Reduction of fluorescence interference in Raman spectroscopy via analyte adsorption on graphitic carbon

Raman spectra of normally fluorescent materials were obtained by quenching the fluorescence through adsorption on carbon surfaces. Energy transfer from the excited state to the carbon surface greatly reduced the fluorescence of rhodamine 6G and the near-IR laser dye IR125, and Raman spectra with high S/N were obtained from the adsorbed molecules. Strong resonance enhancement of Raman scattering was observed, and monolayer films of RGG were observed. RGG exhibited Langmuirian adsorption behavior, with saturation coverage occuring at [approximately] 0.5 mM in methanol. The technique was also successfully applied to bis(methylstyryl)benzene (BMB) and fluoranthene, which are less fluorescent but also much weaker scatterers. Since the laser wavelengths yielding resonance enhancement are in the same range as those producing fluorescence, adsorption to carbon permits elimination of fluorescence without loss of resonance enhancement. The porous graphite substrate has the added advantage of high microscopic surface area, low elastic background, and propensity to adsorb a wide range of organic materials. With care, it was possible to observe Raman features from monolayers of materials having cross sections comparable to that of benzene, such as the 1205 [cm.sup.-1] peak of BMB.