Scanning near field optical microscopy of phase separated regions in a mixed interfacial protein (BSA)-surfactant (Tween 20) film

Scanning near field optical microscopy (SNOM) has been used to visualize ph ase-separated regions in a monolayer protein film. The film was created at an air-water interface during displacement of the protein bovine serum albu min (BSA) by the nonionic surfactant Tween 20. Contrast was generated in th e SNOM images By fluorescent labeling of the protein. The validity of the S NOM methodology has been confirmed by a qualitative and quantitative compar ison of the displacement process as visualized by SNOM and atomic force mic roscopy (AFM) on fluorescently labeled BSA displaced by Tween 20 and AFM da ta on native BSA displaced by Tween 20. The data suggest that SNOM studies of labeled proteins can be used to investigate the behavior of particular p rotein species in complex multicomponent interfacial films.