ARTICLE pubs.acs.org/ac
Soft X-ray Induced Photoreduction of Organic Cu(II) Compounds Probed by X-ray Absorption Near-Edge (XANES) Spectroscopy Jianjun Yang,†,‡ Tom Regier,§ James J. Dynes,§ Jian Wang,§ Jiyan Shi,† Derek Peak,|| Yidong Zhao,^ Tiandou Hu,^ Yingxu Chen,*,† and John S Tse*,‡ †
Institute of Environmental Science and Technology, Zhejiang University, Hangzhou 310029, P.R. China Department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon, Canada S7N 5E2 § Canadian Light Source Inc., University of Saskatchewan, Saskatoon, Canada S7N 0X4 Department of Soil Science, University of Saskatchewan, Saskatoon, Canada S7N 5A8 ^ Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Science, Beijing 100049, P.R. China
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bS Supporting Information ABSTRACT: Photoreduction is a major obstacle for using the X-ray absorption near-edge structure (XANES) fingerprint to perform metal speciation at the molecular level in biological and environmental samples, especially for metalloproteins. In this study, soft X-ray induced photoreduction was observed in organic Cu(II) compounds during XANES measurements in a third-generation synchrotron source. Next Cu L3-edge, O K-edge, and C K-edge XANES spectroscopy, together with the scanning transmission X-ray microscopy (STXM), were used to probe the specific radiation damage processes of Cu acetate with similar local structures to Cu metalloproteins. Breakup of the Cu Cu bond was hypothesized for the initial photoreduction of Cu acetate. The following radiation damage of Cu acetate produced CuO and an organic Cu(I) compound with a CdC bond, and the further photoreduction of the resulting CuO to Cu metal was also demonstrated. Our results indicated the importance of consideration of photoreduction during soft XANES measurements for the solid state compounds with high valence metals. Reducing the radiation dose to ∼0.1 MGy effectively prevented the photoreduction of organic Cu(II) compounds during these measurements. This proposed radiation damage mechanism in Cu acetate may be generally useful in explaining the photoreduction process in Cu metalloproteins.
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ANES spectroscopy is a widely used and element specific probe to study the density of unoccupied states in targeted materials, especially for heterogeneous environmental and biological samples.1 4 However, the interaction of X-ray and materials can generate a large number of electrons (typically 1010 to 1012 per second by synchrotron radiation) and then cause radiation damage on samples, especially for low Z elements. Possible consequences of radiation damage include the breaking of chemical bonds, redox processes, and generation of free radicals as well as long-range rearrangements of molecules in crystals.5 Therefore, XANES may reflect the density of unoccupied states of the damaged products rather than the original sample if significant radiation damage occurs. Organic copper complexes are very common in the environment,6,7 especially in living organisms as metalloproteins mainly responsible for electron transfer.8 Therefore, it is of fundamental importance to characterize Cu oxidation states to reveal its bioavailability and biological functions. Hard X-ray absorption spectroscopy has been used to probe the Cu oxidation state in these samples.9,10 Photoreduction, as one of the major radiation damage processes, has been observed during r 2011 American Chemical Society
studies of organic Cu complexes6,11 and metalloproteins using hard X-ray beams.12,13 However, the mechanism of photoreduction in organic Cu complexes induced by X-ray has not yet been fully understood.14 Photoreduction of these Cu compounds are probably due to the Auger or secondary electrons (typically
