**4. Lignin applications: focus on nanoparticles production**

Lignin is an exciting polymer due to its nontoxicity and bio-based renewable nature, making it a sustainable feedstock that does not compete with food chain production [58], and recent technologies to recover lignin and its conversion into valuable compounds have been reviewed [59]. However, lignin applications on a large scale present some limitations due to lignin's poor solubility in water and complex macromolecule structure [35].

However, many efforts have been made; for example, the *Borregaard company* commercializes tailor-made lignin that can be used in wide-ranging industries, as a binding, dispersing agent, but also as an emulsion stabilizer (https://www.borregaa rd.com/product-areas/lignin-biopolymers/). The *Bloom company* uses aldehydeassisted fractionation to obtain a stabilized lignin, delivering lignin as a biopolymer with known characteristics that can be used for the production of nutraceuticals, food additives, or even cosmetics (https://www.bloombiorenewables.com/).

Despite these good examples of lignin valorization, in both cases, the lignin is tailor-made, but what about the other lignins produced on a laboratory scale but with high diversity? They have not yet reached the market, in particular because the lignin is obtained in low yields, with structural diversity, and some processes also use dangerous solvents. So, the target of many researchers has been to modify the lignin, or to develop lignin nanoparticles (LNPs), that is, to obtain lignin in the form of aqueous nanoparticles with uniform size and shape, showing better characteristics

such as a higher surface area and stability in an aqueous medium, that are non-toxic (in reasonable concentrations), biodegradable (either by bacteria but also fungi), and present bioactive properties, which can replace the commercial lignin [60].

Furthermore, LNP has functional groups that can be chemically modified, increasing their applicability, and can be applied in a wide range of applications [61–92]: (i) encapsulation with biocides or anticancer drugs [62–68], (ii) incorporated in coatings for wood UV protection [69], or in polymeric matrices to enhance mechanical properties (e.g., strength and toughness), barrier properties and thermal stability [70, 71], and (iii) in the agriculture field [72]. **Table 7** presents some LNP applications based on the starting technical lignin.

Still, some limitations have to be overcome, and LNP application at the industrial level has not yet been achieved due to diverse factors. The *first* one is about lignin's variability in nature that is quite heterogeneous (as already mentioned in this chapter); therefore, its isolation will produce a mixture of products, apart from being a challenging task and less economically feasible to purify all the different compounds for further applications [45, 71]. The *second* point is related to the lignin fractionation methodologies that yield complex condensed lignin, which limits its high-value applicability [71, 94], difficulties in attaining lignin in high yield and with good chemical and physical properties (e.g., molecular weight distribution, solubility, reactivity and number of functional groups), and with quality (in relation to the inter-units linkages, condensed lignin is not attractive) [43, 95]. The *third* point is associated with LNP production and their adequate stabilization for different applications [96]. This last point will be developed in the following paragraphs.


#### **Table 7.**

*Possible applications of the different nanoparticles produced from technical lignins. Adapted from Ekielski and Mishra [38].*
