Evanescent field sensors based on waveguide surfaces play an important role where high sensitivity is required. Particularly tantalum pentoxide (Ta2O5) is a suitable material for thin-film waveguides due to its high refractive index and low attenuation. Many label-free biosensor systems such as grating couplers and interferometric sensors as well as fluorescence-based systems benefit from this waveguide material leading to extremely high sensitivity. Some biosensor systems based on Ta2O5 waveguides already took the step into commercialization. This report reviews the various detection systems in terms of limit of detection, the applications, and the suitable surface chemistry.

This paper describes the development of an optical readout system for the real-time analysis of fluorescent-labeled DNA microarrays is described. The system is targeted toward research applications in genomics, agriculture, and life sciences, where the end-point detection of state-of-the-art readout systems does not provide sufficient information on the hybridization process. The hybridization progress of molecules from the liquid phase in a flow cell to immobilized oligonucleotides on a transducer surface can be observed. The excitation of fluorochromes is realized by a semiconductor laser, and the fluorescence emission is collected by a cooled CCD camera. Quantitative data can be extracted from the images for analysis of the microarray. For the signal transduction, the principle of total internal reflection is used. With a multiple internal reflection arrangement, the sensor chip was adapted to the standard microscope slide format and a homogeneous evanescent illumination of the active area of the sensor surface was achieved. An application measurement was carried out with this readout system. The hybridization of Cy5-labeled 30-mer single-stranded oligonucleotides to fully complementary immobilized strands was observed in real time. A kinetic analysis was demonstrated with the recorded data. Melting curves of a 140-mer PCR product from a hemochromatosis patient sample hybridized to immobilized wild-type mutant 15- and 17-mer oligonucleotides were recorded and single-point mutations could be detected.

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Infrared attenuated total reflection (ATR) spectroscopy is a common laboratory technique for the analysis of highly absorbing liquids and solids. However, in a process environment, maintaining a sufficient sample exchange and cleaning of the sensitive surface of the element is a crucial issue. An important industrial application is the measurement of isocyanate concentrations. Isocyanates are necessary for the fabrication of polyurethane materials and are among the chemicals with the highest production volume worldwide. For process applications, narrowband photometers or MEMS spectrometers are more appropriate than the use of bulky FTIR instruments frequently encountered in a laboratory environment. Toluene diisocyanate (TDI) and hexamethylene diisocyanate (HDI) concentrations are measured with a planar ATR photometer setup. Using a miniature Fabry–Perot interferometer (FPI), trace concentrations below 100 ppm (m/m) are detected. By employing an ATR element of the cylindrical shape, sensors can be realized with a smooth surface ideally suited for an automatic cleaning system in a process environment. A laboratory setup with sapphire tubes as ATR elements for incorporation in a liquid flow system is described. Reflection and transmission configurations were investigated. Measurements with acetonitrile as a less toxic substitute showed that with cylindrical ATR sensors’ detection limits for isocyanate concentrations below 100 ppm (m/m) are feasible.

The losses of curved waveguide structures have been analyzed with respect to optimal S -bend design. Directional couplers consisting of curved waveguides fabricated by ionexchange in glass have been investigated. A theoretical model for the coupling ratio with good agreement to the experimental results has been derived.