# Novel Chitosan Derivatives and Their Multifaceted Biological Applications
## 3 Biological Applications of ChDs
### Antimicrobial Applications  
Eight novel ChDs bearing urea groups were synthesized, and their antioxidant activities were explored. In vitro fungicidal activity of these derivatives was further tested against _Phomopsis asparagus_, _Fusarium oxysporum_ f. sp. niveum, _Botrytis cinerea_ and _F. oxysporum_ f. sp. _cucumebrium_. The results showed that most of the derivatives exhibited significant control at concentrations of 1.0 mg mL\({}^{-1}\) and showed low cytotoxicity [106]. Two antimicrobial phenolic chitosan Schiff bases (I) and (II) were synthesized and the antimicrobial activities of Schiff base (I) were significantly higher than Schiff base (II). High concentrations of Schiff base (I) inhibited 99% of Gram-positive bacteria and Schiff base (II) led to 82% inhibition. The cytotoxicity of these derivatives was tested against fibroblast cells [70]. New ChDs bearing guanidinium, N-guanidinium chitosan acetate and N-guanidinium chitosan chloride were synthesized and found to show high antimicrobial activity [75]. _Botrytis cinerea_, _Phytophthora capsaicin_ and _Fusarium solani_ are notorious plant pathogenic fungi that lead to huge losses of crops worldwide. Novel water-soluble functional ChDs were synthesized and tested against the three fungi and the cytotoxicity were estimated in vitro; the ChDs exhibited better antifungal activities and water solubility than chitosan and better biocompatibility [98]. In another study, authors synthesized a novel cationic ChD possessing 1,2,3-triazolium and pyridinium groups via cuprous-catalyzed azide-alkyne cycloaddition (CuAAC) and methylation. The antifungal efficiencies and their biocompatibility were confirmed [99]. [PERSON] et al. [123] investigated the effect of chitosan on SARS-CoV-2. \(\beta\)-ChDs were screened against the HepG2 and MCF-7 (breast) cancer cell lines and found to be effective. They also investigated the ChDs (1a-1j) docking against SARS coronavirus via in silico docking analysis. The results showed this to be a promising alternative antiviral agent for SARS-CoV2. Alkyl groups were introduced into amine groups of chitosan via Schiff's base intermediates to obtain N-alkyl ChDs. The antibacterial activity against S. aureus was explored and was found to increase with the increase in the chain length of the alkyl substituent [125]. Novel N,O-acyl chitosan (NOAC) derivatives were synthesized and their fungicidal activity against _Botrytis cinerea_ and the rice leaf blast fungus _Pyricularia oryzae_ was assessed [88]. CS Schiff bases (CSSBs) (CS-P1, CS-P2, and CS-P3) were synthesized and their antimicrobial activity and antifungal activity was confirmed [82]. Authors have also reported a study using chitosan acylation with linoleic (LA) and dilinoleic acid (DLA) for fabrication of polymer films and further assessment of antimicrobial properties of the polymer films against _E. coli_ and the fungi _Candida albicans_[60]. Several novel ChDs bearing benzenoid/heterocyclic moieties displayed significant enhancement in superoxide-radical scavenging activity and DPPH radical scavenging activity and antifungal activity against plant pathogens, _Colletotrichum lagenarium_ and _Phomopsis asparagi_.