# Fluorescent Composite Cotton Fabric Modified with Crosslinked Chitosan for Theranostic Applications
## 3 Results and Discussion  
### Synthesis and Characterization of Ch-Ni  
The modification of chitosan with 1,8-naphthalimide was carried out in a two-step synthetic route. Initially, 4-nitro-1,8-naphthalic anhydride interacts with the primary amino groups of chitosan at 70 \({}^{\circ}\)C through its anhydride group, forming a strong covalent bond. The nitro group has electron-accepting properties, and, for this reason, in the structure of 4-nitro-1,8-naphthalimide, a donor-acceptor interaction cannot take place, which would cause the polarization of the molecule and the appearance of color and fluorescent emission [31]. For this reason, the nitro group was nucleophilically substituted with an \(N\),\(N\)-dimethylamino group by adding dimethylamine to the reaction mixture at a low temperature for 24 h (Scheme 1). In this, the nucleophilic substitution of the nitro group is favored by both the electron-accepting properties of the carbonyl group of 1,8-naphthalimide and DMF as a solvent, and the reaction proceeds at low temperature [32]. The chemical structure of the modified chitosan was confirmed by \({}^{1}\)H-NMR and IR spectroscopy. \({}^{1}\)H-NMR spectroscopy was used to prove the structure of the chitosan modified with 4-dimethylamino-1,8-naphthalimide (Figure 1). The characteristic signals of the chitosan, such as H7 at 2.06 ppm (the protons from CH\({}_{3}\) of the NHCOCH\({}_{3}\) group), H2 at 3.20 ppm, H3-6 in the range 3.48-4.03 ppm (the overlapping signals at the non-anomeric carbon atoms from in the glucopyranose ring), and H1 at 4.59 ppm (from the anomeric proton), were recorded in the proton NMR spectrum of the modified chitosan Ch-NI. Since the chitosan used has a 90% deacetylation degree of chitin, the signal at 2.77 ppm refers to the H2\({}^{\prime\prime}\) proton, which is offset from the signal at H2. The signals from the structure of the 4-dimethylamino-1,8-naphthalimide fragment are observed in the spectrum also. The characteristic signals for the aromatic protons from the naphthalene nucleus of 1,8-naphthalimide are located in the range of 7.27 \(\div\) 8.66 ppm. The protons from -N(CH\({}_{3}\))\({}_{2}\) are appeared at 3.02 ppm. The signal at 2.86 ppm refers to the H2\({}^{\prime}\) proton and differs from that at H2 due to a polarization change after covalent coupling of the 1,8-naphthalimide fragment to the glucosamide structure. Figure 2 presents the IR spectra of the modified chitosan with 4-dimethylamino-1,8-naphthalimide Ch-NI in the region from 4000 cm\({}^{-1}\) to 400 cm\({}^{-1}\). For comparison, the original unmodified commercial product chitosan was used. Due to the small amount of 1,8-naphthalimide fragments attached to the chitosan molecule, its principal characteristic peaks overlap with those of chitosan. An increase in the intensity of the bands of the modified chitosan compared to the initial one is observed, probably due to the presence of 1,8-naphthalimides and a change in the polarization of the monosaccharide nuclei to which they are attached. The absorption band at 3220-3336 cm\({}^{-1}\) refers to N-H and O-H stretching due to possible intramolecular hydrogen bonds in the chitosan structure. Absorption bands at around 2919 cm\({}^{-1}\) and 2878 cm\({}^{-1}\) are due to C-H symmetric and asymmetric stretching of the monosaccharides ring. The carbonylimide groups (C=O) of the 1,8-naphthalimide structure have two characteristic vibrational bands caused byasymmetric and symmetric vibration. The band of the asymmetric vibration is recorded at 1699 cm\({}^{-1}\), while the band of the symmetric vibration at 1652 cm\({}^{-1}\) is almost overlapped by the absorption band of NHCOCH\({}_{3}\) (C=O stretching of amide I) at about 1646 cm\({}^{-1}\). A band at 1576 cm\({}^{-1}\) corresponds to the N-H bending of the primary amino groups of the chitosan structure [33]. The symmetric deformation vibrations of the -CH\({}_{2}\)- groups have an absorption band at 1422. The band at 1323 cm\({}^{-1}\) is due to the C-N-C from the imid (O=C-NR-C=O) from the 1,8-naphthallimides.