Light scattering microscopy from monolayers and nanoparticles at the air/water interface

Abstract

Light scattering microscopy (LSM) is introduced here as a versatile technique for the study of interfacial films at the air/water interface. Laser light scattered from the interface is collected by a microscope objective and imaged onto a CCD camera. LSM allows determination of the spatial distribution of submicron particles, phase transitions in two-dimensions, and defects. The power of LSM, especially if conducted simultaneously with fluorescence microscopy or Brewster angle microscopy (BAM), is illustrated in three examples. (a) Visualization of the spatial distribution of nanoscale particles with respect to monolayer phases: calcium oxalate crystals were grown underneath dipalmitoyl phosphatidyl choline (DPPC) and dimyristoyl phosphatidyl serine (DMPS) monolayers in the liquid expanded/liquid condensed (LE/LC) phase coexistence. The density of light scatterers was considerably higher in the respective LE than the LC phase, and a gradual migration of particles was observed towards the domain boundaries. A similar behavior was observed for nanocrystals injected underneath lipid monolayers. (b) Probing two-dimensional (2D) protein crystallization processes underneath functionalized lipid monolayers and the spatial distribution of crystal defects: streptavidin was crystallized as a model protein in 2D through coordination of exposed histidines on the protein surface to the monolayer-anchored metal chelator, Cu-DO-IDA. Vacancies were formed within the 2D protein crystals after injection of the soluble metal chelator EDTA, and the spatial distribution of vacancies was probed by LSM. (c) Detection of monolayer topographic transitions, corrugation and nanoscale budding: The phases of DPPC monolayers were studied under isothermal compression. New nanoscale topographic transitions are observed by LSM if DPPC is compressed into the liquid condensed (LC) state far below the collapse pressure.