Jannu R. Casanova-Moreno

Influence of Surface Structure on Single or Mixed Component Self-Assembled Monolayers via in Situ Spectroelectrochemical Fluorescence Imaging of the Complete Stereographic Triangle on a Single Crystal Au Bead Electrode

The use of a single crystal gold bead electrode is demonstrated for characterization of self-assembled monolayers (SAM)s formed on the bead surface expressing a complete set of face centered cubic (fcc) surface structures represented by a stereographic projection. Simultaneous analysis of many crystallographic orientations was accomplished through the use of an in situ fluorescence microscopic imaging technique coupled with electrochemical measurements. SAMs were prepared from different classes of molecules, which were modified with a fluorescent tag enabling characterization of the influence of electrical potential and a direct comparison of the influence of surface structure on SAMs adsorbed onto low index, vicinal and chiral surfaces. The assembly of alkylthiol, Aib peptide and DNA SAMs are studied as a function of the electrical potential of the interface revealing how the organization of these SAMs depend on the surface crystallographic orientation, all in one measurement. This approach allows for a simultaneous determination of SAMs assembled onto an electrode surface onto which the whole fcc stereographic triangle can be mapped, revealing the influence of intermolecular interactions as well as the atomic arrangement of the substrate. Moreover, this method enables study of the influence of the Au surface atom arrangement on SAMs that were created and analyzed, both under identical conditions, something that can be challenging for the typical studies of this kind using individual gold single crystal electrodes. Also demonstrated is the analysis of a SAM containing two components prepared using thiol exchange. The two component SAM shows remarkable differences in the surface coverage, which strongly depends on the surface crystallography enabling estimates of the thiol exchange energetics. In addition, these electrode surfaces enable studies of molecular adsorption onto the symmetry related chiral surfaces since more than one stereographic triangle can be imaged at the same time. The ability to observe a SAM modified surface that contains many complete fcc stereographic triangles will facilitate the study of the single and multicomponent SAMs, identifying interesting surfaces for further analysis.

What happens to the thiolates created by reductively desorbing SAMs? An in situ study using fluorescence microscopy and electrochemistry.

In situ examination of the reductive desorption process for Au microelectrodes modified with a thiol self-assembled monolayer (SAM) using fluorescence microscopy enabled the study of the fate of the desorbed thiolate species. The Bodipy labeled alkyl-thiol SAM, when adsorbed, is not fluorescent due to quenching by the Au surface. Once reductively desorbed, the thiolate molecules fluoresce and their direction and speed are monitored. At moderately negative reduction potentials, the thiolate species hemispherically diffuse away from the microelectrode. Also observed is the influence of a closely positioned counter electrode on the direction of the desorbed thiolate movement. As the potential becomes more negative, the molecules move in an upward direction, with a speed that depends on the amount of dissolved H(2) produced by water reduction. Shown is that this motion is controlled, in large part, by the change in the electrolyte density near the electrode due to dissolved H(2). These results should help in explaining the extent of readsorption at oxidative potentials observed in cyclic voltammetry (CV) reductive desorption measurements, as well as improving the general understanding of the SAM removal process by reductive desorption. The electrogenerated H(2) was also shown to be able to reductively remove the thiol SAM from the Pt/Ir particles that decorate the microelectrode glass sheath.

In Situ Spectroelectrochemical Fluorescence Microscopy for Visualizing Interfacial Structure and Dynamics in Self-assembled Monolayers

In situ analysis of electrochemical interfaces modified with molecular adsorbates using fluorescence microscopy is outlined. The fluorescence intensity from the fluorophore-modified adsorbate is strongly quenched when the separation of the fluorophore from the metal electrode surface is decreased below 200 nm. The theory describing this important characteristic is outlined with emphasis on the lifetime and far-field intensity of the fluorophore as a function of the separation from the metal. A number of examples are given in which fluorescence microscopy is used to study surfaces modified with the self-assembled monolayers (SAMs) composed of either alkylthiols, peptides, or DNA. The ability to interrogate both the lateral and axial distributions of the adsorbed monolayers within the micron scale optical resolutions is highlighted. The influence of the electrode potential (or charge) on the fluorescence images is shown for the reductive or oxidative removal of the adsorbate. The preparation of modified electrode surfaces is also reviewed, illustrating the influence of surface crystallography on the resulting surface modification or thiol exchange processes. Preliminary results of a DNA SAM studied using 2-photon fluorescence lifetime imaging microscopy are presented, demonstrating the measurement of lifetime distributions and its correspondence with the theory. In situ spectroelectrochemical fluorescence microscopy is thus shown to be useful in studying the electrochemical interface in terms of its homogeneity of modification, the structure in the axial direction away from the electrode surface and the influence of charge (or potential) on the dynamics of the interface.

Quantifying the Selective Modification of Au(111) Facets via Electrochemical and Electroless Treatments for Manipulating Gold Nanorod Surface Composition

Manipulating the composition of a mixed alkylthiol self-assembled monolayer (SAM) modified gold surface using both electrochemical and electroless methods is demonstrated. Through the use of fluorophore labeled thiolated DNA and in situ fluorescence microscopy with a gold single crystal bead electrode, a procedure was developed to study and quantify the selective desorption of an alkylthiolate SAM. This method enabled a self-consistent measurement of the removal of the SAM from the 111 surface compared to the 100 surface region at various potentials. A 20-fold increase in the electrochemical removal and replacement of the SAM from the 111 surface over the 100 surface was realized at -0.8 V/AgAgCl. A related procedure was developed for the solution-based electroless removal of the SAM using NaBH4 achieving a similar selectivity at the same potential. Unfortunately, in the electroless process fine control over the reducing potential was difficult to achieve. In addition, working in the presence of O2 complicates the solution potential measurement due to depolarization by the reduction of O2, resulting in a less clear relationship between selectivity and measured solution potential. Interestingly, the electrochemical method was not disturbed by the presence of O2. In preparation for work with Au nanorods, electrochemical measurements were performed in electrolyte that included 1 mM CTAB and was found to not interfere with this method. Preliminary results are promising for using this methodology for treatment of acid-terminated alkylthiol modified Au nanorods.