Journal of Electroanalytical Chemistry 651 (2011) 12–18

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Journal of Electroanalytical Chemistry

journal homepage: www.elsevier .com/locate / je lechem

Iron-tetrasulfophthalocyanine functionalized graphene nanosheets: Attractivehybrid nanomaterials for electrocatalysis and electroanalysis

Nan Li a, Mingfang Zhu a, Meili Qu a, Xia Gao a, Xuwen Li a, Weide Zhang a, Jiaqi Zhang b,⇑, Jianshan Ye a,⇑a College of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, PR Chinab College of Environmental Science and Safety Engineering, Tianjin University of Technology, Tianjin 300071, PR China

a r t i c l e i n f o

Article history:Received 29 July 2010Received in revised form 8 November 2010Accepted 10 November 2010Available online 2 December 2010

Keywords:GrapheneIron-tetrasulfophthalocyanineElectrocatalysisIsoniazidUric acid

1572-6657/$ - see front matter � 2010 Elsevier B.V. Adoi:10.1016/j.jelechem.2010.11.012

⇑ Corresponding authors. Tel.: +86 20 87113241; faE-mail addresses: zhangjqa@sina.com (J. Zhang), js

a b s t r a c t

We report the characteristics and electrochemical sensing features of iron-tetrasulfophthalocyanine(FeTSPc)-functionalized graphene nanosheets (GNs) composites. The noncovalently FeTSPc-functional-ized GNs (GNs–FeTSPc) possess an improved solubility in aqueous solution and the GNs–FeTSPc film elec-trode exhibits an enhanced electrocatalytic activity towards oxidation of isoniazid (INZ) and uric acid(UA). Direct electrochemistry of GNs–FeTSPc nanocomposites shows that GNs could facilitate the electrontransfer between glass carbon electrodes (GCEs) and the electroactive center of FeTSPc. Additionally, acomparative study using different carbon nanomaterials reveals that the functionalized GNs have betterelectrocatalytic ability than that of multi-walled carbon nanotubes (MWCNTs) and graphite while noobvious enhancement is obtained on mesoporous carbons (MPC) and fullerene (C60). Hence, the GNs withunusual structure are suitable for preparing functionalized nanocomposite as a promising electrochem-ical sensing platform.

� 2010 Elsevier B.V. All rights reserved.

1. Introduction

Graphene has recently been attracting considerable attention asa novel monolayer of carbon atoms in a myriad of applications ow-ing to its extraordinary properties [1–8]. However, the insolubilityand irreversible agglomeration of graphene in aqueous solutionwithout dispersing agents is a major obstacle in implementing itswidespread use [9,10]. Such a limitation essentially makes itdifficult to investigate graphene-based electrochemistry and/orelectrochemical sensing applications.

To reduce the tendency of forming insoluble aggregates by largegraphene nanostructures, various materials, such as flexible sidechains [11,12], conjugated-polyelectrolyte [13], aromatic mole-cules [14], pluronic copolymers [15], surfactant [16], dye [9] andso on, have been used to improve the solubility of graphene.Generally, the achievement can be carried out by covalent or non-covalent way. The latter is particularly promising since it enablesattachment of molecules through p–p stacking or hydrophobicinteractions thus, still preserving the intrinsic electronic propertiesof graphene. In addition, electrochemical doping has been widelyexploited to tailor the electronic properties of graphene [14]. How-ever, although the solubility could be improved, the uses of organic

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x: +86 20 87112906.ye@scut.edu.cn (J. Ye).

polymers are poisonous and time consuming. Otherwise, most ofthem could not endow graphene with electroactive properties [9].

Iron-tetrasulfophthalocyanine (FeTSPc), one kind of water-soluble and environmental friendly material, is found to beappropriate as the dispersing agents owing to the excellent ther-mal stability, semiconductivity, well-defined redox activity, andlow cost [17–19]. Moreover, the existence of Fe-xN and Pc activesites has been demonstrated to have good features as analyticalsensors because they can provoke electrocatalysis, increasing thesensitivity and the selectivity of the electrodes [20,21]. FeTSPcand its related complexes have been successfully used as electrodemodifiers in a variety of electrocatalytic studies which include thedetection of oxygen [22,23], nitrite [18,24,25], thiols [26], fuel [27],H2O2 [28], neurotransmitters [29,19], and several pollutants [30].Recently, noncovalently functional graphene nanosheets (GNs)with methylene green was reported to show an enhancement ofelectrocatalytic activity toward the oxidation of NADH [9]. How-ever, so far, there has no report concerning the functionalization,electrochemical characterization, and potential electrochemicalapplications of the graphene-FeTSPc film electrodes.

On the basis of our previous research on reduced grapheneoxide (rGO) [31–33], this paper demonstrates that GNs can befunctionalized with water-soluble FeTSPc by ultrasonic process,as shown in Scheme 1. This method not only provides a facile ap-proach to dispersing GNs in water but also preserves the predom-inant properties of GNs and, most interestingly, the redox propertyas well as the electrocatalytical ability of FeTSPc. The noncovalent

Scheme 1. Proposed schematic diagram of the GNs–FeTSPc nanocomposites.

N. Li et al. / Journal of Electroanalytical Chemistry 651 (2011) 12–18 13

interaction of GNs with FeTSPc through p–p stacking greatlyimproves the solubility of GNs in aqueous solution. What is more,the presence of the GNs enhances the direct electrochemical re-sponse from the FeTSPc center, which is beneficial to improvingthe sensitivity of the resulting electrochemical sensors. The strongsynergy leads to an enhanced electrocatalytic activity towards theoxidation of an important antituberculosis agent, namely isoniazid(INZ) and a common biomolecule, uric acid (UA), indicating a valu-able GNs-based system for electrochemical sensing platform.

2. Experimental

2.1. Materials and reagents

FeTSPc, tetrasulfophthalocyanine (TSPc), C60, and UA were pur-chased from Sigma–Aldrich. N2 (99.5%) was purchased from localgas company. INZ (98%) was received from Guangzhou Qiyun Bio-logical Technological Co. Ltd. (Guangzhou, China). MWCNTs werepurchased from Chengdu Organic Chemicals Co. Ltd. (Chengdu,China). All other chemicals were at least analytical grade and usedwithout further purification. Ultrapurified water (0.07 lS cm�1)was used throughout.

GNs were synthesized by the chemical oxidation–reductiontreatment of graphite [34,35]. Typically, 5 g graphite was addedinto a stirred mixture of H2SO4 (87.5 mL) and HNO3 (45 mL) inan ice-water bath. Then, KClO3 (55 g) was added slowly into themixture and kept stirring for 96 h at room temperature to obtaingraphite oxide. After dried at 80 �C, graphite oxide was exfoliatedin de-ionized water by ultrasonic treatment for 2 h. GNs were ob-tained by reacting with hydrazine monohydrate (1 lL: 3 mg graph-ene oxide) for 24 h at 80 �C finally.

2.2. Apparatus and measurements

Scanning electron microscope (SEM) characterization ofgraphene was performed with a LEO 1530 VP (LEO, Germany) at15 kV. Raman spectra were obtained on a LabRAM Aramis (HJY,France) with the excitation wavelength of 632.8 nm. X-rayphotoelectron spectroscopic (XPS) measurements were preformedwith a Kratos AXis Ultra (DLD). UV–vis absorption spectra wererecorded with a Hitachi 3010 spectrometer (Japan). Eletrochemicalimpedance spectroscopic (EIS) measurements were carried out ona PGSTAT100/FRA2 system (Autolab, Metrohm China Ltd.) in 0.1 MKCl solution containing K3[Fe(CN)6]/K4[Fe(CN)6] (both 5 mM). Pho-tographs were taken with a Canon IXUS105 digital camera. Otherelectrochemical experiments were conducted in a conventionalthree-electrode system using a LK6200 Electrochemical Worksta-tion (BioNano International Singapore Pte. Ltd.). A modified GCE

was used as working electrode. A platinum wire and an Ag/AgCl(3.0 M KCl) electrode were used as counter electrode and referenceelectrode, respectively. Phosphate buffered saline (PBS) (0.1 M, pH7.4) was employed as the supporting electrolyte, which was deaer-ated with N2 gas for 20 min. All experiments were performed atroom temperature (ca. 25 �C).

2.3. Preparation of modified GCEs

The GCEs (3 mm in diameter) were polished with 0.3 and0.05 lm alumina slurries and then ultrasonically cleaned in doubledistilled water. The GNs–FeTSPc nanocomposite used for theexperiment was prepared by a mixture consisting of 5 mg GNsand 1 mg FeTSPc in 5 mL distilled water under ultrasonication for8 h at room temperature. The resulting suspension was filteredwith a Millipore porous filter (0.45 lm, Millipore). The obtainedsample was first thoroughly rinsed with distilled water to removethe non-adsorbed FeTSPc and then dried at 60 �C overnight. Thesame procedure was used to prepare GNs–TSPc, MWCNTs-FeTSPc,MPC-FeTSPc, Graphite-FeTSPc and C60–FeTSPc samples. The nano-composites above were dispersed in distilled water to give a homo-geneous suspension (2 mg mL�1) under ultrasonication. GNs weredispersed into DMF under ultrasonic to obtain a black suspension(2 mg mL�1). FeTSPc aqueous solution was 0.4 mg mL�1. In orderto prepare the modified GCEs, 10 lL of the prepared solutionsabove were dropped onto GCEs respectively to obtain the modifiedelectrodes and then evaporating the solvent under room tempera-ture in air.

3. Results and discussion

3.1. Characteristics of GNs–FeTSPc

The morphologies of the pristine GNs and GNs–FeTSPc areobserved by SEM images (Fig. 1). As shown in Fig. 1A, there arelarge flakes of GNs with slightly scrolled edges. Some GNs flakesfold together, due to the partial aggregation of the GNs. Whilethe nanohybrids show crinkly sheets. The surface is much rougherthan that of GNs (Fig. 1B), which can be attributed to the absorp-tion of FeTSPc on GNs.

Raman spectroscopy is a powerful tool for investigating thestructural changes that occur in carbon materials [36]. As shownin Fig. 2a, the D band around 1288 cm�1 corresponding to sp2 do-mains isolated by oxidized carbon atoms, and G band around1536 cm�1 is attributed to first order scattering of the E2g mode,a characteristic band of crystalline graphite [36]. In comparisonwith FeTSPc, characteristic vibrational peaks of FeTSPc are also evi-dent in the GNs–FeTSPc nanocomposite spectrum (Fig. 2c), thusindicating that FeTSPc has indeed bound to the GNs. Besides, thefeatures of GNs are not disappeared in the GNs–FeTSPc, thus indi-cating that the functionalization of GNs with FeTSPc does not de-stroy the structure of the GNs. Moreover, the intensity ratio ofthe D band to the G band increased from 2.01 to 2.56 after modi-fied with FeTSPc. According to Choi’s report [37], the intensity ratio(ID/IG) is a measure of the amount of disorder present within in thematerial. The increase in the intensity of the D band suggests a de-crease in the average size of the sp2 domains, which is caused bythe increased number of smaller graphitic domains formed duringreduction [34]. The existence of FeTSPc increases the dispersion ofGNs, which is similar to the report from Liu [9].

X-ray photoelectron spectroscopy (XPS) is employed to analyzethe samples of GNs and FeTSPc-modified GNs sheet. Fig. 3 showsthat most of the O groups are successfully removed and thus theO/C ratio in the GNs decreases remarkably. After the reductionby hydrazine the percentage of oxygen decreases to ca. 8.52%,

Fig. 1. Scanning electron micrographs of (A) GNs and (B) GNs–FeTSPc.

Fig. 2. Raman spectra of (a) GNs, (b) FeTSPc and (c) GNs–FeTSPc.

Fig. 3. X-ray photoelectron spectra of (a) GNs and (b) GNs–FeTSPc.

Fig. 4. UV–vis spectra of (a) FeTSPc, (b) GNs, and (c) GNs–FeTSPc. Inset: digitalphotograph showing the aqueous dispersion of FeTSPc (left), GNs (middle) andGNs–FeTSPc (right).

14 N. Li et al. / Journal of Electroanalytical Chemistry 651 (2011) 12–18

which is close to the reported results [38,39]. Moreover, the inter-action between FeTSPc and GNs is revealed that the nanohybridscontain Fe, S and N, indicating that FeTSPc indeed adsorbed onGNs.

The interaction between GNs and FeTSPc can be further charac-terized by UV–vis adsorption spectra. The FeTSPc aqueous solutionpresents an intense absorbance Q-band at 635 nm (Fig. 4a), charac-teristic of the dimeric species [17]. The GNs do not show obviousabsorption (Fig. 4b) from 800 to 450 nm. In the existence of GNs,the GNs–FeTSPc nanocomposites show a decrease of intensity witha red shift from 637 to 650 nm (Fig. 4c), demonstrating theformation of conjugated system and successful adsorption ofFeTSPc onto GNs. Unlike the aggregated pristine GNs (inset, mid-dle), the GNs–FeTSPc nanocomposites greatly enhance dispersityin water and can be stable in the dark for weeks without precipita-tion (inset, right). The excellent dispersion property also indicatesthe interaction between GNs and FeTSPc and makes it availableeither to print, brush or spray-coat the catalyst onto the electrode.

The electron transfer kinetics of a redox probe at the GNs–FeTSPc/GCE is investigated with EIS (Fig. 5). At a bare GCE, theredox process of the probe, FeðCNÞ3�=4�

6 , shows an electron-transferresistance of 320 X (Fig. 5a). After FeTSPc is coated on the elec-trode, the resistance increases dramatically to 1050 X (Fig. 5b),

Fig. 5. Electrochemical spectra of (a) bare GCE and GCEs modified with (b) FeTSPc,(c) GNs and (d) GNs–FeTSPc in 0.1 M KCl solution containing K3[Fe(CN)6]/K4[Fe(CN)6] (both 5 mM).

Fig. 6. Cyclic voltammograms of (A) a bare GCE (a) and GCEs modified with (b)FeTSPc, (c) GNs–TSPc, (d) GNs and (e) GNs–FeTSPc, in N2-saturated PBS (0.1 M, pH7.4) at 100 mV s�1. (B) GNs–FeTSPc modified GCE at different scan rates about 30,80, 100, 200, 300, 400 and 500 mV s�1 (from inner to outer) in 0.1 mol L�1 N2-saturated PBS (pH 7.4). Inset: plots of oxidation and reduction peak current (ip) vs.scan rate (v).

N. Li et al. / Journal of Electroanalytical Chemistry 651 (2011) 12–18 15

suggesting that FeTSPc film blocks the electron exchange betweenthe redox probe and electrode surface. However, the resistancedecreases obviously to 12 X at the GNs coated GCE (Fig. 5c), imply-ing that graphene is excellent electric conducting material andaccelerates electron transfer. Compared to FeTSPc/GCE and bareGCE, the electron-transfer resistance of 160 X at the GNs–FeTSPcmodified GCE is the lowest, indicating that the presence of GNsmake the electron transfer easier (Fig. 5d).

3.2. Electrochemical behavior of GNs–FeTSPc modified GCE

The cyclic voltammograms of bare and FeTSPc modified GCEs inN2-saturated PBS do not show any observable peak (Fig. 6A, curvesa and b). Both GNs–TSPc/GCE and GNs/GCE show a couple of smallredox peaks that resulted from oxycarbide species on the GNssurface (Fig. 6A, curves c and d), whereas GCE modified withGNs–FeTSPc shows another pair of redox peaks (Fig. 6A, curve e),which could be attributed to the FeIII/FeII redox couple in FeTSPc,although they could not be observed at the FeTSPc-coated GCEdue to the high electron-transfer resistance and water-solubility.The result further confirms the synergic effect of GNs and FeTSPc.

Another important note is that GNs modified GCE shows noobvious enhancement in background current (Fig. 6c), which mayattributed to the aggregation of GNs. Contrarily it can be seenfrom Fig. 6e that there is an enhanced background current at theGNs–FeTSPc/GCE, which is caused by the improved solubility andwell dispersion of the nanocomposites [40].

Furthermore, the GNs–FeTSPc modified GCE shows a couple ofstable, symmetrical, and well-defined redox peaks at 0.279 and0.231 V with a anodic and cathodic peak currents of 10.94 lAand 11.07 lA, respectively. The separation of peak potentials (DEp)is 48 mV, indicating a faster electron transfer rate. It is clear to seethat the GNs promote the electron transfer between FeTSPc andGCE.

According to Fig. 6B, another well-shaped pair of redox peaks at�0.5 V are attributed to the electrochemical conversion of the Pcunit [Fe(II)TSPc4�/Fe(II)TSPc6�]. To understand the heterogeneouselectron transfer of FeIII/FeII better, the reduction and oxidationpeak currents of the GNs–FeTSPc modified GCE increase linearlywith the increasing of scan rates up to 500 mV s�1, indicatinga surface-controlled electrode process. (Inset, Fig. 6; linearregression equations: Ipa = 2.25771 + 0.11807v, R = 0.999; Ipc =�2.44399 � 0.11860v, R = 0.998). Otherwise, in the scan ratesranging from 10 to 1000 mV s�1, the liner regression equations of

the Epa and Epc vs. the logarithm of the scan rates are expressedas Epa = 0.3095 + 0.1930 log v and Epc = 0.2023 � 0.1852 log v withR = 0.996 and 0.992, respectively. Based on the slopes of the lines2.303RT/(1 � a) nF and �2.303(RT/anF), the value of a is calculatedas 0.51. The electron transfer rate constant can be obtained basedon Laviron theory [41], which is ks = 6.35 s�1 for GNs–FeTSPc. Thevalue is higher than that reported previously for a GCE electrodemodified with alternated layers of FeTsPc and iron(III) tetra-(N-methyl-pyridyl)-porphyrin (FeT4MPyP) (3.8 ± 0.1 s�1) [42]. It isindicated that the electron transfer ability of FeTSPc has beenimproved due to the existence of GNs. The surface coverage(C/mol cm�2) can be calculated by using the equation C = Q/nFA,where Q is the charge involved in the reaction, F is the Faraday con-stant, n is the number of moles of electrons transferred and A is theexperimentally determined area of the electrode. The amount ofFeTSPc immobilized on GNs is 2.49 � 10�9 mol cm�2 (1.4 � 1015

particles cm�2 �7.14 Å2 per particle), which is much larger thanthe first layer coverage of iron-phthalocyanine on single-walledcarbon nanotubes (1.2 � 10�10 mol cm�2) [43] and that of1.22 � 10�10 mol cm�2 for iron-tetraaminophthalocyanine-single-walled carbon nanotubes [30]. The estimated values indicate mul-tilayer coverage rather than a monolayer coverage expected to bein the order of 10�10 mol cm�2 for MPc molecules [44]. Thus, thehigh surface coverage of FeTSPc on GNs can be a clear indication

16 N. Li et al. / Journal of Electroanalytical Chemistry 651 (2011) 12–18

that the high special surface area (2600 m2 g�1) of GNs offers astraightforward way to increase the number of active sites [45].

Fig. 7 shows the CVs of GNs–FeTSPc under different pH values.With an increase of pH from 4 to 12, the redox potentials of theGNs–FeTSPc modified GCE shift to more negative values and exhi-bit a linear variations with slopes of �67.8 mV pH�1 for the oxida-tion processes. According to Laviron equation [41], the na isestimated to be 0.91, indicating that one electron and one protontransfer is involved in the electrode reaction. Besides, the stabilitymeasurements (50-cycles cyclic voltammograms at 100 mV s�1)are also carried out (not shown). GNs–FeTSPc film shows high sta-bility when submitted to several cycles and there is no obviouschange in the peak current, further exhibiting its potential valuefor sensing application.

3.3. Electrocatalytic oxidation of isoniazid (INZ)

As shown in Fig. 8, when 0.5 mM INZ is added to N2-saturatedPBS (pH 7.4), the electrochemical oxidation at a bare electrode pro-ceeds at a high overpotential (0.81 V) because of slow electrontransfer kinetics and electrode fouling (Fig. 8a) [9]. Although thepeak potential shifts negatively to 0.7 V after using redox-activeFeTSPc, neither the peak current nor background is changed, owing

Fig. 7. Cyclic voltammograms in various pH solutions. Inset: anodic potential vs. pHfrom 4 to 12.

Fig. 8. Cyclic voltammograms of bare GCE (a) and GCEs modified with (b) FeTSPc,(c) GNs, (d) GNs–TSPc and (e) GNs–FeTSPc in N2-saturated PBS (0.1 M, pH 7.4) with(solid line) and without (dotted line) 0.5 mM INZ at 100 mV s�1.

to the difficulty to confine these water-soluble molecules on theelectrode (Fig. 8b). GNs modified GCE shows two peaks with thefirst peak potential at about 0.3 V but no obvious enhancementin peak current (Fig. 8c), which may attributed to the aggregationof GNs. Another small peak was observed in both GNs andGNs–FeTSPc modified GCEs, which may contribute to the two stepreactions during the electrooxidation of INZ [46]. However,GNs–FeTSPc/GCE shows an enhanced background current andanodic peak with the lowest potential (0.294 V) and highestcurrent (63.49 lA) than those at the other modified GCEsabove(Fig. 8e), which indicates that the redox reaction can serveas an electron transfer mediator to facilitate the oxidation of INZwith reduced overpotential and enhanced current response. Thusthe electroactive GNs–FeTSPc nanocomposite possesses an excel-lent synergic effect of GNs and FeTSPc.

Also important to note is that the adsorption of FeTSPc onto GNsactually increases the electrocatalytic activity of FeTSPc toward theoxidation of INZ. The presence of GNs accelerates the electrontransfer and increases the amount of FeTSPc on electrode surface,which acts as a catalyst to further reduce the oxidation potential.Hence, the formation of GNs–FeTSPc nanocomposite not onlyavoids GNs aggregation, but also presents a facile approach to con-finement of FeTSPc on electrode surface. This behavior provides anadvantage for preparation of INZ sensor.

To further confirm the active sites in the oxidation of INZ,GNs–TSPc/GCE is also tested for comparison (Fig. 8d). As antici-pated, without FeIII/FeII center, the peak current is only 23.4 lAat 0.4 V, but higher than that at GNs/GCE, indicating a smallenhancement in electrocatalysis of INZ. Thus both the TSPc unitand FeIII/FeII may be the two active sites in this hybrid catalyst.While the latter part plays the more crucial role in INZ electrooxi-dation than the former part.

3.4. Electrocatalytic oxidation of uric acid (UA)

The oxidation of UA at bare GCE shows a small oxidative peak of10.64 lA at 0.41 V (Fig. 9a). After absorbing either FeTSPc or GNson the GCE, the peak currents enhance to 20.82 lA and 14.79 lAwith potentials at 0.39 V and 0.34 V, respectively (Fig. 9b and c).Obviously GNs decrease the overpotential of UA oxidation. Com-paring with FeTSPc/GCE, the electrocatalysis of GNs–FeTSPc to-ward UA oxidation is further enhanced. It can be emphasizedthat the use of GNs–FeTSPc modified GCE allowed a large enhance-

Fig. 9. Cyclic voltammograms of (a) bare GCE, (b) FeTSPc/GCE, (c) GNs/GCE, (d)GNs–TSPc/GCE and (e) GNs–FeTSPc/GCE in N2-saturated PBS (0.1 M, pH 7.4)without (dotted line) or with (solid line) 0.5 mM UA at the scan rate of 100 mV s�1.

Table 1Comparative studies on electrocatalytic activity of different carbon materials withand without FeTSPc toward the oxidation of INZ and UA.

Modified GCE Epa (V) Ipa (lA)

Carbon material FeTSPc INZ UA INZ UA

GNs With 0.294 0.342 63.49 52.18Without 0.336 0.340 14.70 14.90

MWCNTs With 0.377 0.381 73.11 61.70Without 0.303 0.385 48.78 42.67

MPC With 0.274 0.392 34.10 49.00Without 0.283 0.364 62.10 41.67

Graphite With 0.556 0.366 27.31 23.17Without 0.620 0.495 19.01 10.32

C60 With – – – –Without 0.790 0.340 15.68 13.90

N. Li et al. / Journal of Electroanalytical Chemistry 651 (2011) 12–18 17

ment current density about 50 lA. In addition, the oxidation of UAon GNs–FeTSPc/GCE occurs at lower potential (0.341 V) valuesthan that on bare GCE and FeTSPc modified GCE. This again clearlyindicates that the combination of GNs and FeTSPc allows twiceenhancing the electrocatalytic performances for UA oxidation interms of current intensity. These results again indicate a synergybetween GNs and FeTSPc in electrocatalytic oxidation of UA. TheGNs–FeTSPc nanocomposites may provide anther valuable plat-form for UA determination.

Moreover, GNs–TSPc/GCE also slightly enhances the anodicpeak current of UA (about 28.9 lA at 0.351 V) compared with thatof bare GCE, FeTSPc/GCE and GR/GCE. Both FeIII/FeII and TSPc unitscan affect the oxidation of UA, as what has shown in the oxidationof INZ.

3.5. Comparison between GNs and other carbon materials

The intriguing synergy effect of GNs–FeTSPc has been demon-strated above. However, whether FeTSPc with other carbon mate-rials could also possess the synergy effect hasn’t been investigatedyet. In order to make this question clear, a series of FeTSPc nano-composites: C60–FeTSPc, MWCNTs–FeTSPc, MPC–FeTSPc, andgraphite–FeTSPc are also prepared in the same process and theirelectroactivities to INZ and UA are obtained (as Table 1 shown).The MWCNTs–FeTSPc and graphite/FeTSPc modified GCEs showstable, well-defined and quasi-reversible redox peaks, indicatingthat FeTSPc can still interact with MWCNTs and graphite. The sim-ilar results for the electrooxidation of INZ and UA are also observedon both MWCNTs–FeTSPc and MPC–FeTSPc modified GCEs. Thismay due to the similar active sites in graphene, MWCNTs andMPC, such as edge-plane or edge-plane-like defects on the surfaceof those carbon materials [47]. Graphene is the basic building blockof other important allotropes. It can be stacked to form 3Dgraphite, rolled to form 1D nanotubes, and wrapped to form 0Dfullerenes [48]. However, no obvious peak is obtained from MPC–FeTSPc/GCE and C60–FeTSPc/GCE, which may be attributed to thespecial pore structure of MPC and spherical structure of C60 [49].In addition, C60 film on the electrode may act as inert particles,resulting in a partially blocked electrode surface, which appearsto slow down the rate of electrode transfer [47]. Furthermore, dur-ing the electrooxidation of UA and INZ, GNs–FeTSPc has more neg-atively peak potential compared with MWCNTs–FeTSPc, which canprovide a novel and facile way for GNs application in catalyst andsensors.

4. Conclusion

The results presented here demonstrate the use of grapheneand FeTSPc to form a novel nanocomposite electrode for the

development of electrochemical sensing platform. The strongadsorption of FeTSPc onto GNs not only enhances the solubilityof GNs into aqueous media, but also preserves the predominantproperties of GNs and the redox property of FeTSPc. The excellentelectrocatalytic ability of GNs–FeTSPc lowers the overpotential,and greatly enhances the current response for the oxidation ofINZ and UA. Direct electrochemistry of GNs–FeTSPc nanocompos-ites shows that GNs could facilitate the electron transfer betweensubstrate electrodes and the electroactive center of FeTSPc. Addi-tionally, a comparative study using different carbon nanomaterialsreveals that the functionalized GNs have better electrocatalyticability than that of carbon nanotubes and graphite while no obvi-ous enhancement is obtained on MPC and C60. Hence, the GNs withunusual structure are suitable for preparing functionalized nano-composites and for potential electrochemical sensor applications.

Acknowledgements

The authors gratefully acknowledge the financial support of the863 Program (2008AA06Z311), NSFC (20945004), and ScientificResearch Foundation for Returned Scholars, Ministry of Educationof China.

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