The optical waveguide device can be considered to be an optical bench. As such, a wide variety of optical experiments can, in principle, be carried out either sequentially or, preferentially, simultaneously. Rather than listing the variety of such experiments, for the sake of brevity, just one extension to the DPI technique will be briefly discussed here.
Perhaps the most obvious additional experiment is to measure the extinction coefficient (the losses) of the sensing waveguide. These can be determined by measuring the contrast of the fringe image, which is a ratio-
1.5 mMol CaCI2 3.0 mMol NaCI
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Fig. 10. The response of immobilised tissue transglutaminase to sodium and calcium chloride, demonstrating clear conformational changes with calcium but not with sodium. Dashed lines raw data, Solid lines resolved data, thickness and refractive (Ref) index metric measure of the light passing down the upper (sensing) waveguide, and that passing down the lower (reference) waveguide. This data provides information on the structure of assemblies on the waveguide surface (e. g. nucleated or stochastic structures) and the degree of order (optical anisotropy) that they exhibit. Fringe image contrast has already been shown to provide a very valuable indicator of the early onset of crystallisation. Data relating to extinction coefficients is actively being developed at the present time. It has been demonstrated that the losses are not diffraction limited. Losses have been observed with particulates that are smaller than A/20 (i. e. less than 32 nm). A full analytical solution for waveguide loss is now being sought to provide yet further information on the structure and orientation of proteins and the solid-liquid interface.
The other major area in which rapid developments are being made is the area of biological membrane mimics. There is considerable interest in proteins that reside in membranes and there is a strong requirement for the provision of biologically relevant membrane mimics on sensor surfaces. DPI is a particularly relevant measurement technique, as liposome and bilayer constructs can be monitored on the sensor surface, allowing the engineering of constructs that are similar to those found in nature. Technical progress and surface offerings relating to this area are expected to be dramatic.
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