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

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.

Preparation, Structure, and Properties of PVA–AgNPs Nanocomposites

“…The dichroic effect is very rare; achieving and maintaining such special conditions in a solution requires very precise preparation. There are only a few works describing the dichroic effect of Ag and Au nanoparticles [ 27 , 28 ]. Dichroism is the result of the presence of the different shapes and sizes of nanoparticles in the colloids.…”

Synthesis and Analysis of Polymorphic Silver Nanoparticles and Their Incorporation into the Polymer Matrix

“…Dekker et al studied the dichroic solutions Ag and Au and showed that polydisperse asymmetric nanoparticles were present in all of them [5]. The dichroic effect was observed in AuNPs colloids by Koolo et al and Jakhmola, and both found asymmetric nanoparticles in dichroic colloids [31][32][33]. This leads to the conclusion that the polydispersity and asymmetric shape of the nanoparticles is the main reason for the dichroic behavior.…”

Seed-Mediated Synthesis of Thin Gold Nanoplates with Tunable Edge Lengths and Optical Properties

“…Because the LSPR properties depend heavily on the size and shape of these nanocrystals, it is highly desirable to achieve customizable sizes and shapes toward tunable LSPR bands in the UV–visible near infrared (NIR) region, according to the needs of applications. To date, significant efforts have been devoted to the shape control of Ag and Au nanocrystals, leading to the convenient availability of nanocrystals with diverse shapes, such as spheres [ 12 , 13 ], polyhedra [ 14 , 15 , 16 ], bipyramids [ 17 , 18 ], rods [ 1 , 19 , 20 ], wires [ 21 , 22 ], and plates [ 23 , 24 , 25 , 26 , 27 , 28 , 29 ]. However, the synthesizing of nanocrystals with specific shapes, e.g., thin Au nanoplates, still needs substantial perfecting in terms of the morphological yield and size controllability.…”