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Sensors and Actuators B 208 (2015) 389–398

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

jo ur nal home page: www.elsev ier .com/ locate /snb

ne-step hydrothermal green synthesis of silver nanoparticle-carbonanotube reduced-graphene oxide composite and its application asydrogen peroxide sensor

arnaz Lorestani ∗, Zohreh Shahnavaz, Pooria Mn, Yatimah Alias, Ninie S.A. Manan ∗∗

epartment of Chemistry, Faculty of Science, University Malaya, Kuala Lumpur 50603, Malaysia

r t i c l e i n f o

rticle history:eceived 10 October 2014ccepted 13 November 2014vailable online 22 November 2014

eywords:ydrothermalilver nanoparticlesarbon nanotubeeduced graphene oxideydrogen peroxideensor

a b s t r a c t

A novel sensing composite of silver nanoparticles (AgNPs)-reduced graphene oxide (rGO)-carbon nano-tube (MWCNT) was successfully synthesized by a simple one-step hydrothermal method withoutreducing agent. Mild reduction of GO was carried out under hydrothermal condition. While most con-ventional approaches make use of multistep chemical methods wherein strong reducing agents, suchas hydrazine, hydroquinone, and sodium borohydride are employed, our method provides the notableadvantage of a single-step reaction without employing any toxic solvent or reducing agent by provid-ing a novel green synthetic route to produce the nanocomposites of rGO, carbon nanotube and silver.The results of X-ray diffraction (XRD) and Fourier-transform infrared transmission spectroscopy (FT-IR)confirmed the simultaneous formation of silver nanoparticles in the GO and MWCNT matrix. Field emis-sion scanning electron microscope (FESEM) images and transmission electron microscopy (TEM) showeduniform distribution of nanometer-sized silver nanoparticles and narrow-sized MWCNT on GO sheets,which was achieved using silver ammonia complex as the precursor, instead of the commonly used sil-ver nitrate. The composite exhibited excellent electrocatalytic activity for the reduction of H2O2 with a

fast amperometric response time less than 3 s. The electrocatalytic activity for the reduction was stronglyaffected by the concentration of silver ammonia solution in the nanocomposites, with the best electrocat-alytic activity observed for the composite of 6:1 volume ratios of MWCNT–GO (3:1, v/v) to Ag(NH3)2OH(0.04 M). The corresponding calibration curve for the current response showed a linear detection rangeof 0.1–100 mM (R2 = 0. 9985), while the limit of detection was estimated to be 0.9 �M.

. Introduction

Graphene, a single-layer sheet of sp2-hybridized carbon atomsith high surface area and superior mechanical and electrical prop-

rties, provides an extraordinary platform for preparing compositeanomaterials [1–3]. These specific properties have potential appli-ations in various advanced technologies such as nanoelectronics,ensors, capacitors and composites [4–6]. Nowadays, grapheneheets can be prepared through various techniques, such as chemi-

al, vapor deposition, electrochemical reduction of graphene oxidend thermal or chemical reduction of graphene oxide [7–10].O can be reduced slowly by heat treatment and the process is

∗ Corresponding author. Tel.: +60 379675160.∗∗ Corresponding author. Tel.: +60 379674022 ext 3873; fax: +60 379674193.

E-mail addresses: farnaz.lorestani@siswa.um.edu.my (F. Lorestani),ohreh.shahnavaz@siswa.um.edu.my (Z. Shahnavaz), pooriamn@yahoo.comP. Mn), yatimah70@um.edu.my (Y. Alias), niniemanan@um.edu.my (N.S.A. Manan).

ttp://dx.doi.org/10.1016/j.snb.2014.11.074925-4005/© 2014 Elsevier B.V. All rights reserved.

© 2014 Elsevier B.V. All rights reserved.

named thermal annealing reduction. In the early years of grapheneresearch, rapid heating was usually used to exfoliate graphite oxideto achieve graphene [11].

The reduction of GO can be accomplished using reductants likehydrazine, hydroquinone, NaBH4 and so on, but, the proceedingsare usually time-consuming and contain toxic and dangerousreductants. Thus, it is desirable to use a low-cost, facile and effec-tive approach for preparing soluble reduced graphene oxide (rGO)sheets in high quantity. For these reasons, thermal reduction ofgraphene oxide has the privilege of being a simple, fast and envi-ronmentally friendly approach.

Graphene’s excellent characteristics such as high electrical con-ductivity, excellent chemical tolerance and high surface to volumeratio make it an attractive matrix for composites. Graphene andmulti-walled carbon nanotube (GO–MWCNT) has been proven to

be good a material combination for preparing nanocomposites[12]. Previous studies have shown that GO–MWCNT nanocompos-ite resulted in large electrochemical surface area and fast electrontransfer for sensor applications [13,14]. Decoration of graphene or

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arbon nanotubes with metal nanoparticles could enhance the elec-rochemical properties of the composite compared to using eachonstituent individually. Recently, graphene, or MWCNT compos-tes with Ag, Pt, and Au nanoparticles have been used in sensorpplications [15–19]. Among these materials, silver nanoparticlesAgNPs) demonstrated excellent catalytic activity in sensor fields20]. The sensor performance of composites could be affected by thehape, size, and dispersion of metal particles on them [21]. There-ore, the composite matrix plays an important role in gaining highispersion of AgNPs, as well as its respective size and shape. Forxample, Yang et al. and the Compton group discussed the chem-cal and physical methods that have been employed to fabricategNPs–MWCNT composites [22,23]. Other reported methods toroduce AgNPs–MWCNT composite including templates, chemi-al reagents, irradiations [24,25], electrodeposition method [26],nd synthesis by MWCNT without the addition of any chemi-al (reducing agent) or exposure to irradiation. The AgNPs wereeduced to MWCNT surface, causing a good dispersion of AgNPsn the AgNPs–MWCNT composite and displayed good performanceoward sensing H2O2 at high sensitivity levels [27,28]. The uniformistribution of nanometer-sized silver nanoparticles on GO sheetsould be achieved using silver ammonia complex as the precursor,nstead of silver nitrate which is commonly used [29].

In the present study, we present a simple and facile one-stepethod to prepare an AgNPs–MWCNT–rGO composite for non-

nzymatic H2O2 sensor. The most notable aspect of our study ishe use of an aqueous solution and the omission of harsh chemicalsuch as hydrazine during the synthesis. The reduction of graphenexide and the crystallization of the produced Ag were carried outoncurrently in a single-step reaction by hydrothermal treatment.ilver particles with small diameter were nicely distributed usingn optimized amount of silver ammonia complex in the composite.he AgNPs–MWCNT–rGO composite exhibits an excellent sensitiv-ty, wide linear range and high selectivity toward H2O2 detection.

. Experiment

.1. Chemicals

Graphite flakes were purchased from Ashbury Inc. (NJ, USA).ulfuric acid (H2SO4, 98%), nitric acid (HNO3, 98%), potassiumermanganate (KMnO4, 99.9%), hydrogen peroxide (H2O2, 30%),mmonia solution (NH3, 25%), sodium phosphate dibasic dihydrateNa2HPO4·2H2O), sodium phosphate monobasic NaH2PO4 and sil-er nitrate (AgNO3, 99.7%) were purchased from Sigma–Aldrich.ulti-wall carbon nanotubes (30–50 nm diameter and 15 �m

ength), with >95% purity were obtained from DropSens (Spain).ll aqueous solutions were prepared with doubled distilled water

rom a Millipore system.

.2. Apparatus and electrochemical measurements

.2.1. CharacterizationThe morphology of the samples were characterized by X-ray

owder diffraction (XRD; Philips X’pert system using Cu K� radi-tion), field emission scanning electron microscope (FESEM; FEIova Nano-SEM 400 operated at 10.0 kV), a Fourier transform

nfrared spectrometer (FTIR; Perkin Elmer System 2000 series spec-rophotometer, USA) and transmission electron microscopy (TEM,hilips EM400ST microscopy).

.2.2. Electrochemical measurement

A three-electrode cell was used, consisting of a modified glassy

arbon electrode (GCE) with dropped nanoparticle-graphene-arbon nanotube nanocomposite (AgNPs/MWCNT/rGO) as theorking electrode, Pt coil as the counter electrode and Silver

ators B 208 (2015) 389–398

chloride electrode (Ag/AgCl) as the reference electrode. Cyclicvoltametries were collected between 0.2 and −0.8 V in 0.2 M phos-phate buffer solution (Na2HPO4 and NaH2PO4) at pH 6.5 with a20 mV s−1 scan rate.

2.3. Preparation of composite and biosensor

2.3.1. Preparation of graphene oxide (GO)GO was prepared using a modified Hummers’ method [30].

Probable extra acids and salts were removed from the preparedGO 0.05 wt% suspension by dialyzing it for 6 h. The dialyzed GOsuspension was then centrifuged for 20 min at 3000 rpm to removeunexfoliated GO and dried for 24 h at 50 ◦C.

2.3.2. Synthesis of silver nanoparticle-carbonnanotube-reduced-graphene-oxide nanocomposite(AgNPs–MWCNT–rGO)

Silver-ammonia [Ag(NH3)2OH] solution was obtained by addingammonia (1 wt%) to silver nitrate solution (50 mM) until the pre-cipitates were visually invisible to the naked eye. The concentrationof the synthesized Ag(NH3)2OH was approximately 40 mM. Thewater dispersion of MWCNT and GO (1.0 mg ml−1) were preparedin separate containers. The solution of MWCNT and GO was mixedwith the ratio of 3:1 (v/v). The AgNPs–MWCNT–rGO compositewas prepared by mixing the freshly prepared Ag(NH3)2OH solu-tion with MWCNT–GO solution in three different volume ratios,which were (12:1, v/v), (6:1, v/v), and (3:1, v/v) [31]. All thesolutions were stirred for 30 min. Then, the mixture was placedin an autoclave and heated at 170 ◦C for 4 h, which were laternominated into AgNPs–MWCNT–rGO-1, AgNPs–MWCNT–rGO-2,and AgNPs–MWCNT–rGO-3, respectively. After being cooled toroom temperature naturally, the product was isolated by cen-trifugation, washed several times with pure water and then driedovernight at 50 ◦C. Previously, AgNO3 was used as AgNPs sourcefor most nanocomposites [32,33]. To check the effect of using Ag(NH3)2OH instead of AgNO3, the AgNPs–MWCNT–rGO compos-ite was prepared using the same volume ratio of MWCNT–GO to0.04 M AgNO3 with the volume ratio of 12:1, which was then calledAgNPs–MWCNT–rGO-4.

2.3.3. Synthesis of silver nanoparticle-carbonnanotube-graphene-oxide nanocomposite (AgNPs–MWCNT–GO)

Silver-ammonia [Ag(NH3)2OH] solution was obtained by addingammonia (1 wt%) to silver nitrate solution (50 mM) until com-plete disappearance of precipitate was observed. The concentrationof the synthesized Ag(NH3)2OH was approximately 40 mM.AgNPs–MWCNT–GO was prepared by mixing the freshly preparedAg(NH3)2OH solution with an aqueous solution of MWCNT and GO(1.0 mg ml−1) (3:1, v/v) to Ag(NH3)2OH volume ratios 12.6 and 3[31] under stirring for 4 h. The obtained precipitate was filtered,and then dried overnight at 50 ◦C.

2.3.4. Preparation of modified AgNPs–MWCNT–GO andAgNPs–MWCNT–rGO nanocomposite GCE

To obtain AgNPs–MWCNT–GO and AgNPs–MWCNT–rGOnanocomposite modified glassy carbon electrodes(AgNPs–MWCNT–GO/GCE) and (AgNPs–MWCNT–rGO/GCE),AgNPs–MWCNT–GO nanocomposite (1 mg) of each compositewas dispersed into doubled-distilled water (1 ml) by 30 min mildultrasonication to achieve homogeneous suspension. After that, a

certain amount (10 �l) of prepared suspensions was dropped intothe well-polished GCEs (GCE Metrohm d = 3 mm) and dried at roomtemperature for 24 h to prepare each AgNPs–MWCNT–GO/GCEand AgNPs–MWCNT–rGO/GCE.

F. Lorestani et al. / Sensors and Actuators B 208 (2015) 389–398 391

Fp

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3n

(soM4trsrcAscGdc

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Fig. 2. FTIR spectra of GO (a), rGO (b), MWCNT (c) and AgNPs–MWCNT–rGO com-posite (d).

ig. 1. XRD patterns of MWCNT (a), GO (b), rGO (c) and AgNPs–MWCNT–rGO com-osite (d).

. Results and discussion

.1. Structural characterization of AgNPs–MWCNT–rGOanocomposite

Fig. 1 shows the XRD patterns of MWCNT (a), GO (b), rGOc) and AgNPs–MWCNT–rGO nanocomposite (d). GO (b) has aharp peak at 10.8◦, which proved inter-planer spacing (0.82 nm)f the typical feature of GO (d0 0 2) [34]. The XRD pattern ofWCNT (a) indicates the presence of two peaks at 25.8◦ and

2.8◦ which corresponded to the inter-layer spacing (0.34 nm) ofhe nanotube (d0 0 2) and the d1 0 0 reflecting of the carbon atoms,espectively [35]. After hydrothermal reduction, all of the inten-ities of the related oxygen peaks were clearly decreased in theGO sample (c), compared to GO (b), indicating that the delo-alized p conjugation is restored in our rGO sample [36]. At thegNPs–MWCNT–rGO nanocomposite XRD pattern (d) the inten-ity for the 111, 200, 220 and 311 of Ag (Ref. code: 003-0921)an be obviously seen. High intensity of Ag peaks decreasedO and MWCNT peaks, but they could still be seen. This resultemonstrates that AgNPs–MWCNT–rGO nanocomposite were suc-

essfully fabricated [37].

FTIR spectra for GO, rGO, MWCNT and AgNPs–MWCNT–rGOanocomposites are shown in Fig. 2a–d respectively. For GO

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92 F. Lorestani et al. / Sensors an

Fig. 2a), the board stretching vibrations for O H peak centered at265 cm−1 were recorded. The peaks at 1727, 1623, 1367, and 1056ere assigned to C O stretching, C C stretching bands for aromatic

ings and O H bending, C OH stretching and C O C stretching,espectively. Furthermore, the peaks at 1041 and 1161 cm−1 wereelated to C O vibration of alkoxy or epoxy groups. [38]. The peakt 32,670 cm−1 in the rGO spectrum is attributed to O H groupshat is obviously shows the reduction of O H groups in compare toO by using hydrothermal and the bands at 1610 and 1358 cm−1

re related to C O and C OH, respectively (Fig. 2b). The peak025 cm−1 in the rGO spectrum are recognized as the C O stretch-

ng vibration of the epoxy and alkoxy groups [39]. The existencef MWCNT in the composite could also be confirmed using FTIRpectroscopy (Fig. 2c). A band centered at 1599 cm−1 and a shoul-er at 1350 cm−1 were due to C C stretching vibrations, whilehe band at 1117 cm−1 was associated with C O stretching vibra-ions [40]. For the AgNPs–MWCNT–rGO nanocomposite (Fig. 2d),he peaks at 1066, 1642, 1418, 2853, and 2925 cm−1 were assignedo C O stretching vibrations, C C stretching bands for aromaticings and O H bending, O H deformation, as well as symmetric andsymmetric stretching vibrations of CH2 groups, respectively [41].he broad peak of 3300 cm−1 for AgNPs–MWCNT–rGO nanocom-osite could be attributed to O H stretching vibrations of thebsorbed water molecules [38]. The O H peak of rGO dramati-ally reduced compared to the board stretching vibrations O Heak of GO centered at 3265 cm−1 (Fig. 2a) caused by hydrothermaleduction.

FSEM images and silver nanoparticle size distribution dia-rams of AgNPs–MWCNT–rGO nanocomposites prepared usingifferent volume ratios of MWCNT–GO (1.0 mg ml−1) (3:1, v/v)o Ag(NH3)2OH and (0.04 M) 12.6 and 3 are shown in Fig. 3(and b), (c and d) and (e and f), respectively. The FESEM image ofgNPs–MWCNT–rGO nanocomposite, which was prepared usingWCNT–GO (1.0 mg ml−1) (3:1, v/v) to AgNO3 (0.04 M) volume

atio of 12 is shown in Fig. 3g and its silver nanoparticle size dis-ribution diagram is shown in Fig. 3h. As seen in Fig. 3(a–c), byncreasing the concentration of Ag(NH3)2OH, the size of reducedilver particles increased from 12.73 to 22.00, which means thathe particle diameter and the quantity of reduced silver particlen the nanocomposite matrix also increased, and therefore, someilver agglomeration was seen on the surface of MWCNT and GO.y changing the AgNO3, instead of Ag(NH3)2OH in the mixture, theg particle size increased dramatically from 12.73 (Fig. 3b) to 45.59

Fig. 3h) mean particle diameter. GO with negative charge increasedn number through neutralization by the alkaline Ag(NH3)2OH inrder to absorb more [Ag(NH3)2]+ ions than AgNO3, which leado the creation of more initial nucleation sites [42] and greatertability and resistance to reduction of Ag(NH3)2OH over AgNO3,revention of growing of Ag into large particles[43].

The TEM micrograph in Fig. 4 shows the AgNPs–MWCNT–rGOanocomposite prepared by MWCNT–GO (1.0 mg ml−1) (3:1, v/v)o Ag(NH3)2OH volume in a 6:1 ratio. The existence of high denseilver nanoparticles in MWCNT and GO matrix agreed with the databtained from FESEM and XRD.

.2. Electrochemical impedance spectroscopy ofgNPs–MWCNT–rGO nanocomposite

The electrochemical impedance spectroscopy (EIS) is an efficientechnique to explore the characteristics of interfaces of modifiedlectrodes. After several surface modifications, the conductivity

haracteristics were characterized by EIS. Meaningful discrepancyn the impedance spectra was detected.

One of the main factors in EIS includes Rs or cell resistance, whichonsists of the solution and the electrolyte resistance.

ators B 208 (2015) 389–398

CPE or the constant phase element was modeled as a non-idealcapacitor

CPE = −1(Ciω)˛

The charge existing at a double layer interface is characterized bycapacitance, C and used when the surface is heterogeneous. Rct

represents the resistance of charge transfer.The definition of the Warburg element is

Zw = Rdif ctnh([i�ω]˛)

(i�ω)˛

Rdif is a diffusion resistance of electroactive species, � a time con-stant that depends on the diffusion rate (� = l2/D), where l is theeffective diffusion thickness, and D is the effective diffusion coef-ficient of the species. At “high” frequency, the impedance showsinfinite diffusion behavior (phase angle 45◦) and at “low” frequency,it tends to show the capacitor behavior (phase angle 90◦). ̨ valueof less than 0.5 implies the non-uniformity of the interface (as ithappens with CPE non-ideal capacitance when ̨ < 1) [44].

Fig. 5 presents the Nyquist plot of impedance spectrafor bare and modified GCE electrodes with GO, MWCNT,AgNPs–MWCNT–GO and AgNPs–MWCNT–rGO in 0.1 M KCl solu-tion which contained 1 mM Fe(CN)6

3−/4− (1:1(. For each of theseelectrodes, experiments were carried out three times to minimizeerror.

The obtained EIS spectra of AgNPs–MWCNT–GO andAgNPs–MWCNT–rGO modified GCE electrodes show two areas:a semicircular part related to electron transfer process at highfrequencies, and a linear part related to diffusion control at lowerfrequencies. Rct or electron transfer resistance at the surfaceof the electrode can be assessed by exploiting the semicirclediameter.

The EIS adjusted well with the corresponding circuit shownin Fig. 5. The chi-squared (�2) was minimized at 10−4. The cor-responding circuit considers the EIS characteristics of modifiedelectrodes which are symbolized by C (capacitance) and Rct (resis-tance) attached subsequently with the double layer capacitanceand W (Warburg impedance). The observed Warburg impedancewas a 45◦ straight line, which was recognized in the mid- and low-frequency areas suggesting that the diffusion controlled processoccurred.

As shown in Fig. 5 compared to bare GCE, modified GO on aGCE surface caused the semicircle to rise drastically, implying thatGO performed as an insulating layer that caused the interfacialcharge transfer to be challenging. MWCNT showed behavior akinto GO, but with a difference, in that CNT showed a semicircle withsmaller diameter suggesting better conductivity compared to GO.As it carried noble metal, AgNPs–MWCNT–GO had smaller diametercompared to CNT and GO. As GO hydrothermally reduced to rGO(AgNPs–MWCNT–rGO), the semicircle diameter was smaller anddisplayed higher conductivity compared to other modified elec-trodes. This can be explained by the existence of smaller band gapin rGO, which is ideal for electron conduction and leads to lowerRct [45].

3.3. Electrocatalytic reduction of H2O2 on AgNPs–MWCNT–rGOnanocomposites

All of these composite parts were known to exhibit cat-alytic activity to reduce hydrogen peroxide individually.

The cyclic voltammetric (CVs) of modified GCE with all sil-ver volume ratio of AgNPs–MWCNT–rGO nanocomposites(AgNPs–MWCNT–rGO/GCE) were carried out in 0.2 M phos-phate buffer solution (PBS) at pH 6.5 in the presence of 1 mM

F. Lorestani et al. / Sensors and Actuators B 208 (2015) 389–398 393

F site pv a MW

HAtHr

ig. 3. FESEM images and size distribution diagram of AgNPs–MWCNT–rGO compoolume ratios of 12 (a and b), 6 (c and d), and 3 (e and f) and using the solution with

2O2. As shown in Fig. 6, in comparison to bare GCE, all modified

gNPs–MWCNT–GO/GCE and AgNPs–MWCNT–rGO/GCE elec-

rodes exhibited considerable cathodic peaks for the reduction of2O2, but AgNPs–MWCNT–rGO/GCE electrodes exhibited higher

eduction toward H2O2. Electrodes that were individually modified

repared by using the solution with MWCNT/GO (3:1, v/v) to Ag(NH3)2OH (0.04 M)CNT–GO (3:1, v/v) to AgNO3 (0.04 M) volume ratio of 12 (g and h).

with GO (Fig. 6b), MWCNT (Fig. 6c) and AgNPs (Fig. 6d) did not

show significant reduction effect toward H2O2.

The possible mechanisms of H2O2 reduction by the modifiedGCE electrodes are shown in Scheme 1. The conductivity of rGO andMWCNT facilitate the electron transfer of rGO/GCE (Scheme 1A)

394 F. Lorestani et al. / Sensors and Actuators B 208 (2015) 389–398

F MWC(

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ig. 4. TEM images of MWCNT–GO nanocomposite without Ag (a) and the AgNPs–b).

nd MWCNT/GCE (Scheme 1B) electrodes to reduce the H2O2.gNPs could greatly enhance the electron-transfer reactivity of

he AgNPs–MWCNT–rGO composite thus the reduction of H2O2 isediated by the catalyst effect of AgNPs [46] (Scheme 1D); a com-

arison catalytic effect of these modified electrodes toward H2O2s shown in Fig. 6.

Fig. 7 shows the comparison between the catalytic activitiesf modified AgNPs–MWCNT–rGO/GCE electrodes prepared underimilar conditions using different precursors, i.e. Ag(NH3)2OHnd AgNO3, respectively. The H2O2 reduction ability of modifiedgNPs–MWCNT–rGO/GCE electrodes of the AgNPs/MWCNT/rGO-

(Fig. 7(c)) and AgNPs/MWCNT/rGO-3 (Fig. 7(d)) composites

ere dramatically higher than AgNPs/MWCNT/rGO-1 (Fig. 7(b))

nd AgNPs/MWCNT/rGO-4 (Fig. 7e), which used AgNO3 insteadf Ag(NH3)2OH. The larger available surface area of smaller

ig. 5. Nyquist plots of: (1) bare GCE, (2) GO, (3) CNT, (4) AgNPs–MWCNT–GO and (5) AgNith 0.1 M KCL supporting electrolyte.

NT–rGO-2 with MWCNT/GO (3:1, v/v) to Ag(NH3)2OH (0.04 M) volume ratios of 6

AgNPs on the composite could cause better catalytic activ-ity toward sensing H2O2. In comparison, among the modifiedAgNPs–MWCNT–rGO/GCE electrodes prepared using Ag(NH3)2OH,the AgNPs/MWCNT/rGO-2 composite (Fig. 7c) showed bet-ter reduction activity than the two other volume ratios.One plausible explanation for this phenomenon was that theAgNPs/MWCNT/rGO-2 composite possesses higher silver parti-cle density and lower silver particle size than the other one,which coincidentally agreed with a previous study which observedthat electrocatalytic activity of silver nanoparticles increases withincreasing particle density and decreases with increasing particlesize [47]. The peak reduction for all AgNPs–MWCNT–rGO elec-

trodes increased in current and shifted to positive potential as thesize and density of AgNPs were increased [37,38]. Therefore, the CVprofiles corroborated the observations from the FESEM images.

Ps–MWCNT–rGO and their equivalents circuits in 1 mM Fe(CN)63−/4− (1:1) solution

F. Lorestani et al. / Sensors and Actuators B 208 (2015) 389–398 395

Scheme 1.

FHA

3

r

FHrtM

ig. 6. CVs of various electrodes in 0.2 M PBS (pH 6.5) in the presence of 1.0 mM2O2: bare GCE (a), GO/GCE (b), MWCNT/GCE (c), Ag nanoparticles/GCE (d),gNPs–MWCNT–GO/GCE (e) and AgNPs–MWCNT–rGO/GCE (f).

.4. Optimization of the sensor

The above experimental results show that the electrochemicaleduction of H2O2 can be achieved by using AgNPs–MWCNT–rGO

ig. 7. CVs of various electrodes in 0.2 M PBS (pH 6.5) in the presence of 1.0 mM2O2: bare GCE (a), AgNPs–MWCNT–rGO/GCE prepared by using different volume

atios of MWCNT–GO (3:1, v/v) to Ag(NH3)2OH (0.04 M) of 12, 6, and 3, respec-ively (b–d), and AgNPs–MWCNT–rGO/GCE prepared by using the solution with

WCNT–GO (3:1, v/v) to AgNO3 (0.04 M) volume ratio of 12 (e).

Fig. 8. The effect of applied potential to the current response for 1 mM H2O2 atAgNPs–MWCNT–rGO/GCE electrode in 0.2 M PBS (pH 6.5).

composite modified sensor. The factors which may affect theresponse of the sensor were studied to improve the performance ofthe sensor. Fig. 8 shows the relationship between the applied poten-

tial in chronoamperometry and the reduction current of H2O2. Theaffiliation of the amperometric response on the applied potentialof the AgNPs–MWCNT–rGO/GCE electrode was evaluated between

Fig. 9. The effect of pH of PBS on the current response for 1 mM H2O2 atAgNPs–MWCNT–rGO/GCE electrode.

396 F. Lorestani et al. / Sensors and Actuators B 208 (2015) 389–398

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Fig. 11. Amperometric response of AgNPs–MWCNT–rGO/GCE electrode upon thesuccessive addition of 1 mM H2O2, glucose, glycine, ethanol and ascorbic acid into0.2 M phosphate buffer solution (pH 6.5) with an applied potential 0.35 V under astirring condition.

Table 1The comparison data obtained from this work and literatures regarding the perfor-mance of modified electrode toward the H2O2.

Modify electrode Performance References

LOD (�M) Liner range (mmol−1)

Ag NPs-GN-R/GCE 28 0.1–40 [48]AgNPs/collagen/GCE 0.7 0.005–40.6 [49]Ag NPs–MWCNT/Au 0.5 0.05–17 [50]AgNP/GO/ssDNA/Au 1.9 0.1–20 [51]AgNP/rGO/benzylamine/GCE 31.3 0.1–100 [52]

ig. 10. Steady-state response of the AgNPs–MWCNT–rGO/GCE electrode to consec-tive injection of H2O2 into the stirred 0.2 M PBS (pH 6.5) with an applied potentialf 0.35 V (a) and the corresponding calibration curve (b).

he range of −0.1 to −0.5 V. The current response was graduallyncreased when the applied potential was increased from −0.1o −0.35 V, but after that, the response current decreased. Theeast negative potential that shows a high analyte-dependent cur-ent was a suitable working potential to achieve good selectivity.o, −0.35 V was the best potential for the reduction of H2O2 bygNPs–MWCNT–rGO/GCE modified electrode.

Fig. 9 indicates the effect of the pH value of phos-hate buffer solution on the reduction current of H2O2 ongNPs–MWCNT–rGO/GCE electrode. The current response wasramatically increased with the increasing pH from 4.5 to 6.5 andradually decreased at higher than pH = 6.5. It is recommended thatH = 6.5 was the optimized pH for the electrochemical reduction of2O2.

.5. Amperometric detection of H2O2 at modifiedgNPs–MWCNT–rGO electrode

The typical current-time plot of the AgNPs–MWCNT–rGO withifferent H2O2 concentrations in 0.2 M phosphate buffer solutionf Na2HPO4 and NaH2PO4 at pH 6.5 was studied as shown in Fig. 10t an applied potential of −0.35 V. The current of the working elec-rode increased dramatically up to 95% steady-state current within

s, and demonstrated a fast amperometric response behavior. Theteady-state calibration curve of AgNPs–MWCNT–rGO modifiedlectrode could be prepared from the amperometric response, ashown in Fig. 10. The inset indicates the calibration curve of theensor. The linear steady-state amperometric detection range wasstimated to be from 100 �M to 100 mM with a linear regressionquation of I = 0.833 (mA mM−1) + 2.24 (R2 = 0.9985) of H2O2. Theimit of detection (LOD) and the limit of quantification (LOQ) ofgNPs–MWCNT–rGO coated GC electrode were calculated using

he following equation [36]:

OD = 3SBb

OD = 10SBb

here SB is the standard deviation of the blank solution and b is

he slope of the analytical curve, The limit of detection (LOD) andhe limit of quantification were estimated to be 0.9 �M and 3.2 �Mespectively at a signal-to-noise ratio of 3. Also, the sensor’s stabil-ty was checked by measuring the current response of the modified

ERGO-Ag/GCE 1.6 0.1–100 [31]AgNPs–MWCNT–rGO/GCE 0.9 0.1–100 This work

electrode. After 7 days, only 5.2% of the current signal was lost. Thisillustrates the excellent stability of AgNPs–MWCNT–rGO modifiedelectrode for the purpose of detecting H2O2. The electrode-to-electrode reproducibility was approximated to be in the presenceof 1.0 mM H2O2 in phosphate buffer solution of Na2HPO4 andNaH2PO4 0.2 M (pH 6.5) at three electrodes (AgNPs/MWCNT/rGO)that have been prepared in the same condition, which yielded arelative standard deviation of 4.5%.

3.6. Interference study and real sample analysis

The effect of common interfering species onAgNPs–MWCNT–rGO modified electrode was studied. Fig. 11shows the amperometric response of the modified electrodetoward addition of 1 mM H2O2 and this is followed by glucose,glycine, ethanol and ascorbic acid into 0.2 M phosphate buffersolution (pH 6.5). As can be seen in the figure, the interferingsubstances responded with very weak signals which demonstratethat the modified electrode has a good selectivity toward H2O2.

Table 1 compares the limit of detection and the linear rangeof obtained AgNPs–MWCNT–rGO modified electrode with some ofthe previous voltammetric H2O2 reports [27,31,48–52]. Based onthis comparative analysis, it is found that the AgNPs–MWCNT–rGOmodified electrode is capable of achieving a favorable detectionlimit and linear range to sense H2O2.

4. Conclusion

Silver nanoparticle-carbon nanotube-reduced-graphene-oxidenanocomposite (AgNPs–MWCNT–rGO) was successfully syn-thesized by a simple and environmentally friendly one-step

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ydrothermal method. The advantage of this method is that it didot require any toxic solvent or chemical to reduce the graphitexide. Ag nanoparticles that exist on the composite was synthe-ized from silver ammonia complex instead of silver nitrate wereell-dispersed with small and narrow-sized distributions. ThegNPs–MWCNT–rGO composite exhibited excellent sensitivity forlectrochemical detection of hydrogen peroxide in a cyclic voltam-etry curve. Our present work provides us with a low cost, simple

reparation, one-step, environmentally benign and green syntheticethod of preparing AgNPs–MWCNT–rGO/GCE nanocomposites,hich ultimately works as an effective enzymeless electrochemical2O2 sensor.

cknowledgments

This research is supported by High Impact Research Grant UM-OE UM.C/625/1/HIR/MOE/F00004-21001 from the Ministry of

ducation Malaysia, RP006C-13SUS and PG135-2012B from theniversity of Malaya. The authors thank Mr. Pooya Moslemzadehehrany for valuable discussion.

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iographies

arnaz Lorestani is a Ph.D. candidate at the University of Malaya, after receiv-ng her M.Sc. in Analytical Chemistry, Mashhad, Iran 2011. She also holds a B.Sc.egree in Applied Chemistry from Ferdowsi University, Mashhad, Iran 2007. Her

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research interests include nanomaterials, nanocomposites, chemical sensor andelectrochemical sensors.

Pooria Moozarm Nia is a Ph.D. candidate at the University of Malaya, after receivinghis M.Sc. in Polymer Engineering from Amirkabir University of Technology in 2009.He also holds a B.Sc. degree in polymer engineering from Azad University, Iran.His research interests include corrosion, coatings, chemical and electrochemicalsensors.

Zohreh Shahnavaz is a Ph.D. candidate at the University of Malaya, after receivingher M.Sc. in Electrochemistry from University of Malaya in 2009. She also holds aB.Sc. degree in pure chemistry from University Tarbiat Moallem, Tehran, Iran. Herresearch interests include corrosion, chemical sensor and electrochemical sensors.

Ninie Suhana Abdul Manan is a senior lecturer in Department of Chemistry, Facultyof Science, University of Malaya, Malaysia. She was graduated from Queen’s Univer-sity Belfast, Northern Ireland in 2011 for her Ph.D. Her research interest includeselectrochemistry, ionic liquid, deep eutectic solvents, electrochemical and opticalsensors.

Yatimah Alias is a Professor of Electrochemistry in Department of Chemistry, Fac-

ulty of Science, University of Malaya, Malaysia. She graduated her Ph.D. from EastAnglia University, UK (1998). Her area of expertise is Electrochemistry and Coordina-tion Chemistry and her fields of interests are Electrosynthesis, Polymer Electrolytesand Molecular Assemblies. She is one of the members of the Malaysian AnalyticalSciences Society (ANALISIS) and also the Electrochemical Society.