Conductive and biphasic pulses in tunable resistive pulse sensing.

This experimental study concerns the occurrence of biphasic pulses generated during tunable resistive pulse sensing (TRPS) of 200 nm carboxylate polystyrene spheres. In TRPS, a short-lived pulse in ionic current is observed when an individual colloid passes through a pore which separates two fluid reservoirs. The pulse is conventionally resistive, but conductive pulses are observed under certain experimental conditions, as well as biphasic pulses which include both resistive and conductive components. The experimental variables investigated here include the concentration of the phosphate-buffered saline electrolyte, particle charge, pore size, applied voltage, and the direction of particle motion. The onset upper electrolyte concentration for biphasic pulses in TRPS is ∼50 mM, and the ordering of biphasic pulse components can be interpreted using ionic concentration polarization if the conductive component is generated when the particle is in the ion depletion region. Besides providing fundamental understanding, the results are important for the TRPS technique, which is becoming widely used for particle-by-particle measurements of submicron colloids.