Raman Enhancement Research Articles

Collision Enhanced Raman Scattering (CERS): An Ultra-High Efficient Raman Enhancement Technique for Hollow Core Photonic Crystal Fiber Based Raman Spectroscopy Gas Analyzer.

Raman enhancement techniques are essential for gas analysis to increase the detection sensitivity of a Raman spectroscopy system. We have developed an efficient Raman enhancement technique called the collision-enhanced Raman scattering (CERS), where the active Raman gas as the analyte is mixed with a buffer gas inside the hollow-core photonic-crystal fiber (HCPCF) of a fiber-enhanced Raman spectroscopy (FERS) system. This results in an enhanced Raman signal from the analyte gas. In this study, we first showed that the intensity of the 587 cm-1 stimulated Raman scattering (SRS) peak of H2 confined in an HCPCF is enhanced by as much as five orders of magnitude by mixing with a buffer gas such as helium or N2. Secondly, we showed that the magnitudes of Raman enhancement depend on the type of buffer gas, with helium being more efficient compared to N2. This makes helium a favorable buffer gas for CERS. Thirdly, we applied CERS for Raman measurements of propene, a metabolically interesting volatile organic compound (VOC) with an association to lung cancer. CERS resulted in a substantial enhancement of propene Raman peaks. In conclusion, the CERS we developed is a simple and efficient Raman-enhancing mechanism for improving gas analysis. It has great potential for application in breath analysis for lung cancer detection.

Plasmonic cellulose microfilament assisted SERS detection in microfluidics

Limited by the narrow enhanced area of nanoscale on the metal surface, the sensitivity of surface-enhanced Raman spectroscopy (SERS) detection in solution is usually much lower than the detection in a solid substrate, which is dramatic in microfluidics for online detection. In this work, a cellulose microfilament embraced by Ag nanoparticles, called plasmonic cellulose microfilament, is located in a microchannel for SERS detection in microfluidics. Benefiting from the congestion caused by the plasmonic cellulose microfilament in a microchannel, the trace molecule in the solution is much easier to gather in Ag nanoparticles for Raman enhancement. To obtain high sensitivity, the structure of plasmonic cellulose microfilament is optimized. The SERS spectra collected in this novel microfluidics demonstrate that the plasmonic cellulose microfilament presents a high sensitivity at 10−13 M and good reproducibility in SERS detection. In addition, automatic identification of urea presence or absence was achieved based on deep learning (DL) here. The results show excellent diagnostic accuracy (99 %), which suggests that a fast, label-free urea screening tool can be developed. These results point out this SERS microfluidics with plasmonic cellulose microfilament has a great application prospective in online SERS detection with high sensitivity.

Plasmon-enhanced photoluminescence spectroscopy of a single molecule in the subnanometer cavity

We investigate the influence of quantum tunneling on the plasmon-enhanced electric field and photoluminescence spectroscopy in the subnanometer nanogap between two adjacent Ag nanoparticles using the quantum-corrected method. We analyze the contribution of nonlocal dielectric response at different separation distances to plasmon-enhanced fluorescence using the D-parameters method. Our results demonstrate the high sensitivity of the quantum-induced effect and nonlocal dielectric response to the separation distance. In cases of shorter separation distances, the localized fields start to decrease due to tunneling current short circuit, resulting in a substantial reduction in Raman enhancement. Additionally, the lower quantum yield caused by loss of electron-hole pairs further quenches the fluorescence intensity in the narrowing gaps. We achieve calculated Raman and fluorescence enhancement factors of 13 and 6 orders of magnitude. Under excitation wavelengths of 660 nm, we achieve spatial resolutions of 1.5 nm and 2.2 nm for Raman and fluorescence spectroscopy.

Platform for surface-enhanced Raman scattering in layered quantum materials

Raman spectroscopy has been used to study transition metal dichalcogenides (TMDs), quantum materials with promising properties. However, monolayer (1L) TMDs have limited observable Raman modes due to low light absorption. In this study, we propose a potential platform that can effectively enhance Raman scattering and increase the number of observable Raman modes in 1L TMDs. To enhance Raman scattering in 1L TMDs, a platform was fabricated by forming large-scale periodic arrays of gold micropillars (MPs) using conventional photolithography followed by gold film deposition. 1L WSe2 was transferred onto the Au MPs and Raman scattering was observed. The Raman intensity of 1L WSe2 on Au MPs was 19 times higher than that of 1L WSe2 on SiO2 plates, and Raman modes difficult to detect in typical 1L WSe2 are observed. Confocal Raman spectroscopic mapping revealed that the strong local Raman enhancement at the edge of the Au MP resulted in the amplification of Raman scattering in 1L WSe2 on Au MP. This local field enhancement was theoretically verified using finite difference time domain (FDTD). The platform has industrial advantages and wide applicability due to its low cost, simple process, large controllable area, and short process time.

Gold Nanoparticle‐Coated Bioceramics for Plasmonically Enhanced Molecule Detection via Surface‐Enhanced Raman Scattering

Herein, feasibility of plasmonically enhanced molecule detection via surface‐enhanced Raman scattering for ceramics that are commonly used as bone or tooth replacement materials is evaluated. Open cell foams of Bioglass 45S5, the commercial hydroxyapatite‐based product Bio‐Oss, and bioinert zirconia‐toughened‐alumina (ZTA) are coated with Au nanoparticles via colloidal deposition to introduce plasmonic effects. Depending on the pore size, gold‐functionalized plasmonic porous Bioglass shows effective Raman enhancement factor (eEF) up to , while depositing gold nanoparticles on Bio‐Oss and porous ZTA resulted in eEF of and respectively. The performance of the plasmonic porous bioceramics under simulated biological conditions is examined in situ in the biological medium fetal bovine serum (FBS) and during extended incubation in mineralizing simulated body fluid (SBF). Most notably, the plasmonic porous Bioglass still delivered an eEF around after 28 days of incubation in SBF, indicating promising stability in simulated biological conditions without significant difference in SBF bioactivity before and after Au deposition. Accordingly, the plasmonically enhanced porous bioceramics offer the possibility for real‐time and sensitive molecule detection at SBF and FBS conditions and can be further developed for sensing of specific biomarkers, for example, in the context of osseointegration of bone replacement materials.

Flexible silver ring SERS substrate array fabrication by vortex evaporation method and its application for high sensitive detection of sulfonamides residues in water

Flexible surface-enhanced Raman scattering (SERS) substrates have driven much attention in Raman detection of environmental pollutants due to their unique advantage of sensitivity, low-cost and fast sampling. However, it is still a high challenge for fabricating flexible SERS substrate with high reproducibility and sensitivity. In this study, we developed a novel silver nanoparticle assembly method based on vortex evaporation method to batch assemble multiple silver ring (SR) SERS substrate array on a parafilm. The flexible SR SERS substrates were uniformly covered with active silver nano-islands, which significantly improved the Raman signal intensity of the targets by 126 folds and showed 5 times Raman enhancement compared with the common SERS substrate by direct evaporation method, and also represented the unique bending response properties of flexible substrate. The Raman detection for sulfonamide residues in water with SR SERS substrate represented high sensitivity, which limits of detection for Sulfapyridine and Sulfacloropyridazine were 9.46 and 9.3 × 10−8 mol/L, respectively. The quantitative relationships were obtained in wide linear range of 4 orders with good accuracy and stability. The flexible SR SERS substrate array can be large-scale prepared with good repeatability, stability, and low-cost, which has promising applications for trace contaminants detection in water environment.

Metal–metal bonding, electronic excitations, and strong resonance Raman effect in 2D layered α-MoCl3

Covalent bonding between transition metal atoms is a common phenomenon in honeycomb lattices of layered materials, which strongly affects their electronic and magnetic properties. This work presents a detailed spectroscopic study of α-MoCl3, 2D van der Waals material with covalently bonded Mo2 dimers, with a particular focus on the Mo–Mo bonding. Raman spectra of α-MoCl3 were studied with multiple excitation laser lines chosen in different parts of the absorption spectrum, while polarization measurements aided in the symmetry assignment of the observed modes. Furthermore, far-IR measurements and (Density Functional Theory) DFT phonon computations were performed to complete vibrational assignment. Polarized absorption, PL, and photoelectron spectroscopy supported by DFT calculations were employed to understand the consequences of the Mo–Mo bonding for the electronic structure and the localization/delocalization balance in d3–d3 interactions. A coupling of dimerization-related structural and electronic properties was revealed in the strong resonance Raman enhancement of the Mo–Mo stretching mode at 153 cm−1 when the excitation laser matched the electronic transition between σ-bonding and antibonding orbitals of the Mo2 dimer (σ → σ*). The deeper understanding of the metal–metal bonding and identification of the vibrational and electronic spectroscopic signatures of the dimerization will be of great use for the studies of electron delocalization in magnetic van der Waals materials.

Raman, UV-Vis Absorption, and Fluorescence Spectroelectrochemistry for Studying the Enhancement of the Raman Scattering Using Nanocrystals Activated by Metal Cations.

Raman signal enhancement is fundamental to develop different analytical tools for chemical analysis, interface reaction studies, or new materials characterization, among others. Thus, phenomena such as surface-enhanced Raman scattering (SERS) have been used for decades to increase the sensitivity of Raman spectroscopy, leading to a huge development of this field. Recently, an alternative method to SERS for the amplification of Raman signals has been reported. This method, known as electrochemical surface oxidation-enhanced Raman scattering (EC-SOERS), has been experimentally described. However, to date, it has not yet been fully understood. In this work, new experimental data that clarify the origin of the Raman enhancement in SOERS are provided. The use of a complete and unique set of combined spectroelectrochemistry techniques, including time-resolved operando UV-vis absorption, fluorescence, and Raman spectroelectrochemistry, reveals that such enhancement is related to the generation of dielectric or semiconductor nanocrystals on the surface of the electrode and that the interaction between the target molecule and the dielectric substrate is mediated by metal cations. According to these results, the interaction metal electrode-nanocrystal-metal cation-molecule is proposed as being responsible for the Raman enhancement in Ag and Cu substrates. Elucidation of the origin of the Raman enhancement will help to promote the rational design of SOERS substrates as an attractive alternative to the well-known SERS phenomenon.

Lab-on-Fiber Sensors with Ag/Au Nanocap Arrays Based on the Two Deposits of Polystyrene Nanospheres.

Surface-enhanced Raman spectroscopy (SERS) can boost the pristine Raman signal significantly which could be exploited for producing innovative sensing devices with advanced properties. However, the inherent complexity of SERS systems restricts their further applications in rapid detection, especially in situ detection in narrow areas. Here, we construct an efficient and flexible SERS-based Lab-on-Fiber (LOF) sensor by integrating Ag/Au nanocap arrays obtained by Ag/Au coating polystyrene nanospheres on the optical fiber face. We obtain rich "hot spots" at the nanogaps between neighboring nanocaps, and further achieve SERS performance with the assistance of laser-induced thermophoresis on the metal film that can achieve efficiency aggregation of detected molecules. We achieve a high Raman enhancement with a low detection limitation of 10-7 mol/L for the most efficient samples based on the above sensor. This sensor also exhibits good repeatability and stability under multiple detections, revealing the potential application for in situ detection based on the reflexivity of the optical fiber.

Modulation of photo-induced Raman enhancement in Ag nanoparticles deposited on nanometer-thick TiO2 films. An interplay between plasmonic properties and irradiation energy

Photo-induced enhanced Raman spectroscopy (PIERS) is an innovative technology that offers additional enhancement in Raman signal compared to surface-enhanced Raman spectroscopy (SERS). In this study, we fabricated nanohybrids consisting of silver nanoparticles on an ultra-thin anatase film using a photoreduction method. This approach allowed for the controllable synthesis of SERS and PIERS nanoplatforms, characterized by oval-shaped nanoparticles, yet varying in size and surface coverage, leading to distinct plasmonic properties. A mere 15-minute UV pre-treatment with low photon density already initiated significant charge-transfer processes followed by Raman spectra under non-resonant conditions of the molecule and estimated by enhancement factor in the range of 12–––17. This phenomenon was observed for a molecular monolayer of a thiol derivative. Not only boosting electron migration appeared. This unique interface of the Ag-anatase composite undoubtedly contributed to extended relaxation times of photo-induced enhancement. Furthermore, we investigated how plasmonic and morphological features of the nanoplatforms, in conjunction with UV and Vis illumination, modulated the migration of photoinduced electrons from the semiconductor to the metal. These findings highlighted the variety of processes contributing to the creation of efficient PIERS materials.

A multiscale 3D hotspot-rich nanostructured substrate for biomolecular detection of SARS-CoV-2

The current fabrication methods of surface-enhanced Raman scattering (SERS) chips used for biological detection mostly require antibodies conjugated on nanostructured metals or additionally connected to a reporter, which leads to complicated fabrication processes and increases the cost of these chips. More importantly, only a single-layer (2D) signal source is generated on the substrate of the chip, resulting in poor sensitivity. Herein, we constructed a single-component, multiscale, three-dimensional SERS (M3D-SERS) substrate from silver nanowires (AgNWs) packing. According to our results, the Raman enhancement effect of the M3D-SERS substrate was related to the degree of AgNWs stacking along the z axis. In addition, the light source-dependent plasmonic partition and hotspot formation of the M3D-SERS substrate were evaluated by the finite integration technique to prove that M3D-SERS offers advantages, with isotropic localized surface plasmon resonance as well as homogeneous hotspot distribution, for SERS over its 1D and 2D counterparts. Experimentally, the optimal construction of the M3D-SERS chip was explored and established based on the Raman signal enhancement of bovine serum albumin, and consequently, the efficiency of the M3D-SERS chip in detecting SARS-CoV-2-related biomolecules was investigated based on the detection superiority to biomolecules. This study demonstrates a simple, label-free, pre-treatment-free potential biosensor technology that can be used in healthcare units. Furthermore, in combination with a suitable laser light source, this technology can be applied for efficient detection in point-of-care tests with a handheld spectrometer.

A New Gas Analysis Method Based on Single-Beam Excitation Stimulated Raman Scattering in Hollow Core Photonic Crystal Fiber Enhanced Raman Spectroscopy.

We previously developed a hollow-core photonic crystal fiber (HCPCF) based Raman scattering enhancement technique for gas/human breath analysis. It enhances photon-gas molecule interactions significantly but is still based on CW laser excitation spontaneous Raman scattering, which is a low-probability phenomenon. In this work, we explored nanosecond/sub-nanosecond pulsed laser excitation in HCPCF based fiber enhanced Raman spectroscopy (FERS) and successfully induced stimulated Raman scattering (SRS) enhancement. Raman measurements of simple and complex gases were performed using the new system to assess its feasibility for gas analysis. We studied the gas Raman scattering characteristics, the relationship between Raman intensities and pump energies, and the energy threshold for the transition from spontaneous Raman scattering to SRS. H2, CO2, and propene (C3H6) were used as test gases. Our results demonstrated that a single-beam pulsed pump combined with FERS provides an effective Raman enhancement technique for gas analysis. Furthermore, an energy threshold for SRS initiation was experimentally observed. The SRS-capable FERS system, utilizing a single-beam pulsed pump, shows great potential for analyzing complex gases such as propene, which is a volatile organic compound (VOC) gas, serving as a biomarker in human breath for lung cancer and other human diseases. This work contributes to the advancement of gas analysis and opens alternative avenues for exploring novel Raman enhancement techniques.

Preparation of 3D nano silver trees/sea urchin-like gold and SERS detection of uric acid

In this paper, 3D nano-silver trees/sea urchin-like gold Surface Enhanced Raman Spectroscopy (SERS) substrates were prepared by chemical reduction method, characterized by SEM, EDS, UV–Vis and XRD et.al, and the preparation process was optimized using Rhodamine B (RB) as the Raman signal molecule. The SERS characteristics of Uric Acid on this substrate were investigated and the results showed that the SERS substrate had a Raman enhancement factor of 1.9 × 107 for RB, and also had a significant Raman enhancement effect on Uric Acid. The SERS intensity (ISERS) at 1400 cm−1 showed a good linear relationship with the logarithm value of uric acid concentration (log C) between the range of 5 × 10−4 M to 1 × 10−7 M. The linear fitting equation was ISERS = 890 (log C) + 8066 (R2 = 0.983), and the LOD = 2.8 × 10−8 M. The spiked test was performed in human serum with recoveries ranging from 82.86% to 125.13% and RSD (n = 3) < 3.5%.

Surface structured silver-copper bimetallic nanoparticles by irradiation of excimer laser pulses to bilayer thin films

Optical chips based on plasmonic bimetal nanoparticles are of particular interest for various applications, including optical sensors, as well as plasmon-enhanced fluorescence and plasmon-enhanced Raman scattering due to their extraordinary optical properties. In the paper, the formation of the bimetallic nanoparticles on the BK7 substrate and tuning of the LSPR wavelengths by irradiation of excimer laser pulses (193 nm output wavelength and 15 ns temporal pulse linewidth) to silver-copper thin films deposited by the physical vapor deposition (PVD) method is studied. Applying four and five ArF laser pulses with the fluence of 140 mJ cm−2 leads to the formation of spherical Ag-Cu nanoparticles and clusters, respectively, with several LSPR peaks in the UV and visible ranges. Also, sharp and intense LSPR peaks in the 560 to 700 nm spectral range are observed in the absorption spectra of the produced samples with four laser pulses which indicates the ability of the method for tuning the LSPR wavelength. Furthermore, the highest enhancement in the Raman scattering of R6G solution is provided using this sample as a substrate for Raman spectroscopy. Therefore, the laser-produced spherical Ag-Cu NPs by these parameters of laser irradiation can be a good candidate for applications based on LSPR, and Raman or fluorescence enhancement.

Exploring the surface-enhanced Raman scattering (SERS) activity of gold nanostructures embedded around nanogaps at wafer scale: Simulations and experiments

A unique way of converting free space light into a local electromagnetic field in small spaces is via metallic nanostructuring. In this work fabrication, experimental characterization and simulation of surface-enhanced Raman scattering (SERS) active specimens based on Au nanostructures are discussed. We used displacement Talbot lithography (DTL) to fabricate silicon nano-wedge substrates with Au nanostructures embedded around their apices. After the ion beam etching process, a nanogap is introduced between two Au nanostructures templated over nano-wedges, yielding specimens with SERS characteristics. The Au nanostructures and the nanogaps have symmetric and asymmetric configurations with respect to the wedges. With this nanofabrication method, various wafer-scale specimens were fabricated with highly controllable nanogaps with a size in the order of 6 nm for symmetric gaps and 8 nm for asymmetric gaps. SERS characteristics of these specimens were analyzed experimentally by calculating their analytical enhancement factor (AEF). According to finite-difference time-domain (FDTD) simulations, the Raman enhancement arises at the narrow gap due to plasmonic resonances, yielding a maximum AEF of 6.9 × 106. The results highlight the SERS activity of the nanostructures and ultimately comply with reliable substrates for practical applications.

Trace Detection of Ciprofloxacin in Milk by Label-Free Raman Enhancement using Two-Dimensional Magnesiochromite

Trace Detection of Ciprofloxacin in Milk by Label-Free Raman Enhancement using Two-Dimensional Magnesiochromite

Periodic nanostructures on single-crystal copper for SERS substrate fabricated by using AFM dynamic lithography

The fabrication of periodic nanostructures on noble metals as surface enhanced Raman scattering (SERS) substrate is still challenging through facile and cost-effective process. Dynamic lithography under AFM tapping mode has been applied to the fabrication of nanogrooves on single-crystal copper to reduce the feature size. However, there was a lack of fundamental understanding of the effect of the fabrication of periodic nanostructures due to dynamic lithography and then SERS enhancement. To this end, this study aims to investigate the variation of the energy dissipation and the corresponding fabricated nanogrooves at different laser spot positions. It was found that improper laser position was contributed to the increased energy dissipation and irregular nanogrooves were thus obtained with much larger machined depth. On the contrary, at the optimal laser position, the minimal sensitivity was associated with the maximum voltage value on the photodiode. In this case, regular nanogrooves with controlled feature size can be produced for the fabrication of periodic nanostructures. And checkerboard nanodots with a period of 200 nm were fabricated by the combination of arrays of nanogrooves, which indicated advantages of high stability for AFM dynamic lithography. In addition, rhodamine 6G (R6G) was selected as the detecting target to measure SERS spectra of fabricated substrates. Results indicated the Raman enhancement factors can be promoted due to uniform shape of nanogroove and high resolution in horizon dimension.

A durian-shaped multilayer core-shell SERS substrate for flow magnetic detection of pesticide residues on foods

A new type of durian-shaped Fe3O4@Au@Ag@Au (DFAAA) multilayer core-shell composite was prepared as an efficient surface-enhanced Raman scattering (SERS) substrate. The optimization process and SERS enhancement mechanism of the substrate were further explained with finite-difference time-domain simulation. The dense and uniform spiny array on the DFAAA surface had abundant “hot spots”, greatly improving sensitivity, uniformity and reproducibility, with a Raman enhancement factor of 3.01 × 107 and storage-life of 30 d. A “flow magnetic detection method” was proposed to realize rapid and flexible detection of pesticide residues on the surface of different foods including fish and apple. The limit of detection of malachite green and thiram on the fish and apple surfaces were 0.13 and 0.18 ng/cm2, respectively. With its high SERS performance and good magnetic, the DFAAA possessed great application prospects as a facile SERS substrate for rapid and non-destructive detection of trace pesticide residues on foods.

High sensitivity D-shaped fiber decorated with self-assembly Ag nanoparticles for Raman enhancement via collecting backward and vertical direction scattering signals

High sensitivity D-shaped fiber decorated with self-assembly Ag nanoparticles for Raman enhancement via collecting backward and vertical direction scattering signals

Narrow-Linewidth Pr:YLF Laser for High-Resolution Raman Trace Gas Spectroscopy

Spontaneous Raman gas spectroscopy, which stands out as a versatile chemical identification tool, typically relies on frequency-doubled infrared laser sources to deliver the high power and narrow linewidth needed to achieve chemical detection at trace concentrations. The relatively low efficiency and high complexity of these lasers, however, can make them challenging to integrate into field-deployable instruments. Additionally, the frequency doubling prevents the utilization of circulating laser power for Raman enhancement. A diode-pumped Pr:YLF laser was investigated as an alternative narrow-band light source that could potentially realize a more portable Raman scattering system. When operated with an intracavity etalon, the laser realized a linewidth of 0.5 cm−1 with a green output power of 0.37 W and circulating power of 16 W when pumped with 3.1 W from a blue diode laser. Trace detection at atmospheric pressure with a high degree of spectral discrimination was demonstrated by resolving overlapping N2/CO and CO2/N2O Raman bands in air.