Syntheses of gold and silver dichroic nanoparticles; Looking at the Lycurgus cup colors

Abstract

Metallic nanoparticles are small particles, with dimensions of less than 100 nm, which have unique characteristics. For example, the color of a nanoparticle solution is given by the surface plasmon resonance, in contrast to only absorbance of a molecule. Metallic nanoparticles have been used in the antiquity for the production of colored glass. Many of the medieval cathedral red windows are in fact, stained with gold nanoparticles. However, presenting just a red solution to a class and explaining that the solution contains gold nanoparticles can be disappointing as the red color can be obtained in many different ways without the need of nanoparticles. Something peculiar, like dichroism, can, on the other hand, shine the interest of an observer to the "nano-world". Dichroic nanoparticles show different colors when illuminated from the front (scattering) or from the back (transmission). This effect is unique for nanoparticles and nanostructures in general and can generate the "WOW"effect in class or during public demonstrations. In this article we present a simple synthesis of dichroic silver and gold nanoparticles. Solutions of such nanoparticles are a stunning visual media for demonstrations, outreach and engaging audience in the "nano"world.