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. 2025 Jul 4;29:102730. doi: 10.1016/j.fochx.2025.102730

Biodegradable photosensitive antimicrobial hydrogel film based on curcumin-carbon dots for raw meat preservation

Xiaoqin Pan a,b, Shan Xiao a,b, Yanxue Cai a,b, Zhouyi Xiong a,b, Bo Wang a,b, Xuan Chen a,b, Jihui Wang a,b,⁎
PMCID: PMC12275136  PMID: 40686883

Abstract

Biodegradable photodynamic antimicrobial hydrogel films have gained significant attention for food preservation. Konjac glucomannan (KGM) based hydrogels offer commendable biocompatibility and biodegradability but exhibit low antimicrobial activity. While curcumin (Cur) is widely incorporated into biodegradable antimicrobial packaging, it often compromises transparency and shortens the functional lifespan of hydrogel films. In this study, we report a biodegradable photosensitive film based on KGM, polyvinyl alcohol (PVA), D-sorbitol and Curcumin‑carbon dots (Cur-CDs). The water penetrating in the film show the Fick diffusion behavior, and biodegraded 27.90 % within 4 weeks. Cur-CDs enhanced hydrogen bonding between KGM and PVA, resulting in a smoother film surface, a 17.80° reduction in water contact angle, and a 24.91 % increase in transparency. Besides, the light stability of Cur-CDs in film was significantly enhanced. Accordingly, 1.74 × 104 CFU microbial colony was photodynamic eliminated when applied on pork preservation, showing the promising potential application in food preservation.

Keywords: Curcumin‑carbon dots, Hydrogel, Cross-linking, Biodegradation, Photodynamic antibacterial

Highlights

  • •

    PVA improved the mechanical properties of KGM-based film.

  • •

    Cur-CDs boost film transparency (↑24.91 %) and photostability.

  • •

    Cur-CDs enhanced the crosslinking reaction and thermal stability of film.

  • •

    1.74 × 104 CFU microbial colony were photo-eliminated on pork preservation.

1. Introduction

Biodegradable packaging materials are increasingly favored due to their environmental benefits (Wang et al., 2024). Konjac glucomannan (KGM), has garnered widespread attention for its biodegradability, prebiotic effects, anti-obesity properties, and antioxidant activity (Dodange, Shekarchizadeh and Kadivar, 2024; Sun et al., 2023; Xu et al., 2013). Owing to its high molecular weight and rich oxygen functional groups, KGM based hydrogels exhibit excellent flexibility and mechanical stability (Cao et al., 2024; Hou et al., 2022). Nevertheless, pure KGM hydrogel films suffer from high synesis rate and low strength (Tong et al., 2023). To address these limitation, polyvinyl alcohol (PVA)-noted for its good degradability, biocompatibility, and excellent mechanical strength, which can form a stable skeleton between starch/polysaccharide and PVA-have been used to improve the mechanical properties of KGM-based food hydrogels (Cao et al., 2024; Thomas et al., 2024; Xu et al., 2019). So far, KGM-based hydrogels have demonstrated commendable biocompatibility and biodegradability (Wu et al., 2024).

On the other hand, antimicrobial hydrogel films have been extensively received tremendous attention for long time, especially in the field of food antibacterial preservation (Chang et al., 2023; Ruan et al., 2022). However, KGM exhibits low antimicrobial activity. Additional antimicrobial agents are required for the preparation of antimicrobial hydrogels using KGM (Sun et al., 2023). Owing to its excellent photosensitive property, curcumin (Cur) as a natural polyphenolic compound have been widely used in biodegradable antimicrobial packaging materials (Miao et al., 2024; Roy & Rhim, 2020; Sanchez et al., 2022; Wang et al., 2024). Zangirolami et al. (2022) have demonstrated its strong photodynamic antimicrobial properties when incorporated into films. However, the water solubility and photostability of Cur are poor, leading to lower transparency and shorter lifespan in antimicrobial hydrogel films. In Musso et al.'s study, Cur was just used as antioxidant agent, no antimicrobial activity can be observed (Musso et al., 2017). When Cur was used as monitoring freshness, the transparency was limited (Wang et al., 2024). Food packaging materials prefer high transparency. Hence, one of the objectives of this study is to develop a photocatalytic antimicrobial hydrogel film that achieves high transparency and photostability.

In recent years, nanotechnology has shown great potential in improving the mechanical properties of biodegradable hydrogel films (Tong et al., 2023; Tong et al., 2024). Among them, Carbon dots (CDs), as novel nanoscale quantum dots with rich molecular functional groups on their surface, has been incorporated into hydrogel matrix popularly (Duan et al., 2025). In the gelatin-based film, CDs served as nano crosslinking agents or plasticizers to promote cross-linking effect and improve the mechanical properties of thin hydrogel films (Campalani et al., 2022). Fu et al. (2022) developed a bio-nanocomposite film based on Gelatin/Chitosan incorporated with CDs, utilizing the antibacterial, antioxidant, pH-sensitive and UV shielding properties of CDs. In the previous study of our group, we greatly improved the water solubility and photostability of Cur by preparing Cur-CDs nanocapsule, which have the potential of preparing antibacterial hydrogels with high transmittance and photostability (Pan et al., 2024). Based upon this, we conjecture that by incorporating Cur-CDs into KGM and PVA bases to make hydrogel films, Cur-CDs can not only exert their photodynamic antibacterial effects, but also act as crosslinking agents to improve the mechanical properties of the hydrogel films.

Meanwhile, we noted that D-sorbitol as a plasticizer can enhanced the mechanical and barrier properties of antibacterial films (Cheng et al., 2019). Therefore, in this study we used D-sorbitol as a plasticizer, Cur-CDs as photodynamic antibacterial and crosslinking agent, KGM and PVA as the hydrogel matrix, to prepare biodegradable photosensitive food hydrogel film. Via the characterization of hydrogel film chemical bond change, mechanical properties, swelling behavior, thermal stability, and its photodynamic antibacterial properties, the mechanism of Cur-CDs in the hydrogel film was full studied. In addition, the hydrogel film for fresh cut pork antibacterial preservation was also evaluated.

2. Experimental

2.1. Hydrogel film preparation

Firstly, Cur-CDs was prepared according to our previous study (Pan et al., 2024). Briefly, 2 g citric acid and 2 g chitosan was dissolved with 50 mL pure water in 100 mL Teflon-linedstainless-steel autoclave and heated at 180 °C for 3 h to obtained CDs. After dialysis purification, 1 mL of 0.4 mg/mL Cur ethanol solution was injected into 10 mL 0.5 mg/mL CDs aqueous dispersion and stir in dark environment for 24 h and subsequently purified by freeze centrifugation to obtained Cur-CDs. Then, different hydrogel films were prepared according to followed Table 1 and Fig. 1.

Table 1.

Formulations for preparing different films.

KGM PVA PK PKCC PKC
D-sorbitol (g) 0.8 2 2 2 2
KGM (g) 2 \ 1.5 1.5 1.5
PVA (g) \ 5 3.5 3.5 3.5
Cur-CDs (mg) \ \ \ 6 \
Cur (mg) \ \ \ \ 0.62
Water (mL) 97.2 93 93 93 93
95 °C water bath and stirred for 1 h.
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Note: KGM represents pure KGM; PVA represents pure PVA; PK represents PVA mixed with KGM; PKCC represents PVA mixed with KGM and Cur-CDs; PKC represents PVA mixed with KGM and Cur. All the films were made at the same matrix quality.

Fig. 1.

Fig. 1

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Preparation process diagram of PVA/KGM/Cur-CDs hydrogel film.

For pure KGM film, 25 g mixture was poured onto a 9 × 9 cm2 circular plate and stood for defoaming, while the rest of films were prepared by respectively pouring 10 g of the above mixture onto a 9 × 9 cm2 circular plate and stood for defoaming. Finally, they were placed on the 50 °C blast oven for 6 h to obtain the according films.

2.2. Characterization

Atomic force microscope (AFM) image was taken on Bruker Dimension Icon. CIE color parameters were measured by High-quality portable colorimeter NH310. The transparencies were measured by Spectrophotometer CM-5 (KONICΛ MINOLTΛ), while the Elongation and Young's modulus were measured by TA. XT. PLUSC (SMS), in which the hydrogel film was cut into dumbbell shaped strips (wide side size: 6 × 35 mm; narrow side size: 2 × 10 mm) and tested at a constant speed of 10 mm/min, using 5 N tension sensor. Fourier transform infrared (FTIR) spectrum was conducted on Nicolet IS50, while X-ray diffraction (XRD) analysis was conducted on SMARTLAB 9KW (RIGAKU, Cu-Κα radiation, λ = 0.1545051 nm, scanning area set to 2θ from 10° to 60°). Thermal stability analysis was conducted on TGA-8000 (Perkinelmer TA) under nitrogen atmosphere with a flow capacity of 50 mL/min, heating rate of 10 °C/min from 30 to 600 °C. The scanning electron microscopy (SEM) images were taken on ZEISS EVO MA 15, while the water contact angle was measured by utra high melting temperature contact angle measuring instrument L200HT (KINO).

2.3. Moisture content and swelling property

The moisture content and swelling rate of films were measured by weighing method. Specifically, the film was placed on the 105 °C blast oven to get its constant weight. For moisture content determined, measured the weight of the dry film that was placed in normal room environment for 24 h, and then the moisture content was calculated by Eq. (1):

Moisture content%=Wn−W0/W0×100% (1)

where Wn is the weight of dry film that placed in normal room environment for 24 h, while W0 is the constant weight of film.

After the film was dried to constant weight, it was immersed in to pure water (25 °C). At each certain time interval, removed the film from the water and the excess surface water was removed with filter paper, and weight to calculated the swelling rate according Eq. (2). Meanwhile, the initial behavior of water molecules in the hydrogel was study by the swelling kinetics according to Eq. (3–4)

Swelling rate%=Wt−W0/W0×100% (2)
Swelling rate%=At2+Bt+C (3)
WtWe=Ktn (4)

where Wt represent the weight of film after immersed in water at the tested time interval, We represent the weight of film at its swelling equilibrium state, while K represent swelling rate constant of the film and n is the diffusion coefficient.

2.4. Light stability

One side of the PK, PKC and PKCC film were placed under 30 mins of blue LED light (468 nm) illumination, while the other side of the film were treated with a black light blocking plate to avoid light as comparison.

2.5. Cur release rate

Variable content of Cur was respectively dissolved in 0.5 % V/V Tween-80 solution that consist of 0.01 M PBS solution (pH = 7.0) and recorded the absorption value at 434 nm to make a standard curve. Then, PKC and PKCC films were respectively cut into 2 × 4 cm2 pieces and immersed into 20 mL of the above solution. At each certain time interval, 1 mL of above sample solution was removed for recording absorption value at 434 nm to calculated the Cur concentration and replaced with an equal amount of clean solution to maintain the total volume.

2.6. Biodegradation of hydrogel films

The biodegradation study of hydrogel films was carried out according to Suhasini et al. (2023). Specifically, PVA, KGM, and PKCC films were buried in soil. Before burying, the initial weight of films was recorded. During the biodegradation experiment, water the soil with 20 mL of pure water every 3 days. At certain days interval, the buried films were dug out and took pictures, and the weight of the buried films were recorded after the films were washed and dried to constant weight by oven dry.

2.7. Antibacterial experiment of hydrogel film

All the utensils were sterilized by high-pressure steam sterilization, while the hydrogel films before experiment were sterilized by 20 mins of UV-C light irradiation.

Listeria monocytogenes (L. monocytogenes) was activated by culturing in Luria−Bertani (LB) solution under 37 °C over night. Then, the bacterial LB solution was diluted 1000× with physiological saline. Subsequently, 20 μL bacterial dispersion dripped on circle films with a radius of 3 mm and placed under 468 nm LED light with the intensity of 1 mW/cm2 for 20 mins. After that, 2 mL physiological saline was used for rinsing the above films and 100 μL of rinse solution was used for plate counting method to evaluate the antibacterial effect.

2.8. Antimicrobial preservation of pork

Fresh pork was purchased from local supermarket and was directly exposed to the air at 25 °Cfor 3 h before preservation. The pork was respectively wrapped in commercially available PE plastic wrap, PK film, and PKCC film, and then exposed to 468 nm LED light with the intensity of 1 mW/cm2 for 20 mins before being stored in a refrigerator at 4 °C for preservation. Subsequently, samples were taken every other day and the colony growth was detected using the plate coating method according to National Food Safety Standard of China (GB 4789.2–2022).

2.9. Beef water locking preservation

Fresh beef was purchased from local supermarket and respectively covered with PK and PKCC films, while those exposed to the air treatment was blank comparison. Then, they were stored in a refrigerator at 4 °C for water locking preservation, and were taken every two days for determined the hardness, chewiness and firmness.

2.10. Statistical analyze

The statistical data are analyzed by the SPSS 24 software. The significance of the difference between treatments was analyzed by one-way ANOVA, and the Duncan comparison test was used (P < 0.05). The analysis result and fitted line was plotted by Origin 2022 software.

3. Results and discussion

3.1. Color, mechanical property and swelling behavior

Fig. S1 shown the AFM image of as-prepared Cur-CDs, which demonstrated the successful preparation of Cur-CDs according to our previous study (Pan et al., 2024). Based upon this, Fig. 2a exhibits the digital picture of five thin films with the emblem of Dongguan University of Technology as background. As is shown, the surface of the pure PVA film was smooth with a thickness of 0.082 mm (Fig. S2), while the surface of the pure KGM film was relatively rough with a thickness of 0.068 mm. When PVA was mixed with KGM, the surface of the formed film PK shown a distinct granular texture, with a thickness up to 0.120 mm, which might be due to the insufficient cross-linking reaction. This result is consistent with a previous study that reported a konjac flour/PVA composite film (Lang et al., 2024). Interestingly, the incorporating of Cur-CDs and Cur significantly decrease the thickness of the film, in which of PKCC was 0.071 mm and PKC was 0.087 mm. Both of them are significantly thinner than the film reported by Lang et al. (2024) that used the same matrix. This is likely due to that the Cur-CDs and Cur induced cross-linking reaction are enhanced, which will be deeply analyzed by later thermal gravimetry analysis (TGA) and FTIR results. A4 white paper was used as the standard sample. Apparently, the introducing of Cur-CDs and Cur give the film yellow color, and the △E of PKCC was the biggest (Fig. 2b and c). Consistent with what can be seen with the naked eye in Fig. 2a, the transparency of PK film was the lowest, while that of PVA film was the highest. The incorporating of Cur-CDs and Cur can significantly increase the transparency of the PVA/KGM film, and the improving effect induced by Cur-CDs was better (by 24.91 %), which indicated that the induced cross-linking reaction by Cur-CDs is stronger (Fig. 2d).

Fig. 2.

Fig. 2

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(a) Digital picture of hydrophilic films; (b-d) CIE color parameters and transparency of hydrophilic films; (e-h) elongation, Young's modulus, moisture content and swelling kinetic curves of hydrophilic films (n≧3, P < 0.05 and different letter means that they are significant different).

Then, the elongation rate and Young's modulus of the thin films were measured (Fig. 2e and f). Here the Young's modulus was calculated from the slope of the force–time curve in the linear region, which also is the initial modulus. Although pure PVA film had the highest elongation rate, it also has the highest Young's modulus, suggested it has relatively poor polymer chains' flexibility. This is because the crosslinking density between the PVA and D-sorbitol is too high and hinders the plastic deformation of the film (Rynkowska et al., 2019), which is not conducive to the meat preservation packaging. The crosslinking interaction between PVA and D-sorbitol also will be further studied by later FTIR and TGA. Pure KGM film shown the lowest elongation and negative Young's modulus, which is attributed to the dominant viscoelastic stress relaxation behavior of soft gel. At low crosslinking, the unfolding rate of KGM gel is faster than the elastic shrinkage of the network, and the initial section of the stress-strain curve shows a negative slope (Xue et al., 2023). Compared with pure PVA film, PVA mixed with KGM decreased the elongation rate but also decreased the Young's modulus. It indicated the increment of polymer chains' flexibility, which is conductive to preservation packaging. Following the addition of Cur-CDs, compared with PK, the elongation rate of PKCC further significantly increases and the Young's modulus remained the same, indicating the enhanced flexibility. Similar results were reported (Guo et al., 2024). The Young's modulus of PKC also significantly increases.

By weighing method, the moisture content of the thin films was depicted in Fig. 2g, among which films that with the existent of KGM have significantly higher moisture content. It is attributed to the high-water absorption property of KGM. Here the swelling behavior reflects the movement of water molecules in the film (Xu et al., 2019). And the initial behavior of water molecules in the films were studied by swelling kinetic. As shown in Fig. 2h, owing to that the molecular chains of the dry gel films were tightly stacked before swelling, the swelling ratio of the hydrogel increased quickly at the initial stage. With the further acceleration of the swelling process, the space that can hold water molecules in the films gradually decreases, resulted in gradually slow swelling rate until the swelling equilibrium was reached. KGM film shown remarkable highest swelling rate during the whole process, which reflected the high mobility of molecular chains of KGM (Tang et al., 2024). PVA film exhibits the lowest swelling rate, which revealed that the crosslinking density between the PVA and D-sorbitol is the highest. Because the high crosslinking density reduces the free volume for small water molecular penetrating into the hydrogel matrix of the film. After KGM was mixed with PVA, the swelling rate of the film was between the two, indicating that the mix of KGM decreases the crosslinking intensity of PVA, giving more free volume for small water molecular penetrating. The swelling curves of PKCC and PKC were both lower than PK, suggesting the improved crosslinking intensity of them. Meanwhile, Korsmeyer-Peppas equation was applied and the results can be seen in Table 2 and Fig. S3. The swelling index n reflects the diffusion type of water in the film. All the films shown n ≤ 0.5, conforming to Fick diffusion behavior, which reveal that the water molecular diffusion rate is far less than the relaxation rate of the polymer network (Wu et al., 2019; Xu et al., 2019).

Table 2.

Evaluation of swelling parameters of hydrogel films in water.

Sample k n Adj. R2
PVA 0.72570 ± 0.00383 0.17636 ± 0.00381 0.99835
KGM 0.41295 ± 0.00777 0.49524 ± 0.02705 0.98589
PK 0.71989 ± 0.00704 0.22475 ± 0.01538 0.98074
PKCC 0.76762 ± 0.03711 0.1496 ± 0.03447 0.93002
PKC 0.71813 ± 0.01996 0.20524 ± 0.02971 0.91727
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3.2. Morphology and contact angle

The surface morphology of the hydrophilic films was identified by SEM images. As in Fig. 3, from the cross-section SEM images of the films, there were wrinkles when KGM and PVA were mixed without Cur and Cur-CDs, indicating the insufficient cross-linking reaction. The introducing of Cur and Cur-CDs are conductive to smooth the surface of the hydrophilic films, which was fully demonstrated from the surface SEM images. Especially, the surface smoothing effect of Cur-CDs on the films was much better than that of Cur, which might be attributed to the abundant hydrophilic function groups on Cur-CDs. The average water contact angle of PVA, KGM, PK, PKCC and PKC were respectively 34.83°, 18.66°, 37.92°, 20.11° and 25.61°, revealing that all the films are hydrophilic (Li et al., 2018; Liu et al., 2021). Though the surface of PVA film was smoother than KGM, its contact angle was higher, because the functional groups on the surface of PVA film play a decisive role. When PVA mixed with KGM, its contact angle was instead highest for that the surface roughness had a dominant effect. With the introducing of Cur-CDs, the surface roughness and water contact angle of PKCC decreased, demonstrated that the cross-linking reaction induced by Cur-CDs is more sufficient. This result is consistent with the results in SEM results and is strongly related to the abundant hydrophilic function groups on Cur-CDs (Pan et al., 2024).

Fig. 3.

Fig. 3

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SEM and water contact angle images of the as-prepared hydrophilic films.

3.3. Thermal stability and chemical interaction

To further study the interaction of PVA, KGM, D-Sorbitol, Cur and Cur-CDs, TGA was used. As is well known, the first stage of degradation of all hydrophilic films occurs within 100 °C is assigned to the evaporation of water. In Fig. 4a, peak around 307.69 °C is attributed to the scission of the backbone and degradation of KGM, while peak around 476.26 °C is assigned to the further aromatization volatilization (Li et al., 2023). By contrast, the degradation temperature of the main chain of pure KGM film was lower (Fig. S4a). Pristine PVA exhibited one peak at 209.00 °C (Fig. 4b). However, in Fig. S4b, the second stage of degradation that corresponded to the PVA degradation occurred around 317.22 °C, while the third stage occurs around 440.25 °C is assigned to the release of CO2, CO, CH4 and volatile compounds et al. (Suhasini et al., 2023). The degradation temperature of the main chain of pure PVA film was higher than pristine PVA, again demonstrated the high crosslinking degree of PVA and D-sorbitol. In Fig. 4c-4e, the third stage of degradation that attributed to the PVA and KGM degradation took place at higher temperature than pristine PVA and pristine KGM, strongly suggesting the intense interaction of PVA with KGM. Interestingly, the peaks of fourth stage that originated from PVA occurred in Fig. 4d also was highest, again reveals that the interaction induced by Cur-CDs is more intensive.

Fig. 4.

Fig. 4

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TGA curves(a-e), FTIR spectra(f), and the crosslinking schematic diagram (g) of the as-prepared PKCC films.

In addition, FTIR spectra were taken to analyze the chemical interaction. In Fig. 4f, PK, PKCC and PKC exhibited similar peaks to those of pristine PVA, revealed that PVA is the basic backbone of the composite hydrogels to develop a network structure. The O—H stretching vibration peak locates around 3280 cm−1, while the C—H stretching peak locates around 2900 cm−1(Wu et al., 2024). The peaks at 1644 and 1637 cm−1 are assigned to the adsorbed water, while the peak at 1413 and 1329 cm−1 are respectively attributed to the COO− and C—O bond vibration (Li et al., 2017). In Fig. S5a, peak located at 1080 cm−1 is assigned to the stretching vibration of C—O bond connected to the hydroxyl group of secondary alcohol, while peak at 1010 cm−1 is attributed to the stretching vibration of glycosidic bond (C-O-C). Compared with pristine KGM, the characteristic peak of mannose of KGM film shifted from 853 cm−1 to 871 cm−1(Li et al., 2021), which is assigned to the disorder of molecular chains during film formation. Therefore, compared with pristine KGM, the thermal stability of the KGM film decreased (Fig. 4a and Fig. S4a). In Fig. S5b, compared with pristine PVA, the O—H stretching vibration peak of PVA film shifted from 3284 cm−1 to 3265 cm−1, the C—O band peak shifted from 1028 cm−1 to 1044 cm−1 (Thomas et al., 2024). Besides, a new peak that assigned to the absorbed water at 1644 cm−1 can be seen, while the peak at 835 cm−1 that assigned to the C—H deformation vibration (PVA characteristic peak) weakened (Lang et al., 2024). These changes in FTIR spectra further supported the high crosslinking density between the PVA and D-sorbitol, which resulted highest Young's modulus and lowest swelling rate of PVA film in Fig. 2e and h. As for PK, PKCC and PKC films, the O—H stretching vibration peaks located at 3281 and 3278 cm−1 (more negative than that of pristine KGM and pristine PVA), demonstrated the formation of hydrogen bond between KGM and PVA. The peaks at 1022 cm−1 of PK, PKCC and PKC were between the corresponding peak of pristine KGM (1006 cm−1) and pristine PVA (1028 cm−1), signified the hydrogen bond formation mainly between bridging O band of KGM and C—O band of PVA. In addition, the PVA characteristic vibration peaks at 835 cm−1 negatively shifted in PK, PKCC and PKC films, and the intensity of them were also weakened. And, the peak located at 1717 cm−1 that attributed to the C Created by potrace 1.16, written by Peter Selinger 2001-2019 O bond of PVA disappeared. These changes indicated that, PVA is successfully grafted with KGM. Especially, the peak at 845 cm−1 of PKCC was the weakest, demonstrated that the crosslinking reaction induced by Cur-CDs is the fullest. Meanwhile, the XRD patterns are exhibited. As can be seen in Fig. S6, a broad peak at 2θ 20.0° of KGM films suggested the amorphous state of KGM, while a small peak at 24.2° is corresponded to the (002) reflection plane (Xu et al., 2019). PVA film shown the characteristic semicrystalline peaks at and 19.8°(Gürler et al., 2023). In the XRD patterns of PK, PKCC and PKC films, KGM peak at 24.2° disappear, PVA characteristic peak at 19.8° also weakened, suggesting that the blending of PVA, KGM, and D-sorbitol weaken the intermolecular forces that caused by crosslinking reaction (Xu et al., 2019). Especially, the crystal peak intensity of PKCC film was the lowest, indicating that its crosslinking degree is the highest. Based upon results above, Fig. 4g presents the crosslinking schematic diagram of cling film, in which hydrogen bond is the dominant forces between KGM and PVA induced by Cur-CDs and enhances the thermal stability of the composite film.

3.4. Biodegradation of hydrogel films

To study the biodegradation ability of the films, PVA, KGM, and PKCC films were respectively buried in the soil for 4 weeks (Fig. S7). As can be seen in Fig. 5, due to the erosion inside the soil and the action of microbes, the surface of the KGM and PKCC films rough and accompanied by serious voids, indicating that the structural integrity of the membrane was affected. Within 4 weeks, the loss weight was 58.83 % of KGM film and 27.90 % of PKCC film. In contrast, the internal structure of PVA film is more compact, and the soil erosion effect is limited within 4 weeks, only exhibiting 18.44 % loss weight. The results are consistent with Yan et al. (2021) that believing high hydrophilicity promotes the film degradation rate.

Fig. 5.

Fig. 5

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(a) Photos of different films dug out of the soil at 7 days interval; (b) remained curves of different films buried in the soil.

3.5. Antimicrobial preservation on pork

Photodynamic antibacterial therapy (PDAT) has been widely used in food preservation (Do Prado-Silva et al., 2022; Piksa et al., 2023). Among them, Cur was widely used for preparing antimicrobial films, due to its excellent photodynamic property (Lan et al., 2023). However, the poor light stability restricts its long-term application. As can be seen in Fig. 6a, compared with films that without blue light expose, the Cur characteristic color of PKC film under blue light becomes lighter, signifying the photobleaching of Cur. In contrast, the photobleaching degree of Cur in PKCC film was significantly decreased, which was owing to the photoprotection by CDs in Cur-CDs (Pan et al., 2024). Fig. 6b and c shown the Cur release rate of PKC and PKCC films. The swelling behavior of PKC and PKCC film were similar in Fig. 2h, but the water solubility Cur-CDs is much higher than Cur (Pan et al., 2024). Within 20 min, due to the higher hydrophilicity of PKCC (proved by the water contact angle) and Cur-CDs, the contact area between the PKCC film and water is larger (insert in Fig. 6b), which is consistent with the results in Fig. 3. Therefore, the Cur release rate of PKCC was higher than that of PKC (Fig. 6b). After 20 min but within 5 h, due to the hydrogen bond between Cur and CDs, the Cur release rate of PKCC film is lower than that of PKC film (Fig. 6c).

Fig. 6.

Fig. 6

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(a) Light stability result pictures of the as-prepared hydrophilic films; (b-c) Cur release curves of PKC and PKCC films (n ≥ 3, insert is the picture of them immersed in water); (d) Antibacterial results of films; (e) Results of antibacterial preservation on pork.

Under similar 20 mins of blue LED light illumination, PKCC film shown enhanced anti- L. monocytogenes effects compared to PK and PKC films by killing 1.0 × 103 CFU L. monocytogenes (Fig. 6d), due to the enhanced reactive oxygen species (ROS) generation of Cur-CDs than Cur (Pan et al., 2024). Then, PK and PKCC films were selected to study their antimicrobial preservation effect on pork, using PE film as comparation. Before packaging, the pork was directly exposed to the air at 25 °Cfor 3 h to be initially microbial contaminated, and the contamination situation can be seen in Fig. 6e as 0 day. Subsequently, the pork was wrapped in the films and irradiated under 20 mins of blue LED light. One days after, all the pork shown significant reduced bacterial colony, which might be due to that blue light itself also has antibacterial properties (Hyun & Lee, 2020). Among them, the pork that packaged by PKCC exhibited no bacterial colony (1.74 × 104 CFU microbial colony was eliminated), which is owing to the photodynamic sterilization ability of Cur-CDs. On the 2nd day, PKCC film still exhibited better antibacterial effect.

3.6. Preservation of beef

Moreover, using beef as the experimental subject to measure the water locking preservation effect of the films. As can be seen in Fig. S6a, beef without packaging began to dry and show shrinkage on the 2nd day, with a darker surface color and a reddish black appearance on the sixth day. Covering PK films, the surface of beef began to show darker color and exhibit shrinkage on the 6th day, 4 days prolong than unpackaged beef. As for PKCC film packaged beef, even on the 6th day, it shown no significant changing appearance, demonstrating that PKCC hydrophilic film has an excellent water locking preservation ability. Compared with the unpacked group, the hardness, chewiness and firmness of the beef packaged with PK film and PKCC film did not change significantly on the 6th day (Fig. S8b-S8d), which further proved that the PKCC film prepared in this study has good preservation ability for raw meat.

4. Conclusion

KGM and PVA were used as the hydrogel matrix, while D-sorbitol as plasticizer and Cur-CDs as photosensitizer, to make the degradable antibacterial hydrophilic film. The incorporating of KGM with PVA reducing the Young's modulus of PVA, and the water penetrating showing Fick diffusion behavior. In addition, the introducing of Cur-CDs promoted the crosslinking reaction between KGM and PVA, forming a film with smooth surface and high transparency. Moreover, the thermal stability of the composite film was significant enhanced compared to pure PVA and KGM films. Within 4 weeks, the film can be biodegraded 27.90 %, showing good soil biodegradability. Due to the photoprotection of CDs towards Cur, and the enhanced water solubility, PKCC film show better photostability and quicker Cur released rate within 20 mins than PKC film. Finally, PKCC film show enhanced photodynamic antibacterial effect and raw meat preservation. Nevertheless, the photodynamic antibacterial effect of Cur-CDs doped into the film are significantly lower than it before doping. Further study will focus on improving its antibacterial ability doped into the film.

CRediT authorship contribution statement

Xiaoqin Pan: Writing – original draft, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization. Shan Xiao: Resources, Investigation. Yanxue Cai: Writing – review & editing, Investigation. Zhouyi Xiong: Methodology. Bo Wang: Funding acquisition. Xuan Chen: Methodology. Jihui Wang: Supervision, Software, Funding acquisition.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The work was supported by the National Key Research and Development Program of China (NO. 2023YFD2100205), the Guangdong Provincial Foundation for Basic and Applied Basic Research (NO. 2024A1515110149), the Regional Brand Innovation & Development Institute of Dongguan Premade Cuisine (NO. 2339139), and the Dongguan Food Industry Science and Technology Innovation Center.

Footnotes

Appendix A

Supplementary data to this article can be found online at https://doi.org/10.1016/j.fochx.2025.102730.

Contributor Information

Xiaoqin Pan, Email: panxiaoqin@dgut.edu.cn.

Shan Xiao, Email: xiaoshan@dgut.edu.cn.

Yanxue Cai, Email: caiyanxue@dgut.edu.cn.

Bo Wang, Email: bwang@dgut.edu.cn.

Xuan Chen, Email: chenxuan@dgut.edu.cn.

Jihui Wang, Email: wangjihui@dgut.edu.cn.

Appendix A. Supplementary data

Supplementary material: Supplemental results of AFM, film thickness, swelling curve fitted lines, TGA curves, FTIR spectra, XRD patterns, and beef preservation.

mmc1.docx (31.1MB, docx)

Data availability

Data will be made available on request.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

Supplementary material: Supplemental results of AFM, film thickness, swelling curve fitted lines, TGA curves, FTIR spectra, XRD patterns, and beef preservation.

mmc1.docx (31.1MB, docx)

Data Availability Statement

Data will be made available on request.

Articles from Food Chemistry: X are provided here courtesy of Elsevier

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