Solution-gated organic thin film transistors (SGOTFT) have shown promising applications in biosensors due to the high sensitivity, low working voltage and the simple design of the devices. SGOTFTs normally 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 leads to the working voltages of the devices to be less than 1 V. On the other hand, SGOTFTs 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. Recently, we have developed many types of biosensors based on SGOTFTs. The sensing mechanisms are attributed to the potential changes across solid/electrolyte interfaces induced by electrochemical reactions of analytes or the interactions between the analytes and the devices. Most of the devices have shown ultrahigh sensitivity due to the inherent amplification function of the transistors.

I will introduce several types of SGOTFT-based biosensors for the detection of DNA/RNA, glucose, dopamine, uric acid, cells and proteins, which have been studied by our group recently. One major problem of the previously report SGOTFT-based biosensors is the poor selectivity, which should be solved in practical applications. We found that SGOTFT-based biosensors can show both 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. The devices were successfully used for the selective detections of uric acid level and glucose level in saliva, which renders the devices promising transducers for noninvasive detections of biomarkers in human body.

Since the organic devices are flexible and solution processable, we have developed the devices for wearable applications. SGOTFTs were prepared on plastic or fabric substrates by solution process and showed excellent stability in device performance during bending tests. SGOTFTs prepared on thin fibers were woven together with cotton yarns successfully by using a conventional weaving machine, resulting in flexible and stretchable fabric biosensors with high performance. The fabric sensors showed much more stable signals in the analysis of moving aqueous solutions than planar devices due to a capillary effect in fabrics. The devices were integrated in diapers for real-time analysis of urine and demonstrated glucose levels in urine with high sensitivity. Due to their low working voltages, the fabric devices were remotely operated by using a mobile phone, offering a unique platform for convenient wearable healthcare monitoring.

The analysis of protein biomarkers is of great importance in the diagnosis of diseases. Although many convenient and low-cost electrochemical approaches have been extensively investigated, they are not sensitive enough in the detection of protein biomarkers with low concentrations in physiological environments. We prepared a novel SGOTFT-based biosensor that can successfully detect cancer protein biomarkers with ultrahigh sensitivity. The devices are operated by detecting electrochemical activity on gate electrodes, which is dependent on the concentrations of proteins labelled with catalytic nanoprobes. The protein sensors can specifically detect a cancer biomarker, human epidermal growth factor receptor 2, down to the concentration of 10^-14 g/mL, which is several orders of magnitude lower than the detection limits of previously reported electrochemical approaches. Moreover, the devices can successfully differentiate breast cancer cells from normal cells at various concentrations. This work paves a way for developing highly sensitive and low-cost biosensors for the detection of various protein biomarkers in clinical analysis in the future.

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