Biosensing Phenolic Acids Content in Fruit and Leaf Extracts with a Laccase–Based Biosensor

Abstract: Laccase enzyme is known for its ability to selectively oxidize phenolic antioxidants naturally present in edible plants. In this context, a laccase-based electrochemical biosensor has been developed and optimized to operate in a mixed medium (ethanol–aqueous buffer). The interaction between the enzyme and syringic, caffeic, chlorogenic, ferulic, and p–coumaric acids was examined by cyclic voltammetry, revealing that the first three phenolic acids effectively mediate electron transport between the laccase active site and the electrode surface, while the interaction between the enzyme and either ferulic or p–coumaric acids was not electrochemically detectable. Furthermore, the sensitivity, linear dynamic range, limits of detection, and quantification for the first three acids were determined by amperometry at a constant potential. It was found that both hydroxycinnamic acids – caffeic and chlorogenic – can be sensed at an applied potential of 0.2 V (vs. Ag|AgCl), while syringic acid (a hydroxybenzoic acid) is detectable at –0.3 V. In this context, the content of hydroxycinnamic or hydroxybenzoic acids was biosensed in alcoholic extracts of coffee, wild berries, and grape leaves, and the results were compared with those obtained by chromatography. The data analysis indicated that the presence of other antioxidants (catechins and anthocyanins), might cause an overestimation of analyte levels.