The electrochemically reduced graphene oxide modified glass carbon (ERGO/GC) electrode was prepared in situ from graphene oxide modified glass carbon (GO/GC) electrode. The electro-oxidation of guanine at the ERGO/GC electrode was investigated by cyclic voltammetry and differential pulse voltammetry under controlled pH and concentration conditions. The ERGO/GC electrode showed a significantly enhanced voltammetric response for guanine due to the increase of the electrochemical active surface area and the conductivity of the electrode during the reduction process. The electro-oxidation reaction of guanine was identified to involve two electrons and two protons. The voltammetric peak currents were linearly dependent on the concentration of guanine in the range of 0.5 to 10 mu M. The voltammetric peaks in cell lysates were attributed to the oxidation of guanine in RNA and DNA of cells. The peak current was increased linearly with increase in the amount of cells with a detection limit of 1 x 10(5) cells mL(-1) (S/N = 3). (C) 2014 The Electrochemical Society.