Raman Spectroscopy and Surface Enhanced Raman Scattering (SERS) for the Analysis of Blue and Black Writing Inks: Identification of Dye Content and Degradation Processes.

Daniela Saviello,Maddalena Trabace,Daniela Iacopino,Rodorico Giorgi,Antonio Mirabile,Piero Baglioni,Abeer Alyami

doi:10.3389/fchem.2019.00727

Daniela Saviello, Maddalena Trabace + Show 5 more

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https://doi.org/10.3389/fchem.2019.00727

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Abstract

Raman spectroscopy and Surface Enhanced Raman Scattering (SERS) were applied to the analysis of blue and black writing inks. SERS was performed by application of plasmonic nanopastes constituted by Ag nanoparticles and Au nanorods directly on inks deposited on paper substrates under laser irradiation of 514 nm. It was found that SERS spectra were largely enhanced compared to Raman spectra and that Ag nanopastes produced much larger enhancements than Au nanopastes, due to a combination of plasmonic resonance, charge transfer, and molecular resonance effects. All analyzed writing inks resulted constituted by Crystal Violet and other triarylmethane dye mixtures, to which sometimes phthalocyanine dyes were also added (for example in Bic pens). SERS was also used for the identification of degradation processes occurring in artificially aged blue pens deposited on paper substrates. It was found that color alteration changed from ink to ink and varied from darkening to discoloration to slight fading, depending on the initial chemical composition. For inks containing Crystal Violet, two mechanisms associated to de-methylation and photo-reduction of excited dye to colorless leuco forms were identified.

Highlights

Modern commercial inks are a complex mixture of several dyes and/or pigments constituting up to the 50% of the total ink formulation, carried either in a glycol-based solvent or benzyl alcohol (Bell, 2008; Houck and Siegel, 2010; Houck et al, 2013)
In this work we propose the use of Raman and Surface Enhanced Raman Scattering (SERS) spectroscopy for the analysis of a number of blue and black writing inks on paper substrates
Prior measurements of SERS spectra Raman spectra were recorded for five pens, in order to establish limitations of Raman spectroscopy and assess enhancement generated by subsequent application of SERS

Summary

Introduction

Modern commercial inks are a complex mixture of several dyes and/or pigments constituting up to the 50% of the total ink formulation, carried either in a glycol-based solvent or benzyl alcohol (Bell, 2008; Houck and Siegel, 2010; Houck et al, 2013). Visible, Raman, and infrared spectroscopies have gained popularity as techniques for providing rapid information on the chemical content of the ink mixtures and are increasingly pursued for their rapidity of analysis, little sample preparation and non-destructive analysis. Such approaches are amenable to the investigation of ink-based paper works, whereby the identification of dyes and pigment ink constituents is important to assess dating, originality and to implement correct conservation procedures ensuring long-term preservation. Our group has developed plasmonic probes comprising Ag and Au nanopastes and nanoinks and has used SERS in combination with UV-vis spectroscopy for the analysis of blue and colored Bic pens, increasingly used for artistic purposes (Alyami et al, 2016, 2017, 2018)

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