HyBridGel: a bridge between in-situ and laboratory analyses. A new minimally invasive procedure to identify organic colourants in complex matrices

Dealing with complex matrices can present an imposing challenge when it comes to identifying organic components. For scientists working in the field of cultural heritage, this challenge becomes even more complex, as they handle aged and deteriorated objects, often unique, and are required to address the most demanding questions while adhering to the principles of minimal or non-invasiveness. As awareness of the potential of Surface Enhanced Raman Spectroscopy (SERS) for organic analysis has grown, scientists have become increasingly creative in developing micro-sampling strategies and SERS supports to leverage the technique's ultra sensitivity without posing any risk to the integrity of artifacts. In the context of this Ph.D. thesis, a recent in-situ extraction procedure, which exploits hydrogel’s ability to interact with water solution, was taken and pushed beyond its limits. The direct gel-supported liquid extraction method was employed for the extraction of one of the most elusive organic components, natural dyes, in the worst-case scenario: aged paint layers. The most ambitious goal was to adapt the extraction process into a comprehensive workflow capable of combining both SERS and mass spectrometry to conduct fingerprint investigations of the collected dyes. The gel-supported liquid extraction method was adapted to Arabic gum, tempera, and oil-based paint mock-up containing pigments derived from madder, brazilwood, and indigo. The results demonstrate that the procedure's visible impact and extraction efficiency vary across different paint systems, primarily contingent on the type of binder employed. However, numerous unpredictable factors come into play in real-case scenarios. To address this challenge, the most effective extraction strategies were chosen for experiments involving reduced-scale gels, applied using a Pasteur pipette tip. Once it was confirmed that the gel-supported liquid extraction could successfully extract dyes from various types of aged paint systems without causing any detrimental effects, it was subsequently applied to real case studies, encompassing historical paints and textiles, too. The procedure employed for printing inks from the early 19th century, hindered in non-invasive characterization due to fluorescence, has allowed the identification of two synthetic dyes. Utilizing gels, the extraction of natural dyes derived from madder and indigo was achieved from linen fragments found in Pharaoh Tutankhamun's tomb. Additionally, Kermes and flavonoid dyes were extracted from Hispano-Moresque textiles and identified using both SERS and mass spectrometry.