Zabala Rodríguez, Kevin Johan

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    Pickering liquid crystal emulsions: Influence of mixed-monolayer protected gold nanoparticles on colloidal stability and surfactant-induced ordering transitions
    (2021-04-13) Zabala Rodríguez, Kevin Johan; Acevedo Vélez, Claribel; College of Engineering; Acevedo Rullán, Aldo; Córdova Figueroa, Ubaldo M.; Méndez Román, Rafael; Department of Chemical Engineering; Rivera, Rosita L.
    Liquid crystal (LC) droplets dispersed in aqueous phases (e.g., LC-in-water emulsions) provide a versatile platform for the design of droplet-based LC sensors that can respond to the presence of chemical and biological analytes (i.e., surfactants, proteins, lipids, cells, etc.). One of the challenges of this technology is related to the long-term stability of the system because LC droplets aggregate and coalesce over time. The studies presented in this thesis were initiated as a step towards devising new strategies to stabilize LC droplets by using gold nanoparticles (AuNPs) with tunable surface chemistry as droplet stabilizers. To this end, AuNPs were functionalized with binary mixtures of alkanethiols displaying polar (hydroxyl or amine) and nonpolar (methyl) groups. In initial studies, we characterized the adsorption of chemically functionalized AuNPs onto droplets of the thermotropic LC 4-cyano-4'-pentylbiphenyl (5CB) using polarized light microscopy. Our results showed that AuNPs adsorbed onto 5CB droplets do not trigger a change in the initial LC bipolar configuration. However, when the anionic surfactant sodium dodecyl sulfate was added to the dispersion of AuNP-laden LC droplets, a bipolar-to-radial LC ordering transition was observed. This LC ordering transition is consistent with the self-assembly of the surfactant at the droplet interface, suggesting that nanoparticle adsorption does not interfere with surfactant-induced LC transitions. Subsequently, we characterized LC emulsion stability by monitoring changes in turbidity as a function of time. Our results showed that adsorbed AuNPs can confer colloidal stability to LC emulsions in ways that depend upon nanoparticle surface chemical composition. Finally, we characterized the optical responses of nanoparticle-stabilized LC emulsions when exposed anionic, cationic, or nonionic surfactants (keeping the aliphatic chain length of the surfactant constant). Our results showed that ordering transitions and detection limit of the LC droplets depend upon surfactant head group, suggesting that surfactant interactions with the AuNPs functionalized enhance or delay its recruitment to the LC interface. Overall, the results presented in this thesis demonstrate that surface chemistry can be tailored to enhance nanoparticle adsorption onto LC droplets to produce LC-in-water emulsions with improved colloidal stability, tunable selectivity and sensitivity to analytes.