Polyethylenimine-stabilized silver nanoclusters act as an oxidoreductase mimic for colorimetric determination of chromium(VI). A new and efficient assay is proposed for the photometric determination of Cr6+ by employing polyethylenimine-stabilized Ag nanoclusters (PEI-AgNCs) as an oxidoreductase mimic.  Cr6+ with certain oxidicability is able to specifically react with 3,3',5,5'-tetramethylbenzidine (TMB), giving a color change from colorless to blue indicating the presence of Cr6+.  However, the redox kinetics is so slow that the sensitivity obtained for Cr6+ determination is very poor.  It is interestingly found that PEI-AgNCs can act as an oxidoreductase-like nanozyme to significantly promote the sluggish reaction, making it possible to rapidly detect toxic Cr6+ with remarkably enhanced performance.  With the use of PEI-AgNCs, fast and convenient determination of Cr6+ was realized, with a limit of detection as low as 1.1 μM.  Additionally, the proposed assay exhibited excellent selectivity; other ions, including Cr3+, hardly affected the determination of Cr6+.  Graphical abstract Polyethylenimine-stabilized silver nanoclusters (PEI-AgNCs) act as an oxidoreductase mimic to catalyze the redox reaction of Cr6+ and 3,3',5,5'-tetramethylbenzidine (TMB), enabling the high-performance colorimetric determination of toxic Cr6+.
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