# Novel Chitosan Derivatives and Their Multifaceted Biological Applications
## 3 Biological Applications of ChDs
### Antimicrobial Applications
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Novel Chitosan Derivative Type** & \begin{tabular}{c} **Biological** \\ **Application** \\ \end{tabular} & \begin{tabular}{c} **Bio-Functionality** \\ \end{tabular} & \
\begin{tabular}{c} **Reference** \\ \end{tabular} \\ \hline Amino-pyridine functionalized chitosan quaternary ammonium derivatives & Antioxidant & Stronger radical (DPPH and hydroxyl) scavenging activity than chitosan. & [110] \\ \hline Double quaternized chitosan derivatives & Antioxidant & Stronger radical (DPPH, hydroxyl, and superoxide) scavenging activity. & [111] \\ \hline Chitosan derivatives with & Antioxidant & Higher radical (DPPH, hydroxyl, and superoxide) scavenging activity than chitosan. & [112] \\ \hline Pyridylurea-functionalized chitosan derivatives & Antioxidant & Enhanced antioxidant activity than & \\ \hline \multirow{2}{*}{
\begin{tabular}{c} Pyridylurea-functionalized chitosan derivatives \\ \end{tabular} } & \multirow{2}{*}{Cytotoxicity} & N-pyridylurea chitosan derivatives. & [113] \\ \cline{3-3}  & & Low cytotoxicity to L929 cells. & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: _Cont._Moreover, they also showed low cytotoxicity on L929 cells [107]. A series of 6-O-imidazole-based quaternary ammonium ChDs, were successfully synthesized. Their antioxidant property was evaluated in vitro. Most of the quaternized ChDs with long length alkyl chains and primary amino groups showed \(>\)85% inhibition at 1.0 mg/mL against _Botrytis cinerea_. Another group reported an antimicrobial effect on crop-threatening bacteria by benzyl moiety-grafted or quaternized ChDs. Three novel thiosemicarbazone ChDs were obtained via condensation reaction of thiosemicarbazide chitosan with phenylaldehyde, o-hydroxyphenylaldehyde, and p-methoxyphenylaldehyde, respectively. Antifungal activity against the plant pathogenic fungi _Stemplyium solani_ weber, _Alternaria solani_, _Rhizoctonia solani_, and _Phomopsis asparagi_ Sacc. was demonstrated [102]. [PERSON] et al. [85] reported the synthesis of ChD-inspired heterocyclic anhydride and the antibacterial activity of the ChDs and their use against _S. aureus_ and _B. subtilis_. Three heteroaryl pyrazole derivatives, namely 1-phenyl-3-(thiophene-2-yl)-1H-pyrazole-4-carbaldehyde, 1-phenyl-3-(furan-2-yl)-1H-pyrazole-4-carbaldehyde and 1-phenyl-3-(pyridine-3-yl)-1H-pyrazole-4-carbaldehyde, were synthesized and reacted to form Schiff bases of chitosan. These were tested for their antibacterial effect against Gram-negative bacteria (_Escherichia coli_ and _Klebsiella pneumonia_), Gram-positive bacteria (_Staphylococcus aureus_ and _Streptococcus mutans_) and fungi (_Aspergillus fumigatus_ and _Candida albicans_). The derivatives also showed no cytotoxicity [81]. Novel water-soluble ChDs (TQCSPX) were synthesized; antifungal results indicated that the quaternized derivatives showed enhanced antifungal activity [101]. Four novel coumarin-functionalized ChDs 4a-4d were synthesized via condensation reactions of thiosemicarbazide chitosan with coumarin derivatives. Their antifungal activities against three kinds of phytopathogens: _Fusarium oxysporum f.sp. vasinfectum, Alternaria solani_ [PERSON] _and Fusarium multimforme_ was confirmed [100]. A novel O-quaternary ammonium N-acyl thiourea chitosan (QQCATUCS) with different degrees of substitution was synthesized by [PERSON] et al. [23]. The antimicrobial activities of the ChDs were assessed against _Staphylococcus aureus_, _Escherichia coli_, _Aspergillus niger_, _Pseudomonas aeruginosa_ and _Bacillus subtilis_. Diethyl dithiocarbamate chitosan (EDTCCS) was used against _Alternaria porri_, _Gloesporium thae sinensis [PERSON]_, and _Stemplyium solani_ [PERSON] as tested. Compared with native chitosan, EdTCCS showed a better inhibitory effect [23]. Three novel quaternary ChDs were synthesized by reacting chloracetylLithosan (CACS) with pyridine (PACS), 4-(5-chloro-2-hydroxybenzylideneamino)-pyridine (CHPACS), and 4-(5-bromo-2-hydroxybenzylideneamino)-pyridine (BHPACS).