Time-resolved fluorescence and anisotropy studies of red pigments present in acrylic formulations

Red organic pigments are frequently found in modern paintings and murals based on acrylic formulations. The detection of these molecules is valuable to guide investigations about cultural heritage and for restoration efforts. These studies usually employ microscopic amounts of materials that are obtained through swabbing or micro-sampling. In this contribution we describe the time-resolved emission properties of a set of red pigments with the objective of characterizing their excited state properties and developing strategies to identify their presence through fluorescence lifetime measurements, even in concentrations of the order of 10−9 M. As we show, using different solvent systems, the emission decay measurements can be setup to be a robust identification technique that avoids problems with evaporation or partial solubility. We also show that the sensitivity of these determinations is improved using a confocal type of setup with a high numerical aperture lens to ensure a high photon capture. This setup also allows for the samples to be prepared in microliter level volumes which implies a relatively high concentration of the pigments. In addition, we show that the lifetime measurements can be complemented with determinations of the emission anisotropy decays with the same experimental setup, which provides an additional property specific to each pigment, permitting an accurate differentiation between fluorophores.