Surface-enhanced Raman scattering (SERS) from trinitrotoluene and other nitro-based explosives is important for the
development of a reliable detection scheme exhibiting low false-positive rates. However, the interaction of these
compounds with Ag and Au causes the molecules to orient in ways such that the primary vibrations of the nitro groups,
the main identifying Raman marker of these compounds, are inhibited in addition to causing a reduction in the SERS
response. It has recently been shown that cysteamine, which contains amine functional end groups, will electrostatically
attract the nitro groups of TNT. Therefore, as the thiol functional group of cysteamine chemically bonds this molecule
to the plasmonically-active Au and Ag nanoparticles studied, SERS of TNT can be obtained following the nitro-amine
functional group complex formation. It is observed that the cysteamine adsorbs in one of two configurations on the
metal surface, with the trans configuration consisting of bonding at the S end of the molecule and the cysteamine is
perpendicular to the metal surface, while in the Gauche configuration S bonding occurs, but the molecule bends over
towards the metal film surface, approaching the parallel configuration allowing the amine groups interact with the
surface. We find that the trans configuration is best for the detection of SERS from TNT. Experiments compare well
with DFT calculations of the cysteamine and TNT complex and their adsorption on Ag.
Surface enhanced Raman scattering (SERS) is a powerful technique for the detection of submonolayer coverage of gold
or silver surfaces. The magnitude of the effect and the spectral wavelength of the peak depend on the metal
nanoparticles used and its geometry. In this paper we show that the use of chemicals that bind to gold or silver can lead
to the clustering of nanoparticles. We used well defined Au nanoparticles in our experiments and add cysteamine to
solutions containing the nanoparticles. The plasmonic response of the nanoparticles is measured by transmission
Surface Plasmon Resonance (SPR) spectroscopy. We observed significant changes to the SPR spectra that are
characteristics of close coupled nanoparticles. The time evolution of these changes indicates the formation of gold
nanoparticles clusters. The SERS response of these clustered nanoparticles is observed to red shift from the designed
peak wavelength in the green to the red. In addition, the placement of these clusters on dielectric surfaces shifts the SPR
even more into the red. The experimental results are supported by calculations of the electromagnetic fields using finite
difference methods.
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