G-rich oligodeoxynucleotides (ODNs) have great medical and nanotechnological potential, because they can self-assemble into G-quadruplexes and higher-order nanostructures. The folding properties of d(G)10, d(TG9) and d(TG8T) ODNs were studied using atomic force microscopy (AFM) and voltammetry at carbon electrodes. Single-stranded ODNs, in Na(+) containing solutions and for short incubation times, were detected using AFM as network films and polymeric structures and using voltammetry by the occurrence of only the guanine oxidation peak. G-quadruplexes, in Na(+) containing solutions and long incubation times, or in K(+) containing solutions, were detected using AFM as spherical aggregates and using voltammetry by the decrease of the guanine oxidation peak and the occurrence of the G-quartet oxidation peak. Concerning the self-assembling into higher-order nanostructures, d(G)10 was the only sequence forming G-nanowires observed using AFM, d(TG9) formed short G-based super-structures that adsorbed as rod-like shape aggregates, and d(TG8T) formed no nanostructures, due to the presence of thymine residues at both 5' and 3' ends.