Introduction

The creation and processing of nanocomposite materials with a polymer matrix that possesses bio-safe attributes like biocompatibility and biodegradability may be of relevance in the current contaminated surroundings. Polymer nanocomposites (PNCs) have gained prominence in various scientific disciplines, including biology, electronics, and mechanics, due to their exceptional mechanical, biological, and physical properties [1]. Polymer nanocomposites (PNCs) offer significant advantages over polymers doped with micron-sized nanofillers, including enhanced thermo-mechanical properties such as degradation resistance and dielectric strength. The incorporation of suitable fillers can effectively tailor the mechanical, optical, thermal properties, and antimicrobial activity (ANMAC) of polymers to meet specific requirements [2, 3]. One of the earth’s most prevalent naturally occurring alkaline polysaccharides is chitosan (Cs) biopolymer [4]. The pursuit of innovative polymer electrolytes, biopolymers, or polysaccharides such as Cs, carboxymethyl cellulose, and cellulose, holds promise as host polymer networks [5, 6]. One of the many benefits of biopolymers, particularly Cs, is that they are inexpensive, readily available, and environmentally benign [7]. Additionally, they contain β (1–4)-linked D-glucosamine de-acetylated linear polysaccharide [8, 9]. Two atoms, O and N, in Cs, have a lone pair of electrons [8]. The backbone of Cs contains ether (C–O–C), O–H group, and amine (NH2) that enable potential interactions between the cations of filler and other polymers like PVA, PVP, etc. in terms of polymer blending [10, 11]. PVA possesses a wealth of O-H group with its side chains, imparting excellent water solubility and tunable optical and electrical properties upon doping. Their properties make them a good choice for blending with Cs. PVA, a non-toxic, biocompatible, water-soluble, and biodegradable synthetic polymer, stands as a versatile host polymer for preparing polymer blends with biopolymers [12]. . The O–H groups along the side chains of polyvinyl alcohol facilitate interactions with other polymers and biopolymers containing functional groups like those in chitosan, enabling the formation of miscible polymer blends [8]. It’s possible that using a single biopolymer won’t provide good chemical, optical, or mechanical flexibility. Polymer blending emerges as a promising strategy to improve the mechanical flexibility, optical properties, and ionic conductivity, of electrolytes [13]. Nano-composite films based on Cs/PVA-ZnO exhibit antimicrobial activity [14]. The polymer network’s semi-crystalline phase shows constrained segmental motion of the polymer chains as well as limited free space between and polymer chains the along. The physical properties of polymers are influenced by the nature of chemical of the nanofiller and its interactions with the polymer blend. The properties of polymer matrices can be significantly enhanced by incorporating various nanofillers, such as carbon nanotubes, metal oxides, and rare earth doped ions. The effectiveness of these nanofillers depends on their interaction and dispersion within the polymeric matrices [15]. Due to their special adjustable optical characteristics, bimetallic nanoparticles are utilized extensively [16]. In addition to the need for novel materials in optical and optoelectronic applications, the introduction of novel medications is critically needed because of infections spread by various bacteria [17]. The Co3O4, a versatile inorganic material, has found widespread applications in both bulk and nano forms across various fields, including electrochemical systems, electrochromic devices, water electrolysis processes, high-temperature solar selective absorbers, and rechargeable lithium-ion batteries [18, 19]. The potential formulation of a novel class of bactericidal materials may be made possible by the Co3O4 nanoparticles. Despite the growing interest in cobalt oxide nanoparticles, there is a dearth of research investigating their antibacterial potential. A limited number of recent investigations have demonstrated that Co3O4 NPs exhibit synergistic antibacterial effects and can combat both gram-negative and gram-positive bacteria [19, 20]. V. Gupta et al. [21] explored the antibacterial activity of Co3O4 NPs across a range of concentrations and compared their efficacy to established antibiotics tetracycline and gentamicin. The antibacterial efficacy of these nanoparticles demonstrated their potential to reduce the environmental burden of pathogenic bacteria and combat antibacterial resistance and could be applied in different areas like food packages, water disinfection, medical sciences, etc. This study focuses on the fabrication of Co3O4 NPs as nanofiller to investigate their impact on the physical and chemical properties of PVA/Cs blend. A comprehensive characterization of PVA/Cs blend has been conducted using various techniques to evaluate its structural, optical, thermal, mechanical, and antifungal properties. This extensive investigation will contribute to the development of novel nanocomposites with potential applications in antimicrobial packaging and optoelectronic devices.

Materials and methods

Materials

Polyvinyl alcohol (PVA) (C2H4O)n, 99% purity, and M.W = 14,000 g/mol purched from E-Merck, Germany. Chitosan powder (Cs) has a medium molecular weight, viscosity: 200–800 cP, and 75–85% deacetylated from Sigma-Aldrich. Deionized water (DW) was used as solvent.

Synthesis of Co3O4 Nanoparticles

Co3O4 NPs were prepared via the precipitation of cobalt salt in an alkaline medium [21]. In brief, a 3 M sodium hydroxide solution was used to raise the pH of an aqueous CoCl2 solution (0.6 M) to 10.5 after it had been heated to 80 °C. After two hours of stirring the solution at 80 °C, the precipitates-green cobalt hydroxide were centrifuged and then washed with distilled water and ethanol until the pH became 7. After that, the precipitate was dried for 12 h and finally was annealed for three hours at 150 °C to produce powdered Co3O4 NPs.

Synthesis of PVA/Cs- Co3O4 Nanocomposite Films

To commence the process, 0.2 g powder of PVA was dissolved in 20 ml of DW by heating at 80 °C for four hours, followed by stirring until a transparent solution was obtained. Concurrently, 0.8 g Cs powder was dissolved in a mixture of acetic acid and DW (2:8) by heating and stirring at 50 °C for four hours. Subsequently, both solutions were combined and stirred at RT for eight hours. To prepare the Co3O4 solution, Co3O4 nanoparticles were dispersed in 10 ml of DW using ultrasonication at room temperature for one hour. Subsequently, the desired amount of Co3O4 was added dropwise to the polymer mixture solution and stirred for approximately 12 h to achieve a homogeneous dispersion of Co3O4 nanoparticles in the PVA/Cs (20/80 wt%) matrix. Finally, the uniformly dispersed solution was poured into a glass Petri dish and cast at 50 °C for five days. The resulting smooth and flexible blend and blend/ Co3O4 nanocomposites films were carefully peeled off and stored at room temperature for further characterization.

Antibacterial and Antifungal Activities Assay

Gram-negative bacteria (Escherichia coli) and Gram-positive bacteria (Staphylococcus aureus) were used to assess the produced compounds’ antimicrobial properties. Two fungi were used to assess the compounds’ anti-fungal properties (Aspergillus niger, Candida albicans). After dissolving each chemical in DMSO, a separate solution with a concentration of 1 mg/ml was created. Whatman filter paper discs in the typical size of 5 cm were manufactured, cut, and autoclave-sanitized. The petri plates containing nutrient agar media (agar 20 g + beef extract 3 g + peptone 5 g) seeded with S. aureus, E. coli, C. albicans, and A. niger were aseptically filled with paper discs soaked in the necessary concentration of the complex solution. After 24 h of incubation at 36 °C, the inhibition zones in the petri dishes were measured. The test agent’s antibacterial activity was assessed by measuring the zone of inhibition’s diameter, which was expressed in millimeters (mm). Each treatment was replicated three times, and the average reading was reported [22]. Using the same protocol as previously described, the antifungal Colitrimazole and the common standard antibiotic ampicillin were also tested for antibacterial activity at the same concentrations and solvent combinations.

Characterization Techniques

Using Copper Kα radiation, X-ray diffraction (XRD) analysis was carried out using the Pan analytical X’ Pert PRO XRD system at a wavelength of λ = 1.540 A°. Transmission electron microscopy (JEOL 1200 EX) was utilized to specify the shape and particle size distribution of Co3O4. The prepared films were analyzed at room temperature using FT-IR spectroscopy (Nicolet iS10, USA) in the 4000–500 cm−1 range to identify the functional groups. Using a JASCO 630 Japan ultraviolet–visible (UV–vis) spectrophotometer in the 190–800 nm wavelength range, optical properties were measured at room temperature. Thermogravimetric analysis (TGA) was carried out utilizing an STD-Q600 thermal analyzer (USA) at a heating rate of 10 °C min−1 in N2. To examine the mechanical characteristics of the nanocomposite films, the AMETEK LLOYD (LLOYD-5 KN, London, UK) Universal Testing Machine (UTM) was utilized.

Results and Discussion

XRD

To explore the structural modifications induced by the incorporation of Co3O4 nanoparticles into the PVA/Cs polymer blend, X-ray diffraction (XRD) measurements were conducted. Figure 1a reveals the crystalline structure of the synthesized Co3O4 nanoparticles, as evidenced by the sharp peaks in the XRD pattern. The absence of any additional peaks confirms the purity of the nanoparticles, indicating the successful synthesis of Co3O4 nanoparticles without any impurities. The XRD pattern exhibits a prominent peak at 37.65°, corresponding to the (311) plane of cobalt oxide crystals. Additional well-defined diffraction peaks are observed at 19.0°, 31.91°, 37.65°, 38.68°, 44.89°, 55.72°, 59.61° and 65.82° corresponding to the (111), (220), (311), (222), (400), (422), (511) and (440) planes, respectively with lattice constants for the Co3O4 phase of = 0.521 nm and = 0.324 nm, respectively (JPCDS File No. 36–1451) [23]. The formation of crystalline Co3O4 nanoparticles is confirmed by XRD. The Scherrer equation (D = 0.9λ/β cosθ) was used to determine the crystal sizes of Co3O4, the crystallite size is 12.5 nm [24]. The dislocation density is determined using the following equation [25]:

$$\delta = {\text{1}}/{\text{D}}^{{\text{2}}}$$
(1)

Through this equation, it is observed that the dislocation density value of the pure Co3O4 is 6.4 × 10−3 m−2. Micro strain (ε) value of Co3O4 NPs can be calculated using the formula [26]:

$$\varepsilon = {\text{b}}/{\text{4tan}}\theta$$
(2)

where, β is the full width at half maximum (FWHM). It was observed that the value of the Micro strain (ε) for pure Co3O4 using this equation was 1.62 × 10−3.

Figure 1b illustrates the XRD patterns of PVA/Cs-based polymer blend samples incorporated with varying contents of Co3O4 NPs. The broad peak observed at 2θ = 20.21° for the pure polymer blend film is attributed to its semi-crystalline nature, arising from intramolecular and intermolecular interactions between polymer chains through hydrogen bonding. The XRD peak exhibits broadening and a decrease in intensity upon the introduction of Co3O4 nanoparticles, implying a reduction in the crystallinity of the polymer nanocomposite films [27, 28]. The 1.5%Co3O4/PVA/Cs and 0.5%Co3O4/PVA/Cs samples do not exhibit any peaks in intensity for Co3O4 NPs, as can be seen from the XRD diffractogram, which suggests that the added nanoparticles have dissociated. The broadening of XRD peaks, indicative of increased amorphicity, likely contributes to the higher ionic density and enhanced ionic conductivity observed in this sample [29]. The incorporation of Co3O4 nanoparticles induced slight shifts in the XRD peaks compared to the pure polymer blend, suggesting effective interactions between the nanofiller and the polymer matrix. Notably, the 2.5% PVA/Cs and 3.5% PVA/Cs samples exhibit sharp crystalline peaks at diffraction angles of 31.91°, 37.65°, 38.68°, 44.89°, 55.72°, 59.61° and 65.82°, which can be ascribed to the recrystallization of Co3O4 nanoparticles on the film surface due to ion recombination at higher Co3O4 NPs concentrations and may be due to agglomeration/aggregation of Co3O4NPs in blend [30]. The distinct diffraction peaks observed in samples with higher Co3O4 nanoparticle concentrations are in good agreement with the XRD data of the Co3O4 NPs.

Fig. 1
figure 1

XRD pattern of a Co3O4 nanoparticles and b pure blend and blend/x (wt%) Co3O4 nanocomposites films

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Morphology Analysis by TEM

Transmission electron microscopy (TEM) was used to investigate the structural characteristics of the synthesized Co3O4 NPs as shown if Fig. 2. The TEM image presented in Fig. 2a revealed that the synthesized product exhibits a uniform morphology characterized by approximately spherical nanoparticles with a narrow size distribution. The small dimensions and high surface energy of these nanoparticles lead to a tendency for aggregation, as observed in Fig. 2a. The particle size histogram derived from the TEM image using the using SmartTiff software, shown in Fig. 2b, further confirms the narrow size distribution of the Co3O4 nanoparticles, with diameters predominantly ranging from 5 to 45 nm. The average particle size, determined to be 15.5 nm and a standard deviation of 5.23, is in close agreement with the value calculated using the Scherer equation based on the half-width of the diffraction peaks.

Fig. 2
figure 2

a TEM image of the Co3O4 NPs and b Histogram showing the distribution of the Co3O4 nanoparticles size

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FTIR Spectra

Figure 3 presents the FTIR absorbance spectra of Pure Co3O4 NPs, pure Cs, pure PVA, pristine blend and blend/x(wt%) Co3O4 nanocomposites samples within the wavenumber range of 500 to 4000 cm−1. The Co3O4’s FTIR transmittance spectra were captured within a range of 4000 cm−1–500 cm−1 as shown in Fig. 3a. The intensity of absorption linked to O–H groups and free water in the range of 3350 cm−1 to 3650 cm−1 is directly correlated with the moisture content of Co3O4. The bands ranging from 1370 cm−1 to 1596 cm−1 are indicative of carbonyls (C=O) and a strong three-bond alkyne group (C–C). The peaks 860 cm−1, 696 cm−1, and 620 cm−1 in the fingerprint region show that the sample contains a significant amount of ethers, esters, and carboxylic acids (C–C and C–O bonds) [31]. Figure 3b displays the infrared spectrum of pure Cs. For Cs, the axial stretching of the overlapped O–H and N–H bands is represented by the wide band in the 3100–3600 cm–1 range. Small bands at 2930 cm−1 and 2855 cm−1 are attributed to C–H in both –CH2 and –CH3 bonding, respectively. The typical absorption bands at 1654 cm−1 (amide I), 1588 cm−1 (amide II), and 1422, 1380 cm−1 (–CH, –CH2 bending) [32] are visible in the chitosan spectrum Fig. 3b. The asymmetric vibrations stretching C–O in oxygen are responsible for the band at 1170 cm−1, while the C–O of the ring COH and COC bonds are related to the bands at 1090 cm−1 [33]. The peaks at 3445 cm−1 and 1360 cm−1 for pure PVA were attributed to the PVA film’s OH stretching and bending vibrations. At 2930 cm−1, the peak corresponding to the asymmetric stretching vibration of the CH2 was observed [34]. The unhydrolyzed ester functional group on the PVA backbone had a C=O, which was represented by the peak at 1670 cm−1. The C–O–C asymmetric stretching vibration of the ester group could be responsible for the peak at 1270 cm−1 [35]. With a few exceptions to the PVA/Cs blend film’s distinctive form and absorption band shifts brought about by hydrogen bonding between functional groups, the PVA/Cs blend film’s spectra generally showed the same peaks as those seen in the Cs and PVA films. Therefore, in the studied blend, both intramolecular and intermolecular interactions are possible [36]. The FTIR spectrum of PVA/Cs blend exhibits characteristic bands at 3354 cm−1 and 3293 cm−1, corresponding to the OH and N–H groups stretching vibrations, respectively. C–H symmetric and asymmetric stretching vibrations are attributed to the bands at 3304 cm−1, 2921 cm−1, and 2854 cm−1, respectively. Additionally, peaks assigned to O=C–NHR stretching vibrations of the C=O and vibrations associated with amino group (N–H) appear at 1645 cm−1and 1560 cm−1, respectively. Bands at 1430 cm−1and 1374 cm−1are attributed to C–H bending and C–H wagging vibrations. The C–H wagging vibration band of the acetate residue is observed at 1243 cm−1, and C–O stretching vibration bands are evident at 1088 cm−1 and 1021 cm−1 [14]. The FTIR spectra of blend/Co3O4 nanocomposites with varying Co3O4 nanoparticle loadings reveal characteristic peaks similar to those of the pure blend, as shown in Fig. 3c. The O–H bands and C–H symmetric and asymmetric bands shift towards lower wavenumbers upon Co3O4 nanoparticle incorporation. The peaks corresponding to O=C–NHR stretching and N–H bonding shift to lower wavenumbers, and the peaks at 1645 cm−1and 1560 cm−1 become more intense with increasing nanoparticles loading [37, 38]. Similarly, the peak associated with C–N bending vibrations of Cs and C–OH stretching of PVA shifts to a lower wavenumber at 1090 cm−1, while its intensity decreases significantly, indicating a reduction in the crystallinity of the pure blend [39]. The peak corresponding to the PVA skeletal vibration and the Cs saccharide structure exhibited a shift towards a lower wavenumber and a decrease in intensity upon nanofiller loading, indicating complexation between the functional groups of the nanofiller and the polymer blend (Scheme 1). This demonstrates that Co3O4 NPs and the polymeric chains are compatible.

Scheme 1
scheme 1

The possible mechanism for interaction between PVA/Cs and PVA/Cs/Co3O4 NPs.

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Fig. 3
figure 3

FTIR absorbance spectra of a pure Co3O4 NPs, b pure Cs, pure PVA, and PVA/Cs blend c pure blend and PVA/Cs/x(wt%) Co3O4 nanocomposites films

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UV–Vis Absorption

Ultraviolet-visible (UV–Vis) spectroscopy is a valuable tool for optical characterization. Figure 4 presents the absorption spectra of the pure PVA, Cs, pure blend and the blend doped with Co3O4 nanoparticles. A prominent shoulder band at 240 nm is monitored, which can be ascribed to the semi-crystalline nature of the pure blend and the presence of a C=O-containing structure associated with the polymer blend [40, 41]. These bands are ascribed to electronic transitions within the polymer backbone π–π* [14, 42]. The intensity of the absorption band increases with growing Co3O4 wt% in blend. These findings indicate an interaction between blend and nanofiller. Notably, none of the examined samples exhibited absorption bands in the visible region, confirming their transparency. This figure shows that, for all films, the absorption is roughly unchanged in the spectral range from 400 to 800 nm, but there is a significant change in the ultraviolet from 200 to 400 nm, where the fundamental absorption edge of the blend sample shifts towards a longer wavelength (red shift) in the blend-x wt.%Co3O4 ions nanocomposite samples and this shift increase by growing Co3O4 ions content in the blend demonstrates the complexation between blend and Co3O4 NPs which leads to in crystallinity change [43]. The absence of a well-defined optical absorption edge suggests a low degree of crystallinity in the films. Moreover, the absorbance edge is a defining characteristic of the charge transfer complex (CTC) formation between polymer blend chains and dispersed Co3O4 NPs [44, 45]. This result validates the highly wavelength-controllable absorbance behavior of this PNCs material, suggesting that it might be a good option for developing flexible devices that use spectral shorting in the future.

Fig. 4
figure 4

UV–Vis absorption spectra of a pure PVA, Cs and PVA/Cs blend b pure blend and PVA/Cs/x(wt%) Co3O4 PNCs.

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Determination of Optical band gap(OBG)

The OBG is a crucial method for defining the optical transition in prepared samples, as illustrated by Tauc’s plot [46, 47].

$$(\alpha {\text{hn}})^{{\text{n}}} = {\text{ A }}({\text{h}}\nu - {\text{E}}_{{\text{g}}} )$$
(3)

where A is a constant and the value n indicates the type of optical transformation; n = 2 and 1/2, respectively. Consistent with the findings of Hezma et al. [14], Fig. 5; Table 1 reveal an energy gap (direct and indirect) of 5.52 and 5.07 eV for PVA/Cs film. Nevertheless, the direct energy gap was reduced from 5.52 to 4.01 eV and the indirect energy gap was reduced from 5.07 to 3.56 eV by Co3O4 doping with a concentration ratio of 0.0wt.% and 2.5wt.% of PVA/Cs. This reduce in the optical bandgap can be explained by the formation of Co3O4 in the electronic structure of the PVA/Cs matrix, which is in charge of creating localized states between the band valence and conduction. Soliman et al. observed a decrease in the OBG as the BaTiO3 concentration in the PVA matrix increased. This reduction is attributed to the introduction of defects that disrupt the orderliness of the polymer matrix [48]. Table 1 show that the Edir and Eind values gradually increase with the addition of only 3.5 wt% of Co3O4 to the PVA/Cs bend. The increase in polymer backbone conjugation and electron-donor nature, along with the addition of Co3O4, which splits the valence and conduction bands, are the reasons for this increase in the optical energy gap [49]. For the Co3O4NPs, there are two optical band gaps as shown in Fig. 5c, indicating direct allowed transitions. According to the figure, ΔEg = 0.27 eV separates the first band gap (Eg1), which is 1.52 eV, and the second band gap (Eg2), which is 1.79 eV. Additionally, very similar results for thin films and Co3O4NPs were found [50]. While Eg1 is linked to the beginning of \({\text{O}}^{{{\text{ - 2}}}} {\text{ }} \to {\text{ Co}}^{{{\text{ + 3}}}}\)  excitations, Eg2 is attributed to the interband transition and is considered the basic or genuine band gap energy. The measured Eg1 and Eg2 are less than the bulk values of 2.85 eV and 1.70 eV, respectively. The quantum confinement effects in the nanomaterials could be the cause of this [51].

Fig. 5
figure 5

The dependence of a (αhυ)2, b (αhυ)1/2 and c (αhυ)1/2 on hυ for pure Co3O4 NPs and all the prepared films

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Thermal Analysis by TGA

TGA was utilized to examine the thermal stability of pure blend and Co3O4 NPs combined with PVA/Cs. TGA thermograms of pure blend and Co3O4NPs combined with PVA/Cs thin films are displayed in Fig. 6. Thermal degradation is observed in two steps in the TGA of pure blend and Co3O4/PVA/Cs PNCs. The first weight loss occurred between 25 °C and 113 °C, which is when the water and moisture that had been adsorbed evaporated. The main chain of the PVA/Cs matrix’s breakdown, chemical stability, and intermolecular and intramolecular bonding are all related to the significant weight loss that was seen between 275 and 700 °C [52]. The two weight-loss stages of the pure blend are 113 °C and 275 °C. When Co3O4NPs are added to the PVA/Cs matrix, the weight loss of the nanocomposite films reduced. In the nanocomposits, the weight loss drops from 81% for the pure blend to 38% with the addition of 3.5%Co3O4 NPs. This phenomenon may be explained by the polymer matrix’s increased physicochemical bonding density. It depicts the physical binding density, which describes the quantity of non-electronic-sharing interactions between atoms, such as per unit volume, ionic-based interactions, as well as the chemical bonding density, which is the number of bonds, such as covalent and the weaker interactions involving electron-sharing, among the atoms of the PVA/Cs-Co3O4 nanocomposite per unit volume [53]. At 700 ◦C, no more weight loss was seen. As can be seen from the trend below, the PVA/Cs blend exhibits improved thermal stability as the concentration of Co3O4NPs increases while the percentage weight loss decreases. Furthermore, the TGA curves of the Co3O4/PVA/Cs nanocomposites shift toward higher temperatures as a result of the interaction between the blend and Co3O4NPs [54].

Fig. 6
figure 6

The TGA thermograms of a pure PVA, Cs, and PVA/Cs blend b blend and blend doped with Co3O4 NPs

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Mechanical Properties

The mechanical behavior of pristine blend and PVA/Cs-Co3O4 PNCs films was analyzed employing a Universal Testing Machine (UTM). Figure 7 illustrates the stress-strain curves for pristine blend and its corresponding PVA/Cs-Co3O4 PNCs. The curves reveal that the pure PVA/Cs film exhibits a lower stress-bearing capacity compared to the PVA/Cs-Co3O4 PNCs. The slope of the linear region of the stress-strain curves in Fig. 7 represents the Young’s modulus of the respective samples. The mechanical properties of PVA/Cs-Co3O4 nanocomposites are exhibited in Table 1. Analysis of Young’s modulus, tensile strength, and elongation% are essential to evaluate the suitability of material for food packaging applications. As seen from Table 1, tensile strength, Young’s modulus, and elongation% of all PVA/Cs-Co3O4 nanocomposite films significantly surpass those of the pure blend. The PVA/Cs blend with 2.5 weight% Co3O4 exhibits the highest tensile strength of the composite. Strong interfacial adhesion between the blend chains and Co3O4 nanoparticles accounts for the enhanced tensile strength of the PVA/Cs-Co3O4 nanocomposite [55]. The composite’s tensile strength also increases as the concentration of Co3O4 nanofiller increases up to 2.5 weight%. This suggests that the Co3O4 nanofiller has been dispersed more uniformly, creating a larger interfacial area and improving the blend’s and Co3O4 interfacial adhesion [56]. The Young’s modulus and tensile strength decrease when the concentration of Co3O4 NPs surpasses 2.5wt% because of the inadequate reinforcing effect of filler particles as a result of their aggregation. Poor polymer-filler interaction results from heterogeneous dispersion of aggregated NPs in the blend at higher loadings of Co3O4 nanoparticles [57]. The incorporation of 3.5% Co3O4-PVA/Cs resulted in a decrease in the toughness of the blend, which could be attributed to the increased rigidity and restricted molecular mobility caused by the strong interaction between Co3O4 nanofiller and blend [58]. Strong Co3O4 NP–PVA/Cs matrix interactions improve the material’s Young’s modulus and tensile strength, making it appropriate for food packaging [59].

Fig. 7
figure 7

Stress-strain curves of pure blend and PVA/Cs/x(wt%) Co3O4 PNCs films

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Table 1 The OBG (direct and indirect) values and mechanical properties for pure blend and Co3O4 doped PVA/Cs.
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Antimicrobial Properties

The antimicrobial efficacy of the pristine blend and Co3O4-PVA/Cs PNCs was assessed employing the disc diffusion method against a range of Gram-positive (S. aureus) and Gram-negative (E. coli) bacteria, as well as fungi (C. albicans and A. niger) as shown in Fig. 8. The effectiveness of the samples against microorganisms was assessed by measuring the inhibition zone diameters. The antibacterial activity of a commonly used standard antibiotic, ampicillin, and the antifungal drug, clotrimazole, was also evaluated. The tested PVA showed very little activity against microbial strains compared to chitosan, while chitosan showed properties slightly lower than those shown by pure blend. The Fig. 8 demonstrates that the PVA/Cs blend without Co3O4 nanoparticles exhibited limited antibacterial activity against both Gram-negative and Gram-positive bacterial strains. The addition of Co3O4 to the PVA/Cs blend demonstrated exceptional antibacterial efficacy against all bacteria tested. Additionally, the diameter of the clear zone grew with the increasing amount of Co3O4 in the polymer blend. For instance, the antibacterial activity against S. aureus, assessed by the inhibition zone diameter, demonstrated a positive correlation with the increasing content of Co3O4 NPs in the polymer blend. Similarly, a positive correlation between clear zone diameters and Co3O4 NPs content was observed for other tested bacteria. The polymer blend’s antibacterial activity against S. aureus, E. coli, C. albicans, and A. niger increases when Co3O4 NPs are added. 3.5% Co3O4-PVA/Cs nanocomposite film exhibited the highest antibacterial activity against A. niger, with a zone of inhibition 31 mm. It is evident that when the concentration of Co3O4NPs rises, so does the zone of inhibition. The increased composition of Co3O4NPs in the 3.5% Co3O4-PVA/Cs nanocomposite may be the cause of the enhancement in its activity. Gram-positive S. aureus bacteria were more strongly inhibited by the Co3O4-PVA/Cs nanocomposite films than E. coli bacteria. This difference in the composition and structure of the bacteria’s cell walls may be the cause of this. The microorganisms growth was effectively inhibited by the increase in weight of Co3O4NPs, the similar results were reported by Akhavan et al. [60]. There are potential differences in the observations that are reported, because the bacterial cell wall structure varies [1]. The Co3O4-PVA/Cs nanocomposites may specifically target the thick peptidoglycan layer of E. coli, increasing their antibacterial potential [61]. Several methods have been proposed to explain PVA/Cs-Co3O4’s antibacterial activity, as illustrated in Fig. 9: (i) The polymer absorbs onto the target bacteria due to electrostatic attractions between its positively-charged chains and the negatively-charged bacterial cell walls, disrupting the cell wall [62]. (ii) Reactive oxygen species (ROS) production [63, 64]. (iii) One possible explanation for the inhibitory activity of nanoparticles could be their ability to enter microbes and cause harm through interactions with the phosphorus and sulfur groups found in DNA and proteins [65, 66]. Table 2 lists earlier research on polymer nanocomposites and how they affected S. aureus and E. coli. These findings suggest that nanocomposites’ appropriate composition can be used to customize their potential antibacterial profile for use in food packaging applications.

Fig. 8
figure 8

Shows the antibacterial inhibition area diameter of Cs/PVA-Co3O4 PNCs films

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Fig. 9
figure 9

Mechanism of antibacterial activity of Cs/PVA-Co3O4 nanocomposites

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Table 2 lists earlier research on polymer nanocomposites and how they affected S. aureus and E. coli
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Conclusions

Effective synthesis of Co3O4 nanoparticles using cobalt salt has been accomplished. The existence of Co3O4 NPs and the increased amorphous content of the polymer blend are validated by XRD analysis. The FTIR analysis confirms that the interactions between PVA/Cs blend functional groups and nanofiller. With the addition of nanoparticles, the intensity of the UV–visible absorption spectra increases. Because more energy states were introduced between the valence and conduction bands as the dopant concentration increased, the optical absorption edge and optical band gaps (both direct and indirect) showed a decreasing trend. The improved optical properties of the 2.5% Co3O4-PVA/Cs nanocomposite indicate that it may be a promising material for optoelectronic applications. The TGA analysis of PVA/Cs-Co3O4 nanocomposites revealed that the addition of Co3O4 nanoparticles enhanced the thermal stability of PVA/Cs. The Young’s modulus, tensile strength, and elongation% of the blend were all improved by the Co3O4 nanoparticles, according to the results. Co3O4 nanoparticles integrated with PVA/Cs film demonstrated outstanding antibacterial activity against fungi (A. niger and C. albicans) as well as Gram-negative (E. coli) and Gram-positive (S. aureus) bacteria. Additionally, the Co3O4-PVA/Cs nanocomposite’s potential antibacterial profile can be tailored for food packaging and biomedical applications with the help of an appropriate composition.