The objective of ATR-FTIR analysis was to investigate the registered peaks corresponding to the functional groups (OH-, C-O and C-H) indicating the presence of the phenols acid, terpenoids or flavonoids compounds and observing the influence of plant extracts on each PVA formulation (Table S1, See Supplementary Information).

Regarding the ISO Standard 3515, the identification of individual components of L. angustifolia are as follows: 1,8-cineole, 0–15%; limonene, 0–0.5%; trans-b-ocimene, 2–6%; cis-b-ocimene, 4–10%; 3-octanone, 0–2%; camphor, 0–0.5%; linalool, 25–38%; linalyl acetate, 25–45%; terpinen-4-ol, 2–6%; lavandulol minimum, 0.3%; lavandulyl acetate minimum, 2.0% and α-terpineol, 0–1%. Thus, the major constituents are linalool, linalyl acetate, terpinen-4-ol and camphor [13].

According to Beiji et al. [23], in fresh peppermint, 36 components were identified in total that accounting for 93.84% of the total volatile. Nonetheless, the main components generally reported of the Mentha piperita extracts were the phenolic acids, flavonoids, terpenoids (menthol and carvone) or terpenes (1,8-cineole, menthone, menthyl acetate, linalool, limonene and pinene), respectively.

Additionally, Elhendawy et al. [19] described the hemp (Cannabis sativa) extras as classes with numerous active compounds such as amino acids, fatty acids and steroids, which belong to primary metabolites, while cannabinoids, flavonoids, stilbenoids, lignans, terpenoids and alkaloids belongs to secondary metabolites. The main important compounds are the cannabinoids such as Δ9-trans-tetrahidrocanabinol (Δ9-THC), Cannabidiol (CBD), Cannabigerol (CBG) and Cannabinol (CBN), flavonoids (apigenin, luteolin), terpenoids, noncannabinoid phenols and steroids [19].

With respect to Verbena officinalis extract, Kubica et al. noted that it contains two phenylpropanoid glycosides (verbascoside and isoverbascoside), 6 phenolic acids (protocatechuic, chlorogenic, vanillic, caffeic, ferulic and rosmarinic acids), iridoids (hastatoside, verbalin and aucubin), flavonoids (luteolin, apigenin, pedalitin, scutellarein) and terpenoids [28]. The main components of Verbena officinalis extract are iridoids, phenols or flavonoids.

The Salvia officinalis extract is characterized by Pavlic et al. as presenting mainly terpenoids (such as α- and β-thujone, camphor and 1,8-cineole) and a polyphenolic fraction that includes phenolic acids (rosmarinic, ferulic and caffeic acid), flavonoids (luteolin, apigenin and naringenin) and diterpenes (carnosic acid, carnosol, rosmanol, epirosmanol and isorosmanol) [41].

The obtained FTIR spectra of plant extracts were analyzed by defining the registered peaks corresponding to the functional groups (OH-, C-O, C-H) and by indicating the presence of the phenols acid, terpenoids or flavonoids compounds in their structure. This spectrum is described in Figure 2. For all investigated extracts, a broad band that corresponds to –OH groups of alcohols and phenols is registered in the range of 3500 to 3000 cm⁻¹ [42]. The highest intensity was observed in hemp plant extracts (Cannabis sativa L.). Toward 2970–2800 cm⁻¹, we have found peaks with different intensity (2925 and 2853 cm⁻¹ for Lavandula angustifolia, 2923 and 2850 cm⁻¹ for Mentha piperita, 2919 and 2851 cm⁻¹ for hemp plant (Cannabis sativa L.), 2920 and 2850 cm⁻¹ for Verbena officinalis and 2922 and 2851 cm⁻¹ for Salvia officinalis). Their presence was connected with the ν(C–H) stretching vibration of symmetric and asymmetric aliphatic groups [43,44,45]. Moreover, the band carbonyl groups ν(C=O) at different peaks (1733 cm⁻¹ for CE, 1709 cm⁻¹ for VE, 1712 cm⁻¹ for ME, 1732 cm⁻¹ for LE, 1728 cm⁻¹ for SE) indicated the presence of monoterpenes such as thujones, camphor, tannins or flavonoid structure [46,47,48,49]. The different intensity peaks in the range of 1570 to 1230 cm⁻¹ were assigned to the alkenes and methyl groups [42]. Furthermore, the large peaks situated in the region 1164–1042 cm⁻¹ (1140 and 1091 cm⁻¹ for CE; 1169 and 1071 cm⁻¹ for VE, 1073 and 1048 cm⁻¹ for ME, 1167 and 1076 cm⁻¹ for LE) emphasize the presence of ν(C-O-C) stretch vibration and ν(C-O) stretch vibration from flavonoids [44,50,51]. The highest intensity was found in hemp plant extract (Cannabis sativa L.). The obtained FTIR spectra identified different classes of compounds.

The spectra of the PVA and PVA loaded with plant extracts are present in the Figure 3.

The analysis shows that the characteristic peaks of poly(vinyl alcohol) and plant extracts have almost similar functional groups, but the intensities of the peaks are different. Thus, the large band between 3500 and 3200 cm⁻¹ was attributed to the stretching vibrations ν(O-H) and intermolecular hydrogen bonding vibration [52,53]. Two bands with maxima at 2938 cm⁻¹ and 2858 cm⁻¹ are the asymmetric and symmetric stretching vibrations ν(C-H) of -CH₂ groups. At 1733 cm⁻¹, it was found the characteristic band of the residual acetate peak and the peaks from the region 1418–1236 cm⁻¹ were attributed to the bending vibrations δ(C-H) and δ(O-H) [53,54]. Furthermore, the stretching vibration ν(C-O-C) was located at 1141 cm⁻¹ [4,53,55], and the stretching vibration ν(C-O) was found at 1092 cm⁻¹ [53]. The presence of plant extracts within the PVA films were evidenced by different features, which proves the existence of intermolecular interactions between them and poly(vinyl alcohol).

In the spectrum of films loaded with Lavandula angustifolia extract (PVA-LE), the broad peak corresponding to stretching vibrations ν(O-H), shifts to 3322 cm⁻¹ indicated the presence of linalool and terpene-4-ol alcohols in the film [43,56]. The stretching vibration ν(C-H) of methyl from the components terpene-4-ol, linalool and linalyl acetate are manifested by changes in the peaks locations toward lower frequencies such as 2919 cm⁻¹ and 2854 cm⁻¹ [43].

The presence of carbonyl groups ν(C=O) of the ester components such as linalyl acetate, camphor and lavandulyl acetate [43,46,56] are further confirmed by the band at 1733 cm⁻¹ and the stretching vibration ν₍C-O) of these esters at 1246 cm⁻¹ or 1079 cm⁻¹. Additionally, the peak associated with ν(C=C) stretching vibrations in components such as linalool, linalyl acetate and terpene-4-ol was found at 1602 cm⁻¹ [14,43]. The bands at 858 and 658 cm⁻¹ represent the bending vibration δ(C-H) such as that in camphor [46,56]. Moreover, different functional groups are presented in PVA film loaded with Mentha piperita extract (PVA-ME). The peak that is attributed to phenols and hydrogen-bonded alcohol was shifted toward 3315 cm⁻¹, and those with a higher intensity corresponded to the ν(C–H) stretching vibration of terpenoids such as menthol were shifted toward 2919, 2851 cm⁻¹, respectively [44]. Different changes on the stretching vibration band of carbonyl group ν(C=O) characteristic to the menthol esters and terpenes such as camphor, limonene, carvone and pinene were observed [47]. The band is more defined at 1733 cm⁻¹, broadening the shoulder of stretching vibration ν(C=C) at 1644.1 cm⁻¹. The observed stretch vibration ν(C–O) peaks of alcohol, ethers and ester were founded at 1082 and 1048 cm⁻¹, and the peak that went toward to 838 cm⁻¹ highlighted the bending vibration δ(C–H) of alkenes [44].

Regarding the PVA film loaded with hemp (Cannabis sativa L.) extract (PVA-CE), a broad band of phenol ν(O-H) stretch vibration shifts to 3320 cm⁻¹ and some sharp peaks attributed to the asymmetric and symmetric ν(C-H) stretches were situated toward 2919 and 2851 cm⁻¹ [45]. The spectra indicated the presence of cannabinoids components such as Δ9-trans-tetrahidrocanabinol (Δ9-THC), Cannabidiol (CBD) and flavonoids such as luteolin and apigenin, etc. The typical carbonyl band was at 1733 cm⁻¹, and the pronounced absorption peak of the stretching vibration ν(C=C) was defined at 1629 cm⁻¹; they were assigned to cannabinoids components [48]. The peaks from the region 1164–1042 cm⁻¹, characteristic to flavonoids, ν₍C-O-C) asymmetric stretch vibration and ν(C-O) stretch vibration were shifted to 1161 cm⁻¹ and 1091 cm⁻¹, respectively.

As respects the PVA film loaded with Verbena officinalis extract (PVA-VE), the band of ν(O-H) stretch vibration attributed to phenols compounds such as phenols acids (protocatechuic, chlorogenic and vanillic) or verbascoside shifted to 3319 cm⁻¹ [57]. Peaks of different heights are situated at 2920 cm⁻¹ and 2851 cm⁻¹, respectively. Their presence was connected with the asymmetric and symmetric stretching vibrations of ν(C–H) of alkanes. The analysis highlighted the common wavenumber for carbonyl structure ν(C=O) bonds at 1732 cm⁻¹ from the flavonoid structure such as luteolin, while the stretching vibration ν(C=C) was found at 1631 cm⁻¹, respectively, and the stretching vibration ν(C-O) bond shifted to 1086 cm⁻¹ from the iridoids structure [50]. Additionally, the bending vibration of δ(C-H) was situated to lower wavenumbers at 948 cm⁻¹ [57].

Furthermore, the PVA film loaded with Salvia officinalis extract (PVA-SE) relates a broad peak at 3359 cm⁻¹ due to the presence of hydroxyl groups, from the phenolic compounds such as rosmarinic acid [11,31]. The asymmetric and symmetric ν(C-H) stretch peaks situated at 2920 cm⁻¹ and 2851 cm⁻¹ are narrower and better contoured, suggesting the presence of interactions between the structures, whereas the bands within 1461–1366 cm⁻¹ were assigned to the symmetric, asymmetric bending of CH₃ and CH₂, respectively [49]. The sharp band of stretching vibration of ν₍C=O) was shifted to 1735 cm⁻¹. The authors attributed this peak to camphor [49]. The peaks from the region 1164–1042 cm⁻¹ had different heights at 1169 cm⁻¹ and 1082 cm⁻¹ and were attributed to 1,8-cineole [58]. In addition, the peak positioned at about 722 cm⁻¹ was assigned to ν(C-C-O) symmetric ether stretch [49]. In the literature, others authors [59] has also concluded that the sage species contains mainly camphor and 1,8-cineole.

The ATR-FTIR spectrum of PVA films loaded with plant extracts confirms the presence of functional groups such as phenols acids and flavonoids, which are widely reported for their antibacterial property. The compatibility between the poly(vinyl alcohol) and the plant extracts was mainly the result of some hydrogen bond formation. These bonds reveal the probability of antibacterial activity.