In Situ Formed 3,3',5,5'-Tetramethylbenzidine-Cu(I/II) Charge-Transfer Complex Intermediates Promoting Colorimetric Assay of Cr.

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 Cu/Cr-promoting strategy for TMB-based colorimetric detection of Cr. By introducing Cu, 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 Cr-mediated TMB oxidation. TMC intermediates also played a pivotal role in HO-participated TMB oxidation, clarifying the secondary responsibility of reactive oxygen species frequently caused by Fenton-like reactions. Leveraging the synergetic capacity between Cu and Cr for TMB oxidation, we demonstrated sensitive and specific colorimetric detections for Cr with a limit of detection of 0.006 μM. With its convenient operation and rapid responsiveness, this strategy successfully enabled the practical detection of Cr 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.