Modification of Indium Tin Oxide Electrodes with Nucleic Acids:  Detection of Attomole Quantities of Immobilized DNA by Electrocatalysis

Indium tin oxide electrodes were modified with DNA, and the guanines in the immobilized nucleic acid were used as a substrate for electrocatalytic oxidation by Ru(bpy)3 3+ (bpy = 2,2‘-bipyridine). Nucleic acids were deposited onto 12.6-mm2 electrodes from 9:1 DMF/water mixtures buffered with sodium acetate. The DNA appeared to denature in the presence of DMF, leading to adsorption of single-stranded DNA. The nucleic acid was not removed by vigorous washing or heating the electrodes in water, although incubation in phosphate buffer overnight liberated the adsorbed biomolecule. Acquisition of cyclic voltammograms or chronoamperomograms of Ru(bpy)3 2+ at the modified electrodes produced catalytic signals indicative of oxidation of the immobilized guanine by Ru(III). The electrocatalytic current was a linear function of the extent of modification with a slope of 0.5 μA/pmol of adsorbed guanine; integration of the current−time traces gave 2.2 ± 0.4 electrons/guanine molecule. Use of long DNA strands therefore gave steep responses 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, giving a sensitivity of 44 amol/mm2.