Title

Controlling electrode reactivity: Applications in biomembrane mimetics and molecular and ion sensors

Date of Award

1994

Availability

Article

Degree Name

Doctor of Philosophy (Ph.D.)

First Committee Member

Angel E. Kaifer, Committee Chair

Abstract

Part I. The first examples of molecular recognition at the electrode solution interface are demonstrated through the design and self-assembly of supported monolayers containing preformed binding sites. Two different systems were addressed. In particular two different hosts were employed, both of them derivatives of the very well known molecular receptor $\beta$-cyclodextrin and Cyclobis(paraquat-p-phenylene). The tailor-made hosts are immobilized, via sulfur derivatization, at the surface of gold bead electrodes.The thiolated $\beta$-cyclodextrin derivative, (1), chemisorbed strongly to the gold substrate with a submonolayer coverage. Since the development of a functional monolayer required a compact monolayer, a procedure was devised in which the intermolecular gaps were sealed leaving the molecular cavities unperturbed. The interfacial properties of the resulting modified electrode were found to be controlled by the recognition properties of the immobilized cyclodextrin which follow closely those observed for the parent receptor in the solution phase. As a result the modified surface was capable of molecular recognition and inclusion complex formation with appropriate guests in the contacting solution.The disulfide cyclobis(paraquat-p-phenylene) derivative, 2$\sp4+$, was designed and synthesized to have the ability of surface chemisorption on gold electrodes and to maintain the receptor properties of the parent cyclophane derivative. In analogy to 1,2$\sp4+$ was found to chemisorb on the gold electrodes in submonolayer coverages. After sealing the defective sites with an appropriate n-alkanethiol, the interfacial assembly was found to be capable of molecular recognition, binding and inclusion complex formation with aromatic guests in the contacting aqueous solution. Charge transfer interactions between the electron deficient cavity of 2$\sp4+$ and the guest controlled the electrode reactivity yielding a modified electrode selective towards electron rich substrates.Part II. Glassy carbon electrodes are modified through a casting procedure with multilayers films of different natural and synthetic lipids in pure or composite assemblies with cholesterol. The incorporation of redox probes inside the thick lipid film yields a membrane mimetic assembly suitable for the study of electron transfer in a biologically relevant environment. The effect of cholesterol doping, alkyl chain length and degree of unsaturations was addressed. The fluidity of the lipid assembly in the cast films was found to control the extent of the electrochemical conversions of the incorporated redox probes. Likewise the effect of the cholesterol addition was found to depend on the fluidity of the cast lipid film. Its effect was only observed on crystalline lipid films.Part III. Different ferrocenyl-based ionophores were studied to assess their binding affinities for various cations and their ability to undergo redox switching. The ferrocenyl cryptand 10, was found to be the first efficient redox-switchable synthetic ionophore for alkali and silver cations. The ferrocenyl moiety acts as a donor group in the binding of the silver ion, a fact to which the unexpectedly efficient binding is ascribed. As a result ligand 10 was found to be capable of binding the ion in an aqueous environment. This phenomenon was exploited and led to the development of an amperometric sensor for the analytical determination of silver in solution.

Keywords

Chemistry, Analytical; Chemistry, Physical

Link to Full Text

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