E.G. Wrigglesworth, J.H. Johnston
Victoria University of Wellington,
Keywords: dichroic effect, gold, nanoparticles, polymer, nanocomposites
Summary:Gold nanoparticles have long been utilised for their attractive colours. A well-known example is the Lycurgus cup, dated at the 4th century, which appears red when viewed in transmitted light and green in reflected light . This unique optical phenomenon, known as the dichroic or Lycurgus cup effect, has been attributed to the presence of gold, silver and copper nanoparticles in the glass of the cup. However due to the highly precious nature of the cup, no detailed studies have been carried out on characterising the nanoparticles or their distribution in the glass to understand this effect. Gold nanoparticles undergo a localised surface plasmon resonance absorption due to the resonance interaction of electromagnetic radiation with the conduction band electrons of the metal surface. The wavelengths of visible light that are not absorbed in this process correspond to the colour of the nanoparticles in transmitted light. Nanogold is also effective at scattering visible light of certain wavelengths and this gives us the colour in reflected light. It is well understood that larger nanoparticles scatter a greater proportion of light (and absorb less light), compared to smaller nanoparticles . However, despite this effect being observed a number of times, the ability of certain nanoparticles to transmit one wavelength of light but scatter a different wavelength is poorly understood . We have successfully demonstrated this dichroic effect in aqueous gold nanoparticle colloids through a novel synthesis, which we present here along with a characterisation of the nanoparticles and the ensuing dichroic effect. We use a specific stabilising agent which has enabled us to control the extent of the effect through the generation of nanoparticles having somewhat complex shapes and a planar type morphology. Decreasing the concentration of the stabilising agent has resulted in a colour change in transmitted light through from red to purple/grey, and a significant increase in the scattering of orange/brown coloured light, thereby causing the nanoparticles to exhibit dichroic properties. These colloids contrast sharply with the methods commonly used to synthesise gold nanoparticles which usually produce colloids that are red in colour and do not significantly scatter light, i.e. the dichroic effect is not observed. It has been found that the size and shape of the nanoparticles can be correlated with the observed dichroic optical properties of the colloids, with high scattering being attributed to the large, relatively ‘planar’ nanoparticles observed via transmission electron microscopy. The implication of these findings on our understanding of the dichroic effect will be discussed. This improved understanding has further enabled the transfer of these nanoparticles from an aqueous into an acrylic system with complete conservation of the dichroic property. The production of dichroic polymer materials has novel applications in design and other fields. References:  I. Freestone, N. Meeks, M. Sax, Gold Bull. 40 (2007) 270.  P.K. Jain, K.S. Lee, I.H. El-Sayed, M.A. El-Sayed, J. Phys. Chem. B 110 (2006) 7238.  B. Karmakar, T. Som, S.P. Singh, M. Nath, T. Indian Ceram. Soc. 69 (2010) 171.