Resistive-Pulse Detection of Multilamellar Liposomes

The resistive-pulse method was used to monitor the pressure-driven translocation of multilamellar liposomes with radii between 190 and 450 nm through a single conical nanopore embedded in a glass membrane. Liposomes (0% and 5% 1,2-dioleoyl-<i>sn</i>-glycero-3-phospho-l-serine (sodium salt) in 1,2-dilauroyl-<i>sn</i>-glycero-3-phosphocholine or 0%, 5%, and 9% 1,2-dipalmitoyl-<i>sn</i>-glycero-3-phospho­(1′-<i>rac</i>-glycerol) (sodium salt) in 1,2-dipalmitoyl-<i>sn</i>-glycero-3-phosphocholine) were prepared by extrusion through a polycarbonate membrane. Liposome translocation through a glass nanopore was studied as a function of nanopore size and the temperature relative to the lipid bilayer transition temperature, <i>T</i>_c. All translocation events through pores larger than the liposome, regardless of temperature, show translocation times between 30 and 300 μs and current pulse heights between 0.2% and 15% from the open pore baseline. However, liposomes at temperatures below the <i>T</i>_c were captured at the pore orifice when translocation was attempted through pores of smaller dimensions, but squeezed through the same pores when the temperature was raised above <i>T</i>_c. The results provide insights into the deformation and translocation of individual liposomes through a porous material

Tunable Negative Differential Electrolyte Resistance in a Conical Nanopore in Glass

Liquid-phase negative differential resistance (NDR) is observed in the <i>i–V</i> behavior of a conical nanopore (∼300 nm orifice radius) in a glass membrane that separates an external <i>low-conductivity</i> 5 mM KCl solution of dimethylsulfoxide (DMSO)/water (v/v 3:1) from an internal <i>high-conductivity</i> 5 mM KCl aqueous solution. NDR appears in the <i>i–V</i> curve of the negatively charged nanopore as the voltage-dependent electro-osmotic force opposes an externally applied pressure force, continuously moving the location of the interfacial zone between the two miscible solutions to a position just inside the nanopore orifice. An ∼80% decrease in the ionic current occurs over less that a ∼10 mV increase in applied voltage. The NDR turn-on voltage was found to be tunable over a ∼1 V window by adjusting the applied external pressure from 0 to 50 mmHg. Finite-element simulations based on solution of Navier–Stokes, Poisson, and convective Nernst–Planck equations for mixed solvent electrolytes within a negatively charged nanopore yield predictions of the NDR behavior that are in qualitative agreement with the experimental observations. Applications in chemical sensing of a tunable, solution-based electrical switch based on the NDR effect are discussed