Impedance sensing of DNA binding drugs using gold substrates modified with gold nanoparticles

Interfacial interactions between immobilized DNA probes and DNA-specific sequence binding drugs were investigated using impedance spectroscopy toward the development of a novel biosensing scheme. The impedance measurements are based on the charge-transfer kinetics of the [Fe(CN)₆]^3-/4- redox couple. Compared to bare gold surfaces, the immobilization of DNA and then the DNA-drug interaction on electrode surfaces altered the capacitance and the interfacial electron resistance and thus diminished the charge-transfer kinetics by reducing the active area of the electrode or by preventing the redox species from approaching the electrode. Electrochemical deposition of gold nanoparticles on a gold electrode surface showed significant improvement in sensitivity. DNA-capped gold nanoparticles on electrodes act as selective sensing interfaces with tunable sensitivity due to higher amounts of DNA probes and the concentric orientation of the DNA self-assembled monolayer. The specificity of the interactions of two classical minor groove binders, mythramycin, a G-C specific-DNA binding anticancer drug, netropsin, an A-T specific-DNA binding drug and an intercalator, nogalamycin on AT-rich DNA-modified substrate and GC-rich DNA-modified substrate are compared. Using gold nanoparticle-deposited substrates, impedance spectroscopy resulted in a 20-40-fold increase in the detection limit. Arrays of deposited gold nanoparticles on gold electrodes offered a convenient tool to subtly control probe immobilization to ensure suitably adsorbed DNA orientation and accessibility of other binding molecules.