Single Crystal Gold Electrodes Research Articles

Engineering DNA Nanocube SAM Scaffolds for FRET-Based Biosensing: Interfacial Characterization and Sensor Demonstration.

Decorating a gold surface with molecular-level control over the positioning of DNA probes was demonstrated using a self-assembled monolayer (SAM) of wireframe DNA nanocube structures. The DNA nanocubes were specifically adsorbed and oriented using thiol-modified DNA on one face of the cube. The DNA nanocube SAM had a uniform coverage over the gold single crystal bead electrode with a separation of 20-30 nm measured by AFM. The face of the nanocube furthest from the gold surface was designed to hybridize with two different sequences of a 50 base single-stranded DNA probe that was modified with a fluorophore. The first 20 bases were hybridized with the DNA nanocube. One of a pair of FRET fluorophores was used for each probe strand. The dimensions of the nanocube controlled the relative spacing between these fluorophores. When the DNA probes were single-stranded, a FRET signal was observed. FRET decreased to background levels when a complementary DNA target was hybridized to either probe, resulting in a turn-off sensor with little cross-talk between the individual hybridization events. Hybridization isotherms for one target gave KA = 170 pM and a detection limit <50 pM. In addition, the DNA nanocube SAM was configured to be used as a turn-on NeutrAvidin sensor using biotinylated DNA targets hybridized to each probe resulting in an increase in FRET. We show that the wireframe DNA nanocube can be an effective scaffold for preparing biosensors with controlled separation between surface-bound probes facilitating precise sensor surface design and enabling a wide range of sensing modalities with more than one signal available for correlative confirmation of the target binding.

Self-assembled monolayers of O-(2-Mercaptoethyl)-O′-methyl-hexa(ethylene glycol) (EG7-SAM) on gold electrodes. Effects of the nature of solution/electrolyte on formation and electron transfer blocking characteristics

The self-assembly of O-(2-Mercaptoethyl)-O’-methyl-hexa(ethylene glycol) (EG7) on gold substrates produces monolayers whose structure should depend primarily on the solvents used for their formation. Although this should be also the case for any self-assembled monolayer (SAM) system, the presence of oxygen atoms in the EG7 chains brings about specific interactions with the solvent molecules other than just the van der Waals interactions taking place with alkanethiols. In this work we investigate the effects of the nature of the solutions for EG7-SAM formation in the reductive desorption processes using gold substrates either polycrystalline or single crystal electrodes. The patterns obtained in polycrystalline substrates are compared to the peaks observed at gold single crystal electrodes and it has been found that the main peaks correspond to the molecules desorbed from the different gold facets contained in the polycrystalline substrate. These single peaks are in fact composed of at least two contributions that can be explained as the presence of domains where the EG7 molecules are organized with different orientations, giving place to intermolecular interactions of different magnitude. The blocking properties of these films are strongly determined by the nature of the electrolyte used for the analysis and not by the solvent used in their formation. Although no experimental evidence of the specific interaction and/or retention inside the film of Na+ or phosphate or both ions is obtained, a specific effect that accounts for an electron transfer rate constant of an order of magnitude lower than the obtained with other electrolytes is observed. These results can contribute to increase the understanding of the relationship between the EG7-SAMs structure and functionalities.

Voltammetric investigation of anodic and cathodic processes at Au(hkl)|ionic liquid interfaces

• The electrochemical behaviors of 8 ionic liquids (ILs) were studied on Au( hkl ). • Pyrrolidinium-based ILs had the widest electrical double-layer region. • The potential windows depended on Au( hkl ) and the ions comprising the ILs. • The desorption/re-adsorption of iodine for 2 iodide-based ILs depended on Au( hkl ). • In terms of the peak potential for iodine desorption: Au(100) < Au(111) < Au(110) . In this work, the effect of the crystallographic orientation of gold on the interfacial processes of six non-halide-based ionic liquids (ILs) and two iodide-based ILs was systematically investigated by linear sweep voltammetry (LSV), cyclic voltammetry (CV), and double potential step chronocoulometry (DPSCC). The cation and anion of the IL affected both the width of the electrical double-layer (EDL) region ( E dl ) and the electrochemical potential window ( E pw ) of the ILs. The shape of the CV profile in the EDL region was unique for each Au( hkl )|IL interface. The reductive desorption and re-adsorption of iodine adatoms on gold in 1-alkyl-3-methylimidazolium iodide ([C n mim][I], n = 4 or 6) were also surface-sensitive. The reduction peak current density was proportional to the scan rate, suggesting that the reaction involves adsorbed iodine. The amount of iodine adatoms desorbed during the initial cathodic scan was identical to the monolayer coverage of iodine on gold single crystal electrodes. The peak potential for reductive desorption was estimated to follow the ensuing order from negative to positive: [C 6 mim][I] < [C 4 mim][I] and Au(100) < Au(111) < Au(110) . Following reductive desorption of iodine, the cations were partly attached to the gold surface. The adsorption strength of [C n mim] + followed an identical order: Au(100) < Au(111) < Au(110) .

On the behavior of CTAB/CTAOH adlayers on gold single crystal surfaces

The behavior of adsorbed CTAB and CTAOH on gold single crystal electrodes has been studied in solutions with different pH values. For the different single crystal electrodes, the adsorbed adlayer formed by CTA+ cations is in contact with the surface when the electrode charge is negative. As the surface charge becomes positive, the adlayer detaches from the surface and water molecules permeate through it, giving rise to characteristic peaks in the voltammogram. Charge and laser-induced jump temperature measurements show that the composition of the adlayer contains not only the CTA+ cations but also anions, which are required to stabilize the adlayer. In alkaline solutions, the higher solubility of CTAB/CTAOH in alkaline solutions leads to the partial desorption of the adlayer when these species are not present in the solution. When CTAB or CTAOH are dissolved in the alkaline cell solution, the adlayer is strongly bonded to the surface in the whole potential window due to the negative charge of the surface.

Modeling the Gouy–Chapman Diffuse Capacitance with Attractive Ion–Surface Interaction

The interfacial capacitance of a metal electrode in contact with a dilute electrolyte is generally expected to follow the behavior predicted by the Gouy–Chapman–Stern model. Recent experiments [Angew. Chem. Int. Ed. 2020, 59, 711], however, have shown that a deviation from the Gouy–Chapman behavior is observed even in dilute electrolytes on platinum and gold single-crystal electrodes. Such deviations are usually attributed to an interaction between the surface and the electrolyte ions. However, a quantitative model showing that the strong deviations from the Gouy–Champan behavior observed for Pt can be ascribed to such an effect is still lacking, particularly as other experimental observables do not indicate a strong ion adsorption. Here, we propose a double-layer model that is capable of reproducing the main experimental findings in a simple and (in parts) analytical way. The analytical model thereby includes the attractive ion–surface interaction via an additional capacitive element connected in parallel to the Gouy–Chapman capacitance. By comparing the model predictions to experiment, we subsequently infer characteristics of the ion–surface interaction. In particular, we find that the model predicts the attractive interaction to be weak (weaker than a typical chemical bond formed when contact adsorbing) and that the interaction has to be very similar for all ions. Furthermore, for a good agreement with experiment, ion-size effects are suggested to play a role in determining the potential of minimum capacitance.

Surface-confined redox-active monolayers of a multifunctional anthraquinone derivative on nanoporous and single-crystal gold electrodes

We have investigated self-assembled molecular monolayers (SAMs) of a novel thiol-bearing anthraquinone derivative AQ(OMe) (1-amino-4-((3-mercapto-5-(methoxycarbonyl)phenyl)amino)-9,10-dioxo-9,10-dihydroanthracene-2-sulfonic acid) with multifarious protonation sites, on nanoporous gold (NPG) electrode surfaces. We employ cyclic voltammetry, electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy (XPS). Single-crystal Au(111)-electrode surfaces area used as a reference. XPS exhibits S2p3/2 peaks at 161.1 and 162.1 eV, verifying well-defined AQ(OMe) Au-S bound SAMs. The mid-point potential (E1/2) of the SAMs shifts negatively with increasing pH with a slope of −55 mV pH−1, indicative of a 2e2H redox reaction. The anthraquinone group exhibits quasi-reversible behavior over a wide range of pH values (from2.8to9.0) and a sigmoidal electron transfer (ET) rate constant (kapp) with a maximum value at pH 5.5, which is likely to be close to the isoelectric point. The electrochemically addressable surface coverage, determined from the voltammetric peak areas, decreases with increasing pH. The apparent surface coverage on NPG is more than 13 times higher than that on Au(111), attributed mainly to the larger surface area and different structural packing modes, and possibly also to favorable terrace edges on NPG, but – somewhat unexpectedly – with little effect on kapp. The new anthraquinone derivative offers perspectives for gentle immobilization and efficient mediated interfacial electrochemical ET of redox proteins and other complex biomolecules.

Measuring and Controlling the Local Environment of Surface-Bound DNA in Self-Assembled Monolayers on Gold When Prepared Using Potential-Assisted Deposition

DNA self-assembled monolayers (SAMs) were prepared using potential-assisted deposition on clean gold single-crystal bead electrodes under a number of conditions (constant or square-wave potential perturbations in TRIS or phosphate immobilization buffers with and without Cl-). The local environment around the fluorophore-labeled DNA tethered to the electrode surface was characterized using in situ fluorescence microscopy during electrochemical measurements as a function of the underlying surface crystallography. Potential-assisted deposition from a TRIS buffer containing Cl- created DNA SAMs that were uniformly distributed on the surface with little preference to the underlying crystallography. A constant (+0.4 V/SCE) or a square-wave potential perturbation (+0.4 to -0.3 V/SCE, 50 Hz) resulted in similar DNA-modified surfaces in TRIS immobilization buffer. Deposition using a square-wave potential without Cl- resulted in lower DNA surface coverage. Despite this, the local environment around the DNA in the SAM appears to be densely packed. This implies the formation of clusters of densely packed DNA in the SAM. This effect was also demonstrated when depositing from a phosphate buffer. DNA clusters were significantly reduced when Cl- was present in the buffer. Clusters were most prevalent on the low-index plane surfaces (e.g., {111} and {100}) and less on the higher-index planes (e.g., {210} or {311}). A mechanism is proposed to rationalize the formation of DNA-clustered regions for deposition using a square-wave potential perturbation. The conditions for creating clusters of DNA in a SAM or for preventing these clusters from forming provide an approach for tailoring the surfaces used for biosensing.

Molecular Rheology of a Nanometer-Confined Ionic Liquid

Molecular mechanisms in the shear of an ionic liquid in nanometer-scale confinement were investigated by atomic force microscopy with a laterally oscillating tip. On a single-crystal gold electrode, the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][NTf2]) exhibits molecular ordering in a simple cubic structure upon confinement by the nanometer-sized asperity. The strength of shear resistance decreases exponentially with increasing number of confined molecular layers. The dependence of lateral forces and of dissipation on the applied electrochemical potential confirms a mechanism in the electrolubrication by ionic liquids, namely, the change of the slippage plane from the interface with the electrode into the ionic liquid for increasing surface potential.

Spatially Resolved Carrier Dynamics at MAPbBr3 Single Crystal-Electrode Interface.

Despite the rapid progress in organic-inorganic halide perovskites (OIHPs) for applications such as solar cells and detectors, knowledge of coupling between electronic and ionic charge carrier dynamics is so far limited. While the presence of dual-conduction channels is widely accepted, the precise physical mechanisms governing the impact of electronic (e.g. electrons and holes) and ionic conduction, especially interface phenomena, remain uncertain. The lack of understanding stems largely from the lack of appropriate tools to capture the electrochemical dynamics on the length scales of the local inhomogeneities present (e.g., interfaces, grain boundaries, space charge regions) and time scales over which the coupled dynamics take place. Here, we implement Kelvin probe force microscopy (KPFM) to explore the charge carrier dynamics at the methylammonium lead tribromide (MAPbBr3) single crystal-gold electrode interface. In this work, the temporal dynamics of the electric field and charge carrier distribution at the electrode interface is spatially visualized by time-resolved KPFM mapping. The results demonstrate an interplay of several phenomena, including charge injection, recombination, and ion migration, leading to an unbalanced charge dynamic at MAPbBr3 single crystal-electrode interface under forward and reverse bias conditions explaining the origin of the current-voltage hysteresis in these devices. We contrast the bias-assisted charge dynamics under both illuminated and dark conditions, providing a comprehensive picture of overall carrier dynamics and interface properties in a MAPbBr3 single crystal with lateral symmetric Au electrodes. Remarkably, illumination leads to the formation of a wider space charge region due to accumulation of negative charges (both electrons and halide ions) at the positive electrode, which can effectively screen the external electric field leading to lower charge extraction efficiency. The results suggest that the choice of contact or interfacial engineering can control the performance of OIHP devices without requiring modification of the material's bulk properties.

Influence of argon ion beam etching and thermal treatment on polycrystalline and single crystal gold electrodes Au(100) and Au(111)

The effect of argon ion beam etching and subsequent annealing on the surface morphology and electrochemical response of poly- and monocrystalline gold electrodes was studied with the help of scanning electron microscopy, cyclic voltammetry and measurements of under-potential deposition of lead. While the samples were cleaned and restructured by the argon ion beam the following thermal treatment produced stable and very smooth highly ordered surfaces. The specific behaviour of polycrystalline and monocrystalline gold surfaces is discussed.

Spectroelectrochemical and Density Functional Theory Study of Squaric Acid Adsorption and Oxidation at Gold Thin Film and Single Crystal Electrodes

The adsorption and oxidation of squaric acid (H2C4O4, H2SQ) at gold single crystal and thin-film electrodes with preferential (111) orientation were studied spectroelectrochemically in perchloric acid solutions. The existence of reversible adsorption-desorption processes in the double-layer region is reflected by structure-sensitive voltammetric features. Infrared reflection absorption spectroscopy experiments carried out with Au(111) and Au(100) single-crystal surfaces in 10 mM H2SQ solutions show potential-dependent adsorbate bands at ca. 1780-1785 and 1511-1577 cm-1 for potentials below 1.00 V RHE. The increasing sensitivity of the attenuated total reflection (ATR)-surface-enhanced infrared reflection absorption (SEIRA) experiments allows the detection of similar features for much lower H2SQ concentrations. According to density functional theory (DFT) calculations, these bands can be assigned to adsorbed squarate anions which are bonded to the gold surfaces in a bidentate configuration through two oxygen atoms, with the molecular plane perpendicular to the metal surface. For 10 mM H2SQ solutions, additional bands are detected in the ATR-SEIRA spectra at ca. 1630 cm-1 both in water and deuterium oxide solutions. Even if this frequency fits with one of the vibrational modes of adsorbed bisquarate, DFT calculations provide an alternative explanation for this potential-dependent feature that could be ascribed to collective vibrational modes of adsorbed squarate appearing at high adsorbate coverage. The existence of in-phase and out-of-phase contributions under these conditions would also explain the broadening and/or splitting of the observed bands. DFT calculations also show that squaric acid molecules adsorb very weakly at the gold surfaces and can be discarded as the origin of the observed infrared bands. The external reflection infrared spectra obtained for gold single-crystal electrodes in the H2SQ oxidation region show bands for dissolved carbon dioxide molecules as the main product. Bands for adsorbed bicarbonate anions formed from carbon dioxide are detected in the ATR-SEIRA spectra.

(Invited) Electrocatalysis of Hydrogen Evolution on Single Crystal Gold Electrodes Decorated By Palladium and Rhodium Nanoislands

Single crystal gold electrodes, Au(hkl), modified by submonolayer deposit of highly active metals from platinum group are convenient bimetallic model systems for fundamental studies of the electrocatalytic activity of various nanostructures for fuel cell reactions. Preparation of such bimetallic electrodes was carried out by spontaneous deposition method, which was monitored by chronopotentiometric measurement of open circuit potential. Surface characterization of the modified bimetallic electrodes, as well as base substrate electrodes, were performed ex situ by atomic force microscopy Simultaneously recorded topographic and phase images provided insight into different characteristics of the bimetallic surfaces, such as the dimensions of the deposited islands, occurrence of the preferential deposition along specific surface sites and the substrate coverage with deposited islands. Phase AFM images display an obvious chemical contrast between two materials, independent of the surface topography enabling thus quite accurate estimation of the surface coverage, particularly important in the cases when the conventional ways are not accurate enough or not applicable. X-ray photoelectron spectroscopy has shown the oxidation state of the surface constituents of the as prepared bimetallic electrodes. Pronounced catalytic activity for hydrogen evolution reaction (HER) of gold single crystals decorated by palladium nanoislands with submonolayer coverage has been reported so far for different Pd/Au(111) nanostructures in acid [1] and alkaline [2] media. Rh/Au(111) bimetallic electrodes, obtained by the same spontaneous deposition method have shown even better catalytic activity for HER in acid solution [3]. The activity was in agreement to the one theoretically predicted for Au(111) surface covered with Rh overlayers, which was comparable to that of Pt(111) [4]. It was shown that among different Rh/Au(111) nanostructures, the one with 50% Rh coverage has shown the highest activity for HER in acid media, starting at approx. 100 mV more positive potential than the most active Pd/Au(111) [1], and very close to the activity of Pt(111), although not exceeding it. Similar order of activity has been reported for HER in alkaline solution [2,5]. Comparison of the catalytic activity of the most active Pd/Au(111) and Rh/Au(111) in acid and alkaline media is shown in Fig. 1. The research is extended to the use of Au(100) electrode as the substrate for the same foreign metals deposition. The comparison of the catalytic activity of bimetallic surfaces consisting of Pd or Rh islands spontaneously deposited with the same coverage on differently oriented gold single crystals have shown that those bimetallic surfaces having Au(111) as substrate is more active than the ones deposited on Au(100) as substrate. The improved catalytic activity of the hereby explored bimetallic electrodes was interpreted by means of geometric and electronic effects of the substrates, which are responsible for different electrocatalytic properties with respect to both constitutive metals. We belive that these results contribute to the current search for the efficient catalyst for HER. [1] M.Smiljanić, I. Srejić, B. Grgur, Z. Rakočević, and S. Štrbac, Electrocatal. 3, 369 (2012). [2] M.Smiljanić, I. Srejić, B. Grgur, Z. Rakočević, and S. Štrbac, Electrochim. Acta 88, 589 (2013). [3] M.Smiljanić, I. Srejić, B. Grgur, Z. Rakočević, and S. Štrbac, Electrochem. Commun. 28, 37 (2013). [4] G. Soldano, E.N. Schulz, D.R. Salinas, E. Santos, and W. Schmickler, Phys. Chem. Chem. Phys. 13, 16437 (2011). [5] S. Štrbac, M. Smiljanić, and Z. Rakočević, J. Electrochem. Soc. 163, D3027 (2016). Figure 1

Redox Potentials and Electronic States of Iron Porphyrin IX Adsorbed on Single Crystal Gold Electrode Surfaces.

Metalloporphyrins are active sites in metalloproteins and synthetic catalysts. They have also been studied extensively by electrochemistry as well as being prominent targets in electrochemical scanning tunneling microscopy (STM). Previous studies of FePPIX adsorbed on graphite and alkylthiol modified Au electrodes showed a pair of reversible Fe(III/II)PPIX peaks at about -0.41 V (vs NHE) at high solution pH. We recently used iron protoporphyrin IX (FePPIX) as an intercalating probe for long-range electrochemical electron transfer through a G-quadruplex oligonucleotide (DNAzyme); this study disclosed two, rather than a single pair of voltammetric peaks with a new and dominating peak, shifted 200 mV positive relative to the ≈-0.4 V peak. Prompted by this unexpected observation, we report here a study of the voltammetry of FePPIX itself on single-crystal Au(111), (100), and (110) and polycrystalline Au electrode surfaces. In all cases the dominating pair of new Fe(III/II)PPIX redox peaks, shifted positively by more than 200 mV compared to those of previous studies appeared. This observation is supported by density functional theory (DFT) which shows that strong dispersion forces in the FePPIX/Au electronic interaction drive the midpoint potential toward positive values. The FePPIX spin states depend on interaction with the Au(111) interface, converting all the Fe(II)/(III)PPIX species into low-spin states. These results support electrochemical evidence for the nature of the electronic coupling between FePPIX and Au-surfaces, and the electronic states of adsorbate molecules, with a bearing also on recent reports of magnetic FePPIX/Au(111) interactions in ultrahigh vacuum (UHV).

Surface Structure Sensitivity of CO2 Electroreduction on Low‐Index Gold Single Crystal Electrodes in Ionic Liquids

Surface Structure Sensitivity of CO₂ Electroreduction on Low‐Index Gold Single Crystal Electrodes in Ionic Liquids

Ordered SiO2 cavity promoted formation of gold single crystal nanoparticles towards an efficient electrocatalytic application

A [111] facet dominated gold single crystal electrode with improved electrocatalytic ability for the oxidation of ethanol and nitrite.

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.

DFT and spectroelectrochemical study of cyanate adsorption on gold single crystal electrodes in neutral medium

The adsorption of cyanate anions at Au(111) and Au(100) single crystal electrodes has been studied spectroelectrochemically in neutral solutions. Potential-dependent in situ InfraRed Reflection Absorption spectra obtained below the onset of cyanate oxidation were compared with previously published data and analyzed on the basis of periodical Density Functional Theory (DFT) calculations. The calculated adsorption energies for cyanate and related species suggest that cyanic and isocyanic acid adsorb weakly at the studied gold surfaces and, thus, seems not to be at the origin of any of the adsorbate bands in the experimental infrared spectra collected in the cyanate-containing solutions. The latter features can be clearly ascribed to the asymmetric OCN stretching of N-bonded cyanate species. The observation of absorption bands in a wide spectral region, including features above 2200cm−1, agrees with the coexistence of N-bonded cyanate species with different adsorption sites and tilting angles. DFT calculations have revealed that although these adspecies can have significantly different frequencies, their adsorption energies are rather close. In addition, the existence of collective in-phase vibrations at relatively high cyanate coverages also contributes to the widening of the absorption bands.

Surface X-ray diffraction studies of single crystal electrocatalysts

In this article we review the contribution of surface X-ray diffraction (SXRD) to the topic of electrocatalysis. Based on several key examples it is shown how SXRD measurements elucidate the atomic structure at the polarised solid-liquid interface which can be used to develop a fundamental understanding of specific electrocatalytic reactions. The review begins with a discussion of single crystal gold electrodes and how the interplay of different adsorbates affects the lifting and formation of the reconstruction. This has given insight into the mechanism of the oxidation reactions on Au(hkl) surfaces. The second part of the review highlights the results obtained on Pt(hkl) single crystal surfaces, specifically the information obtained from SXRD measurements regarding the adsorption and oxidation of carbon monoxide. This includes the effects of anion adsorption, metal underpotential deposition and temperature changes. The third section gives a brief overview of bimetallic surfaces and how SXRD gives crucial information regarding both reactivity and the stability-reactivity relationship. The final section presents an outlook on current and future advances, highlighting the possibility of accessing structural information about the liquid side of the interface and the aim of performing faster SXRD measurements to probe the time domain in electrochemical reactions. The development of experimental setups to study both reactive non-noble metal electrodes and surfaces with specifically engineered atomic geometries and composition for enhanced electrocatalytic reactivity is also described.

On the electrochemical behavior of formamidine disulfide on gold electrodes in acid media

The adsorption and reactivity of formamidine disulfide (FDS) were studied at gold single crystal and thin film electrodes in perchloric acid solutions and the results of cyclic voltammetry and in situ infrared spectroscopy (SNIFTIR and ATR-SEIRAS) experiments compared with those previously obtained with thiourea (TU). In addition, optimized geometries and theoretical harmonic vibrational frequencies were obtained for adsorbed FDS, TU and thioureate from plane-wave DFT calculations using periodic surface models. These results were compared in the case of TU and thioureate to those previously obtained with a model Au cluster surface with (111) orientation. In the case of FDS, weak adsorption takes place with the S–S bond essentially parallel to the metal surface. No specific bands for adsorbed FDS can be identified in the experimental ATR-SEIRA spectra. These latter spectra suggest that adsorbed thioureate species are spontaneously formed upon dissociative adsorption of FDS when dosing this molecule at open circuit. Some of the adsorbed thioureate species undergo reductive protonation giving rise to a mixed adlayer formed by adsorbed thioureate and thiourea in a surface process which is faster when a controlled potential of 0.70V is applied. In agreement with the observations reported when dosing TU, the ratio between TU and thioureate adsorbates is found to depend on the electrode potential, being higher for potentials close to the hydrogen evolution limit and decreasing for higher potential values.

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.