Metal-coated magnetic nanoparticles for surface enhanced Raman scattering studies

We report the optimization and usage of surfactantless, water dispersible Ag and Au-coated g\boldsymbol\gamma–Fe₂_{\boldsymbol 2}O₃_{\boldsymbol 3} nanoparticles for applications in surface-enhanced Raman scattering (SERS). These nanoparticles, with plasmonic as well as super paramagnetic properties exhibit Raman enhancement factors of the order of 10⁶ (10⁵) for Ag (Au) coating, which are on par with the conventional Ag and Au nanoparticles. Raman markers like 2-naphthalenethiol, rhodamine-B and rhodamine-6G have been adsorbed to these nanoparticles and tested for nonresonant SERS at low concentrations. Further, to confirm the robustness of Ag-coated nanoparticles, we have performed temperature-dependent SERS in the temperature range of 77–473 K. The adsorbed molecules exhibit stable SERS spectra except at temperatures $\boldsymbol >$\boldsymbol >323 K, where the thermal desorption of test molecule (naphthalenethiol) were evident. The magnetic properties of these nanoparticles combined with SERS provide a wide range of applications.     

Investigation into a surface plasmon related heating effect in surface enhanced Raman spectroscopy

The irreversible loss of surface enhanced Raman spectroscopy (SERS) intensity derived from electrostatically immobilized Au colloidal substrate is studied. The total intensity of the SERS scattering from crystal violet molecules adsorbed onto the substrate was monitored for up to 240 s under continuous laser irradiation. The rate of signal decay was found to depend upon the thickness of the liquid layer over the coated substrate. On the basis of this observation, we propose a plausible mechanism in which surface plasmon related heating induced a small but significant lateral particle diffusion in closely spaced Au particles. The result of the shifting is an increase in the average interparticle distance, which subsequently reduces the electromagnetic coupling between the Au nanoparticles, and in turn causes a reduction in the SERS activity. Finally, a three-state model is proposed and shown to satisfactorily describe the observed decays.     

Nanofabrication of densely packed metal-polymer arrays for surface-enhanced Raman spectrometry

A key element to improve the analytical capabilities of surface-enhanced Raman spectroscopy (SERS) resides in the performance characteristics of the SERS-active substrate. Variables such as shape, size, and homogeneous distribution of the metal nanoparticles throughout the substrate surface are important in the design of more analytically sensitive and reliable substrates. Electron-beam lithography (EBL) has emerged as a powerful tool for the systematic fabrication of substrates with periodic nanoscale features. EBL also allows the rational design of nanoscale features that are optimized to the frequency of the Raman laser source. In this work, the efficiency of EBL fabricated substrates are studied by measuring the relative SERS signals of Rhodamine 6G and 1,10-phenanthro-line adsorbed on a series of cubic, elliptical, and hexagonal nanopatterned pillars of ma-N 2403 directly coated by physical vapor deposition with 25 nm films of Ag or Au. The raw analyte SERS signals, and signals normalized to metal nanoparticle surface area or numbers of loci, are used to study the effects of nanoparticle morphology on the performance of a rapidly created, diverse collection of substrates. For the excitation wavelength used, the nanoparticle size, geometry, and orientation of the particle primary axis relative to the excitation polarization vector, and particularly the density of nanoparticles, are shown to strongly influence substrate performance. A correlation between the inverse of the magnitude of the laser backscatter passed by the spectrometer and SERS activities of the various substrate patterns is also noted and provides a simple means to evaluate possible efficient coupling of the excitation radiation to localized surface plasmons for Raman enhancement.     

In situ Raman monitoring of competitive adsorption of Ag and Au nanoparticles onto a poly(4-vinyl pyridine) surface

The competitive adsorption of citrate-capped Ag and Au nanoparticles (~25 nm in diameter) onto a poly(4-vinyl pyridine) (P4VP) surface has been investigated by means of Raman scattering spectroscopy. The P4VP film prepared on a glass slide was too thin for its normal Raman spectrum to be observed, but the Raman peaks of P4VP could be detected upon the adsorption of Ag and/or Au nanoparticles onto the film, due to the surface-enhanced Raman scattering (SERS) effect associated with the localized surface plasmon of Ag and/or Au nanoparticles. Neither quartz crystal microbalance nor atomic force microscopy (AFM) nor scanning electron microscopy (SEM) methodologies can distinguish between Ag and Au nanoparticles during their adsorption onto P4VP, but it is possible through Raman scattering spectroscopy because Ag (though not Au) nanoaggregates are SERS active at 514.5 nm excitation, while both Ag and Au nanoaggregates are SERS active at 632.8 nm excitation. Coupled with the AFM data, we were thus able to infer that about 120 Ag nanoparticles per 1 μm(2) were adsorbed, along with 60 Au nanoparticles per 1 μm(2), onto the P4VP film over a period of 1.5 h from a 1 : 1 mixture of Ag and Au sols at 1.6 nM each.     

十八胺作为还原剂和包裹剂制备具有SERS活性的金纳米线

采用化学还原方法,在有机相中利用十八胺作为还原剂和包裹剂,在油酸的辅助下,合成了具有一维结构的金纳米线,并将制得的金纳米线用作基底,在其表面获得了罗丹明B分子的SERS光谱,实验结果表明,金纳米线具有良好的SERS活性,有望应用于高灵敏度光学检测。     

Characterization of gold nanoparticle binding to microtubule filaments

Microtubule (MT) protein filaments were used as templates for fabricating Au nanowires as a bottom-up approach for fabricating building blocks for future integrated circuits. Photochemical reduction methods were employed to form Au nanoparticles which bind and uniformly cover the MT filaments. Synthesis of the MT-templated Au nanowires was characterized using UV/vis spectroscopy and transmission electron microscopy (TEM). In addition, binding between the MT filaments and Au nanoparticles was investigated using surface enhanced Raman spectroscopy (SERS) and X-ray photoelectron spectroscopy (XPS) to establish the nature of the binding sites. A variety of functional groups were identified by SERS to interact with the Au including imidazole, sulfur, aromatic rings, amine, and carboxylate. The imidazole ring in the histidine is the most prominent functional group for Au binding. The results from these studies provide better understanding of the binding between Au and the biotemplate and give insight concerning methods to improve Au coverage for MT-templated Au nanowires.     

Growth of Ag, Au, Cu, and Pt nanostructures on surfaces by micropatterned laser-image formations

Silver, gold, copper and platinum nanoparticles (NPs) were grown on surfaces in the form of patterns by the exposure of laser radiation onto droplets of metal ion solutions and the aid of a reducing agent. The generation of patterns from metallic NPs was achieved by combining induced growth of NPs and nanostructures by laser incidence directly on surfaces and optical image formation techniques for transferring the patterns. Near-UV (363.8 nm) and visible (532 nm) laser wavelengths were used for the laser-induced growth of NPs into microstructures on glass, quartz, stainless steel, silicon, and gold-on-silicon substrates. The sizes of the patterns formed were on the micrometer scale and the sizes of the transferred patterns were on the millimeter scale. The patterns formed were generated by optical transference of image and interference of laser beams. Ag and Au substrates were highly active in surface enhanced Raman spectroscopy (SERS). The enhanced Raman activity was measured for SERS probe molecules: 9H-purin-6-amine (adenine) and 1,2-bis (4-pyridyl)-ethane analytes on Ag and Au substrates, respectively. The enhancement factors obtained were 1.8×10(5) and 6.2×10(6), respectively.     

A High‐Sensitivity and Low‐Power Theranostic Nanosystem for Cell SERS Imaging and Selectively Photothermal Therapy Using Anti‐EGFR‐Conjugated Reduced Graphene Oxide/Mesoporous Silica/AuNPs Nanosheets

A high‐sensitivity and low‐power theranostic nanosystem that combines with synergistic photothermal therapy and surface‐enhanced Raman scattering (SERS) mapping is constructed by mesoporous silica self‐assembly on the reduced graphene oxide (rGO) nanosheets with nanogap‐aligned gold nanoparticles (AuNPs) encapsulated and arranged inside the nanochannels of the mesoporous silica layer. Rhodamine 6G (R6G) as a Raman reporter is then encapsulated into the nanochannels and anti‐epidermal growth factor receptor (EGFR) is conjugated on the nanocomposite surface, defined as anti‐EGFR‐PEG‐rGO@CPSS‐Au‐R6G, where PEG is polyethylene glycol and CPSS is carbon porous silica nanosheets. SERS spectra results show that rGO@CPSS‐Au‐R6G enhances 5 × 10⁶ magnification of the Raman signals and thus can be applied in the noninvasive cell tracking. Furthermore, it displays high sensitivity (detection limits: 10^?8m R6G solution) due to the “hot spots” effects by the arrangements of AuNPs in the nanochannels of mesoporous silica. The highly selective targeting of overexpressing EGFR lung cancer cells (A549) is observed in the anti‐EGFR‐PEG‐rGO@CPSS‐Au‐R6G, in contrast to normal cells (MRC‐5). High photothermal therapy efficiency with a low power density (0.5 W cm^?2) of near‐infrared laser can be achieved because of the synergistic effect by conjugated AuNPs and rGO nanosheets. These results demonstrate that the anti‐EGFR‐PEG‐rGO@CPSS‐Au‐R6G is an excellent new theranostic nanosystem with cell targeting, cell tracking, and photothermal therapy capabilities.     

Near-Field Surface-Enhanced Raman Imaging of Dye-Labeled DNA with 100-nm Resolution

Raman chemical imaging on a scale of 100 nm is demonstrated for the first time. This is made possible by the combination of scanning near-field optical microscopy (SNOM or NSOM) and surface-enhanced Raman scattering (SERS), using brilliant cresyl blue (BCB)-labeled DNA as a sample. SERS substrates were produced by evaporating silver layers on Teflon nanospheres. The near-field SERS spectra were measured with an exposure time of 60 s and yielded good signal-to-noise ratios (25:1). The distinction between reflected light from the excitation laser and Raman scattered light allows the local sample reflectivity to be separated from the signal of the adsorbed DNA molecules. This is of general importance to correct for topographic coupling that often occurs in near-field optical imaging. The presented data show a lateral dependence of the Raman signals that points to special surface sites with particularly high SERS enhancement.     

Silver nanocrystal-modified silicon nanowires as substrates for surface-enhanced Raman and hyper-Raman scattering

Metal catalyzed, CVD-grown silicon nanowires decorated by chemical assembly of closely spaced Ag nanocrystals were modified with the well-known "silver mirror" reaction and investigated as substrates for surface-enhanced Raman (SERS) and hyper-Raman (SEHRS) spectroscopy. Four chromophores were examined: Rhodamine 6G, crystal violet, a cyanine dye, and a cationic donor-acceptor substituted stilbene. After soaking the substrates overnight in 10(-4) M aqueous chromophore solutions, all four chromophores gave good-quality SERS spectra in < or =60 s using <1 microW of 458-nm cw laser power, and SEHRS spectra are obtained in < or =120 s using <1 mW of mode-locked 916-nm laser power. Results from this substrate are compared with those on colloidal silver nanoparticles deposited as a film, as well as surfaces grown by the silver mirror reaction.     

Fabrication of a Au nanoporous film by self-organization of networked ultrathin nanowires and its application as a surface-enhanced Raman scattering substrate for single-molecule detection

Due to its demonstrated usefulness in fields such as trace analysis, biodiagnosis, and in vivo study, surface-enhanced Raman scattering (SERS) has received renewed interest in recent years. Development of SERS substrates is of great importance as the SERS intensity and reproducibility depend strongly on the SERS substrates. In this paper we report the fabrication of Au nanoporous film (NPFs) by self-organization of networked ultrathin Au nanowires for use as SERS substrates. The acquired Au NPFs display controllable thickness, low relative density, and considerable specific surface area. Furthermore, this self-organization of nanowires not only provides abundant junctions between nanowires, 5-20 nm nanopores, and three-dimensional nanowells, but also makes nanopores/nanogaps down to 1-2 nm. These nanoscale characteristics result in a high spatial density of hotspots with Raman enhancement factors up to 10(9). Combined with the uniformity and high purity, our Au NPF provides high-quality substrates for SERS sensing.     

R6G on graphene: high Raman detection sensitivity, yet decreased Raman cross-section

Several recent studies have demonstrated the use of single and few-layer graphene as a substrate for the enhancement of Raman scattering by adsorbed molecules in a method termed graphene-enhanced Raman spectroscopy (GERS). Here we determine the resonance Raman scattering cross-section for the dye molecule rhodamine 6G (R6G) adsorbed on bilayer graphene. For the 1650 cm(-1) R6G mode, we obtain a cross-section of 5.1 × 10(-24) cm(2)·molecule(-1), a greater than 3-fold reduction from the previously reported solution value. We show that the absorption spectrum of adsorbed R6G can be measured using micro-optical contrast spectroscopy, and we find that detuning of the molecular resonance explains the decreased Raman scattering cross-section. We find no evidence for a graphene Raman enhancement process. We also study the graphene thickness dependence of the adsorbed R6G Raman signal and show that a model incorporating electromagnetic interference effects can qualitatively explain the decrease in signal with increasing graphene thickness.     

Uniform gold nanoarray formed by controlled IP6 micelles for chemical mapping

A uniform Au nanoarray is successfully formed at an indium tin oxide (ITO) glass surface modified with well-distributed inositol hexakisphosphoric (IP(6)) micelle layers by controlling the pH of the medium at 10. When Rhodamine 6G (R6G) and 2-mercaptopyridine (2-MPy) are used as the Raman probes, the uniform Au nanoarray presents a sound surface enhanced Raman scattering (SERS) efficiency and a reproducible Raman signal in two dimensions. The relative standard deviation (RSD) of Raman intensities of R6G or 2-MPy on the uniform Au nanoarray recorded by point to point is less than 12%, which is beneficial to its application for chemical mapping or imaging. A case of Raman point-mapping for onion epidermis is demonstrated in the present work. A uniform IP(6)-Au nanoarray might be mass-produced by this protocol.     

Gold nanorods with finely tunable longitudinal surface plasmon resonance as SERS substrates

Advances in nanophotonics have shown the potential of colloidal metal nanoparticles with sharp tips, such as rods, to focalize plasmonic electromagnetic fields. We report on the synthesis of Au nanorods via a seed mediated approach and the influence of silver ions on the aspect ratio of the Au nanorods. The longitudinal surface plasmon resonance (LSPR) of the Au nanorods was successfully tuned with the concentration of silver ions. The surface enhanced Raman scattering (SERS) effect of 2-aminothiophenol (2-ATP) as a probe molecule on Au nanorods was systematically studied by varying the longitudinal surface plasmon resonance of the nanorods. The highest electromagnetic enhancement was observed when the longitudinal surface plasmon resonance of the Au nanorods overlapped with the laser excitation wavelength. The variation of the SERS enhancement factor with the longitudinal surface plasmon resonance and laser excitation lines is also discussed in detail.     

In-situ partial sintering of gold-nanoparticle sheets for SERS applications

A new, versatile substrate design for surface-enhanced Raman spectroscopy (SERS) is introduced that provides better illumination and collection efficiency than other solid substrates. It uses sheets of 5 nm diameter gold nanoparticles that are draped by drying-mediated self-assembly onto 100 nm thick silicon nitride membranes. During laser illumination, partial in-situ sintering of the nanoparticles into larger structures with tiny gaps (≈2 nm) greatly increases the SERS enhancement factor. The detection of 1 pM of p-mercaptoaniline and 1 fg of 2,4-dinitrotoluene is demonstrated. The use of self-assembled nanoparticle sheets furthermore makes it possible to perform SERS detection in situ on top of a probe solution droplet.     

Nanosphere molecularly imprinted polymers doped with gold nanoparticles for high selectivity molecular sensors

We report the first attempt of using molecularly imprinted polymers (MIPs) in the shape of nanoparticles that were doped with gold nanoparticles (AuNPs) for surface enhanced Raman scattering (SERS)-based sensing of molecular species.Specifically,AuNPs doped molecularly imprinted nano-spheres (AuNPs@nanoMIPs) were synthesized by one-pot precipitation polymerization using Sudan Ⅳ as the template for the SERS sensing.The AuNPs@nanoMIPs were characterized by various modes of scanning transmission electron microscopy (STEM) that showed the exact location of the AuNPs inside the MIP particles.The effects of Au concentration and solution stirring on the shape and the polydispersity of the particles were studied.Significant enhancement of the Raman signals was observed only when the MIP particles were doped with the AuNPs.The SERS signal improved significantly with increase in the Au concentration inside the AuNPs@nanoMIPs.Selectivity measurements of the Sudan Ⅳ imprinted AuNPs@nanoMIPs carried out with different Sudan derivatives showed high selectivity of the AuNPs-doped MIP particles.     

SERS‐Active‐Charged Microgels for Size‐ and Charge‐Selective Molecular Analysis of Complex Biological Samples

Surface‐enhanced Raman scattering (SERS) provides a dramatic increase of Raman intensity for molecules adsorbed on nanogap‐rich metal nanostructures, serving as a promising tool for molecular analysis. However, surface contamination caused by protein adsorption and low surface concentration of small target molecules reduce the sensitivity, which severely restricts the use of SERS in many applications. Here, charged microgels containing agglomerates of gold nanoparticles (Au NPs) are designed using droplet‐based microfluidics to provide a reliable SERS substrate with molecular selectivity and high sensitivity. The limiting mesh size of hydrogel enables the autonomous exclusion of large proteins and the charged matrix concentrates oppositely charged small molecules through electrostatic attraction. As nanogaps among Au NPs in the agglomerates enhance Raman intensity, Raman spectrum of the adsorbed molecules is selectively measured with high sensitivity in the absence of interruption from adhesive proteins. Therefore, the SERS‐active‐charged microgels can be used for direct analysis of pristine biological samples without the pretreatment steps of separation and concentration, which are commonly a prerequisite for Raman analysis. For the purpose of demonstration, a direct detection of fipronil sulfone with partial negative charges, a metabolite of toxic insecticide, dissolved in eggs using the positively charged microgels without any pretreatment of the samples, is shown.     

基于蝶翅鳞片三维结构的Au/SnO2纳米复合材料制备及其表面增强拉曼散射性能研究

利用巴黎翠凤蝶前翅翅鳞片作为模板,通过前驱体浸泡后烧结的方法制备出具有原始蝶翅鳞片准周期三维结构的SnO2(金红石相),并采用化学沉积的方法在已制备的SnO2上沉积Au纳米颗粒,合成出Au/SnO2纳米复合材料.通过SEM、XRD以及TEM等表征方法检测并分析该材料形貌结构和成分组成.采用罗丹明6G(R6G)作为分析物,测试该材料的表面增强拉曼散射光谱.通过材料形貌结构图及UV-Vis漫反射光谱谱图,分析该基底的表面增强拉曼散射机理.该基底所具有的三维结构为拉曼信号增强提供大量"热点",而基底材料中SnO2和Au纳米颗粒为拉曼增强效应提供协同作用.良好的拉曼性能以及较低的制备成本表明,该新型表面增强拉曼散射基底具有一定的应用前景.     

Nanoscale roughness on metal surfaces can increase tip-enhanced Raman scattering by an order of magnitude

We studied the influence of nanosteps on signal intensity in gap-mode tip-enhanced Raman spectroscopy (TERS). A benzenethiol monolayer adsorbed on an Au substrate was investigated. The correlation between the TERS signal and the local topography on the substrate shows that a 2 nm high sharp step on the Au surface can significantly increase the enhancement. Furthermore, theoretical models were built, and the numerical simulation results were consistent with our experimental results. The findings provide evidence that nanoscale roughness can play a crucial role in the "hot sites" corresponding to single-molecule surface-enhanced Raman spectroscopy (SERS).     

High-density silver nanoparticle film with temperature-controllable interparticle spacing for a tunable surface enhanced Raman scattering substrate

The formation of high-density silver nanoparticles and a novel method to precisely control the spacing between nanoparticles by temperature are demonstrated for a tunable surface enhanced Raman scattering substrates. The high-density nanoparticle thin film is accomplished by self-assembling through the Langmuir-Blodgett (LB) technique on a water surface and transferring the particle monolayer to a temperature-responsive polymer membrane. The temperature-responsive polymer membrane allows producing a dynamic surface enhanced Raman scattering substrate. The plasmon peak of the silver nanoparticle film red shifts up to 110 nm with increasing temperature. The high-density particle film serves as an excellent substrate for surface-enhanced Raman spectroscopy (SERS), and the scattering signal enhancement factor can be dynamically tuned by the thermally activated SERS substrate. The SERS spectra of Rhodamine 6G on a high-density silver particle film at various temperatures is characterized to demonstrate the tunable plasmon coupling between high-density nanoparticles.