Do you remember “glow-in-the-dark” stickers? I loved the ones I had on my room’s ceiling when I was a kid, and I always asked how can they shine without any batteries! Everyone has seen a photoluminescent object, but have you ever asked why a pigment or dye “glows”? Photoluminescence is a property of some materials that emit photons (light) after the absorption of a given energy of radiation. The phenomena occur in different ways, including fluorescence and phosphorescence, displayed in Figure 1.
Figure 1. Scheme of photoluminescence phenomena. The absorbed radiation can produce more than one excited state depending on the energy of the radiation. The emission can follow two pathways: fluorescence when it goes from the excited state to the lower energy state. On the other hand, when an internal energy transfer occurs from the initial excited state to a second internal excited state (triplet in the image) and later the molecule goes back to the lower state by emitting light, it is called phosphorescence.
Photoluminescence occurs when a molecule absorbs a part of the radiation spectrum, which causes the molecule to enter an excited state. After removing the excitation source, the molecule then “relaxes” by reverting to the low energy state. This process produces fluorescence when the relaxation pathway goes directly from the excited state to the lower energy state. On the other hand, when an internal energy transfer occurs, the molecule goes from the excited state to a secondary excited state with a lower energy and then to the lower energy state. This process is called phosphorescence. The photoluminescence phenomena can be observed in different analytical techniques such as fluorescence spectroscopy and X-ray fluorescence (XRF, see also) depending on the energy of the excitation.
A 2010 paper in Accounts of Chemical Research by Romani, Clementi, Miliani, and Favaro presents a summary of the possible applications of fluorescence spectroscopy to the analysis of artwork. The authors present four different examples of analyzing artworks with a portable fluorometer, which is one of the many techniques used by MOLAB. The portable fluorometer consists of a xenon lamp as an excitation source and a monochromator for the selection of the excitation wavelength. The source is connected to an optical fiber, which directs the light to the surface of the object. Some filters and a detector collect the fluorescence signals from the object. This setup allows for fluorescence to be performed in a non-invasive way. It may also be used for imaging studies, giving information about the spatial distribution of analytes of interest.
Materials used in works of art—such as binders, pigments, and dyes—exhibit fluorescence. In order to use fluorescence spectroscopy to study artworks, it is necessary to previously characterize the materials present in the object to know the possible presence or absence of photoluminescence, the maximum absorbance of the objects, and the optimal excitation energy required for the experiment. For example, fluorescence spectroscopy is a good technique for studying the recipes used in the preparation of lake pigments or dyeing textiles. The use of different mordants produces a shift in the maximum emission of dyes, and in particular cases, such as the use of alizarin for dyeing wool in the absence of mordant, there is no fluorescence from the dye.
The cases presented in the paper show the potential of fluorescence studies for the accurate characterization of several materials. In the case of the mural paintings form Domus Aurea in Rome, they were able to identify Egyptian blue in a mixture with red anthraquinone dyes to obtain purple color. They have also analyzed tapestries designed by Raphael at the Vatican Museums, as displayed in Figure 2. They were able to identify the characteristic fluorescence of wool fibers at 450 nm. An emission peak at 624-630 nm in red-purple areas was attributed to the presence of orcein, the main constituent of a red-purple organic dye obtained from lichens. A weak band at 750 nm also indicated the presence of indigo, a blue organic pigment, in the blue areas. Orcein and indigo were also identified during the analysis of the Book of Kells, a medieval manuscript exhibited at the Trinity College Library of Dublin.
Figure 2. Left: MOLAB portable fluorimeter at work on the tapestry Earthquake in Filippi. In all the cases the excitation radiation was 350 nm and the fluorescence of wool fiber present a maximum emission at 450 nm. Right: Top, emission spectra (624-630 nm) compared to a sample of wool dyed with orcein. Middle, emission spectra in the blue area (750 nm), the black spectrum is the emission of the wrap. Bottom, emission spectra of green regions with two emission one centered at 500 nm and a weak one at 750 nm indicating a mixture of indigo and with some yellow dye.
Not only organic colorants produce fluorescence; inorganic semiconductor pigments such as zinc white and cadmium yellow can also exhibit fluorescence. In this case, the fluorescence emission is not produced by the relaxation of an excited state but by the transition of electrons from the valence band (bound to the atom) to the conduction band (where the electrons can freely move). Zinc white produces a characteristic emission in the ultraviolet. For the cadmium yellow, the emission is in the near infrared. The authors characterize these pigments used in Victory Boogie Woogie by Piet Mondrian.
Fluorescence is now a commonplace technique to study artists’ materials. Recent publications show the improvements of the technique and the continued applicability of fluorescence spectroscopy for the study of cultural heritage. For example, research by Nevin et al. explores the use of time-resolved fluorescence spectroscopy. Monitoring fluorescence in a time-resolved manner can offer useful information to better understand pigment degradation mechanisms and add complementary information about the excited states and relaxation pathways. This information can be correlated to the data obtained with other techniques such as chromatography, FTIR, or SERS.
All figures reproduced/adapted with permission from: “Romani, Aldo, Clementi, Catia, Miliani, Costanza, et al., Fluorescence Spectroscopy: A Powerful Technique for the Noninvasive Characterization of Artwork in Accounts of Chemical Research, 2010, American Chemical Society. Copyright 2019 American Chemical Society”