The present paper describes the preparation of new modified surfaces for electrodes based on guanine/thiocytosine and gold nanoparticles. The gold nanoparticles were analyzed by UV-vis spectroscopy and transmission electron microscopy (TEM) and it was found that they have diameters between 30 and 40 nm. The layers were characterized by specular reflectance infrared spectroscopy (FTIR-RAS) and by atomic force microscopy (AFM). The thickness of layers was found to be approximately 30 nm for TC layers and 300 nm for GU layers. Every layer was characterized as electrochemical sensor (by cyclic voltammetry) both for uric acid and ascorbic acid determinations, separately and in their mixture. The modified sensors have good calibration functions with good sensitivity (between 1.145 and 1.406 mA cm(-2)/decade), reproducibility (<1%) and good long-term stability (3 determinations in an interval time of 3 months). The best responses were obtained for gold_thiocytosine (Au_TC) and gold_guanine (Au_GU) layers. (C) 2012 Elsevier Ltd. All rights reserved.