Optical biosensors for real-time measurement of analytes in blood plasma

The preparation of assemblies consisting of multiple molecular layers of bovine serum albumin (BSA), monoclonal antibodies against horseradish peroxidase (anti-HRP), and monoclonal antibodies against methotrexate (anti-MTT), as well as interaction of the assemblies with human blood plasma were observed using a grating coupler and Young interferometer (YI). The assemblies could be arranged according to decreasing amounts of nonspecific deposits bound irreversibly to them from blood plasma as follows-an adsorbed antibody monolayer saturated with adsorbed BSA, antibody multilayers linked with polycations, antibodies covalently immobilized on a BSA layer densely crosslinked with glutaraldehyde (GA), slightly crosslinked BSA double layer, slightly crosslinked antibody double layers. The occurrence of human serum albumin (HSA), human fibrinogen (Fg), IgG, and IgM in the plasma deposits was studied by binding the respective antibodies. IgG, IgM, and Fg were detected in plasma deposits on the immobilized assemblies while the composition of a plasma deposit on the unmodified sensor surface reflected roughly the plasma composition containing mainly adsorbed HSA and Fg. A crosslinked anti-HRP double layer was immobilized on a waveguiding branch of YI and a similar anti-MTT double layer was immobilized on the other branch. The sensor response to blood plasma was fairly decreased owing to a compensation of the respective optical changes in the two branches, in which a similar non-specific adsorption took place. The addition of HRP or MTT to plasma induced specific responses of the corresponding branches.