Abstract

Polyphenolic compounds have the ability to selectively bind metals and certain proteins, properties that can be harnessed for sensing applications. Their analyte binding affinity is partially influenced by the proximity and conformational freedom of the individual phenol moieties. This makes the ability to control the organization of phenol groups an important consideration for the development of phenolic-functionalized thin film coatings. We investigated the approach of using a Langmuir monolayer, pre-organized at the air-water interface to control the lateral spacing upon deposition onto a solid surface. The self-assembly of phenolic surfactants is determined by the strong headgroup interactions: the hydroxyl groups can hydrogen bond with neighboring phenols and the aromatic ring can form π-interactions. We find that the strong, directional interactions lead to the creation of very rigid films that we attribute in large part to the presence of an organized and crystalline phenol headgroup layer. Modification of the subphase pH caused deprotonation of the phenol hydroxyls, which disrupt the organization of the monolayer in two ways: first by increasing the inter-domain charge repulsion and, at high pH, by disrupting the hydrogen bond network yielding a new lateral organization.

In order to deposit the film onto solid support for chemisorb to the surface using gold-thiol chemistry, an ω-thiolated phenolic surfactant was used. The impact of the ω-thiolation on the behavior of the phenol film was investigated. The thiol-terminated surfactant initially forms a film with the same structure and organization as the methyl-terminated surfactant but this phase was found to be metastable and convert to a bent conformation, with both the phenol and the thiol tethered to the water surface, over time. The rate of this interconversion between the condensed phase and bent phases could be controlled using the subphase pH, however high pH can also induce multilayer formation. We demonstrated that spreading the film over a subphase that contains binding analytes of interest, such as poly-L-proline and copper ions, can be used as a strategy to pre-organize the surfactants into the optimal lateral spacing for subsequent binding.