Creating an ac-based capacitive sensor, as opposed to dc voltage or current based sensor, sides-steps concentration and chemical consumption issues encountered in battery-like reactions.

The carbon material, described as a ‘graphene foam’ by the researchers, is from Integrated Graphene of Stirlingshire, Scotland.

Integrated Graphene makes a standard sensing structure (see photo) on a flexible substrate with a central 4mm disc diameter electrode of the foam surrounded by a gap and then an annulus of the same material.

Developed in an earlier international science collaboration*, the sensitive molecule is a ‘pyrene-appended boronic acid’, whose molecules attach to the carbon surface at one end, and react with lactic acid at the other (they also react with glucose).

Measurement are made with the sensor immersed in a buffer liquid.

“When lactate binds to the sensor, it causes a change in the quantum capacitance of the carbon foam,” according to Integrated Graphene. “The foam therefore detects low levels of lactic acid, without consuming it, by measuring changes in electrical charging.”

The capacitance is due to a double-layer capacitor that forms from cations and anions. It is modified, speculates the team, as charges redistribute when the lactic acid attaches.