Quantitative Electrochemical SERS of Flavin at a Structured Silver Surface

Surface-enhanced Raman spectra (SERS) are reported for nicotinamide adenine dinucleotide (NAD+, oxidised form) adsorbed on colloidal silver at concentrations between 10–3 and 10–5 mol dm–3, and using various excitation wavelengths. Adenosine diphosphate (ADP) gave an SER spectrum similar to that of NAD+, except for the absence of the nicotinamide SER band observed for NAD+ at 1030 cm–1. NADH (reduced form of NAD+) did not give an SER spectrum at the alkaline pHs necessary to prevent decomposition. No SER spectrum was obtained for NAD+ when bound to the enzyme GAPDH; the NAD+ is too deeply buried in the protein for effective surface enhancement. The applicability of the surface selection rules arising from the electromagnetic enhancement mechanism to determine NAD+ orientation at the silver surface is discussed in the light of the probable contribution to SERS intensity from a charge-transfer mechanism. While the selection rules do not, in this case, appear useful for determining NAD+ orientation at silver, wavenumber shifts do indicate surface interaction with N1 of adenine. The general surface geometry of NAD+ adsorbed on silver at low ( < 10–4 mol dm–3) concentrations is deduced as being extended, with phosphate groups binding directly to silver, and ribose, nicotinamide and adenine moieties all in close proximity to the surface.

SERS of the bis(4,5-dimercapto-1,3-dithiole-2-thionato) nickelate(III) complex ion and of its inclusion compound in γ-cyclodextrin adsorbed on a silver electrode

Raman and surface-enhanced Raman spectra of chrysin, apigenin and luteolin

Electrochemical surface-enhanced Raman scattering (SERS) of the cruciform system 1,4-bis((E)-2-(pyridin-4-yl)vinyl)naphthalene (bpyvn) was recorded on nanostructured silver surfaces at different electrode potentials by using excitation laser lines of 785 and 514.5 nm. SERS relative intensities were analyzed on the basis of the resonance Raman vibronic theory with the help of DFT calculations. The comparison between the experimental and the computed resonance Raman spectra calculated for the first five electronic states of the Ag2-bpyvn surface complex model points out that the selective enhancement of the SERS band recorded at about 1600 cm−1, under 785 nm excitation, is due to a resonant Raman process involving a photoexcited metal-to-molecule charge transfer state of the complex, while the enhancement of the 1570 cm−1 band using 514.5 nm excitation is due to an intramolecular π→π* electronic transition localized in the naphthalenyl framework, resulting in a case of surface-enhanced resonance Raman spectrum (SERRS). Thus, the enhancement of the SERS bands of bpyvn is controlled by a general chemical enhancement mechanism in which different resonance processes of the overall electronic structure of the metal-molecule system are involved.

Surface structures and electronic properties of hypophosphiteon Ni(111) and Ag(111) surfaces were investigated by means of density functional theory at B3LYP/6‐31++G(d,p) level. The most stable structure was that in which theadsorbs with its two P–O bonds faced to the substrate surface. The results of the Mulliken population analysis showed that because of the subtle difference of electron configuration, the adsorption energy was larger on the Ni surface than on the Ag surface, and the amounts of both donation and back donation were larger on the Ni(111) surface than on the Ag(111) surface. There were more negative Mulliken charge transfer from H2PO−2to substrate clusters on Ni surface than on Ag surface and more positive Mulliken charges on P atom in Ni4H2PO−2than in Ag4H2PO−2, which means that P atom in Ni4H2PO−2is easily attacked by a nucleophile such as OH−. Thus, H2PO−2is more easily oxidated on Ni(111) surface than on Ag(111) suface. These results indicated that the silver surface is inactive for the oxidation reaction of the hypophosphite anion.

A spectroelectrochemical investigation of the interaction of diisobutyldithiophosphinate with copper, silver and gold surfaces: II. Electrochemistry and Raman spectroscopy

Structure and composition of the Ag (111) surface as a function of electrode potential in aqueous halide solutions

Electrochemistry and electrochemical reactions are increasingly important in the transition to a sustainable chemical industry. The electron transfer that drives such reactions takes place within nanometers of the electrode surface, and follow-up chemical reactions take place within the diffusion layer. Hence, understanding electrochemical reactions requires time-, potential-, and spatially resolved analysis. The confocal nature of Raman spectroscopy provides high spatial resolution, in addition to detailed information on molecular structure. The intrinsic weakness of nonresonant Raman scattering, however, is not sensitive enough for relatively minor changes to the solution resulting from reactions at the electrode interface. Indeed, the limit of detection is typically well above the concentrations used in electrochemical studies. Here, we show that surface-enhanced Raman scattering (SERS) and resonance Raman (rR) spectroscopy allow for spatially and time-resolved analysis of solution composition at (<1-2 nm) and near (within 5 μm) the electrode surface, respectively, in a selective manner for species present at low (<1 mM) concentrations. We show changes in concentration of species at the electrode surface, without the need for labels, specific adsorption, or resonance enhancement, using a SERS-active gold electrode prepared readily by electrochemical surface roughening. A combination of smooth and roughened gold electrodes is used to distinguish between surface and resonance enhancement using the well-known redox couples ferrocene and 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS). We discuss the impact of specific adsorption on the spectral analysis with the ruthenium(II) polypyridyl complex, [Ru(bpy)3]2+. The dual function of the electrode (surface enhancement and electron transfer) in the analysis of solution processes is demonstrated with the reversible oxidation of TMA (4,N,N-trimethylaniline), where transient soluble species are identified in real time, with rapid spectral acquisition, making use of localized enhancement. We anticipate that this approach will find use in elucidating electro(catalytic) reactions at electrode interfaces.

Surface enhanced Raman (SER) spectra were obtained from nickel electrodes immersed in borate buffer solution (pH 8.4) at +600, and +900 mV(SCE) using electrodeposited gold particles to produce the surface enhancement effect. Comparison with earlier experiments performed using silver particles to obtain SERS indicates that the spectra of the nickel passive films are identical regardless of whether silver or gold is used. At +100 mV, the nickel passive film consists of amorphous ; is not observed. At +600 mV, amorphous persists, and Au‐O forms on the gold particles. The SER spectrum of the transpassive film formed on nickel at +900 mV consists of peaks located at 480 and 560 cm−1, and it is speculated that the species responsible for the Raman scattering resembles . Upon returning the potential to +100 mV from +900 mV, reforms, accompanied by . This is the first example of the use of gold particles to explore the passive film of nickel and the first time the SER spectrum of the transpassive film on nickel has been measured.

SERS and in situ SERS spectroscopy of riboflavin adsorbed on silver, gold and copper substrates. Elucidation of variability of surface orientation based on both experimental and theoretical approach

Adsorption of carbendazim pesticide on plasmonic nanoparticles studied by surface-enhanced Raman scattering

We report borrowed surface-enhanced Raman scattering (SERS) at Pd and Pt films deposited on Au sphere segment void (SSV) SERS substrates and show that ultrathin, pinhole-free Pd and Pt films could be easily achieved by choosing constant-current deposition for the Pd deposition and under-potential deposited Cu-mediated deposition for the Pt deposition. The surface enhancement achieved from the Pd and Pt films was reproducible, owing to the ordered SSV structure of their surfaces, and borrowed additional enhancement from the long-range electromagnetic (EM) fields generated by the underlying Au SSV substrate. SERS experiments and theoretical simulations indicate that the distance over which the EM fields generated by a concave Au SSV surface can be transferred to the surface of the transition-metal films is approximately an order of magnitude larger than that for a convex sphere surface. This longer range of the EM fields allowed deposition of thicker, more robust Pd and Pt films while retaining the borrowed enhancement. These properties make such Pd and Pt films attractive as SERS surfaces to reveal surface species and reactive intermediates on these two typical electrocatalyst materials.

Galactose oxidase (GalOD) was immobilized on self-assembled monolayers of thiols on silver and gold surfaces using trans-stilbene (4,4′-diisothiocyanate)-2,2′disulphonic acid (DIDS) as the bridging compound. DIDS is the symmetrical bifunctional reagent that reacted with the amine moiety of the thiol and with primary amino groups of enzyme. The Raman measurement revealed that onto cysteamine-modified silver and gold electrodes, bands corresponding to the galactose oxidase (about 694, 1076, 1274 cm—1 on Au and 762, 1058, 1274 cm–1 on Ag ) appeared and clearly demonstrated its immobilization onto Au and Ag surfaces. Simultaneously, we have also observed changes in the ratio of trans–gauche conformers of adsorbed cysteamine molecules. Layers revealing high content of trans conformer are transformed into layers composed mainly of cysteamine molecule in gauche conformation after galactose oxidase adsorption. These observations deliver a strong support for enzyme immobilization on cysteamine-modified gold and silver surfaces. The surface plasmon resonance experiment gave a surface coverage of ~8.4 × 107 g/cm2 for gold electrode modified cysteamine using DIDS chemistry and 1.1 × 107 g/cm2 for the cysteamine only modified gold substrate and demonstrated that galactose oxidase layers immobilized with DIDS coupling reagent are quite stable and cannot be easily removed from the surface by treatment with a buffer solution. The surface plasmon resonance results indicated that in this method, a multilayer of galactose oxidase have been immobilized. Our new method of covalent attachment of enzymes seems to be quite promising as a new way of manufacturing biosensors. Copyright © 2012 John Wiley & Sons, Ltd.

Oxygen adsorption on Ag(110): density functional theory band structure calculations and dynamical simulations

Surface-enhanced Raman scattering of DNA molecules on silver electrodes modified with silver nanoparticles