A Cu(0)-methylene blue clock reaction for sensitive Cr³⁺ detection in acidic media

Clock reactions, characterized by abrupt and time-controlled color transitions, are attractive platforms for analytical sensing, but methylene blue (MB)-based systems under acidic conditions typically rely on external reducing agents or nanostructured catalysts. Herein, we report a simple Cu(0) wire-catalyzed MB clock reaction that operates in acidic media (pH 2.0) without any added reducing agent or shape-controlled nanoparticles. The system exhibits reversible color transitions between MB and leuco-MB, governed by interfacial electron transfer and Cu redox cycling. Notably, at elevated MB concentrations (100 mg L^-1), repeated clock cycles lead to the gradual formation of a brown precipitate, which can compromise analytical performance. Mechanistic investigations (SEM, EDS, and XPS) reveal that the precipitate originates from interactions among Cu species (Cu⁰/Cu⁺/Cu²⁺), Cl^-, and MB, with partial incorporation of MB into CuCl-based solids during redox cycling. Strategies including acid cleaning of the Cu wire and lowering MB concentration effectively suppress precipitate formation and improve system stability. Under optimized conditions, the clock reaction enables quantitative detection of Cr³⁺via modulation of reaction kinetics, affording a linear response with a limit of detection of 3.22 µmol L^-1. The method also demonstrates excellent selectivity and robustness in real water samples. This work establishes a reducing-agent-free MB clock platform and provides new insights into Cu-mediated interfacial redox processes, offering a simple, cost-effective, and practical strategy for metal ion sensing.