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Given that hexavalent chromium ions (Cr(VI)) are among the most biologically toxic heavy metal ions in the environment, it would be desirable to develop approaches that are easy-to-use, practical, and cost-effective for monitoring Cr(VI) content in environmental matrices. Herein, we report a colorimetric assay for Cr(VI) detection based on the oxidase-like activity of a series of nanoscale substituted transition metal vanadates (STMVs) with the general formula M3V2O8, where M represents divalent metal cations such as Ni2+, Cu2+, and Zn2+. The structural and physicochemical characterisations indicated that mesoporous Cu3V2O8 and Ni3V2O8 with roughly nano-spherical morphologies were successfully formed with monoclinic and orthorhombic phases, respectively. In addition, micro/mesoporous Zn3V2O8 had uniform hexagonal nanosheets with an orthorhombic crystalline phase. The UV-vis spectroscopy showed that Cu3V2O8 and Ni3V2O8 oxidised O-phenylenediamine (OPD) to 2,3-diaminophenazine (DAP); however, Zn3V2O8 did not exhibit the desired activity. Under the optimised reaction conditions, a simple, fast, and low-cost colorimetric sensor was designed based on the outstanding oxidase-like activity of Cu3V2O8 (Vmax = 0.25 µM s−1) to detect Cr(VI). The Cu3V2O8/OPD system operated selectively for Cr(VI) against other ions, and the lowest detectable [Cr(VI)] within the linear range of 2–32 μM was 0.24 μM, which is lower than the standard Cr(VI) levels (0.96 µM) for drinking water set by the World Health Organisation (WHO). This colorimetric technique was also applicable for assessing Cr(VI) in untreated tap water and Caspian Sea water samples. The analytical results showed that mesoporous Cu3V2O8 reliably developed a sensitive colorimetric system with thermal stability for the determination of toxic ions in water sources based on an intrinsic oxidase mimic. [ABSTRACT FROM AUTHOR]