Molecular interaction and synergistic effects of silver nanoparticles capped by thesinic acid in sodium alginate films: Toward active preservation and intelligent freshness monitoring

Abstract

This study reported a multifunctional sodium alginate (SA) film enhanced with silver nanoparticles (AgNPs) for pork preservation and freshness monitoring. Spectroscopic analyses confirmed that thesinic acid (CBDA-11) served as a dual bio-reductant and capping ligand, facilitating the green synthesis of stable AgNPs. Those nanoparticles established a dense cross-linked network with the SA matrix, achieving an extraordinary tensile strength of 101 MPa while enhancing water vapor barriers and UV–vis shielding (>99.86% at 200–280 nm). The films exhibited potent antibacterial efficacy and robust antioxidant activity (84.54% ABTS scavenging). In practical applications, SA-CBDA-11-Ag films effectively retarded myoglobin oxidation, delayed protein degradation, and significantly inhibited volatile basic nitrogen accumulation and microbial proliferation, extending pork shelf life to 8 days. Furthermore, the film functioned as a colorimetric sensor, showing a pH-dependent visual transition for freshness monitoring. This work highlighted the potential of AgNPs-based films as advanced active-intelligent systems for sustainable food preservation.

Graphical abstract

Highlights

• •Biogenic AgNPs were synthesized using thesinic acid (CBDA-11) as a reducing and capping agent.
• •Incorporation of CBDA-11-AgNPs enhanced the mechanical properties and UV-shielding performance of the SA-based films.
• •CBDA-11-AgNPs endowed the SA films with excellent antioxidant and antibacterial activities.
• •The composite film significantly extended the shelf-life of pork by maintaining quality and inhibiting microbial growth.
• •pH-dependent chromogenic behavior enables visual monitoring of pork freshness.

1. Introduction

Meat and meat products, particularly fresh pork, are highly susceptible to spoilage due to their high moisture, protein, and lipid content (Ren et al., 2021). During storage, lipid oxidation and microbial growth lead to discolouration, off-flavours and the formation of alkaline substances, thereby impairing nutritional value and sensory appeal (Chandra Varsha et al., 2026). More critically, the prevalence of foodborne pathogens and the emergence of antimicrobial resistance (AMR) pose persistent threats to public health, demanding advanced intervention strategies beyond traditional preservation (Chen et al., 2023). Commercially available meat preservation films are primarily made from non-biodegradable petroleum-based polymers, which tend to accumulate in the environment after disposal (Chauhan et al., 2024). More importantly, the function of these inert materials is limited to serving as a passive barrier, without the ability to participate in food preservation actively (Xu et al., 2025). Specifically, they cannot provide sustained release of antibacterial or antioxidant agents to extend shelf life, nor can they offer real-time information on the freshness of packaged meat. These functional shortcomings not only contribute to food waste but also increase the safety risk of consumers inadvertently consuming spoiled products. Consequently, the development of multifunctional packaging that integrates active protection with intelligent sensing, capable of addressing both sustainability and commercial safety requirements, has emerged as a transformative direction for the food industry (Zhan et al., 2026).

To overcome the aforementioned shortcomings of traditional inert materials, sodium alginate (SA), a biocompatible and biodegradable polysaccharide, provides an ideal matrix for the development of multifunctional food packaging (Metha et al., 2024). However, SA films are highly hydrophilic and lack intrinsic antioxidant or antimicrobial properties, making them ineffective for high-moisture meat preservation (Luan et al., 2025; Wang et al., 2024). Recent studies have demonstrated that incorporating nanomaterials as additives can reduce the hydrophilicity and improve the barrier properties of SA-based films (Mousavi et al., 2026; Shi et al., 2024). In fact, nanomaterials are most commonly used as antimicrobial agents to incorporate into SA-based films for the fabrication of active packaging systems (Li et al., 2022). These nanomaterials include silver nanoparticles, gold nanoparticles, copper nanoparticles and zinc oxide nanoparticles, among others (Lieu et al., 2024). In addition to being used in food packaging, they are also widely used in food manufacturing, the early diagnosis and treatment of cancer, and the removal of contaminants (Al-Toriahi et al., 2023; Dejene, 2025; Ramezani et al., 2024; Wang et al., 2025). Among them, silver nanoparticles (AgNPs) are the most widely used synthetic nanoparticles (Azooz et al., 2025). Their high antimicrobial activity and facile fabrication process have attracted considerable attention.

AgNPs possess broad-spectrum antimicrobial activity and exhibit high efficacy against common pathogenic bacteria, including Bacillus, Staphylococcus, Klebsiella and Pseudomonas (Lieu et al., 2024). Notably, unlike antibiotics that act via a single pathway, AgNPs combat pathogenic bacteria through multiple synergistic mechanisms, which helps overcome bacterial resistance (Patel et al., 2024). However, traditional methods for synthesizing AgNPs often involve toxic chemicals, require expensive equipment and are energy-intensive, which may lead to environmental contamination and potential health risks (Khursheed et al., 2023). To address these issues, researchers have increasingly focused on green synthesis strategies. Plant-derived polyphenols serve as ideal biological reducing and capping agents for the green synthesis of AgNPs, endowing them with enhanced stability and biocompatibility (Yang et al., 2024). Beyond their reducing capacity, polyphenols also possess inherent antioxidant and antibacterial activities. They can work synergistically with AgNPs to further enhance the antibacterial and food preservation properties of the composite system (Zhang & Jiang, 2020). Sun et al. reported that polyphenol-rich kiwi peel extracts (PE) can reduce and stabilize AgNPs. The as-synthesized Ag@PE NPs significantly improved the mechanical properties of SA-based films and endowed the films with excellent antioxidant and antibacterial activities (Sun et al., 2021). Khan et al. incorporated curcumin-mediated AgNPs into pullulan films. The composite films maintained the texture and physicochemical properties of broiler meat and enhanced its oxidative stability during 14 days of storage (Khan et al., 2022). However, there are few studies on the application of SA-based films incorporated with AgNPs for meat preservation. Furthermore, developing active packaging with intelligent freshness monitoring functions remains a major challenge for SA/AgNPs composite films.

This work innovatively utilized a plant-derived polyphenol, thesinic acid (4,4′-dihydroxy-α-truxillic acid, CBDA-11), to fabricate multifunctional SA-based films. Compared to common single polyphenols, CBDA-11 possesses a distinct, multi-branched phenolic acid backbone with optimal redox potentials, which could facilitate the rapid and uniform reduction of silver ions under mild conditions (Liu et al., 2013; Song et al., 2006; Zhang et al., 2025). Furthermore, its spatial hindrance and abundant peripheral carboxyl/hydroxyl groups offer superior steric stabilization, inhibiting AgNPs aggregation without the aid of toxic synthetic surfactants (Yuan et al., 2020). CBDA-11 was hypothesized to provide robust chemical stability to AgNPs while imparting strong UV-shielding, antioxidant and antibacterial capacity to the SA matrix. Crucially, we found the pH-dependent chromogenic behavior of the resulting SA-CBDA-11-Ag film, which allows the film to function as a visual indicator for meat freshness by responding to volatile amines. The physicochemical properties and bioactivities of the SA-CBDA-11-Ag films were systematically characterized, and their effectiveness in retarding protein degradation and preserving the quality of pork was comprehensively validated. This work developed SA-CBDA-11-Ag intelligent films that combine preservation and visual freshness monitoring, providing new insights for expanding the practical application of AgNPs composite films in meat active packaging.

2. Materials and methods

2.1. Materials

SA (viscosity: 200 ± 20 mPa·s) was purchased from Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). 0.1 M silver nitrate (AgNO₃) standard titration solution was purchased from Guangzhou Howei Pharmaceutical Technology Co., Ltd. (Guangdong, China). CBDA-11 was purchased from Taiyicheng Biochemical Pharmaceutical Co., Ltd. (Hubei, China). 1,1-Diphenyl-2-picrylhydrazyl (DPPH) and 2,2′-azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) were purchased from Shanghai Macklin Biochemical Technology Co., Ltd. Luria-Bertani (LB) agar medium, LB broth medium, and Mueller-Hinton (MH) broth medium were supplied by Changde BKMAM Biotechnology Co., Ltd. (Hunan, China). Sodium hydroxide was acquired from Meryer Biochemical Technology Co., Ltd. (Shanghai, China). Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli) were purchased from Shandong Geyan Biotechnology Co., Ltd. (Shandong, China). Pork was obtained from Jilin Huazheng Agribusiness Development Co., Ltd. (Jilin, China).

2.2. Preparation and characterization of CBDA-11-AgNPs

2.2.1. Preparation of CBDA-11-AgNPs

CBDA-11 (100 mg) was weighed into a centrifuge tube, and 5 mL of ultrapure water was added for preliminary mixing. The pH value of the solution was slowly adjusted to 7.0 with 0.5 M NaOH, and it was then sonicated for 1 min until CBDA-11 was completely dissolved. AgNO₃ solution was slowly added to the centrifuge tube with a molar ratio of CBDA-11 to Ag of 1:3, and the mixture was stirred continuously at room temperature for 30 min. The resultant sample was centrifuged at 8000 rpm for 15 min. After the supernatant was discarded, the precipitate was washed three times with ultrapure water and then three times with ethanol, sequentially. Next, the washed precipitate was finally placed in a constant-temperature oven and dried at 60 °C for 24 h to obtain CBDA-11-AgNPs powder.

2.2.2. Particle size and ζ-potential of CBDA-11-AgNPs

Particle size and ζ-potential were measured using a dynamic light scattering (DLS) instrument (Zetasizer Nano-ZS, Malvern Instruments, UK).

2.2.3. Transmission electron microscopy (TEM) and scanning electron microscope (SEM) analysis

TEM (JEM-2100F, JEOL, Japan) and SEM (Hitachi SU5000, Hitachi Ltd., Japan) were used to observe the surface morphology of the particles. The samples were dispersed in ultrapure water and ultrasonicated for 30 min at room temperature to prevent particle aggregation. Subsequently, a drop of the suspension was placed onto a copper grid for TEM observation and onto a silicon wafer for SEM analysis, respectively, and dried before imaging.

2.2.4. Fourier transform infrared (FTIR) and ultraviolet-visible (UV–vis) analysis

The FTIR analysis of CBDA-11-AgNPs was conducted using an FTIR Spectrometer (Nicolet iS5, Thermo Scientific, USA) via the KBr pellet method. Additionally, a UV–vis spectrophotometer (UV-2700, Shimadzu, Japan) was employed to investigate UV–vis spectra of the CBDA-11-AgNPs.

2.2.5. X-ray photoelectron spectroscopy (XPS)

XPS measurements were performed to analyze the chemical composition and elemental valence states of the CBDA-11-AgNPs using a Thermo Scientific Escalab 250 Xi spectrometer (Thermo Scientific, USA). Next, the XPS spectra were processed using Avantage software (Thermo Scientific, USA). The binding energy positions were calibrated using the C 1 s peak at 284.8 eV.

2.3. Preparation and characterization of SA-CBDA-11-Ag films

2.3.1. Preparation of SA-CBDA-11-Ag films

Antibacterial films containing 0.5%, 1%, 2%, and 4% (w/v) CBDA-11-AgNPs were prepared, with AgNP-free SA films serving as the blank control. Five replicate samples were prepared for each formulation. Specifically, 0 g, 0.0125 g, 0.025 g, 0.05 g, and 0.1 g of CBDA-11-AgNPs were individually added to the SA solution (2.5 g of SA dissolved in 250 mL of ultrapure water) supplemented with 0.75 g of glycerol. The mixtures were vigorously stirred at 400 rpm for 1 h to form homogeneous solutions, which were then transferred to clean petri dishes and dried in an oven at 37 °C for 24 h. The obtained films were named SA, SA-CBDA-11-Ag-0.5, SA-CBDA-11-Ag-1, SA-CBDA-11-Ag-2, and SA-CBDA-11-Ag-4.

2.3.2. Atomic force microscopy (AFM)

The microstructure of SA-CBDA-11-Ag films was characterized by AFM. Images were acquired in tapping mode using an atomic force microscope (Dimension Icon, Bruker, Germany) at a scan rate of 1 Hz. Film roughness (Ra and Rq) was analyzed using Bruker Nanoscope Analysis 3.0 software.

2.3.3. Attenuated total reflection-Fourier transform infrared (ATR-FTIR) and UV–vis analysis

ATR-FTIR spectra of SA-CBDA-11-Ag films were recorded using a Fourier transform infrared spectrometer (Nicolet iS5, Thermo Scientific, USA) over 4000–400 cm⁻¹, with a resolution of 4 cm⁻¹ and 32 scans. For UV–vis analysis, films were cut into strips (1 cm × 3 cm) and placed on the transmission surface of a transparent quartz cuvette. UV–vis transmission spectra (200–800 nm) were recorded on a UV–Vis spectrophotometer (UV-2700, Shimadzu, Japan) at 1 nm intervals, with air as the reference.

2.3.4. X-ray diffraction (XRD)

XRD measurements were conducted on an X-ray diffractometer (D8 Advance, Bruker, Germany). The diffraction patterns were recorded over a 2θ range of 5°–50° at a scanning rate of 5° min⁻¹.

2.3.5. Thickness, water content (WC), and moisture absorption (MA)

The average surface thickness of three replicate films per formulation was measured at 7 random points using a digital micrometer (resolution: 0.001 mm). WC and MA were determined via high-temperature dehydration to remove free water. Samples were accurately weighed (recorded as W₀), dried at 105 °C for 3 h (weighed and recorded as W₁), then stored in a humid environment (25 °C, 75% relative humidity) for 2 days (weighed and recorded as W₂). WC and MA were calculated using eqs. (1), (2) (Yan et al., 2022).

$$\mathrm{WC}\left(\%\right)=\frac{{\mathrm{W}}_0-{\mathrm{W}}_1}{{\mathrm{W}}_0}$$ (1)

$$\mathrm{MA}\left(\%\right)=\frac{W_2-W_1}{W_1}$$ (2)

2.3.6. Water vapor permeability (WVP)

WVP was evaluated via the cup method. Films (2 cm × 2 cm) were sealed over ampoules (inner diameter: 12.6 mm, depth: 40 mm) containing 3 g anhydrous CaCl₂, then placed in desiccators with saturated NaCl solution (70 ± 2% relative humidity). Weight changes were recorded every 4 h for 4 days. WVP (× 10⁻¹⁰ g·m⁻¹·s⁻¹·Pa⁻¹) was calculated by eq. (3) (do Evangelho et al., 2019).

$$\mathrm{WVP}=\frac{\left(\mathit{\Delta m}\times \mathrm{d}\right)}{\left(\mathit{\Delta t}\times \mathrm{A}\times \mathit{\Delta P}\right)}$$ (3)

Where Δm (g) = weight increment of the ampoule; d (m) = average film thickness; A (m²) = permeation area; Δt (s) = time interval; ΔP = vapor pressure deficit (2400 Pa for saturated NaCl at 25 °C).

2.3.7. Mechanical properties

SA-CBDA-11-Ag films were cut into strips measuring 0.9 cm × 4 cm. Their tensile strength (TS) and elongation at break (EAB) were measured using an INSTRON materials testing machine in accordance with GB 1040–2006, at a test speed of 5 mm/min.

2.3.8. Thermal stability

Simultaneous thermal analysis (STA) was employed to evaluate the thermal stability of SA-CBDA-11-Ag films. Approximately 5 mg of film samples were analyzed using a thermogravimetric analyzer (SDT Q600, TA Instruments, USA) under a nitrogen atmosphere. Scans were performed at a heating rate of 20 °C/min within the temperature range of 30–600 °C. The thermogravimetric analysis (TGA), differential thermogravimetric (DTG), and differential scanning calorimetry (DSC) curves of SA-CBDA-11-Ag film samples were recorded simultaneously.

2.3.9. Biodegradability

The biodegradability of SA-CBDA-11-Ag films was evaluated via soil burial assays. Briefly, SA films and SA-CBDA-11-Ag films (3 cm × 3 cm) were buried in fresh soil at a depth of 5 cm (Lade et al., 2023). Samples were retrieved weekly, rinsed with water, oven-dried at 80 °C for 12 h, then precisely weighed (do Evangelho et al., 2019). This experiment was conducted continuously for 4 weeks, and the degradation curves were plotted based on weight loss rate and degradation time.

2.3.10. Antioxidant capacity

The antioxidant capacity of SA-CBDA-11-Ag films was evaluated using DPPH and ABTS assays. 1 cm × 1 cm film pieces were dissolved in 3 mL of ultrapure water, reacted with DPPH/ABTS working solutions at a 1:3 ratio, and thoroughly mixed in the dark for 30 min. The absorbance was measured at wavelengths of 517 nm and 734 nm by a UV–vis spectrophotometer (UV-2700, Shimadzu, Japan). The free radical scavenging activity was calculated as follows.

$$\text{Radical scavenging ability}\left(\%\right)=\left(1-\mathrm{A}/{\mathrm{A}}_0\right)\times 100$$ (4)

Where A and A₀ denote the absorbance of solutions with and without the film samples, respectively.

2.3.11. Antibacterial property

The antibacterial property of SA-CBDA-11-Ag films was evaluated by the agar disk diffusion method. Films of different concentrations were cut into 6 mm diameter disks and sterilized under UV irradiation for 30 min on each side. Sterile LB agar (30 mL) was poured into petri dishes and allowed to solidify. The pre-cultured E. coli and S. aureus were diluted to approximately 10⁵ CFU/mL as the standard inoculation concentration. Aliquots (10 μL) of each bacterial suspension were evenly spread onto the agar plates. The cut films were placed on the inoculated agar plates and incubated at 37 °C for 24 h. Inhibition zones were measured using a ruler (Zhang, Zhang, et al., 2025).

2.3.12. pH sensitivity to ammonia and acetic acid

The SA-CBDA-11-Ag-2 film was selected as a representative sample, cut into 1 cm × 1 cm squares, and affixed to the lids of petri dishes containing either 25% (v/v) aqueous ammonia or 50% (v/v) acetic acid solution using transparent tape. There was no direct contact between the films and the liquids, enabling fumigation of the film samples with volatilized ammonia gas and acetic acid vapor. Photographs were captured at 0, 5, 10, 20, 60 and 120 min.

2.4. Application of active films in pork preservation

The practical application efficacy of SA-CBDA-11-Ag films was evaluated via pork tenderloin preservation tests. The SA-CBDA-11-Ag-4 films, exhibiting optimal comprehensive performance, were selected as representative samples. Pork tenderloin pieces (3 g each) were randomized into three groups with three replicates per group. The meat pieces were wrapped in SA films, SA-CBDA-11-Ag-4 films, or left unwrapped. All samples were stored at 4 °C. At days 0, 2, 4, 6, 8, and 10, random samples were taken to measure weight and pH, with photographic records of their condition. The content of total volatile basic nitrogen (TVB-N) in the samples was determined by the micro-diffusion method, in accordance with the Chinese national standard GB 5009.228–2016 (Leng et al., 2021). Additionally, on day 10, 100 mg of each sample was homogenized with 10 mL sterile saline. The supernatants were serially diluted, and 10 μL aliquots were plated to count colonies on agar plates.

Volatile compounds in pork tenderloin were analyzed by headspace gas chromatography–mass spectrometry (HS-GC–MS) using a TRACE 1310 gas chromatograph coupled to an ISQ 7000 single quadrupole mass spectrometer (Thermo Fisher Scientific, USA). Detailed analytical conditions are provided in the Supplementary Materials.

The CIE L*, a*, b* color parameters of the SA-CBDA-11-Ag film were measured at predetermined storage intervals, with the film sample collected on day 0 set as the reference group. The total color difference (ΔE) of the film during storage was calculated via the standard formula:

$$∆\mathrm{E}=\sqrt{{\left(L_t^{\ast }-L_0^{\ast }\right)}^2+{\left(a_t^{\ast }-a_0^{\ast }\right)}^2+{\left(b_t^{\ast }-b_0^{\ast }\right)}^2}$$ (5)

Where $L_0^{\ast }$, $a_0^{\ast }$ and $b_0^{\ast }$ represent the initial color values of the film at day 0; $L_t^{\ast }$, $a_t^{\ast }$ and $b_t^{\ast }$ correspond to the color parameters at storage time t.

2.5. Statistical analysis

All experiments were repeated three times, and all data were expressed as mean ± standard deviation (SD). Statistical analyses were conducted using SPSS software (Version 25, IBM Corp., USA). All statistical data were subjected to one-way analysis of variance (ANOVA), with statistical significance defined at the p < 0.05 level. Data graphical visualization was performed using Origin software (Version 2025, OriginLab Corporation, USA). The Pearson correlation coefficient (r) was used to evaluate the correlations between the color changes of the indicator film and TVB-N values during pork storage.

3. Results and discussion

3.1. Characterization of CBDA-11-AgNPs

As shown in Fig. 1a, upon the reaction of CBDA-11 with AgNO₃, the color of the mixture solution changed to brown. Fei et al. and Sun et al. both reported that AgNPs prepared using polyphenolic reducing agents exhibit a brown hue (Fei et al., 2014; Sun et al., 2021). This color change may be attributed to the reduction of Ag⁺ ions to Ag⁰ nanoparticles, suggesting the successful synthesis of CBDA-11-AgNPs.

Fig. 1.

(a) Optical photographs during the synthesis of CBDA-11-AgNPs (left: CBDA-11 solution; right: CBDA-11-AgNPs suspension). (b) Particle size and (c) ζ-potential plots of CBDA-11-AgNPs. (d-e) TEM images of CBDA-11-AgNPs with lower (scale: 500 nm) and higher (scale: 200 nm) magnification. (f) The FTIR spectra of CBDA-11 and CBDA-11-AgNPs. (g-h) SEM images of CBDA-11-AgNPs with lower (scale: 5.00 μm) and higher magnification (scale: 2.00 μm). (i) The UV–vis spectra of CBDA-11 and CBDA-11-AgNPs. (j) XPS spectra of survey, (k) Ag 3d and (l) C 1s of CBDA-11-AgNPs.

The particle size distribution and ζ-potential characteristics of CBDA-11-AgNPs are presented in Fig. 1b and c. The composite particles exhibited a unimodal size distribution with a particle size of approximately 137 nm and a polydispersity index (PDI) of about 0.35. This particle size is close to that of AgNPs derived from Tinospora cordifolia leaf extract (168 nm) (Lekkala et al., 2025). Furthermore, the CBDA-11-AgNPs exhibited a negative ζ-potential of −25.4 mV, which is attributable to the conversion of phenolic hydroxyl groups (−OH) in CBDA-11 to phenoxide anions (—O⁻) following the reduction reaction, thereby imparting a negative surface charge to the AgNPs (Abdel-Aty et al., 2023). Given that the |ζ| value ranged from 10 to 30 mV, the nanoparticle dispersion exhibited relative stability, albeit with a tendency toward mild aggregation.

Fig. 1d and e display TEM images of CBDA-11-AgNPs at different magnifications. The nanoparticles exhibited a relatively uniform spherical morphology. However, a tendency toward aggregation was also observed. Notably, Fig. 1e illustrates the formation process of CBDA-11-AgNPs. Initially, CBDA-11 captured Ag⁺ ions via electrostatic interactions. Subsequently, the phenolic hydroxyl groups on CBDA-11 reduced Ag⁺ to Ag⁰, generating small AgNP units. Finally, these small units self-assembled to form CBDA-11-AgNPs (Sun et al., 2021). During the reaction, a portion of CBDA-11 self-assembled into a sheet-like structure, which prompted the synthesized CBDA-11-AgNPs to bind firmly to the sheet surface. This observation was further corroborated by SEM images (Fig. 1g and h).

Fig. 1f presents the FTIR spectra of CBDA-11 and CBDA-11-AgNPs. The absorption peaks at 800–860 cm⁻¹ were attributed to out-of-plane C—H bending vibrations of para-disubstituted benzene rings; those at 1200–1320 cm⁻¹ to C—O stretching vibrations of carboxyl groups. A methyl bending vibration was observed at ∼1375 cm⁻¹, while multiple peaks at 1450–1600 cm⁻¹ corresponded to benzene ring skeletal stretching vibrations. A broad peak at 3200–3500 cm⁻¹ was assigned to O—H stretching vibrations of hydrogen-bonded hydroxyl groups. Notably, compared with CBDA-11, the hydroxyl absorption band of CBDA-11-AgNPs at approximately 3460 cm⁻¹ exhibited evident broadening and a slight shift. This phenomenon is attributed to the oxidation of phenolic hydroxyl groups in CBDA-11 by Ag⁺, which is consistent with previous reports on the biosynthesis of AgNPs using polyphenols (Fei et al., 2014; Sun et al., 2021). Overall, the distinct spectral differences between CBDA-11 and CBDA-11-AgNPs confirm the interaction between CBDA-11 and Ag, further supporting the successful formation of CBDA-11-AgNPs.

Fig. 1i displays the UV–Vis spectra of CBDA-11 and CBDA-11-AgNPs. CBDA-11 showed a significant UV absorption peak at 276 nm, which is attributed to the π → π* electronic transition of the aromatic rings. However, in the UV spectrum of CBDA-11-AgNPs, this absorption peak blue-shifted to 262 nm. This phenomenon may be ascribed to the coordination between the oxygen atoms of the phenolic hydroxyl/carboxyl groups in CBDA-11 and Ag⁺/Ag⁰, which perturbs the electron cloud density of the chromophores and alters the energy levels of the π → π* transition. Furthermore, the spectrum of CBDA-11-AgNPs exhibited a distinct absorption band within the 400–500 nm wavelength range, corresponding to the localized surface plasmon resonance (LSPR) of CBDA-11-AgNPs. This feature is consistent with the characteristic optical properties of AgNPs (Abdel-Aty et al., 2023; Corciovă et al., 2024).

XPS analysis serves as one of the key pieces of evidence to confirm the formation of CBDA-11-AgNPs, as it enables the direct characterization of the silver valence state in the composite nanoparticles. As shown in Fig. 1j, the XPS survey spectrum confirmed the presence of Ag, C, and O elements in CBDA-11-AgNPs. Two main peaks were observed in the Ag 3d spectrum of CBDA-11-AgNPs at 368.79 eV (Ag 3d₅/₂) and 374.76 eV (Ag 3d₃/₂) (Fig. 1k), which are characteristic of Ag⁰ (metallic state) and consistent with the reported values of 368.8 eV and 374.7 eV (Hu et al., 2024). In addition, the C 1s spectrum exhibited three distinct peaks at 284.8 eV (C–C/C=C), 286.22 eV (C—O), and 288.37 eV (O–C=O), corresponding to the carbon skeleton and oxygen-containing functional groups, such as phenolic hydroxyl and carboxyl groups of CBDA-11, respectively (Fig. 1l). Collectively, these results demonstrated the successful synthesis of CBDA-11-AgNPs.

3.2. Morphology and optical properties of SA-CBDA-11-Ag films

CBDA-11 is a water-insoluble thesinic acid, which exhibits acidic properties due to its carboxyl groups, and it turns water-soluble upon reaction with NaOH. SA, a water-soluble polysaccharide polymer, was selected as the film matrix. Accordingly, SA-CBDA-11-Ag composite films were prepared via an aqueous solution casting route (Fig. 2a). As shown in Fig. 2b, both the pure SA film and SA-CBDA-11-Ag films showed smooth surfaces under visual observation. With increasing CBDA-11-AgNPs content, the SA-based films changed in color from colorless to purplish-black.

Fig. 2.

(a) Schematic illustration of the preparation process for SA-CBDA-11-Ag films. (b) Optical photographs, (c) 2D AFM images, and (d) 3D AFM images of the SA film and SA-CBDA-11-Ag films with different CBDA-11-AgNPs loadings (0.5, 1, 2 and 4).

AFM was employed to characterize the surface topography of the pure SA film and SA-CBDA-11-Ag films, from which the average roughness (Ra) and root-mean-square roughness (Rq) were determined. As shown in Figs. 2c-d, the incorporation of CBDA-11-AgNPs initially increased the surface roughness compared with that of the pure SA film (Ra = 19.7 nm, Rq = 28.1 nm). However, when the CBDA-11-AgNPs loading exceeded 1%, the film's surface roughness gradually decreased with further increasing nanoparticle content. This may be attributed to the fact that at lower CBDA-11-AgNPs loadings, particles remain dispersed within the SA matrix without forming a continuous cross-linked network. Conversely, at higher CBDA-11-AgNPs concentrations, enhanced hydrogen bonding interactions between CBDA-11-AgNPs and SA molecules restrict disordered aggregation of molecular chains, thereby promoting surface levelling of the film.

FTIR can characterize the chemical composition, molecular structure and surface modification of materials, providing insights into film properties. The FTIR spectra of the film samples are shown in Fig. 3a. The peak at 1024 cm⁻¹ was assigned to the stretching vibrations of C—O—C ether bonds and C—O single bonds on the pyranose ring of SA. The absorption bands at 1596 cm⁻¹ and 1408 cm⁻¹ was attributed to asymmetric and symmetric stretching vibrations of C—O bonds in COO⁻, respectively. Both pure SA films and SA-CBDA-11-Ag composite films exhibited a broad absorption band ranging from 3000 to 3500 cm⁻¹, assigned to stretching vibrations of hydroxyl groups. Owing to the low loading of CBDA-11-AgNPs, no significant change in peak positions was observed, indicating that CBDA-11-AgNPs had a minimal impact on the overall SA film structure (Ruan et al., 2024). These results demonstrate that no new covalent chemical bonds are formed between CBDA-11-AgNPs and the SA matrix, and the interfacial interaction between the two components is dominated by hydrogen bonding, which is consistent with the findings reported by Li et al. (Li, Feng, et al., 2024).

Fig. 3.

(a) ATR-FTIR spectra, (b) transmittance plots, (c) XRD patterns and (d) stress-strain curves of the SA film and SA-CBDA-11-Ag films with different CBDA-11-AgNPs loadings (0.5, 1, 2 and 4).

UV radiation can induce photochemical reactions in food, leading to deterioration in color, texture, flavour and nutritional quality (Tripathi et al., 2024). UV-shielding capacity is a critical parameter for evaluating the practical application potential of the films. As presented in Fig. 3b, the pure SA film exhibited high transmittance in the UV region of 200–400 nm. This observation is consistent with previous findings that SA films possess extremely limited UV-shielding performance (Chen et al., 2021). With the increase of CBDA-11-AgNPs loading, the UV-shielding efficiency of SA-CBDA-11-Ag composite films gradually improved, which was markedly superior to that of pure SA films. Notably, the SA-CBDA-11-Ag-4 film achieved UV shielding rates exceeding 99.86% and 96.58% in the UVC (200–280 nm) and UVB (280–320 nm) regions, respectively. This outstanding performance was mainly attributed to the intrinsic ultraviolet absorption capacity of the phenolic conjugated structure of CBDA-11. In the UVA region (320–400 nm), the UV shielding rate of the SA-CBDA-11-Ag-4 film still reached as high as 89.21%. Zhang et al. reported that the UV shielding rate of CBDA-11-Ca/SA crosslinked films exceeded 50% within the 320–375 nm range (Zhang et al., 2025). The difference in UVA protective capacity between the two film systems originated from the characteristic LSPR absorption of AgNPs at 300–400 nm, which was consistent with the UV–Vis spectra of CBDA-11-AgNPs measured in this work (Fig. 1i). Thus, the SA-CBDA-11-Ag composite films hold great promise for UV-shielding applications in preventing packaged food spoilage.

XRD was used to characterize the crystal structure, phase composition and grain size of the films. As shown in Fig. 3c, the SA film exhibited a broad diffraction peak at 2θ ≈ 23^o, indicating an amorphous phase of SA, consistent with published results (Gholizadeh et al., 2018). In the XRD patterns of SA-CBDA-11-Ag-2 and SA-CBDA-11-Ag-4 films, additional weak diffraction peaks appeared at identical positions of 27.84^o, 32.25^o, and 46.29^o. The intensities of these peaks increased with increasing concentration. In addition, a weak peak at 37.91^o was slightly visible in the pattern of the SA-CBDA-11-Ag-4 film. Among these, the peaks at 37.91^o and 46.29^o can be attributed to the (111) and (200) planes of the cubic phase of AgNPs (Ramachandraiah et al., 2017; Venkatesan et al., 2017). The peaks at 27.84^o and 32.25^o may originate from the organic capping layer on the AgNPs surface or incompletely reduced CBDA-11 residues, as similarly reported in other AgNPs systems (Iswarya et al., 2025; You et al., 2024). The high similarity between the XRD patterns of SA-CBDA-11-Ag films and the SA film indicated good compatibility between CBDA-11-AgNPs and SA. Compared to the SA film, the peak positions of other samples exhibited slight shifts, which may be attributed to the cross-linking between CBDA-11 (the shell of AgNPs) and SA, resulting in a more tightly packed molecular chain arrangement within the film (Zhang et al., 2025).

3.3. Physicochemical properties of SA-CBDA-11-Ag films

Table 1 shows that the film thickness increased gradually with the rising loading of CBDA-11-AgNPs, while the WC, MA and WVP decreased significantly. This phenomenon can be attributed to the cross-linking interaction between CBDA-11 and SA, which triggers network shrinkage of the film's internal structure via intermolecular forces (Ciannamea et al., 2016). Furthermore, Shankar and Rhim reported that the incorporation of AgNPs reduced the WVP of composite films. This is mainly because the impermeable nanoparticles dispersed within the polymer matrix can prolong and tortuous the diffusion pathways of water vapor (Shankar & Rhim, 2017). These results demonstrate that CBDA-11-AgNPs can remarkably improve the water resistance of SA-based films. Accordingly, SA-CBDA-11-Ag composite films possess great application potential in the field of food packaging.

Table 1.

Thickness, WC, MA, and WVP of each film.

Film	Thickness (μm)	WC (%)	MA (%)	WVP (×10−10 g·m−1·s−1·Pa−1)
SA	45.4 ± 3.66d	19.95 ± 1.34a	23.66 ± 2.83a	1.478 ± 0.06a
SA-CBDA-11-Ag-0.5	47.4 ± 3.66cd	18.53 ± 2.14ab	20.72 ± 2.76a	1.408 ± 0.26a
SA-CBDA-11-Ag-1	51.6 ± 2.08bc	17.03 ± 2.49ab	18.39 ± 1.53a	1.257 ± 0.01b
SA-CBDA-11-Ag-2	53.2 ± 2.78ab	14.90 ± 0.79b	17.97 ± 2.22a	1.058 ± 0.03c
SA-CBDA-11-Ag-4	58.8 ± 2.60a	9.47 ± 1.17c	11.47 ± 1.26b	0.578 ± 0.08d

The stress-strain curves of the film samples are presented in Fig. 3d. According to previous reports, the tensile strength of SA films containing AgNPs generally varies from 4 to 16 MPa, depending on the formulation and processing parameters employed (Mousavi et al., 2026; Susilowati et al., 2022). In this study, compared with the pure SA film, the mechanical properties of CBDA-11-AgNPs modified films significantly improved with increasing CBDA-11-AgNPs content. Among all samples, the SA-CBDA-11-Ag-4 film exhibited the highest load-bearing capacity (101 MPa), though its flexibility was marginally lower than that of the SA-CBDA-11-Ag-2 film. This may be attributed to the dense network structure formed by hydrogen bonding between the phenolic groups of CBDA-11 and SA chains, which results in a reduction in intermolecular spacing. This enhancement in film rigidity limits further improvements in flexibility at high CBDA-11-AgNPs concentrations. This trend is highly consistent with the findings of Ruan et al. on CBDA-10-SA films (Ruan et al., 2024). Together, both works confirm that CBDA derivatives, as crosslinkers, can significantly enhance the mechanical properties of SA-based films.

To investigate the thermal stability of the films, simultaneous thermal analysis was conducted. TGA, DSC and DTG curves of SA-CBDA-11-Ag films are shown in Fig. 4a-e. The thermal behavior of all films can be divided into two main weight-loss stages: in the first stage, from room temperature to 150 °C, a slow mass loss was observed in the curves, accompanied by a broad endothermic region in the DSC curves. This phenomenon is attributed to the evaporation of free water and bound water within the films. The second stage consists primarily of a thermal decomposition region concentrated between 200 and 300 °C. All films exhibited a distinct DTG weight loss peak at approximately 220 °C, accompanied by a corresponding endothermic peak in the DSC curve, which can be attributed to the thermal decomposition of polysaccharides in the SA-based films as the temperature increased (Roy & Rhim, 2020). The DTG weight-loss peaks of the films incorporating CBDA-11-AgNPs shifted slightly toward higher temperatures, indicating that CBDA-11-AgNPs can improve the thermal stability of the films to a certain extent by forming a cross-linked network with SA.

Fig. 4.

(a-e) TGA-DSC spectra of the SA film and SA-CBDA-11-Ag composite films (0.5, 1, 2 and 4) in the temperature range of 30–600 °C. (f) Weight loss curves and (g) optical photographs of the SA film and SA-CBDA-11-Ag films with different CBDA-11-AgNPs loadings (0.5, 1, 2 and 4) during soil degradation.

3.4. Biodegradability of SA-CBDA-11-Ag films

A 4-week soil degradation assay was carried out to visually characterize the biodegradability of SA-based films containing different concentrations of CBDA-11-AgNPs. As shown in Fig. 4g, both the SA films and SA-CBDA-11-Ag films exhibited varying degrees of degradation over time. After four weeks of burial, obvious shrinkage and breakage were observed on the surface of all films. Fig. 4f records the weight loss percentage of the films as a function of burial time. The weight loss of the SA films reached 57.75% after four weeks of burial, indicating that SA-based films possessed favorable biodegradability in soil and could effectively alleviate white pollution (Zhang et al., 2024). All films shared a consistent overall degradation trend, but the weight loss percentage gradually decreased with the increasing concentration of CBDA-11-AgNPs. This can be mainly attributed to two aspects: on the one hand, CBDA-11-AgNPs can form a cross-linked network skeleton with SA, which enhances the structural stability of SA-based films, maintains the film integrity to a certain extent during soil degradation, and slows down the degradation rate (Ruan et al., 2024). On the other hand, AgNPs exert a certain inhibitory effect on microorganisms, which also contributes to the delayed degradation of the films. These results confirmed that the incorporation of CBDA-11-AgNPs didn't compromise the excellent biodegradability of SA-based films, but also improved the structural stability of the films to a certain extent, which is consistent with the results of the previous TGA-DSC analysis.

3.5. Functional properties of SA-CBDA-11-Ag films

The antioxidant activity of SA-based films evaluated by ABTS and DPPH assays is presented in Fig. 5b. With the increase of CBDA-11-AgNPs content, the ABTS radical scavenging capacity of the films was significantly enhanced, increasing from 2.52% for SA to 84.54% for SA-CBDA-11-Ag-4. Similarly, the DPPH radical scavenging capacity of the films was also significantly improved after the addition of CBDA-11-AgNPs, rising gradually from 21.28% for SA to 36.41% for SA-CBDA-11-Ag-4. The polyphenolic structure of CBDA-11 contains abundant phenolic hydroxyl groups, which act as strong electron-donating groups to reduce ABTS/DPPH radicals, thereby exerting antioxidant effects.

Fig. 5.

(a) Schematic diagram of the antibacterial mechanism of SA-CBDA-11-Ag films. (b) Antioxidant activities of the SA film and SA-CBDA-11-Ag films with different CBDA-11-AgNPs loadings (0.5, 1, 2 and 4) against DPPH and ABTS. (c-d) Inhibition zones of the SA film and SA-CBDA-11-Ag films with different CBDA-11-AgNPs loadings (0.5, 1, 2 and 4) against E.coli and S.aureus. (e) Color responses of SA-CBDA-11-Ag-2 films after exposure to acetic acid and ammonia (0–120 min).

The antibacterial effect of SA-based films was tested using the inhibition zone method, and the results are shown in Fig. 5c-d. After 24 h of culture, in the E.coli group and S.aureus group, the bacteria at the bottom of the films in the blank group and low-concentration groups (0.5% and 1%) were turbid, with colonies appearing at the edges. In the E.coli group, a distinct inhibition zone was observed on the 2% film, with an average diameter of 18.18 mm, while the average diameter of the inhibition zone generated by the 4% film reached 21.82 mm. A similar trend was observed for the S.aureus group: the average diameter of the inhibition zone on the 2% film was 17.66 mm, while that on the 4% film was 19.89 mm. It can be seen that the blank group and low-concentration groups (0.5% and 1%) had almost no antibacterial effect, while the 2% film began to show antibacterial effect, and the antibacterial effect became stronger with the increase of CBDA-11-AgNPs. Moreover, CBDA-11-AgNPs exhibited a stronger inhibitory effect on E. coli than on S. aureus, which is consistent with the report on the antibacterial activity of Ag@PE NPs (Sun et al., 2021). The difference in antibacterial efficacy between these two strains can be attributed to the differences in bacterial composition and metabolic characteristics between Gram-negative and Gram-positive bacteria (Zhang et al., 2020).

Two volatile substances, aqueous ammonia and acetic acid, were employed to construct alkaline and acidic environments around the film, respectively, for investigating the film's visual responsiveness to different pH conditions. As shown in Fig. 5e, the SA-CBDA-11-Ag-2 film rapidly changed color from grayish purple to tan within merely 5 min of exposure to ammonia vapor. In contrast, the film underwent a slight color change with prolonged exposure to acetic acid vapor, which was far less pronounced than that observed in the ammonia vapor environment. Such a color transition may be synergistically driven by the structural transformation of CBDA-11 and plasmonic optical shifts of AgNPs. Specifically, when exposed to an ammonia environment, volatile ammonia molecules penetrate into the hydrophilic SA matrix and hydrolyze to generate hydroxyl ions (OH⁻). This alkaline microenvironment triggers the deprotonation of the phenolic hydroxyl groups on the CBDA-11 capping layer (Li et al., 2024). This deprotonation alters the ligand's conjugated electronic state and the LSPR of the embedded AgNPs (Buccolieri et al., 2018; Zhan et al., 2026), leading to a visible color transition from grayish purple to tan. These observations indicate that the SA film modified with CBDA-11-AgNPs is sensitive to ambient alkaline variations, thus verifying the pH-indicating potential of SA-CBDA-11-Ag composite films.

3.6. Application of SA-CBDA-11-Ag films in pork preservation

Meat packaging constitutes a pivotal element in safeguarding its quality and safety, whilst also contributing to extended shelf life (Ren et al., 2022). Based on the previous test results of UV-shielding capacity, water resistance, tensile mechanical properties, antioxidant activity and antibacterial performance of the films, the SA-CBDA-11-Ag-4 film exhibited the most outstanding performance among all the aforementioned physicochemical and functional indicators. Therefore, the SA-CBDA-11-Ag-4 film with optimal comprehensive properties was selected for pork preservation packaging in this study. To evaluate the impact of incorporating CBDA-11-AgNPs on the intrinsic properties of the SA matrix, pure SA film packaging (SA group) and unpackaged samples (Control group) were selected as controls. It should be noted that a comparison with commercial packaging materials such as PE film will provide a more direct reference for practical industrial applications which will be conducted in our future scale-up trials.

Fig. 6a illustrates the visual changes in pork samples from different treatment groups during 10 days of refrigerated storage. All samples initially exhibited a bright red color, which gradually underwent browning and eventually turned brownish-red, a phenomenon attributed to the oxidation of oxymyoglobin and deoxymyoglobin to brown metmyoglobin (Cao et al., 2019). The Control group showed surface drying, darkening, and progressive shrinkage starting from day 4. Pork packaged in the SA film began losing moisture and shrinking from day 6. In contrast, pork packaged in the SA-CBDA-11-Ag-4 film consistently maintained the best visual appearance, most likely due to the film's low water vapor transmission rate preventing moisture loss (Han et al., 2025). Furthermore, the SA-CBDA-11-Ag-4 group exhibited the least variation in color and gloss from day 0 to day 10. This may be attributed to the phenolic hydroxyl groups in CBDA-11-AgNPs, which may have delayed myoglobin oxidation in the pork, consistent with findings from ABTS and DPPH tests.

Fig. 6.

(a) Appearance changes of pork tenderloin from the Control, SA, and SA-CBDA-11-Ag-4 groups during 10-day storage at 4 °C (The embedded photograph depicts the condition of the films at the corresponding time point). (b-d) Changes in weight, pH value and TVB-N value of pork tenderloin from the Control, SA, and SA-CBDA-11-Ag-4 groups during 10-day storage at 4 °C. (e) Colony images and (f) TVC values of each group of pork tenderloin from the Control, SA, and SA-CBDA-11-Ag-4 groups after 10-day storage at 4 °C.

As shown in Fig. 6b, the weight loss rate of pork tenderloin across all groups gradually increased over time. The control group showed a weight loss rate of 46.98% by day 10, which was higher than that of the SA group (35.30%) and the SA-CBDA-11-Ag-4 group (33.72%). This phenomenon can be attributed to the SA-CBDA-11-Ag-4 film, which features the lowest WVP. It can more efficiently inhibit the outward migration of moisture inside the samples, thus minimizing weight loss during refrigerated storage.

pH is one indicator for assessing the freshness of pork. Typically, when the pH exceeds 6.4, the meat is degraded (Zhang, Zhou, et al., 2020). In the experiment, the pH curve of pork tenderloin in each group showed a trend of initial decline followed by subsequent increase (Fig. 6c). This phenomenon arises because post-slaughter muscle cells continue anaerobic respiration, breaking down muscle glycogen into acidic metabolites such as lactic acid. The progressive accumulation of lactic acid within muscle tissue causes a temporary decrease in pH (Li et al., 2024). As storage time extends, proteins within the meat are broken down by proliferating microorganisms, producing alkaline compounds such as ammonia and amines, which causes the pH to rebound (Cao et al., 2019). On day 10, the pH value of the SA-CBDA-11-Ag-4 group was 6.78, significantly lower than that of other groups (p < 0.05). This may be attributed to the effective inhibition of microbial proliferation by CBDA-11-AgNPs within the film, thereby delaying the generation of alkaline substances. According to Zhou et al., the in-situ growth of AgNPs enables packaging materials with sustained antimicrobial activity, which significantly retards the elevation of pH, total viable count (TVC), and TVB-N values in pork during storage (Zhou et al., 2026).

TVB-N values serve as a crucial indicator for assessing meat freshness. During fresh meat storage, proteins in the meat gradually decompose under the action of enzymes and bacteria, producing alkaline nitrogen-containing compounds such as ammonia and amines, leading to an increase in TVB-N values (Zhang et al., 2020). Changes in TVB-N levels during storage for each group of pork tenderloin are shown in Fig. 6d. The Control group exhibited the fastest rate of increase in TVB-N over time, followed by the SA group. From day 4 to day 10, the TVB-N levels in the SA-CBDA-11-Ag-4 group were significantly lower than those in all other groups (p < 0.05). According to Chinese National Standard GB 2707–2016, meat is considered spoiled when TVB-N exceeds 15 mg/100 g. By day 6 of refrigerated storage, the Control group's TVB-N content reached 18.93 mg/100 g, exceeding the spoilage threshold. Although the SA group's TVB-N content (15.46 mg/100 g) was lower than the Control group, it had also spoiled by this stage, consistent with observable changes in pork appearance. In contrast, the SA-CBDA-11-Ag-4 group did not spoil until the 10th day, with a TVB-N content of 15.39 mg/100 g. This indicates that under identical storage conditions, SA-CBDA-11-Ag-4 film packaging can retard pork spoilage.

Fig. 6e-f display the colony photographs and TVC of pork tenderloin samples from different treatment groups after 10 days of storage. A TVC value of 6 (lg CFU g⁻¹) is generally regarded as the acceptable upper limit for fresh meat products (Chen et al., 2019; Chen et al., 2024). The Control group exhibited the highest TVC value, reaching 10.27 (lg CFU g⁻¹), far exceeding 6 (lg CFU g⁻¹), indicating that the unwrapped fresh meat had undergone significant microbial proliferation during refrigeration and had already suffered severe spoilage. The SA group exhibited marginally superior antimicrobial efficacy compared to the Control group. This phenomenon stems from the SA film's excellent airtight properties, which minimise pork exposure to ambient air, thereby reducing contamination risk from environmental microorganisms. However, moisture accumulation during storage stages still facilitated microbial proliferation. Among all experimental groups, the SA-CBDA-11-Ag-4 group showed the most significant preservation effect, with a TVC value of 6.80 (lg CFU g⁻¹) in the corresponding pork samples, which was slightly higher than the spoilage threshold. This outcome stems from the SA-CBDA-11-Ag-4 film retaining the physical barrier properties of SA films while incorporating CBDA-11-AgNPs with potent antibacterial activity. These nanoparticles directly inhibit bacterial growth and proliferation within the pork's immediate environment. Consequently, the SA-CBDA-11-Ag films developed in this study demonstrate significant application potential within the field of food preservation packaging.

HS-GC-MS was adopted to determine the changes in volatile compounds of pork under different treatments during storage (Table S1, Supplementary Materials). After 10 days of storage, hexanal (3.99%) was only detected in the Control group. Hexanal is a key volatile marker of lipid oxidation in pork, which indicates that direct exposure to air aggravates lipid oxidation of pork (Zhang et al., 2025). Meanwhile, dimethyl disulfide (1.36%) and dimethyl trisulfide (2.01%) were also detected in the Control group, suggesting that composite spoilage combining oxidative deterioration and microbial metabolism occurred under this treatment. Sulfur-containing compounds are generally produced by spoilage microorganisms through the metabolism of sulfur-containing amino acids, and they are characteristic products in the later spoilage stage of meat (Casaburi et al., 2015). In the SA group, the relative peak areas of dimethyl disulfide (8.66%) and dimethyl trisulfide (3.31%) were much higher than those of other samples, which proves that sulfur-producing spoilage bacteria proliferated massively under SA packaging, and the spoilage had developed into an advanced stage characterized by massive accumulation of sulfur-containing malodorous substances (Del Blanco et al., 2017). In the SA-CBDA-11-Ag-4 group, although trimethylamine presented the highest peak area (84.82%), dimethyl disulfide (1.09%) and dimethyl trisulfide (1.49%) were both at the lowest levels, and no hexanal was detected. These results revealed that the SA-CBDA-11-Ag composite film effectively inhibited lipid oxidation and the metabolic activity of spoilage microorganisms such as sulfur-producing bacteria, limiting protein degradation mainly to the early enzymatic hydrolysis stage without further progressing to the middle and late severe spoilage driven by massive microbial proliferation.

In Section 3.5, we investigated the alkaline pH responsiveness of the SA-CBDA-11-Ag film (Fig. 5e), suggesting the film may exhibit a color response to TVB-N released from pork tenderloin during storage. As anticipated, the SA-CBDA-11-Ag film can indicate pork spoilage (Fig. 6a). The film's color transitioned from grey to brown, progressively deepening, consistent with the color change observed in the SA-CBDA-11-Ag film subjected to ammonia fumigation. Figs. 7a-c show the color parameter variations of the SA-CBDA-11-Ag-4 film during pork storage. The film's L* value declined continuously, while the a* and b* values increased gradually throughout the storage period. After 10 days of storage, the TVB-N in pork from the SA-CBDA-11-Ag-4 group exceeded the limit. Correspondingly, the film's L* value decreased to 34.80 ± 2.280 (p < 0.05), while the a* and b* values increased to 15.00 ± 1.225 (p < 0.05) and 28.00 ± 1.871 (p < 0.05), respectively. The ΔE value was calculated using L*, a*, and b* values (Fig. 7d). Generally, human eyes can clearly distinguish color differences when the ΔE value exceeds 5, which serves as an indicator for evaluating the color of freshness indicators (Lee et al., 2024). During pork storage, the ΔE value of the SA-CBDA-11-Ag-4 film increased continuously and exceeded 30 on day 10 (p < 0.05). These results indicated that color variations of the SA-CBDA-11-Ag film could visually reflect pork freshness.

Fig. 7.

Color parameters of the indicator film during 10 days of pork storage: (a) L*, (b) a*, (c) b*. (d) Total color difference (ΔE) of the indicator film during 10 days of pork storage. (e) Linear fitting relationship between ΔE of SA-CBDA-11-Ag-4 intelligent film and TVB-N values of pork tenderloin during storage. (f) Pearson correlation heatmap of L*, a*, b*, ΔE of SA-CBDA-11-Ag-4 film and TVB-N values of pork tenderloin. Numerical values in the lower triangle are correlation coefficients (r); asterisks in upper circles denote *** p < 0.001. Blue and red colors represent negative and positive correlations, respectively, with a coefficient range of −1 to 1. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Pearson correlation matrix analysis revealed strong pairwise correlations between the film's color indices and TVB-N values of pork (Fig. 7f). The L* values showed a significant negative correlation with TVB-N (r = −0.96, p < 0.001). By contrast, the a* and b* values showed significant positive correlations with TVB-N (r = 0.92 and r = 0.84, p < 0.001). Importantly, the ΔE values had a significant positive correlation with TVB-N (r = 0.95, p < 0.001). A linear regression analysis was subsequently performed to quantify the linear relationship between the film's ΔE and TVB-N values in pork (Fig. 7e). The fitted linear equation was TVB-N = 0.37ΔE + 2.42. The R² value reached 0.8989, indicating that approximately 89.89% of the variation in TVB-N values in pork can be explained by the ΔE values of the SA-CBDA-11-Ag film. Combined with the results of the correlation heatmap, this strong positive linear relationship confirmed that the SA-CBDA-11-Ag film could sensitively and quantitatively reflect the degree of pork spoilage. Therefore, this composite film holds great potential as a visual freshness indicator for meat packaging.

4. Conclusions

In this study, a novel active and intelligent packaging film platform was successfully fabricated by incorporating CBDA-11-capped silver nanoparticles (CBDA-11-AgNPs) into an SA matrix. Here, CBDA-11 was used as a reducing agent to prepare CBDA-11-AgNPs via a green synthesis method. The incorporation of CBDA-11-AgNPs enhanced the tensile strength, water vapor barrier, and UV barrier properties of the SA-based film. Moreover, the CBDA-11-AgNPs endowed the film with obvious antioxidant and antibacterial activities. A major highlight of this work is the pH-responsive chromogenic behavior of the SA-CBDA-11-Ag film, which enables its dual role as both an active protective layer and a non-destructive visual sensor for freshness monitoring. In practical applications, this composite film effectively slowed the rate of pork spoilage and enabled visual monitoring of pork freshness. In summary, the SA-CBDA-11-Ag film serves as an environmentally friendly alternative to traditional plastic packaging and provides a highly promising strategy for the integrated design of active and intelligent food packaging systems.

Foundation

The financial support was from the College Students' Innovation and Entrepreneurship Training Program of Jilin University (202510183325).

CRediT authorship contribution statement

Miao Chen: Writing – review & editing, Writing – original draft, Visualization, Validation, Investigation, Formal analysis, Data curation. Panyao Ruan: Writing – original draft, Visualization, Validation, Investigation, Formal analysis, Data curation. Sinuo Wu: Writing – review & editing, Validation, Formal analysis. Shanshan Li: Validation, Investigation, Formal analysis. Changjun Xin: Validation, Formal analysis. Dongmeng Zhang: Validation, Formal analysis. Zhihan Wang: Writing – review & editing, Visualization, Supervision, Resources, Funding acquisition, Conceptualization. Yongsheng Wang: Methodology, Formal analysis, Data curation. Xiao Liu: Writing – review & editing, Visualization, Supervision, Resources, Funding acquisition, Conceptualization.

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.

Appendix A. Supplementary data

Supplementary material

Data availability

No data was used for the research described in the article.