Fluorine-labeling as a diagnostic for thiol-ligand and gold nanocluster self-assembly

An ω-fluorine-labeled oxyethylene thiol ligand, F(CH₂CH₂O)₂CH₂CH₂SH, was synthesized, characterized and incorporated into mixed self-assembled monolayers with CH₃(OCH₂CH₂)₃SH onto a planar gold substrate and onto 2 nm gold nanoclusters. The fluorine-labeled nanocluster was self-assembled onto gold substrates using alkane dithiol (HS(CH₂)_nSH; n = 5, 8, 11) and oxyethylene dithiol (HS(CH₂CH₂O)_nCH₂CH₂SH; n = 1, 2, 3) linking agents with equivalent chain lengths for comparative purposes. X-ray photoelectron spectroscopy (XPS) was used to track the fluorine-label in the self-assembly operations and to evaluate the effectiveness of the dithiols. For adequate XPS sensitivity at least 10% of the monolayer-forming molecules should be functionalized with this fluorine-label. In the comparative self-assembly of the fluorine-labeled gold nanoclusters in chloroform solution, the alkane dithiols were observed to be the more effective linking agents. This effectiveness correlates with the XPS analysis of alkane dithiols self-assembling onto the gold substrates with a higher packing density and with a larger fraction of molecules having one thiol group as opposed to two bonded to the gold surface. The oxyethylene dithiols self-assemble with a smaller packing density and a smaller fraction of molecules with an unbonded thiol group available for self-assembly.