Raman Spectroscopy Systems Research Articles

High-Throughput Raman Spectroscopy by Horizontally Shifted Collection Fibers.

Optical fiber probe-based Raman spectroscopy systems are widely used for in situ measurements ranging from material characterization to biomedical applications. However, small Raman cross sections necessitate the use of high-power lasers or long exposure times that limit Raman's larger application to multiple research fields. This limitation can be overcome by collecting more Raman photons through additional collection fibers with taller detectors. This system configuration requires replacement of the detector and modification of the spectrograph to incorporate larger optical components, making it a costly and cumbersome option. In probe-based Raman systems, a typical detector image shows stacked collection fibers on the vertical axis and Raman spectra on the horizontal axis. While the vertical pixels are fully packed with multiple collection fibers, horizontal pixels have broad silent regions due to the narrow bandwidth of Raman peaks, potentially wasting valuable detector pixels. Here, we propose a new approach utilizing horizontally shifted collection fibers rather than vertically stacked ones. We designed and fabricated a novel collection fiber bundle that has horizontally shifted optical fibers in two vertical lines at the spectrograph entrance. This custom-made fiber bundle was incorporated into the imaging spectrograph to provide multiple horizontally shifted spectra on the detector. Through deconvolution, the original spectra can be recovered with an improved detection limit from greater photon collection. We demonstrate an enhanced limit of detection on various bioanalytes, such as glucose, urea, and lactate. Further, we applied the probe to measure tissue Raman spectra and successfully decomposed them into basis spectra, demonstrating the potential application of high-throughput in vivo tissue diagnosis. Our approach provides a simple, cost-effective, and universal method to increase the throughput without modifying existing Raman spectrometers.

Library-Based Raman Spectral Identification Using Multi-Input Hybrid ResNet.

Raman spectroscopy is widely used for its exceptional identification capabilities in various fields. Traditional methods for target identification using Raman spectroscopy rely on signal correlation with moving windows, requiring data preprocessing that can significantly impact identification performance. In recent years, deep-learning approaches have been proposed to leverage data augmentation techniques, such as baseline and additive noise addition, in order to overcome data scarcity. However, these deep-learning methods are limited to the spectra encountered during training and struggle to handle unseen spectra. To address these limitations, we propose a multi-input hybrid deep-learning model trained with simulated spectral data. By employing simulated spectra, our method tackles the challenges of data scarcity and the handling of unseen spectra encountered in traditional and deep-learning methods. Experimental results demonstrate that our proposed method achieves outstanding identification performance and effectively handles spectra obtained from different Raman spectroscopy systems.

Performance assessment of probe-based Raman spectroscopy systems for biomedical analysis.

We present a methodology for evaluating the performance of probe-based Raman spectroscopy systems for biomedical analysis. This procedure uses a biological standard sample and data analysis approach to circumvent many of the issues related to accurately measuring and comparing the signal quality of Raman spectra between systems. Dairy milk is selected as the biological standard due to its similarity to tissue spectral properties and because its homogeneity eliminates the dependence of probe orientation on the measured spectrum. A spectral dataset is first collected from milk for each system configuration, followed by a model-based correction step to remove photobleaching artifacts and accurately calculate SNR. Results demonstrate that the proposed strategy, unlike current methods, produces an experimental SNR that agrees with the theoretical value. Four preconfigured imaging spectrographs that share similar manufacturer specifications were compared, showing that their capabilities to detect biological Raman spectra widely differ in terms of throughput and stray light rejection. While the methodology is used to compare spectrographs in this case, it can be adapted for other purposes, such as optimizing the design of a custom-built Raman spectrometer, evaluating inter-probe variability, or examining how altering system subcomponents affects signal quality.

Advanced Raman Spectroscopy Based on Transfer Learning by Using a Convolutional Neural Network for Personalized Colorectal Cancer Diagnosis

Advanced Raman spectroscopy (RS) systems have gained new interest in the field of medicine as an emerging tool for in vivo tissue discrimination. The coupling of RS with artificial intelligence (AI) algorithms has given a boost to RS to analyze spectral data in real time with high specificity and sensitivity. However, limitations are still encountered due to the large amount of clinical data which are required for the pre-training process of AI algorithms. In this study, human healthy and cancerous colon specimens were surgically resected from different sites of the ascending colon and analyzed by RS. Two transfer learning models, the one-dimensional convolutional neural network (1D-CNN) and the 1D–ResNet transfer learning (1D-ResNet) network, were developed and evaluated using a Raman open database for the pre-training process which consisted of spectra of pathogen bacteria. According to the results, both models achieved high accuracy of 88% for healthy/cancerous tissue discrimination by overcoming the limitation of the collection of a large number of spectra for the pre-training process. This gives a boost to RS as an adjuvant tool for real-time biopsy and surgery guidance.

Backscattering and transmission Raman spectroscopy systems in the quantitative analysis concentration of acetone solutions

Raman spectroscopy is a set of techniques based on Raman scattering properties, widely applied to analyze the composition of various substances. The techniques consist of 1) backscattering, which collects Raman signals scattered from the surface of a sample; and 2) transmission, which collects Raman signals transmitted by the surface of a sample in the opposite direction of a light source (which triggers less fluorescence than backscattering). In this paper, we will measure the Raman spectrum of acetone solution by the transmission Raman spectroscopy and the backscattering Raman spectroscopy systems that we set up. Those accessories we use are a 532-nanometer diode laser with 100 milliwatt power as the light source, a focusing lens, an objective lens used for amplifying the Raman signals scattered from the sample, and a long-pass filter used to block light with a wavelength shorter than 532 nanometers. For the experimental samples, there are acetone solutions each prepared at 1M, 3M, 5M, 7M, 9M, 11M, and 13M. In the results of this experiment, we found that the intensity of Raman peaks for each Raman shift of acetone and the molar concentrations of acetone measured by both systems have a linear function in the backscattering Raman system and have a quadratic function in the transmissions Raman system because in backscattering system have noise by fluorescence effect more than in transmission system and it might be the effect of the Beer-Lambert law.

Thermal maturity assessment of marine source rocks integrating Raman spectroscopy, organic geochemistry and petroleum systems modeling

A core sample set, comprising twenty-eight carbonaceous mudrocks from ten exploration wells was analysed by standard organic geochemical methods (programmed pyrolysis, elemental analysis) and a novel Raman spectroscopic technique. The kerogen of the studied rocks is classified as amorphous marine, sulfur-rich, oil-prone organic matter with predominantly algal precursor material. The sample set covers a wide range of thermal maturation from premature to late mature. Hydrogen index, Tmax and atomic H/C ratios vary from 52 to 708 mg HC/g rock, from 426 to 488 °C, and from 0.58 to 1.35, respectively. Raman spectral features, expressed as Raman band separation (RBS), correlate with geochemical maturity parameters and with results from basin modeling. The combined use of independent maturity parameters, like programmed pyrolysis, elemental analysis and Raman spectroscopy, improves the calibration of a regional basin model in the Eastern Arabian Basin, where direct maturity measurements from vitrinite reflectance are deficient.

Research work in the Microscope-based Nanoscale Physics Lab

Assistant Professor Sangmin An is head of the Microscope-based Nanoscale Physics Laboratory (MNPL), Jeonbuk National University, South Korea. He and his team are using atomic force microscope (AFM)-based technology as the basis of their work to make advances in materials science on the atomic and molecular levels. The researchers are combining their AFM-based technology with the scanning tunnelling microscope (STM), Raman spectroscopy systems and scanning electron microscope (SEM). Using this variety of techniques is enabling the team to drive progress in their field and also assisting with advances in semiconductor technology, including improving surface roughness measurement technology, which directly impacts consumer electronics. An and the team are also focused on nanoscale 3D printing with a view to overcoming limitations associated with the resolution of 3D printing. The researchers have fabricated nanopipettes through laser irradiation of perforated glass or quartz using a mechanical puller. The nanopipettes are then attached to a crystal oscillator to enable 3D printing at the nanoscale. The team has developed a process in which physical properties of materials can be measured directly using AFM + in situ Raman spectroscopy. By combining AFM-based nanoscale 3D printing technology and optical apparatus, it is possible to measure the physical, electrical, chemical, and optical properties of target materials. An and the team are also performing investigations related to Kelvin probe force microscope (KPFM) and friction measurement, as well as improving AFM technology for AFM and quartz tuning fork AFM (QTF-AFM) systems.

On-Line Monitoring of Gas-Phase Molecular Iodine Using Raman and Fluorescence Spectroscopy Paired with Chemometric Analysis.

Molten salt reactors (MSRs) have the potential to safely support green energy goals while meeting baseload energy needs with diverse energy portfolios. While reactor designers have made tremendous strides with these systems, licensing and deployment of these reactors will be aided through the development of new technology such as on-line and remote monitoring tools. Of particular interest is quantifying reactor off-gas species, such as iodine, within off-gas streams to support the design and operational control of off-gas treatment systems. Here, the development of advanced Raman spectroscopy systems for the on-line analysis of gas composition is discussed, focusing on the key control species I2(g). Signal response was explored with two Raman instruments, utilizing 532 and 671 nm excitation sources, as a function of I2(g) pressure and temperature. Also explored is the integration of advanced data analysis methods to enable real-time and highly accurate analysis of complex optical data. Specifically, the application of chemometric modeling is discussed. Raman spectroscopy paired with chemometric analysis is demonstrated to provide a powerful route to analyzing I2(g) composition within the gas phase, which lays the foundation for applications within molten salt reactor off-gas analysis and other significant chemical processes producing iodine species.

A high-resolution miniaturized ultraviolet spectrometer based on arrayed waveguide grating and microring cascade structures

Miniaturized ultraviolet spectrometer is promising for on-site accurate detections. In this work, a high-resolution on-chip ultraviolet spectrometer is designed and analyzed. MgF2 and Al2O3 are selected as the substrate and core material of the waveguide, respectively, resulting in a low bending loss of 1.5 dB/cm at a radius of 3 μm and wavelength of 278 nm. The spectrometer is designed as a cascaded structure composed of ultra-compact multi-diffraction order arrayed waveguide gratings and low-loss microrings. The cascaded structure exhibits a high wavelength resolution of 0.048 nm (the spectral resolution of the structure is 6.8 cm−1, quality factor is 5790) and low insertion loss of 3.98 dB∼7.55 dB, comparable to the performance of the lens-based ultraviolet Raman spectroscopy systems. This work may pave the way for the development of ultra-compact chip-based ultraviolet spectrometers.

Data consistency and classification model transferability across biomedical Raman spectroscopy systems

AbstractSurgical guidance applications using Raman spectroscopy are being developed at a rapid pace in oncology to ensure safe and complete tumor resection during surgery. Clinical translation of these approaches relies on the acquisition of large spectral and histopathological data sets to train classification models. Data calibration must ensure compatibility across Raman systems and predictive model transferability to allow multi‐centric studies to be conducted. This paper addresses issues relating to Raman measurement standardization by first comparing Raman spectral measurements made on an optical phantom and acquired with nine distinct point probe systems and one wide‐field imaging instrument. Data standardization method led to normalized root‐mean‐square deviations between instruments of 2%. A classification model discriminating between white and gray matter was trained with one point probe system. When used to classify independent data sets acquired with the other systems, model predictions led to >95% accuracy, preliminarily demonstrating model transferability across different biomedical Raman spectroscopy instruments.

Standoff Raman spectroscopy for architectural interiors from 3-15 m distances.

Portable and mobile Raman spectroscopy systems are increasingly being adopted in in situ non-invasive examination of artworks given their high specificity in material identification. However, these systems typically operate within centimeter range working distances, making the examination of large architectural interiors such as wall paintings in churches challenging. We demonstrate the first standoff Raman spectroscopy system for in situ investigation of historic architectural interior at distances > 3 m. The 780 nm continuous wave laser-induced standoff Raman system was successfully deployed for the in situ examination of wall paintings, at distances of 3-15 m, under ambient light. It is able to identify most common pigments while maintaining a very low laser intensity to avoid light induced degradation. It is shown to complement our current method of standoff remote surveys of wall paintings using spectral imaging.

Molybdenum nanoparticles generation by pulsed laser ablation and effects of oxidation due to aging

Molybdenum oxides (MoOx) nanoparticles (NPs) have great optical and electronic features that make them suitable for potential applications such as surface enhanced Raman spectroscopy (SERS) and energy systems. However, there are very few papers that report the synthesis of MoOx NPs by using the laser ablation of solids in liquids (LASL) technique and they lack the explanation for the oxidation process. This work reports on the generation of NPs composed of molybdenum trioxide hydrated (MoO3 · xH2O) (x = 1, 2) by using this method and its oxidation due to aging. A picosecond Nd:YAG laser was used and the per pulse laser fluence was varied from 5 to 20 J/cm2. Spherical NPs were obtained with average diameters from 48 to 141 nm, respectively. The absorption evolution of the obtained colloids was characterized by optical absorption spectroscopy, TEM was used to study the MoOx NPs morphology, size and structure and Raman spectroscopy to determine the material chemical composition.

High Spectral Resolution Raman Measurements Using Light-Emitting Diode as Excitation Based on Weighted Spectral Reconstruction Method

Raman spectroscopy is a spectroscopic technique that can provide rich biochemical information about the sample through the inelastic scattering between monochromatic light and molecular vibrations, thus a narrow bandwidth and costly laser is typically required. Although the replacement of laser to light-emitting diode (LED) can reduce the cost significantly, spectral resolution degrades seriously and a large amount of Raman fingerprint information is lost due to the overlaps of Raman peaks. In this study, a weighted spectral reconstruction method was developed to retrieve high quality Raman spectrum from the Raman measurements using LED as the excitation, in which the LED based Raman measurements were numerically synthesized from the Raman spectra acquired by the conventional laser-based Raman spectroscopy systems. The proposed weighted spectral reconstruction method was numerically test on the synthesized LED based Raman measurements from 25 agar phantoms, 50 blood serum samples, and 56 cell samples, respectively. According to the results, the restored Raman spectra are in excellent agreement with the conventional Raman spectra, and the differences are always less than 2% in all test samples. Therefore, Raman measurements using LED as excitation combined with weighted spectral reconstruction method demonstrate significant potential in accurate and high spectral resolution Raman measurements, which promotes a cost-effective Raman setup while preserve the most important spectral and biochemical information.

Development and characterization of a handheld hyperspectral Raman imaging probe system for molecular characterization of tissue on mesoscopic scales

Raman spectroscopy is a promising cancer detection technique for surgical guidance applications. It can provide quantitative information relating to global tissue properties associated with structural, metabolic, immunological, and genetic biochemical phenomena in terms of molecular species including amino acids, lipids, proteins, and nucleic acid (DNA). To date in vivo Raman spectroscopy systems mostly included probes and biopsy needles typically limited to single-point tissue interrogation over a scale between 100 and 500 microns. The development of wider field handheld systems could improve tumor localization for a range of open surgery applications including brain, ovarian, and skin cancers. Here we present a novel Raman spectroscopy implementation using a coherent imaging bundle of fibers to create a probe capable of reconstructing molecular images over mesoscopic fields of view. Detection is performed using linear scanning with a rotation mirror and an imaging spectrometer. Different slits widths were tested at the entrance of the spectrometer to optimize spatial and spectral resolution while preserving sufficient signal-to-noise ratios to detect the principal Raman tissue features. The nonbiological samples, calcite and polytetrafluoroethylene (PTFE), were used to characterize the performance of the system. The new wide-field probe was tested on ex vivo samples of calf brain and swine tissue. Raman spectral content of both tissue types were validated with data from the literature and compared with data acquired with a single-point Raman spectroscopy probe. The single-point probe was used as the gold standard against which the new instrument was benchmarked as it has already been thoroughly validated for biological tissue characterization. We have developed and characterized a practical noncontact handheld Raman imager providing tissue information at a spatial resolution of 115 microns over a field of view >14 mm2 and a spectral resolution of 6 cm-1 over the whole fingerprint region. Typical integration time to acquire an entire Raman image over swine tissue was set to approximately 100 s. Spectra acquired with both probes (single-point and wide-field) showed good agreement, with a Pearson correlation factor >0.85 over different tissue categories. Protein and lipid content of imaged tissue were manifested into the measured spectra which correlated well with previous findings in the literature. An example of quantitative molecular map is presented for swine tissue and calf brain based on the ratio of protein-to-lipid content showing clear delineations between white and gray matter as well as between adipose and muscle tissue. We presented the development of a Raman imaging probe with a field of view of a few millimeters and a spatial resolution consistent with standard surgical imaging methods using an imaging bundle. Spectra acquired with the newly developed system on swine tissue and calf brain correlated well with an establish single-point probe and observed spectral features agreed with previous finding in the literature. The imaging probe has demonstrated its ability to reconstruct molecular images of soft tissues. The approach presented here has a lot of potential for the development of surgical Raman imaging probe to guide the surgeon during cancer surgery.

Marangoni Convection Assisted Single Molecule Detection with Nanojet Surface Enhanced Raman Spectroscopy.

Many single-molecule (SM) label-free techniques such as scanning probe microscopies (SPM) and magnetic force spectroscopies (MFS) provide high resolution surface topography information, but lack chemical information. Typical surface enhanced Raman spectroscopy (SERS) systems provide chemical information on the analytes, but lack spatial resolution. In addition, a challenge in SERS sensors is to bring analytes into the so-called "hot spots" (locations where the enhancement of electromagnetic field amplitude is larger than 103). Previously described methods of fluid transport around hot spots like thermophoresis, thermodiffusion/Soret effect, and electrothermoplasmonic flow are either too weak or detrimental in bringing new molecules to hot spots. Herein, we combined the resonant plasmonic enhancement and photonic nanojet enhancemnet of local electric field on nonplanar SERS structures, to construct a stable, high-resolution, and below diffraction limit platform for single molecule label-free detection. In addition, we utilize Marangoni convection (mass transfer due to surface tension gradient) to bring new analytes into the hotspot. An enhancement factor of ∼3.6 × 1010 was obtained in the proposed system. Rhodamine-6G (R6G) detection of up to a concentration of 10-12 M, an improvement of two orders of magnitude, was achieved using the nanojet effect. The proposed system could provide a simple, high throughput SERS system for single molecule analysis at high spatial resolution.

Modulated Raman Spectroscopy for Enhanced Cancer Diagnosis at the Cellular Level.

Raman spectroscopy is emerging as a promising and novel biophotonics tool for non-invasive, real-time diagnosis of tissue and cell abnormalities. However, the presence of a strong fluorescence background is a key issue that can detract from the use of Raman spectroscopy in routine clinical care. The review summarizes the state-of-the-art methods to remove the fluorescence background and explores recent achievements to address this issue obtained with modulated Raman spectroscopy. This innovative approach can be used to extract the Raman spectral component from the fluorescence background and improve the quality of the Raman signal. We describe the potential of modulated Raman spectroscopy as a rapid, inexpensive and accurate clinical tool to detect the presence of bladder cancer cells. Finally, in a broader context, we show how this approach can greatly enhance the sensitivity of integrated Raman spectroscopy and microfluidic systems, opening new prospects for portable higher throughput Raman cell sorting.

Investigation of Durability of Optical Coatings in Highly Purified Tritium Gas

Anti-reflection coated windows are part of Raman spectroscopy systems for tritium analytics in the KATRIN experiment and fusion-related applications. Damages of such windows were observed after three months of exposure to highly purified tritium gas in the LOOPINO facility. In this work, the origin of the damages was investigated, identified and eliminated. Coating samples manufactured by various physical vapor deposition methods have been tested for durability by exposure to pure tritium gas and subsequent visual inspection. Electron beam deposited coatings showed indications for damage after 17 days of tritium exposure in contrast to samples manufactured by ion assisted deposition or sputtering. An improved coating layout of the sample cell is presented for reliable long-term monitoring of tritium gas using Raman spectroscopy.

Quantitative analysis of multiplex-components and double stranded DNA by wide-range surface-enhanced Raman spectroscopy based on ordered Ag/Si nanowire arrays

Quantitative analysis by metal-nanoparticle surfaces enhanced Raman spectroscopy (SERS) systems is usually associated with complicated complementary techniques, such as microfluidics, internal standards and multivariate analysis, due to the poor signal reproducibility originating from narrow inter-particle nanogaps as hot spots. In this work, we report on direct and quantitative multiplex detection on a SERS substrate made of an ordered silver film-coated silicon nanowire (Ag/SiNW) array. Compared with the nanogap-based SERS substrate, the Ag/SiNWs are hexagonally packed with an inter-wire distance of 150 nm, and the whole surface of the silver film functions as an active site with a wide-ranging enhancing field, which make the substrates capable of detecting multiplex components and large molecules with much improved spot-to-spot reproducibility over the substrate. Quantitative measurements of a single component yields a coefficient of determination (R2) of over 0.99, while univariate calibration of two interfering components yield an R2 above 0.96. Significantly, univariate calibration of double stranded DNA yields an R2 of 0.994. The excellent reproducibility of our SERS substrate highly simplifies quantitative multiplex detection and thus exhibits great potential for practical sensing applications, especially for multiplex detection as well as for the detection of large bio-molecules.

Remote Continuous Wave and Pulsed Laser Raman Detection of Chemical Warfare Agents Simulants and Toxic Industrial Compounds

This study describes the design, assembly, testing and comparison of two Remote Raman Spectroscopy (RRS) systems intended for standoff detection of hazardous chemical liquids. Raman spectra of Chemical Warfare Agents Simulants (CWAS) and Toxic Industrial Compounds (TIC) were measured in the laboratory at a 6.6 m source-target distance using continuous wave (CW) laser detection. Standoff distances for pulsed measurements were 35 m for dimethyl methylphosphonate (DMMP) detection and 60, 90 and 140 m for cyclohexane detection. The prototype systems consisted of a Raman spectrometer equipped with a CCD detector (for CW measurements) and an I-CCD camera with time-gated electronics (for pulsed laser measurements), a reflecting telescope, a fiber optic assembly, a single-line CW laser source (514.5, 488.0, 351.1 and 363.8 nm) and a frequency-doubled single frequency Nd:YAG 532 nm laser (5 ns pulses at 10 Hz). The telescope was coupled to the spectrograph using an optical fiber, and filters were used to reject laser radiation and Rayleigh scattering. Two quartz convex lenses were used to collimate the light from the telescope from which the telescope-focusing eyepiece was removed, and direct it to the fiber optic assembly. To test the standoff sensing system, the Raman Telescope was used in the detection of liquid TIC: benzene, chlorobenzene, toluene, carbon tetrachloride, cyclohexane and carbon disulfide. Other compounds studied were CWAS: dimethylmethyl phosphonate, 2-chloroethyl ethyl sulfide and 2-(butylamino)-ethanethiol. Relative Raman scattering cross sections of liquid CWAS were measured using single-line sources at 532.0, 488.0, 363.8 and 351.1 nm. Samples were placed in glass and quartz vials at the standoff distances from the telescope for the Remote Raman measurements. The mass of DMMP present in water solutions was also quantified as part of the system performance tests.

A combined laser-induced breakdown and Raman spectroscopy Echelle system for elemental and molecular microanalysis

Raman and laser-induced breakdown spectroscopy is integrated into a single system for molecular and elemental microanalyses. Both analyses are performed on the same ~ 0.002 mm 2 sample spot allowing the assessment of sample heterogeneity on a micrometric scale through mapping and scanning. The core of the spectrometer system is a novel high resolution dual arm Echelle spectrograph utilized for both techniques. In contrast to scanning Raman spectroscopy systems, the Echelle–Raman spectrograph provides a high resolution spectrum in a broad spectral range of 200–6000 cm − 1 without moving the dispersive element. The system displays comparable or better sensitivity and spectral resolution in comparison to a state-of-the-art scanning Raman microscope and allows short analysis times for both Raman and laser induced breakdown spectroscopy. The laser-induced breakdown spectroscopy performance of the system is characterized by ppm detection limits, high spectral resolving power (15,000), and broad spectral range (290–945 nm). The capability of the system is demonstrated with the mapping of heterogeneous mineral samples and layer by layer analysis of pigments revealing the advantages of combining the techniques in a single unified set-up.