Gold nanoparticles form mirror that can be switched on and off

To generate metamaterials with unusual optical properties not found in nature, scientists have used nanoscale building blocks like metal nanoparticles. But tuning those optical properties in real time has been challenging.

Researchers at Imperial College London have now created a switchable liquid mirror with gold nanoparticles. Changing the voltage makes the device flip between being a reflective mirror and a transparent window.

Anthony R. Kucernak, Joshua B. Edel, Alexei A. Kornyshev, and coworkers made their liquid mirror from 16-nm gold nanoparticles capped with 12-mercaptododecanoic acid (Nat. Mater. 2017, DOI: 10.1038/nmat4969). The nanoparticles assemble into a dense monolayer at the interface between immiscible aqueous and organic electrolyte solutions.

The density of the nanoparticles at the interface depends on the electrolyte concentrations and on the pH of the aqueous solution. The researchers dynamically control the assembly process by applying an electric potential between electrodes in the two solutions. The nanoparticles assemble tightly into a mirror at −200 mV and disassemble into a window at 200 mV.

The researchers demonstrated their device’s reflective and transmissive states by showing that when the mirror is turned on, it reflects a coin held in front of it. When turned off, the device makes paper money held behind it become visible.

“The interesting aspect of these results is the onset of reversible metallic behavior,” says David Fermin, a chemistry professor at the University of Bristol. But he thinks that these mirrors will be more useful for studying the structure of liquids at interfaces than as practical devices.