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

Light scattering microscopy (LSM) is introduced here as a versatile techniq ue for the study of interfacial films at the air/water interface. Laser lig ht scattered from the interface is collected by a microscope objective and imaged onto a CCD camera. LSM allows determination of the spatial distribut ion of submicron particles, phase transitions in two-dimensions, and defect s. The power of LSM, especially if conducted simultaneously with fluorescen ce microscopy or Brewster angle microscopy (BAM), is illustrated in three e xamples. (a) Visualization of the spatial distribution of nanoscale particl es with respect to monolayer phases: calcium oxalate crystals were grown un derneath dipalmitoyl phosphatidyl choline (DPPC) and dimyristoyl phosphatid yl 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 particl es was observed towards the domain boundaries. A similar behavior was obser ved for nanocrystals injected underneath lipid monolayers. (b) Probing two- dimensional (2D) protein crystallization processes underneath functionalize d lipid monolayers and the spatial distribution of crystal defects: strepta vidin was crystallized as a model protein in 2D through coordination of exp osed histidines on the protein surface to the monolayer-anchored metal chel ator, 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 tra nsitions, corrugation and nanoscale budding: The phases of DPPC monolayers were studied under isothermal compression. New nanoscale topographic transi tions are observed by LSM if DPPC is compressed into the liquid condensed ( LC) state far below the collapse pressure. (C) 2000 Elsevier Science B.V. A ll rights reserved.