A crystalline composite nanostructure based on graphene and gold nanoparticles (denoted as Gr-Au) was synthesized in a single-step process by Radio Frequency catalytic Chemical Vapour Deposition (RF-cCVD) over an Au-x/MgO catalytic system (where x =3 wt.% and represents the amount of metal loaded in the catalyst). After preparation, the composite was chemically oxidized with a mixture of sulfuric/nitric acid (3:1 vol.), being subsequently denoted as Gr-Au-OH. No massive exfoliation of graphene layers occurred during oxidation, so the degree of crystallinity was preserved, as proved by X-ray powder diffraction (XRD) measurements. For both samples, the crystalline domain had a mean value of approximately 2.25 nm, corresponding to about 6 graphitic layers. The electro-catalytic properties of Gr-Au and Gr-Au-OH composites were tested by modifying two gold electrodes with the same amount of each material (denoted Au/Gr-Au and Au/Gr-Au-OH, respectively) and subsequently employed for the electrochemical analysis of guanine. A significant decrease in the electrochemical oxidation potential of guanine (similar to 100 mV) was obtained in both cases. Tafel analysis proved that the modified electrodes have a large value for the exchange current density (approximately 10-7A) which is one order of magnitude larger than that corresponding to a bare gold electrode. Consequently, these composite materials greatly enhance the transfer of electrons from solution to electrode. However, the major drawback of the Gr-Au-OH nanostructure is the large capacitive current induced by the oxygen functional groups and observed in all cyclic voltammetric measurements, which considerably diminishes the sensitivity of the modified electrode. (C) 2013 Elsevier Ltd. All rights reserved.