(176h) Tunable Mirrors Made From Gold Nanoparticle Assembly At the Oil-Water Interface

Nanoparticle self-assembly has opened the door to the design of materials with unique optical, electronic, and chemical properties. The interface between two immiscible liquids is a promising platform for nanoparticle assembly due to the rapid correction of defects afforded by the dynamic fluid interface, and optical applications afforded by the ability of light to access both sides of the interface. In particular, assembly of a two-dimensional array of metal nanoparticles at the oil-water interface enables the optical reflectance of the interface to be tuned according to the diameter of and interparticle spacing between the particles. Here we demonstrate the reversible or switchable assembly of reflective array of nanoparticles at a liquid interface. The films described herein possess both reversibility and mirror-like reflectance. Specifically, we show that 5 and 10 nm diameter gold nanoparticles, functionalized with an ion-pair, assemble spontaneously into pH-sensitive reflective sheets at the oil-water interface. The assembly occurs across a wide range of pH values (pH < 12) and more importantly, can be switched off and on reversibly by shifting the pH above and below pH 12, respectively.  We address the role of the surface functionality on the reversible assembly and optical properties of the film at the oil-water interface. We also demonstrate control over the interfacial assembly based on an applied electrical potential.