Raman Enhancement Factor Research Articles

Raman enhancement of large-area silver grating arrays based on self-assembled polystyrene microspheres

Two types of Ag grating arrays as surface-enhanced Raman scattering (SERS) were fabricated using the monolayer self-assembled polystyrene (PS) as a template, using the silver film and monocrystalline silicon wafer as the substrates, imprinting different thicknesses of silver (50 nm, 75 nm, 100 nm, 125 nm and 150 nm) on the template, then removing PS. Rhodamine 6G (R6G) was used as a probe to characterize the performance of Raman enhancement. Experimental results showed that two structures can obtain a Raman enhancement factor of more than 107 at arbitrary deposition thicknesses. The insensitivity of two arrays induced by incident polarization and the difference in near-field and far-field simulation were also compared. These novel SERS substrates can achieve considerable uniformity, and the relative standard deviation (RSD) of the characteristic peak calculated at 1650 cm−1 were about 9.2% and 9.5%, respectively.

Double-frequency metallic grating over large areas with ultrahigh Raman enhancement

Raman spectroscopy reveals molecular vibration and rotation modes and directly reflects the molecular structure of analytes. Surface enhanced Raman spectroscopy may be applied to practical applications because of the greater Raman scattering cross section. This paper proposes and demonstrates a large-area double frequency metallic grating based on surface enhanced Raman scattering substrate which is cost effective. By exciting two coupled resonance modes, a strong local electromagnetic field was formed. The Raman enhancement factor up to 108 and the minimum detectable concentration down to 10−15 M were achieved.

Performance and Shelf-Life of Electrospun Ag-doped WO3 Nanofiber-Based SERS Substrates

Recent advances in Ag-doped WO3 nanofibers have provided an opportunity to fabricate high quality and low-cost surface-enhanced Raman scattering (SERS) substrates. This work studied the performance and shelf-life of Ag-doped WO3 nanofiber-based substrates. Methylene blue (MB) molecules were used as analytical references. The prepared substrates were stored in ambient air at room temperature for the duration of 12 months. The Raman enhancement factor (EF) of freshly prepared sample was calculated to be 6.2 × 105. In the first 6 months, a small decrease in EF was observed. The EF was found to significantly decrease after samples were stored for 12 months. After 12 months, the relative standard deviations (RSD) of Raman signals changed from 10.35% to 50.29%, indicating non-uniform Raman enhancements at different locations of the sample. These suggested agglomerations of Ag nanoparticles, which deteriorated the Raman enhancement effect, after samples were stored for a prolonged period.

Synthesis and defect engineering of molybdenum oxides and their SERS applications.

Surface-enhanced Raman scattering (SERS) spectroscopy has been developed into a cross-disciplinary analytical technology through exploring various materials' Raman vibrational modes with ultra-high sensitivity and specificity. Although conventional noble-metal based SERS substrates have achieved great success, oxide-semiconductor-based SERS substrates are attracting researchers' intensive interest due to their merits of facile fabrication, high uniformity and tunable SERS characteristics. Among all the SERS active oxide semiconductors, molybdenum oxides (MoOx) possess exceptional advantages of high Raman enhancement factor, environmental stability, recyclable detection, etc. More interestingly, the SERS effect of the MoOx SERS substrates may involve both the electromagnetic enhancement mechanism and the chemical enhancement mechanism, which is determined by the stoichiometry and morphology of the material. Therefore, the focus of this review will be on two critical points: (1) synthesis and material engineering methods of the functional MoOx material and (2) MoOx SERS mechanism and performance evaluation. First, we review recent works on the MoOx preparation and material property tuning approaches. Second, the SERS mechanism and performance of various MoOx substrates are surveyed. In particular, the performance uniformity, enhancement factor and recyclability are evaluated. In the end, we discuss several challenges and open questions related to further promoting the MoOx as the SERS substrate for monitoring extremely low trace molecules and the theory for better understanding of the SERS enhancement mechanism.

Fabrication of 3D surface-enhanced Raman scattering (SERS) substrate via solid-state dewetting of sputtered gold on fumed silica surface

Raman spectroscopy is a potent and widespread optical analytical technique thanks to its non-invasive and high-specification for the detection of targeted molecules. However, for the case of trace detection, it is common that a weak Raman signal is easily swamped by noise and thus unable to be resolved. Here, we demonstrated a facile fabrication of a three-dimensional surface enhanced Raman spectroscopy (SERS) substrate, based on low-vacuum sputtering of gold nanofilm on hierarchically rough fumed silica monolayers deposited by layer-by-layer self-assembly technique. Due to the much lower surface energy of the silica-air heterostructure compared to metallic materials, deposited gold layers dewetted the surface spontaneously, forming nano-sized spherical gold particles without the requirement of an extra annealing process. Plasmonic effects were studied through optical absorption measurements, while the surface morphology and topography were examined using SEM and AFM for various sputtering durations. Furthermore, the enhancement of Raman spectrum was investigated for 10−4 M of methylene blue (MB), using 532 nm and 0.57 mW excitation laser. An initial Raman enhancement factor of 17 was observed at 1645 cm−1 peak, even with yet to be optimized fabrication procedures.

Electrical Tuning of MoOx/Ag Hybrids and Investigation of their Surface‐Enhanced Raman Scattering Performance

Surface‐enhanced Raman scattering (SERS) technology is a cutting‐edge analytical tool for molecule detection. Attractive SERS performance has been achieved on noble metal nanostructures; however these substrates usually suffer from difficulties of direct adjusting of the physical structures to achieve tunable SERS performance. Studies on semiconductor oxides have revealed that attractive SERS performance can be obtained on them, but strategies of engineering material properties for SERS performance improvement still pose a challenge. Here, an electrically programmable SERS substrate is prepared by depositing hydrothermally synthesized MoOx/Ag hybrids within electrodes as the SERS active region. In the experiment, an electrical field is applied on the electrodes to regulate Ag+ ion migration and redeposition in the MoOx solid electrolyte. Through adjusting the leakage current level, the size of the Ag nanoparticles in the MoOx/Ag hybrids is electrically controlled. The SERS performance of the substrate is evaluated using rhodamine 6G as the Raman reporter. The results evidence that Raman enhancement factors of 1.13 × 105, 4.75 × 105, and 1.04 × 106 can be obtained by programming the leakage current level to 10−7, 10−5, and 10−3 A, respectively. A maximum detection limit of 10−8 m is achieved on the 10−3 A substrate.

Plasma-Enabled Amorphous TiO2 Nanotubes as Hydrophobic Support for Molecular Sensing by SERS.

We devise a unique heteronanostructure array to overcome a persistent issue of simultaneously utilizing the surface-enhanced Raman scattering, inexpensive, Earth-abundant materials, large surface areas, and multifunctionality to demonstrate near single-molecule detection. Room-temperature plasma-enhanced chemical vapor deposition and thermal evaporation provide high-density arrays of vertical TiO2 nanotubes decorated with Ag nanoparticles. The role of the TiO2 nanotubes is 3-fold: (i) providing a high surface area for the homogeneous distribution of supported Ag nanoparticles, (ii) increasing the water contact angle to achieve superhydrophobic limits, and (iii) enhancing the Raman signal by synergizing the localized electromagnetic field enhancement (Ag plasmons) and charge transfer chemical enhancement mechanisms (amorphous TiO2) and by increasing the light scattering because of the formation of vertically aligned nanoarchitectures. As a result, we reach a Raman enhancement factor of up to 9.4 × 107, satisfying the key practical device requirements. The enhancement mechanism is optimized through the interplay of the optimum microstructure, nanotube/shell thickness, Ag nanoparticles size distribution, and density. Vertically aligned amorphous TiO2 nanotubes decorated with Ag nanoparticles with a mean diameter of 10-12 nm provide enough sensitivity for near-instant concentration analysis with an ultralow few-molecule detection limit of 10-12 M (Rh6G in water) and the possibility to scale up device fabrication.

Large-Scale Flexible Surface-Enhanced Raman Scattering (SERS) Sensors with High Stability and Signal Homogeneity

Flexible surface-enhanced Raman scattering (SERS) sensors have attracted great attention as a portable and low-cost technique for chemical and bio-detecting. However, flexible SERS sensors tend to suffer low signal spatial homogeneity due to their uneven active plasmonic nanostructures (hot-spots) distribution, and quick degradation of SERS sensors' sensitivity due to low adhesion of hot-spots and flexible substrates during fast sampling. Herein, a large area (20×20 cm2) polyimide (PI) based SERS sensor is exploited for trace detection with high signal homogeneity and stability. The SERS sensor is constructed from PI through in situ growth of silver and gold core-shell nanoparticles (Ag@Au NPs) based on chemical reduction and galvanic replacement process. Benefiting from the abundant carboxyl groups on the surface cleavaged PI, densely and uniformly distributed Ag@Au NPs are successfully prepared on the film under ambient conditions. The high Raman enhancement factor (up to 1.07 × 107) and down detection capability of nanomolar (10-9 M) detection limits are obtained for this flexible SERS sensor. The uniform Raman signals in the random region show well signal homogeneity with a low variation of 8.7%. Moreover, the flexible SERS sensor exhibited superior efficiency durability after storage 30 days even after mechanical stimuli (bending or torsion) 500 cycles. The pesticide thiram (TMTD) residue has been rapidly traced by direct sampling from the apple surface, and a sensitivity of 10 ng/cm2 for TMTD was achieved. These findings show that the PI-based SERS sensor is a very strong candidate for broad and simple utilization of flexible SERS for both laboratory and commercial applications in chemical and biomolecule detections.

Rapid label-free SERS detection of foodborne pathogenic bacteria based on hafnium ditelluride-Au nanocomposites

Two-dimensional (2D) nanomaterials have captured an increasing attention in biophotonics owing to their excellent optical features. Herein, 2D hafnium ditelluride (HfTe[Formula: see text], a new member of transition metal tellurides, is exploited to support gold nanoparticles fabricating HfTe2-Au nanocomposites. The nanohybrids can serve as novel 2D surface-enhanced Raman scattering (SERS) substrate for the label-free detection of analyte with high sensitivity and reproducibility. Chemical mechanism originated from HfTe2 nanosheets and the electromagnetic enhancement induced by the hot spots on the nanohybrids may largely contribute to the superior SERS effect of HfTe2-Au nanocomposites. Finally, HfTe2-Au nanocomposites are utilized for the label-free SERS analysis of foodborne pathogenic bacteria, which realize the rapid and ultrasensitive Raman test of Escherichia coli, Listeria monocytogenes, Staphylococcus aureus and Salmonella with the limit of detection of 10 CFU/mL and the maximum Raman enhancement factor up to [Formula: see text]. Combined with principal component analysis, HfTe2-Au-based SERS analysis also completes the bacterial classification without extra treatment.

Preparation of a monolayer array of silica@gold core-shell nanoparticles as a SERS substrate

In this work, we report the preparation of monolayer silica@gold core-shell nanoparticle (SiO2@Au NP) nanostructures, which combines spin-coating, nanoscale masking and in situ reduction methods. Using this approach, a potential SERS substrate based on a composite particles of SiO2@Au NP monolayer array was fabricated. The surface morphology and optical properties of the samples were performed by scanning electron microscopy (SEM) and ultraviolet-visible (UV–vis) spectrophotometry. To study the SERS properties of the substrate, crystal violet (CV) was selected as the probe molecule. The Raman activity of characteristic vibrational bands of CV showed better when the excitation wavelength was close to the plasma peak of the SiO2@Au NPs. In addition, the Raman signal of the probe molecule on the SERS substrate presented good homogeneity, reproducibility and stability, also reproduce the SERS spectra after three weeks. Furthermore, the Raman enhancement factor (1.4×107), the detection limit (1×10−8 mol/L) and the relative standard deviation (RSD, 14.07 %) were evaluated. Our approach has potential advantage in the fabrication of viable solid SERS substrates that provides a possibility for commercialization used in nondestructive testing, food testing and detection of low-concentration.

Unravelling the Thickness Dependence and Mechanism of Surface-Enhanced Raman Scattering on Ti3C2TX MXene Nanosheets

MXenes have attracted great attention as promising substrates for surface-enhanced Raman scattering (SERS) applications. However, the underlying SERS mechanism has not been a focus of any investigation. Herein, we report the first systematic experimental study on the SERS activity of titanium carbide (Ti3C2TX) nanosheets with thicknesses ranging from 5 to 120 nm, using methylene blue (MB) as a probe molecule. The experimental and mathematical modeling results show that the Raman enhancement factor (EF) increases monotonically with the increasing thickness of Ti3C2TX nanosheets; however, it falls drastically around a sheet thickness of 0.8 and 1.0 μm under 532 and 633 nm laser excitations, respectively. The Raman EF reaches a maximum value around a thickness of 2.0 μm, suggesting that a maximum EF can be achieved with a 2.0 μm-thick Ti3C2TX film substrate. The thickness dependence of the Raman enhancement can be accounted for by the adsorption and intercalation of MB molecules into the interlayer spacing of Ti3C2TX. Furthermore, by combining experimental observations and numerical calculation, we confirm that the charge-transfer mechanism is dominantly responsible for Raman enhancement on Ti3C2TX. Additionally, we report an observation of resonance coupling of charge transfer and molecular transition as a contributing factor to the higher EF obtained with a 633 nm laser excitation. Taken together, these findings have significant implications for cost and performance optimization in designing MXene-based SERS substrates for next-generation chemical and biological sensing platforms.

Facile synthesis of metal-phenolic-coated gold nanocuboids for surface-enhanced Raman scattering.

Metal-phenolic networks (MPNs) have been exploited to be a versatile coating film to fabricate core-shell structure due to their general adherent properties. Herein, gold nanocuboid (GNCB) wrapped by MPNs (GNCB at MPNs) are prepared by a facile encapsulation method for surface-enhanced Raman scattering (SERS) analysis. The MPN coating not only reshapes the electric field distribution around the nanostructures but also allows the substrate to adsorb more analytes, both of which contribute to the superior SERS activity of GNCB at MPNs. The SERS signals induced by plasmonic nanostructures increase four- to sixfold after MPN coating, reaching a maximum Raman enhancement factor calculated to be 9.47×108. Moreover, the core-shell SERS substrate also demonstrates improved biocompatibility (∼fivefold increase) that facilitates the reliable SERS analysis of cancer cells and further diverse biomedical applications.

Mixed-Dimensional Heterostructure Material-Based SERS for Trace Level Identification of Breast Cancer-Derived Exosomes.

Raman spectroscopy has capability for fingerprint molecular identification with high sensitivity if weak Raman scattering signal can be enhanced by several orders of magnitudes. Herein, we report a heterostructure-based surface-enhanced Raman spectroscopy (SERS) platform using 2D graphene oxide (GO) and 0D plasmonic gold nanostar (GNS), with capability of Raman enhancement factor (EF) in the range of ∼1010 via light–matter and matter–matter interactions. The current manuscript reveals huge Raman enhancement for heterostructure materials occurring via both electromagnetic enhancement mechanism though plasmonic GNS nanoparticle (EF ∼107) and chemical enhancement mechanism through 2D-GO material (EF ∼102). Finite-difference time-domain (FDTD) simulation data and experimental investigation indicate that GNS allows light to be concentrated into nanoscale “hotspots” formed on the heterostructure surface, which significantly enhanced Raman efficiency via a plasmon–exciton light coupling process. Notably, we have shown that mixed-dimensional heterostructure-based SERS can be used for tracking of cancer-derived exosomes from triple-negative breast cancer and HER2(+) breast cancer with a limit of detection (LOD) of 3.8 × 102 exosomes/mL for TNBC-derived exosomes and 4.4 × 102 exosomes/mL for HER2(+) breast cancer-derived exosomes.

Plasmon-exciton coupling for nanophotonic sensing on chip.

The monolayer graphene-noble metallic nanostructure hybrid system with excellent optical characteristic, which is deserved pay attentions in the study of surface-enhanced Raman scattering spectroscopy. In this work, a hybrid sandwich structure is designed to transfer single-layer graphene to the surface of discs substrate covered by silver film and assembly of the dense Au nanoparticles (AuNPs). Blu-ray disc has a cycle density of approximately 5.7 times that of DVD-R due to the different storage capacities of these optical discs. In the research, enhancement effects have been explored for two different periodic grating structures. Compared to spectra of Si/G structure, Graphene Raman spectra from Blu-grating/AuNPs/G structure and Blu-grating/G/AuNPs enhancement multiples at the 2D peak position possesses different Raman responses of 1.09 and 2.51 times, respectively. The sandwich hybrid structure of Ag grating/graphene/AuNPs obtains a Raman enhancement factor (EF) of 6.2×108 for Rhodamine 6G and surface-enhanced Raman Scattering(SERS) detection limit of 0.1 nM. These findings can be attributed to the electric field enhancement of the hybrid structure and the chemical enhancement of graphene. This study provides a new approach for SERS detection and offers a new technique for designing SERS sensors with grapheme-plasmon hybrid structures.

A facile periodic porous Au nanoparticle array with high-density and built-in hotspots for SERS analysis

Porous noble metal nanoparticles have attracted great attention in surface enhanced Raman scattering (SERS) applications due to the presence of high-density and built-in hotspots. Herein, we develop a facile route to fabricate large-area, periodic porous Au nanoparticle (PGNP) array based on ultra-thin alumina membrane (UTAM) incorporated with annealing and dealloying techniques. The size and ligament of the PGNP can be easily tailored by changing the thickness of the deposited Au/Ag film and the dealloying time. The PGNP array exhibits excellent uniformity and high sensitivity in SERS analysis due to the periodic structure and the enormous built-in hotspots. A large Raman enhancement factor (EF) of 1.4 × 107 is achieved with a relative standard deviation (RSD) less than 6.6%. Furthermore, experimental results and numerical simulations demonstrate that this PGNP array substrate exhibit polarization independence. The facile and controllable fabrication strategy makes the substrate promising for practical applications in many SERS-based qualitative and quantitative analyses.

Supported Ultra-Thin Alumina Membranes with Graphene as Efficient Interference Enhanced Raman Scattering Platforms for Sensing.

The detection of Raman signals from diluted molecules or biomaterials in complex media is still a challenge. Besides the widely studied Raman enhancement by nanoparticle plasmons, interference mechanisms provide an interesting option. A novel approach for amplification platforms based on supported thin alumina membranes was designed and fabricated to optimize the interference processes. The dielectric layer is the extremely thin alumina membrane itself and, its metallic aluminum support, the reflecting medium. A CVD (chemical vapor deposition) single-layer graphene is transferred on the membrane to serve as substrate to deposit the analyte. Experimental results and simulations of the interference processes were employed to determine the relevant parameters of the structure to optimize the Raman enhancement factor (E.F.). Highly homogeneous E.F. over the platform surface are obtained, typically 370 ± (5%), for membranes with ~100 nm pore depth, ~18 nm pore diameter and the complete elimination of the Al2O3 bottom barrier layer. The combined surface enhanced Raman scattering (SERS) and interference amplification is also demonstrated by depositing ultra-small silver nanoparticles. This new approach to amplify the Raman signal of analytes is easily obtained, low-cost and robust with useful enhancement factors (~400) and allows only interference or combined enhancement mechanisms, depending on the analyte requirements.

Theoretical study of surface-enhanced Raman scattering mechanism of scandium-doped copper/silver clusters

Rare earth metals exhibit strong chemical activity and have many unique properties in the aspects of magnetic susceptibility, photo-absorption, catalytic activity and electrical property. Precious metals have strong chemical stability and great surface-enhanced Raman scattering (SERS) enhancing activity, providing a good platform for detecting SERS signals from molecules. Combining precious metals with rare earth metals could form new composite materials, providing more possibilities for SERS substrates. In this work, the SERS and absorption spectra of the probe molecule adsorbed on scandium-doped silver/copper clusters are theoretically simulated by time-dependent density functional theory. The contributions of charge-transfer (CT) enhancement and electromagnetic enhancement are treated uniformly in calculations based on a short-time approximation for the Raman scattering cross-section, and distinguished by using visualization of electron transitions. The largest Raman enhancement factor of the probe molecule adsorbed on Sc@Cu7 and Sc@Ag7 alloy clusters could reach the order of 105, due to the enhancement of resonance excitation to the CT transition. The factors influencing SERS are systematically investigated, including the composition of the substrate, local chemical environment of the binding site, form of electron transition, oscillator strength of excitation and excitation wavelength.

Polymeric micro-reactors mediated synthesis and assembly of Ag nanoparticles into cube-like superparticles for SERS application

Hierarchical anisotropic superparticles derived from the controlled assembly of individual plasmonic nanoparticles have been recognized as promising building blocks for surface-enhanced Raman spectroscopy (SERS) nanosensors. Thus, the protocols that precisely control the near-field coupling of individual plasmonic nanoparticles and the diffusion of highly diluted analytes to the plasmonic surface are of great interests. Herein, we have synthesized a series of aromatic amphiphilic block copolymers named as polyarylene ether nitrile (amPEN in short), containing a hydrophobic backbone of aromatic moieties with hydrophilic sulfonate/carboxyl and cyano groups in the side chain. Next, the surfactant stabilized water-in-oil (W/O) reverse emulsion is constructed to induce the confined self-assembly of amPEN, leading to the formation of functional micro-reactors for the in-situ synthesis and organization of silver nanoparticles, which finally contribute to the generation of sub-micrometer cube-like plasmonic superparticles (PSPs). Furthermore, the SERS nanosensors are created by drop-casting of synthesized PSPs on silicon substrate followed by depositing an additional thin layer of silver. Finally, the optimized SERS nanosensors with a Raman enhancement factor of 108 are able to detect chlorpyrifos pesticide down to 10-10 M thanks to the presence of abundant Raman hot-spots on the synthesized PSPs and the effective analyte enrichment derived from π-π interaction between pesticide molecules and aromatic backbone of amPEN.

Tailoring a novel Au nanodot arrays on graphene substrate for a highly active Surface-Enhanced Raman Scattering (SERS)

In this work, gold nanodots (Au NDs) were arrayed on the thin graphene (GR) layer, prepared by chemical vapor deposition, by focused ion beam technique. Following, GR/Au NDs substrates was evaluated of Surface-Enhanced Raman Scattering (SERS) by using rhodamine 6G (R6G) as a molecular probe. As expected, GR/Au NDs significantly enhanced the SERS signals. The limit of detection for R6G was as low as 10−12 M and the Raman enhancement factor was as large as 108 times. GR/Au NDs SERS-active substrate showed a promising technique to enhance the trace detection and quantitative detection of organic contaminants.

Roadmap for single-molecule surface-enhanced Raman spectroscopy

In the near future, single-molecule surface-enhanced Raman spectroscopy (SERS) is expected to expand the family of popular analytical tools for single-molecule characterization. We provide a roadmap for achieving single molecule SERS through different enhancement strategies for diverse applications. We introduce some characteristic features related to single-molecule SERS, such as Raman enhancement factor, intensity fluctuation, and data analysis. We then review recent strategies for enhancing the Raman signal intensities of single molecules, including electromagnetic enhancement, chemical enhancement, and resonance enhancement strategies. To demonstrate the utility of single-molecule SERS in practical applications, we present several examples of its use in various fields, including catalysis, imaging, and nanoelectronics. Finally, we specify current challenges in the development of single-molecule SERS and propose corresponding solutions.