Target-triggered UCNPs/Fe3O4 @PCN-224 assembly for the detection of cadmium ions by upconversion fluorescence and magnetic relaxation switch dual-mode immunosensor

A dual-mode immunosensor based on upconversion fluorescence (UCFL) and magnetic relaxation switch (MRS) was developed for the anti-interference and sensitive detection of cadmium ion (Cd2+). In this strategy, upconversion nanoparticles (UCNPs) and magnetic porous coordination network nanoparticles (Fe3O4 @PCN224) were employed to form the UCNPs/Fe3O4 @PCN-224 assembly through immune recognition of antigen and antibody. The fluorescence of UCNPs was quenched by the Fe3O4 @PCN-224 due to good adsorption quenching abilities of Fe3O4 @PCN-224. The relaxation performance of the Fe3O4 @PCN-224 was enhanced by the assembling UCNPs on the surface of Fe3O4 @PCN-224, which causing difficulty access of water molecular into the porous structured Fe3O4 @PCN-224. Owing to the anti-background fluorescence interference of UCNPs, the UCFL mode solved the interference of pigment fluorescence in pellucid or colored samples, such as red wine. Due to the quick pre-treament capability of Fe3O4 @PCN-224 at the magnetic field, the MRS mode overcame the disturbance of turbid and opaque samples, such as egg and soymilk. The presence of Cd2+ could affect the assembly process of UCNPs and Fe3O4 @PCN-224. Therefore, the Cd2+ could be detected by UCFL and MRS dualmode immunosensor. The limits of detection of UCFL and MRS modes for detection of Cd2+ were 0.038 ng/mL and 0.16 ng/mL, respectively. This study provided a valuable information on the development of dual-mode immunosensor for the detection of Cd2+ in foods and had a broad application prospect in rapid detection.