Stress at the solid-liquid interface of self-assembled monolayers on gold investigated with a nanomechanical sensor

The interfacial stress of self-assembled monolayers on Au exposed to buffer s of various pH values and ionic strengths is measured as a function of the liquid environment. The method uses two thiol-modified Au-covered silicon cantilevers and a differential method to compensate for thermal and refract ive index changes of the liquid environment. Increasing pH and ionic streng th leads to a bending, i.e., a compressive stress, of a mercaptohexadecanoi c acid-covered cantilever compared to a hexadecanethiol-covered reference c antilever. In addition, the interfacial stress is found to be highly depend ent on the surface density of the ionizeable mercaptohexadecanoic acid mole cules when they are coadsorbed with hexadecanethiols on Au.