Analytica Chimica Acta 852 (2014) 28–36

A highly sensitive electrochemical sensor for simultaneousdetermination of hydroquinone and bisphenol A based on the ultrafinePd nanoparticle@TiO2 functionalized SiC

Long Yang a,1, Hui Zhao b,1, Shuangmei Fan a, Bingchan Li a, Can-Peng Li a,*a School of Chemical Science and Technology, Yunnan University, Kunming 650091, PR Chinab Laboratory for Conservation and Utilization of Bio-resource, Yunnan University, Kunming 650091, PR China

H I G H L I G H T S G R A P H I C A L A B S T R A C T

� TiO2–SiC was successfully preparedby a facile generic in situ growthstrategy.

� Ultrafine Pd NPs with a uniform sizeof �2.3 nm monodispersed onTiO2–SiC surface.

� Electrochemical simultaneous deter-mination of HQ and BPA was estab-lished.

� Ultrafine metal NPs@metal oxide–SiCmay be extended to other applica-tions.

The illustration of Pd@TiO2–SiC nanohybrids simultaneous sensing hydroquinone and bisphenol A by anelectrochemical strategy.

A R T I C L E I N F O

Article history:Received 11 June 2014Received in revised form 6 August 2014Accepted 11 August 2014Available online 21 August 2014

Keywords:Titanium dioxide–silicon carbideUltrafine palladium nanoparticlesHydroquinoneBisphenol AElectrochemical performanceSimultaneous determination

A B S T R A C T

A titanium dioxide–silicon carbide nanohybrid (TiO2–SiC) with enhanced electrochemical performance wassuccessfully prepared through a facile generic in situ growth strategy. Monodispersed ultrafine palladiumnanoparticles (Pd NPs) with a uniform size of �2.3 nm were successfully obtained on the TiO2–SiC surfacevia a chemical reduction method. The Pd-loaded TiO2–SiC nanohybrid (Pd@TiO2–SiC) was characterizedby transmission electron microscopy and X-ray diffractometry. A method for the simultaneouselectrochemical determination of hydroquinone (HQ) and bisphenol A (BPA) using a Pd@TiO2–SiCnanocomposite-modified glassy carbon electrode was established. Utilizing the favorable properties ofPd NPs, the Pd@TiO2–SiC nanohybrid-modified glassy carbon electrode exhibited electrochemicalperformance superior tothose of TiO2–SiC and SiC. Differential pulse voltammetrywas successfully used tosimultaneously quantify HQ and BPA within the concentration range of 0.01–200 mM under optimalconditions. The detection limits (S/N = 3) of the Pd@TiO2–SiC nanohybrid electrode for HQ and BPA were5.5 and 4.3 nM, respectively. The selectivity of the electrochemical sensor was improved by introducing10% ethanol tothe buffer medium. The practicalapplication of the modified electrodewas demonstrated bythe simultaneous detection of HQ and BPA in tap water and wastewater samples. The simple andstraightforward strategy presented in this paper are important for the facile fabrication of ultrafine metalNPs@metal oxide–SiC hybrids with high electrochemical performance and catalytic activity.

ã 2014 Elsevier B.V. All rights reserved.

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Analytica Chimica Acta

journal homepa ge: www.elsev ier .com/locate /aca

* Corresponding author. Tel.: +86 871 65031119; fax: +86 871 65031119.E-mail address: lcppp1974@sina.com (C.-P. Li).

1 These authors contributed equally to this work.

http://dx.doi.org/10.1016/j.aca.2014.08.0370003-2670/ã 2014 Elsevier B.V. All rights reserved.

L. Yang et al. / Analytica Chimica Acta 852 (2014) 28–36 29

1. Introduction

Phenol compounds are highly abundant in nature as they arethe essential raw materials and byproducts of numerous chemicalindustries. Especially, some of these compounds can disrupt theendocrine system of humans, cause abnormal sexual developmentand decrease the average number of human spermatozoa [1].Bisphenol A (BPA), for example, is an estrogenic environmentaltoxin widely used to produce plastics; ubiquitous human exposureto this chemical presents potential risks to public health becauseBPA has been linked to a variety of adverse effects, includingimpaired brain development, sexual differentiation, behavior, andimmune function, which could extend to future generations [2].Hydroquinone (HQ) can cause fatigue, headache, tachycardia, andkidney damage in humans [3]. These compounds are extremelyharmful to animals, plants, and aquatic environments even at verylow concentrations. Therefore, development of methods for BPAand HQ detection is an important undertaking.

Several methods, such as capillary electrophoresis, gas or liquidchromatography, chemiluminescence detection, and fluorescencedetection, have been proposed to determine phenolic compounds.However, these methods have several disadvantages, such asexcessive time consumption, complicated sample pretreatments,low sensitivity, and complex manipulations. Electrochemicaldetection is an attractive alternative to these techniques becauseit features high sensitivity, instrument simplicity, low cost,feasibility of miniaturization, and ease of use [4].

Over the past few years, noble metals with ultrafine sizes haveattracted substantial attention because their large surface areas andhigh number of edge and corner atoms significantly enhance thecatalytic properties of noble metal nanocomposites [5,6]. Therefore,the synthesis of small metal particles with high accessible surfaceareas is a worthwhile endeavor. Unfortunately, surface energiesincrease with decreasing noble metal particle size, leading to seriousaggregationof smallparticles[7,8]. Toovercomethisaggregation, themetal particles must be anchored to suitable supports [9,10].

Numerous carbon support materials, such as carbon nanotubes[11,12], graphene [8,10,13,14], mesoporous carbons [15], porouscarbon [16], macroporous carbon [17], and carbon nanowires [18],have been applied as metal particle supports. Unfortunately, whilethese carbon supports present large surface areas, high electricalconductivities, and large pore structures, the corrosion caused byelectrochemical oxidation, which decreases catalyst durability andreliability, limits their wider application [9]. As such, alternativesupport materials must be explored. A variety of non-carbonsupport materials, such as alloys [19], nitrides [20], carbides [21],mesoporous silica [22], conducting polymers [23], and metaloxides [24], have been previously proposed. Among thesematerials, silicon carbide (SiC), a wide band gap semiconductor,is considered a promising metal particle support because of itshigh-temperature stability, hardness, and chemical inertness [9].SiC is formed through covalent bonding of Si and C atoms in atetrahedral form, in which Si (or C) is the central atom. The highmechanical and chemical stabilities of this material are attributedto its extremely short bond length [25]. Moreover, non-carbonsupport materials usually have lower background currentscompared with carbon-based materials. These characteristicssuggest that SiC could be an ideal substrate for the growth andanchorage of metal and metal-oxide nanoparticles for high-performance electrocatalytic or electrochemical devices.

Commercially available SiC is less electronically conductive(around 10�6 S cm�1) and has a fairly low specific surface area [9].Therefore, considerable research is necessary to develop SiC into avaluable catalyst support for electrocatalytic or electrochemicalapplications. Some studies suggest that heterostructures such asmetal oxide nanocomposites or metal oxide–SiC nanohybrids,

which possess superior or novel functional properties, are idealcandidates as supports for noble metal nanoparticles because oftheir specific geometries and mechanical properties [26]. Theseheterostructures exhibit electronic, mechanical, thermal, andoptical properties that significantly differ from those of theirconstituent components and can thus present more versatilefunctions than individual materials when used for nanoscaledevices [26,27].

Titanium dioxide (TiO2) is the most widely investigatedmaterial among all of the metal oxides because of its uniqueproperties and promising applications in variety of fields, includingphotocatalysis [28], supercapacitors [29], lithium ion batteries[30], solar cells [31], sensors [32], and biosensors [33]. TiO2 ischemically stable in both acidic and alkaline solutions and has highcatalytic activity for the reduction of several small organicmolecules [34]. TiO2–SiC heterostructures can be obtained throughan in situ growth method, and desirable properties, such as largespecific surface area. In addition high catalytic activity can beexpected from the final products. However, only a limited numberof studies have demonstrated the photocatalysis application of theTiO2–SiC heterostructure [35–38]. The TiO2–SiC heterostructure, asa noble metal nanoparticle support, is rarely investigated forelectrochemical sensing or biosensing applications.

Numerous studies on the assembly of noble metal nano-particles on graphene surfaces have been recently reported[8,10,39,40]. In particular, palladium (Pd)-based catalysts haveattracted considerable attention because of their lower cost andbetter resistance to carbon monoxide compared with platinumcatalysts. For instance, Chen et al. [8] reported that clean and well-dispersed Pd nanoparticles (NPs) can be strongly anchored onto agraphene oxide (GO) surface by mixing GO and a K2PdCl4 aqueoussolution without addition of other reductants or surfactantsbecause GO directly acts as a reductant. Tang and co-workers [10]demonstrated that gold clusters could be grown on reduced GOsheets without addition of any protecting molecule or reductant.However, carbon-based materials usually present high backgroundcurrents, which are disadvantageous in any detection system,particularly for electrochemical sensing or biosensing applications.Therefore, fabrication of ultrafine monodispersed metal NPsloaded onto non-carbon support materials, such as TiO2–SiCheterostructures, for electrochemical sensing or biosensingplatforms is highly desirable.

In the present study, a TiO2–SiC nanohybrid was fabricatedthrough a facile generic in situ growth method. The preparation ofultradispersed Pd NPs by anchoring Pd NPs onto the TiO2–SiCsurface was demonstrated to prevent the aggregation of NPs.Utilizing the favorable properties of Pd NPs, the Pd-loaded TiO2–SiCnanohybrid (Pd@TiO2–SiC)-modified glassy carbon electrode (GCE)exhibited electrochemical performance superior to those ofTiO2–SiC and SiC in the simultaneous detection of HQ and BPA.

2. Materials and methods

2.1. Materials

SiC waspurchased from NanjingAipureiNano-Material Company(Nanjing, China). Titanium isopropoxide (TTIP), isopropyl alcohol,PdCl2 were obtained from Sigma Chemical Co. (St. Louis, MO, USA).All other reagents were of analytical grade. Phosphate buffer (PBS,0.1 M, pH 6.0) was used as working solution. All aqueous solutionswere prepared with deionized water (DW, 18 MV cm�1).

2.2. Apparatus

Differential pulse voltammetry (DPV) experiments wereperformed with a CHI 660E Electrochemical Workstation from

30 L. Yang et al. / Analytica Chimica Acta 852 (2014) 28–36

Shanghai Chenhua Instrument (Shanghai, China) and conductedusing a three-electrode system, with the modified GCE asworking electrode, a platinum wire as the counter electrode, asaturated calomel electrode (SCE) as the reference electrode. Themorphologies of the prepared samples were characterized by aJEM 2100 transmission electron microscopy (TEM, JEOL, Japan).Fourier transform infrared (FTIR) study was performed overthe wavenumber, range of 4000–400 cm�1 by a Thermo FisherScientific Nicolet IS10 (Thermo Fisher, USA) FTIR impact 410spectrophotometer using KBr pellets. BET surface areas weremeasured by nitrogen adsorption/desorption using a NOVA 2000egas sorption analyzer (Quantachrome Corp., USA). X-raydiffractometry (XRD) spectra were obtained by using a RigakuTTR III X-ray diffractometer (Rigaku, Japan). High performanceliquid chromatography (HPLC) was performed on an Agilent model1200 series with UV detector.

2.3. Synthesis of TiO2–SiC and Pd@TiO2–SiC

SiC was hydroxyl functionalized through a hydrothermalreaction of glucose. Briefly, 100 mg of SiC and 400 mg of glucosewere dispersed in 60 mL of DW via sonication, the suspension wastransferred to an autoclave and kept at 180 �C for 8 h. Afterreaction, the autoclave was cooled naturally. The suspension wascentrifuged and washed with DW and alcohol for three times,respectively. SiC��OH was obtained by freeze-drying. TiO2–SiC wasobtained by an in situ growth method as follows: briefly, TTIP(0.2 mL), isopropyl alcohol (3.0 mL), and nitric acid (2.0 M, 9.0 mL)were mixed under stirring at room temperature for 1 h. By addingDW to the mixture until a final volume of 50 mL was reached. Then200 mg of SiC��OH was added into the above mixture understirring for 2 h. Afterwards, this mixture was heated at 65 �C for24 h under stirring. After cooling to room temperature, theresulting suspension was centrifuged and washed with DW forthree times, dried in an oven at 60 �C to obtain TiO2–SiC. TiO2 NPswere obtained by using the same procedure in the absence ofSiC��OH. The Pd@TiO2–SiC nanohybrid was prepared as follows:TiO2–SiC (5.0 mg), polyethylene glycol 400 (0.1 mL), sodium citrate(0.01 M, 1.0 mL), and PdCl2 aqueous solution (0.01 M, 0.5 mL) weredispersed into 5.0 mL of DW via sonication, and then the mixturewas stirred with a magnetic stirrer for 0.5 h at room temperature.Three milliliter of 25.0 mM sodium borohydride solution wasadded dropwise and stirred for 0.5 h and shaken for an additional2 h. After centrifuging and washing with DW for three times, theresulting Pd@TiO2–SiC nanohybrid was obtained by freeze-drying.Pd@TiO2 and Pd@SiC were obtained by using the same process.

Scheme 1. The illustration of the Pd@TiO2–SiC na

Pd NPs were obtained by using the same process in the absence ofTiO2 NPs, SiC��OH, or TiO2–SiC.

2.4. Fabrication of modified electrodes

GCE (3 mm in diameter) was polished with 0.3 and 0.05 mmAl2O3 powder respectively and subsequently sonicated in ethanoland DW to remove the physically adsorbed substance and dried inair. The Pd@TiO2–SiC nanohybrid was dissolved in DW at aconcentration of 0.5 mg mL�1 with the aid of ultrasonic agitationfor 30 min, resulting in a homogeneous suspension. Similarly0.5 mg mL�1 TiO2–SiC and SiC homogeneous suspensions were alsoobtained. To prepare the Pd@TiO2–SiC modified electrode, 5 mL ofthe Pd@TiO2–SiC suspension was dropped onto the electrodesurface and dried at room temperature. The obtained electrode wasnoted as Pd@TiO2–SiC/GCE. For comparison, a similar procedurewas used to prepare Pd/GCE, SiC/GCE, TiO2–SiC/GCE, Pd@TiO2/GCE,and Pd@SiC/GCE.

2.5. Electrochemical measurements

DPV was applied in 0.1 M pH 6.0 PBS containing a certain amountof HQ and BPA from 0.2 to 0.9 V with a pulse amplitude of 0.05 V and apulse width of 0.05 s. All the measurements were carried out at roomtemperature. The illustration of the Pd@TiO2–SiC nanohybridssimultaneous sensing HQ and BPA was showed in Scheme 1.

2.6. Preparation and determination of real samples

When performed for practical applications, the tap water andwastewater samples were filtered by a 0.22 mm membrane and thepH was adjusted to 6.0. A certain amount of the samples wasdiluted with 0.1 M pH 6.0 PBS containing 10% ethanol fordetermination. A known amount of the pure BPA and HQ wasadded to the samples to study the recovery under the optimalconditions. The recovery results were obtained by using the relatedcalibration equation for three repeated measurements. HPLCmethod was used to detect BPA and HQ in the samples ascomparison.

3. Results and discussion

3.1. Characterization of TiO2–SiC and Pd@TiO2–SiC

The Pd@TiO2–SiC nanohybrid was strategically designed andfabricated through a multistep approach. The morphologies and

nohybrids simultaneous sensing HQ and BPA.

L. Yang et al. / Analytica Chimica Acta 852 (2014) 28–36 31

microstructures of Pd@TiO2–SiC and related materials wereinvestigated by transmission electron microscopy (TEM). Fig. 1Ashows that the SiC NPs have particle sizes of approximately30–40 nm. Fig. S1 illustrates TEM and high-resolution TEM(HRTEM) images of SiC��OH. The surface of the SiC NPs is notablycoated by amorphous carbon after the hydrothermal reaction inglucose aqueous solution. The typical HRTEM image shown inFig. S1B illustrates that SiC��OH has a core–shell structure with ashell of �4.0 nm thickness. Compared with pure SiC, SiC��OHshowed numerous surface hydroxyl groups that can react with

Fig. 1. TEM images of SiC (A), TiO2–SiC (B), and P

organochlorosilanes and alkoxides to generate covalently bondedorganic–inorganic derivatives, which anchor metal or metal oxideparticles [41,42]. Fig.1B shows that �15 nm TiO2 NPs are associatedwith SiC NPs. Fig. 1C–F shows that ultrafine Pd NPs with a uniformsize of �2.3 nm are monodispersed well on the TiO2–SiC surface.

Fig. S2A shows the low-temperature nitrogen adsorption–desorption isotherms of TiO2–SiC, which clearly Type IV adsorptiveisothermal curves; here, the adsorptive capacity sharply increaseswhen the relative pressure ranges from 0.80 to 0.95 and thehysteresis loop is observed during desorption. The shapes of the

d@TiO2–SiC at different magnifications (C–F).

32 L. Yang et al. / Analytica Chimica Acta 852 (2014) 28–36

isotherms and the hysteresis loop suggest that the sample has amesoporous structure. The isotherm showed continuous increaseseven at relative pressures above 0.9, which indicates that thesample also possesses several macroporous structures. The BETsurface area of TiO2–SiC is 30.5 m2g�1, which is higher than thepreviously reported value [21]. These results illustrate thatTiO2–SiC is suitable for anchoring metal nanoparticles. The crystalstructures of SiC, TiO2–SiC, and Pd@TiO2–SiC were investigatedthrough XRD, as shown in Fig. S2B. The XRD pattern of SiC showedwell-defined peaks at 35.64�, 41.34�, 60.03�, 71.82�, and 75.33�

(2u), indicating the formation of the SiC NP cubic phase. Based onthe cubic phase crystal structure of SiC, the crystal form of SiC wasconcluded to be b-SiC (3C–SiC). Compared with that of SiC, theXRD pattern of TiO2–SiC presented two weak peaks at 27.26� and54.24� (2u), which could be attributed to the formation of theTiO2 NP rutile phase. The XRD pattern of Pd@TiO2–SiC showedwell-defined peaks at 40.13�, 46.68�, 68.14�, 82.05�, and 86.64�

(2u), which indicates the formation of the Pd NP cubic phase. Thecharacteristic diffraction peaks of the Pd NPs confirmed thepresence of Pd NPs in the Pd@TiO2–SiC nanohybrids.

Fig. 2. (A) DPVs obtained for the oxidation of 1.0 mM HQ at bare GCE in 0.1 MpH 6.0 PBS (a), 1.0 mM BPA at bare GCE in 0.1 M pH 6.0 PBS containing 10% ethanol(b), and mixture containing 1.0 mM HQ and 1.0 mM BPA at bare GCE in 0.1 MpH 6.0 PBS containing 10% ethanol (c). Pulse width: 0.05 s; amplitude: 0.05 V.(B) DPVs obtained for the oxidation of mixture containing 200.0 mM HQ and200.0 mM BPA at bare GCE (a), SiC/GCE (b), Pd/GCE (c), TiO2–SiC/GCE (d),Pd@TiO2/GCE (e), Pd@SiC/GCE (f), Pd@TiO2–SiC (g) in 0.1 M pH 6.0 PBS containing10% ethanol. Pulse width: 0.05 s; amplitude: 0.05 V; accumulation potential:�0.2 V; accumulation time: 200 s.

3.2. Electrochemical behavior of HQ and BPA at modified electrodes

The electrochemical behaviors of BPA and HQ toward themodified electrodes were investigated using DPV. Fig. 2A showsthe oxidation processes of (a) 1.0 mM HQ on the bare GCE in 0.1 M pH6.0 PBS, (b) 1.0 mM BPA on the bare GCE in 0.1 M PBS (pH 6.0)containing 10% ethanol, and (c) a mixture of 1.0 mM HQ and 1.0 mMBPA onthe bare GCE in 0.1 M PBS(pH 6.0)containing 10%ethanol.Thetwo well-defined peaks at 0.20 and 0.44 V correspond to HQ and BPA,respectively; these peaks are well separated and show a potentialdifference of 240 mV, which is an adequately large window forsimultaneously determining the concentrations of mixed solutionsof HQ and BPA. Fig. 2B shows the oxidation processes of mixturescontaining 200.0mM HQ and 200.0 mM BPA on the bare GCE (a),SiC/GCE (b), Pd/GCE (c), TiO2–SiC/GCE (d), Pd@TiO2/GCE (e),Pd@SiC/GCE (f), Pd@TiO2–SiC (g) in 0.1 M PBS (pH 6.0) containing10% ethanol. The oxidation peaks of HQ and BPA were observed at0.20 and 0.44 V, respectively. The oxidation currents of HQ and BPAon the SiC-modified GCE improved by approximately 20% comparedwith that on the bare GCE. This improvement indicates that SiCsignificantly promotes electron transport and communicationbetween the solution and electrode, which is consistent with theresults of previous studies [25,43]. The excellent adsorptive capacityof SiC can also lead to high current responses. The oxidation currentsof HQ and BPA on the Pd NPs-modified GCE improved byapproximately 30% compared with that on the bare GCE due tothe excellent catalytic activities of the Pd NPs. On the TiO2–SiC/GCE,the HQ and BPA oxidation currents significantly increased comparedwith those on the bare and SiC-modified GCEs, which indicates thatthe high surface area and adsorptive capacity of the TiO2–SiCcomposite increase the effective electrode area and improvecatalytic activity for HQ and BPA oxidation. In the case ofPd@TiO2/GCE and Pd@SiC/GCE, the HQ and BPA oxidation currentsremarkably increased compared with those on the SiC, Pd, andTiO2–SiC-modified GCEs, which could be ascribed to the synergisticeffects between Pd NPs and TiO2 or SiC. Oxidation currentsincreased dramatically when the Pd@TiO2–SiC nanohybrid wasimmobilized onto the GCE surface. This result may be attributedto the synergistic effect between the three component. Andthe remarkable conductivities and electrocatalytic activities ofultrafine-monodispersed Pd NPs may be the main contribution thatcan amplify the electrochemical signals [8,44].

3.3. Optimization of experimental conditions

The acidity of the detection medium can affect the rate of masstransport to the electrode surface, especially when the redox processinvolves several protons, as in the present study. The effect of pH onthe current response of the Pd@TiO2–SiC/GCE toward 200.0 mM HQand 200.0 mM BPA was investigated within the pH range of 4.0–8.0.Fig. 3A shows that the oxidation peak current gradually increaseswith increasing pH from 4.0 to 6.0. This phenomenon may beattributed to the high concentration of protons in solution that canreplace the molecules of phenol compounds on adsorption sites onthe Pd@TiO2–SiC/GCE surface. Further increases in pH beyond8.0 resulted in a decrease in the oxidation peak current becausehydroxyl anions may prevent phenol compounds from accessingadsorption sites on the Pd@TiO2–SiC/GCE surface. The peakpotentials of HQ and BPA and pH show linear relationshipswith regression equations of EP (V) = �0.064 pH + 0.6024 andEP (V) = �0.062 pH + 0.8416, respectively (Fig. 3B). Shifts of 64 mVfor HQ and 62 mV for BPA per pH unit are close to the theoreticalvalue of 57.6 mV per pH unit [45], which indicates that electrontransfer is accompanied by an equal number of protons in theelectrode reaction. Therefore, pH 6.0 was selected as the optimalsolution pH for the simultaneous determination of HQ and BPA.

Fig. 4. (A) DPV curves obtained for of the oxidation of 1 mM HQ and 1 mM BPA atPd@TiO2–SiC/GCE for different concentrations 0.00, 0.01, 0.1, 0.5, 1, 3, 5, 50, 100, 150,200 mM. Pulse width: 0.05 s; amplitude: 0.05 V; accumulation potential: �0.2 V;accumulation time: 200 s. (B) Calibration plots of the oxidation current atPd@TiO2–SiC/GCE versus concentration of HQ under optimal conditions.(C) Calibrationplotsof theoxidationcurrentatPd@TiO2–SiC/GCEversusconcentrationof BPA under optimal conditions.

Fig. 3. (A) Effect of detection medium pH on the oxidation peak currents of 200.0mMHQ and 200.0mM BPA at Pd@TiO2–SiC/GCE in 0.1 M PBS pH 6.0 PBS containing 10%ethanol by DPV. Pulse width: 0.05 s; amplitude: 0.05 V; accumulation potential:�0.2 V; accumulation time: 200 s. (B) Variation of the peak potentials vs. pH of thedetection medium.

L. Yang et al. / Analytica Chimica Acta 852 (2014) 28–36 33

The accumulation step is a simple and effective strategy toenhance sensitivity; thus, the effects of accumulation time andpotential on the Pd@TiO2–SiC/GCE oxidation peak currents wereinvestigated. The oxidation peak currents of 1.0 mM HQ and1.0 mM BPA were compared at different accumulation times. Fig. S3shows that oxidation peak currents gradually increase withincreasing accumulation times of up to 200 s and then level offthereafter, which indicates that accumulation of HQ and BPA on thePd@TiO2–SiC/GCE can rapidly reach saturation. The impact ofaccumulation potential on the HQ and BPA oxidation peak currentswas also investigated. Fig. S4 shows that the highest oxidation peakcurrent is achieved at �0.2 V. Therefore, in the present study, theaccumulation step was performed at �0.2 V for 200 s.

3.4. Simultaneous determination of HQ and BPA using DPV

DPV was used to determine HQ and BPAunderoptimal conditionsbecause the technique is highly sensitive and has a low detectionlimit. Fig. 4A shows the DPV curves of HQ and BPA on thePd@TiO2–SiC/GCE under different solution concentrations. Theoxidation peak currents increased with increasing HQ and BPAconcentration. Fig. 4B shows the corresponding calibration curve forHQ. The oxidation currents were proportional to HQ concentrationsbetween 0.01 and 5 mM and between 5 and 200 mM with a detection

limit of 5.5 nM (S/N = 3). The corresponding regressionequations were calculated as I (mA) = 0.44C (mM) + 0.74 andI (mA) = 0.012C (mM) + 2.843 with correlation coefficients of0.994 and 0.997, respectively. Fig. 4C shows the correspondingcalibration curve for BPA. Here, the oxidation currents were also

Fig. 5. The oxidation currents of 100.0 mM HQ and 100.0 mM BPA in the absence (a)and presence of 10 mM various interferents (from (b) to (g)): phenol, catechol,resorcinol, 2-nitrophenol, 3-nitrophenol, and 4-nitrophenol. Pulse width: 0.05 s;amplitude: 0.05 V; accumulation potential: �0.2 V; accumulation time: 200 s.

Table 1Comparison of proposed sensor for detection of HQ and BPA with others.

Sample Electrode Method Liner range(mM)

Detection limit(mM)

Refs.

HQ IL/CPE DPV 10–1500 4 [46]MWNTs/GCE LSV 2–100 0.6 [47]PANI/MnO2/GCE DPV 0.2–100 0.13 [4]CMK-3-Nafion/GCE DPV 0.5–25 0.1 [48]Electroreduced GO/GCE DPV 6–200 0.2 [49]Gold nanostructure/GCE DPV 2.5–800 0.5 [50]Carbon nanofibers/GCE DPV 1–200 0.4 [51]Carbon NPs–chitosan/GCE DPV 0.8–100 0.2 [52]Nitrogen-doped CNT/GCE DPV 10–1000 1.2 [53]Thermally RGO/GCE DPV 1–500 0.75 [54]Pd@TiO2–SiC/GCE DPV 0.01–5, 5–200 0.0055 This work

BPA MCM-41/CPE DPV 0.088–0.22 0.038 [55]N-GS/CS/GCE Amperometry 0.01–1.3 0.005 [56]ITO DPV 5–120 0.29 [57]Thionine-tyrosinase/CPE Amperometry 0.15–45 0.15 [58]CoPc-CPE DPV 0.088–12.5 0.01 [59]PAMAM–Fe3O4/GCE Amperometry 0.01–3.07 0.005 [60]Au NPs/MoS2/GCE CV 0.05–100.0 0.005 [61]Tyr-SF-MWNTs-CoPc/GCE Amperometry 0.05–3.0 0.03 [62]CS-Fe3O4/GCE DPV 0.05–30.0 0.008 [63]Arg-G/GCE DPV 0.005–40.0 0.001 [64]C60/GCE SWV 0.074–0.23 0.0037 [3]Pd@TiO2–SiC/GCE DPV 0.01–5, 5–200 0.0043 This work

34 L. Yang et al. / Analytica Chimica Acta 852 (2014) 28–36

proportional to the concentration of BPA between 0.01 and 5 mMand between 5 and 200 mM with a detection limit of 4.3 nM(S/N = 3). The corresponding regression equations were calculatedas I (mA) = 0.45C (mM) + 0.59 and I (mA) = 0.013C (mM) + 2.798 withcorrelation coefficients of 0.995 and 0.998, respectively. Theperformance of the proposed Pd@TiO2–SiC modified electrodewas compared with those of other electrodes reported in previousstudies. The Pd@TiO2–SiC/GCE exhibited a lower detection limit andwider linear range than other electrodes as shown in Table 1; thus,the modified electrode fabricated in the present studycan be used tosimultaneously detect HQ and BPA in solution with high sensitivity.

3.5. Selectivity, reproducibility, and stability

Common interferences in the simultaneous detection of HQand BPA, including those attributed to phenol, catechol, resorcinol,2-nitrophenol, 3-nitrophenol, and 4-nitrophenol, were studiedusing the Pd@TiO2–SiC-modified GCE. The oxidation currents of100.0 mM HQ and 100.0 mM BPA on the Pd@TiO2–SiC/GCE werecompared with signals obtained in the presence of 10-foldconcentrations of the interfering species. Fig. 5 shows that thepresence of the interferences specified above exert no evidenteffect on the HQ and BPA responses in the present system. The highselectivity of the modified electrode may be attributed to severalreasons. Fig. S5 shows that three oxidation peaks at �0.05, 0.15,and 0.9 V may be observed for 4-nitrophenol in 0.1 M PBS at pH 6.0.However, no peaks at �0.05 and 0.15 V were observed in 0.1 M PBScontaining 10% ethanol at pH 6.0. The peak observed at 0.9 V wasrelatively distant from the HQ and BPA oxidation peaks. In addition,the 2-nitrophenol and 3-nitrophenol peaks were parallel to that of4-nitrophenol. The oxidation peaks of resorcinol and phenol, at0.75 and 1.0 V, respectively, are far from the oxidation peaks of HQand BPA. No significant interference from common ions, such asCa2+, Mg2+, Fe3+, Zn2+, Br�, SO4

2�, and NO3�, was observed, even at

100-fold excess concentrations. Therefore, the modified electrodemay be reliably used for the simultaneous and quantitativedetection of both HQ and BPA even at ambient conditions.

To evaluate the fabrication reproducibility of the Pd@TiO2–

SiC/GCE sensor, the oxidation peak currents of 200 mM HQ and200 mM BPA on 10 similar Pd@TiO2–SiC/GCEs were compared. All

of the modified electrodes exhibited similar electrochemicalresponses with a relative standard deviation (RSD) of 2.1%, whichindicates satisfactory reproducibility.

Successive cyclic potential scans of 50 cycles and long-termstorage assays were used to examine the stability of thePd@TiO2–SiC/GCE sensor. The long-term stability experiment wasintermittently performed (every 5 d). The initial peak currentdecreased by 4.6% after 50 continuous cycle scans. When thePd@TiO2–SiC/GCE sensor was not in use, it was stored in arefrigerator at 4 �C. Initial responses of over 95.6% and 88.4% weremaintained after storage for 15 and 30 d, respectively. These findingsindicate that the Pd@TiO2–SiC-based sensor is fairly stable.

3.6. Real sample analysis

To evaluate the feasibility of the Pd@TiO2–SiC/GCE sensor forreal sample analysis, the Pd@TiO2–SiC/GCE sensor was used to

Table 2Determination of HQ and BPA in tap-water and wastewater samples (n = 3).

Sample Added (mM) Founded (mM) RSD (%) Recovery (%) HPLC (mM)

HQ BPA HQ BPA HQ BPA HQ BPA HQ BPA

Tap-water 5.0 5.0 4.93 5.18 3.4 4.9 98.6 103.6 4.84 5.0610.0 10.0 10.26 9.74 3.1 4.6 102.6 97.4 9.91 9.88

Wastewater 5.0 5.0 5.27 5.11 3.8 3.9 105.4 102.2 5.35 5.2610.0 10.0 10.30 10.24 2.9 3.3 103.0 102.4 10.52 9.92

L. Yang et al. / Analytica Chimica Acta 852 (2014) 28–36 35

detect HQ and BPA in tap water and wastewater samples using thestandard addition method. Results showed recoveries rangingfrom 97.4% to 105.4% and RSDs ranging from 2.9% to 4.9% (Table 2).For comparison, the HPLC method was also used to detect HQ andBPA in the samples. It shows that the results are in agreement withthe present method. These findings demonstrate that thePd@TiO2–SiC-based sensor fabricated in this study has practicalapplications.

4. Conclusions

A TiO2–SiC nanohybrid was successfully fabricated through afacile generic in situ growth method. Ultrafine monodispersedPd NPs were anchored onto a TiO2–SiC surface through achemical reduction process using polyethylene glycol and sodiumcitrate as the dispersant and stabilizing agent, respectively.Utilizing the favorable properties of Pd NPs, the Pd@TiO2–SiCnanohybrid-modified GCE exhibited electrochemical performancesuperior to those of TiO2–SiC and SiC in the simultaneous detectionof HQ and BPA. The simple, inexpensive, and mass-productivestrategy presented in this study provides a suitable alternative forthe fabrication of various ultrafine metal NPs@metal oxide–SiChybrids that may be used in sensing and biosensing applications,food safety, and environmental monitoring.

Acknowledgements

This work was supported by the National Natural ScienceFoundation of China (31160334) and the Natural ScienceFoundation of Yunnan Province (2012FB112,2014RA022), People’sRepublic of China.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.aca.2014.08.037.

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