Progress in plasmonic engineering of surface-enhanced Raman-scattering substrates toward ultra-trace analysis

Artificial Intelligence for Surface-Enhanced Raman Spectroscopy

Surface-enhanced Raman spectroscopy (SERS), well acknowledged as a fingerprinting and sensitive analytical technique, has exerted high applicational value in a broad range of fields including biomedicine, environmental protection, food safety among the others. In the endless pursuit of ever-sensitive, robust, and comprehensive sensing and imaging, advancements keep emerging in the whole pipeline of SERS, from the design of SERS substrates and reporter molecules, synthetic route planning, instrument refinement, to data preprocessing and analysis methods. Artificial intelligence (AI), which is created to imitate and eventually exceed human behaviors, has exhibited its power in learning high-level representations and recognizing complicated patterns with exceptional automaticity. Therefore, facing up with the intertwining influential factors and explosive data size, AI has been increasingly leveraged in all the above-mentioned aspects in SERS, presenting elite efficiency in accelerating systematic optimization and deepening understanding about the fundamental physics and spectral data, which far transcends human labors and conventional computations. In this review, the recent progresses in SERS are summarized through the integration of AI, and new insights of the challenges and perspectives are provided in aim to better gear SERS toward the fast track.

Metallic and Non-Metallic Plasmonic Nanostructures for LSPR Sensors

Localized surface plasmonic resonance (LSPR) provides a unique scheme for light management and has been demonstrated across a large variety of metallic nanostructures. More recently, non-metallic nanostructures of two-dimensional atomic materials and heterostructures have emerged as a promising, low-cost alternative in order to generate strong LSPR. In this paper, a review of the recent progress made on non-metallic LSPR nanostructures will be provided in comparison with their metallic counterparts. A few applications in optoelectronics and sensors will be highlighted. In addition, the remaining challenges and future perspectives will be discussed.

Recent Progress in the Application of Metal Organic Frameworks in Surface-Enhanced Raman Scattering Detection

Metal-organic framework (MOF) compounds are centered on metal ions or metal ion clusters, forming lattices with a highly ordered periodic porous network structure by connecting organic ligands. As MOFs have the advantages of high porosity, large specific surface area, controllable pore size, etc., they are widely used in gas storage, catalysis, adsorption, separation and other fields. SERS substrate based on MOFs can not only improve the sensitivity of SERS analysis but also solve the problem of easy aggregation of substrate nanoparticles. By combining MOFs with SERS, SERS performance is further improved, and tremendous research progress has been made in recent years. In this review, three methods of preparing MOF-based SERS substrates are introduced, and the latest applications of MOF-based SERS substrates in biosensors, the environment, gases and medical treatments are discussed. Finally, the current status and prospects of MOF-based SERS analysis are summarized.

Charge transfer effect: a new assignment of the abnormal optical absorption band of gold nanoparticles

As a significant accompanying phenomenon of surface-enhanced Raman scattering (SERS), the addition of foreign molecules to colloidal gold or silver nanoparticles results in a new abnormal optical absorption (AOA) band, which usually appears in the long-wavelength region. The assignment of this AOA band has long been debated as an important issue that is desired to be addressed in the SERS field, which is crucial for a clear understanding of the SERS enhancement mechanism and beneficial to surface plasmonics. In this study, both the calculated and measured optical absorptions of gold nanoparticle monomers and dimers as well as their interactions with adsorbed molecules, showed that the AOA band in the long-wavelength region which was assigned to the characteristic longitudinal localized surface plasmon resonance (LSPR) effect of gold nanoparticle chain aggregates in conventional SERS electromagnetic theory, should be attributed to the charge-transfer resonance absorption from gold nanoparticles to adsorbed molecules. This was further confirmed by the corresponding SERS effects. As the excitation wavelength at 785 nm was resonant with the broad AOA band centered at 750 nm, the SERS peaks of the adsorbed pyridine molecules could be dramatically enhanced due to the charge-transfer resonance effect. In contrast, under an excitation wavelength of 532 nm, the SERS peaks appeared very weak, although the excitation wavelength was resonant with the LSPR absorption band of the individual gold nanoparticles.

Measurement of trace bisphenol A in drinking water with combination of immunochromatographic detection technology and SERS method

Sensitive and selective detection of target analyte is very important in many fields such as commodity inspection and quality monitoring. In this work, based on the principle of competitive immunoassay, surface-enhanced Raman spectroscopy (SERS) was used to establish a rapid and highly sensitive method for the detection of trace amounts of bisphenol A in water. Here, Raman molecule 5,5-dithiobis-2-nitrobenzoic acid and anti-BPA antibody were conjugated with Au (core)@Ag (shell) nanoparticle to serve as SERS nanoprobe. After the SERS nanoprobe is combined with the substance to be tested, it uses the siphon effect to pass through the test line and the charging line on the test strip. And the Raman test was performed on the T line with a Raman spectrometer. The detection limitation was 0.1 pg/mL. Compared with the reported gas chromatography, liquid chromatography, fluorescence analysis, and other detection methods, SERS ICA does not demand complicated sample preparation procedures, and has the advantages of simple detection methods, quick results, High sensitivity, good specificity, and low technical demands for laboratory environment and testers. In addition, Raman spectrometers have gradually developed to be portable, making it easier to meet the needs of on-site rapid and highly sensitive detection, and will show broad prospects for applications in the fields of biomedical diagnosis and food safety monitoring.

A New Covalent Organic Framework of Dicyandiamide-Benzaldehyde Nanocatalytic Amplification SERS/RRS Aptamer Assay for Ultratrace Oxytetracycline with the Nanogold Indicator Reaction of Polyethylene Glycol 600

In this paper, dicyandiamide (Dd) and p-benzaldehyde (Bd) were heated at 180 °C for 3 h to prepare a new type of stable covalent organic framework (COF) DdBd nanosol with high catalysis. It was characterized by molecular spectroscopy and electron microscopy. The study found that DdBd had a strong catalytic effect on the new indicator reaction of polyethylene glycol 600 (PEG600)-chloroauric acid to form gold nanoparticles (AuNPs). AuNPs have strong resonance Rayleigh scattering (RRS) activity, and in the presence of Victoria Blue B (VBB) molecular probes, they also have a strong surface-enhanced Raman scattering (SERS) effect. Combined with a highly selective oxytetracycline (OTC) aptamer (Apt) reaction, new dual-mode scattering SERS/RRS methods were developed to quantitatively analyze ultratrace OTC. The linear range of RRS is 3.00 × 10-3 -6.00 × 10-2 nmol/L, the detection limit is 1.1 × 10-3 nmol/L, the linear range of SERS is 3.00 × 10-3-7.00 × 10-2 nmol/L, and the detection limit is 9.0 × 10-4 nmol/L. Using the SERS method to analyze OTC in soil samples, the relative standard deviation is 1.35-4.78%, and the recovery rate is 94.3-104.9%.

Coupling enhancement mechanisms, materials, and strategies for surface-enhanced Raman scattering devices

Surface-enhanced Raman scattering (SERS) has become one of the most sensitive analytical techniques for identifying the chemical components, molecular structures, molecular conformations, and the interactions between molecules. However, great challenges still need to be addressed until it can be widely accepted by the absolute quantification of analytes. Recently, many efforts have been devoted to addressing these issues via various electromagnetic (EM), chemical (CM), and EM-CM hybrid coupling enhancement strategies. In comparison with uncoupled SERS devices, they offer key advantages in terms of sensitivity, reproducibility, uniformity, stability, controllability and reliability. This review provides an in-depth analysis of coupled SERS devices, including coupling enhancement mechanisms, materials and approaches. Finally, we also discuss the remaining bottlenecks and possible strategies for the development of coupling-enhanced SERS devices in the future.

Improve optical properties by modifying Ag nanoparticles on a razor clam SERS substrate

Irregular substrates are inappropriate for enhancing surface enhanced Raman scattering (SERS) due to their poor performances in terms of uniformity, enhancement performance, and polarization characteristics. However, in this work, we purposely employed a natural biological razor clam material with messy and irregular structures to improve the SERS. The rough surface was achieved by magnetron sputtering Ag nanoislands on the prism layer of the razor clams, and the Ag nanoparticles were treated using the method of oil-water interface self-assembly to form relatively uniform structures. Compared to the substrate without Ag nanoparticles, the presented substrate has better reproducibility, polarization-independence, and higher SERS intensity, and the detect limitation of R6G can be decreased from 10^-12 M to 10^-18 M. The ultrasensitive detection of thiram gives our structures potential for high sensitivity biosensors.

Silver nanoparticles-decorated Preyssler functionalized cellulose biocomposite as a novel and efficient catalyst for the synthesis of 2-amino-4H-pyrans and spirochromenes

Silver nanoparticles-decorated Preyssler functionalized cellulose biocomposite (PC/AgNPs) was prepared and fully characterized by FTIR, UV–vis, SEM, and TEM techniques. The preparation of PC/AgNPs was studied systematically to optimize the processing parameters by Taguchi method using the amount of PC, reaction temperature, concentration of silver nitrate and pH of medium. Taguchi’s L9 orthogonal (4 parameters, 4 level) was used for the experimental design. The SEM analysis confirmed the presence of the Preyssler as a white cloud as well as spherical AgNPs on the surface of cellulose. The formation of AgNPs on the surface was observed by changing of the color from yellow to deep brown and confirmed by UV–vis spectroscopy. The best yield of AgNPs forming was obtained in pH 12.5 at 80 ºC in 20 min. TEM analysis confirmed the formation of spherical AgNPs with a size of 50 nm, at the 1% wt. loading of Preyssler. This easily prepared PC/AgNPs was successfully employed as an efficient, green, and reusable catalyst in the synthesis of a wide range of 2-amino-4H-pyran and functionalized spirochromene derivatives via a one-pot, multicomponent reaction. The chief merits realized for this protocol were the utilization of commercially available or easily accessible starting materials, operational simplicity, facile work-up procedure, obtaining of high to excellent yields of the products and being done under green conditions. The catalyst could be easily separated from the reaction mixture and reused several times without observing any appreciable loss in its efficiency.

Phosphorylation-Dependent SERS Readout for Activity Assay of Protein Kinase A in Cell Extracts

Protein kinases are key regulators of cell function, the abnormal activity of which may induce several human diseases, including cancers. Therefore, it is of great significance to develop a sensitive and reliable method for assaying protein kinase activities in real biological samples. Here, we report the phosphorylation-dependent surface-enhanced Raman scattering (SERS) readout of spermine-functionalized silver nanoparticles (AgNPs) for protein kinase A (PKA) activity assay in cell extracts. In this assay, the presence of PKA would phosphorylate and alter the net charge states of Raman dye-labeled substrate peptides, and the resulting anionic products could absorb onto the AgNPs with cationic surface charge through electrostatic attraction. Meanwhile, the Raman signals of dyes labeled on peptides were strongly enhanced by the aggregated AgNPs with interparticle hot spots formed in assay buffer. The SERS readout was directly proportional to the PKA activity in a wide range of 0.0001-0.5 U·μL^-1 with a detection limit as low as 0.00003 U·μL^-1. Moreover, the proposed SERS-based assay for the PKA activity was successfully applied to monitoring the activity and inhibition of PKA in real biological samples, particularly in cell extracts, which would be beneficial for kinase-related disease diagnostics and inhibitor screening.

Fabrication of liquid–liquid self-assembled Ag arrays on disposable screen-printed electrodes and their application in the identification and analysis of the adsorption behavior of organic carboxylates through in situ electrochemical surface-enhanced Raman scattering

A disposable Ag array@screen-printed electrode (SPE) was fabricated for the identification and analysis of the adsorption behavior of organic carboxylates in in situ electrochemical surface-enhanced Raman scattering (EC-SERS).

Facile synthesis of silver-rich Au/Ag bimetallic nanoparticles with highly active SERS properties

A one-pot route to the synthesis of Au/Ag bimetallic nanoparticles with a high SERS activity for detection of proteins at low concentration.

Extraordinary optical fields in nanostructures: from sub-diffraction-limited optics to sensing and energy conversion

Along with the rapid development of micro/nanofabrication technology, the past few decades have seen the flourishing emergence of subwavelength-structured materials and interfaces for optical field engineering at the nanoscale. Three remarkable properties associated with these subwavelength-structured materials are the squeezed optical fields beyond the diffraction limit, gradient optical fields in the subwavelength scale, and enhanced optical fields that are orders of magnitude greater than the incident field. These engineered optical fields have inspired fundamental and practical advances in both engineering optics and modern chemistry. The first property is the basis of sub-diffraction-limited imaging, lithography, and dense data storage. The second property has led to the emergence of a couple of thin and planar functional optical devices with a reduced footprint. The third one causes enhanced radiation (e.g., fluorescence), scattering (e.g., Raman scattering), and absorption (e.g., infrared absorption and circular dichroism), offering a unique platform for single-molecule-level biochemical sensing, and high-efficiency chemical reaction and energy conversion. In this review, we summarize recent advances in subwavelength-structured materials that bear extraordinary squeezed, gradient, and enhanced optical fields, with a particular emphasis on their optical and chemical applications. Finally, challenges and outlooks in this promising field are discussed.

Spiers Memorial Lecture : Introductory lecture: Hot-electron science and microscopic processes in plasmonics and catalysis

In these introductory remarks we discuss the generation of nonequilibrium electrons in metals, their properties, and how they can be utilized in two emerging applications: for extending the capabilities of photodetection (left), and for photocatalysis (right), lowering the barriers of chemical reactions.

Improving the Surface-Enhanced Raman Scattering Performance of Silver Nanodendritic Substrates with Sprayed-On Graphene-Based Coatings

This study examines the improvements in surface-enhanced Raman scattering (SERS) performance achieved when silver nanodendritic structures are coated with various graphene-based materials, namely graphene oxide (GO), reduced graphene oxide (rGO), and graphene nanoplatelets (GNPs). The tests are performed on our unique SERS-active substrates, prepared on the surface of planar microelectrode chips using an electric field-guided Ag nanoparticle assembly process. The graphene-based materials are introduced into the substrate by means of an in-house spray-coating technique. The SERS enhancement effect of each coating is examined as a function of spray nozzle passes (N) and optimal values are identified for each coating type. The enhancements found for GO, rGO, and GNP (6⁻9 graphene layers thick) coatings are 2.3 (N = 25), 2.5 (N = 5), and 1.6 (N = 1), respectively. Additionally, in comparison with their uncoated counterparts, substrates coated with rGO (N = 5) are shown to enhance the intensity of the methamphetamine (5 ppb) spectrum in artificial saliva by approximately 3-fold. Overall, it can be concluded that the introduction of GO or rGO to the SERS substrate using spray-coating, a simple and also scalable method, can produce substantial SERS performance enhancement.

Controllable self-assembled plasmonic vesicle-based three-dimensional SERS platform for picomolar detection of hydrophobic contaminants

Hydrophobic contaminants in food and the environment seriously threaten human health. The ultrasensitive detection of these pollutants can minimize their damage. However, current ultrasensitive sensing strategies are limited to solid substrate-based surface-enhanced Raman spectroscopy (SERS) detection. Herein, we report a controllable and reproducible solution-based SERS detection platform for the direct and ultrasensitive detection of hydrophobic contaminants by using self-assembled three-dimensional plasmonic vesicles. To this end, amphiphilic gold nanoparticles (AuNPs) tethered with linear block copolymer (BCP) of polystyrene-b-poly (ethylene oxide) (PS-b-PEO) were designed, which display dual functions for improving detection sensitivity, including serving as building blocks for the construction of plasmonic vesicles to yield large numbers of hot-spots for SERS enhancement, and providing hydrophobic PS layers to enrich and concentrate target hydrophobic molecules for direct SERS detection with hydrophobic interaction. By modulating the AuNP size and the length of BCP chains, the ultrahigh detection sensitivity, down to the picomolar level, was obtained via using 80 nm AuNPs tethered with BCP of PEO45-b-PS900-SH. In addition, the proposed method exhibits excellent reproducibility, universality, practicability, as well as multiplexing detection capacity in actual contaminant-spiked soil samples. Briefly, the designed self-assembled plasmonic vesicle-based SERS platform provides an ideal generic methodology for the ultrasensitive detection of hydrophobic contaminants that can greatly accelerate on-site testing in food and environmental monitoring.

Surface-Enhanced Raman Scattering Spectral Imaging for the Attomolar Range Detection of Crystal Violet in Contaminated Water

A series of nanocomposites based on polyamide (NL16, PA) filter membranes containing metal nanoparticles (NPs) have been prepared by filtration under reduced pressure of the metal colloids. The ensuing materials were then investigated as substrates for surface-enhanced Raman scattering (SERS) imaging studies envisaging the spectroscopic detection of vestigial organic pollutants dissolved in contaminated water. The organic dye crystal violet (CV) was used here as a model pollutant because it is a hazardous compound present in certain effluent waters. Moreover this compound is well-known for its strong SERS activity, which is clearly advantageous in the context of material development for SERS. Indeed, several preparative strategies were employed to prepare PA-based composites, and the impact on SERS detection was investigated. These include the use of chemical and morphological distinct plasmonic NPs (Ag, Au), a variable metal load and changing the order of addition of the analytical specimens. These studies demonstrate that the parameters employed in the fabrication of the SERS substrates have a strong impact on the Raman signal enhancement. The use of Raman imaging during the fabrication process allows establishing improvements that translate to better performances of the substrates in the analyte detection. The results have been interpreted by considering an integrated set of operational parameters that include the affinity of CV molecules to the substrate, amount and dispersion of NPs in the PA membranes, and the detection method.  Noteworthy the use of SERS analysis assisted with Raman imaging allowed achieving a detection limit for CV as low as 100 aM in ultrapure water and 10 fM in real samples.

A silver/graphene oxide nanocomposite film as a flexible SERS substrate for reliable quantitative analysis using high-speed spiral scanning spectrometry

Excellent uniformity (∼1.5% RSD) in SERS signals was obtained from an Ag/GO decorated adhesive tape on a simple in-house cylindrical scanning system. The calibration curve for the quantitative analysis of CV shows reliable linearity ranging from 75 nM to 50 μM. This novel method is promising to be an adept tool for universal quantitative analysis and be used complementarily with the conventional Raman mapping method for a more time efficient and reliable analysis.

Fabrication of Semiconductor ZnO Nanostructures for Versatile SERS Application

Since the initial discovery of surface-enhanced Raman scattering (SERS) in the 1970s, it has exhibited a huge potential application in many fields due to its outstanding advantages. Since the ultra-sensitive noble metallic nanostructures have increasingly exposed themselves as having some problems during application, semiconductors have been gradually exploited as one of the critical SERS substrate materials due to their distinctive advantages when compared with noble metals. ZnO is one of the most representative metallic oxide semiconductors with an abundant reserve, various and cost-effective fabrication techniques, as well as special physical and chemical properties. Thanks to the varied morphologies, size-dependent exciton, good chemical stability, a tunable band gap, carrier concentration, and stoichiometry, ZnO nanostructures have the potential to be exploited as SERS substrates. Moreover, other distinctive properties possessed by ZnO such as biocompatibility, photocatcalysis and self-cleaning, and gas- and chemo-sensitivity can be synergistically integrated and exerted with SERS activity to realize the multifunctional potential of ZnO substrates. In this review, we discuss the inevitable development trend of exploiting the potential semiconductor ZnO as a SERS substrate. After clarifying the root cause of the great disparity between the enhancement factor (EF) of noble metals and that of ZnO nanostructures, two specific methods are put forward to improve the SERS activity of ZnO, namely: elemental doping and combination of ZnO with noble metals. Then, we introduce a distinctive advantage of ZnO as SERS substrate and illustrate the necessity of reporting a meaningful average EF. We also summarize some fabrication methods for ZnO nanostructures with varied dimensions (0–3 dimensions). Finally, we present an overview of ZnO nanostructures for the versatile SERS application.

Highly Sensitive, Uniform, and Reproducible Surface-Enhanced Raman Spectroscopy Substrate with Nanometer-Scale Quasi-periodic Nanostructures

We introduce a simple and cost-effective approach for fabrication of effective surface-enhanced Raman spectroscopy (SERS) substrates. It is shown that the as-fabricated substrates show excellent SERS effects in various probe molecules with high sensitivity, that is, picomolar level detection, and also good reliability. With a SERS enhancement factor beyond 10⁸ and excellent reproducibility (deviation less than 5%) of signal intensity, the fabrication of the SERS substrate is realized on a four-inch wafer and proven to be effective in pesticide residue detection. The SERS substrate is realized first through the fabrication of quasi-periodic nanostructured silicon with dimension features in tens of nanometers using superaligned carbon nanotubes networks as an etching mask, after which a large amount of hot spots with nanometer gaps are formed through deposition of a gold film. With rigorous nanostructure design, the enhanced performance of electromagnetic field distribution for nanostructures is optimized. With the advantage of cost-effective large-area preparation, it is believed that the as-fabricated SERS substrate could be used in a wide variety of actual applications where detection of trace amounts is necessary.

Surface-Enhanced Raman Spectroscopy: A Facile and Rapid Method for the Chemical Component Study of Individual Atmospheric Aerosol

A simple and rapid method for detecting chemical components of individual aerosol particles on Klarite substrate with surface-enhanced Raman spectroscopy (SERS) is described. For both single simulated aerosol particles and ambient atmospheric particles, this new analytical method promotes the enhancement factor of the Raman signal. The spectra of ammonium sulfate and naphthalene particles at the microscopic level are enhanced by a factor of 6 and therefore greatly improve the detection of the chemical composition of an individual aerosol particle. When aerosol particles are found over a microscopic domain, a set of Raman spectra with chemical information can be obtained via SERS mapping. The maps illustrate the distribution of organic or inorganic species on the SERS substrate. This constitutes a facile and rapid method to study aerosol particles. This new method allows the analysis of chemical composition in single aerosol particles, demonstrating the power of SERS to probe the ambient atmospheric particles and to study the formation of aerosol particles.

Self-assembly of silver nanoparticles as high active surface-enhanced Raman scattering substrate for rapid and trace analysis of uranyl(VI) ions

A facile surface-enhanced Raman scattering (SERS) substrate based on the self-assembly of silver nanoparticles on the modified silicon wafer was obtained, and for the first time, an advanced SERS analysis method basing on this as-prepared substrate was established for high sensitive and rapid detection of uranyl ions. Due to the weakened bond strength of OUO resulting from two kinds of adsorption of uranyl species ("strong" and "weak" adsorption) on the substrate, the ν₁ symmetric stretch vibration frequency of OUO shifted from 871cm^-1 (normal Raman) to 720cm^-1 and 826cm^-1 (SERS) along with significant Raman enhancement. Effects of the hydrolysis of uranyl ions on SERS were also investigated, and the SERS band at ~826cm^-1 was first used to approximately define the constitution of uranyl species at trace quantity level. Besides, the SERS intensity was proportional to the variable concentrations of uranyl nitrate ranging from 10^-7 to 10^-3molL^-1 with an excellent linear relation (R²=0.998), and the detection limit was ~10^-7molL^-1. Furthermore, the related SERS approach involves low-cost substrate fabrication, rapid and trace analysis simultaneously, and shows great potential applications for the field assays of uranyl ions in the nuclear fuel cycle and environmental monitoring.

Spectroscopic Detection of Glyphosate in Water Assisted by Laser-Ablated Silver Nanoparticles

Glyphosate is one of the most widely used herbicides in the world. Its safety for both human health and aquatic biomes is a subject of wide debate. There are limits to glyphosate’s presence in bodies of water, and it is usually detected through complex analytical procedures. In this work, the presence of glyphosate is detected directly through optical interrogation of aqueous solution. For this purpose, silver nanoparticles were produced by pulsed laser ablation in liquids. Limits of detection of 0.9 mg/L and 3.2 mg/L were obtained with UV-Vis extinction and Surface Enhanced Raman spectroscopies, respectively. The sensing mechanism was evaluated in the presence of potential interferents as well as with commercial glyphosate-based herbicides.

Surface regeneration and signal increase in surface-enhanced Raman scattering substrates

Regenerated surface-enhanced Raman scattering (SERS) substrates allow users the ability to not only reuse sensing surfaces, but also tailor them to the sensing application needs (wavelength of the available laser, plasmon band matching). In this review, we discuss the development of SERS substrates for response to emerging threats and some of our collaborative efforts to improve on the use of commercially available substrate surfaces. Thus, we are able to extend the use of these substrates to broader Army needs (like emerging threat response).

Highly sensitive pressure switch sensors and enhanced near ultraviolet photodetectors based on 3D hybrid film of graphene sheets decorated with silver nanoparticles

Highly sensitive pressure switch sensors and enhanced near ultraviolet photodetectors based on 3D Ag/GNs hybrid film were proposed and fabricated, and this work presented a simple route to obtain high performance devices.

Enhanced detection of explosives by turn-on resonance Raman upon host–guest complexation in solution and the solid state

Being colour coordinated allows turn on detection of nitroaromatics by combining molecular recognition with resonant enhancement of the Raman spectra.

Graphene/TiO2 hybrid layer for simultaneous detection and degradation by a one-step transfer and integration method

During the past decades, researchers have made great efforts towards an ideal surface enhanced Raman spectroscopy (SERS) substrate.

Formation Regularities of Plasmonic Silver Nanostructures on Porous Silicon for Effective Surface-Enhanced Raman Scattering

Plasmonic nanostructures demonstrating an activity in the surface-enhanced Raman scattering (SERS) spectroscopy have been fabricated by an immersion deposition of silver nanoparticles from silver salt solution on mesoporous silicon (meso-PS). The SERS signal intensity has been found to follow the periodical repacking of the silver nanoparticles, which grow according to the Volmer-Weber mechanism. The ratio of silver salt concentration and immersion time substantially manages the SERS intensity. It has been established that optimal conditions of nanostructured silver layers formation for a maximal Raman enhancement can be chosen taking into account a special parameter called effective time: a product of the silver salt concentration on the immersion deposition time. The detection limit for porphyrin molecules CuTMPyP4 adsorbed on the silvered PS has been evaluated as 10(-11) M.

Controlled-layer and large-area MoS2 films encapsulated Au nanoparticle hybrids for SERS

In this work, a facile and effective method for controlled-layer and large-area MoS₂ films encapsulated Au nanoparticle hybrids is developed. With accurate Ar plasma treatment time control, the large-area MoS₂ layers can be obtained from monolayer to trilayer. The fabricated MoS₂@Au NPs with higher surface area exhibit excellent Raman enhanced effect for aromatic organic molecules (rhodamine 6G and crystal violet) and achieve the best when the monolayer MoS₂@AuNPs was obtained. The limit of detection is found to be as low as 1 × 10^-10 M. The MoS₂@AuNPs was characterized by SEM, EDS, AFM, Raman spectroscopy, UV-Vis, XRD and HRTEM.

The application of nanoparticles in diagnosis and theranostics of gastric cancer

Gastric cancer is the fourth most common cancer and the second leading cause of cancer related death worldwide. For the diagnosis of gastric cancer, apart from regular systemic imaging, the locoregional imaging is also of great importance. Moreover, there are still other ways for the detecting of gastric cancer, including the early detection of gastric cancer by endoscopy, the detection of gastric-cancer related biomarkers and the detection of circulating tumor cells (CTCs) of gastric cancer. However, conventional diagnostic methods are usually lack of specificity and sensitivity. Nanoparticles provide many benefits in the diagnosis of gastric cancer. Besides, nanoparticles are capable of integrating the functions of diagnosis and treatment together (theranostics). In this paper, we reviewed the applications of nanoparticles in diagnosis and theranostics of gastric cancer in the above mentioned aspects.

Chemically Roughened Solid Silver: A Simple, Robust and Broadband SERS Substrate

Surface-enhanced Raman spectroscopy (SERS) substrates manufactured using complex nano-patterning techniques have become the norm. However, their cost of manufacture makes them unaffordable to incorporate into most biosensors. The technique shown in this paper is low-cost, reliable and highly sensitive. Chemical etching of solid Ag metal was used to produce simple, yet robust SERS substrates with broadband characteristics. Etching with ammonium hydroxide (NH₄OH) and nitric acid (HNO₃) helped obtain roughened Ag SERS substrates. Scanning electron microscopy (SEM) and interferometry were used to visualize and quantify surface roughness. Flattened Ag wires had inherent, but non-uniform roughness having peaks and valleys in the microscale. NH₄OH treatment removed dirt and smoothened the surface, while HNO₃ treatment produced a flake-like morphology with visibly more surface roughness features on Ag metal. SERS efficacy was tested using 4-methylbenzenethiol (MBT). The best SERS enhancement for 1 mM MBT was observed for Ag metal etched for 30 s in NH₄OH followed by 10 s in HNO₃. Further, MBT could be quantified with detection limits of 1 pM and 100 µM, respectively, using 514 nm and 1064 nm Raman spectrometers. Thus, a rapid and less energy intensive method for producing solid Ag SERS substrate and its efficacy in analyte sensing was demonstrated.

Ultrathin Carbon Film Protected Silver Nanostructures for Surface-Enhanced Raman Scattering

In this article, ultrathin carbon film protected silver substrate (Ag/C) was prepared via a plasma-enhanced chemical vapor deposition (PECVD) method. The morphological evolution of silver nanostructures underneath, as well as the surface-enhanced Raman scattering (SERS) activity of Ag/C hybrid can be tuned by controlling the deposition time. The stability and reproducibility of the as-prepared hybrid were also studied.

Boron Nitride Nanosheet-Veiled Gold Nanoparticles for Surface-Enhanced Raman Scattering

Atomically thin boron nitride (BN) nanosheets have many properties desirable for surface-enhanced Raman spectroscopy (SERS). BN nanosheets have a strong surface adsorption capability toward airborne hydrocarbon and aromatic molecules. For maximized adsorption area and hence SERS sensitivity, atomically thin BN nanosheet-covered gold nanoparticles have been prepared for the first time. When placed on top of metal nanoparticles, atomically thin BN nanosheets closely follow their contours so that the plasmonic hot spots are retained. Electrically insulating BN nanosheets also act as a barrier layer to eliminate metal-induced disturbances in SERS. Moreover, the SERS substrates veiled by BN nanosheets show an outstanding reusability in the long term. As a result, the sensitivity, reproducibility, and reusability of SERS substrates can be greatly improved. We also demonstrate that large BN nanosheets produced by chemical vapor deposition can be used to scale up the proposed SERS substrate for practical applications.

A rapid method to authenticate vegetable oils through surface-enhanced Raman scattering

Vegetable oils are essential in our daily diet. Among various vegetable oils, the major difference lies in the composition of fatty acids, including unsaturated fatty acids (USFA) and saturated fatty acids (SFA). USFA include oleic acid (OA), linoleic acid (LA), and α-linolenic acid (ALA), while SFA are mainly palmitic acid (PA). In this study, the most typical and abundant USFA present with PA in vegetable oils were quantified. More importantly, certain proportional relationships between the integrated intensities of peaks centered at 1656 cm⁻¹ (S₁₆₅₆) in the surface-enhanced Raman scattering spectra of different USFA were confirmed. Therefore, the LA or ALA content could be converted into an equivalent virtual OA content enabling the characterization of the USFA content in vegetable oils using the equivalent total OA content. In combination with the S₁₆₅₆ of pure OA and using peanut, sesame, and soybean oils as examples, the ranges of S₁₆₅₆ corresponding to the National Standards of China were established to allow the rapid authentication of vegetable oils. Gas chromatograph-mass spectrometer analyses verified the accuracy of the method, with relative errors of less than 5%. Moreover, this method can be extended to other detection fields, such as diseases.

Nanosensors for early cancer detection and for therapeutic drug monitoring

The use of nanotechnology for drug delivery in cancer therapy has raised high expectations. Additionally, the use of nanomaterials in sensors to extract and detect tumor specific biomarkers, circulating tumor cells, or extracellular vesicles shed by the tumor holds the promise to detect cancer much earlier and hence improve long-term survival of the patients. Moreover, the monitoring of the anticancer drug concentration, which has a narrow therapeutic window, will allow for a personalized dosing of the drug and will lead to improved therapeutic outcome and life quality of the patient. This review will provide an overview on the use of nanosensors for the early diagnosis of cancer and for the therapeutic drug monitoring, giving some examples. We envision nanosensors to make significant improvements in the cancer management as easy-to-use point-of-care devices for a broad population of users.

Advances in explosives analysis--part II: photon and neutron methods

The number and capability of explosives detection and analysis methods have increased dramatically since publication of the Analytical and Bioanalytical Chemistry special issue devoted to Explosives Analysis [Moore DS, Goodpaster JV, Anal Bioanal Chem 395:245-246, 2009]. Here we review and critically evaluate the latest (the past five years) important advances in explosives detection, with details of the improvements over previous methods, and suggest possible avenues towards further advances in, e.g., stand-off distance, detection limit, selectivity, and penetration through camouflage or packaging. The review consists of two parts. Part I discussed methods based on animals, chemicals (including colorimetry, molecularly imprinted polymers, electrochemistry, and immunochemistry), ions (both ion-mobility spectrometry and mass spectrometry), and mechanical devices. This part, Part II, will review methods based on photons, from very energetic photons including X-rays and gamma rays down to the terahertz range, and neutrons.

SERS detection of arsenic in water: A review

Arsenic (As) is one of the most toxic contaminants found in the environment. Development of novel detection methods for As species in water with the potential for field use has been an urgent need in recent years. In past decades, surface-enhanced Raman scattering (SERS) has gained a reputation as one of the most sensitive spectroscopic methods for chemical and biomolecular sensing. The SERS technique has emerged as an extremely promising solution for in-situ detection of arsenic species in the field, particularly when coupled with portable/handheld Raman spectrometers. In this article, the recent advances in SERS analysis of arsenic species in water media are reviewed, and the potential of this technique for fast screening and field testing of arsenic-contaminated environmental water samples is discussed. The problems that remain in the field are also discussed and an outlook for the future is featured at the end of the article.

Plasmonic properties of triangle-shaped silver trimers selectively fabricated by near-field photo-reduction using an apertured cantilever for an atomic force microscope

On an AgNO3 crystal, an equilateral or a right-angle triangle-shaped Ag trimer was selectively fabricated through near-field photo-reduction and observed in situ by using an apertured cantilever coupled with an atomic force microscope. By using the different triangle-shaped Ag trimers, irradiation wavelength and polarization dependence of surface-enhanced Raman scattering were investigated.

Self-assembled plasmonic templates produced by microwave annealing: applications to surface-enhanced Raman scattering

Perhaps the simplest method for creating metal nanoparticles on a substrate is by driving their self-assembly with the thermal annealing of a thin metal film. By properly tuning the annealing parameters one hopes to discover a recipe that allows the pre-determined design of the NP arrangement. However, thermal treatment is known for detrimental effects and is not really the manufacturer's route of choice when it comes to large-scale applications. An alternative method is the use of microwave annealing, a method that has never been applied for metal processing, due to the high reflectance of microwave radiation at the surface of a metal. However, in this work we challenge the widely used nanostructuring methods by proving the microwave's annealing ability to produce plasmonic templates, out of extremely thin metal films, by simply using a domestic microwave oven apparatus. We show that this process is generic and independent of the deposition method used for the metal and we further quantify the suitability of these plasmonic templates for use in surface-enhanced Raman scattering applications.

Surface-enhanced Raman imaging of cell membrane by a highly homogeneous and isotropic silver nanostructure

Label-free chemical imaging of live cell membranes can shed light on the molecular basis of cell membrane functionalities and their alterations under membrane-related diseases. In principle, this can be done by surface-enhanced Raman scattering (SERS) in confocal microscopy, but requires engineering plasmonic architectures with a spatially invariant SERS enhancement factor G(x, y) = G. To this end, we exploit a self-assembled isotropic nanostructure with characteristics of homogeneity typical of the so-called near-hyperuniform disorder. The resulting highly dense, homogeneous and isotropic random pattern consists of clusters of silver nanoparticles with limited size dispersion. This nanostructure brings together several advantages: very large hot spot density (∼10(4) μm(-2)), superior spatial reproducibility (SD < 1% over 2500 μm(2)) and single-molecule sensitivity (Gav ∼ 10(9)), all on a centimeter scale transparent active area. We are able to reconstruct the label-free SERS-based chemical map of live cell membranes with confocal resolution. In particular, SERS imaging is here demonstrated on red blood cells in vitro in order to use the Raman-resonant heme of the cell as a contrast medium to prove spectroscopic detection of membrane molecules. Numerical simulations also clarify the SERS characteristics of the substrate in terms of electromagnetic enhancement and distance sensitivity range consistently with the experiments. The large SERS-active area is intended for multi-cellular imaging on the same substrate, which is important for spectroscopic comparative analysis of complex organisms like cells. This opens new routes for in situ quantitative surface analysis and dynamic probing of living cells exposed to membrane-targeting drugs.

Hybrid nanostructures for SERS: materials development and chemical detection

This review focuses on recent developments in hybrid and nanostructured substrates for SERS (surface-enhanced Raman scattering) studies. Thus substrates composed of at least two distinct types of materials, in which one is a SERS active metal, are considered here aiming at their use as platforms for chemical detection in a variety of contexts. Fundamental aspects related to the SERS effect and plasmonic behaviour of nanometals are briefly introduced. The materials described include polymer nanocomposites containing metal nanoparticles and coupled inorganic nanophases. Chemical approaches to tailor the morphological features of these substrates in order to get high SERS activity are reviewed. Finally, some perspectives for practical applications in the context of chemical detection of analytes using such hybrid platforms are presented.