Electrochemistry of Cytochrome c on Self-Assembled Phosphonic Acid Terminated Alkanethiols on Gold

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Title: Electrochemistry of Cytochrome c on Self-Assembled Phosphonic Acid Terminated Alkanethiols on Gold
Author: Quan, Lan Tu
Advisors: Tatyana I. Smirnova, Committee Member
Edmond F. Bowden, Committee Chair
Morteza G. Khaledi, Committee Member
Abstract: The synthesis of 10-mercaptodecanylphosphonic acid (C10PA) and self-assembly of C10PA on gold electrodes are reported. The electrochemical characterization of cytochrome c (cyt c) adsorption on the C10PA self-assembled monolayer (SAM) and on mixed hydroxyl(OH)/phosphonic acid (PA) terminated SAMs is also described, and a comparison is made to the pure carboxylic acid (COOH) SAM. On the pure C10PA SAM, no electroactivity for adsorbed cyt c was obtained. This lack of electroactivity could be due to a lack of adsorption resulting from hydrogen binding among the diprotic phosphonic acid groups, which could act to limit ionization needed for electrostatic cyt c binding at the interface. Alternatively, it could be that cyt c does bind strongly to this SAM, but becomes denatured or oriented in an unfavorable position for electron transfer to take place. On the other hand, cyt c electroactivity was obtained on the mixed OH/PA SAMs. In the mixed SAM, the OH terminated alkanethiol serves as a hydrophilic diluent, spacing out the phosphonic acid groups and thus limiting their ability to engage in mutual hydrogen bonding. Studies undertaken to examine the effects of OH alkanethiol chain length, the mole ratio of OH/PA, and self-assembly time yielded an optimal set of conditions for cyt c electroactivity, namely a 10:1 ratio C11OH/C10PA mixed SAM prepared with a 2 hour assembly time. Under those conditions, the redox properties of adsorbed cyt c were determined and compared with results for the widely studied COOH SAM. The formal potentials obtained for horse cyt c for both the C11OH/C10PA mixed SAM and the C10COOH pure SAM were approximately 230 mV vs NHE. The surface coverage for both types of monolayer were ~ 10 pmol/cm2. The electron transfer rate constants obtained were 17.7 s-1 and 12.7 s-1 for C11OH/C10PA mixed SAM and C10COOH pure SAM, respectively. Overall, the redox properties of cyt c on C11OH/C10PA mixed SAM were found to be similar to those obtained for a pure C10COOH SAM of comparable thickness.
Date: 2005-01-27
Degree: MS
Discipline: Chemistry
URI: http://www.lib.ncsu.edu/resolver/1840.16/1492


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