Application of ionic liquids and enzymes for the removal of proteinaceous layers from polychrome of works of art and evaluation of the cleaning effectiveness

Abstract

Dissertação Apresentada na Faculdade de Ciências e Tecnologia da Universidade Nova de Lisboa para obtenção do grau de Mestre em Ciências da Conservação, especialização em PinturaA novel use of ionic liquids as alternative solvents for enzymes in cleaning treatments for the removal of proteinaceous materials from painted or gilded surfaces is presented. The ionic liquids are potentially green solvents to be applied in restoration treatments being also called designer solvents, because of their peculiar properties which can be adjusted by selecting different cationanion combinations. Two ionic liquids were selected: IL1)1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4])and IL2) 1-ethyl-3-methylimidazolium ethylsulfate ([EMIM][EtSO4]). Formulations were prepared with these ionic liquids and two different proteases: one acid (pepsin) and one alkaline (from Aspergillus sojae). Additionally aqueous gel formulations were prepared with these enzymes for reference purpose. A third enzyme provided by the Bromatology Department at the Faculty of Pharmacy from the Porto University was tested only in gel formulation in order to assess its potential use in cleaning treatments. To understand the enzyme activity of these formulations and predict their ability as cleaning agents, analyses were performed with ultraviolet–visible (UV-Vis) spectroscopy and highperformance liquid chromatography (HPLC) prior cleaning; and with matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF-MS) after cleaning. These formulations were tested on mock-up samples prepared in accordance with documented and historical sources of artistic techniques of egg tempera and oil painting, and gilding. A non-invasive non-destructive multi-scale analytical protocol was carried out for cleaning effectiveness evaluation and surface characterization before and after treatment. Different surface analytical techniques were adopted to this purpose: stereomicroscopy (SM), optical microscopy (OM) with visible and fluorescence light, atomic force microscopy (AFM), scanning electron microscopy (SEM) and electron dispersive spectroscopy (EDS) and colorimetry (CIE L*a*b* system). The surface analytical protocol proved to be adequate, not only, for monitoring the cleaning process but also for complete characterization of the surface, before and after treatment, including information on the presence of residues and possible surface deterioration. It was also proved that the formulations of enzymes combined with ILs can be used successfully for the removal of proteinaceous material as alternatives to gel formulations. More studies should be conducted to determine the most suitable IL or group of ILs, the main concern should focus on improving aspects such as compatibility with other surface materials, and possible long-term effects of residues after cleaning