A portable shifted excitation Raman difference spectroscopy system: device and field demonstration

In this paper, we present a portable shifted excitation Raman difference spectroscopy (SERDS) system applied in outdoor experiments. A dual‐wavelength diode laser emitting at 785 nm is used as excitation light source. The diode laser provides two individually controllable excitation lines at 785 nm with a spectral distance of about 10 cm⁻¹ for SERDS. This monolithic light source is implemented into a compact handheld Raman probe. Both components were developed and fabricated in‐house. SERDS measurements are performed in an apple orchard, and apples and green apple leafs are used as test samples. For each excitation wavelength, a single Raman spectrum is measured with 50 mW at the sample. Strong background interference from ambient daylight and laser‐induced fluorescence obscure the Raman signals. SERDS efficiently separates the wanted Raman signals from the disturbing background signals. For the Raman spectroscopic investigations of green leafs, one accumulation with an exposure time of 0.2 s was used for each excitation wavelength to avoid detector saturation. An 11‐fold improvement of the signal‐to‐background noise is achieved using SERDS. The results demonstrate the suitability of the portable SERDS system for rapid outdoor Raman investigations. Copyright © 2016 John Wiley & Sons, Ltd.     

Rapid shifted excitation Raman difference spectroscopy with a distributed feedback diode laser emitting at 785 nm

A distributed feedback (DFB) laser diode emitting at 785 nm was tested and applied as a light source for shifted excitation Raman difference spectroscopy (SERDS). Due to the physical properties of the laser diode, it was possible to shift the emission wavelength by 8 cm^-1 (0.5 nm) required for our SERDS measurements by simply changing the injection current. The internal grating ensured single mode operation at both wavelength with the frequency stability of ±0.06 cm^-1 (0.004 nm) required for high resolution Raman spectroscopic applications. The shifted spectra were used for calculating enhanced Raman spectra being obscured by a strong scattering background. A 16 dB (≈38 fold) improvement of the signal-to-background noise S̄/σ_B was demonstrated using blackboard chalk as a sample. The tunable DFB laser is a versatile excitation source for SERDS, which could be used in any dispersive Raman system to subtract fluorescence contributions and scattering background. PACS 82.80.Gk; 42.55.-f; 42.64.Fi     

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.     

双波长LD模块的拉曼光谱测试系统

荧光干扰是拉曼光谱检测过程中常见的干扰因素之一,而移频激发法是一种有效的克服荧光干扰的检测手段。移频激发法利用两个波长相近的激光分别激发被测物质,并将获得的拉曼光谱进行差谱。由于两次激发的荧光背景相同,而拉曼特征峰会产生平移,因此可有效地消除荧光背景的干扰,进而利用一定的算法还原拉曼特征峰。移频激发法的关键在于两个激发光波长的稳定性,不稳定的波长差将严重影响对拉曼特征峰的还原效果。本文研制了一种拉曼光谱测试系统,该系统的双波长LD模块能够产生两个波长稳定的激发光(分别为784.7和785.8 nm),满足移频激发法的测试要求。影响激发光波长稳定性的因素主要是光功率和温度,本系统中对这两个因素均进行了实时的监控,以保证激发光波长的稳定。系统的硬件部分主要包括ARM主控板、双波长LD模块及其驱动电路、温度控制板、数字光开关、光谱检测光路和光纤探头(两个高功率的蝶形封装激光器);软件部分可自动获取被测物质的拉曼光谱图,并对其进行后续的处理。在稳定性测试实验中,对系统驱动电源电流和激光器温度的稳定性均进行了测试。测试结果显示,电流波动范围小于0.01 mA、温度变化范围小于0.004 ℃,能够有效地保证激发光波长的稳定性。最后,对某品牌花生油进行了拉曼光谱检测,并对检测结果进行了处理,获得了良好的效果。     

Naturally grown Ag nanoparticles on quartz substrates as SERS substrate excited by a 488 nm diode laser system for SERDS

A silver nanoparticle ensemble was prepared under ultrahigh vacuum (UHV) conditions by Volmer–Weber growth on a quartz substrate for surface-enhanced Raman scattering (SERS) investigations of pyrene molecules. To tune the surface plasmon resonance frequency in the vicinity of the excitation wavelength of 488 nm of the diode laser, the morphology of the silver nanoparticles was optimized. The substrates were mounted in a flow-through cell as part of the optical Raman set-up. A microsystem diode laser generates two slightly different emission wavelengths (λ=487.61 nm and λ=487.91 nm) with a spectral width <10 pm and an optical power of 20 mW, i.e. SERS experiments are possible but also shifted excitation Raman difference spectroscopy (SERDS) can be carried out. For trace analysis of pyrene in water we demonstrate SERS/SERDS experiments which lead to a limit of detection of 2 nmol/l for pyrene. These results suggest that with silver nanoparticle ensembles excited at their plasmon resonance at 488 nm combined SERS/SERDS measurements can be effectively performed for in-situ trace analysis of pollutant chemicals in water.     

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.     

Dual‐wavelength monolithic Y‐branch distributed Bragg reflection diode laser at 671 nm suitable for shifted excitation Raman difference spectroscopy

A dual‐wavelength monolithic Y‐branch distributed Bragg reflection (DBR) diode laser at 671 nm is presented. The device is realized with deeply etched surface DBR gratings by one‐step epitaxy. A maximum optical output power of 110 mW is obtained in cw‐operation for each laser cavity. The emission wavelengths of the device are 670.5 nm and 671.0 nm with a spectral width of 13 pm (0.3 cm⁻¹) and a mean spectral distance of 0.46 nm (10.2 cm⁻¹) over the whole operating range. Together with a free running power stability of ± 1.1% this most compact diode laser is ideally suited as an excitation light source for portable shifted excitation Raman difference spectroscopy (SERDS).     

Raman spectroscopy with LED excitation source

We propose and experimentally demonstrate a novel instrument arrangement, which allows for the collection of Raman spectra with a broadband light source. This is achieved by spatially dispersing the optical spectrum in the focal plane and confocally reimaging the Raman signal, which originates from different locations, onto the entrance slit of an imaging spectrometer. Using this approach and broadband radiation derived from a commercially available LED, we acquired high signal‐to‐noise spectra with a spectral resolution limited by the spectral resolution of a spectrometer. Copyright © 2013 John Wiley & Sons, Ltd.     

Application of a likelihood ratio approach in solving a comparison problem of Raman spectra recorded for blue automotive paints

Evidential values of Raman spectra recorded for solid and metallic blue car paints were evaluated using visual comparison as well as a statistical approach, i.e. likelihood ratio (LR) test supported by Empirical Cross Entropy (ECE) results. Raman spectra were obtained using a Renishaw inVia spectrometer equipped with a confocal Leica microscope and a near infrared semiconductor laser (785 nm) as an excitation source. When a visual comparison was performed for solid paints, in 17.7% of comparisons (53 pairs out of 300) the samples were indistinguishable, whereas for metallic paints this value was 8.5% of such comparisons (37 pairs out of 435). Comparing the spectra using an LR approach was based on variables being the areas under the most significant Raman spectra bands of pigments present in the analysed samples. Proposed LR models delivered low false positive and false negative rates (for many models lower than 10%), and the ECE plots confirmed that their performance was much better than visual comparison. Copyright © 2015 John Wiley & Sons, Ltd.     

An original approach for Raman spectroscopy analysis of radioactive materials and its application to americium‐containing samples

An approach for Raman measurements of highly radioactive samples is presented here. The innovative part of this approach lies in the fact that no single part of the Raman equipment is in direct contact with the radioactive sample, as the sample is sealed in an alpha‐tight capsule. Raman analysis is effectively performed through the optical‐grade quartz window closing the capsule. This allows performing micro‐Raman measurements on radioactive samples with no limitations on the laser source wavelength, polarisation mode, spectrometer mode and microscope mode (provided the focal length of the microscope objective is greater than the thickness of the quartz window and with sub mg samples). Some example results are shown and discussed. In particular, some spectral features of americium‐containing oxide nuclear fuel specimens are presented. Raman spectra clearly reveal in these specimens the presence of abundant oxygen defects induced in the fcc fluorite lattice by trivalent americium. In order to complete the analysis the Raman spectrum of pure americium dioxide was also measured with a lower energy excitation source compared with previous research. The current results seem to be consistent with the possible occurrence of a photolysis process induced by the Raman laser, resulting in the formation of hyperstoichiometric americium sesquioxide Am₂O_{3 + z}. Such a photolytic process is deemed to be unavoidable when visible lasers are used as excitation sources for the Raman analysis of americium dioxide. © 2015 The Authors Journal of Raman Spectroscopy Published by John Wiley & Sons, Ltd.     

一种用多色光作为激发源产生拉曼光谱的新算法

探讨用多色光代替激光作为拉曼光源的新型拉曼光谱仪的可能性。根据拉曼光谱原理, 并通过数学分析, 发现当用多色光照射在样品上时, 所得到的散射光经散射频率校正后在不同频率上的强度分布可以写成样品的Raman-Rayleigh联合散射谱和激发光光源的功率谱的卷积。利用傅里叶变换算法,有可能从多色光照射样品所得到的散射光谱中导出样品的拉曼光谱。基于上述原理,可能发展出不用激光的新一代拉曼光谱仪。     

Trace analysis of polycyclic aromatic hydrocarbons using calixarene layered gold colloid film as substrates for surface‐enhanced Raman scattering

A surface‐enhanced Raman scattering (SERS) active substrate for the detection of polycyclic aromatic hydrocarbons (PAHs) was developed, which used 25, 27‐dimercaptoacetic acid‐26, 28‐dihydroxy‐4‐terbutyl calix[4]arene (DMCX) to functionalize a gold colloid film. This SERS‐active substrate prepared by self‐assembly method exhibits a high sensitivity, especially for the detection of PAHs. With the use of this SERS‐active substrate and with the application of the shifted excitation Raman difference spectroscopy (SERDS) technique, Raman signals of pyrene and anthracene in aqueous solutions at low concentration level (500 pM) can be obtained. Moreover, because PAHs are blocked from being directly adsorbed on gold colloid by DMCX and the photochemical reactions of adsorbates are avoided, the Raman bands of PAHs adsorbed on DMCX‐fuctionalized gold colloid film can be one‐to‐one correspondence with those of solid PAHs, and additionally, this SERS‐active substrate can be easily cleaned and reused. The obtained results demonstrate that the DMCX‐functionalized gold colloid films prepared by self‐assembly method have great potential to be developed to an in situ PAHs detection substrate. Copyright © 2012 John Wiley & Sons, Ltd.     

Test of an optical parametric oscillator (OPO) as a compact and fast tunable stokes source in coherent anti-stokes raman spectroscopy (CARS)

An optical parametric oscillator (OPO), as a novel kind of broadband Stokes source, is employed for coherent anti-Stokes Raman spectroscopy (CARS). Compared to the conventional dye laser configuration OPO-CARS offers practical advantages. The tunable OPO allows a fast and comfortable frequency tuning. The excitation bandwidth of about 35 cm^–1 (FWHM) limits the spectral range of effective and stable single pulse CARS generation but can be used to enhance selected spectral structures.     

多波长抽运宽带光纤拉曼放大器的数值模拟与优化

采用平均功率分析算法对多波长抽运宽带光纤拉曼放大器进行了细致的数值模拟，并根据多抽运拉曼增益谱的特点和不同抽运波长对增益斜率的不同影响提出了一种新的有效的抽运优化算法，并利用这种方法对10波长抽运的光纤拉曼放大器进行了优化，获得了70nm带宽上增益谱波动低于0．5dB的结果。     

以PAA为模板制备SERS基底及对三聚氰胺的检测

采用热蒸镀的方法直接在多孔氧化铝(porous anodic alumina,PAA)模板上蒸镀几微米的银膜,然后在HCl溶液中溶解掉模板,得到表面具有纳米尺度规则结构的银膜作为表面增强拉曼散射(surface-en-hanced Raman spectra,SERS)基底,并在该基底上测量了吡啶溶液(0.01 mol.L-1)的增强拉曼光谱,发现平均增强因子大于105。与直接在载玻片上蒸镀的银膜相比,具有纳米尺度规则结构银膜的增强效果提高了30倍。改变激发光功率测量吡啶的拉曼光谱,和普通拉曼散射一样,增强拉曼光谱的峰值强度随激发光强度线性变化,并在该基底上测量了三聚氰胺的拉曼光谱,发现在1 mW的激发功率下对于三聚氰胺的检出限为2.5 mg.L-1。     

A line‐scan hyperspectral Raman system for spatially offset Raman spectroscopy

Spatially offset Raman spectroscopy (SORS) is a technique that can obtain subsurface layered information by collecting Raman spectra from a series of surface positions laterally offset from the excitation laser. Currently optical fiber probes are used as major tools in SORS measurement, which are either slow (single fiber probe with mechanical movement) or restricted in selecting offset range and interval (fiber probe array). This study proposes a new method to conduct SORS measurement based on a newly developed line‐scan hyperspectral Raman imaging system. A 785‐nm point laser was used as an excitation source. A detection module consisting of an imaging spectrograph and a charge‐coupled device camera was used to acquire line‐shape SORS data in a spectral region of −592 to 3015 cm⁻¹. Using a single scan, the system allowed simultaneous collection of a series of Raman spectra in a broad offset range (e.g. 0–36 mm in two sides of the incident laser) with a narrow interval (e.g. 0.07 mm). Four layered samples were created by placing butter slices with thicknesses of 1, 4, 7, and 10 mm on top of melamine powder, providing different individual Raman characteristics to test the line‐scan SORS technique. Self‐modeling mixture analysis (SMA) was used to analyze the SORS data. Raman spectra from butter and melamine were successfully retrieved for all four butter‐on‐melamine samples using the SMA method. The line‐scan SORS measurement technique provides a flexible and efficient method for subsurface evaluation, which has potential to be used for food safety and quality inspection. Copyright © 2015 John Wiley & Sons, Ltd.     

Structure correlation of silylated dicarboxylic acid monomer and its respective oligomeric polyamide-imide using experimental and theoretical vibrational spectra

Correlation between theoretical and experimental (infrared and Raman spectroscopies) vibrational spectra of two compounds, both with a silyl group present in their main chain and with an optically active structure (L-valine) as side group, was performed. These compounds are based in a chiral dicarboxylic acid monomer and its respective polyamide-imide, oligomer that was previously synthesized by a direct polycondensation. Spectra were recorded in the region comprised between 500 and 4000?cm^?1 for infrared and Raman analysis. The Raman spectra were obtained through a 1064?nm laser as excitation source.

Theoretical models were carried out in order to find the optimal molecular geometry of the analyzed systems, with a complete assignment of their vibrational spectra. The Raman experimental data obtained with a Nd:YAG laser for this kind of silylated organic compounds, and the comparison between these results with the theoretical data is a useful advance in the polymer synthesis field, which can be used as reference for subsequent studies.     

Synthesis of SiC/graphene nanosheet composites by helicon wave plasma

We report an approach to the rapid, one-step, preparation of a variety of wide-bandgap silicon carbide/graphene nanosheet(Si C/GNSs) composites by using a high-density helicon wave plasma(HWP) source. The microstructure and morphology of the Si C/GNSs are characterized by using scanning electron microscopy(SEM), Raman spectroscopy, x-ray diffraction(XRD), x-ray photoelectron spectroscopy(XPS), and fluorescence(PL). The nucleation mechanism and the growth model are discussed. The existence of Si C and graphene structure are confirmed by XRD and Raman spectra.The electron excitation temperature is calculated by the intensity ratio method of optical emission spectroscopy. The main peak in the PL test is observed at 420 nm, with a corresponding bandgap of 2.95 e V that indicates the potential for broad application in blue light emission and ultraviolet light emission, field electron emission, and display devices.     

Influence of surface plasmon resonance wavelength on SERS activity of naturally grown silver nanoparticle ensemble

We present experimental results to quantify and optimize the surface‐enhanced Raman scattering (SERS) activity of naturally grown silver nanoparticles. Ag nanoparticle ensembles with mean equivalent radii ranging from 10.6 to 20.3 nm were prepared under ultrahigh vacuum conditions by Volmer–Weber growth on quartz plates. A tuning of the localized surface plasmon polariton resonance wavelength from 453 to 548 nm was performed by varying the morphology of the silver nanoparticles. The dependence of the SERS activity on the plasmon resonance wavelength was investigated with a Raman set‐up containing a microsystem light source with an emission line at 488 nm. Shifted excitation Raman difference spectroscopy was applied to remove the fluorescence‐based background from the SERS spectra of pyrene in water using two slightly different emission wavelengths (487.61 and 487.91 nm) of the microsystem light source. We demonstrate that the Raman activities for all SERS substrates are available in the nanomolar range in a water sample. However, the Raman activity crucially depends on the plasmon resonance wavelength of the nanoparticle ensembles. Although for an on‐resonance ensemble the limit of detection for pyrene in water is very low and was estimated to be 2 nmol/L, it increases rapidly to several tens of nanomol for slightly off‐resonance ensembles. Hence, the highest SERS activity was obtained with a nanoparticle ensemble exhibiting a plasmon resonance wavelength at 491 nm, which almost coincides with the excitation wavelengths. Copyright © 2012 John Wiley & Sons, Ltd.     

Noise reduction in Raman spectra: Finite impulse response filtration versus Savitzky–Golay smoothing

Application of the finite impulse response (FIR) filtration technique for the removal of spectral noise and background broadband deformations from the Raman spectra is tested. Optimal parameters of FIR filters are found and their effectiveness is compared with the Savitzky–Golay (SG) smoothing procedure. The FIR filtration is found to be an effective procedure to treat the whole Raman spectra, but high computing power is needed. Copyright © 2007 John Wiley & Sons, Ltd.