Two-Dimensional Diffusion of Prodan on Self-Assembled Monolayers Studied by Fluorescence Recovery after Photobleaching

Fluorescence recovery after photobleaching (FRAP) is used to determine diffusion constants for the dye molecule 6-propionyl-2-dimethylaminonaphthalene (prodan) on silica and on a variety of organic monolayers grafted onto silica. Similar diffusion constants of ∼2 × 10-7 cm2/s are found for prodan on chlorotrimethylsilane (CTS) based self-assembled monolayers and on silanated poly(ethylene glycol) based monolayers. The similar diffusion constants are intriguing given the different surface free energies of these surfaces. No dye recovery is observed on octadecyltrichlorosilane (OTS) based monolayers or on clean silica surfaces. No significant change in the diffusion constant is observed with varying relative humidity on any of the surfaces. A mechanism for the diffusion of dye on these four surfaces is discussed. A simple surface energy argument may explain why diffusion is extremely limited on both very hydrophobic and very hydrophilic surfaces, but is allowed on surfaces with intermediate polarities. Additionally, we suggest, in the case of CTS vs OTS based monolayers, that the presence of disorder in the CTS based monolayer is important for rapid diffusion. Similar concepts are presented for the other systems examined.