Calix[4]arene-Functionalised Silver Nanoparticles as Hosts for Pyridinium-Loaded Gold Nanoparticles as Guests

A series of lipophilic ~5 nm gold nanoparticles (AuNPs) having a mixed organic layer consisting of 1-dodecanethiol and 1-(11-mercaptoundecyl) pyridinium bromide was synthesised by reacting in toluene tetraoctylammonium bromide-stabilised AuNPs with different mixtures of the two thiolate ligands. A bidentate omega-alkylthiolate calix[4]arene derivative was instead used as a functional protecting layer of ~3 nm silver nanoparticles (AgNPs). The functionalised nanoparticles (NPs) have been all characterised by transmission electron microscopy (TEM), and by UV-visible and X-ray photoelectron spectroscopy (XPS). Recognition of the pyridinium moieties loaded on the AuNPs by the calix[4]arene units immobilised on the AgNPs was demonstrated in solution of weakly polar solvents by UV-visible titrations and DLS measurements. The extent of the Au-AgNPs aggregation, evidenced through low-energy shift of their surface plasmon bands (SPB), was strongly dependent on the loading of the pyridinium moieties present in the organic layer of the AuNPs. Extensive aggregation between dodecanethiol-capped AuNPs and the Ag calix[4]arene-functionalised ones was also promoted by the action of a simple N-octyl pyridinium difunctional supramolecular linker. This linker can either interdigitate through its long fatty tail in the organic layer of the dodecanethiol-capped AuNPs, and simultaneously interact via its pyridinium moiety with the calix[4]arene units at the surface of the modified AgNPs