Structural and electronic properties of protein/thiolate-protected gold nanocluster with "staple" motif: A XAS, L-DOS, and XPS study.

Following the recent breakthrough of total structural determination of a Au-thiolate nanocluster [P. Jadzinsky et al., Science 318, 430 (2007)], extensive interests have been stimulated to unveil (or revisit) the structure-property relationship of various thiolate-Au nanostructures in light of the new finding of -SR-(Au-SR)(x)- "staple" motif. Here, we present experimental x-ray absorption spectroscopy (XAS) and x-ray photoelectron spectroscopy (XPS) results on the local structure and electronic properties of thiolate-protected Au nanocluster encapsulated in bovine serum albumin (Au-BSA) together with theoretical calculation of projected local density of states (l-DOS) of Au(25)(SR)(18) model cluster. Analysis of the Au L(3)-edge extended x-ray absorption fine structure (EXAFS) of Au-BSA suggested that the nanocluster is Au(25) with Au-thiolate "staple" motif. X-ray absorption near-edge structure (XANES) and Au 4f XPS were used to probe the electronic behavior of Au-BSA. The Au d-electron density of Au-BSA was found to decrease by 0.047 e(-) relative to that of the bulk. A self-consistent real space Green's function approach implemented in ab initio FEFF8 program was used to calculate the l-DOS of Au(25)(SR)(18) and other model clusters from a site-specific perspective. The theoretical results are in good agreement with the experimental d-DOS data of Au-BSA and, importantly, systematically illustrate the effect of Au-thiolate "staple" motif on the electronic behavior of Au(25)(SR)(18). The present work sheds light on the structure-property relationship of thiolate-protected Au(25) from both experimental and theoretical perspectives and illustrates the usefulness of XAS/l-DOS method in such studies.