Evanescent-wave IR spectroscopy of single-bilayer membranes coated on chalcogenide fibers: sensitivity improvements using a diamond rod coupler between fiber and source

By means of evanescent wave absorption spectroscopy of single-bilayer biological membranes coated on optical fibers, we have measured such phenomena as lipid phase transitions and incorporation into the bilayer of small antibiotic peptides. A key innovation which has resulted in a substantial increase in sensitivity is our use of a 500 μm diameter diamond rod as a direct optical coupling device between an IR blackbody source and the 500 μm optical fiber. One of the rod's ends is in direct contact with the hot SiC source, while the other abuts the cleaved end of the AsSeTe fiber. The fiber passes through a custom-built Langmuir-Blodgett trough, then into the emission port of our spectrometer where its output is collimated into the interferometer. An important property of our method of coupling light into the fiber is that it allows the excitation of a very wide range of the fiber's optical modes. This has allowed us to obtain excellent evanescent-wave spectra of liquids even using fibers with the protective plastic coating still on them.