Article pubs.acs.org/JPCC
Permanganate-Route-Prepared Electrochemically Reduced Graphene Oxides Exhibit Limited Anodic Potential Window Chee Shan Lim, Chun Kiang Chua, and Martin Pumera* Division of Chemistry & Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371 Singapore S Supporting Information *
ABSTRACT: Graphene-related materials have been of significant interest in the field of electrochemical sensing and biosensing. Four main methods of synthesizing large quantities of graphene from graphite via graphene oxide have been used to date, using either chlorate (Staudenmaier and Hofmann methods) or permanganate (Hummers and Tour methods) oxidants and strong mineral acids to generate graphite oxide with subsequent reduction. In electrochemical applications, electrochemical reduction is often used to prepare reduced graphenes so as to eliminate any electrochemically reducible groups on the surface of graphene oxides which could interfere with the analytical signals. Here, we show that electrochemical reduction of oxygen-containing groups at graphene oxide surfaces indeed results in materials without inherent electrochemistry for chloratebased graphene oxides; however, permanganate-based electrochemically reduced materials exhibited significant limitation in the anodic region, starting from ∼+0.1 V (vs Ag/AgCl). The effect of the anodic potential windows was studied with uric acid, ascorbic acid, and dopamine, and it was evident that the oxidation signals of the analytes performed on permanganate-based reduced graphene oxides were superposed on the background signals, resulting in wider peaks and larger oxidation currents. Given the fact that the permanganate route (Hummers method) has been most widely used for preparation of graphene oxide, we wish to warn the electrochemical community and to emphasize that the method used for preparation of these reduced graphene materials should be considered in advance as it may be interfering with the response of some compounds. the mixture of sulfuric acid and fuming nitric acid.9 Hofmann then eliminated the use of fuming nitric acid in the production of graphite oxide in 1937.10 Potassium chlorate used in these two syntheses was then substituted by potassium permanganate by Hummers in 1958.11 The three methods have been widely used in the synthesis of graphite oxide, until another alternative synthesis was elucidated by Tour’s research group in 2010.12 This alternative route uses concentrated phosphoric acid instead of the classical nitric acid in the previous syntheses, with permanganate as the oxidant. Graphene oxides are known to possess large numbers of oxygen functionalities with a carbon:oxygen (C:O) ratio of approximately 2:1.9,11,12 These functionalities include carbonyl, carboxyl, epoxyl, hydroxyl, and peroxyl groups which are electroactive in nature.13−15 As a result, graphene-related materials exhibit inherent electrochemical behaviors which should be accounted for when analyses are performed with these materials.16 Such inherent electrochemical behavior of graphene oxides can result in limitations in their potential window.17 Electrochemical potential windows are ranges of potentials over which the electrode materials and solvents used are stable, when they are neither oxidized nor reduced. The
1. INTRODUCTION Graphene materials can be fabricated via either the “top-down” approach or the “bottom-up” growth routes.1 The former method is normally favored because of its prospective use in large-scale production, which is essential for industrial purposes. The “top-down” approach first requires chemical oxidation to convert graphite to graphite oxide before the latter can be reduced to a series of chemically modified graphene oxides through either thermal exfoliation or the sono-chemical process.2 Graphene oxides can be produced from the ultrasonication of graphite oxides, which can be modified to electrochemically reduced or chemically reduced graphene oxides by the application of a reduction potential or the addition of a suitable reducing agent, respectively. These chemically modified graphene materials have displayed great potential in electrochemical sensing, enhancing the detection limits and sensitivities of electrochemical systems.3−6 Graphite oxide is a crucial intermediate in the production of chemically modified graphenes from graphite.7 It retains the lamellar structure of graphite and yet possesses larger spacing between layers because of the existence of oxygen-containing groups.8 Over the past century, graphite oxide has been synthesized using different methods as shown in Figure 1, and analyses have been carried out to compare the electrochemical properties of these graphite oxides. Staudenmaier reported a method which made use of potassium chlorate as an oxidant in © 2014 American Chemical Society
Received: June 30, 2014 Revised: August 19, 2014 Published: September 29, 2014 23368
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Figure 1. Schematic representation of preparation of graphite oxide via oxidation of graphite.
typical examples of electrode materials with limited potential windows are mercury and gold electrode surfaces. This is because mercury dissolves in solutions where the potential is above 0 V against SCE and oxidation of gold surfaces tends to occur at higher anodic potentials.18 Both scenarios limit the detection window of the electrochemical system. Graphene oxides prepared by different methods not only exhibit different degrees of oxidation but also contain varied amounts of oxygen functionalities.19 Previous studies have examined the cathodic potential windows of graphene oxides prepared via the Staudenmaier (chlorate oxidant) method.16 Until recently, it was understood that graphene oxides are stable in anodic (that is, potentials in positive, oxidation region) potential windows.13,15 We have noted that this is true for graphene oxides prepared by chlorate methods (Staudenmaier, Hofmann); such graphene oxides exhibit chemically irreversible reduction (cathodic) waves. In contrast, it is noteworthy that graphene oxides prepared by permanganate methods (Hummers, Tour) display inherent electrochemistry in both cathodic and anodic regions. Moreover, this electrochemical behavior is proven to be chemically reversible.20 Here we wish to study the electrochemical windows of the graphene oxides prepared by the permanganate (Hummers, Tour) and chlorate (Staudenmaier, Hofmann) oxidants in both cathodic and anodic regions and the influence of the limited electrochemical window on the detection of several important compounds.
computer. The analyzer is governed by General Purpose Electrochemical Systems Version 4.9 software (Eco Chemie). Cyclic voltammetry experiments were performed in a 5 mL electrochemical cell using a three-electrode configuration at room temperature. A platinum electrode served as an auxiliary electrode; an Ag/AgCl electrode was used as a reference electrode. All electrochemical potentials in this report are stated versus the Ag/AgCl reference electrode, and all measurements were carried out at a scan rate of 0.1 V s−1. 2.3. Procedure. The preparation of the four types of graphite oxides are listed below. Staudenmaier Method.9 Sulfuric acid (17.5 mL, 95−98%) and nitric acid (9 mL, >90%) were added to a reaction flask containing a magnetic stir bar. After the mixture was cooled at 0 °C for 15 min, graphite (1 g) was added to the mixture under vigorous stirring to avoid agglomeration. A homogeneous dispersion is hence obtained. Potassium chlorate (11 g) was gradually added to the mixture (over 15 min) at 0 °C to prevent sudden surge in temperature and the formation of chlorine dioxide gas, an explosive gas at high concentrations. Following the complete dissolution of potassium chlorate, the reaction flask was loosely capped to allow evolution of gas, and the mixture was stirred vigorously for 96 h at room temperature. Upon completion of the reaction, the mixture was poured into ultrapure water (1 L) and filtered. GO was then re-dispersed and washed continually in HCl solutions (5%) to remove sulfate ions. The graphene oxide was washed with ultrapure water until a neutral pH of the filtrate was obtained. The GO slurry was then dried in a vacuum oven at 50 °C for five days before use. Hofmann Method.10 Sulfuric acid (17.5 mL, 95−98%) and nitric acid (9 mL, 63%) were added to a reaction flask containing a magnetic stir bar. The mixture was cooled at 0 °C for 15 min before graphite (1 g) was added to the mixture under vigorous stirring to avoid agglomeration and obtain a homogeneous dispersion. Potassium chlorate (11 g) was subsequently added to the mixture slowly (over 15 min) at 0 °C to prevent sudden surge in temperature and the formation of chlorine dioxide gas, an explosive gas at high concentrations. After the complete dissolution of potassium chlorate, the reaction flask was loosely capped to allow evolution of gas, and the mixture was stirred vigorously for 96 h at room temperature. Upon completion of the reaction, the mixture
2. EXPERIMENTAL SECTION 2.1. Materials. Hydrochloric acid (35%, p.a.), hydrogen peroxide (30%, p.a.), nitric acid (68%, p.a.), potassium chlorate (99%, p.a.), potassium permanganate (99.5%, p.a.), sulfuric acid (98%, p.a.), N,N-dimethylformamide (DMF), potassium phosphate monobasic, sodium phosphate dibasic, potassium chloride, sodium chloride, uric acid, ascorbic acid, dopamine, and 2,4-dinitrotoluene were purchased from Sigma-Aldrich, Singapore. Glassy carbon (GC) electrodes with a diameter of 3 mm were obtained from Autolab, The Netherlands. Milli-Q water with a resistivity of 18.2 MΩcm was used throughout the experiments. 2.2. Apparatus. All cyclic voltammetry measurements were performed with a μAutolab type III electrochemical analyzer (Eco Chemie, The Netherlands) connected to a personal 23369
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was poured into ultrapure water (1 L) and filtered. GO was then re-dispersed and washed repeatedly in HCl solutions (5%) to eliminate sulfate ions. The GO was washed with ultrapure water until a neutral pH of the filtrate was obtained. The GO slurry was then dried in a vacuum oven at 50 °C for five days before use. Hummers Method.11 Graphite (0.5 g) was stirred with sulfuric acid (23.0 mL, 95−98%) for 20 min at 0 °C prior to the addition of NaNO3 (0.5 g) portion-wise. The mixture was left to stir for 1 h. KMnO4 (3 g) was then added portion-wise at 0 °C. The mixture was subsequently heated to 35 °C for 1 h. Following which, water (40 mL) was added into the mixture, which resulted in the rise of the temperature of the mixture to 90 °C. The temperature was maintained at 90 °C for 30 min before additional water (100 mL) was added into the mixture. This was followed by a slow addition of H2O2 (30%, ca. 10 mL). The warm solution was filtered using a RC membrane (0.22 mm) and washed with warm water (100 mL). The solid was then washed with a copious amount of water until a neutral pH was achieved. The materials was kept in the oven at 50 °C for five days before use. Tour Method.12 Graphite (0.3 g) was added to a mixture of concentrated H2SO4/H3PO4 (9:1) before KMnO4 (1.8 g) was added to the reaction mixture. The reaction mixture was heated to 50 °C and stirred continuously for 12 h. The mixture was then cooled to room temperature and poured into ice water. Hydrogen peroxide (30%, 0.5 mL) was added to the mixture which was subsequently filtered and washed with water, HCl (30%), ethanol, and diethyl ether. The solid product obtained was stored in the oven at 50 °C for 5 days before use. Glassy carbon electrode surfaces were polished with 0.05 μm alumina powder on a polishing cloth before a new measurement was performed. A suspension of the desired graphene material was prepared with a concentration of 1 mg mL−1 in DMF. After sonication of 1 h, 1 μL of the required suspension was then deposited onto the electrode surface. Immobilization of the graphene material onto the electrode surface is complete after evaporation of the solvent at room temperature. A randomly distributed film on the glassy carbon electrode surface is formed, and the measurement can be performed subsequently. Electrochemically reduced graphene oxide is formed by applying a potential of −1.6 V for 300 s to a graphene oxide modified glassy carbon electrode in a 50 mM phosphate buffer solution (pH 7.2). This applied potential is able to eliminate most of the oxygen-containing groups, such as peroxyl,21,22 aldehyde,23 and epoxyl,24,25 which can be electrochemically reduced at −0.7, −1.0, and −1.1 V, respectively (vs Ag/AgCl). Three repeated experiments were performed each time, using 3 different electrode units to ensure the reproducibility of each measurement. All voltammetry measurements were performed in a 50 mM phosphate buffer solution with pH 7.2.
explosives-related compound, 2,4-dinitrotoluene, using cyclic voltammetry. 3.1. Anodic Potential Window. Figure 2 shows the electrochemical windows of bare GC and the four ERGO
Figure 2. Cyclic voltammograms of PBS background electrolyte (50 mM, pH 7.2) on bare GC, ERGO-ST, ERGO-HO, ERGO-HU, and ERGO-TO. Conditions: scan rate, 100 mV s−1. No analyte is added.
surfaces in the anodic region. Cyclic voltammetric scans were recorded in the background electrolyte from −0.2 V in the anodic direction and reversed at +1.0 V. The bare GC, ERGOST, and ERGO-HO surfaces did not display any oxidation or electrochemical reactions, as shown by the absence of anodic peaks. It is, however, evident that the ERGO-HU and ERGOTO surfaces produced apparent oxidation peaks, starting from −0.042 and −0.014 V, and reaching a maximum at +0.266 V and +0.340 V, respectively. Presence of these oxidation peaks can be attributed to oxygen functionalities that are present only in the permanganate-based graphene.20 The permanganatebased GOs have a larger proportion of carbonyl (i.e., quinonelike) and carboxyl groups as compared to the chlorate-oxidized GOs (where epoxy and likely peroxy groups are predominant). While epoxy/peroxy groups are irreversibly reduced (in the chemical sense of the word), the quinone-like groups can be reduced to hydroquinone and oxidized back reversibly (in the chemical sense of the word), giving rise to the varied intrinsic electrochemistry among the surfaces.20 As a result, the potential windows of the permanganate-based graphene oxides are limited in the anodic region, starting from 0 V. The intrinsic oxidation signals might coincide with that of other compounds, resulting in overlapping or superposition of peaks, which can jeopardize the accuracy of the results. The consequences of the innate signals were studied via oxidation of uric acid, ascorbic acid, and dopamine and are reported in the following text. Oxidation of uric acid was first studied using the four electrode surfaces, and the oxidation peaks are compared in Figure 3. It can be observed in Figure 3A,B that the ERGOs prepared using chlorate oxidant, ERGO-ST and ERGO-HO, do not exhibit any inherent electrochemical response (dashed lines). The oxidation of uric acid at these surfaces began at +0.207 V and +0.194 V with the peak maximums at +0.398 (±0.003) V and +0.391 (±0.002) V, respectively (Figure 3, red lines). As for the permanganate-based ERGOs, the outcome was completely different. Panels C and D of Figure 3 display an
3. RESULTS AND DISCUSSION Cathodic and anodic potential windows of the electrochemically reduced graphene oxides from Staudenmaier (ERGO-ST), Hofmann (ERGO-HO), Hummers (ERGO-HU), and Tour (ERGO-TO) are investigated in the following text. The effect of the inherent electrochemical behaviors of ERGOs prepared by permanganate or chlorate oxidants will be demonstrated through the oxidation of three important biomolecules in the field of electrochemical sensing, uric acid, ascorbic acid and dopamine, as well as the reduction of a nitroaromatic 23370
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Figure 3. Cyclic voltammograms of 10 mM uric acid on graphene oxides prepared by (A) Staudenmaier, (B) Hofmann, (C) Hummers, and (D) Tour methods against background electrolyte. Conditions: 50 mM PBS background electrolyte; pH 7.2; scan rate, 100 mV s−1.
inherent electrochemistry of the ERGOs synthesized by the Hummers and Tours methods, respectively, each with an intrinsic electrochemical peak in the anodic region, starting ∼+0.100 V (vs Ag/AgCl) with a maximum obtained at +0.280 V and +0.350 V, respectively, and the end of peak at about +0.500 V (dashed lines in Figure 3C,D). The potential of this inherent peak coincides with the oxidation potential of many compounds, including biomarkers such as uric acid. The oxidation of uric acid on ERGO-HU originated from +0.222 V before reaching a maximum at +0.410 (±0.003) V. Oxidation of uric acid on ERGO-TO started at +0.062 V and peaked at +0.440 (±0.011) V. All four surfaces displayed an oxidation peak potential lower than that of the bare GC, which initiated at +0.222 V before the maximum is attained at +0.469 (±0.016) V (Figure S1A of Supporting Information). The existence of inherent oxidation signals in the permanganatebased graphene oxides has proven to result in larger oxidation currents and broader peaks as compared to those of the chlorate-based graphenes. This is due to the fact that at permanganate-based ERGO surfaces, there are major contributions from the inherent electrochemical signals of the reduced graphene surfaces. To further affirm the limitation of the ERGOs prepared using permanganate oxidant in the anodic window (but not to those prepared with chlorate oxidant), oxidation of ascorbic acid was
studied at these surfaces. As displayed in Figure 4, the chloratebased ERGOs did not show inherent anodic peaks in blank phosphate buffer (Figure 4A,B, dashed lines) but the permanganate-based ERGOs did (Figure 4C,D). Among the four graphene surfaces, the oxidation of ascorbic acid at permanganate-based graphene oxides displayed peak potentials slightly lower than that of the oxidation of ascorbic acid at chlorate-based graphene oxides; however, it should be noted that on permanganate-based ERGO surfaces, the resulting voltammograms are superposition of the signals originating from both the inherent electrochemistry of ERGO and the oxidation of ascorbic acid. The peak potentials for ERGO-HU and ERGO-TO were at +0.310 (±0.014) V and +0.281 (±0.018) V, respectively, and +0.357 (±0.014) V and +0.361 (±0.010) V for ERGO-ST and ERGO-HO, respectively, showing that the permanganate-based graphene oxides oxidized ascorbic acid at potentials lower than that of chlorate-based graphene oxides. Oxidation of ascorbic acid on the permanganate-based graphene surfaces displayed larger anodic currents and wider peaks than those of the chlorate-oxidized surfaces. The onsets of oxidation on ERGO-HU and ERGOTO were at −0.164 and −0.158 V, respectively. These are much earlier than the oxidation on ERGO-ST and ERGO-HO, which were at −0.108 and −0.124 V, respectively. The larger currents and wider peaks are direct results of superposition of 23371
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Figure 4. Cyclic voltammograms of 10 mM ascorbic acid on graphene oxides prepared by (A) Staudenmaier, (B) Hofmann, (C) Hummers, and (D) Tour methods against background electrolyte. Conditions: 50 mM PBS background electrolyte; pH 7.2; scan rate, 100 mV s−1.
the inherent electrochemistry of ERGO and ascorbic acid oxidation peaks. The oxidation peak of ascorbic acid on bare GC began at +0.079 V and reached its maximum at +0.542 (±0.020) V, as illustrated in Figure S1B of Supporting Information. Finally, we wish to demonstrate the electrochemical constraint of ERGOs prepared by the permanganate route on oxidation of dopamine (Figure 5). Electrochemical responses of the surfaces in blank buffer (in absence of the analyte) were recorded and showed as dashed lines. For the chlorate-based ERGOs, oxidation of dopamine on ERGO-ST began at +0.017 V and reached its maximum at +0.361 (±0.015) V, while that on ERGO-HO started from +0.009 V to a maximum of +0.363 (±0.007) V. On the other hand, ERGO-HU generated an oxidation peak beginning from −0.276 V to +0.324 (±0.014) V; the oxidation peak of ERGO-TO surface started from −0.293 V until a maximum of +0.318 (±0.011) V was reached. Figure 5 also provides yet another justification of the larger oxidation currents and wider oxidation peaks exhibited by the ERGO-HU and ERGO-TO surfaces. The wide peaks and large oxidation currents displayed by these two surfaces, together with the early onset of peaks, is attributed to the superposition of the dopamine oxidation signals onto the background responses. In addition, it is also apparent that the first peak of the “double peak” seen on the cyclic voltammograms of
ERGO-HU and ERGO-TO originated from the inherent background electrolyte peaks. For comparison, bare GC surface exhibited an oxidation potential higher than that of the graphene surfaces, with its oxidation peak initiating at +0.045 V before peaking at +0.461 (±0.019) V (Figure S1C of Supporting Information). Therefore, analyses of the three biomolecules suggest that the potential windows of ERGO-HU and ERGO-TO are limited in the anodic region, whereas the chlorate-based graphene oxides have a larger electrochemical window in the same region. 3.2. Cathodic Potential Region. Though inherent peaks were observed for ERGO-HU and ERGO-TO in the anodic potential region above +0.0 to +1.0 V, the cathodic potential window is of equal importance as well. Detection of explosives in trace quantities has been highly emphasized in terms of national security and protection. Hence, reduction analyses were performed on 2,4-DNT in the range from +0.0 to −1.4 V to establish the detection abilities of the graphene oxides in the cathodic region. Figure 6 exhibits the reduction signals of the bare GC and four ERGO surfaces in the background electrolyte, phosphate buffer solution after they have been electrochemically reduced. Inherent reduction signals were not observed after the graphene surfaces have undergone electrochemical reduction 23372
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Figure 5. Cyclic voltammograms of 10 mM dopamine on graphene oxides prepared by (A) Staudenmaier, (B) Hofmann, (C) Hummers, and (D) Tour methods against background electrolyte. Conditions: 50 mM PBS background electrolyte; pH 7.2; scan rate, 100 mV s−1.
reduction signals from electrochemical experiments will not be influenced by the background signals. Figure 7 illustrates the reduction of 2,4-DNT on the four ERGO surfaces. The chlorate-based ERGOs were able to generate three distinct reduction signals of 2,4-DNT, which is a result of the multistep electrochemical reduction of nitro groups.26,27 The reduction on ERGO-ST began from −0.380 V, before producing peaks at −0.566 (±0.008) V, −0.703 (±0.001) V, and −0.850 (±0.006) V subsequently. As for ERGO-HO, reduction started from −0.311 V with three distinct reduction signals at −0.591 (±0.017) V, −0.706 (±0.009) V, and −0.850 (±0.004) V. For permanganate-based ERGOs, the reduction signal of 2,4-DNT on ERGO-HU began from −0337 V and produced two observable peaks at −0.610 (±0.005) V and −0.850 (±0.021) V. A small peak was also noticed at −0.713 V. Finally, 2,4-DNT at ERGO-TO generated two peaks at −0.605 (±0.001) V and −0.791 (±0.005) V, with the reduction signal beginning at −0.330 V. For comparison, a reduction of 2,4-DNT on the bare GC was recorded (Figure S1D of Supporting Information); it occurred from −0.385 V and eventually displayed two peaks at −0.623 (±0.019) V and −0.830 (±0.014) V. Therefore, it is evident that the intrinsic signals of the ERGO electrode surfaces are insignificant in the cathodic region.
Figure 6. Cyclic voltammograms of PBS background electrolyte (50 mM, pH 7.2) on bare GC, ERGO-ST, ERGO-HO, ERGO-HU, and ERGO-TO. Conditions: scan rate, 100 mV s−1. No analyte is added.
at −1.6 V for 5 min (see Experimental Section). The absence of reduction signals shows that the electrochemical reduction potential and duration were sufficient to eliminate all the reducible oxygen-containing groups. Hence, all subsequent 23373
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Figure 7. Cyclic voltammograms of 10 mM 2,4-DNT on graphene oxides prepared by (A) Staudenmaier, (B) Hofmann, (C) Hummers, and (D) Tour methods against background electrolyte. Conditions: 50 mM PBS background electrolyte; pH 7.2; scan rate, 100 mV s−1.
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4. CONCLUSION
ASSOCIATED CONTENT
S Supporting Information *
The electrochemical potential windows of electrochemically reduced graphene oxides synthesized from four different methods have been examined. It is apparent that the electrochemical potential windows of the permanganate-based ERGOs are limited in the anodic region because of their inherent electrochemistry. This limitation has proven to have an effect on the detection of some biomolecules, impeding the usability of the permanganate-based graphene and accuracy of the signals in the anodic region because of the likelihood of superposition. On the contrary, chlorate-based ERGOs did not show a limited anodic potential window. In the reversed direction, none of the graphene oxides are constrained in the cathodic region. The absence of reduction signals is largely attributable to the electrochemical reduction applied onto the surfaces, which was capable of eliminating reducible oxygencontaining groups that could possibly result in inherent reduction signals in the background electrolyte. The issues raised here would likely play a role for amperometric measurements as well. We wish to alert the electrochemistry community and to show that the oxidation route used for preparation of reduced grapheme oxides has strong implications on their electrochemical properties.
Cyclic voltammograms of 10 mM uric acid, ascorbic acid, dopamine, and 2,4-DNT on bare glassy carbon electrode against background electrolyte. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Fax: (65) 6791-1961. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS M.P. acknowledges Tier 2 Grant (MOE2013-T2-1-056; ARC 35/13) from Ministry of Education, Singapore.
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REFERENCES
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