Programmable Redox Behavior of a π-Conjugated Schiff Base Mediator across Electrocatalytic and Enzymatic Catechol Sensing Platforms

Redox-programmable mediators enable context-adaptive signal transduction in electrochemical biosensors. We report a π-conjugated Schiff-base mediator (PT) formed by condensing thionine with 1,10-phenanthroline-2,9-dicarbaldehyde; the phenothiazine donor and phenanthroline acceptor create a rigid, electronically delocalized scaffold. PT switches function with potential and enzymatic context: (i) an oxygen-reduction electrocatalyst at -0.6 V following a two-electron pathway; (ii) a redox relay in a polyphenol oxidase (PPO) sensor at -0.1 V, mediating o-benzoquinone–electrode transfer and achieving a 9.79 nM LOD; and (iii) a conductive matrix for laccase at +0.23 V that preserves O₂ turnover, affording high selectivity and a wide linear range (1 μM-3 mM; LOD = 0.821 μM). Mechanistic data confirm that the applied bias and enzyme identity dictate PT’s operative role, evidencing molecular-level adaptability. This π-engineered framework provides a general route to tunable electron mediation and multi-mode catechol sensing across electrocatalytic and enzymatic interfaces.