Probing the Influence of X-rays on Aqueous Copper Solutions Using Time-Resolved in Situ Combined Video/X-ray Absorption Near-Edge/Ultraviolet−Visible Spectroscopy

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Abstract
Time-resolved in situ video monitoring and ultraviolet−visible spectroscopy in combination with X-ray absorption near-edge spectroscopy (XANES) have been used for the first time in a combined manner to study the effect of synchrotron radiation on a series of homogeneous aqueous copper solutions in a microreactor. This series included both non biologically relevant (pyridine, bipyridine, neocuproine, terpyridine, dimethylpyridine, ammonia, ethylenediamine, and 1,10-phenanthroline) and biologically relevant (histidine, glycine, and imidazole) ligands. It was found that when water is present as solvent, gas bubbles are formed under the influence of the X-ray beam. At the liquid−gas interface of these bubbles, in particular cases colloidal copper nanoparticles are formed. This reduction process was found to be influenced by the type of copper precursor salt (SO42-, NO3-, and Cl-), the ligands surrounding the copper cation, and the redox potential of the copper complexes (ranging between +594 and −360 mV). In other words, in some cases, no reduction was encountered (e.g., ammonia in the presence of SO42- and NO3-), whereas in other cases reduction to either Cu+ (neocuproine with SO42-) or Cu0 (e.g., histidine and imidazole both with SO42-, NO3-, and Cl-) was observed. These results illustrate the added value of video spectroscopy for the interpretation of in situ XANES studies. Not only do the results give an illustration of the parameters that are important in the redox processes that occur in biological systems, they also show the potential problems associated with studying catalytic processes in aqueous solutions by XANES spectroscopy.

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