3.3.2. FTIR Analysis of Indicator Films

The absorption peaks of the FIIR spectra of the film-forming materials and the bi-layer films are shown in Figure 2E. The band at 3356 cm−<sup>1</sup> corresponds to the OH stretching vibration of the hydroxyl structure. It occurred in all the spectra but with lower peak intensity changes [24]. The peaks at 2289 and 2901 cm−<sup>1</sup> were due to the C-H and -CH2 stretching vibrations of alkane groups [25]. The absorption band that appears at 1637 cm−<sup>1</sup> of the anthocyanins was ascribed to the C=C stretching from the aromatic ring frame of the butterfly bean flower anthocyanin, which is related to the flavonoid fingerprint spectra and was also found in all the bi-layer films [26]. The absorption peaks of carrageenan at 1242 and 943 cm−<sup>1</sup> were associated with the C=O of the glycoside bond and the S=O of the sulfate ester group, respectively [27]. The other major band at 1049 cm−<sup>1</sup> was attributed to the C-O-C stretching vibration of the conjugated carbonyl group [28]. In the case of the A-CBAL films, all the peaks presented similar positions with minor intensity changes to the control film (A-CBA). The results indicated that there was no chemical interaction between the liposomes and anthocyanins.
