Vapor deposition method for sensitivity studies on engineered surface-enhanced Raman scattering-active substrates

The design and optimization of a vapor-phase analyte deposition method for limit of detection (LOD) studies on engineered surface-enhanced Raman scattering (SERS)-active substrates is presented. The vapor deposition method was designed to overcome current challenges in quantitative analysis of lithographically produced SERS substrates that are relatively small (hundreds of square micrometers). A custom-built flow cell was used to deposit benzenethiol from the vapor phase onto SERS-active Ag thin films, as the control substrates, and nanoaperture arrays that were generated by electron-beam lithography. The surface coverage of benzenethiol as a function of time was monitored using the ring stretching mode 1070-cm(-1) band and the trend was fit to Langmuir adsorption kinetics. The method was deemed reliable based on agreement between the LOD determined on the control substrates and previously reported values for those substrates. Application of the new method to a 20 x 20 microm(2) nanoaperture array yielded a LOD of 4.2 +/- 0.3 amol.     

Analytical technique for label-free multi-protein detection based on Western blot and surface-enhanced Raman scattering

We have developed a new analytical procedure for label-free protein detection designated "Western SERS", consisting of protein electrophoresis, Western blot, colloidal silver staining, and surface-enhanced Raman scattering (SERS) detection. A novel method of silver staining for Western blot that uses a silver colloid, an excellent SERS-active substrate, is first proposed in the present study. During the process of silver staining, interactions between proteins and silver nanoparticles result in the emergence of SERS of proteins. In the present study, we use myoglobin (Mb) and bovine serum albumin (BSA) as model proteins. From different protein bands on a nitrocellulose (NC) membrane, we have observed surface-enhanced resonance Raman scattering (SERRS) spectra of Mb and SERS spectra of BSA. The proposed technique offers dual advantages of simplicity and high sensitivity. On one hand, after the colloidal silver staining, we can detect label-free multi-proteins directly on a NC membrane without digestion, extraction, and other pretreatments. On the other hand, the detection limit of the Western SERS is almost consistent with the detection limit of colloidal silver staining, and the SERRS detection limit of Mb is found to be 4 ng/band. This analytical method, which combines the technique of protein separation with SERS, may be a powerful protocol for label-free protein detection in proteomic research.     

Minimally invasive surface-enhanced Raman scattering detection with depth profiles based on a surface-enhanced Raman scattering-active acupuncture needle

To obtain depth profiles of surface-enhanced Raman scattering (SERS) information in living systems, a SERS-active needle was structured by acupuncture needles, gold nanoshells (GNSs), and polystyrene, which were used as carriers, SERS-active elements to be absorbed on the carriers, and coatings to protect the absorbed GNSs from being erased during insertion, respectively. The SERS-active needle is minimally invasive for entering and exiting the body. The interspaces between the GNSs became vessels to collect diffused fluids at different depths after a SERS-active needle was inserted into an agarose gel, and the SERS intensity profile on the SERS-active needle coincided with the concentration profile of Nile Blue A (NBA) in the gel. SERS detection in vitro avoided the signal attenuation in gels, and the SERS detection at different spots of the SERS-active needle provided a depth profile of the NBA molecule in the gel. In vivo experiments of NBA and 6-mercaptopurine confirmed that the SERS-active needle could collect fluids in living systems easily with minimal invasion and provide information about depth profiles of target molecules in tissues.     

Electromigrated nanoscale gaps for surface-enhanced Raman spectroscopy

Single-molecule detection with chemical specificity is a powerful and much desired tool for biology, chemistry, physics, and sensing technologies. Surface-enhanced spectroscopies enable single-molecule studies, yet reliable substrates of adequate sensitivity are in short supply. We present a simple, scaleable substrate for surface-enhanced Raman spectroscopy (SERS) incorporating nanometer-scale electromigrated gaps between extended electrodes. Molecules in the nanogap active regions exhibit hallmarks of very high Raman sensitivity, including blinking and spectral diffusion. Electrodynamic simulations show plasmonic focusing, giving electromagnetic enhancements approaching those needed for single-molecule SERS.     

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.     

Flow-based autocorrelation studies for the detection and investigation of single-particle surface-enhanced resonance Raman spectroscopic events

We report on the characterization and detection of single metallic nanoparticles using a combination of correlation spectroscopy and surface-enhanced resonance Raman spectroscopy (SERRS). Minimizing the number of independent characterization steps is critical to efficiently perform such an analysis. In this article, we improve upon conventional diffusion-limited approaches by implementing a flow-based system with high temporal resolution detection. The benefits of flow over diffusion measurements allow for a higher throughput of detected events resulting in shorter analysis times. The nanoparticles are sized using their rotational diffusion time calculated with a modified autocorrelation function. Experiments are performed using Au nanoparticles labeled with the reporter molecule malachite green isothiocyanate on a custom-built Raman spectrometer.     

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.     

Use of a sample translation technique to minimize adverse effects of laser irradiation in surface-enhanced Raman spectrometry

Surface-enhanced Raman spectroscopy (SERS) has proven to be a very powerful tool in the analysis of a wide range of compounds. However, continuous irradiation of the laser beam over the SERS substrate can promote the gross decomposition of the sample analytes and significantly broaden and diminish the intensities of observed spectral bands. In addition, the incident radiation can promote thermal or photolytic fragmentation of analytes, thereby altering the observable bands and possibly leading to a misinterpretation of analytical data. Finally, chemical or morphological changes in the SERS substrate are possible. This work presents the use of a sample translation technique (STT) as a means to minimize these adverse effects. By spinning the sample rapidly, the effective residence time of analytes and substrate within the irradiated zone is dramatically decreased without reduction of spectral acquisition time or the density of analyte in the zone. The technique is studied by acquiring SERS spectra of Naproxen USP, riboflavin, folic acid, Rhodamine 6G, and 4-aminothiophenol using silver islands on glass and silver-poly(dimethylsiloxane) composite substrates under various spinning and stationary conditions. In all cases, spectra show improvements upon spinning at laser powers as low as 4.2 (+/- 0.1) mW. Specific differences in the appearance of the spectra and the potential use of STT for improved SERS qualitative and quantitative determinations are presented.     

New strategy for ready application of surface-enhanced resonance Raman scattering/surface-enhanced Raman scattering to chemical analysis of organic films on dielectric substrates

Dropping of appropriately concentrated AgNO3 and NaBH4 solutions, as well as laser-ablated Ag sols, onto organic molecules results in the formation of aggregated Ag nanoparticles that can induce surface-enhanced Raman scattering (SERS) for the molecules. The addition of flocculating agents such as alkali halides can further increase the Raman signals. We demonstrate in this work that Raman spectra can be obtained even for 0.01 monolayers of R6G on Si simply by spreading silver nanoparticles and/or fabricating Ag nanoparticles and nanoaggregates at the gaps and vacant sites of R6G molecules. The application prospect of the present methodology is extremely high, not only because of its simplicity but also because of the fact that the observation of vibrational spectra is one of the most incisive methods for understanding the chemical and physical phenomena on a variety of surfaces.     

Simplified protocol for detection of protein-ligand interactions via surface-enhanced resonance Raman scattering and surface-enhanced fluorescence

A simple and effective protocol for detections of protein-protein and protein-small molecule interactions has been developed. After interactions between proteins and their corresponding ligands, we employed colloidal silver staining for producing active substrates for surface-enhanced Raman scattering (SERS) and surface-enhanced fluorescence (SEF). Tetramethylrhodamine isothiocyanate (TRITC) and Atto610 were used for both Raman and fluorescent probes. We detected interactions between human IgG and TRITC-anti-human IgG, and those between avidin and Atto610-biotin by surface-enhanced resonance Raman scattering (SERRS) and SEF. The detection limits of the proposed SERRS-based method are comparable to those of the proposed SEF-based one, 0.9 pg/mL for anti-human IgG and 0.1 pg/mL for biotin. This protocol exploits several advantages of simplicity over other SERS and SEF-based related methods because of the protein staining-based strategy for silver nanoparticle assembling, high sensitivity from SERRS and SEF, and high stability in photostability comparing to fluorescence-based protein detections. Therefore, the proposed method for detection of protein-ligand interactions has great potential in high-sensitivity and high-throughput chip-based protein function determination.     

Directional emission of surface-enhanced Raman scattering based on a planar-film plasmonic antenna

A planar-film plasmonic antenna for surface-enhanced Raman scattering (SERS) with good emission directivity (divergence angle < 3°) was realized on a Kretschmann prism configuration with Raman-active analytes as emitters. The simulated results of finite-difference time-domain method show the emission efficiency, the directivity and the gain of the planar-film antenna were expected to be 50%, 300 and 22 dB, respectively. Angle-resolved spectroscopy was used to characterize its properties in SERS. The experimental results show that the SERS signal of analytes was remarkably enhanced when a laser excited this planar-film plasmonic antenna at the resonance angle. Meanwhile, the radiation of SERS was concentrated in a small region in space. The planar-film antenna with high gain coefficient can be a promising light harvesting and emitting device. The good emission directivity allows high collection efficiency. This advantage opens up interesting prospects in the applications for plasmon-enhanced spectroscopy and single-phonon detections.     

Visualizing chromatographic separation of metal ions on a surface-enhanced Raman active medium

Metal ion carboxylato complexes possess ion-specific carboxylate Raman bands. Using this attribute we follow the chromatographic separation of a microliter aliquot of an initially equimolar solution of Pb(2+) and Hg(2+) using the surface-enhanced Raman spectroscopy spectra of their carboxylato complexes as unique identifiers. A glass capillary whose interior is lined with a dense layer of gold nanoparticles treated with 4-mercaptobenzoic acid simultaneously acts as a separation medium and an efficient SERS reporter of the step-by-step separation process. The observed adsorption-desorption equilibrium along the capillary is shown to conform with theory. Although Hg(2+) complexes with COO(-) much more strongly than Pb(2+), it is the Pb(2+) that survives the separation process as the sole surface species. We show that this is because so much mercury is taken out of solution during early separation steps that the surface equilibrium is ultimately driven toward adsorbed Pb(2+).     

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.     

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.     

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.     

Characterization of silane-modified immobilized gold colloids as a substrate for surface-enhanced Raman spectroscopy

Immobilized gold colloid particles coated with a C-18 alkylsilane layer have been characterized as a substrate for surface-enhanced Raman scattering (SERS) studies of adsorption onto hydrophobic surfaces. Atomic force microscopy images, optical extinction spectra, and SERS measurements are reported as a function of accumulation of gold colloid on glass. As the metal particles become increasingly aggregated on the surface, the SERS enhancement increases until the plasmon resonance shifts to wavelengths longer than the excitation laser. The gold colloid substrates are stable and exhibit reproducible SERS enhancement. When octadecyltrimethoxysilane is self-assembled over the gold, the metal surface is protected from exposure to solution-phase species, as evidenced by the inhibition of chemisorption of a disulfide reagent to the overcoated gold surface. The results show that interactions with gold can be blocked by a silane layer so as not to significantly influence physisorption of molecules at the C-18/solution interface. The SERS enhancement from these C-18-overcoated gold substrates is reproducible for different films prepared from the same colloidal suspension; the substrates are also stable with time and upon exposure to laser irradiation.