Extraction of Lycopene from Tomato Peels Using Supercritical Carbon Dioxide and Conjugation of the Extracted Lycopene with TiO ₂ Nanoparticles

The tomato processing industry is one of the most widespread food manufacturing sectors globally, generating substantial amounts of residue, including tomato skins, peels, seeds, and vascular tissues. These residues still retain valuable bioactive compounds (e.g., carotenoids like lycopene), essential for food, pharmaceutical, and nutraceutical applications. Currently, traditional solvent extraction is the most common method for retrieving these compounds from tomato residue. However, this approach has notable disadvantages, including high solvent consumption and difficulties in utilizing leftover biomass. To address these issues, innovative technologies have introduced modifications to process configurations and techniques that alter or break down plant cells, significantly improving compound recovery. Supercritical fluid extraction offers an effective method for enhancing the value of tomato residues before disposal, as it is an environmentally friendly approach, particularly when carbon dioxide is used as the extraction solvent. The novelty of the present work is the specific optimization and integration of this green extraction technique with the subsequent conjugation of the extracted lycopene to TiO₂ nanoparticles (TiO₂NPs). This is the first report demonstrating the conjugation of supercritical CO₂-extracted lycopene with TiO₂NPs, effectively eliminating the use of organic-solvent-derived carotenoids traditionally used in such conjugates. The supercritical CO₂-extraction (scCO₂) was employed at various times (2, 3, and 4 h), temperatures (40-80 °C), and pressures (25, 30, and 35 MPa). Among the tested protocols, the treatment of tomato at 50 °C and 30 MPa for 2 h showed the highest yield based on the UV-vis and HPLC results. For the potential pharmaceutical benefits of these extracts, lycopene was directly conjugated with TiO₂NPs, using a mild and green approach (TiO₂NPs-lycopene) and characterized using different techniques, including HPLC, UV-vis, FTIR-ATR, FESEM-EDS, and DLS to assess the successful loading of 95.0 ± 2.1% of lycopene on the TiO₂NPs. The FESEM results exhibited the grain-like TiO₂NPs-lycopene particles with a 60-80 nm size distribution, indicating size-controlling effect of the lycopene on the TiO₂NPs during the conjugation process and the presence of the organic molecule layer on the surface. TiO₂NPs-lycopene represents a promising candidate for antimicrobial studies due to the potential synergic effect of TiO₂NPs and lycopene in a single nanoplatform. The results show the importance of scCO₂-based techniques, as valid alternatives to the conventional methods that exploit organic solvents, to prepare the TiO₂NPs-carotenoids conjugates. To the best of our knowledge, this is the first study to report the conjugation of TiO₂NPs with lycopene, offering a novel approach to enhance their antioxidant and photoprotective properties.