Surface-enhanced Raman Scattering Enhancement Research Articles

Additional Important Considerations in Surface-Enhanced Raman Scattering Enhancement Factor Measurements

Despite considerable progress, the issues for precise evaluation of the surface-enhanced Raman scattering (SERS) substrate enhancement factor (EF) have not yet been completely solved until now. Herein, we verify that key aspects of probe molecules including initial concentration, footprint, and diffusion kinetics need to be taken into account for the experimental SERS EF measurement. The three factors collectively affect the surface coverage and thereby the determination of the number of probe molecules on the SERS substrate. We recommend that the measurement of the SERS EFs should be performed with complete surface coverage of probe molecules at steady-state equilibrium using relatively high initial concentrations. It has been demonstrated for the first time that the SERS EFs are independent of the initial concentration of probe molecules when exceeding the threshold initial concentration for complete surface coverage. This study highlights the importance of probe molecules in terms of diffusion kinetics and surface coverage for the accurate determination of the SERS EFs, which has yet to be addressed in the literature.

Hybrid Enhancement of Surface-Enhanced Raman Scattering Using Few-Layer MoS2 Decorated with Au Nanoparticles on Si Nanosquare Holes.

By combining the excellent biocompatibility of molybdenum disulfide (MoS2), excellent surface-enhanced Raman scattering (SERS) activity of Au nanoparticles (Au NPs), and large surface area of Si nanosquare holes (NSHs), a structure in which MoS2 is decorated with Au NPs on Si NSHs, was proposed for SERS applications. The NSH structure fabricated by e-beam lithography possessed 500 nm of squares and a depth of approximately 90 nm. Consequently, a few-layer MoS2 thin films (2–4 layers) were grown by the sulfurization of the MoO3 thin film deposited on Si NSHs. SERS measurements indicated that MoS2 decorated with Au NPs/Si NSHs provided an extremely low limit of detection (ca. 10−11 M) for R6G, with a high enhancement factor (4.54 × 109) relative to normal Raman spectroscopy. Our results revealed that a large surface area of the NSH structure would probably absorb more R6G molecules and generate more excitons through charge transfer, further leading to the improvement of the chemical mechanism (CM) effect between MoS2 and R6G. Meanwhile, the electromagnetic mechanism (EM) produced by Au NPs effectively enhances SERS signals. The mechanism of the SERS enhancement in the structure is described and discussed in detail. By combining the hybrid effects of both CM and EM to obtain a highly efficient SERS performance, MoS2 decorated with Au NPs/Si NSHs is expected to become a new type of SERS substrate for biomedical detection.

Numerical investigation of plasmon sensitivity and surface-enhanced Raman scattering enhancement of individual TiN nanosphere multimers

Titanium nitride (TiN) nanoparticles have recently been considered as potential candidate plasmonic materials; such materials support localized surface plasmon resonances (LSPRs) and show excellent thermal stability with a high melting point. The electromagnetic (EM) field coupling and gap distance between components of individual TiN nanosphere multimers are critical parameters affecting their plasmonic sensitivity and surface-enhanced Raman scattering (SERS) performance, both of which are numerically investigated by the finite element method. It is demonstrated that the fractional shifts of both the dipolar LSPR wavelength and the refractive index sensitivity factor S follow the universal ‘plasmon ruler’ behavior, which is explained well in terms of EM field distribution. The response of the obtained S to is also presented and elucidated in terms of the optical response of the dielectric constants of TiN. The maximum S and SERS enhancement (excited by three normally available lasers in experiments) are also predicted; both are comparable to the values for Au dimeric nanoparticles. The present work holds great promise for the development of non-noble metal plasmonic materials in both SERS and plasmonic sensing applications.

A facile surface enhanced Raman scattering substrate based on silver deposited sandpaper

Flexible surface enhanced Raman scattering (SERS) substrate was prepared by modification of sandpaper with silver nanoparticles. Under 633 nm excitation wavelength, the SERS enhancement effect of sandpapers which were treated with silver nanoparticles were evaluated by collecting Raman signals of probed molecules. The results demonstrate that the SERS enhancement effect of white (12,000 meshes) is better than that of pink (8000 meshes) sandpaper under the same condition; when the concentration of probe molecules is [Formula: see text] mol/L, white sandpaper has the best SERS enhancement; the Raman scattering spectrum has better signal when the silver sol is 15 [Formula: see text].

Bias Voltage Induced Surface-Enhanced Raman Scattering Enhancement on the Single-Molecule Junction

We have studied surface-enhanced Raman scattering (SERS) of aminobenzenethiol (ABT) and benzenedithiol (BDT) single molecules bridging Au electrodes (single-molecule junction) at different bias vol...

Core–Shell Gold@Silver Nanorods of Varying Length for High Surface-Enhanced Raman Scattering Enhancement

We report an effective method to produce surface-enhanced Raman scattering (SERS) substrates with improved enhancement efficiency, high uniformity, greater chemical and physical stability, and absolute synthetic reproducibility. Nanobipyramidal gold (Au) core and silver (Ag) shell nanorods (NRs) with variable length were prepared simply by varying the Ag precursor amount during growth, and we successfully prepared NRs of lengths from 200 to 1200 nm and studied their formation mechanism and morphology using transmission electron microscopy (TEM) and their optical properties by using UV–vis–near-IR (NIR) and Raman spectroscopy. Here we have utilized the properties of both Au and Ag nanoparticles by synthesizing Au@Ag core–shell NRs for high SERS enhancement. NRs have stronger absorption and large scattering cross sections for electromagnetic radiation, and variation of the aspect ratios of these NRs leads to a broad-band surface plasmon tuning in the vis–NIR region. The relative SERS enhancement efficiencie...

Effect of surface density silver nanoplate films toward surface-enhanced Raman scattering enhancement for bisphenol A detection

This paper reports a study on surface-enhanced Raman scattering (SERS) phenomenon of triangular silver nanoplate (NP) films towards bisphenol A (BPA) detection. The NP films were prepared using self-assembly technique with four different immersion times; 1 hour, 2 hours, 5 hours, and 8 hours. The SERS measurement was studied by observing the changes in Raman spectra of BPA after BPA absorbed on the NP films. It was found that the Raman intensity of BPA peaks was enhanced by using the prepared SERS substrates. This is clearly indicated that these SERS silver substrates are suitable to sense industrial chemical and potentially used as SERS detector. However, the rate of SERS enhancement is depended on the distribution of NP on the substrate surface.

Surface-enhanced Raman Scattering Enhancement Factors for RNA Mononucleotides on Silver Nanoparticles

Surface-enhanced Raman Scattering Enhancement Factors for RNA Mononucleotides on Silver Nanoparticles

Surface-enhanced Raman scattering enhancement factor distribution for nanoparticles of arbitrary shapes using surface integral equation method

Poggio-Miller-Chang-Harrington-Wu-Tsai (PMCHWT) surface integral equation method is applied for the first time to accurately estimate the surface-enhanced Raman scattering (SERS) enhancement factor distribution for arbitrary nanoparticles and nano-aggregates. It is the first time in literature that the distributions of SERS enhancement factors of nanoparticles of a large variety are reported. It is shown that not every SERS substrate exhibits a long-tail distribution as a dimer consisting of two spheres in close proximity. Generic methods are proposed to evaluate the performance of nanoparticles on SERS substrates. A cumulative distribution is proposed to examine the contributions of hot and warm spots around the nanoparticles. It is used to identify the importance of warm spots on a SERS substrate. A parameter q is proposed to describe the likelihood of a randomly positioned molecule that can be activated. This study provides guidance and insights for the optimization of SERS substrate fabrication techniques.

Exploring the Chemical Enhancement of Surface-Enhanced Raman Scattering with a Designed Silver/Silica Cavity Substrate

Silver nanoparticles were assembled onto the bottom of closed-packed silica cavity using polystyrene (PS) spheres as template. Charge transfer between the adsorbed 4-aminothiophenol (PATP) and the silver nanoparticles was studied using surface-enhanced Raman spectroscopy with 514, 633, 785, and 1064 nm excitation, and compared to that of the immobilized silver nanoparticles without the modification of silica cavity. Using the concept of degree of charge transfer, we directly observed the additional chemical enhancement without a deliberate distinction between electromagnetic (EM) enhancement and chemical enhancement. It was demonstrated that the negative charges of the silica could induce the formation of the dipole in the nanoparticles, thus enlarging the electron density at the sites where probe molecules adsorbed, and leading to higher charge transfer from the metal to the adsorbed PATP molecules. We also proposed another model to further elucidate the relationship between the electron density and the ...

UV/Ozone-Oxidized Large-Scale Graphene Platform with Large Chemical Enhancement in Surface-Enhanced Raman Scattering

We fabricated a highly oxidized large-scale graphene platform using chemical vapor deposition (CVD) and UV/ozone-based oxidation methods. This platform offers a large-scale surface-enhanced Raman scattering (SERS) substrate with large chemical enhancement in SERS and reproducible SERS signals over a centimeter-scale graphene surface. After UV-induced ozone generation, ozone molecules were reacted with graphene to produce oxygen-containing groups on graphene and induced the p-type doping of the graphene. These modifications introduced the structural disorder and defects on the graphene surface and resulted in a large chemical mechanism-based signal enhancement from Raman dye molecules [rhodamine B (RhB), rhodamine 6G (R6G), and crystal violet (CV) in this case] on graphene. Importantly, the enhancement factors were increased from ∼10(3) before ozone treatment to ∼10(4), which is the largest chemical enhancement factor ever on graphene, after 5 min ozone treatment due to both high oxidation and p-doping effects on graphene surface. Over a centimeter-scale area of this UV/ozone-oxidized graphene substrate, strong SERS signals were repeatedly and reproducibly detected. In a UV/ozone-based micropattern, UV/ozone-treated areas were highly Raman-active while nontreated areas displayed very weak Raman signals.

Wrinkled Nanoporous Gold Films with Ultrahigh Surface-Enhanced Raman Scattering Enhancement

Amplified by plasomonic nanostructured metals, Raman intensity of organic molecules and biomolecules can be dramatically improved, particularly at "hot spots" where intense electromagnetic fields are produced in the vicinity of narrow nanogaps between metallic nanostructures. Therefore, developing new substrates with a high density of "hot spots" has been the recent topic of intense study. Here we report wrinkled nanoporous gold films that contain abundant Raman-active nanogaps produced by deformation and fracture of nanowire-like gold ligaments. This novel nanostructure yields ultrahigh surface enhanced Raman scattering for molecule detection.

Surface-Enhanced Raman Scattering Enhancement by Aggregated Silver Nanocube Monolayers Assembled by the Langmuir−Blodgett Technique at Different Surface Pressures

The surface-enhanced Raman scattering spectrum of poly(vinyl) pyrrolidone (PVP) molecules capping 50 nm silver nanocubes assembled in a monolayer via the Langmuir−Blodgett (LB) technique is studied at different surface pressures (at different nanoparticle densities). The observed correlation between the intensity of different SERS bands of the PVP with the nanoparticle density and those of the extinction intensity of the localized surface plasmon resonance bands (as a measure of the surface plasmon field) suggests the following: (1) the observed SERS enhancement results mostly from surface fields of the aggregated nanoparticles; and (2) the relative intensities of the different Raman bands are in reasonable agreement with those expected from the electromagnetic mechanism of enhancement. Besides the large surface plasmon field used in the Raman scattering enhancement, the broad SPR band of the aggregated nanocubes provides an additional advantage in the analytical applications of Raman spectroscopy.

The effects of Au aggregate morphology on surface-enhanced Raman scattering enhancement

We have identified empirically a relationship between the surface morphology of small individual aggregates (<100 Au nanoparticles) and surface-enhanced Raman scattering (SERS) enhancement. We have found that multilayer aggregates generated greater SERS enhancement than aggregates limited to two-dimensional (2D) or one-dimensional structures, independent of the number of particles. SERS intensity was measured using the 730 cm(-1) vibrational mode of the adsorbed adenine molecule on 75 nm Au particles, at an excitation wavelength of 632.8 nm. To gain insight into these relationships and its mechanism, we developed a qualitative model that considers the collections of interacting Au nanoparticles of an individual aggregate as a continuous single entity that retains its salient features. We found the dimensions of the modeled surface features to be comparable with those found in rough metal surfaces, known to sustain surface plasmon resonance and generate strong SERS enhancement. Among the aggregates that we have characterized, a three 75 nm nanoparticle system was the smallest to generate strong SERS enhancement. However, we also identified single individual Au nanoparticles as SERS active at the same wavelength, but with a diameter twice in size. For example, we observed a symmetric SERS-active particle of 180 nm in diameter. Such individual nanoparticles generated SERS enhancement on the same order of magnitude as the small monolayer Au aggregates, an intensity value significantly stronger than predicted in recent theoretical studies. We also found that an aspect of our model that relates the dimensions of its features to SERS enhancement is also applicable to single individual Au particles. We conclude that the size of the nanoparticle itself, or the size of a protrusion of an irregularly shaped single Au particle, will contribute to SERS enhancement provided that its dimensions satisfy the conditions for plasmon resonance. In addition, by considering the ratio of the generated intensities of typical 2D Au aggregates to the enhancement of individual SERS-active particles, a value of approximately 2 is determined. Its moderate value suggests that it is not the aggregation effect that is responsible for much of the observed SERS enhancement but the surface region associated with the SERS-active site.

On the Mechanism of Chemical Enhancement in Surface-Enhanced Raman Scattering

On the Mechanism of Chemical Enhancement in Surface-Enhanced Raman Scattering