Opponent: Professor Kishore Paknikar, Indian Institute of Technology, Bombay
Supervisor: Professor Joydeep Dutta, Tillämpad fysik
Marine resources play a crucial role in supporting the blue economy and addressing global challenges to achieve some of the UN’s Sustainable Development Goals (SDGs). However, the marine ecosystem is adversely affected by plastic pollution, biofouling, and corrosion; thus, there is an urgent need for sustainable solutions. Chitosan, derived from chitin, the second most abundant biopolymer on Earth after cellulose, offers promising potential due to its unique biodegradable, water-soluble, antimicrobial, and film-forming properties. This thesis summarizes the sustainable extraction of chitosan, chemical modification of chitosan derivatives, and the application of chitosan coatings for antimicrobial, UV-filtering, corrosion prevention, and marine antifouling. In the first part of the thesis, extraction of chitosan from food waste was carried out using green solvents as a sustainable solution for circular economy. Further, the antimicrobial activity of chitosan was enhanced by developing nanocomposites with core-shell ZnO@SnOx particles, which demonstrated a significant potential in food packaging application. To achieve even greater antimicrobial efficacy and UV-blocking capabilities, chitosan was chemically grafted with benzophenone-3 (BP-3), a plant extract known for its UV filtering properties. The antimicrobial activity of the obtained chitosan-BP-3 coatings was evaluated against both Gram-negative and Gram-positive bacteria, and the hydroxyl group on benzophenone-3 is found playing a crucial role in the antimicrobial effectiveness. Continuous irradiation tests showed that the coating possesses long-term UV-blocking effect. Biofouling is a process about the settlement of micro- and macro-organisms on any substance immersed in water, leading to significant economic losses due to increased drag, material degradation, and maintenance costs in marine and industrial applications. To address this challenge, polyethylene glycol (PEG)-grafted chitosan coatings with varied chain length of PEG were synthesized, aiming to reduce microbial and diatom adhesion. This was achieved by modulating the brush effect and hydrophilicity of the surface coatings. Moreover, ZnO-Ag nanoparticles embedded in the coating has shown playing a bactericidal role under light or in the dark. On the other hand, it is urgent to find alternatives to the prevalent but environmentally unfriendly epoxy resin in anticorrosive coatings and prevent the embedded anti-corrosion agents from discharging into water. Multifunctional chitosan coatings have emerged as a promising solution for marine corrosion prevention and antifouling applications. For the first time, corrosion inhibitor, 2-mercaptobenzotriazole (MBT), was chemically linked to chitosan. In addition, new anti-corrosion mechanism was proposed based on the chitosan-MBT coatings. The corrosion resistance of the chitosan-MBT coating was improved by 40 times compared with the chitosan and MBT mixture coating. This bifunctional chitosan-MBT coating had also exhibited superior antifouling effect against settlement of mussels for 48 h. The work presented in this thesis highlights the versatility and potential of chitosan as an emerging biomaterial for sustainable development in the fields of antimicrobial, food packaging, marine antifouling, and corrosion prevention.