Polymer indicator displacement assay (PIDA) with boronic acid receptors on graphene foam electrodes for self-optimised impedimetric lactic acid determination

Synthetic organic receptor molecules are employed based on boronic acids attached to graphene to provide functionality and selectivity in competitive analyte binding. Here, a new electrochemical sensor concept based on a surface redox polymer indicator displacement (avoiding traditional solution redox indicators) is proposed and demonstrated on graphene foam electrodes. A pyrene-derivatised boronic acid chemo-receptor for lactic acid is adsorbed onto graphene foam and coated with poly-nordihydroguaiaretic acid (poly-NHG) as an electrochemically active polymer indicator. When bound to the boronic acid, poly-NHG remains redox silent. Dynamic displacement with lactic acid results in a concentration-dependent Faradaic current response. Effects of pH and detection methodology (voltammetry, chronoamperometry, and impedance spectroscopy) are investigated. Self-optimised impedimetric sensing based on the interfacial electron transfer resistance is demonstrated. While lactic acid sensing in human serum is possible, the sensor response is lower. Surface-active components from human serum are shown to modify the sensor response without affecting performance. Lactic acid sensing in artificial sweat at pH 4.7 is shown to result in a Langmuirian binding curve with apparent binding constant K = 40 M-1.