Abstract: In response to the stringent maximum residue limits (MRLs) established by major countries and international organizations globally for sulfonamides (SAs) in animal-derived foods, this study addressed the inherent bottlenecks of conventional large-scale instrumental analysis methods (HPLC, LC-MS/MS), which include costly equipment, cumbersome sample pretreatment, lengthy detection cycles, strict temperature requirements, and a reliance on highly trained personnel. Furthermore, existing rapid detection methods (ELISA, LFIA) that rely on optical signals often suffer from limitations such as signal instability and high false-positive rates. Therefore, this research aimed to develop a novel technology that was highly sensitive, robust against interference, and suitable for the rapid, on-site screening of high-throughput samples. Concurrently, traditional magnetic relaxation switching (MRS) sensors face significant challenges in detecting small-molecule hazards; due to the scarcity of antigenic determinants on the target's surface, these sensors exhibit minimal changes in aggregation state, low signal conversion efficiency, and limits of detection (LODs) that struggle to meet the strict requirements of trace residue monitoring. To overcome these limitations, this study proposed the construction of an innovative quantity-mediated magnetic relaxation sensing strategy. Given the negligible magnetic background noise inherent in food matrices, magnetic sensing techniques offer extensive and promising applications in the detection of food hazards owing to their superior signal-to-noise ratios. This study innovatively proposed and established a novel quantity-mediated magnetic relaxation switching (nMRS) sensing strategy based on the coupling of "magnetic separation and competitive immunoassay". Specifically, a dual-size immunomagnetic probe system was designed and introduced, consisting of antibody-modified small magnetic nanoparticles (10E6-MNP30 nm) and antigen-coated large magnetic beads (BSA-MNP 1000 nm). Based on the principle of competitive immunoassay, the small-molecule SAs in the testing system compete with the large magnetic probes to bind to the small immunomagnetic probes. Subsequently, utilizing the significant differential separation characteristics of the large and small magnetic beads under a magnetic field, the small magnetic probes bound to the large magnetic beads are rapidly captured and removed from the solution. This process ingeniously transformed the subtle changes in the small molecule aggregation state-which are difficult to induce in traditional MRS-into a significant change in the quantity of the remaining small magnetic nanoparticles in the solution. By measuring the change in the transverse relaxation time (T2) of water protons caused by the remaining magnetic nanoparticles using a portable NMR relaxometer, the quantitative detection of the target concentration is achieved. Experimental results demonstrated that under optimal conditions, the limit of detection (LOD) of the established nMRS method for SAs was as low as 0.149 ng/mL. This sensitivity was approximately 20-fold higher than those of both the traditional state-mediated MRS method and the conventional icELISA method. Furthermore, in the detection of real complex meat matrices such as chicken, duck, and pork, the LOD of this method was only 0.34～2.76 μg/kg, which is far below the statutory MRL of 100 μg/kg. Meanwhile, benefiting from the effective washing and enrichment of complex matrices by the magnetic separation step, the spiked recovery rates in actual samples ranged from 76.7% to 119%, with low coefficients of variation (CV). Moreover, the quantitative detection results for real positive samples were highly consistent with the gold-standard LC-MS/MS method. The nMRS sensing strategy developed in this study provided a highly efficient, accurate, and low-cost novel strategy for the on-site, real-time, and rapid screening of veterinary drug residues in animal-derived foods. It held significant practical implications and academic value for ensuring food safety, enhancing public health monitoring, and addressing technical barriers to international trade.

Key words: sulfonamides; immunoassay; food contaminants; relaxation time; magnetic relaxation sensing; veterinary drugs