## 3 Results and Discussion
### Functional Groups Analysis  
Infrared (see Figure 2) and thermal stability (see Figure 3) analyses of PL-N, PL-DB, PL-H, and PL-HUS nanofibers were performed to demonstrate the efficiency of bleaching, hydrolysis acid, and ultrasound treatments. Regarding PL-N, two absorption zones were observed (see Figure 2): 600 cm\({}^{-1}\)-1750 cm\({}^{-1}\) and 2800 cm\({}^{-1}\)-3400 cm\({}^{-1}\). The absorption band found at 3332 cm\({}^{-1}\) corresponded to the stretching of the hydroxyl groups (-OH). The peak at 2860 cm\({}^{-1}\) corresponded to the C-H stretching present in cellulose, hemicellulose, and lignin [26]. Meanwhile, the peak at 1739 cm\({}^{-1}\) corresponded to the C=O vibrations in the ester groups (-COO- and H\({}_{3}\)CO-) characteristic of hemicellulose, or in the ester bond of the carboxylic group (-COOH) characteristic of the ferulic and p-coumaric acids belonging to lignin [26; 27]. The peak at 1240 cm\({}^{-1}\) was related to the -C-O-C- vibration found in the bonds between the aromatic ring and methoxy groups in lignin [28]. The peaks at 1647 cm\({}^{-1}\), 1562 cm\({}^{-1}\), and 1460 cm\({}^{-1}\) referred to the C=C stretching in the aromatic rings in lignin [24; 25; 26; 28]. Finally, the peak at 1028 cm\({}^{-1}\) corresponded to the C-O-C stretching of the \(\beta\)-1,4-glycosidic ring bonds between the D-glucose units in cellulose [24].  
Figure 1: Scanning electron micrographs (SEM) of (**a**) native (PL-N) and (**b**) delignified–blanched (PL-DB) nanofibers from pineapple leaves.  
Figure 2 also includes infrared spectra of PL-DB, PL-H, PL-HUS, and CC. Figure 2 shows that the bands around 1240 cm\({}^{-1}\), 1647 cm\({}^{-1}\), and 1740 cm\({}^{-1}\) disappeared after the delignification-bleaching process (see PL-DB in Figure 2), which was proof of the removal of lignin and hemicellulose during the treatment [29]. Additionally, Figure 2 shows the disappearance of the peak around 2848 cm\({}^{-1}\) for PL-DB, PL-H, and PL-US. According to [PERSON] et al. [30], a peak around 2850 cm\({}^{-1}\) corresponds to the C-H stretching vibration corresponding to the methyl (-CH\({}_{3}\)) and methoxy (O-CH\({}_{3}\)) groups that form the aromatic ring of lignin. Consequently, a disappearance of this peak after treatment could be attributed to lignin removal.  
Meanwhile, FTIR spectra of PL-HUS (see Figure 2) show a peak around 1000 cm\({}^{-1}\), which was related to the C-O-C vibration of the \(\beta\)-1,4-glycosic bond between the D-glucose units in the cellulose [24]. Figure 2 also shows an increase in the intensity of the bands around 1000 cm\({}^{-1}\), 3300 cm\({}^{-1}\), and 2900 cm\({}^{-1}\), which is typical of the functional groups of cellulose, which were prominent in the PL-H and PL-HUS spectra. In these last ones, the appearance of bands at 1100 cm\({}^{-1}\), 1300 cm\({}^{-1}\), and 890 cm\({}^{-1}\) was also noted, which became more intense due to the effect of the treatment with H\({}_{2}\)SO\({}_{4}\) and ultrasound.  
Figure 3: (**a**) Thermogravimetric and (**b**) derivative thermogravimetric analysis of native (PL-N), delignified–blanched (PL-DB), hydrolyzed (PL-H), and ultrasound (PL-HUS) nanofibers from pineapple leaves, along with the commercial microcrystalline cellulose (CC).  
Figure 2: Infrared spectrum (FITR) of native (PL-N), delignified–blanched (PL-DB), hydrolyzed (PL-H), and ultrasound (PL-HUS) nanofibers from pineapple waste, along with the commercial microcrystalline cellulose (CC).  
Additionally, a band around 1400 cm\({}^{-1}\) and the increase in the intensity of the bands around 1360 cm\({}^{-1}\) in the PL-HUS spectrum were evident, which agreed with that reported by [PERSON] et al. [24], who indicated that sharp signals around 1426 cm\({}^{-1}\)-897 cm\({}^{-1}\) and 1370 cm\({}^{-1}\)-2900 cm\({}^{-1}\) reflect the cellulose crystalline band.