Accurately measuring small shifts in biological markers, like proteins and neurotransmitters, or harmful chemicals in the water supply can identify critical problems before they have a chance to impact patients or the environment. While some existing sensors can monitor the microscopic matter behind these issues, they often have limitations. A primary example is a device known as a field-effect transistor - a tiny component that controls the flow of electrical current in a system - that struggles to remain stable when exposed to liquid.
Researchers at Penn State have designed a new type of field-effect transistor that can facilitate responsive and versatile sensing, even in liquid-rich environments like the human body. Sensors built with the team's transistors were up to 20 times more sensitive to various chemical and biological signals, like hazardous chemicals in water or the levels of dopamine in the brain, than other sensors built with comparable transistor designs. The team published their work in npj 2D Materials and Applications.
The technology is based on graphene, a two-dimensional (2D) material that is conductive and highly sensitive to its environment despite being only a few atoms thick. Field-effect transistors used in biosensors have traditionally been constructed with silicon, but are increasingly built with 2D materials like graphene. However, according to Aida Ebrahimi, Thomas and Sheila Roell Early Career Associate Professor of Electrical Engineering and corresponding author of the paper, when immersed in liquid, these field-effect transistors face signal drift - the sensor's readings gradually shift over time, even when the inputs measured remain the same, consequently lowering accuracy.
"Aside from signal drift, these devices struggle with electrical leakage and the instability caused by sweeping, a common measurement technique that substantially impacts their reliability over time," said Ebrahimi, who also holds appointments in biomedical engineering and in materials science and engineering. "This makes it difficult to apply these transistors in biointerfaces, like implantable devices, or in any interaction that interfaces with fluid."
Field-effect transistors essentially work like a tap in a sink, explained Vinay Kammarchedu, an electrical engineering doctoral candidate and first author on the paper. When the tap - or gate, in the language of electronics - is open, the field-effect transistor allows current to flow freely through a system. When the tap or gate closes, the flow stops. However, taking measurements with conventional sensors requires constantly adjusting that tap up and down. According to Kammarchedu, this constant shifting causes instability in the system, leading to inaccurate readings.
"We adjusted the design to have two gates rather than one, allowing us to have independent control over the amount of current flowing through the system," Kammarchedu said. "Using two gates, we can keep the current running through the system constant, removing a primary cause of signal drift. On top of that, we added a feedback system to one of the gates to more accurately track the impact that molecules have on the sensor's voltage."
