The presence and quantity of potassium (K+) ions in liquids has been sensed with high resolution and selectivity using graphene field effect transistors (GFET). Quick and reliable measurement of ions at very low concentrations is critical to a range of applications, including food and beverage production, pharmaceutical production, clinical applications, and water quality monitoring.
The most attractive technology for ion sensing employs ion selective electrodes (ISEs). When specific ions bind to an ISE, they create an electric potential whose strength depends on ion concentration. The effect can be amplified by integration with field effect transistors (FET), resulting in low noise, small device size and planar geometry, ideally suited to integration with electronics for real-time, on-site measurement. Now, researchers have demonstrated a high-performance graphene FET integrated with a standard ISE for potassium sensing. Potassium ions were detected at concentrations as low as 10-9 M, equivalent to 39 ng/L. The GFET provided a low-noise channel, resulting in ~10 times better resolution (precision) than state-of-the-art commercial devices. The work was published in the journal Sensors & Actuators: B. Chemical.
Image: GFET used for potassium ion sensor.
Using commercially available chemicals, researchers from Canada and Spain deposited a potassium ionophore, an ISE that is selective to potassium absorption, onto a GFET on a standard SiO2/Si substrate. The graphene was protected from environmental degradation by encapsulation in a thin (8 nm) layer of parylene. The ionophore was formed after the GFET was mounted on a printed circuit board. The graphene sensor was made to have a large sensing area (~ 0.4 cm2) which is important for reducing device noise.
Sensing was performed both in carefully prepared analyte solutions and with realistic specimens. The sensor had excellent selectivity between K+ and other ions such as Na+, Ca2+, Mg2+, and NH4+. The results of potassium sensing also matched excellently with results obtained from a commercial sensor, accurately detecting concentrations of K+ in grape, lemon and orange juice, milk, and bovine and sheep blood. This practical and sensitive device can also be used for detection of other ions by utilizing a different ISE. The researchers envision arrays of sensors with preferential selectivity to a variety of ions for accurately inferring ion concentrations in complex solutions.