Monolayer-Protected Gold Nanoparticles Functionalized with Halogen Bonding Capability- An Avenue for Molecular Detection Schemes

The use of functionalized nanoparticles (NPs) andtheir aggregation in the presence of a targeted analyte is a well-established molecular detection strategy predicated on harnessingspecific molecular interactions to the NP periphery. Molecules ableto specifically interact with the functionalized NPs alter the uniqueoptical and electrochemical properties of the NPs as a function ofinterparticle spacing. While many intermolecular interactions havebeen successfully exploited in this manner in conjunction withaqueous NP systems, the use of non-aqueous NPs in the samecapacity is significantly less explored. A fundamental interactionthat has not been previously investigated in NP schemes is halogenbonding (XB). XB is an orthogonal, electrostatic interactionbetween a region of positive electrostatic potential (delta+) on ahalogen atom (i.e., XB donor) and a negative (delta-) Lewis base (XB acceptor) molecule. To couple XB with NP systems, ligandsfeaturing a molecular structure that promotes XB interactions need to be identified, optimized, and synthesized for subsequentattachment to NPs. Herein, density functional theory (DFT) and NMR techniques are used to identify a strong XB-donor moiety(-C6F4I) and a synthetic scheme for a thiolate ligand featuring that functionality is devised and executed with high purity/yield(78%). Ligand-exchange reactions allow functionalization of non-aqueous alkanethiolate-protected gold NPs or monolayer-protectedclusters (MPCs) with the XB-donor ligands. Functionalized MPCs (f-MPCs), within both assembledfilms and in solution, areshown to engage in XB interactions with target XB-acceptor molecules. Molecular recognition events, including induced aggregationof the f-MPCs, are characterized with UV-vis spectroscopy, cyclic voltammetry, TEM imaging, and diffusion-ordered spectroscopyNMR with limits of detection of 50-100 nM for strong XB acceptors. While fundamental exploration of XB interactions is ongoing,this study represents a step toward utilizing XB within molecular detection schemes, an application with implications forsupramolecular chemistry, forensic, and environmental chemical sensing.