Laser Transmission Spectroscopy and applications to liposome studies

We describe the implementation of precision laser transmission spectroscopy (LTS) for sizing nanoparticles in suspension. Our apparatus incorporates a tunable laser and balanced optical system which measures light transmission over a wide (210 -- 2300 nm) wavelength range with high precision and sensitivity. Spectral inversion was employed to determine both the particle size distribution and absolute density of particles with diameters over a total range of 5 to 3000 nm. LTS has a dynamic range of $\sim$ $10^{3}$ particles/mL to $\sim$ $10^{10}$ particles/mL ($5x10^{-8}$ vol.\% to 0.5 vol.\%). Currently, LTS is being applied as a tool to investigate the behavior of liposomes, dipalmitoylphosphatidycholine (DPPC) and dipalmitoylphosphatidylserine (DPPS), under the presence of fusing and de-aggregating agents. Our measurements indicate a maximum diameter of 400 nm for liposomes suspended in solution after fusion.