Highly efficient peroxidase-like activity of a metal–oxide-incorporated CeO2–MIL(Fe) metal–organic framework and its application in the colorimetric detection of melamine and mercury ions via induced hydrogen and covalent bonds

Abstract

The illegal addition of melamine to dairy products and the contamination of water with mercury (Hg²⁺) are serious threats to human health. Hence, herein, a highly sensitive colorimetric sensor for the visual detection of melamine and Hg²⁺ ions has been developed using a metal–oxide-in-MOF nanomaterial (CeO₂–MIL (Fe)) as a peroxidase mimic. Highly mono-dispersed CeO₂–MIL (Fe) was synthesised via a facile hydrothermal process. The CeO₂–MIL (Fe) exhibited outstanding peroxidase activity, and can catalyze the oxidation of TMB (3,3′,5,5′-tetramethylbenzidine) by H₂O₂, resulting in the development of blue-coloured oxidation products within 5 min. In the presence of melamine, the H₂O₂ interacts with melamine to form melamine–H₂O₂via H-bonding. Due to the uptake of H₂O₂ by melamine, the catalytic oxidation reaction was halted, and the blue TMB oxidation product became pale. The relative change in the absorption intensity at 652 nm was proportional to the concentration of melamine in the linear range of 0–0.1 μM and the detection limit was found to be 8 nM. Subsequently, when Hg²⁺ ions were added to the above solution, the Hg²⁺ ions reacted with melamine via strong covalent bonding to form a Hg²⁺–melamine covalent complex, causing the release of H₂O₂, which again strongly oxidised the TMB to give the blue-coloured oxidation product. Furthermore, the comparative change in the absorption intensity at 652 nm was dependent on the concentration of Hg²⁺ ions in the linear range of 0–6 nM, and a detection limit of 2 nM was achieved. The suggested system has several advantages including greater simplicity, good selectivity, naked-eye detection and cost-effectiveness without using any complicated detection procedure. This technique was successfully utilized to identify melamine in real foods and Hg²⁺ ions in real water samples, yielding high recovery rates.