Surface-enhanced Raman Scattering Efficiency Research Articles

Mechanism Switch in Surface-Enhanced Raman Scattering: The Role of Nanoparticle Dimensions.

Surface-enhanced Raman scattering (SERS) is renowned for amplifying Raman signals, with electromagnetic mechanism (EM) enhancement arising from localized surface plasmon resonances and chemical mechanism (CM) enhancement as a result of charge transfer interactions. Despite the conventional emphasis on EM as a result of plasmonic effects, recent findings highlight the significance of CM when noble metals appear as smaller entities. However, the threshold size of the noble metal clusters/particles corresponding to the switch in SERS mechanisms is not clear at present. In this work, the VSe2-xOx/Au composites with different Au sizes are employed, in which a clear view of the SERS mechanism switch is observed at the Au size range of 16-21 nm. Our findings not only provide insight into the impact of noble metal size on SERS efficiency but also offer quantitative data to assist researchers in making informed judgments when analyzing SERS mechanisms.

Toward microfluidic SERS and EC-SERS applications via tunable gold films over nanospheres

Many promising applications of surface-enhanced Raman scattering (SERS), such as microfluidic SERS and electrochemical (EC)-SERS, require immersion of plasmonic nanostructured films in aqueous media. Correlational investigations of the optical response and SERS efficiency of solid SERS substrates immersed in water are absent in the literature. This work presents an approach for tuning the efficiency of gold films over nanospheres (AuFoN) as SERS substrates for applications in aqueous environment. AuFoN are fabricated by convective self-assembly of colloidal polystyrene nanospheres of various diameters (300–800 nm), followed by magnetron sputtering of gold films. The optical reflectance of the AuFoN and Finite-Difference Time-Domain simulations in both water and air reveal the dependence of the surface plasmon band on nanospheres’ diameter and environment. SERS enhancement of a common Raman reporter on AuFoN immersed in water is analyzed under 785 nm laser excitation, but also using the 633 nm line for the films in air. The provided correlations between the SERS efficiency and optical response in both air and water indicate the best structural parameters for high SERS efficiency and highlight a route for predicting and optimizing the SERS response of AuFoN in water based on the behavior in air, which is more practical. Finally, the AuFoN are successfully tested as electrodes for EC-SERS detection of the thiabendazole pesticide and as SERS substrates integrated in a flow-through microchannel format. The obtained results represent an important step toward the development of microfluidic EC-SERS devices for sensing applications.

Does the chemical contribution have a secondary role in SERS?

It is an established understanding that the electromagnetic contribution (plasmon-mediated enhancement of a laser and scattered local electromagnetic fields) is the main actor in surface enhanced Raman scattering (SERS), with the so-called chemical (molecule-related) contribution assuming only, if any, a supporting role. The conclusion of our comprehensive experimental resonant study of a broad range of nanosphere lithography based metallic substrates, with covalently attached 4-mercaptobenzoic acid monolayers used as a probe (standard molecules that are non-resonant in solution), is that this accepted understanding needs to be revised. We present a detailed resonant SERS study of metal-film-over-nanosphere (MFON) substrates that is done by both scanning the laser wavelength and tuning the plasmon response through the nanosphere diameter, which is varied from 500 to 900 nm. Far and local field properties are characterized through measures of optical reflectivity and SERS efficiency, respectively, and are supported by numerical simulations. We demonstrate that SERS intensity depends indeed on the electromagnetic mechanism, determined by the plasmonic response of the system, but we observe that it is also strongly defined by a chemical resonant contribution related to a metal-to-ligand electronic transition of the covalently bound probe molecule. Optimum amplification occurs when the plasmon modes intersect with the ligand-to-metal chemical resonance, contributing synergically both mechanisms together. Quite notably, however, the largest SERS signal is observed when the laser is tuned with the metal-to-ligand transition, and typically does not follow the wavelength dependence of the plasmon modes when varying the nanosphere size. The same general trend is observed for other nanosphere lithography based substrates, including sphere segment void cavities and hexagonally ordered triangular nanoparticles, using either Ag or Au as the plasmonic metal, and also with a commercial substrate (Klarite). Interestingly, this extensive comparative investigation shows in addition that MFON substrates are significantly better than these other studied plasmonic substrates in terms of Raman intensity and homogeneity. We conclude that a deep understanding of both electromagnetic and chemical mechanisms is necessary to fully exploit these substrates for analytical applications.

Unifying stability and plasmonic properties in hybrid nanoislands: Au–Ag synergistic effects and application in SERS

In plasmon-based devices, the choice of metal for the nanostructures often leads to an undesirable tradeoff between either the superb performance of Ag or the stability of Au. There is a great interest in combining Au and Ag in a way that preserves both their favourable properties, while also exploiting synergistic effects to boost surface-enhanced Raman scattering (SERS) performance beyond the capabilities of a single metal. To address this issue, a large-area nanofabrication procedure based on galvanic replacement was used to create hybrid Au–Ag nanoislands with a diverse morphology and highly tunable (through visible to NIR) plasmon resonance. Their stability was comprehensively studied and shown to be complex to achieve. However, in the right conditions, Au–Ag nanoislands with silver-like plasmonic properties were fabricated, and shown to be stable over a period of at least 5 months. Moreover, their SERS efficiency considerably outperformed even that of Ag nanoislands. Computational investigation helped explain the advantageous properties and provided insights for future sensor design. Detection of the model molecules was achieved in the nanomolar range and in mild conditions (low laser power), indicating great potential for biomedicine sensing. Beyond that, nanoislands with simultaneous stability and good plasmonic properties can find application in a wide variety of photonic devices.

Silver nanoparticles embedded temperature-sensitive nanofibrous membrane as a smart free-standing SERS substrate

To develop a smart free-standing surface enhanced Raman scattering (SERS) substrate, silver nanoparticles (AgNPs) embedded temperature-sensitive nanofibrous membrane was fabricated by electrospinning their aqueous solution containing the copolymer poly(N-isopropylacrylamide-co-N-hydroxymethylacrylamide), followed by heat treatment to form crosslinking structure within its constituent nanofibers. To avoid negative effect of the additive like stabilizer and the reactant like reductant on their SERS efficiency, the AgNPs were in-situ synthesized through reducing Ag+ ions dissolved in the polymer solution by ultraviolet irradiation. The prepared hybrid nanofibrous membrane with high stability in aqueous medium can reach its swelling or deswelling equilibrium state within 15 seconds with the medium temperature changing between 25 °C and 50 °C alternately. When it was used as a free-standing SERS substrate, 10-12 mol/L of 4-nitrothiophenol in aqueous solution can be detected at room temperature, and elevating detection temperature can further lower its low detection limit. Since its generated SERS signal has desirable reproducibility, it can be used as SERS substrate for quantitative analysis. Moreover, the hybrid membrane as SERS substrate is capable of real-time monitoring the reduction of 4-nitrothiophenol into 4-aminothiophenol catalyzed by its embedded AgNPs, and the detected intermediate indicates that the reaction proceeds via a condensation route.

Relationship between the nano-structure of GaN surfaces and SERS efficiency: Chasing hot-spots

This work reviews different types of surface structures formed on selectively etched GaN substrates for surface-enhanced Raman scattering (SERS) measurements. A one-to-one correlation between nano-structure and SERS efficiency, represented by the enhancement factor (EF), is shown by comparing SEM images with maps of Raman intensity. In this way, the location of hot-spots (H-Ss) is disclosed, leading to the selection of an optimal technology for manufacturing efficient and uniform SERS platforms. GaN platforms with nano-structures containing bunched pillars and pyramids covered with an Au-Ag plasmonic layer reproducibly show an EF of up to 106 for 4-mercaptobenzoic acid (pMBA) test molecules. Platforms with pits (inverted pyramids) yield an EF of one order of magnitude lower.

A spectroscopic sensing platform for MARCKS protein monolayers

We developed a highly sensitive silicon platform, suitable to assess the molecular organization of protein samples. Prototype platforms were obtained using different electrochemical protocols for the electrodeposition of Ag-nanoparticles onto the hydrogenated silicon surface. A platform with high Surface Enhanced Raman Scattering efficiency was selected based on the surface coverage and the number density of particles size distribution. The performance of the platform was determined by studying the interaction of Myristoylated Alanine-Rich C Kinase Substrate (MARCKS) protein with the substrate according to its molecular organization. The chemical and structural characteristics of MARCKS molecules were examined under two configurations: i) a disordered distribution given by a MARCKS solution drop deposited onto the platform and, ii) a compact monolayer transferred to the platform by the Langmuir-Blodgett method. Raman spectra show vibrational bands of Phenylalanine and Lysine residues specific for the protein effector domain, and evidence the presence of alpha helix structure in both configurations. Moreover, we distinguished the supramolecular order between the compact monolayer and random molecular distribution. The platforms containing Ag-nanoparticles are suitable for studies of protein structure and interactions, advancing a methodological strategy for our long term goal, which is to explore the interaction of proteins with model membranes.

High-Performance Real-Time SERS Detection with Recyclable Ag Nanorods@HfO2 Substrates.

Ag nanorods coated with an ultrathin HfO2 shell (Ag NRs@HfO2) were prepared for the synthesis of a versatile, robust, and easily recyclable surface-enhanced Raman scattering (SERS) substrate. This substrate maximizes the high melting point of the HfO2 shell and thus ensures the excellent plasmonic efficiency of Ag NRs. Therefore, it possesses extraordinary thermal stability and SERS activity, which could act as a reusable and cost-effective SERS detector. After SERS detection, the regeneration of Ag NRs@HfO2 was achieved by annealing the substrate within several seconds. This procedure led to the thermal release of adsorbed molecules and resulted in a refreshed substrate for subsequent measurements. The composite substrate maintained its SERS efficiency well during multiple "detection-heating" cycles, hence demonstrating the stability and recyclability of Ag NRs@HfO2. Furthermore, in addition to revealing the feasibility of SERS sensing in liquids, Ag NRs@HfO2 also provided continuous real-time monitoring of vapor-phase samples at ultralow concentrations. This work provides a robust and renewable SERS sensor with advantages of high sensitivity, stability, cost effectiveness, and easy operation, which can be implemented for both aqueous and gaseous analyte detection and is thus an intriguing candidate for practical applications in environmental, industrial, and homeland security sensing fields.

Preparation of monodisperse bimetallic nanorods with gold nanorod core and silver shell and their plasmonic property and SERS efficiency

In this study, monodisperse bimetallic nanorods with gold (Au) nanorod core and silver (Ag) shell (Au@AgNRs) were synthesized through seed‐mediated growth process by reduction of AgNO3 using Au nanorods with narrow size and shape distribution as seeds. With increasing the used amount of AgNO3, the Ag shell thickness of their lateral facets is raised faster than that of their two tips, leading to a decrease of their aspect ratios. Four plasmon bands are observable on the extinction spectra of Au@AgNRs, which are attributed to the longitudinal dipolar plasmon mode, transverse dipolar plasmon mode, and octupolar plasmon mode of the core‐shell structured bimetallic nanorods, respectively. As their Ag shell thickness increases, their longitudinal plasmon band blue‐shifts notably with the transverse plasmon band blue‐shifting and the two octupolar plasmon bands red‐shifting slightly, due to the decrease of their aspect ratios and enhancement of Ag plasmon resonance contribution. When used as surface‐enhanced Raman scattering (SERS) substrate for probing minute amounts of 4‐mercaptobenzoic acid in aqueous solution, Au@AgNRs have much stronger SERS activity than Au nanorods, and the obtained Raman signals are highly reproducible arising from their excellent monodispersity. Their SERS activity is remarkably increased with their Ag shell thickness thanks to the enhancing surface electric field and the chemical enhancement associated with electronic ligand effect. Copyright © 2014 John Wiley & Sons, Ltd.

In situ SERS monitoring of photocatalytic organic decomposition using recyclable TiO2-coated Ag nanowire arrays

Recently, semiconductor and noble metal complex nanomaterials have attracted ever-increasing attention because of the realization of multiple functionalities in a single entity. In this study, Ag–TiO2 core–shell nanocomposites were synthesized by hydrolysis of butyl titanate on the surface of Ag nanowires. Due to efficient electron transfer at the Ag–TiO2 interface, the as-prepared nanocomposites exhibit much higher photocatalytic activity than bare TiO2 films, as demonstrated by the enhanced photodegradation rate of R6G and methyl blue molecules. In the meanwhile, such nanocomposites serve as a high-sensitivity surface-enhanced Raman scattering (SERS) substrate for in situ monitoring of the photocatalytic decomposition reaction, and the substrate is recyclable due to its self-cleaning function. Full-wave numerical calculations reveal that the improved photocatalysis and SERS efficiencies are attributed to the largely enhanced electromagnetic near-field in the nanocomposites. Our results point out that bifunctional semiconductor-metal hybrids hold great promise for simultaneously detecting and decomposing organic pollutants in the environment.

Fabrication of Metal Half-Shells Using Colloidal Particle Monolayer and Their Application in Surface-Enhanced Raman Scattering

Three types of Au shells, an isolated half-shell, one-dimensional strings of shells, and two-dimensional films, were fabricated by using a monolayer of polystyrene (PS) particles with diameters of 213, 560, and 1360 nm. The three types of Au shells that were removed from the PS particle monolayer and the as-deposited Au shells that adhered to PS particles were modified with 4-mercaptopyridine for use as platforms for surface-enhanced Raman scattering (SERS). We examined the effects of the shapes and sizes of Au shells on their SERS efficiency and found that the Au shells exhibited strong SERS signals and that Au shells prepared by using 560-nm PS particles were the most suitable platform for SERS at both 632.8- and 785-nm excitations. Further, we found that SERS enhancements depended on the shape of Au shells and on whether Au shells adhered to PS particles or not.

Nanostructured Ag Platforms as Biosensors of Nucleobase Chains

Nanostructured Ag platforms have been obtained by simple chemical procedure and characterized by AFM (atomic force microscopy) measurements, for use in biosensing by means of SERS (surface-enhanced Raman scattering) spectroscopy. The SERS efficiency of these substrates has been verified by microRaman measurements on small RNA chains with different nucleobase content, showing sensitivity near attomole level. It is our opinion that these metal substrates may be widely used as appropriate sensors for detecting biomolecules in many applications concerning medical diagnostics, pharmacological research and nanomaterials technology.

Fabrication of nanostructured silver substrates for surface-enhanced Raman spectroscopy

Four nanostructured Ag substrates have been fabricated with different surface morphologies and tested with surface-enhanced Raman scattering (SERS) experiments by adsorption of adenine. Their SERS efficiency has been compared and related to the surface roughness resulting from atomic force microscopy measurements. Chemical etching of silver by thiourea/Fe(III)nitrate produces homogeneously roughened plates, exhibiting the largest three-dimensional surface and the best SERS enhancement. They mostly exhibit surface protrusions with sizes around 200 nm, thus matching the best condition for obtaining SERS enhancement by laser excitation at 785 nm. This is quite important in the case of biomolecules, whose samples often present strong fluorescence bands, which usually are not observed with red-shifted exciting lines. Moreover, these Ag platforms, owing to their uniform nanostructured surfaces, are suitable for obtaining reproducible results from microRaman investigation. In conclusion, the present nanofabrication of Ag surfaces allows obtaining SERS-active substrates, which combine high reproducibility and sensitivity and can be successfully employed in the molecular recognition of different organic ligands or biomolecules like nucleic acids and proteins.

Surface Enhanced Raman Scattering from Porous Gold Nanofibers of Different Diameters

Porous gold nanofibers of different diameters from 43 to 219 nm were fabricated using electrochemical deposition techniques. Gold-silver alloy were electrochemically deposited in the form of nanofibers within the porous alumina templates of various diameters and only a silver phase was chemically removed using nitric acid. Field-emission scanning electron microscope images of the resulting nanofibers show a high-quality nanoporous network with homogeneous pores. A notable surface-enhanced Raman scattering (SERS) has been observed for all porous gold nanofibers of which scattering efficiencies are distinctly higher than that of the smooth solid gold nanofibers without porosity. As the diameter of porous gold nanofibers decreases, the observed SERS efficiency gradually increases. Controlled fabrication of lateral width of gold nanofibers reveals promising application for high efficient and stable molecular sensing platforms.

Optimization of porous silicon preparation technology for SERS applications

A series of porous silicon samples prepared at different etching parameters, namely etchant composition, etching time and current density, was investigated as substrates for surface-enhanced Raman scattering (SERS). Silver nanostructures were deposited on porous silicon by immersion plating method and Rhodamine 6G was used as analyte. The relation between the etching parameters, morphology of porous silicon surface and its SERS efficiency after silver deposition is examined. We show that a high HF content in the etchant allows the formation of a film with close-packed silver nanocrystals, which possess strong surface enhancement properties.

SERS Study of Alizarin Red Dye on Colloidal Silver Nanoparticles

AbstractThe present paper deals with surface enhanced Raman scattering (SERS) study of an organic dye Alizarin Red on colloidal silver nanoparticles. The colloidal solution of nanoparticles is synthesized by pulsed laser ablation of silver rod in pure deionized water using focused out put of 1064nm wavelength of Nd:YAG laser having 35mJ/ pulse energy. Sodium chloride is used as aggregating agents. It is observed that SERS enhancement factor varies strongly depending on concentration of used aggregating agent (especially on the concentration of Cl- ions). These changes in SERS efficiency of colloid are reflected through changes on the absorption spectra. The possible mechanism of SERS is also discussed.

Gold Nanostars For Surface-Enhanced Raman Scattering: Synthesis, Characterization and Optimization

The controlled synthesis of high-yield gold nanostars of varying sizes, their characterization and use in surface-enhanced Raman scattering (SERS) measurements are reported for the first time. Gold nanostars ranging from 45 to 116-nm in size were synthesized in high-yield, physically modeled and optically characterized using transmission and scanning electron microscopy and UV-Visible absorption spectroscopy. The nanostar characterization involved both studying morphology evolution over time and size as a function of nucleation. The nanostars properties as substrates for SERS were investigated and compared with respect to size. As the overall star size increases, so does the core size, the number of branches and branch aspect ratio; the number of branch tips per star surface area decreases with increasing size. The stars become more inhomogeneous in shape, although their yield is high and overall size remains homogeneous. Variations in star size are also accompanied by shifts of the long plasmon band in the NIR region, which hints towards tuning capabilities that may be exploited in specific SERS applications. The measured SERS enhancement factors suggest an interesting correlation between nanostar size and SERS efficiencies, and were relatively consistent across different star samples, with the enhancement factor estimated as 5×103 averaged over the 52-nm nanostars for 633-nm excitation.

Self‐Assembled Au Nanoparticles as Substrates for Surface‐Enhanced Vibrational Spectroscopy: Optimization and Electrochemical Stability

Three-dimensional nanostructured metallic substrates for enhanced vibrational spectroscopy are fabricated by self-assembly. Nanostructures consisting of one to 20 depositions of 13 nm-diameter Au nanoparticles (NPs) on Au films are prepared and characterized by means of AFM and UV/Vis reflection-absorption spectroscopy. Surface-enhanced polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS) is observed from Au NPs modified by the probe molecule 4-hydroxythiophenol. The limitation of this kind of substrate for surface-enhanced PM-IRRAS is discussed. The surface-enhanced Raman scattering (SERS) from the same probe molecule is also observed and the effect of the number of Au-NP depositions on the SERS efficiency is studied. The SERS signal from the probe molecule maximizes after 11 Au-NP depositions, and the absolute SERS intensities from different batches are reproducible within 20%. In situ electrochemical SERS measurements show that these substrates are stable within the potential window between -800 and +200 mV (vs. Ag/AgCl/sat. Cl(-)).

Surface-Enhanced Raman Scattering Spectroscopy as a Sensitive and Selective Technique for the Detection of Folic Acid in Water and Human Serum

Surface-enhanced Raman scattering (SERS) is shown to give linear and sensitive concentration-dependent detection of folic acid using silver nanoparticles created via ethylene-diaminetetraacetic acid (EDTA) reduction. Optical detection by SERS overcomes the primary limitation of photodissociation encountered during the application of other shorter wavelength ultraviolet (UV)/near-UV techniques such as fluorescence based microscopy. The SERS approach in water-based samples was demonstrated and optimized using several longer wavelengths of excitation (514.5, 632.8, and 785 nm). Excitation in the green (514.5 nm) was found to achieve the best balance between photodissociation and SERS efficiency. Linear concentration dependence was observed in the range of 0.018 to 1 microM. The importance of folic acid in a clinical setting and the potential applications of this technique in a biological environment are highlighted. We demonstrate the potential to transfer this technique to real biological samples by the detection of folic acid in human serum samples by SERS.

Modelling of the near‐field of metallic nanoparticle gratings: localized surface plasmon resonance and SERS applications

We numerically study the influence of the shape of gold nanostructures on the spectral position of their localized surface plasmon resonance and on Surface Enhanced Raman Scattering efficiency. Calculations are performed using a Finite-Difference Time-Domain (FDTD) method, whose accuracy for dispersive media is enhanced through the use of a Drude-Lorentz model. The relevance of this method is then pointed out by comparing the calculations with experimental data for both the plasmon resonance and the Raman signal enhancement. The influence of a thin layer of water is also investigated.