When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.
Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.
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 phenols group 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 disrupted 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 where it will chemisorb to the surface using gold-thiol chemistry, an ω-thiolated phenolic surfactant was used. The impact of its impact ω-thiolation on the behavior of the phenol film was investigated. The thiolated surfactant initially forms a film with same structure and organization as the methyl-terminated surfactant but this phase was found to be metastable converted to a bola form conformation, with both the phenol and the thiol tethered to the water surface, over time. This rate of the interconversion between the condensed and bola phases could be controlled using the subphase pH, however this can also induce multilayer formation. We demonstrate that spreading the film over a subphase that contain 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.