**3. Chitosan-Based Nanoparticles**

Chitosan is a polysaccharide, with 2-deoxy-2-(acetylamino) glucose units bonded by 1,4-glycosidic linkages; it is prepared by partial N-deacetylation of chitin (Figure 3). Chitin has good mechanical strength, is biocompatible, bioactive and biodegradable, but has limited utilization due to low solubility. Therefore, it is converted into chitosan by deacetylation in the presence of hydroxide at high temperatures [38]. Chitin is extracted from marine organisms such as lobsters and molluscs, and from crab shells, insects, yeast, and fungi. A long polymeric chain is composed of glycosidic linkages. Chitosan is insoluble in sulfuric acid and phosphoric acid but soluble in organic solutions with a pH lower than 6.5, such as acetic acid, citric acid, and tartaric acid. Due to its solubility, chitosan is present in films, hydrogels, pastes, nanoparticles, and nanofilms [39,40]. Chitosan has a greater degree of deacetylation and molecular weight; the variation in the size of a particle and aggregation is dependent upon the degree of deacetylation and molecular weight [3]. Recently, chitosan and its derivatives have been considered as the best vehicle in the pharmaceutical field due to their biocompatibility, and their non-carcinogenic, non-toxic, antibacterial properties. Chitosan offers a large range of options for industries and scientists for the generation of modified and novel drug delivery systems. Chitosan acts as an auxiliary agent in the therapeutic application for tissue engineering, wound dressing, and sliming. Protonation of the amino group in an acidic medium results in cation formation. Chitosan exhibits unique behavior because of its cationic nature [38,41]. Modification of chitosan and the stability of drugs delivered using chitosan decrease the adverse effects of diseases and increase the biocompatibility of drugs for various diseases [42].

**Figure 3.** Structure of chitosan.

Chitosan nanoparticles widely act as a potential carrier for therapeutic applications. The cationic nature of chitosan is useful for the development of a drug delivery system. One of the major benefits of chitosan nanoparticles is their rapid uptake across the cell membrane due to the presence of amine groups. Complexation of chitosan with anionic charged polymers results in an interesting gelation property [43]. Methods generally applied for the preparation of chitosan nanoparticles include the microemulsion method, ionic gelation, and micro-emulsion solvent diffusion method [3,38]. In the microemulsion method, chitosan nanoparticles are prepared, and micellar droplets are cross-linked in the presence of glutaraldehyde. Surfactant (hexane mixture) is added to the chitosan acetic/glutaraldehyde solution. Surfactant helps in the formation of chitosan nanoparticles smaller than 100 nm under continuous stirring to complete cross-linking between the amine group of chitosan with glutaraldehyde. By applying a low pressure, the excess organic solvent is removed. The excess organic solvent used, the complexity of the washing process, and the amount of time this method takes is its main drawback [3,44]. Chitosan nanoparticles prepared using the inotropic gelation method depend upon electrostatic interaction between the amino group of chitosan and the polyanions group such as triphosphate in an aqueous medium [45–48]. Firstly, the chitosan is dissolved in acetic acid in the presence of stabilizers such as poloxamer. So, nanoparticles are formed under continuous stirring. The size of the nanoparticle depends upon the ratio of chitosan-to-stabilizer [3]. The emulsion solvent diffusion method is based on the addition of an organic phase into a chitosan solution containing a stabilizer at a higher temperature and pressure with constant stirring. Nanoparticles are formed by the addition of water to an organic solvent. This method of nanoparticle formation is better for hydrophobic-based drug delivery systems. This method also has some drawbacks, such as greater shear force and harsh process conditions [49]. Chitosan is one of the most extensively studied biopolymers because chitosan possesses some ideal properties for polymeric carriers for nanoparticles. The properties are given in Table 1.

Current studies have shown a massive variation of applications and novel modifications of chitosan, thereby increasing its overall value. Drug encapsulation using chitosan with tripolyphosphate was first reported by Bodmeier et al. [50,51] who used an inotropic gelation process, which results in the formation of chitosan nanoparticles. Chitosan (100–150 nm) nanoparticles were loaded with Rosuvastatin drug encapsulated in polyvinyl alcohol (PVA)/Sodium alginate (SA) core using an ionic gelation method. The release behavior of the drug was observed within 24 h, and chitosan particles delivered significant results in the drug release process. This biocompatible and biodegradable drug delivery system was a suitable choice for replacing the various doses of the drug Rosuvastatin [51]. In 1998, Alonso et al. synthesized the chitosan nanoparticle with the development of inter-and intra-molecular interaction between the chitosan amino group and tripolyphosphate (TPP). This method produced a high yield of chitosan nanoparticles [43]. Thandapani Gomathi et al., in 2017, synthesized chitosan nanoparticles with TPP-loaded drugs with letrozole (LTZ) for cancer treatment. Characterization techniques such as SEM, TEM, FTIR, XRD, and TGA results showed the optimum results. Additionally, the prepared formulation was evaluated in vitro to determine its biodegradability, hemocompatibility, and serum stability. The preliminary studies supported the assertion that chitosan nanoparticle synthesis had

biocompatibility and hemocompatible applications, and could act as an essential pharmaceutical excipient for letrozole [52]. Synthesis techniques for chitosan nanoparticles are given in Figure 4.

**Table 1.** Some ideal properties of polymeric nanocarriers [3].
