Dual fluorometric biosensor based on a nanoceria encapsulated metal organic framework and a signal amplification strategy of a hybridization chain reaction for the detection of melamine and Pb2+ ions in food samples

Abstract

The increased need for melamine and Pb²⁺ ion detection systems that are versatile, ultra-sensitive, and easy to use is highly significant. Here, we report a new nanoceria encapsulated metal organic framework (CeO₂@NH₂-MIL(Fe)) based fluorescent biosensor for the detection of melamine using a hybridization chain reaction (HCR) amplification strategy. In this approach, the CeO₂@NH₂-MIL(Fe) acts as an effective fluorescence quenching material to decrease the background signal, and three types of DNA, for example, FAM attached to two hairpin probes (HDNA 1 and HDNA 2) and helper DNA are used for target recognition and as a signal reporter. In the absence of melamine, the two hairpin and helper DNAs are adsorbed on the surface of CeO₂@NH₂-MIL(Fe) and the fluorescence emission was quenched by a FRET and PET mechanism. However, upon the addition of melamine, the HCR between the two HDNAs is initiated by the melamine with the assistance of the helper DNA via T–melamine–T hydrogen-bond coordination. The double stranded products of the HCR are released by the CeO₂@NH₂-MIL(Fe) surface and the fluorescence is recovered. Subsequently, the addition of Pb²⁺ unwound the HCR product to form a covalent melamine–Pb²⁺ complex, leading to an intensive decrease of the FAM emission. The proposed sensing strategy led to a low detection limit of 7.6 nM melamine and 1.3 nM Pb²⁺ ions, which is acceptably sensitive for practical use. In addition, this sensing method exhibits high selectivity for the melamine and Pb²⁺ when compared to other interfering molecules, and it was applied to the detection of melamine in real food samples such as raw milk, milk powder and a protein beverage, with high accuracy and good recovery results.