Determining an Equivalent Circuit Model for DNA Electrochemistry with Electrochemical Impedance Spectroscopy

DNA electrochemistry has proven beneficial for understanding fundamental charge transport features of DNA, DNA-protein interactions, enzymatic kinetics, and DNA-damaging anticancer drug activity. Still, fundamental insight on the overall electrical and electrochemical behavior of DNA electrochemistry is elusive due to the interplay of ionic and electronic effects. Electrochemical Impedance Spectroscopy (EIS) has been proven to be a useful tool in analyzing the ionic and electronic features of electrochemical systems. Here, we use EIS with equivalent circuit modeling to develop a fundamental equivalent circuit to represent our DNA electrochemistry system. When analyzing a working electrode modified with electrochemically active DNA monolayers and a mercaptohexanol backfilling agent, key capacitive and diffusive elements were uncovered. A modeled parallel-plate capacitor with 6.7-nm spacing matched to the height of the DNA monolayer revealed a solution dielectric constant of 76, consistent with an aqueous buffer solution. A capacitor attributed to double layer formation yielded a characteristic spacing of 0.9 nm, closely matching the ionically-blocking mercaptohexanol agent. A modeled Warburg diffusion element produced a diffusion constant of 6.2 x 1010 cm2/s, consistent with sodium ion diffusion across the DNA monolayer. Comparing to a DNA-free control revealed that the negatively charged DNA monolayer drew approximately triple the ionic charge to the electrode. surface. Additional mechanistic insight was revealed through protein studies with DNA helicases. These experiments provide a different outlook on the ionic and electronic features of DNA, giving a better understanding of how DNA reacts to charge.