Glutathione-Protected Gold Clusters Revisited:  Bridging the Gap between Gold(I)−Thiolate Complexes and Thiolate-Protected Gold Nanocrystals

Abstract

Small gold clusters (approximately 1 nm) protected by molecules of a tripeptide, glutathione (GSH), were prepared by reductive decomposition of Au(I)-SG polymers at a low temperature and separated into a number of fractions by polyacrylamide gel electrophoresis (PAGE). Chemical compositions of the fractionated clusters determined previously by electrospray ionization (ESI) mass spectrometry (Negishi, Y. et al. J.Am. Chem. Soc. 2004, 126, 6518) were reassessed by taking advantage of freshly prepared samples, higher mass resolution, and more accurate mass calibration; the nine smallest components are reassigned to Au10(SG)10, Au15(SG)13, Au18(SG)14, Au22(SG)16, Au22(SG)17, Au25(SG)18, Au29(SG)20, Au33(SG)22, and Au39(SG)24. These assignments were further confirmed by measuring the mass spectra of the isolated Au:S(h-G) clusters, where h-GSH is a homoglutathione. It is proposed that a series of the isolated Au:SG clusters corresponds to kinetically trapped intermediates of the growing Au cores. The relative abundance of the isolated clusters was correlated well with the thermodynamic stabilities against unimolecular decomposition. The electronic structures of the isolated Au:SG clusters were probed by X-ray photoelectron spectroscopy (XPS) and optical spectroscopy. The Au(4f) XPS spectra illustrate substantial electron donation from the gold cores to the GS ligands in the Au:SG clusters. The optical absorption and photoluminescence spectra indicate that the electronic structures of the Au:SG clusters are well quantized; embryos of the sp band of the bulk gold evolve remarkably depending on the number of the gold atoms and GS ligands. The comparison of these spectral data with those of sodium Au(I) thiomalate and 1.8 nm Au:SG nanocrystals (NCs) reveals that the subnanometer-sized Au clusters thiolated constitute a distinct class of binary system which lies between the Au(I)-thiolate complexes and thiolate-protected Au NCs.

Functionalized Au22 Clusters: Synthesis, Characterization, and Patterning

“…The laser desorption mass spectrum of clusters such as Au 25 does not give the integral features of the cluster core along with the ligands (36,41). However, in favorable cases, the MALDI MS spectra show characteristic features.…”

“…The 1 H NMR spectrum is given in the Supporting Information ( Figure S1). Au 22 was characterized by a series of measurements such as UV/vis, steadystate fluorescence, and XPS (36). Figure 1A shows the UV/ vis spectra of Au 25 , Au 22 , and H 2 TPPOASH.…”

“…The PL spectrum of H 2 TPPOASH was taken in toluene, whereas that of the clusters was taken in DI water. The PL studies of Au 25 were done in great detail and have been reported recently (36)(37)(38). Emission of Au 25 occurs in the range of 600-800 nm with an emission maximum of 700 nm at 535 nm excitation.…”

“…Mass spectrometry confirmed that Au 28 (SG) 16 was the most abundant species in the synthesis (5). In the recent past, a series of -SG-protected gold clusters with well-defined compositions have been synthesized (36). The different fractions were separated utilizing poly(acrylamide) gel electrophoresis (PAGE).…”

“…The molecular compositions of these clusters were characterized by electrospray ionization mass spectrometry (ESI-MS). Tsukuda et al (36) characterized the elemental composition of all of the separated entities by ESI-MS and confirmed that the most abundant species is Au 25 (SG) 18 and not Au 28 (SG) 16 , as suggested originally (5). The ligand-exchange reaction of phosphine-stabilized gold clusters (Au 11 ) with GSH gave only Au 25 SG 18 in large yield, suggesting that it is the most stable one among the thiolated clusters from this class (37).…”