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Fig. 12 Simpli fi ed schematic of the production of silver-doped lignin nanoparticles. 129
coating for membranes, improving the treatment of oily wastewater. 139 Gu et al. used lignosulfonates and poly- ethyleneimine on a polysulfone membrane, which successfully repelled the adsorption of proteins. 15 While the preparation is straight-forward, the long-term stability of such coatings also must be demonstrated. Both the lignin and (poly-)cation were water-soluble, so the coating would be pointless if washed away with the retentate. 3.3.4. Packaging applications. Packaging applications can bene t from lignin-containing surfaces in various regards. Improvements in the water-resistance, water and oxygen barrier, and mechanical strength of cellulose-based substrates have been reported. 35,107,108,140 A more detailed summary of these materials is given in Section 3.3.1. The lignin may also serve as an oxygen scavenger. To implement this, several authors have formulated coatings that include both lignin and laccase enzyme. 113 – 115 At last, the antibacterial and UV-shielding prop- erties of lignin have been mentioned as bene cial contribu- tors. 83,107,110 While the studies demonstrate feasibility on a technological level, there are other factors that must be considered as well. Long-term stability and migration of the coatings is rarely addressed, despite this being a crucial parameter in food packaging. In other words, one must be sure that no detrimental substances are transferred to the food. Some foods release both water and fat, for which lignin is in theory a good match, as it is soluble in neither. Packaging of non-foods generally poses less harsh requirements. The requirements on price per volume are greater; however, the pricing of technical lignin should be competitive. 3.3.5. UV-protection. The polyaromatic backbone of lignin provides extended absorption at sub-visible wavelengths of light. UV-shielding applications hence draw on one of the intrinsic properties of lignin. One example would be the development of natural sunscreens via hydroxylation of tita- nium oxide particles, during which lignosulfonates were added. 141 Unsurprisingly, it was concluded that the lignin enhanced the UV-blocking e ff ect of the titanium oxide particles. Another publication explored sunscreens, where nanoparticle- size lignin was added to commercial lotion formulations. 142
and bio-adhesives, where the colloidal lignin acted as the scaf- fold intended for the synthesis of bio-compatible particles. However, no assessment of biocompatibility of the generated complexes was performed. Hence, the question remains whether this approach may be suitable for biomedical applications. According to Dominguez-Robles, there are various addi- tional biomedical applications, in which lignin could be promising, e.g. , as hydrogels, nanoparticles and nanotubes, for wound healing and tissue engineering. 135 The interest in lignin for biomedical applications was also emphasized by the increasing amount of publications related to lignin applied as a functional material for tissue engineering, drug delivery and pharmaceutical use. 135,136 However, the reported studies on lignin for biomedical applications is still limited and various challenges will need to be overcome to advance in this area. These are especially regarding relevant assessment of lignins toxicological pro le and biocompatibility. Not to mention the large variability of lignins when it comes to the source of lignin, the fractionation processes and posterior modi cation, which may a ff ect the chemical structure, homogeneity, and purity. 3.3.3. Wastewater treatment. Di ff erent researchers have formulated lignin-based materials, which were designed for the puri cation of dye containing wastewater. 16,102,137 Suitability for such applications is principally derived from the chemical similarities, which exist between lignin and many dyes, i.e. , an abundance of heteroatoms and aromatic moieties. Such adsorbents can be produced via deposition on silica gel, 102 coating onto carbon particles, 16 carbonization, 43 sorption and co-precipitation. 138 Development of such materials is generally positive, as it draws on the unique composition of lignin. A di ff erent technological approach within the same area would be membranes. Layer-by-layer assembly is a frequently used technique, in which polyanionic lignosulfonates or sulfonated Kra lignin are combined with a polycation. Multiple bilayers are made by stepwise application of the poly- electrolytes, e.g. , by immersion in a solution of polyanion rst, followed by drying and immersion in a solution of polycation. This approach was used by Shamaei et al. as an antifouling
12542 | RSCAdv. , 2023, 13 , 12529 – 12553
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