An Integrated Optical Biosensor (IOBS)

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Abstract
We have demonstrated an integrated optical biosensor (IOBS) employed as differential refractometer with a resolution down to nC.=5 × 10−5. The minimum volume required for measurements is V=Lx∗Ly∗Lz=1∗10−4 microlitres where Lx∗Ly=1mm∗0.1mm is the illuminated grating area and Lz is the penetration depth of the evanescent field of the guided mode into the cover medium. The sensor described was successfully used to monitor enzyme activities by measuring the change of refractive index of a substrate converted by an enzyme. Because the sensor responds not only to refractive index changes of a cover medium but also to thickness changes of absorbed layers it should be possible to measure biological interactions such as between antigen/antibody, inhibitor/enzyme or receptor/cell by coupling of either respective biomolecule onto the waveguiding film. Thus we have shown in preliminary experiments that on coupling covalently to the grating rabbit anti-human-IgG (both of similar molecular weight around 150 000) a pronounced attenuation of the intensity of the guided mode was observed due to the increased adlayer thickness (to be published). It should be possible to miniaturize the unit using channel waveguides fabricated by a combination of photolithography and etching techniques with the aim to obtain implantable clinical integrated optical biosensors. The sensitivity can be further improved electronically by measuring directly the intensity of the guided mode using a second grating as an output coupler. This will increase the signal to noise ratio substantially.