Electrophoresis and Electroomosis Influence Local Ionic Concentrations and Shape of Ion Current Pulses in Resistive-Pulse Based Detection

Abstract

Passage of single molecules and particles through pores is the basis of resistive-pulse sensing. The relative amplitude of the resistive pulse, expressed as a percentage change of the baseline current, carries information on the particle size; thus in order to assure high, easy to detect signals, it is often preferred to perform these experiments at high ionic strengths of the background electrolyte. In cases when particle agglomeration becomes a problem, solutions with low salt content (e.g. 10 mM KCl) must be used to enhance electrostatic repulsion. We performed systematic studies on the effects of ionic strength on the detection of negatively charged polystyrene particles in single polymer pores. We show the shape of the resistive pulse is influenced by the background electrolyte concentration. In all studied solutions, a particle's entrance caused a current decrease, which was used to calculate the effective size of the particles. In salt concentrations up to 400 mM, the current decrease was followed by a current increase whose amplitude decreased with increasing KCl concentration. The current increase occurred at the end of the translocation event, and its magnitude sometimes exceeded the amplitude of the current blockage. The current increase was observed only in cases when the particles moved electrophoretically through a pore whose walls carried charge of the same sign as the particles. The charged walls induced the process of electroosmosis, which acted in the opposite direction to the electrophoretic particle movement. The performed continuum modeling revealed a critical importance of electroosmosis in the formation of transient increase of ionic concentrations caused by an exiting particle, recorded as an increase of ionic current.