In Situ Formed\n3,3′,5,5′-Tetramethylbenzidine-Cu(I/II)\nCharge-Transfer Complex Intermediates Promoting Colorimetric Assay\nof Cr<sup>6+</sup>

Developing a synthesis-free, multifunctional, and effective\ncolorimetric\nassay system based on 3,3′,5,5′-tetramethylbenzidine\n(TMB) oxidation is attractive yet challenging. Herein, we established\na synergetic Cu<sup>2+</sup>/Cr<sup>6+</sup>-promoting strategy for\nTMB-based colorimetric detection of Cr<sup>6+</sup>. By introducing\nCu<sup>2+</sup>, critical TMB·<sup>+</sup>···Cu(I/II)···TMB\ncharge transfer complex (TMC) intermediates were in situ formed to\nreduce the activation energy of TMB oxidation, thereby accelerating\nCr<sup>6+</sup>-mediated TMB oxidation. TMC intermediates also played\na pivotal role in H<sub>2</sub>O<sub>2</sub>-participated TMB oxidation,\nclarifying the secondary responsibility of reactive oxygen species\nfrequently caused by Fenton-like reactions. Leveraging the synergetic\ncapacity between Cu<sup>2+</sup> and Cr<sup>6+</sup> for TMB oxidation,\nwe demonstrated sensitive and specific colorimetric detections for\nCr<sup>6+</sup> with a limit of detection of 0.006 μM. With\nits convenient operation and rapid responsiveness, this strategy successfully\nenabled the practical detection of Cr<sup>6+</sup> in real water samples.\nThis work not only enhances the understanding of the internal acceleration\nmechanism in colorimetric sensing but also provides valuable opportunities\nto advance synthesis-free detection platforms.