Fluorescence Lifetime Phasor Analysis and Raman Spectroscopy of Pigmented Organic Binders and Coatings Used in Artworks

João Luis Lagarto,Alice Dal Fovo,Jana Striova,Diego Quintero Balbas,Sara Mattana,Riccardo Cicchi,Raffaella Fontana,Antonina Chaban

doi:10.3390/app12010179

João Luis Lagarto, Alice Dal Fovo + Show 6 more

Open Access

https://doi.org/10.3390/app12010179

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Abstract

Fluorescence analysis of materials used as binders and coatings in artworks is often hampered by numerous factors, leading to uncertainties in data interpretation. Fluorescence lifetime (FL) measurements enable improvement of the specificity with respect to steady-state measurements by resolving the decay dynamics of the fluorophore emissions. In this work, layers of natural resin, oil, and wax—in pure form, pigmented, in mixtures, and spread on different substrates—were analyzed using a compact, portable, fiber-based FL instrument. FL data were processed via the phasor method and integrated with Raman spectroscopy to obtain complementary chemical information on the different substances. It was observed that the τ-phase of the mixtures is affected by both the pigments and the dispersing medium, and that the presence of the metal substrate contributes to changes in the FL of mixtures. The results obtained with our portable FL system combined with Raman spectroscopy pave the way for a systematic study of a larger number of materials for future in situ applications on works of art.

Highlights

Fluorescence is a photoinduced luminescence (PL) emission from a molecule’s singlet excited state
UV-excited fluorescence has long been applied as a simple photographic technique for the non-invasive qualitative examination of artworks to assess the presence of intrinsically luminescent materials typically employed in cultural heritage (CH) [2,3,4,5]
Time-resolved photoluminescence (TRPL) spectroscopy and fluorescence lifetime (FL) spectroscopy and imaging are enable to enhance the specificity of fluorescence measurements by resolving the emission decay dynamics of fluorophores characterized by overlapping emission spectra but different fluorescence lifetimes

Summary

Introduction

Fluorescence is a photoinduced luminescence (PL) emission from a molecule’s singlet excited state. The presence of non-fluorescent pigments dispersed in protein- and oil-based organic binders can strongly affect the emissions of the latter [14,16,17], sometimes obliterating them with optical absorption or quenching effects and, producing false-negative results [12]. These factors may significantly hamper the application of fluorescence spectroscopy in CH studies, especially when the goal is the identification and mapping of coating materials. Similar decay kinetics are characteristic of the long-chain organic molecules that make up resins, waxes, binders, and glues, which are highly influenced by several factors, including pH, temperature, and molecular flexibility [15]

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