Polystyrene beads as probes of the surface-enhanced Raman scattering response characteristics of silver nanorod arrays.

Polystyrene microspheres were used to interrogate the surface-enhanced Raman scattering (SERS) response of silver nanorod (AgNR) array substrates. It has been suggested that enhancement between nanorods is significantly greater than that at the top due to contributing electromagnetic fields from each nanostructure. To test this theory, two different sized fluorescent polystyrene microspheres were used. The SERS intensity of beads small enough to fit within the array was compared with that of larger beads confined to rest on top of the array. Location of the beads within the array was established using optical fluorescence and scanning electron microscopy. The findings presented herein suggest that evaporation of the sample produces a nonuniform distribution of scatterers across the AgNR array and that the enhancement found for beads located on top of the AgNRs was significantly greater than that for beads located within the array.

Limitations of surface enhanced Raman scattering in sensing DNA hybridization demonstrated by label-free DNA oligos as molecular rulers of distance-dependent enhancement.

This article presents a critical evaluation of silver nanorod arrays as substrates for assaying nucleic acid hybridization by surface enhanced Raman scattering (SERS). SERS spectra acquired on complementary oligos, alone or in combination, contain the known spectral signatures of the nucleotides that comprise the oligo; however, no signature bands characteristic of the hybrid were observed. Spectra acquired on an oligo with a 5'- or 3'-thiol were distinctly different from that acquired on the identical oligo without a thiol pendant group suggesting a degree of control over the orientation of the oligo on the nanorod surface. A set of oligos consisting of adenine tracts in a polycytosine chain served as molecular rulers to probe the distance dependence of the SERS enhancement. Using these, we have identified the point at which the characteristic bands for the nucleotides that comprise the oligo disappear from the spectrum. These findings suggest that the applicability of SERS for label-free detection of nucleic acid hybridization is limited to short oligos of less than nine nucleotides.

Removal of surface contamination and self-assembled monolayers (SAMs) from silver (Ag) nanorod substrates by plasma cleaning with argon.

Surface contamination of surface-enhanced Raman (SERS)-active metallic substrates has been a limitation to the utility of SERS as an analytical technique, potentially affecting surface coverage, spectral reproducibility, and analytical limits of detection. We have developed a simple and versatile cleaning method for SERS-active Ag nanorod arrays that consists of a short (4 min) exposure of the substrate to an Ar(+) plasma in a low-pressure environment. The findings presented here demonstrate that this cleaning procedure essentially eliminates organic background contamination. This procedure works equally well for self-assembled monolayers of thiolates that strongly adsorb onto Au and Ag surfaces. For SERS-active surfaces composed of arrays of Ag nanorods prepared by oblique-angle vapor deposition, we investigated the (1) Raman band intensities, (2) nanorod morphology via scanning electron microscopy, and (3) surface hydrophobicity via static contact angle measurements, as a function of exposure time of the Ag nanorods to the Ar(+) plasma. Short (4 min) exposure to Ar(+) plasma eliminated background contamination but decreased the observed SERS intensity for re-adsorbed analytes by approximately a factor of 2 while leaving the nanorod morphology essentially unchanged. Prolonged exposure to Ar(+) plasma (>10 min) resulted in substantial morphological changes of the Ag nanorod lattice and led to a decrease in the observed SERS intensities by a factor of 10. The results presented here suggest that Ar(+) plasma cleaning is an efficient process for removing carbonaceous and organic contamination as well as thiolate monolayers from SERS-active Ag surfaces, as long as the plasma conditions and exposure times are carefully monitored.

Surface-enhanced Raman scattering of bacterial cell culture growth media.

The application of surface-enhanced Raman spectroscopy (SERS) to characterizing bacteria is an active area of investigation. Micro- and nano-structured SERS substrates have enabled detection of pathogens present in biofluids. Several publications have focused on determining the spectral bands characteristic of bacteria from different species and cell lines. In this report, the spectra of fifteen commonly used bacterial growth media are presented. In many instances, these spectra are similar to published spectra purportedly characteristic of specific bacterial species. The findings presented herein suggest that bacterial fingerprinting by SERS requires further examination.

Patterned silver nanorod array substrates for surface-enhanced Raman scattering.

A novel method for batch fabrication of substrates for surface-enhanced Raman scattering (SERS) has been developed. A modified platen that fits in a commercial electron beam evaporator enables the simultaneous deposition of Ag nanorod arrays onto six microscope slides by glancing angle deposition. Following removal of substrates from the evaporator, patterned wells are formed by contact printing of a polymer onto the surface. Well dimensions are defined by penetration of the polymer into the nanorod array and subsequent photochemical curing. Inherent advantages of this method include: (1) simultaneous production of several nanorod array substrates with high structural uniformity, (2) physical isolation of nanorod arrays from one another to minimize cross contamination during sample loading, (3) dimensional compatibility of the patterned array with existing SERS microscope, (4) large SERS enhancement afforded by the nanorod array format, (5) small fluid volumes, and (6) ease of use for manual delivery of fluids to each element in the patterned array. In this article, the well-to-well, slide-to-slide, and batch-to-batch variability in physical characteristics and SERS response of substrates prepared via this method is critically examined.