Selective Epitaxial Growth of MOF-on-MOF-Derived Core–Shell Fe₂O₃/CoFe₂O₄ Nanocomposites for Efficient Electrochemical Detection of Multiple Heavy Metal Ions

Combining two metal–organic frameworks (MOFs) with different compositions and structures is an effective way to grow MOF-on-MOF materials. MOF-on-MOF-derived Fe2O3/CoFe2O4 nanocomposites were constructed by epitaxial growth method and high-temperature heat treatment. The morphology and structure of Fe2O3/CoFe2O4 were modulated by varying different organic ligands, and their effects on the detection properties of Pb2+, Cu2+, Hg2+, and Cd2+ were explored. Co3O4 microspheres acted as stabilizing seeds, and typical samples of Fe2O3/CoFe2O4-2 nanocomposites showed spherical nanoparticles of the core–shell type with an average diameter of about 55–60 nm. Meanwhile, compared with individual MOF derivatives, the surface of MOF-on-MOF derivative materials had higher electrical conductivity and active surface area by high-temperature heat treatment, thus promoting efficient adsorption of a wide range of heavy metal ions (HMIs). Furthermore, Fe2O3/CoFe2O4-2/GCE exhibited excellent electrochemical detection performance for the simultaneous detection of Pb2+, Cu2+, Hg2+, and Cd2+ with low limit of detection (LOD) of 2.98, 8.56, 4.44, and 5.40 nM, respectively. By two-by-two and three-by-three assays, promoting and inhibiting effects of different HMIs were observed on the target ions. This work is significant for the development of high-performance MOF-on-MOF-derived Fe2O3/CoFe2O4 nanocomposites for real-time environmental monitoring.