Interactions of iodoperfluorobenzene compounds with gold nanoparticles

Understanding the interactions of small molecules with gold nanoparticles is important for controlling their surface chemistry and, hence, how they can be used in specific applications. The interaction of iodoperfluorobenzene compounds with gold nanoparticles was investigated by UV-Vis difference spectroscopy, surface enhanced Raman spectroscopy (SERS) and Synchrotron X-ray photoelectron spectroscopy (XPS). Results from UV-Vis difference spectroscopy demonstrated that iodoperfluorobenzene compounds undergo charge transfer complexation with gold nanoparticles. SERS of the small molecule-gold nanoparticle adducts provided further evidence for formation of charge transfer complexes, while Synchrotron X-ray photoelectron spectroscopy provided evidence of the binding mechanism. Demonstration of interactions of iodoperfluorobenzene compounds with gold nanoparticles further expands the molecular toolbox that is available for functionalising gold nanoparticles and has significant potential for expanding the scope for generation of hybrid halogen bonded materials.     

In situ surface-enhanced Raman scattering and X-ray photoelectron spectroscopic investigation of coenzyme Q10 on silver electrode

In this study, coenzyme Q(10) (CoQ(10)) has been investigated by in situ near-infrared Fourier transform surface-enhanced Raman scattering (NIR-FT-SERS) spectroelectrochemistry and angle-resolved X-ray photoelectron spectroscopy (AR-XPS) on silver surface. The surface adsorption behavior of the coenzyme Q(10) radical intermediate could be monitored by potential-dependent SERS technique. At the applied potential lower than -0.30 V vs. SCE, the radical intermediate CoQ(10)H˙ stands perpendicularly on the silver surface with both oxygen atoms of the aromatic ring and isoprenoid side chains. When the applied potential is more positive than -0.30 V vs. SCE or at open circuit potential, the quinone ring (benzene ring) of reduced form of coenzyme Q(10) (CoQ(10)H(2)) adopts a face-on surface configuration on the surface. The responsible mechanism for the potential-dependent SERS spectra is presented. Moreover, the adsorption conformation of CoQ(10) has been further confirmed by AR-XPS at the silver surface.     

Investigation of the effects of the local environment on the surface-enhanced Raman spectra of striped gold/silver nanorod arrays

The effects of the local environment on surface-enhanced Raman scattering (SERS) spectra utilizing gold, silver, and gold/silver striped nanorod array substrates was investigated. The arrays were fabricated using an electrochemical metal deposition into an anodic aluminum oxide template. The analyte chosen for this study was p-nitroso-N,N-dimethylaniline (p-NDMA), which has an electronic structure that is highly sensitive to its surrounding environment. Changes in the peak positions and peak ratios were used to probe the influence of water and the striping pattern on the SERS signal of p-NDMA. We present the results of the fabrication and characterization of the nanorod array substrates, as well as SERS spectra of p-NDMA in both polar and nonpolar environments and SERS spectra on a variety of striped nanorod arrays. The Raman data suggests that the p-NDMA molecule exists in a more polarized state when bound to the gold as compared to the silver rods. We have attempted to use these differences to determine whether the SERS signal predominantly arises from the tips of the rods or from the interior of the array.     

Adsorption mechanisms of sulfathiazole on gold,silver and copper surfaces studied by SERS

Surface-enhanced Raman scattering (SERS) of sulfathiazole was studied in gold, silver and copper colloids as well as on a gold plate. SERS spectra of sulfathiazole in gold and silver colloids indicated chemisorption of molecules on the metal nanoparticles through the amide nitrogen, with the phenyl moiety orthogonally placed and the thiazole ring almost parallel positioned towards the metal surface. Although selectively enhanced phenyl bands pointed to a very similar position of the sulfathiazole molecules on the copper colloid, a chemical bonding was not implied. Unlike adsorption mechanisms and position of the molecules on the colloid metal surfaces, a sideway adsorption of sulfathiazole on the gold plate was proposed. Hereby, both, the amide nitrogen and the thiazole nitrogen were considered responsible for approaching of sulfathiazole to the gold enhancing surface.     

Surface-enhanced vibrational spectroscopy of B vitamins: what is the effect of SERS-active metals used?

Surface-enhanced Raman scattering (SERS) spectroscopy and surface-enhanced infrared absorption (SEIRA) spectroscopy are analytical tools suitable for the detection of small amounts of various analytes adsorbed on metal surfaces. During recent years, these two spectroscopic methods have become increasingly important in the investigation of adsorption of biomolecules and pharmaceuticals on nanostructured metal surfaces. In this work, the adsorption of B-group vitamins pyridoxine, nicotinic acid, folic acid and riboflavin at electrochemically prepared gold and silver substrates was investigated using Fourier transform SERS spectroscopy at an excitation wavelength of 1,064 nm. Gold and silver substrates were prepared by cathodic reduction on massive platinum targets. In the case of gold substrates, oxidation–reduction cycles were applied to increase the enhancement factor of the gold surface. The SERS spectra of riboflavin, nicotinic acid, folic acid and pyridoxine adsorbed on silver substrates differ significantly from SERS spectra of these B-group vitamins adsorbed on gold substrates. The analysis of near-infrared-excited SERS spectra reveals that each of B-group vitamin investigated interacts with the gold surface via a different mechanism of adsorption to that with the silver surface. In the case of riboflavin adsorbed on silver substrate, the interpretation of surface-enhanced infrared absorption (SEIRA) spectra was also helpful in investigation of the adsorption mechanism.     

Spectral investigations on 1,5-dipiperidino anthraquinone

Optical absorption, fluorescence emission and surface enhanced Raman spectra of 1,5-dipiperidino anthraquinone (1,5-DPAQ) have been examined to elucidate the nature of molecule in different environments. Fluorescence emission and optical absorption measurements show that the polarity of the solvent plays a vital role through intermolecular hydrogen bonding and reorientational motion of solvent molecule around excited state fluorophore. Anisotropy measurement gives the angle between absorption and emission transition dipoles. The vibrational features observed in surface enhanced Raman spectroscopy (SERS) suggest that the molecules are chemisorbed. The adsorption of the molecule is through pi electrons in the anthraquinone ring and the axial lone pair of electrons of nitrogen. The orientation of the molecule is found to be 'flat-on'.     

Chemically bound gold nanoparticle arrays on silicon: assembly, properties and SERS study of protein interactions

A highly reproducible and facile method for formation of ordered 2 dimensional arrays of CTAB protected 50 nm gold nanoparticles bonded to silicon wafers is described. The silicon wafers have been chemically modified with long-chain silanes terminated with thiol that penetrate the CTAB bilayer and chemically bind to the underlying gold nanoparticle. The silicon wafer provides a reproducibly smooth, chemically functionalizable and non-fluorescent substrate with a silicon phonon mode which may provide a convenient internal frequency and intensity calibration for vibrational spectroscopy. The CTAB bilayer provides a potentially biomimetic environment for analyte, yet allows a sufficiently small nanoparticle separation to achieve a significant electric field enhancement. The arrays have been characterized using SEM and Raman spectroscopy. These studies reveal that the reproducibility of the arrays is excellent both between batches (<10% RSD) and across a single batch (<5% RSD). The arrays also exhibit good stability, and the effect of temperature on the arrays was also investigated. The interaction of protein and amino acid with the nanoparticle arrays was investigated using Raman microscopy to investigate their potential in bio-SERS spectroscopy. Raman of phenylalanine and the protein bovine pancreatic trypsin inhibitor, BPTI were studied using 785 nm excitation, coincident with the surface plasmon absorbance of the array. The arrays exhibit SERS enhancements of the order of 2.6 x 10(4) for phenylalanine, the standard deviation on the relative intensity of the 1555 cm(-1) mode of phenylalanine is less than 10% for 100 randomly distributed locations across a single substrate and less than 20% between different substrates. Significantly, comparisons of the Raman spectra of the protein and phenylalanine in solution and immobilized on the nanoparticle arrays indicates that the protein is non-randomly orientated on the arrays. Selective SERS enhancements suggest that aromatic residues penetrate through the bilayer inducing conformational changes in the protein.     

Closely adjacent gold nanoparticles linked by chemisorption of neutral rhodamine 123 molecules providing enormous SERS intensity

Addition of neutral R123 molecules (10(-7) M) to an as-prepared gold nanoparticles (AuNPs) suspension generated flocculates that are a small number of closely adjacent particles. Formation of AuNP flocculates was evidenced by the coupled localized plasmon peak at 720-750 nm. The AuNP flocculates provided pronounced SERS spectra of adsorbed neutral R123 molecules (SERS-A) as anticipated by FDTD (Finite Difference Time Domain) simulations. The observed SERS spectra are significantly different from those of cationic R123(+) molecules (SERS-B), which electrostatically adsorbed on Cl(-)-treated AuNPs. The difference is not simply due to deprotonation but reflects a distinct difference in adsorption nature between neutral R123 and cationic R123(+) molecules. Indeed neutral R123 molecules exclusively gave an Au-N stretching band at 202 cm(-1), showing the chemisorption on Au surfaces through lone pair electrons at the amino groups. The different adsorption nature is further evidenced by the observation that cationic R123(+) molecules adsorbed on as-prepared (without NaCl addition) AuNP flocculates gave both SERS-A and SERS-B spectra. Thus, the cationic R123(+) molecules form the flocculates both by chemisorption and electrostatic adsorption owing to modest surface charge on as-prepared AuNPs.     

Surface enhanced Raman spectra of isomeric acetylpyridines adsorbed on silver sol

Raman spectra of 2-, 3- and 4-acetylpyridines (AP) are obtained in bulk phase, in aqueous solution and in the adsorbed state on colloidal silver particles. Addition of the acetylpyridines on the Ag-sol results in aggregation of the silver particles showing characteristic charge transfer (CT) bands. Significant surface enhancement of the Raman bands are observed. Both the estimated enhancement factor and the absorption maxima of the CT bands are in inverse parallel relationship with the electron density on the nitrogen atom as reflected by the Hammett σ values of the substituents. It is inferred that the charge transfer interactions between the adsorbates and the metal particles contribute to the enhancement mechanism. This is further substantiated by the concentration dependence of enhancement. A classical electromagnetic contribution is demonstrated by the Raman excitation frequency dependence of SERS. The results further show that the molecules are adsorbed on the metal surface through the nitrogen atom. Appearance of some out-of-plane modes in the SERS spectra suggests that the pyridyl ring planes are not perpendicular to the metal surface, but are tilted.     

Gold particle interaction in regular arrays probed by surface enhanced Raman scattering

Lithographically designed two-dimensional arrays consisting of gold nanoparticles deposited on a smooth gold film are used as substrate to examine the SERS effect of the trans-1,2-bis (4-pyridyl) ethylene molecule. These arrays display two plasmon bands instead of the single one observed for the same arrays of particles but deposited on indium tin oxide coated glass. Laser excitation within the short wavelength band does not bring about any SERS spectrum, while excitation within the long wavelength band yields SERS spectra with a gain per molecule rising up to 10(8). The simultaneous investigation of extinction and Raman spectra of arrays exhibiting various topography parameters enables us to suggest an interpretation for both the occurrence of the two plasmon resonances and for the high Raman enhancement. We suggest to assign the short wavelength band to a plasmon wave propagating at the gold glass interface and the long wavelength one to an air/gold surface plasmon mode modified by particle-particle interaction.     

Surface-enhanced hyper-Raman scattering (SEHRS) on Ag film over Nanosphere (FON) electrodes: surface symmetry of centrosymmetric adsorbates

Electrochemical surface-enhanced hyper-Raman scattering (SEHRS) and surface-enhanced Raman scattering (SERS) of centrosymmetric molecules on Ag film over nanosphere (AgFON) electrodes are presented. The SEHR spectra of trans-1,2-bis(4-pyridyl)ethylene (BPE) at different potentials (vs Ag/AgCl) are presented for the first time, and a reversible potential tuning of the SEHR spectra of BPE is demonstrated. The SEHRS and SERS techniques were used to determine to what extent either site symmetry reduction or field gradient effects dictate the origin of the observed vibrational spectra. It is found that the SEHR and SER spectra for the molecules studied were distinctly different at all frequency regions at a fixed voltage, suggesting that centrosymmetry is largely retained upon adsorption to the AgFON surface and that field gradient effects are negligible. This work also shows that the SEHR spectra clearly depend on potential, whereas the SER spectra are essentially independent of potenial. It is determined that the combination of changes in deltaGads and the presence of coadsorbed counterions are responsible for altering the local symmetry of the adsorbate and only SEHRS has the sensitivity to detect these changes in the surface environment. Thus, SEHRS is a uniquely useful spectroscopic tool that is much more sensitive to the local adsorption environment than is SERS.     

Photoelectron spectra of the allyl amines

The Hel photoelectron spectra of the three allyl amines show low ionization potential bands arising from nitrogen lone-pair n electrons and ethylenic π-bond electrons. Analysis of the spectra using MINDO/3 calculations and comparisons with ionization data of related molecules shows that π-π interactions are considerable, but n-π interactions are small. The π-π splitting in triallyl amine is consistent with a near-planar C₃ symmetry structure for the gas phase molecule.     

Surfactantless synthesis of multiple shapes of gold nanostructures and their shape-dependent SERS spectroscopy

A useful method for the synthesis of various gold nanostructures is presented. The results demonstrated that flowerlike nanoparticle arrays, nanowire networks, nanosheets, and nanoflowers were obtained on the solid substrate under different experimental conditions. In addition, surface-enhanced Raman scattering (SERS) spectra of 4-aminothiophenol (4-ATP) on the as-prepared gold nanostructures of various shapes were measured, and their shape-dependent properties were evaluated. The intensity of the SERS signal was the smallest for the gold nanosheets, and the flowerlike nanoparticle arrays gave the strongest SERS signals.     

Controlled fabrication of nanopillar arrays as active substrates for surface-enhanced Raman spectroscopy

Highly ordered gold nanopillar arrays were fabricated using anodized aluminum oxide (AAO) templates. Nanopillars with a dimension of 110 +/- 15 nm in vertical height and 75 +/- 10 nm in base diameter were formed with a density of 150 microm(-2). The ordered nanopillar arrays give reproducible surface-enhanced Raman scattering (SERS) at a detection limit of 10(-8) M using thionine as probing molecules. The enhancement by the Au nanopillar arrays was comparable with or better than that of dispersed gold nanoparticle SERS substrates. This work demonstrates a new technique for producing highly ordered and reproducible SERS substrates potentially applicable for chemical and biological assay.     

A surface enhanced Raman spectroscopy study of aminothiophenol and aminothiophenol-C60 self-assembled monolayers: evolution of Raman modes with experimental parameters

P-aminothiophenol (PATP) is a well-known molecule for the preparation of self-assembled monolayers on gold via its thiol functional group. After adsorption, it has been demonstrated that this molecule is anchored to gold through its thiol group, and standing nearly upright at the surface with the amino functional group on top. This molecule has been extensively studied by surface enhanced Raman spectroscopy but its exact SERS spectrum remains unclear. Here, we demonstrate that it can be strongly affected by at least two experimental parameters: laser power and layer density. Those features are discussed in terms of a dimerization of the PATP molecules. The free amino group affords the adsorption of other molecules such as C(60). In this case, a complex multilayer system is formed and the question of its precise characterisation remains a key point. In this article, we demonstrate that surface enhanced Raman spectroscopy combined with x ray photoelectron spectroscopy can bring very important information about the organization of such a self-assembled multilayer on gold. In our study, the strong evolution of Raman modes after C(60) adsorption suggests a change in the organization of aminothiophenol molecules during C(60) adsorption. These changes, also observed when the aminothiophenol layer is annealed in toluene, do not prevent the adsorption of C(60) molecules.     

Bimetallic gold–silver nanoplate array as a highly active SERS substrate for detection of streptavidin/biotin assemblies

The silver-modified gold nanoplate arrays as bimetallic surface-enhanced Raman scattering (SERS) substrates were optimized for the surface-enhanced Raman detection of streptavidin/biotin monolayer assemblies. The bimetallic gold–silver nanoplate arrays were fabricated by coating silver nanoparticles uniformly on the gold nanoplate arrays. Depending on silver nanoparticle coating, the localized surface plasmon resonance (LSPR) peak of the bimetallic gold–silver nanoplate arrays blue-shifted and broadened significantly. The common probe molecule, Niel Blue A sulfate (NBA) was used for testing the SERS activity of the bimetallic gold–silver nanoplate arrays. The SERS intensity increased with the silver nanoparticle coating, due to a large number of hot spots and nanoparticle interfaces. The platforms were tested against a monolayer of streptavidin functionalized over the bimetallic gold–silver nanoplate arrays showing that good quality spectra could be acquired with a short acquisition time. The supramolecular interaction between streptavidin (strep) and biotin showed subsequent modification of Raman spectra that implied a change of the secondary structure of the host biomolecule. And the detection concentration for biotin by this method was as low as 1.0 nM. The enhanced SERS performance of such bimetallic gold–silver nanoplate arrays could spur further interest in the integration of highly sensitive biosensors for rapid, nondestructive, and quantitative bioanalysis, particularly in microfluidics.     

表面增强拉曼光谱研究以4,4'-联吡啶为标记分子的免疫检测

通过表面增强拉曼光谱(SERS)研究了标记分子4,4'-联吡啶在金溶胶上的吸附行为, 并将其与山羊抗小鼠IgG结合, 获得SERS标记免疫金溶胶. 在固相基底上组装抗体, 两者组装得到固相抗体-抗原-标记抗体“三明治”结构. 在单组分和双组分体系中借助抗体上标记金纳米粒子所带的SERS信号达到免疫检测的目的.     

表面增强拉曼光谱研究以4,4''''-联吡啶为标记分子的免疫检测

通过表面增强拉曼光谱(SERS)研究了标记分子4,4'-联吡啶在金溶胶上的吸附行为,并将其与山羊抗小鼠IgG结合,获得SERS标记免疫金溶胶.在固相基底上组装抗体,两者组装得到固相抗体-抗原-标记抗体“三明治”结构.在单组分和双组分体系中借助抗体上标记金纳米粒子所带的SERS信号达到免疫检测的目的.     

Thin films of Ag nanoparticles prepared from the reduction of AgI nanoparticles in self-assembled films

A novel method for the preparation of thin films of Ag nanoparticles is reported. Using mercaptoacetic acid as the stabilizing agent, AgI nanoparticles were prepared in aqueous solution. And based on electrostatic interactions, the thiol-passivated AgI nanoparticles were assembled in a self-assembled film by alternative deposition with a cationic polyelectrolyte. Then the AgI nanoparticles in the composite film were reduced by NaBH(4), which resulted in the formation of a thin film of Ag nanoparticles. UV-visible spectra and X-ray photoelectron spectroscopy data confirmed the transformation from AgI to Ag. Atomic force microscopy (AFM) showed that the formed Ag nanoparticles distributed on the film homogeneously. Surface-enhanced Raman spectroscopy (SERS) measurement indicated that the prepared thin films could be used as effective SERS substrates. The reduction process was also carried out by UV light at selective surface regions, which resulted in the formation of patterned nanoparticle arrays.     

Ag/ZrO2-NT/Zr hybrid material: A new platform for SERS measurements

In this paper, we present recent results of our attempts to produce nanoporous zirconia, as well as our investigations of a hybrid material consisting of nanoporous zirconia loaded with Ag-nanoparticles, Ag-n/ZrO₂-NT/Zr, which could be used as an active SERS substrate. The Zr-based hybrid material, as our investigations have shown, is an active and stable substrate in SERS investigations aimed at detecting various organic molecules: mercaptobenzoic acid, pyridine and two different dyes – rodhanine derivatives. The SERS spectra for the probe molecules adsorbed on silver nanoparticles on a ZrO₂-NT/Zr platform display characteristic intensity ratios different from those measured on previously studied nanoporous substrates based on Ti and Al, which ensure a different (alternative) interaction between the investigated adsorbate and adsorbent. In order to characterize our new substrate we used high-resolution SEM and surface analytical techniques: XPS (X-ray photoelectron spectroscopy) and SERS (surface enhanced Raman spectroscopy).