Two-dimensional condensation of benzalkonium chloride at the mercury electrode and its relation to DNA imaging using scanning force microscopy

Abstract

We show two important properties of the cationic detergent benzalkonium chloride (BAC): (a) formation of condensed layers of BAC at the mercury electrode, and (b) ability of BAC to facilitate anchoring of DNA at the mica surface which makes it possible to visualize individual DNA molecules by scanning force microscopy (SFM). The adsorption behaviour of BAC (in the absence of DNA) was studied by ac voltammetry using a hanging mercury dropping electrode (HMDE) and a dropping mercury electrode (DME) at various temperatures and BAC concentrations. Two pits were observed on the capacitance-potential curve, one located at the potential of zero charge (pzc) and the other in a narrow potential range around −1.7 V. The pit at the pzc corresponds to a condensed layer formed by the BAC molecules which may be adsorbed via the hydrophobic alkyl chain pointing towards the electrode surface. The pit at the negatively charged electrode indicates a condensed layer in which the BAC molecules may be adsorbed in a different way, probably via the positively charged nitrogen. In both orientations the BAC molecules may have their benzene rings perpendicular to the electrode surface with stabilization of the condensed phase by the stacking interactions between the benzene rings. The condensation of BAC molecules on the electrode surface is stimulated by sulphate anions and hindered by bromide anions. Our results suggest that the adsorption behaviour of BAC at the liquid | liquid interface at negative charge densities can be used as a model for DNA + BAC spreading on mica as used in SFM.