Characterization of polycyclic aromatic hydrocarbons using Raman and surface‐enhanced Raman spectroscopy

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous atmospheric pollutants and food contaminants, which exhibit potent carcinogenicity, mutagenicity, and teratogenicity. Vibrational spectroscopy techniques, especially Raman spectroscopy and surface‐enhanced Raman spectroscopy (SERS), can be potentially used as an alternative technique to liquid and gas chromatography in PAH analysis. However, there is limited information on the intrinsic Raman and SERS fingerprints of PAHs. In this study, we have acquired the Raman and SERS spectra of seven PAH compounds and compared their experimental spectra with theoretical Raman spectra calculated by density function theory (DFT). The vibrational modes corresponding to the Raman peaks have also been assigned using DFT. Characteristic Raman and SERS peaks have been identified for five PAH compounds, and the limits of detection were estimated. Such information could be useful for developing SERS assays for simple and rapid PAH identification. Copyright © 2014 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman spectroscopy coupled with dendritic silver nanosubstrate for detection of restricted antibiotics

Surface‐enhanced Raman spectroscopy (SERS) coupled with dendritic silver nanosubstrates was used in this study for rapid detection and characterization of restricted antibiotics. Dendritic silver nanosubstrates were prepared through a simple replacement reaction and stored in deionized water for months. SERS methods with near‐IR excitation at 785 nm using silver nanosubstrates were evaluated for detection of three restricted antibiotics (i.e. enrofloxacin, ciprofloxacin, and chloramphenicol) prepared in standard solutions. SERS was capable of identifying and characterizing three antibiotics quickly and accurately. Silver dendrites exhibit satisfactory and consistent performance with an analytical enhancement factor of ∼10⁴. The limit of detection and limit of quantification for antibiotics could reach the level of 20 ppb. Silver dendrites can be kept in deionized water for up to 6 months with no signs of degradation in SERS performance. These results demonstrate a great potential of using SERS coupled with silver dendrites for rapid detection, classification, and quantification of chemical contaminants. Copyright © 2009 John Wiley & Sons, Ltd.     

Detection of biologically active diterpenoic acids by Raman Spectroscopy

Three poorly detectable, biologically active diterpenoic acids, kaurenoic, abietic, and gibberellic acid, were studied by using different modes of Raman spectroscopy. Because of their structural similarities, in the absence of strongly polarizable groups, conventional Raman spectroscopy is not suitable for their unambiguous identification, especially not in solution. We attempted to increase the sensitivity by applying UV‐resonance Raman spectroscopy and surface‐enhanced Raman spectroscopy (SERS) techniques. The UV‐Raman spectra of the three compounds in ethanol/water 50:50 showed only very few enhanced Raman lines. SERS spectra with 514‐nm excitation with Ag colloids were also relatively weak. The best SERS spectra were obtained with 785‐nm excitation on a novel nanostructured substrate, ‘black silicon’ coated with a 400‐nm gold layer. The spectra showed clear differences, and these ‘fingerprints’ would be suitable for the unambiguous identification of these diterpenoic acids. Copyright © 2009 John Wiley & Sons, Ltd.     

不同粒径、超均匀球形金纳米粒子合成及其表面增强拉曼散射效应研究

以氯金酸为原料,抗坏血酸为还原剂,柠檬酸钠为保护剂,用化学还原(种子生长)法制备了不同粒径、超均匀的球形金纳米粒子溶胶,并通过紫外可见吸收光谱(UV-Vis)和扫描电子显微镜(SEM)进行表征。结果表明,随着金纳米粒子粒径的增大,其UV-Vis光谱中的吸收峰发生红移并出现四极峰。为进一步研究金纳米粒子表面增强拉曼散射(SERS)效应的作用机理并优化其灵敏度,我们以罗丹明6G(R6G)为探针分子,对不同粒径的金纳米粒子进行SERS表征,发现R6G的SERS信号随着金纳米粒子的增大先增强后减弱。当金纳米粒子的平均粒径达到120 nm时,产生最强SERS信号增强,增强因子约为1.1×107。三维时域有限差分法(3D-FDTD)理论模拟纳米粒子阵列电磁场分布结果与实验数据的趋势一致。     

Surface enhanced Raman spectroscopy of aromatic compounds on silver nanoclusters

Surface enhanced Raman spectroscopy (SERS) has been used to characterize multilayers of three different aromatic compounds in the proximity of silver nanoclusters. SERS of mercapto benzoic acid (MBA), which adsorbs onto the silver nanoclusters through the sulfur moiety, exhibits frequency shifts in comparison to the Raman spectrum of crystalline MBA. Conversely, benzoic acid and benzophenone that adsorb through the oxygen species lack these frequency shifts, and show only a typical SERS enhancement. We employed X-ray photoelectron spectroscopy (XPS), to probe the nature of the binding between the silver and the three different aromatic compounds. Thereafter, we assigned the major Raman peaks of all three molecules to specific molecular vibrations. Overall, this enables us to determine the origin of the observed shifts in the SERS spectrum of MBA and similar molecules.     

Silver nanoparticles synthesized by direct photoreduction of metal salts. Application in surface‐enhanced Raman spectroscopy

A simple synthesis method of silver nanoparticles and its application as an active surface‐enhanced Raman spectroscopy (SERS) colloid are presented in this work. The photoreduction of AgNO₃ in presence of sodium citrate (NaCit) was carried out by irradiation with different light sources (UV, white, blue, cyan, green, and orange) at room temperature. The evaluation of silver nanoparticles obtained as a function of irradiation time (1–24 h) and light source was followed by UV‐visible absorption spectroscopy. This light‐modification process results in a colloid with distinctive optical properties that can be related to the size and shape of the particles. The Ag colloids, as prepared, were employed as active colloids in SERS. Pyridine and caffeine were used as test molecules. Copyright © 2008 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman spectroscopy for quantitative measurement of lactic acid at physiological concentration in human serum

Lactic acid is a simple and effective indicator for estimating physiological function. Rapid and sensitive detection of lactic acid is very useful in clinical diagnosis. However, the concentration of lactic acid in the physiological state is too low to be detected using traditional Raman spectroscopy. We applied silver colloidal nanoparticles‐mediated surface‐enhanced Raman spectroscopy (SERS) for rapid identification and quantification of lactic acid. The standard SERS spectra of lactic acid were defined and the 1395 cm⁻¹ band intensity was used for quantification from 0.3 to 2 mM (R² = 0.99). In clinical blood sample measurement, the ultrafiltration (cutoff value 5 kDa) can efficiently reduce background fluorescence to improve SERS performance. We established identical and optimal procedure by adjusting reaction time and volume ratio of serum and nanoparticles to obtain high SERS reproducibility. Finally, we showed that silver colloidal nanoparticles‐mediated SERS technique was successfully applied to detect lactic acid at physiological concentrations in the blood. Copyright © 2010 John Wiley & Sons, Ltd.     

Detection of melamine in liquid milk using surface‐enhanced Raman scattering spectroscopy

Melamine, a nitrogen‐rich chemical, has recently caused enormous economic losses to the food industry due to the cases of milk products adulterated by melamine. This has led to an urgent need of rapid and reliable methods for detection of melamine in food. In this study, surface‐enhanced Raman scattering (SERS) spectroscopy was used to detect melamine in liquid milk. The sample preparation with liquid milk is very easy; it has to be only diluted with double‐distilled water followed by centrifugation. By using a silver colloid, at least a 10⁵‐fold enhancement of the Raman signal was achieved for the measurement of melamine. The limit of detection by this method was 0.01 µg ml⁻¹ for melamine standard samples. Based on the intensity of the Raman vibrational bands normalised to that of the band at 928 cm⁻¹ (CH₂), an external standard method was employed for quantitative analysis. The linear regression square (R²) of the curve was 0.9998; the limit of quantitation using this approach was 0.5 µg ml⁻¹ of melamine in liquid milk; the relative standard deviation was ≤10%; and recoveries were from 93 to 109%. The test results for SERS were very precise and as good as those obtained by liquid chromatography/tandem mass spectrometry. The method was simple, fast(only needs about 3 min), cost effective, and sensitive for the detection of melamine in liquid milk samples. Therefore, it is more suitable for the field detection of melamine in liquid milk. Copyright © 2010 John Wiley & Sons, Ltd.     

SERS detection of polychlorinated biphenyls using β‐cyclodextrin functionalized gold nanoparticles on agriculture land soil

A highly sensitive surface‐enhanced Raman scattering (SERS) platform for the selective trace analysis of persistent organic pollutant (POP) such as polychlorinated biphenyl (PCBs) was reported based on β‐CD modified gold nanoparticles (AuNPs) with the real environmental sample of polluted soil. The synthesized gold nanoparticles were characterized using UV–vis spectroscopy, Fourier transform infrared spectroscopy (FTIR), X‐ray diffraction (XRD) and transmission electron microscopy (TEM). In polluted soil the presentation of PCB is confirmed by using GC‐MS. It is further verified and confirmed by using SERS. When the contaminated soil was added to the system, the binding of soil with β‐CD resulted in the aggregation of AuNPs, and excellent Raman signal was obtained which can reflect the isomers of polychlorinated biphenyls. Copyright © 2015 John Wiley & Sons, Ltd.     

Application of surface‐enhanced Raman spectroscopy (SERS) to the analysis of natural resins in artworks

Surface‐enhanced Raman spectroscopy (SERS) is rapidly growing as an analytical technique for the detection of extremely low concentrations of analytes. The analysis of natural resins from artworks is often restricted by sample size constraints in general, and Raman spectroscopy in particular is hampered by fluorescence when using visible irradiation wavelengths. This work demonstrates that SERS is able to overcome interference from fluorescence in such samples using the incident wavelength 514.5 nm, to allow collection of SERS spectra from extremely small samples. Characterisation of the natural resin surface coating from a painting by Tiepolo is discussed. Copyright © 2008 John Wiley & Sons, Ltd.     

Experimental and statistical analysis methods for peptide detection using surface‐enhanced Raman spectroscopy

Surface‐enhanced Raman spectroscopy (SERS) has the potential to make a significant impact in biology research due to its ability to provide information orthogonal to that obtained by traditional techniques such as mass spectrometry (MS). While SERS has been well studied for its use in chemical applications, detailed investigations with biological molecules are less common. In addition, a clear understanding of how methodology and molecular characteristics impact the intensity, the number of peaks, and the signal‐to‐noise of SERS spectra is largely missing. By varying the concentration and order of addition of the SERS‐enhancer salt (LiCl) with colloidal silver, we were able to evaluate the impact of these variables on peptide spectra using a quantitative measure of spectra quality based on the number of peaks and peak intensity. The LiCl concentration and order of addition that produced the best SERS spectra were applied to a panel of synthetic peptides with a range of charges and isoelectric points (pIs) where the pI was directly correlated with higher spectral quality. Those peptides with moderate to high pIs and spectra quality scores were differentiated from each other using the improved method and a hierarchical clustering algorithm. In addition, the same method and algorithm was applied to a set of highly similar phosphorylated peptides, and it was possible to successfully classify the majority of peptides on the basis of species‐specific peak differences. Copyright © 2008 John Wiley & Sons, Ltd.     

Sensitive Raman spectroscopy of lipids based on drop deposition using DCDR and SERS

Raman spectroscopy is widely used for study of lipids and membrane models. A severe limitation of this technique lies in the low Raman cross section requiring high sample concentrations. We report sensitive detection of synthetic 1,2‐dimyristoyl‐3‐trimethylammonium‐propane (DMTAP) lipid employing two Raman techniques with improved sensitivity: drop coating deposition Raman (DCDR) and surface‐enhanced Raman scattering (SERS) spectroscopies. DCDR provided well‐reproducible DMTAP spectra without considerable loss of its solution properties if measured from the ‘coffee ring’ pattern of a drop dried on a SpectRIM^TM plate. DMTAP was detected at ~10 μM initial solution concentration, which is about three orders of magnitude lower than that for conventional Raman spectroscopy. Moreover, SERS spectra from dried ring of Ag hydrosol/DMTAP system were obtained down to ~0.3 μM DMTAP concentration, which means that sensitivity of SERS is about five orders of magnitude higher than that of conventional Raman spectroscopy. In contrast to the DCDR technique, good SERS spectra of DMTAP were obtained only from some spots of the ring containing big nanoparticle aggregates, and the structural properties of DMTAP were significantly perturbed by adsorption on the Ag nanoparticles. Copyright © 2013 John Wiley & Sons, Ltd.     

Raman spectroscopy of dipyrrins: nonresonant,resonant and surface‐enhanced cross‐sections and enhancement factors

Detailed studies of the mechanism of surface‐enhanced (resonance) Raman spectroscopy (SE(R)RS), and its applications, place a number of demands on the properties of SERS scatterers. With large Raman cross‐sections, versatile synthetic chemistry and complete lack of fluorescence, free dipyrrins meet these demands but the Raman and SE(R)RS spectroscopy of free dipyrrins is largely unknown. The first study of the Raman spectroscopy of free dipyrrins is therefore presented in this work. The nonresonant Raman, resonant Raman and surface‐enhanced Raman spectra of a typical meso aryl‐substituted‐dipyrrin are reported. Absolute differential cross‐sections are obtained for excitation wavelengths in the near infrared and visible region, in solution phase and for dipyrrin adsorbed on the surface of silver nanoparticles. Raman enhancement factors for SERRS and resonance Raman are calculated from the observed differential cross‐sections. The magnitudes of the resonantly enhanced cross‐sections are similar to those recently reported for strong SERS dyes such as Rhodamine 6G and Crystal Violet. Free dipyrrins offer the advantages of existing SERS dyes but without the drawback of strong fluorescence. Free dipyrrins should therefore find applications in all areas of Raman spectroscopy including fundamental studies of the mechanisms of SERS and bioanalytical and environmental applications. Copyright © 2011 John Wiley & Sons, Ltd.     

On‐site detection of phosgene agents by surface‐enhanced Raman spectroscopy coupled with a chemical transformation approach

Phosgene and its analogs are greatly harmful to the public health, environmental safety and homeland security as widely used industrial substances with extremely high toxicity. In order to rapidly evaluate the emergency risk caused by these chemicals, a new highly sensitive method based on surface‐enhanced Raman spectroscopy (SERS) technique for measurement of phosgene agents was developed for the first time. Coupled with a chemical transformation approach, the highly toxic phosgene was conveniently converted to a SERS‐sensitive probe, i.e. iodine (I₂), with low toxicity or non‐toxicity. The characteristic SERS peak in 459 cm⁻¹ was used for quantitation and was presumed as a formation of triiodide anion (I₃⁻), which was induced in an iodide (I⁻)‐aggregation Au NPs system. The total measurement can be completed in ~20 min with the limits of detection of ~60 µg/l (phosgene) and ~30 µg/l (diphosgene), respectively, on a portable Raman spectrometer. This work is the first report of SERS measurement on phosgene and diphosgene in a quantitative level. This method is expected to meet the requirements of on‐site detection of phosgene agents, promote emergency responses and raise more opportunities for the portable SERS applications. Copyright © 2015 John Wiley & Sons, Ltd.     

Adjustment and control of SERS activity of metal substrates by pressure

Metal pellets of silver and copper for surface‐enhanced Raman scattering (SERS) spectroscopy were prepared by compression with different pressures. It was found that the SERS activity of the pellet could be controlled by pressure. Enhanced Raman scattering properties of the metal pellets in the presence of adsorbed 4‐mercaptobenzoic acid (4‐MBA) with excitation at 632.8 or 514 nm could be obtained by choosing proper pressure of pellatization. The SERS peak intensity of the band at ∼1584 cm⁻¹ of 4‐MBA adsorbed on the metal pellets varies as a function of applied pressure, and which is about 1.2–32 times greater than when it is adsorbed on silver and copper particles. The calculated results of three‐dimensional finite‐difference time‐domain method (3D‐FDTD) are in good agreement with the experimental data. Moreover, no spurious peaks appear in the SERS spectra of the samples because no other chemicals are involved in the simple preparation process of the metal pellets, which will facilitate its use as an SERS‐active substrate for analytical purposes. In summary, SERS‐active metal pellets can be produced simply and cost effectively by the method reported here, and this method is expected to be utilized in the development of SERS‐based analytical devices. Copyright © 2009 John Wiley & Sons, Ltd.     

Applications of Raman spectroscopy in detection of water quality

Water pollution is hazardous to the health of humans and other organisms, and detection of pollutants in aquatic environments is of primary importance for water quality monitoring. Raman spectroscopy offers an effective tool for qualitative analysis and quantitative detection of contaminants in a water environment. This article focuses on applications of Raman spectroscopy for detection of water quality. In this article, various Raman spectroscopy techniques employed for water quality detection are presented based on the types of pollutants: organics, inorganics, and biological contaminants. Additionally, the relevant detection parameters are reviewed, such as detection materials, limit of detection, detection range, peak positions, and selectivity. Furthermore, the advantages and limitations of various Raman spectroscopy techniques are summarized. Finally, the future development of Raman spectroscopy for detection of water quality is discussed.     

SERS signal response and SERS/SERDS spectra of fluoranthene in water on naturally grown Ag nanoparticle ensembles

We present experimental results of the time‐dependent Raman signal response of fluoranthene adsorbed on a naturally grown Ag nanoparticle ensemble, which serves as surface enhanced Raman scattering (SERS) substrate. In addition, SERS characteristics such as the concentration‐dependent calibration curves and the limit of detection (LOD) for fluoranthene in distilled water will be shown. The SERS substrate was prepared by Volmer–Weber growth under ultrahigh vacuum condition and exhibits a plasmon resonance wavelength at 491 nm. For the measurement of SERS signal response and SERS/shifted excitation Raman difference spectroscopy spectra of fluoranthene in water, experimental Raman setup containing a microsystem light source with two emission wavelengths (487.61 nm and 487.91 nm) was used. We experimentally demonstrate that the maximum SERS intensity is achieved 9 min after changing the analyte concentration from 0 nmol/l to 600 nmol/l. This response time is explained by a time‐dependent adsorption of the probe molecules onto the nanoparticles. The LOD for fluoranthene in water was evaluated applying shifted excitation Raman difference spectroscopy (SERDS) at different molecule concentrations. For SERDS, two emission wavelengths of a prototype microsystem light source have been used for Raman excitation. The experimental results reveal that the LOD for the probe molecules is very low. Experimentally, we have detected a fluoranthene concentration of only 4 nmol/l, which is very close to our estimated LOD of 2 nmol/l. Thus, the presented Raman setup, with a SERS substrate, whose plasmon resonance coincides with the excitation wavelength for SERS measurements, is well suited for in‐situ trace detection of pollutant chemicals in water. Copyright © 2013 John Wiley & Sons, Ltd.     

Resonance Raman spectroscopy of carbon nanotubes: pressure effects on G-mode

We use 488 and 568 nm laser Raman spectroscopy under high pressure to selectively follow evolution of Raman G-mode signals of single-walled carbon nanotubes (SWCNTs) of selected diameters and chiralities ((6, 5) and (6, 4)). The G-mode pressure coefficients of tubes from our previous work are consistent with the thick-wall tube model. Here we report the observation of well-resolved G-minus peaks in the Raman spectrum of SWCNTs in a diamond-anvil cell. The pressure coefficients of these identified tubes in water, however, are unexpected, having the high value of over 9 cm^?1 GPa^?1 for the G-plus and the G-minus, and surprisingly the shift rates of the same tubes in hexane have clearly lower values. We also report an abrupt increase of G-minus peak width at about 4 GPa superposed on a continuous peak broadening with pressure.     

Visible laser–induced photoreduction of silver 4‐nitrobenzenethiolate revealed by Raman scattering spectroscopy

We have investigated the photochemical characteristics of silver 4‐nitrobenzenethiolate (Ag‐4NBT) by means of Raman spectroscopy. When Ag‐4NBT is irradiated with an argon ion laser at 514.5 nm, its Raman spectrum changes over time, resulting in the production of 4NBT‐capped silver nanoparticles. The surface‐enhanced Raman scattering (SERS) spectrum of 4NBT adsorbed on those Ag nanoparticles is subsequently converted to that of 4‐aminobenzenethiol (4ABT). These surface‐induced photoreduction characteristics were investigated by monitoring the growth of Raman peaks of 4ABT as a function of the laser exposure time. Water vapor or ambient conditions were more effective than vacuum conditions for the photoreduction of 4NBT to 4ABT. Nonetheless, the occurrence of photolysis even under vacuum conditions suggests that the benzene ring hydrogen atoms might be the H‐atom source of the nitro‐to‐amine group conversion although in ambient conditions water or solvent molecules trapped inside the Ag‐4NBT should be the primary H‐atom source and facilitate the transfer of electrons, as well as the diffusion of Ag atoms to form highly SERS‐active nanoaggregates. Copyright © 2009 John Wiley & Sons, Ltd.     

Detection and analysis of cyclotrimethylenetrinitramine (RDX) in environmental samples by surface‐enhanced Raman spectroscopy

Techniques for rapid and sensitive detection of energetics such as cyclotrimethylenetrinitramine (RDX) are needed both for environmental and security screening applications. Here we report the use of surface‐enhanced Raman scattering (SERS) spectroscopy to detect traces of RDX with good sensitivity and reproducibility. Using gold (Au) nanoparticles (∼90–100 nm in diameter) as SERS substrates, RDX was detectable at concentrations as low as 0.15 mg/l in a contaminated groundwater sample. This detection limit is about two orders of magnitude lower than those reported previously using SERS techniques. A surface enhancement factor of ∼6 × 10⁴ was obtained. This research further demonstrates the potential for using SERS as a rapid, in situ field screening tool for energetics detection when coupled with a portable Raman spectrometer. Copyright © 2010 John Wiley & Sons, Ltd.