New research shows that graphene field effect transistors can be used to selectively detect ions in a liquid solution. The work, just published in Nature Communications, paves the way to applications such as genome sequencing, medical diagnostics, environmental monitoring, and industrial process control.

State of the art technology for detecting and resolving ions in solution relies on ion sensitive field effect transistors (ISFETs). Standard ISFETs are made of silicon, due to the ease of technological processing, however silicon ISFETs have some drawbacks that hinder their performance in real-life scenarios.

Image: Sketch of sensor in multianalyte solution.

To achieve selectivity to different ionic species, ISFETs that are selective to specific ions are assembled into arrays and post-processing is used to estimate ion concentration. Since many ISFETs are packed on small areas to implement selectivity, each ISFET has to be made small, which leads to low-frequency noise that is prominent in silicon. Increasing the size of individual ISFETs leads to loss of resolution, which imposes a tradeoff that limits practical use.

The present research, reported by teams in Canada and Spain, overcomes the tradeoff by using graphene instead of silicon as the ISFET channel. Graphene has high carrier mobility even in large-area devices, which enables construction of a single large sensor for multiple ionic species. Post-processing of the transistor signal enables the measuring of concentration of K⁺, Na⁺, NH₄⁺, NO₃^-, SO₄^2-, and Cl^- ions down to concentrations lower than 10^-5 M in a multianalyte solution. These ions were chosen due to their prominence in agriculture runoff, hence the importance of their detection in water quality monitoring.

Image: Selective graphene ion sensor monitoring duckweed ion uptake. From Fakih, I., Durnan, O., Mahvash, F. et al. Selective ion sensing with high resolution large area graphene field effect transistor arrays. Nat Commun 11, 3226 (2020). Creative Commons 4.0 license.

Practical graphene ISFET use was demonstrated by monitoring the uptake of ions by duckweed in an aquarium over a period of three weeks. The researchers tracked, with high precision and selectivity, the concentration of seven different ionic species over time after adding plant nutrients to the aquarium. This novel work demonstrates that large-area graphene ISFETs can be fabricated from wafer scale graphene by a facile method, yielding ISFETs with a high signal-to-noise-ratio and high-resolution sensing. Graphene ISFETs hence overcome poor selectivity typically associated with ISFETs made of other materials and can be applied to real-life scenarios in environmental sensing.