Near-field infrared imaging of molecular changes in cholesteryl oleate by free electron laser infrared ablation

Abstract

We have applied infrared near-field scanning optical microscopy (IR–NSOM) to enable evaluation of detailed molecular changes in cholesteryl oleate, a primary cause of arteriosclerosis. In our IR–NSOM, a wide wavelength range of 2.9–6.7 μm is achieved by use of an optical parametric amplifier and an apertured cantilever. IR irradiation from a free-electron laser (FEL) tuned to a 5.75 μm wavelength induced molecular structural changes and caused cholesteryl oleate to decompose to cholesterol and fatty acids in the FEL irradiated areas. The IR–NSOM images at two different wavelengths, 5.75 and 5.3 μm, with a 2 μm apertured cantilever probe successfully identified areas of molecular change in cholesteryl oleate beyond the diffraction limit of IR microspectroscopy. In-depth molecular structure changes were also evaluated by the IR–NSOM and we demonstrated that the FEL irradiation induced subsurface molecular structure changes throughout cholesteryl oleate in the irradiated areas.