Composites of ZIF-67 MOF Nanostructures and CoFe₂O₄ Magnetic Nanospheres Both Decorated with Ag Nanoparticles as SERS Carboxylesterase 1 Sandwich Assay Immunosensors

In this work, we have fabricated a surface-enhanced Raman scattering (SERS) magnetic immunosensor using a sandwich structure consisting of an SERS tag, a supporting substrate with magnetic properties, and a target. On the one hand, the SERS tag consists of AgNPs, a metal–organic framework called the zeolite imidazolate framework-67 (ZIF-67), and R6G, which acts as a Raman reporter. Due to the self-selective SERS-enhancing properties of ZIF-67 and better adsorption to analytes, SERS tags with this structure can further enhance the sensing capabilities. On the other hand, after surface functionalization with the human carboxylesterase 1 (hCE1) antibody, the nanostructure containing CoFe 2 O 4 as the magnetic core and Ag NPs as the shell was used as the magnetic support substrate. The introduction of the CoFe 2 O 4 magnetic core can not only realize the rapid separation of biological samples to simplify pre-treatment procedures but also stabilize and uniformly distribute the AgNPs and increase the Raman “hot spots”, thereby further enhancing the SERS intensity and improving the reproducibility. Not only that, the SERS signal can be further enhanced by the synergistic effect generated by the plasmonic coupling of the CoFe 2 O 4 magnetic core–Ag satellite nanostructure (CoFe 2 O 4 /Ag). hCE1 in HepG-2 cell lysates and culture supernatants was rapidly determined using the constructed SERS magnetic immunosensor, and hCE1 released from acetaminophen-treated HepG-2 cells into the extracellular medium was also monitored in real time. The limit of detection was as low as 3.6 pg/mL, and the linear response covered a range of hCE1 concentrations (1.0 ng/mL–1.0 mg/mL) spanning 3 orders of magnitude. This strategy can be used for the quantitative detection of complex trace proteins or enzymes in biological samples.