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.     

Plasma reduction of silver compounds for fabrication of surface-enhanced Raman scattering substrates

Several silver compounds were reduced by low-pressure air plasma to produce porous nanostructured surfaces as surface-enhanced Raman scattering (SERS) substrates. This method is advantageous because substrates are easy to prepare and the silver metal surface is inherently clean without spectroscopic background. Silver compounds were melted into 1-2 mm slugs on quartz slides and plasma treated for different lengths of time. Silver chloride was found to be the best compound to make reproducible and stable SERS substrates. SERS activity of the substrates was tested using L-tryptophan, 4-mercaptobenzoic acid, and adenine.     

Dynamic rastering surface-enhanced Raman scattering (SERS) measurements on silver nanorod substrates

Surface-enhanced Raman spectra of a thiol-modified biotin derivative on oblique-angle-deposited silver nanorod (AgNR) array substrates were measured using both static and rotating rastering methods. We find that the rotating rastering method has a strong tendency to decrease the point-to-point relative standard deviation (RSD) compared to static measurements as well as decrease the effects of cumulative excitation exposure. The AgNR substrates treated with the modified biotin typically demonstrate intra-substrate RSDs of <10%, with an average RSD of ～3% when the rastering radius r=1 mm. The quantitative studies on the relationship between rastering radius, sampling area, and rastering frequency show that only the rastering radius appears to have significant effect on the measured RSD. Our results demonstrate that under the proper measurement and sample preparation conditions, the Ag nanorod substrates are very uniform.     

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.     

Polarization-dependent surface-enhanced Raman scattering from a silver-nanoparticle-decorated single silver nanowire

Single silver nanowires produced by DC electrodeposition in porous anodic alumina templates were surface-functionalized with the bifunctional molecule 4-aminobenzenthiol (ABT) then exposed to aqueous silver nanoparticles resulting in a silver nanoparticle-decorated silver nanowire. The polarization dependent surface-enhanced Raman (SERS) signal from this system showed significant intensity anisotropy when measured at a midsection of the nanowire, where the largest SERS intensities were observed when the incident light was polarized perpendicular to the nanowire's long axis but was almost isotropic near the tip of the nanowire. The observed effects were accounted for in terms of the electromagnetic fields concentrated in the collection of hot spots created through the ABT-linker-driven nanoparticle-nanowire self-assembly process.     

Gold/palladium and silver/palladium colloids as novel metallic substrates for surface-enhanced Raman scattering

New surface-enhanced Raman scattering (SERS) substrates, composed of gold or silver colloidal nanoparticles doped with palladium, were prepared. These novel colloids are stable and maintain a satisfactory SERS efficiency, even after long aging. The interest in doping the coinage metal nanoparticles with palladium is due to the well-known catalytic activity of this metal. Transmission electron microscopy (TEM) and ultraviolet-visible absorption spectroscopy were used to characterize the shape and size of the metal particles. It was found that these bimetallic colloidal nanoparticles have a core-shell structure, with gold or silver coated with palladium clusters.     

New surface-enhanced Raman spectroscopy substrates via self-assembly of silver nanoparticles for perchlorate detection in water

Perchlorate (ClO4-) has recently emerged as a widespread contaminant in drinking water and groundwater supplies in the United States, and a need exists for rapid detection and monitoring of this contaminant. In this study, surface-enhanced Raman spectroscopy (SERS) was studied as a means of ClO4- detection, and new sol-gel-based SERS substrates were developed by self-assembly of silver colloidal nanoparticles with various functionalized silane reagents. These substrate materials were tailored to allow detection of ClO4- in water with improved sorptivity, stability, and sensitivity and with the ability to detect ClO4- at concentrations as low as 10(-6) M (or 100 microg/L) with good reproducibility. Similar techniques were used to fabricate capillary SERS flow cells by assembling functionalized silver nanoparticles capable of attracting ClO4- to the SERS surface or the internal wall of glass capillaries. These capillary flow cells could be readily configured to allow for in situ, nondestructive detection of ClO4- via fiber optics.     

Angle-resolved surface-enhanced Raman scattering on metallic nanostructured plasmonic crystals

Surface-enhanced Raman scattering is an ideal tool for identifying molecules from the "fingerprint" of their molecular bonds; unfortunately, this process lacks a full microscopic understanding and, practically, is plagued with irreproducibility. Using nanostructured metal surfaces, we demonstrate strong correlations between plasmon resonances and Raman enhancements. Evidence for simultaneous ingoing and outgoing resonances in wavelength and angle sheds new light on the Raman enhancement process, allowing optimization of a new generation of reproducible Raman substrates.     

A reusable surface-enhanced Raman scattering (SERS) substrate prepared by atomic layer deposition of alumina on a multi-layer gold and silver film

A thermally stable, reusable surface-enhanced Raman scattering (SERS) substrate consisting of a gold/silver bi-layer film with a protective alumina coating is reported. The film is synthesized by thermally evaporating sequential layers of gold and silver followed by coating an ultra-thin alumina layer using atomic layer deposition. The use of gold as the foundational layer improves the thermal stability of the metal bi-layer film while providing the additional ability to tune the SERS response. Deposition of the thin alumina overlayer on the bi-layer film creates a SERS substrate capable of enduring multiple high-temperature exposures to 400 °C with minimal loss of enhancement capabilities. We demonstrate the multi-use capability of the substrate by measuring the SERS spectrum of rhodamine 6G followed by a thermal treatment at 400 °C to remove the analyte. A representative substrate was used to acquire SERS spectra of rhodamine 6G up to five repeat measurements, thus establishing the reusability of this relatively simple, inexpensive, and stable substrate.     

Aligned silver nanorod arrays for surface-enhanced Raman scattering

Silver nanorods were prepared by a seed-mediated growth approach, and self-assembled into two-dimensional ordered arrays on glass substrates. The polarization-dependent optical responses of the rods were measured, which indicated ordered alignment. These arrays were evaluated as potential surface-enhanced Raman spectroscopy (SERS) substrates using trans-bis(4-pyridyl)ethylene molecules. The SERS signals were observed to be enhanced with the increase of the aspect ratio of the Ag nanorod, and this was mainly attributed to the local field enhancement. The lateral arrangement of the Ag nanorod arrays was also partially responsible for the SERS enhancement.     

Ultraviolet-visible and surface-enhanced Raman scattering spectroscopy studies on self-assembled metalloporphyrin films on organic monolayer-modified ultra-thin silver substrates

A self-assembled monolayer (SAM) film of 5,10,15,20-tetra-(para-chlorophenyl)-porphyrin terbium (or lutetium) hydroxy compound (TbOH/LuOH) fabricated on a silver substrate premodified with a SAM of 4-mercaptopyridine (PySH) was studied by ultraviolet-visible (UV-Vis) spectroscopy and surface-enhanced Raman scattering (SERS) spectroscopy. PySH can modify the substrate and deliver its pyridyl group pointing out from the silver surface. Thus, we can investigate the effects of the central metals of the metalloporphyrins in the formation of the composite films. For the TbOH-PySH composite film, a new absorption band arising from TbOH appears at 425 nm, and a band at 512 nm due to the PySH-modified silver substrate shifts to a longer wavelength (538 nm). The results suggest that TbOH is successfully assembled on the top of PySH-modified silver film and that there is an interaction between TbOH and PySH in the film. The frequency shifts and relative intensity changes of bands due to PySH in the SERS spectra imply the coordination of the pyridyl group on Tb in the SAM. As for the LuOH-PySH composite film, its SERS spectrum shows bands arising from both the LuOH and PySH moieties, indicating that LuOH is assembled on the PySH-modified silver film. Furthermore, a band at 1221 cm(-1) due to the in-plane C-H bending mode of PySH disappears, implying that the pyridyl moiety of PySH becomes more parallel to the silver surface upon the formation of the LuOH-PySH composite film. Additionally, an absorption band at 515 nm shifts to a longer wavelength (541 nm) and becomes broad upon the formation of the composite film, suggesting an interaction between LuOH and PySH in the film. By comparing the spectral changes between the two self-assembled composite films, we find that the central metal is crucial in the formation of the composite films. The structure and orientation of the composite films depend on the central metal of the metalloporphyrin compounds.     

Functionalizing metal nanostructured film with graphene oxide for ultrasensitive detection of aromatic molecules by surface-enhanced Raman spectroscopy

Surface-enhanced Raman spectroscopy (SERS) as a powerful analytical tool has gained extensive attention. Despite of many efforts in the design of SERS substrates, it remains a grand challenge for creating a general substrate that can detect diverse target analytes. Herein, we report our attempt to address this issue by constructing a novel metal-graphene oxide nanostructured film as SERS substrate. Taking advantages of the high affinity of graphene oxide (GO) toward aromatic molecules and the SERS property of nanostructured metal, this structure exhibits great potential for diverse aromatic molecules sensing, which is demonstrated by using crystal violet (CV) with positive charge, amaranth with negative charge, and neutral phosphorus triphenyl (PPh(3)) as model molecules.     

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.     

Ultraviolet-visible and surface-enhanced Raman scattering spectroscopy studies on self-assembled films of ruthenium phthalocyanine on organic monolayer-modified silver substrates

A self-assembled film of ruthenium phthalocyanine (RuPc) fabricated on a silver substrate pre-modified with a monolayer of 4-mercaptopyridine (PySH) or 1,4-bis[2-(4-pyridyl)ethenyl]-benzene (BPENB) was studied by ultraviolet-visible (UV-Vis) and surface enhanced Raman scattering (SERS) spectroscopy. PySH or BPENB was used as a ligand to link RuPc since they not only modify the silver substrate, but also deliver a pyridyl group pointing out from the silver surface. Therefore, we can explore the relationship between the structure and orientation of metallophthalocyanine and the substrate modified by the two kinds of organic-monolayers with the different conjugates and molecular lengths. UV-Vis bands due to the organic-monolayer (PySH or BPENB) modified silver films shift to longer wavelengths and a new band arising from the metallophthlocyanine appears, suggesting the binding of RuPc to PySH/BPENB, as well as the interaction between the marcocycle of RuPc and the ring of PySH/BPENB. Vibrational bands arising from both the RuPc and PySH/BPENB moieties appear clearly in the SERS spectra of the RuPc-PySH/BPENB composite films, indicating that RuPc is successfully assembled on the top of PySH/BPENB film. The shifts and relative intensity changes of bands due to PySH or BPENB in the SERS spectra imply the binding of the metallophthalocyanine to the pyridyl group in the composite films. Furthermore, the comparison of the SERS spectra revealed that the orientations of PySH and BPENB in the two kinds of composite films are different; the BPENB moiety in the RuPc-BPENB composite film is more perpendicular to the silver surface compared with the PySH moiety in the RuPc-PySH composite film.     

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-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.     

Flexible surface-enhanced Raman scattering-active substrates based on nanofibrous membranes

Surface-enhanced Raman scattering (SERS) has emerged as an excellent analytical tool for the effective detection and fingerprint identification of various chemicals. Recently, significant progress has been made in the fabrication of SERS-active substrates using simple, inexpensive, and affordable methods. The full potential of universal SERS diagnostics will likely be realized with the development of approaches and devices capable of effectively detecting analytes on various surfaces as well as in multicomponent media. In addition, the combination of implantable or wearable SERS-active substrates and remote portable devices enables real-time diagnostics that ideally fit the concept of personalized medicine. In this paper, we summarize recent achievements in fabricating flexible SERS substrates made of cellulose paper, polymer membranes, and textile fibrous films. Emphasis is placed on the in-situ extraction and detection of various chemicals in real-world surfaces and complex media using flexible nanofibrous SERS platforms. The potential SERS applications and future perspectives in on-site diagnostics are also discussed.

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Detection of anions by normal Raman spectroscopy and surface-enhanced Raman spectroscopy of cationic-coated substrates

The use of normal Raman spectroscopy and surface-enhanced Raman spectroscopy (SERS) of cationic-coated silver and gold substrates to detect polyatomic anions in aqueous environments is examined. For normal Raman spectroscopy, using near-infrared excitation, linear concentration responses were observed. Detection limits varied from 84 ppm for perchlorate to 2600 ppm for phosphate. In general, detection limits in the ppb to ppm concentration range for the polyatomic anions were achieved using cationic-coated SERS substrates. Adsorption of the polyatomic anions on the cationic-coated SERS substrates was described by a Frumkin isotherm. The SERS technique could not be used to detect dichromate, as this anion reacted with the coatings to form thiol esters. A competitive complexation method was used to evaluate the interaction of chloride ion with the cationic coatings. Hydrogen bonding and pi-pi interactions play significant roles in the selectivity of the cationic coatings.     

Atomic layer deposition of platinum thin films on anodic aluminium oxide templates as surface-enhanced Raman scattering substrates

Platinum thin films are deposited on anodic aluminium oxide (AAO) templates by atomic layer deposition (ALD). The highly ordered island-like platinum nanostructures formed on the AAO template produce a high Raman scattering signal because of the periodical hexagonal arrangement. As an illustration, dramatic enhancement is achieved using Rhodamine 6G (R6G) as a molecular probe.Field-emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM) show that the gap between the island-like structures is below 10 nm. Owing to activation by the incident laser beam, the localized electromagnetic field on the platinum island surface can be dramatically enhanced by the sub-10 nm regime subsequently amplifying the Raman signal. Finite-difference time-domain (FDTD) calculation matches the experimental phenomena suggesting that the excellent surface-enhanced Raman scattering (SERS) characteristics of the platinum structure arise from the high density and abundance of hot spots. Because the platinum film is inert in air, the SERS enhancing substrate can be used reliably in many trace chemical and biological detection applications.     

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.