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 extract labeled with membrane-bound 3,3-dioctadecyloxacarbocyanine (DiO) fluorescent molecules. Here, a nanopipette serves as a waveguide to direct excitation light to the resistive-pulse sensing zone at the end of the nanopipette tip. Individual DiO-labeled liposomes (>50 nm radius) were multipassed back and forth through the orifices of glass nanopipettes 110-to-150 nm radius via potential switching to obtain sub-nanometer 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 multi-vesicle liposomes emit greater fluorescence compared to unilamellar liposomes, consistent with all lipid membranes of the multi-vesicle liposomes containing DiO. Fluorescent and non-fluorescent 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 m thickness just outside of the nanopipette orifice was observed during resistive-pulse sensing, resulting in ‘on/off’ fluorescent behavior during liposome multipassing. Our results provide a path forward to simultaneously characterize the size of biologically relevant nanoparticles (e.g., extracellular vesicles) and the presence of fluorescently labeled surface proteins.