Wang et al. Nanoscale Research Letters 2012, 7:161 http://www.nanoscalereslett.com/content/7/1/161 1,599 cm-1 (curve d). This phenomenon was not observed on the C-rGO. Unlike the hydrazine reduction, no extra element was introduced into E-rGO during the process of electroche- mical reduction. The Raman spectroscopy of E-rGO is similar to the pristine graphene, which results in the G band shift to 1,587 cm-1. But after being kept in air for 1 day, E-rGO might be doped by the moisture or physi- sorbed oxygen [53]. This p-doping was reflected by the upshift of G band [53]. In order to reduce the effect of p-doping to E-rGO, the freshly prepared E-rGO was immediately used for the following experiments. In order to evaluate the charge transfer properties of E-rGO and C-rGO, cyclic voltammograms (CV) were performed in [Ru(NH3)6]3+/2+ solution (Figure 3A). The peak-to-peak separations (ΔEp) of C-rGO (solid line) and E-rGO (dashed line) are 82 and 78 mV, respectively. In addition, the difference between peak current of E- rGO and C-rGO is approximately 6 μA. This result indicates that the surface negative charge on the surface of E-rGO and C-rGO has similar effects on the electron transfer property of redox system [Ru(NH3)6]3+/2+. Since the GO films used for preparation of C-rGO and E-rGO are the same, the current instead of current density was used here to compare the conductivity of C-rGO and E- rGO. To probe negative charge amount present on the C- rGO and E-rGO surface, the C-rGO and E-rGO were evaluated by CV using [Fe(CN)6]3-/4- as indicators (Fig- ure 3B). It has been reported that the voltammetry of [Fe(CN)6]3-/4- is sensitive to the surface functional groups, such as hydrogen and oxygen-containing groups, present in the carbon-based electrode [54]. Therefore, in our experiment, [Fe(CN)6]3-/4- was chosen as an indica- tor to explore the properties of E-rGO and C-rGO. Fig- ure 3B represents the typical CV curves of C-rGO (solid Page 4 of 7 line) and E-rGO (dashed line) in 5 mM [Fe(CN)6]3-/4- solution containing 0.1 M KCl. The ΔEp of C-rGO and E-rGO are 82 and 73 mV, respectively. These ΔEp values indicate that the microstructural defects and den- sity of electronic states near Fermi potential between C- rGO and E-rGO is different [54]. Since only partial functional groups of GO have been reduced, negatively charged functional groups remain on the surface of rGO. As [Fe(CN)6]3-/4- is also negatively charged, the repulsion force between rGO and [Fe(CN)6]3-/4- reflects the amount of the negatively charged functional groups in rGO. From Figure 3B, it can be seen that the currents for C-rGO and E-rGO are 19 and 23 μA, respectively. Therefore, the amount of negatively charged groups in C-rGO is higher than those in E-rGO, confirming that the efficiency of hydrazine vapor reduction is lower than that of electrochemical reduction. In order to compare the electrocatalytic activity of C- rGO and E-rGO, the detection of DA was performed by using C-rGO (Figure 4A) and E-rGO (Figure 4B), respectively. Usually, two factors are considered in the detection of DA. One is the interference from other electroactive species such as AA and UA [54]. Both AA and UA can be electrochemically oxidized at the poten- tial close to DA, resulting in an overlapped oxidation peak. The other is that the concentration of AA is higher than that of DA in the neuronal regions [54]. In the pure AA, DA, or UA solution, the similar peaks were observed at C-rGO (Figure 4A) and E-rGO (Figure 4B). Such electrocatalysis of E-rGO and C-rGO towards DA is probably due to the edge-plane-like defects on rGO surface that might provide many active sites and accelerate electron transfer between the electrode and species in solution [55]. However, in the mixture of AA, DA, and UA, the obtained results are quite different. With E-rGO, well-defined oxidation peaks for AA, DA, Figure 3 CV curves of C-rGO (solid line) and E-rGO (dashed line). In (A) 1.0 mM [Ru(NH3)6]3+/2+ (1:1) containing 0.1 M KCl and (B) 5 mM [Fe (CN)6]3-/4- (1:1) containing 0.1 M KCl.