Voltammetric characterization of DNA intercalators across the full pH range: anthraquinone-2,6-disulfonate and anthraquinone-2-sulfonate.

The use of anthraquinone and its derivatives, notably the sulfonate and disulfonate salts, for the detection of DNA via electrochemical techniques, has been the focus of a number of recent articles. This study provides a quantitative model of the two redox systems of anthraquinone-2,6-disulfonate and anthraquinone-2-sulfonate, over the full aqueous pH range (0-13); the model is based upon the theoretical "scheme of squares" for a 2H(+), 2e(-) system, as first proposed by Jacq (Jacq, J. J. Electroanal. Chem. 1971, 29, 149-180). The effect of pH and ionic strength on the observed cyclic voltammetry was investigated experimentally. The variation of the electrochemical response with proton concentration was modeled through use of the commercially available simulation software, DIGISIM; the system was successfully fitted with attention to voltammetric peak height, position, width, and shape. The model demonstrates how the pK(a) values of the anthraquinone intermediates dominate the observed pH dependence of the voltammetry. At high pH (above pH 12), a simple EE process is found to occur. As the pH decreases, the formation of other protonated species becomes possible; this not only causes a Nernstian shift in the measured electrochemical potential for the redox couple but also results in changes in the mechanistic pathway. At pH 10, an EECC process dominates, as the pH is further lowered into the range 4-7, the overall mechanism is an ECEC process, and finally a CECE mechanism operates at around pH 1 and below. This work provides physical insight into the complex mechanistic pathways involved and will aid the future development of more sophisticated and accurate anthraquinone based DNA sensors.