3.1. Fourier transform infrared (FTIR)
FTIR spectra of chitosan, PVP, PVA, unfilled and filled PVP/PVA/chitosan blends with different AgNP contents were shown in Figure (1). The spectra revealed the characteristic peaks of bending and stretching vibrations for the functional groups found in the prepared films. Table 2 shows the peak positions of the FTIR absorption bands and their assignments for the produced films.
In the case of pure PVA, the broadness and absorbance at about 3265 cm− 1 are attributed to the stretching vibration of the hydroxyl groups (OH) of PVA [18, 19]. The asymmetric CH2 stretching vibration is responsible for the peak at 2938 cm− 1. The bands at 1710 and 1661 cm− 1 were assigned to the C = O and C = C stretching modes [19, 20]. The absorption peak at 1420 cm− 1 was attributed to the CH2 symmetrical bending, while the peak at 1327 cm− 1 is assigned to (CH + OH) bending [21]. The C–O stretch of the carbonyl groups in the PVA backbone corresponds to the band at about 1088 cm− 1. The peak at 842 cm− 1 has been assigned to C-C stretching vibrations with moderate absorption. The OH wagging is responsible for the peak at 593 cm− 1, whereas the CH2 rocking is responsible for the peak at 917 cm− 1 [20].
For PVP, the absorbance observed at 3396 cm− 1 are attributed to (OH) stretching [22], while the absorbance at 1437 cm− 1 and 843 cm− 1 correspond respectively to the CH2 scissoring vibrations and CH2 bending. C = O stretching and C–N stretching are given to the peaks at 1644 cm− 1 and 1017 cm− 1, respectively [23–25].
Chitosan spectra showed a peak at 3290 cm− 1 is attributed to OH stretching, while at 2875 cm− 1 is attributed to CH2 stretching. The vibrational mode of amide C═O stretching was observed at 1654 cm− 1 [26]. C–O–C stretching is assigned to the peak at 1151 cm− 1, while NH out-of-plane bending is assigned to the peak at 561 cm− 1 [27].
In the PVP/PVA/chitosan blend sample, most intensities of the absorption peaks differed irregularly from their values in the individual polymers. Furthermore, the hydroxyl groups of the blend showed a decrease in intensity and a shift to a lower wavenumber 3280 cm− 1 because of the hydrogen linking formation between the OH group (of PVA and chitosan) with the C = O of PVP, ensuring the miscibility of the prepared blend [28, 29]. In the spectra of the PVP/PVA/chitosan blend filled with AgNPs we could not observe any specific bands for AgNPs in the range between 4000 − 400 cm− 1. Since their absorption occurs in far-infrared only that is in the range (400 − 100 cm− 1) [30]. We, therefore, discuss the AgNPs incorporation in the PVP/PVA/chitosan blend and the interaction between the bonds of the blend and AgNPs.
It may be visualized that in figure (1_b&c) there is new small broadband at 3746 cm− 1 and a small sharp peak at 414 cm− 1 with increasing in AgNPs content. For the highest content of AgNPs filling (H green and H laser-ablated), the peak at 2859 cm− 1 reappeared which is one of the characteristic peaks of chitosan (CH2 symmetric stretching). There is also a marked change in intensity with a small shift in most peaks in the major characteristic bands of the PVP/PVA/chitosan blend compared to that filled with various contents of AgNPs, which mean that the AgNPs have highly interacted with hydrogen bonds in PVP/PVA/chitosan spectra.
Table (2): FTIR absorption bands and their assignments for a) pure PVA, b) pure PVP, c) Chitosan and d) unfilled and filled PVP/PVA/chitosan blend with AgNPs.
Band assignment
|
Wavenumber (cm− 1)
|
Ref.
|
PVA
|
PVP
|
Chitosan
|
Blend
|
OH stretching
|
3265
|
3396
|
3290
|
3280
|
[18, 19, 22, 31, 32]
|
N–H stretching of secondary amide
|
|
|
2974
|
|
[32]
|
CH2 asymmetric stretching
|
2938
|
2949
|
|
2933
|
[19, 20, 23, 31]
|
CH2 symmetric stretching
|
2908
|
2859
|
2875
|
|
[19, 20, 22, 27, 33]
|
C = O stretching
|
1710
|
1644
|
1645
|
1644
|
[19, 20, 22, 24, 33]
|
C = C stretching
|
1661
|
|
|
|
[19, 20]
|
N-H bending (amide II band)
|
|
|
1563
|
|
[32, 33]
|
characteristic vibration of C = N (pyridine ring)
|
|
1493
|
|
|
[23, 31]
|
CH2 bending
|
1420
|
1461, 1422, 1372, 843
|
1416
|
1422
|
[20, 23, 24, 27, 31, 33–35]
|
CH2 scissoring vibrations
|
|
1437
|
|
|
[23, 31]
|
CH wagging
|
1374, 1238
|
1287
|
|
1287
|
[19, 20, 23, 31]
|
(CH + OH) bending
|
1327
|
|
|
|
[20]
|
C-C stretching
|
1140, 842
|
1217, 933
|
|
842
|
[19, 20, 24, 36]
|
Asymmetric bridge oxygen stretching (glycosidic linkage)
|
|
|
1151
|
|
[33]
|
C-O stretching
|
1088
|
|
|
1088
|
[20]
|
C–O bending
|
476
|
|
1044
|
|
[20, 27, 33]
|
C–N stretching vibrations
|
|
1017
|
881
|
|
[25, 27]
|
CH2 rocking
|
917
|
733
|
|
|
[20, 24]
|
OH wagging
|
593
|
|
|
|
[20]
|
N-H bending
|
|
|
561
|
|
[27]
|
CO wagging
|
417
|
|
|
|
[20]
|
C–N bending
|
|
645
|
|
|
[31]
|
N-C = O bending
|
|
|
568
|
568
|
[37]
|
3.2. X-ray diffraction (XRD)
Figure (2a) explains the XRD for pure chitosan, PVP, PVA, and the blend, while Figure (2b) reveals the XRD of blend films filled with different concentrations of AgNPs in the extented 2θ range between 3° and 70°. The XRD of the pure PVA reveals diffraction peaks at 2θ = 19.4° and 40.4° due to the partially crystalline nature of the PVA structure supported by intermolecular and intramolecular hydrogen bonds. PVA exhibits as well a low intensity diffraction peak at 2θ = 22.7o [38]. The chitosan film has a broad diffraction peak at 2θ = 23o, while PVP has very broad diffraction peaks at 2θ = 11° and 22°, confirming the amorphous nature of the chitosan and PVP films examined [39].
For the unfilled PVP/PVA/chitosan blend, it can be seen that the intensity of the peak at 2θ = 19.4° has reduced and become broader. This means that adding PVP and chitosan to PVA reduces the semi-crystalline nature of PVA and indicates high compatibility between the components of the blend.
Filling the blend with AgNPs also reduces the peak intensity at 2θ = 19.4°, which is based on the interaction between the filler and the blend, resulting in less intermolecular interaction between the blend chains and a decrease in the degree of crystallization [22]. This amorphous nature is responsible for the higher ionic diffusion resulting in higher ionic conductivity [40]. This behavior indicates that the PVA/PVP/chitosan blend matrix has been structurally modified as a result of the AgNPs filling and confirms the results obtained in the FTIR studies (section 3.1).
Moreover, it can be seen from the same figure that after filling with a high content of ablated AgNPs, a new peak is generated at ~ 38o, which can be attributed to the Face centred cubic (fcc) structure of the embedded AgNPs, which corresponds to h k l parameters (111) [41].
3.3. Transmission electron microscopy (TEM)
To ensure the size and morphology of the produced AgNPs, transmission electron microscopy (TEM) was used. Figures (3) displays the transmission electron microscopic images for the purely green and laser-ablated AgNPs in aqueous solution, revealing that the nanoparticles are roughly rectangular, randomly distributed, and have diameters ranging from 30 to 100 nm.
3.4. UV/Vis. absorption and optical studies
Figure (4) displays the UV/visible absorption spectra of the pristine, blend, and nanocomposite film samples.
The spectra of pure chitosan show an absorption band at 208 nm, which can be assigned to the chromatic functional groups (NHCOR) and/or the presence of chromophore groups (C = C) or (C = O) [42], while pure PVA shows an absorbance peak at ~ 200 nm, that is attributed to an n → π* transition. The PVA transition is associated with the carbonyl groups (C = O) related to ethylene unsaturation (C = C) of the (CH = CH)CO– type. The presence of carbonyl functionalities might be because of residual acetate groups remaining after the PVA fabrication with the aid of using hydrolysis of polyvinyl acetate or oxidation throughout fabrication and processing [43].
After adding AgNPs (Fig. 5), the nanocomposite samples give a significant increase in the absorbance values at 200 nm with a gradual red shift to longer wavelengths as the AgNPs content increase. This can be assigned to the interaction between polymeric matrices and the added AgNPs affecting the calculated optical bandgap [44] associated with the crystallinity change in the nanocomposite, as discussed earlier in Section 3.2. In addition, a peak begins to appear at λmax = 427 nm, and its intensity continuously increases with the increase of AgNPs concentration. The appearance of this peak in the visible range is caused by the surface plasmon resonance (SPR) nature of the AgNPs inserted in a dielectric medium, which results from the collective excitation of the conduction band electrons in the nanoparticle. The presence of these beaks also indicates that the fabricated blend can be used as a good cabbing agent for the AgNPs. The increase in λmax ( and hence decrease in its optical bandgap energy) mean that the particle size increases as the filler concentration increase [45], indicating that the laser-ablated Ag nanoparticles have a greater size than the obtained from green prepared Ag nanoparticles.
The optical energy gap (\({\text{E}}_{\text{g}}\)) can be estimated using the Mott and Davis equation [46], which analyzes the spectral dependence of the absorption coefficient near the absorption edge.
$${\alpha }= \frac{\text{A} (\text{h}{\upsilon } - {\text{E}}_{\text{g}} {)}^{\text{r}}}{\text{h}{\upsilon }} \left(1\right)$$
where A is a constant associated with the electronic transition probability, \(\text{h}{\upsilon }\) is the energy of the incident photons, and the power \(\text{r}\) is related to the transition behavior and equal to 2 or ½ for the allowed direct and indirect transition, respectively.
Figures 6a and 6b show a plot of (αhυ)2 and (αhυ)1/2 with hυ for the prepared samples. Extrapolating the linear fit lines in these data points on the hυ axis yields the optical band gap values, which are listed in table 3.
It is evident that the calculated optical bandgaps decrease with increasing AgNPs content and are assigned to the role of AgNPs in modulating the structure caused by the formation of variable polaronic and defect levels that are related to localized states density \(\text{N}\left(\text{E}\right)\) [47].
At lower energies below the fundamental absorption edge, spectral data revealed the appearance of an elongated tail that correlates with the localized states in the valence band tail, attributed to defects formed by AgNPs, and extends to states in the conduction band. In such a case, the absorption coefficient \({\alpha }\) can be entirely attributed to the energy tail width \({\Delta }\text{E}\) and the photon energy which is thermal vibration in the lattice [20] and can be determined by Urbach formula [48].
$$\alpha ={\alpha }_{o}exp\left(\frac{\text{h}{\upsilon }}{{\Delta }E}\right) \left(2\right)$$
where \({{\alpha }}_{\text{o}}\)is a constant
Urbach energy \(\left({\Delta }\text{E}\right)\) values can be obtained by plotting the logarithm of the absorption coefficient against photon energy \(\left(\text{h}{\upsilon }\right)\) as shown in Figure (7) and listed in Table 3. For the pure blend, the value of \({\Delta }\text{E}\)is 0.16 eV and that of the filled samples is increased up to 0.23 eV.
The increase in Urbach energy confirms the increase in the width of localized states within the bandgap and hence, is responsible for bandgap decay of the filled samples.
Table (3): indirect optical energy gab Egid direct optical energy gab Egd, and Urbach Energy \({\Delta }\mathbf{E}\) of the prepared blend filled with AgNPs
Sample
|
\({\text{E}}_{\text{g}\text{i}\text{d} }\left(\text{e}\text{V}\right)\)
|
\({\text{E}}_{\text{g}\text{d} }\left(\text{e}\text{V}\right)\)
|
\({\Delta }\text{E} \left(\text{e}\text{V}\right)\)
|
Blend
|
4.94
|
5.25
|
0.16
|
L Ag green
|
4.93
|
5.24
|
0.16
|
L Ag ablated
|
4.91
|
5.21
|
0.17
|
H Ag green
|
4.87
|
5.20
|
0.18
|
H Ag ablated
|
4.85
|
5.19
|
0.23
|
3.5. Antibacterial studies
Antibacterial characteristics of the studied samples containing both high and low-level dopant of both green synthesized and silver ablated nanoparticles were performed against two gram-positive and two gram-negative bacteria namely E. Coli, Pseudomonas aeuroginosa, S. aureus, and B. Cereus. The minimum inhibition zone (MIZ) agar diffusion method was used. Polymeric samples with nearly equal diameter were seeded in the bacterial medium and incubated at 37°C for 24h. Sample concentrations and their corresponding inhibition zone in (mm) were listed in the table (4).
Table (4) Sample concentrations and their corresponding inhibition zone in (mm)
Sample
|
E. coli
|
Pseudomonas aeuroginosa
|
S. aureus
|
B. cereus
|
Blend
|
12
|
12
|
11
|
11
|
Green L
|
13
|
14
|
13
|
12
|
Green H
|
14
|
15
|
15
|
14
|
Laser L
|
15
|
17
|
17
|
16
|
Laser H
|
15
|
17
|
18
|
17
|
Obtained data shows the effect of adding silver nanoparticles to the polymeric matrix. It was noticed that the inhibition zone in the case of samples containing silver nanoparticles is generally increasing and the result appears promising and with higher value, in the case of ablated laser, this may be attributed to both the size and monodisperse distribution of the nanoparticle. It was also clear that the effect of all samples against gram-positive bacteria is slightly increasing than that of gram-negative bacteria as shown in Fig. 8. Such behavior can be considered in terms of the rapid generation of free radicals through redox reaction while ROS can react directly with membrane components including proteins, lipids, and DNA that are scavenged by antioxidants resulting in oxidative stress in bacterial cells [
49].