Host–guest chemistry with water-soluble gold nanoparticle supraspheres

Abstract

The uptake of molecular guests, a hallmark of the supramolecular chemistry of cages and containers, has yet to be documented for soluble assemblies of metal nanoparticles. Here we demonstrate that gold nanoparticle-based supraspheres serve as a host for the hydrophobic uptake, transport and subsequent release of over two million organic guests, exceeding by five orders of magnitude the capacities of individual supramolecular cages or containers and rivalling those of zeolites and metal-organic frameworks on a mass-per-volume basis. The supraspheres are prepared in water by adding hexanethiol to polyoxometalate-protected 4 nm gold nanoparticles. Each 200 nm assembly contains hydrophobic cavities between the estimated 27,400 gold building blocks that are connected to one another by nanometre-sized pores. This gives a percolated network that effectively absorbs large numbers of molecules from water, including 600,000, 2,100,000 and 2,600,000 molecules (35, 190 and 234 g l ) of para-dichorobenzene, bisphenol A and trinitrotoluene, respectively.H ost-guest phenomena that involve the uptake of gases and small molecules are associated with the supramolecular chemistry of soluble capsules, cages and containers [1][2][3][4][5] or, alternatively, with heterogeneous reactions of porous solid-state materials such as zeolites 6 and metal-organic frameworks 7 . Only now, however, are organized assemblies of metal nanoparticles beginning to serve in a similar capacity as hosts for molecular guests. In organic solvents, for example, light-induced dipoledipole interactions between gold nanoparticles with azobenzenefunctionalized thiolate ligands were recently used to entrap and catalyse the reactions of polar and alkylaromatic substrates 8 . By design, substrates were entrapped during nanoparticle aggregation to overcome the poor uptake and diffusion of molecular guests into colloidal nanocrystal assemblies. However, the uptake of molecular guests, a trait of supramolecular and solid-state chemistry, has so far not been achieved for colloidal nanoparticle assemblies.For that to occur, a thermodynamically favourable driving force must be combined with a porous structure whose interior architecture features host cavities, along with pathways for the effective diffusion of molecular guests from the exterior (bulk solution) to host domains deeply buried in the assembly's interior. If this were achieved, colloidal metal-nanoparticle assemblies could emerge as a new class of functional nano-engineered structures that are uniquely positioned between supramolecular containers and porous solid-state materials.We report a new class of functional assemblies, wherein the hydrophobic effect 9,10 drives the spontaneous uptake of alkyl and alkylaromatic guests 11 by porous 200 nm diameter supraspheres whose host capacities are orders of magnitude larger than those of individual cages or containers. On a mass-per-volume basis, the level of uptake rivals those of zeolites and metal-organic frameworks, which, for methane at elevated pressures, r...

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