Towards a Rational Design of Biosensors: Engineering Covalently Grafted Interfacial Adlayers as a Testbed Platform for Electrochemical Detection of Epinephrine.

The performance of electrochemical (bio)sensors is fundamentally determined by the precise engineering of interfacial layers that govern (bio)analyte-surface interactions. However, elucidating structure-function relationships remains challenging due to the complex architecture of modern sensors and the irregular nanoscale morphology of many high-performance materials. In this study, we present a strategy for designing custom functional interfaces as well-defined platforms for probing interfacial processes. Focusing on epinephrine (EP) detection as an important representative of catecholamines, we compare the interfacial behavior of two carboxy-functionalized electrodes-grafted with either para-aminobenzoic acid (PAB) or 3,4,5-tricarboxybenzenediazonium (ATA)-against atomically flat highly oriented pyrolytic graphite (HOPG) as a control. While both modifiers introduce carboxyl groups, PAB forms disordered multilayers that inhibit surface responsiveness, whereas ATA yields an ultrathin monolayer with accessible COOH groups. Electrochemical analysis reveals that ATA-HOPG significantly enhances EP detection at sub-micromolar levels, facilitated by electrostatic interactions between surface-bound COO- and protonated EP and its redox products. These results demonstrate that nanoscale control of diazonium grafting is crucial for optimizing bioanalyte recognition. More broadly, this work highlights how molecular-level surface engineering on high-quality carbon substrates can serve as a test-bed platform for the rational design of advanced electrochemical sensing interfaces.