A multiple sweep voltammetry is effectively applied to the electrochemical assembly of polypyridyl cobalt(III) complexes [Co(phen)(2)(tatp)](3+) and [Co(phen)(3)](3+) (where phen = 1,10-phenanthroline and tatp = 1,4,8,9-tetra-aza-triphenylene) on a glassy carbon electrode modified with DNA or sodium dodecyl sulfate (SDS)-dispersed multi-walled carbon nanotubes (MWCNTs). The DNA-MWCNTs and SDS-MWCNTs are found to impact redox reactions of [Co(phen)(2)L](3+) (L = phen or tatp) adsorbed on the electrode surfaces. The anionic surfactant (SDS) facilitates the adsorption of [Co(phen)(2)L](3+) on the MWCNT surfaces via a fully-packed pi-pi stacking mode. An excess amount of DNA as the dispersant attached to the surface of MWCNTs interferes with the pi-pi stacking between MWCNTs and [Co(phen)(2)L](3+), representing a DNA-bridged binding mode. Moreover, the DNA-MWCNT or SDS-MWCNT modified electrodes are suitable for the detection of 6-mercaptopurine (6-MP) using [Co(phen)(2)(tatp)](3+) as voltammetric probes. The proposed sensor exhibits a high selectivity to 6-MP (detection limit = 0.05 mu M) and a good linear response in the range from 0.2 mu M to 0.2 mM. (C) 2011 Elsevier B.V. All rights reserved.