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Electrochemical characterization of self-assembled monolayers on gold substrates derived from thermal decomposition of monolayer-protected cluster films

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Abstract

Networked films of monolayer-protected clusters (MPCs), alkanethiolate-stabilized gold nanoparticles, can be thermally decomposed to form stable gold on glass substrates that are subsequently modified with self-assembled monolayers (SAMs) for use as modified electrodes. Electrochemical assessment of these SAM-modified gold substrates, including double-layer capacitance measurements, linear sweep desorption of the alkanethiolates, and diffusional redox probing, all show that SAMs formed on gold supports formed from thermolysis of MPC films possess substantially higher defect density compared to SAMs formed on traditional evaporated gold. The density of defects in the SAMs on thermolyzed gold is directly related to the strategies used to assemble the MPC film prior to thermolysis. Specifically, gold substrates formed from thermally decomposing MPC films formed with electrostatic bridges between carboxylic acid-modified MPCs and metal ion linkers are particularly sensitive to the degree of metal exposure during the assembly process. While specific metal dependence was observed, metal concentration within the MPC precursor film was determined to be a more significant factor. Specific MPC film linking strategies and pretreatment methods that emphasized lower metal exposure resulted in gold films that supported SAMs of lower defect density. The defect density of a SAM-modified electrode is shown to be critical in certain electrochemical experiments such as protein monolayer electrochemistry of adsorbed cytochrome c. While the thermal decomposition of nanoparticle film assemblies remains a viable and interesting technique for coating both flat and irregular shaped substrates, this study provides electrochemical assessment tools and tactics for determining and controlling SAM defect density on this type of gold structure, a property critical to their effective use in subsequent electrochemical applications.

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Notes

  1. As in other reports, films assembled with zinc ions serving as the linking bridge required the deprotonation of the MPC’s carboxylic acid functional groups by adjusting the basicity of the metal ion dipping solution with the addition of 10–50 µL of 1 mM KOH to the 10 mL vial.

  2. MHA was not available commercially at reasonable prices at the time of this study and had to be synthesized according to the procedure described in the Supplementary Materials.

  3. Since this study is focused on the general assessment of overall SAM quality, we make no differentiation between pinhole defects, where solvent is able to contact gold, and point defects in the SAM, where the gold is not exposed but solvent can approach the electrode closer than the average thickness of the SAM. We use the term “defect density” to describe both types of defects [33].

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Acknowledgments

We gratefully acknowledge the National Science Foundation (CHE-1401593), Henry Dreyfus Teacher-Scholar Award Program, and Virginia’s Commonwealth Health Research Board for generously supporting this research. Student support was also provided by Richmond’s School of Arts & Sciences Undergraduate Research Committee and the Department of Chemistry Puryear-Topham Fund. Likewise, we acknowledge significant contributions from other student researchers in the group including Nick Poulos, D.J. Tognarelli, Katey Reighard, Kris Gerig, Morgan Vargo, Anne Galyean, Michael Freeman, and Debbie Campbell-Rance. We also would like to thank Aaron Rothrock and Dr. Mark Schoenfisch of the University of North Carolina at Chapel Hill for performing XPS measurements on our materials and Dr. Robert Miller of the University of Richmond for his assistance in synthesizing MHA. Special thanks is given to Drs. Tammy Leopold, Rene Kanters, Diane Kellogg, and Will Case, as well as Russ Collins, Phil Joseph, Mandy Mallory, and LaMont Cheatham—all of whom make undergraduate research possible at the University of Richmond.

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Authors and Affiliations

  1. Department of Chemistry, Gottwald Center for the Sciences, University of Richmond, Richmond, VA, 23173, USA

    Michael C. Leopold, Tran T. Doan, Melissa J. Mullaney, Andrew F. Loftus & Christopher M. Kidd

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  1. Michael C. Leopold
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  2. Tran T. Doan
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Correspondence to Michael C. Leopold.

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Leopold, M.C., Doan, T.T., Mullaney, M.J. et al. Electrochemical characterization of self-assembled monolayers on gold substrates derived from thermal decomposition of monolayer-protected cluster films. J Appl Electrochem 45, 1069–1084 (2015). https://doi.org/10.1007/s10800-015-0880-6

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