Abstract: Apparatus for the determination of a chemical or biochemical species comprises a resonant optical biosensor (1-4) disposed in a light path between a pivotally-mounted source (5) of monochromatic light and a stationary detector (12) adapted to monitor some characteristic of the light. There is provided means (7) for causing pivotal motion of the light source (5) so as to vary the angle of incidence of the light on the sensor (1-4). Also provided is means for monitoring the instantaneous angle of incidence. The means for varying the angle of incidence of the light on the sensor may be a cam arrangement (7) acting on a pivoting member (6) carrying the light source (5), and the means for monitoring the instantaneous angle of incidence of the light on the sensor (1-4) may comprise means for monitoring the number of steps performed by a stepper motor driving the cam arrangement (7).

Abstract: A sensor, particularly a resonant optical biosensor based on the principle of frustrated total reflection, includes an optical structure comprising: a) a cavity layer (3) of transparent dielectric material of refractive index n.sub.3, b) a dielectric substrate (1) of refractive index n.sub.1, and c) interposed between the cavity layer (3) and the substrate (1), a dielectric spacer layer (2) of refractive index n.sub.2. The arrangement is such that the optical structure may be illuminated by a beam of incident radiation, internal reflection occurring at the interface between the substrate (1) and the spacer layer (2). The device is characterised in that the cavity layer (3) or the spacer layer (2) absorbs at the wavelength of the incident radiation. The cavity layer (3) or the spacer layer (2) comprises a material which is either absorbing or is doped with an absorbing species, most preferably a fluorophore.

Abstract: A long-range SPR sensor includes: (a) a source of electromagnetic radiation, (b) an optical structure including a block of material transparent to the electromagnetic radiation, a spacer layer of dielectric material, a metallic layer, and a sensitive layer capable of reaction with a sample to be tested, and (c) apparatus for monitoring radiation from the source which is incident upon the block and which is internally reflected at the interface between the block and the spacer layer. A thin layer of dielectric material of high refractive index is interposed between the metallic layer and the layer of sensitive material. The provision of the thin layer of dielectric material between the metal layer and the sample under test enables the advantages of long-range SPR to be more fully realized, notably a very sharp resonance with corresponding improvements in sensitivity.