**5. Conclusions**

This chapter includes an overview on lignin and lignin nanoparticles. Lignin is a complex natural polymer with characteristics that contribute to the development and growth of plants. Lignin provides rigidity, hydrophobicity, and protection against biotic agents and plays an important role in the plant's adaptability to the environment. Plants can produce different monomers as a response to stress (e.g., drought, high or low temperature, and light).

As a natural polyphenol polymer, lignin presents high variability associated with the diversity of plants, which creates great challenges for its recovery and valorization.

There are several methods to deconstruct the biomass and obtain the so-called *technical lignin*. These approaches present some advantages, the major one being to obtain a pulp (a valuable product). However, from the lignin point of view, there are several disadvantages: lignin is highly condensed, difficult to recover and process for subsequent valorization. Aiming to change this, a new perspective has been used, the *lignin-first* approach. Here, lignin is obtained first to prevent adverse reactions and be easier to valorize under the scope of the biorefinery. The *lignin-first* approach includes different methods also due to lignin's natural diversity, and it is still hard to attain lignin both in high yield and with good characteristics for different purposes.

The production of lignin nanoparticles (LNPs) started, at the beginning of 2000, because the nano-size improves the lignin characteristics (e.g., increased antioxidant activity). LNPs can be produced by different methods, where a diverse range of sizes and shapes can be obtained and tested for multiple applications. However, there are some aspects that still have to be improved so that LNP can be scaled up (e.g., costs of lignin isolation and LNP production, use of more green solvents).

Undoubtedly, lignin is an unfinished theme for research due to its chemical characteristics and endless potential applications.
