Improved discrimination of pen inks on document by surface-enhanced Raman substrate fabricated by magnetron sputtering

Abstract

For questioned document examinations, conventional Raman spectroscopy often encounter with weak signal intensity and/or fluorescence interfering. The surface enhanced Raman spectroscopic (SERS) technique can efficiently accommodate the issues, by greatly improving the Raman intensity by several orders of magnitude. However, colloidal SERS fabricated by chemical reduction method practically suffer from native contaminant and uncontrollable aggregation on document surface. In this work, we report an alternative surface-enhanced Raman scattering technique for analysis of pen inks on written document by utilizing film-based Ag nanorod SERS substrate. Highly improved discriminating powers (DP) were obtained from SERS substrates. The pen samples consisted of ninety-five blue and black ballpoint pens, of oil-, water-, and gel-based, indiscriminately acquired from local sources in Thailand. The Raman spectroscopic (RS) and SERS analyses were performed with the confocal Raman spectrometer at 785 nm excitation wavelength. The spectral data were directly collected from both the written pen inks and the traces of the inks with the SERS substrate. The results could categorized the pen ink samples into either the RS or SERS active/inactive results, and further calculated and analyzed as the discriminating powers (DP). The results indicated that the proposed SERS method was highly effective at distinguishing the ballpoint pens on the written documents with the obtained DP values at 0.79 and 0.92 for the blue and black pen samples, respectively.