Ion and plasma based treatments for enhanced chemical speciation of metals in ferritin

• Focused ion beam and oxygen plasma etching are used for controlled disruption of ferritin.
• The detection gain of iron by EDX upon focused ion beam treatment increased by 300%.
• XPS detection of Fe in ferritin after oxygen plasma etching was enhanced by ∼15.

Ferritin is the main protein devoted to iron storage in living systems. Several evidences show that ferritin can bind in vivo metals other than iron, such as aluminium. Moreover, recent studies demonstrated that ferritin iron core composition differs between physiological and pathological brain ferritins. The precise evaluation of chemical species present within the ferritin mineral core and their chemical coordination is therefore of paramount importance. X-ray photoelectron spectroscopy and energy dispersive X-ray spectroscopy are powerful tools to perform this chemical analysis: however the protein shell surrounding the mineral core reduces significantly the amount of electrons/photons detected, thus reducing the potentiality of these techniques.In this framework we investigated the use of physical treatments, namely focused ion beam milling and oxygen plasma etching, in order to overcome the limitations imposed by the presence of the protein shell. We demonstrated that the combined use of focused ion beam milling and energy dispersive X-ray spectroscopy provides an enhancement of iron detection by a factor of about two. As for the X-ray photoelectron spectroscopy technique, it has an intrinsic low capability of probing the core metal in ferritin due to the electron escape depth value that is of the order of the protein shell thickness. Using a pre-treatment in oxygen plasma the detection of iron was enhanced by a factor of more than 15. Therefore, the disruption of the protein shell by plasma etching or ion milling emerges as a fundamental prerequisite to perform an accurate chemical characterization of the ferritin core.

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