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Nanosheets and Capsids from Precise Gold Nanoclusters

December 24, 2016


Assembling of colloidal particulate building blocks into 2D and 3D structures, referred to as superlattices (SLs),1-3 has been contributing to many practical applications.4-7 The essential criterion for constructing these structures is the synthesis of monodisperse nanoparticles. Although there have been enormous efforts,8-9 controlling the size and homogeneity of nanoparticles is still a challenging task due to the difficulty originated from the huge number of atoms as well as lacking a strong structure.

Recently, a collaborative work10 between research groups from University of Jyväskylä and Aalto University published on Angewandte Chemie International Edition revealed a novel type of supracolloidal self-assembly structure made possible by gold nanoclusters, resulting in 2D crystals and 3D capsids. Different from generally accepted pathway which normally use nanoparticles as building blocks, this work started with atomically precise nanoclusters, i.e. Au102(pMBA)44, (pMBA = p-mercaptobenzoic acid) which guaranteed the unparalleled monodispersity towards successful assembling. The gold clusters linked with each other efficiently with the aid of pMBA-based hydrogen bonding, and formed template-free 2D nanosheets and closed spherical capsids.

There are two key points for the assembly in this work. 1) The cluster is not spherical and the ligands point towards the equatorial plane, favoring the inter-cluster hydrogen bonding; 2) It is easy to control the carboxylic acid deprotonation by adjusting solvent condition. The gold clusters were firstly synthesized based on a previous method in which tetrachloroauric(III) acid was reduced by sodium borohydride in the presence of ligands in aqueous solution.11-12 Sodium hydroxide was then used to partially deprotonate the ligands. With coexistence of neutral carboxylic acid groups and deprotonated carboxylate groups, clusters were dialyzed against methanol to generate template-free nanosheets, which was a great advancement compared to conventional solvent casting method requiring a substrate.

Spherical capsids constructed by monolayer of clusters were efficiently prepared by mixing aqueous cluster solution with methanol. The capsid structures were preserved by hydrogen bonding dimerization as well as the electrostatic interactions between cations and negatively charged carboxylates. It was noteworthy that the hydrogen bonding did not only serve as inter-cluster connections, but also enabled joining of different capsids, shedding light on the possibility to design lightweight framework materials.

Looking into the future, the reported assembling method could be used for various number of gold atoms such as the popular Ag25 and Au38 etc.13 As different clusters possess quite distinctive properties, it is feasible to realize adjustment of size, specific structures, and stability of assembled 2D and 3D structures. As the closest counterpart of gold among other noble metal systems, atomically precise silver nanoclusters have also been successfully synthesized in recent years.14-20 With broad applications in many areas such as water filtration,21 sensing,22 catalysis,23-24 textile industry,25 and biology,26-27 it can be expected that assembling silver nanoclusters into higher dimensional structures will gain increasing attention and achieve fast development soon after.


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