Experimental and Theoretical Studies on Sustainable Synthesis of Gold Sol Displaying Dichroic Effect.

Anshuman Jakhmola,Maurizio Celentano,Raffaele Vecchione,Valentina Onesto,Francesco Gentile,Paolo Netti

doi:10.3390/nano11010236

Abstract

Gold nanoparticles depending on their shape and mixtures of multiple shapes can exhibit peculiar optical properties, including the dichroic effect typical of the Lycurgus cup, which has puzzled scientists for a long time. Such optical properties have been recently exploited in several fields such as paint technology, sensors, dichroic polarizers, display (LCD) devices, laser applications, solar cells and photothermal therapy among others. In this article, we have demonstrated a simple room temperature one-pot synthesis of gold sol displaying a dichroic effect using a slow reduction protocol involving only trisodium citrate as a reducing agent. We found that the dichroic gold sol can be easily formed at room temperature by reducing gold salt by trisodium citrate below a certain critical concentration. The sol displayed an orangish-brown color in scattered/reflected light and violet/blue/indigo/purple/red/pink in transmitted light, depending on the experimental conditions. With minor changes such as the introduction of a third molecule or replacing a small amount of water in the reaction mixture with ethanol, the color of the gold sol under transmitted light changed and a variety of shades of red, pink, cobalt blue, violet, magenta and purple were obtained. The main advantage of the proposed method lies in its simplicity, which involves the identification of the right ratio of the reactants, and simple mixing of reactants at room temperature with no other requirements. TEM micrographs displayed the formation of two main types of particles viz. single crystal gold nanoplates and polycrystalline faceted polyhedron nanoparticles. The mechanism of growth of the nanoplates and faceted polyhedron particles have been described by an enhanced diffusion limited aggregation numerical scheme, where it was assumed that both trisodium citrate and the gold ions in solution undergo a stochastic Brownian motion, and that the evolution of the entire system is regulated by a principle of energy minimization. The predictions of the model matched with the experiments with a good accuracy, indicating that the initial hypothesis is correct.

Highlights

The outstanding optical properties of colloidal solutions of coinage metals have been known to mankind since ancient times [1,2]
The proposed method to produce gold sol displaying a dichroic effect is environmentfriendly and further simplifies the Turkevich method [38] as the particles are synthesized under ambient conditions, saving unnecessary energy and water wastage during the refluxing process associated with the synthesis at boiling water temperatures
We have demonstrated the synthesis of dichroic gold sol at room temperature by an ecofriendly route

Summary

Introduction

The outstanding optical properties of colloidal solutions of coinage metals have been known to mankind since ancient times [1,2]. Nanomaterials 2021, 11, 236 and copper, absorb and scatter light very efficiently; this results in many types of vibrant colors [3,4]. These optical properties of metal nanoparticles were exploited by artists for centuries and were often used to stain glass and ceramics. Modern scientific evaluation of colloidal gold began much later with the pioneering work of Michael Faraday in the 1850s, when he accidentally created a ruby red color with gold salts [6] He found out that the color was due to the miniature gold particles, when he studied their light-scattering properties, which are collectively called the Faraday–Tyndall effect [7]. An impressive example of the optical effect of metal nanoparticles is the glass piece from the 4th century, the mysterious

Methods

Results

Conclusion