Improved voltammetric peak separation and sensitivity ...

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Electrochemistry Communications 12 (2010) 596–599

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Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom

Improved voltammetric peak separation and sensitivity of uric acid and ascorbic acid at nanoplatelets of graphitic oxide Jen-Lin Chang a, Kuo-Hsin Chang b,c, Chi-Chang Hu b, Wan-Ling Cheng a, Jyh-Myng Zen a,* a

Department of Chemistry, National Chung Hsing University, 250 Kuo-Kuang Road, Taichung 402, Taiwan Department of Chemical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan c Department of Chemical Engineering, National Chung Cheng University, Chiayi 621, Taiwan b

a r t i c l e

i n f o

Article history: Received 26 January 2010 Received in revised form 8 February 2010 Accepted 9 February 2010 Available online 13 February 2010 Keywords: Nanoplatelets of graphitic oxide Edge plane Uric acid Ascorbic acid Screen-printed electrode

a b s t r a c t In this study, much improved voltammetric peak separation and sensitivity of uric acid and ascorbic acid was observed at a screen-printed carbon electrode modiﬁed with nanoplatelets of graphitic oxide (GO). Electrochemical sensing of uric acid and ascorbic acid was further used to explore the role of oxygen functionalities and edge plane sites on electrocatalysis at GO. We successfully apply a microwave-assisted hydrothermal elimination method to remove the oxygen-containing functional groups from the GO surface. The edge plane on GO can be retained and the density of oxygen-containing functional groups can be easily controlled by varying the microwave treatment temperature. Using this platform, such discrimination to peak separation is attributed to the very distinct behavior of uric acid and ascorbic acid to form hydrogen bonds with oxo-surface groups (especially COOH group) at GO. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction In our earlier studies we have demonstrated that the preanodized screen-printed carbon electrode (designated as SPCE*) acts more or less like an edge plane graphite electrode with improved electrochemical activity and lower overpotential for the detection of various analytes [1–4]. Insights into the understanding of the contribution from oxygen functionalities and edge-plane-like sites to the SPCE* was compared on electroanalysis of dopamine, uric acid (UA) and ascorbic acid (AA) at an oxygen plasma-treated SPCE, at which the contribution of the edge plane effect was much less than that of SPCE* [5]. Although both electrodes are effective in voltammetric peak separation with improved sensitivity, an extra negative shift of 200 mV in the oxidation potential of AA and no extra shift in the oxidation potential of UA were observed at the SPCE*. Considering the fact that oxygen plasma treatment can induce different oxygen functionalities to those of preanodization, it is thus hard to explain correctly of this peak increase and shifting phenomena. Indeed, the role that these surface functional groups play in facilitating or hindering electron transfer to various redox species in solution at multi-walled carbon nanotubes (MWCNTs) remains rather unclear. Some reports claim that redox catalysis by oxo-surface groups can enhance the rate of electron

* Corresponding author. Tel.: +886 4 22850864; fax: +886 4 22854007. E-mail address: [email protected] (J.-M. Zen). 1388-2481/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2010.02.008

transfer in certain cases [6,7] and some evidence that the converse is true [8,9]. Previously, Wildgoose and co-workers explored the inﬂuence of edge-plane defects and oxygen-containing surface groups on the voltammetry of acid-treated, annealed and super-annealed multiwalled carbon nanotubes (MWCNTs) [10]. Note that the acid-treated, annealed and super-annealed MWCNTs represent different surface conditions of a large number of edge-plane-like sites heavily decorated with surface functional groups, little functional groups but leaving some undecorated edge-plane-like sites exposed and the original, pristine, almost edge-plane defect-free structure, respectively. The high temperature annealing treatment can not only remove the surface functional groups but also reduce the density of defects (i.e., the edge plane). The results provide evidence that edge-plane-like sites are the electroactive sites on MWCNTs with high electrocatalytic activity toward several biological molecules. They also recommend caution in using redox probes to characterize carbon and in particular, MWCNT surfaces. Recently, microwave-assisted solvothermal reduction of exfoliated graphene oxide was reported to produce chemically modiﬁed graphene nanosheets within a short reaction time of 5– 15 min at relatively low temperatures (180–300 °C) [11]. We found that this microwave-assisted hydrothermal elimination method can remove the oxygen-containing functional groups from the nanoplatelets of graphitic oxide (GO) surface and still maintain the edge plane on GO. The density of oxygen-containing functional groups can be easily controlled by varying the microwave

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treatment temperature. Since the edge-plane-like sites are in fact the electroactive sites, a platform to retain the edge plane and/or with tunable oxygen-containing functional groups should be useful to study the peak increase and shifting phenomena of UA and AA. Furthermore, graphene oxide is of great interest because effective exposure of the edge plane may provide a unique ability for chemical sensing [12–14]. It is hence interesting to study the role of oxygen functionalities and edge plane sites on the voltammetry of GO.

2. Experimental UA and AA were obtained from Sigma and used as received without further puriﬁcation. All other chemicals used are of ACScertiﬁed reagent grade. Aqueous solutions were prepared with doubly distilled deionized water. A 0.1 M, pH 7.4 phosphate buffer solution (PBS) was used in all electrochemical studies. Nanographite platelets (N008-100-N) with ca. 100 nm in thickness (Angstron Materials LLC, USA) were used as the raw material to prepare the GO. The preparation of GO generally follows the modiﬁed Hummer’s method with supersonic exfoliation [15,16]. The GO solutions were heated from room temperature to 125 and 200 °C and kept at the speciﬁed temperature with an air-ﬂow cooling for 5 min at a constant power of 100 W in a microwave reactor (CEM, USA). The resulted GO at 125 and 200 °C were denoted as G125 and G200, respectively. Electrochemical measurements were performed with a BAS 50 W electrochemical workstation in a three-electrode cell assembly. A bare-SPCE or GO-modiﬁed SPCEs working electrode, an Ag/

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AgCl, 3 M KCl reference electrode and a platinum auxiliary electrode were used to complete the cell setup. The SPCE with a working area of 0.2 cm2 and a conductive track radius of 2.5 mm was purchased from Zensor R&D (Taichung, Taiwan). The measured average resistance was 85.64 ± 2.10 X/cm. The GO-modiﬁed SPCEs were fabricated by casting a 5 lL aliquot of the prepared GO solution on the SPCE. The solvent was allowed to evaporate at room temperature in the air for 30 min. The density of surface functional groups was measured by an Xray photoelectron spectrometer (XPS, Kratos Analytical AXIS Ultra DLD) under Al monochromator (hm = 1486.69 eV) irradiation. The ratio of surface functional group was obtained from the area ratio of ﬁtting peaks by the XPSPeak 4.1 program with Shirley baseline and asymmetric Lorentzian–Gaussian sum function. The edgeplane exposure (i.e., ID/IG ratio as the index) was determined by high-resolution micro-Raman analysis using Lab RAM HR Raman Microscope (HOROBA, France).

3. Results and discussion Fig. 1 shows typical Raman and XPS C-1s core-level spectra of GO (a), G125 (b) and G200 (c). In all cases, two principal spectral peaks corresponding to the D-band radial breathing mode and the G-band tangential stretching mode are observed at ca. 1350 cm 1 and ca. 1600 cm 1, respectively, in agreement with the literature [17]. The relative intensities of D and G bands (i.e., ID/IG) provide the evidence for edge-plane exposure. Most importantly the very similar values of ID/IG (i.e., 0.97, 0.95 and 0.98) for all samples indicate that the edge-plane structures on these GO-derived mate-

Fig. 1. Raman and XPS spectra of GO (a), G125 (b) and G200 (c).

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Fig. 2. Linear scan voltammograms at bare-SPCE (a), GO-SPCE (b), G125-SPCE (c) and G200-SPCE (d) for 1 mM AA + 500 lM UA at a scan rate of 2 mV/s.

rials are not signiﬁcantly changed by the microwave-assisted hydrothermal elimination treatment (i.e., at low temperatures in a short period). Overall, the microwave-method has been proved effective to remove the surface functional groups of GO without changing its original edge-plane exposure. As can be seen from the XPS C-1s core-level spectra of GO, G125 and G200, effective removal of these oxygen-containing functional groups was conﬁrmed from the relative proportion of the C–C bond (284.5 eV) versus C–O bonds (286.7 and 287.6 eV) in the deconvoluted XPS spectra. A signiﬁcantly reduced amount of COOH (288.6 eV), i.e., GO (9.9%) ? G125 (7.7%) ? G200 (6.3%), was attained with increasing the microwave-assisted hydrothermal temperature. This result is attributable to the formation of supercritical water under hydrothermal conditions to act as a reducing agent to deoxygenate the oxo-surface functional groups [18,19]. Accordingly, the density of functional groups on GO can be easily controlled by varying the temperature of the microwave-assisted hydrothermal elimination method. Fig. 2 shows the linear scan voltammograms of 2 mM AA + 500 lM UA at SPCE, GO-SPCE, G125-SPCE and G200-SPCE, respectively. Similar to our previous study on SPCE*, much improved voltammetric peak separation and sensitivity was obtained at all three GO-based electrodes. Meanwhile, a large shift in the oxidation potential of AA in cathodic direction was observed with the extent of potential shift to increase from 19 mV ? 58 mV ? 158 mV for G200-SPCE ? G125-SPCE ? GO-SPCE, respectively. While virtually the same peak potential was observed for UA at all three GO-based electrodes. Further investigation was made to the transport characteristics of UA and AA. The LSV current response of UA was found to be linearly proportional to the scan rate, which illustrated that the process was adsorption-controlled. Supplementary evidence for the adsorptive behavior of UA was demonstrated by the following experiment. When switched to a medium containing only pH 7.4 PBS after being used in measuring an UA solution, virtually the same voltammetric peak signal was observed. On the other hand, the transport characteristic of AA was found to be more of diffusion-controlled. Indeed the observed

Fig. 3. A schematic diagram illustrating the proposed mechanism for the improved voltammetric peak separation and sensitivity of UA and AA at GO.

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transport characteristics is quite consistent with the difference in voltammetric peak shape of UA and AA at GO. Compared to the overlapped voltammetric peaks of UA and AA at a bare-SPCE, lower overpotential and increased sensitivity for both analytes reveals proper function of the edge-plane-like sites on GO. As to peak separation, such discrimination is attributed to the very distinct ability of AA and UA to form hydrogen bonds with oxo-surface groups (especially COOH group), as illustrated in Fig. 3. Note that the inﬂuence of the hydrogen bond is determined by a different strength between hydrogen-bond donor and hydrogenbond acceptor. The correlation between hydrogen bond and molecular structure is reported that the hydrogen-bond acceptor strength of amide group is generally stronger than that of ester group [20,21]. A possible explanation for the potential shift could be proposed as follows. In an oxidation step, a capto-dative stable radical intermediate could be formed by removing one electron and one hydronium ion [22]. The result of potential shift observed for AA indicates that a direct chemical interaction between oxosurface groups and AA is involved in the reaction center during the oxidation step. The hydrogen-bonding interaction between oxo-surface groups and oxygen atom of carbonyl group at C-1 in AA will increase the proton acidity of hydroxyl group at C-3 (Fig. 3). The more acidic a proton is, the easier oxidation occurs. Therefore, a shift of the oxidation potential of AA in cathodic direction was observed. However, no such a large potential shift was observed for UA. It is because the hydrogen bond between oxosurface groups and the oxygen atom of the carbonyl group at C-2 in UA is formed, whose interaction is far from the reaction center and thus no strong interfering is involved in the oxidation step. 4. Conclusions The present study demonstrates the unique properties of GO to improve electrochemical monitoring of UA and AA. Raman spectra and XPS analyses show the evidences for the presence of edge plane sites and also the change of surface oxygen-containing functionalities on GO. Microwave-assisted hydrothermal treated GO provides a platform to retain the edge plane with tunable oxygen-containing functional groups to study the peak increase and shifting phenomena of UA and AA. The major suffering from the

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overlapped oxidation potentials of UA and AA could be overcome through the very distinct ability of them to form hydrogen bonds with GO. The strength of hydrogen bond as well as the distance of interaction site to the reaction center involved in the oxidation step is the two combined factors to inﬂuence the voltammetric peak separation of UA and AA. Acknowledgements This work was supported by National Science Council of Taiwan and in part by the Ministry of Education, Taiwan under the ATU plan and the Boost Program of NTHU. References [1] J.-C. Chen, H.-H. Chung, C.-T. Hsu, D.-M. Tsai, A.S. Kumar, J.-M. Zen, Sens. Actuators, B 110 (2005) 364. [2] J.-C. Chen, A.S. Kumar, H.-H. Chung, S.-H. Chien, M.-C. Kuo, J.-M. Zen, Sens. Actuators, B 115 (2006) 473. [3] C.-C. Yang, A.S. Kumar, J.-M. Zen, Anal. Biochem. 338 (2005) 278. [4] K.S. Prasad, J.-C. Chen, C. Ay, J.-M. Zen, Sens. Actuators, B 123 (2007) 715. [5] K.S. Prasad, G. Muthuraman, J.-M. Zen, Electrochem. Commun. 10 (2008) 559. [6] I. Dumitrescu, N.R. Wilson, J.V. Macpherson, J. Phys. Chem. C 111 (2007) 12944. [7] A. Chou, T. Boecking, N.K. Singh, J.J. Gooding, Chem. Commun. 7 (2005) 84232. [8] C.E. Banks, X. Ji, A. Crossley, R.G. Compton, Electroanalysis 18 (2006) 2137. [9] X. Ji, C.E. Banks, A. Crossley, R.G. Compton, ChemPhysChem 7 (2006) 1337. [10] A.F. Holloway, G.G. Wildgoose, R.G. Compton, L. Shao, M.L.H. Green, J. Solid State Electrochem. 12 (2008) 1337. [11] A.V. Murugan, T. Muraliganth, A. Manthiram, Chem. Mater. 21 (2009) 5004. [12] C.H. Lu, H.H. Yang, C.L. Zhu, Z. Chen, G.N. Chen, Angew. Chem., Int. Ed. 48 (2009) 4785. [13] M. Zhou, Y. Zhai, S. Dong, Anal. Chem. 81 (2009) 5603. [14] M. Pumera, Chem. Rev. 9 (2009) 211. [15] S. Stankovich, D.A. Dikin, G.H.B. Dommett, K.M. Kohlhaas, E.J. Zimney, E.A. Stach, R.D. Piner, S.T. Nguyen, R.S. Ruoff, Nature 442 (2006) 282. [16] C.N.R. Rao, A.K. Sood, K.S. Subrahmanyam, A. Govindaraj, Angew. Chem., Int. Ed. 48 (2009) 7752. [17] S. Park, J. An, R.D. Piner, I. Jung, D. Yang, A. Velamakanni, S.T. Nguyen, R.S. Ruoff, Chem. Mater. 20 (2008) 6592. [18] C. Yao, Y. Shin, L.Q. Wang, C.F. Windisch Jr., W.D. Samuels, B.W. Arey, C. Wang, W.M. Risen Jr., G.J. Exarhos, J. Phys. Chem. C 111 (2007) 15141. [19] Y. Zhou, Q. Bao, L.A.L. Tang, Y. Zhong, K.P. Loh, Chem. Mater. 21 (2009) 2950. [20] R.W. Taft, M. Berthelot, C. Laurence, A.J. Leo, Chem. Tech. (1996) 20. [21] H. Asanuma, S. Gotoh, T. Ban, M. Komiyama, Chem. Lett. (1996) 681. [22] H.G. Viehe, R. Merenyi, L. Stella, Z. Janousek, Angew. Chem., Int. Ed. 18 (1979) 917.

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