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Electrochimica Acta 128 (2014) 138–148

Contents lists available at ScienceDirect

Electrochimica Acta

j our na l ho me pa g e: www.elsev ier .com/ locate /e lec tac ta

raphenated polyaniline-doped tungsten oxide nanocompositeensor for real time determination of phenanthrene

luwakemi Tovidea, Nazeem Jaheeda, Nurali Mohameda, Ezo Nxusania,hristopher E. Sundaya, Abebaw Tsegayea, Rachel F. Ajayia, Njagi Njomoa,lamulo Makelanea, Mawethu Bilibanaa, Priscilla G. Bakera, Avril Williamsb,ibulelo Vilakazi c, Robert Tshikhudoc, Emmanuel I. Iwuohaa,∗

SensorLab, Department of Chemistry, University of Western Cape, Moderddam Road, Bellville, Cape Town 7535, South AfricaDepartment of Biological and Chemical Sciences, University of the West Indies, Cave Hill BarbadosNanotechnology Innovation Centre, Advanced Materials Division, MINTEK, Randburg, South Africa

r t i c l e i n f o

rticle history:eceived 24 July 2013eceived in revised form7 December 2013ccepted 19 December 2013

a b s t r a c t

A graphenated polyaniline/tungsten oxide (PANI/WO3/GR) nanocomposite sensor was prepared by elec-tropolymerisation of a mixture of aniline monomer and tungsten oxide on a graphene-modified glassycarbon electrode (GCE). The PANI/WO3/GR/GCE nanocomposite electrode was tested as a sensor forthe determination of phenanthrene. The direct electro-oxidation behaviour of phenanthrene on thePANI/WO3/GR modified GCE was carefully investigated by cyclic voltammetry. The results indicated that

vailable online 4 January 2014

eywords:raphenated-polyanilineancompositeolyaromatic hydrocarbonnvironmental pollutants

the PANI/WO3/GR/GCE sensor was more sensitive to phenanthrene (with a dynamic linear range of 1.0 -6.0 pM and a detection limit of 0.123 pM.) than GCE, PANI/GCE or PANI/WO3/GCE. The sensor exhibitedexcellent reproducibility and long-term stability. The sensor exhibits lower detection sensitivity than theWHO permissible level of 1.12 nM phenanthrene in wastewater.

© 2014 Elsevier Ltd. All rights reserved.

. Introduction

The growing concern about the environment and the healthf every individual has resulted in the search for an effectiveethod of determination and quantification of contaminants in

rder for pollution free environment. Polyaromatic hydrocarbonsPAHs) are among the persistent organic pollutants (POPs) thatas been recognized as teratogenic, mutagenic and carcinogenic

n nature [1], and to have deleterious effect on both aquatic orga-isms and humans, through natural and anthropogenic activities,ia industrial, agricultural and domestic waste [2]. PAHs are alass of several individual chemical compounds defined to be com-osed of more than one fused aromatic rings, commonly found inetroleum fuels, incomplete combustion of coal, tar products andther forms of organic materials [3]. Anthracene, phenanthrene,uoranthene, benz[a]anthracene and benz[a]pyrene to mention a

ew, are some examples of PAH [4]. Because of their profusion, its not astonishing that they are able to have their way into theood chain in every level [5]. They are potentially serious health

∗ Corresponding author. Tel.: +27 21 959 3054.E-mail address: eiwuoha@uwc.ac.za (E.I. Iwuoha).

013-4686/$ – see front matter © 2014 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2013.12.134

risk and therefore become attention for greater focus of research. Inrecent times the regulations for PAHs have been enacted and antici-pated in Europe as well as by the world health organization (WHO)for monitoring [6,7]. Presently, instrumental methods of analysisinvolving chromatographic (TLC, GC, HPLC spectroscopic [8], (UV-Vis, IR, MS) or coupled techniques (GC-MS) [9] are heavily reliedupon for environmental analysis. These instrumental techniquesare usually expensive, not easily amenable to on-site applications,require extensive pre-treatment stages before analyte quantifica-tion and they fail to indicate whether the compounds are accessiblefor assimilation by living organisms. Herein, a simple, less timeconsuming, low cost and reliable electrochemical sensor is beingproposed for real time determination of PAHs.

Intrinsically conducting polymers (ICPs) are useful materialsfor applications in electronics, electrochemistry and sensors [10].Among the ICPs, polyaniline (PANI) has attracted great attentiondue to a good combination of properties; reasonable stability, lowcost, eases of synthesis, environmental and thermal stability aswell as adequate electrical conductivity. There have been numer-

ous attempts to apply high conductivity, electrochromic, catalytic,sensor, redox and other properties of this PANI to different practicalneeds. Direct [11] application is, however, greatly limited becauseof processibility limitations and its intractable nature. Of recent,

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he application of PANI has been widened through the forma-ion of composites or blends with common polymers [12,13] andther particulate dispersants like WO3, SiO2 and SnO2 to form PANIanocomposites with better processibility [14].

Considering inorganic materials for the fabrication of sensor,emiconductive metal oxides are of great attention owing to theirnteresting properties e.g. ZnO2 [15,16], TiO2 [17]. Among thenown transition metals oxide that has been widely studied, tung-ten oxide (WO3) has gained a lot of recognition due to its intrinsicroperties. It can exhibit different oxidation states which results in

ts different forms of oxides, e.g., WO2 and WO3. Tungsten oxideWO3), with a high percentage of oxide and as an n-type semi-onductor, that has been reported to have a band gap of 2.8 eV,s economically stable and its band gap has the ability to absorb

ithin the solar spectrum. WO3 semiconductor is a promising can-idate for a sensor, and because of the high surface-to-volume ratios well as advantage of its high sensitivity performance, it has alsoound suitable applications in devices such as photochromic [18],as sensors [19], optochromic, optical devices, electrochromic [20],nergy-saving devices e.g. smart windows in cars and buildings21].

Recently, graphene-based nanomaterials have shown fascinat-ng applications in electrochemical sensors and biosensors, whichrovide an effective sensing platform for small biomolecules [22].wing to the extraordinary electronic transport property andigh electrocatalytic activity of graphene, a single atom of two-imensional sp2-hybridized carbon, the electrochemical reactionsf analyte are greatly promoted on graphene film, resulting innhanced voltammetric response [23]. In addition, graphene has

high quality alternative to carbon nanotubes (CNTs) and car-on nanofibers in composite applications. An outstanding electricalnd thermal conductivity is exhibited compared to CNTs, and thepecific surface area is twice that of CNTs [24]. The electrochemi-al properties of graphene can be well modified by incorporationith other functional nanomaterials such as catalyst nanoparticle

r polymer to produce resourceful electrochemical sensing per-ormance [11]. Doped form of polyaniline has been recognised as

promising and cost effective electrocatalytic sensor due to themproved redox activity and conductivity [25–29].

In this work, WO3 was prepared from a solution of an inex-ensive tungstic acid (WO3 precursor) and peroxide. The problemf precipitation usually encountered in WO3, is eliminated and

homogenous solution of WO3 is prepared to achieve a highurface-volume-ratio and thin film of tungsten oxide, this is thenncorporated with PANI and graphene nanosheets to improvehe performance of the nanocomposite. Therefore, a polyani-ine/tungsten oxide/graphene (PANI/WO3/GR) nanocomposite haseen prepared by electrochemical deposition of a mixture of WO3nd PANI on a graphene modified glassy carbon electrode. Thelectrochemical sensor fabricated has exhibited an excellent per-ormance in the detection of polyaromatic hydrocarbon in aqueous

edium which serves as the working electrolyte, and which maytand as a challenge because of the hydrophobic nature of pol-aromatic hydrocarbons instead of the general detection in organicedium e.g. acetonitrile.

. Experimental

.1. Reagents

Natural graphite powder (microcrystal grade, 99.9995%) (Metal

ase) UCP -1–M grade, ultra “F” purity purchased from Alfa Aesaras used for graphite oxide preparation using Hummers methodith some modification [30,31]. Analytical grade aniline (99%)as obtained from Aldrich Chemical, Gillingham, England and

Acta 128 (2014) 138–148 139

purified by distillation under reduced pressure prior to use forchemical synthesis. Phenanthrene (99%), hydrochloric acid (HCl,37%), hydrogen peroxide (H2O2, 30% wt water solution), sulphuricacid (H2SO4), sodium nitrite (NaNO3), potassium permanganate(KmNO4), tungstic acid were obtained from Sigma-Aldrich. Allchemicals used in this study were analytical reagent grade andwere used without further purification. 1 M HCl was used as thesupporting electrolyte. Ultra pure water (Millipore) was used forall preparations.

2.2. Apparatus

Cyclic voltammetric (CVs) and square wave voltammetric(SWVs) measurements were carried out using a BAS 100 W inte-grated, automated electrochemical workstation from BioAnalyticalSystems (BAS) Technicol, Stockport, UK. A conventional three-electrode cell system was used, which consisted of a glassycarbon (GCE), an Ag/AgCl (saturated NaCl) and a platinum wireas working, reference and counter electrodes, respectively. Aque-ous hydrochloric acid was used as the supporting electrolyte. Allexperimental solutions were purged with high purity argon gasduring measurements and experiments were carried out at con-trolled room temperature (25 ◦C). The detailed structural propertiesof the prepared materials was evaluated using Fourier transforminfrared spectrometer (FT-IR, Perkin Elmer Spectrum 100), X-raydiffraction (XRD) measurement (Phillips X-ray diffractometer withCu-K� radiation). A tapping-mode atomic force microscope (AFM)(Veeco Nanoman V) was employed to evaluate the morphologyof graphene, with special emphasis on estimating its thickness.The silicon tip [antimony (n) doped] had a curvature radius of2.5–3.5 �M, a force constant of 1–5 N m−1 and a resonance fre-quency of 60–100 kHz. The samples for AFM were prepared bydrop coating the graphene/water (5 �L) dispersion onto a siliconwafer. High resolution transmission electron microscopy imageswere taken on a Tecnai G2 F20X-Twin MAT 200KV HRTEM fromFEI (Eindhoven, Netherlands) and the high resolution scanningelectron microscopy images were taken using LEO 1450 HRSEM3OKV instrument equipped with EDX and WDS. Raman spectrawere recorded on a Dilor XY Raman spectrometer with a CoherentInnova 300 Argon laser with a 532 nm laser excitation. Electro-chemical impedance spectra (EIS) measurements were performedusing Volta Lab PGL 402 from Radiometer Analytical (Lyon, France)in a solution containing 1 M HCl. UV-Vis spectra and measurementswere recorded over a range of 350-700 nm using 3 cm3 quartzcuvettes with Nicolette Evolution 100 Spectrometer (Thermo Elec-tron Corporation, UK). Polishing pads obtained from Buehler, IL,USA and Alumina micro powder (1.0, 0.3 and 0.05 �m aluminaslurries) were used for polishing the GCE.

2.3. Material synthesis

Graphene oxide was synthesized from graphite powder by themodified Hummers method [31,32]. Concentrated sulphuric acid(50 mL) was gradually added to a 500 mL volumetric flask contain-ing mixture of powdered graphite flakes (2.0 g) and sodium nitrate(1.0 g) at room temperature. The solution was then cooled to 0 ◦Cin an ice bath, while vigorous agitation was maintained; potassiumpermanganate (7.0 g) was added to the suspension for duration ofabout 30 min. The rate of addition was carefully controlled to pre-vent the temperature of the suspension from exceeding 20 ◦C, theice bath was then removed, and the suspension was brought toroom temperature, where it was maintained for 30 min. The tem-

perature was then raised to 35 ◦C in a water bath, and then stirredwith a Teflon coated magnetic stirring bar for 12 h. As the reac-tion progressed, the slurry gradually thickened with diminishingeffervescence. The mixture was then cooled in ice bath, followed

1 imica Acta 128 (2014) 138–148

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y addition of excess deionised water to the mixture and treatedith hydrogen peroxide (30%) until gas evolution ceased, and also

o reduce the residual permanganate and manganese dioxide toolourless soluble manganese sulphate. The resultant brownishellow suspension was filtered off and thoroughly washed severalimes with warm deionised water/conc HCl mixed solvent (9/1 vol-me ratio) in other to remove all sulphate ions, followed by distilledater until neutral pH of the filtrate. The precipitate was then driednder reduced pressure at 60 ◦C for 24 h to obtain graphite oxideGO).

.4. Preparation of graphene

100 mg of graphite oxide (GO) powder was dispersed in 100 mLf deionised water to be exfoliated into graphene oxide sheets byltrasonication for 1 h and to form a stable aqueous dispersion.00 mg of NaBH4 was added to the dispersion under magnetic stir-ing, and the mixture was continuously stirred at 125 ◦C for 3 h.inally a black precipitated solid was obtained and isolated by fil-ration, then dried under vacuum at 60 ◦C [33].

.5. Preparation of graphene modified electrode andanocomposite films

The bare GCE was polished with 1.0, 0.3 and 0.05 �M, aluminaowder, rinsed with distilled water followed by ultrasonicationith ethanol, and deionised water respectively, and dried at room

emperature. Graphene (1 mg) was dispersed in ethanol (1 mL) andltrasonicated for 30 min to get homogeneous suspension. Then

�L of the suspension was drop coated onto the surface of freshlyolished glassy carbon electrode (GCE) and dried at room temper-ture, resulting in graphene modified electrode (GR-GCE). Prioro electrodeposition, deposition solution was prepared, consist-ng of 0.2 M tungstic acid, 1 M HCl, 0.08 M H2O2. The solution wastirred at 60 ◦C for 1 h, after which 0.1 M aniline was added tohe solution. The resulting solution was electrodeposited on theraphene modified electrode. The cyclic voltammetry electrodepo-ition was carried out by scanning the potential from -0.6 to +1.0 Vt a potential scan rate of 50 mV s−1. After 10 voltammetry cycles,he PANI/WO3/GR electrode formed was then removed for furtherharacterisation and application. PANI/WO3 composite film wasrepared at the same condition and in the absence of grapheneo get PANI/WO3 electrode. Cyclic voltammetry polymerisation ofniline on the glassy carbon electrode was also carried out in a solu-

ion of 0.1 M aniline and 1 M HCl at a scanning potential of between0.6 V and +1.0 V, potential scan rate of 50 mV s−1 as PANI/GCElectrode.

Scheme 1.

Scheme 1. Schematic diagram of the deposition of PA

Fig. 1. XRD pattern of (a) graphite, (b) graphite oxide and (c) graphene.

3. Results and Discussion

3.1. Surface morphology and structural characterisation

Fig. 1 shows the XRD patterns of (a) graphite, (b) graphite oxideand (c) graphene. The interlayer distance of graphite exhibits astrong peak (002) at 26.55◦ which can be attributed to a van derWaal’s layer of carbon atoms tightly packed together. After oxi-dation, it becomes expanded as result of the displacement of thecarbon atoms and the presence of the oxygen containing functionalgroups and water (H2O) molecules [34]. The (002) peak disap-pears and an additional peak corresponding to diffraction peak ofgraphite oxide (001) is observed at 9.65◦. At reduction, the inter-layer distance returns showing the disappearance of the diffractionpeak of graphite oxide, and replaced by a broadened diffractionpeak at 23.81◦ and lowered in intensity compared to graphite andgraphite oxide. This suggested that most of the oxygen functionalgroups have been removed [35,36].

Fourier Transform Infrared (FT-IR) spectroscopy was employedto characterise the strutures of the synthesized graphite oxide andgraphene. No distint peak are detected in graphite. However onoxidation, apperance of collection of peaks comfirms the formationof graphite oxide. The appearance of the absorption peaks on thegraphite oxide sheet can be confirmed by the collections of func-tional groups corresponding to: C = O (1,735 cm−1), aromatic C = O

(1,602 cm−1), carboxyl C-O (1,416 cm−1), epoxy C-O (1,265 cm−1),alkoxy C-O (1,047 cm−1) and hydroxyl O-H (3,390 cm−1). This is inagreement to the reported data [37]. However, the intensity of theabsorption bands decreased tremendously in the graphene sheet.

NI/WO3 film on graphene modified electrode.

O. Tovide et al. / Electrochimica Acta 128 (2014) 138–148 141

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can be attributed to the presence of tungsten oxide (WO3), anda characteristic peak of PANI film around 300 nm attributing tothe benzenoid �-�* transition. However, PANI/WO3 composite filmhas been studied to exhibits UV − vis transmission spectra similar

Fig. 2. The FT-IR spectra of (a) graphite, (b) graphene and (c) graphite oxide.

Fig. 2.Atomic Force Microscopy (AFM) was also performed on

raphene to characterize the degree of exfoliation. Fig. 3 repre-ents the AFM topography image of graphene, wherein severalraphene sheets were randomly deposited on the silicon substrate.he graphene surface was slightly rough and this could be due tohe existence of some functional groups. The cross sectional viewcross the plain area of the sheet gave an estimated height of 1.4 nmhich is consistent with reported literature [38].

Fig. 4.Raman spectroscopy is generally used to determine the trans-

ormation in the structural pattern during the chemical synthesisf the graphite oxide to graphene and also to identify the number ofayer of graphene [39]. In the Raman spectral, graphene (a) showedhree peaks, G-mode at 1583 cm−1 due to stretching of the C-Conding in the in-plane vibration of graphitic materials and typi-al of sp2 carbon system, D- mode appeared at around 1345 cm−1,sually caused by disordered structure in sp2–hybridized carbonystems according to literature [40]. Graphite oxide (GO) showed a-mode shift at 1604 cm−1 (b) compared to graphene. The oxida-

ion of graphite brings about the intensity ratio of ID/IG of D bandnd G band of GO to be 0.973 as a result of the oxygen functionalroups in the sp3 graphitic structure. After reduction, the intensityatio ID/IG increased to 1.073 confirming the formation of graphenend with indication that a new structure has been formed in the sp2

omain [39,40]. The appearance of the third peak at the 2D bandn the region of 2861 cm−1 in the graphene spectrum is typical of

ll sp2 carbon system and which determines the number of layerf graphene. The 2D peaks revealed a noteworthy change in shapeompared to graphite oxide and also showed an intense and shaper

Fig. 3. AFM image of graphene sheets on silicon

Fig. 4. Raman spectra of graphene (a) and graphite oxide (b).

peak compared to multi layer graphene which is broad and wideaccording to literature [41].

The UV-vis spectra of PANI, PANI/WO3, PANI/WO3/GR and GR inDMF are shown in fig. 5. The inset is the spectrum of GR demon-strating absorption at 270 nm. The spectrum of PANI showed anabsorption bands at 326 nm which correspond to the benzenoid�-�* transition and at the 620 nm due to �-�* transition ofquinine-imines group of a typical polyaniline[42,43], PANI/WO3showed an appearance of an absorption peak at 452 nm which

Fig. 5. UV-vis spectra of PANI, PANI/WO3/GR and PANI/WO3 with the inset spectrumof GR.

substrate together with the height profile.

142 O. Tovide et al. / Electrochimica

Table 1Band gap energy values calculated from the UV-vis. spectra in Fig. 5.

Electrode Wavelength/nm Band gap energy/keV

PANI/GCE 602 0.002014

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of graphene, with the selected area electron diffraction (SAED)

PANI/WO3/GCE 452 0.002763PANI/WO3/GR/GCE 633 0.0019

o PANI especially at positive potentials [44]. This might have con-ributed to the merged of the broadened peak at around 620 nm forANI film in the present of WO3 and appearance of the absorptioneak at 452 nm.

The spectrum of PANI/WO3/GR revealed absorptions at aboutame region as that of PANI but more broadened and slightlyhifting towards long wavelength, (633 nm) which can also bettributed to the merging of the absorption of WO3. Also GR inhe PANI/WO3/GR composite appeared to be merged with the ben-enoid �-�* transition and with increase in intensity of the peaknd a slight shift to lower wavelength of 294 nm. However theroadening of the WO3 band and appearance of merged band ofR in the composite of PANI/WO3/GR film revealed the presencend interaction of the individual particles in the composite. Theand gap energy for the different modified composite on the elec-rodes, based on the UV-vis spectra in Fig. 5 was calculated usinghe formula:

and gap energy (Eg) = hC/�

here h is the plank constant (6.66 × 10−34 J s), C is the speed ofight (3.0 × 108 m) and � is the wavelength maximum absorbance

f a specific composite (nm) [45].

From the calculated data in Table 1, it can be observed thatANI/WO3/GR composite has the lowest band gap energy value0.0019 keV), this is an indication that graphene has great influence

Fig. 6. Typical SEM images of: (a) PANI; (b) WO3; (c) PANI/WO3; (d) GR

Acta 128 (2014) 138–148

on the PANI/WO3 film, thereby reducing the bang gap energyand thus higher in conductivity. However, in agreement with thegeneral characteristics of the activity of different types of nanoma-terials in which the band gap energy of an insulator is usually large(> 4 eV), semiconductor (< 3 eV), PANI/WO3/GR has also showed theactivity of a semiconductor [46].

The typical morphologies of the resulting PANI, WO3,PANI/WO3, GR and PANI/WO3/GR composites were observed byHRSEM as exhibited in Fig. 6. Fig. 6a showed a porous fibrillarand rod like irregular structure of PANI while WO3 display fusedlarge particles in Fig. 6b. The microstructure of PANI/WO3 film inFig. 6c showed smaller granules of WO3 with rough porous surfaceof aggregates within the PANI film. An overlapped thin, crumpledand wrinkle sheets closely associated together can be observed forgraphene in Fig. 6d. However, the composite film of PANI/WO3/GRrevealed a porous surface of micro aggregates of WO3 and PANIenveloped by a thin film of graphene with a large surface area andthe EDX analysis consisting of the elemental composition of thecomposite as shown in Fig. 6e and 6f. The structure and compo-sition of the composite was further performed using transmissionelectron microscopy (TEM) and EDX. A flat transparent layers thinsheet can be observed for graphene in Fig. 7a, with the selected areaelectron diffraction (SAED) pattern showing some crystallinity. Theappearance of a long rod like shapes of flakes of PANI nanofibers canbe observed in Fig. 7b. As shown in Fig. 7c, the PANI/WO3 film con-sist of compact WO3 particles on the surface of PANI nanofibers,possibly due to the agglomeration of the nanoparticle. The result-ing composite in Fig. 7d revealed the distribution of nanoparticleof WO3 and PANI nanofibers on and between the thin sheets

pattern exposing the crystallinity. The presence of the different ele-ments and composition can be observed from the EDX analysis inFig. 7e.

; and (e) PANI/WO3/GR. (f) The EDX spectrum of PANI/WO3/GR.

O. Tovide et al. / Electrochimica Acta 128 (2014) 138–148 143

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ig. 7. TEM image and SAED pattern of graphene (a); TEM image of PANI (b); TEMpectrum of PANI/WO3/GR/GRE (e).

.2. Electrochemical characterisation of modified films on glassyarbon electrode.

The CV behaviour of PANI films grown on the glassy elec-rode at 50 mV s−1 is shown in Fig. 8a. The cyclic voltammogramevealed that the polymerisation current increases as the numberf voltammetric cycles increases, confirming that the polymer is

e PANI/WO3 (c); and TEM image and SAED pattern of PANI/WO3/GR (d); and EDX

conducting. Moreover the redox peaks (A/A’, B/B’, C/C’) were alsoformed and their peak currents increased with increase in the num-ber of potential scans, confirming that the film is conducting and

electroactive. The three anodic and cathodic redox peaks in the CVsare characteristically typical of the electrosynthesis of polyanilineand correspond to leucomeraldine, emeraldine and pernigranilineredox states of the polymer. The redox peaks A/A’ and C/C’ indicate

144 O. Tovide et al. / Electrochimica Acta 128 (2014) 138–148

Fig. 8. Cyclic voltammograms for the electrodeposition of PANI (a), PANI/WO3 (b) and PANI/WO3/GR films (c) on GCE in 1 M HCl: potential window = -0.6 to +1.0 V; and scanrate = 50 mV s−1. Fig. 8 (d) is the overlay of the CV’s of GCE, PANI/GCE, PANI/WO3/GCE and PANI/WO3/GR/GCE performed in 1 M HCl. Scan rate: 50 mV s−1.

Table 2Electrochemical parameters of PANI/WO3/GR, PANI/WO3 and PANI.

Electrode Redox couple Epa/V Epc/V Ipa/�A Ipc/�A Ipa/Ipc Eo′ /V

PANI/WO3/GR A/A′ 0.253 0.075 634.13 406 .51 1.5 0.164B/B′ 0.598 0.521 488.02 468.00 1.04 0.5595C/C′ 0.823 0.733 538.03 -760.05 0.71 0.778

PANI/WO3 A/A′ 0.211 0.086 301.60 192.60 1.5 0.1485B/B′ 0.543 0.533 119.20 107.13 1.1 0.538C/C′ 0.792 0.747 200.39 -369.77 0.5 0.7695

PANI A/A′ 0.230 0.79 272.44 108.09 2.4 0.1549B/B′ 0.548 0.548 76.96 74.71 1.0 0.548C/C′ 0.802 0.767 126.50 -184.94 0.6 0.7845

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he conversion of leucomeraldine to emeraldine and emeraldine toernigraniline, respectively. The middle redox peak B/B’ could beeferred to as the dimers or benzoquinone/hydroquinone couplef the polymeric chains but with a lesser amount of intensity [47].ig. 8a–8c display the cyclic voltammograms of PANI, PANI/WO3nd PANI/WO3/GR films on a glassy carbon electrode respectively.hree anodic and cathodic corresponding peaks can be observedith increase in the peak currents as the potential scan increases.

he three redox peaks have been assigned accordingly [48]. Fromhe data in Table 2, the anodic peak current (Ip,a) from each of thehree redox couples A/A’ B/B’ and C/C’ can be observed to increaseith the cathodic peak potential (Ip,c), shifting anodically for each

f the electrodes. PANI/WO3/GR possessed the highest peak currentompared to PANI/WO3 and PANI electrode. And the Ip,a/Ip,c ratio is

unity’ for all the couples from the experimental data. It can also bebserved that the Ip,a/Ip,c ratio drops within the range of 0.5 and 0.7or C/C’ in each of the electrode. This change can be attributed tohe efficiency of the polymer when in the form of leucomeraldine

radical cation or pernigraniline radical cation, thus revealing somecharge transportation [48].

For the electrodeposition of PANI/WO3, Fig. 8b, the response issimilar to that of pure PANI film, except for the oxidation of themonomer occurring at a slightly lower potential and an enhance-ment in the peak current. The response in PANI/WO3 film couldbe attributed to the WO3 particles agglomeration shown in Fig. 6c(SEM), which can likely reduce the expected and more effective sur-face area of the film by incorporating with PANI. The response in thepresence of graphene modified GCE is shown in Fig. 8c. The threeanodic peaks are more prominent and higher than in Fig. 8a and 8b,and the cathodic peaks shifting more positively. The remarkableincrease in peak current of PANI/WO3/GR film can be associated tothe exceptional properties of graphene which include its high cat-

alytic property and large surface area. Also, the distribution of thepolymer matrix and interfacial adhesion due to bonding betweenPANI/WO3 and the graphene thin layer can be confirmed by themorphology analysis from TEM and SEM. The voltammogram in

O. Tovide et al. / Electrochimica Acta 128 (2014) 138–148 145

Fig. 9. Nyquist plots of the EIS measurement of (a) GCE, (b) PANI/WO3/GR/GCE, (c)Pu

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Table 3Charge transfer resistance values for the components of PANI/WO3/GR/GCE sensor.

Electrode Rct/�

GCE 9.637 × 103

PANI/WO3/GCE 2.831 × 103

F(

ANI/WO3/GCE and (d) PANI/GCE in 1 M HCl. Inset is the Randle’s equivalent circuitsed for fitting the Nyquist plots.

ig. 8d obviously revealed the differences in the peak enhancementor the different electrodes.

.3. Electrochemical impedance spectroscopy ofANI/WO3/GR/GCE

Electrochemical impedance spectra (EIS) were used to investi-ate the performance of the modified electrodes being an effectiveethod of probing the features of a surface modified electrode

49–51]. The measurement was performed in 1 M HCl aqueousolution with a frequency range of 100 kHz to 0.1 Hz of which themplitude of the alternative voltage was 10 mV, as shown in Fig. 9.he y-axis represents the negative number of the imaginary partf the measured impedance while the x-axis represents the realart of the measured impedance. The diagram (Nyquist plots: realart of the impedance Z(Re) vs. imaginary part Z(Im)) representshe charge transfer dynamics of the modified electrodes in whichs is the ohmic internal or solution resistance, CPE is the capaci-ance phase element for the semiconductor-electrolyte interface,

(Warburg impedance) is the mass transfer element and Rct is theharge transfer resistance across the interface [52]. For a reactionhat is reversible, the Nyquist plot usually exhibits two regions:i) a semicircle at a high frequency region which corresponds to

lectron charge transfer process from which the electron-transferesistance value can be measured directly, and (ii) a straight lineection at low frequency region of the plot, which represents the

ig. 10. Scan rate dependence of the CV’s of PANI/WO3/GCE electrode performed in 1 M

b).

PANI/GCE 3.371 × 103

PANI/WO3/GR/GCE 1.433 × 103

diffusion-limited transport of the redox species from the electrolyteto the electrode interface.

The Rct results of the modified electrodes are listed in Table 3.The bare GCE gave an Rct value of 9.637 × 103 �, exhibiting a linearNyquist behaviour at lower frequencies similar to the characteris-tics of a diffusion process. After the modification of GCE with PANIfilm, the Rct value was reduced as was also the case with when GCEwas modified with PANI/WO3 film with values of 3.371 × 103 �and 2.831 × 103 �, respectively. The modified electrodes exhibitedlarge semicircles and a straight lines in the lower frequency region,indicating a sign of modification and charge resistance transfer buthigh diffusion system. The Rct value of of PANI/WO3/GR GCE is1.433 × 103 �, which is lesser than that of both PANI and PANI/WO3,indicating that the GR-modified electrode in the presence of PANIand WO3 has high interfacial charge transfer resistance, i.e. it isfacile to electron transfer and high conductivity than PANI/GCEand PANI/WO3/GCE which was consisted with cyclic voltammetryresults. More so, graphene has the ability of creating a nucleatingsite for polyaniline and the coating of the surface of polyanilinewith tungsten oxide which could possibly contribute to the syner-gic effect and more interaction thus, the more enhancements andmore conductivity and catalytic effect of the PANI/WO3/GR/GCE.

Fig. 10a showed the response of PANI/WO3/GR electrode tochange in scan rate. The peak current increases as the scanrate are increased. Fig. 10b displays the plot of the anodic peakcurrents as a function of potential scan rate for peak A ofFig. 10a. The linear relationship of the plot confirmed that thenanocomposite was electroactive, conducting and confined to thesurface of the electrode. This was further confirmed by plot oflog of peak current against log of scan rate which gave a slopeof 1 as expected for the electrochemistry of surface-adsorbedspecies.

3.4. Electrochemical responses of the PANI/WO3/GR/GCEphenanthrene sensor

Cyclic voltammograms of PANI/GCE, PANI/WO3/GCE andPANI/WO3/GR/GCE in 1 M HCl in the presence and absence of5.0 × 10−3 nM phenanthrene are recorded in Fig. 11. The CV’s of

HCl (a), and a plot of the scan rate dependence of its anodic peak (peak A) currents

146 O. Tovide et al. / Electrochimica Acta 128 (2014) 138–148

Fig. 11. Cyclic voltammograms of PANI/WO3/GR/GCE in 1 M HCl before and after the addition of 5.0 × 10−3 nM phenanthrene (a). Overlay of the CV’s of PANI/WO3/GR/GCE,PANI/WO3/GCE and PANI/GCE obtained for 1 M HCl containing 5.0 × 10−3 nM phenanthrene (b). All CV’s were obtained at a scan rate of 30 mV s−1.

F or in

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eiaPtiwet

Fs

ig. 12. Scan rate dependence cyclic voltammograms of PANI/WO3/GR/GCE sensependence of its anodic peak currents (b).

ach of the electrode systems are overlaid in Fig. 11b for compar-son. The PANI/WO3/GR electrode exhibited a high increase in thenodic peak current revealing a catalytic property compared to theANI/WO3 and PANI modified electrodes. This may be attributed tohe large surface area and good conductivity of PANI/WO3/GR mod-

fied electrode as result of the incorporation of graphene nanofiller

hich have a high catalytic effect [22]. Phenanthrene actuallyxhibits a remarkable enhanced anodic peak response comparedo the other electrodes.

ig. 13. Cyclic voltammograms of the PANI/WO3/GR/GCE in 1 M HCl in the presence of pheensor (b).

1 M HCl containing 5.0 × 10−3 nM phenanthrene (a); and a plot of the scan rate

The effect of scan rate on the response of 5.0 × 10−3 nM phenan-threne was investigated with PANI/WO3/GR modified electrode in1 M HCl. Fig. 12a show the cyclic voltammogram of the modifiedelectrode at different scan rates upon the addition of 5.0 × 10−3

nM phenanthrene. The plot of the anodic peak versus the square

root of scan rate over the range of 10 and 200 mV s−1 is shownin Fig. 12b. The anodic peak current increased linearly with scanrate in accordance to the equation of Ip,a = 1.3 ×10−3 + 7.01 × 10−4

x v½/mV½ s−½; (r2) = 0.999. Thus the electrochemical process was

nanthrene at 30 mV s−1 (a). A calibration plot for PANI/WO3/GR/GCE phenanthrene

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imited by the rate of diffusion of phenanthrene from the solution tohe surface of the electrode. Also observed was the oxidation peakotential shifting to more positive potentials, thereby confirminghat the peak currents were diffusion-controlled [53].

Cyclic voltammetric response of the electrocatalytic oxidation ofifferent concentration of phenanthrene at the PANI/WO3/GR mod-

fied electrode in 1 M HCl can be observed in Fig. 13a. An increase inhe anodic peak current occurs at every addition of phenanthrene,ndicating the oxidation of phenanthrene and also confirming theatalytic behaviour of the modified electrode. A plot of the con-entration versus anodic current is shown in Fig. 13b, with theinearity ranging from 1.0 to 6.0 pM and a correlation coefficient of.9806. The limit of detection (LOD) was determined to be 0.123 pM.he dynamic linear range and detection limit values are by abouthree orders of magnitude lower than the WHO permissible levelf 0.2 �g L−1 for PAHs in wastewater [7], which for phenanthrene is.12 nM. And when compared with other PAH sensors, the sensor

s more sensitive than other reported PAH detection systems suchs Ag-Au alloy nanoparticles/overoxidized-polypyrrole compositeensor (LOD(anthracene) = 0.167 �M) [54], dendritic polythio-hene electro-catalytic sensor (LOD(phenanthrene) = 19.0 nM) [55]nd liquid phase microextraction (LOD(phenanthrene) = 4.488 nM)56].

.5. Reproducibility, stability and interference studies

One of the basic means of testing the performance of anlectrode is by stability and reproducibility measurements. Theeproducibility of the PANI/WO3/GR modified electrode was inves-igated with 5.0 × 10−3 nM phenanthrene in 1 M HCl solution. Thelectrode gave a relative deviation of 1.8% for 3 successive measure-ent (n = 3) showing its reproducibility. The storage stability of theodified electrode was also examined with 5.0 × 10−3 nM phenan-

hrene and the oxidation current was monitored every 3 days for 2eeks using CV. About 15.69% decrease was observed in the peak

urrent and was able to retain 84.31% of phenanthrene initial sen-itivity at 4 ◦C. Further investigation was made to check for possiblenterferences of inorganic ions which may be present in water sam-les. The effect of the following ions: Mn2+, Cl−, SO4

2− and NO3−;id not affect the response for 5.0 × 10−3 nM of the phenanthrene.

. Conclusion

The PANI/WO3/GR/GCE sensor exhibited a high sensitivity forhenanthrene detection with a dynamic linear range and detec-ion limit of 1.0 - 6.0 pM and 0.123 pM, respectively. The dynamicinear range and detection limit values are by about three orders of

agnitude lower than the WHO permissible level of 0.2 �g L−1 forAHs in wastewater [7], which for phenanthrene is 1.12 nM. Theensor exhibited good reproducibility, long term storage stabilitynd excellent sensitivity. This method, apart from being sensitive,an be scaled up into a reactor that provides an environmentallyriendly method for the degradation of PAHs with minimal energyequirement.

cknowledgment

The authors acknowledge the South African Water Researchommission (WRC) and the National Research Foundation (NRF)f South Africa for financial support.

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