Cu-PyC MOF with oxidoreductase-like catalytic activity boosting colorimetric detection of Cr(VI) on paper

Hexavalent chromium ions (Cr(VI)) are highly toxic and prone to bioaccumulation in the natural environment. This study sought to develop a facile and cost-effective approach to on-site monitoring Cr(VI) content in diverse environmental samples. We synthesized copper-based MOF nanozymes (Cu-PyC MOF) and developed a simple colorimetric method by which to detect Cr(VI) on paper-based devices. The colorimetric product is obtained using the MOF nanozyme as a catalyst, Cr(VI) as an oxidant, 3,3',5,5'-tetramethylbenzidine (TMB) as a chromogenic substrate to produce a bluish-green color observable by the naked eye or a UV-visible spectrophotometer. After Cu-PyC MOF was suspended in an acidic buffer, it remained stable and showed oxidoreductase-like catalytic activity, which in turn boosts Cr(VI) reduction. The proposed scheme enables the detection of Cr(VI) within 3 min over a linear range of 0.5-50 μM with a low detection limit of 0.051 μM. The proposed nanozyme-based colorimetric method exhibits excellent reusability and selectivity over other 27 interference ions and can be used in real sample analysis.