Raman Excitation Wavelength Research Articles

Spectroscopic investigation of oxidation in GaSe 2D layered materials

GaSe, a two-dimensional layered metal monochalcogenide, has recently attracted growing interest due to its unique electronic properties and potential technological applications. In this study, we investigate the oxidation mechanisms and properties of GaSe exposed to air for different durations, with the intensive use of Raman spectroscopy, combined with atomic force microscopy (AFM), photoluminescence (PL), and X-ray photoelectron spectroscopy (XPS). Raman analysis reveals the oxidation of GaSe, resulting in the formation of a thin layer comprising Ga2Se3, Ga2O3, and amorphous selenium. Utilizing these signatures, oxidation is then tracked. Raman spectroscopy reveals that GaSe layer becomes oxidized almost immediately after exposure to air. However, the oxidation is a self-limiting process, taking roughly 15 min to construct an 8 Å thick layer of Ga₂O₃. XPS analysis shows a good agreement with Raman analysis. The polarized Raman study suggests that the Ga₂Se₃ and Ga₂O₃ layers tend to reach an oriented structural state over time. In ambient conditions, the intensity of all Raman modes and the luminescence decreases, linked to reduction in GaSe thickness. By using various Raman excitation wavelengths, we highlight the depth-dependent oxidation dynamics in this 2D layered GaSe material.

Highly sensitive and stable SERS sensing on superhydrophobic ladder-like nanostructures

A new type of plasmonically active Laser-Induced Periodic Surface Structures (LIPSS) is fabricated on the stainless-steel surface. In particular, these LIPSS appear as ladder-like periodic patterns whose periodicity can also be controllable at a nanoscale. Here we demonstrated the ability to use these structured surfaces for sensitive SERS (Surface Enhance Raman Spectroscopy) signal measurements. Also, these nanostructured substrates exhibit hydrophobic nature with a contact angle of ∼147 ± 2. The hydrophobic nature of the surface confines the test samples to a small area without spreading, which increases the sensitivity of SERS signals by several folds. These SERS measurements are demonstrated using standard methylene blue (MB) dye with a 532 nm Raman excitation wavelength. Structural uniformity over a large area allows it to qualify as a reliable SERS platform. The random signal measurement across a large sample showed only a 7 % relative standard deviation. The lowest detectable concentration between 10 fM to 100 fM, with an enhancement factor of 2 × 109 for 10 fM, is demonstrated by drop casting the silver nano particles on the substrate. The substrate provides excellent thermal stability to the analyte due to better heat conduction, which reduces laser exposure-induced photodegradation of samples. Also, the lowest detectable concentration of picric acid is 10 µM. The demonstrated SERS active substrates can be used ubiquitously for all sensing applications with great sensitivity, greater repeatability, and increased photodegradation threshold energy. They get better with aging after fabrication and can be scaled to industrial productions.

Structure-Resolved Monitoring of Single-Wall Carbon Nanotube Functionalization from Raman Intermediate Frequency Modes.

Single-wall carbon nanotubes (SWCNTs) can be covalently modified to generate useful changes in their spectroscopic and photophysical properties. We report here a new method to monitor the extent of such functionalization reactions for different nanotube structures. Raman spectra are analyzed to find the intensities of structure-specific intermediate frequency mode (IFM) features in the range of ca. 350 to 650 cm-1, which are induced by introduction of sp3 defects. The IFM frequencies are found to depend on both the nanotube diameter and Raman excitation wavelength. The growth of IFM features is accompanied by a decrease in RBM intensities, so the IFM to RBM intensity ratio can provide a sensitive, structure-specific measure of nanotube functionalization.

Multiwavelength Raman spectroelectrochemical study of poly(N-methylaniline): An overview


 
 
 Raman spectroscopic study of poly(N-methylaniline) as a thin layer deposited at a electrode was performed in an electrochemical system. A broad range of Raman excitation wavelengths from UV (325 nm) through blue (442 nm) and red (633 nm) to NIR (785 nm) were used. Two limiting redox forms, a reduced (leucoemeraldine) and a fully oxidized one, were considered as obtained at controlled electrochemical potential conditions, and two solution pH of 2.0 and 8.0 were selected to ensure the existence of protonated and deprotonated forms. The Raman spectra obtained and discussed show a strong enhancement for the reduced form at UV and blue laser line excitations, whereas the Raman enhancement for an oxidized form was observed for red and NIR laser line excitations. The main Raman features for the combinations of the reduced and oxidized, and protonated or deprotonated states of poly(N-methylaniline) were analyzed and compared.
 
 

New Insights on the Raman and SERS Spectra of Luteolin under Different Excitation Conditions: Experiments and DFT Calculations

We have studied, by density functional theory, the interaction between luteolin and Ag, devising two complexes where an Ag14 cluster faces two different sites of the molecule. The two sites are identified as quinoid-like and cathecol-like, and the complexes are CPLX1 and CPLX2, respectively. Raman and SERS spectra of luteolin were measured at different excitation wavelengths. Luteolin solid samples from different suppliers have different Raman spectra, possibly associated with different arrangements in the solids. These spectra are well reproduced by our DFT calculations. Assignment of the vibrational modes of luteolin and of the two luteolin–Ag14 complexes is obtained thanks to decomposition of the normal coordinates in terms of internal coordinates. The calculated Raman spectrum for CPLX1 seems to better reproduce the experimental SERS spectra. CPLX2 furnishes a spectrum still resembling that of luteolin in the high frequency region and is possibly responsible for some weak bands in the 1400–1700 cm−1 range that cannot be accounted by the CPLX1 system. SERS spectra are dependent on the Raman excitation wavelength. The calculation of the electronic spectrum suggests the presence of charge-transfer states, which might be responsible for the changes in the SERS spectra.

Excited-state resonance Raman spectroscopy probes the sequential two-photon excitation mechanism of a photochromic molecular switch.

Some diarylethene molecular switches have a low quantum yield for cycloreversion when excited by a single photon, but react more efficiently following sequential two-photon excitation. The increase in reaction efficiency depends on both the relative time delay and the wavelength of the second photon. This paper examines the wavelength-dependent mechanism for sequential excitation using excited-state resonance Raman spectroscopy to probe the ultrafast (sub-30fs) dynamics on the upper electronic state following secondary excitation. The approach uses femtosecond stimulated Raman scattering (FSRS) to measure the time-gated, excited-state resonance Raman spectrum in resonance with two different excited-state absorption bands. The relative intensities of the Raman bands reveal the initial dynamics in the higher-lying states, Sn, by providing information on the relative gradients of the potential energy surfaces that are accessed via secondary excitation. The excited-state resonance Raman spectra reveal specific modes that become enhanced depending on the Raman excitation wavelength, 750 or 400nm. Many of the modes that become enhanced in the 750nm FSRS spectrum are assigned as vibrational motions localized on the central cyclohexadiene ring. Many of the modes that become enhanced in the 400nm FSRS spectrum are assigned as motions along the conjugated backbone and peripheral phenyl rings. These observations are consistent with earlier measurements that showed higher efficiency following secondary excitation into the lower excited-state absorption band and illustrate a powerful new way to probe the ultrafast dynamics of higher-lying excited states immediately following sequential two-photon excitation.

Plasmonic Superstructure Arrays Fabricated by Laser Near-Field Reduction for Wide-Range SERS Analysis of Fluorescent Materials.

Surface-enhanced Raman scattering (SERS) enables trace-detection for biosensing and environmental monitoring. Optimized enhancement of SERS can be achieved when the energy of the localized surface plasmon resonance (LSPR) is close to the energy of the Raman excitation wavelength. The LSPR can be tuned using a plasmonic superstructure array with controlled periods. In this paper, we develop a new technique based on laser near-field reduction to fabricate a superstructure array, which provides distinct features in the formation of periodic structures with hollow nanoclusters and flexible control of the LSPR in fewer steps than current techniques. Fabrication involves irradiation of a continuous wave laser or femtosecond laser onto a monolayer of self-assembled silica microspheres to grow silver nanoparticles along the silica microsphere surfaces by laser near-field reduction. The LSPR of superstructure array can be flexibly tuned to match the Raman excitation wavelengths from the visible to the infrared regions using different diameters of silica microspheres. The unique nanostructure formed can contribute to an increase in the sensitivity of SERS sensing. The fabricated superstructure array thus offers superior characteristics for the quantitative analysis of fluorescent perfluorooctanoic acid with a wide detection range from 11 ppb to 400 ppm.

Leveraging Multiple Raman Excitation Wavelength Systems for Process Monitoring of Nuclear Waste Streams

Leveraging Multiple Raman Excitation Wavelength Systems for Process Monitoring of Nuclear Waste Streams

Atypical Green Luminescence from Raw Cassia Siamea Extract: A Comparison with Red Emitting Tinospora Cordifolia.

Optical and electrochemical properties from Cassia and Giloy leaves' raw extract have been studied, and they show similar properties as UV absorber but different emission properties, under UV excitation, even though they appear the same in natural light. Giloy and Cassia extracts show red and green luminescence, respectively, under UV excitation. Like the appearance, their redox properties are also similar, which shows that both can act as antioxidants. Raman spectroscopy and excitation wavelength dependent photoluminescence data have been compared. The difference in relative emission intensities have been explained based on the presence of corresponding color centers in different ratios in the two leaves.

Excellent surface enhanced Raman properties of titanate nanotube-dopamine-Ag triad through efficient substrate design and LSPR matching

In this work, a novel triad nanocomposites containing trititanate nanotubes, Ag nanoparticles and dopamine were prepared and the SERS properties were measured experimentally. Control over the size and position of the Ag nanoparticles in the TiNT-dop-Ag system, along with the localised surface plasmon resonance (LSPR) of the Ag particles matching the Raman excitation wavelength, gives a SERS enhancement greater than 6 orders of magnitude. Electromagnetic modelling was used to provide a theoretical basis for these results.

The active role of the support in propane ODH over VOx/CeO2 catalysts studied using multiple operando spectroscopies

Because of the relevance of supported vanadia catalysts for the oxidative dehydrogenation (ODH) of alcohols and short alkanes, a detailed mechanistic understanding, including the role of the support, is of great importance. In this work, we address the ODH of propane over ceria-supported vanadia (VOx/CeO2) catalysts, especially regarding the active participation of ceria and the synergy effect between ceria and vanadia. A combination of operando multi-wavelength Raman-, operando UV-Vis- and operando IR spectroscopy is applied, including the targeted use of different Raman excitation wavelengths to selectively enhance vibrational features through resonance from vanadia (514 nm) and the ceria support (385 nm). Our operando results show that surface lattice oxygen from ceria is crucial for the selective oxidation of propane while the structure of vanadia seems to stay unchanged during the reaction. Rather vanadia structures play an indirect role in increasing the overall selectivity of the reaction. The surface oxygen defects of ceria are affected by the interaction with vanadia monomeric species in direct proximity, owing to an irreversible occupancy of the vacancies by vanadia monomers, which slows down the overall oxygen dynamics and thereby decreases the amount of CO2 produced. Dimeric and oligomeric vanadia species weaken the adsorption of propane onto the ceria surface, further increasing the selectivity. Our mechanistic insights demonstrate the synergy effect between vanadia and ceria in oxidation reactions and provide an experimental basis for a detailed understanding of the role of the support in VOx/CeO2 and other supported vanadia catalysts.

Cicada Wing Inspired Template-Stripped SERS Active 3D Metallic Nanostructures for the Detection of Toxic Substances.

This article introduces a bioinspired, cicada wing-like surface-enhanced Raman scattering (SERS) substrate based on template-stripped crossed surface relief grating (TS-CSRG). The substrate is polarization-independent, has tunable nanofeatures and can be fabricated in a cleanroom-free environment via holographic exposure followed by template-stripping using a UV-curable resin. The bioinspired nanostructures in the substrate are strategically designed to minimize the reflection of light for wavelengths shorter than their periodicity, promoting enhanced plasmonic regions for the Raman excitation wavelength at 632.8 nm over a large area. The grating pitch that enables an effective SERS signal is studied using Rhodamine 6G, with enhancement factors of the order of 1 × 104. Water contact angle measurements reveal that the TS-CSRGs are equally hydrophobic to cicada wings, providing them with potential self-cleaning and bactericidal properties. Finite-difference time-domain simulations are used to validate the nanofabrication parameters and to further confirm the polarization-independent electromagnetic field enhancement of the nanostructures. As a real-world application, label-free detection of melamine up to 1 ppm, the maximum concentration of the contaminant in food permitted by the World Health Organization, is demonstrated. The new bioinspired functional TS-CSRG SERS substrate holds great potential as a large-area, label-free SERS-active substrate for medical and biochemical sensing applications.

Wavelength-dependent label-free identification of isolated nontuberculous mycobacteria using surface-enhanced Raman spectroscopy

We investigated the effect of Raman excitation wavelengths on the surface-enhanced Raman spectroscopy (SERS)-based identification of isolated nontuberculous mycobacteria (NTM). The SERS spectra with 3 commonly used excitation wavelengths, 532, 638, and 785 nm, were compared across 6 representative NTM species that primarily cause human NTM infections in Korea and the United States; these species were identified. The statistical differences among NTM SERS spectra at each Raman excitation wavelength were verified using 1-way analysis of variance, and the 6 NTM species were identified using principal components-linear discriminant analysis with leave-one-out cross validation. The identification accuracies with aromatic amino acid biomarkers were 99.3%, 91.3%, and 90.7% for 532, 638, and 785 nm, respectively. We believe that the proposed SERS protocol with aromatic amino acid biomarkers at the 532-nm Raman excitation wavelength will enable fast and accurate identification of NTM compared to previous identification methods.

Excitation wavelength-dependent SERS and DFT study to probe Herzberg-Teller selection rules on charge-transfer effect.

In the present work, Herzberg-Teller selection rules on the charge-transfer (CT) effect in surface-enhanced Raman spectroscopy (SERS) are explored for the 3,4,9,10-perylene tetracarboxylic diimide (PTCDI) adsorbed on the Ag nano-island film (AgNIF) using several Raman excitation wavelengths. The UV/VIS/NIR spectrum of PTCDI adsorbed on the AgNIF indicates that excitation wavelengths of 514.5 nm, 633 nm, and 785 nm are in resonance with CT states of the complex, PTCDI adsorbed on the AgNIF. This CT resonance results in intensity enhancement of non-totally symmetric vibrational modes with b1 symmetry in SERS. The three resonances (molecular, localized surface plasmon, and charge transfer) are observed for SERS with 514.5 nm. The totally symmetric Raman bands with symmetry a1 exhibit maximum enhancement in SERS with a 514.5 nm wavelength and suggest the maximum electromagnetic mechanism in SERS with 514.5 nm. Few Raman-forbidden modes and silent modes are allowed in the SERS spectra due to the reduced symmetry of PTCDI in the proximity of the AgNIF. Moreover, density functional theory computation is also carried out to calculate vibrational modes and electronic transitions.

(Invited) Hybrid Plasmonic Nanomaterials for Uranyl Sensing

Introduction Reproducible detection of uranyl, an important biological and environmental contaminant, from complex matrices by surface-enhanced Raman scattering (SERS) is successfully achieved using hybrid plasmonic nanoparticles. Traditionally, non-specific binding of interfering species limits detects of molecules such as uranyl. Herein, this is overcome using materials design and rigorous sample analysis workflow design. Synergistic approaches for uranyl isolation and SERS detection is promising for real-world sample detection and eliminates the need of radioactive tracers and extensive sample pretreatment steps.Uranium, a radioactive material with a long half-life, accumulates in the environment in its oxidative form uranyl, which can contaminate soil and water [1]. Importantly, uranyl forms complexes with anions and cations thereby influencing solubility, toxicity, and fate of these heavy metal species. Furthermore, uranyl speciation varies with pH. The resulting complex speciation complicates detection and/or requires significant sample pretreatment. As such, methods that are capable of identifying trace uranyl species in complex samples are needed.Herein, hybrid plasmonic nanomaterials and experimental workflow implementation will be used to establish a rigorous protocol for uranyl detection [2,3]. Namely, localized surface plasmon resonance spectroscopy (LSPR), Raman spectroscopy [4], and surface enhanced Raman scattering (SERS) [5,6] serve as label-free and near real-time methods for identifying uranium species in complex aqueous solutions. Experimental Approaches A rigorous protocol for spectral analysis will be shown using Raman spectroscopy and aqueous uranium samples. Raman excitation wavelength, pH, and coordinating ions are systematically varied. The spectral analysis results are rigorously validated using uranyl speciation models. Next, plasmonic nanomaterials are used to enhance the Raman signals for trace detection of low (and high) abundant species. Finally, an approach that promotes the reproducible detection of uranyl using SERS will be shown. All in all, the developed protocol provides an accurate and routine analysis of Raman spectra for uranyl species identification and relative abundance elucidation. These advances are expected to provide a straight-forward approach for uranium species identified using hybrid plasmonic nanomaterials. Results and Conclusions Plasmonic nanomaterials offer many advantages over traditional methodologies in applications ranging from catalysis, sensing, and imaging. Despite these strengths, challenges arise including for both direct and indirect SERS detection of uranyl include nanoparticle stability, SERS spectral complexity, and variations in SERS intensities and vibrational frequencies. All of these challenges depend on intra- and intermolecular interactions at the plasmonic metal interface as well as the plasmonic nanomaterial stability in complex matrices.Uranyl, unfortunately, exhibits poor affinity to traditional SERS substrates. Methods to promote molecule-metal interactions often lead to nanoparticle instability. Herein, gold nanostars and gold coated silver nanospheres are used. Implications of nanoparticle architecture are investigated for maximizing SERS responses and detectability of uranyl. Nanoparticle architecture and surface potential drive these interactions.To promote detectability in complex matrices, both polymer and silica surface chemistries will be evaluated to initially “screen” unwanted interfering species for adsorbing to the metal surface while also permitted the adsorption and detection of uranyl for SERS detection. Finally, solution conditions are used to identify key features that rationally promote uranyl-surface interactions without compromising the physical stability of the nanostructures. Solution composition induces changes in nanoparticle architecture and/or adsorption processes, yet systematic responses are observed. As a result, we expect these studies will broaden the scope of SERS and plasmonic-based assays as small molecules with weak affinity to metal nanostructures will be more readily detected.While uranyl is a difficult molecule to detect in complex matrices, hybrid nanomaterial design and carefully designed experimental approaches facilitate detectability in a reproducible manner. By using polymers and silica hybrid materials, influences from interfering species are minimized and solution impacts reduced. Future advances in further reducing detection limits and integrating these materials with rapid sampling platforms have the potential to lead to a revolutionary sensor for heavy metal detection. Acknowledgments Research reported in this publication was supported by the National Institute of Environmental Health Sciences of the National Institutes of Health under award number R01ES027145 and the National Science Foundation, (CHE-1707859).

Effects on surface-enhanced Raman scattering from copper nanoparticles synthesized by laser ablation

In this work, we report the synthesis of Cu nanoparticles (NPs) by laser ablation technique using 532 nm and 1064 nm wavelength in ethylene glycol. The optical and morphological studies of as-prepared NP colloids were characterized using UV–Visible absorption , photoluminescence spectrometers and transmission electron microscope (TEM) techniques. Cu NPs synthesized at 532 nm and 1064 nm were spherical with an average size of nm and nm respectively. Furthermore, surface-enhanced Raman scattering (SERS) studies were performed on methylene blue (MB) using Cu NPs as SERS platform. In addition, the effects of size, shape and localized surface plasmon resonance (LSPR) of Cu NPs as well as the Raman excitation wavelength used for SERS measurement were investigated and discussed to give insight into the factors affecting their SERS enhancement factor.

Double resonant plasmonic lattices for Raman studies.

We demonstrate radiation induced enhancement of both the in coupling of Raman excitation wavelength and Raman signal in plasmonic nanoparticle lattices. Rectangular nanoparticle lattices show two independently controllable lattice resonances, which we tune to be resonant with both the Raman excitation wavelength and the Raman transitions of rhodamine 6G molecules. We demonstrate that these narrow and intense resonances produced by the nanoparticle lattices allow for Raman transition specific enhancements. The system allows for independent tuning of both resonance conditions, enabling an efficient and versatile platform for Raman studies of various molecules.

Probing optical resonances of silicon nanostructures using tunable-excitation Raman spectroscopy.

Optical materials with a high refractive index enable effective manipulation of light at the nanoscale through strong light confinement. However, the optical near field, which is mainly confined inside such high-index nanostructures, is difficult to probe with existing measurement techniques. Here, we exploit the connection between Raman scattering and the stored electric energy to detect resonance-induced near-field enhancements in silicon nanostructures. We introduce a Raman setup with a wavelength-tunable laser, which allows us to tune the Raman excitation wavelength and thereby identify Fabry-Pérot and Mie type resonances in silicon thin films and nanodisk arrays, respectively. We measure the optical near-field enhancement by comparing the Raman response on and off resonance. Our results show that tunable-excitation Raman spectroscopy can be used as a complimentary far-field technique to reflection measurements for nanoscale characterization and quality control. As proof-of-principle for the latter, we demonstrate that Raman spectroscopy captures fabrication imperfections in the silicon nanodisk arrays, enabling an all-optical quality control of metasurfaces.

Investigation of the Factors Influencing the Surface-Enhanced Raman Scattering Activity of Silver Nanoparticles

Surface-enhanced Raman scattering (SERS) is one of the most effective methods for applications in optical sensors and chemical analysis. However, control of the optimal conditions of substrates and the selection of chemical probes are two major challenges which have yet to be solved. In this work, using a simple seeded growth method, nanoparticles with different sizes and shapes were produced. The SERS enhancement factor for each nanoparticle size and shape obtained was evaluated based on three different analytes, methylene blue, Nile blue A and Rhodamine B. We found a maximum enhancement factor on the order of 106 for the case of silver nanorods and Rhodamine B. Considering the SERS performance for silver nanospheres, we observed a systematic increase in the sequence methylene blue-Nile blue A-Rhodamine B. The reason behind the enhanced efficiency is that the maximum of the surface plasmon resonance band of Rhodamine B is the closest to the Raman excitation wavelength. The study also demonstrates that a decrease in size of spherical nanoparticles can lead to an increased enhancement, resulting from a larger surface area for a smaller particle size. Compared with silver nanospheres, silver nanorods yielded a better SERS enhancement factor, as a result of shape anisotropy which significantly enhance the local field hotspots. For low concentration, the intensity of Raman bands increases linearly with increasing dye concentration, which could be useful for applications involving chemical sensors.

In-line whole blood fractionation for Raman analysis of blood plasma.

Blood plasma evaluation has high significance in clinical diagnostics. Current schemes involve the preparation of blood plasma by centrifugation of whole blood followed by electrochemical or spectroscopic analysis. However, centrifugation is often too time-consuming for application in clinical emergency and point-of-care settings. We propose to combine microfluidic, instantaneous plasma fractionation with localized spectroscopic methods for in-line analysis. As an example, we present confocal Raman spectroscopy in fractionated plasma domains at two different Raman excitation wavelengths. Resonance Raman spectroscopy with laser excitation at 408 nm allows the specific detection of free hemoglobin in blood plasma at concentrations above 22 mg dl-1 (level of detection). Consequently, we are able to accurately resolve the range of clinical relevance regarding hemolysis. At near-infrared excitation (785 nm) we furthermore demonstrate the acquisition of characteristic Raman spectra of fractionated blood plasma in the microfluidic setting. These spectra can serve as starting point for a multi-parameter regression analysis to quantify a set of blood plasma parameters from a single Raman spectrum. The combined microfluidics and Raman spectroscopy method is non-destructive and has a whole blood consumption of less than 100 μl per hour. It thus allows for continuous in-line blood plasma monitoring.