Date: Oct 01, 2012 Author: David Bradley Source: ezine (
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Raman significance
A team has demonstrated two significant benefits of a new approach called dynamic surface enhanced Raman spectroscopy (DSERS), Credit: American Chemical Society/Wyoming team
A team has demonstrated two significant benefits of a new approach called dynamic surface enhanced Raman spectroscopy (DSERS), which shows how normal Raman and instrumentation interference can be reduced in SERS.
The researchers, based at the University of Wyoming in Laramie and colleagues at Snowy Range Instruments, have found a way to avoid much of the noise and interference intrinsic to a SERS analysis by adopting a dynamic approach to their studies of gold nanoparticles.
The team has demonstrated proof of principle for their approach by testing it on shelled nanoparticles. This verified that they could retrieve strong spectra even at the very low concentrations at which solvent effects would interfere with the signals. They also revealed an additional benefit whereby the dynamic approach allows them to distinguish between different types of molecules adhering to the nanoparticles. Overall the approach could open up new areas for SERS particularly in nanotechnology.
The team explains how in standard Raman spectroscopy, the signal-to-noise ratio is boosted by integration of signal in the wells of the charge-coupled device used as the detector. They point out that appropriate cooling and readout circuitry gives users optical detection that follows a Poisson distribution for noise-limited spectra where signal strength is equal to the intensity. Spectra are averaged but this means that all photons are treated equally regardless of source, which means fluorescent particles, stray light, impurities and light from sample containers is all added into the milieu.
Colloids of gold or silver nanoparticles have been used to improve SERS assays. Now, the Wyoming team has exploited a statistical approach to obtain dynamic SERS data with colloidal suspensions, which they say allows them to differentiate between their signals and the indiscriminate fluorescence and matrix Raman signals by exploiting the difference in the particles' sizes compared with the molecular matrix.
SERS with distinction
"We have demonstrated two significant benefits of dynamic surface enhanced Raman spectroscopy (DSERS) measurements," the team explains. The first of these is thus the removal of instrumental and normal Raman interferences. "Our first example of shelled nanoparticles at very low concentrations confirmed the benefit of DSERS for removal of an overwhelmingly strong solvent spectral interference," they add. "The second benefit, site selection, was demonstrated with 4-mercaptopyridine on bare gold nanoparticles to observe a small population of molecules that were spectroscopically unique from the large population of molecules on the particles." They further explain that the DSERS spectrum arises because of an excess of a small population of molecules adsorbed on the nanoparticle ensemble, therefore allowing them to distinguish between functionalised nanoparticles even if the particles are the same size.
"The solvent removal component of our work is already being used in our (Snowy Range Instruments). It is quite useful in detecting the spectra from nanoparticles in overwhelming large solvent signals," Carron told SpectroscopyNOW.
