Micro- and Nanoparticle Translocation through a Solid-State Membrane Pore Thinner than their Diameters

Abstract

Micron-sized particles have been detected by the resistive-pulse or Coulter counting technique since the 1950's (Coulter, US Pat. 2656508) with 90 nm particle detection reported in 1970 (DeBlois and Bean, Rev. Sci. Inst. 41, 909). Due to the challenges of fabricating submicron pores in those times, the pores used were all several microns in length. Nanofabrication has advanced significantly since then, and in this poster we examine the interesting case of particles translocating through a membrane pore thinner than those particles.With micron-long, high aspect ratio pores, a spherical particle will be entirely contained within the pore for the majority of the time it translocates. Thus, all but the beginning and end of the observed signal is due to the entire particle. In addition, pore resistance dominates over access resistance. In contrast, with low-aspect ratio pores thinner than the particle, the signal reflects the interactions of the particle with both the access resistance volumes and the pore volume. The signal is a complex combination of these interactions, that changes as the particle moves through the pore. Another advantage of shorter pores is that identical particles passing through longer pores give smaller modulations in current, because the proportional change in resistance due to the particle is smaller.We use a focused ion beam to drill nanopores tens to hundreds of nanometers in diameter in silicon nitride membranes, with thicknesses in the same range. Polystyrene microspheres are driven through these pores by an applied electric field. As they pass through, they modulate the flux of ions through the pore, and thus the current flowing between the electrodes. We present experimental and simulated studies of particle translocation, aimed at determining the optimum system for analyzing virus particles using this method.