Surface enhanced Raman scattering from porous gold nanofibers of different diameters

Porous gold nanofibers of different diameters from 43 to 219 nm were fabricated using electrochemical deposition techniques. Gold-silver alloy were electrochemically deposited in the form of nanofibers within the porous alumina templates of various diameters and only a silver phase was chemically removed using nitric acid. Field-emission scanning electron microscope images of the resulting nanofibers show a high-quality nanoporous network with homogeneous pores. A notable surface-enhanced Raman scattering (SERS) has been observed for all porous gold nanofibers of which scattering efficiencies are distinctly higher than that of the smooth solid gold nanofibers without porosity. As the diameter of porous gold nanofibers decreases, the observed SERS efficiency gradually increases. Controlled fabrication of lateral width of gold nanofibers reveals promising application for high efficient and stable molecular sensing platforms.     

Immuno-surface-enhanced coherent anti-stokes Raman scattering microscopy: immunohistochemistry with target-specific metallic nanoprobes and nonlinear Raman microscopy

Immunohistochemistry (IHC) is one of the most widely used staining techniques for diagnostic purposes. The selective localization of target proteins in tissue specimens by conventional IHC is achieved with dye- or enzyme-labeled antibodies in combination with light microscopy. In this contribution, we demonstrate the proof-of-principle for IHC based on surface-enhanced coherent Raman scattering for contrast generation. Specifically, antibody-labeled metallic nanoshells in conjunction with surface-enhanced coherent anti-Stokes Raman scattering (SECARS) microscopy are employed for the selective, sensitive, and rapid localization of the basal cell protein p63 in normal prostate tissue. Negative control experiments were performed in order to confirm the selective binding of the target-specific metal nanoprobes and to disentangle the role of plasmonic (metal) and molecular (Raman reporter) resonances in this plasmon-assisted four-wave mixing technique.     

Surface enhanced Raman scattering spot tests: a new insight on Feigl's analysis using gold nanoparticles

Traditional Feigl's spot tests can be greatly improved with the aid of gold nanoparticles and Raman probes, by monitoring the changes in the surface enhanced Raman scattering (SERS) of the analytes directly applied on a filter paper previously impregnated with the selective organic reagent. As a proof of concept, diphenylthiocarbazone (dithizone) was treated with citrate stabilized gold nanoparticles and employed in paper spot tests for a variety of transition and heavy metal ions. Below 10(-5) mol L(-1), only mercury(II) ions were able to displace the dithizone molecules from the "coordination shell" of the gold nanoparticles, leading to a systematic decay of the Raman signals. Because of the huge enhancement of the dithizone vibrational peaks, the SERS spot tests allowed the detection of picograms of Hg(2+) ions.     

Crossed two-beam coherent anti-stokes Raman spectroscopy in dispersive media

Coherent anti-Stokes Raman spectroscopy (CARS) is a nonlinear optical wave mixing process that is used in gas-phase systems to determine the energy distribution of the probed species (usually N2) and, through a fitting procedure, the temperature giving rise to it. CARS signal strengths are maximized when the phase matching condition is met. Because gases are generally non-dispersive, this phase matching condition can be found geometrically as a function of the crossing angles between the CARS beams and their wavelengths. In addition, perfect phase matching in non-dispersive media occurs automatically for collinear beams. To improve spatial resolution, however, intersecting the laser beams is desirable. Being a third-order process, phase matching for CARS in gases typically requires three input laser beams. This paper discusses and demonstrates the issues of phase matching for CARS when the medium is dispersive, and the ability for CARS phase matching to occur with only two crossed laser beams (one pump and one probe). This two-beam X-CARS in dispersive media can be used as an alignment tool for gas-phase CARS and may be relevant as a simpler diagnostic in high-pressure environments. The paper also discusses the effects of non-ideal phase matching in dispersive and non-dispersive media.     

Background-free coherent anti-stokes Raman scattering of gas- and liquid-phase samples in a mesoporous silica aerogel host

Efficient time-resolved coherent anti-Stokes Raman scattering (CARS) of atmospheric nitrogen and ethanol trapped in a nanoporous silica aerogel matrix is demonstrated. Silica aerogel hosts are attractive for analytical CARS spectroscopy due to their high porosity/low density, low refractive index, and low scattering cross-section. Differences between the resonant and nonresonant parts of the nonlinear optical susceptibilities lead to much longer relaxation times for analytes compared to the matrix. Time-resolved CARS can then be used to obtain a nearly background-free measurement at characteristic vibrations of the analyte. These results demonstrate the potential of this approach for rapid, sensitive, background-free analyses of analytes entrapped in the aerogel pores, which may be advantageous for some environmental, chemical, and biological sensing applications.     

Surface enhanced Raman scattering of neurotransmitter release in neuronal cells using antibody conjugated gold nanoparticles

This study demonstrated the potential feasibility of using antibody-conjugated gold nanoparticles as highly sensitive and homogeneous sensing probes for biological monitoring of neurotransmitters in neuronal cells. Bands at 1152 and 1322 cm(-1) were also similar to SERS of metal catecholates, and could be assigned to catechol ring vibration and carbon-oxygen stretches.     

Surface enhanced Raman scattering on long-range ordered noble-metal nanocrescent arrays

Long-range ordered noble-metal nanocrescent arrays of different sizes and shapes have been successfully fabricated by using both nanoimprint lithography and e-beam lithography techniques. Large surface enhanced Raman scattering (SERS) enhancements in the detection of rhodamine 6G (R6G) molecules on these arrays have been observed and attributed to the enhancement of the local electromagnetic (EM) fields near individual nanocrescents. Electromagnetic enhancement factors for crescents of different shapes are computed using the discrete dipole approximation and compared with experimental measurements of the R6G Raman intensities. It is found that the maximum values of SERS intensity appear at an intermediate value of the crescent eccentricity and the observed behaviour is related to the spatial distributions of the enhancement of the local EM field (hot spots).     

Spatial control of coherent anti-stokes emission with height-modulated gold zig-zag nanowires

Intrinsic coherent anti-Stokes emission is observed in lithographically patterned gold nanowires. Polarization dependent measurements reveal that the nanostructure's anti-Stokes response is polarized in the direction of the transverse surface plasmon resonance of the wire. We have used specially fabricated gold nanozigzag wires that are modulated in height between 20 and 80 nm to demonstrate tuning of the plasmon polarizability through control of wire height. Stronger anti-Stokes emission is shown to correlate with structures that support higher plasmon polarizability, underlining the primary role of the transverse plasmon resonance in the generation of anti-Stokes radiation from gold nanostructures. Our results also point out that a potential surface-enhanced coherent anti-Stokes Raman scattering (CARS) assay for detecting the vibrational response of surface-tethered molecules needs to include a mechanism for separating the molecular response from the strong intrinsic anti-Stokes emission of the metallic nanosubstrate.     

Binding of aromatic isocyanides on gold nanoparticle surfaces investigated by surface-enhanced Raman scattering

The adsorption structure and binding of phenyl isocyanide (PNC), 2,6-dimethyl phenyl isocyanide (DMPNC), and benzyl isocyanide (BZI) on gold nanoparticle surfaces have been studied by means of surface-enhanced Raman scattering (SERS). PNC, DMPNC, and BZI have been found to adsorb on gold assuming a standing geometry with respect to the surfaces. The presence of the nu(CH) band in the SERS spectra denotes a vertical orientation of the phenyl ring of PNC, DMPNC, and BZI on Au. The lack of a substantial red shift and significant band broadening of the ring breathing modes implied that a direct ring pi orbital interaction with metal substrates should be quite low. For PNC, the band ascribed to the C-NC stretching vibration was found to almost disappear after adsorption on Au. On the other hand, the C-NC band remained quite strong for DMPNC after adsorption. This result suggests a rather bent angle of C-N[triple bond]C: for the nitrogen atom of the NC binding group on the surfaces, whereas a linear angle of C-N[triple bond]C: should be more favorable on gold surfaces due to an intramolecular steric hindrance of its two methyl groups. SERS of BZI on gold nanoparticles also supports a bent angle of :C[triple bond]N-CH2 for its nitrogen atom, suggesting a preference of sp3 (or sp2) hybridization for the nitrogen atom.     

Quantitative enhanced Raman scattering of labeled DNA from gold and silver nanoparticles

Surface-enhanced resonance Raman scattering (SERRS) from silver nanoparticles using 514.5-nm excitation has been shown to offer huge potential for applications in highly sensitive multiplexed DNA assays. If the technique is to be applied to real biological samples and integrated with other methods, then the use of gold nanoparticles and longer wavelengths of excitation are desirable. The data presented here demonstrate that dye-labeled oligonucleotide sequences can be directly detected by SERRS using gold nanoparticles in a quantitative manner for the first time. The performance of gold and silver nanoparticles as SERRS substrates was assessed using 514.5-, 632.8-, and 785-nm excitation and a range of 13 commercially available dye-labeled oligonucleotides. The quantitative response allowed the limit of detection to be determined for each case and demonstrates that the technique is highly effective, sensitive, and versatile. The possibility of excitation at multiple wavelengths further enhances the multiplexing potential of the technique. The importance of effectively combining the optical properties of the nanoparticle and the dye label is demonstrated. For example, at 632.8-nm excitation, the dye BODIPY TR-X and gold nanoparticles make a strong SERRS combination with very little background fluorescence. This study allows the choice of nanoparticle and dye label for particular experimental setups, and significantly expands the applicability of enhanced Raman scattering for use in many disciplines.     

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.     

In situ analysis of living embryonic stem cells by coherent anti-stokes Raman microscopy

Embryonic stem cells (ESC), derived from preimplantation embryos, are defined by their ability to both self-renew and differentiate into all of the cells and tissues of a mature animal. Efforts to develop methods for in vitro culture of ESC for research or eventual therapeutic applications are hampered by the lack of rapid, nondestructive assays for distinguishing ESC from other (differentiated) cells within a growing culture. Coherent anti-Stokes Raman scattering (CARS) microscopy is shown here to be a sensitive and nondestructive method for identifying mouse ESC based on selective observation of specific molecular vibrations believed to be spectroscopic markers indicating the differentiated vs undifferentiated states of such cells. The nonlinear nature of CARS also permits imaging with subcellular resolution, potentially offering a means by which chemical changes accompanying the early stages of differentiation may be associated with certain intracellular compartments (e.g., nucleus, cytoplasm, membranes). A novel exposure/collection configuration is described, which yields high collection efficiency and low interference from nonresonant background components.     

Surface enhanced Raman scattering of silver sensitized cobalt nanoparticles in metal-dielectric nanocomposites

We report the preparation of a new type of nanocomposite containing cobalt and silver nanoparticles organized in parallel layers with a well controlled separation. This arrangement allows the observation of an enhanced low-frequency Raman signal at the vibration frequency of cobalt nanoparticles excited through the surface plasmons of silver nanoparticles. Numerical simulations of the electric field confirm the emergence of hot spots when the separation between silver and cobalt nanoparticles is small enough.     

In vivo characterization of atherosclerotic plaque depositions by Raman-probe spectroscopy and in vitro coherent anti-stokes Raman scattering microscopic imaging on a rabbit model

Visualization as well as characterization of inner arterial plaque depositions is of vital diagnostic interest, especially for the early recognition of vulnerable plaques. Established clinical techniques provide valuable visual information but cannot deliver information about the chemical composition of individual plaques. Here, we employ Raman-probe spectroscopy to characterize the plaque compositions of arterial walls on a rabbit model in vivo, using a miniaturized filtered probe with one excitation and 12 collection fibers integrated in a 1 mm sleeve. Rabbits were treated with a cholesterol-enriched diet. The methodology can improve the efficiency of animal experiments and shows great potential for applications in cardiovascular research. In order to further characterize the plaque depositions visually, coherent anti-Stokes Raman scattering (CARS) microscopy images have been acquired and are compared with the Raman-probe results.     

Improved scanning range for coherent anti-stokes Raman spectroscopy using a tunable optical parametric oscillator

Coherent anti-Stokes Raman spectroscopy (CARS) is a well-known form of nonlinear spectroscopy that has been used for a wide range of specialized quantitative applications. From an analytical chemist's point of view, however, conventional CARS is impractical as a tool for qualitative and quantitative analyses because the scan range is too short to produce complete vibrational spectra. This paper introduces a new technique, synchronously scanned optical parametric oscillator (OPO) CARS, that improves the potential for using nonlinear spectroscopy as an analytical technique in both gas- and condensed-phase samples. First, it uses a broadly tunable OPO to increase the scan range. Second, phase matching problems that limit scans in condensed-phase CARS are reduced by using both the signal and the idler beams in a synchronous scanning manner. Finally, this synchronous scanning method generates an output signal that remains fixed at a single wavelength (single-wavelength detection). Advantages of single-wavelength detection include reduction of stray light, simplicity, and elimination of the need for wavelength calibration of the detection optics. Results are presented on neat and mixed samples in gas and condensed phases.     

Surface-enhanced Raman scattering on gold nanotrenches and nanoholes

Dependent effects on edge-to-edge distance and incidence polarization in surface-enhanced Raman Scattering (SERS) were studied in detection of 4-mercaptopyridine (4-MPy) molecules absorbed on gold nanotrenches and nanoholes. The gold nanostructures with controllable size and period were fabricated using electron-beam lithography. Large SERS enhancement in detection of 4-MPy molecules on both nanostructred substrates was observed. The SERS enhancement increased exponentially with decrease of edge-to-edge distance for both the nanotrenches and nanoholes while keeping the sizes of the nanotrenches and nanoholes unchanged. Investigation of polarization dependence showed that the SERS enhancement of nanotrenches was much more sensitive to the incidence polarizations than that of nanoholes.     

Refractive effects in coherent anti-Stokes Raman scattering microscopy

Coherent anti-Stokes Raman scattering (CARS) microscopy with high sensitivity and high three- dimensional resolution has been developed for the vibrational imaging of chemical species. Due to the coherent nature of the CARS emission, it has been reported that the detection of epi-CARS and forward-CARS (F-CARS) signals depends on the size and shape of the sample. We investigate theoretically and experimentally the effects on the CARS signal of refractive index mismatches between the sample and its surroundings. Backward-CARS and F-CARS signals are measured for different polystyrene bead diameters embedded in different refractive index solvents. We show that index mismatches result in a backward-reflected F-CARS signal that generally dominates the experimentally backward-detected signal. Simulations based on geometrical and wave optics comparing forward- and backward-detected signals for polystyrene beads embedded in different index solvents confirm our findings. Furthermore, we demonstrate that the maxima of forward- and backward-detected signals are generated at different positions along the optical axis in the sample if refractive index mismatches are present between the sample and its surroundings.     

Development of high-resolution n(2) coherent anti-stokes Raman scattering for measuring pressure, temperature, and density in high-speed gas flows

Mean and instantaneous measurements of pressure, temperature, and density have been acquired in an optically accessible gas cell and in the flow field of an underexpanded sonic jet by use of the high-resolution N(2) coherent anti-Stokes Raman scattering (CARS) technique. This nonintrusive method resolves the pressure- and temperature-sensitive rotational transitions of the nu = 0 ? 1 N(2) Q-branch to within Domega = 0.10 cm(-1). To extract thermodynamic information from the experimental spectra, theoretical spectra, generated by a N(2) spectral modeling program, are fit to the experimental spectra in a least-squares manner. In the gas cell, the CARS-measured pressures compare favorably with transducer-measured pressures. The precision and accuracy of the single-shot CARS pressure measurements increase at subatmospheric conditions. Along the centerline of the underexpanded jet, the agreement between the mean CARS P/T/rho measurements and similar quantities extracted from a Reynolds-averaged Navier-Stokes computational fluid dynamic simulation is generally excellent. This CARS technique is able to capture the low-pressure and low-temperature conditions of the M = 3.4 flow entering the Mach disk, as well as the subsonic conditions immediately downstream of this normal shock.     

Surface-enhanced Raman scattering of dopamine on self-assembled gold nanoparticles

Dopamine, a potent neurotransmitter in the brain, influences a variety of motivated behaviors and plays a major role in Parkinson's disease. In this study, the Raman signal of dopamine was detected on a fabricated nanoparticle-immobilized glass surface by surface-enhanced raman spectroscopy (SERS). Amine-modified glass was prepared by the self-assembly of amine-terminated silane on substrate, followed by the deposition of gold nanoparticles. The gold nanoparticles deposited on the glass surface were functionalized by anti-dopamine or dopamine. The antigen-dopamine was captured by antibody-assembled gold substrate and detected by SERS. The optical properties and morpology of the glass substrate with immobilized gold nanoparticles were analyzed by scanning electron microscopy and UV-VIS absorption spectroscopy. The Raman spectrum of dopamine displayed broad bands at 1267, 1331, 1158, 1478, 1578 and 1584 cm(-1). The strongest peaks in the spectra (at 1267 and 1478 cm(-1)) were identified as phenolic carbon-oxygen and phenyl C=C stretches, respectively. A working curve of the SERS signal constructed from cathecol ring vibration versus antigen-dopamine concentration was obtained at 1478 cm(-1), and the non-optimized detection limit for anti-dopamine surface antigen was as low as 1 ng/ml. These results suggest that SERS-based immunosensor can be a promising tool for the detection and screening of neurotransmitters.     

Increased efficiency of vacuum ultraviolet generation by stimulated anti-stokes Raman scattering with stokes seeding

Stimulated anti-Stokes Raman scattering in molecular hydrogen allows for the generation of continuously tunable narrow-bandwidth radiation down to the transmission limit of vacuum ultraviolet (VUV) window materials. Simultaneous irradiation of UV-pump radiation (in this application, dye laser radiation of wavelength lambda = 372 nm) and of radiation whose wavelength corresponds to the first Stokes component allows a considerable increase in efficiency-by nearly 2 orders of magnitude in the far VUV. The additional Stokes radiation is generated in a simple manner during the passage of the unfocused pump radiation through a high-pressure Raman cell that precedes the VUV Raman cell.