Strong Surface-enhanced Raman Scattering Effect Research Articles

ZnO nanoparticles functionalized SWCNTs as highly sensitive SERS substrate for heavy metal ions detection

This report presents the most effective and novel approach to fabricate a three-dimensional (3D) Surface-Enhanced Raman Scattering (SERS) substrate for detection of heavy metal ions in the environment. Firstly, Single Wall Carbon Nanotubes (SWCNTs) are synthesized by Plasma Enhanced Chemical Vapor Deposition (PECVD) technique at low temperature. Then, the surface of SWCNTs is functionalized by Zinc Oxide nanoparticles (ZnO NPs) using thermal evaporation technique to achieve ZnO@SWCNTs nanocomposite film as a SERS substrate. The ZnO@SWCNTs with 3D nanostructure increases the coverage of plasmonic nanostructure. The effect of intertube and interparticle coupling generates the multitude ‘hot spots’ with a strong SERS effect. Different characterization techniques- Field Emission Scanning Electron Microscope (FESEM), X-Ray Diffraction (XRD), UV–Vis and Raman spectroscopy were performed. For detection of heavy metal ions, the Raman spectra are recorded for samples of different metal ions- Pb2+, Hg2+, Cd2+, Cu2+, and As2+. The proposed substrate demonstrates high sensitivity, selectivity and very low Limit of detection of the order of 0.225 nM with high stability and repeatability for Pb2+. Therefore, the proposed ZnO@SWCNTs nanocomposite SERS substrate can be used as a powerful tool for the detection of heavy metal ion- Pb2+ in aqueous solutions for a green environment.

Amorphous Ni(OH)2 nanocages as efficient SERS substrates for selective recognition in mixtures

Surface-enhanced Raman scattering (SERS) technology has become one of the most powerful tools for identifying trace analytes due to its outstanding merit of fingerprint spectrum. It has been found that noble metal (Ag, Au) materials can produce obvious surface plasmon resonance under visible light illumination, which can bring out a strong SERS effect. However, due to poor stability and non-selectivity, its practical applications are seriously hindered, especially for the detection of polluted water contaminating mixture organic pollutants. Herein, we designed an amorphous Ni(OH)2 nanocages (a-Ni(OH)2 NCs) substrate that exhibits an excellent SERS performance with good spectral stability and selectivity, which is attributed to the strong interfacial photo-induced charge transfer (PICT) mechanism between a-Ni(OH)2 NCs and molecules. Due to the effective vibrational coupling of a-Ni(OH)2 NCs and molecule under laser irradiation, the PICT transitions between a-Ni(OH)2 NCs and molecules evidently increase the molecular polarizability tensor and magnify molecular Raman scattering cross-section, resulting in a remarkable SERS activity. Besides, in wastewater containing several nitrogen and phosphorus pollutants, we successfully identified methylene blue and crystal violet molecules by using a-Ni(OH)2 NCs due to their selective recognition function. This research points out a new application direction for SERS technology in water pollution detection based on non-metal substrates, which widely broadens application prospects of SERS spectroscopy.

Synthesis of Large-Area MoSe2 Monolayer Film for Surface-Enhanced Raman Scattering Analysis

Ultrathin two-dimensional transition metal dichalcogenides represent a promising new class of materials with surface-enhanced Raman scattering (SERS) activities. Previous work mainly focused on the SERS effects of transition metal dichalcogenide crystals with sizes of micrometer scale. The enhancement effect is usually nonuniform on the SERS substrates. In this paper, we report a chemical vapor deposition method for the preparation of centimeter-scale MoSe2 films with uniform and high SERS activity. The molybdenum precursors are supplied in a “face-to-face” manner from a silica gel plate to the growth substrate, which guarantees uniform nucleation and growth of the monolayer MoSe2. We demonstrate that the monolayer MoSe2 film has strong SERS effect, which can be attributed to charge transfer between MoSe2 and the probe molecules. The detection limit of Rhodamine B on monolayer MoSe2 is determined to be 10[Formula: see text][Formula: see text]mol/L, and the Raman enhancement factor is estimated to be [Formula: see text]. Due to its atomic uniformity and chemical stability, the MoSe2 film can serve as an ideal two-dimensional SERS material for detection of various molecules.

Surface-enhanced Raman scattering from semiconductor and graphene quantum dots coupled to metallic-film-on-nanosphere substrates

In this article, we perform surface-enhanced Raman scattering (SERS) with a periodically corrugated metallic substrate obtained by evaporating an Ag film onto an artificial opal crystal, which is a self-assembled lattice of SiO2 600-nm spheres. These metallic-film-on-nanosphere (MFON) surfaces are characterized by scanning electron microscopy for different Ag film thicknesses ranging from 44 to 620 nm. Raman scattering spectra are measured for two different types of quantum dots, CdTeSe quantum dots (QDs) and graphene quantum dots (GQDs). We are able to show that enhancement of the Raman spectra can be obtained for all MFON substrates and for both types of QDs. For both CdTeSe and graphene quantum dots, the strongest SERS effect was observed in samples with 135 nm Ag thickness. This is attributed to the formation of hot spots in the nanogap dips of the Ag film.

Comparison of the efficiency of generation of Raman radiation by various Raman reporters connected via DNA linkers to different plasmonic nano-structures

In this contribution we compared the efficiency in generation of Raman radiation in various conditions by three commonly used Raman reporters: carboxytetramethylrhodamine (TAMRA), cyanine 3 (Cy3) and cyanine 5 (Cy5) conjugated to model ssDNA adsorbed on gold and silver nanoparticles. Although such experiments are described in the literature as surface-enhanced Raman scattering (SERS) measurements, we found that in many cases, it is significantly more important to match the frequency of the excitation radiation to the resonant frequency of the Raman reporter than to the resonant frequency of the plasmonic system, when strong SERS effect is observed. For example, the relative intensity of the Raman radiation of TAMRA-labelled DNA on 20 nm gold nanoparticles measured using green excitation laser line (in this case SERS enhancement is very low) is significantly higher than the relative intensity of the Raman radiation recorded in experiments in which red excitation radiation has been used (in this case strong SERS enhancement is observed). Specific interactions between the metal nanoparticles and the deposited labelled DNA are also very important, for example, when green excitation laser line is used, Raman radiation generated from TAMRA-labelled DNA on gold is much higher compared to the TAMRA-labelled DNA on silver although silver nanoparticles are highly SERS active in these conditions, whereas gold nanoparticles are practically SERS inactive. Also, structures formed by model ssDNA with attached Raman reporters on silver and gold surfaces are probably different. Because in SERS sensors containing immobilised DNA there is usually a possibility of changing of many construction details, our studies may give some useful hints how to prepare such sensors which would work more effectively. The values of the increase in the efficiency of Raman scattering due to the resonance effect in comparison to the non-resonance conditions for adsorbed TAMRA and Cy3 labelled DNA have been also estimated.

Sub-nanomolar detection of biogenic amines by SERS effect induced by hairy Janus silver nanoparticles

Surface enhanced Raman scattering (SERS) is largely used as a transduction method for analytes detection in liquid and vapor phase. In particular, SERS effect was promoted by a plethora of different metal and semiconducting nanoparticles (NPs) and silver and gold nanoparticles appear particularly suitable for this application. Nevertheless, silver nanoparticles intrinsic propensity to aggregate in large clusters reduces the possibility to use naked nanoparticles in SERS applications, for this reason they are usually functionalized with organic molecules. This approach inhibits the aggregation process but, on the other hand, reduces the surficial area of the NPs able to interact with the analyte molecules. In the present work, we propose a simple method to obtain surficial anisotropic Janus silver nanoparticles: octadecylamine was used to stabilize the nanoparticles and to promote the deposition of the silver nanoparticles on a solid substrate. The AgNPs/octadecylamine nanostructures showed the typical “hairy” Janus morphology and a strong SERS effect was observed when two biogenic amines, i. e. 2-phenylethylamine and tyramine, were fluxed on the solid film. SERS phenomenon was studied as a function both of the chemical structure of the fluxed amine and of the distance between the aromatic moiety and the nanoparticle allowing to propose the AgNPs/octadecylamine Janus nanoparticles as an active layer for the detection of phenylethylamine and tyramine in picomolar concentration.

The oxidant and laser power-dependent plasmon-driven surface photocatalysis reaction of p-aminothiophenol dimerizing into p,p′-dimercaptoazobenzene on Au nanoparticles

Recently, plasmon-driven surface photocatalysis (PDSPC) reactions have attracted more and more attention by means of surface-enhanced Raman scattering (SERS) because we can in situ monitor the reaction process and determine the final products and their quantities by the real-time SERS spectrum. In this work, self-assembly AuNPs with both high catalytic activity and strong SERS effect were used as a bifunctional platform for in situ monitoring of PDSPC reactions. p-Aminothiophenol (PATP), a famous model molecule, was selected as a probe molecule and FeCl3 and NaClO were selected as oxidants. In this way, oxidation reaction of PATP dimerizing into p,p′-dimercaptoazobenzene (DMAB) has been investigated by SERS, and the results show that oxidant and laser power can alter the conversion rate of the reaction. This work provides a novel approach for controlling PDSPC reaction rate, which may be useful for understanding the mechanism of PDSPC reactions.

Surface-enhanced Raman scattering properties of multi-walled carbon nanotubes arrays-Ag nanoparticles

A three-dimensional (3D) surface-enhanced Raman scattering (SERS) substrate is fabricated by decorating multi-walled carbon nanotubes (MWNTs) arrays with silver nanoparticles (AgNPs) on silicon, via magnetron sputtering and annealing method. The MWNTs-AgNPs hybrids with 3D structure increase the coverage of plasmonic nanostructures. The effect of inter-particle and inter-tube coupling creates a multitude of hot spots, with a strong SERS effect. The structure of MWNTs-AgNPs hybrid material is confirmed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Furthermore, Raman scattering from MWNTs, which is significantly enhanced in such hybrids, is systematically investigated in all samples. Surface-enhanced Raman scattering property is proved using 4-mercaptobenzoic acid (4-MBA) as standard analyte and the detection concentration level is down to 10−10 M. Additionally, the stability and recyclability of MWNTs-AgNPs substrates have been studied, positioning SERS mapping within an area of 10 × 10 μm2 is carried out to investigate the uniformity; time-course SERS mapping is performed to study the influence induced by molecule solution evaporation process. Finally, finite-difference time-domain (FDTD) method is utilized to simulate E-field distribution of the hybrid structures with multilayer AgNPs decorated on MWNTs. The results show a Raman scattering enhancement factor (EF) of 107.

A New SERS Substrate Based on TiO<sub>2</sub> Nanorods Thin Film Assembled Gold Nanoparticles

Surface enhanced Raman scattering (SERS) has been proved to be a highly sensitive method to detect organic molecules at very low concentrations. In recent years, many researchers have reported that 1-dimension semiconductor nanomaterials assembled noble metal nanoparticles can get a strong SERS effect. In this paper, we succeeded to synthesize TiO2 nanorod thin films on fluorine-doped tin oxide (FTO) glass with hydrothermal synthesis which were able to be used as SERS substrates. Gold nanoparticles were assembled to TiO2 nanorod thin films using the physical sputtering method and the citrate reduction method, respectively. The field emission scanning electron microscope (FESEM) images show that the later method could achieve the more desirable and uniform distribution of gold nanoparticles. Rhodamine 6G (R6G) was chosen as the probe molecule to study the SERS performance of our novel SERS substrates. Raman scattering measurement proved that the substrates were able to enhance Raman signals by several orders of magnitude and could be applied to biochemical detection. The whole fabrication process was facile and cost-effective, and the SERS activity and reproducibility of the substrates were pretty good.

Pore-size tuning and optical performances of nanoporous gold films

Through pulse electrochemical dealloying (PED) and chemical dealloying (CD), a variety of nanoporous gold (NPG) films of 100nm thickness with a wide range of pore size (4–140nm) were prepared. The coarsening mechanism of pore and the effect of residual Ag on optical performances were investigated. The pore size was tuned by controlling the dissolution rate of active Ag atoms and surface diffusion rate of noble Au atoms along the alloy/solution interfaces. The maximum pore size of 140nm was obtained by surfactant-assisted chemical dealloying (SA-CD) and the minimum pore size of 4nm was achieved by PED. It is found that the surface diffusivity of gold in PED is about 0.4–2×10−20m2s−1, four orders of magnitude smaller than that in SA-CD (2.4×10−16m2s−1). Finally, optical performances of the NPG films were investigated by UV–vis and surface-enhanced Raman scattering (SERS) spectra. The results reveal that the NPG film with a pore size of 8nm fabricated by PED exhibits higher surface-plasmon absorption and the strongest SERS effect.

Monitoring plasmon-driven surface catalyzed reactions in situ using time-dependent surface-enhanced Raman spectroscopy on single particles of hierarchical peony-like silver microflowers.

Investigating the kinetics of catalytic reactions with surface-enhanced Raman scattering (SERS) on a single particle remains a significant challenge. In this study, the single particle of the constructed hierarchical peony-like silver microflowers (SMFs) with highly roughened surface led to the coupling of high catalytic activity with a strong SERS effect, which acts as an excellent bifunctional platform for in situ monitoring of surface catalytic reactions. The kinetics of the reaction of 4-nitrothiophenol (4-NTP) dimerizing into 4,4'-dimercaptoazobenzene (DMAB) was investigated and comparatively studied by using the SERS technique on a single particle of different morphologies of SMFs. The results indicate that a fully developed nanostructure of a hierarchical SMF has both larger SERS enhancement and apparent reaction rate constant k, which may be useful for monitoring and understanding the mechanism of plasmon-driven surface catalyzed reactions.

Site-Specific Growth of Au–Pd Alloy Horns on Au Nanorods: A Platform for Highly Sensitive Monitoring of Catalytic Reactions by Surface Enhancement Raman Spectroscopy

Surface-enhanced Raman scattering (SERS) is a highly sensitive probe for molecular detection. The aim of this study was to develop an efficient platform for investigating the kinetics of catalytic reactions with SERS. To achieve this, we synthesized a novel Au-Pd bimetallic nanostructure (HIF-AuNR@AuPd) through site-specific epitaxial growth of Au-Pd alloy horns as catalytic sites at the ends of Au nanorods. Using high-resolution electron microscopy and tomography, we successfully reconstructed the complex three-dimensional morphology of HIF-AuNR@AuPd and identified that the horns are bound with high-index {11l} (0.25 < l < 0.43) facets. With an electron beam probe, we visualized the distribution of surface plasmon over the HIF-AuNR@AuPd nanorods, finding that strong longitudinal surface plasmon resonance concentrated at the rod ends. This unique crystal morphology led to the coupling of high catalytic activity with a strong SERS effect at the rod ends, making HIF-AuNR@AuPd an excellent bifunctional platform for in situ monitoring of surface catalytic reactions. Using the hydrogenation of 4-nitrothiophenol as a model reaction, we demonstrated that its first-order reaction kinetics could be accurately determined from this platform. Moreover, we clearly identified the superior catalytic activity of the rod ends relative to that of the rod bodies, owing to the different SERS activities at the two positions. In comparison with other reported Au-Pd bimetallic nanostructures, HIF-AuNR@AuPd offered both higher catalytic activity and greater detection sensitivity.

Positively Charged Active Sites for the Adsorption of Five-Membered Heterocycles on Silver Colloids

The adsorption of isothiazole on silver colloids was studied by surface-enhanced Raman scattering (SERS) spectroscopy in comparison with other five-membered heterocycles. The experimental results coupled with those obtained by density functional theory calculations for molecule/metal complexes evidenced the presence of positively charged active sites on the surface of the silver nanoparticles, along with a close relation between ligand basicity and strong SERS effect.

Polarized SERS study of an individual Ag nanowire with bulb humps

Polarization-dependent surface enhanced Raman scattering (SERS) of an individual Ag nanowire (NW) with bulb humps (including nm-scaled bulb humps on the body part and a bulb NW-tip) was investigated. Strong SERS effect was observed from the bulb NW-tip, which exhibited cos 2 θ dependence relative to the polarization angle θ of the incident laser. Such dependence is similar to that of the NW-body with bulb humps, but different from that of the other NW-tip with crown shape. Their different polarized SERS behaviors along with the bulb hump effect on SERS were discussed.

Gold Nanorod Arrays with Good Reproducibility for High-Performance Surface-Enhanced Raman Scattering

We have attempted to develop surface-enhanced Raman scattering (SERS) substrates by the use of two-dimensional (2D) Au nanorod arrays and to characterize the SERS-active sites of the Au nanostructures. We prepared two different types of 2D Au nanorod arrays by means of the anodic aluminum oxide (AAO) template-assisted nanofabrication. In the template-embedded array (type I) an Au nanorod fills up the AAO pore, while in the annularly spaced array (type II) an Au nanorod keeps approximately 20 nm away from the pore wall. The strongest SERS effect was observed for both types of substrates with an Au nanorod diameter of approximately 66 nm. With respect to the bare AAO template, the NaOH-etched (unetched) type I substrate with the 66 nm Au nanorod showed 115-fold (63-fold) enhancement in Raman scattering, corresponding to a SERS enhancement factor of 10(7)-10(9). For the unetched and ordered Au naonorod arrays, the SERS-active sites localized around the rod end, whereas for the NaOH-etched and partially aggregated Au nanorods, hot spots for SERS were found in the interstices between rod tips. The type II substrate allowed the effect of rod length on the SERS response to be investigated, and the SERS response was observed to vary very little with the rod length increase (250-1000 nm), indicating that majority of the signal originates at the rod end. A comparison between the analytes all-trans-beta-carotene and pyridine suggests that, for both types of substrates, the electromagnetic enhancement is predominant over the chemical enhancement. This work demonstrates that the unetched type I substrate is highly SERS effective, for which the fabrication protocol is advantageous in its simplicity and reproducibility.

Electrochemically induced flowerlike gold nanoarchitectures and their strong surface-enhanced Raman scattering effect

The authors report the fabrication of gold flowerlike nanoarchitectures (FNs) on indium tin oxide coated glass substrate using a low-cost electrochemical strategy at a proper current density and polyvinylpyrrolidone concentration in electrolyte. The FNs are grown and built with the blocks of two-dimensional flakelike nanostructures. Importantly, such FNs show strong surface-enhanced Raman scattering (SERS) effect associated with their geometries, which is much stronger than that from other particle films. The minimum concentration of detectable rhodamine 6G molecules can be down to 10−12M. These FNs with strong SERS effect could be used in chemical analysis, biosensors, and nanodevices with molecule-level detection.