Pores with Undulating Opening Diameter can Determine Particles by Size and Shape

Abstract

The resistive-pulse technique has been applied to detect and size particles as they traverse through a pore. The pulse, defined as a change in current with respect to the baseline, carries information about the particle's volume. Pores characterized with an undulating opening diameter could be used to identify the shape of a passing object using the resistive-pulse technique. The experiments were performed with 12 micrometer long pores in polyethylene terephthalate (PET) prepared by the track-etching technique. Due to the laminar structure of PET films, the subsequent wet chemical etch formed longitudinal irregularities along the pore wall. The topography of each pore is examined nondestructively using polystyrene particles and observed as a series of peaks within a pulse. We studied the effects of the pore undulations on differently shaped particles by comparing pulses of polystyrene spherical beads with pulses created by silica rod-shaped particles with lengths of 590 and 1950 nm and diameter of ∼200 nm. To understand the undulating diameter's role, we also passed both particles through a smooth, 30 μm long polycarbonate (PC) pore. We found that the rods’ passage induced averaging of the pulses so that a smaller number of sub-peaks was observed compared to the pulses of the spheres. The degree of the pulse averaging could be correlated with the rod length. Pulses of beads and rods were virtually indistinguishable in the smooth PC pores, emphasizing the importance of the pore undulations’ ability to distinguish shapes. In studying non-spherical particles, we discovered that the rod's pulse amplitude was larger than expected from volume exclusion. This suggests that the pulse amplitude of non-spherical particles depends not only on particle volume, but also on its angular orientation and rotational motion as it travels across the pore.