Catalytic effects of silver plasmonic nanoparticles on the redox reaction leading to ABTS˙+ formation studied using UV-visible and Raman spectroscopy.

ABTS (2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)) is a compound extensively employed to evaluate the free radical trapping capacity of antioxidant agents and complex mixtures such as biological fluids or foods. This evaluation is usually performed by using a colourimetric experiment, where preformed ABTS radical cation (ABTS˙+) molecules are reduced in the presence of an antioxidant causing an intensity decrease of the specific ABTS˙+ UV-visible absorption bands. In this work we report a strong effect of silver plasmonic nanoparticles (Ag NPs) on ABTS leading to the formation of ABTS˙+. The reaction of ABTS with Ag NPs has been found to be dependent on the interfacial and plasmonic properties of NPs. Specifically, this reaction is pronounced in the presence of spherical nanoparticles prepared by the reduction of silver nitrate with hydroxylamine (AgH) and in the case of star-shaped silver nanoparticles (AgNS). On the other hand, spherical nanoparticles prepared by the reduction of silver nitrate with citrate apparently do not react with ABTS. Additionally, the formation of ABTS˙+ is investigated by surface-enhanced Raman scattering (SERS) and the assignment of the most intense vibrational bands of this compound is performed. The SERS technique enables us to detect this radical cation at very low concentrations of ABTS (∼2 µM). Altogether, these findings allow us to suggest the use of ABTS/Ag NPs-systems as reliable and easy going substrates to test the antioxidant capacity of various compounds, even at concentrations much lower than those usually used in the spectrophotometric assays. Moreover, we have suggested that ABTS could be employed as a suitable agent to investigate the interfacial and plasmonic properties of the metal nanoparticles and, thus, to characterize the nanoparticle metal systems employed for various purposes.

Adsorption of linear aliphatic α,ω-dithiols on plasmonic metal nanoparticles: a structural study based on surface-enhanced Raman spectra.

The adsorption mechanism of linear aliphatic α,ω-dithiols with chain lengths of 6, 8 and 10 carbon atoms on silver and gold nanoparticles has been studied by surface-enhanced Raman scattering (SERS) spectroscopy. SERS spectra provided the structural marker bands of these compounds and they were employed to obtain information about the adsorption and coordination mechanism, the orientation, conformational order, and packing of the aliphatic chains of dithiols on the metal nanoparticle surface. The effect of the type of metal (silver or gold) and the extent of surface coverage on all the above mentioned properties is discussed. It was found that the adsorption of dithiols on Au nanoparticles leads to a more disordered structure of the aliphatic chains of dithiols in comparison with the adsorption on Ag nanoparticles. The interaction through both thiol groups makes the adsorption of dithiols on metal surfaces substantially different from that of monothiols; in particular, the orientation of dithiols is perpendicular, while monothiols adopt a tilted orientation. Dithiols may act as linkers between metal nanoparticles and induce the formation of nanogaps with a controllable interparticle distance. The nanogaps thus formed are able to produce hot spots exhibiting a large intensification of electromagnetic field in these points which has been proved by the observation of intense SERS spectra of dithiols until a concentration of 10(-8) M, corresponding to a large Raman enhancement factor of 5 × 10(6).

Effect of pH on the chemical modification of quercetin and structurally related flavonoids characterized by optical (UV-visible and Raman) spectroscopy.

In this work we report the study of the chemical modifications undergone by flavonoids, especially by quercetin (QUC), under alkaline conditions by UV-visible absorption, Raman and surface-enhanced Raman scattering (SERS) spectroscopy, the study was performed in aqueous solution and also on Ag nanoparticles (AgNPs). Several processes are involved in the effect of alkaline pH both in solution and on AgNPs: autoxidation affecting mainly the C-ring of the molecule and giving rise to the molecular fragmentation leading to simpler molecular products, and/or the dimerization and further polymerization leading to species with a higher molecular weight. In addition, there exists a clear structure-instability correlation concerning mainly particular groups in the molecule: the C3-OH group in the C-ring, the catechol moiety in the B-ring and the C2=C3 bond also existing in the C-ring. QUC possesses all these groups and exhibits high instability in alkaline solution. The SERS spectra registered at different pH revealed a change in the dimerization protocol of QUC going from the A- and C-rings-like-condensation to B-ring-like-condensation. Increasing the knowledge of the chemical properties of these compounds and determining the structure-activity relationship under specific environmental factors allow us to improve their beneficial properties for health as well as the preservation of Cultural Heritage objects, for example, by preventing their degradation.

Multicomponent direct detection of polycyclic aromatic hydrocarbons by surface-enhanced Raman spectroscopy using silver nanoparticles functionalized with the viologen host lucigenin.

Silver nanoparticles (NPs) functionalized with the molecular assembler bis-acridinium dication lucigenin (LG) have been used as a chemical sensor system to detect a group of polycyclic aromatic hydrocarbon (PAH) pollutants in a multicomponent mixture by means of surface-enhanced raman scattering (SERS). The effectiveness of this system was checked for a group of PAHs with different numbers of fused benzene rings, namely anthracene, pyrene, triphenylene, benzo[c]phenanthrene, chrysene, and coronene. In order to determine the host capacity of this sensor system, the self-assembly of the LG viologen on a metallic surface has been checked by analyzing SERS intensities of PAH bands at different LG concentrations. The NP-LG-analyte affinity is derived from the analysis of PAH band intensities at different concentrations of pollutants, the adsorption isotherm of each PAH on NP-LG cavities has been studied, and the corresponding adsorption constants have been evaluated. The limit of detection at trace-level concentration is confirmed by the presence of their characteristic fingerprint vibrational bands. The SERS spectra of PAH mixtures confirm that LG viologen dication shows a higher analytical selectivity to PAHs constituted by four fused benzene rings, mainly pyrene and benzo[c]phenanthrene, in agreement with their higher affinity which is also related to their better fit into the intermolecular LG cavities. As a conclusion, SERS spectra recorded on modified NP-LG surfaces are a powerful chemical tool to detect organic pollutants.

Analysis of biomolecules in cochineal dyed archaeological textiles by surface-enhanced Raman spectroscopy.

SERS spectroscopy is successfully employed in this work to reveal different components integrating the cochineal colorant employed for dying archaeological textile samples from the Arica Region in North Chile. This analysis was done by in-situ experiments that does not imply the material (colorant and biomolecules) extraction. The spectroscopic analysis of the archaeological textiles by SERS reveals the presence of bands attributed to carminic acid and nucleobases, mainly adenine and guanine. The identification of these biomolecules was also verified in raw cochineal extract and in cochineal dyed replica wool fibers fabricated by us following ancient receipts. The effect of Al on the complexation of carminic acid and other biomolecules was also tested in order to understand the changes induced by the metal interaction on the colorant structure. This study revealed that Al can also complex biomolecules existing in the cochineal extract. In particular, guanine residue seems to interact strongly with the metal, since SERS bands of this residue are enhanced. Furthermore, a theoretical analysis on the interaction of carminic acid and a silver surface was also performed in order to better understand the interaction mechanism between carminic acid and a metal surface that leads to the final SERS spectrum. The results of the present work will be very useful in the identification of different molecules and metal complexes that may be forming part of the cochineal colorant found in archaeological materials.

Adsorption of beta-adrenergic agonists used in sport doping on metal nanoparticles: a detection study based on surface-enhanced Raman scattering.

The adsorption of beta(2)-adrenergic agonist (ßAA) drugs clenbuterol, salbutamol, and terbutaline on metal surfaces has been investigated in this work by means of surface-enhanced Raman scattering (SERS). To assist in this investigation, a previous vibrational (IR and normal Raman) characterization of these drugs was performed, supported by ab initio density functional theory calculations. The application of SERS was aimed to apply this highly sensitive technique, based on localized surface plasmon resonance, in the detection of ßAA at trace concentrations and as a possible alternative method which can be postulated in routine antidoping analysis. The adsorption of these drugs was studied in depth at different experimental conditions: on Au and Ag, at different pHs, and with varying adsorbate concentration. Moreover, plasmon resonance spectroscopy was employed to investigate the adsorption of these drugs on the metal nanoparticles as well as their aggregation. It was found that the adsorption of these molecules is more effective on gold nanoparticles and at acidic pH, based on the direct interaction of the aromatic or aliphatic moieties through ionic or coordination bonds with the metal. These drugs followed a Langmuir adsorption model from which the adsorption constant and the limit of detection can be determined.

Carbon nanotube bundles as molecular assemblies for the detection of polycyclic aromatic hydrocarbons: surface-enhanced resonance Raman spectroscopy and theoretical studies.

In this work surface-enhanced resonance Raman spectroscopic experiments have demonstrated that metallic single-walled carbon nanotubes can be used as chemical assemblies between the pyrene analyte and the silver colloidal surface. Pyrene has been detected at concentrations lower than 10(-9) M by use of the 514.5 nm excitation laser line. A charge transfer from the surface to the nanotube characterizes the nanotube-silver surface interaction. The pyrene-nanotube interaction occurs through a pi-pi electronic stacking. Extended Hückel calculations based on a simplified molecular model for the analyte/nanotube/surface system support the experimental conclusions. The nanotube-pyrene distance is 3.4 A, and the most probable orientation for pyrene is confirmed to be plane parallel to the nanotube surface. An energy transfer from the silver surface to the nanotube/analyte system is verified.

Degradation of curcumin dye in aqueous solution and on ag nanoparticles studied by ultraviolet-visible absorption and surface-enhanced Raman spectroscopy.

The chemical degradation of curcumin (CU) in aqueous solution and on silver nanoparticles was studied by means of ultraviolet (UV)-visible absorption and surface-enhanced Raman (SERS) spectroscopy at different pH levels and upon light irradiation. CU undergoes a chemical degradation in aqueous solution mainly when the pH is increased. The CU degradation is catalytically enhanced in the presence of Ag nanoparticles. In general, CU degradation implies two steps: (1) the breakdown of the interring chain connecting the two CU aromatic side rings, producing smaller phenolic compounds rich in carboxylate groups, and (2) polymerization of the resulting phenolic products, giving rise to phenolic polymeric products. The degradation-polymerization mechanism can be modulated depending on the experimental conditions. The chemical photoproducts resulting from the visible light irradiation are similar to the polycatechol products obtained when catechol is adsorbed on Ag nanoparticles.

Surface-enhanced Raman spectroscopy of chernozem humic acid and their fractions obtained by coupled size exclusion chromatography-polyacrylamide gel electrophoresis (SEC-PAGE).

A humic acid extracted from a chernozem soil was fractionated combining size exclusion chromatography and polyacrylamide electrophoresis (SEC-PAGE). Three fractions named A, B, and C+D, with different electrophoretic mobilities and molecular sizes (MS), were obtained and subsequently characterized by thermochemolysis and surface-enhanced Raman spectroscopy (SERS). The data confirmed that fraction A, with the higher MS, was more aliphatic than fractions B and C+D and, in turn, fractions with lower MS (B and C+D) denoted an enrichment in lignin residues. These structural features explain conformational changes when varying the pH in the humic fraction A and indicated that combination of the two techniques is a good approach for characterizing humic substances.

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

Surface-Enhanced Raman Spectra (SERS) of methyl N-(1H-benzimidazol-2-yl)carbamate (MBC), usually named carbendazim, have been recorded on silver colloids at different pH values. In order to identify the neutral, protonated or deprotonated species of MBC that originate the SERS, the vibrational wavenumbers of these three isolated forms and linked to a silver atom have been predicted by carrying out DFT calculations. The results indicate that the active SERS species in the studied pH range correspond to the neutral MBC and its deprotonated ion in the amidate form. According to theoretical calculations, neutral MBC is linked to the metal through the imidazolic nitrogen atom, while the deprotonated MBC could be linked through the imidazolic nitrogen together with the amidic nitrogen atom or the carbonyl oxygen atom. Both adsorbed species, neutral and deprotonated, have the benzimidazolic ring orientated almost perpendicular to the silver surface and no molecular reorientation has been detected. pH of the bulk controls the relative abundance of the neutral MBC and its amidate anion which can be monitored through the intensities of the SERS bands recorded at about 1230 and 1270cm(-1). These two key bands correspond to the in-plane NH deformation of amidic and imidazolic groups, respectively.

Surface enhanced vibrational (IR and Raman) spectroscopy in the design of chemosensors based on ester functionalized p-tert-butylcalix[4]arene hosts.

Surface-enhanced IR (SEIR) and Raman scattering (SERS) have been employed to study the adsorption of ester functionalized tert-butyl calix[4]arenes on Ag and Au nanostructured surfaces as well as their complexes with pyrene. The influence of adsorption and complexation with pyrene on the host calixarene structure was tested for two different calixarene molecules bearing carboethoxy groups (CH(3)CH(2)COOCH(2)-) in the low rim at positions 1,3- and 1,2,3,4-. The results obtained with SEIR were compared to those obtained with SERS, to better understand the interaction mechanism of the studied calixarenes with the metallic surfaces and the ligand as well as to investigate the structure/selectivity relationship of these two surface techniques in the analysis of recognition problems in which these ester functionalized calixarene molecules are involved.

Surface-enhanced micro-Raman detection and characterization of calix[4]arene-polycyclic aromatic hydrocarbon host-guest complexes.

Surface-enhanced micro-Raman spectroscopy (micro-SERS) was used to detect traces of the hazardous pollutant polycyclic aromatic hydrocarbons (PAHs) pyrene and benzo[c]phenanthrene deposited onto a calix[4]arene-functionalized Ag colloidal surface. High spectral reproducibility and very low molecular detection limits (10(-8) M) were obtained by using 25,27-carboethoxy-26,28-hidroxy-p-tert-butylcalix[4]arene as host molecule. Films of immobilized aggregated Ag nanoparticles, obtained by chemical reduction with hydroxylamine, were prepared by direct adhesion on a glass surface. The influence of the aggregation degree of the initial Ag nanoparticles on the micro-SERS detection effectiveness was checked. Different relative concentrations of the host (calixarene receptor) and the guest (PAHs) were attempted in order to optimize detection of the pollutant. The obtained results indicated that the detection limit is much lower in the case of benzo[c]phenanthrene than in pyrene when exciting with the 785 nm line of a diode laser. A detailed interpretation of the Raman spectra was accomplished in order to obtain more information about the interaction mechanism of the host-guest complex, which could be useful in the future for the design of powerful detection systems.

Surface-Enhanced Vibrational Study (SEIR and SERS) of Dithiocarbamate Pesticides on Gold Films.

Surface-enhanced infrared (SEIR) and surface-enhanced Raman scattering (SERS) spectroscopies are applied to the study of the adsorption on Au films of the dimethyldithiocarbamate derivative fungicides thiram and ziram. The specificity and sensitivity of both techniques in relation to the detection and surface stability of the above compounds is analyzed comparatively. We have found that both fungicides undergo a breakdown when adsorbed on Au films, although this breakdown takes place to a different extent; thiram seems to be less stable than ziram. The sensitivity of infrared techniques is higher, although a lower enhancement in relation to SERS is observed. Whereas the SEIR technique allows the detection of all the adsorbed fungicide forms, SERS displays a high sensitivity toward only certain adsorbed molecules, those that undergo a strong adsorption induced by the fungicide breakdown.

Interaction of hypericin with serum albumins: surface-enhanced Raman spectroscopy, resonance Raman spectroscopy and molecular modeling study.

Surface-enhanced Raman spectroscopy, resonance Raman spectroscopy and molecular modeling were employed to study the interaction of hypericin (Hyp) with human (HSA), rat (RSA) and bovine (BSA) serum albumins. The identification of the binding site of Hyp in serum albumins as well as the structural model for Hyp/HSA complex are presented. The interactions mainly reflect: (1) a change of the strength of H bonding at the N1-H site of Trp; (2) a change of the Trp side-chain conformation; (3) a change of the hydrophobicity of the Trp environment; and (4) a formation of an H-bond between the carbonyl group of Hyp and a proton donor in HSA and RSA which leads to a protonated-like carbonyl in Hyp. Our results indicate that Hyp is rigidly bound in IIA subdomain of HSA close to Trp214 (distance 5.12 A between the centers of masses). In the model presented the carbonyl group of Hyp is hydrogen bonded to Asn458. Two other candidates for hydrogen bonds have been identified between the bay-region hydroxyl group of Hyp and the carbonyl group of the Trp214 peptidic link and between the peri-region hydroxyl group of Hyp and the Asn458 carbonyl group. It is shown that the structures of the Hyp/HSA and Hyp/RSA complexes are similar to, and in some aspects different from, those found for the Hyp/BSA complex. The role of aminoacid sequence in the IIA subdomains of HSA, RSA and BSA is discussed to explain the observed differences.

Interaction of antimalarial drug quinacrine with nucleic acids of variable sequence studied by spectroscopic methods.

The interaction of antimalarial drug quinacrine (QA) with polynucleotides is studied by UV-visible absorption, fluorescence and surface-enhanced Raman spectroscopy (SERS). The polynucleotides employed for such a study were calf thymus DNA, poly(A).poly(T), poly(A).poly(U), poly(C).poly(G) and poly(dG-dC).poly(dG-dC). Absorption and fluorescence spectra of QA complexes indicate that an interaction with the biomolecule is taking place, although different interaction mechanisms are probable depending on the sequence. The SERS spectra also reflect spectral changes which depend on the polymer sequence and that can be correlated to those observed by fluorescence, with the advantage of the detailed structural information provided by this vibrational technique. QA interacts with polynucleotides through its diprotonated form and by ring stacking. The strength of such interaction is extremely sequence dependent, thus suggesting different interaction mechanisms in each case. The SERS technique allows the simultaneous study of those polynucleotide moieties that are directly involved in the interaction thanks to the short-range character of the SERS spectroscopy. The interaction of QA with the above nucleic acids lead to a different change in the chain stability and flexibility which is further related to the different denaturation tendency of the polymer in the presence of the metal surface.

Interaction of antiviral and antitumor photoactive drug hypocrellin A with human serum albumin.

Absorption, resonance Raman, surface-enhanced Raman spectroscopy and differential scanning microcalorimetry were employed to study the interaction of hypocrellin A with human serum albumin. The identification of the binding place for hypocrellin A as well as the model for the albumin-hypocrellin A complex are proposed. In this model hypocrellin A interacts with albumin through more than one binding site placed on the protein surface. This model of non-specific interaction could explain why the absorption spectrum of hypocrellin A does not change in the presence of albumin and why the presence of the drug does not change significantly the thermodynamic parameters of the protein, while the Raman spectra show evident changes concerning both the protein and the drug structure. Even if hypocrellin A does not interact with an interior binding site, it can affect deeply the general albumin structure.

Adsorption of a cholesteric liquid crystal polyester on silver nanoparticles studied by surface-enhanced Raman scattering and micro Raman spectroscopy.

A Raman study of the adsorption of thermotropic cholesteric liquid crystal polyester PTOBDME ([C(34)H(36)O(8)](n)) on Ag surfaces is presented in this work. The affinity and adsorption mechanism of this polymer was tested on Ag metal colloids and on Ag films obtained by direct immobilization of the colloidal nanoparticles. We have first studied the structure of PTOBDME suspended in several solvents in order to identify the Raman bands used as structural markers. The adsorption of the polymer leads to a deep conformational change involving both the main chain and the aliphatic side chain. The interaction of polymers like PTOBDME with metals could be interesting in the formation of functionalized surfaces, providing them with specific physicochemical properties with possible applications in recognition phenomena, which can be easily characterized by Raman spectroscopy.

Quantitative estimation of peat, brown coal and lignite humic acids using chemical parameters, 1H-NMR and DTA analyses.

Humic acids extracted from peats (P), brown coals (BC) and lignites (L), were characterized using different (chemical, 1H-nuclear magnetic resonance spectroscopy and differential thermal analysis) techniques. Fourteen variables were obtained from these analyses and only five were selected because uncorrelated in multiple partial correlation. The chosen variables were C concentration, aliphatic and aromatic components and the heat of reaction of the second exothermic peak. The multivariate discriminant analysis was performed on these variables and a discriminant function was obtained which was able to efficiently separate the P, BC and L. This function enables simple predictions on samples of unknown origin. The straightforward method proposed and the results obtained are discussed.

Vibrational study of the salicylate interaction with metallic ions and surfaces.

Infrared and Raman spectroscopy are used in this work to study the metallic complexes of salicylic acid with silver and copper, comparing the interaction between salicylate and the cations (Ag+ and Cu2+) in the metal complexes with the SERS spectra when adsorbed on colloidal metal surfaces of the same metals. The salicylate complexes with the above metals were compared to those of Na+, Fe3+ and Al3+ cations. A different interaction mechanism is deduced for salicylate in the metal complex and when adsorbed on the metal surface.

Interaction of soil humic acids with herbicide paraquat analyzed by surface-enhanced Raman scattering and fluorescence spectroscopy on silver plasmonic nanoparticles.

A study of the interaction between paraquat (methyl viologen) and humic acids, extracted from a soil amended over 30 years with crop residues, cow slurries and cattle manure, was carried out by two emission spectroscopies based on plasmonic effects: surface-enhanced Raman scattering (SERS) and surface-enhanced fluorescence (SEF). To carry out this study Ag nanoparticles were used. The complex formation was tested by analyzing the effect of the herbicide on humic acids, and by varying experimental parameters such as the pH and the laser excitation wavelength. The study of the vibrational bands led to infer information about the interaction mechanism of paraquat with humic acids and to find a correlation between this interaction and the humic acids structural modification induced by the different amendments added to soil.