) equipped with GPES,version 4.9. A conventional three-electrode system wasemployed containing bare or modied GCE (GR-2S/N, Iran,Tehran, diameter 2 mm), an Ag/AgCl (3 mol L�1 KCl) electrode(Metrohm, Switzerland), and a platinum rod (IV-EL/EB-2200,Ivium, Eindhoven, The Netherlands) as working, referenceand counter electrodes, respectively. Prior to the electro-chemical experiments, high-purity argon gas (99.999%) waspurged into the electrolytic cell solutions for 10 minute andcovered by the Ar-atmosphere during the experiments. Allexperiments were done at ambient temperature.

2.4. Preparation of (bdpy)3[SiW11O39Ni(H2O)]$nH2O ((bdpy)SiW11Ni)

First, a solution of 14.0 g of HSiW (4.72 mmol) in 50 mL distilledwater at 95 �C was prepared. Then, a hot solution of 1.1 gNi(NO3)2$6H2O (6.05 mmol) in 10 mL distilled water was addeddrop by drop, followed by a solution of 15.0 g CH3COOK in15 mL hot distilled water (pH regulated to 7 with acetic acid).The stirred solution was kept at 95 �C for one minute and wasquickly ltered. Then, the ltrate was cooled to 50 �C, and anequal volume of methanol was added. In continuous, solid bdpyionic liquid was gradually added into the solution until no moreprecipitate formed. Finally, the light green powders of (bdpy)

Scheme 1 Schematic diagram of stepwise electrode modification proce

© 2021 The Author(s). Published by the Royal Society of Chemistry

SiW11Ni were ltered, well washed with ethanol and diethylether, and dried at room temperature (yield � 85%).

2.5. Preparation of (bdpy)SiW11Ni/GO

GO was obtained from graphite powder using modiedHummer's method.74 A suspension of 30 mg GO in 10 mL ofdistilled water was prepared, followed by sonication (30 min) toform a uniform suspension. Then 1.5 g of (bdpy)SiW11Ni (0.34mmol) was added. Aer sonication (20 min), the mixture waspoured into a 100 mL Teon-lined stainless-steel autoclave. Theautoclave was transferred to an oven at 180 �C for 24 h. Theobtained black precipitation ((bdpy)SiW11Ni/GO) was washedwith distilled water and dried at 60 �C for 12 h in a vacuum oven.

2.6. Fabrication of (bdpy)SiW11Ni/P-ERGO/GCE

Before the electrode modication, the bare GCE was cleaned bypolishing with 0.30 mm, 0.10 mm, and 0.05 mm a-Al2O3 slurriesto elimination the adsorbed impurities. The polished electrodewas sonicated in deionized water, ethanol, and deionized waterbath, respectively, each for 5 minute. Then the GCE was treatedwith 0.5 mol L�1 H2SO4 solution by repeated cycling in thepotential range from�1.0 to 1.0 V (vs. Ag/AgCl) until a stable CVwas obtained, followed by the rinsing with deionized water andnatural drying at room temperature. Next, 10 mg of as-prepared(bdpy)SiW11Ni/GO nanocomposite was dispersed in 5 mL ofethanol and sonicated for 30 minutes to get a homogenouslydispersed solution. 5 mL of the (bdpy)SiW11Co/GO suspensionwas drop-casted on the cleaned bare GCE, and the solvent wasremoved under room temperature and washed with deionizedwater. Then, electrochemical reduction of GO to RGO was per-formed in 0.1 mol L�1 H3PO4 containing various concentrationsof bdpy (0.5, 1.0, 5.0, and 10.0 mmol L�1) using chro-noamperometry in the potential of�1.5 V at different times (30,60, 90, and 120 s). A better analytical signal obtained in thepresence of 1.0 mmol L�1 bdpy and at time 60 s. Hence,

sses.

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1.0 mmol L�1 bdpy and time 60 s were used as optimum factorsin the fabrication of the modied working electrode in allstudies. For comparison, SiW11Ni/GCE, bdpy/GCE, (bdpy)SiW11Ni/GCE, and P-ERGO/GCE were prepared by the samemethod. The schematic illustration of the preparation proce-dure of the as-prepared modied electrode is shown in Scheme1.

2.7. Electrochemical procedure

The electrochemical measurements were carried out in a 90 mLelectrochemical cell, containing 5 mL of HClO4 (0.1 mol L�1, pH1.5) solution and various concentrations of IO3

�. All the solu-tions within the cell were purged with high-pure argon for 10minute before to the electrochemical analysis. The CV, squarewave voltammetry (SWV), and amperometry techniques wereused for the investigation of electrochemical behavior andelectrocatalytic activity of the modied electrode in IO3

�

determination.

Fig. 1 XRD patterns of (a) SiW11Ni, (b) (bdpy)SiW11Ni, (c) bdpy, (d) GO,and (e) (bdpy)SiW11Ni/GO.

3. Results and discussion3.1. Characterization

The carbon, nitrogen, silicon, tungsten, and nickel content of(bdpy)SiW11Ni were analyzed by elemental analysis. Calculatedvalues [%]: C: 13.96; N: 2.32; Si: 0.78; W: 56.02; Ni: 1.63.Experimental values [%]: C: 13.75; N: 2.38; Si: 0.72; W: 52.71; Ni:1.67. The obtained results approve that experimental valuesagree with the theoretical values calculated based on formula(bdpy)3SiW11Ni. Moreover, the molar ratios of Si : W : Ni ¼1 : 11.2 : 1.1 prove that the Keggin structure of SiW11Ni hasbeen retained throughout the functionalization with bdpy.

XRD analysis is done for the crystalline structure investiga-tion of the (bdpy)SiW11Ni/GO. The XRD patterns of SiW11Ni,(bdpy)SiW11Ni, bdpy, GO, and (bdpy)SiW11Ni/GO are presentedin Fig. 1. As displayed in Fig. 1b, (bdpy)SiW11Ni shows (001),and (002) diffraction peaks, which are located at 8.3�, and 16.5�,respectively of the SiW11Ni (Fig. 1a). Other obvious character-istic peaks, at 15.4�, and 20.2� can be credited to the bdpy ionicliquid (Fig. 1c). Fig. 1d is the XRD pattern of GO. Two relativelybroad peaks are observed at 9.1� and 26.4�, which correspond to(001) and (002) reections of GO and graphite existent in thestructure of GO, respectively.75,76 In Fig. 1e, the characteristicXRD peaks of (bdpy)SiW11Ni/GO appeared at nearly similarlocations without a signicant shi in its peaks positioncompared to those of SiW11Ni. Additionally, the characteristicpeaks at 26.4� (002), conrmed that (bdpy)SiW11Ni was immo-bilized on the GO surface. However, the peak (001) was notobserved due to the overlap with the peak of SiW11Ni, which hasa higher crystallinity.

The TGA/DTA diagrams for bdpy, SiW11Ni, and (bdpy)SiW11Ni/GO hybrid composite under the air atmosphere areshown in Fig. 2.

As shown in Fig. 2A, bdpy shows a signicant weight loss ofabout 61.3% at the temperature range of 250–320 �C (3rd peakof the DTA curve) that can be credited to the decomposition ofpyridine rings.

8996 | RSC Adv., 2021, 11, 8993–9007

The thermal analysis of SiW11Ni shows a slight weight loss(about 3.4%) at temperatures below 150 �C (1st peak of DTA),which may be related to eliminating adsorbed water moleculesinto SiW11Ni crystal lattice. A weight loss (about 13.2%) isobserved in 150 to 384 �C temperature range during two steps(2nd and 3rd peak of DTA), which can be attributed to theremoval water of crystallization from the crystal lattice ofSiW11Ni. Above 400 �C, no signicant change in mass isobserved, indicating its high thermal stability up to 800 �C(Fig. 2B).

As shown in Fig. 2C, the thermal stability of the (bdpy)SiW11Ni/GO is entirely more than the parent bdpy but is slightlyless than the original SiW11Ni. Below 250 �C, no major changein weight loss was observed which conrms that the amount ofwater in the hybrid material was reduced but not eliminated. A

© 2021 The Author(s). Published by the Royal Society of Chemistry

Fig. 2 TGA-DTA curves of (A) bdpy, (B) SiW11Ni, and (C) (bdpy)SiW11Ni/GO in the temperature range of 25–800 �C at a heating rate of10 �C min�1.

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slight weight loss (0.9%) has appeared at 25–50 �C temperaturerange. Above 250 �C, two steps in weight loss 6.2% at 261 �C,and 14% at the temperature range of 331–557 �C are observed,correlated to the oxidative decomposition of bdpy, and GO,respectively.

The ATR-FTIR and UV-vis studies were also carried out, asshown in ESI (Fig. S1, Table S1, and Fig. S2).†

The TEM images of the GO and (bdpy)SiW11Ni/GO atdifferent magnications were presented in Fig. 3. TEM imagesof (bdpy)SiW11Ni/GO exhibit that (bdpy)SiW11Ni nanosphereswere dispersed homogeneously on the GO surface. Indeed,such homogeneous dispersal of the (bdpy)SiW11Ni nano-spheres on the surface of GO enhanced the electrochemicalactivity.

3.2. FE-SEM/EDS characterization of (bdpy)SiW11Ni/P-ERGO/GCE

The surface morphology of the (bdpy)SiW11Ni/P-ERGO/GCE wascharacterized by FE-SEM. The FE-SEM images of the bare GCE(Fig. 4A(a and b)) have smooth and mirror surface, which wasconverted to heterogeneously and crudely surface aer themodication with (bdpy)SiW11Ni/P-ERGO (Fig. 4B(a and b)).Furthermore, the EDS of the bare and modied GCE are dis-played in Fig. 4A and B(c), respectively. The EDS spectra andelemental mappings approve the elemental composition, whichconrms the presence of C, and O in the bare GCE (Fig. 4A(c–e)),

© 2021 The Author(s). Published by the Royal Society of Chemistry

and W, C, O, N, P, Si and, Ni elements on the (bdpy)SiW11Ni/P-ERGO/GCE (Fig. 4B(c–j)).

3.3. Optimization of coating parameters

To gain a modied electrode with a rich electrochemicalbehavior, optimizing of experimental conditions is signicantlynecessary. Some essential factors, including type, and concen-tration of electrolyte additive were studied. These factors couldhave an impact on the electrochemical activity of the modiedelectrode. The following experiments were examined using CVin 0.1 mol L�1 H3PO4 supporting electrolyte at the potential of�1.5 V, and the deposition time 60 s.

3.3.1. Electrolyte additive type effect. Fig. 5A, shows theeffect of electrolyte additive type (without additive, SiW11Niadditive, and bdpy additive), on the electrochemical behavior ofthemodied electrode. Themodied surface, which is preparedin bdpy electrolyte additive (curve (c)), shows two-step redoxpeaks with less peak-to-peak separations (DEp) and more fara-daic peak currents (ip) with respect to the case without additive(curve (a)). In contrast, no peak is observed in a similar potentialrange at the modied electrode in the presence of SiW11Niadditive (curve (b)). This phenomenon proves that bdpy ionicliquid, due to high ionic conductivity, can improve the electrontransfer between the modier and the electrode surface.

3.3.2. Electrolyte additive concentration effect. Theoptimum concentration of bdpy additive was studied at

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Fig. 3 TEM images of (A) GO (a and b) and (B) (bdpy)SiW11Ni/GO (a–d) at different magnifications.

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different values of 0.5, 1.0, 5.0, and 10.0 mmol L�1 (Fig. 5B). Themaximum peak currents of the modied electrode appeared inthe presence of 1.0 mmol L�1 bdpy additive (curve (b)). The CVsignal of the modied electrode increased with bdpy concen-tration up to 1.0 mmol L�1 and then decreased (compare curve(b) with curve (c), (d)). This situation is related to the viscosity of

8998 | RSC Adv., 2021, 11, 8993–9007

bdpy ionic liquid. The viscosity is an essential parameter toevaluate the performance of an IL electrolyte, which directlyinuences the ions diffusion. Generally, high viscosity causesinferior performance. The interactions of the IL systems arethrough the interaction of cations and anions, leading toa signicant limitation of IL electrolyte with high-viscosity.

© 2021 The Author(s). Published by the Royal Society of Chemistry

Fig. 4 (A) FE-SEM images (a and b), EDS pattern (c) and EDS element mapping (d and e) of the bare GCE and (B) FE-SEM images (a and b), EDSpattern (c) and EDS element mapping (d–j) of (bdpy)SiW11Ni/P-ERGO/GCE.

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3.4. Electrochemical behavior of (bdpy)SiW11Ni/P-ERGO/GCE

The electrochemical properties of the bare GCE, (bdpy)SiW11Ni/GCE, and (bdpy)SiW11Ni/P-ERGO/GCE were studied in0.1 mol L�1 HClO4 electrolyte using CV technique (Fig. 6A(a–c)

© 2021 The Author(s). Published by the Royal Society of Chemistry

respectively). To better comparison, the CVs of the P-ERGO/GCE, bdpy/GCE, and SiW11Ni/GCE were displayed in inset ofFig. 6A(d–f) respectively.

However, at the (bdpy)SiW11Ni/GCE, and (bdpy)SiW11Ni/P-ERGO/GCE appeared two pairs of pseudo-reversible peakswith the half-wave potentials (E1/2 ¼ (Epa + Epc)/2) of �529 (I–I0),

RSC Adv., 2021, 11, 8993–9007 | 8999

Fig. 5 Effect of (A) electrolyte additive type, (B) concentration of bdpyadditive in 0.1 mol L�1 H3PO4; potential of �1.5 V; deposition time 60,on the electrochemical behavior of (bdpy)SiW11Ni/P-ERGO/GCE.

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and �709 (II–II0) mV respectively, that correspond to the twosequential one-electron transfer processes of W6+ / W5+ /

W4+. Also, as shown in Fig. 6A, there is a threefold increase ofpeak currents in the (bdpy)SiW11Ni/P-ERGO/GCE (curve (c))compared to the (bdpy)SiW11Ni/GCE (curve (b)), and bothfactors conrm that the P-ERGO can promote the electrontransfer between the GCE surface and the (bdpy)SiW11Ni ina more effective way. In contrast, at the bare GCE (curve (a)),bdpy/GCE (curve (e)), and SiW11Ni/GCE (curve (f)), no signi-cant peak is observed under identical experimental conditions.At the P-ERGO/GCE (curve (d)), a broad redox peak appearedthat was not in the potential range of electrochemical activity of(bdpy)SiW11Ni.

3.4.1. Scan rate effect on the electrochemical behavior of(bdpy)SiW11Ni/P-ERGO/GCE. The CVs of the (bdpy)SiW11Ni/P-ERGO/GCE in HClO4 (0.1 mol L�1) at various scan rates (20–300 mV s�1) were displayed in Fig. 6B. As shown, the cathodicpeak current (peak I) increases linearly with increasing scan ratefrom 20 to 300 mV s�1 (inset of Fig. 6B) indicating the electro-chemical behavior of this modied electrode is surface-controlled. Furthermore, the potential of peaks did not

9000 | RSC Adv., 2021, 11, 8993–9007

signicantly shi with increasing scan rate, suggesting that theelectron transfer kinetic was very fast on the modied electrodesurface.

3.4.2. pH effect on the electrochemical behavior of (bdpy)SiW11Ni/P-ERGO/GCE. The effect of buffer pH on the electro-chemical behavior of the (bdpy)SiW11Ni/P-ERGO/GCE wasstudied in 0.1 mol L�1 acetate buffer at different pH values (1.5–6.0), using SWV, as shown in Fig. 6C. It is clearly seen that withincreased pH, the cathodic peak current (ipc) reduced until nosignicant peak was observed at pH 6, and peak (II) potential(Ep) shis negatively with a linear relationship of Epc vs. pH: Epc(V) ¼ �0.052pH � 0.6896 (R2 ¼ 0.9976) with the slope of 52 mV/pH. The relationship between peak potential and pH, Nernstequation,77 is described in eqn (1).

Ep ¼ 0:0592m

npHþ b (1)

where Ep is the peak potential (V), m and n are the number ofprotons and electrons participating in the electrochemicalreaction, respectively, and b is the intercept of the equation.From the straight line in the inset of Fig. 6C. The slope of 52mV/pH is close to the theoretical slope value of 59 mV/pH,which means the number of protons and electrons transferredin the redox reaction of the modier were equal.

3.5. Redox behavior of (bdpy)SiW11Ni/P-ERGO/GCE inpresence of [Fe(CN)6]

3�/4�

The interfacial electron transfer properties of the bare andmodied GCEs were evaluated by [Fe(CN)6]

3�/4� as a redoxcouple probe. The electron transfer kinetics of the Fe(CN)6

3�/4�

at the interface of the modied GCEs depends on the electronicstructure and thickness of the modier.27 As shown in Fig. 7A,curve a displays an intact pair of redox peaks at the bare GCEwith a DEp 102 mV and ipc 2.18 mA in Fe(CN)6

3�/4� solution in10 mmol L�1 PBS (pH 3) containing 0.1 mol L�1 KCl. Theimmobilization of SiW11Ni or P-ERGO on the GCE leads toa signicant increase in DEp and decrease of faradaic peakcurrents (ip) (Fig. 7A(b and d)). The SiW11Ni or P-ERGO lmserves as a barrier layer and blocks the electron transferbetween [Fe(CN)6]

3�/4� and the surface of GCE. However,a smaller DEp supplemented by an increase of ip is found for thebdpy/GCE (Fig. 7A(c)). It indicates that the bdpy ionic liquid canimprove the efficiency and rate of electron transfer at the GCEsurface. The (bdpy)SiW11Ni/GCE shows a greater DEp than thebare GCE but smaller DEp than the parent SiW11Ni (Fig. 7A(e)).The better reversible behavior of (bdpy)SiW11Ni versus SiW11Nican be credited to the presence of the bdpy ionic liquid, whichowns excellent electrical conductivity.

Aer modication of electrode by (bdpy)SiW11Ni/P-ERGOa meaning increase of DEp and decrease of ip compared withthe bare GCE and also (bdpy)SiW11Ni due to the presence of P-ERGO can be observed in Fig. 7A(f). The obtained quantitativeinformation from these voltammograms is illustrated in Table1.

The obtained CVs of (bdpy)SiW11Ni/P-ERGO/GCE in0.5 mmol L�1 [Fe(CN)6]

3�/4� at different pHs (pH 3–9) are

© 2021 The Author(s). Published by the Royal Society of Chemistry

Fig. 6 (A) CVs obtained on the bare GCE (a), (bdpy)SiW11Ni/GCE (b), (bdpy)SiW11Ni/P-ERGO/GCE (c), P-ERGO/GCE (d), bdpy/GCE (e), andSiW11Ni/GCE (f) in HClO4 (0.1 mol L�1, pH 1.5) and scan rate of 100 mV s�1, (B) CV of the (bdpy)SiW11Ni/P-ERGO/GCE in HClO4 (0.1 mol L�1, pH1.5) at different scan rates from 20 to 300 mV s�1. The inset shows variations of the cathodic peak I currents with scan rates, and (C) SWVs of the(bdpy)SiW11Ni/P-ERGO/GCE in acetate buffer (0.1 mol L�1) at different pHs from pH 1.5 to pH 6.0; scan rate 50 mV s�1. The inset shows thevariation of the cathodic peak I potentials with pHs.

Table 1 Electrochemical parameters extracted from CVs on the bare andmodified GCEs in the presence of [Fe(CN)6]3�/4� (0.5 mmol L�1) in PBS

(10.0 mmol L�1, pH 3) containing KCl (0.1 mol L�1) as supporting electrolyte (Fig. 7A)

Electrode DE/mV � SDa ipc/mA � SDa ipa/mA � SDa

Bare GCE 102 � 0.10 2.18 � 0.12 2.04 � 0.15SiW11Ni/GCE 124 � 0.11 2.01 � 0.09 1.89 � 0.06bdpy/GCE 78 � 0.09 2.20 � 0.14 2.14 � 0.08P-ERGO/GCE 431 � 2.21 0.60 � 0.08 0.39 � 0.04(bdpy)SiW11Ni/GCE 115 � 0.83 1.18 � 0.21 1.05 � 0.06(bdpy)SiW11Ni/P-ERGO/GCE 130 � 1.23 1.98 � 0.09 1.75 � 0.06

a SD: standard deviation (from 3 data point).

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presented in Fig. 7B. As shown in Fig. 7B, with pH increasing,the ips decreases and the DEp increases, so that at pHs above 7,almost no peak is observed. This behavior is described asfollows: pH-metric titration graphs using the drawing of varia-tion of the DEp and ip of (bdpy)SiW11Ni/P-ERGO/GCE versus pHshows a surface pKa of 5.6, and resulting, a positive charge-stateof the surface at lower pH values (inset of Fig. 7B). Therefore, itcan be supposed that the decreases in DEp, as well as theincreases in ips of the modied surface at acidic solutions havebeen affected by electrostatic interaction between the positively

© 2021 The Author(s). Published by the Royal Society of Chemistry

charged the surface of (bdpy)SiW11Ni/P-ERGO/GCE and[Fe(CN)6]

3�/4� redox probe.The effectively electroactive surface area of the (bdpy)

SiW11Ni/P-ERGO/GCE was evaluated by performing CVmeasurement at the (bdpy)SiW11Ni/P-ERGO/GCE in 0.5 mmolL�1 [Fe(CN)6]

3�/4� solution in 10.0 mmol L�1 PBS, pH 3 PBS, pH3 containing 0.1 mol L�1 KCl at different scan rates in the rangeof 10–150 mV s�1 (Fig. 7C). According to the Randles–Sevcikequation,78,79 the surface area was calculated by eqn (2).

RSC Adv., 2021, 11, 8993–9007 | 9001

Fig. 7 CVs obtained in the presence of 0.5 mmol L�1 [Fe(CN)6]3�/4� as a redox probe, in PBS solution (10.0 mmol L�1, pH 3) containing

0.1 mol L�1 KCl as supporting electrolyte, (A) on the bare GCE (a), SiW11Ni/GCE (b), bdpy/GCE (c), P-ERGO/GCE (d), (bdpy)SiW11Ni/GCE (e), and(bdpy)SiW11Ni/P-ERGO/GCE (f); scan rate: 50 mV s�1. (B) On the (bdpy)SiW11Ni/P-ERGO/GCE at different pHs. The inset shows the variation ofnormalized ipa and DEp extracted fromCVs of (bdpy)SiW11Ni/P-ERGO/GCE. Normalized ipa and DEp are (ipa(max)� ipa)/ipa(max) and (DEp(max)� DEp)/DEp(max), respectively. (C) On the (bdpy)SiW11Ni/P-ERGO/GCE at different scan rates from 10 to 150 mV s�1. The inset shows variations of theanodic peak currents with square root of scan rates.

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ip ¼ 2.69 � 105n3/2AeffD1/20 n1/2Cp (2)

where, ip is the peak current (A), n is the number of transferredelectrons, Aeff is the effective surface area (cm2), D0 is thediffusion coefficient of [Fe(CN)6]

3�/4� (7.6 � 10�6 cm2 s�1), n1/2

is the square root of scan rate (V s�1)1/2, and Cp is the concen-tration of the probe solution (mol L�1). The effective surfacearea of the (bdpy)SiW11Ni/P-ERGO/GCE is calculated to be 0.588cm2 according to the calibration equation of ip ¼ 0.2163n1/2 +0.2850 (R2¼ 0.9951) shown in inset of Fig. 7C, which is nearly 18times larger than the bare electrode (0.0314 cm2). However, thehigh electroactive surface area of the (bdpy)SiW11Ni/P-ERGO/GCE led to enhancing the electrocatalytic activity, which isconrmed in the following part.

3.6. Electrocatalytic properties of (bdpy)SiW11Ni/P-ERGO/GCE

The electrocatalytic activity of the (bdpy)SiW11Ni/P-ERGO/GCEfor IO3

� electro-reduction was investigated by CV technique in0.1 mol L�1 HClO4 aqueous solution (pH 1.5) containingdifferent concentrations of IO3

� (Fig. 8A). With the increasingconcentrations of IO3

�, the cathodic peak current at about

9002 | RSC Adv., 2021, 11, 8993–9007

�0.45 V (peak I) of the nanocomposite modied electrodegradually increased, whereas the corresponding oxidation peakcurrent decreased, suggesting that (bdpy)SiW11Ni/P-ERGO/GCEwould have a potential ability in the detection of IO3

�. However,the CV method, due to essential defects such as low sensitivityand a high limit of detection (LOD), is insufficient for quanti-tative chemical analysis. Therefore, SWV and amperometrytechniques, which benet from low LOD and high sensitivity(small inuence of background in analytical signal), were usedfor the sensitive measurement of IO3

� at (bdpy)SiW11Ni/P-ERGO/GCE.

3.6.1. Determination of IO3� by SWV. The SWV responses

to IO3� of the (bdpy)SiW11Ni/P-ERGO/GCE in 0.1 mol L�1 HClO4

(pH 1.5) were recorded in Fig. 8B. Calibration curve for quan-tication of IO3

� was obtained by plotting the dependence ofpeak I current vs. concentration of IO3

� ([IO3�]) under optimum

experimental conditions (Fig. 8C). The proportional increase ofthe ip values with increasing [IO3

�] was observed in the twolinear ranges of 10–400 mmol L�1 and 400–1000 mmol L�1 withlinear regression equations of ip (mA) ¼ 0.0055[IO3

�] (mmol L�1)+ 0.1149 (R2 ¼ 0.9947), and ip (mA) ¼ 0.0014[IO3

�] (mmol L�1) +

© 2021 The Author(s). Published by the Royal Society of Chemistry

Fig. 8 (A) CVs of the (bdpy)SiW11Ni/P-ERGO/GCE with different concentrations of IO3� in HClO4 aqueous solution (0.1 mol L�1, pH 1.5); scan

rate 50 mV s�1. (B) SWV responses for the determination of IO3� in the concentration ranges of 0–1000 mmol L�1 in HClO4 aqueous solution

(0.1 mol L�1, pH 1.5) at (bdpy)SiW11Ni/P-ERGO/GCE; scan rate 50 mV s�1. (C) Corresponding calibration plot for IO3� (peak I). (D) Current–time

responses of (bdpy)SiW11Ni/P-ERGO/GCE in HClO4 aqueous solution (0.1 mol L�1, pH 1.5) at �0.40 V with an increasing of IO3� concentration.

(E) Corresponding calibration plot of the absolute value of steady-state currents obtained against concentrations of IO3�.

Table 2 Comparative characteristics of the proposed sensor and some other electrodes for the determination of IO3�

Modied electrode Sweep mode

Potential/V(vs.Ag/AgCl)

Linear range/mM LOD/mM

Sensitivity/mAmM�1 Ref.

PMo12-doped sol–gel lm on GCE Amperometry �0.24 5–6000 1 8 80CoW11Co/PVP

a/TiO2/GCE Amperometry �0.50 2–280 0.8 N.R.b 81Fe(III)P/MWCNTs/GCE Amperometry �0.15 10–4000 2.5 1.5 82P2Mo18/OMCc/GC Amperometry 0.00 1.13–6250 0.377 1.51 83PMo12 modied CILEd using [C8Py][PF6] Amperometry 0.08 10–1000 2.6 6.3 84GCE/MWCNTs/[C8Py][PF6]-PMo12 Amperometry 0.00 20–200 15 14 85Ag/PMo12/PBz

e/GCE Amperometry �0.03 N.R. 1.45 0.57 86Attapulgite/PANIf/PMo12/GCE Amperometry 0.00 2–520 0.53 300 87[PEIg/PSSh-Cu@AgNPs/PEI/P8W48]5/ITO

i Amperometry �0.50 0.1–300 0.04 5.52 88Fe(II)-NClinj-CPEk SWV 0.47 4–100 0.64 67 89Carbon nanoparticles-poly(ortho-aminophenol)-modiedelectrode

Amperometry N.R. 500–6500 10 17.14 90

PdNFl/TiO2NTm/Ti Amperometry �0.85 8.0–95.2 3.4 8.5 � 103 91

95.2–666.6 5.0 � 102

666.6–2065.0 1.0 � 102

(bdpy)SiW11Ni/P-ERGO/GCE SWV �0.47 10–400 2.4 �10�3

5.50 Thiswork

400–1000 9.3 �10�2

1.40

Amperometry �0.40 10–1600 4.7 �10�4

28.10

a Poly(4-vinylpyridine). b N.R.—not recorded. c Ordered mesoporous carbon. d Carbon ionic liquid electrode. e Polybenzidine. f Polyaniline.g Poly(ethylenimine). h Poly(sodium-p-styrenesulfonate). i Indium tin oxide. j Clinoptilolite nanoparticles. k Carbon paste electrode. l Palladiumnanoowers. m Titanium oxide nanotubes.

© 2021 The Author(s). Published by the Royal Society of Chemistry RSC Adv., 2021, 11, 8993–9007 | 9003

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Fig. 9 (A) Stability investigation under potential cycling of (bdpy)SiW11Ni/P-ERGO/GCE in HClO4 (0.1 mol L�1, pH 1.5) over 200 scans atthe scan rate 50 mV s�1. The inset shows variations of the cathodicpeak I currents versus the potential scan numbers, (B) SWV responsesof the (bdpy)SiW11Ni/P-ERGO/GCE to 500 mmol L�1 IO3

� in thepresence of different interference ions in HClO4 aqueous solution(0.1 mol L�1, pH 1.5); scan rate 50 mV s�1.

Table 3 Effect of interference ions on the detection of 500 mmol L�1

IO3�

Interference ions½Interference ion�

½IO3��

Relative signalchange (%)

Na+, K+ 600 �0.95Cu2+, Mg2+, Ca2+ 500 �5.67Cl� 2.0 +5.15I� 2.0 +5.21IO4

� 2.0 +6.51NO2

� 20 +9.96NO3

� 50 +1.96PO4

3� 100 +7.58SO4

2� 100 +2.18ClO3

� 50 +7.82BrO3

� 20 +7.91

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1.786 (R2 ¼ 0.9914) and limit of detections (LOD, S/N ¼ 3) of0.238 nmol L�1, and 0.937 nmol L�1, respectively.

3.6.2. Determination of IO3� by amperometry. Amperom-

etry was also applied to examine the sensing characteristics ofthe (bdpy)SiW11Ni/P-ERGO/GCE for determination of IO3

�

(Fig. 8D). As the IO3� was added into the stirred 0.1 mol L�1

HClO4 aqueous solution (pH 1.5) at an optimum potential of�0.40 V, dynamic stairs of the current with quick steady-statewere obtained, and the currents increased along with thecontinuous addition of IO3

�. This can be due to the rapiddiffusion of the IO3

� small molecules from the solution to thesurface of the modied electrode and the better electrontransfer rate of the modier, which can play an important role.The calibration curve presented in Fig. 8E indicated that IO3

�

sensor based on (bdpy)SiW11Ni/P-ERGO/GCE displayed a linearresponse range from 10 to 1600 mmol L�1 with a linear regres-sion equation of ip (mA) ¼ 0.0281[IO3

�] (mmol L�1) � 0.0249 (R2

¼ 0.9999), LOD of 4.7 � 10�4 mmol L�1, LOQ of 1.6 � 10�3 mmolL�1, and sensitivity of 28.1 mA mmol L�1. The LOD and LOQ

9004 | RSC Adv., 2021, 11, 8993–9007

were estimated by using the following equations, LOD ¼ 3Sb/m,and LOQ¼ 10Sb/m, where Sb is the standard deviation of twentyresponses of the blank solution and m is the slope of theregression line. However, at higher concentrations of 1600 mmolL�1, the noise of the amperometric signal increases, and thecurrent stabilization take longer and longer until no straightline is achieved anymore.

The linear calibration range, LOD, and sensitivity of thesuggested sensor for IO3

� determination are compared withthose in previous literatures80–91 (Table 2). The results provedthat (bdpy)SiW11Ni/P-ERGO/GCE is an excellent platform forIO3

� detection.The electrocatalytic behavior of the nanocomposite modied

GCE towards IO3� can be explained by the following mecha-

nism, that is according to previous literature:80,81

3H4SiWV4W

VI7 Ni(H2O)O39

6� + IO3� / 3H2SiW2VW9VI-

Ni(H2O)O396� + I� + 3H2O (3)

3H6SiW6VW5VINi(H2O)O396� + IO3

� / 3H4SiW4VW7VI-

Ni(H2O)O396� + I� + 3H2O (4)

3.7. Stability, repeatability, and reproducibility

Achieving a high stability sensor or biosensor is one of the maingoals of the analytical electrochemistry. The POMs-basedmodied electrodes are ordinarily stable in aqueous solutionsin acidic pH. The stability of the (bdpy)SiW11Ni/P-ERGO/GCEwas evaluated by recording successive CVs at the potentialrange of +1.0 to �0.9 V in 0.1 mol L�1 HClO4 solution at 50 mVs�1 and monitoring the changes in peak currents. As is shownin Fig. 9A, the cathodic peak I current of the modied electrodedisplays a small change with the current degradation of only7.2% aer 200 cycles.

Furthermore, the long-term stability experiment using SWVwas also performed by keeping the (bdpy)SiW11Ni/P-ERGO/GCEin the air at ambient conditions. The results showed that thepeak I current of the modied electrode stayed at 96.12% of itsprimary current aer ten days, 93.57% aer 20 days, and

© 2021 The Author(s). Published by the Royal Society of Chemistry

Table 4 Recovery and reliability obtained by SWV using (bdpy)SiW11Ni/P-ERGO/GCE for the determination of IO3� at its various concentrations

in the real samples

Sample Originally/mM Standard method Spiked/mM Found/mM Recovery (%) � SDa (n ¼ 3)

1b NDc — 30.00 31.59 105.30 � 1.2— 70.00 69.02 98.60 � 1.6

2d ND — 20.00 21.12 105.60 � 1.3— 50.00 47.39 94.78 � 1.4

3e 50 56 50.00 109.80 109.80 � 1.270.00 112.74 93.95 � 1.5

a Standard deviation. b Mineral water, from local market. c Not detected. d Tap water. e Commercial edible iodized salt, from local market.

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91.67% aer one month, indicating the excellent long-termstability of the suggested sensor.

To validate the repeatability of the (bdpy)SiW11Ni/P-ERGO/GCE, four different voltammetry tests with one modied GCEsurface were repeated in the presence of 500 mmol L�1 IO3

�.Furthermore, four (bdpy)SiW11Ni/P-ERGO/GCEs were preparedunder the same conditions to compare the response currents ofIO3

�. The values of relative standard deviation (RSD) of theresponse currents for the repeatability and reproducibility were3.7% and 7.8%, respectively, which demonstrated that themodied electrode has excellent repeatability andreproducibility.

3.8. Interference study

The selective response is an essential characteristic for sensorsthat probe the sensor-selective response to an analyte in thepresence of other species. For this purpose, the potentialinterferences for the IO3

� detection on this sensor were studiedby adding various potential interferents into HClO4

(0.1 mol L�1, pH 1.5) aqueous solution through the SWVmethod. The results are displayed in Table 3, and Fig. 9B. Theseresults revealed an excellent selectivity of the (bdpy)SiW11Ni/P-ERGO/GCE nanocomposite to detect IO3

� in complicatedmedia.

3.9. Analytical applications for real sample

The analytical applicability of the (bdpy)SiW11Ni/P-ERGO/GCEwas validated by measuring IO3

� content present in themineral water (Dasani Mineral Water Co., Iran), tap water, anda commercial edible iodized salt (sample 1, 2, 3, respectively).The samples 1, and 2 were directly prepared in the HClO4

solution (0.1 mol L�1, pH ¼ 1.5) without further treatment. Forpreparation of the sample 3, 15 mg of commercial iodized saltwas dissolved in 100 mL deionized water. Then, it was diluted27 times with 0.1 mol L�1 HClO4 (pH ¼ 1.5) and then pouredinto the electrochemical cell without any pretreatment. Thecontent of iodate in the solution was determined by the iodo-metric titration method as a standard method according to theprocedure given in the previous literature.92 The measurementswere performed using the standard addition method, by SWV inthe potential range from 0 to �1.0 V, scan rate 50 mV s�1.

The standard solutions of iodate were injected into the realsamples to check the recovery percent of added iodate (found

© 2021 The Author(s). Published by the Royal Society of Chemistry

value/intrinsic value). The IO3� contents of iodized salt (sample

3) were compared with the values obtained by iodometrictitrationmethod as shown in Table 4, the obtained results are ingood agreement with the results of the standard method withhigh accuracy and precision (recovery close to 100.00%).Therefore, the proposed modied electrode can be efficientlyapplied in detecting trace-level IO3

� in real samples. The SWV ofthe (bdpy)SiW11Ni/P-ERGO/GCE for the determination of IO3

�

in these real samples was shown in the ESI (Fig. S3).†

4. Conclusions

A new hybrid organic–inorganic nanocomposite based on POMsand RGO has been fabricated, characterized, and reported asa capable electrocatalyst in the electrochemical detection of theiodate ion. The modied electrode revealed an excellent elec-trocatalytic activity towards the electro-reduction of iodate ion.The high stability, good repeatability and reproducibility, aswell as simplicity of electrode preparation are the features of theproposed sensor. Furthermore, the modied electrode hasspecic characteristics such as low LOD (4.7 � 10�4 mmol L�1),high sensitivity (28.1 mA mM�1), good selectivity, and widelinear range (10–1600 mmol L�1) compared to other surfaces.Also, this sensor has been effectively applied for the monitoringof iodate in the real samples. All of these benecial propertiesare favorable for high-performance iodate sensors.

Conflicts of interest

There are no conicts to declare.

Acknowledgements

The authors gratefully acknowledge the nancial support of thiswork by the Research Council of the University of Hormozgan.

References

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3 M. Chi, Z. Zhu, L. Sun, T. Su, W. Liao, C. Deng, Y. Zhao,W. Ren and H. Lu, Appl. Catal., B, 2019, 259, 118089.

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