Thermal Stability of Mono-, Bis-, and Tris-Chelating Alkanethiol Films Assembled on Gold Nanoparticles and Evaporated Flat Gold

The thermal stability of SAMs generated from the adsorption of n-octadecanethiol (n-C18), 2-hexadecylpropane-1,3-dithiol (C18C2), 2-hexadecyl-2-methylpropane-1,3-dithiol (C18C3), and 1,1,1-tris(mercaptomethyl)heptadecane (t-C18) on colloidal gold and evaporated flat gold was investigated. The optical extinction of the monolayer-protected nanoparticles (MPCs) was monitored as a function of thermal stress by using ultraviolet-visible (UV-vis) spectroscopy, which revealed that the evolution of the surface plasmon resonance varied with the nature of the adsorbate. Specifically, MPCs functionalized with monodentate n-C18 showed the fastest red shift of the surface plasmon resonance while those functionalized with tridentate t-C18 showed the slowest red shift, with those derived from the bidentates C18C2 and C18C3 falling in between, Suggesting a correlation between film stability and the degree of chelation. In separate studies, X-ray photoelectron spectroscopy (XPS) was used to evaluate the desorption of the monolayers on both colloidal gold and flat gold as a function of thermal stress. In these studies, SAMs generated from monodentate n-C18 showed the fastest desorption while SAMs generated from tridentate t-C18 showed the slowest desorption, with those derived from the bidentates C18C2 and C18C3 falling in between, again suggesting a correlation between film stability and the degree of chelation. As a whole, the following trend in thermal stability was observed: t-C18 > C18C2 approximate to C18C3 > n-C18.