Synthesis and Stability of Monolayer-Protected Au38 Clusters

Abstract

A synthesis strategy to obtain monodisperse hexanethiolate-protected Au38 clusters based on their resistance to etching upon exposure to a hyperexcess of thiol is reported. The reduction time in the standard Brust-Schiffrin two-phase synthesis was optimized such that Au38 were the only clusters that were fully passivated by the thiol monolayer which leaves larger particles vulnerable to etching by excess thiol. The isolated Au38 was characterized by mass spectrometry, thermogravimetric analysis, optical spectroscopy, and electrochemical techniques giving Au38(SC6)22 as the molecular formula for the cluster. These ultrasmall Au clusters behave analogously to molecules with a wide energy gap between occupied (HOMO) and unoccupied levels (LUMO) and undergo single-electron charging at room temperature in electrochemical experiments. Electrochemistry provides an elegant means to study the electronic structure and the chemical stability of the clusters at different charge states. We used cyclic voltammetry and scanning electrochemical microscopy to unequivocally demonstrate that Au38 can be reversibly oxidized to charge states z = +1 or +2; however, reduction to z = -1 leads to desorption of the protecting thiolate monolayer. Although this reductive desorption of thiol from the cluster surface is superficially analogous to electrochemical desorption of planar self-assembled monolayers (SAMs) from macroscopic electrodes, the molecular details of the process are likely to be complicated based on the current view that the thiolate monolayer in clusters is in fact composed of polymeric Au-S complexes.

Temperature-Dependent Emission of Monolayer-Protected Au38 Clusters

“…7,14,26,27 For comparison, a room temperature emission spectrum is plotted in the same figure (red line). The low-energy side of this emission band is at approximately the same energy as the absorption onset.…”

“…[1][2][3][4][5] The synthesis has reached the stage where clusters with a defined structural formula of Au(RS) can be routinely isolated. [6][7][8][9][10][11][12][13][14] These advances in synthesis and purification lead to the recent breakthrough in the crystal structure determination of the cluster. 15 It revealed that the monolayer is far from simple involving Au-thiolate oligomers that form a shell around the Au core in a "divide and protect motif".…”

Luminescent Ag7 and Ag8 Clusters by Interfacial Synthesis

“…These include Au 8 , [4] Au 18 , [5] Au 25 , [6] Au 38 , [7] and so on. Au 11 has also been the subject of recent research.…”

Towards Rational Design of Nanoparticle Catalysis in Ionic Liquids

“…For example, Au 15 , Au 18 , Au 25 [92], Au 38 [93] and Au 144 NCs [94] can all be prepared, and some of them have been fully characterized by single crystal X-ray diffraction [95]. These clusters break down the barriers between molecular systems and NP systems and are potential catalysts with unusual properties.…”