Wang et al. Nanoscale Research Letters 2012, 7:161 http://www.nanoscalereslett.com/content/7/1/161 Both the electrochemical reduction [20,46-49] and hydrazine vapor reduction [31,40-43,50] can reduce GO to get electrochemically reduced GO (E-rGO) and che- mically reduced GO (C-rGO), referred to as E-rGO and C-rGO, respectively. In this work, after a systematic study, we found that although the morphologies of sin- gle-layer E-rGO and C-rGO adsorbed on solid sub- strates are quite similar, their components and especially their electrocatalysis towards dopamine (DA) are quite different. Our experiment results showed that the amount of oxygen-containing functional groups in C-rGO is more than that in E-rGO, and the electrocata- lysis of E-rGO towards DA is better than that of C-rGO. Experimental details Nature graphite was purchased from Bay Carbon (Bay City, Michigan, USA) and used for synthesizing GO. 3- Aminopropyltriethoxysilane (APTES), H2O2 (30%), H2SO4 (98%), phosphate buffered saline, K4[Fe(CN)6] (99.9%), K3[Fe(CN)6] (99%), hexaammine ruthenium (III) chloride (98%), dopamine hydrochloride (99%), uric acid (UA) (≥ 99%), and hydrazine hydrate (98%) were purchased from Sigma-Aldrich (Milwaukee, WI) and used as received. Ascorbic acid (AA) (≥ 99.5%, Fluka, Sigma-Aldrich), HCl (37%, Merck, Darmstadt, Ger- many), NH3·H2O (28%, J. T. Baker, Phillipsburg, NJ, USA), NaCl (99.5%, Merck, Darmstadt, Germany) were used as received. Toluene was purified from a solvent purification system (PS-400-5, innovative technology Inc, USA). Indium tin oxide (ITO) (10 ohm/sq, thick- ness, 0.7 mm) was purchased from Kintec Company (Hong Kong, China). Ultrapure Milli-Q water (Milli-Q System, Millipore, Billerica, MA, USA) was used in all experiments. GO was synthesized based on our previous report [31]. The obtained GO powder was dispersed in water with a certain concentration by sonication to get GO aqueous solution. The adsorptions of single-layer GO on APTES-modified glassy carbon electrode (GCE) [46], ITO [46], and SiO2 substrates [31,50], referred to as GCE-APTES-GO, ITO-APTES-GO, and SiO2-APTES- GO, respectively, were prepared based on our previous reports. The obtained GCE-APTES-GO and ITO- APTES-GO electrodes were scanned in 0.5 M NaCl solution saturated with N2 from 0.7 to -1.1 V at a scan rate of 50 mV s-1 [46]. The GCE-APTES-GO, ITO-APTES-GO, and SiO2- APTES-GO electrodes were reduced by hydrazine vapor at 80°C overnight based on our previous report [31]. Atomic force microscopy (AFM) images were obtained by using Dimension 3100 (Veeco, CA, USA) in tapping mode using Si tip (Veeco, resonant frequency, 320 kHz, Spring constant, 42 N m-1) under ambient conditions with a scanning rate of 1 Hz and scanning line of 512. Page 2 of 7 Raman spectra were recorded on a WITec CRM200 confocal Raman microscopy system using 633 nm laser with an air cooling charge-coupled device as detector (WITec Instrument Corp, Germany). Before measure- ment, the instrument was calibrated by silicon wafer. All electrochemical measurements were carried out in a conventional three-electrode cell using CHI 660C Electrochemical Workstation (CHI instrument, Austin, TX, USA). In the experiment, GCE or ITO, platinum electrode, and Ag/AgCl (saturated KCl) electrode were used as working, counter, and reference electrodes, respectively. All the potentials shown in this work refer to the Ag/AgCl (saturated KCl) electrode. Results and discussion The experiment process is shown in Figure 1. The sin- gle-layer GO film was adsorbed on the 3-aminopropyl- triethoxysilane (APTES)-modified GCE, referred to as GCE-APTES-GO [20,46]. After GCE-APTES-GO was reduced by the electrochemical method [20,46] and hydrazine vapor reduction [31], the obtained products are referred to as E-rGO and C-rGO, respectively. As proof-of-concept, DA was chosen to compare the elec- trocatalysis properties of E-rGO and C-rGO. In order to study the GO and rGO films by AFM and Raman spectroscopy, single-layer GO was assembled on APTES-modified ITO and SiO2 substrates, referred to as ITO-APTES-GO and SiO2-APTES-GO, respectively, which then was reduced by electrochemical method and hydrazine vapor reduction (see the Experimental details). Figure S1 (see Additional file 1) shows AFM images of GO and rGO films. Although a few overlaps between single-layer GO or rGO films showed up, the morphologies of the GO films before and after reduc- tion have not shown much difference. The previous study reported that that new C-N bond appeared in C-rGO [39]. But after the electrochemical reduction, no additional element was incorporated into the product of E-rGO [20,46]. We believe that this dif- ference arises from the different reduction methods. The mechanism of hydrazine vapor reduction of the functional groups, e.g. ketone, in GO might follow the probable route as shown in Figure S2 (See Additional file 1) [51], which is different from the mechanism of the electrochemical reduction of GO proposed by Dong et al. who suggested that the hydrogen ions played an essential role in the electro-reduction process [48]. In the process of hydrazine vapor reduction, the new C-N bond was introduced. But during the electrochemical reduction, the aromatic ketone, alcohol, benzylic alcohol, phenol, and aromatic carboxylic acid groups in GO might be reduced by the electrons from the extra power [52], and no new element was incorporated since no reducing agent, e.g. hydrazine, was used.