Charged Particles Modulate Local Ionic Concentrations and Cause Formation of Positive Peaks in Resistive-Pulse-Based Detection

Abstract

We study the effect of electrolyte concentration on the shape of ion current pulses in resistive-pulse sensing. We show that electrokinetic passage of several hundred nanometers in diameter charged polystyrene particles through a micropore leads to formation of current increase when the particles exit the pore. The particle entrance, as reported before, causes formation of the current decrease, which is a measure of the particle size. Formation of the double peak, i.e., current decrease followed by a current increase, is especially pronounced if the resistive-pulse experiments are carried out in KCl concentrations below 200 mM. In order to explain the pulse shape, experiments were designed in which the particles passed through the pore only by either electroosmosis or electrophoresis. The presented experiments and modeling indicate that while both electroosmosis and electrophoresis affect the ion current pulse, formation of the positive peak is mainly determined by the latter effect and the charged state of the particle. The importance of the findings for resistive-pulse analysis is discussed.