New insights in the enzymatic upgrade of lignin: from added value compounds to tunable nanoparticles

In this thesis the development of a strategy to perform the enzymatic modification of technical lignins in order to obtain both monomeric high added value compounds and high molecular weight oxidised lignins that can be used for the preparation of lignin-based nanomaterials is discussed. The enzyme used for the investigation was a transgenic laccase of bacterial origin characterised by a higher catalytic activity and a higher pH stability than the natural enzymes. After developing a protocol for the enzymatic modification of technical lignins (Kraft and Organosolv), the candidate focused on the mechanistic aspect of the laccase-catalysed oxidation of these substrates. The approach followed involved the analyses of the effects of laccases on both pristine technical lignins and their fractions. Depolymerisation forming high value products (e.g. vanillin) as well as polymerisation mechanisms were investigated. Then, the laccase-induced modification of Kraft lignin properties was exploited to prepare lignin nanoparticles with tailored size and hydrophobicity. Finally, a strategy to favour the deposition of Kraft lignin nanoparticles onto lignin-rich nanofibrillated cellulose via the use of laccase was developed. This methodology permitted to prepare films with improved mechanical and thermal properties with respect to the starting nanofibrillated cellulose.