Submicron fluorescence, topology, and compliance measurements of phase-separated lipid monolayers using tapping-mode near-field scanning optical microscopy

Tapping-mode near-field scanning optical microscopy (TM-NSOM) is used to probe the structure and compliance properties of phase separated L-alpha-dipalmitoylphosphatidylcholine (DPPC) lipid monolayers doped with 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (diIC(18)) fluorescent dye. Near-field fluorescence images of monolayers deposited in the solid condensed/liquid condensed coexistence region reveal a threadlike network of liquid condensed lipid connecting the larger liquid condensed domains. Simultaneously collected TM-NSOM topology images of the lipid monolayer show small height differences of 5 Angstrom between the two lipid phases. Sample compliance measurements taken by monitoring the phase of the TM-NSOM tip resonance show that the two lipid phases exhibit distinctly different mechanical properties. Comparison of the near-field fluorescence, topography, and compliance measurements on DPPC monolayers reveals structures suggestive of the presence of lipid microdomains.