The application of graphite/carbon electrodes modified by a mercury layer for the detection of a picomolar quantity of oligodeoxynucleotides (ODN) is described. The electrochemical characterisation of the Hg-modified graphite/carbon electrode surfaces was performed by cyclic voltammetry in presence of 100 VM Cd(II) ions. An optical microscope was used for the visualization of the surface morphology of the bare and Hg-modified graphite/carbon electrodes. The ODN analysis was performed by the following procedure: (i) the chemical step includes the acidic hydrolysis of the ODN (hODN) sample by 0.5 M HClO4 for a time t = 30 min at 75 degrees C. During this step only the purine bases are released from the ODN-chain; (ii) the first electrochemical step includes a potential-controlled reduction of the copper ions (Cu(II)) and accumulation of the purine base residue-Cu(I) complex (hODN-Cu(I) complex) on the Hg-modified graphite/carbon surface; (iii) the second electrochemical step represents the cathodic stripping of the electrochemically accumulated hODN-Cu(I) complex from the electrode surface resulting in a current peak on the voltammogram. The proposed electrochemical method can be used for the determination of different ODN lengths in the analysed solution. The stripping current peak of the electrochemically accumulated hODN-Cu(I) complex increased linearly with the length of ODN used. The detection of acid hydrolysed (A),so ODN by the above-mentioned procedure is possible down to 250 and 500 pM (related to the monomer) for a 0.4 mu m mercury layer modified pyrolytic graphite (0.4 mu m Hgm-PGEb) and a glassy carbon electrode (0.4 mu m Hgm-GCE), respectively. For the shortest (A)(10) ODN a detectable value of 1 nM on the 0.4 mu m Hgm-PGEb was observed. Our measurements confirm that Hg-modified graphite/carbon electrodes in the presence of the copper ions are suitable for sensitive electrochemical detection of ODN. (c) 2004 Elsevier B.V. All rights reserved.