Conventional Raman Spectrometer Research Articles

Portable Hadamard-Transform Raman Spectrometer: A Powerful Analytical Tool for Point-of-Care Testing.

A portable Hadamard-transform Raman spectrometer with excellent performance was fabricated consisting of a 785 nm laser, an optical filter, an optical system, a control system, and a signal processing system. As the core of the spectrometer, the optical system was composed of a slit, collimator, optical grating, reflector, digital micromirror devices (DMD), lens system, and InGaAs photodetector. Compared with a conventional dispersive Raman spectrometer, the proposed Raman spectrometer adopted the DMD and corresponding controlling device (DLPC350 control chip) to collect the Raman spectrum. Thus, in our design, the gratings are fixed, while the full Raman spectrum was collected by the deflection of the micromirror. This design can greatly improve the vibration resistance ability of the spectrometer since the gratings are not rotating during the spectrum collecting. More importantly, Hadamard-transform was used as signal processing technology, which has the ability of faster calculation, the merits of high energy input, single detector multichannel simultaneous detection (imaging) ability, and high signal-to-noise ratio (SNR). Hence, the Hadamard-transform portable Raman spectrometer has the potential to be applied in the field of point-of-care testing (POCT).

Point-and-shoot: portable Raman and SERS detection of organic gunshot residue analytes

Raman spectroscopy is one of many tools available to verify the molecular composition of an analyte. Due to its non-destructive nature and its ability to accurately discern differences in closely related molecular structures, it has become invaluable in many fields, including its potential for forensic gunshot residue detection. Firearm-related fatalities in the United States continue to rise and many of them remain unsolved. This necessitates tools that are better equipped to aid in the investigation of firearm-related crimes, capable of high-throughput analysis yet remaining sensitive to give accurate and valuable information. The comparative downside of Raman spectroscopy to neighboring techniques like mass spectrometry and nuclear magnetic resonance is its lower sensitivity. Surface-enhanced Raman spectroscopy allows for the benefits of Raman spectroscopy alongside the added lower limit of detection with the simple application of nanoparticles, typically gold. Unfortunately, these benefits have seen little on-site application due to the difficulty of translating methods from conventional tabletop Raman spectrometers to point-and-shoot portable Raman spectrometers which have even lower sensitivities (higher limits of detection). Herein, we outline a versatile methodology for the detection of 6 organic gunshot residue components (diphenylamine (DPA), ethyl centralite (EC), 2,4-dinitrotoluene (2,4-DNT), 2-nitrodiphenylamine (2-nDPA), 4-nitrodiphenylamine (4-nDPA), and N-nitrosodiphenylamine (N-nDPA)) in liquid-phase that allows us to detect millimolar concentrations of these analytes. Furthermore, we report calculated vibrational assignments for these 6 analytes in solution, alongside detailed peak-by-peak analyses on a portable instrument. We showed signal enhancement and lower LODs through our data processing as well as a proof-of-concept SERS enhancement in a complex liquid-phase matrix, with an increased sensitivity of 700% when using SERS.

Decomposition of the anisotropic strain in 3D-structure GaN layers using Raman spectroscopy

Strain engineering for gallium nitride has been studied by many researchers to improve the performance of various devices (i.e., light-emitting diodes, laser diodes, power devices, high electron mobility transistors, and so on). Further miniaturization of gallium nitride devices will clearly continue in the future, and therefore an accurate understanding of the strain state in the devices is essential. However, a measurement technique for axially resolved evaluation of the strain in microareas has not yet been established. In this study, we revealed that the anisotropic strain state induced in c-plane growth gallium nitride is linked to the split state of Raman peaks, which were measured with z(xx)z¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$z(xx)\\overline{z }$$\\end{document} and z(yx)z¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$z(yx)\\overline{z }$$\\end{document} polarized configurations. The anisotropic strain state in c-plane gallium nitride was induced in the 3D-structure by epitaxial lateral overgrowth, which enabled successful performance of our work. This result allowed us to axially decompose the strain in c-plane gallium nitride through Raman spectroscopy and establish a measurement technique for axially resolving the strain. This measurement technique is feasible using a conventional Raman spectrometer. Furthermore, the method was indicated to be applicable to all wurtzite-type crystals, including gallium nitride, silicon carbide, and aluminum nitride. Our work provides a new perspective for understanding the complex strain state in microareas for wurtzite materials. Comprehending the strain state, which strongly affects device performance, will help promote the research and development of III-V semiconductor devices.

Reliable and easy-to-use SERS spectroscopy probe using a tapered opto-fluidic photonic crystal fiber.

Surface enhanced Raman spectroscopy (SERS) is one of the most sensitive biosensing techniques that offers label free detection for a variety of applications. Generally, SERS spectroscopy is performed on nano-functionalized planar substrates with plasmonic structures or colloidal nanoparticles. Recently, photonic crystal fibers (PCFs) have gained great interest for SERS based bio sensing applications due to the immense advantages such as improved sensitivity, flexibility and remote sensing capability that it offers compared to the planar substrates. However, the use of PCF based biosensors demand the alignment of it under a microscope, which can affect the reliability of SERS measurements and could be restrictive for practical end use applications. Herein, we aim to develop a tapered suspended core PCF fiber (Tapered-SuC-PCF) that represents an improvement in coupling efficiency and measurement reliability as well as it opens the way to the development of an easy-to-use bio-sensing probes with a plug and play option with conventional Raman spectrometers. We have fabricated several samples of the optimized tapered-SuC-PCF and demonstrated its superior SERS performance compared to standard SuC-PCF fibers with 2 µm core diameter. An excellent SERS measurement reliability is demonstrated using such a fiber in a plug and play type system demonstrating its versatility for practical end use applications.

Rapid and non-destructive spatially offset Raman spectroscopic analysis of packaged margarines and fat-spread products

Spatially offset Raman spectroscopy (SORS) is a novel technique capable of measuring samples through the original packaging and recovering the spectra without the contribution of surface layers. Here, a portable SORS equipment was used to measure 62 samples of margarines and fat spreads through the original plastic container. Chemometric tools were used to analyse the data obtained. A total of 25 classification models were developed based on: (i) geographical origin, (ii) vegetable oils and (iii) some significant minor constituents present in the samples. Partial least squares-discriminant analysis (PLS-DA), support vector machine (SVM) and soft independent modelling of class analogy (SIMCA) were used for model classification. Quantitative analysis using the partial least squares regression (PLSR) method was also performed to determine the total fat content. In parallel, a benchtop conventional Raman spectrometer was used to analyse the same samples, develop the models with the same training and validation sets in order to compare the results. The calculated classification performance metrics showed better classification models from SORS data than conventional Raman spectroscopy (CRS), highlighting the one-class SIMCA models for margarines containing phytosterols, olive oil or linseed oil. These models exhibited very high predictability (performance parameters with values equal to or higuer than 0.8, 0.9 and 1, respectively). The quantitation model developed from SORS exhibited a higher R2 than from CRS data, and prediction errors below 5% from SORS versus errors between 5 and 13% from CRS data.These results reveal the ability of SORS to avoid the influence of fluorescence, a major drawback when analysing Raman spectra, but also the potential of the technique as a fast, non-destructive and non-invasive analytical technique in the field of food analysis. In conclusion, the tandem 'SORS-chemometrics' has been shown to be a potential tool in the food quality and food authentication fields. Thus, it is necessary to perform further investigations in this field in order to advance the knowledge of this technique and to be able to develop new methods of rapid analysis.

Probing plasma-treated graphene using hyperspectral Raman.

Raman spectroscopy provides rich optical signals that can be used, after data analysis, to assess if a graphene layer is pristine, doped, damaged, functionalized, or stressed. The area being probed by a conventional Raman spectrometer is, however, limited to the size of the laser beam (∼1 µm); hence, detailed mapping of inhomogeneities in a graphene sample requires slow and sequential acquisition of a Raman spectrum at each pixel. Studies of physical and chemical processes on polycrystalline and heterogeneous graphene films require more advanced hyperspectral Raman capable of fast imaging at a high spatial resolution over hundreds of microns. Here, we compare the capacity of two different Raman imaging schemes (scanning and global) to probe graphene films modified by a low-pressure plasma treatment and present an analysis method providing assessments of the surface properties at local defects, grain boundaries, and other heterogeneities. By comparing statistically initial and plasma-treated regions of graphene, we highlight the presence of inhomogeneities after plasma treatment linked to the initial state of the graphene surface. These results provided statistical results on the correlation between the graphene initial state and the corresponding graphene-plasma interaction. This work further demonstrates the potential use of global hyperspectral Raman imaging with advanced Raman spectra analysis to study graphene physics and chemistry on a scale of hundreds of microns.

Compact high‐sensitivity symmetrical integrated optical system for the detection of Raman scattering with dual channels

AbstractWe present here a compact symmetrical integrated optical system for the high‐sensitivity handheld Raman spectrometer. This symmetric optical system is of higher sensitivity of Raman spectrum detection by coupling collection optics to the spectrometer directly, reflecting excitation laser incident at 45° while efficiently passing the longer Raman‐shift wavelength with two long‐pass dichroic laser‐flat beam‐splitters placed in parallel. Three aspherical collecting lens with high‐numerical aperture were used to collect Raman scattering. Four protected silver plane mirrors, two right‐angle mirrors, and an aspherized achromatic focusing lens were used to focus Raman scattering into the slit on the collecting optical path. The Raman scattering light from the entrance slit were collimated to a blazed holographic reflective grating by an achromatic doublet lens, and the two diffraction lights with different orders were focused onto the two different surface of charge‐coupled device with two broadband dielectric coating concave mirrors, respectively. The coma and astigmatism of this optical system are decreased by optimizing the optical structure parameters. The finer Raman spectrum of samples can be obtained through the second order diffraction spectrum acquisition channel, whereas the first order diffraction channel covers almost all the Raman spectrum range. The tested spectrum of Benzoyl peroxide indicates that this symmetric integrated optical system with three collecting channels and double‐detecting channels has a higher sensitivity, higher signal‐to‐noise ratio, and finer Raman spectrum than the conventional portable Raman spectrometer with independent probe and single detection channel.

Lightweight Raman spectroscope using time-correlated photon-counting detection

Raman spectroscopy is an important tool in understanding chemical components of various materials. However, the excessive weight and energy consumption of a conventional CCD-based Raman spectrometer forbids its applications under extreme conditions, including unmanned aircraft vehicles (UAVs) and Mars/Moon rovers. In this article, we present a highly sensitive, shot-noise-limited, and ruggedized Raman signal acquisition using a time-correlated photon-counting system. Compared with conventional Raman spectrometers, over 95% weight, 65% energy consumption, and 70% cost could be removed through this design. This technique allows space- and UAV-based Raman spectrometers to robustly perform hyperspectral Raman acquisitions without excessive energy consumption.

Optical characterization and confocal fluorescence imaging of mechanochromic acrylate polymers

The development of mechanochromic molecules has opened new pathways for the study of localized stress and failure in polymers. Their application as stress or temperature diagnostics, however, requires suitable measurement techniques capable of detecting the force- and temperature-sensitive chemical species with high spatial resolution. Confocal imaging techniques offer excellent spatial resolution but the energy input during these measurements can itself affect the activation state of the mechanochromic species. Here, we present a systematic study of the effects of laser-based imaging on the activation and fluorescence behavior of mechanochromic spiropyran (SP) integrated into poly(methyl acrylate) (PMA) and poly(methyl methacrylate) matrices using a confocal Raman microspectrometer. Localized stress and temperature activation were studied by means of high-rate compressive loading and dynamic fracture. Laser illumination of SP in PMA revealed a strong excitation wavelength- and power-dependence. Suitable correction functions were established and used to account for the observed laser effects. The presented study demonstrates that confocal imaging using conventional Raman spectrometers is a powerful characterization tool for localized stress analysis in mechanochromic polymers, offering quantifiable information on the activation state with high spatial resolution. However, laser-mechanophore interactions must be well understood and effects of laser excitation and exposure times must be taken into consideration when interpreting the obtained results.

Photophoretic trapping-Raman spectroscopy for single pollens and fungal spores trapped in air

Photophoretic trapping-Raman spectroscopy (PTRS) is a new technique for measuring Raman spectra of particles that are held in air using photophoretic forces. It was initially demonstrated with Raman spectra of strongly-absorbing carbon nanoparticles (Pan et al. [44] (Opt Express 2012)). In the present paper we report the first demonstration of the use of PTRS to measure Raman spectra of absorbing and weakly-absorbing bioaerosol particles (pollens and spores). Raman spectra of three pollens and one smut spore in a size range of 6.2–41.8µm illuminated at 488nm are shown. Quality spectra were obtained in the Raman shift range of 1600–3400cm−1 in this exploratory study. Distinguishable Raman scattering signals with one or a few clear Raman peaks for all four aerosol particles were observed within the wavenumber region 2940–3030cm−1. Peaks in this region are consistent with previous reports of Raman peaks in the 1600–3400cm−1 range for pollens and spores excited at 514nm measured by a conventional Raman spectrometer. Noise in the spectra, the fluorescence background, and the weak Raman signals in most of the 1600–3400cm−1 region make some of the spectral features barely discernable or not discernable for these bioaerosols except the strong signal within 2940–3030cm−1. Up to five bands are identified in the three pollens and only two bands appear in the fungal spore, but this may be because the fungal spore is so much smaller than any of the pollens. The fungal spore signal relative to the air-nitrogen Raman band is approximately 10 times smaller than that ratio for the pollens. The five bands are tentatively assigned to the CH2 symmetric stretch at 2948cm−1, CH2 Fermi resonance stretch at 2970cm−1, CH3 symmetric stretch at 2990cm−1, CH3 out-of-plane end asymmetric stretch at 3010cm−1, and unsaturated =CH stretch at 3028cm−1. The two dominant bands of the up-to-five Raman bands in the 2940–3030cm−1 region have a consistent band spacing of 25cm−1 in all four aerosols. Finally we discuss improvements to the PTRS that should provide a system which can trap a higher fraction of particle types and obtain Raman spectra over a larger range (e.g., 200–3600cm−1) than those achieved here.

Surface-enhanced Raman spectra of medicines with large-scale self-assembled silver nanoparticle films based on the modified coffee ring effect

We report here a simple and innovative method to prepare large-scale silver nanoparticle films based on the controlled coffee ring effect. It is demonstrated that the films can be used as surface-enhanced Raman scattering probes to detect low-concentration medicines. Silver nanoparticles with the average size about 70 nm were prepared by reduction of silver nitride. In our experiment, the coffee ring effect was controlled by tilting the substrates during the deposition of silver nanoparticle films. Silver nanoparticle films were spontaneously formed on the surface of silicon substrates at the temperatures about 50°C based on the solvent evaporation and the coffee ring effect. The microstructure of the films was investigated using the scanning electron microscope and atomic force microscope. The surface roughness of the films is found as small as 20 nm. Then, the films were exposed to aqueous solutions of medicine at different concentrations. A comparison with a Raman spectra measured with a conventional Raman spectrometer showed that the Raman signal can be detected in the solution with concentrations as low as 1 × 10−5 M, and the enhancement factor achieved by the silver nanoparticle film can at least reach to 1.08 × 104. Our experimental results indicate that this technique is promising in the production of large-scale silver nanoparticle films for the surface-enhanced Raman scattering. These may be utilized in biochemical and trace analytical applications.

Tunable Plasmons in Shallow Silver Nanowell Arrays for Directional Surface-Enhanced Raman Scattering

The purpose of this article is to improve the collection efficiency of surface-enhanced Raman scattering (SERS) further to increase SERS detection sensitivity in trace detection. To achieve this, a silver nanowell array substrate was designed based on its tunable propagating surface plasmons. This substrate supported directional surface plasmon coupling emission and could guide SERS to the vertical direction of the substrate. Silver nanoparticles were assembled on the shallow silver nanowell array to contribute localized surface plasmons for higher electromagnetic enhancement. Spatial SERS radiation patterns on the silver nanoparticle assembled nanowell array substrate were simulated by the finite-difference time-domain method and recorded by a self-made 3D angle-resolved Raman spectrometer. The results showed that SERS signals were strong and unidirectional in space. The half divergence angle of the SERS pattern was about 10°, which would facilitate SERS collection by using a conventional backscattering Raman spectrometer. This silver nanowell array is supposed to be an applicable configuration to many systems that require high collection efficiency like single-molecule SERS detection and tip-enhanced Raman spectroscopy.

Identification of Explosive Substances Through Improved Signals Obtained by a Portable Raman Spectrometer

ABSTRACT The development of devices to detect explosive substances in situ with characteristics such as easy portability, simple operation, and quick response is of high interest nowadays. Raman spectroscopy meets most of these requirements, allowing for the identification of volume and trace amounts of an unknown substance. In this paper we report the characterization of explosive substances such as ANFO and pentolite, both in their pure forms as well as mixed with other substances, such as sugar and sodium hydrogen carbonate, using a portable Raman Enwave Optronics EZRaman-model M, whose signal is sent and received by a head coupled to an optical fiber. The obtained spectra have been validated by comparison with those obtained by a conventional Raman spectrometer with high resolution. We discuss the advantage of using miniaturized laptops running with a 785-nm laser, making a correction of the signal response to obtain optimized spectral fingerprints with minimum noise, in order to design an autocalibrated, easily portable device that performs the function of identification of the mentioned substances.

Raman spectroscopy with an integrated arrayed-waveguide grating

An integrated arrayed-waveguide grating fabricated in silicon-oxynitride technology is applied to Raman spectroscopy. After its validation by reproducing the well-known spectrum of cyclohexane, polarized Raman spectra are measured of extracted human teeth containing localized initial carious lesions. Excellent agreement is obtained between the spectra of healthy and carious tooth enamel measured with our integrated device and spectra recorded using a conventional Raman spectrometer. Our results represent a step toward the realization of compact, hand-held, integrated spectrometers, e.g. for the detection of dental caries at an early stage.

Wavelength selection-based nonlinear calibration for transcutaneous blood glucose sensing using Raman spectroscopy.

While Raman spectroscopy provides a powerful tool for noninvasive and real time diagnostics of biological samples, its translation to the clinical setting has been impeded by the lack of robustness of spectroscopic calibration models and the size and cumbersome nature of conventional laboratory Raman systems. Linear multivariate calibration models employing full spectrum analysis are often misled by spurious correlations, such as system drift and covariations among constituents. In addition, such calibration schemes are prone to overfitting, especially in the presence of external interferences that may create nonlinearities in the spectra-concentration relationship. To address both of these issues we incorporate residue error plot-based wavelength selection and nonlinear support vector regression (SVR). Wavelength selection is used to eliminate uninformative regions of the spectrum, while SVR is used to model the curved effects such as those created by tissue turbidity and temperature fluctuations. Using glucose detection in tissue phantoms as a representative example, we show that even a substantial reduction in the number of wavelengths analyzed using SVR lead to calibration models of equivalent prediction accuracy as linear full spectrum analysis. Further, with clinical datasets obtained from human subject studies, we also demonstrate the prospective applicability of the selected wavelength subsets without sacrificing prediction accuracy, which has extensive implications for calibration maintenance and transfer. Additionally, such wavelength selection could substantially reduce the collection time of serial Raman acquisition systems. Given the reduced footprint of serial Raman systems in relation to conventional dispersive Raman spectrometers, we anticipate that the incorporation of wavelength selection in such hardware designs will enhance the possibility of miniaturized clinical systems for disease diagnosis in the near future.

Factors Determining the Stability, Resolution, and Precision of a Conventional Raman Spectrometer

We verified the performance of a conventional Raman spectrometer, which is composed of a 30 cm single polychromator, a Si based charge-coupled device (CCD) camera, and a holographic supernotch filter. For that purpose, the time change of the peak positions of Raman spectra of naphthalene and fluorescence spectra of ruby (Cr-doped Al(2)O(3)) were monitored continually. A time-dependent deviation composed of two components was observed: a monotonous drift up to 0.4 cm(-1) and a periodic oscillation with a range of 0.15 cm(-1). The former component was stabilized at approximately 2000 s after the CCD detector was cooled, indicating that incomplete refrigeration of the CCD detector induced the drift. The latter component synchronized with the periodic oscillation of the room temperature, indicating that thermal expansion or contraction of the whole apparatus induced this oscillation. The implemental deviation is reduced when measurements are conducted using a sufficiently cooled CCD detector at a constant room temperature. Moreover, the effect of the room temperature oscillation is lowered in a spectrum acquired over a duration that is longer than one cycle of this oscillation. Applying the least squares fitting method to carefully measured spectra enhanced the precision of the determination of the peak position to 0.05 cm(-1) using the spectrometer with pixel resolution of 1.5 cm(-1).

Intensity Considerations in Liquid Core Optical Fiber Raman Spectroscopy

This paper develops expressions for the Raman power emitted by liquid core optical fiber (LCOF) sample cells in six simple excitation/collection geometries and the fraction of that power that can be utilized in a conventional Raman spectrometer. From these expressions a “figure of merit” is developed that can be used to predict the relative intensity enhancement provided by LCOFs having different inside diameters and loss characteristics. For the apparatus used here, we show theoretically and experimentally that the figure of merit takes the simple form (α d)−1 where α and d are, respectively, the loss coefficient and inside diameter of the LCOF. Given the varying optical quality of Teflon®-AF LCOFs currently in use, the analysis presented here should be useful in optimizing the performance of LCOF accessories in Raman and related spectroscopic applications.

On the reliability of the Raman spectra of boron-rich solids

The problem of qualitatively different Raman spectra of boron-rich solids obtained by conventional and FT (Fourier-transform) Raman spectroscopy has been solved. Because of the high absorption coefficient in the spectral range of the fundamental absorption the penetration depth at the energy of the 514.5 nm line of the Ar laser (2.41 eV) frequently used in conventional Raman spectrometers is so small that the spectra are essentially determined by the oxidized sample surfaces or by the strongly distorted Beilby layers. The photon energy of the Nd:YAG laser (1.16 eV) used in FT-Raman spectrometers is beyond the absorption edge, and therefore scattering essentially takes place within the bulk of the sample. Hence, FT-Raman spectra are much more bulk-determined than conventional Raman spectra.

Surface-enhanced Raman scattering of halide ions, pyridine and crystal violet on colloidal silver with near-infrared excitation: low-wavenumber vibrational modes

Abstract Surface-enhanced Raman scattering from halide ions, pyridine and crystal violet with and without coexistence of halide ions (Cl−, Br−, I−) in the low-wavenumber region is studied by using a conventional scanning Raman spectrometer and near-infrared excitation. It is found that there exist at least two active Raman modes in the wavenumber region between 200 and 300 cm−1 for pyridine adsorbed on colloidal silver in the presence of chloride ions as well as that the halide ions affect also the ring breathing modes of adsorbed pyridine. Enhancement and shift in band positions of the ring breathing modes decrease in the order Cl− > Br− > I−. The observed band at about 240 cm−1 is assigned to the Ag-Cl stretching vibration whereas a band at about 221 cm−1 is due to the surface complex of the adsorbed pyridine. We also found that more than one Raman mode in the low-wavenumber region are simultaneously active for adsorbed crystal violet on colloidal silver in the presence of chloride ions. The experimental results indicate that the Ag-Cl stretching vibration accounts for the Raman band at about 240 cm−1 and that the Raman mode at about 210 cm−1 is specific for crystal violet.

Raman spectroscopy using a fiber optic probe with subwavelength aperture

Raman spectroscopy with subwavelength spatial resolution of a diamond sample was recorded using a tapered fiber optical probe in conjunction with a conventional Raman spectrometer. The experiment demonstrates the potential of suboptical wavelength resolution analytical spectroscopy. The tapered fiber optical probe with an aperture of around 100 nm, served as the means for delivering pump radiation while simultaneously collecting the Stokes radiation from the diamond specimen. Comparing the magnitude of the Raman scattering measured with the submicron single mode fiber probe to similar signals obtained with a nontapered probe made of the same type of fiber, illustrates the potential increase in effective optical aperture resulting from the close approach of the fiber to the surface.