A novel biosensor based on a planar water gated organic field effect transistor using biotin functionalized polythiophene

In this thesis, a novel affinity type biological sensor based on a planar water-gated field effect transistor (WGOFET) is introduced and investigated. The liquid electrolyte layer in the WGOFET is used as the gate insulator and presented the opportunity to add water soluble biomolecules such as streptavidin to the transistor. It has a simple and low-cost fabrication process and can be integrated to microfluidic channels. It works below 1 volt. Devices are fabricated on a glass wafer by patterning the same Cr/Au (5nm/50nm) thin film to form source, drain and gate electrodes. Functionalized polythiophene polymers with biotin were synthesized and dissolved in chlorobenzene with a ratio of 5 mg/mL-1. The polymer solution is then spin coated on the wafer to form the active layer of the device. Streptavidin solutions with three different concentrations of 0.4, 1 and 2.5 μg/mL are prepared and put as a droplet into the O-ring placed over the active region of the device. A cover glass is used to confine the droplet. IDS-VDS and IDS-VGS curves of the devices are measured at specific intervals for 30 minutes. A typical on-off current ratio of 220, threshold voltage of -0.2 V and mobility of 3x10-3 cm2V-1s-1 are measured. On-current of 270 nA is achieved at -0.5 V VDS and -1 V VGS. The on-current value drops to 80 nA after 30 minutes of specific interaction between streptavidin and biotin molecules. The on-current values drop with the increased concentrations of streptavidin.