Sensing soluble molecules through SERS substrates in one-step procedure: Unrevealing the Meiji woodblock printing...

Ultrasensitive SERS substrates allowed us to detect complex mixtures of coloring components from Meiji Japanese woodblock prints (1868-1912). In museum settings, compositional analyses have limitations due to restrictions to sampling advised by conservators and curators for the adequate preservation of the objects. An additional layer of complexity is brought by the high heterogeneity of heritage materials, usually not resolved with commercial portable instruments. High-performance instruments for in situ analyses are seldom available in museums. Furthermore, the chambers of most instruments for morphological or chemical characterization accommodate small samples rather than large or medium-sized objects. The innovative sampling strategy herein proposed comprises the gentle touch-dry removal of small coloring molecules weakly bound to the surface of heritage objects, transferred through a silicone sampler to planar SERS substrates with selected solvents in a one-step procedure. The analytical protocol reduces the amount of sample necessary for reliable identification of color components down to nanograms. The selectivity of the solvents combined with the geometry of the planar SERS sensing devices produces reliable signals for molecular identification, with no need for incision or wetting of the printed material. Further, 3D Raman imaging allowed us to reach an unprecedented degree of molecular discrimination, advancing previously available minimally-invasive instrumental methods used in heritage science research. The validation with historical inks from Meiji woodblock prints led to the identification of soluble synthetic azo β-naphthols, barium sulfonic lakes, purple anilines, Prussian blue, glass arsenic sulfides and other traditional coloring media.