Increased Stability and Complexity of Glycol-Terminated Self-Assembled Monolayers and their Use in Supported Lipid Bilayers

Date of Award

Spring 5-15-2012

Author's School

Graduate School of Arts and Sciences

Author's Department

Chemistry

Degree Name

Doctor of Philosophy (PhD)

Degree Type

Dissertation

Abstract

Self-assembled monolayers (SAMs) terminated with a poly(ethylene glycol) moiety are able to resist non-specific protein and cell adhesion, which has made them an important class of surfaces for biochemical studies on solid substrates. Gold-thiolate SAMs have been used extensively for biochemical studies at a surface, since they are easy to form, easy to pattern, and gold is a biocompatible substrate. However, for patterned cell culture, typical glycol-terminated gold-thiolate SAMs are only stable for a maximum of a week, which does not allow for long-term biochemical processes to be studied. Increasing the intramolecular forces between the SAM monomers, by either hydrogen bonding or dipole-dipole interactions, has been demonstrated to increase pattern fidelity for up to five weeks in culture. Furthermore, it was found that gold topology plays a major role in the stability of these patterned cell-culture substrates.

Additionally, increasing the control over the attachment of multiple proteins on a surface would allow for the generation of more complex protein maps. In order to control the relative concentration of two proteins on a surface, two pairs of hetero-functional glycol molecules were synthesized. Each pair contained a surface-reactive molecule and a protein-reactive molecule that could be coupled together by either a triazole ring or oxime bond. Varying the ratio of the surface reactive linkers then controls the ratio of two proteins binding to a surface, assuming that the two surface-reactive linkers have a similar affinity to couple to the surface. Functionalization of carboxylic acid-terminated SAMs with the two pairs of linkers was confirmed by reflective infrared spectroscopy. However, when these linkers were used to bind two different extracellular matrix proteins to the surface for cell attachment and growth, there was no observed attachment of cells.

Glycol-terminated SAMs also provide an ideal cushion for the formation of a supported lipid bilayer (SLB), since they allow the bilayer to maintain its maximum fluidity due to the inertness of the glycol-terminated surface. Due to this inertness, typical methods for SLB formation do not work for SLB formation on a glycol-terminated SAM. Microcontact printing was used to pattern attachment sites for lipid vesicles to adhere and subsequently rupture to form SLBs on glycol-terminated SAMs. The formation and fluidity of these patterned SLBs were confirmed by a variety of surface-characterization techniques.

The ability to form SLBs on a glycol-terminated SAM raises questions about how this occurs mechanisticly. Imaging surface plasmon resonance (SPRi) was used to monitor the rate of formation of the supported lipid monolayer (SLM) for the alkane-terminated regions of the monolayer and SLB formation for the glycol-terminated regions of the monolayer. The data supported a simultaneous attachment and rupture mechanism for SLM formation, while SLB formation occurs by attachment at the interface of the two SAM regions, followed by a slower spreading process over the glycol-terminated SAM.

Ligand-gated ion channels directly couple small molecule binding to an electrical signal and making them a model system for biosensing, however instability of electrophysiology measuring has prevented their use as biosensors. SLBs provide a more stable lipid bilayer compared to traditional black lipid membranes, whose bilayer's are formed over a small aperture, but creating a SLB device to measure ion-channel activity is not trivial. The strategy for SLB formation on patterned monolayers was utilized to create SLBs on glycol-terminated SAM gold microelectrodes that were surrounded by aluminum oxide which was coated with an alkane-terminated SAM. A double lift-off procedure was used to fabricate the microelectrode device, and SLB formation was monitored electrically. Furthermore, the pore-forming peptide alamethicin was inserted into the SLB and its activity was monitored using a voltage-clamp experiment.

Language

English (en)

Chair and Committee

Joshua A Maurer

Committee Members

John-Stephen Taylor, Kevin Moeller, Viktor Gruev, Garland Marshall, Jay Ponder

Comments

Permanent URL: https://doi.org/10.7936/K7MS3QPJ

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