Supramacromolecular organization of gold nanocrystals capped with amphiphilic hyperbranched polyethyleneimine

Abstract

The supramolecular structural aspects of hetero-assemblies constituted of gold nanoparticles capped with amphiphilic unimolecular micelles were studied using synchrotron-based small angle X-ray scattering (SAXS). Experimental results revealed that straightforward transfer of citrate-capped Au nanoparticles from an aqueous environment to a toluenic solution of amphiphilic hyperbranched polymers results in the spontaneous integration of the nanocrystals into the extended hydrophilic domains of self-assembled supramolecular structures. In this way, we were able to self-organize metal–polymer nanoarchitectures in solution displaying interesting thermoactive functions, i.e.: hybrid assemblies exhibiting negative thermal expansion coefficients. We consider that this strategy has potentiality to realize self-organized supramolecular hetero-assemblies as it provides an alternative methodology to spontaneously integrate nanoscale building blocks into preformed supramolecular objects.

Introduction

The combined use of colloidal nanocrystals and amphiphilic polymers as building blocks for the creation of hybrid nanoassemblies has found incredible resonance within the soft matter research community in recent years. In this context, one of the ongoing challenges for supramolecular materials science is the development of flexible strategies and protocols that can facilitate the assembly of inorganic and polymeric building blocks into organized functional structures [1]. Self-assembly of gold nanoparticles into two-dimensional (2D) or three-dimensional (3D) arrangements has granted access to a myriad of structured nanoscale materials amenable to multiple technological applications [2]. Performance of these assemblies strongly depends on the aggregation state, both in terms of overall size and interparticle distance [3]. As such, devising strategies to control nanoparticle assembly in solution and in the solid state is mandatory to harness the fascinating properties of nanoparticles in practical applications like biosensing or electronic/optoelectronic devices [4], [5]. In this way, polymer scaffolds can be exploited not only to stabilize the NP suspension but also to induce ordering or to confer functionality to the metal clusters. Rotello and co-workers pioneered the use of dendrimers as integral components in NP assembly [6]. Electrostatic interaction between Au NPs and PAMAM dendrimers enabled fine control over interparticle distance and provided a route to create thin films of Au NPs spaced by PAMAM dendrimers [7]. Recently, Tang et al. [8] demonstrated that hyperbranched unimicelles, i.e.: dendritic structures with micelle-like properties [9], consisting of palmitic acid-modified hyperbranched polyethylenimine can be successfully employed to transfer citrate-protected Au NPs from water into organic solvents without the aid of other compounds.. In contrast to perfectly branched dendrimers, that are monodisperse “nano-objects” prepared through multistep synthesis [10], hyperbranched polymers are prepared in a one-pot synthesis leading to heterogeneous products with a distribution in molar mass and branching [11]. Even so, they also display unique properties as manifested by their globular structure and large number of terminal groups [12]. Up to now there are no studies revealing the presence of supramolecular order and the formation of superstructural assemblies in solutions of Au NPs capped with amphiphilic hyperbranched polymers. Similarly, the integration and processing of such supramolecular systems into thin-film configurations remain unexplored. Under this perspective, we have explored the supramolecular structural aspects of hetero-assemblies constituted of gold nanoparticles capped with amphiphilic unimolecular micelles generated by straightforward phase transfer protocols typically used in nanoparticle synthesis (Fig. 1). Our experimental evidence reveals the spontaneous integration of the gold nanocrystals into self-assembled unimicelle domains, thus leading to the self-organization metal–polymer nanoarchitectures exhibiting thermoactive functions.