Multilayer enhanced gold film over nanostructure surface-enhanced Raman substrates

We have developed a novel class of gold multilayer, surface-enhanced Raman scattering (SERS) substrates that are capable of enhancing SERS signals by 15.3-fold over conventional gold film over nanostructure (GFON) SERS substrates, making them comparable in sensitivity to optimized silver film over nanostructure (SFON) substrates, while providing the long-term stability obtained from gold. They are fabricated by depositing 10 A thick silver oxide islands on conventional GFON substrates, followed by deposition of a second continuous gold layer. The silver oxide layer acts as a dielectric spacer between the two continuous gold films and produces significantly enhanced SERS signals, as compared to optimized single layer substrates of the same geometry or comparable substrates prepared by deposition of silver islands that are not oxidized. In addition to the enhanced sensitivity of these multilayer substrates, they also exhibit long SERS active shelf-lives (i.e., months), with no measurable degradation in SERS enhancement, and relative standard deviations in SERS enhancement of less than 5.2% across the substrate's surface.     

Preparation of tetrapod-like nanostructure array and used as the surface-enhanced Raman scattering substrates

Nanostructure surface composed of tetrapod-like structure with the period of about 2 microm has been made by simple nanoprinted method, the collodion was poured directly on the surface of the template and tore off after it became solidification, and then a layer of gold was sputtered on the collodion surface. The electric field distributions of the tetrapod-like structure with different size and period have been simulated using finite element method. Surface-enhanced Raman spectra mapping image gotten from the substrates illustrated the uniform enhancement property of it. The diluted aqueous solution of Rhodamine 6G (50 nM) was used to test the enhanced property of substrates for trace detection and good quality spectra has been gotten. The spectra of melamine with the concentration of 1 mg/L (7.9 microM) on the substrates were gotten to show the enhanced ability for molecule with small Raman scattering cross section.     

Fabrication of rice-like gold nanoparticles and application in surface-enhanced Raman scattering

A simple method for the synthesis of rice-like gold nanoparticles using gold nanorods (GNRs) as precursors in the aqueous phase was exploited. The method used in this work involves eroding GNRs with potassium ferricyanide in the aqueous phase. Surface plasmon resonance (SPR) bands of the resulting nanoparticles present a notable blue-shift from 670 to 570 nm with increasing amounts of potassium ferricyanide, and subsequently the shape of the resulting nanoparticles can be readily controlled. Most importantly, the SPR response is an almost linear function of the quantity of potassium ferricyanide added. The synthesis of the resulting nanoparticles with various aspect ratios has been extensively studied and is well established. The surface-enhanced Raman scattering (SERS) intensity enhancement of the adsorbate on the surface of these gold nanoparticles was also studied.     

Novel Ag-Cu substrates for surface-enhanced Raman scattering

Ag dendrites were deposited on rough Cu plate by a simple galvanic displacement process between Ag ion and Cu under room temperature. Surface-enhanced Raman scattering (SERS) performances have been studied using Rhodamine 6G (R6G) probe molecules on this kind of Ag-Cu substrates. The high SERS enhancements are attributed to the highly branched Ag dendritic nanostructures and Ag nanoparticles formed on the trunks, branches, and even leaves.     

Carbon nanowalls amplify the surface-enhanced Raman scattering from Ag nanoparticles

We report surface-enhanced Raman scattering (SERS) from Ag nanoparticles decorated on thin carbon nanowalls (CNWs) grown by microwave plasma chemical vapor deposition. The Ag morphology is controlled by exposing the CNWs to oxygen plasma and through the electrodeposition process by varying the number of deposition cycles. The SERS substrates are capable of detecting low concentrations of rhodamine 6G and bovine serum albumin, showing much higher Raman enhancement than ordinary planar HOPG with Ag decoration. The major factors contributing to this behavior include: high density of Ag nanoparticles, large surface area, high surface roughness, and the underlying presence of vertically oriented CNWs. The relatively simple procedure of substrate preparation and nanoparticle decoration suggests that this is a promising approach for fabricating ultrasensitive SERS substrates for biological and chemical detection at the single-molecule level, while also enabling the study of fundamental SERS phenomena.     

Comparison of surface-enhanced resonance Raman scattering from unaggregated and aggregated nanoparticles

The effect of excitation frequency and state of aggregation on the sensitivity obtained in ultratrace analysis using colloidal suspensions of silver nanoparticles and surface-enhanced resonance Raman scattering (SERRS) detection is explored to define suitable conditions for quantitative analysis. Two structurally similar dyes, only one of which causes aggregation, were used as analytes without the use of external aggregating agents, thus simplifying the surface chemistry and removing a major source of error. Addition of the nonaggregating dye caused no change in particle charge or size and no time-dependent aggregation as measured by zeta potential and particle size analysis. The most intense single-particle scattering was obtained using excitation at the wavelength of the plasmon resonance. Molecular resonance added approximately 2 orders of magnitude in sensitivity. Addition of the aggregating dye caused a reduction in surface charge of the particles and initiated a time-dependent aggregation process. However, constant SERRS with time is obtained at some excitation wavelengths probably because a constant number of clusters active at these wavelengths is maintained in the dynamic aggregation process. The additional enhancement caused by aggregation and molecular resonance is spread over a range of excitation frequencies. However, electronic spectra suggested that plasmon resonance enhancement would be effective at the longest wavelength of excitation used (785 nm), but there was a significant drop in intensity this far away from the absorbance maximum of the dye (429 nm). Thus, sensitive analysis using suspensions of single nanoparticles is feasible provided the excitation frequency used is close to that of the plasmon resonance frequency. Aggregation adds only an enhancement of approximately 6 in the experiments performed since only some particles in aggregates will have an active plasmon at any one wavelength, but the range of excitation wavelengths at which good enhancement is obtained is wider giving more flexibility if more complexity.     

Optical probes for biological applications based on surface-enhanced Raman scattering from indocyanine green on gold nanoparticles

We report surface-enhanced Raman scattering (SERS) studies on indocyanine green (ICG) on colloidal silver and gold and demonstrate a novel optical probe for applications in living cells. In addition to its own detection by the characteristic ICG SERS signatures, the ICG gold nanoprobe delivers spatially localized chemical information from its biological environment by employing SERS in the local optical fields of the gold nanoparticles. The probe offers the potential to increase the spectral specificity and selectivity of current chemical characterization approaches of living cells and biomaterials based on vibrational information.     

Spectroscopic tags using dye-embedded nanoparticles and surface-enhanced Raman scattering

Surface-enhanced Raman scattering is capable of providing rich vibrational information at the level of single molecules and single nanoparticles, but the practical applications of this enormous enhancement effect are still a challenge. Here we report a new class of dye-embedded core-shell nanoparticles that are highly efficient for surface Raman enhancement and could be used as spectroscopic tags for multiplexed detection and spectroscopy. The core-shell particles contain a metallic core for optical enhancement, a reporter molecule for spectroscopic signature, and an encapsulating silica shell for protection and conjugation. A surprising finding is that organic molecules with an isothiocyanate (-N=C=S) group or multiple sulfur atoms are compatible with silica encapsulation. In comparison with fluorescent dyes and quantum dots, enhanced Raman probes contain a built-in mechanism for signal amplification and provide rich spectroscopic information under ambient experimental conditions.     

Silver-coated zeolite crystal films as surface-enhanced Raman scattering substrates

Silver-coated zeolite A and zeolite NaX crystal films prepared by vacuum deposition were investigated as surface-enhanced Raman scattering (SERS) substrates. The substrates were active for the enhancement of Raman scattering from uranyl ions. A detection limit of 10(-5) M for uranyl was obtained using silver-coated zeolite A films. One advantage of these zeolite-based substrates is that the negatively charged microporous framework provides the selectivity for adsorption based on static electric charges. The SERS effects of positively charged uranyl ions and neutrally charged benzoic acid were compared. For the zeolite A substrate, there was a 100-times-greater sensitivity.     

A nanoforest structure for practical surface-enhanced Raman scattering substrates

A nanoforest structure for surface-enhanced Raman scattering (SERS) active substrates is fabricated and analyzed. The detailed morphology of the resulting structure can be easily controlled by modifying the process parameters such as initial gold layer thickness and etching time. The applicability of the nanoforest substrate as a label-free SERS immunosensor is demonstrated using influenza A virus subtype H1N1. Selective binding of the H1N1 surface antigen and the anti-H1 antibody is directly detected by the SERS signal differences. Simple fabrication and high throughput with strong in-plane hot-spots imply that the nanoforest structure can be a practical sensing component of a chip-based SERS sensing system.     

Silver-embedded zeolite crystals as substrates for surface-enhanced Raman scattering

This paper reports the preparation of a type of Ag-embedded zeolite crystals as surface-enhanced Raman spectroscopy (SERS) substrates by chemical reduction of Ag⁺-exchanged ZSM-5. Ag⁺ ions were loaded into the zeolite framework by ion exchange. Then the exchanged-Ag⁺ ions were reduced and metallic silver clusters formed inside the zeolite channel. The resulting Ag-embedded zeolite crystals are characterized by using a number of techniques including X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy to confirm silver formed inside the crystal channel. The fabricated Ag-embedded ZSM-5 zeolite substrates displayed strong and reproducible SERS activity for different Raman probe molecules such as Tris(2,2′-bipyridyl) ruthenium(II) chloride (RuBpy) and rhodamine 6G (R6G). Since silver embedded into the zeolite channel without changing the crystal surface property, the Ag–ZSM-5 zeolite crystal can be used to prepare different SERS-active substrate (SERS-tags), in which different probe molecules may be detected. Such Ag-embedded zeolite substrate would be useful in chemical and biological sensing and in the development of SERS-based analytical devices.     

Seed-mediated synthesis of branched gold nanoparticles with the assistance of citrate and their surface-enhanced Raman scattering properties

We report an easy synthesis of highly branched gold particles through a seed-mediated growth approach in the presence of citrate. The addition of citrate in the growth solution is found to be crucial for the formation of these branched gold particles. Their size can be varied from 47 to 185?nm. The length of the thumb-like branch is estimated to be between about 5 and 20?nm, and changes slightly as the particle size increases. Owing to these obtuse and short branches, their surface plasmon resonance displays a marked red-shift with respect to the normal spherical particles. These branched gold particles exhibit stronger SERS activity than the non-branched ones, which is most likely related to these unique branching features.     

Polarization dependence of surface-enhanced Raman scattering in gold nanoparticle-nanowire systems

We study the polarization dependence of surface-enhanced Raman scattering (SERS) in coupled gold nanoparticle-nanowire systems. The coupling between the continuous nanowire plasmons and the localized nanoparticle plasmons results in significant field enhancements and SERS enhancements comparable to those found in nanoparticle dimer junctions. The SERS intensity is maximal when the incident light is polarized across the particle and the wire, and the enhancement is remarkably insensitive to the detailed geometrical structures of the nanoparticles.     

Linker-molecule-free gold nanorod layer-by-layer films for surface-enhanced Raman scattering

This paper reports a methodology for synthesizing and ordering gold nanorods into two-dimensional arrays at a water/hexane interface. This preparation method allows the systematic control of the nanoparticle film thickness. An investigation into the thickness-dependent surface-enhanced Raman scattering (SERS) of the adsorbed molecules revealed the nanorod (NR) films to have 1 order of magnitude stronger SERS enhancement than the nanosphere (NS) under similar experimental conditions. The exposed surface areas of the prepared NR and NS films were analyzed using electrochemical methods, and it was found that they had comparable exposed surface areas. Therefore, the order of magnitude difference in the enhancement factor was not due to the differences in surface area but to their intrinsic difference in the optical coupling of each film. The difference was attributed to the high density of junction points with the NR films in comparison with the corresponding NS films. Scanning emission microscopy showed that the NR films have line contacts with each other but the NS films have point contacts, which can explain the difference in SERS intensity between the NR and NS films.     

Ammonium bicarbonate reduction route to uniform gold nanoparticles and their applications in catalysis and surface-enhanced Raman scattering

A new protocol for the synthesis of nearly monodisperse gold nanoparticles with controllable size is described. The pathway is based on the reduction of AuCl₄⁻ by ammonium bicarbonate in the presence of sodium stearate under hydrothermal conditions. The particle sizes could be easily tuned by regulating the reaction conditions including precursor concentration, reaction temperature and growth time. A tentative explanation for the reduction and growth mechanism of uniform gold nanoparticles has been proposed. The as-prepared gold particles showed good catalytic activity for the reduction of 4-nitrophenol to 4-aminophenol by excess NaBH₄, and a surface-enhanced Raman scattering (SERS) study suggested that the gold nanoparticles exhibited a high SERS effect on the probe molecule Rhodamine 6G.      

Improving nanoprobes using surface-enhanced Raman scattering from 30-nm hollow gold particles

We report the development of nanoprobes that exploit the surface-enhanced Raman scattering (SERS) from nonaggregated, hollow, gold nanospheres (HGNSs). The homogeneity of the HGNSs leads to a nearly 10-fold improvement in signal consistency over standard silver SERS substrates, which translates into a significant increase in sensitivity and dynamic range for the model application of pH sensing. Moreover, the small size (30-nm diameter) of these SERS-active nanoparticles represents a major step in advancing sensing technology based on SERS, making this technology more amenable to intracellular sensing.     

Silver nanowire layer-by-layer films as substrates for surface-enhanced Raman scattering

In this paper, the fabrication of highly stable, surface-enhanced Raman scattering (SERS) active dendrimer/silver nanowire layer-by-layer (LBL) films is reported. Ag nanowires, approximately 100 nm in diameter, were produced in solution and transferred, using the LBL technique, onto a single fifth-generation DAB-Am dendrimer layer on a glass substrate. The Ag nanowires, and the resulting LBL films were characterized using UV-visible surface plasmon absorbance, while the LBL films were further characterized by atomic force microscopy measurements and surface-enhanced Raman and resonance Raman scattering of several analytes. The dendrimer was found to effectively immobilize the Ag nanowires with increased control over spacing and aggregation of the particles. These films are shown to be excellent substrates for SERS/SERRS measurements, demonstrating significant enhancement, and trace detection capability. Several trial analytes were tested using a variety of excitation energies, and results confirmed effective enhancement of Raman signals throughout the visible range (442-785 nm) with different molecules. Analytes were deposited onto the enhancing Ag nanowire LBL films surface using both casting and Langmuir-Blodgett monolayer transferring techniques.     

Preparation of silver nanoparticles by photo-reduction for surface-enhanced Raman scattering

A substrate for surface-enhanced Raman scattering (SERS) has been developed. Based on the surface-catalyzed reduction of Ag⁺ by citrate on the silver nanoparticles surface under light irradiation, small silver seeds on a quartz slide can be enlarged. The optical properties and characteristics of the silver films have been investigated by ultraviolet-visible spectroscopy, scan electron microscope and atomic force microscopy (AFM). The results indicate that the particle size and shape are different at different reduction time. At the first 3 h, some triangular and hexagonal nanoparticles formed; with the reduction proceeding, the shape of the silver particles became irregular and the size became larger. The silver films obtained are very suitable as SERS active substrate. The relationship between SERS intensity and the reduction time has been investigated for 1,4-bis[2-(4-pyridyl)ethenyl]-benzene molecule adsorbed on the silver film. The SERS intensity reached a maximum at 8 h reduction. The AFM measurements indicate that roughness features with an average size of 100 nm are present on the surface, which yielded the strongest SERS signal. Pyridine was used as a probe molecule to investigate the enhancement factor (EF) of the silver films. According to the formalism of Tian and co-workers, the EF of the silver films is estimated to be 3.4 × 10⁵. The silver film that can remain active for more than 50 days would seem to be suitable for various analytical applications.     

Shape-controlled synthesis and application in surface-enhanced Raman scattering of novel palladium nanoparticles

Palladium nanoparticles (PdNPs), as a platinum substitute, have opened many new opportunities for future catalytic applications, especially in fuel cell reactions, because of its comparable catalytic properties. In the present study, a simple and “green” procedure to the synthesis of PdNPs with cauliflower-like and polyhedron-like shapes is demonstrated. The novel PdNPs were prepared by the reduction of palladium chloride with ascorbic acid in the presence of chitosan served as a capping agent, whose shapes can be effectively controlled by varying reaction temperatures. Surface-enhanced Raman scattering (SERS) investigation demonstrated that the more irregular PdNPs have the stronger SERS activities. We believe that the PdNPs with unique shapes may find practical applications for electronics, photonics, fuel cells, and biofuel cells.     

Highly sensitive and selective determination of iodide and thiocyanate concentrations using surface-enhanced Raman scattering of starch-reduced gold nanoparticles

In this report, we propose a novel technique for the determination of the concentrations of iodide and thiocyanate by surface-enhanced Raman scattering (SERS) of starch-reduced gold nanoparticles. Starch-reduced gold nanoparticles show an intrinsic Raman peak at 2125 cm(-1) due to the -C≡C- stretching mode of a synthesized byproduct. Because of the high adsorptivity of iodide on a gold surface, the intensity of the SERS peak at 2125 cm(-1) decreases with an increase in the iodide concentration. Thiocyanate also strongly adsorbs on a gold surface, and a new peak appears at around 2100 cm(-1), attributed to the -C≡N stretching vibration in a SERS spectrum of starch-reduced gold nanoparticles. These two peaks were successfully used to determine the iodide and thiocyanate concentrations separately, even in their mixture system. The detection limit of this technique for iodide is 0.01 μM with a measurement range of 0.01-2.0 μM, while the detection limit of this technique for thiocyanate is 0.05 μM with a measurement range of 0.05-50 μM. This technique is highly selective for iodide and thiocyanate ions without interference from other coexisting anions such as other halides, carbonate, and sulfate.