Small gold clusters (<1 nm) protected by a glutathione (GSH) monolayer were fractionated into six components by polyacrylamide gel electrophoresis, and their chemical compositions were investigated by electrospray ionization mass spectroscopy. The results demonstrate isolation of a series of magic-numbered gold clusters, Au18(SG)11, Au21(SG)12, Au25+/-1(SG)14+/-1, Au28(SG)16, Au32(SG)18, and Au39(SG)23. Their optical absorption spectra are highly structured with clear absorption onsets, which shift toward higher energies with reduction of the core size. These molecular-like gold clusters exhibit visible photoluminescence. The results reported herein provide helpful guidelines or starting points for further experimental and theoretical studies on structures, stabilities, and optical properties of monolayer-protected gold clusters.

Carboxylate-modified gold nanoparticles have been synthesized in a single-phase system based on the reduction of hydrogen tetrachloroaurate(III) by sodium borohydride in methanol using mercaptosuccinic acid (MSA) as the stabilizing thiol ligand. Five samples with diameters of 10.2, 10.8, 12.8, 19.4, and 33.6 Å have been prepared as water-redispersible powders through decreasing the initial MSA/HAuCl4 molar ratio from 2.5 to 0.5. These samples were characterized by X-ray diffraction, transmission electron microscopy, elemental analysis, thermogravimetric analysis, ζ-potential measurement, Fourier transform infrared spectroscopy, and UV−vis spectroscopy. The results show that a large number of the particles are fcc single crystals with the polyhedral morphology of a truncated octahedral motif and that a self-assembled monolayer of thiolates has indeed formed through the adsorption of mercapto groups on the gold particle surface; the maximum packing density of the thiolates is 15.23 Å2 per mercapto group. The surface structures of the powders are clearly defined; MSA on the particle surface exists in the form of sodium carboxylate, and one MSA unit combines with one H2O molecule.

We have succeeded for the first time in preparing a pair of gold nanocluster enantiomers protected by optically active thiols: D- and L-penicillamine (D-Pen and L-Pen). Circular dichroism (CD) spectroscopy confirmed the mirror image relationship between the D-Pen-capped and the L-Pen-capped gold nanoclusters, suggesting that the surface modifier acts as a chiral selector, and that the nanoclusters have well-defined stereostructures as common chiral molecules do. No CD signals could be obtained when the gold nanoclusters were synthesized by using a racemic mixture (rac-Pen). These chiroptical properties were investigated for the three separated fractions of each of the gold nanoclusters (D-Pen-capped, L-Pen-capped, or rac-Pen-capped clusters) by polyacrylamide gel electrophoresis (PAGE). Each fractioned component has the mean diameter of 0.57, 1.18, or 1.75 nm that was determined by a solution-phase small-angle X-ray scattering. With a decrease in the mean cluster diameter, optical activity or anisotropy factors gradually increased. On the basis of the kinetic and the structural considerations, the origins of large optical activity of the gold nanocluster enantiomers are discussed.