Raman Intensity Research Articles

Surface-enhanced Raman scattering of a gold core-silver shell-sponge substrate for detection of thiram and diquat.

Aiming at the difficulty of traditional pesticide sampling, a low-cost and convenient flexible surface enhanced Raman scattering (SERS) gold core-silver shell-sponge (Au-Ag-sponge) substrate was synthesized by chemical reduction. The SERS substrate consisted of Au-AgNPs and a melamine sponge. The sponge had a rich open pore structure, which could well "capture" Au-AgNPs, generating a large number of "hot spots". The SERS enhancement activity of the flexible substrate was characterized with rhodamine 6G (R6G) Raman probe molecules. The substrate showed good activity to 10-12 M rhodamine 6G with an enhancement factor (EF) of 7.72 × 106. Applying this substrate to the qualitative and quantitative detection of pesticide residues, the results showed that the Raman intensity was well related to the concentration of pesticide solution with the range of 0.1-10 mg L-1 of thiram and 1-10 mg L-1 of diquat. Furthermore, the substrate was analyzed by finite difference time domain (FDTD) simulation and the results were in good agreement with the experimental results. The reason for the difference in Raman signals of pesticide molecules on the same substrate was the different binding modes of Au-AgNPs on the sponge. Finally, we pointed out the advantages of flexible substrates in the field of pesticide residues, as well as future opportunities and challenges.

MOLECULAR STRUCTURE, EXPERIMENTAL AND THEORETICAL (HF, DFT) SPECTRAL ANALYSIS OF 3,4,5- TRIMETHOXYANILINE

Experimental infrared and laser Raman spectra of 3,4,5-trimethoxyaniline were recorded in the first part of our investigation. Theoretically, using Hartree-Fock and density functional theory, structural properties and vibrational frequencies were estimated. The Hartree-Fock level spectral analysis was conducted using the HF/6-31+G(d,p) and HF/6-311++G(d,p) approaches. The B3LYP/6-31+G(d, p) technique was implemented to accomplish the DFT analysis. Comprehensive details of infrared, Raman intensity, scaled frequencies, reduced masses, and force constants are also presented in this paper. The vibrational experimental and theoretical findings are compared in the final section of our investigation using the 3D Gauss view visualization program. This paper also creates and provides Raman and IR spectra of 3,4,5-trimethoxy aniline using HF and DFT methodology.

A ratiometric SERS sensor with one signal probe for ultrasensitive and quantitative monitoring of serum xanthine.

Xanthine can be converted into uric acid, and a high concentration of xanthine in the human body can cause many diseases. Therefore, it is important to develop a sensitive, simple, and reliable approach for measuring xanthine in biological liquids. Hence, a ratiometric surface-enhanced Raman spectroscopy (SERS) sensing strategy with one signal probe was exploited for reliable, sensitive, and quantitative monitoring of serum xanthine. 3-Mercaptophenylboronic acid (3-MPBA) was used as a typical reference with a Raman peak at 996 cm-1. First, 3-MPBA was bound to gold nanoflowers@silica (GNFs@Si) through Au-S bonds. Xanthine oxidase (XOD) catalyzed the oxidation of xanthine into H2O2 on GNFs@Si. Afterward, the obtained H2O2 further reduced 3-MPBA to 3-hydroxythiophenol (3-HTP) accompanied by the emergence of a new Raman peak at 883 cm-1. Meanwhile, the Raman intensity at 996 cm-1 remained constant. Therefore, the ratio of I883/I996 increased with the increasing of xanthine concentration, thus realizing quantitative detection of xanthine. As a result, a ratiometric SERS sensor for the detection of xanthine was proposed with a detection limit of 5.7 nM for xanthine. The novel ratiometric SERS sensor provides a new direction for analyzing other biomolecules with high sensitivity and reliability.

Machine Learning of polymer types from the spectral signature of Raman spectroscopy microplastics data

The tools and technology that are currently used to analyze chemical compound structures that identify polymer types in microplastics are not well-calibrated for environmentally weathered microplastics. Microplastics that have been degraded by environmental weathering factors can offer less analytic certainty than samples of microplastics that have not been exposed to weathering processes. Machine learning tools and techniques allow us to better calibrate the research tools for certainty in microplastics analysis. In this paper, we investigate whether the Raman shift values are distinct enough such that well studied machine learning (ML) algorithms can learn to identify polymer types using a relatively small amount of labeled input data when the samples have not been impacted by environmental degradation. Several ML models were trained on a well-known repository, Spectral Libraries of Plastic Particles (SLOPP), that contain Raman shift and intensity results for a range of plastic particles, then tested on environmentally aged plastic particles (SloPP-E) consisting of 22 polymer types. After extensive preprocessing and augmentation, the trained random forest model was then tested on the SloPP-E dataset resulting in an improvement in classification accuracy of 93.81% from 89%.

Non-destructive distinction between geogenic and anthropogenic calcite by Raman spectroscopy combined with machine learning workflow.

Here, we demonstrate, for the first time, the possibility of distinguishing between geogenic and anthropogenic calcite in a non-destructive and effective way. Geogenic calcite derives from natural sedimentary and metamorphic rocks whereas anthropogenic calcite is formed artificially due to the carbonation process in mortars and plaster lime binders. Currently, their distinction is a major unaddressed issue although it is crucial across several fields such as 14C dating of historical mortars to avoid contamination with carbonate aggregates, investigating the origins of pigments, and studying the origins of sediments, to name a few. In this paper, we address this unmet need combining high-resolution micro-Raman spectroscopy with data mining and machine learning methods. This approach provides an effective means of obtaining robust and representative Raman datasets from which samples' origins can be effectively deduced; moreover, a distinction between sedimentary and metamorphic calcite has been also highlighted. The samples, chemically identical, exhibit systematic and reliable differences in Raman band positions, band shape and intensity, which are likely related to the degree of structural order and polarization effects.

Layer-by-layer coating of natural diatomite with silver nanoparticles for identification of circulating cancer protein biomarkers using SERS.

Surface-enhanced Raman scattering (SERS) is an emerging spectroscopy technique for detecting and characterizing chemical or biological structures in the vicinity of plasmonic nanostructures. Colloidal, solid, and flexible nanostructures are widely used in SERS experiments to enhance the Raman intensity. The nanostructure used in SERS is one of the main influencing parameters and a growing research area. Fabrication of simple and cheap SERS substrates with a high enhancement factor is desired. In this study, we fabricated a reproducible, cheap, and flexible SERS active strip by coating natural diatomite (biosilica) with silver nanoparticles (AgNPs) using the layer-by-layer assembly method and the fabricated strip is used for the label-free identification of circulating cancer protein biomarkers. SERS active strips were fabricated having different numbers of AgNP layers on natural diatomite and comprehensive characterization techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), and UV/vis absorption spectrophotometry were used. SERS activities of the strips depending on the number of layers were evaluated using 4-aminothiophenol (4-ATP) and rhodamine 6G (Rh6G) molecules. We found that the SERS intensity is strongly dependent on the number of AgNP layers, with the maximum SERS intensity obtained from the strip with 5 layers of AgNPs, having a 2.0 × 105 enhancement factor. The strip with the highest SERS activity was used for the label-free identification of circulating cancer protein biomarkers (HER2, CA15-3, PSA, MUC4, and CA27-29). The results demonstrate that the fabricated strip can help in the effective label-free identification of circulating protein biomarkers and open new directions for SERS-based label-free biosensing applications.

Localized vibrational characteristics of biphenylene strips resulting in length-dependent Raman spectra.

The length dependence of the Raman spectra and vibrational properties of biphenylene strips are explored using density functional theory. The Raman intensity of two bands increases and decreases with length due to the enlarging and shrinking of the proportion of effective vibrating units. The red shift of vibrational modes is observed with the increase in length, owing to the various vibrational characteristics of the effective vibrating units. More importantly, a linear relationship between the energy gap and the wavenumber of the shifting Raman bands is obtained. The results allow us to interpret the length-dependence of the Raman spectra from the perspective of localized vibrational characteristics and suggest that Raman spectroscopy can be used as a convenient method to determine the energy gap of nanomaterials.

Effect of precursor concentration on stoichiometry and optical properties of spray pyrolyzed nanostructured NiO thin films

In this study, spray pyrolysis was used to produce nanostructured NiO thin films from high purity nickel acetate (Ni(CH3COO)2.3H2O) precursors on pre-heated ultrasonically cleaned soda-lime glass substrates. The metallic constituent concentrations in the films were varied, and the precursors were produced in distilled water at various molarities ranging from 0.1 to 0.4 M. In the study, the field-emission scanning electron microscope (FESEM) results strongly confirmed adherence of the films to the glass substrate at 350 °C. The presence of Ni and O in the samples was confirmed using Rutherford backscattering spectroscopy (RBS), X-ray diffractometry (XRD) and energy dispersive X-ray spectroscopy (EDX). For the 0.1 M NiO thin films, the thickness was approximately 43 nm, and for the 0.2 M, 0.3 M, and 0.4 M films, the thickness was 46 nm, 47 nm, and 49 nm, respectively. The XRD findings were supported by the increased Raman intensity peaks with increased precursor concentration, which confirmed the films' improved crystallinity. For the same number of passes of films deposition, as the molar concentration increases, the films thickness increases. The amount of nickel in NiO thin films increases as the molarity increases, but the amount of oxygen in NiO thin films decreases as the molarity increases. It was discovered that as molarity increases, the optical transmittance decreases and the optical band gap narrows. The qualities of NiO discovered in this study suggest the films’ potentials for usage as window layer and buffer material in thin film solar cells.

Anisotropy and thermal properties in GeTe semiconductor by Raman analysis.

Low-symmetric GeTe semiconductors have attracted wide-ranging attention due to their excellent optical and thermal properties, but only a few research studies are available on their in-plane optical anisotropic nature that is crucial for their applications in optoelectronic and thermoelectric devices. Here, we investigate the optical interactions of anisotropy in GeTe using polarization-resolved Raman spectroscopy and first-principles calculations. After determining both armchair and zigzag directions in GeTe crystals by transmission electron microscopy, we found that the Raman intensity of the two main vibrational modes had a strong in-plane anisotropic nature; the one at ∼88.1 cm-1 can be used to determine the crystal orientation, and the other at ∼124.6 cm-1 can reveal a series of temperature-dependent phase transitions. These results provide a general approach for the investigation of the anisotropy of light-matter interactions in low-symmetric layered materials, benefiting the design and application of optoelectronic, anisotropic thermoelectric, and phase-transition memory devices based on bulk GeTe.

Abnormal vibrational anisotropy and thermal properties of a two-dimensional GeAs semiconductor.

Anisotropy in a crystal structure plays a striking role in determining the optical, electrical and thermal properties of the condensed matter. Here, we investigated in-plane vibrational anisotropy in a two-dimensional (2D) van der Waals (vdW)-layered GeAs narrow-gap semiconductor by combining microstructural characterization and polarization Raman spectroscopy. Interestingly, not only the intensities but also the Raman shifts in all modes evolved periodically with different symmetries as the polarization angle changed continuously, which could be well-analyzed using the Raman tensors and further interpreted from the phonon dispersion relations. More importantly, the temperature-dependent Raman intensities of the Raman modes in the range from 83 K to 823 K gave a thermal-related uniform constant, based on which key parameters, including the thermal expansion coefficient, Grüneisen constant and quasi-particle lifetime, could be directly derived, which were in line with the calculated predictions. This investigation provides a comprehensive understanding of structure-dependent optical anisotropy in 2D vdW-layered GeAs and suggests a new idea for exploring the thermal properties of related materials using temperature-dependent Raman spectroscopy.

Multiplexing potential of NIR resonant and non-resonant Raman reporters for bio-imaging applications.

Multiplexed imaging, which allows for the interrogation of multiple molecular features simultaneously, is vital for addressing numerous challenges across biomedicine. Optically unique surface-enhanced Raman scattering (SERS) nanoparticles (NPs) have the potential to serve as a vehicle to achieve highly multiplexed imaging in a single acquisition, which is non-destructive, quantitative, and simple to execute. When using laser excitation at 785 nm, which allows for a lower background from biological tissues, near infrared (NIR) dyes can be used as Raman reporters to provide high Raman signal intensity due to the resonance effect. This class of imaging agents are known as surface-enhanced resonance Raman scattering (SERRS) NPs. Investigators have predominantly utilized two classes of Raman reporters in their nanoparticle constructs for use in biomedical applications: NIR-resonant and non-resonant Raman reporters. Herein, we investigate the multiplexing potential of five non-resonant SERS: BPE, 44DP, PTT, PODT, and BMMBP, and five NIR resonant SERRS NP flavors with heptamethine cyanine dyes: DTTC, IR-770, IR-780, IR-792, and IR-797, which have been extensively used for biomedical imaging applications. Although SERRS NPs display high Raman intensities, due to their resonance properties, we observed that non-resonant SERS NP concentrations can be quantitated by the intensity of their unique emissions with higher accuracy. Spectral unmixing of five-plex mixtures revealed that the studied non-resonant SERS NPs maintain their detection limits more robustly as compared to the NIR resonant SERRS NP flavors when introducing more components into a mixture.

Construction of a novel nano-enzyme for ultrasensitive glucose detection with surface-enhanced Raman scattering

Fabricating more sensitive, stable and low-cost nanomaterials for the detection of glucose is important for the disease diagnosis and monitoring. Herein, we established a nanocomposite (polypyrrole bridging GO@Au@MnO2) as a novel surface-enhanced Raman scattering (SERS) nanoprobe for the quantitative detection of glucose in trace serum. Each component in the nanocomposites played an irreplaceable role in SERS detection of glucose. Polypyrrole (PPy) could act as Raman signal and extra SERS signal molecules didn’t need to be introduced; Graphene oxide (GO) and gold nanoparticles (Au NPs) could enhance Raman signal of PPy; Au NPs also acted as glucose oxidase, which can oxidize glucose to produce gluconic acid and hydrogen peroxide(H2O2); Manganese oxide (MnO2) further enhanced Raman signal of PPy and responded to hydrogen peroxide, which will induce the decrease of Raman intensity of PPy. Thus, glucose can be quantified according to Raman signal output of PPy, which displayed a liner range from 1 to 10 μM, with detectable limit of 0.114 μM. Because of the merits in sensitivity, convenience and versatility, the novel method shows large potential space for disease-related substance detection in the future.

Experimental and DFT Quantum Chemical Studies on Structural, Vibrational and Molecular Properties of Some Substituted 4-Phenylphenols

Fourier Transform infrared spectra (4000–400 cm−1) and Fourier Transform Raman spectra (4000–50 cm−1) were recorded for 2-chloro-4-phenylphenol (CP); 2-nitro-4-phenylphenol (NP); and 2-amino-4-phenylphenol (AP). 1H and 13C NMR spectra, along with UV-Vis spectra of the three samples were also measured. Quantum chemical calculations, at the level of DFT/B3LYP/6-311++G(d,p) theory were implemented to study their ground state geometry, vibrational wave numbers, infrared and Raman intensities, 1H and 13C NMR spectra, frontier molecular orbital parameters, NLO behavior, NBO properties, thermodynamic quantities, rotational constants and MESP behavior. TD-DFT variant was employed to simulate electronic transitions of these molecules. Observed and calculated vibrational frequencies agreed with an rms error 7.44, 8.98 and 6.97 cm−1, the corresponding RMSD values being 7.09, 9.39 and 6.59 cm−1 for CP, NP and AP, respectively. Experimental chemical shifts concurred, with their theoretical counterparts, with RMSD value, 0.19, 0.29 and 0.56 ppm for 1H NMR; and 6.34, 6.28 and 5.39 ppm for 13C NMR, respectively, in CP, NP and AP. This kind of agreement was also true for absorption maxima (λ max) of their electronic transitions in solution form. Frontier molecular orbitals were found useful to understand origin of electronic transition maxima and chemical reactivity of the three molecules which was supported by NBO analyses. The computations showed that the three molecules were potentially good for developing NLO materials. MESP investigations showed that the most reactive sites are at oxygen atoms in the three molecules. Moreover, we also made an attempt to understand the effect of deactivating (Cl, NO2) and activating (NH2) groups on certain properties of the three molecules. Presence of intra-molecular hydrogen bond was predicted in CP, NP and AP.

MoS2/Au/Ag Nanostructures for Ratiometric Surface-Enhanced Raman Scattering Determination of Pesticide Residues

Surface-enhanced Raman scattering (SERS) is a promising method for rapid and sensitive detection of pesticide residues. Herein, a three-dimensional MoS2/Au/Ag nanostructure was developed by employing composite systems containing semiconductors, noble metal nanoparticles, and internal standard. The MoS2 nanoflowers provide abundant anchors for the loading of plasmonic nanoparticles so as to generate intense electromagnetic “hot spots,” and the Au-MBN/Ag nanostructures can increase the accuracy of SERS detection with the calibration ability of the internal standard molecules (4-mercaptobenzonitrile, 4-MBN). Therefore, the MoS2/Au/Ag nanostructures integrate the advantages of high-enhancement SERS substrates and high-accuracy ratiometric Raman detection. Moreover, the nanostructures were fabricated via a facile procedure and showed high sensitivity, accuracy, and reproducibility in SERS detection. The nanostructures also showed good linear fitting between the ratios of Raman intensity and logarithmic concentrations of analytes in ratiometric SERS analysis. In addition, thiabendazole (TBZ) in apple juice was successfully detected by using the as-prepared SERS substrates with a limit of detection down to 0.1 ppm in practical applications. To conclude, the MoS2/Au/Ag nanostructures showed excellent SERS activity in ratiometric analysis and the substrate is expected to have great potential for the quantitative detection of low-concentration pesticide residues.

Graphene-Au nanosphere composite arrays and their enhanced SERS performance

The effective regulation of the absorption properties of Au nanosphere arrays, as well as effective enhancement of its SERS performance, are of great significance for its applications in the field of optical sensing. In this paper, the polyol synthesis method was used to fabricate monolayer and bilayer periodic Au nanosphere arrays, onto which monolayer graphene was coated, and the optical properties of the obtained composite structures were investigated. The results showed that the coverage of graphene enhances the absorption peak intensities of the monolayer and bilayer Au nanoarrays by up to 32.92%, and the resonance peaks were red-shifted by up to 17 nm. The Raman signals of graphene-covered monolayer and bilayer Au nanosphere composite arrays are 180 and 280 times stronger than that of pristine graphene, due to the combination of chemical enhancement and electromagnetic enhancement. And the detection sensitivity of R6G molecules by graphene-bilayer Au nanosphere composite arrays is 10−6 M. In addition, the staggered stacking of bilayer Au nanosphere arrays can provide more plasmonic hotspots, resulting in higher Raman intensity of the substrate. These results will pave the way for the fabrication of highly efficient SERS substrates by providing various promising candidates.

Raman imaging monitors the time-resolved response of A. thaliana to the artificial inhibition of PSII

The short-term (0–96 h) response of A. thaliana to the oxidative stress induced by PSII inhibitor metribuzin was examined using Raman spectroscopy. Whole leaves of wildtype (WT, Col-0) and ros1 mutant were scanned and changes in carotenoids were examined. Strong differences in Raman intensity distributions between WT and ros1 were observed. A stronger decrease of carotenoid v1(C=C) band intensity across the leaf was observed in ros1 after 48 h of exposure to metribuzin. It can be assumed that higher sensitivity to oxidative stress in ros1 mutant results in significantly faster degradation of carotenoids.

TNAs/g-C3N4/AuNPs heterojunction used as integrated device of photocatalytic degradation and SERS detection

Novel TiO2 nanotube arrays (TNAs) involved heterojunctions are widely used as photocatalyst for pollutants, while the residues are usually detected by other method. A device which integrates photocatalytic degradation and enhancing detection signals could increase the atom utilization and decrease detection cost. In this work, a novel TNAs/g-C3N4/AuNPs heterojunction was prepared and used as photocatalyst and SERS substrate simultaneously. Dye of crystal violet was successfully degraded, and the concentration of residue was detected by Raman spectroscopy. There was a good linear correlation between the Raman intensity and the concentration of crystal violet from 10−10 M to 10−6 M, and the limit of detection (LoD) was 8.79 × 10−11 M. Moreover, the prepared heterojunctions were used on wastewater and biological staining detection. In the applications of photocatalytic degradation and detection, the time-dependent SERS intensities demonstrated the photocatalytic degradation achieved more than 95% in 20 min, and almost completed in 25 min, and the heterojunctions showed good recyclability in at least 10 cycles. For SERS property, the original enhancement factor (EF) was calculated to be 7.43 × 106, then the EF descended slightly and remained with less than 90% attenuation after 10 cycles.

Time-resolved spectroscopic mapping of vibrational energy flow in proteins: Understanding thermal diffusion at the nanoscale.

Vibrational energy exchange between various degrees of freedom is critical to barrier-crossing processes in proteins. Hemeproteins are well suited for studying vibrational energy exchange in proteins because the heme group is an efficient photothermal converter. The released energy by heme following photoexcitation shows migration in a protein moiety on a picosecond timescale, which is observed using time-resolved ultraviolet resonance Raman spectroscopy. The anti-Stokes ultraviolet resonance Raman intensity of a tryptophan residue is an excellent probe for the vibrational energy in proteins, allowing the mapping of energy flow with the spatial resolution of a single amino acid residue. This Perspective provides an overview of studies on vibrational energy flow in proteins, including future perspectives for both methodologies and applications.

Fabrication of SERS Substrates Based on Porous- Si Nanostructures and Silver Nanoparticles for Application in Identification of Carbendazim

In this work, we present a quick and simple approach for fabrication of active-surface-enhanced Raman scattering (SERS) substrates based on silver nanoparticles (AgNPs) and porous silicon nanostructures (denoted by Si/Ag substrate). SERS substrates were fabricated by a 3-stage-process including the deposition of Au nanoparticles on Si wafer, etching for porous Si wafer, and coating Ag NPs on porous Si to produce SERS-active layer. To control the uniformity and cleanliness of the SERS substrate, sputtering technique was used to coat Au and AgNPs. The absorption spectra of the Si/Ag substrates exhibited a strong surface plasmon resonance absorption band of AgNPs at 425 nm. Using Si/Ag substrates to detect carbenzadim (CBZ) residue dispersed in acetone has shown that Si/Ag substrates morphology greatly affected the Raman intensity of carbendazim which opens up a promising and potential approach using SERS spectroscopy for food, environment and especially pharmaceutical applications.
 Keywords: Raman, SERS, plasmon, nanoparticles, carbendazim.
 

A general approach to hybrid platform of Au nanoparticles on monolayer semiconductor for ultrasensitive Raman enhancement of 2D materials and molecule detection

The hybrid platform combining two-dimensional (2D) materials and noble metals has great potential to further enhance the performance of surface enhanced Raman spectroscopy (SERS). However, the fabrication process of metallic nanostructures may bring damage to 2D materials and also lack of general applicability. Here, we develop an approach for the fabrication of Au nanoparticles (AuNPs) on monolayer semiconductor through gold mediated mechanical exfoliation followed by a controllable etching in KI/I2 solution. The proposed method and as-prepared hybrid SERS platform have been proved to have several advantages. (1) The fabrication of Au nanoparticles is mild and non-substrate-selective, which can be generally adapted to various substrate (e.g., ReSe2, MoS2 and PdSe2). (2) The hybrid platform of AuNPs/2D materials can effectively enhance the Raman signals of the underneath 2D materials. For example, the Raman intensity of monolayer PdSe2 in hybrid substrate is enhanced by about four times compared with that of bare PdSe2. (3) The hybrid platform can dramatically enhance the detection limits of R6G molecule down to picomolar, approximately three to four orders of magnitude lower than that of bare 2D material counterpart. Our work provides a general approach for the fabrication of high-performance SERS hybrid substrate.