Spontaneous Assembly of Gold Nanostars onto Magnetotactic Bacteria: Morphological Characterization and SERS Response of Magneto-Plasmonic Biohybrids.

Magnetotactic bacteria (MTB) are a distinct group of microorganisms that naturally biomineralize nanoparticles in the form of magnetite (Fe3O4) or greigite (Fe3S4). These nanocrystals, known as magnetosomes, give MTB natural magnetic properties and the ability to orient precisely in external magnetic fields. In parallel, gold nanoparticles are extensively used in biomedicine due to their high biocompatibility, chemical stability, and the possibility of functionalization with various biomolecules. The exposure of MTB to plasmonic nanoparticles is only now beginning to be seen as a promising avenue of research, one that could pave the way for the self-assembly of magneto-plasmonic structures. In our study, the combination of MTB (MS-1 strain) with symmetrical gold nanostars (AuNS) or silica-coated gold nanostars (AuNS-SiO2) was initially conceived as "possible interaction", but it was unclear whether the two would "recognize" each other. As expected, scanning electron microscopy (SEM) analysis showed that there is indeed a visible association. The nanostars were observed at multiple locations on the bacterial surface, including regions corresponding to the cell body and polar areas, suggesting a more complex anchoring preference than we had anticipated. The AuNS strongly enhance the Raman spectra of MTB via surface-enhanced Raman scattering (SERS) and provide better image contrast for MTB in SEM imaging. This shows that the use of gold nanostars is not only effective but also opens up new horizons for exploring the optical and structural properties of MTB. Such observations provide the foundation for future research on the spontaneous assembly of MTB-plasmonic nanoparticles as potential components in biosensing and bioimaging platforms. The SERS method could provide a more sustainable and efficient framework for combining MTB with gold nanoparticles in advanced biomedical applications.