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

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.

Nano-Au/C Catalysts for the Selective Oxidation of Glucose into Gluconate

Au/C catalyst used for the liquid phase selective oxidation of glucose to gluconate was prepared by gold sol method that includes the preparation of metallic colloid by citrate reduction and subsequent immersion by carbon support. The influence of gold particles of different size on the activity of Au/C catalyst was studied. The results show that the catalyst with good dispersed spherical gold particles has higher catalytic activity. After 16 runs, the conversion of glucose selective oxidation can still achieve over 96% under mild reaction conditions.

Surface-Enhanced Raman Scattering of Aromatic Sulfides in Aqueous Gold Sol

A surface-enhanced Raman scattering (SERS) study has been performed for benzyl phenyl sulfide (BPS) and dibenzyl sulfide (DBS) in aqueous gold sol to examine the feasibility of the occurrence of surface-induced photoreactions of aromatic sulfides. Although BPS and DBS were reported to be decomposed into mercaptides on the silver surface by the 514.5 nm radiation, the molecules were found not to undergo such reactions on the gold surface. Such a difference in behavior was speculated to be associated with the different adsorption strength of BPS and DBS on gold and silver. The absorption bands of BPS and DBS seemed to be substantially broadened on silver so that the low-energy tails of the broadened absorption bands were in resonance with the 514.5 nm radiation, while the electronic absorption bands were not broadened sufficiently on the gold surface to absorb visible radiation.