Quantification of particle number concentration in liposomal suspensions by Laser Transmission Spectroscopy (LTS)

Laser Transmission Spectroscopy (LTS) is an experimental technique able to determine the particle number concentration and the size of colloidal suspensions by a single measurement of the transmittance of a laser beam through the suspension of particles as a function of the wavelength. In this protocol, we show that LTS represents a unique and powerful tool to investigate suspensions of liposomes, where the precise quantification of the number concentration is particularly relevant for the complete definition of the colloidal properties of the suspension. We study a model formulation of Soy-PC:Chol liposomes and we validate LTS results by comparison with High-Performance Liquid Chromatography determination of lipid mass. Then LTS protocols is applied to state-of-art liposomal nanocarrier suspensions. We explain details of data analysis to obtain the particle number concentration by using the Lambert-Beer law and by calculating the extinction cross section, within the framework of Mie theory for spherical vesicles. We also determine the liposome radius and compare it with the hydrodynamic radius measured by Dynamic Light Scattering. As future perspective, we aim to extend LTS analysis to other nanostructures with different geometries and to contribute to the development of new quantitative strategies for the accurate characterization of nanocarriers and other nanoparticles.