DESIGN OF HIGHLY STABLE LOW-DENSITY SELF-ASSEMBLED MONOLAYERS USING THIOL-YNE CLICK REACTION FOR THE STUDY OF PROTEIN-SURFACE INTERACTIONS

Abstract

OF DISSERTATION DESIGN OF HIGHLY STABLE LOW-DENSITY SELF-ASSEMBLED MONOLAYERS USING THIOL-YNE CLICK REACTION FOR THE STUDY OF PROTEIN-SURFACE INTERACTIONS Protein adsorption on solid surfaces is a common yet complicated phenomenon that is not fully understood. Self-assembled monolayers have been utilized in many studies, as well-defined model systems for studying protein-surface interactions in the atomic level. Various strategies, including the use of single component SAMs[1, 2], and using mixtures of alkanethiolates with varying chain length and terminal functional group [3-5] have been used to effectively control the surface wettability and determine the effect of surface composition and wettability on protein adsorption. In this dissertation we report key new findings on the effect of surface density of functional groups on protein adsorption phenomenon. In the first phase of this research, we developed a novel approach for preparation of low-density self-assembled monolayers (LD-SAMs) on gold surfaces, based on radical-initiated thiol-yne click chemistry. This approach provides exceptional adsorbate stability and conformational freedom of interfacial functional groups, and is readily adapted for low-density monolayers of varied functionality. The resulting monolayers have two distinct phases: a highly crystalline head phase adjacent to the gold substrate, and a reduced density tail phase which is in contact with the environment. First, we investigated the feasibility of the proposed chemistry in solution-phase. In this approach, we synthesized “Y” shaped carboxylate-terminated thiol adsorbates via radical-initiated thiol-yne reaction. The LD-SAMs were then prepared through immersion of gold substrates into the solution of synthesized adsorbate molecules in hexane. The chemical structuring and electrochemical properties of resultant LD-SAMs were analyzed and compared with those of analogous traditional well-packed monolayers, using techniques such as. Characterization results indicated that resulting LD-SAMs have a lower average crystallinity, and higher electrochemical stability compared to well-packed monolayers. In addition, using a three-electrode system, we were able to show a reversible change in LD-SAM surface wettability in response to an applied voltage. This remodeling capacity confirms the low density of the surface region of LD-SAM coatings. The second area of work was focused on using the developed chemistry in solidphase. The solid-phase approach minimized the required synthesis steps in solution-phase method and used the photo-initiated thiol-yne click-reaction for grafting of acidterminated alkynes to thiol-terminated monolayers on a gold substrate to create similar LD-SAMs as what were prepared through solution-phase process. We characterized the resulting monolayers and compared them to analogous well-packed SAMs and the also low-density monolayers prepared through the solution phase approach. The results confirmed the proposed two-phase structure with a well-packed phase head phase and a loosely-packed tail phase. In addition, the electrochemical studies indicated that the resultant monolayers were less stable than the monolayers prepared via solution-phase, but they are significantly more stable than typical well-packed monolayers. The lower stability of these monolayers were attributed to the partial desorption of adsorbates from the gold substrate due during the grafting process. Building on the established chemistry, we studied the effect of lateral packing density of functional groups in a monolayer on the adsorption of Bovine serum albumin protein. We used surface plasmon resonance spectroscopy (SPR) and spectroscopic ellipsometry to evaluate BSA adsorption on carboxylate-, hydroxyl-, or alkylterminated LD-SAMs. For the LD-SAMs, the magnitude of protein adsorption is consistently higher than that of a pure component, well-packed SAM for all functionalities studied. In addition, it was seen that the magnitude of BSA adsorption the LD-SAMs was consistently higher than that of a pure component, well-packed SAM for all functionalities studied. The difference of protein adsorption on LD-SAMs and SAMs can not be associated to difference in lateral packing density, unless we eliminate the impact of other contributing factors in protein adsorption such as surface energy. In order to better understand the impact of packing density on protein-surface interactions, we prepared the mixed SAMs of (carboxylate/alkyl) and (hydroxyl/alkyl) with matching surface energy as the carboxylate and hydroxyl terminated LD-SAMs. It was found that the energy-matched mixed SAMs of carboxylate and hydroxyl functionality adsorbed more protein than the LD-SAMs. However, an opposite trend was seen for the alkyl surfaces, where surface energies are comparable for LD-SAMs and pure component SAMs, indicating that BSA proteins have higher affinity for methylterminated LDSAMs than well-packed SAMs.