The shining brightness of daylight fluorescent pigments: Raman and SERS study of a modern class of painting materials

Raman and surface enhanced Raman spectroscopy (SERS) are established techniques allowing for fast and sensitive identification of dyes and pigments. To date, Raman has only limitedly been applied to the identification of daylight fluorescent pigments, despite this modern class of materials has been adopted by several significant artists (e.g. Frank Stella, Peter Halley, Andy Warhol, and Keith Haring). In this study, seven commercial daylight fluorescent pigments from Radiant were investigated by Raman and SERS spectroscopy. A new type of silver colloid, consisting mainly of rod-like silver nanostructures guaranteeing a high SERS enhancement, has been employed here for the first time in cultural heritage studies. As the number of fluorescent dyes suitable for the manufacturing of daylight fluorescent pigments is limited, these materials are often constituted by a mixture of dyes. Since SERS is not able to physically separate multiple dyes in mixtures, to reveal the complex composition of these materials the coupling of SERS with a separation technique proved to be mandatory. Thin-layer chromatography, being a cost-effective and fast procedure, was chosen for this purpose. The identification of each component was made possible by the comparison of SERS spectra from standard fluorescent dyes, thus increasing the available SERS database for modern materials. Lastly, the analytical protocol was applied to a modern artwork called "Rosso verde nero", made in 1968 by the Italian artist Carla Accardi, who used to paint on plastic surfaces with fluorescent colours.