## 2 Chitosan Modifications
### Formation of Nanoparticles
A stoichiometric PE complex contains equal amounts of each opposite charge. In the case of CS, various PE complexes can be formed when the positively charged CS surface interacts with anionic biopolymers such as sodium alginate (SA), pectin, carboxymethylcellulose, chondroitin sulfate, dextran sulfate, etc. [80]. (iii) Drying:  
Spray-drying and supercritical drying are often used for the production of CS particles. These methods are relatively fast, simple and reproducible, allowing for highly stable CS particles of various sizes to be obtained [81]. Spray-drying is a comprehensive, convenient, one-step method for producing CS micro-/NPs loaded with heat-sensitive substances  
Figure 8: CS crosslinked with TPP via ionotropic gelation [71]. such as pigments, medicines, essential oils, flavorings and proteins [82]. CS particles produced while using spray-drying have a smooth surface, spherical shape and narrow size distribution; they also demonstrate the high stability of an immobilized drug [83]. The size of the resulting particles is affected by the rate of the sprayed flow, the degree of CS crosslinking and the air flow rate [84]. 4. Emulsification: CS NPs can be obtained using nanoemulsion. Emulsification processes usually require high mechanical energy to produce small droplets. The decrease in energy need is possible while using water-oil mixture with the addition of surfactants [85]. First, the dispersion of CS is prepared in a solution of glacial acetic. Then, a portion of hydrophobic solvent (e.g., cyclohexane, cyclohexanol, soybean oil, etc.) is added to the CS solution under continuous stirring. The resulting mixture is treated with a surfactant, e.g., Triton X-100 or Tween, at stirring until the mixture becomes translucent, indicating the formation of an emulsion containing NPs [86]. Another option is to premix the surfactant with one of the solutions. The use of an ultrasound and an increase in reaction temperature may contribute to the process of NP/nanocapsule formation. Gel-forming agents such as chlorides, TPP or polycations can be also added to enhance CS NP formation [87]. NPs or nanocapsules formed using the emulsification method have high drug-loading efficiency and high bioavailability [88]. However, the mandatory use of strong crosslinking agents causes the issue of their incomplete removal [89]. 5. Self-assembly: Self-assembled NPs can be formed in aqueous media from amphiphilic polymers [90]. Though CS is not an amphiphilic biopolymer, self-assembled CS NPs can be obtained through the structural modification of CS, allowing them to bind compounds such as fatty acids, phthaloyls, polyesters, cholesterol, etc., which causes CS' self-aggregation [91; 92; 93]. Quaternary CS derivatives are often used to produce self-assembled CS NPs [94] used as carriers for hydrophobic substances. According to the most recent data, there are four main types of CS-containing NP structures (Figure 9) [95; 96]. Thus, CS has many advantages for producing materials for adsorption and catalysis. However, the goal-oriented design and synthesis of chemically modified CS derivatives is a complex task. Depending on the type of functional moiety attached to the carbohydrate and also on the type of inorganic component, new adsorbents and catalysts with improved properties can be obtained, which requires further systematic effort.