Surface structural effects on specific adsorption of oxoanions on gold single crystal electrodes

A surface chemistry perspective on SERS: revisiting the basics to push the field forward

Surfaces are well known to be complex entities that are extremely difficult to study, and any phenomenon that is related to them is consequently challenging to approach. Moving from the bulk to the nanoscale adds a further layer of complexity to the problem. Because SERS relies on surfaces at the nanoscale, a rigorous understanding of the chemical phenomena that concur in the observation of the SERS signal is still limited or disorganized at best. Specifically, the lack of understanding of the chemical properties of nanoparticle surfaces has direct consequences on the development of SERS-based devices, causing a widespread belief that SERS is an inherently unreliable and fundamentally irreproducible analytical technique. Herein, we discuss old and new literature from SERS and related fields to accompany the reader through a journey that explores the chemical nature and architecture of colloidal plasmonic nanoparticles as the most popular SERS-active surfaces. By examining the chemistry of the surface landscape of the most common SERS colloids and the thermodynamic equilibria that characterize it, we aim to paint a chemically realistic picture of what a SERS analyst deals with on a daily basis. Thus, our goal for this review is to provide a centralized compilation of key, state-of-the-art surface chemistry information that can guide the rational development of analytical protocols and contribute an additional path through which our community can continue to advance SERS as a reliable and robust analytical tool.

Electrochemical and Non-Electrochemical Pathways in the Electrocatalytic Oxidation of Monosaccharides and Related Sugar Alcohols into Valuable Products

In this contribution, we review the electrochemical upgrading of saccharides (e.g., glucose) and sugar alcohols (e.g., glycerol) on metal and metal-oxide electrodes by drawing conclusions on common trends and differences between these two important classes of biobased compounds. For this purpose, we critically review the literature on the electrocatalytic oxidation of saccharides and sugar alcohols, seeking trends in the effect of reaction conditions and electrocatalyst design on the selectivity for the oxidation of specific functional groups toward value-added compounds. Importantly, we highlight and discuss the competition between electrochemical and non-electrochemical pathways. This is a crucial and yet often neglected aspect that should be taken into account and optimized for achieving the efficient electrocatalytic conversion of monosaccharides and related sugar alcohols into valuable products, which is a target of growing interest in the context of the electrification of the chemical industry combined with the utilization of renewable feedstock.

Mapping Electrochemical Heterogeneity at Gold Surfaces: A Second Harmonic Imaging Study

Designing efficient catalysts requires correlating surface structure and local chemical composition with reactivity on length scales from nanometers to tens of microns. While much work has been done on this structure/function correlation on single crystals, comparatively little has been done for catalysts of relevance in applications. Such materials are typically highly heterogeneous and thus require methods that allow mapping of the structure/function relationship during electrochemical conversion. Here, we use optical second harmonic imaging combined with cyclic voltammetry to map the surface of gold nanocrystalline and polycrystalline electrodes during electrooxidation and to quantify the spatial extent of surface reconstruction during potential cycling. The wide-field configuration of our microscope allows for real-time imaging of an area ∼100 μm in diameter with submicron resolution. By analyzing the voltage dependence of each pixel, we uncover the heterogeneity of the second harmonic signal and quantify the fraction of domains where it follows a positive quadratic dependence with increasing bias. There, the second harmonic intensity is mainly ascribed to electronic polarization contributions at the metal/electrolyte interface. Additionally, we locate areas where the second harmonic signal follows a negative quadratic dependence with increasing bias, which also show the largest changes during successive cyclic voltammetry sweeps as determined by an additional correlation coefficient analysis. We assign these areas to domains of higher roughness that are prone to potential-induced surface restructuring and where anion adsorption occurs at lower potentials than expected based on the cyclic voltammetry.

Analysis of Membrane Protein Deinsertion-Associated Currents with Nanoneedle-Supported Bilayers to Discover Pore Formation Mechanisms

Analysis of the pore formation mechanisms of biological nanopores can provide insight into pore-forming peptide-induced diseases and into the characterization of nanopores employed in sensing methods. Evaluation of pore formation mechanisms is typically performed using microscopy including atomic force microscopy, transmission electron microscopy, as well as electrically via channel current measurements using a patch-clamp amplifier. However, due to the relatively low temporal resolution of the above-mentioned microscopy techniques and the low analysis accuracy of the channel current measurements, new analytical methods are required. Here, we describe a new analytical strategy to measure and analyze both ionic currents associated with biological nanopore insertion and deinsertion into and out of lipid bilayers to determine pore formation mechanisms for several representative proteins. The current changes associated with protein deinsertion are monitored as the lipid membrane leaflets are pulled apart-a unique phenomenon enabled by our gold nanoneedle measurement probe. This deinsertion current analysis (DiCA) is performed using a gold nanoneedle-supported lipid bilayer at which a bilayer membrane is formed by bringing together two lipid monolayers on the surface of the nanoneedle and at the interface of an aqueous solution and a lipid/oil mixture. The lipid bilayer can be pulled apart by removing the nanoneedle from this interface. In this study, we demonstrate the determination of pore formation mechanisms for four different pore-forming proteins and peptides-α-hemolysin, streptolysin O, alamethicin, and amyloid β 1-42 using DiCA. As a result, we successfully discern the pore formation mechanism, either addition or expansion, for each protein/peptide by analyzing the ratio and magnitude of insertion and deinsertion current events.

Electrochemical and spectroelectrochemical characterization of an iridium-based molecular catalyst for water splitting: turnover frequencies, stability, and electrolyte effects

We present a systematic electrochemical and spectroelectrochemical study of the catalytic activity for water oxidation of an iridium-N-dimethylimidazolin-2-ylidene (Ir-NHC-Me2) complex adsorbed on a polycrystalline gold electrode. The work aims to understand the effect of the electrolyte properties (anions and acidity) on the activity of the molecular catalyst and check its stability toward decomposition. Our results show that the iridium complex displays a very strong dependence on the electrolyte properties such that large enhancements in catalytic activity may be obtained by adequately choosing pH and anions in the electrolyte. The stability of the adsorbed compound was investigated in situ by Surface Enhanced Raman Spectroscopy and Online Electrochemical Mass Spectrometry showing that the catalyst exhibits good stability under anodic conditions, with no observable evidence for the decomposition to iridium oxide.

Electrochemical modification of surface morphology of Au/Ti bilayer films deposited on a Si prism for in situ surface-enhanced infrared absorption (SEIRA) spectroscopy

Surface-enhanced infrared absorption (SEIRA)-active Au/Ti bilayer films sputter deposited on Si substrates have been prepared by an electrochemical annealing (ECA) treatment for the first time. The application of Au/Ti bilayer films on Si substrates to the spectroscopic technique is a promising alternative to the conventional technique using directly deposited Au films on Si substrates, offering excellent adhesive durability of the deposited metal films. However, Au/Ti bilayer films have never been selected for the spectroscopy technique because the films in the as-prepared state exhibit relatively smooth surface morphology: the excitation of the localized surface plasmon is vital to achieving SEIRA enhancements but could hardly be observed on the smooth morphology. It is shown by ex situ scanning tunneling microscopy measurements that the unfavorable smooth morphology of the as-prepared Au/Ti bilayer films can be modified by the ECA treatment to a reasonably rough, island-structure morphology similar to that of the conventional SEIRA-active Au films. In situ infrared absorption spectroscopy of adsorbed sulfate anions has been conducted on the Au/Ti bilayer film both before and after ECA treatment. The spectroscopy measurements demonstrate that the SEIRA activity of the film after being subjected to the treatment is significantly improved so that the technique could detect adsorbates on the film electrodes even with the submonolayer coverage. As an additional benefit, the ECA treatment has brought about a substantial increase in the fraction of Au(111) domains on the polycrystalline Au film surfaces. Accordingly, this approach enables us to prepare SEIRA-active Au films having sufficient adhesion to the Si substrates as well as the highly preferred (111) orientation.

On the effects of ion-wall chemical association on the electric double layer: A density functional approach for the restricted primitive model at a charged wall

We describe a density functional theory for the restricted primitive model of ionic fluid at a charged wall with active sites to which ions can bond. The theory is an extension of our recent approach [Pizio et al., J. Chem. Phys. 121, 11957 (2004)] and is focused in the effects of specific adsorption of ions on the wall, besides the electrostatic phenomena. In order to solve the problem, we use the first-order thermodynamic perturbation theory of chemical association developed by Wertheim [J. Chem. Phys. 87, 7323 (1987)]. The microscopic structure of the electric double layer and the amount of adsorbed charge are investigated. Also, the temperature dependence of capacitance is analyzed. The capacitance depends on the kind of ions that form associative bonds with the surface sites and is determined by a net charge acting on the diffuse layer. The shape of the temperature dependence of capacitance essentially depends on the association energy and the density of bonding sites.

Determination of oxoanions in river water by capillary electrophoresis

A new capillary electrophoresis (CE) procedure was developed for simultaneous determination of ten oxoanions (CrO4(2-), SeO4(2-), MoO4(2-), WO4(2-), VO4(3-), SeO3(2-), As04(3-), TeO3(2-), TeO4(2-), and AsO3(3-)) which were baseline-separated from each other and from the interfering UV absorbing anions (NO3- and NO2-) commonly found in environmental water samples. The new background electrolyte system developed contained 5 mM potassium phosphate and 0.007 mM octadecyltrimethylammonium hydroxide, pH 11.2. The optimized working conditions were electrokinetic sampling at -5 kV for 10 s, running voltage at -15 kV with 5 microA current, and detection wavelength at 205 nm. No interference was observed for non-UV-absorbing anions and UV-absorbing anions up to 20 and 10 times higher concentrations respectively. The speed of analysis was fast, with a complete CE run within 6 min. Wide linear ranges (1-2,000 microg/L), good repeatability in migration time (relative standard deviation RSD 0.55-2.8%), satisfactory precision in peak area (RSD 3.8-5.6%) and peak height (RSD 3.9-5.3%) measurement, and detection limits (1-25 microg/L) sufficiently sensitive to detect oxoanions found in environmental water samples were obtained. The reliability of the CE procedure developed had been established by recovery test and parallel method determination using atomic absoprtion spectrophotometry for real river water sample.