Magnetic relaxation switching sensor based on self-generated porous Fe3O4 with controllable apparent diffusion coefficient for highly sensitive detection of caffeine

Magnetic relaxation switch (MRS) sensors based on magnetic nanoparticles have received much attention in the field of environmental monitoring due to their rapid response and resistance to complex matrix interference. However, conventional MRS (cMRS) sensors constructed using magnetic nanoparticles have low sensitivity due to the lower relaxation rate of the magnetic probe. In this study, self-generated porous Fe 3 O 4 were employed in the MRS sensors for detection of caffeine, and the regulation of transverse relaxation performance by porous structure has been explored. The abundant porous structure of self-generated porous Fe 3 O 4 restricted the diffusion of the surrounding water molecules, and the controllable pore size can effectively adjust the apparent diffusion coefficient (ADC), as a result, improving the transverse relaxation properties. The developed MRS sensor exhibits a wide linear detection range from 0.5 to 100 ng/mL and a low limit of detection (LOD) of 0.047 ng/mL for caffeine. The LOD is 12 times lower than that of cMRS sensors. Consequently, the MRS sensor was further applied to the detection of caffeine in surface water samples. The results were consistent with those detected using high-performance liquid chromatography, demonstrating its superior anti-interference ability and the significant potential in environmental monitoring and food safety domains.