NIR-dye encoded gold nanoparticles (GNP) are rapidly emerging as contrast agents in many bio-imaging/sensing applications. The coding process is usually carried out without control or a clear understanding of the metal–liquid interface properties which, in contrast, are critical in determining the type and extension of dye–metal interaction. In this paper, we investigated the effect of gold surface composition on the adsorption of indocyanine green (ICG) on GNP, simulating the surface conditions of gold nanorods on citrate-capped gold nanospheres. These substrates allowed a careful control of the metal–liquid interface composition and, thus, detailed absorption and fluorescence concentration studies of the effects of each individual chemical in the colloidal solution (i.e.bromide anions, cetyl trimethylammonium ions and Ag+ ions) on the ICG–gold interaction. This study reveals the drastic effect that these experimental parameters can have on the ICG adsorption on GNP.