Surface-enhanced Raman Spectra Research Articles

Electric-Field-Induced Connectivity Switching in Single-Molecule Junctions.

SummaryThe manipulation of molecule-electrode interaction is essential for the fabrication of molecular devices and determines the connectivity from electrodes to molecular components. Although the connectivity of molecular devices could be controlled by molecular design to place anchor groups in different positions of molecule backbones, the reversible switching of such connectivities remains challenging. Here, we develop an electric-field-induced strategy to switch the connectivity of single-molecule junctions reversibly, leading to the manipulation of different connectivities in the same molecular backbone. Our results offer a new concept of single-molecule manipulation and provide a feasible strategy to regulate molecule-electrode interaction.

USING MOLECULAR ELECTROSTATIC POTENTIALS AND FRONTIER ORBITALS FOR THE SURFACE-ENHANCED RAMAN INTERPRETATION OF FLUOXETINE

Raman and surface-enhanced Raman (SERS) spectra of (N-methyl-3-phenyl-3-[4-(trifluoromethyl)phenoxy)propane-1-amine hydrochloride, fluoxetine, have been recorded. Density functional theory with the B3LYP functional was used for optimization of the ground state geometry, calculation of the Raman normal modes of this molecule and the modelling of the SERS effect. Calculated geometrical parameters of fluoxetine fit well with the experimental ones. Based on the recorded data, the DFT results and a normal coordinate analysis based on a scaled quantum mechanical (SQM) force field approach, a complete vibrational assignment of fluoxetine as well as its adsorption behavior on a silver surface (using SERS selection rules) is derived.

Atmospheric pressure plasma jet: A complete tool for surface enhanced Raman spectroscopy substrates preparation

In this paper, a new method for fabrication of surface enhanced Raman spectroscopy (SERS) substrates by atmospheric pressure plasma jet (APPJ) has been introduced. Arc free cold plasma is generated at atmospheric pressure by using atmospheric pressure plasma jet system with 120 W and 200 W RF power and 3 L/min and 5 L/min argon flow. SERS substrates are prepared by two-step treatment process. In first step, PMMA substrates are roughened by 15 min plasma treatment using APPJ and in second step, spin coated Ag/GO suspension (on roughened PMMA substrates) is again treated with APPJ for 1 min. Treated PMMA samples have been characterized by AFM and XPS to observe the morphology, roughness and the chemical changes on surface. Variation of the roughness with the variation of vertical distance has also been analysed. Treated spin coated Ag/GO samples are characterized by Raman and FESEM to observe the surface enhanced Raman spectra and the morphological changes, respectively. Plasma jet coming out form APPJ system has also been characterized by optical emission spectroscopy to identify the various species in the plasma jet and for electron energy measurement. The gas temperature of the plasma jet has also been measured.

Preparation, Characterization of Silver-Nanoparticles from Seed Coat Exudates of Celastrus paniculatus Willd. and Their Bactericidal and Biofilm Inhibition Effects

We are reporting a novel approach for preparing silver nanoparticles with seed coat exudates of Celastrus paniculatus Willd, a medicinal plant traditionally used for the treatment of skin diseases, fever, leprosy and neurological disorders. HPTLC study revealed that aqueous, soluble seed coat exudates contain phenolics, alakaloids and flavonoids. The formation, crystalline nature and morphology of the nanoparticles were identified by UV–Vis spectroscopy, X-ray Diffraction (XRD) analysis, Transmission Electron Microscopy (TEM) and selected-area electron diffraction (SAED). Functional group stretching of aqueous soluble extracts was identified by using FTIR. Results revealed that nanosilver particles are spherical, range in size from 10 to 82 nm and crystalize in face-centered cubic structures. Surface-enhanced Raman spectra analysis showed that AgNP are capped with bioactive molecules from exudates and that they may act as precursors of the reduction of silver nitrate from the metallic state (Ag+) to the atomic state (Ag0). We also examined the minimal inhibitory concentration for bacteria Escherichia coli and Bacillus subtilis using a resazurin color assay. Nanosilver strongly inhibited the bacterial growth, leading to MIC values of 40µg/ml and 60µg/ml for the bacteria, respectively. The colony screening method and inhibition kinetics of biofilm formation in the Klebsiella pneumoniae strain were also studied using the tube method and a quantitative microplate assay. SEM analysis and quantification of the EPS revealed a fivefold decrease in concentration in treated compared to untreated. The inhibition response was duly reflected in SEM images.

Experimental and Theoretical Study of Surface-Enhanced Raman Spectra of Sulfadiazine Adsorbed on Nanoscale Gold Colloids.

Sulfadiazine, as a class of antibiotics, has been widely used in the world for decades; however, its surface-enhanced Raman spectra (SERS) on gold colloids are obviously different from ordinary Raman spectra in the solid powder and liquid solution. To explore the reasons for such significant differences, we used density functional theory calculations and normal-mode analysis to investigate the effects of the configuration, conformation, protonation, hydrogen-bonding interaction, and adsorption configurations of sulfadiazine on gold clusters to check these different effects on the vibrational assignments. Our calculated results can be summarized as two points. First, the Raman spectra strongly depend on the configuration, conformation, protonation, and hydrogen bonding of sulfadiazine. Second, the wagging vibration displays a significant vibrational frequency shift and a very strong SERS peak responsible for the observed SERS signal when sulfadiazine is adsorbed on gold clusters through the terminal amino group. This is different from another adsorption configuration through two oxygen atoms of the -SO2NH- group on gold clusters. Finally, we further investigate the potential energy surfaces along the wagging vibration and the binding interaction of -NH2 adsorbed on different sites of gold surfaces.

Construction of Sensitive Surface‐Enhanced Raman Spectroscopy Chip with Sub‐Nanometer WO3−x Nanowire‐Based Superstructures for Multicomponent Detection

AbstractExploring nonmetallic plasmonic materials is desirable yet challenging. For a long time, free‐plasmonic semiconductor is difficult to be used in high‐sensitive surface‐enhanced Raman spectrum (SERS) due to poor enhanced factor and high‐fluorescent background. Herein, it is reported that a nonmetallic plasmonic SERS chip, constructed with a highly ordered sub 1 nm WO3−x nanowire‐based superstructure, shows multicomponent detection capability. The densely arranged nanowires enable a strong localized surface plasmon resonance due to the high charge carriers' density induced by oxygen vacancies in the nanowires. In addition to the usual charge‐transfer enhancement, such nonmetallic WO3−x displays a unusual dramatic plasmon‐enhanced SERS sensing performance with a detection limit of 10−10 m and a maximum Raman enhanced factor of 8.3 × 107. The substrate fluorescence signals are efficiently quenched, making the SERS spectra more distinguishable. More importantly, the prepared WO3−x SERS chip through micro–nano printing realizes the separation and detection of multicomponents simultaneously.

Jahn-Teller reconstructed surface of the doped manganites shown by means of surface-enhanced Raman spectroscopy

We report direct evidence of the theoretically predicted electron-rich surface of doped perovskite manganites ${\mathrm{La}}_{0.7}{A}_{0.3}{\mathrm{MnO}}_{3}\phantom{\rule{4pt}{0ex}}(A=\text{Ca}$, Sr) by means of surface-enhanced Raman spectroscopy. The required Au nanoparticles were grown on top of thin manganite films by the metalorganic aerosol deposition technique, which provides a stable oxygen atmosphere and prevents deoxygenation effects. The acquired surface-enhanced Raman spectra of rhombohedral ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_{3}$ thin films reveal the symmetry-forbidden Jahn-Teller stretching modes demonstrating the predicted symmetry breaking at the surface and fallback into the orthorhombic (Pnma) structure. This results in a unique Jahn-Teller reconstructed surface structure, which limits the capabilities of mixed-valence manganites in spintronic applications, but will be highly favorable for catalytic reactions.

The fabrication of a high-sensitivity surface-enhanced Raman spectra substrate using texturization and electroplating technology

A method for preparing surface-enhanced Raman scattering substrate with high sensitivity is presented in this study. The new type of substrate was fabricated by depositing silver nanoparticles, using electroplating, onto the surface of boron-doped single silicon with a texture of inverted pyramid arrays prepared by the technology of texturization. In order to evaluate the enhancement ability of the substrate, a test of Raman scattering with Rhodamine 6G (R6G) as probe molecules was performed and the results showed a high sensitivity, good uniformity and stability. Relay on the enhancement by the substrate, the Raman scattering signal of R6G with a high signal-to-noise ratio could be detected out at a rather low concentration level of 10−18 M. The performance of the substrate is also simulated and illustrated using the software COMSOL Multiphysics. The electric field is greatly enhanced on the surface of silicon wafer with inverted pyramid morphology, especially in pits. According to the electromagnetic enhancement theory, the calculated Raman enhancement factor is as high as 1011, which strongly supports the experimental data. This work may offer a novel and practical method for Raman spectroscopy application in trace analysis.

Erratum to: Identification of Bacteria by Surface-Enhanced Raman Spectra after Peroxide Treatment

Erratum to: Identification of Bacteria by Surface-Enhanced Raman Spectra after Peroxide Treatment

Advanced Nanomaterials for Pollutant Sensing and Environmental Catalysis

In this chapter we give a comprehensive introduction to noble metal–based sensing platforms, including gold nanoparticles, silver nanoparticles, and their alloys and assembles in the application of environmental detection. Noble-metal nanoparticles have shown great potential applications in environmental sensing due to their unique physical and chemical properties, including good conductivity, easy functionalization with a range of ligands, large electronic field enhancement, fluorescence quenching and catalytic behavior. Colorimetric sensing toward heavy metal ions detection, fluorescence-based sensing toward biomolecules analysis, surface-enhanced Raman spectrum sensing platform and electrochemical sensors have been introduced in terms of working principles, and research progress through the classic example.

ReaxFF MD Simulations of Peptide-Grafted Gold Nanoparticles.

Functionalized gold nanoparticles have critical applications in biodetection with surface-enhanced Raman spectrum and drug delivery. In this study, reactive force field molecular dynamics simulations were performed to study gold nanoparticles, which are modified with different short-chain peptides consisting of amino acid residues of cysteine and glycine in different grafting densities in the aqueous environment. Our study showed slight facet-dependent peptide adsorption on a gold nanoparticle with the 3 nm core diameter. Peptide chains prefer to adsorb on the Au(111) facet compared to those on other facets of Au(100) and Au(110). In addition to the stable thiol interaction with gold nanoparticle surfaces, polarizable oxygen and nitrogen atoms show strong interactions with the gold surface and polarize the gold nanoparticle surfaces with an overall positive charge. Charges of gold atoms vary according to their contacts with peptide atoms and lattice positions. However, at the outmost peptide layer, the whole functionalized Au nanoparticles exhibit overall negative electrostatic potential due to the grafted peptides. Moreover, simulations show that thiol groups can be deprotonated and subsequently protons can be transferred to water molecules and carboxyl groups.

Surface-enhanced Raman scattering on nanodiamond-derived carbon onions

Annealing nanodiamonds (ND) at high temperatures up to 1700 °C is a common method to synthesize carbon onions. The transformation from NDs to carbon onions is particularly interesting because of carbon onions’ potential in the field of tribology and their application in ultra-charge/discharge devices. In this paper, a novel surface-enhanced Raman scattering technique that involves coating the sample with nanoscopic gold particles is proposed to characterize the NDs after different annealing treatments. Conventional Raman and surface-enhanced Raman spectra were obtained, and the changes of peak parameters as the function of annealing temperature were evaluated. It was found that the widths of the D and the G peaks decreased with increasing annealing temperature, reflecting an improved order in the sp2-hybridized carbon during the transformation from NDs to carbon onions. After annealing at 1700 °C, the sp2-carbon was highly ordered, indicating desirable electrical conductivity and lubricity. With increasing annealing temperature, the D peak showed a blue shift of almost 30 cm−1, while the G peak merely shifted by 5 cm−1. For annealing temperatures above 1100 °C, an increase of intensity ratio ID/IG was observed. Compared to the uncoated area, red shifts of 0.5–2 cm−1 and of 5–9 cm−1 for the G and D peaks, respectively, were detected for the gold-coated area, which was due to the coupling of the plasmons and the phonons of the samples.

Effect of Alkylamines on Morphology Control of Silver Nanoshells for Highly Enhanced Raman Scattering.

Morphology control of the surface of a nanostructure is a key issue in modulating its surface plasmon resonance and scattering properties. Here, we studied the effect of alkylamines on morphology control during the one-step fabrication of silver nanoshells (NSs) for highly enhanced Raman scattering. Various types of alkylamines were used to study the effects of chain length, existence of hydroxyl groups, and degree of alkyl chains on the surface morphology of silver NSs. The alkylamines influenced the silver ion reduction and the growth of silver domains, resulting in distinctive morphology changes. The optical properties of the silver NSs of different surface morphologies were characterized by surface-enhanced Raman spectra. Especially, when long alkylamines were used, intense and uniform surface-enhanced Raman scattering signals were obtained at the visible and near-infrared (NIR) region, and their enhancement factor was ∼107. To detect cancer biomarkers in vivo, as a feasibility test, silver NSs were modified to highly NIR-active nanoprobes and successfully applied to detect colon cancer without causing nonspecific interactions. Our one-step fabrication method of silver NSs is simple and can overcome various hurdles of morphology control and can be extended to other metal nanostructures of controlled surface morphologies or shape.

Simultaneous detection of different probe molecules using silver nanowires as SERS substrates.

Metallic silver nanowires with high yield were synthesized using a modified seed mediated approach at room temperature. Ribbon-like nanostructures were obtained when the concentration of NaOH was lower and further increase of NaOH transformed it into long nanowires. These nanowires possess high aspect ratio, with length and diameter ~6.5 μm and 17 nm respectively. The surface enhanced Raman scattering activity of these nanowires was tested with three different probe molecules viz., crystal violet, malachite green and nile blue chloride using visible (514.4 nm) and near-infrared (784.8 nm) excitation lines. The minimum detection limits for crystal violet and nile blue chloride molecules were found to be down to 10-7 M with good linear responses, as evidenced by values of correlation coefficients, indicating their potential for a variety of applications such as sensing. Principal component analysis was performed with the surface enhanced Raman spectra in order to discriminate the dye molecules and their mixture, simultaneously. The first two principal components, which provided 69.80 and 27.93% of the total data variance, could be conveniently represented as a two dimensional PCA score plot. The score plot showed clear clustering of probe molecules and their mixture. The relative contribution of wavenumbers to each of the two principal components was identified by plotting the PCA loading matrix. These results further promote possibilities of quantification of multiplexed SERS detection and analysis.

Metallic and plasmonic MoO2 monocrystalline ultrathin mesoporous nanosheets for highly sensitive and stable surface-enhanced Raman spectroscopy.

Structure and size control are always considered as effective routes to enhance sensitive performance. Different from common polycrystalline porous nanosheets, herein, the general synthesis of monocrystalline ultrathin porous nanosheets (MUPNSs) by a facile decompressing decomposition (DPD) route is reported. Accompanied with the properties inherited from MUPNSs, i.e., high adsorption and strong plasma resonance, these MoO2 MUPNSs presented high sensitive activity as a surface enhanced Raman spectrum (SERS) substrate. The SERS sensitivity of MoO2 MUPNSs was about 1000 times higher than MoO2 polycrystalline ultrathin porous nanosheets (PUPNSs).

Identification of Bacteria by Surface-Enhanced Raman Spectra after Peroxide Treatment

Surface-enhanced Raman spectroscopy (SERS) is a prospective method for the detection and identification of microorganisms. Hereby we describe the method for the registration of bacterial SERS spectra, including the mild oxidation sample treatment for the additional amplification of SERS signal. We also propose the spectral data processing method for the calculation of distinguishing criteria for the bacterial spectra.

Charge-transfer surface-enhanced resonance Raman spectra of benzene-like derivative compounds under the effect of an external electric field.

Since the discovery of surface-enhanced resonance Raman scattering (SERS), elucidating the charge-transfer (CT) mechanism has been a challenging and controversial process. Different theoretical models have been proposed to explain the effect of applied electrode potential on SERS-CT, but achieving a high-quality conserved trend of experimental observations and explaining the nature of the selective enhancement of the signals is not a trivial task and the results and conclusions are still in dispute. We investigated recently the performance of time-dependent excited-state gradient approximation under the effects of a uniform finite electric field in a simulation of the experimental spectra of pyridine on an Ag electrode. The singular patterns of the experimental spectra for symmetric and non-symmetric benzene-like derivative compounds and the consistent trends of enhancements of their signals under various electrode potentials motivated us to extend our simulation studies to 4-methylpyridine, pyrazine and pyrimidine molecules on silver metal clusters. For these molecules, selective enhancement and de-enhancement of totally symmetric (υ6a, υ9a and υ8a) and non-totally symmetric (υ6b and υ8b) modes upon changing the field were obtained and matched well with experimental observations. The selective enhancement of each signal in a zero field was explained by means of excited-state vector gradients and excited-state charge density difference for the S0→ SCT transition. On-field calculations showed slight perturbations of the geometries and electronic structures of the molecules. These on-field calculations also directly affected the magnitude of specific excited-state vector gradients and dimensionless displacements, and moreover the patterns of the spectra. The results of this investigation provided insight into the nature of the selective enhancements of signals and may help researchers propose the selection rules of SERS-CT.

Controllable preparation and high properties of fluorescence and surface enhanced raman spectra encoded poly(glycidyl methacrylate) microsphere

Controllable preparation and high properties of fluorescence and surface enhanced raman spectra encoded poly(glycidyl methacrylate) microsphere

The application of surface enhanced Raman scattering to the detection of asphaltic petroleum in sediment extracts: deconvolving three component-mixtures using look-up tables of entire surface enhanced Raman spectra

The detection of asphaltic petroleum by surface enhanced Raman scattering (SERS) is uncomplicated, except in instances where the petroleum has been mixed with other components that also exhibit a strong Raman effect.

Molecular origin of surface-enhanced Raman spectra of E. coli suspensions excited at 532 and 785 nm using silver nanoparticle sols as sers substrates

Research into the molecular origin of surface-enhanced Raman spectra (SERS) of bacteria is a crucial step in assessing the future of SERS-based discrimination and identification of bacteria in clinical analysis, food quality control, etc. Previous studies have revealed that at 785 nm excitation wavelength SERS of bacterial cells placed on a solid surface functionalized with in-situ grown aggregated gold nanoparticles covered with SiO2 originate from a mixture of 6 purine derivatives (adenine, guanine, AMP, hypoxanthine, xanthine, and uric acid) that are released by the cells into the medium. The aim of the present work was to investigate whether such interpretation is possible with a different class of SERS substrates: silver nanoparticle sols at excitation wavelengths of 785 and 532 nm. The suspension of the Escherichia coli DH5α strain was used as a model bacterium. Sols of silver nanoparticles were obtained by reducing silver nitrate in the presence of alkaline hydroxylamine hydrochloride. Number-weighted mean hydrodynamic diameter of the particles was 43±2 nm. We confirm that at both excitation wavelengths the spectra can be best described as a superposition of 4 purine derivatives: adenine, guanine, hypoxanthine, and xanthine. Importantly, we have discovered that 1) the spectra of the purine mixture are characteristic of viable cells only; 2) due to the variations in the concentrations of purine metabolites released by the cells into the surrounding medium the spectra of a bacterial strain can vary significantly when a silver nanoparticle sol is used as a SERS substrate.