Electrochemical detection of the immobilization and hybridization of unlabeled linear and hairpin DNA on gold

Unlabeled hairpin and linear DNA probes have been immobilized on polycrystalline gold electrodes by forming mixed SAMs of thiolated single-stranded oligonucleotides (HS-ssDNA) and a spacer thiol (4-mercaptobutan-1-ol) by spontaneous coadsorption. Chronocoulometry was used to determine the amount of DNA present in the mixed SAM. It was found that linear ssDNA achieves a higher surface concentration compared to hairpin ssDNA under the same deposition conditions. The hybridization behaviour of these SAMs was analysed by impedance spectroscopy in the presence of [Fe(CN)6]3−/4−. Addition of the complementary oligonucleotide to the linear ssDNA modified gold electrode resulted in an increase of the electron transfer resistance Rct, while it decreased in the case of the hairpin DNA probe. These results are due to the smaller surface coverage and the conformational change of the DNA hairpin probe upon hybridization. Hybridization experiments with a non-complementary sequence resulted in no change of the impedance spectra.