SPRbased biosensors

Several optical biosensors are commercially available, all exploiting the evanescent wave and using SPR or other, wave-guiding techniques for detection. They include the BIAcore (Biacore AB, Uppsala, Sweden), IASYS (Affinity Sensors, Cambridge, UK), IBIS (Intersens Instruments, Utrecht, Netherlands) and BIOS-1 (Artificial Sensing Instruments, Zurich, Switzerland). The vast majority of published applications have used several different models of the BIAcore system and therefore the following descriptions are based on these instruments.

The BIAcore has a light source that produces a wedge of monochromatic light in order to provide a wide range of incident angles, all above the critical value for total internal reflection. The light passes through a prism and is focused on a small area (1.4x0.16 mm) on the surface of a disposable "sen-sorchip'. The latter consists of a glass support on to which a thin gold film is attached. For most applications, a carboxymethylated dextran layer (100 nm thick) is fixed on to the gold surface in order to provide a hydrophilic environment for immobilization of one of the interacting components (such as an antibody, peptide or any other macromolecule). The sensorship is sealed against a micro-fluidic deliver;, cartridge, so that the dextran layer is present within a flowcell of precise dimensions (60 nl volume), through which solutions can be passed under conditions of constant flow and temperature. Several methods are available for immobilizing molecules on the dextran layer; the most commonly employed uses standard succinimide chemistry for the covalent immobilization of proteins through free amino groups. Indirect methods may also be used, such as immobilizing streptavidin to capture biotinylated molecules.

The principle of detection is demonstrated in Figure 1 (as an example, antibody molecules are shown immobilized on the dextran layer). With buffer flowing through the flowcell, the medium close to the sen-sorchip surface has a certain refractive index which depends on the amount and size of the immobilized antibody and the composition of the buffer. SPR occurs at a certain angle, HI, of incident light and is detected as an intensity 'dip' in the reflected light by

Figure 1 Schematic outline of the principle of SPR measurements using the BIAcore biosensor (not in scale). See text for further details.

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