Modification of indium tin oxide electrodes with nucleic acids: Detection of attomole quantities of immobilized DNA by electrocatalysis

Indiun tin oxide electrodes were modified with DNA and the guanines in the immobilized nucleic acid were used as a substrate for electrocatalytic oxid ation by Ru(bpy)(3)(3+) (bpy = 2,2'-bipyridine). Nucleic acids were deposit ed onto 12.6-mm(2) electrodes from 9:1 DMF/water mixtures buffered with sod ium acetate. The DNA appeared to denature in the presence of DMF, leading t o adsorption of single-stranded DNA. The nucleic acid was not removed by vi gorous washing or heating the electrodes in water, although incubation in p hosphate buffer overnight liberated the adsorbed biomolecule. Acquisition o f cyclic voltammograms or chronoamperomograms of Ru(bpy)(3)(2+) at the modi fied electrodes produced catalytic signals indicative of oxidation of the i mmobilized guanine by Ru(III). The electrocatalytic current was a linear fu nction of the extent of modification with a slope of 0.5 mu A/pmol of adsor bed guanine; integration of the current-time traces gave 2.2 +/- 0.4 electr ons/guanine molecule, Use of long DNA strands therefore gave steep response s in terms of the quantity of adsorbed DNA strand. For example, electrodes modified with a 1497-bp PCR product from the HER-2 gene produced detectable catalytic currents when as little as 550 amol of strand was adsorbed, givi ng a sensitivity of 44 amol/mm(2).