Colorimetric quantification of chromium (VI) ions based on oxidoreductase-like activity of Fe₃O₄

Chromium (VI) is a highly toxic metal ion that is usually emitted to the environment through different activities like metal plating, leather tanning, wood preservatives treatment, pigment production and mining. Therefore, rapid and simple routine method of detection is highly demanding. Here, we develop a simple method based on the oxidoreductase-like activity of Fe₃O₄ for colorimetric determination of Cr⁶⁺. Simple co-precipitation method was employed to produce Fe₃O₄. The colorless 3, 3′, 5, 5′-tetramethylbenzidine (TMB) solution can be oxidized to deep blue colored oxTMB in the presence of Fe₃O₄, and Cr⁶⁺ rapidly. On account of this we determine Cr⁶⁺ using Fe₃O₄ as oxidoreductase catalyst. The method avoids utilization of H₂O₂ which can compromise the repeatability of measurements due to its instability. The method can detect Cr⁶⁺ with a wide linear range of concentration and low limit of detection as small as 34.65 nM that enables determination of the minimum allowable concentration of Cr⁶⁺ in drinking water. The assay shows excellent selectivity against potentially interfering metal ions and anions. The method has also employed for quantification of Cr⁶⁺ in borehole water samples.