Introduction

Foodborne pathogenic bacteria are one of the main factors affecting food safety, particularly in areas such as food processing, storage, transportation, and sales. To ensure food hygiene and safety, the role of food packaging is crucial. Antimicrobial cling films can inhibit the growth of foodborne pathogens, improving food quality and safety, while extending the shelf life of food. As a result, these films have garnered significant attention from the food packaging industry. However, the environmental pollution caused by traditional plastic cling films has become an increasingly prominent issue, and the search for biodegradable alternative materials has become a major focus for researchers. Natural biopolymers have attracted considerable interest due to their abundant availability, biodegradability, and ease of processing1.Common natural biopolymers include proteins (such as collagen, casein, gelatin, and zein)2,3,4, polysaccharides (such as starch, chitosan, carrageenan, and cellulose)5,6,7,8, and lipids (such as fatty acids, stearic acid, and beeswax)9,10,11.

Carrageenan is a natural, linear sulfated polysaccharide obtained from red seaweeds. It is composed of alternate units of D-galactose and 3,6-anhydro-galactose joined by α−1,3 and β−1,4-glycosidic linkages12,78,79,80. Based on the position and number of ester sulfate groups, these polysaccharides are classified into κ-carrageenan, i-carrageenan, and λ-carrageenan et al.13. Among them, κ-carrageenan possesses excellent film-forming properties and has been widely developed as a food packaging material14. Carrageenan is commonly used as a multifunctional packaging film to extend the shelf life of food products due to its outstanding film-forming ability, biocompatibility, biodegradability, high gelling ability, and barrier properties15. Studies have shown that the incorporation of antimicrobial agents into carrageenan films is a promising method to enhance their antimicrobial effect16,17,18.

Polyoxometalates (POMs) are metal-oxygen cluster compounds formed by the coordination of transition metals with oxygen, including various structures such as Keggin, Well-Dawson, Anderson, Waugh, and others.Due to their unique structure and excellent antimicrobial19,20and enzyme inhibition21,22properties, POMs, particularly the Keggin and Well-Dawson types, have important applications in food preservation23,81,82,83,84,85,86. Fang JQ et al.24 found that Dawson P2Mo17Ni could effectively inhibit the activity of polyphenol oxidase, slowing down the blackening and spoilage processes of South American white shrimp, thereby prolonging its shelf life. Xing R et al.25 found that manganese-substituted PMo11Mn was able to better maintain the flavor and taste quality of grape berries, reduce the weight loss and browning rates of fresh grapes, inhibit the degradation of titratable acid content, and effectively prolong the shelf life of grapes.

The Keggin-structured Na3PW12O40 exhibits excellent antibacterial properties.Lv BL et al.26studied the antibacterial activity of POMs (PW12, H3PW12) against Escherichia coli (E.coli) and S.aureus, and found that PW12 exhibited the strongest antibacterial activity against both bacteria.Wang L et al.27found that Na3PW12O40 demonstrated certain antibacterial effects against E.coli, S.aureus, yeast, and Aspergillus niger, with the most notable effect against S.aureu.

Based on the excellent properties of POMs, researchers have incorporated POM as a functional additive into film-forming substrates to prepare functionalized antibacterial composite films28.Xu L et al.29used silver phosphotungstate as an antimicrobial agent and polyvinyl alcohol (PVA) as a film-forming substrate to prepare silver phosphotungstate/PVA composite films via the casting method. They studied the mechanical properties, WVP, and antimicrobial properties of the composite films. The results showed that with an increase in the silver phosphotungstate content, the mechanical properties of the composite film gradually improved, and the film exhibited enhanced antimicrobial effects against S.aureus and E. coli.

When preparing composite films, compatibility between POMs and bio-based matrices is essential30,93,94,95,96. The good water solubility of Na3PW12O40 facilitates its uniform dispersion within the carrageenan matrix, ensuring homogeneous distribution of the antimicrobial agent and enhancing the overall functional performance of the packaging material. Additionally, Na3PW12O40 exhibits high thermal stability, commercial availability31,32,86,87,88,89,90, and ease of synthesis, with inexpensive and readily accessible raw materials. Based on its outstanding antimicrobial efficacy, stability, and compatibility, Na3PW12O40 was selected as a functional additive for the fabrication of carrageenan-based composite films with enhanced antimicrobial activity.

Research on the direct application of Keggin-type POMs in food packaging remains in its early stages. Enderle A G and colleagues33,97,98,99developed hybrid films by integrating POM-IL with poly(methyl methacrylate) which can function as surface coatings or packaging materials for ready-to-eat foods. These hybrid films effectively limit the proliferation and transmission of pathogenic microorganisms, thereby reducing the risk of microbial spread through surface contact. Multilayer films combining POMs with methylene blue have also demonstrated antibacterial activity against E.coli, whereas methylene blue alone exhibits limited efficacy, further underscoring the potential application of POM-based systems in food packaging34,91,92.

This study incorporated Keggin-type POM into κ-carrageenan and prepared Carr/POM films via the casting method. The effects of the POM content on the mechanical properties, physical properties, thermal stability, and microstructure of the films were thoroughly investigated.Additionally, antibacterial tests were conducted to further investigate the antibacterial properties of the films.

Materials and methods

Materials and reagents

κ-Carrageenan was purchased from Aladdin, glycerol was purchased from GENERAL-REAGENT, Shanghai Titan Technology Co. and sodium phosphotungstate hydrate (Na3PW12O40.xH2O) was purchased from Sinopharm Chemical Reagent Co. E.coli (ATCC 25922) and S.aureus (ATCC 6538) were purchased from Beijing Preservation Biotechnology Co. Nutrient broth medium (NB, CN120054-250 g) and nutrient agar medium (NA, CN230275-250 g) were purchased from Qingdao Haibo.

Synthesis of carrageenan-based keggin polyoxometalate (Na3PW12O40) antibacterial films (carrageenan/Na3PW12O40 films)

κ-Carrageenan powder (1%, W/V) and glycerol (1%, V/V) were dissolved in sterile water and magnetically stirred at 80 °C for 30 min to obtain a transparent and homogeneous film-forming solution. Subsequently, Na3PW12O40 was dissolved in 5 mL of sterile water to produce a yellow transparent solution. This solution was then magnetically stirred with the κ-Carrageenan film-forming solution at 60 °C for 20 min to obtain Carrageenan/Na3PW12O40 film solutions of different concentrations (0, 1, 2, 4, 8 mg/mL), labeled as Carr, Carr/POM-1, Carr/POM-2, Carr/POM-3, and Carr/POM-4, respectively. The films were prepared using the tape-casting method. Fifteen milliliters of the film-forming solution were poured into a sterile petri dish with a diameter of 90 mm and subjected to ultrasonic treatment for 10 min to remove air bubbles. Finally, the films were dried in a hot-air circulation oven at 35 °C for 24 h. The prepared films were stored at room temperature (25 ± 1 °C) and 50% relative humidity.

Characterizations

The film thickness was measured using a micrometer (DL9325). Five random points were selected on each film, and the average value of the measurements was used as the film thickness. The mechanical properties of the films were tested using a universal testing machine (MTS E43.104) manufactured by Shenzhen Yinfei Electronic Technology Co., Ltd. The film samples were cut into dimensions of 1 cm×3 cm and tested according to GB/T453-2002 standards. The WVP of the films was measured using the W3-031 WVP testing system (W010B) from Jinan Languang Electromechanical Technology Co., Ltd. The films were placed in the device under conditions of 38 ± 0.6 °C and 90 ± 2% relative humidity (RH) for 24 h to obtain the WVP, which was used to evaluate the films’ barrier properties against water vapor. The surface color indices (L, a, b values) of the films were measured using a colorimeter (NR110) manufactured by Guangdong Sanenshi Technology Co., Ltd. The device was calibrated with white paper, and measurements were taken at three different locations on each film sample. The film color was represented using the CIE Lab color system, which evaluates its appearance in terms of brightness (L), red/green (a), and yellow/blue (b) values. The color difference (∆E) was calculated using the corresponding formula.

$${\rm \Delta\: E = \sqrt{(\Delta L)^2 + (\Delta a)^2 + (\Delta b)^2}}$$

The infrared spectra of the films were recorded using a Thermo Nicolet iS10 Fourier Transform Infrared (FT-IR) spectrometer (USA). The scanning wavenumber range was 4000–400 cm−1, with a resolution of 4 cm−1. The surface microstructure of the films was examined using a Zeiss Gemini Sigma 300 scanning electron microscope (SEM) equipped with an Oxford Xplore30 energy dispersive spectroscopy system. The films were cut, sputter-coated with gold, and analyzed under the SEM. The thermal stability of the films was assessed using a TGA55 thermogravimetric(TG) analyzer (TA Instruments, USA). The samples underwent TG analysis under a nitrogen atmosphere, heated from 30 °C to 800 °C at a rate of 10 °C/min. The changes in sample mass with increasing temperature were recorded to obtain the TG and derivative thermogravimetric (DTG) curves. The crystalline characteristics of the films were analyzed using an XRD-3X X-ray diffractometer (Beijing Purkinje General Instrument Co., Ltd.). The film samples were cut to appropriate sizes, placed into the circular grooves of a glass slide, flattened, and analyzed using Cu-Kα radiation. Following UV irradiation for 30 min, the films were punched into circular discs with a diameter of 6 mm. These discs were placed on agar plates inoculated with E. coli and S. aureus, incubated at 37 °C for 24 h, and then removed to record and photograph the inhibition zone diameters.Biodegradation of Carr and Carr/POM-4 films was assessed using a protocol adapted from Rech et al.35. Each film sample was wrapped in gauze and buried 10 cm deep in soil maintained at 25 °C and 80% relative humidity. The samples were retrieved daily, photographed, and examined to assess the extent of degradation.

Results and discussion

Analysis of thickness measurement results

The effect of Na3PW12O40 incorporation on the thickness of Carr/POM films was systematically investigated. As shown in Fig. 1, the thickness of pure carrageenan films without POMs was 0.038 ± 0.001 mm. The addition of a low concentration of POMs (1 mg/mL) had negligible effects on film thickness. However, as the concentration of POMs increased, the film thickness increased significantly (p < 0.05), likely due to the incorporation of POMs, which increased the solid content or the internal gaps between polymer chains and glycerol in the film matrix28,36. The incorporation of bee pollen extract and honey extract into κ-carrageenan-based films also resulted in the increase in film thickness37.

An increase in film thickness typically influences its flexibility, particularly affecting tensile strength and deformability. While increased thickness may enhance the structural integrity of the film, excessive thickness may compromise its flexibility under certain conditions. Incorporating small amounts of additives can improve the ductility and flexibility of the film by promoting intermolecular cross-linking, such as hydrogen bonding38. However, the excessive incorporation of additives may cause aggregation and the formation of stress concentration points, leading to brittle fracture and reduced flexibility of the film39.

The barrier properties of films, particularly their WVP, are critical for a wide range of practical applications, particularly in food packaging systems. An increase in film thickness generally leads to enhanced barrier properties. Although increased thickness extends the diffusion path of water molecules, studies suggest that the densification of the film structure is the primary factor responsible for the reduced WVP40,41. In the design of food packaging materials, a careful balance between thickness and flexibility must be achieved to meet practical performance requirements.

Fig. 1
figure 1

Effect of Na3PW12O40 Concentration on Carr/POM Thickness.

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Analysis of tensile strength and elongation at break results

The mechanical properties of the films are commonly represented by tensile strength and elongation at break. Tensile strength reflects the mechanical resistance of the films, while elongation at break indicates their flexibility. The mechanical properties of the films are summarized in Table 1. As shown in the table, the elongation at break increased significantly following the addition of POM, thereby enhancing the flexibility of the films. This improvement is attributed to strong intermolecular interactions between an optimal amount of POM and carrageenan42.The tensile strength of the films varied with the POM content. As the POM content increased, both the tensile strength and elongation at break of the composite films initially increased and then decreased. Carr/POM-2 exhibited the highest tensile strength and elongation at break, which is due to the fact that when an appropriate amount of POM is added, the crosslinking degree between POM and carrageenan increases, resulting in higher membrane tensile strength. However, as the amount of POM exceeds the optimal level, the intermolecular and intramolecular interactions of carrageenan decrease43, leading to a reduction in membrane tensile strength. The tensile strength of Carr/POM-4 is markedly lower than that of pure carrageenan, mainly due to the agglomeration of POM within the carrageenan matrix at elevated POM concentrations, resulting in a diminished tensile strength of the composite film29.This finding is corroborated by SEM images of Carr/POM-4, which reveal POM aggregation on the film surface at elevated concentrations. Compared with other carrageenan-based films or films containing POM, Carr/POM has higher mechanical properties, as shown in Table 2.

Table 1 Effect of Na3PW12O40 concentration on tensile strength and elongation at break of carr/pom Films.
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Table 2 Mechanical property data of different types of composite Films.
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Analysis of WVP results

WVP is commonly used to assess the water resistance of films, with lower WVP values indicating better water resistance. The WVP results of the films are presented in Table 3. The WVP of Carr/POM films was lower than that of pure carrageenan films. As the POM content increased, the WVP of the composite films decreased, suggesting that POM significantly enhanced the water vapor barrier properties of carrageenan films. Hydrogen bonding interactions between Na3PW12O40 and κ-carrageenan result in the formation of a dense intermolecular network. This network structure significantly reduces the availability of hydrophilic groups for water vapor adsorption, thereby decreasing the WVP of the film48. Another contributing factor is the increased surface roughness of the film, which becomes more pronounced as higher amounts of Na3PW12O40 are incorporated into the matrix. The aggregation of Na3PW12O40 particles obstructs the microchannels for water vapor diffusion within the film matrix49, further reducing the WVP. These observations are consistent with the results obtained from SEM characterization.

Table 3 Effect of Na3PW12O40 concentration on WVP of carr/pom Films.
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Analysis of color measurement results

Table 4 presents the L, a, b, and ΔE values of carrageenan and Carr/POM films. As shown in Fig. 2, pure carrageenan films without POM were nearly colorless and transparent. Upon the incorporation of POM, Carr/POM films exhibited blue and green hues, as indicated by the negative values of a and b. Furthermore, as the concentration of POM increased, the color of the composite films darkened, as evidenced by the decrease in L values. With increasing POM content, the absolute value of a increased, signifying a more pronounced green color in the composite films. The ΔE value of Carr/POM films was higher than that of pure carrageenan films (p < 0.05), suggesting that Carr/POM films were darker in color50. The absolute value of b for the composite films was greater than that for pure carrageenan films, indicating that the composite films were bluer, as shown in Fig. 2.

Table 4 L, a, b, and ΔE values of Carrageenan and carr/pom Films.
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Fig. 2
figure 2

Morphology of pure carrageenan film and Carr/POM films incorporated with Na3PW12O40 at different concentrations.

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Analysis of fourier transform infrared (FT-IR) spectroscopy results

The intermolecular interactions between the carrageenan matrix and POM were investigated using FT-IR spectroscopy, as illustrated in Fig. 3. The FT-IR spectrum of pure carrageenan exhibited several characteristic peaks in the range of 500–3700 cm−1. A broad absorption peak centered at 3330 cm−1, within the range of 3000–3600 cm−1, corresponded to the O-H stretching vibration. The peak at 1643 cm−1 was attributed to the bending vibration of water molecules. Peaks at 2942 cm−1 and 2880 cm−1 corresponded to the asymmetric and symmetric stretching vibrations of methylene (− CH2) groups. The peak at 916 cm−1 represented the C − O stretching vibration of 3,6-anhydro-D-galactose. The peaks at 842 cm−1 and 1217 cm−1 were attributed to the C-O-S stretching vibration and the S-O asymmetric stretching vibration, respectively13. Additionally, carrageenan displayed characteristic sulfate-related vibrations, with the peak at 842 cm−1 corresponding to the vibration of D-galactose-4-sulfate, and the peak at 1217 cm−1 representing the asymmetric stretching vibration of sulfate (O = S = O). Peaks at 1030 cm−1 and 1156 cm−1 were assigned to the C-O stretching vibration in glycosidic bonds and the asymmetric stretching vibration of C − O−C, respectively8,51.

The FT-IR absorption peaks of Na3PW12O40 included P-Oa, W-Od, W-Ob-W, and W-Oc-W, which are characteristic of typical Keggin-type POMs52. Among these, the peak at 793 cm−1 corresponded to the asymmetric stretching vibration of W-Oc-W, while the peak at 968 cm−1 was attributed to the W-Od stretching vibration. The P-Oa and W-Ob-W stretching vibration peaks of Na3PW12O40 may overlap with the vibration peaks of carrageenan53,54.

When Na3PW12O40 was incorporated into the films, changes in the position and intensity of FT-IR peaks were observed. As the Na3PW12O40 content increased in the composite films, the absorption peaks at 968 cm−1 and 793 cm−1 became more pronounced, while the peak at 842 cm−1 weakened. The absorption peaks at 968 cm−1 and 793 cm−1 are characteristic of Na3PW12O40, further confirming the interaction between Na3PW12O40 and carrageenan. This interaction is likely attributed to the terminal and bridging oxygen atoms on the POM surface forming hydrogen bonds with the –OH groups of carrageenan30. Additionally, as the POM content increased, the intensity of the peak at 842 cm−1, a characteristic peak of carrageenan, decreased. This is primarily because the relative amount of carrageenan in the composite films decreased with the increasing POM content.

Fig. 3
figure 3

FT-IRspectroscopy of pure carrageenan film and Carr/POM films incorporated with Na3PW12O40 at different concentrations.

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SEM analysis

The surface morphology of the films was examined using scanning electron microscopy (SEM). As shown in Fig. 4, the surface of the pure carrageenan film was smooth and uniform, indicating good compatibility between κ-carrageenan and glycerol. The surfaces of Carr/POM-1, Carr/POM-2, and Carr/POM-3 films exhibited no significant differences compared to the pure carrageenan film, suggesting that, at low concentrations (≤ 4 mg/mL), POM was well integrated into the κ-carrageenan film matrix. However, as the POM concentration increased, the surface roughness of the films gradually intensified, and cracks began to form. This phenomenon was particularly noticeable at high POM concentrations (8 mg/mL) and attributed to the aggregation of POM, which led to a less smooth film surface. Similar changes in surface smoothness were observed when Dawson-type K6[Mo18O62P2] was incorporated into κ-carrageenan films28.

Fig. 4
figure 4

SEM micrographs of pure carrageenan film and Carr/POM films incorporated with Na3PW12O40 at different concentrations.

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TG-DTG analysis

TG analysis was conducted to investigate the effect of polyoxometalate (POM) concentration on the thermal stability of the films. The TG and DTG curves of pure carrageenan films and Carr/POM films are presented in Fig. 5. These curves revealed three stages of weight loss for all film samples. The first stage (60–120 °C) was attributed to the evaporation of free water, which can be ascribed to the hydrophilic nature of carrageenan55. The second stage (215–235 °C) was primarily due to the degradation of glycerol in the films56,57. Previous studies58have shown that potassium tungstophosphate and similar POMs exhibit excellent thermal stability, remaining stable without decomposition below 800 °C. The third stage (230–800 °C) was associated with the decomposition of carrageenan59,60. Compared to pure carrageenan films, the third decomposition stage of Carr/POM composite films occurred at a relatively lower temperature, indicating that Keggin-type POMs reduce the thermal stability of carrageenan films, likely due to diminished interactions among POM, glycerol, and the κ-carrageenan matrix61. In the first decomposition stage, the weight loss of pure carrageenan films was lower than that of Carr/POM composite films, possibly due to the presence of POM in the composite films. In the second decomposition stage, pure carrageenan films experienced greater weight loss than Carr/POM composite films. As the POM content increased, the weight loss gradually decreased, likely because the incorporation of POM reduced the glycerol mass fraction in the films, leading to less weight loss in the second stage.

Fig. 5
figure 5

TG (A) and DTG (B) curves of pure carrageenan film and Carr/POM films incorporated with Na3PW12O40 at different concentrations.

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XRD analysis results

X-ray diffraction (XRD) analysis was conducted to examine the crystalline characteristics of carrageenan-POM films, as illustrated in Fig. 6. Due to the amorphous nature of carrageenan, pure carrageenan films did not exhibit any distinct crystalline peaks, as previously reported62. The characteristic diffraction peak of Keggin-structured POMs was observed at 2θ = 26.4 °63,64. The XRD patterns of all film samples showed similar diffraction features, with a broad peak around 2θ = 20 °, which can be attributed to the semi-crystalline nature of the films65,66. Similar diffraction peaks of pure carrageenan films were observed in previous studies67.

Furthermore, as the POM content increased, the intensity of the diffraction peaks diminished, likely due to interactions between POM, carrageenan, and glycerol, which disrupted the original crystalline structure of carrageenan and reduced its crystallinity. A similar phenomenon was observed upon the addition of rutin essential oil (RGEO) to chitosan68.The interaction between chitosan and RGEO slightly disrupted the original crystalline structure of chitosan, resulting in a decrease in the crystallinity of the chitosan film69,70. XRD analysis indicated good compatibility between carrageenan, POM, and glycerol.

Fig. 6
figure 6

XRD curves of pure carrageenan film and Carr/POM films incorporated with Na3PW12O40 at different concentrations.

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Antibacterial performance of carr/pom films

As shown in Fig. 7, in the investigation of the antibacterial performance of κ-carrageenan and its composite films with Na3PW12O40 against E.coli and S.aureus, the samples were incubated at 37 °C for 24 h.The results indicated that neither the κ-carrageenan film nor the Carr/POM film exhibited significant antibacterial activity against E.coli, as shown in Table 5. Under identical incubation conditions, neither the κ-carrageenan film nor the Carr/POM film containing low concentrations of Na3PW12O40 demonstrated significant antibacterial activity against S.aureus. However, when the concentration of Na3PW12O40 reached 4 mg/mL, a clear inhibition zone formed around the Carr/POM composite film, suggesting a marked enhancement in its antibacterial activity.Furthermore, as the Na3PW12O40 concentration increased to 8 mg/mL, the diameter of the inhibition zone further expanded, demonstrating that the antibacterial effect of the composite film is strongly correlated with the concentration of Na3PW12O40 in the film. Following antibacterial testing, the films underwent dissolution. Future research will aim to enhance the stability of the films under high-humidity conditions, thereby broadening their range of applications.

The antibacterial experiment demonstrated that the Carr/POM film exhibited a significantly stronger antibacterial effect against S.aureus compared to E.coli. This difference is speculated to arise from the fact that E.coli is a Gram-negative bacterium with a complex cell wall structure that is difficult to disrupt, whereas S.aureus is a Gram-positive bacterium with a relatively weaker and more susceptible cell wall structure71.

The antimicrobial mechanism of Keggin-type POMs against S.aureus is believed to involve several pathways, including disruption of the cell membrane, interference with the bacterial respiratory chain, and the generation of reactive oxygen species (ROS)19,72. POMs exhibit antibacterial activity primarily by interacting electrostatically with the oppositely charged components of microbial cell walls, which leads to the leakage of intracellular contents73,79,93. Furthermore, Keggin-structured POMs possess redox activity, allowing them to oxidize key electron carriers, thereby disrupting the bacterial respiratory system, impairing ATP synthesis, and inflicting irreversible damage to the cells. POMs can also directly oxidize bacterial proteins, lipids, and other biomolecules, generating ROS, or indirectly increase ROS levels by depleting glutathione, thereby leading to cellular oxidative damage19.

In Carr/POM films, carrageenan itself typically does not possess significant antibacterial properties and mainly serves as the film matrix74, whereas POM is incorporated as the antibacterial agent. This observation aligns with previous studies and is further evidenced by the absence of antimicrobial activity in pure carrageenan films, as demonstrated in antibacterial assays28,75. In the chitosan/POM system, given that chitosan exhibits intrinsic antibacterial properties, the antibacterial efficacy of the chitosan/POM composite is predominantly attributed to the synergistic interaction between chitosan and POM. The electrostatic attraction between positively charged chitosan (CS) and negatively charged bacterial membrane components not only enhances membrane permeability but also promotes contact between POM and the bacterial membrane, thereby augmenting the antibacterial efficacy of the composite30,100,101,102,103,104,105.

Future research will further explore the synergistic effects between POMs and other antibacterial agents. Silver nanoparticles modified with H3PW12O40 induce severe physical damage to both Gram-negative (E.coli) and Gram-positive (S.aureus) bacterial cells. However, their antibacterial efficacy is notably stronger against E. coli. The antibiotic activity of silver ions (Ag+) appears to potentiate the effect of H3PW12O40, with their synergistic interaction markedly enhancing antibacterial efficacy. Silver nanoparticles disrupt bacterial cell walls and generate high concentrations of reactive oxygen species (ROS), thereby stabilizing and facilitating the delivery of POMs into bacterial cells19. Additionally, chitosan–polyoxometalate (CS–POM) nanocomposites, fabricated with POMs and chitosan as the primary matrix, exhibit robust synergistic antibacterial activity against E. coli. The synergistic antibacterial mechanism between CS and H5PMo10V2O40 involves CS-mediated membrane destabilization and POM-induced oxidation of electron transport chain substrates30. Chen D et al. employed layer-by-layer self-assembly to construct two multilayer films incorporating Keggin-type POMs (α-[SiW12O40]4−/α-[PMo12O40]3−) and methylene blue, both exhibiting substantial antibacterial activity against E. coli34. This study establishes a foundation for the development of multilayer composite film architectures, which may be further optimized to improve antibacterial performance.

Fig. 7
figure 7

The representative images of bacteriostatic circle of Carr/POM anti-bacterial films against E. coli (A) and S. aureus(B).

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Table 5 The average diameters of bacteriostatic circle of the carr/pom anti-bacterial films incorporated with Na3PW12O40 at different concentrations.
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Biodegradability of carr/pom anti-bacterial film in soil

Biodegradation analysis of the Carr/POM antimicrobial films is presented in Fig. 8. As shown in Fig. 8A, the pure carrageenan film remained largely intact during the first two days, exhibiting no significant structural damage. However, degradation began on the third day, with visible fragmentation occurring and progressing to complete disintegration by day seven. Similarly, Fig. 8B shows that the Carr/POM-4 film maintained its structural integrity on the first day, with no apparent damage, though a noticeable fading in color was observed. From the second day onward, the film developed significant cracks and ultimately underwent full degradation by the seventh day. These results demonstrate that Carr/POM films possess excellent biodegradability in soil, highlighting their potential as environmentally friendly antimicrobial materials. This observation is consistent with previous studies, which have reported favorable biodegradability of carrageenan-based films76,77.

Fig. 8
figure 8

Degradation of Carr and Carr/POM-4 film in soil.

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Conclusion

In the present study, Carr/POM films were fabricated by incorporating the Keggin-type polyoxometalate Na3PW12O40 into κ-carrageenan via the solution casting method. The mechanical strength, thermal stability, microstructure, and antibacterial properties of the composite films were thoroughly investigated. The results demonstrated interactions between Na3PW12O40 and carrageenan. The incorporation of Na3PW12O40 enhanced the water resistance of the carrageenan films and modified their microstructure. In antibacterial assays, Carr/POM films exhibited robust antibacterial activity against S.aureus, especially when the Na3PW12O40 concentration reached 8 mg/mL, resulting in the most significant inhibition. The incorporation of an appropriate amount of POMs into κ-carrageenan to prepare composite antibacterial materials can significantly enhance its inhibitory effect against common pathogenic microorganisms in food.This composite material demonstrates significant potential for the development of antibacterial materials and holds broad application prospects in the field of food packaging.

However, large-scale production of Na3PW12O40 may encounter challenges related to raw material availability and process optimization, particularly in maintaining product consistency and controlling production costs. Owing to the limited hydrophilicity of the prepared Carr/POM films, future research will aim to develop preservative films with improved stability. Furthermore, identifying POMs with superior antibacterial properties will be central to developing preservative films exhibiting broad-spectrum antibacterial activity. The long-term effects of POMs on food safety warrant further investigation to validate their feasibility in food packaging applications. Moreover, research on the in vivo activity of POMs remains insufficient. Whether POMs generate harmful metabolites during bodily metabolism and their subsequent impact on the internal environment remains unclear. The degradation behavior and environmental impact of POMs also necessitate further study, particularly concerning their potential ecological toxicity in discarded packaging materials.