Deep-UV surface-enhanced resonance Raman scattering of adenine on aluminum nanoparticle arrays

We report the ultrasensitive detection of adenine using deep-UV surface-enhanced resonance Raman scattering on aluminum nanostructures. Well-defined Al nanoparticle arrays fabricated over large areas using extreme-UV interference lithography exhibited sharp and tunable plasmon resonances in the UV and deep-UV wavelength ranges. Theoretical modeling based on the finite-difference time-domain method was used to understand the near-field and far-field optical properties of the nanoparticle arrays. Raman measurements were performed on adenine molecules coated uniformly on the Al nanoparticle arrays at a laser excitation wavelength of 257.2 nm. With this technique, less than 10 amol of label-free adenine molecules could be detected reproducibly in real time. Zeptomole (~30,000 molecules) detection sensitivity was readily achieved proving that deep-UV surface-enhanced resonance Raman scattering is an extremely sensitive tool for the detection of biomolecules.     

Transition metal-coated nanoparticle films: vibrational characterization with surface-enhanced Raman scattering

The utilization of surface-attached gold nanoparticles as templates for generating Pt-group particles displaying near-optimal surface-enhanced Raman scattering (SERS) characteristics is described. Essentially epitaxial transition metal coatings down to the monolayer level can be prepared, most readily by the spontaneous replacement of an electrochemically deposited copper layer by the desired Pt-group metal. The and essentially pinhole-free nature of the coated nanoparticles is demonstrated from the form of the SER spectra for chemisorbed carbon monoxide and ethylene. The potential of the present strategy for synthesizing relatively monodispersed "core-shell" nanoparticles using a myriad of coating materials, also displaying SERS activity, is pointed out.     

Three-dimensional Pt-coated Au nanoparticle arrays: applications for electrocatalysis and surface-enhanced Raman scattering

This study demonstrates a novel approach to synthesis methods for core-shell nanoparticle assembly using nanoparticle trapping at an interface and subsequent transfer onto a substrate for electrochemical ultrathin layer coating. The transferred nanoparticle array can have a tunable surface area depending on the number of transferred layers. Subsequently coating the surface with Pt-group metals that behave as an ultrathin film provides electrocatalytic activities with respect to a variety of chemical reactions, depending on the properties of the selected coating materials. The transferred 3D Au nanoparticle arrays act as a high-surface-area platform for the diversity of overlayer materials. The resulting 3D core-shell nanoparticle films could be utilized as a highly active electrocatalysis and Raman scattering substrate. The approach provides a versatile and convenient synthesis route to new nanoporous material with tailorable pore structure and material properties through bottom-up assembly.     

Gold nanoparticle embedded, self-sustained chitosan films as substrates for surface-enhanced Raman scattering

In this work, self-sustained, biocompatible, biodegradable films containing gold nanostructures have been fabricated for potential application in nanobioscience and ultrasensitive chemical and biochemical analysis. We report a novel synthesis of gold nanoparticles mediated by the biopolymer chitosan. Self-supporting thin films are formed from the resultant gold-chitosan nanocomposite solutions and characterized by UV-visible surface plasmon absorption, transmission electron microscopy, atomic force microscopy, infrared absorption, and Raman scattering measurements. Results demonstrate control over the size and distribution of the nanoparticles produced, which is promising for several applications, including the development of biosensors. As a proof of principle, we demonstrate that gold-chitosan films can be employed in trace analysis using surface-enhanced Raman scattering.     

等离激元激励表面增强拉曼光谱检测技术与仪器

本文总结了近年来基于传播型表面等离激元（Propagafingsurfaceplasmons,PSPs）参与的表面增强拉曼（Surface—enhancedRamanscattering,SERS）技术和仪器方面的研究进展．内容主要包括3部分：（1）基于PSPs激励拉曼散射的装置和技术,包括在消逝场下激发PSPs共振增强拉曼的原理与装置、与表面等离子体共振（Surfaceplasmonresonance,SPR）传感技术的联用及新型结构的长程等离激元激励拉曼技术的研究进展;（2）通过引入局域型表面等离激元（Localizedsurfaceplasmons,LSPs）进一步增强SERS,进而实现PSPs-LSPs共同增强拉曼的超灵敏检测技术,包括在消逝场激发的PSPs基础上,增加纳米粒子实现的PSPs与LSPs共同增强拉曼的原理、装置,以及用该方法进行生物体系的免疫识别检测,此外,还在微纳周期结构上实现了PSPs与LSPs共同激励拉曼;（3）基于PSPs耦合的定向SERS技术,包括在消逝场结构和周期结构上实现SERS定向耦合发射以达到高激发和高收集效率的新技术．     

The theory of surface-enhanced Raman scattering

By considering the molecule and metal to form a conjoined system, we derive an expression for the observed Raman spectrum in surface-enhanced Raman scattering. The metal levels are considered to consist of a continuum with levels filled up to the Fermi level, and empty above, while the molecule has discrete levels filled up to the highest occupied orbital, and empty above that. It is presumed that the Fermi level of the metal lies between the highest filled and the lowest unfilled level of the molecule. The molecule levels are then coupled to the metal continuum both in the filled and unfilled levels, and using the solutions to this problem provided by Fano, we derive an expression for the transition amplitude between the ground stationary state and some excited stationary state of the molecule-metal system. It is shown that three resonances contribute to the overall enhancement; namely, the surface plasmon resonance, the molecular resonances, as well as charge-transfer resonances between the molecule and metal. Furthermore, these resonances are linked by terms in the numerator, which result in SERS selection rules. These linked resonances cannot be separated, accounting for many of the observed SERS phenomena. The molecule-metal coupling is interpreted in terms of a deformation potential which is compared to the Herzberg-Teller vibronic coupling constant. We show that one term in the sum involves coupling between the surface plasmon transition dipole and the molecular transition dipole. They are coupled through the deformation potential connecting to charge-transfer states. Another term is shown to involve coupling between the charge-transfer transition and the molecular transition dipoles. These are coupled by the deformation potential connecting to plasmon resonance states. By applying the selection rules to the cases of dimer and trimer nanoparticles we show that the SERS spectrum can vary considerably with excitation wavelength, depending on which plasmon and/or charge-transfer resonance is excited.     

An ionic surfactant-mediated Langmuir-Blodgett method to construct gold nanoparticle films for surface-enhanced Raman scattering

A gold nanoparticle film for surface-enhanced Raman scattering (SERS) was successfully constructed by an ionic surfactant-mediated Langmuir-Blodgett (LB) method. The gold film was formed by adding ethanol to a gold colloid/hexane mixture in the presence of dodecyltrimethylammonium bromide (DTAB). Consequently, gold nanoparticles (AuNPs) assembled at the water/hexane interface due to the decrease in surface charge density of AuNPs. Since DTAB binds the gold surface by a coulombic force, rather than a chemical bonding, it is easily replaced by target molecules for SERS purposes. The SERS enhancement factor of the 80 nm gold nanoparticle film was approximately 1.2 × 10(6) using crystal violet (CV) as a Raman dye. The SERS signal from the proposed DTAB-mediated film was approximately 10 times higher than that from the octanethiol-modified gold film, while the reproducibility and stability of this film compared to an octanethiol-modified film were similar. This method can also be applied to other metal nanostructures to fabricate metal films for use as a sensitive SERS substrate with a higher enhancement factor.     

Nanostructured Ag surface fabricated by femtosecond laser for surface-enhanced Raman scattering

Femtosecond laser was employed to fabricate nanostructured Ag surface for surface-enhanced Raman scattering (SERS) application. The prepared nanostructured Ag surface was characterized by field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The FESEM images demonstrate the formation of nanostructure-covered femtosecond laser-induced periodic surface structure, also termed as ripples, on the Ag surface. The AFM images indicate that the surface roughness of the produced nanostructured Ag substrate is larger than the untreated Ag substrate. The XRD and XPS of the nanostructured Ag surface fabricated by femtosecond laser show a face centered cubic phase of metallic Ag and no impurities of Ag oxide species. The application of the produced nanostructured Ag surface in SERS was investigated by using rhodamine 6G (R6G) as a reference chemical. The SERS intensity of R6G in aqueous solution at the prepared nanostructured Ag surface is 15 times greater than that of an untreated Ag substrate. The Raman intensities vary linearly with the concentrations of R6G in the range of 10(-8)-10(-4)M. The present methodology demonstrates that the nanostructured Ag surface fabricated by femtosecond laser is potential for qualification and quantification of low concentration molecules.     

Role of O2 in inducing intensive fluctuations of surface-enhanced Raman scattering spectra

Confocal Raman microscopic measurements were performed on silver electrodes covered with hydrogenated amorphous carbon (a-C:H). When short accumulation time was used, the subsequently measured surface-enhanced Raman scattering (SERS) spectra exhibited fluctuations. As previously reported for other systems, the intensity of fluctuations of SERS spectra significantly decreases if O2 was removed from the ambient medium. In this contribution we show that intensive SERS fluctuations can be also observed for a-C:H/Ag samples immersed in the deoxygenated electrolyte after applying a negative potential pulse to the silver electrode. It means that the O2-mediated Burstein mechanism of SERS fluctuations, which has been previously proposed to explain the SERS O2 effect, is not adequate for these results. We suggest that oxygen chemisorbed on the silver surface decreases the average strength of the interaction between a-C:H clusters and the metal surface (and hence the speed of movement of a-C:H clusters across the metal surface) and that the SERS O2 effect should be rather explained using the "classical" model of SERS fluctuations, in which fluctuations are interpreted as a result of the thermally activated diffusion of carbon segments in and out of the SERS "hot spots". A numerical algorithm for modeling of the fluctuations of SERS intensity has been proposed, and some example simulations of SERS fluctuations have been carried out. For the first time, strongly fluctuating bands due to the stretching vibrations of significantly weakened C-H bonds have been identified.     

On the theory of surface-enhanced Raman scattering

A model for Raman scattering from molecules chemisorbed on surfaces is proposed. It is shown that part of the en hancement may be due to charge-transfer excitations between the metal and the adsorbed molecules.     

Relating surface-enhanced Raman scattering signals of cells to gold nanoparticle aggregation as determined by LA-ICP-MS micromapping

The cellular response to nanoparticle exposure is essential in various contexts, especially in nanotoxicity and nanomedicine. Here, 14-nm gold nanoparticles in 3T3 fibroblast cells are investigated in a series of pulse-chase experiments with a 30-min incubation pulse and chase times ranging from 15 min to 48 h. The gold nanoparticles and their aggregates are quantified inside the cellular ultrastructure by laser ablation inductively coupled plasma mass spectrometry micromapping and evaluated regarding the surface-enhanced Raman scattering (SERS) signals. In this way, both information about their localization at the micrometre scale and their molecular nanoenvironment, respectively, is obtained and can be related. Thus, the nanoparticle pathway from endocytotic uptake, intracellular processing, to cell division can be followed. It is shown that the ability of the intracellular nanoparticles and their accumulations and aggregates to support high SERS signals is neither directly related to nanoparticle amount nor to high local nanoparticle densities. The SERS data indicate that aggregate geometry and interparticle distances in the cell must change in the course of endosomal maturation and play a critical role for a specific gold nanoparticle type in order to act as efficient SERS nanoprobe. This finding is supported by TEM images, showing only a minor portion of aggregates that present small interparticle spacing. The SERS spectra obtained after different chase times show a changing composition and/or structure of the biomolecule corona of the gold nanoparticles as a consequence of endosomal processing.     

Spectroscopic analysis of L-histidine adsorbed on gold and silver nanoparticle surfaces investigated by surface-enhanced Raman scattering

The adsorption of l-histidine on gold (Au) and silver (Ag) nanoparticle surfaces has been comparatively analyzed by means of surface-enhanced Raman scattering (SERS). The SERS spectra of l-histidine on Ag were found to be quite different from those on Au, indicating dissimilar adsorption structures depending on metal substrates. Most peaks of l-histidine on Ag appeared to be due to coordination via the carboxylate (COO(-)) group with an imidazole ring of fairly upright geometry, whereas on Au it was assumed to adsorb with a rather flat geometry. A density functional theory (DFT) calculation was performed at the level of B3LYP/LANL2DZ to estimate the energetic stability of the binding of the imidazole ring and the carboxylate group of l-histidine with the Ag and Au atoms, respectively. Based on the DFT calculation, the carboxylate group of l-histidine was predicted to bind more favorably to Ag than to Au, and this was in line with our SERS spectral analysis.     

Adsorption of -histidine on copper surface as evidenced by surface-enhanced Raman scattering spectroscopy

The adsorption of -histidine on a copper electrode from H₂O- and D₂O-based solutions is studied by means of surface-enhanced Raman scattering (SERS) spectroscopy. Different adsorption states of histidine are observed depending upon pH, potential, and the presence of the SO²⁻₄ and Cl⁻ ions. In acidic solutions of pH 1.2 the imidazole ring of the adsorbed histidine remains protonated and is not involved in the chemical coordination with the surface. The SO²⁻₄ and Cl⁻ ions compete with histidine for the adsorption sites. In solutions of pH 3.1 three different adsorption states of histidine are observed depending on the potential. Histidine adsorbs with the protonated imidazole ring oriented mainly perpendicularly to the surface at potentials more positive than −0.2 V. Transformation of that adsorption state occurs at more negative potentials. As this takes place, histidine adsorbs through the α-NH₂ group and the neutral imidazole ring. The Cl⁻ ions cause the protonation and detachment of the α-NH₂ group from the surface and the formation of the ion pair NH⁺₃ … Cl⁻ can be observed. In the neutral solution of pH 7.0 histidine adsorbs through the deprotonated nitrogen atom of the imidazole ring and the α-COO⁻ group at E ≥ −0.2 V. However, this adsorption state is transformed into the adsorption state in which the α-NH₂ group and/or neutral imidazole ring participate in the anchoring of histidine to the surface, once the potential becomes more negative. In alkaline solutions of pH 11.9 histidine is adsorbed on the copper surface through the neutral imidazole ring.     

Nanoparticle-mirror sandwich substrates for surface-enhanced Raman scattering

Sandwich surface-enhanced Raman scattering (SERS) substrates (3S) utilizing coupling between continuous metal films and plasmonic particles were fabricated using silver mirrors, electrochemically roughened films, and various sizes of silver nanoparticles. The effect of excitation wavelength and nanoparticle size on SERS spectra of poly(vinylpyridine), selected as a model compound, was studied to determine the optimum conditions for the strongest SERS signal. The Raman enhancement resulted from the plasmon coupling of silver nanoparticles to the underlying continuous film as well as the lateral plasmon coupling between the silver nanoparticles. The formation of the charge transfer complex was also observed. The 3S configuration was used to obtain SERS spectra of dipicolinic acid (DPA), a chemical signature for Bacillus anthracis.     

Self-assembled silver nanochains for surface-enhanced Raman scattering

Surface-enhanced Raman scattering (SERS) integrates high levels of sensitivity with spectroscopic precision and has tremendous potential for chemical and biomolecular sensing. The key to the wider application of Raman spectroscopy using roughened metallic surfaces is the development of highly enhancing substrates for analytical purposes. Here, we demonstrate a simple strategy for self-assembling silver nanochains on glass substrates for sensitive SERS substrates. The chain length of short Ag nanochains can be controlled by adjusting the concentration of cetyltrimethylammonium bromide (CTAB) and 11-mercaptoundecanoic acid (MUA). CTAB with appropriate concentration serves as the "glue" that can link the {100} facets of two neighboring Ag nanoparticles. MUA is found to be effective in "freezing up" the aggregation of Ag short chains and preventing them from further aggregating into a long chainlike network structure. The surface plasmon bands can be tuned over an extended wavelength range by controlling the length of the nanochains. The Ag monolayer, mainly composed of four-particle nanochains, exhibited the maximum SERS enhancement factor of around 2.6 x 108, indicating that a stronger SERS enhancement can be obtained in these interstitial sites of chainlike aggregated Ag nanoparticles.     

Silver nanovoid arrays for surface-enhanced Raman scattering

Highly ordered silver nanovoid arrays are fabricated on porous anodic alumina membranes to produce robust and cost-efficient surface-enhanced Raman scattering (SERS) substrates. Plasmonic tunability can be accomplished by adjusting the topography with different anode voltages. Evenly distributed plasmonic fields, high average enhancement factor, and excellent ambient stability due to the natural protective layer are some of the unique advantages, and the silver nanovoid arrays are applicable to sensing devices.     

Multiobjective evolutionary optimisation for surface-enhanced Raman scattering

In most optimisation experiments, a single parameter is first optimised before a second and then third one are subsequently modified to give the best result. By contrast, we believe that simultaneous multiobjective optimisation is more powerful; therefore, an optimisation of the experimental conditions for the colloidal SERS detection of l-cysteine was carried out. Six aggregating agents and three different colloids (citrate, borohydride and hydroxylamine reduced silver) were tested over a wide range of concentrations for the enhancement and the reproducibility of the spectra produced. The optimisation was carried out using two methods, a full factorial design (FF, a standard method from the experimental design literature) and, for the first time, a multiobjective evolutionary algorithm (MOEA), a method more usually applied to optimisation problems in computer science. Simulation results suggest that the evolutionary approach significantly out-performs random sampling. Real experiments applying the evolutionary method to the SERS optimisation problem led to a 32% improvement in enhancement and reproducibility compared with the FF method, using far fewer evaluations.     

Raman,surface-enhanced Raman scattering and DFT study of para-nitro-aniline

Raman spectra of para-nitro-aniline (pNA), a molecule with high applicability potential in molecular electronics, were recorded in solid state and in ethanol solution. Complete assignment of the experimental spectra was made by using the B3LYP/6-31G(d) theoretical results. The calculated molecular electrostatic potential shows a high negative charge localized on the nitro group of pNA and the surface-enhanced Raman scattering (SERS) spectrum of pNA adsorbed to colloidal silver particles denote the molecule interaction with the silver surface mainly through the nitro group. However, theoretical results obtained by modeling the pNA–4Ag complex also suggest the adsorption of pNA through the amino group or a flattened orientation of pNA with respect to the silver surface.     

The role of cavity sites in surface-enhanced Raman scattering

Thermal desorption spectra (TDS) of pyridine from silver films deposited in ultra high vacuum are reported. Marked differences in the TDS are seen depending on the deposition conditions and the thermal history of the films, which have been correlated with surface-enhanced Raman scattering (SERS). These results as well as some of the observations in electrochemical systems are discussed in light of the recent Xe probe analysis carried out by Albano et al.     

Convective assembly of bacteria for surface-enhanced Raman scattering

A sample preparation method based on convective assembly for "whole-microorganism" identification using surface-enhanced Raman scattering (SERS) is developed. With this technique, a uniform sample can easily be prepared with silver nanoparticles. During the deposition process, bacteria and nanoparticles are assembled to form a unique well-ordered structure with great reproducibility. The SERS spectra acquired from the samples prepared with this technique have better quality and improved reproducibility for SERS spectra obtained from the same sample and limited variation due to the consistent sample preparation. E. coli, a Gram-negative bacilli, and Staphylococcus cohnii, a Gram-positive coccus, are studied as model bacteria.