Modified monolayer electrodes for electrochemical and piezoelectric analysis of substrate-receptor interactions: Novel immunosensor electrodes

Monolayer-modified Au-electrodes were used to analyze electrochemically host-guest binding interactions of biomaterials. Two configurations to sense the binding of an antibody and a lectin to the complementary substrate monolayer are addressed. In one configuration, a fluorescein monolayer was assembled on an Au-electrode and binding of the complementary anti-fluorescein antibody Flc-Ab was followed by the examination of electrode insulation by the antibody towards a solubilized redox probe, Fe(CN)^3-₆/Fe(CN)^4-₆. The extent of electrode insulation is controlled by the Flc-Ab concentration in the sample and the electrode responds amperometrically to Flc-Ab concentrations as low as 0.71 μM. The second configuration applies a redox-modified protein to analyze competitively the protein itself. An Au-electrode was modified by an α-D-mannopyranose monolayer, and a bipyridinium-modified concanavalin A was used to analyze concanavalin A (Con. A). Competitive binding of the redox-modified Con. A and the analyzed Con. A to the monolayer-modified electrode occurred, and the amperometric response was inversely proportional to the Con. A concentration. Quartz crystals coated with Au-electrodes were applied for the piezoelectric QCM analyses of Flc-Ab and Con. A. The crystal electrodes are modified with a fluorescein antigen monolayer. The Flc-Ab was sensed by the changes in the crystal frequencies as a result of the antibody association to the electrode. Flc-Ab at a concentration as low as 5 ngml^-1 was detected. The series of monosaccharides α-D-mannopyranose, β-D-glucose or α-D-glucose was assembled onto the Au-electrodes of the quartz crystals and used as a sensing interface for concanavalin A. The α-D-mannopyranose monolayer revealed high affinity for the binding of Con. A, whereas the β-D-glucose monolayer showed lower affinity for the protein, and the α-D-glucose monolayer lacked association to Con. A. The monolayer-modified quartz crystal electrodes revealed specificity for the respective complementary proteins.