Immobilization of Gold Nanorods onto Acid-Terminated\nSelf-Assembled Monolayers via Electrostatic Interactions

Abstract

We report the immobilization of gold nanorods onto self-assembled monolayers (SAMs) of 16-mercaptohexadecanoic acid (16-MHA). The simple two step protocol involves formation of a SAM of 16-MHA molecules onto gold-coated glass slides and subsequent immersion of these slides into the gold\nnanorod solution. The nanorods, formed by a seed-mediated, surfactant-assisted synthesis protocol, are\nstabilized in solution due to surface modification by the surfactant cetyltrimethylammonium bromide\n(CTAB). Attractive electrostatic interactions between the carboxylic acid group on the SAM and the positively\ncharged CTAB molecules are likely responsible for the nanorod immobilization. UV−vis spectroscopy has\nbeen used to follow the kinetics of the nanorod immobilization. The nature of interaction between the gold\nnanorods and the 16-MHA SAM has been probed by Fourier transform infrared spectroscopy (FTIR). The\nsurface morphology of the immobilized rods is studied by scanning electron microscopy (SEM) and atomic\nforce microscopy (AFM) measurements. SEM was also used to determine the density of the immobilized\nnanorods as a function of the pH of immobilization. Control over the surface coverage of the immobilized\ngold nanorods has been demonstrated by simple pH variation. Such well-dispersed immobilized gold nanorods\nwith control over the surface coverage could be interesting substrates for applications such as surface-enhanced Raman spectroscopy (SERS).