Probing paper ageing by label-free fluorescence lifetime microscopy.

Paper ageing is a complex and irreversible process that critically affects materials ranging from packaging to cultural heritage artefacts. While both invasive and non-invasive analytical techniques exist to assess the deterioration of paper, many approaches still present limitations in terms of sensitivity, spatial resolution, or applicability to historical artefacts. Here, we present a fully optical, non-invasive method to monitor oxidation in paper using label-free Fluorescence Lifetime Imaging Microscopy (FLIM). We show that the intrinsic fluorescence lifetime of paper changes with oxidation and that these shifts correlate strongly with FTIR indicators of ageing. Validation was performed on both modern and historical samples subjected to controlled oxidation. This approach represents the first step toward a new diagnostic methodology for monitoring the degradation of paper-based materials in sensitive contexts such as heritage science.