Design for supported planar waveguides for obtaining mid-IR evanescent-wave absorption spectra from biomembranes of individual cells

We have been developing miniature planar Ge waveguides to detect mid-IR evanescent-wave absorption spectra from the cell membranes of individual intact frog eggs, 1.5-mm in diameter, from Xenopus laevis, with the aim of detecting and analyzing transient conformational changes of voltage-gated ion channel proteins in the membrane. Here we use waveguide optical theory to calculate optimal dimensions for a germanium waveguide to be used as a multiple-internal-reflection ATR element for this purpose. We assume that light from a standard broad-band IR source is coupled efficiently into and out of a planar Ge waveguide, and then onto a small-area MCT detector, by using an IR microscope with a high-numerical-aperture objective. To increase the coupling efficiency even further, we assume that we can fabricate the waveguide with a gradual 7-fold-tapering to the tiny dimensions needed in the sensing region. Then, assuming that ∼10⁷ ion channel proteins in an occyte can be made to contact an area of a planar Ge waveguide up to ∼200 μm in diameter, we calculate that voltage-gated structural changes in these channel proteins should produce absorbance change signals of ∼10^-6 if the waveguide sensor thickness is set near the optimal thickness of ∼1 μm and the sensor region length is limited to 100 μm. If such a sensor can be fabricated, we calculate that detection of the predicted voltage-gated absorbance changes with a commercial FT-IR microscope should be possible after ∼20 min of signal averaging.