Gd³⁺-nanoparticle-enhanced multivalent biosensing that combines magnetic relaxation switching and magnetic separation

In this work, we developed a multivalent magnetic biosensing strategy by integrating magnetic separation and magnetic relaxation switching (MRS) where Gd³⁺-loaded magnetic nanoparticles acted as the probe. As a transition metal ion, Gd³⁺ has multiple unpaired electrons in the d-orbitals that can induce a strong fluctuating magnetic field and thus can reduce the transverse relaxation time (T₂), contributing to a strong magnetic signal. By loading Gd³⁺ onto magnetic nanoparticles, we prepared a multivalent magnetic probe that combined magnetic separation and MRS for the signal readout. This multivalent sensing technique simplified the procedures and greatly enhanced the detection sensitivity of conventional MRS assays. A sensitive detection of ractopamine in real samples has been demonstrated with this multivalent sensing technique. The magnetic probe enabled the detection of ractopamine in a linear range from 0.1 to 100 ng/mL and the limit of detection was 20 pg/mL, a 25-fold enhancement in the sensitivity compared with conventional MRS assays. This Gd³⁺-nanoparticle-mediated MRS biosensor is a potential magnetic platform to detect trace levels of targets in complex samples.