The effect of aqueous solution in Raman spectroscopy

In Raman detection, the most popular solution for the samples is tri-distilled water. But the effect of aqueous solution is barely studied in Raman spectroscopy. In fact Raman spectroscopy of solid-state and liquid-state are obvious different. In addition, FWHM of Raman spectral peaks also change evidently. In this paper, several samples were selected for the experiment; including sodium nitrate, sodium nitrite, glucose and caffeine. By comparing the Raman spectroscopy of samples at different concentrations, it is found that the concentration of the sample can affect the strength of Raman spectroscopy, but it can hardly impact FWHM of Raman spectral peaks. By comparing the Raman spectroscopy of liquid-state with the Raman spectroscopy of solid-state, it is observed that the FWHM of some Raman spectral peaks varied obviously; that may be because when the sample was dissolved into the water, the crystal lattice structure was broken, and for some samples atom form became ion form in aqueous solution. Those structural variations caused the variation of the FWHM. The Raman spectroscopy of caffeine aqueous solution at very low concentration was also detected and analyzed. Compared with the Raman spectra of solid-state samples, it is found that some Raman spectral peaks disappeared when the sample was dissolved in water. It is possible that the low concentration of the sample result in the weakening of Raman signals and the disappearing of some weak Raman spectral peaks. Then Ag nanoparticles were added into the caffeine aqueous solution, the results suggest that surface enhanced Raman spectroscopy (SERS) not only can enhance the Raman spectral signal, but also can reduce the effect of aqueous solution. It is concluded that the concentration of sample only affects the strength of Raman spectroscopy; the aqueous solution can affect the FWHM of Raman spectral peaks; and SERS can reduce the effect of aqueous solution.