Simplicity, sensitivity and versatility of optical sensors based on competitive immunoassays using antibody-antigen reactions are achieved by solid-state, single-step reactions which permit accurate sensitive qualitative and quantitative information to be obtained without human participation. All of the chemistry-biochemistry is an inherent part of the sensor. A direct reaction occurs when the sample (antigen) is brought in contact with the sensor. The sensitivity of the competitive immunoassay optical sensor is controlled and increased by selecting a tag for the antigen or altering the attachment of a tag to an antigen so that the binding of tagged antigen to an antibody is decreased relative to the binding of untagged antigen to the antibody. The user can vary size, molecular weight and geometric configuration of the tagged antigen. This can be accomplished by selecting the proper tag or by attaching the indicator material to the antigen through a spacer or by attaching the tag directly to the antigen and attaching a compound of proper molecular weight and size elsewhere. Pretreatment of the substrate to which the antibody-bound tagged antigen is immobilized to block the surface from unwanted interferences and the use of optical isolation increases sensitivity. If the tag cannot be attached to the antigen, it is attached to the antibody. Attaching the controlled size, molecular weight tag to either the antigen or antibody permits the analysis of species not normally measureable by competitive immunoassay, thus increasing the versatility of the method.
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