Selective and sensitive colorimetric detection of Cr(VI) using porous M₃V₂O₈ (M= Ni, Cu, and Zn) nanostructures as oxidase mimics

ABSTRACT

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 (V_max = 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.

KEYWORDS:

• Cr(VI)
• colorimetric sensor
• kinetics
• nanozyme
• oxidase-like

Acknowledgments

Financial support from the University of Guilan is gratefully acknowledged.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Authorship contribution statement

Fahime Vetr: Writing – original draft; Conceptualisation; Methodology; Formal analysis; Investigation Hadi Fallah Moafi: Supervision; Resources Zeinab Moradi-Shoeili: Project administration; Validation; Writing – review & editing; Conceptualisation; Methodology; Supervision