Robust SERS enhancement factor statistics using rotational correlation spectroscopy

Decoration of Ag nanoparticle on MXene sheets for SERS studies

MXene sheets with the unique electrical and optical properties show the excellent potential for surface-enhanced Raman spectroscopy (SERS) applications. In this study, we chose Ti3C2Tx, a type of MXene, to decorate silver nanoparticles (Ag NPs) on the ultrathin two-dimensional (2D) MXene sheets. The designed Ag-MXene substrates with SERS activity showed high sensitivity, high stability, and reproducibility. The SERS signal was enhanced by the synergistic contribution of both charge-transfer (CT) and surface plasmon resonance (SPR) involving the Ag NPs and the MXene sheets. Due to the strong interaction between the probe molecules and Ag NPs which provided the nanoscale gap, the substrate exhibited remarkable SERS performance. A novel experimental strategy was developed to facilitate the controlled synthesis of noble metal NPs and MXene sheets and provide insights for further improving the practical applications of these materials in SERS detection.

Ag decoration on Na2Ti3O7 nanowires for improved SERS and PHE performance

Na2Ti3O7 (NTO) is recognized as an authenticated promising photocatalyst and surface-enhanced Raman scattering (SERS) active material, although its performance is limited by its high carrier recombination rate, wide band gap and inadequate utilization of visible light. In this study, to solve these issues, sea urchin-shaped NTO nanowires directly grown on a substrate were fabricated, and then Ag nanoparticles were decorated on NTO nanowires using sputtering equipment. The as-prepared Ag-NTO substrates exhibited different morphologies and high SERS activity, which was confirmed by finite-difference time-domain (FDTD) simulations, showing that appropriate Ag decoration can bring more nanogaps and thus enhance the electromagnetic field (EM) contribution. We visualized the charge transfer (CT) mechanism in SERS and further investigated the catalytic hydrogen production process similarly induced by photogenerated CT. The optimal SERS substrate (Ag-NTO-3) was adopted to verify the photocatalytic hydrogen evolution (PHE) activity, and the hydrogen evolution rate of Ag-NTO-3 was 106.7 μmol h-1 (twice that of pristine NTO). Photoelectrochemical measurements and photoluminescence (PL) analysis were used to elucidate the potential enhancement mechanisms for the photocatalytic performance and CT process. This study can provide a valuable reference for performance and mechanism studies of SERS substrates and photocatalysts.

Plasma Exosome Analysis for Protein Mutation Identification Using a Combination of Raman Spectroscopy and Deep Learning

Protein mutation detection using liquid biopsy can be simply performed periodically, making it easy to detect the occurrence of newly emerging mutations rapidly. However, it has low diagnostic accuracy since there are more normal proteins than mutated proteins in body fluids. To increase the diagnostic accuracy, we analyzed plasma exosomes using nanoplasmonic spectra and deep learning. Exosomes, a promising biomarker, are abundant in plasma and stably carry intact proteins originating from mother cells. However, the mutated exosomal proteins cannot be detected sensitively because of the subtle changes in their structure. Therefore, we obtained Raman spectra that provide molecular information about structural changes in mutated proteins. To extract the unique features of the protein from complex Raman spectra, we developed a deep-learning classification algorithm with two deep-learning models. Consequently, controls with wild-type proteins and patients with mutated proteins were classified with high accuracy. As a proof of concept, we discriminated the lung cancer patients with mutations in the epidermal growth factor receptor (EGFR), L858R, E19del, L858R + T790M, and E19del + T790M, from controls with an accuracy of 0.93. Moreover, the protein mutation status of the patients with primary (E19del, L858R) and secondary (+T790M) mutations was clearly monitored. Overall, our technique is expected to be applied as a novel method for companion diagnostic and treatment monitoring.

Highly sensitive SERS behavior and wavelength-dependence charge transfer effect on the PS/Ag/ZIF-8 substrate

In this work, the monodisperse polystyrene colloidal particles/Ag/zeolite imidazole framework (PS/Ag/ZIF-8) substrate was successfully prepared and served as SERS active substrate. The composition, structure and morphology of the PS/Ag/ZIF-8 substrates were studied by XRD, SEM, UV-Vis and XPS measurements. The main finding of this study was that the as-prepared PS/Ag/ZIF-8 substrate could exhibit outstanding SERS property when 4-mercaptobenzoic acid (4-MBA) was used as the SERS probes. The SERS mechanism was attributed to the combined effect of the electromagnetic enhancement and chemical enhancement (CT). In addition, the SERS behavior of the sandwich PS/Ag/ZIF-8 substrate exhibit a laser wavelength-dependence CT effect with changing the laser source (473 nm, 514 nm, 633 nm and 785 nm). The wavelength-dependence CT mechanism were discussed briefly in the article. The results showed that the chemical interaction in the structure is a necessary condition for occurrence of the CT. The CT process can be evaluated quantitatively by the charge transfer degree (ρ_CT). Moreover, the enhancement factor (EF) of 1.23 × 10⁶ was obtained with 4-MBA probes adsorbed on the synthesized PS/Ag/ZIF-8 substrate. More importantly, our research may open the door for developing the SERS substrate research with the well-studied metal-organic frameworks nanostructures materials.

Controlled synthesis of a PS/Au/ZIF-8 hybrid structure as a SERS substrate for ultrasensitive detection

A PS/Au/ZIF-8 hybrid was successfully prepared and used as a SERS active substrate. Meanwhile, the hybrid SERS substrate exhibited good SERS reproducibility, and we successfully obtained an ultrahigh enhancement factor of 1.67 × 10⁶.

Preparation of Hierarchically Assembled Silver Nanostructures based on the Morphologies of Crystalline Peptide-Silver(I) Complexes

The preparation of a hierarchically assembled Ag nanostructures based on a nanocrystalline assembly was demonstrated using an Ag(I) complex of a dipeptide (AspDap). By heating under N₂ gas, a spherical assembly of a nanocrystalline dipeptide-Ag(I) complex (diameter 4-5 μm), which has a morphology similar to the assembled structure of the dipeptide, was transformed to an assembly of Ag nanostructures, where the micrometre-order crystalline morphology was maintained. In addition, detailed scanning electron microscopy studies revealed that Ag nanoparticles (diameter ca. 10 nm) were formed on the surface of the Ag nanostructure. When the Ag(I) ions were reduced to Ag(0), this phenomenon exhibited surface dependence due to the anisotropic two-dimensional Ag(I) arrangement in the crystals. Thermogravimetric measurements and X-ray photoelectron spectroscopy revealed that the reduction proceeds in a stepwise manner around 200-250 °C, together with the removal of primary and secondary carboxylic groups in the dipeptide. Comparison with the heating process of the crystalline Ag(I) complex of β-alanine indicated that stepwise reduction is key for maintaining the original micrometre-order morphology.

Conformational Changes and Charge Transfer in Biomolecules Resolved Using Dynamic Enhanced Raman Correlation Spectroscopy

In this contribution, we report that conformational changes of molecules that are often buried in a wide-distributed Gaussian distribution can be discerned by analyzing the dynamics of specific Raman lines. We investigate the pertinence of the auto- and cross-correlation functions applied to the dynamics of three Raman lines of an amino acid, the tryptophan. The cross-correlation between intensity and the Raman band is an indicator of the charge transfer during the diffusion limited reaction of tryptophan and the gold surface. The Péclet number Pe can provide a valuable indicator of the convective and/or diffusive features of each Raman band. Adsorption induced conformation changes can be identified using the autocorrelation of the multiples states within the Raman band centered at 1550 cm^-1.

In Situ Identification of Spherical Ag Monomers and Dimers at Zeptomole Adsorbate Concentrations by Surface-Enhanced Raman Scattering Correlation Spectroscopy

We have developed a bright optical setup, especially optimized for surface-enhanced Raman scattering (SERS) correlation spectroscopy. We show that the brightness of the experiment combined with the correlation approach permits in situ access to the size and shape of the SERS-active aggregates in solution despite a very weak SERS signal of the adsorbed molecules. As a result, we demonstrate that dimers and larger SERS-active aggregates can be identified through the temporal fluctuations of the SERS signal of only a few hundred of adsorbed molecules, that is, at zeptomole adsorbate concentrations. Monomers covered by a monolayer of MBA were also identified. These results open a way for single nanoparticle sensing, for single-molecule SERS-active aggregate characterization, or for quantitative monitoring of functionalization processes on metallic objects.

Predictions on the SERS enhancement factor of gold nanosphere aggregate samples

A boundary element method simulation is used to accurately predict the SERS EFs of gold nanoparticle aggregates via their experimental extinction spectra.

Watching Visible Light-Driven CO2 Reduction on a Plasmonic Nanoparticle Catalyst

Photocatalytic reduction of carbon dioxide (CO₂) by visible light has the potential to mimic plant photosynthesis and facilitate the renewable production of storable fuels. Accomplishing desirable efficiency and selectivity in artificial photosynthesis requires an understanding of light-driven pathways on photocatalyst surfaces. Here, we probe with single-nanoparticle spatial resolution the dynamics of a plasmonic silver (Ag) photocatalyst under conditions of visible light-driven CO₂ reduction. In situ surface-enhanced Raman spectroscopy captures discrete adsorbates and products formed dynamically on single photocatalytic nanoparticles, most prominent among which is a surface-adsorbed hydrocarboxyl (HOCO*) intermediate critical to further reduction of CO₂ to carbon monoxide (CO) and formic acid (HCOOH). Density functional theory simulations of the captured adsorbates reveal the mechanism by which plasmonic excitation activates physisorbed CO₂ leading to the formation of HOCO*, indicating close interplay between photoexcited states and adsorbate/metal interactions.

Highly robust, uniform and ultra-sensitive surface-enhanced Raman scattering substrates for microRNA detection fabricated by using silver nanostructures grown in gold nanobowls

Highly sensitive and reproducible surface enhanced Raman spectroscopy (SERS) requires not only a nanometer-level structural control, but also superb uniformity across the SERS substrate for practical imaging and sensing applications. However, in the past, increased reproducibility of the SERS signal was incompatible with increased SERS sensitivity. This work presents multiple silver nanocrystals inside periodically arrayed gold nanobowls (SGBs) via an electrochemical reaction at an overpotential of -3.0 V (vs. Ag/AgCl). The gaps between the silver nanocrystals serve as hot spots for SERS enhancement, and the evenly distributed gold nanobowls lead to a high device-to-device signal uniformity. The SGBs on the large sample surface exhibit an excellent SERS enhancement factor of up to 4.80 × 10⁹, with excellent signal uniformity (RSD < 8.0 ± 2.5%). Furthermore, the SGBs can detect specific microRNA (miR-34a), which plays a widely acknowledged role as biomarkers in diagnosis and treatment of diseases. Although the small size and low abundance of miR-34a in total RNA samples hinder their detection, by utilizing the advantages of SGBs in SERS sensing, reliable and direct detection of human gastric cancer cells has been successfully accomplished.

In situ synthesis of Au-shelled Ag nanoparticles on PDMS for flexible, long-life, and broad spectrum-sensitive SERS substrates

A simple, fast, one-step fabrication of silver nanoparticles with atomically thin gold coatings on polydimethylsiloxane affords oxidation-resistant and highly sensitive surface enhanced Raman scattering (SERS) substrates.

Plasmonic enhancement of SERS measured on molecules in carbon nanotubes

We isolated the plasmonic contribution to surface-enhanced Raman scattering (SERS) and found it to be much stronger than expected. Organic dyes encapsulated in single-walled carbon nanotubes are ideal probes for quantifying plasmonic enhancement in a Raman experiment. The molecules are chemically protected through the nanotube wall and spatially isolated from the metal, which prevents enhancement by chemical means and through surface roughness. The tubes carry molecules into SERS hotspots, thereby defining molecular position and making it accessible for structural characterization with atomic-force and electron microscopy. We measured a SERS enhancement factor of 10⁶ on α-sexithiophene (6T) molecules in the gap of a plasmonic nanodimer. This is two orders of magnitude stronger than predicted by the electromagnetic enhancement theory (10⁴). We discuss various phenomena that may explain the discrepancy (including hybridization, static and dynamic charge transfer, surface roughness, uncertainties in molecular position and orientation), but found all of them lacking in enhancement for our probe system. We suggest that plasmonic enhancement in SERS is, in fact, much stronger than currently anticipated. We discuss novel approaches for treating SERS quantum mechanically that appear promising for predicting correct enhancement factors. Our findings have important consequences on the understanding of SERS as well as for designing and optimizing plasmonic substrates.

Large Format Surface-Enhanced Raman Spectroscopy Substrate Optimized for Enhancement and Uniformity

Gratings have been widely investigated both theoretically and experimentally as surface-enhanced Raman spectroscopy (SERS) substrates, exhibiting, under appropriate circumstances, increased far-field extinctions and near-field intensities over those of an appropriately equivalent number of isolated particles. When the grating order transitions from evanescent to radiative, narrow resonance peaks are observed in the extinction spectrum whose properties can be manipulated by controlling the grating's geometric parameters. Here we report the application of the architectural principles of grating fabrication using a square two-dimensional array of gold-coated nanostructures that achieves SERS enhancements of 10(7) uniformly over areas of square centimeters. The high-performance grating substrates were fabricated using commonly available foundry-based techniques that have been chosen for their applicability to large-scale wafer processing. Additionally, we restricted ourselves to a parametric regime that optimizes SERS performance in a repeatable and reproducible manner.

Rational design of Raman-labeled nanoparticles for a dual-modality, light scattering immunoassay on a polystyrene substrate

Background

Surface-enhanced Raman scattering (SERS) is a powerful light scattering technique that can be used for sensitive immunoassay development and cell labeling. A major obstacle to using SERS is the complexity of fabricating SERS probes since they require nanoscale characterization and optical uniformity. The light scattering response of SERS probes may also be modulated by the substrate used for SERS analysis. A typical SERS substrate such as quartz can be expensive. Polystyrene is a cheaper substrate option but can decrease the SERS response due to interfering Raman emission peaks and high background fluorescence. The goal of this research is to develop an optimized process for fabricating Raman-labeled nanoparticles for a SERS-based immunoassay on a polystyrene substrate.

Results

We have developed a method for fabricating SERS nanoparticle probes for use in a light scattering immunoassay on a polystyrene substrate. The light scattering profile of both spherical gold nanoparticle and gold nanorod SERS probes were characterized using Raman spectroscopy and optical absorbance spectroscopy. The effects of substrate interference and autofluorescence were reduced by selecting a Raman reporter with a strong light scattering response in a spectral region where interfering substrate emission peaks are minimized. Both spherical gold nanoparticles and gold nanorods SERS probes used in the immunoassay were detected at labeling concentrations in the low pM range. This analytical sensitivity falls within the typical dynamic range for direct labeling of cell-surface biomarkers using SERS probes.

Conclusion

SERS nanoparticle probes were fabricated to produce a strong light scattering signal despite substrate interference. The optical extinction and inelastic light scattering of these probes was detected by optical absorbance spectroscopy and Raman spectroscopy, respectively. This immunoassay demonstrates the feasibility of analyzing strongly enhanced Raman signals on polystyrene, which is an inexpensive yet non-ideal Raman substrate. The assay sensitivity, which is in the low pM range, suggests that these SERS probe particles could be used for Raman labeling of cell or tissue samples in a polystyrene tissue culture plate. With continued development, this approach could be used for direct labeling of multiple cell surface biomarkers on strongly interfering substrate platforms.

Electronic supplementary material

The online version of this article (doi:10.1186/s13036-015-0023-y) contains supplementary material, which is available to authorized users.

Biotags Based on Surface-Enhanced Raman Can Be as Bright as Fluorescence Tags

Surface enhanced Raman spectroscopy (SERS) is a powerful analytical technique that has been proposed as a substitute for fluorescence for biological imaging and detection but is not yet commercially utilized. The reason lies primarily in the lower intensity and poor reproducibility of most metal nanoparticle-based tags as compared to their fluorescence-based counterparts. Here, using a technique that scrupulously preserves the same number of dye molecules in both the SERS and fluorescence measurements, we show that SERS-based biotags (SBTs) with highly reproducible optical properties can be nanoengineered such that their brightness is at least equal to that of fluorescence-based tags.

Hierarchical porous plasmonic metamaterials for reproducible ultrasensitive surface-enhanced Raman spectroscopy

Hierarchical porous plasmonic metamaterials consisting of periodic nanoholes with tunable diameter and uniformly distributed mesopores over the bulk are developed as a new class of 3D surface-enhanced Raman spectroscopy (SERS) substrates. This multiscale architecture not only facilitates efficient cascaded electromagnetic enhancement but also provides an enormous number of Raman-active binding sites, exhibiting excellent reproducibility and ultrasensitive detection of aromatic molecules down to 10(-13) M.

Prospects for plasmonic hot spots in single molecule SERS towards the chemical imaging of live cells

Single molecule surface enhanced Raman scattering (SM-SERS) is a highly local effect occurring at sharp edges, interparticle junctions and crevices or other geometries with a sharp nanoroughness of plasmonic nanostructures (“hot spots”) for an analyte detection.

Antitags: SERS-encoded nanoparticle assemblies that enable single-spot multiplex protein detection

Simultaneous detection of multiple proteins on a single spot can be efficiently achieved by using multiplexed surface-enhanced Raman spectroscopy (SERS)-encoded nanoparticle 'antitags' consisting of poly(ethylene glycol) (PEG)-protected silver dimers (and higher aggregates) and antibody-tagging entities. The effective SERS-based multivariate deconvolution approach guarantees an accurate and successful distinguishable identification of single and multiple proteins in complex samples. Their potential application in multiplexed SERS bioimaging technology can be easily envisaged.

Surface enhanced Raman correlation spectroscopy of particles in solution

Surface enhanced Raman correlation spectroscopy (SERCS) is shown as a label-free, chemically specific method for monitoring individual polymer beads and lipid vesicles interacting with a 2-D planar surface enhanced Raman (SERS) substrate in solution. The enhancement afforded by the SERS substrate allows for spectral data to be acquired in series at rates between 31 and 83 Hz. Auto- and cross-correlation of spectral data facilitates the measurement of diffusion constants for particles ranging in radius from 50 to 500 nm while discriminating signal associated with the target analyte from extraneous fluctuations. The measured diffusion coefficients are on the order of 10(-10)-10(-11) cm(2)/s, a factor of 40 times slower than predicted from the Stokes-Einstein equation, suggesting that particles are experiencing hindered diffusion at the surface. The enhanced signals appear to originate from particles less than 5 nm of the SERS substrate, consistent with adsorption to the surface. This work provides a means to measure and monitor surface interactions and demonstrates the utility and limits of SERS detection in solution over planar SERS substrates.

Persistent misconceptions regarding SERS

SERS is some 35 years old, and the subject of over 11 000 articles. The field of Plasmonics, and large aspects of Metamaterials are clearly based on concepts that became current as a result of SERS. Despite this, a number of persistent, fuzzy ideas about the origin of the enhancement in SERS continue to be current even among SERS researchers, leading to the external impression that SERS is uniquely poorly understood. Six such ideas are discussed.

Simultaneous SERS detection of copper and cobalt at ultratrace levels

We report a SERS-based method for the simultaneous and independent determination of two environmental metallic pollutants, Cu(ii) and Co(ii). This was achieved by exploiting the coordination-sensitive Raman bands of a terpyridine (TPY) derivative for detecting transition metal ions. Changes in the vibrational SERS spectra of dithiocarbamate anchored terpyridine (TPY-DTC) were correlated as a function of each metal ion concentration, with limits of detection comparable to those of several conventional analytical methods. Simultaneous detection of ultratrace levels of Co(ii) in the presence of high Cu(ii) concentration was also demonstrated, supporting the potential of this sensing strategy for monitoring potable water supplies.

Advances in superresolution optical fluctuation imaging (SOFI)

We review the concept of superresolution optical fluctuation imaging (SOFI), discuss its attributes and trade-offs (in comparison with other superresolution methods), and present superresolved images taken on samples stained with quantum dots, organic dyes, and plasmonic metal nanoparticles. We also discuss the prospects of SOFI for live cell superresolution imaging and for imaging with other (non-fluorescent) contrasts.

Molecularly-mediated assemblies of plasmonic nanoparticles for Surface-Enhanced Raman Spectroscopy applications

In recent years, Surface-Enhanced Raman Spectroscopy (SERS) has experienced a tremendous increase of attention in the scientific community, expanding to a continuously wider range of diverse applications in nanoscience, which can mostly be attributed to significant improvements in nanofabrication techniques that paved the way for the controlled design of reliable and effective SERS nanostructures. In particular, the plasmon coupling properties of interacting nanoparticles are extremely intriguing due to the concentration of enormous electromagnetic enhancements at the interparticle gaps. Recently, great efforts have been devoted to develop new nanoparticle assembly strategies in suspension with improved control over hot-spot architecture and cluster structure, laying the foundation for the full exploitation of their exceptional potential as SERS materials in a wealth of chemical and biological sensing. In this review we summarize in an exhaustive and systematic way the state-of-art of plasmonic nanoparticle assembly in suspension specifically developed for SERS applications in the last 5 years, focusing in particular on those strategies which exploited molecular linkers to engineer interparticle gaps in a controlled manner. Importantly, the novel advances in this rather new field of nanoscience are organized into a coherent overview aimed to rationally describe the different strategies and improvements in the exploitation of colloidal nanoparticle assembly for SERS application to real problems.

Label-free in situ detection of individual macromolecular assemblies by surface enhanced Raman scattering

We demonstrate label-free detection of lipid vesicles and polystyrene beads freely diffusing in aqueous solution using surface enhanced Raman scattering (SERS). The signals observed enable real-time identification and monitoring of individual particles interacting with the SERS substrate. SERS is demonstrated as a label-free method capable of monitoring transient species in solution on the millisecond time scale.