3.2.2. IR Spectroscopy

OH stretching of zein and HZ particles was at 3318 cm−<sup>1</sup> but changed to 3316 and 3405 cm−<sup>1</sup> in HZ-chi and HZ-chi/pec particles, indicating hydrogen bonding among zein, chitosan and pectin (Figure 4B,C). Likewise, amide I and II absorption band of zein changed from 1658 and 1541 cm−<sup>1</sup> to 1656 and 1536 cm−<sup>1</sup> in HZ particles, to 1658 and 1542 cm−<sup>1</sup> in HZ-chi particles, and to 1654 and 1635 cm−<sup>1</sup> in HZ-chi/pec particles. The variations of stretching and bending vibrations of C=O and N-H indicate that hydrophobic and electrostatic interactions occurred during the fabrication of HZ, chitosan and chitosan/pectin coated HZ particles [29]. The distinct absorption peaks of quercetin IR spectrum at 3386, 1655, 1599, 1373, 1257 and 1160 cm−<sup>1</sup> (Figure 4A) signify O-H, C=O, C=C, C-OH, C-O-C

and C-OH (B ring), respectively [34,44]. The absorption bands were clearly seen with lower intensities in the physical mixture of quercetin, zein, chitosan and pectin (Figure 4A). However, the characteristic absorption bands of quercetin were invisible in HZ, HZ-chi and HZ-chi/pec particles, indicating the entrapment of quercetin in the particles and limited starching and bending of various bonds [42]. The encapsulation of quercetin resulted in the variation of OH stretching of HZ, HZ-chi and HZ-chi/pec particles (Figure 4A,B), suggesting the flavonoid interaction with wall materials [45]. Upon encapsulation of quercetin in hollow zein particle, the amide I peak of zein was unchanged, whereas the amide II showed a redshift to 1537, 1543 and 1539 cm−<sup>1</sup> in HZ, HZ-chi and HZ-chi/pec particles (Figure 4A). These shifts in the amide II result from hydrophobic, electrostatic and hydrogen bonding of quercetin with wall materials [46,47].

**Figure 4.** IR spectra of quercetin, mixture (quercetin, zein, chitosan and pectin), quercetin-loaded HZ−chi and HZ−chi/pec particles (**A**), HZ, HZ−chi and HZ−chi/pec blank particles (**B**), and raw materials zein, chitosan and pectin (**C**).
