We use colloidal An to enhance the DNA immobilization amount on a g-Old electrode and ultimately lower the detection limit of our electrochemical DNA biosensor. Self-assembly of approximately 16-nm diameter colloidal Au onto a cysteamine modified gold electrode resulted in an easier attachment of an oligonucleotide with a mercaptohexyl group at the 5 ' -phosphate end, and therefore an increased capacity for nucleic acid detection. Quantitative results showed that the surface densities of oligonucleotides on the Au colloid modified gold electrode were approximately (1-4) x 10(14) molecules cm(-2). Hybridization was induced by exposure of the ssDNA-containing gold electrode to ferrocenecarboxaldehyde labeled complementary ssDNA in solution. The detection limit is 5 x 10(-10) mol 1(-1) of complementary ssDNA, which is much lower than our previous electrochemical DNA biosensors. The Au nanoparticle films on the An electrode provide a novel means for ssDNA immobilization and sequence-specific DNA detection.