Building better biosensors from the molecule up

Organic electrochemical transistors (OECTs) are innovative devices that could be made into implantable biosensors, or used to analyze biological samples to help diagnose illnesses. KAUST researchers have studied how the semiconducting polymers inside OECTs interact with ions in the samples — a crucial step in designing materials that tune the performance of these devices for particular applications^[1]

“The technology is rapidly advancing, and some OECT-based probes have already been used in clinical research settings, particularly in neuroscience for the brain–electronic device interfacing,” says Sahika Inal, who led the research. In biosensing applications, she explains, OECTs could operate directly in blood, sweat, or saliva; they might also be used to detect viruses or other pathogens in liquid samples.

OECTs contain a polymer film that can carry a current between two electrodes. These polymers are known as organic mixed ionic-electronic conductors. The film touches a water-based sample containing various ions, which are atoms or molecules bearing positive or negative charges. A third electrode delivers a voltage that drives one type of ion (for example, positive ions, known as cations) from the water into the polymer film, which increases the flow of current, producing a signal.

Researchers previously assumed the other ions in the electrolyte (negative ions, known as anions) were bystanders in this process and did not affect the device’s performance. But the KAUST team have now found that is not always true — these anions sometimes have a profound effect on how well OECTs perform.

The researchers made OECTs from two different semiconductive polymers. The first, P-100, is based on molecules with a water-repelling backbone, but with side chains that attract water molecules and the ions they carry. The second polymer, BBL, lacks these side chains.

The team tested the devices with water containing positive sodium ions and one of five different anions. With the BBL polymer, the type of anion made no difference to the OECT’s performance because all but the very largest anions infiltrate BBL films equally well. In contrast, the P-100 device showed considerable variation in performance. With anions based on a single atom – such as chloride or bromide – the P-100 device had a relatively good sensitivity. However, with larger anions, the performance declined by 90 percent over the course of an hour.

“By understanding these interactions more deeply, researchers can better tailor OECTs for specific applications, whether that means optimizing for stability, selectivity, signal amplification, or biocompatibility,” says team member David Ohayon, now at the National University of Singapore.

For instance, OECTs designed to measure ions involved in biological processes should be able to clearly differentiate between those ions. Other applications might require OECTs to operate independently of any ions present. “This is exactly why it is so crucial to understand how different ions affect device performance,” says Ohayon.

“The research may have wider implications for other devices,” adds Inal. “These findings could also inform the development of organic-based supercapacitors and batteries, where understanding how ions interact with the polymer matrix can help optimize charge storage and improve the stability of organic electrodes.”