Iron tetrasulfophthalocyanine functionalized graphene nanosheets: attractivehybrid nanomaterials for electrocatalysis and electroanalysis

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  • 1. Journal of Electroanalytical Chemistry 651 (2011) 12–18 Contents lists available at ScienceDirect Journal of Electroanalytical Chemistry journal homepage: functionalized graphene nanosheets: Attractivehybrid nanomaterials for electrocatalysis and electroanalysisNan 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 Chinaa r t i c l e i n f o a b s t r a c tArticle history: We report the characteristics and electrochemical sensing features of iron-tetrasulfophthalocyanineReceived 29 July 2010 (FeTSPc)-functionalized graphene nanosheets (GNs) composites. The noncovalently FeTSPc-functional-Received in revised form 8 November 2010 ized GNs (GNs–FeTSPc) possess an improved solubility in aqueous solution and the GNs–FeTSPc film elec-Accepted 10 November 2010 trode exhibits an enhanced electrocatalytic activity towards oxidation of isoniazid (INZ) and uric acidAvailable online 2 December 2010 (UA). Direct electrochemistry of GNs–FeTSPc nanocomposites shows that GNs could facilitate the electron transfer between glass carbon electrodes (GCEs) and the electroactive center of FeTSPc. Additionally, aKeywords: comparative study using different carbon nanomaterials reveals that the functionalized GNs have betterGrapheneIron-tetrasulfophthalocyanine electrocatalytic ability than that of multi-walled carbon nanotubes (MWCNTs) and graphite while noElectrocatalysis obvious enhancement is obtained on mesoporous carbons (MPC) and fullerene (C60). Hence, the GNs withIsoniazid unusual structure are suitable for preparing functionalized nanocomposite as a promising electrochem-Uric acid ical sensing platform. Ó 2010 Elsevier B.V. All rights reserved.1. Introduction polymers are poisonous and time consuming. Otherwise, most of them could not endow graphene with electroactive properties [9]. Graphene has recently been attracting considerable attention as Iron-tetrasulfophthalocyanine (FeTSPc), one kind of water-a novel monolayer of carbon atoms in a myriad of applications ow- soluble and environmental friendly material, is found to being to its extraordinary properties [1–8]. However, the insolubility appropriate as the dispersing agents owing to the excellent ther-and irreversible agglomeration of graphene in aqueous solution mal stability, semiconductivity, well-defined redox activity, andwithout dispersing agents is a major obstacle in implementing its low cost [17–19]. Moreover, the existence of Fe-xN and Pc activewidespread use [9,10]. Such a limitation essentially makes it sites has been demonstrated to have good features as analyticaldifficult to investigate graphene-based electrochemistry and/or sensors because they can provoke electrocatalysis, increasing theelectrochemical sensing applications. sensitivity and the selectivity of the electrodes [20,21]. FeTSPc To reduce the tendency of forming insoluble aggregates by large and its related complexes have been successfully used as electrodegraphene nanostructures, various materials, such as flexible side modifiers in a variety of electrocatalytic studies which include thechains [11,12], conjugated-polyelectrolyte [13], aromatic mole- detection of oxygen [22,23], nitrite [18,24,25], thiols [26], fuel [27],cules [14], pluronic copolymers [15], surfactant [16], dye [9] and H2O2 [28], neurotransmitters [29,19], and several pollutants [30].so on, have been used to improve the solubility of graphene. Recently, noncovalently functional graphene nanosheets (GNs)Generally, the achievement can be carried out by covalent or non- with methylene green was reported to show an enhancement ofcovalent way. The latter is particularly promising since it enables electrocatalytic activity toward the oxidation of NADH [9]. How-attachment of molecules through p–p stacking or hydrophobic ever, so far, there has no report concerning the functionalization,interactions thus, still preserving the intrinsic electronic properties electrochemical characterization, and potential electrochemicalof graphene. In addition, electrochemical doping has been widely applications of the graphene-FeTSPc film electrodes.exploited to tailor the electronic properties of graphene [14]. How- On the basis of our previous research on reduced grapheneever, although the solubility could be improved, the uses of organic oxide (rGO) [31–33], this paper demonstrates that GNs can be functionalized 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- ⇑ Corresponding authors. Tel.: +86 20 87113241; fax: +86 20 87112906. inant properties of GNs and, most interestingly, the redox property E-mail addresses: (J. Zhang), (J. Ye). as well as the electrocatalytical ability of FeTSPc. The noncovalent1572-6657/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.doi:10.1016/j.jelechem.2010.11.012
  • 2. N. Li et al. / Journal of Electroanalytical Chemistry 651 (2011) 12–18 13 was used as working electrode. A platinum wire and an Ag/AgCl (3.0 M KCl) electrode were used as counter electrode and reference electrode, respectively. Phosphate buffered saline (PBS) (0.1 M, pH 7.4) was employed as the supporting electrolyte, which was deaer- ated with N2 gas for 20 min. All experiments were performed at room temperature (ca. 25 °C). 2.3. Preparation of modified GCEs The GCEs (3 mm in diameter) were polished with 0.3 and 0.05 lm alumina slurries and then ultrasonically cleaned in double distilled water. The GNs–FeTSPc nanocomposite used for the experiment was prepared by a mixture consisting of 5 mg GNs and 1 mg FeTSPc in 5 mL distilled water under ultrasonication for Scheme 1. Proposed schematic diagram of the GNs–FeTSPc nanocomposites. 8 h at room temperature. The resulting suspension was filtered with a Millipore porous filter (0.45 lm, Millipore). The obtained sample was first thoroughly rinsed with distilled water to removeinteraction of GNs with FeTSPc through p–p stacking greatly the non-adsorbed FeTSPc and then dried at 60 °C overnight. Theimproves the solubility of GNs in aqueous solution. What is more, same procedure was used to prepare GNs–TSPc, MWCNTs-FeTSPc,the presence of the GNs enhances the direct electrochemical re- MPC-FeTSPc, Graphite-FeTSPc and C60–FeTSPc samples. The nano-sponse from the FeTSPc center, which is beneficial to improving composites above were dispersed in distilled water to give a homo-the sensitivity of the resulting electrochemical sensors. The strong geneous suspension (2 mg mLÀ1) under ultrasonication. GNs weresynergy leads to an enhanced electrocatalytic activity towards the dispersed into DMF under ultrasonic to obtain a black suspensionoxidation of an important antituberculosis agent, namely isoniazid (2 mg mLÀ1). FeTSPc aqueous solution was 0.4 mg mLÀ1. In order(INZ) and a common biomolecule, uric acid (UA), indicating a valu- to prepare the modified GCEs, 10 lL of the prepared solutionsable GNs-based system for electrochemical sensing platform. above were dropped onto GCEs respectively to obtain the modified electrodes and then evaporating the solvent under room tempera-2. Experimental ture in air.2.1. Materials and reagents 3. Results and discussion FeTSPc, tetrasulfophthalocyanine (TSPc), C60, and UA were pur- 3.1. Characteristics of GNs–FeTSPcchased from Sigma–Aldrich. N2 (99.5%) was purchased from localgas company. INZ (98%) was received from Guangzhou Qiyun Bio- The morphologies of the pristine GNs and GNs–FeTSPc arelogical Technological Co. Ltd. (Guangzhou, China). MWCNTs were observed by SEM images (Fig. 1). As shown in Fig. 1A, there arepurchased from Chengdu Organic Chemicals Co. Ltd. (Chengdu, large flakes of GNs with slightly scrolled edges. Some GNs flakesChina). All other chemicals were at least analytical grade and used fold together, due to the partial aggregation of the GNs. Whilewithout further purification. Ultrapurified water (0.07 lS cmÀ1) the nanohybrids show crinkly sheets. The surface is much rougherwas used throughout. than that of GNs (Fig. 1B), which can be attributed to the absorp- GNs were synthesized by the chemical oxidation–reduction tion of FeTSPc on GNs.treatment of graphite [34,35]. Typically, 5 g graphite was added Raman spectroscopy is a powerful tool for investigating theinto a stirred mixture of H2SO4 (87.5 mL) and HNO3 (45 mL) in structural changes that occur in carbon materials [36]. As shownan ice-water bath. Then, KClO3 (55 g) was added slowly into the in Fig. 2a, the D band around 1288 cmÀ1 corresponding to sp2 do-mixture and kept stirring for 96 h at room temperature to obtain mains isolated by oxidized carbon atoms, and G band aroundgraphite oxide. After dried at 80 °C, graphite oxide was exfoliated 1536 cmÀ1 is attributed to first order scattering of the E2g mode,in de-ionized water by ultrasonic treatment for 2 h. GNs were ob- a characteristic band of crystalline graphite [36]. In comparisontained by reacting with hydrazine monohydrate (1 lL: 3 mg graph- with FeTSPc, characteristic vibrational peaks of FeTSPc are also evi-ene oxide) for 24 h at 80 °C finally. dent in the GNs–FeTSPc nanocomposite spectrum (Fig. 2c), thus indicating that FeTSPc has indeed bound to the GNs. Besides, the2.2. Apparatus and measurements features of GNs are not disappeared in the GNs–FeTSPc, thus indi- cating that the functionalization of GNs with FeTSPc does not de- Scanning electron microscope (SEM) characterization of stroy the structure of the GNs. Moreover, the intensity ratio ofgraphene was performed with a LEO 1530 VP (LEO, Germany) at the D band to the G band increased from 2.01 to 2.56 after modi-15 kV. Raman spectra were obtained on a LabRAM Aramis (HJY, fied with FeTSPc. According to Choi’s report [37], the intensity ratioFrance) with the excitation wavelength of 632.8 nm. X-ray (ID/IG) is a measure of the amount of disorder present within in thephotoelectron spectroscopic (XPS) measurements were preformed material. The increase in the intensity of the D band suggests a de-with a Kratos AXis Ultra (DLD). UV–vis absorption spectra were crease in the average size of the sp2 domains, which is caused byrecorded with a Hitachi 3010 spectrometer (Japan). Eletrochemical the increased number of smaller graphitic domains formed duringimpedance spectroscopic (EIS) measurements were carried out on reduction [34]. The existence of FeTSPc increases the dispersion ofa PGSTAT100/FRA2 system (Autolab, Metrohm China Ltd.) in 0.1 M GNs, which is similar to the report from Liu [9].KCl solution containing K3[Fe(CN)6]/K4[Fe(CN)6] (both 5 mM). Pho- X-ray photoelectron spectroscopy (XPS) is employed to analyzetographs were taken with a Canon IXUS105 digital camera. Other the samples of GNs and FeTSPc-modified GNs sheet. Fig. 3 showselectrochemical experiments were conducted in a conventional that most of the O groups are successfully removed and thus thethree-electrode system using a LK6200 Electrochemical Worksta- O/C ratio in the GNs decreases remarkably. After the reductiontion (BioNano International Singapore Pte. Ltd.). A modified GCE by hydrazine the percentage of oxygen decreases to ca. 8.52%,
  • 3. 14 N. Li et al. / Journal of Electroanalytical Chemistry 651 (2011) 12–18 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: digital photograph showing the aqueous dispersion of FeTSPc (left), GNs (middle) and GNs–FeTSPc (right). Fig. 1. Scanning electron micrographs of (A) GNs and (B) GNs–FeTSPc. which is close to the reported results [38,39]. Moreover, the inter- action between FeTSPc and GNs is revealed that the nanohybrids contain Fe, S and N, indicating that FeTSPc indeed adsorbed on GNs. The interaction between GNs and FeTSPc can be further charac- terized by UV–vis adsorption spectra. The FeTSPc aqueous solution presents an intense absorbance Q-band at 635 nm (Fig. 4a), charac- teristic of the dimeric species [17]. The GNs do not show obvious absorption (Fig. 4b) from 800 to 450 nm. In the existence of GNs, the GNs–FeTSPc nanocomposites show a decrease of intensity with a red shift from 637 to 650 nm (Fig. 4c), demonstrating the formation of conjugated system and successful adsorption of FeTSPc onto GNs. Unlike the aggregated pristine GNs (inset, mid- dle), the GNs–FeTSPc nanocomposites greatly enhance dispersity in water and can be stable in the dark for weeks without precipita- tion (inset, right). The excellent dispersion property also indicates the interaction between GNs and FeTSPc and makes it available either 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, the redox process of the probe, FeðCNÞ3À=4À , shows an electron-transfer 6 resistance of 320 X (Fig. 5a). After FeTSPc is coated on the elec- Fig. 2. Raman spectra of (a) GNs, (b) FeTSPc and (c) GNs–FeTSPc. trode, the resistance increases dramatically to 1050 X (Fig. 5b),
  • 4. N. Li et al. / Journal of Electroanalytical Chemistry 651 (2011) 12–18 15Fig. 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).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, curves Fig. 6. Cyclic voltammograms of (A) a bare GCE (a) and GCEs modified with (b)a and b). Both GNs–TSPc/GCE and GNs/GCE show a couple of small FeTSPc, (c) GNs–TSPc, (d) GNs and (e) GNs–FeTSPc, in N2-saturated PBS (0.1 M, pHredox peaks that resulted from oxycarbide species on the GNs 7.4) at 100 mV sÀ1. (B) GNs–FeTSPc modified GCE at different scan rates about 30,surface (Fig. 6A, curves c and d), whereas GCE modified with 80, 100, 200, 300, 400 and 500 mV sÀ1 (from inner to outer) in 0.1 mol LÀ1 N2-GNs–FeTSPc shows another pair of redox peaks (Fig. 6A, curve e), saturated PBS (pH 7.4). Inset: plots of oxidation and reduction peak current (ip) vs. scan rate (v).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. the Epa and Epc vs. the logarithm of the scan rates are expressed Another important note is that GNs modified GCE shows no as Epa = 0.3095 + 0.1930 log v and Epc = 0.2023 À 0.1852 log v withobvious enhancement in background current (Fig. 6c), which may R = 0.996 and 0.992, respectively. Based on the slopes of the linesattributed to the aggregation of GNs. Contrarily it can be seen 2.303RT/(1 À a) nF and À2.303(RT/anF), the value of a is calculatedfrom Fig. 6e that there is an enhanced background current at the as 0.51. The electron transfer rate constant can be obtained basedGNs–FeTSPc/GCE, which is caused by the improved solubility and on Laviron theory [41], which is ks = 6.35 sÀ1 for GNs–FeTSPc. Thewell dispersion of the nanocomposites [40]. value is higher than that reported previously for a GCE electrode Furthermore, the GNs–FeTSPc modified GCE shows a couple of modified with alternated layers of FeTsPc and iron(III) tetra-(N-stable, symmetrical, and well-defined redox peaks at 0.279 and methyl-pyridyl)-porphyrin (FeT4MPyP) (3.8 ± 0.1 sÀ1) [42]. It is0.231 V with a anodic and cathodic peak currents of 10.94 lA indicated that the electron transfer ability of FeTSPc has beenand 11.07 lA, respectively. The separation of peak potentials (DEp) improved due to the existence of GNs. The surface coverageis 48 mV, indicating a faster electron transfer rate. It is clear to see (C/mol cmÀ2) can be calculated by using the equation C = Q/nFA,that the GNs promote the electron transfer between FeTSPc and where Q is the charge involved in the reaction, F is the Faraday con-GCE. stant, n is the number of moles of electrons transferred and A is the According to Fig. 6B, another well-shaped pair of redox peaks at experimentally determined area of the electrode. The amount ofÀ0.5 V are attributed to the electrochemical conversion of the Pc FeTSPc immobilized on GNs is 2.49 Â 10À9 mol cmÀ2 (1.4 Â 1015unit [Fe(II)TSPc4À/Fe(II)TSPc6À]. To understand the heterogeneous particles cmÀ2 %7.14 Å2 per particle), which is much larger thanelectron transfer of FeIII/FeII better, the reduction and oxidation the first layer coverage of iron-phthalocyanine on single-walledpeak currents of the GNs–FeTSPc modified GCE increase linearly carbon nanotubes (1.2 Â 10À10 mol cmÀ2) [43] and that ofwith the increasing of scan rates up to 500 mV sÀ1, indicating 1.22 Â 10À10 mol cmÀ2 for iron-tetraaminophthalocyanine-single-a surface-controlled electrode process. (Inset, Fig. 6; linear walled carbon nanotubes [30]. The estimated values indicate mul-regression equations: Ipa = 2.25771 + 0.11807v, R = 0.999; Ipc = tilayer coverage rather than a monolayer coverage expected to beÀ2.44399 À 0.11860v, R = 0.998). Otherwise, in the scan rates in the order of 10À10 mol cmÀ2 for MPc molecules [44]. Thus, theranging from 10 to 1000 mV sÀ1, the liner regression equations of high surface coverage of FeTSPc on GNs can be a clear indication
  • 5. 16 N. Li et al. / Journal of Electroanalytical Chemistry 651 (2011) 12–18that the high special surface area (2600 m2 gÀ1) of GNs offers a to the difficulty to confine these water-soluble molecules on thestraightforward way to increase the number of active sites [45]. electrode (Fig. 8b). GNs modified GCE shows two peaks with the Fig. 7 shows the CVs of GNs–FeTSPc under different pH values. first peak potential at about 0.3 V but no obvious enhancementWith an increase of pH from 4 to 12, the redox potentials of the in peak current (Fig. 8c), which may attributed to the aggregationGNs–FeTSPc modified GCE shift to more negative values and exhi- of GNs. Another small peak was observed in both GNs andbit a linear variations with slopes of À67.8 mV pHÀ1 for the oxida- GNs–FeTSPc modified GCEs, which may contribute to the two steption processes. According to Laviron equation [41], the na is reactions during the electrooxidation of INZ [46]. However,estimated to be 0.91, indicating that one electron and one proton GNs–FeTSPc/GCE shows an enhanced background current andtransfer is involved in the electrode reaction. Besides, the stability anodic peak with the lowest potential (0.294 V) and highestmeasurements (50-cycles cyclic voltammograms at 100 mV sÀ1) current (63.49 lA) than those at the other modified GCEsare also carried out (not shown). GNs–FeTSPc film shows high sta- above(Fig. 8e), which indicates that the redox reaction can servebility when submitted to several cycles and there is no obvious as an electron transfer mediator to facilitate the oxidation of INZchange in the peak current, further exhibiting its potential value with reduced overpotential and enhanced current response. Thusfor sensing application. the electroactive GNs–FeTSPc nanocomposite possesses an excel- lent synergic effect of GNs and FeTSPc.3.3. Electrocatalytic oxidation of isoniazid (INZ) Also important to note is that the adsorption of FeTSPc onto GNs actually increases the electrocatalytic activity of FeTSPc toward the As shown in Fig. 8, when 0.5 mM INZ is added to N2-saturated oxidation of INZ. The presence of GNs accelerates the electronPBS (pH 7.4), the electrochemical oxidation at a bare electrode pro- transfer and increases the amount of FeTSPc on electrode surface,ceeds at a high overpotential (0.81 V) because of slow electron which acts as a catalyst to further reduce the oxidation potential.transfer kinetics and electrode fouling (Fig. 8a) [9]. Although the Hence, the formation of GNs–FeTSPc nanocomposite not onlypeak potential shifts negatively to 0.7 V after using redox-active avoids GNs aggregation, but also presents a facile approach to con-FeTSPc, neither the peak current nor background is changed, owing finement of FeTSPc on electrode surface. This behavior provides an advantage 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 lA at 0.4 V, but higher than that at GNs/GCE, indicating a small enhancement in electrocatalysis of INZ. Thus both the TSPc unit and 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 of 10.64 lA at 0.41 V (Fig. 9a). After absorbing either FeTSPc or GNs on the GCE, the peak currents enhance to 20.82 lA and 14.79 lA with 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-Fig. 7. Cyclic voltammograms in various pH solutions. Inset: anodic potential vs. pH ward UA oxidation is further enhanced. It can be emphasizedfrom 4 to 12. that the use of GNs–FeTSPc modified GCE allowed a large enhance-Fig. 8. Cyclic voltammograms of bare GCE (a) and GCEs modified with (b) FeTSPc, Fig. 9. Cyclic voltammograms of (a) bare GCE, (b) FeTSPc/GCE, (c) GNs/GCE, (d)(c) GNs, (d) GNs–TSPc and (e) GNs–FeTSPc in N2-saturated PBS (0.1 M, pH 7.4) with GNs–TSPc/GCE and (e) GNs–FeTSPc/GCE in N2-saturated PBS (0.1 M, pH 7.4)(solid line) and without (dotted line) 0.5 mM INZ at 100 mV sÀ1. without (dotted line) or with (solid line) 0.5 mM UA at the scan rate of 100 mV sÀ1.
  • 6. N. Li et al. / Journal of Electroanalytical Chemistry 651 (2011) 12–18 17Table 1 development of electrochemical sensing platform. The strongComparative studies on electrocatalytic activity of different carbon materials with adsorption of FeTSPc onto GNs not only enhances the solubilityand without FeTSPc toward the oxidation of INZ and UA. of GNs into aqueous media, but also preserves the predominant Modified GCE Epa (V) Ipa (lA) properties of GNs and the redox property of FeTSPc. The excellent Carbon material FeTSPc INZ UA INZ UA electrocatalytic ability of GNs–FeTSPc lowers the overpotential, GNs With 0.294 0.342 63.49 52.18 and greatly enhances the current response for the oxidation of Without 0.336 0.340 14.70 14.90 INZ and UA. Direct electrochemistry of GNs–FeTSPc nanocompos- MWCNTs With 0.377 0.381 73.11 61.70 ites shows that GNs could facilitate the electron transfer between Without 0.303 0.385 48.78 42.67 substrate electrodes and the electroactive center of FeTSPc. Addi- MPC With 0.274 0.392 34.10 49.00 tionally, a comparative study using different carbon nanomaterials Without 0.283 0.364 62.10 41.67 reveals that the functionalized GNs have better electrocatalytic Graphite With 0.556 0.366 27.31 23.17 ability than that of carbon nanotubes and graphite while no obvi- Without 0.620 0.495 19.01 10.32 ous enhancement is obtained on MPC and C60. Hence, the GNs with C60 With – – – – unusual structure are suitable for preparing functionalized nano- Without 0.790 0.340 15.68 13.90 composites and for potential electrochemical sensor applications.ment current density about 50 lA. In addition, the oxidation of UA Acknowledgementson GNs–FeTSPc/GCE occurs at lower potential (0.341 V) valuesthan that on bare GCE and FeTSPc modified GCE. This again clearly The authors gratefully acknowledge the financial support of theindicates that the combination of GNs and FeTSPc allows twice 863 Program (2008AA06Z311), NSFC (20945004), and Scientificenhancing the electrocatalytic performances for UA oxidation in Research Foundation for Returned Scholars, Ministry of Educationterms of current intensity. These results again indicate a synergy of China.between GNs and FeTSPc in electrocatalytic oxidation of UA. TheGNs–FeTSPc nanocomposites may provide anther valuable plat-form for UA determination. References Moreover, GNs–TSPc/GCE also slightly enhances the anodic [1] M. Zhou, Y.M. Zhai, S.J. Dong, Anal. Chem. 81 (2009) 5603–5613.peak current of UA (about 28.9 lA at 0.351 V) compared with that [2] A.K. Geim, K.S. Novoselov, Nat. Mater. 6 (2007) 183–191.of bare GCE, FeTSPc/GCE and GR/GCE. Both FeIII/FeII and TSPc units [3] K.S. Kim, Y. Zhao, H. Jang, S.Y. Lee, J.M. Kim, K.S. Kim, J.H. Ahn, P. Kim, J.Y. 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