The prospect of ultra-sensitive detection of molecular species, particularly those of
energetic materials, has prompted the present research initiative. The combination of
metal surface nano-technology and Raman spectroscopy has given rise to ‘Surface
Enhanced Raman Spectroscopy’ (SERS). This is a very sensitive technique and has
proved to be capable of detecting a single molecule.
SERS was demonstrated by recording Raman spectra of the sample molecules adsorbed
on various specially prepared SER-active surfaces both in the form of a colloidal
suspension and on the solid roughened surfaces. Using a gold colloidal suspension,
pyridine has been detected down to 10-11 molar (M) concentration. A silver slab was
roughened to a dimension of a nano-scale by etching in nitric acid solution to make
SER-active surface. Pentaerythritol Tetranitrate (PETN) explosive was detected using
this surface after its 10-2 M solution was dropped, dried and washed (of any residue)
from the surface. Lithographically engineered silver structures in the form of nanoarrays
having a number of silver structures of approximately 106 in a region of 0.1 mm2
have been used for SERS. The major noise contribution to the scattering from
impurities in an ordinary glass substrate has been eliminated by replacing glasses as
substrates with pure quartz discs. The headspace vapours from peroxide explosives,
Triacetone Triperoxide (TATP) and Hexamethylene Triperoxide Diamine (HMTD),
were detected at approximately 70 parts per million (ppm) and 0.3 ppm concentrations
respectively using a portable commercial Raman Spectrometer. PETN was also detected
from its headspace vapour at about 18 parts per trillion (ppt) in spite of it having a much
lower vapour pressure.
The possibility of desorption of adsorbed molecules from a nano-structured surface by
laser irradiation has been demonstrated experimentally with the aim of reusability of
SER-active surfaces. Also demonstrated was the enhancement in Raman intensity
through resonance Raman effect spectroscopy for the future use in surface enhanced
resonance Raman spectroscopy (SERRS)
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