Reactive capture of gold nanoparticles by strongly physisorbed monolayers on graphite.

Anthracene Diels Alder adducts (DAa) bearing two long side chains (H-(CH(2))(22)O(CH(2))(6)OCH(2)-) at the 1- and 5-positions form self-assembled monolayers (SAMs) at the phenyloctane-highly oriented pyrolytic graphite (HOPG) interface. The long DAa side chains promote strong physisorption of the monolayer to HOPG and maintain the monolayer morphology upon rinsing or incubation in ethanol and air-drying of the substrate. Incorporating a carboxylic acid group on the DAa core enables capture of 1-4 nm diameter gold nanoparticles (AuNPs) provided (i) the monolayer containing DAa-carboxylic acids is treated with Cu(2+) ions and (ii) the organic coating on the AuNP contains carboxylic acids (11-mercaptoundecanoic acid, MUA-AuNP). AuNP capture by the monolayer proceeds with formation of Cu(2+)-carboxylate coordination complexes. The captured AuNP appear as mono- and multi-layered clusters at high coverage on HOPG. The surface density of the captured AuNPs can be adjusted from AuNP multi-layers to isolated AuNPs by varying incubation times, MUA-AuNP concentration, the number density of carboxylic acids in the monolayer, the number of MUA per AuNP, and post-incubation treatments.