Solution-gated transistors have shown promising applications in biosensors due to the high sensitivity, low working voltage and the simple design of the devices. Solution-gated transistors normal have no gate dielectric and the gate voltages are applied directly on the solid/electrolyte interfaces or electric double layers near the channel and the gate, which lead to very low working voltages (about 1 V) of the transistors. On the other hand, the devices can be easily prepared by solution process or other convenient methods because of the much simpler device structure compared with that of a conventional field effect transistor with several layers. Many biosensors can be developed based on the detection of potential changes across solid/electrolyte interfaces induced by electrochemical reactions or interactions. The devices normally can show high sensitivity due to the inherent amplification function of the transistors. Here, I will introduce several types of biosensors studied by our group recently, including DNA, glucose, dopamine, uric acid, cell, and bacteria sensors, based on solution-gated organic electrochemical transistors or graphene transistors. The biosensors show high sensitivity and selectivity when the devices are modified with functional nano-materials (e.g. graphene, Pt nanoparticles) and biomaterials (e.g. enzyme, antibody, DNA) on the gate electrodes or the channel. Furthermore, the devices are miniaturized successfully for the applications as sensing arrays. It is expected that the solution-gated transistors will find more important applications in the future. Some representative works are presented as follows.Figure 1 shows an organic electrochemical transistor (OECT) based on poly(3,4-ethylenedioxythiophene): poly(styrene sulfonic acid) (PEDOT:PSS) integrated in a flexible microfluidic system. We find that the device performance is not influenced by the bending status of the device. A novel label-free DNA sensor is developed using the OECT with single-stranded DNA probes immobilized on the gate electrode. The device can successfully detect a complementary DNA target down to 1 nM. The detection limit is extended to 10 pM by pulse-enhanced hybridization process of DNA in the microfluidic channel. Therefore OECTs are excellent candidates for flexible and low-cost biosensors.Figure 2 shows a solution-gated graphene transistor with graphene as both channel and gate electrodes. The device is used as a dopamine sensor with the detection limit down to 1nM, which is three orders of magnitude better than that of conventional electrochemical measurements. The sensing mechanism is attributed to the change of effective gate voltage applied on the transistors induced by the electro-oxidation of dopamine at the graphene gate electrodes. The interference from glucose, uric acid and ascorbic acid on the dopamine sensor is characterized. The selectivity of the dopamine sensor is dramatically improved by modifying the gate electrode with a thin Nafion film by solution process. This work paves the way for developing many other biosensors based on the solution-gated graphene transistors by specifically functionalizing the gate electrodes. Because the devices are mainly made of graphene, they are potentially low cost and ideal for high-density integration as multifunctional sensor arrays.

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