Using Polymer Field Effect Transistors as Sensors

While our conscious perception is limited to five senses, our bodies contain biological sensors with many more functions. We might be unaware of them, but the body would not be able to function without them. For example, we “sense” glucose and oxygen levels, hormones, neurotransmitters, cytokines, toxins, nutrients, and pheromones, and can differentiate between our own cells and alien cells, and fight off the latter with our immune systems.

Scientists often are inspired by nature and draw the ideas from extremely complex natural signaling systems. Compared to those found in nature, human-made sensors are much simpler, usually measuring just one or a few parameters and making us consciously aware of them. We can easily measure temperature, humidity, pressure, oxygen levels, salinity, light and UV intensity, electric and magnetic fields, radiation, and more with sensors of increased complexity.

While chemical sensors measure the chemical composition of their environments, biosensors either measure or use biological molecules, which is responsible for the sensors’ high selectivity and specificity. The modern challenge in the biosensing field is to be able to detect analytes with the highest sensitivity (sometimes down to a single molecule) and do it in real time — that is, instantly. In addition, the ideal biosensor has to be miniature and lightweight, like the ones in wearable electronics. Electrochemical sensors that convert a signal into simple electrical output are best for these purposes, with so-called “field effect transistors,” or FETs, offering the best sensitivity through signal amplification. Semiconductor-based biosensors can be miniaturized without sensitivity loss, allowing easy integration into the required electronics.

The number of research publications on field effect transistors has been growing exponentially over the past decade (already exceeding 1,000), with approximately half dedicated to biological and chemical detection. These studies include a wide range of semiconducting materials, including organic polymer semiconductors, which are good for sensing because they change their conducting properties depending on the environment.

Compared with silicon FETs, organic thin film FETs (OFETs) show lower charge carrier mobility but better performance in terms of flexibility, biocompatibility, large area, and solution processability. And, organic semiconductor films can be deposited on flexible substrates such as polyethylene terephthalate (PET) using various fabrication approaches, such as thermal evaporation, spin-coating, and screen and inkjet printing, allowing scalability and low cost of the resulting sensors. OFET sensors increasingly have studied for environmental sensing, biopathogen detection, and defenseapplications, as well as medical, health, and human performance monitoring.

Although different configurations of OFETs are used for different applications, a most common OFET is a thin, flexible, layered structure built from (bottom to top) a support, a gate electrode, a dielectric film, a semiconductor film, and two electrodes (source and drain) on top of semiconductor. When a source-drain voltage is applied, a channel current flows through the organic semiconductor layer due to the charge carrier transport. The sensing is performed on the level of the semiconductor, which is exposed to the target analytes. The channel currents can be changed by the changes at the sensing surface (charge doping or analyte trapping), as well as modulated due to field effect doping by the gate voltage, applied across the gate insulator.

As many environmental and biomedical measurements have to be performed in corrosive environments, such as ground water, salt water, or body fluids, OFET sensors have to be very stable in these environments. Miniature OFET chips can be incorporated into flow or microfluidic devices or even implanted in the body for real-time detection of analytes in solution.

A recent breakthrough has been reported for detecting mercury in seawater using OFET biosensor. The team of scientists from Stanford University led by Zhenan Bao designed and tested an OFET sensor, using a stable semiconducting conjugated polyisoindigo-based polymer with siloxane side-chains, synthesized by their collaborators from Peking University. Here is how the article in Nature Communications describes it:

In recent decades, the susceptibility to degradation in both ambient and aqueous environments has prevented organic electronics from gaining rapid traction for sensing applications. Here we report an organic field-effect transistor sensor that overcomes this barrier using a solution-processable isoindigo-based polymer semiconductor. More importantly, these organic field-effect transistor sensors are stable in both freshwater and seawater environments over extended periods of time. The organic field-effect transistor sensors are further capable of selectively sensing heavy-metal ions in seawater. This discovery has potential for inexpensive, ink-jet printed, and large-scale environmental monitoring devices that can be deployed in areas once thought of as beyond the scope of organic materials.