Harnessing Interfacial Oxidation‐State in MoS ₂– Mo ₂ CT x MXene Heterostructures Enables Ultrasensitive and Selective Dopamine Sensing

Dopamine is a neurotransmitter whose dysregulation underlies neurodegenerative disorders such as Parkinson's and Alzheimer's diseases, yet its reliable electrochemical detection remains challenging due to ultralow physiological concentrations and interference from coexisting electroactive species. Addressing this challenge requires sensing interfaces that move beyond conventional conductivity enhancement and instead exploit control over interfacial chemistry and electronic structure. Here, we introduce an oxidation‐state‐engineered Mo 2 CT x –MoS 2 heterostructure as a sensing interface for ultrasensitive dopamine detection, directly fabricated on a flexible and low‐cost aluminium foil substrate via a one‐step hydrothermal process. Unlike prior MXene–TMDC systems that treat MXenes as passive conductive scaffolds, ethylene glycol (EG)/KOH treatment in our design suppresses Mo 6 + formation and preserves low‐valence molybdenum species and hydroxyl terminations on Mo 2 CT x , thereby regulating dopamine adsorption and interfacial charge‐transfer kinetics. The resulting heterostructure combines the chemically active surface of Mo 2 CT x with defect‐rich metallic MoS 2 sheets, creating a stable and responsive electrochemical interface. Consequently, the flexible sensor achieves femtomolar‐level sensitivity with a broad linear detection range from 1 mM to 1 fM, excellent selectivity against common interferents, and robust reproducibility. Supported by density functional theory calculations and electrochemical impedance spectroscopy analyses, this work establishes oxidation‐state control at MXene–TMDC interfaces as a decisive design principle for ultrasensitive biosensing.