Role of Conducting Polymers on Potentiometric and Floating Gate Field Effect Transistor (FGFET) Array-Based Immunoassays

Semiconductor-based diagnostics have revolutionized genome sequencing since the application of ion-sensitive field effect systems for the monitoring of pH changes during the polymerization of DNA. Similar charge-dependent changes can be monitored during protein-protein binding as in antibody-antigen reaction-based immunoassays. Organic semiconductors have several beneficial effects with such CMOS FET arrays. First, the organic semiconducting layer makes the covalent conjugation of the biospecific ligands on the gate surface very convenient: We have successfully used both thiolation and glutaraldehyde linkage, especially with polyaniline layer on the floating gate. Interfacial electrochemical polymerization gives the important control of surface area with the nanofiber formation for improved sensitivity. Depending on the device architecture, the band-gap engineering for optimal Schottky barrier performance or the additional capacitive effects, both give amplified threshold voltage changes.

We have designed and manufactured autonomic floating gate (FG)  field-effect transistor (FET) arrays and used them for monitoring biological reactions The advantage of FGFET array is that with a floating gate, the threshold voltage doesn't need to be “scanned”, but only the shifts in the threshold can be monitored. We compared the FET array in both IS (with reference electrode) and FGFET architecture to follow pH changes and bindings of cationic and anionic polyelectrolytes, always with a conducting polymer layer coated on the floating gate.  Further on, we followed the change in the threshold voltage during protein binding on the surface, and also the effect of enzymatic substrate reacting with the conjugated enzyme as in the traditional ELISA test.