Aqueous iron interacts as strong as solid iron

Advances spectroscopy research: HZB scientists come up with new method for examining the structure of metal ions-complexes in solution

HZB's Young Investigator Group for Functional Materials in Solution headed by Prof. Dr. Emad Aziz has already applied the new method in iron ions dissolved in aqueous solution. Their findings have now been published in the Journal of Physical Chemistry Letters (DOI: 10.1021/jz300403n).

The researchers used X-ray radiation – generated by HZB's own electron storage ring BESSY II – to examine iron ions in aqueous solution. "We measured the absorption strength of the X-rays from our Fe 2+ and oxygen ions in the liquid micro-beam" explains Malte Gotz, who performed the experiments as part of his graduate research. "From here, we were able to draw conclusions regarding the electronic structure of the iron ions and further more to investigate the interaction of iron ions with the water solvent, " says Gotz.

The researchers used a new approach to measuring X-ray absorption of liquids. "Oxygen, which, along with iron ions, is also present in the solution, turns out to play a rather important role. If X-ray light is used to irradiate – and thereby the oxygen that is present in the water will absorb this radiation, and will end up emitting light for a brief period of time. You might compare it to the glow-in-the-dark of a clock," Gotz explains. If you now reduce the amount of incoming radiation by having a different material – in this case ionic iron absorbs it, it will directly reduce the amount of radiation emitted by the oxygen. "This in turn allows us to measure the absorption strength of ionic iron," says Gotz.

According to Emad Aziz, by definition, any measurement obtained at the free fluid stream is highly accurate. "A major advantage of our protocol is the fact that besides measuring only the signal from our fluid stream – without having to account for any artifacts induced by the surrounding container – we are also measuring a continuously fresh liquid sample," Aziz explains. In their studies the scientists  found that iron ions suspended in the solution  interact strongly with the solvent; a conclusion drawn by the strong 'Coster Kroenig decay process’  observed in the liquid system, which were thus far observed only in  solid iron. "We concluded that ions interact more strongly with water than was previously thought," says Aziz.

Our next step is to apply the new method to biological functional materials where the transition metals play key biological functions- such as oxygen-carrying iron in human blood. New and deep insights into these catalysts’ structure and function are the challenge of our scientific research.