Normal Raman Spectra Research Articles

Surface‐enhanced Raman scattering of Cis‐(dicyanomethylene) squarate ion on a silver substrate

AbstractThere has been increasing interest in the use of squaraines with different physicochemical properties. However, their applications in nanoplasmonics are limited. For that, detailed information on the interactions of squaraines with plasmonic nanostructures is required. In this work, we have studied the adsorption of the squaraine cis‐dicyanomethylene squarate (CDSQ) on the surface of Ag nanoparticles (AgNP). We have combined normal Raman, surface‐enhanced Raman scattering (SERS), and DFT calculations to get insights into the configuration of its adsorption. DFT calculations have been carried out using simplified models of different adsorption geometries for comparison with the results of the SERS experiments. A comparison between the relative intensities of the normal Raman and SERS spectra indicates that CDSQ is adsorbed perpendicular to the AgNP surface. The predicted spectra suggest that CDSQ is chemically adsorbed on the silver surface through one of its N atoms.

Probing the Adsorption Sites of 5‐amino‐2‐mercaptobenzimidazole Conformations on Colloidal Gold Nanoparticles Investigated by SERS and DFT

AbstractGold nanoparticles (AuNP) have shown great potential in biomedical applications due to their unique and tunable properties. Understanding the ligand conformation on AuNP has profound interest in nanoparticle research leading to a precise drug delivery system. Colloidal AuNPs were synthesized using trisodium citrate as reducing agent. Normal Raman spectrum (NRS) in solid and aqueous phase identifies that 5‐amino‐2‐mercaptobenzimidazole (5A2MBI) mainly exist as thione form. Surface enhanced Raman scattering (SERS) investigation of 5A2MBI depending on pH was performed to explore the adsorption mechanism of thiolate and thione conformations on AuNPs. Density functional theory (DFT) calculations were used to find the band assignments and molecular electrostatic potential (MEP) mapping to identify the electron rich site where metal can interact preferentially. Using SERS spectral analysis and DFT results, surface adsorption scheme was proposed. At basic pH, 5A2MBI adsorbs on AuNP as thiolate form bidentately via Sulfur and nitrogen atom of imidazole ring with tilted geometry. However, in acidic condition the thione form adsorbs on nanosurface with perpendicular orientation.

Physical insight into optical spectroscopy of chiral covalent organic pillars as molecular nanotubes

In this paper, we theoretically investigate chiral covalent organic pillars synthesized by connecting rim-desymmetrized macrocycle, stimulated by recent experimental synthesis [Nat. Synth. 2023, 2, 395–402]. Transition densities reveal physical mechanism of spectral intensity in one-photon absorption (OPA) and two-photon absorption (TPA), and solvent effect strongly influence the spectral profile and intensity in TPA, but little influence the spectrum in OPA. The intra-unit and inter-unit charge transfer are visualized in five phenylenediamine linkers. The chirality of covalent organic pillars is revealed by electronic circular dichroism (ECD) spectroscopy, and interpreted by the visualized transition electric and magnetic dipole moments. Molecular vibrational information is demonstrated by normal and resonance Raman spectra and vibrational modes. The visualization of static and dynamic (hyper) polarization can reveal the physical mechanisms of polarization dependent normal and resonance Raman scattering. Our results provide deeper understanding on the physical mechanism of chiral covalent organic pillars as molecular nanotubes.

Infrared Raman spectroscopy enables noninvasive biochemical assessment of skin tissue and the thermal stability

Raman-active modes of human skin and pork belly have been studied systematically by a near-infrared Raman spectrometer with an exciting laser of 1064nm. The main components and quantitative determination of pork belly are extracted by fitting the Raman spectra with the normalized Raman spectra of biochemical reagents such as collagen, elastin, triolein, fibronectin, fibrin, and hyaluronic acid. It demonstrates that the main components and quantity are various at different locations of pork belly, while the main components of human skin are similar to those of pig skin. In a further step, the evolution of the heating time-dependent Raman modes of isolated pig skin has been investigated for the mechanism of burnt skin. One can find that the spatial structure and main components of skin have an excellent thermal stability in the temperature range from -120 to 200∘C, which is confirmed by the temperature dependent Raman spectra of isolated pig skin, microporous acellular dermal matrix (MADM) as well as their corresponding biochemical reagents (collagen, elastin, triolein, etc.). These results help understand the mechanism of the living skin burnt by fire or hot water, and supplies an alternative technology for surgeons to diagnose the depth of a burn injury in time.

Spectral Physics of Stable Cu(III) Produced by Oxidative Addition of an Alkyl Halide.

In this paper, we theoretically investigated spectral physics on Cu(III) complexes formed by the oxidative addition of α-haloacetonitrile to ionic and neutral Cu(I) complexes, stimulated by recent experimental reports. Firstly, the electronic structures of reactants of α-haloacetonitrile and neutral Cu(I) and two kinds of products of Cu(III) complexes are visualized with the density of state (DOS) and orbital energy levels of HOMO and LUMO. The visually manifested static and dynamic polarizability as well as the first hyperpolarizability are employed to reveal the vibrational modes of the normal and resonance Raman spectra of two Cu(III) complexes. The nuclear magnetic resonance (NMR) spectra are not only used to identify the reactants and products but also to distinguish between two Cu(III) complexes. The charge difference density (CDD) reveals intramolecular charge transfer in electronic transitions in optical absorption spectra. The CDDs in fluorescence visually reveal electron-hole recombination. Our results promote a deeper understanding of the physical mechanism of stable Cu(III) produced by the oxidative addition of an alkyl halide.

Activated carbon@silver nanoparticles conjugates as SERS substrate for capturing malathion analyte molecules for SERS detection

AbstractMalathion is one of the commonly used organophosphate pesticides known to attack the central nervous system, posing a risk to humans and other animals upon exposure. The surface enhanced Raman spectroscopy (SERS) has been identified as an indispensable tool for chemical and biomolecular sensing. In this work, the facile fabrication of activated carbon (AC)‐based colloidal SERS active substrate dubbed AC@AgNPs was designed by trapping AgNPs on the surface of AC for detection of varying concentrations of malathion. Apart from the higher concentrations of malathion, the rest of the normal Raman spectra of malathion standard solutions exhibited weak Raman signals. The intensity of peaks for 0.47 mg L−1 were nearly non‐existent which is an indication that the malathion pesticide could only be detected up to 0.95 mg L−1 when using silica wafer. On the contrary, all the SERS spectra of malathion in wheat extracts adsorbed on AC@AgNPs substrate exhibited strong Raman signals. Quantitative analysis of malathion was performed by regression models developed using PLSR built with the raw spectra (no pretreatment), SNV‐PLSR, and SNV‐CARS‐PLSR. The model with the most remarkable performance was established by using SNV‐PLSR with r = 0.9869 and RPD = 4.61. This research shows that the proposed method can rapidly detect malathion residues in wheat, suggesting that it could be adopted for production process monitoring of other related food products to guarantee their safety for human and animal consumption.

Label-Free identification of fentanyl analogues using surface-enhanced Raman scattering and machine learning algorithm

The identification and analysis of drug abuse is crucial for controlling and combating this global public health problem. In this regard, surface-enhanced Raman spectroscopy (SERS) is a highly sensitive detection technique that is fast, non-destructive, and widely used in various fields. In this study, Ag@Au nanowires (Ag@Au NWs) were synthesized using the seed growth method. The SERS performance of Ag@Au NWs was evaluated using R6G as a probe molecule, and the minimum detection concentration of R6G was found to be 10−9 M. Similarly, the minimum detection concentration of fentanyl was observed to be 10−7 M. Furthermore, the study also involved performing routine Raman detection on 10 fentanyl analogs that were structurally similar. Additionally, density functional theory (DFT) calculations were carried out to distinguish between these substances. The Raman spectra calculated by DFT provided vibrational frequencies and normal mode assignments for each species. By comparing normal Raman spectra, SERS and DFT calculated spectrogram data, we assigned characteristic peaks of 10 substances. Finally, principal component analysis was used to reduce the dimension of the data, and support vector machine were further used to classification of SERS spectral data of the above 10 substances, with an accuracy of 96.4%.

Electrochemical Surface-Enhanced Raman Spectroscopy for Aniline Adsorbed on Silver Electrodes: A DFT Study of the Anharmonic Effects of Amino Wagging Vibrational Modes

Aniline molecules exhibit a special amino wagging vibrational mode on the surface of the silver electrode. The corresponding Raman peak not only displays enhanced Raman signals in surface-enhanced Raman spectroscopy (SERS) experiments but also shows a shift toward higher wavenumbers. This work adopts a molecule-metal cluster model to investigate the normal Raman spectra of aniline molecules and their SERS spectra adsorbed on silver electrode surfaces by density functional theory (DFT) approaches. Based on the harmonic approximation, the influence of anharmonic effects on the normal Raman and SERS spectra of aniline is further considered. Our calculated results of the anharmonic Raman spectra of free aniline molecules show that their anharmonic effect mainly comes from the amino wagging vibrational mode. Then, the anharmonic Raman spectra of aniline binding to neutral and positively charged silver clusters were calculated and compared with the harmonic Raman spectra. It was found that the coupling effect between vibrational modes also greatly affects the Raman spectral characteristics. Finally, the excited state energy gap and pre-resonance Raman spectra of the aniline–silver cluster complexes are studied, and it is believed that there may be a charge-transfer enhancement mechanism.

Cladosporium cladosporioides (strain Clc/1): a candidate for low-density polyethylene degradation

BackgroundPlastic is one of the most widely used materials worldwide in various fields, including packaging and agriculture. Its large quantities require proper disposal and for this reason more and more attention is paid to the issue of degrading plastic. Thanks to the production of non-specific enzymes, fungi are able to attack complex and recalcitrant xenobiotics such as plastics. In recent years, several spectroscopic methods were used to study the plastic degradation ability of different fungal species. Among these, Fourier transform infrared (FT-IR) and FT-Raman spectroscopy techniques are the most used. Surface-enhanced Raman scattering (SERS) spectroscopy is a powerful technique which uses metal nanoparticles (NPs) to enhance the Raman signal of molecules adsorbed on the NPs surface. In this work, the isolation of different fungi from field-collected plastic debris and the ability of these isolates to growth and colonizing the low-density polyethylene (LDPE) were explored by using scanning electron microscope (SEM), attenuated total reflectance-Fourier transform infrared (ATR-FTIR) and SERS spectroscopies.ResultsForty-seven fungal isolates belonging to 10 genera were obtained; among them only 11 were able to grow and colonize the LDPE film. However, after 90 days trial, only one isolate of Cladosporium cladosporioides (Clc/1) was able to carry out the initial degradation of the LDPE film. In particular, based on SEM observations, small cavities and depressed areas of circular shape were visible in the treated samples. Additionally, ATR-FTIR, normal Raman and SERS analyses supported the structural changes observed via SEM. Notably, ATR-FTIR and normal Raman spectra showed a significant decrease in the relative intensity of the methylene group bands. Similarly, the SERS spectra of LDPE after the fungal attack, confirmed the decrease of methylene groups bands and the appearance of other bands referring to LDPE polyphenolic admixtures.ConclusionsThese results suggest that Cladosporium cladosporioides Clc/1 is able to carry out an initial degradation of LDPE. Moreover, combining ATR-FTIR, Raman and SERS spectroscopies with SEM observations, the early stages of LDPE degradation can be explored without any sample pretreatment.Graphical

Experimental and DFT investigations of SERS Profile of a Bioactive Hydroxybenzamide Derivative

AbstractSurface enhanced Raman scattering (SERS) experiments were used to evaluate the adsorption and orientation of 5‐bromo‐N‐[4‐bromo‐3‐(trifluoromethyl)phenyl]‐2‐hydroxybenzamide (BTB) adsorbed on silver colloid and the results were compared to normal Raman spectra of BTB. Density functional theory (DFT) analysis was used to theoretically validate the observed spectra of BTB. BTB and BTB‐Ag6 molecular structures were optimized at first. The BTB molecule is adsorbed with the Ag6 with energy of −10.67 eV through atoms, O9, F22 and Br17 atoms. The charge transfer interaction between Ag6 clusters and BTB, which lead to BTB adsorption on Ag6 metal clusters, is confirmed by the Frontier Molecular Orbitals (FMOs) study. The BTB chemisorbed on silver with a tilted orientation, as confirmed by SERS spectral analysis and the accompanying results were theoretically justified.

Plastic spectral interference in the biological characterization by Raman or SERS spectroscopy

AbstractRaman spectroscopy and surface‐enhanced Raman spectroscopy (SERS) have been widely applied for characterizing biological samples. Nowadays, it is routine to use plastic products for biological sample preparation and analysis during spectral acquisition. However, plastic containers produce undesirable signals severely interfering with the spectra of biological samples, which is neglectable and misleading. Herein, we unveiled the physical and chemical interference of plastic products in the process of Raman spectroscopy characterization. Firstly, we revisited a representative case of interference from polystyrene plastics on normal Raman and SERS spectra of cysteine and demonstrated its physical interference due to misfocusing. Furthermore, we testified SERS spectra of aqueous solution stored in several plastic materials commonly used in biological experiments. The spectra indicate strong spectral interference from the leaching chemicals. To address these problems, we proposed a series of strategies to avoid interference from plastic products in the Raman and SERS characterization of biological samples.

A density functional theory study of van der Waals interaction in carbon nanotubes

AbstractIn this article, we investigate the effect of van der Waals force in zigzag carbon nanotubes (CNTs) including single‐wall CNT (SWCNT) and double‐walled CNT (DWCNT) structures with several interaction configurations. The solid‐state density functional theory is employed to calculate the geometric optimization, normal mode frequencies, and IR and Raman spectra with the periodic boundary condition. For SWCNTs, we find that the Raman intensity is not affected by the tube diameter or the electronic structure. The IR absorption, however, increases with the tube diameter. We find that the close metallicity of the electronic structure has a significant impact on the IR simulations. When the van der Waals force is applied outside the CNTs at a distance longer than 3.0, the effect on Raman spectra is minimal but some effects can still be confirmed by IR absorption. When the van der Waals force acts inside the CNTs, the effect on the spectrum can be observed, especially at a distance of 2.8 Å, both IR and Raman can be significantly enhanced in many modes.

Effect of the Gas Temperature on Agglomeration of Au Nanoparticles Synthesized by Spark Discharge and Their Application in Surface-Enhanced Raman Spectroscopy

In this work, we have, for the first time, experimentally verified the hypothesis of reducing the agglomeration rate of aerosol nanoparticles produced by spark discharge upon decreasing the carrier gas temperature in the range of 24 °C to –183 °C. The synthesis of nanoparticles was implemented as a result of spark ablation of electrodes manufactured from Au with a purity of 99.998% installed in a specially designed gas chamber dipped into liquid nitrogen (−196 °C) to cool down the carrier gas supplied through one of hollow electrodes. It follows from the analysis of transmission electron microscopy images that both the average size of primary nanoparticles and the degree of their sintering become lower if the gas is cooled. For example, in the case of using nitrogen as a carrier gas, the average size of primary nanoparticles decreases from 9.4 nm to 6.6 nm as the gas temperature decreases from 24 °C to –183 °C. This also causes the aggregates to become more branched, manifested by the reduction in their solidity from 92% to 76%. The agglomeration model of Feng based on Smoluchowski theory was employed to calculate particle size distributions that were found to be consistent with the experimental data. The gold nanoparticles synthesized at room and cryogenic temperatures of the carrier gas (N₂, Ar + H₂, He) were used to pattern plasmonic nanostructures on ceramic alumina substrates by using aerosol jet printing technology for the purpose of demonstrating the possibility of their application in surface-enhanced Raman spectroscopy (SERS). The SERS enhancement factor was estimated at 2 × 106 from the analysis of SERS and normal Raman spectra of 1,2-bis(4-pyridyl)ethylene used as an analyte.

Prediction of electronic and optical properties of monoclinic 1T’-phase OsSe2 monolayer using DFT principles

Following the discovery and characterization of transition metal dichalcogenides (TMDs) monolayers, which has created interest in last few years, it would seem natural to explore new possibilities of monolayers. Thus, using the density functional theory (DFT), we investigated the structural, electronic, vibrational and thermodynamic properties, and optical absorption of osmium selenide monolayer (monoclinic 1T’-phase OsSe 2 monolayer). We perform the calculations using the approaches based on the generalized gradient (GGA) and the local density (LDA) approximations for the optimized structure with the minimum energy, as well as we use the hybrid exchange–correlation functional HSE06 recommended in the literature for bandgap energy estimation. An indirect bandgap E g = 0 . 85 eV, E g = 0 . 95 eV and E g = 1 . 80 eV was obtained within the LDA-CAPZ, GGA-PBE and HSE06 level of calculation, respectively. The vibrational normal modes, infrared and Raman spectra were obtained and assigned in the frequency range of 0 − 400 c m − 1 together with the phonon dispersion relation. In addition, the optical absorption was shown to be sensitive to the plane of polarization of the incident light mainly in the range of UV radiation. The thermodynamic potentials and specific heat at constant volume were calculated and analyzed. These results indicated that the monoclinic 1T’-phase OsSe 2 structure could be potentially synthesized and bringing new potential technological applications.

Surface-enhanced Raman scattering of DNA molecules on silver electrodes modified with silver nanoparticles

High-quality surface-enhanced Raman spectra (SERS) of calf thymus DNA, apple single-stranded DNA, and rice single-stranded DNA are obtained by using rough silver electrodes modified with silver nanoparticles as the substrate. The appearance of fruitful Raman-active modes enables to identify the fine vibrational modes of DNA molecules of which the normal Raman spectra are not easy to be measured. It is concluded that the silver electrode modified by silver nanoparticles is a highly sensitive SERS substrate for DNA analysis, due to the uniformly aligned silver nanoparticles in combination with additional contributions of electrode potentials. Furthermore, the SERS spectra of these DNA molecules at varying potential conditions also provide information on the adsorption behavior of these DNA molecules on the silver. It is anticipated that the DNA molecules may tend to lie flat on the nanosilver surface; with increasing negative potentials, the spectra exhibit dramatic fluctuation, which could be associated with interface oxidation and changes of the adsorption behaviors in view of the different hydrophobicity/hydrophilic nature and electrostatic interactions of the phosphate base and the electrode surface.

Synthesis and stockpile of polyynes in paraffin as well as extraction for preparing single-walled carbon nanowires (LLCCs@SWCNTs)

The polyynes are extremely unstable and reactive, and their long-term storage is still a challenge. We first used paraffin as an organic solvent in a submerged electric arc for preparing polyynes, placed them in solid paraffin at room temperature, and assessed their storage stability. One month late, we extracted the polyynes from paraffin, and separated the extracted polyynes by HPLC for obtaining single isolated polyynes. The polyynes extracted from solid paraffin were also purified and filled into open-ended single-walled carbon nanotubes (SWCNTs) to obtain polyynes@SWCNTs samples, which were heated to prepare single-walled carbon nanowires (SWCNWs, LLCCs@SWCNTs), made of long linear carbon chains (LLCCs) inserted inside SWCNTs. The 4L band at 7250 cm−1 was observed in the normal Raman spectra of SWCNWs for the first time. This work solves the problem of long-term storage of polyynes, and opened new avenues for exploring physical properties and applications of polyynes, LLCCs and SWCNWs.

Surface-Enhanced Raman Spectroscopy (SERS) for characterization SARS-CoV-2

We used SERS with silver nanoparticles (AgNPs) as the active substrate to develop a, simple, quick, and accurate method for the detection and characterization SARS-CoV-2 without the need for RNA isolation and purification. Inactivated SARS-CoV-2 was used. The SERS signals were more than 105times enhanced than the normal Raman (NR) spectra. The SERS spectra of SARS-CoV-2 fingerprint revealed pronounced intensity signals of nucleic acids; aromatic amino acid side chains: 1007 cm−1 (Phe marker), 1095 cm−1 (CN and PO2− markers), 1580 cm−1 (Tyr, Trp markers). Vibrations of the protein main chain: 1144 cm−1 (CN and NH2 markers), 1221 cm−1 (CN and NH markers), 1270 cm−1 (NH2 marker), 1453 cm−1 (CHCH2 marker). All of these biomolecules could be adsorbed on the AgNPs surface's dense hot patches. The intensity of the SERS band varied with the concentration of SARS-CoV-2, with a virus detection limit of less than 103 vp/mL and RSDs of 20 %.

SERS chemical enhancement of 2,4,5-trichlorophenoxyacetic acid absorbed on Au20, its Au-Ag and Au-Cu bimetallic clusters: a DFT study

The surface-enhanced Raman scattering (SERS) phenomenon of 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) adsorbed on Au20 pyramidal cluster is studied by Density Functional Theory. All possible adsorption configurations between the adsorbate and the Au20 clusters are evaluated and the vibrational assignments of normal Raman and SERS spectra are analyzed. The result shows that the most stable adsorption configuration of the adsorbate on Au20 cluster is formed by the interaction of O13 (-COOH group) and Au atom. The SERS activities of 2,4,5-T adsorbed on 1:1 Au-Ag and Au-Cu bimetallic clusters are also investigated. For bimetallic clusters, the substitution of 1, 4 or 10 Au atoms by Ag/Cu is performed by replacing the respective number of Ag/Cu atoms in 1, 2 and 3 parallel layers of the Au20 cluster. While the Raman scattering activity of the Au10Ag10 cluster is comparable to that of the Au20 cluster, it is remarkably higher than the Au10Cu10 cluster. This observation is explained by the molecule-to-cluster charge transfer process. The obtained results are hoped to contribute to better design of SERS-based analytical sensors for mobile and on-site applications.

Surface-Enhanced Raman Spectroscopy for the Investigation of Chromogenic Motion Picture Films: A Preliminary Study

In the present work, surface-enhanced Raman spectroscopy (SERS) is proposed for the identification of dyes in chromogenic films. These substances, which are generated within the film itself during the colour development process, are first studied on reference molecules synthesised for the purpose and, subsequently, on dyes extracted from a chromogenic film. SERS spectroscopy proved to be an efficient technique for their investigation, providing a proof of concept for its applicability for further studies on this complex topic. SERS spectra of the reference molecules were also compared with normal Raman spectra, acquired with a spectrometer based on SSE™ technology. Furthermore, the excellent SERS properties of anisotropic nanomaterials, such as silver nanostars, have been confirmed.

Interlayer Coulomb interaction in twisted bilayer graphene nanofragments characterized by the vibrational mode of Gr+ band

Tip enhanced Raman spectra of AB-stacked bilayer graphene (BLG) and twisted bilayer graphene (TwBLG) nanofragments have been studied by using density functional theory. Different from a normal Raman spectrum of BLG, a unique Raman band Gr+ is observed in its tip enhanced Raman spectrum and assigned as a split of the G band. We attribute this split to the nonuniformity distribution of the charge on carbon atoms. Compared with BLG, the Gr+ band intensity of TwBLG is dramatically enhanced at a small twist angle and decreases with the angle increasing. Interlayer Coulomb interaction represented in the Gr+ mode of TwBLG matches well with the atom vibration strength distribution of the Gr+ band at different twist angles, and thereby the properties of the Gr+ band can be tuned by the twisted angle. The results may help to further understand the Raman spectra of TwBLG and provide deep insights for exploring vibrational modes of two-dimensional nanomaterials.