Sensing lactic acid with a graphene-like foam capacitor

University of Bath researchers have created a lactic acid sensor using double-layer capacitance created when a particular sensitive molecule is attached to a graphene-like porous electrode structure and immersed in a buffer solution.

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.

The measurement is by electrochemical impedance spectroscopy – measuring current and voltage amplitude and phase at different frequencies – then calculating the capacitance when the system is approximated to a series RC circuit.

Capacitance varied from 60 to 110μF, with a useful log relationship, with lactic acid concentration between 0.1 and 100mM.

A drawback is that it only works in slightly acid conditions (~4 pH) and not in the body’s 6-7 pH, and it is not particularly reversible – both of which might be amenable to modification of the receptor molecule, according to the research team.

The work is published in ACS Sensors as ‘Pyrene-appended boronic acids on graphene foam electrodes provide quantum capacitance-based molecular sensors for lactate‘, which can be read in full without payment.

*This particular pyrene-appended boronic acid was developed by the Universities of Bath, Kitakyushu and Birmingham, and Xiamen University.