Surface‐enhanced Raman scattering on colloid gels originated from low molecular weight gelator

Surface‐enhanced Raman scattering (SERS) on silver and gold colloid gels formed by a low molecular weight organic gelator, bis‐(S‐phenylalanine) oxalyl amide, was obtained. Strong Raman signals dominate in the SERS spectra of hydrogels containing silver nanoparticles prepared by citrate and borohydride reduction methods, whereas broad bands of low intensity are detected in the spectra of gold colloid gels. Resemblance between Raman spectrum of the crystalline substance and the SERS spectra of the silver nanoparticle–hydrogel composites implies the electromagnetic nature of the signal enhancement. A change in Raman intensity of the benzene and amide II bands caused by an increase in temperature and concentration indicates that the gelling molecules are strongly attached through the benzene moieties to the metal nanoparticles while participating in gel formation by intermolecular hydrogen bonding between the adjacent oxalyl amide groups. Transmission electron microscopy reveals a dense gel structure in the close vicinity of the enhancing metal particles for both silver colloid gels. Copyright © 2008 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman scattering of protoberberine alkaloids

Quaternary protoberberine alkaloids are a class of natural dyes characterized by bright colors ranging from yellow to orange. As they present a strong fluorescence emission, their analysis by Raman spectroscopy is limited to specific techniques such as Fourier transform (FT)‐Raman and spectral shift Raman techniques such as shifted subtracted Raman difference spectroscopy (SSRDS) and shifted excitation Raman difference spectroscopy (SERDS). In a previous article, we successfully used surface‐enhanced Raman scattering (SERS) in the analysis of the alkaloid dye berberine in an ancient textile. The examination of the Raman and SERS spectra of berberine in combination with density functional theory (DFT) calculations indicated a flat adsorption geometry of the molecule on the Ag surface. In this article we extend that work to the study of related protoberberine alkaloids, palmatine, jatrorrhizine, and coptisine. The same adsorption geometry as in berberine was deduced. We found that the four alkaloids, although minimally different in their chemical structures, could be differentiated by the position of marker bands. Those bands are the most enhanced ones in the SERS spectra, which appear in the 700–800 cm⁻¹ region. Copyright © 2008 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 and density functional theory studies of bis(4‐aminophenyl)sulfone

Raman and surface‐enhanced Raman scattering (SERS) spectra of dapsone by using colloidal silver nanoparticles have been recorded. Density functional theory was used for the optimization of ground state geometries and simulation of the vibrational spectrum of this molecule. The SERS spectrum with a large silver cluster as a model metallic surface was simulated for the first time. Taking into account the experimental and calculated Raman as well as the SERS normal modes and the corresponding assignments, along with the modeling of the free dapsone and the one in the presence of the colloidal silver nanoparticles, the importance of the sulfone group on the SERS effect in dapsone was inferred. Copyright © 2009 John Wiley & Sons, Ltd.     

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 scattering and DFT calculations studies of 3,3′‐diethylthiatri‐ carbocyanine iodide

As an infrared Raman probe, the molecule 3,3′‐diethylthiatricarbocyanine iodide (DTTC) has received much attention in the past decades due to its potential applications in Raman imaging, single‐cell detection, cancer diagnosis, and surface‐enhanced Raman scattering (SERS). In this work, ordinary Raman, SERS, and theoretical Raman spectra were investigated to estimate the DTTC suspension. More specifically, the original gold nanospheres (60 nm diameter) and gold nanorods were encoded with DTTC and stabilized with a layer of thiol–polyethylene glycol as Raman reporter; SERS data were also obtained from the samples. Hartree–Fock theory and density functional theory (DFT) calculation were applied to calculate the optimized Raman spectra of DTTC in water on the B3LYP/6‐31G level. Subsequently, the obtained experimental spectra from DTTC were carefully compared with the theoretically calculated spectra, and good agreement was obtained between the theoretical and experimental results.The bands between 500 and 3100 cm⁻¹ in the ordinary Raman and SERS spectra were assigned as well. This work will facilitate the development of ultrasensitive SERS probes for advanced biomedical imaging applications. Copyright © 2011 John Wiley & Sons, Ltd.     

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 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 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.     

Surface‐enhanced Raman scattering spectroscopy for potential noninvasive nasopharyngeal cancer detection

Combining membrane electrophoresis with surface‐enhanced Raman scattering (SERS) spectroscopy, the serum proteins were first purified and then mixed with silver nanoparticles to perform SERS spectral analysis. Therefore, the spectral signatures were enhanced to high‐fidelity SERS signatures because of the purification procedure of the first step. We used the method to analyze blood plasma samples from nasopharyngeal cancer patients (n = 43) and healthy volunteers (n = 33) for cancer detection. Principle component analysis of the SERS spectra revealed that the data points for the cancer group and the normal group form distinct, completely separated clusters with no overlap. Therefore, the nasopharyngeal cancer group can be unambiguously discriminated from the normal group, i.e., with both diagnostic sensitivity and specificity of 100%. These results are very promising for developing a label‐free, noninvasive, and reliable clinical tool for rapid cancer detection and screening. Copyright © 2011 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.     

Identification of berberine in ancient and historical textiles by surface‐enhanced Raman scattering

The highly fluorescent natural dye berberine can be easily identified in microscopic textile samples by surface‐enhanced Raman spectroscopy employing citrate‐reduced Ag colloid. The ordinary Raman (OR) and SERS spectra of berberine are presented and discussed in the light of a DFT calculation. Using FT‐Raman and FT‐SERS we could reliably compare relative intensity shifts and investigate the adsorption geometry of berberine on Ag nanoparticles. The significant enhancement in the FT‐SERS spectrum of the out‐of‐plane ring system bending deformation mode at 729 cm⁻¹ relative to a group of in‐plane vibrations at around 1500 cm⁻¹ was interpreted as evidence of a ‘flat‐on’ adsorption geometry. SERS was successfully used to identify berberine in silk fiber samples coated with colloidal Ag following a pretreatment with HCl vapor. The SERS method allowed us to detect berberine in a microscopic sample of a single silk fiber from a severely degraded and soiled 17th Century Chinese textile fragment. Copyright © 2007 John Wiley & Sons, Ltd.     

In situ microanalysis of organic colorants by inkjet colloid deposition surface‐enhanced Raman scattering

We present a new method for the minimally invasive in situ identification of inks and colorants on documents and other objects on the basis of the deposition of silver colloid nanodroplets on a region of interest in the objects to be tested using inkjet technology. By adapting commercially available thermal and piezoelectric inkjet heads, volumes of silver colloid in the 60–220 picoliters range (corresponding to impact diameters in the range of 50–150 µm) can be delivered onto substrates with great accuracy and precision. We demonstrate that the instantaneous superheating of the colloid in the thermal print head does not adversely affect the surface‐enhanced Raman spectroscopy efficiency of the Ag nanoparticles. Furthermore, by mounting a compact piezoelectric inkjet head within a large‐stage Raman microscope, we developed an instrument where all phases of surface‐enhanced Raman spectroscopy microanalysis are integrated, with great advantages in sample handling, spatial accuracy, and colloid delivery reproducibility. The approach can be considered functionally nondestructive as the amount of silver delivered, and the area affected are too small to be detected by visual observation, and in most conditions, even by optical microscopy. The method was successfully applied to the analysis of textile fibers, gel pen ink writing on paper, and a Japanese woodblock print dating to the end of the 19th century. Copyright © 2013 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman scattering study of Ag@PPy nanoparticles

Surface‐enhanced Raman scattering (SERS) spectra of Ag@polypyrrole (PPy) nanoparticles with both 488 and 1064 nm excitation were investigated. Experimental results as well as theoretical analysis demonstrated that electromagnetic (EM) enhancement and charge transfer (CT) both rebounded to the SERS effect of Ag@PPy nanoparticles. When near‐IR excitation (1064 nm) was used for the SERS measurements, the contribution from CT was amplified relative to that from EM because the energy of the near‐IR excitation is far from the surface plasmon resonance of the nanosized Ag particles. The increased doping level of PPy, leading to optimal energy matching between the Fermi levels of the Ag nanoparticles and the energy levels in PPy molecules, could obviouslyenhance the SERS signal. These results suggested that the SERStechnique wasan effective tool for investigating the doping effect and interface interaction in metal‐conductingpolymer composite nanoparticles. In particular, the SERS technique with near‐IR excitation could give more information regarding the contribution of the charge‐transfer mechanism to the spectral enhancement of this kind of system. Copyright © 2010 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.     

Application of surface‐enhanced Raman scattering in cell analysis

In surface‐enhanced Raman scattering (SERS), the scattered intensity is drastically increased due to a resonant interaction with surface plasmons of coin metals. SERS is a nondestructive spectroscopic method applied also to biomedical samples. It inherits the advantages of normal Raman spectroscopy and at the same time overcomes the inherent low sensitivity problem. These properties endow SERS with exciting opportunities to be a successful analytical tool for cell analysis. SERS can be used to detect only molecules located on or close to the metallic nanostructures which can support surface plasmon resonances for the enhancement of the Raman signals. Therefore, these metallic nanostructures play a key role in the application of SERS in cell analysis. By incorporating the SERS substrates into the biosamples, molecular structural probing and cellular imaging become possible. In the past decade, analysts worldwide have developed many schemes to study the chemical changes and component distribution in cells by using SERS. In this paper, the application of SERS in cell analysis is reviewed. Copyright © 2011 John Wiley & Sons, Ltd.     

Experimental and theoretical Raman and surface‐enhanced Raman scattering study of cysteine

Raman spectra in solid and 1 M solution of L ‐cysteine and surface‐enhanced Raman scattering (SERS) spectra of this molecule in the zwitterionic form, by using colloidal silver nanoparticles, have been recorded. Density functional theory with the B3LYP functional was used for the optimizations of the ground state geometries and simulation of the vibrational spectrum of this amino acid. The SERS spectrum with a large silver cluster as a model metallic surface was simulated for the first time. Taking into account the experimental and calculated Raman and SERS vibrations and the corresponding assignments, as well as a comparison of force constants and geometrical parameters between the free zwitterion cysteine and the one in the presence of the colloidal silver nanoparticles, we can confirm the presence of gauche (P_H) and trans (P_N) rotamers in the solid state, the formation of a S S bond in the solution state, the dissociation of the peptide bond and mixing of rotamers because of the SERS effect, and the relative importance of the interaction of sulphyldryl, NH₃⁺, and carboxylate groups with the metallic surface. Copyright © 2009 John Wiley & Sons, Ltd.     

DFT study and quantitative detection by surface‐enhanced Raman scattering (SERS) of ethyl carbamate

Ethyl carbamate (EC), a potentially toxic compound, is found in alcoholic beverages and fermented foodstuff. A combined experimental and theoretical study of Raman on EC is reported in this work for the first time. The Raman bands observed for EC in solid phase are characteristic for the carbonyl group, C―C, C―H and N―H stretching and deformation vibrations. These spectral features coupled with a pKa study allowed establishing the neutral species of EC present in the aqueous solutions experimentally tested at different concentrations. In addition, by performing a density functional theory study in the gas phase, the calculated geometry, the harmonic vibrational modes, and the Raman scattering activities of EC were found to be in good agreement with our experimental data and helped establish the surface‐enhanced Raman scattering (SERS) behavior and EC adsorption geometry on the silver surfaces. The Raman peak at 1006 cm⁻¹, assigned to the υ_s(CC) + ω(CH) modes, the strongest and best reproducible peak in the SERS spectra, was used for a quantitative evaluation of EC. The limit of detection, which corresponds to a signal‐to‐noise ratio equal to 3, was found to be 2 × 10⁻⁷ M (17.8 µg l⁻¹). SERS spectra obtained by using hydroxylamine hydrochloride‐reduced silver nanoparticles provide a fast and reproducible qualitative and quantitative determination of EC in aqueous solution. Copyright © 2013 John Wiley & Sons, Ltd.     

pH‐sensing nanostar probe using surface‐enhanced Raman scattering (SERS): theoretical and experimental studies

Local pH environment has been considered to be a potential biomarker for tumor diagnosis because solid tumors contain highly acidic environments. A pH‐sensing nanoprobe based on surface‐enhanced Raman scattering (SERS) using nanostars under near‐infrared excitation has been developed for potential biomedical applications. To theoretically investigate the effect of protonation state on SERS spectra of p‐mercaptobenzoic acid (pMBA), we used the density functional theory (DFT) with the B3LYP functional to calculate Raman vibrational spectra of pMBA‐Au/Ag complex in both protonated and deprotonated states. Vibrational spectral bands were assigned with DFT calculation and used to investigate SERS spectral changes observed from experiment when varying pH value between five and nine. The SERS peak position of pMBA at ~1580 cm⁻¹ was identified to be a novel pH‐sensing index, which has small but noticeable downshift with pH increase. This phenomenon is confirmed and well‐explained with theoretical simulation. The study demonstrates that SERS is a sensitive tool to monitor minor structural changes due to local pH environment, and DFT calculations can be used to investigate Raman spectra changes associated with minor differences in molecular structure. Copyright © 2013 John Wiley & Sons, Ltd.