A facile high‐performance SERS substrate based on broadband near‐perfect optical absorption

Plasmonic systems based on metal nanoparticles on a metal film with high optical absorption have generated great interests for surface‐enhanced Raman scattering (SERS). In this study, we prepare a broadband‐visible light absorber consisting Au nanotriangles on the surface of a continuous optically opaque gold film separated with a dielectric SiO₂ layer, which is a typical metal‐insulator‐metal (MIM) system, and demonstrate it as an efficient SERS substrate. The MIM nanostructure, prepared using nanosphere lithography with a very large area, shows a broadband with absorption exceeding 90% in the wavelength regime of 630–920 nm. We observe an average SERS enhancement factor (EF) as large as 4.9 × 10⁶ with a 22‐fold increase compared to a single layer of Au nanotriangles directly on a quartz substrate. A maximum SERS EF can be achieved by optimizing the thicknesses of the dielectric layer to control the optical absorption. Owing to the simple, productive, and inexpensive fabrication technique, our MIM nanostructure could be a potential candidate for SERS applications. Copyright © 2015 John Wiley & Sons, Ltd.     

Multifunctional Plasmonic Co‐Doped Fe2O3@polydopamine‐Au for Adsorption,Photocatalysis, and SERS‐based Sensing

A new type of multifunctional plasmonic nanoparticles, cobalt‐doped Fe₂O₃@polydopamine‐Au (Co‐Fe₂O₃@PDA‐Au), is fabricated via coating PDA through self‐polymerization onto Co‐Fe₂O₃ and further loading gold nanoparticles by in situ reduction onto the surface of PDA shell. Benefiting from the universal adhesive ability of PDA and negative zeta potetntial of the composite, the Co‐Fe₂O₃@PDA‐Au shows strong adsorptivity for cationic dyes. The presence of gold nanoparticle with the diameter of 15 nm in the Co‐Fe₂O₃@PDA‐Au system promotes surface‐enhanced Raman scattering (SERS) activity with an impressive detection limit of 1 × 10^?6 m . Thanks to the synergistic effect of the light harvesting of PDA, the surface plasmon resonance of Au, and the electron conductibility of PDA and Au, the Co‐Fe₂O₃@PDA‐Au exhibits an enhanced photocatalytic activity comparing with unmodified Co‐Fe₂O₃. All the above‐mentioned functions enable Co‐Fe₂O₃@PDA‐Au to be a multifunctional material system for various applications toward environmental pollutants.     

Single‐molecule detection of thionine on aggregated gold nanoparticles by surface enhanced Raman scattering

We report observations of single‐molecule detection of thionine and its dynamic interactions on aggregated gold nanoparticle clusters using surface enhanced Raman scattering (SERS). Spectral intensities were found to be independent of the size of Au nanoparticles studied (from 17 to 80 nm) at thionine concentration below 10⁻¹² M or at single‐molecule concentration levels. Raman line separations and, in particular, spectral fluctuations and blinking were also observed, suggesting temporal changes in single molecular motion and/or arrangements of thionine on Au nanoparticle surfaces. In contrast, by using dispersed Au nanoparticles, only ensemble SERS spectra could be observed at relatively high concentrations (> 10⁻⁸ M thionine), and spectral intensities varied with the size of Au nanoparticles. Copyright © 2007 John Wiley & Sons, Ltd.     

Single‐molecule surface‐enhanced Raman scattering of fullerene C60

We achieved single‐molecule surface‐enhanced Raman scattering (SM‐SERS) spectra from ultralow concentrations (10⁻¹⁵ M) of fullerene C₆₀ on uniformly assembled Au nanoparticles. It was found that resonant excitation at 785 nm is a powerful tool to probe SM‐SERS in this system. The appearance of additional bands and splitting of some vibrational modes were observed because of the symmetry reduction of the adsorbed molecule and a relaxation in the surface selection rules. Time‐evolved spectral fluctuation and ‘hot spot’ dependence in the SM‐SERS spectra were demonstrated to result from the single‐molecule Raman behavior of the spherical C₆₀ on Au nanoparticles. Copyright © 2010 John Wiley & Sons, Ltd.     

Detection and analysis of cyclotrimethylenetrinitramine (RDX) in environmental samples by surface‐enhanced Raman spectroscopy

Techniques for rapid and sensitive detection of energetics such as cyclotrimethylenetrinitramine (RDX) are needed both for environmental and security screening applications. Here we report the use of surface‐enhanced Raman scattering (SERS) spectroscopy to detect traces of RDX with good sensitivity and reproducibility. Using gold (Au) nanoparticles (∼90–100 nm in diameter) as SERS substrates, RDX was detectable at concentrations as low as 0.15 mg/l in a contaminated groundwater sample. This detection limit is about two orders of magnitude lower than those reported previously using SERS techniques. A surface enhancement factor of ∼6 × 10⁴ was obtained. This research further demonstrates the potential for using SERS as a rapid, in situ field screening tool for energetics detection when coupled with a portable Raman spectrometer. Copyright © 2010 John Wiley & Sons, Ltd.     

Fabrication of periodical Ag–Au compound nanostructure films with controllable Ag nanoparticle aggregate patterns: a study on surface‐enhanced Raman scattering

The Ag–Au compound nanostructure films with controllable patterns of Ag nanoparticle (NP) aggregates were fabricated. A strategy of two‐step synthesis was employed toward the target products. Firstly, the precursor Au NP (17 nm) films were synthesized as templates. Secondly, the Ag NPs (45 nm) were deposited on the precursor films. Three types of Ag NP aggregates were obtained including discrete Ag NPs (discrete type), necklace‐like Ag NP aggregates (necklace type), and huddle‐like Ag NP aggregates (huddle type). The surface‐enhanced Raman scattering (SERS) property was studied on these nanostructures by using the probing molecule of rhodamine 6G under the excitation laser of 514.5 nm. Interestingly, the different types of samples showed different enhancement abilities. A statistical method was employed to assess the enhancement. The relative enhancement factor for each Ag NP was estimated quantitatively under the ratio of 1 : 25 : 18 for the discrete‐type, necklace‐type, and huddle‐type samples at the given concentration of 10⁻⁸ mol/l. This research shows that the enhancement ability of each Ag NP is dependent on the aggregate morphology. Moreover, the different enhancement abilities displayed different limit detection concentrations up to 10⁻⁸, 10⁻¹¹, and 10⁻⁹ mol/l, separately. The understanding of the relationship between the defined nanostructures and the SERS enhancement is very meaningful for the design of new SERS substrates with better performance. Copyright © 2015 John Wiley & Sons, Ltd.     

A three‐dimensional surface‐enhanced Raman scattering substrate: Au nanoparticle supramolecular self‐assembly in anodic aluminum oxide template

A three‐dimensional surface‐enhanced Raman scattering (SERS) substrate via the self‐assembly of properly sized Au nanoparticles in anodic aluminum oxide templates was designed and prepared. Au nanoparticles first underwent hydrophobic surface modification. Then, the hydrophobic Au nanoparticles self‐assembled, aggregated and formed many hot spots in the anodic aluminum oxide templates through a supramolecular interaction. We chose thiophenol as a probe molecule to evaluate the SERS enhancement ability of this three‐dimensional substrate. The enhancement factor was calculated to be 4.6 × 10⁶ under the radiation of a 785‐nm laser. By further comparing SERS signals from different points on the same substrate, we confirmed that this substrate possessed good reproducibility and could be applied for SERS detection. Copyright © 2011 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman spectroscopy on novel black silicon‐based nanostructured surfaces

Two different black silicon nanostructured surfaces modified with thin gold layers were tested for analytical signal enhancement with Surface‐Enhanced Raman Spectroscopy (SERS). The relationship between the thicknesses of the gold layers and the analytical signal enhancement was studied. Also, effects of Ti and Ti/Pt adhesion layers underneath the gold layers on the analytical signal enhancement were tested. An enhancement factor of 7.6 × 10⁷ with the excitation laser 785 nm was achieved for the tested analyte, Rhodamine 6G, and non‐resonance SER spectra were recorded in a 5 s acquisition mode. Such an enhancement enables to achieve a detection limit down to 2.4 pg of Rhodamine 6G on a black silicon‐based nanosurface coated with a 400‐nm‐thin layer of gold. Copyright © 2009 John Wiley & Sons, Ltd.     

Gold‐Cluster‐Based Dual‐Emission Nanocomposite Film as Ratiometric Fluorescent Sensing Paper for Specific Metal Ion

A dual‐emission ratiometric fluorescent sensing film for metal ion detection is designed. This dual‐emission film is successfully prepared from chitosan, graphitic carbon nitride (g‐C₃N₄), and gold nanoclusters (Au NCs). Here, it is shown that the g‐C₃N₄ not only serves as the fluorescence emission source, but also enhances the mechanical and thermal stability of the film. Meanwhile, the Au NCs are adsorbed on the surface of chitosan film by the electrostatic interaction. The as‐prepared dual‐emission film can selectively detect Cu²⁺, leading to the quench of red fluorescence of Au NCs, whereas the blue fluorescence from g‐C₃N₄ persists. The ratio of the two fluorescence intensities depends on the Cu²⁺ concentration and the fluorescence color changes from orange red to yellow, cyan, and finally to blue with increasing Cu²⁺ concentration. Thus, the as‐prepared dual‐emission film can be worked as ratiometric sensing paper for Cu²⁺ detection. Furthermore, the film shows high sensitivity and selectivity, with low limit of detection (LOD) (10 ppb). It is observed that this novel gold‐cluster‐based dual‐emission ratiometric fluorescent sensing paper is an easy and convenient way for detecting metal ions. It is believed that this research work have created another avenue for the detection of metal ions in the environment.     

A hybrid substrate for surface‐enhanced Raman scattering spectroscopy: coupling metal nanoparticles to strong localised fields on a micro‐structured surface

Electromagnetic coupling between localised plasmons on metal nanoparticles and the strong localised fields on a micro‐structured surface is demonstrated as a means to increase the enhancement factor in surface‐enhanced Raman scattering (SERS) spectroscopy. Au nanoparticles of diameter 20 nm were deposited on a micro‐structured Au surface consisting of a periodic array of square‐based pyramidal pits (Klarite). The spectra of 4‐aminothiophenol (4‐ATP) were compared before and after deposition of Au nanoparticles on the micro‐structured surface. The addition of Au nanoparticles is shown to provide significantly higher signal intensities, with improvements of the order of ∼10³ per molecule compared with spectra obtained from the micro‐structured substrate alone. This hybrid approach offers promise for combining nanoparticles with micro‐ and nano‐structured surfaces in order to design SERS substrates with higher sensitivities. Copyright © 2011 John Wiley & Sons, Ltd.     

Mobility Enhancement of P3HT‐Based OTFTs upon Blending with Au Nanorods

The mobility enhancement of organic thin‐film transistors based on poly(3‐hexylthiophene) (P3HT) by incorporating gold nanorods (Au NRs) is reported. Through varying the doping concentration and surface modifier of the Au NRs in P3HT matrix, the P3HT/Au composite with 0.5 mg mL^?1 pyridine‐capped Au NRs exhibits a hole mobility of 0.059 cm² V^?1 s^?1, this value is seven times higher than that of pristine P3HT. This remarkable improvement of mobility originates from the enhanced crystallinity and optimized orientation of P3HT after doping with Au NRs. In addition, the appropriate surface modification can produce more‐efficient hole conduction of Au NRs.     

Gold coatings on polyethyleneterephthalate nano-patterned by F2 laser irradiation

In this work we present periodic surface structures generated by linearly polarized F₂ laser light (157 nm) on polyethyleneterephthalate (PET). Atomic force microscopy was used to study the topological changes induced by the laser irradiation. The laser irradiation induces the formation of periodic ripple structures with a width of ca 130 nm and a height of about 15 nm in the fluence range 3.80-4.70 mJ/cm² and the roughness of the polymer surface increases due to the presence of these periodic structures. Subsequently, the laser modified PET foils were coated with a 50 nm thick gold layer by sputtering. After Au deposition on the PET foils with ripple structure, the roughness of surface decreases in comparison to PET with ripples without Au coating. For 50 nm thick Au layers, the ripple structure is not directly transferred to the gold coating, but it has an obvious effect on the grain size of the coating. With considerably thinner Au layers, the ripple structures are smoothened but preserved.     

Comparison of SERS effectiveness of copper substrates prepared by different methods: what are the values of enhancement factors?

Surface‐enhanced Raman scattering (SERS) spectroscopy is an analytical method for the detection of low amounts of analytes adsorbed on an appropriate coinage metal (Au, Ag, Cu) surface. Generally, the values of the enhancement factor are the highest on silver, lower on gold and relatively very low on copper. In this study, we have focused on the estimation of the enhancement factors of copper surface/substrates formed by different preparation procedures. The SERS activity of large electrochemically prepared substrates and colloidal systems is compared. The surface morphology of the large substrates was studied using scanning electron microscopy and atomic force microscopy. The size distribution of colloidal nanoparticles was monitored by dynamic light scattering. The values of enhancement factor are in both cases more than 10⁵ for the FT‐SERS spectra, demonstrating the fundamental role of nanostructured copper as a substrate material at the excitation wavelength (1064 nm) used. Copyright © 2011 John Wiley & Sons, Ltd.     

Gold‐coated zinc oxide nanowire‐based substrate for surface‐enhanced Raman spectroscopy

Zinc oxide nanowires with two distinct morphologies were synthesized on silicon substrates using a simple thermal evaporation and vapor transport method in an oxidizing environment. The as‐synthesized nanowires were coated with gold to allow excitation of surface plasmons over a broad frequency range. SERS studies with near‐IR excitation at 785 nm showed significant enhancement (average enhancement > 10⁶) with excellent reproducibility to detect monolayer concentrations of 4‐methylbenzenethiol (4‐MBT) and 1,2‐benzendithiol (1,2‐BDT) probe molecules. The Raman enhancement showed a strong dependence on the gold film thickness, and the peak enhancement was observed for a ∼40‐nm‐thick film. The Raman enhancement was stronger for randomly oriented nanowires compared to aligned ones suggesting the importance of contributions from the junctions of nanowires. Copyright © 2009 John Wiley & Sons, Ltd.     

Performances of different metals in optical fibre-based surface plasmon resonance sensor

The capability of various metals used in optical fibre-based surface plasmon resonance (SPR) sensing is studied theoretically. Four metals, gold (Au), silver (Ag), copper (Cu) and aluminium (Al) are considered for the present study. The performance of the optical fibre-based SPR sensor with four different metals is obtained numerically and compared in detail. The performance of optical fibre-based SPR sensor has been analysed in terms of sensitivity, signal-to-noise (SNR) ratio and quality parameter. It is found that the performance of optical fibre-based SPR sensor with Au metal is better than that of the other three metals. The sensitivity of the optical fibre-based SPR sensor with 50 nm thick and 10 mm long Au metal film of exposed sensing region is 2.373 μm/RIU with good linearity, SNR is 0.724 and quality parameter is 48.281 RIU^− 1. The thickness of the metal film and the length of the exposed sensing region of the optical fibre-based SPR sensor for each metal are also optimized.     

Surface characteristics of Ag‐doped Au nanoparticles probed by Raman scattering spectroscopy

We have examined the surface characteristics of Ag‐doped Au nanoparticles (below 5 mol% of Ag) by means of the surface‐enhanced Raman scattering (SERS) of 2,6‐dimethylphenylisocyanide (2,6‐DMPI) and 4‐nitrobenzenethiol (4‐NBT). When Ag was added to Au to form ∼35‐nm‐sized alloy nanoparticles, the surface plasmon resonance band was blue‐shifted linearly from 523 to 517 nm in proportion to the content of Ag up to 5%. In the SERS spectra of 2,6‐DMPI, the N‐C stretching peak also shifted almost linearly from 2184 to 2174 cm⁻¹ when the Ag content was 5 mol% or less; the peak then remained the same as that of the pure Ag film. The potential variation of the SERS spectrum of 2,6‐DMPI in an electrochemical environment, as well as the effect of organic vapor, also showed a similar tendency. From the SERS of 4‐NBT, we confirmed the occurrence of a surface‐induced photoreaction converting 4‐NBT to 4‐aminobenzenethiol, when Ag was added to Au to form alloy nanoparticles. The photoreaction induction ability also increased linearly with the Ag content, reaching a plateau level at 5 mol% of Ag. All these observations suggest that the surface content of Ag should increase almost linearly as a function of the overall mole fraction of Ag and, once the Au/Ag nanoparticles reach 5 mol% of Ag, their surfaces are fully covered with Ag, showing the same surface characteristics of pure Ag nanoparticles. Copyright © 2011 John Wiley & Sons, Ltd.     

Portable smart films for ultrasensitive detection and chemical analysis using SERS and SERRS

Metallic nanostructures, much smaller than the wavelength of visible light, which support localized surface plasmon resonances, are central to the giant signal enhancement achieved in surface‐enhanced Raman scattering (SERS) and surface‐enhanced resonance Raman scattering (SERRS). Plasmonic driven SERS and SERRS is a powerful analytical tool for ultrasensitive detection down to single molecule detection. For all practical SERS applications a key issue is the development of reproducible and portable SERS‐active substrates, where the most widely used metals for nanostructure fabrication are silver and gold. Here, we report the fabrication of a ‘smart film’, containing gold nanoparticles (AuNPs), produced by in situ reduction of gold chloride III (Au⁺³) in natural rubber (NR) membranes for SERS and SERRS applications. The composite films (NR/AuNP membranes) show characteristic plasmon absorption of Au nanostructures, which notably do not influence the mechanical properties of the NR membranes. The term ‘smart film’ has to do with the fact that the SERS substrate (smart film) is flexible and standalone, which allows one to take it anywhere and to dip it into solutions containing the analyte to be characterized by SERS or SERRS technique. Besides, the synthesis of the AuNPs at the surface of NR films is much simpler than making an Au colloid and cast it onto a substrate surface or preparing an Au evaporated film. Copyright © 2012 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman scattering from gold‐coated germanium oxide nanowires

We utilized bulk‐synthesized nanowires (NWs) of germanium dioxide as nanoscale structures that can be coated with noble metals to allow the excitation of surface plasmons over a broad frequency range. The NWs were synthesized on substrates of silicon using gold‐catalyst‐assisted vapor–liquid–solid (VLS) growth mechanism in a simple quartz tube furnace setup. The resulting NWs have diameters of ∼100–200 nm, with lengths averaging ∼10–40 µm and randomly distributed on the substrate. The NWs are subsequently coated with thin films of gold, which provide a surface‐plasmon‐active surface. Surface‐enhanced Raman scattering (SERS) studies with near‐infrared (NIR) excitation at 785 nm show significant enhancement (average enhancement > 10⁶) with good uniformity to detect submonolayer concentrations of 4‐methylbenzenethiol (4‐MBT), trans‐1,2‐bis(4‐pyridyl)ethylene (BPE), and 1,2‐benzendithiol (1,2‐BDT) probe molecules. We also observed an intense, broad continuum in the Raman spectrum of NWs after metal coating, which tended to diminish with the analyte monolayer formation. Copyright © 2008 John Wiley & Sons, Ltd.     

Nanostructuring of thin Au films by means of short UV laser pulses

The particle size distribution, morphology and optical properties of the Au nanoparticle (NP) structures for surface enhanced Raman signal (SERS) application are investigated in dependence on their preparation conditions. The structures are produced from relatively thin Au films (10–20 nm) sputtered on fused silica glass substrate and irradiated with several pulses (6 ns) of laser radiation at 266 nm and at fluencies in the range of 160–412 mJ/cm². The SEM inspection reveals nearly homogeneously distributed, spherical gold particles. Their initial size distribution of the range of 20–60 nm broadens towards larger particle diameters with prolonged irradiation. This is accompanied by an increase in the uncovered surface of the glass substrate and no particle removal is observed. In the absorption profiles of the nanostructures, the broad peak centred at 546 nm is ascribed to resonant absorption of surface plasmons (SPR). The peak position, halfwidth and intensity depend on the shape, size and size distribution of the nanostructured particles in agreement with literature. From peak intensities of the Raman spectra recorded for Rhodamine 6G in the range of 300–1800 cm⁻¹, the relative signal enhancement by factor between 20 and 603 for individual peaks is estimated. The results confirm that the obtained structures can be applied for SERS measurements and sensing.     

Au@SiO2 core/shell nanoparticle assemblage used for highly sensitive SERS‐based determination of glucose and uric acid

The use of Au@SiO₂ core/shell nanoparticle (NP) assemblage with highly sensitive surface‐enhanced Raman scattering (SERS) was investigated for the determination of glucose and uric acid in this study. Rhodamine 6G dye molecules were used to evaluate the SERS enhancement factor for the synthesized Au@SiO₂ core/shell NPs with various silica shell thicknesses. The enhancement of SERS signal from Rhodamine 6G was found to increase with a decrease in the shell thickness. The core/shell assemblage with silica layer of 1–2 nm over a Au NP of ~36 nm showed the highest SERS signal. Our results show that the SERS technique is able to detect glucose and uric acid within wide concentration ranges, i.e. 20 ng/dL to 20 mg/dL (10⁻¹²–10⁻³ M) and 16.8 ng/dL to 2.9 mg/dL (10⁻¹¹–1.72 × 10⁻⁴ M), respectively, with associated lower detection limits of ~20 ng/dL (~1.0 × 10⁻¹² M) and ~16.8 ng/dL (~1.0 × 10⁻¹¹ M). Our work offers a low‐cost route to the fabrication of agile sensing devices applicable to the monitoring of disease progression. Copyright © 2013 John Wiley & Sons, Ltd.