Surface-Enhanced Resonance Raman Scattering and Near-Infrared Fourier Transform Raman Scattering as in Situ Probes of Ink Jet Dyes Printed on Paper

The characterization of ink jet dyes and inks printed onto paper surfaces is of importance for the development of ink jet ink formulations and for use in forensic science. Raman spectroscopy is an effective and informative probe for this purpose if problems associated with fluorescence are overcome. A comparison is made here between two effective techniques, surface-enhanced resonance Raman scattering (SERRS) and near-infrared Fourier transform Raman scattering (NIR-FTR). SERRS provides a good method for obtaining in situ measurements using a visible laser system. It is fast (1–10 s accumulations) and requires very low laser powers (<1 mW). However, it requires the addition of a small dot of silver colloid to the paper surface in order to produce the enhancement. NIR-FTR is noninvasive and simpler to use. However, much higher laser powers are required (>200 mW), and the spectral accumulation time is significantly longer. Both methods overcome fluorescence effectively in most samples studied; however, each is more effective than the other with selected inks and paper substrates. SERRS involves resonant enhancement of the chromophore of the dye in contact with the silver surface, and consequently it is the dye chromophore which is uniquely identified. With NIR-FTR, signals from the paper and the filler are also observed. Comparable spectral patterns with clear, explicable differences are obtained from each method, indicating in particular that SERRS spectra can be interpreted without recourse to specific surface selection rules. The combination of the two techniques provides some information on the electronic as well as the vibrational properties of the dyes in situ.