Simultaneous Multipass Resistive-Pulse Sensing and Fluorescence Imaging of Liposomes.

Simultaneous multipass resistive-pulse sensing and fluorescence imaging have been used to correlate the size and fluorescence intensity of individual E. coli lipid liposomes composed of E. coli polar lipid extracts labeled with membrane-bound 3,3-dioctadecyloxacarbocyanine (DiO) fluorescent molecules. Here, a nanopipet serves as a waveguide to direct excitation light to the resistive-pulse sensing zone at the end of the nanopipet tip. Individual DiO-labeled liposomes (>50 nm radius) were multipassed back and forth through the orifices of glass nanopipets' 110-150 nm radius via potential switching to obtain subnanometer sizing precision, while recording the fluorescence intensity of the membrane-bound DiO molecules. Fluorescence was measured as a function of liposome radius and found to be approximately proportional to the total membrane surface area. The observed relationship between liposome size and fluorescence intensity suggests that multivesicle liposomes emit greater fluorescence compared to unilamellar liposomes, consistent with all lipid membranes of the multivesicle liposomes containing DiO. Fluorescent and nonfluorescent liposomes are readily distinguished from each other in the same solution using simultaneous multipass resistive-pulse sensing and fluorescence imaging. A fluorescence "dead zone" of ∼1 µm thickness just outside of the nanopipet orifice was observed during resistive-pulse sensing, resulting in "on/off" fluorescent behavior during liposome multipassing.

Single-Molecule Electrical Currents Associated with Valinomycin Transport of K.

A quantitative description of ionophore-mediated ion transport is important in understanding ionophore activity in biological systems and developing ionophore applications. Herein, we describe the direct measurement of the electrical current resulting from K+ transport mediated by individual valinomycin (val) ionophores. Step fluctuations in current measured across a 1,2-diphytanoyl-sn-glycero-3-phosphocholine (DPhPC) bilayer suspended over a ∼400 nm radius glass nanopore result from dynamic partitioning of val between the bilayer and torus region, effectively increasing or decreasing the total number of val present in the membrane. In our studies, approximately 30 val are present in the membrane on average with a val entering or leaving the bilayer approximately every 50 s, allowing measurement of changes in electrical current associated with individual val. The single-molecule val(K+) transport current at 0.1 V applied potential is (1.3 ± 0.6) × 10-15 A, consistent with estimates of the transport kinetics based on large val ensembles. This methodology for analyzing single ionophore transport is general and can be applied to other carrier-type ionophores.