Gold Films Deposited over Regular Arrays of Polystyrene Nanospheres as Highly Effective SERS Substrates from Visible to NIR

Ordered zirconium dioxide nanotubes covered with an evaporated gold layer as reversible, chemically inert and very efficient substrates for surface-enhanced Raman scattering (SERS) measurement

The deposition of a layer of plasmonic metal on a surface of highly ordered nanostructured oxide is one of the important methods of preparation of substrates for surface-enhanced Raman scattering (SERS) measurements. In this contribution we describe formation of SERS substrates by the deposition of a gold layer on ordered ZrO₂ nanotubes. The influence of various experimental parameters on the structure of formed composites and the achievable SERS enhancement factor has been analysed. Like commonly used SERS substrates formed by the deposition of plasmonic metals on TiO₂ nanotubes, gold-covered ZrO₂ nanotubes also could be used as reversible SERS platform after water rinsing: there is no any significant decrease in the SERS activity of the substrate even after 20 radiation-induced cleaning cycles. Moreover, SERS substrates formed on ZrO₂ nanotubes are significantly more stable in strongly acidic media than the previously developed SERS substrates based on ordered TiO₂ nanotubes.

Shape and deposition angle control of silver film-over-nanosphere SERS substrates

Thin metallic films on dielectric nanospheres are demonstrated to have a high potential for the fabrication of cost-effective SERS substrates. In addition to the morphological advantages that nanospheres offer for attaining a high density of hot spots, possessing shape adjustability by uncomplicated thermal treatment makes them an attractive platform for tuneable SERS substrates. Furthermore, when combined with the oblique angle metal deposition technique, adjustable gaps at a high density and adjustable shape of metal films, such as Ag films, can be achieved on nanospheres. Applying small changes in deposition angle can provide means for fine adjustment of the Raman enhancement factor (EF), resulting in EF up to 10⁸measured using crystal violet dye molecule as a Raman analyte. This practice paves the way for the fabrication of high EF SERS substrates at a reasonable cost using a monolayer of self-organized nanosphere patterns. An ultra-thin Ag film coated at 5° tilt is shown to be an excellent substitute for a film deposited at 0° with double the thickness. There is a strong agreement between the experimental results and finite-elements-method-based Maxwell simulations exhibiting expected field enhancements up to 10⁹at a tilt angle of 5°.

Thermal crowning mechanism in gold-silica nanocomposites: plasmonic-photonic pairing in archetypal two-dimensional structures

A close-packed monolayer of a two-dimensional periodic array of Silica nanospheres (SNs) with gold (Au) crowning, forming a long-ranged archetypal plasmonic-photonic nanocomposite, has been achieved. We investigate the thermal crowning mechanism in such a nanocomposite using electron microscopy and X-ray diffraction techniques. Pre- and post-annealing morphological features reveal gold crowning on top of SNs, at different annealing temperatures for various thicknesses of the sputter-deposited gold. In situ grazing incidence X-ray diffraction was employed to structurally characterize the reconstruction in the Au-layer as a function of the annealing temperature. Finite element methods were used to simulate the interaction between the paired nanocomposites and the incident electromagnetic radiations to elucidate the crowning and nanodrop formation mechanism. This study provides an insight into real-time morphological and structural changes of a dewetting plasmonic film over a photonic basis and explores a robust, reliable, and scalable route to fabricate coupled nanocomposites. Such nanocomposites allow prospective applications in optoelectronics, sensing, catalysis, and surface-enhanced Raman spectroscopy by exploiting the plasmonic-photonic pairing in archetypal two-dimensional structures.

Breaking the symmetry of nanosphere lithography with anisotropic plasma etching induced by temperature gradients

We report a novel anisotropic process, termed plasma etching induced by temperature gradients (PE-TG), which we use to modify the 3D morphology of a hexagonally close-packed polystyrene sphere array. Specifically, we combined an isotropic oxygen plasma (generated by a plasma cleaner) and a vertical temperature gradient applied from the bottom to the top of a colloidal mask to create an anisotropic etching process. As a result, an ordered array of well-defined and separated nano mushrooms is obtained. We demonstrate that the features of the mushrooms, namely the hat size and their intrinsic undercut, as well as the pillar diameter and height, can be easily tuned by adjusting the main parameters of the process i.e. the temperature gradient and etching time, or the spheres' size. We show that PS mushroom arrays can be used as nanostructured templates to fabricate plasmonic arrays, such as gold-capped nano mushrooms and ultra-small nanoapertures, by using vertical and oblique gold sputtering deposition respectively. PE-TG reveals a new, cheap and facile approach to produce plasmonic nanostructures of great interest in the fields of molecular sensing, surface-enhanced Raman scattering (SERS), energy harvesting and optoelectronics. We study the optical properties of the Au-capped nano mushroom arrays and their performance as biosensing platforms by performing SERS measurements.

Thickness-Dependent NIR LSPR of Curved Ag/TiS2 Bilayer Film

We demonstrated that the localized surface plasmon resonance (LSPR) features of Ag/TiS₂ nanostructures were dependent on the sublayer thickness. The Ag/TiS₂ bilayer film was obtained by the self-assembly method and magnetron sputtering. The thickness was controlled by changing the sputtering time when the sputtering powers were the same. When the Ag thickness decreased from 50 nm to 5 nm, the LSPR was tuned from the visible region to the Near Infrared (NIR) region. When the TiS₂ thickness decreased from 60 nm to 2 nm, the LSPR shifted from the IR to NIR region. Analysis showed the thickness changes of Ag and TiS₂ resulted in the changed carrier density, which led to the thickness-dependent shift of the LSPR.

Hydroquinone-Based Fabrication of Gold Nanorods with a High Aspect Ratio and LSPR Greater than 850 nm to Be Used as a Surface Plasmon Resonance Platform for Rapid Detection of Thiophanate Methyl

The use of gold nanorods (AuNRs) as surface-enhanced Raman scattering (SERS) substrates has gained much attraction due to their remarkably aspect-ratio-dependent plasmonic properties. In this report, we described the development of AuNRs with a high aspect ratio and longitudinal surface plasmon resonance (LSPR) >850 nm through a hydroquinone-based fabrication with minor modifications. The synthesis started with the reduction of chloroauric acid (HAuCl4) by sodium borohydride (NaBH4) to make gold nanoseeds from which AuNRs were grown with the aid of silver nitrate (AgNO3), HAuCl4, cetyltrimethylammonium bromide (CTAB), and hydroquinone (HQ). Scanning electron microscopy coupled with energy-dispersive X-ray (SEM-EDX), Transmission electron microscope (TEM), X-ray diffraction (XRD) and Ultra-violet-Visible spectroscopy (UV-Vis) were performed to study the shape, size, and structural and optical properties of AuNRs, respectively. The results showed that AuNRs with high aspect ratios (AR > 3) were single crystals with a heterogenous size distribution, and that the growth of Au nanoseeds into AuNRs took place along the [001] direction. AuNRs exhibited two plasmon resonance peaks at 520 nm and 903 nm, while gold nanoseeds had only a plasmon resonance peak at 521 nm. The as-synthesized AuNRs also showed SERS effects for thiophanate methyl, a broad-spectrum fungicide, with the limit of detection down to 5 mg/L of the fungicide. AuNR-coated glass can serve as a SERS-based sensing platform for rapid detection of thiophanate methyl with high sensitivity and reproducibility.

Patterned Superhydrophobic SERS Substrates for Sample Pre-Concentration and Demonstration of Its Utility through Monitoring of Inhibitory Effects of Paraoxon and Carbaryl on AChE

We describe a patterned surface-enhanced Raman spectroscopy (SERS) substrate with the ability to pre-concentrate target molecules. A surface-adsorbed nanosphere monolayer can serve two different functions. First, it can be made into a SERS platform when covered by silver. Alternatively, it can be fashioned into a superhydrophobic surface when coated with a hydrophobic molecular species such as decyltrimethoxy silane (DCTMS). Thus, if silver is patterned onto a latter type of substrate, a SERS spot surrounded by a superhydrophobic surface can be prepared. When an aqueous sample is placed on it and allowed to dry, target molecules in the sample become pre-concentrated. We demonstrate the utility of the patterned SERS substrate by evaluating the effects of inhibitors to acetylcholinesterase (AChE). AChE is a popular target for drugs and pesticides because it plays a critical role in nerve signal transduction. We monitored the enzymatic activity of AChE through the SERS spectrum of thiocholine (TC), the end product from acetylthiocholine (ATC). Inhibitory effects of paraoxon and carbaryl on AChE were evaluated from the TC peak intensity. We show that the patterned SERS substrate can reduce both the necessary volumes and concentrations of the enzyme and substrate by a few orders of magnitude in comparison to a non-patterned SERS substrate and the conventional colorimetric method.

Gold Film over SiO2 Nanospheres-New Thermally Resistant Substrates for Surface-Enhanced Raman Scattering (SERS) Spectroscopy

Surface-enhanced Raman scattering (SERS) sensors are constructed from metallic plasmonic nanostructures providing high sensitivity and spectral reproducibility. In many cases, irradiation of the SERS substrate by the laser beam leads to an increase of the local temperature and consequently to thermal degradation of metallic nanostructure itself and/or adsorbed analyte. We report here a "bottom-up" technique to fabricate new thermally resistant gold "film over nanosphere" (FON) substrates for SERS. We elaborated the simple and straightforward method of preparation of homogeneously and closely packed monolayer of SiO₂ nanoparticles (50 nm in diameter) and covered it by a thin (20 nm) layer of magnetron-sputtered gold. The spectral testing using biologically important molecules (methylene blue, cationic porphyrin, and fungicide 1-methyl-1H-benzimidazole-2-thiol) proved a sensitivity and reproducibility of our AuSiO₂ substrates. The main advantage of such SERS-active substrates is high thermal stability and low intensity of background and signal of graphitic carbon.

Plasmonic nanomaterial structuring for SERS enhancement

Au island over nanospheres (AuIoN) structures featuring a three-dimensional (3D) nanostructure on a two-dimensional (2D) array of nanospheres with different adhesion layers were fabricated as surface-enhanced Raman scattering (SERS) substrates.

Combinatorial Thin Film Sputtering Au xAl1- x Alloys: Correlating Composition and Structure with Optical Properties

The Au-Al alloy system was investigated via a combinatorial thin film sputtering method for its potential as a plasmonic material. Au _xAl_{1- x} combinatorial libraries were cosputtered from Au and Al elemental targets and the composition, phase, and dielectric function of a ∼350 nm film was determined using energy dispersive spectroscopy (EDS), grazing incidence X-ray diffraction (GIXRD), and spectroscopic ellipsometry, respectively. The phase evolution and optical properties were analyzed after annealing various compositions under a vacuum. The phases present matched the expected phases based on the published Al-Au binary phase diagram at all compositions. Interestingly, the mixed phase Al-AuAl₂ region showed the most optical tunability, where a maximum in the real part of the dielectric function progressively shifted to higher energy for increasing gold concentration. For almost pure AuAl₂, the imaginary component is largely reduced in the visible range and is comparable to that of pure Al in the UV region. A 20-nm-thick film with composition Au_0.74Al_0.26 was studied using a (scanning) transmission electron microscope with an in situ laser heating system. The structures of the as-deposited and laser annealed films were determined using selected area diffraction and the bulk plasmon of AuAl₂ and Al realized with electron energy loss spectroscopy. Last, the Au-rich solid solution region was investigated as a surface enhanced Raman spectroscopy (SERS) substrate using the benezenethiol (BT) molecule. Good SERS intensity was maintained up to 30% Al addition where enhancements of 10⁵ to 10⁷ were still observed.

Plasmonic Photovoltaic Cells with Dual-Functional Gold, Silver, and Copper Half-Shell Arrays

Solid-state photovoltaic cells based on plasmon-induced charge separation (PICS) have attracted growing attention during the past decade. However, the power conversion efficiency (PCE) of the previously reported devices, which are generally loaded with dispersed metal nanoparticles as light absorbers, has not been sufficiently high. Here we report simpler plasmonic photovoltaic cells with interconnected Au, Ag, and Cu half-shell arrays deposited on SiO₂@TiO₂ colloidal crystals, which serve both as a plasmonic light absorber and as a current collector. The well-controlled and easily prepared plasmonic structure allows precise comparison of the PICS efficiency between different plasmonic metal species. The cell with the Ag half-shell array has higher photovoltaic performance than the cells with Au and Cu half-shell arrays because of the high population of photogenerated energetic electrons, which gives a high electron injection efficiency and suppressed charge recombination probability, achieving the highest PCE among the solid-state PICS devices even without a hole transport layer.

Silver Alginate Hydrogel Micro- and Nanocontainers for Theranostics: Synthesis, Encapsulation, Remote Release, and Detection

We have designed multifunctional silver alginate hydrogel microcontainers referred to as loaded microcapsules with different sizes by assembling them via a template assisted approach using natural, highly porous calcium carbonate cores. Sodium alginate was immobilized into the pores of calcium carbonate particles of different sizes followed by cross-linking via addition of silver ions, which had a dual purpose: on one hand, the were used as a cross-linking agent, albeit in the monovalent form, while on the other hand they have led to formation of silver nanoparticles. Monovalent silver ions, an unusual cross-linking agent, improve the sensitivity to ultrasound, lead to homogeneous distribution of silver nanoparticles. Silver nanoparticles appeared on the shell of the alginate microcapsules in the twin-structure as determined by transmission electron microscopy. Remote release of a payload from alginate containers by ultrasound was found to strongly depend on the particle size. The possibility to use such particles as a platform for label-free molecule detection based on the surface enhanced Raman scattering was demonstrated. Cytotoxicity and cell uptake studies conducted in this work have revealed that microcontainers exhibit nonessential level of toxicity with an efficient uptake of cells. The above-described functionalities constitute building blocks of a theranostic system, where detection and remote release can be achieved with the same carrier.

On-chip surface-enhanced Raman spectroscopy using nanosphere-lithography patterned antennas on silicon nitride waveguides

A hybrid integration of nanoplasmonic antennas with silicon nitride waveguides enables miniaturized chips for surface-enhanced Raman spectroscopy at visible and near-infrared wavelengths. This integration can result in high-throughput SERS assays on low sampling volumes. However, current fabrication methods are complex and rely on electron-beam lithography, thereby obstructing the full use of an integrated photonics platform. Here, we demonstrate the electron-beam-free fabrication of gold nanotriangles on deep-UV patterned silicon nitride waveguides using nanosphere lithography. The localized surface-plasmon resonance of these nanotriangles is optimized for Raman excitation at 785 nm, resulting in a SERS substrate enhancement factor of 2.5 × 10⁵. Furthermore, the SERS signal excited and collected through the waveguide is as strong as the free-space excited and collected signal through a high NA objective.

Gold nanodome SERS platform for label-free detection of protease activity

Surface-enhanced Raman scattering provides a promising technology for sensitive and selective detection of protease activity by monitoring peptide cleavage. Not only are peptides and plasmonic hotspots similarly sized, Raman fingerprints also hold large potential for spectral multiplexing. Here, we use a gold-nanodome platform for real-time detection of trypsin activity on a CALNNYGGGGVRGNF substrate peptide. First, we investigate the spectral changes upon cleavage through the SERS signal of liquid-chromatography separated products. Next, we show that similar patterns are detected upon digesting surface-bound peptides. We demonstrate that the relative intensity of the fingerprints from aromatic amino acids before and after the cleavage site provides a robust figure of merit for the turnover rate. The presented method offers a generic approach for measuring protease activity, which is illustrated by developing an analogous substrate for endoproteinase Glu-C.

Morphology Effects of Cap-shaped Silver Nanoparticle Films as a SERS Platform

In this paper, we evaluate randomly adsorbed cap-shaped silver nanoparticles for applications to surface-enhanced Raman spectroscopy, SERS. They were prepared by depositing silver on top of surface-adsorbed monodisperse SiO2 nanospheres, in a manner similar to the method for preparing metal film on nanosphere, MFON, but one major difference lies in the fact that nanospheres are randomly adsorbed rather than as a close-packed MFON. With random MFON, it is possible to incorporate nanospheres with more than one size. Mixing has been found to increase SERS performance. More specifically, by using 50 and 100 nm nanospheres, we found that substrates containing both types outperform substrates prepared from 100% of either 50 or 100 nm nanospheres. As evaluated by spectrophotometry, this increase could not be attributed to an increase in the extinction coefficient of the substrate at the irradiation wavelength of SERS measurements.

Large-area, lithography-free, low-cost SERS sensor with good flexibility and high performance

Cost-effective, sensitive and bio-compatible surface-enhanced Raman spectroscopy (SERS) substrate has been in high demand since the Raman spectrum was designated as a significant tool for analyzing the composition of liquids, gases and solids in 1998 [1]. In this research, we presented the design, fabrication and characterization of an improved gold-based SERS substrate. With fine tuning of the SiO2 thickness we achieved a 3.391 times improvement and achieved an enhancement factor of 1.55 * 10(7) which is 15 times better than the current gold-standard Klarite substrate. Such improvement is ascribed to the localized surface plasmon resonance (SPR) and propagating SPR, which is proved by full-wave finite-difference time-domain simulations.

Surface enhanced Raman scattering based reaction monitoring of in vitro decyclization of creatinine → creatine

Raman signatures of decyclization of creatinine to creatine appear after 120 min at pH 8, 60 min at pH 10 and 30 min at pH 12. Signature of reversibility at later stages of the reaction.

Plasmonic nanostructures for surface enhanced spectroscopic methods

The development within the last five years in the field of surface enhanced spectroscopy methods was comprehensively reviewed.

Size effect of gold on Ag-coated Au nanoparticle-embedded silica nanospheres

Ag-coated Au nanoparticle (NP)-embedded silica nanospheres (SiO₂@Au@Ag NSs) were prepared using three different Au NPs of 2.5, 7 and 15 nm diameter to investigate their optical properties.

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.

Quantitative SERS analysis of azorubine (E 122) in sweet drinks

Considering both the potential effects on human health and the need for knowledge of food composition, quantitative detection of synthetic dyes in foodstuffs and beverages is an important issue. For the first time, we report a fast quantitative analysis of the food and drink colorant azorubine (E 122) in different types of beverages using surface-enhanced Raman scattering (SERS) without any sample preparation. Seven commercially available sweet drinks (including two negative controls) with high levels of complexity (sugar/artificial sweetener, ethanol content, etc.) were tested. Highly uniform Au "film over nanospheres" (FON) substrates together with use of Raman signal from silicon support as internal intensity standard enabled us to quantitatively determine the concentration of azorubine in each drink. SERS spectral analysis provided sufficient sensitivity (0.5-500 mg L(-1)) and determined azorubine concentration closely correlated with those obtained by a standard HPLC technique. The analysis was direct without the need for any pretreatment of the drinks or Au surface. Our SERS approach is a simple and rapid (35 min) prescan method, which can be easily implemented for a field application and for preliminary testing of food samples.

Reproducible surface-enhanced Raman quantification of biomarkers in multicomponent mixtures

Direct and quantitative detection of unlabeled glycerophosphoinositol (GroPIns), an abundant cytosolic phosphoinositide derivative, would allow rapid evaluation of several malignant cell transformations. Here we report label-free analysis of GroPIns via surface-enhanced Raman spectroscopy (SERS) with a sensitivity of 200 nM, well below its apparent concentration in cells. Crucially, our SERS substrates, based on lithographically defined gold nanofeatures, can be used to predict accurately the GroPIns concentration even in multicomponent mixtures, avoiding the preliminary separation of individual compounds. Our results represent a critical step toward the creation of SERS-based biosensor for rapid, label-free, and reproducible detection of specific molecules, overcoming limits of current experimental methods.

Self-assembled plasmonic nanoring cavity arrays for SERS and LSPR biosensing

Self-assembled plasmonic nanoring cavity arrays are formed alongside the curvature of highly packed metallic nanosphere gratings. The sub-10-nm gap size is precisely tuned via atomic layer deposition and highly ordered arrays are produced over a cm-sized area. The resulting hybrid nanostructure boosts coupling efficiency of light into plasmons, and shows an improved SERS detection limit. These substrates are used for SERS detection of the biological analyte, adenine, followed by concurrent localized surface plasmon resonance sensing.

Tuning surface plasmons in interconnected hemispherical Au shells

We present a new approach for making interconnected hemispherical shells by stripping Au from templates of anodized aluminum, where the metal thickness can be adjusted without affecting the outer radius of curvature, film roughness and the sharpness of the hemisphere contact areas. This provides increased understanding of the surface plasmon resonances (SPRs) observed for Film-On-Nanospheres (FONs) by decoupling these parameters, which are coupled in the case of FONs. Investigating the influence of the shell thicknesses on the spectral positions of SPRs for FONs involves a dielectric core with a fixed radius encased by a metal film with adjustable thickness. By performing linear reflection spectroscopy, we demonstrate a wide tunability of the SPR by tailoring the inner hemisphere diameter, while keeping the outer diameter fixed. Deposition of extra Au on top of thick, previously stripped hemispherical shells isolates optical response contributions from Au grain- and island-mediated roughness, and unsharpening contact areas in form of decreasing LSPR quality factor. Two-photon luminescence scanning optical microscopy of shells with different thicknesses, applying several different laser wavelengths, is exploited to map local electromagnetic hot spots and correlate the high field enhancements with the linear reflection spectroscopy measurements.

Tunable surface plasmon resonance and strong SERS performances of Au opening-nanoshell ordered arrays

Au opening-nanoshell ordered arrays with tunable local surface plasmon resonance (SPR) property have been fabricated based on sputtering deposition onto monolayer colloidal crystal. The changes in local SPR peak for the arrays can be well tuned from visible to near-infrared region with decreasing of the spacing between two neighbor opening-nanoshells. It has been revealed that the changes of SPR peak originate from the electromagnetic coupling between two adjacent Au opening-nanoshells. This study is important to design and fabricate surface-enhanced Raman scattering substrates with high activity and practical application.

Surface-enhanced Raman spectroscopy (SERS): progress and trends

Surface-enhanced Raman spectroscopy (SERS) combines molecular fingerprint specificity with potential single-molecule sensitivity. Therefore, the SERS technique is an attractive tool for sensing molecules in trace amounts within the field of chemical and biochemical analytics. Since SERS is an ongoing topic, which can be illustrated by the increased annual number of publications within the last few years, this review reflects the progress and trends in SERS research in approximately the last three years. The main reason why the SERS technique has not been established as a routine analytic technique, despite its high specificity and sensitivity, is due to the low reproducibility of the SERS signal. Thus, this review is dominated by the discussion of the various concepts for generating powerful, reproducible, SERS-active surfaces. Furthermore, the limit of sensitivity in SERS is introduced in the context of single-molecule spectroscopy and the calculation of the 'real' enhancement factor. In order to shed more light onto the underlying molecular processes of SERS, the theoretical description of SERS spectra is also a growing research field and will be summarized here. In addition, the recording of SERS spectra is affected by a number of parameters, such as laser power, integration time, and analyte concentration. To benefit from synergies, SERS is combined with other methods, such as scanning probe microscopy and microfluidics, which illustrates the broad applications of this powerful technique.

Surface-enhanced Raman scattering of a Ag/oligo(phenyleneethynylene)/Ag sandwich

α,ω-Dithiols are a useful class of compounds in molecular electronics because of their ability to easily adsorb to two metal surfaces, producing a molecular junction. We have prepared Ag nanosphere/oligo(phenyleneethynylene)/Ag sol (AgNS/OPE/Ag sol) and Ag nanowire/oligo(phenyleneethynylene)/Ag sol (AgNW/OPE/Ag sol) sandwiches to simulate the architecture of a molecular electronic device. This was achieved by self-assembly of OPE on the silver nanosurface, deprotection of the terminal sulfur, and deposition of Ag sol atop the monolayer. These sandwiches were then characterized by surface-enhanced Raman scattering (SERS) spectroscopy. The resulting spectra were compared to the bulk spectrum of the dimer and to the Ag nanosurface/OPE SERS spectra. The intensities of the SERS spectra in both systems exhibit a strong dependence on Ag deposition time and the results are also suggestive of intense interparticle coupling of the electromagnetic fields in both the AgNW/OPE/Ag and the AgNS/OPE/Ag systems. Three previously unobserved bands (1219, 1234, 2037 cm(-1)) arose in the SER spectra of the sandwiches and their presence is attributed to the strong enhancement of the electromagnetic field which is predicted from the COSMOL computational package. The 544 cm(-1) disulfide bond which is observed in the spectrum of solid OPE but is absent in the AgNS/OPE/Ag and AgNW/OPE/Ag spectra is indicative of chemisorption of OPE to the nanoparticles through oxidative dissociation of the disulfide bond.

Towards multiple readout application of plasmonic arrays

In order to combine the advantages of fluorescence and surface-enhanced Raman spectroscopy (SERS) on the same chip platform, a nanostructured gold surface with a unique design, allowing both the sensitive detection of fluorescence light together with the specific Raman fingerprint of the fluorescent molecules, was established. This task requires the fabrication of plasmonic arrays that permit the binding of molecules of interest at different distances from the metallic surface. The most efficient SERS enhancement is achieved for molecules directly adsorbed on the metallic surface due to the strong field enhancement, but where, however, the fluorescence is quenched most efficiently. Furthermore, the fluorescence can be enhanced efficiently by careful adjustment of the optical behavior of the plasmonic arrays. In this article, the simultaneous application of SERS and fluorescence, through the use of various gold nanostructured arrays, is demonstrated by the realization of a DNA detection scheme. The results shown open the way to more flexible use of plasmonic arrays in bioanalytics.

Investigation on the second part of the electromagnetic SERS enhancement and resulting fabrication strategies of anisotropic plasmonic arrays

In general, the electromagnetic mechanism is understood as the strongest contribution to the overall surface-enhanced Raman spectroscopy (SERS) enhancement. Due to the excitation of surface plasmons, a strong electromagnetic field is induced at the interfaces of a metallic nanoparticle leading to a drastic enhancement of the Raman scattering cross-section. Furthermore, the Raman scattered light expierences an emission enhancement due to the plasmon resonances of the nanoantennas. Herein, this second part of the electromagnetic enhancement phenomenon is investigated for different Raman bands of crystal violet by utilizing the anisotropic plasmonic character of gold nanorhomb SERS arrays. We aim at evaluating the effects of localized and propagating surface plasmon polariton modes as well as their combination on the scattered SERS intensity. From that point of view, design and fabrication strategies towards the fabrication of SERS arrays for excitation wavelengths in the visible and near-infrared (NIR) spectral region can be given, also using a double-resonant electromagnetic enhancement.

Plasmonic modes of metallic semishells in a polymer film

The symmetry-broken geometry and variation of metal composition of semishells induce new plasmonic properties. A system of separated metallic semishells embedded in a poly(dimethylsiloxane) polymer film provides an ideal platform to investigate the localized surface plasmon resonance modes of semishells. We demonstrate experimentally that silver, gold, copper, and aluminum semishells can offer distinct plasmonic responses due to the wide range of their material parameters. Numerical calculations combined with the plasmon hybridization theory render us a clear understanding and assignment of the plasmonic modes of the semishells.

Third-order optical nonlinearity of gold nanoparticle arrays embedded in a BaTiO3 matrix

Au:BaTiO(3) composite films comprising hexagon-shaped Au nanoparticle arrays covered with BaTiO(3) matrix were fabricated by double-layer nanosphere lithography and pulsed laser deposition technique. The optical nonlinearity of the composite film was determined using the Z-scan technique at a wavelength of 532 nm and a pulse duration of 25 ps. The third-order nonlinear optical susceptibility, chi(3), was found to be 2.9x10(-8) esu, which is comparable with the best values in metal-dielectric films comprising randomly distributed spherical particles but with much higher metal concentrations. The local electric field enhancement in and near the particles was investigated using the dipole discrete approximation method.

Improving surface-enhanced Raman scattering effect using gold-coated hierarchical polystyrene bead substrates modified with postgrowth microwave treatment

We report a novel postgrowth microwave heating implementation by selectively modifying hierarchical polystyrene (PS) bead substrates coated with gold (Au) films to effectively improve the surface-enhanced Raman scattering (SERS) effect on the analytes. The SERS signal of probe molecule rhodamine 6G (Rh 6G) on the microwave-treated Au-PS substrates can be improved by 10-fold, while the detection limit of Rh 6G in concentration can be enhanced by two orders of magnitude compared to the as-growth substrates. The high-quality SERS spectrum of saliva can also be acquired using the modified substrates, demonstrating the potential for the realization of the high-performance SERS substrates for biomedical applications.

Effects of changes in the interparticle separation induced by alkanethiols on the surface plasmon band and other properties of nanocrystalline gold films

Effects of changing the interparticle separation on the surface plasmon bands of ultrathin films of gold nanoparticles have been investigated by examining the interaction of alkanethiols of varying chain length on nanocrystalline gold films generated at the organic-aqueous interface. Adsorption of alkanethiols causes blue-shifts of the surface plasmon adsorption band, the magnitude of the shift being proportional to the chain length. The disordered nanocrystals thus created (lambdamax, 530 m) are in equilibrium with the ordered nanocrystals in the film (lambdamax, 700 m) as indicated by an isosbestic point around 600 nm. Long chain thiols disintegrate or disorder the gold films more effectively, as demonstrated by the increased population of the thiol-capped gold nanocrystals in solution. The rate of interaction of the thiols with the film decreases with the decreasing chain length. The effect of an alkanethiol on the spectrum of the gold film is specific, in that the effects with long and short chains are reversible. The changes in the plasmon band of gold due to interparticle separation can be satisfactorily modeled on the basis of the Maxwell-Garnett formalism. Spectroscopic studies, augmented by calorimetric measurements, suggest that the interaction of alkanethiols involves two steps, the first step being the exothermic gold film-thiol interaction and the second step includes the endothermic disordering process followed by further thiol capping of isolated gold particles.