One-step Pd-NPs/Tyrosinase/Nafion biosensor for dopamine detection in the low μM range: ORR interference, defect-rich surface chemistry effects, and sensing pathway differentiation

A one-step (OS) biosensor for low micromolar (μM) dopamine (DA) detection was developed via co-electrodeposition from multiple palladium-based electrolyte containing tyrosinase (Tyr) and Nafion (Naf). The resulting OS biosensor modified with hybrid Pd-NPs/Tyr/Naf layer enabled DA detection in synthetic urine below 10 μM, with recovery rates of ∼98–112 %. A key challenge was signal interference caused by indirect redox reactions between Pd-oxides (present on the surface of the electrodeposited Pd-NPs) and free DA, as well as from the oxygen reduction reaction (ORR). This issue was addressed by promoting the formation of non-stoichiometric PdOₓ (x < 1) through mild heating of the electrolyte during electrodeposition. This defect-rich surface suppressed ORR and non-enzymatic, ORR-mediated DA interactions, while retaining the catalytic activity of Pd-NPs toward dopamine-quinone as an enzymatic product. Beyond its practical value, this work provides a framework for the controlled synthesis of OS-designed biosensing layers for enzymatic neurotransmitter detection at low μM levels, while minimizing interference from the intrinsic catalytic activity of electrodeposited Pd-NPs.