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Nano-Sized Magnets Enclosed Within Bacteria Individually Characterised for the First Time
This discovery has the potential to enhance the development of hyperthermia cancer treatment, where the magnetic nanoparticles are steered and heated up by magnetic fields to locally destroy cancer cells.

To further explore the potential of the usage of magnetic nanoparticles in nanoparticle therapeutics, a cross-disciplinary research group at Universidad del País Vasco, Leioa, Spain, has been studying magnetotactic bacteria, which are prokaryotic organisms that can synthesise and store magnetic iron oxide nanoparticles in chain-like structures inside their cells.

The largest benefits of using magnetic nanoparticles for biomedical applications include their small size, which at diameters of around 50 nanometers (100 times smaller than blood cells), are similar to that of biomolecules like proteins and nucleic acids, allowing for favourable interactions between the particles and the biomolecules. Furthermore, the ferromagnetic nature of these nanoparticles allows for their manipulation by magnetic fields, allowing them to be moved and heated to higher temperatures. These properties allow the magnetic nanoparticles to be good candidates for their usage as diagnosis agents in cardiovascular diseases, as heating and destruction agents in hyperthermia cancer treatment, or for targeted magnetic cell delivery in regenerative medicine.

However, before these magnetic nanoparticles can be used in any treatment, the various magnetic properties of these particles have to be determined to a high enough precision and accuracy, such that further applications can be properly designed around them, yielding a higher probability of successful treatment. Unfortunately, due to the weakness of signals from the magnetic nanoparticles, previous observations of these magnetic properties have only been values averaged over thousands of such structures, which allows only for a general understanding of the characteristic parameters of the nanoparticles.

Fortunately, in a collaboration with a team of physicists led by Sergio Valencia at Helmholtz-Zentrum Berlin, one of the team members, Spanish physicist Lourdes Marcano, successfully developed a new method to yield better data. It uses data obtained from magnetic imaging at the scanning transmission X-ray microscope MAXYMUS at BESSY II, a third-generation synchrotron radiation source that provides extremely bright X-ray light, in conjunction with existing theoretical models. “We can now obtain precise information on the magnetic properties of several individual nanomagnets in a simultaneous way,” said Marcano.

Using a magnetic imaging technique with resolution in the nanometers, like X-ray photoemission electron microscopy and scanning transmission X-ray microscopy (STXM) under axi-asymmetric magnetic fields, data is collected and fitted onto a theoretical model based on Stoner–Wohlfarth formalism.

This technique allows for the accurate determination of the magnetic anisotropy of the nanoparticles inside the bacterium. Magnetic anisotropy is an important known parameter as it describes how a magnetic nanoparticle reacts to external magnetic fields applied in an arbitrary direction, which is important for the control over the mobility of the nanoparticles.

“Actually, magnetic imaging of magnetic nanoparticles inside a biological cell with enough spatial resolution requires the use of X-ray microscopes. Unfortunately, this is only possible at large-scale research facilities, like BESSY II, providing sufficiently intense X-ray radiation. In the future, however, with the development of compact plasma X-ray sources, this method could become a standard laboratory technique,” said Sergio Valencia.

Source: Marcano et al. (2022). Magnetic Anisotropy of Individual Nanomagnets Embedded in Biological Systems Determined by Axi-asymmetric X-ray Transmission Microscopy, ACS Nano, 16(5), 7398–7408.

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