LAUSR

In the resistive-pulse technique, individual particles passing through a pore cause transient changes of the transmembrane current, and the amplitude of the current change corresponds to the object size. This method has been applied to detect cells, viruses, molecules (e.g. DNA and proteins) and particles. With the special structure of track-etched polycarbonate pores, i.e. narrower entrances and cylindrical interior, the events of highly carboxylated charged polystyrene particles can show a current decrease when a particle enters the pore, a flat region in the center part, and a current increase when the particle exits the pore. The current increase results from concentration polarization as well as ionic sourcing effects, which depend on the particle surface charge density. Thus, the characteristics of the obtained resistive-pulse signal carry information not only on the size of the objects, but also on dynamic changes of the particle surface charge via protonation/deprotonation of surface carboxyl groups. Experiments were performed with a pH gradient set across the pore, so that the particles while translocating would change their effective surface charge due to the dependence of protonation of carboxyl groups on the local proton concentration. Analysis of the current increase at the end of the event revealed that the process of deprotonation/protonation required longer time than the transit time. This finding was unexpected, because the transit times were tens of milliseconds thus significantly longer then the chemical reaction time estimated based on proton diffusion. Our experiments also indicated that a longer pore and a higher pH gradient led to a more complete protonation/deprotonation in the pore during transit. Using our finding, we designed a protocol to trap individual particles in a pore for as long as 3 seconds. The experiments are important for developing new methods which can be used to in situ characterize and precisely control single molecules and particles.