Engineered Zn-doped magnetite nanoparticles as high-performance tracers for ultrasensitive atomic magnetometry.

Atomic magnetometry (AM) enables ultrasensitive magnetic biosensing under weak fields, yet its performance is often limited by a persistent mismatch between AM operating principles and the magnetic nanoparticle (MNP) tracers. Here we develop AM-specific tracers by engineering Zn-doped iron oxide nanoparticles (IONPs) with size control and surface modification to balance colloidal stability and high remanence. The optimal dopamine-modified 19 nm Zn-doped IONPs exhibit a remanence signal approximately 100 times higher than that of VivoTrax, enabling pT-level AM detection from as few as 160 labeled 4T1 and MCF-7 cells with only about 7 ng of tracer. Moreover, an AM-based small-animal system achieves precise subcutaneous detection of small tumor cell clusters in rats. This work identifies composition tuning, size regulation, and surface engineering as critical parameters to maximize the nanoscale remanence of tracers for AM, offering a robust blueprint for future advancements in ultra-sensitive biosensing.