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A key element to improve the analytical capabilities of surface-enhanced Raman spectroscopy (SERS) resides in the performance characteristics of the SERS-active substrate. Variables such as shape, size, and homogeneous distribution of the metal nanoparticles throughout the substrate surface are important in the design of more analytically sensitive and reliable substrates. Electron-beam lithography (EBL) has emerged as a powerful tool for the systematic fabrication of substrates with periodic nanoscale features. EBL also allows the rational design of nanoscale features that are optimized to the frequency of the Raman laser source. In this work, the efficiency of EBL fabricated substrates are studied by measuring the relative SERS signals of Rhodamine 6G and 1,10-phenanthro-line adsorbed on a series of cubic, elliptical, and hexagonal nanopatterned pillars of ma-N 2403 directly coated by physical vapor deposition with 25 nm films of Ag or Au. The raw analyte SERS signals, and signals normalized to metal nanoparticle surface area or numbers of loci, are used to study the effects of nanoparticle morphology on the performance of a rapidly created, diverse collection of substrates. For the excitation wavelength used, the nanoparticle size, geometry, and orientation of the particle primary axis relative to the excitation polarization vector, and particularly the density of nanoparticles, are shown to strongly influence substrate performance. A correlation between the inverse of the magnitude of the laser backscatter passed by the spectrometer and SERS activities of the various substrate patterns is also noted and provides a simple means to evaluate possible efficient coupling of the excitation radiation to localized surface plasmons for Raman enhancement.

The presence of aromatic compounds in water is an important topic in environmental sciences. Silver-polydimethylsiloxane nanocomposites (Ag-PDMS) have recently been demonstrated as promising substrates for the detection of model environmental pollutants via surface-enhanced Raman spectroscopy (SERS). This work discusses how different variables such as pH and matrix composition can affect the sorption and SERS activity of these chemicals. The results show that the conjugate base of weak acids can interact more efficiently with the substrate, leading to an increased signal at higher pH, while amino-aromatic compounds interact more efficiently at a lower pH. The sorption of these chemicals is an essential step in the process and has been attributed to the absorption of the analyte into the PDMS followed by its adsorption to the metallic surface. In addition, the presence of moderate concentrations (1 x 10(-4) M) of a supporting electrolyte such as nitrate or fluoride can improve the sorption of 4-hydroxybenzoic acid to the Ag-PDMS nanoparticles. Other ions such as phosphate and chloride cause rapid oxidation of the substrates even at concentrations as low as 1 x 10(-5) M. The effect of these variables in the analysis of real samples is presented. The potential use of liquid chromatography for isolating the model pollutants from detrimental matrix components in nat- ural waters is also shown.

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.

Surface-enhanced Raman scattering (SERS) is employed to obtain distinctive spectra of compounds that are efficiently separated by capillary electrophoresis (CE) and deposited onto planar SERS-active substrates. A simple method is described that explains how to prepare SERS-active substrates by depositing a silver-colloid solution onto frosted-glass microscope slides, using a high-efficiency nebulizer. Scanning electron micrographs reveal a layered coating of fairly uniform-sized, 100-nm silver nanoparticles with interstitial spaces ranging from a few to tens of nanometers. The on-column separation is monitored by laser-induced fluorescence, while electrofilament depositing the CE effluent onto a moving SERS-substrate. Subsequently, the SERS spectra and off-column electropherograms are obtained with a simple confocal Raman spectrometer. The test compounds used to demonstrate this technique include compounds of biological significance: benzyloxyresorufin, riboflavin, and resorufin. CE and Raman conditions are evaluated to determine their affects on the SERS signals. An average off-column efficiency of 100,000 plates/m and a signal reproducibility of 11% relative standard deviation were achieved. Characteristic spectra with major Raman bands exhibiting signal-to-noise ratios of greater than 3 were obtained for a 3.2-nL injection of 10(-6) M (706 fg) resorufin. Forming a self-assembled monolayer (SAM) on the substrate increases the sensitivity of the SERS technique and decreases the on-substrate broadening. Calibration plots for both plain- and SAM-SERS substrates are demonstrated.