Advanced Sensor Technology: Highly Responsive and Versatile, Even in Liquid Environments

Andrew Corselli

The team fit sensors built with their new field-effect transistor design onto integrated circuit boards, like the one pictured here, in order to test sensing accuracy and sensitivity. They found that their approach facilitates sensors that are not only responsive, but highly resistant to the signal drift issues that had faced previous designs. (Image: Jaydyn Isiminger / Penn State)

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.

Here is an exclusive Tech Briefs interview, edited for length and clarity, with Aida Ebrahimi and Vinay Kammarchedu, Corresponding and First Authors on the paper, respectively.

Tech Briefs: What was the biggest technical challenge you faced while developing this sensing technology?

Ebrahimi & Kammarchedu: The primary hurdle we encountered was gate leakage when the dual-gate sensors were immersed in liquid environments. While using local high-k back gates successfully reduces effective oxide thickness and suppresses electrical leakage in dry settings, liquid environments introduce severe complications. The back-gate electrode area has to be carefully minimized to avoid faradaic currents caused by oxide defects. Microscopic processing defects that act as harmless insulators in the air can suddenly become active leakage pathways for ions once placed in a solution. Additionally, the dielectric material itself is vulnerable to electrochemical degradation and water-assisted etching under bias, which leads to device failure. We believe this severe technical challenge is a key reason why dual-gate graphene field-effect transistors haven't been widely adopted in research or industry until now. To ultimately overcome this, we optimized our oxide layer and refined our fabrication protocols to eliminate as many microscopic defects as possible. Most importantly, we successfully minimized the back-gate area. By drastically reducing this footprint, we effectively cut off the active leakage pathways for ions and suppressed the faradaic currents, which finally allowed us to achieve stable, reliable sensor operation in liquid environments.

Aida Ebrahimi, left, and Vinay Kammarchedu have developed an improved field-effect transistor design that can power incredibly sensitive and resilient sensors. (Image: Jaydyn Isiminger / Penn State)

Tech Briefs: Can you explain in simple terms how it works please?

E&K: Think of a standard field-effect transistor like a water tap in a sink. When the tap — or gate, as we call it in electronics — is open, electrical current flows freely through the system. When the gate closes, the flow stops. To take measurements with conventional sensors, you have to constantly adjust that tap up and down, which causes instability and leads to inaccurate readings. To solve this, we designed a system with two gates instead of one, giving us independent control over the amount of current flowing through the system. Using two gates allows us to keep the current running constantly, which eliminates a major cause of signal drift. We then added a feedback system to one of the gates to precisely track how molecules impact the sensor's voltage. Because the top gate has 10 times the electrical capacitance of the bottom gate, it is incredibly sensitive to the environment, while the bottom gate acts as a stiff electronic counterbalance. This relationship amplifies the signals. If there is a tiny chemical change at the sensor's surface, we see it multiplied by 10 in our measurements, allowing us to clearly identify very minor chemical readings.

Tech Briefs: Do you have any updates you can share?

E&K: We have successfully tested the platform's response to gas-phase volatile organic compounds. Specifically, we utilized our Differential Mode Fixed (DMF) configuration to detect isopropyl alcohol. Regarding commercialization, The Pennsylvania State University has filed a provisional patent application that covers this feedback-driven, dual-gated sensing platform. As for future materials, our use of scalable materials and straightforward electronics makes this platform readily adaptable to other 2D materials moving forward. Currently, we are working on designing experiments to bring this to fruition, which includes optimizing the sensors to identify volatile organic compounds associated with Parkinson's disease and exploring how our system operates with different 2D materials.

Tech Briefs: Do you have any advice for researchers aiming to bring their ideas to fruition?

E&K: Our study is a testament to the power of interdisciplinary collaboration and adaptability. We successfully brought this idea to fruition by merging expertise across electrical engineering, biomedical engineering, and materials science to overcome the longstanding limitations of single gate sensors. My biggest piece of advice is to stay flexible and think outside the box: originally, we were using these devices for another mechanism of sensing, but we had to pivot to this novel, feedback mechanism to achieve the stability and sensitivity we needed.

Tech Briefs: Is there anything else you’d like to add that I didn’t touch upon?

E&K: We would love to highlight how scalable and practical our system is. Our architecture successfully bridges the gap between nanoscale materials and practical, portable diagnostic tools. We have successfully integrated multiple sensors directly into custom circuit boards. We can integrate tens of sensors and measure each one independently without any electrical interference. By stacking arrays of these circuit boards, we can easily scale up the number of sensors in a system while keeping the devices themselves incredibly small. We also want to add that for decades, federal support for research has fueled this exact type of innovation, and recent federal funding cuts threaten our progress in solving real problems that impact human health and safety.