Diamond magnetic microscopy of malarial hemozoin nanocrystals

Magnetic microscopy of malarial hemozoin nanocrystals was performed using optically detected magnetic resonance imaging of near-surface diamond nitrogen-vacancy centers. Hemozoin crystals were extracted from $Plasmodium$-$falciparum$-infected human blood cells and studied alongside synthetic hemozoin crystals. The stray magnetic fields produced by individual crystals were imaged at room temperature as a function of applied field up to 350 mT. More than 100 nanocrystals were analyzed, revealing the distribution of their magnetic properties. Most crystals ($96\%$) exhibit a linear dependence of stray field magnitude on applied field, confirming hemozoin's paramagnetic nature. A volume magnetic susceptibility $\chi=3.4\times10^{-4}$ is inferred using a magnetostatic model informed by correlated scanning electron microscopy measurements of crystal dimensions. A small fraction of nanoparticles (4/82 for $Plasmodium$-produced and 1/41 for synthetic) exhibit a saturation behavior consistent with superparamagnetism. Translation of this platform to the study of living malaria-infected cells may shed new light on hemozoin formation dynamics and their interaction with antimalarial drugs.