Single molecule diffusion on hard, soft and fluid surfaces

Molecular diffusion on a surface is often considered as a thermal energy-activated process of molecular hopping between adjacent adsorption sites on a surface and simply determined by molecule–surface interaction. In this work, we report distinct diffusive dynamics of probe molecules on methyl-terminated self-assembled monolayer (SAM), polymer brush layer and lipid bilayer by using fluorescence correlation spectroscopy (FCS) at a single-molecule level. We have observed that despite weaker molecule–surface interaction, the surface diffusion of probe molecules on soft polymer brush surface and fluid lipid bilayer can be much slower than that on hard SAM surface, suggesting a strong impact of interfacial dynamics of the underlying coating on molecular surface diffusion. To further examine the coupling of thermal activated molecular hopping and soft surface dynamics, we have investigated the diffusion of probe molecules on polymer brushes of varied grafting density and thickness, where the molecule–surface interaction remains nearly the same, yet the adopted conformations and dynamics of surface-grafted PNIPAM chains vary considerably; it is striking to observe that the diffusion of probe molecules could be further retarded on PNIPAM brush surfaces of lower grafting density or higher brush thickness, thereby exhibiting the presence of an optimal brush thickness range to facilitate fast surface diffusion of adsorbed probe molecules. All the observations combined lead to a general model by taking the dynamics of underlying surface layers into account to elucidate the molecular diffusion mechanism on varied surfaces.