Researchers from the US and Germany have developed a new polymeric material that allows organic thin-film transistors (OTFTs) to operate stably in water [Roberts et al., Proc. Natl. Acad. Sci. (2008) doi: 10.1073/pnas.0802105105]. The advance could be a boon for low-cost, disposable chemical and biological sensors.

OTFTs are attractive for sensing applications because they can be fabricated on large-area, flexible substrates and have active layers that can be tuned to detect a variety of different analytes. Exposing OTFTs to a variety of solvents in the vapor phase produces a change in the device current — which is straightforward to detect. But the high operating voltages, degradation, and delamination of OTFTs under humid or aqueous conditions have limited their use as sensors in real applications.

Zhenan Bao's team at Stanford University, together with colleagues from the Max Planck Institute for Polymer Research, have created OTFTs that operate at low voltage and is stable under water.

The device relies on a new cross-linked polymer gate dielectric and a stable organic semiconductor. “We successfully cross-linked poly(4-vinylphenol) or PVP with commercially available dianhydride molecules at relatively low temperatures, yielding well-insulated films with high capacitance,” explains Bao.

To test the sensing capabilities of the OTFT, the researchers then constructed an elastomeric flow cell directly on the surface. The OTFT is sensitive to trinitrobenzene down to 300 ppb, glucose down to 10 ppm, and cystine down to 100 ppb.

“OTFTs can be used to detect low concentrations of chemicals in a complex environment without encapsulation,” says Bao. The researchers are now working on a variety of other interesting analytes including the explosive trinitrotoluene, a chemical warfare nerve agent, and DNA.

It is a big plus for OTFTs to be able to interact chemically with many different analytes, says Ananth Dodabalapur of the University of Texas at Austin. “[The results] are very interesting in that low voltages are used to operate the organic transistor, which is very helpful in avoiding ionic currents,” he adds. “This is an important advance.”