Abstract
Liposomes are widely used as drug delivery carriers and as cell model systems. Here, we measure the dielectric properties of individual liposomes adsorbed on a metal electrode by in-liquid scanning dielectric microscopy in force detection mode. From the measurements the lamellarity of the liposomes, the separation between the lamellae and the specific capacitance of the lipid bilayer can be obtained. As application we considered the case of non-extruded DOPC liposomes with radii in the range ~ 100–800 nm. Uni-, bi- and tri-lamellar liposomes have been identified, with the largest population corresponding to bi-lamellar liposomes. The interlamellar separation in the bi-lamellar liposomes is found to be below ~ 10 nm in most instances. The specific capacitance of the DOPC lipid bilayer is found to be ~ 0.75 µF/cm2 in excellent agreement with the value determined on solid supported planar lipid bilayers. The lamellarity of the DOPC liposomes shows the usual correlation with the liposome's size. No correlation is found, instead, with the shape of the adsorbed liposomes. The proposed approach offers a powerful label-free and non-invasive method to determine the lamellarity and dielectric properties of single liposomes.
Highlights
Liposomes consist of single or multiple concentric lipid bilayers encapsulating an aqueous compartment
In the present work we demonstrate that the lamellarity of single liposomes can be determined in a noninvasive and label-free way by using scanning dielectric microscopy in force detection mode operated in liquid environment [26]
The magnitude measured in in-liquid Scanning Dielectric Microcopy (SDM) is the oscillation amplitude of the cantilever at the modulation frequency, Aωmod, from where the ac electric force acting on the cantilever at this frequency, Fωmod, is obtained
Summary
Liposomes consist of single or multiple concentric lipid bilayers encapsulating an aqueous compartment. Lamellarity, i.e. the number of concentric lipid bilayers in a liposome, is among the Lamellarity can be determined at the population level by using techniques such as Nuclear Magnetic Resonance, X-ray small angle scattering and fluorescence spectroscopy [16, 17]. These techniques provide the average lamellarity of the liposomes’ population. Lamellarity can be determined, at the single liposome level by considering imaging techniques such as cryo-electron
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