Relationship between surface-enhanced Raman scattering and the dielectric properties of aggregated silver films

Adsorption of dipeptide L-alanyl-L-tryptophan on gold colloidal nanoparticles studied by surface-enhanced Raman spectroscopy

Surface adsorption of a dipeptide L-alanyl-L-tryptophan (Ala-Trp) on gold nanoparticles reduced by citrate (CT) and borohydride (BH) ions was investigated by a surface-enhanced Raman scattering (SERS) technique. Two distinct SERS spectra of Ala-Trp depending on the types of gold nanoparticles were observed, and the vibrational assignments were based on the density functional theory simulations and the previous SERS results of Trp. Ala-Trp mainly adsorbs through the amine group on CT gold nanoparticles with a perpendicular orientation of the indole ring to the surface. In contrast, the adsorption occurs via the π electrons of the indole ring on the BH gold surfaces while maintaining a flat geometry of the indole ring to the surface. The amide I band of Ala-Trp was observed only with the CT gold colloids in acidic and neutral conditions where partial surface adsorption via the amide group is expected.

Surface-enhanced Raman scattering of coumarin 343 on silver colloidal nanoparticles

Surface-enhanced Raman scattering (SERS) of coumarin 343 (C343) adsorbed on silver colloidal nanoparticles reduced by sodium citrate was investigated and the surface adsorption geometry of C343 on Ag was sought by optimizing C343-Ag complexes for neutral and deprotonated C343 molecules in the DFT simulations. The SERS of C343 showed a number of spectral changes upon solution pH change. We found that deprotonated C343 adsorbs on the Ag nanoparticles through the carboxylate group keeping a perpendicular geometry to the surface. When protonated, the adsorption geometry of C343 is changed into more or less flat to the surface as the cyclic ester group becomes a preferred surface adsorption site.

Label-free nanometer-resolution imaging of biological architectures through surface enhanced Raman scattering

Label free imaging of the chemical environment of biological specimens would readily bridge the supramolecular and the cellular scales, if a chemical fingerprint technique such as Raman scattering can be coupled with super resolution imaging. We demonstrate the possibility of label-free super-resolution Raman imaging, by applying stochastic reconstruction to temporal fluctuations of the surface enhanced Raman scattering (SERS) signal which originate from biomolecular layers on large-area plasmonic surfaces with a high and uniform hot-spot density (>10¹¹/cm², 20 to 35 nm spacing). A resolution of 20 nm is demonstrated in reconstructed images of self-assembled peptide network and fibrilated lamellipodia of cardiomyocytes. Blink rate density is observed to be proportional to the excitation intensity and at high excitation densities (>10 kW/cm²) blinking is accompanied by molecular breakdown. However, at low powers, simultaneous Raman measurements show that SERS can provide sufficient blink rates required for image reconstruction without completely damaging the chemical structure.

Optimizing electromagnetic enhancement of flexible nano-imprinted hexagonally patterned surface-enhanced Raman scattering substrates

The production of inexpensive, large-scale, uniform substrates for surface-enhanced Raman scattering (SERS) is a key to popularize its usage in chemical and biological detection. We demonstrate a flexible nano-imprinted hexagonally patterned SERS-active substrate. Its electromagnetic enhancement factor was optimized by the thickness adjustment of its silver over-coated film. The experimental data show a good correspondence with the theoretical prediction. Such substrate was shown to exhibit high uniformity and reproducibility with a variation of less than 2%, offering a potential of greatly exploiting such substrate in infield biocide monitoring.

Femtosecond laser-nanostructured substrates for surface-enhanced Raman scattering

We present a new type of surface-enhanced Raman scattering (SERS) substrate that exhibits extremely large and uniform cross-section enhancements over a macroscopic (greater than 25 mm2) area. The substrates are fabricated using a femtosecond laser nanostructuring process, followed by thermal deposition of silver. SERS signals from adsorbed molecules show a spatially uniform enhancement factor of approximately 10(7). Spectroscopic characterization of these substrates suggests their potential for use in few or single-molecule Raman spectroscopy.

Enhanced fluorescence of Cy5-labeled DNA tethered to silver island films: fluorescence images and time-resolved studies using single-molecule spectroscopy

Methods that increase the total emission per fluorophore would provide increased sensitivity and a wider dynamic range for chemical analysis, medical diagnostics, and in vivo molecular imaging. The use of fluorophore-metal interactions has the potential to dramatically increase the detectability of single fluorophores for bioanalytical monitoring. The fabrication and single-molecule analysis of fluorophore-labeled DNA molecules tethered to silver island films are described in this article. The single-molecule spectroscopic method reveals some insightful information on the behaviors of single molecules, rather than an ensemble of molecules. Analysis of fluorescence images, intensity profiles, total emitted photons, and lifetime distributions reveals some of sample heterogeneities. Investigations of time-dependent emission characteristics of single molecules indicate that the total number of emitted photons on the silvered surface is more than 10 times greater than on free labeled DNA molecules on a glass substrate. In addition, time-correlated single-photon counting results reveal the reduced lifetimes of single molecules tethered to silver island films.

Adaptive silver films for detection of antibody-antigen binding

Antibody-antigen binding events at a monolayer protein concentration have been demonstrated on nanostructured adaptive silver films (ASFs) using surface-enhanced Raman scattering (SERS) and luminescence-based assays. It is shown that proteins stabilize and restructure the ASF to increase the SERS signal while preserving antigen-binding activity. Evidence for antibody-antigen binding on the ASF substrates is the distinct SERS spectral changes of the surface-bound antibody or antigen without special tags. The activity of the surface-bound proteins and their practical application are validated by independent immunochemical assays. Results are presented to demonstrate that these surfaces can be extended to protein arrays with detection applications distinct from current SERS, fluorescence, or luminescence methods.

Light-induced specular-reflectivity suppression at a gallium/silica interface

The reflectivity of a gallium/silica interface formed on an optical flat or at the tip of a cleaved optical fiber can be reduced in a reversible fashion when the interface is excited by a few milliwatts of laser power. This phenomenon occurs at temperatures just below gallium's melting point. We believe that the effect can be attributed to light-induced structuring at the interface.

Surface effects on Raman scattering from Sb deposited on Ag-island films

Surface-enhanced Raman scattering from crystal vibrational modes of a Sb film rather than from vibrational modes of individual molecules has been observed for the first time to our knowledge. A ~20x enhancement of the longitudinal-optical mode (150 cm(-1)) in the Raman spectra of crystalline Sb films deposited on silver-island films was obtained. The excitation profiles, as well as the dependence of Raman intensity on the thickness of Ag films, were studied.

Excitation spectra of surface-enhanced Raman scattering on silver-island films

Both the spectral dependence and the magnitude of the excitation spectra of surface-enhanced Raman scattering are found to be quantitatively related to the absorption spectra for silver-island films. This relationship is maintained even when the local electromagnetic enhancement at the islands is varied by coating the film with a layer of absorbing dye molecules. The results illustrate a simple experimental way to determine independently the role of the electronic plasma resonances in the enhancement process on these films and shed new insights into the details of the electromagnetic interactions responsible for surface-enhanced Raman scattering.

Surface-enhanced Raman scattering from silver particles on polymer-replica substrates

Evaporation of silver onto polymer microstructures produces a particle system for which optical characterization (e.g., absorbance) is possible. Measurements of surface-enhanced Raman scattering from these surfaces are compared with the particle-plasmon resonance model, including radiation-damping effects. The results provide evidence for the correlation between the Raman-excitation resonance and the optical properties of the silver system, supporting the electromagnetic model of surface Raman enhancement.