Gold nanoparticles in novel green deep eutectic solvents: self-limited growth, self-assembly & catalytic implications

In this article, we are reporting the catalytic activity of gold nanoparticles (AuNPs) dispersed in a novel eco-friendly Deep Eutectic Solvent (DES; choline chloride and urea) at room temperature. A low energy sputter deposition method is employed to synthesize AuNPs on the DES surface. This is a simple, economical and clean method for producing monodisperse gold nanoparticles by a self-limited growth mechanism. Detailed Small Angle X-ray Scattering (SAXS), UV-Vis and Transmission Electron Microscopy (TEM) investigations show the formation of spherical AuNPs of 5 nm in diameter. Moreover, these particles have a strong tendency to self-assemble at the DES surface as well as into the bulk. This is a new addition to a novel generation of gold based catalysts. The catalytic activity of Au nanoparticles in DES is revealed quantitatively by analyzing the apparent conversion reaction rate constant K_app of para-nitrophenol to paraaminophenol in the presence of sodium borohydride. Catalytic activity reported in the present article ismuch faster (k_app = 0.34 s^-1) than the rate reported earlier (k_app = 0.030 min^-1) by Chirea et al. for gold nanowire networks [Langmuir, 2011, 27, 3906]. In agreement with the literature, we explain the higher catalytic activity in terms of a larger surface area of AuNPs in a non-self-assembled state.