A novel assay for the electrochemical detection of guanine based on carbon nanotubes paste electrodes (CNTPEs) modified with cobalt phthalocyanine (CoPc) has been investigated. The results indicated that the modification of a CNTPE with this compound results in amplification of the guanine oxidation response in contrast to that on the unmodified CNTPE. The electrochemical behavior of the modified electrode and the mechanism of the oxidation of guanine were investigated using cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The methods parameters were optimized. A detection limit of 1.3 x 10(-7)mol L-1 was obtained for guanine using the electrocatalytic oxidation signal corresponding to the Co(II)/Co(III) redox process. This modified electrode was further applied for determination of single-stranded DNA by differential pulse voltammetry with a detection limit of 9.86 x 10(-8) mol L-1. The advantages of convenient fabrication, low-cost detection, short analysis time and combination with nanotechnology for increasing the sensitivity make the modified electrodes worthy of special emphasis in the nonlabeled detection of DNA hybridization reaction and in the development of DNA based biosensors for toxic chemicals, toxins and pathogens determination. (C) 2011 Elsevier B.V. All rights reserved.