Resolving femtogram-scale cellular nanomagnetism via high-gradient magnetophoresis of spheroids in microfluidic chips

Quantifying trace amounts of magnetic iron in living cells remains a major challenge in cellular biology and nanomedicine. Existing approaches such as magnetometry or elemental analysis either require large cell populations or cannot discriminate endogenous iron from magnetic phases, preventing measurements at the femtogram-per-cell scale. Here, we introduce a high-gradient magnetophoresis approach combined with multicellular spheroids to achieve femtogram-level sensitivity in intact 3D cellular assemblies. By exploiting the scaling of magnetic force with spheroid volume and hydrodynamic drag with spheroid diameter, magnetophoretic motion is physically amplified, enabling quantification of magnetic moments corresponding to as little as ∼0.4 femtograms of iron per cell. The method further enables detection of rare magnetically labeled cells within heterogeneous spheroids, with a response that scales linearly with nanoparticle dose and the fraction of labeled cells. Remarkably, cells incubated with soluble, non-magnetic iron salts also exhibit measurable magnetophoretic displacement, despite the absence of detectable signals by VSM, suggesting the emergence of biosynthesized magnetic nanostructures. Altogether, this work establishes spheroid-based high-gradient magnetophoresis as a structurally-preserving platform capable of resolving femtogram-scale cellular magnetism, providing a powerful tool to investigate iron metabolism, cellular heterogeneity, and endogenous biomagnetic phenomena in complex biological systems.