In Situ Formed 3,3′,5,5′-Tetramethylbenzidine-Cu(I/II) Charge-Transfer Complex Intermediates Promoting Colorimetric Assay of Cr6+

Developing a synthesis-free, multifunctional, and effective colorimetric assay system based on 3,3′,5,5′-tetramethylbenzidine (TMB) oxidation is attractive yet challenging. Herein, we established a synergetic Cu2+/Cr6+-promoting strategy for TMB-based colorimetric detection of Cr6+. By introducing Cu2+, critical TMB·+···Cu(I/II)···TMB charge transfer complex (TMC) intermediates were in situ formed to reduce the activation energy of TMB oxidation, thereby accelerating Cr6+-mediated TMB oxidation. TMC intermediates also played a pivotal role in H2O2-participated TMB oxidation, clarifying the secondary responsibility of reactive oxygen species frequently caused by Fenton-like reactions. Leveraging the synergetic capacity between Cu2+ and Cr6+ for TMB oxidation, we demonstrated sensitive and specific colorimetric detections for Cr6+ with a limit of detection of 0.006 μM. With its convenient operation and rapid responsiveness, this strategy successfully enabled the practical detection of Cr6+ in real water samples. This work not only enhances the understanding of the internal acceleration mechanism in colorimetric sensing but also provides valuable opportunities to advance synthesis-free detection platforms.