Polyoxometalate‐Engineered Building Blocks with Gold Cores for the Self‐Assembly of Responsive Water‐Soluble Nanostructures

Abstract

The controlled assembly of gold nanoparticles (AuNPs) with the sizeo fq uantum dots into predictable structures is extremely challenging as it requires the quantitatively and topologically precise placement of anisotropic domains on their small, approximately spherical surfaces.W e herein address this problem by using polyoxometalate leaving groups to transform 2nmd iameter gold cores into reactive building blocks with hydrophilic and hydrophobic surface domains whose relative sizes can be precisely tuned to give dimers,c lusters,a nd larger micelle-like organizations.U sing cryo-TEM imaging and 1 HDOSY NMR spectroscopy, we then providea nu nprecedented "solution-state" picture of howt he micelle-like structures respond to hydrophobic guests by encapsulating them within 250 nm diameter vesicles whose walls are comprised of amphiphilic AuNP membranes.T hese findings provide av ersatile new option for transforming very small AuNPs into precisely tailored building blocks for the rational design of functional water-soluble assemblies. Supramolecular structures comprising gold nanoparticles (AuNPs) have potential applications in medical imaging, photodynamic therapy,drug delivery,catalysis,and optical or electronic devices. [1] Fundamental to the rational design and synthesis of these superstructures is the development of AuNP building blocks with anisotropic ligand shells programmed to react in ap redictable fashion. This,i nt urn, requires precise control over the topological organization of diverse ligand-shell domains on the surfaces of the AuNPs. [2] While considerable progress has been made in modifying the surfaces of relatively large or anisotropic gold cores, [3] this is not the case for very small, approximately spherical AuNPs, for which precise control over the formation of ligand-shell domains remains exceedingly difficult. [4] This is particularly true in water, the solvent of choice for numerous applications of AuNP superstructures in green chemistry and medicine.Important advances in this direction include the surface modification of AuNPs to produce amphiphilic Janus particles [5] on solid templates [6] and at liquid-air [7] or liquidliquid [8] interfaces.T oovercome the low yields associated with the limited surface areas of these interfaces,m ethods have been developed to combine mixtures of different ligands at the particle surface, [9] with spontaneous phase separation [2d,10] leading to anisotropic ligand domains.N evertheless,t he synthesis of amphiphilic AuNPs in which both the relative sizes and locations of different ligand-shell domains are quantitatively controlled remains as erious obstacle to the design of well-defined AuNP building blocks.In this regard, polyoxometalate (POM) ligand shells provide finely tunable control over the reactions of thiolates with spherical AuNPs in water. [11] Forexample,POM ligands were recently used to direct the assembly of 200 nm AuNP supraspheres,w hich served as soluble hosts for over two million hydrophobic guests,anuptake capacity rivalling those of z...