Chiral by Nature: Chiroptical Activity in Fe3O4 Magnetic Nanoparticles from Bacterial Biomineralization

The coexistence of chirality and magnetism in nanomaterials has been observed with metallic complexes but remains rarely achieved in magnetic oxides. Here, we identify biologically induced chiroptical activity in the magnetotactic bacterium Magnetospirillum gryphiswaldense strain MSR-1, which biomineralizes Fe3O4 crystals within a prokaryotic organelle known as a magnetosome. Natural and magnetic circular dichroism spectra were recorded for wild-type MSR-1, a magnetosome-deficient mutant (ΔmamAB), and the magnetosomes extracted from the bacterial cells. While chiroptical signals appeared when the particles remained within the bacterial matrix, the isolated nanoparticles did not demonstrate detectable natural optical activity. Subsequent surface modification of the extracted optically inactive nanoparticles with enantiopure aspartic acid produced chiroptical responses. These observations suggest that the biological environment of magnetosomes imposes chiral asymmetry on magnetic nanoparticles and that surface functionalization revealed optical activity. The results collectively describe a biologically mediated induction of chiroptical signals in Fe3O4 nanoparticles.