SMIRNOV ET AL. VOL. XXX ’ NO. XX ’ 000–000 ’ XXXX www.acsnano.org A C XXXX American Chemical Society Gold Metal Liquid-Like Droplets Evgeny Smirnov, †,§ Michea ´ l D. Scanlon, †,§ Dmitry Momotenko, † Heron Vrubel, † Manuel A. Me ´ ndez, † Pierre-Francois Brevet, ‡ and Hubert H. Girault * ,† † Laboratoire d'Electrochimie Physique et Analytique, Ecole Polytechnique Fédérale de Lausanne, Station 6, CH-1015 Lausanne, Switzerland and ‡ Institut Lumière Matière, ILM UMR CNRS 5306, Universite Claude Bernard Lyon, 110 Rue Ada Byron, 69622 Villeurbanne Cedex, France. § E. Smirnov and M. D. Scanlon contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. F ilms and coatings of nanoparticles (NPs) are key ingredient components in many emerging technologies due to their distinctive optoelectrical, 1,2 biological 3 and magnetic 4 properties. Their remarkable utility has sparked huge interest in their po- tential applications as liquid mirrors, 5 optical filters, 6 sensors, 7 catalysts, 8 anticorrosion 9 and antireflective 10 films, dialysis size selective membranes, 11 photovoltaic light harvesters 12 and antibacterial surfaces, 13 among others. The interface between two immiscible liquids, i.e., oil and water, is an extremely attractive scaffold at which to self-assemble NP films because of its defect-free pristine nature (facilitating reproducibly), transpar- ency (advantageous for optical applications), self-healing dynamism (allowing self-assembly errors to be corrected rapidly) and mechanical flexibility (permitting planar, curved or 3D deformations). 14,15 Self-assembly at liquidÀ liquid interfaces is a classical bottom-up tech- nique to produce 2D and 3D arrays and films of particles, especially metallic NPs. 14,16,17 Since the pioneering work of Yogev and Efrima in 1988, 18 who described the form- ation of metal liquid-like films (MeLLFs), many methods have been introduced to self-assemble metallic NPs at airÀliquid and liquidÀliquid interfaces. Reported ap- proaches include the addition of ethanol or methanol to the interfacial region, 19,20 the use of salts or “promoters” 21,22 and covalent cross-linking interactions. 11 An interesting approach by Han and co-workers involved displacing the stabilizing citrate ligands from the surface of colloidal gold nano- particles (AuNPs) with either fullerene (C 60 ) molecules 23 or carbon nanotubes (CNTs). 24 With this approach they formed dense gold nanocomposite films at waterÀdiethyl ether interfaces. Their proposed mechanism of film formation raised the possibility of charge transfer from the ligand (CNT or C 60 ) to the AuNPs during the adsorption process. The end result of citrate displacement fol- lowed by charge transfer was the reduction in charge density on the surface of the AuNPs, a prerequisite for dense gold film formation. As discussed in more detail vide supra, it is possible that the CNTs and C 60 molecules provide a “lubricating inter- facial glue” layer that binds the AuNPs at the interface, and indeed, the majority * Address correspondence to hubert.girault@epfl.ch. Received for review July 4, 2014 and accepted September 3, 2014. Published online 10.1021/nn503644v ABSTRACT Simple methods to self-assemble coatings and films encompassing nanoparticles are highly desirable in many practical scenarios, yet scarcely any examples of simple, robust approaches to coat macroscopic droplets with continuous, thick (multilayer), reflective and stable liquid nanoparticle films exist. Here, we introduce a facile and rapid one-step route to form films of reflective liquid-like gold that encase macroscopic droplets, and we denote these as gold metal liquid-like droplets (MeLLDs). The present approach takes advantage of the inherent self-assembly of gold nanoparticles at liquidÀliquid interfaces and the increase in rates of nanoparticle aggregate trapping at the interface during emulsification. The ease of displacement of the stabilizing citrate ligands by appropriate redox active molecules that act as a lubricating molecular glue is key. Specifically, the heterogeneous interaction of citrate stabilized aqueous gold nanoparticles with the lipophilic electron donor tetrathiafulvalene under emulsified conditions produces gold MeLLDs. This methodology relies exclusively on electrochemical reactions, i.e., the oxidation of tetrathiafulvalene to its radical cation by the gold nanoparticle, and electrostatic interactions between the radical cation and nanoparticles. The gold MeLLDs are reversibly deformable upon compression and decompression and kinetically stable for extended periods of time in excess of a year. KEYWORDS: gold nanoparticles . self-assembly . liquidÀliquid interface . optical filter . liquid mirror ARTICLE