Plasmonic colloidal pastes for surface-enhanced Raman spectroscopy (SERS) of historical felt-tip pens

Abstract

Surface-enhanced Raman spectroscopy (SERS) has been identified as a suitable technique for the analysis of colorants in works of art. Herein, the application of SERS to the identification of dye compositions in historical felt-tip pens is reported, which is of paramount importance for the development of appropriate conservation protocols for historical drawings. In this study, three pens (pink, green, and blue colors) belonging to the film director Federico Fellini were analyzed. SERS measurements were performed directly on the pen lines drawn on a commercial paper by the deposition of Ag colloidal pastes, which allowed fast in situ dye identification without the need for extraction or hydrolysis treatments. Eosin Y was identified as the only dye present in the pink pen ink, whereas erioglaucine was found to be the main dye component in green and blue pen inks. SERS also resulted in highly efficient identification of the individual dyes erioglaucine, crystal violet, and rhodamine present as a mixture in the blue pen ink. The high SERS sensitivity was ascribed to the plasmonic effects and efficient quenching of the fluorescence interference of dyes. A comparison with contemporary pen inks highlighted minor differences in the chemical composition. These results prove that SERS can be used as a fast and sensitive analytical tool for ink analysis that provides invaluable support for the general assessment of the date, provenance, and originality of the historical drawings as well as for the development of preventive conservation protocols.

Scheme 1. Schematic of sample preparation for in situ SERS measurements of historical drawings: (a) analytical sample constituted by ink coloured paper; (b) formation of nanopaste by centrifugation of Ag nanoparticle solutions; (c) loading of Ag nanopaste into a micro-syringe; (d) droplet deposition of Ag nanopaste on the analytical surface; (e) formation of SERS-active areas on the analytical surface; and (f) SERS measurement.

Fig. 1. (a) Optical image of the pink Tombow ABT 813 felt-tip pen line written on a commercial paper. Inset: an image of the entire line. (b) Optical image of the pink line on a paper with deposited Ag plasmonic paste. (c) SEM image of the pink-colored paper with deposited Ag plasmonic paste. (d) High-magnification SEM image of the paper-deposited Ag colloidal paste showing the morphology of individual particles.

Fig. 2. Raman (a) and SERS (b) spectra of lines written on paper with pink Tombow ABT 813 (pink line), green Caran D'Ache Fibralo series 100 (green line) and blue Caran D'Ache Fibralo series 100 (blue line).

Fig. 3. SERS spectrum of the blue Caran D'Ache felt-tip pen line (blue line) and Gaussian fits of identified dye components erioglaucine (black line), crystal violet (grey line), and rhodamine (pink line).

Fig. 4. Comparison between the historical and contemporary commercial felt-tip pens. SERS spectra of (a) pink Tombow ABT 813 (pink line) and pink Tombow 755 (black line); (b) green Caran D'Ache Fibralo 100 (green line) and green Caran D'Ache 185 (black line); and (c) blue Caran D'Ache Fibralo 100 (blue line) and blue Caran D'Ache 185.