Surface‐enhanced Raman scattering of benzenesulfonamide and sulfanilamide adsorbed on silver nanoparticles

The Surface‐enhanced Raman scattering of benzenesulfonamide and sulfanilamide adsorbed on silver sols was studied. On the basis of the noticeable shifts observed for wavenumbers of the ν_s(OSO), ν(CS), and ν(SN) vibrations with respect to the Raman spectra of the solids and the ionic solutions, we conclude that these molecules are adsorbed on silver nanoclusters at pH ≥ 7 with the aminosulfonyl groups partially deprotonated. The benzenesulfonamide links to the metal through the nitrogen atom of the corresponding azanion, while the sulfanilamide interacts in turn through the nitrogen atoms of the –NH₂ and –SO₂NH^– groups in the para‐position. Additionally, it was found that the most enhanced surface‐enhanced Raman scattering bands, especially the 8a;ν_ring mode, are related to the presence of the charge transfer mechanism. Copyright © 2011 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman scattering of molecules adsorbed on Co‐doped ZnO nanoparticles

Transition‐metal‐doped semiconductor nanoparticles (NPs) have been well studied for their optical and catalytic properties but seldom studied by surface‐enhanced Raman scattering (SERS). In this paper, transition‐metal‐doped semiconductor NPs are investigated for their SERS property. Four groups of Co‐doped (0.5, 1, 3, and 5%) ZnO (Co ZnO) NPs and pure ZnO NPs were synthesized and studied. When 4‐mercaptobenzoic acid was used as probing molecule, significant SERS signals were obtained on all the five samples. Moreover, it is very interesting to observe a relationship between the Co‐doping concentration and enhancement of the SERS signals. SERS intensities first increase with doping concentration (up to 1%), and then decrease with further increase in doping concentration (up to 5%). Charge transfer (CT) is considered to be the main contribution to this phenomenon. Different CT ratios from substrates to molecules seem to induce different intensities of the SERS signals. In our experiments, the crystalline defects of Co ZnO NPs caused by the Co dopant affect the CT ratios. A possible mechanism of CT from the valance band of Co ZnO NPs to the lower unoccupied molecular orbital of the molecules via energy of the surface states is suggested. X‐ray photoelectron spectra, UV vis spectra, and Raman spectra were used to characterize the structure and defects in Co ZnO NPs. Copyright © 2011 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman scattering studies of carbon nanotubes using Ag‐core Au‐shell nanoparticles

Surface‐enhanced Raman scattering from carbon nanotube bundles adsorbed with plasmon‐tunable Ag‐core Au‐shell nanoparticles (Ag@Au nps) was carried out for the first time. By utilizing nanoparticles whose plasmon resonance peak (541, 642 nm) closely matches the commonly used Raman excitation sources (532, 632.81 nm), we can observe a large enhancement in the Raman signatures of carbon nanotubes. We obtain greater enhancement in the Raman signal for the above case when compared to nanotubes adsorbed with conventional Ag, Au or other ‘off resonant’ Ag@Au nps. The power‐dependent SERS experiment on single‐walled nanotubes (SWNTs) with resonant Ag@Au nps reveals a linear behavior between the G‐band intensity and the photon flux density, which is in agreement with the vibrational pumping model of SERS. The observed enhancement by resonance matching is pronounced for carbon nanotubes and may lead to insights into understanding nanotube–nanoparticle interaction. Copyright © 2009 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman scattering study of L‐tryptophan

Surface‐enhanced Raman scattering (SERS) spectra of tryptophan (Trp) were obtained. A unique SERS spectrum of Trp, corresponding to the most stable conformation and orientation on the metal surface, is observed after a stabilization period. The Trp molecules interact with the surface through both the carboxylate and amino groups; the aliphatic moiety is close to the surface. The pyrrole ring of the indole moiety is farther from the surface than the benzene fragment. The observed spectra vary depending on both the preparation of the silver colloid and the aggregation time. The interpretation of the experimental results is supported by theoretical treatment of the molecule on the silver surface. Copyright © 2008 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman scattering of hydroxyproline

Surface‐enhanced Raman scattering (SERS) spectra of hydroxyproline and one deuterated analogue are reported. In this work, we tackled the problem of SERS reproducibility by employing gold colloids instead of the usual silver sols to achieve plasmon enhanced Raman scattering. We slightly modified modified a previously published procedure to obtain to obtain the colloid, and concentrated the gold particles by centrifugation. The SERS spectra show distinctive bands of hydroxyproline, assigned by comparison to normal Raman spectra and density functional theory calculations. Repeated measurements using this procedure showed reproducible SERS spectra. Copyright © 2011 John Wiley & Sons, Ltd.     

Improved surface‐enhanced Raman scattering properties of TiO2 nanoparticles by Zn dopant

In this paper, pure and Zn‐doped TiO₂ nanoparticles (NPs) with various content of Zn were prepared by a sol–hydrothermal method and were employed as active substrates for surface‐enhanced Raman scattering (SERS). On the 3% Zn‐doped TiO₂ substrate, 4‐mercaptobenzoic acid(4‐MBA) molecules exhibit a higher SERS intensity by a factor of 6, as compared with the native enhancement of 4‐MBA adsorbed on undoped TiO₂ NPs. Moreover, the higher SERS activity was still observed on the 3% Zn‐doped TiO₂ NPs at temperature even up to 125 °C. These results indicate that an appropriate amount of Zn doping can improve the SERS performances of TiO₂ SERS‐active substrates. The introduction of Zn dopant can enrich the surface states (defects) of TiO₂ and improve the separation efficiency of photo‐generated charge carriers (electrons and holes) in TiO₂, according to measurements of X‐ray diffraction, UV‐visible diffuse reflectance spectroscopy, and photoluminescence, which are responsible for the influence of Zn dopant on the improved SERS performances of TiO₂ NPs. Copyright © 2009 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman scattering sensing on black silicon

Reactive ion etching was used to fabricate black‐Si over the entire surface area of 4‐inch Si wafers. After 20 min of the plasma treatment, surface reflection well below 2% was achieved over the 300–1000 nm spectral range. The spikes of the black‐Si substrates were coated by gold, resulting in an island film for surface‐enhanced Raman scattering (SERS) sensing. A detection limit of 1 × 10^?6 M (at count rate > 10² s^?1 . mW^?1) was achieved for rhodamine 6G in aqueous solution when drop cast onto a ～ 100‐nm‐thick Au coating. The sensitivity increases for thicker coatings. A mixed mobile‐on‐immobile platform for SERS sensing is introduced by using dog‐bone Au nanoparticles on the Au/black‐Si substrate. The SERS intensity shows a non‐linear dependence on the solid angle (numerical aperture of excitation/collection optics) for a thick gold coating that exhibits a 10 times higher enhancement. This shows promise for augmented sensitivity in SERS applications.     

Surface‐enhanced Raman scattering spectroscopy via gold nanostars

Anisotropic metallic nanoparticles (NPs) have unique optical properties, which lend them to applications such as surface‐enhanced Raman scattering (SERS) spectroscopy. Star‐shaped gold (Au) NPs were prepared in aqueous solutions by the seed‐mediated growth method and tested for Raman enhancement using 2‐mercaptopyridine (2‐MPy) and crystal violet (CV) probing molecules. For both molecules, the SERS activity of the nanostars was notably stronger than that of the spherical Au NPs of similar size. The Raman enhancement factors (EFs) for 2‐MPy on Au nanostars and nanorods are comparable and estimated as greater than 5 orders of magnitude. However, the enhancement for CV on nanostars was significantly higher than for nanorods, in particular at CV concentrations of 100 nM or lower. This article is a US Government work and is in the public domain in the USA. Published in 2008 by John Wiley & Sons, Ltd.     

Surface‐enhanced Raman spectroscopy of 4‐tert‐butylpyridine on a silver electrode

We report the observation of large surface‐enhanced Raman scattering (SERS) (10⁶) for 4‐tert‐butylpyridine molecules adsorbed on a silver electrode surface in an electrochemical cell with electrode potential set at − 0.5 V. A decrease in electrode potential to − 0.3 V was accompanied by a decrease in relative intensities of the vibrational modes. However, there were no changes in vibrational wavenumbers. Comparison of both normal solution Raman and SERS spectra shows very large enhancement of the intensities of a₁, a₂, and b₂ modes at laser excitation of 488 nm. Enhancement of the non‐totally symmetric modes indicates the presence of charge transfer as a contributor to the enhancement. Copyright © 2011 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman scattering characterization of monohydroxylated polymethoxyflavones

Polymethoxyflavones (PMFs) belong to a unique class of flavonoids mainly found in citrus fruits. Characterization of different PMFs is important to further understand and apply these compounds as functional ingredients in food. The objective of this study is to characterize three monohydroxylated PMFs using surface‐enhanced Raman spectroscopy (SERS) and to determine the role of hydroxylation in their SERS behaviors. Serial concentrations of 3′‐hydroxylnobiletin (3HN), 4′‐hydroxylnobiletin (4HN), and 5‐hydroxylnobiletin (5HN) were incubated with silver dendrites for SERS analysis. Results demonstrated that three PMFs exhibited significantly different SERS behaviors. 5HN produced saturation peak intensity at relative low concentration (0.05 mM), while 3HN and 4HN produced saturation peak intensity at much higher concentrations (0.5 and 1 mM, respectively) according to principal component analysis. Below saturation, 5HN had the highest peak intensity, while 3HN had the lowest peak intensity. After reaching saturation, 4HN and 5HN had similar relative peak intensities that were much greater than 3HN. The HPLC analysis revealed that 36.13 ± 1.06% of 5HN, 18.40 ± 3.31% of 4HN, and 9.66 ± 0.94% of 3HN were bound to silver. Based on these results, we speculated that different positions of hydroxylation of PMFs were critical for determining spatial conformation of PMFs on binding sites, resulting in different binding affinities and saturation points, therefore their SERS behaviors. This study first reported that the position of hydroxylation in the monohydroxylated PMFs was crucial for their interactions with silver dendrites and provided valued information for further applying SERS for molecular characterization of flavonoids. Copyright © 2016 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman scattering study of the red dye laccaic acid

FT‐Raman and surface‐enhanced Raman scattering (SERS) spectroscopy were applied to the study of lac dye, a highly fluorescent anthraquinone red dye. The SERS spectra were obtained at different pH values, on Ag nanoparticles prepared by chemical reduction with citrate and hydroxylamine, and at several excitation wavelengths, in order to find the best experimental conditions for the detection of the lac dye. The lower detection limit was achieved using nanoparticles prepared by reduction with hydroxylamine, excitation at 514.5 nm, and slightly acidic pH conditions, thus exploiting a combination of factors including lower electrostatic repulsion between dye and nanoparticles and resonance Raman enhancement. A comparison between the adsorption of laccaic acid (LA) and carminic acid (CA), another anthraquinone red dye, was also done, based on the SERS spectra of both dyes. Copyright © 2007 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman spectroscopy on novel black silicon‐based nanostructured surfaces

Two different black silicon nanostructured surfaces modified with thin gold layers were tested for analytical signal enhancement with Surface‐Enhanced Raman Spectroscopy (SERS). The relationship between the thicknesses of the gold layers and the analytical signal enhancement was studied. Also, effects of Ti and Ti/Pt adhesion layers underneath the gold layers on the analytical signal enhancement were tested. An enhancement factor of 7.6 × 10⁷ with the excitation laser 785 nm was achieved for the tested analyte, Rhodamine 6G, and non‐resonance SER spectra were recorded in a 5 s acquisition mode. Such an enhancement enables to achieve a detection limit down to 2.4 pg of Rhodamine 6G on a black silicon‐based nanosurface coated with a 400‐nm‐thin layer of gold. Copyright © 2009 John Wiley & Sons, Ltd.     

DFT study on the influence of electric field on surface‐enhanced Raman scattering from pyridine–metal complex

The influence of a static external electric field on surface‐enhanced Raman scattering is investigated by calculating the Raman spectra and excited state properties of pyridine–Au₂₀ complex with the density functional theory and time‐dependent density functional theory method. The external electric field with orientation parallel (positive) or antiparallel (negative) to the permanent dipole moment is respectively applied on the complex. This field slightly changes the equilibrium geometry and polarizabilities, which results in shifted vibration frequencies and selectively enhanced Raman intensities. The changes of charge transfer (CT) excited states in response to the electric field are visualized by employing the charge difference densities. Further, the energy of charge transfer transition is tuned by electric field to be resonant or not with the incident light, leading to the Raman intensities are enhanced or not enhanced. At the same time, the intensities of vibration modes are sensitive to the orientation of the field. The positive electric field enhances the totally symmetric ring breathing mode (~1009 cm⁻¹) but suppresses the trigonal ring breathing mode (~1051 cm⁻¹). On the contrary, the mode at 1051 cm⁻¹ is more enhanced than the mode at 1009 cm⁻¹ when the negative electric field is applied on the complex. The Raman spectra could be modulated by tuning the strength and direction of the electric field. Copyright © 2013 John Wiley & Sons, Ltd.     

Adsorption study of 4‐MBA on TiO2 nanoparticles by surface‐enhanced Raman spectroscopy

In this paper, the adsorption of 4‐mercaptobenzoic acid (4‐MBA) on TiO₂ nanoparticles was studied mostly by surface‐enhanced Raman spectroscopy (SERS) and UV‐vis spectroscopy, at different pH values as well as under different temperatures and concentrations. The results show that the 4‐MBA molecules are bonded to the TiO₂ surface both through the sulfur atoms and COO⁻ groups at neutral or alkaline pH, but only through the sulfur atom at acidic pH. Furthermore, the 4‐MBA molecules possess high adsorptive stability on TiO₂ at a comparatively high temperature (150 °C). Concentration‐dependent SERS experiments show that the saturation concentration for 4‐MBA adsorbed on TiO₂ is about 10⁻³ M in natural case (pH = 6). Copyright © 2009 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman scattering from polystyrene on gold clusters

Surface‐enhanced Raman scattering (SERS) constitutes a spectroscopic method of rapidly growing importance, and polystyrene is a widely used compound of great industrial importance. In this work, SERS data were obtained from polystyrene samples prepared by vapor deposition of gold and plasma‐induced polymerization of styrene gas. A thorough examination of this data is presented. The relationships between sample preparation parameters, gold‐cluster morphology, and SERS intensity were elucidated. Using Wilson's notation, vibrations were assigned to all bands between 250 and 1750 cm⁻¹ in the ordinary Raman and SERS spectra of polystyrene. The correct assignment of these bands would be a significant achievement because they have been controversial in the literature for ∼30 years. Our assignments were made by reviewing the literature and comparing the assignments found there to spectral data acquired during this study; they were confirmed using density functional theory (DFT) calculations performed on the styrene monomer. The orientation of polystyrene's phenyl ring, relative to the gold surface, was determined. It has been suggested that reactions involving silver catalyze polystyrene degradation during SERS, but we found that silver is not necessary for the degradation to occur. Copyright © 2009 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman scattering from gold‐coated germanium oxide nanowires

We utilized bulk‐synthesized nanowires (NWs) of germanium dioxide as nanoscale structures that can be coated with noble metals to allow the excitation of surface plasmons over a broad frequency range. The NWs were synthesized on substrates of silicon using gold‐catalyst‐assisted vapor–liquid–solid (VLS) growth mechanism in a simple quartz tube furnace setup. The resulting NWs have diameters of ∼100–200 nm, with lengths averaging ∼10–40 µm and randomly distributed on the substrate. The NWs are subsequently coated with thin films of gold, which provide a surface‐plasmon‐active surface. Surface‐enhanced Raman scattering (SERS) studies with near‐infrared (NIR) excitation at 785 nm show significant enhancement (average enhancement > 10⁶) with good uniformity to detect submonolayer concentrations of 4‐methylbenzenethiol (4‐MBT), trans‐1,2‐bis(4‐pyridyl)ethylene (BPE), and 1,2‐benzendithiol (1,2‐BDT) probe molecules. We also observed an intense, broad continuum in the Raman spectrum of NWs after metal coating, which tended to diminish with the analyte monolayer formation. Copyright © 2008 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman scattering assessment of DNA from leaf tissues adsorbed on silver colloidal nanoparticles

In this work, the surface‐enhanced Raman scattering (SERS) spectra of seven genomic DNAs from leaves of chrysanthemum (Dendranthema grandiflora Ramat.), common sundew (Drosera rotundifolia L.), edelweiss (Leontopodium alpinum Cass), Epilobium hirsutum L., Hypericum richeri ssp. transsilvanicum (Čelak) Ciocârlan, rose (Rosa x hybrida L.) and redwood (Sequoia sempervirens D. Don. Endl.), respectively, have been analyzed in the wavenumber range 200–1800 cm⁻¹. The surface‐enhanced Raman vibrational modes for each of these cases, spectroscopic band assignments and structural interpretations of genomic DNAs are reported. A high molecular structural information content can be found in the SERS spectra of these DNAs from leaf tissues. Based on this work, specific plant DNA–ligand interactions or accurate local structure of DNA might be further investigated using surface‐enhanced Raman spectroscopy. Besides, this study will generate information which is valuable in the development of label‐free DNA detection for chemical probing in living cell. Copyright © 2013 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman scattering of pyridine‐functionalized multi‐walled carbon nanotubes

Carbon nanotubes (CNTs) have attracted great attention for their potential use in many applications because of their intrinsic properties. The importance other than the impact of the application of CNT‐embedded membranes in the area of water technology development is immense. In this context, the identification and quantification of CNTs in aqueous resources during relevant water purification processes can be proven of high significance. Surface‐enhanced Raman scattering (SERS) potentially has the sensitivity required for trace analysis and has been previously used for CNT identification on solid substrates. A thorough study for the identification and quantification of small concentrations of multi‐walled CNTs (MWCNTs) in water suspensions via SERS has been performed. The functionalization of MWCNTs with pyridine groups seems to favor the surface enhancement of relevant Raman signal. This study constitutes the first step of a work in progress for the characterization of CNTs at quite low concentration range by SERS in any water suspension. It is based on an ex ante functionalization of the CNTs by pyridine, demonstrating the potential of the method. Our long‐term aim is its general application built, however, in an ex post relevant functionalization of the CNTs in any aqueous solution. Copyright © 2014 John Wiley & Sons, Ltd.     

P-thiocresol 吸附在银纳米粒子表面系统的表面增强拉曼散射和表面增强共振拉曼散射的增强研究

文中从实验和计算两方面报道了在514.5 nm激发光下P-Thiocresol吸附在银胶表面系统的表面增强拉曼散射(SERS).文中分析了它的增强机制,发现增强主要来自于电磁场增强.如果考虑距离为2nm的两个银纳米粒子的耦舍效应,两粒子之间的SERS的电磁场增强为7.16 × 107.静态化学增强亦起到部分增强作用,它的增强倍数为6.所以,总的SERS增强,包括静态化学增强和电磁场增强,是Gtotal=Gsc ×GEM=4.4×108.我们也理论地研究了此系统的表面增强共振拉曼散射(SERRS).当激发光与P-Thiocresol-Ag3系统的激发态共振时,电荷转移机制(化学增强)也将起到重要作用,最强的增强可迭106.我们使用电荷密度将激发光下p-Thlocresol和Ag团簇问的电荷转移结果可视化,这是电荷转移的直接理论证据.对于SERRS增强,包括电荷转移和电磁场增强机制,能达到1013.     

Surface‐enhanced Raman spectroscopy of indanthrone and flavanthrone

The normal Raman and surface‐enhanced Raman scattering (SERS) spectra of flavanthrone and indanthrone were obtained at several excitation wavelengths. The spectral assignments were aided by density functional calculations. Since both molecules have very high symmetry (C_2h) including a center of inversion, we expect that the modes of u symmetry will be forbidden in the normal Raman spectrum. However, proximity to the surface causes special SERS enhancement of several of the b_u modes, along with somewhat weaker enhancement of the a_u and b_g modes. Copyright © 2009 John Wiley & Sons, Ltd.