Abstract
In historical paintings, the detection of low amounts of pigments and dyes by Raman spectroscopy can sometimes be challenging, in particular for fluorescent dyes. This issue can be overcome by using SERS (surface-enhanced Raman spectroscopy) which takes advantage of the properties of nanostructured metal surfaces to quench fluorescence and enhance Raman signals. In this work, silver nanostars (AgNSs) are applied for the first time to real art samples, in particular to painting cross-sections, exploiting their effective SERS properties for pigment identification. The case study is the Madonna della Misericordia of the National Gallery of Parma (Italy). Cross-sections were analyzed at first by optical microscopy, SEM-EDS, and micro-Raman spectroscopy. Unfortunately, in some cross-sections, the application of conventional Raman spectroscopy was hindered by an intense background fluorescence. Therefore, AgNSs were deposited and used as SERS-active agent. The experimentation was successful, allowing us to identify a modern dye, namely copper phthalocyanine. This result, together with the detection of other modern pigments (titanium white) and expert visual examination, allowed to reconstruct the painting history, postdating its realization from the 15th century (according to the Gallery inventory) to 19th century with a heavy role of recent (middle 20th century) restoration interventions.
Highlights
Nowadays the scientific approach plays a key role in the conservation-restoration of cultural heritage, which is necessary to sort out conservation issues and to answer diagnostic inquiries
In historical paintings, the detection of low amounts of pigments and dyes by Raman spectroscopy can sometimes be challenging, in particular for fluorescent dyes. This issue can be overcome by using SERS which takes advantage of the properties of nanostructured metal surfaces to quench fluorescence and enhance Raman signals
In some cross-sections, the application of conventional Raman spectroscopy was hindered by an intense background fluorescence
Summary
Nowadays the scientific approach plays a key role in the conservation-restoration of cultural heritage, which is necessary to sort out conservation issues and to answer diagnostic inquiries. The analysis of the composition of a work of art follows a precise analytical protocol which starts from the visual analysis (optical microscopy in visible and UV light, IR reflectography), it involves the use of elemental and molecular non-destructive techniques (X-rays fluorescence spectroscopy), scanning electron microscopy (SEM), attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy, Raman spectroscopy, up to eventually resorting to micro-destructive ones (gas chromatography and mass spectrometry, high performance liquid chromatography) Since they are composite materials, works of art must be studied in their three-dimensionality in order to take into account all the layers underneath the superficial one. Drops of colloidal solutions were deposited on holey-carbon copper grids and let dry at room temperature before analysis
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