DNA duplex regions of a spot on a DNA microarray were successfully visualized by scanning electrochemical microscopy (SECM) coupled with ferrocenyl naphthalene diimide as an electrochemically active DNA hybrid indicator. correlation of a normalized steady-state current with the normalized tip-substrate distance suggested that the tip current was generated by direct contact of the DNA duplex layer on the DNA spot bound to this ferrocenyl naphthalene diimide with the tip (not meaning the direct physical touching of the surface). Single nucleotide polymorphisms (SNPs) of Arg or Pro type of cancer suppression gene p53 were detected by using this system. (c) 2005 Elsevier Ltd. All rights reserved.