Surface titration of DNA-modified gold electrodes with a thiol-tethered anthraquinone

The adsorption kinetics of an anthraquinone derivative bearing a two-carbon amido side chain terminated with a thiol (9,10-anthraquinone-2-carboxylic acid (2-mercaptoethyl) amide) have been examined on bare gold electrodes and on gold electrodes that had been modified with SAMs of 6-mercaptohexanol and thiol-tagged DNA 20-mers. This ‘thiol-tethered AQ’ formed stable SAMs on gold electrodes that exhibited reversible voltammetric waves, with a 60 mV per pH dependence for the 2e−/2H+ quinone/hydroquinone couple, and that persisted on the electrode without change upon immersion in solutions of 2-mercaptoethanol. While the formation and maximum surface coverage of the thiol-tethered AQ SAMs was influenced by the pH and ionic strength of the deposition solution, the voltammetry was not markedly dependent on the presence of DNA in solution. Similarly, when a gold electrode was modified with either a mixture of disulfide end-tagged single-strand (ss) DNA 20-mers and the thiol-tethered AQ or a mixture of end-tagged double-strand (ds) DNA 20-mers and the thiol-tethered AQ, the voltammetry of the quinone couple was unchanged. Differentiation between the presence of either ss-DNA or ds-DNA on the electrode surface was achieved by titration of the free surface area with the thiol-tethered AQ after denaturation of the surface duplex in pure buffer. In the case of ds-DNA, but not for the ss-DNA modified electrode, a marked increase in the surface coverage of the thiol-tethered AQ was observed.