Metal Ions Selectively Activate Emissive Centers in Carbon Dots, Triggering Color‐Tunable Phosphorescence for Visual Detection

ABSTRACT Achieving real‐time visual monitoring of heavy metal ions with high sensitivity remains a fundamental challenge due to the inherent limitations of fluorescence quenching‐based sensors, including poor signal‐to‐noise ratios and insufficient color variation. Here, we report a paradigm‐shifting strategy that unlocks color‐tunable “off‐on” phosphorescence in carbon dots (CDs) through selective metal ion activation of dual‐emissive centers. The CDs feature unique dual‐emissive phosphorescent centers: a green‐emitting core state and a red‐emitting surface state. Upon selective coordination with Pb 2+ , a dual regulatory mechanism is activated: the heavy‐atom effect from Pb 2+ enhances phosphorescence from both centers via spin–orbit coupling enhancement, while Pb 2+ ‐induced partial aggregation preferentially suppresses the red surface‐state emission. This competitive enhancement‐suppression dynamics produces a vivid phosphorescence color shift from red to green with increasing Pb2+ concentration, achieving an ultralow detection limit of 58.9 n m . We further integrated the test paper with a smartphone‐based digital readout system, enabling objective, quantitative on‐site analysis.