Introduction

Nowadays, one of the most important applications of the active packaging system is antibacterial packaging for food production. It is a system of packaging that can control the growth and inhibition of the activity of microorganisms, which can make food contamination. Some of the nanoparticles (NPs) have excellent antibacterial activity and can interact with the cell membrane of bacteria because of their high surface-of-volume ratio. Hence, NPs have been recognized as nanofillers for making nanocomposite (NC) films to enhance their packaging performance [1, 2].

Among all of the polymers, poly(vinyl alcohol) (PVA) is introduced as a semi-crystalline polymer that has specific characteristics such as high chemical resistance, the capability to form the film, the good mechanical strength, solubility in hot and cold water, and biodegradability. This polymer has been widely considered in a variety of fields including textile, industry, adhesive, fuel cells, and so on. PVA contains a large number of OH functional groups that produce hydrogen bonding and can interact well with organic and inorganic materials [3,4,5].

The variety of NPs and their usage have been dramatically increased in various fields due to their specific features. Such as TiO2 that is considered in fabrication of wave-absorbing composites [6] or Fe3O4, which is attracted the attention of researchers because of its strong magnetic and low electrical conductivity properties [7]. Different NPs have been used for the reinforcement of PVA NC films such as TiO2 [8], Ag [9], PbO [10], Fe3O4 [11], and so on. Among them, zinc oxide (ZnO) nanoscale has been considered for its superior features. These crystalline NPs in addition to being non-toxic, have the antibacterial trait and photocatalytic capability, and these bio-safe NPs have a large band gap and high excitation binding energy, which make them susceptible to use in the field of catalyst, solar cells, and optoelectronics [12, 13].

Fernandesa et al. [14] prepared PVA NC films with various percentages of ZnO NPs. Under inert atmosphere, they observed that the pure PVA films were thermally more stable than the PVA/ZnO NC films, and the existence of ZnO enhanced the crystallinity of the PVA NC films but could not improve the violence of the NC films. The solvent casting process was applied to prepare PVA and its NC films. At 20% mole of ZnO NPs, the crystallinity decreased owing to the variation in crystallite size and aggregation [15]. Ahangar et al. [16] created PVA-ZnO NC films by casting solution procedure. They observed that the increment of ZnO NPs into the PVA biopolymer reduced the transparency of the NC films and upturned the antibacterial activities, but NPs tended to accumulate at a point in the NC films.

For better dispersion of NPs and preventing their aggregation, researchers have been tried to modify the surface of NPs with other molecules. Tang et al. [17] used the sonochemistry method to graft ZnO NPs with poly(methacrylic acid) (PMAA). The thermogravimetric analysis (TGA) showed that the modified ZnO NPs with PMMA reduced the agglomeration of ZnO NPs. Chakradhar et al. [18] described an eco-friendly approach to modify ZnO NPs with stearic acid (ZnO@SA). They found that the surface modification developed a superhydrophobic property of the NPs’ surface; thus, this feature can be used to build micro/nanoscale devices. Other materials, which can be used for the modification of ZnO NPs are bovine serum albumin [19], diacids based on amino acid [20], vitamin B1 [21], citric acid [22], β-cyclodextrin [23], etc.

The vitamin B family contains folic acid (FA) (also known as vitamin B9), is an orange-yellow crystalline powder and an artificial form of folate. FA is slightly soluble in water and is susceptible to light and heat. This vitamin is easily found in many foods, such as broccoli, orange, peas, and so on. Functional groups in this tasteless and odorless substance are imino, amino, hydroxyl, imide, carboxyl, and tertiary amine, which can easily form intermolecular hydrogen bonding. [24, 25]

A few studies have shown the modification of ZnO NPs with FA. Muhammad et al. and also Ma et al. [26, 27] used amine groups to synthesize the ZnO-NH2 quantum dots (QDs). Toxic solvents such as 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide and dimethyl sulfoxide were used to activate the carboxylic (COOH) groups of FA. Finally, the achieved products were ZnO-FA QDs that were used for targeted cancer cell imaging.

In this paper, we used the sonochemical method as a green and safe technique to modify NPs and prepare PVA NCs. The sonication method leads up better dispersion of ZnO-FA NPs in the polymeric matrix. By this method, it does not require much temperature to carry out effective attractions, also these attractions even have the potential to occur at ambient temperatures by utilizing the energy of ultrasonic waves. The mode of operation is such that vibrations occur through the ultrasonic wave and then propagation into the initial fluid through which bubbles form in the fluid. The bubbles are created, grown, and reached a critical size; inside the bubbles of critical size, localized regions with high pressure and high temperature (about 5000 K), with stability within the nanosecond. Eventually, an explosion of a series of bubbles produces a shock wave. The energy generated by this shock wave is used to break down covalent bonds, homogenize, perform specific chemical reactions, especially the synthesis of NPs, organic materials, and so forth. [28, 29]

In this study, firstly we endeavored to apply the ultrasound technique to develop the attributes of PVA as a polymeric matrix with ZnO-FA NPs. In the second step, the characterization and properties of PVA/ZnO-FA NC films were evaluated. At last, the antibacterial activity of the pure PVA and PVA/ZnO-FA NC film 8 wt% were assessed against Staphylococcus aureus (S. aureus, ATCC 12600) and Escherichia coli (E. coli, ATCC 9637) bacteria using the liquid medium microdilution method.

Experimental

Practical materials

The white powder of ZnO NPs (average particle size = 25–30 nm) with a specific surface area > 60 m2.g−1 was acquired from Neutrino Company (Tehran, I. R. Iran). FA was procured from Acros Organics Company (Belgium). PVA with Mw =145,000 g.mol−1 and N,N′-Carbonyldiimidazole (CDI) were acquired from Merck Chemical Company (Darmstadt, Germany). Deionized water (DIW) was received from the Kowsar Company (Isfahan, I.R. Iran).

Characterization methods and instruments

For preparing the modified NPs, the TOPSONICS ultrasonic liquid processor (Tehran, I. R. Iran) with the power of 100 W and frequency wave of 20 kHz was used. The NC films were rigged with ultrasound output waves from the UP-100 series of TOPSONICS ultrasonic liquid processor (Tehran, I. R. Iran) by the power of 400 W and frequency 20 kHz. Both devices were equipped with a probe. All the FT-IR spectra were obtained, using a Jasco-680 spectrometer (Tokyo, Japan), which has a resolution of 4 cm−1 and reports the wavenumber in the range of 4000–400 cm−1. For powder samples, the potassium bromide (KBr) pellets were provided, which the ratio of sample to KBr was 30:100. To study the crystallinity structures of ZnO-FA NPs and PVA/ZnO-FA NC films, X-ray diffraction (XRD) patterns were gathered by the Philips X’Pert MPD diffractometer (Germany) with 2θ = 10–80° and the rate of 0.05°/min using Cu Kα source and a beam of λ = 1.540 nm. The UV–Vis spectra of the specimens were obtained using Jasco V-570 spectrophotometer (Tokyo, Japan) in the range of 25 to 800 nm. To peruse the thermal behavior of the samples, they first were dried in a vacuum pump at the temperature range of 50–60 °C, and then the corresponding test was carried out at 25–800 °C using a STA6000 Waltman (USA) apparatus and the speed of applied heat was 20 °C.min−1 under an argon atmosphere. The micrographs of field emission scanning electron microscopy (FE-SEM) exhibited the dispersal of ZnO-FA NPs on the surface of the PVA matrix and their morphology was collected by HITACHI (S-4160). By using Philips (CM 120, Netherlands) microscope with a voltage 100 kV, the images of transmission electron microscopy (TEM) were achieved. To obtain TEM images, modified NPs and NC films were dispersed in DIW by ultrasonic irradiation, next a drop of supernatant was placed on copper grid and dried. Then the images were taken. The size of NPs was measured by applying the Digimizer 4.6.1.0 software, and the results were calculated and examined via the SPSS Statistics 17.0 software. The specimens were cut into size 40 mm × 20 mm and then the test of tensile was done by the related device [Testometric Universal Testing Machine M350/500 (UK)]. The speed of this test was adjusted at 20 mm.min−1. Wetting characteristics of the prepared NC films were recorded using digital microscopy camera U-VISION MV500 that was manufactured in China. For the elemental analysis of the specimens, the energy dispersive X-ray (EDX) spectroscopy was accomplished by way of Tescan Mira II spectrometer (Czech Republic). Precise measurement of specific surface area and porosity was performed with a BELSORP MINI II (Japan).

Production of coating ZnO NPs with FA

In the first step, 20 mg (453 × 10−7 mol) of FA was dispersed into 8 mL of DIW and agitated. After 2 h, by adding 7 mg (413 × 10−7 mol) of CDI and stirring for 5 h, a bright yellow solution was seen. This process was performed to activate FA. At the second step, 100 mg of ZnO NPs were added up to 8 mL of DIW and stirred for 2 h, then were exposed to ultrasonic waves for 5 min and power of 60 W (7 s on and 3 s off) for the dispersion of NPs. After that, the suspension of NPs was added to the solution of FA. After stirring overnight, the suspension was exposed to ultrasonic irradiation with a power of 70 W and a duration of 30 min. This suspension was washed with distilled water and centrifuged for 8 min for four times to remove impurities. Then the resulting sediment was dried at ambient temperature (Scheme 1).

Scheme 1
scheme 1

A schematic illustration of ZnO-FA NPs and PVA/ZnO-FA NC films

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Assembling of the PVA/ZnO-FA NC films

To prepare the PVA stock solution, 300 mg of PVA powder was dissolved in the 50 mL of DIW at 95 °C for 2 h. To fabricate the PVA NC films, three distinct quantities of the dried ZnO-FA NP powders were used. In this way, the amounts of 2, 5, and 8 wt% of the modified ZnO NPs (respect to the PVA weight) separately were added to the PVA solution and agitated magnetically for 2 h with high rapidity. After that, to obtain a perfect homogeneity, this suspension was ultrasonicated for 30 min at a power of 70 W. Immediately after disclosure under ultrasound, the mixture was poured into two clean and dry plastic Petri dishes into an equal volume and then were placed under the clean hood to vaporize the solvent, and the NC films were created.

For the preparation of the NC films with better transparency, the dried NC films were crushed and added to the 50 mL of DIW and then were heated at 95 °C for 30 min at the same time. After that, ultrasonication was applied on the suspension for 15 min at a power of 70 W. At the following step, the blend in the Petri dishes turned into NC films after 2 days (Scheme 1).

Antibacterial assays

The antibacterial activity of the pure PVA and PVA/ZnO-FA NC film 8 wt% against S. aureus and E. coli was assessed by using liquid medium microdilution methods like the quantitative analysis. The standardized bacteria solution was prepared by fresh inoculation colonies of E. coli and S. aureus strains into 5 mL nutrient broth medium (QUELAB, England) and incubation at 37 °C overnight. After that, the suspensions were diluted to approach an absorbance value of 0.1–0.2 (λmax = 625 nm). Afterward, the prepared samples were made up to 10 mg and appended on to the fabricated bacterial suspensions and incubated with gentle agitation at 37 °C and the speed of 80 rpm.min−1 on a shaker platform (Eppendorf Thermo-mixer, Germany) for 24 and 48 h. The UV–Vis spectrophotometer was applied to obtain the absorbance value of all samples at the determined time. The following equation was used to calculate the bacterial inhibition percentage:

Bacterial inhibition (%) = [(Ic − Is)/Ic] × 100 (1).

IC: the absorption extent of the control of bacteria suspension.

IS: the absorption extent of the bacteria suspension included the different samples [30].

Results and discussion

Pure ZnO NPs have low stability in water and easily accumulate; the reason for this phenomenon is the presence of hydroxyl groups on their surface, which have relatively high surface energy [31]. Three methods have been developed to improve the stability of NPs, such as (i). ball milling (ii) mechanical fluid shearing, and (iii). ultrasonic agitation [32].

The ultrasonic method and the modifier agent were used in this research to improve the dispersion of ZnO NPs. FA as a surface-modifying agent with carboxylic acid and amine functional groups, can establish the hydrogen bonds and the electrostatic interactions with the hydroxyl groups of the NPs and lead to breaking the agglomerates.

Figure 1(a) shows the stability of ZnO-FA NPs in DIW. The ZnO-FA NPs’ suspension was stable for more than 10 days, but for the pure ZnO NPs under as same conditions as the modified ZnO, as soon as the container placed, sedimentations started, and complete phase separation was observed after 3 h, which shows the instability of the pure ZnO NPs [Fig. 1(b)]. This increment in the stability time can be attributed to the positive and undeniable impact of FA as a factor for surface modification.

Fig. 1
figure 1

Dispersion stability of ZnO-FA NPs in DIW (a) and the pure ZnO NPs (b)

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Excellent dispersion stability of ZnO-FA NPs in the PVA solution after 30 min sonication effect can be observed in Fig. S1. While, if only magnetic stirrer was used, unstable dispersion and fast precipitation of the particles in the container would have been observed. Hence, the significant efficacy of ultrasonic irradiation well is realized for stabilizing a polymer solution containing ZnO-FA NPs.

By increasing the concentration of ZnO-FA NPs in the polymeric matrix, the transparency of the NC films negligibly diminished, and their color turned to more yellowish (Fig. 2). Figure 3 shows the flexibility and non-brittle characteristics for PVA/ZnO NC film 5 wt%.

Fig. 2
figure 2

Visual transparency of the pure PVA (a) and PVA/ZnO-FA NC films of 2 wt% (b), 5 wt% (c), and 8 wt% (d)

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Fig. 3
figure 3

Flexibility image of PVA/ZnO-FA NC film 5 wt%

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Investigation of the FT-IR spectra

Figure 4(a) indicates the spectrum of the pure FA, which the existence of two absorption bands in 3543 and 3414 cm−1 represent the stretching frequencies of the O–H in the acidic functional group and N–H bond, respectively. The stretching vibrations of the C–H and C=O bonds are appeared at 2925 and 1694 cm−1, respectively, and the absorption band in 1605 cm−1 is associated with the N–H bending vibration. The absorption bands of the phenyl ring and pyridine in the FA structure are illustrated in 1697 and 1483 cm−1, respectively [33, 34]. The pure ZnO NPs demonstrate the unique absorption band within the range of 590–400 cm−1. Also, due to the absorption of water molecules on the surface of the NPs, the characteristics of hydroxyl groups are shown in 3434 and 1638 cm−1, which are belonged to the O-H stretching and bending vibrations, respectively [Fig. 4(b)] [35]. For the ZnO-FA NPs, a broad band is observed in 3366 cm−1, that ascribed to the hydroxyl groups. The absorption bands in 2922 and 2852 cm−1 are allocated to the asymmetric and symmetric aliphatic C–H stretch vibrations of –CH2, respectively, which proved the presence of FA in the ZnO NPs structures. Owing to the hydrogen bonding between the ZnO NPs and the modifier agent, the band of the carbonyl group is detected in 1607 cm−1, and the certain absorption band in 438 cm−1 is an evidence of the existence of ZnO NPs. Due to the formation of carboxylate salt, the symmetric and asymmetric stretching vibrations are associated with it appeared at 1510 and 1449 cm−1 [Fig. 4(c)] [36, 37].

Fig. 4
figure 4

FT-IR analysis of the pure FA (a), pure ZnO NPs (b), and ZnO-FA NPs (c)

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In Fig. 5, the FT-IR spectra of the pure PVA and its fabricated PVA/ZnO-FA NC films are shown. The main characteristic bands of the pure polymer are observed at 3436, 2940, 1653, and 840 cm−1, which are arranged in sequence the stretching vibrations of OH, the asymmetric stretching CH2, the C=O stretching, the C–H bending and the C–C stretching vibrations. The differences between the pure PVA and the fabricated PVA/ZnO-FA NC films are the existence of the absorption band in 424 cm−1, because of the stretching vibration of Zn–O and appearance of a low-intensity band in 1607 cm−1, that may be due to the existence of C=O groups in the structure of FA [38].

Fig. 5
figure 5

FT-IR analysis of the pure PVA (a), and PVA/ZnO-FA NC films of 2 wt% (b), 5 wt% (c), and 8 wt% (d)

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Examination of the XRD diagrams

Figure 6 depicts the XRD diagrams of the pure ZnO NPs, pure FA, and modified ZnO NPs. As seen in Fig. 6(a), reflection of (100), (002), (101), (102), (110), (103), (200), (112), and (201) revealed the hexagonal wurtzite form of the ZnO NPs [39]. The XRD diagram of the FA shows the crystallinity form for it [Fig. 6(b)]. To study the trace of FA on the crystalline structure of the ZnO NPs, the XRD diagram of the ZnO NPs was studied before and after the modification process, and there is no variation in the crystalline form of ZnO NPs after surface modification. A peak broadening was observed in the pattern of modified NPs, which is an indication of the effect of FA on the broadening of peaks and reducing the crystallinity of ZnO NPs [Fig. 6(c)].

Fig. 6
figure 6

The XRD diagrams of the pure ZnO NPs (a), pure FA (b), and ZnO-FA NPs (c)

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The XRD diagram of the pure PVA indicates two regions, amorphous and crystalline phases, which are explanatory of the semi-crystalline form that is associated with the hydrogen bonding in the polymer chain [Fig. S2(a)] [40]. In XRD diagrams of PVA/ZnO-FA NC films of 2, 5, and 8 wt%, new peaks with the 2θ values at 32, 34 and 36 are appeared. The intensity of these peaks increases by an increment of ZnO-FA NPs in NC films [Fig. S2(b-d)].

FE-SEM explorations

By moving a set of electron beams on the surface of the specimen and its bombarding, some photons and electrons remove from the specimen and release to the detector in which are transformed into a signal and finally a wide variety of information from the sample surface is provided [41].

By examination of the surface morphology of the ZnO-FA NPs by the FE-SEM, micrographs at various magnifications indicate a uniform spherical form for the NPs (Fig. 7). Regarding the FE-SEM micrograph for the pure PVA, a smooth surface is observed [Fig. 8(a)]. In literature studies, it was designated that ZnO NPs aggregated in the polymer matrix, but the bright and halo points in FE-SEM micrographs clarify the existence of ZnO-FA NPs in the polymer, and it well illustrated the progressive effects of ultrasonic waves and FA as a modifier in order to disperse NPs in the PVA [Fig. 8(b)] [16, 42].

Fig. 7
figure 7

The FE-SEM micrographs of the ZnO-FA NPs at several magnifications

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Fig. 8
figure 8

The FE-SEM micrographs of the pure PVA (a), and PVA/ZnO-FA NC films of 2 wt% (b), 5 wt% (c), and 8 wt% (d)

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TEM studies

Using TEM, the structural and morphological aspects of the ZnO-FA NPs and PVA/ZnO-FA NC films were inspected, and the particle size of the ZnO-FA was estimated in the absence and the presence of a polymeric matrix using the corresponding histograms.

As can be observed from Fig. 9, NPs have a spherical structural morphology. In Fig. 9(c) at a magnification of 20 nm, a core-shell morphology is seen around the spherical NPs, which confirms the presence of the FA on the surface of the ZnO NPs. The average particle size for the pure ZnO NPs was reported between 20 and 30 nm, while for the modified ZnO NPs, the size has been increased to about 44 nm. This view submitted that NPs were well-functionalized with FA.

Fig. 9
figure 9

The TEM images of the ZnO-FA NPs at various magnifications (a-c)

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For TEM analysis, PVA/ZnO-FA NC film 5 wt% was selected because of its better dispersion. The TEM images of this NC film in different magnifications present the good and uniform distribution of the ZnO-FA NPs in the PVA. Also, the size of the ZnO-FA NPs reduced with insertion in the PVA. It implied the establishment of H–bonding between OH groups from the backbone of PVA with functional groups in ZnO NPs and FA (Fig. 10). All the evidence points out to the undeniable power of ultrasound radiations to fracture the NPs accumulation and make them better and more homogeneous in the pure PVA, which cannot be achieved with the conventional methods.

Fig. 10
figure 10

TEM images of PVA/ZnO-FA NC film 5 wt% at various magnifications (a-c)

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UV–visible absorption spectroscopy

In order to investigate the optical features of NPs and the PVA/ZnO-FA NC films, their UV–Vis spectra of them are studied (Figs. S3 and S4). The pure ZnO NPs have a specific peak which is seen at 365 nm [43]. The absorption peaks for the pure FA reveal in 280 nm (π-π* transition) and 360 nm (n-π* transition) [44]. Increasing the absorption was observed for the modified ZnO-FA NPs, which is owing to the presence of FA on the surface of the NPs.

The absorption peak at 260 nm is related to the n-π* electron transition for the pure PVA, while in the NC films of PVA/ZnO-FA, the absorption bands of NPs and FA can be detected. The absorption intensity increments with an increase in the percentages of ZnO-FA NPs but no shift for the wavelength is seen (Fig. S4).

EDX inspections

The attendance of elements such as carbon (C), oxygen (O), and nitrogen (N) are recognized, which proposes that the ZnO NPs were modified by FA (Table 1 and Fig. S5). The presence of sulfur (S) with a minimal amount (2.15 wt%) in this outcome may be related to the use of sulfur compounds in the synthesis of FA [45].

Table 1 Kinds of elements and their percentages in ZnO-FA NPs
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The results for PVA/ZnO-FA NC film is accessible (Table 2 and Fig. S6). An increase in the weight percentage of C from 18.28 wt% in the ZnO-FA NPs to 54.69 wt% in the NC film can well be pointed to the presence of PVA.

Table 2 The results of the EDX outcomes of PVA/ZnO-FA NC film 5 wt%
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Mechanical performance

In order to examine the mechanical properties of NC films, a rectangular piece of the sample was sealed between the two metal jaws, while the lower jaw was constant, the top moved upward at a constant velocity (20 mm.min−1) until the sample tears [46]. The outcomes of this test are summarized in Fig. 11 and Table 3. Mechanical performance is affected by factors such as the amount, size, morphology of NPs, quality, functionality, and distribution of them on the surface of the polymeric matrix and interactions between the polymer and modified ZnO NPs. Moreover, The agglomeration of NPs, trapping gas in the process, which could damage the structure of NPs, may contribute to the degradation of mechanical properties [47]. The amount of the Elastic modulus (E-modulus) for NC films are generally lower than the pure polymer when the concentration of ZnO-FA NPs increased; this may be due to the large amounts of NPs in the polymer matrix, which may change the general condition and effects on the structure of the polymer, and its interaction with the polymer disrupting the interaction between the polymer chains. Also, NPs is more stiffness than a polymer and easily could nest within the space of polymer chains and as limitation factors exert constraint into the molecular mobility of the polymer chains. Therefore, the rough nature of ZnO NPs can disable NC films to be enough flexible. Consequently, by adding smaller amounts of nanoparticles in this case, mechanical strength might be improved [16].

Fig. 11
figure 11

The Stress-Strain curves of the pure PVA (a), and PVA/ZnO-FA NC films of 2 wt% (b), 5 wt% (c), and 8 wt% (d)

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Table 3 The mechanical characterization of the pure PVA and fabricated PVA/ZnO-FA NC films
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Table 4 The results and images of CA for pure PVA and prepared NC films [mean (standard deviation)].
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BET surface area analysis

This analysis is used to study the specific surface area of the NPs by using the adsorption-desorption (ADS-DES) process of nitrogen gas (N2) at 77 K. [48]. The data about the surface area of the pure ZnO NPs and the ZnO-FA NPs has dropped from 41 down to 21 m2.g−1, which presented successful modification of the NPs’ surface. Actually, FA has covered pores of the sample and decreased the surface area. Figure 12 displays N2 ADS-DES isotherm curves for ZnO-FA, which the observed isotherm is compatible with type IV isotherm and the type of hysteresis loop is H3; these commentaries confirm mesoporous structure [49,50,51].

Fig. 12
figure 12

N2 ADS-DES isotherm curve of ZnO-FA NPs

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Wettability

The contact angle (CA) test is used to check the hydrophilic and hydrophobic properties of the samples. (i) The measurements of a liquid droplet in an air atmosphere, (ii) the measurements of a liquid droplet under liquid conditions, and (iii) the measurements of the air bubble under liquid conditions are three categories that are used to quantify the CA. When the amount of CA is below 90°, referred to as the hydrophilic trait, but for the hydrophobic feature, the CA value is upper than 90°. For this reason, with a syringe containing food color, three droplets were placed in different regions of NC films and after a short time, a shot took from the drop on the polymer surface, and this analysis repeated three times. The CA images for the obtained NC films as well as the average amounts of CA and standard deviations are listed in Table 4. To compare these data, the least significant difference (LSD) and One-Way analysis of variance (ANOVA) tests were operated. According to CA images and the outcomes in Table 4, NPs-containing NC films have a hydrophilic property. This feature is reduced by increasing the amount of the ZnO-FA NPs in the polymer, but for the NC film with 8 wt% of ZnO-FA NPs, the angle between surface and droplet decreased. The reason for this phenomenon may be due to the agglomeration of the NPs in the polymer at this percentage. The value of ρ reported 0, which means values reflected statistically significant [52, 53].

Thermal properties

In the TGA curve of pure ZnO NPs, the minor reduction is perceived, which is associated with the elimination of water that physically absorbed on the surface of NPs, but there is no significant reduction in weight in the temperature range [54]. For ZnO-FA NPs, the thermogram presents a mass loss in three main stages. The first stage is related to the dehydration of the sample. The decomposition of FA becomes evident in the second step in the range of 200–650 °C. In the third step, the organic samples are pyrolyzed and produced black carbon, thus, the observed mass loss in the range of 700–800 °C is relevant to the formation of carbon black and ash. (Fig. 13) [55] According to the TGA curves of the pure ZnO NPs and ZnO-FA NPs, approximately 13% of FA was grafted on to the surface of ZnO NPs.

Fig. 13
figure 13

The TGA curves of the pure ZnO NPs (a) and ZnO-FA NPs (b)

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In Fig. 14 the thermal stability behavior of the pure polymer and fabricated PVA/ZnO-FA NC films are reviewed. In all curves, three principal mass losses are seen. In the temperature range of 50–170 °C, the observed mass loss is ascribed to the vaporization of the remaining solvent and the weakly physical and strong chemical bound water. A massive drop in the area of 230–370 °C is related to the decay of the hydroxyl groups as a side chain in the polymer chain, and the third stage of mass occurred in the area of 370–480 °C showed carbonation which means the fracture of the C–C backbone of the PVA [56, 57].

Fig. 14
figure 14

The TGA curves of the pure PVA (a), and PVA/ZnO-FA NC films of 2 wt% (b), 5 wt% (c), and 8 wt% (d)

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Table 5 displays temperature information when the mass loss is 10% (T10%) and 50% (T50%) in the NC films, and also the percentage of residual weight at 800 °C [char yield (CY)]. It shows that the thermal resistance of the NC films measures up to the pure PVA has significantly increased, according to the information of Table 5. Concerning the data of DTG curves (Fig. S7), Tmax (the temperature where a sudden mass loss of the specimen happened) can be achieved. The values of Tmax are raised to 16 °C, 40 °C, and 50 °C for PVA/ZnO-FA NC films 2, 5, and 8 wt% compared to the pure PVA, respectively. There is also an increase in the CY values for NC films which have ZnO-FA NPs. These results inferred strong interactions between PVA and ZnO-FA NPs, which caused the movement of polymer chains near the surface to be decreased [58].

Table 5 The consequences obtained from the thermal stability analysis
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Antibacterial activity

The antibacterial property of the PVA/ZnO-FA NC film 8 wt% was assessed against E. coli and S. aureus as Gram-negative and Gram-positive bacteria for 24 and 48 h, respectively. As shown in Fig. 15, pure PVA has no antibacterial property and cannot inhibit bacterial growth, but the addition of ZnO-FA NPs to the PVA matrix can improve the antibacterial property, and over time, bacterial growth is stopped further. The bacterial inhibition by PVA/ZnO-FA NC film 8 wt% is because of the presence of metal oxides such as ZnO NPs which carry the positive charges, whereas the microorganisms move the negative charges. This difference in the type of charges leads to electrostatic interactions between ZnO NPs and organisms, which causes oxidation and finally death of organisms [59]. The percentage of bacterial inhibition tis tabulated in Tables 6 and 7.

Fig. 15
figure 15

Antibacterial activity of control sample, pure PVA, and PVA/ZnO-FA NC film 8 wt% against E. coli (A) after 24 (a) and 48 h (b), and S. aureus (B) after 24 (a) and 48 h (b)

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Table 6 The percentages of bacterial inhibition against E. coli
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Table 7 The percentages of bacterial inhibition against S. aureus
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The results show the higher activity of PVA/ZnO-FA NC film 8 wt% against S. aureus in contrast with E. coli. The discrepancy in the cell structure, physiology, metabolism, and the degree of contact of organisms with ZnO NPs may be the reasons for the difference in the resistance against the Gram-negative and the Gram-positive bacteria. As an example, S. aureus has more carboxyl and amine groups on its cell surface, so more dependencies exist between ZnO NPs and these groups [60, 61].

Conclusions

In the present study, the first effort was made to amend the properties of PVA by placing ZnO NPs in the polymeric matrix. While for better dispersion, FA was applied as a modifying agent. The most significant tool which was utilized in this investigation was an ultrasonic device. The yielded ultrasonic waves not only reduce the procedure time but also allow to do reactions that are not possible with conventional approaches. In the dispersion stability test, FA had an impressive role in stabilizing ZnO NPs. Also, sonication waves had remarkably influence on the good distribution of the modified ZnO NPs in the solution of the polymeric matrix. According to the previous results, the obtained data from FT-IR and EDX analyses are easily verified that, as expected, the ZnO NPs with FA have been modified. As well as the shifts of band locations in the FT-IR spectrum can be explained hydrogen and electrostatic interactions. The thermal properties were checked, and the obtained outcomes presented that PVA/ZnO-FA NC films 5, and 8 wt% have better thermal stability because there was a dramatic increase in Tmax, which showed a rise of 40 and 50 °C compared with pure PVA, respectively. The spherical structural morphology and the uniform distribution in the polymeric matrix for the ZnO-FA NPs were observed. The mechanical performance of NC films can be improved by optimizing the amounts of NPs. The water CA showed that the polymeric matrix which was occupied with 5 wt% of ZnO-FA NPs has a stronger hydrophobic property than the rest. This feature is because of the firm hydrogen interactions between the agent groups of the modified NPs and polymer, which diminished the number of accessible hydroxyl groups in the NC films and, as a result, decreased the hydrophilicity property. Overall, FA as a modifying agent, and ultrasound irradiation, can be named as useful factors in the non-agglomeration of the NPs. PVA/ZnO-FA NC film 8 wt% showed the good resistance against E. coli and S. aureus bacteria that this feature is highly regarded in the manufacture of food packaging construction industry, and textile industry, by employing this method known as the green method, other NC films with other properties could be prepared and utilized in the different technologies.