Development of an active agar aerogel with dual antioxidant and UV-blocking activities using chlorine-doped graphene quantum dots

Abstract

Active packaging is of great interest in the modern food industry due to increasing shelf life and enhancing food quality. The importance of this technology increases when natural polymers are used in the construction of active packages. Development of a natural, biodegradable, and dual-active film was aimed in this study. So, agar aerogel containing different amounts of chlorine-doped graphene quantum dots (Cl-GQDs) was prepared. Cl-GQDs had excitation-dependent fluorimetry behavior due to the zigzag edges of graphene. The mean diameter of spherical nanoparticles of Cl-GQDs was about 12 nm, according to HR-TEM images. The results of Raman and ATR-FTIR confirmed that chlorine was well-doped on the GQD structure. Cl-GQDs showed high UV-absorption capability and very strong antioxidant activity (94.31 %), which maintained these activities after incorporation into the agar aerogel. The doped chlorine was responsible for the capacity to charge transfer of GQDs. BET and SEM results showed that adding Cl-GQDs to agar caused a porous structure. Finally, different types of agar aerogels containing Cl-GQDs can be used considering the intended application of aerogel. Agar aerogel containing 20 % Cl-GQDs is suggested if a porous aerogel with good thermal insulation properties is considered. However, agar aerogel containing 1 % Cl-GQDs is suitable as an active film. In conclusion, while Cl-GQDs hold promise as sustainable and multifunctional food packaging materials, their potential toxic effects must be thoroughly evaluated. Future studies should explore migration, potential interactions with specific food matrices, and long-term safety to ensure consumer protection.

1. Introduction

In recent years, global market demands for fresh and healthy foods have grown, leading to an increased popularity of biodegradable materials in food packaging, agriculture, and other sectors [1,2]. As a result, many researchers are dedicating their efforts to modifying traditional packaging materials and designing new polymer composites derived from natural sources. The low-cost production of these biodegradable polymer materials has reduced the reliance on synthetic polymers, yielding positive impacts both environmentally and economically.

Active packages contain active molecules such as antimicrobial, anti-ultraviolet (UV) rays, antioxidant compounds, even colorants, and nutrients that can be released into the food during storage time [3,4]. Anti-UV and antioxidant active packaging are very important to preserve food and to protect and increase the shelf life of perishable foods by protecting them from environmental effects and external pollution [5]. Hydroxyl radicals (•OH), singlet oxygen (¹O₂), and superoxide anion (•O₂⁻), are examples of reactive oxygen species (ROS) that are produced from molecular oxygen, which contains one or more unpaired electrons. These unpaired electrons make ROS highly reactive, playing a significant role in food spoilage, oil decomposition, polymer degradation, and biological damage. Consequently, antioxidants are necessary to maintain a balanced ROS content. Protection against low-temperature oxidation involves either eliminating ROS or preventing their formation, which results from the reduction of molecular oxygen in the presence of ultraviolet radiation or active metal catalysts. ROS can be scavenged by carbon nanomaterials, including functionalized carbon dots, fullerenes, and carbon nanotubes [6]. The mechanism by which carbon nanomaterials protect against oxidation is the formation of radical adducts at sp² carbon sites, which cause spin delocalizing across the structure of conjugated graphene and radical destruction.

The goal of investigations on biopolymers is to discover natural precursors as an alternative to conventional oil-based or synthetic materials for the production of aerogels [7]. Aerogels are specific materials with high porosity, thermal insulation, low density, and very lightweight, which can be transparent and are mainly mesoporous solids [8]. Aerogels are defined as microporous solid gels containing gas as the dispersed phase [9]. Air is the main component of this material (more than 95 % of their volume is air) which is surrounded by a solid, fiber-like network prepared from various precursors [10]. This network can be prepared in such a light way that the density of the resulting aerogels is only three times that of air [11].

As an instance of integration of carbon nanomaterials into biodegradable polymer composites, Chen et al. used 2D graphene oxide (GO) sheets and agar as precursors to synthesize a 3D aerogel. Agar was chosen to enhance the immobilization of GO in the construction of the 3D microstructure because it contains a large number of functional groups containing oxygen. GO and the groups containing abundant oxygen on the surface of the agar both react through the dehydration reaction and form a hydrogel by hydrogen bonding [12].

Graphene is one of the new nanomaterials that have recently entered the scientific and industrial fields. Graphene with its unique properties is a two-dimensional analog of graphite [13,14]. Carbon has four valence electrons in its structure. Three of them participate in the σ bond with their nearest neighbors and create sp² hybrids in a honeycomb structure [15]. The fourth valence electron exists in an orbital upright to the one-dimensional (1D) plane, which creates a non-local π bond [16]. The high movement of this electron inside graphene sheets (owing to its unique geometric structure) gives graphene significant physicochemical properties and biocompatibility. It can be used in disease diagnosis, antibacterial, and antiviral substances, targeting cancer, tissue engineering, and carrier for drug delivery [17,18].

Graphene quantum dots (GQDs) possess a graphene network which their size is less than 100 nm with a thickness of fewer than ten layers [19]. GQDs have unique properties, including distinctive fluorescence characteristics. While two-dimensional graphene is converted into zero-dimensional GQDs, this transformation results in the emergence of quantum properties and edge effects within GQDs. As the dimensions of the graphene crystal transition to a nanometric scale, the distribution of electrons undergoes a significant change due to the crystal boundary [20]. Notably, GQDs exhibit different physicochemical properties compared to other carbon-based materials, such as carbon dots (CDs), carbon nanotubes (CNT), fullerene, graphene, etc., because of their special edge effects and quantum property [19,21].

It can be expected that the antioxidant properties of GQDs will increase by introducing a high level of defects to these materials through doping by different heteroatoms. Chlorine (Cl) can be chosen as a doping element due to its relatively larger radius and higher electronegativity than carbon atoms, resulting in facilitating electron transfer between free radicals and Cl-GQDs [6]. Cl-GQDs offer optimal visible light emission using UV light excitation, good water dispersibility, and excitation-dependent photoluminescence [22]. Chlorine doping significantly improves the UV-blocking properties of GQDs by introducing new energy levels that absorb a broader range of UV wavelengths. Additionally, the electron-withdrawing nature of chlorine enhances their antioxidant activity [6]. Furthermore, Cl-GQDs can be easily synthesized using simple, cost-effective methods and have demonstrated good biocompatibility and low toxicity, rendering them suitable for various applications [23]. The combination of these advantages makes Cl-GQDs a promising choice for developing active agar aerogels with dual UV-blocking and antioxidant properties. Wang et al. produced Cl-GQDs utilizing an electrochemical technique. They stated that Cl-GQDs effectively inhibit free radicals, which can be attributed to the higher defect sites caused by chlorine doping. These Cl-GQDs can reduce ROS and dye degradation. They showed that Cl-GQDs had seven times better radical scavenging than the undoped GQDs [6]. Li et al. synthesized chlorine-doped graphene quantum dots by hydrothermal method. In their method, fructose was used as a carbon precursor, and hydrochloric acid as a chlorine precursor. They observed that Cl-GQDs emit several colors when they are excited by different wavelengths of light due to the addition of energy levels after the addition of chlorine in the GQD structure [24].

To the best of our knowledge, the combination of agar aerogel films with Cl-GQDs has not yet been explored as a biodegradable polymer composite for active packaging of food products. Consequently, this study seeks to synthesize and assess the antioxidant and anti-UV activities of agar aerogels incorporated with chlorine-doped graphene quantum dots, aiming for application in packaging various foods, particularly those with oil content. To achieve the goals, Cl-GQDs were synthesized using the hydrothermal method and subsequently integrated into agar aerogels at specific concentrations. Following synthesis, the produced films were thoroughly evaluated and analyzed in terms of their structure, antioxidant, anti-UV capabilities, and migration properties (Fig. 1).

Fig. 1.

Schematic of the development of active agar/Cl-GQDs aerogel.

2. Materials and methods

DPPH (2,2-diphenyl-1-picrylhydrazyl) radical was prepared from Sigma-Aldrich (St. Louis, MO, USA). Fructose (for biochemistry grade), hydrochloric acid fuming 37 % (HCl), and agar powder (food grade) were obtained from Merck (Darmstadt, Germany).

2.1. Cl-GQDs synthesis

In order to synthesize Cl-GQDs, hydrothermal method was used. one gram fructose and 25 μl HCl were completely dissolved in 15 ml distilled water. In the next step, the solution was autoclaved at 170 °C for 4 h. Then the solution was centrifuged at 10,000 rpm for 20 min at ambient temperature to obtain clear and yellow Cl-GQDs solution [24].

2.2. Characterization of Cl-GQDs

2.2.1. Optical properties

The absorption spectrum of prepared Cl-GQDs was measured by a UV/Vis spectrophotometer (Hach Lange, USA) in the range of 200–700 nm. To explore the emission spectrum of Cl-GQDs, a spectrofluorometer (Shimadzu RFe5000, Japan) with an excitation wavelength between 450 and 600 nm was utilized.

2.2.2. Morphology

The morphology of Cl-GQDs was investigated using a high-resolution transmission electron microscope (HR-TEM). The voltage of the device was 200 kV. The particle size of the Cl-GQDs was measured using ImageJ analysis software, and the particle size distribution diagram was drawn. The crystal plates of the sample were evaluated using fast Fourier transform (FFT) images.

2.2.3. Crystalline structure

The crystalline structure of Cl-GQDs was evaluated by an X-ray diffractometer (Asenware AW-XDM300, China). Further details about the instrument and measurement parameters can be found in Ref. [25]). Berg's law and equation (1) were used to determine the distance of crystal plates in the structure.

$$\mathrm{\lambda }=2\mathrm{d}\sin (\mathrm{\theta })$$ (1)

where λ is the wavelength of X-ray (0.154 nm), d is the gap between adjacent plates or layers, and θ is the diffraction angle.

2.2.4. Spectroscopic characterization and elemental analysis

Fourier transform infrared spectroscopy (FTIR) (Brucker Tensor 27, Germany) was exploited to approve the formation of Cl-GQDs. Further details about the instrument and measurement parameters can be found in Ref. [25]. Raman test was performed at a laser wavelength of 785 nm to avoid fluorescence interference. Energy dispersive spectroscopy (EDS) (TESCAN, Czechia) analysis was also used to evaluate the doping of chlorine in the structure of graphene sheets.

2.2.5. Antioxidant activity

The antioxidant activity of Cl-GQDs was analyzed by DPPH radical scavenging method [26]. 100 μl of Cl-GQDs solution was mixed with 1.5 ml DPPH ethanolic solution (0.1 mM). Ethanol was used to prepare the control sample. Solutions were kept for 30 min at room temperature in a completely dark space. The absorption of solutions was measured at a wavelength of 515 nm. Finally, the inhibition efficiency was calculated according to the decrease in absorption intensity using equation (2).

$$\text{DPPH}\text{scavenging}\text{activity}(\%)=\frac{{\text{Abs}}_{\text{control}}-{\text{Abs}}_{\text{sample}}}{{\text{Abs}}_{\text{control}}}\times 100$$ (2)

2.3. Preparation of agar aerogel containing Cl-GQDs

Agar solution (1 % wt%) was first prepared to prepare agar aerogel (control aerogel). After reaching the temperature of the solution to 55 °C, the prepared solution was homogenized for 15 min at 10,000 rpm. Then, the solution was transferred to the plate and allowed to set the agar gel. After, it was dried in a freeze-dryer for 24 h. Aerogels containing Cl-GQDs were prepared by adding certain volumes of Cl-GQDs solution to the agar solution to make aerogels with four different concentrations of Cl-GQDs (1, 5, 10, and 20 % v/v). Other steps of preparing aerogel containing Cl-GQDs were similar to the control aerogel.

2.4. Characterization of agar aerogel containing Cl-GQDs

2.4.1. Color

The color of the surface of aerogels was determined using a colorimeter (ZE6000, Japan) and was characterized by three L*, a*, and b* indicators [27].

2.4.2. Scanning electron microscopy (SEM)

SEM (Philips, XI30, the Netherlands) was used to examine the morphology of the prepared aerogels. The aerogel samples containing different percentages of Cl-GQDs were broken in liquid nitrogen to obtain cross-section images. Before imaging, samples were covered with an ultrathin layer of gold (thickness of approximately 5–6 nm) for 500 s to improve the conductivity during imaging.

2.4.3. BET

Brunauer-Emmett-Teller (BET) test (Belsorp Mini II, Japan) was used to measure the specific area, diameter, and volume of holes in aerogels containing different percentages of Cl-GQDs. The samples were placed at a temperature of 120 °C for 2 h.

2.4.4. Migration

The migration of Cl-GQDs nanoparticles from the aerogel to the food simulant was investigated. For oil-in-water emulsions and foods having free fat on the surface, food simulant D2, which includes vegetable oil, should be used. The test was done with food simulant D2 (olive oil) [28]. Aerogel samples were prepared as sheets with dimensions of 1 × 1 cm². The pieces of the aerogels were immersed in 9.2 g olive oil and placed at 40 °C (standard static test) for 10 days [27]. The graphene nanoparticles transferred from the aerogels to the food simulant were investigated by evaluating the fluorometric spectrum of the olive oil after 10 days of exposure to the aerogels.

2.4.5. Antioxidant activity

Extracts of aerogels were first prepared. Aerogel samples were cut into small segments and submerged in distilled water, followed by shaking for 24 h at ambient temperature. Then the final solution was centrifuged at 4000 rpm for 10 min, and the supernatant was used to evaluate the antioxidant activity of aerogels. One ml of prepared aerogel extract solutions was mixed with 1 ml DPPH ethanolic solution (0.1 mM). Distilled water was used to prepare the control solution. The absorbance of both samples was read at 515 nm after keeping the solutions in the dark space for 30 min at room temperature. The antioxidant activity of aerogels was calculated using equation (2).

2.4.6. UV-blocking

Visible-ultraviolet spectrometer (Hach Lange, USA) was used to check the absorption of UV rays by the aerogels in UVA, UVB, and UVC ranges. The percentage of UV-blocking of the films in the three ranges of UVA, UVB, and UVC was calculated using equations (3), (4), (5).

$$\mathrm{T}(\text{UVA})=\frac{\underset{330}{\overset{400}{\int }}T\lambda \times d\lambda }{\underset{330}{\overset{400}{\int }}d\lambda \times t}$$ (3)

$$\mathrm{T}(\text{UVB})=\frac{\underset{280}{\overset{330}{\int }}T\lambda \times d\lambda }{\underset{280}{\overset{330}{\int }}d\lambda \times t}$$ (4)

$$\mathrm{T}(\text{UVC})=\frac{\underset{200}{\overset{280}{\int }}\mathrm{T}\lambda \times \mathrm{d}\lambda }{\underset{200}{\overset{280}{\int }}\mathrm{d}\lambda \times \mathrm{t}}$$ (5)

where t is the thickness of the aerogel, and Tλ is the transmission rate of the aerogel at the λ wavelength.

2.4.7. Chemical and crystalline structure analysis

FTIR-ATR and XRD tests of aerogels were performed similarly to the Cl-GQDs solution.

2.5. Statistical analysis

For the statistical analysis of the samples, a completely randomized design format was used. The mean data comparison test was done using the least significant difference (LSD) at the 95 % confidence level using SAS software.

3. Results and discussion

3.1. Characterization of Cl-GQDs

As a result of the hydrothermal process, fructose molecules lose their water to form carbon-carbon bonds, which are the basic unit of the graphene structure. Throughout the formation of single-layer graphene quantum dots, hydrogen atoms of a fructose molecule interact with the hydroxyl groups of a neighbor fructose molecule, which leads to the formation of water molecules. As a result, carbon atoms covalently interact with each other. The chlorine in HCl also releases a water molecule and is placed on the edges of the single-layer graphene structure, and finally, a chlorine-doped graphene quantum dot is formed.

HR-TEM was used to investigate the size and morphology of Cl-GQDs. The size distribution diagram of Cl-GQD particles is shown in Fig. 2a. The average diameter of nanoparticles is 12.42 ± 1.1 nm. According to Fig. 2b, Cl-GQD particles are spherical and single [29]. The crystalline structure of Cl-GQDs was confirmed by HR-TEM images shown in Fig. 2c and d.

Fig. 2.

(a) Particle size distribution diagram of Cl-GQDs, (b) HR-TEM image of Cl-GQDs, (c) FFT image of the selected region (red square), (d) HR-TEM image of Cl-GQDs, and (e) line profile analysis of the diffraction fringe.

FFT (Fig. 2c) shows a selected region (red square section) of a single Cl-GQDs. The arrangement of the bright points in relation to the central point confirms the crystalline structure of the graphene quantum dot. According to Fig. 2d, the size of the lattice parameter in the resulting Cl-GQDs structure is 0.24 nm which is equal to the lattice parameter of the graphite and it is clear that chlorine doping does not affect the lattice parameter of the GQDs [23,29]. Based on the line profile analysis of the diffraction pattern or inverse FFT (Fig. 2e), the distance between 10 atomic layers is 2.411 nm, demonstrating an average distance of 0.24 nm between each layer. This value (0.24 nm) is in good agreement with the reported d-spacing by other researchers [6,30].

The XRD pattern obtained from Cl-GQDs is shown in Fig. 3a. There are diffraction peaks around 2θ = 22° (related to the crystalline plane (002)) and at 2θ = 38° (related to the crystalline plane (100)) [31]. The distance between the synthesized Cl-GQDs plates was calculated to be about 0.402 nm, which is more than the gap between the plates in graphite (0.331 nm) [32]. It is because of the forming of functional groups on the surface of graphene. The distance between the plates obtained in this research is consistent with other studies that have synthesized GQD by different methods (0.481–0.340 nm). Different distances in graphite and graphene quantum dots are influenced by C–H, O–H, and C-OR groups at the edges of GQDs [10].

Fig. 3.

XRD pattern (a), FTIR-ATR spectrum (b), Raman spectrum (c), EDS spectrum of the Cl-GQDs (d) and absorption spectrum of Cl-GQDs (e).

The ATR-FTIR was performed to recognize the functional groups on the surface of Cl-GQDs. The obtained spectrum is shown in Fig. 3b. The wide band at 3350 cm⁻¹ is related to O–H stretching vibrations. This shows that there are high amounts of hydroxyl groups on the surface of Cl-GQDs, which makes Cl-GQDs have a hydrophilic property [33]. The 1639 cm⁻¹ band is related to the C O stretching vibration. The 1074 cm⁻¹ band is also associated with C–O stretching vibration. The 648 cm⁻¹ band corresponds to the C–Cl stretching vibration showing that chlorine is well-doped on the graphene quantum dot structure.

The Raman spectrum obtained from the Cl-GQDs is displayed in Fig. 3c. The characteristic bands of graphene, which are D, 2D, and G, are located at 1307 cm⁻¹, 2567 cm⁻¹, and 1587 cm⁻¹, respectively [34,35]. The 809 cm⁻¹ band is related to C–Cl, which confirms the presence of chlorine in the resulting graphene quantum dot structure. The 1026 cm⁻¹ band relates to C–OH, and the 1307 cm⁻¹ band correlates with the D band. D band is related to defects in the structure and substituted heteroatoms [35]. The 1587 cm⁻¹ band belongs to the G band. The G band indicates the sp² structure in the compound [36]. The 1710 cm⁻¹ band relates to two C O and C C bonds. The 1816 cm⁻¹ band is associated with acid halides, which are created due to the presence of hydrochloric acid in the structure. The 2411 cm⁻¹ band ascribes to C C O. The band 2567 cm⁻¹ relates to the 2D band. It should be noted that graphene and graphite have the same 2D band [37]. The 2823 cm⁻¹ band relates to C–CH₃, the 2932 cm⁻¹ band relates to C–H, and the 3055 cm⁻¹ band associates with = CH₂ [38].

EDS was employed to determine the elements of the produced Cl-GQDs. As depicted in Fig. 3d, the results indicated the presence of C (44.18 %), O (53.71 %), and Cl (2.12 %), confirming the Cl doping in the GQDs structure [24].

Fig. 3e shows the absorption spectrum of the Cl-GQDs. Three peaks at wavelengths 247, 300, and 381 nm can be seen in the absorption spectrum of the Cl-GQDs solution. The 247 nm peak is related to the π–π* transition and is due to the presence of the aromatic ring in the structure of Cl-GQDs [39]. The peaks at 300 nm and 381 nm are related to the n-π* transition due to C C and C O bonds, respectively. The resulting Cl-GQDs solution had a high absorption capability in the wavelength range of 200 to about 380 nm, which corresponds to the UV spectrum. Absorption decreases rapidly from about 380 to 700 nm (visible spectrum of light) due to the transparency of the Cl-GQDs solution in the visible light. The intense absorption in the UV range shows the high performance of the fabricated Cl-GQDs in absorbing UV radiation [40].

Photoluminescence (PL) is affected by the size of dots, excitation wavelength, functionalization or oxidation, pH during synthesis, degree of surface doping with heteroatoms, and solvent polarity [41]. The fluorescence spectra of the synthesized Cl-GQDs in the excitation wavelengths (λEx) from 450 to 600 nm are displayed in Fig. 4. The emission peak position of Cl-GQDs shifts to the right, and the intensity of the emission wavelengths (λEm) increases with the excitation wavelength increasing. The highest emission intensity (λ_Em max) was at 550 nm. So, it can be said that Cl-GQDs have excitation-dependent fluorimetry behavior. This characteristic of GQDs may be attributed to the zigzag edges of graphene, which has a carbene-like structure, in which the triplet ground state (σ₁Π₁) is the most common, and emission occurs from the carbene-like triplet ground state [29].

Fig. 4.

Emission spectrum of Cl-GQDs under different excitation wavelengths.

The Cl-GQDs solution had a very strong antioxidant activity, so the color of the solution quickly changed from purple to yellow. The antioxidant activity of the Cl-GQDs was 94.31 %, which decreased by solution dilution, so the dilution of 10⁻⁴ had no antioxidant activity. As mentioned earlier, the carbon atoms in the graphene quantum dots sheet have covalent bonds with three electrons and form a strong network. The position of the fourth valence electron is not fixed on each carbon atom and moves and can be at the edges or defects in the structure of GQDs.

The fraction of carbons with sp² crystalline structure within Cl-GQDs is considerably high, resulting in the inhomogeneous spreading of electron density in the hexagonal carbon ring. This circumstance leads to charge transfer from GQDs to free radicals and increases the antioxidation feature. Charge transfer of GQDs can be further improved with doped chlorine. There is a noticeable difference between the electronegativity of doped Cl (3.16) [42] and the carbon atom (2.55) in the structure [43]. Consequently, covalently doped Cl atoms become electron donors and make net negative charges next to carbon atoms within the structure. As a result, the chemical activity of Cl-GQDs is increased, enhancing the electron exchange between Cl-GQDs and free radicals. This condition causes a higher rate of free radical passivation or higher antioxidation property of Cl-GQDs compared to pure GQDs [6].

3.2. Characteristics of agar/Cl-GQDs aerogel

The images of the final aerogels are shown in Fig. 5. The color parameters of the prepared aerogels are presented in Table 1. The brightness of aerogels decreases by increasing the amount of Cl-GQDs in the aerogels. However, a* and b* parameters of the aerogels increase by Cl-GQDs increasing. These results show that the redness and yellowness of aerogels increase, and the transparency of aerogels decreases by adding Cl-GQDs solution to agar aerogels, which is due to the color of the Cl-GQDs solution.

Fig. 5.

Agar aerogels containing different amounts of Cl-GQDs.

Table 1.

Color parameters of aerogels containing different amounts of Cl-GQDs.

Color parameters	Agar aerogel	Agar/1 % Cl-GQDs aerogel	Agar/5 % Cl-GQDs aerogel	Agar/10 % Cl-GQDs aerogel	Agar/20 % Cl-GQDs aerogel
L*	83.843 ± 0.219a	83.273 ± 0.092a	74.443 ± 0.315b	70.420 ± 1.864c	68.606 ± 0.691c
a*	−0.376 ± 0.125e	−0.966 ± 0.208d	0.800 ± 0.065c	2.466 ± 0.289b	5.603 ± 0.265a
b*	3.303 ± 0.065e	8.040 ± 0.632d	9.070 ± 0.710c	14.33 ± 0.081b	21.193 ± 0.470a

Different small letters indicate significant differences between samples (p < 0.05).

Fig. 6a shows the FTIR spectra of agar aerogel as the control sample and the aerogel containing 20 % Cl-GQDs. In the agar aerogel spectrum, the band at about 3389 cm⁻¹ is related to O–H stretching. The 2922 cm⁻¹ band is attributed to O–CH₃ groups. The band at about 1641 cm⁻¹ is due to the vibrational stretching of the conjugated peptide bond created by the amine (N–H) and acetone (C O) groups, which is related to the protein impurities in the agar. The band at 1369 cm⁻¹ is assigned to sulfate ester (SO₄²⁻), whose existence may be due to the variety of agar species and extraction method. The bands of 1038 cm⁻¹ and 929 cm⁻¹ are related to 3,6-anhydrogalactose (C₆H₁₀O₅) bridges. In the aerogel containing 20 % Cl-GQDs, all bands related to agar are visible without change. Also, the stretching vibration of C–Cl and the stretching vibration of C–O of Cl-GQDs can be seen at 644 cm⁻¹ and 1032 cm⁻¹, respectively, in agar/20 % Cl-GQDs aerogel, which confirms the presence of Cl-GQD in the aerogel. The results show that the Cl-GQDs are well incorporated in the agar aerogel structure, and the lack of disappearance or appearance of new bands indicates that the Cl-GQDs are physically incorporated into the agar aerogel structure, and there is no chemical interaction between Cl-GQDs and agar [33].

Fig. 6.

FTIR spectra of aerogels (a), XRD pattern of aerogels (b), and deconvoluted spectrum of Agar/20 % Cl-GQDs aerogel (c).

The XRD pattern of agar/20 % Cl-GQDs aerogel, agar aerogel, and Cl-GQDs solution are shown in Fig. 6b and c. Two peaks can be observed in the XRD pattern of the agar/20 % Cl-GQDs aerogel. The first peak is at 2θ = 19°, which is related to agar [44], and the second peak is at 2θ = 25.2°, which is associated with Cl-GQDs. The results confirm the presence of Cl-GQDs in the structure of the produced agar aerogel.

The surface and cross-section SEM images of agar aerogels comprising various Cl-GQDs contents are shown in Fig. 7. The surface of the agar aerogel is almost uniform, and very few pores are on it. The cross-section image of this aerogel shows its layered and porous structure caused by freeze-drying. The surface and cross-section images of the agar containing 1 % Cl-GQDs show partial structure destruction, which could be due to the placement of Cl-GQDs in the agar network, which reduced the cohesion of the agar network. The structure destruction became more intense as the amount of Cl-GQDs increased, so the layers were destroyed, and large holes were created in the agar aerogel containing 20 % Cl-GQDs. The severity of the structure destruction in agar/20 % Cl-GQDs aerogel is so high that even the surface has become completely porous.

Fig. 7.

SEM images of agar aerogels containing 0 (a), 1 (b), 5 (c), 10 (d), and 20 % (e) Cl-GQDs. Left images are from the surface and right images are from the cross section of the films.

The amount of specific surface area (A), the average diameter of pore size (d_p), and total pore volume (V_p) of the agar aerogels containing different amounts of Cl-GQDs were analyzed by BET, and the results are reported in Table 2.

Table 2.

BET analysis of aerogels containing different amounts of Cl-GQDs.

Sample	A (m2/g)	Vp (cm3/g)	dp (nm)
Agar aerogel	4.17 ± 0.30d	0.014 ± 0.001c	13.87 ± 0.53b
Agar/1 % Cl-GQDs aerogel	1.21 ± 0.11e	0.003 ± 0.001e	12.62 ± 0.50bc
Agar/5 % Cl-GQDs aerogel	5.99 ± 0.23c	0.009 ± 0.001d	6.48 ± 0.51d
Agar/10 % Cl-GQDs aerogel	10.60 ± 0.30b	0.029 ± 0.001b	11.29 ± 0.26c
Agar/20 % Cl-GQDs aerogel	20.50 ± 0.41a	0.089 ± 0.003a	18.77 ± 0.44a

Different small letters indicate significant differences between samples (p < 0.05).

Specific surface area of agar aerogel was 4.172 (m²/g), which decreased after adding 1 % Cl-GQDs, while the specific surface area increased in samples containing 5, 10, and 20 % Cl-GQDs. The total pore volume of agar aerogel was 0.014 (cm³ g⁻¹), which decreased in the sample containing 1 % Cl-GQDs but increased in the 5, 10, and 20 % Cl-GQDs samples. The average hole diameter of the agar aerogel was 13.866 (nm), which decreased in the sample of 1 and 5 % Cl-GQDs but increased with increasing the amount of Cl-GQDs to 10 and 20 %. According to the SEM images, the BET results can be well interpreted. The control sample is in the form of continuous layers placed on top of each other, which creates a regular and porous layered structure. However, adding 1 % Cl-GQDs to agar aerogel destroyed the regular layered structure of the agar aerogel, and as a result, the porosity of the structure decreased. Therefore, the structure changed from a layered state to a cavity state, and the average diameter of the holes decreased. So, the total volume of pores and specific surfaces were lower than the control sample. However, adding more Cl-GQDs to agar caused a higher porous structure. So, the specific surface area, the total pore volume, and the average diameter of the produced aerogels increased. Finally, different types of agar aerogels containing Cl-GQDs can be used considering the intended application of aerogel. Agar aerogel containing 20 % Cl-GQDs is suggested if a porous aerogel with good thermal insulation properties is considered. However, agar aerogel containing 1 % Cl-GQDs is suitable as a UV-blocking film. Chen et al. (2017) prepared 3D agar/graphene oxide (AGO) composite aerogel by freeze-drying method. According to the BET results, they found that pure agar aerogel had a low specific surface area, and it increased by GO content increasing.

The migration test was used to evaluate the transfer of Cl-GQDs nanoparticles from agar aerogels containing Cl-GQDs to oily media. The fluorescence intensities of olive oils after 10 days of immersion of the aerogel samples at 40 °C (standard static test) are presented in Fig. 8. The fluorescence intensity of olive oil increased by the Cl-GQDs increase in the aerogels due to the highly porous structure of aerogels containing higher amounts of Cl-GQDs (according to the results of BET and SEM) [10]. Fig. 8 shows that the fluorescence strength of the Cl-GQDs solution was much higher than that of all the aerogels, as the aerogels containing 1 and 5 % Cl-GQDs had only 6 and 8 % of the fluorescence intensity of Cl-GQDs solution, respectively.

Fig. 8.

Fluorescence intensity of Cl-GQDs solution and different aerogels immersed in oil after 10 days.

The results of the antioxidant activity of the aerogels comprising various Cl-GQDs contents are presented in Fig. 9. The antioxidant activity of the agar aerogel is 12.43 %. There are some sulfate groups in the structure of agarose. Therefore, it seems that the antioxidant activity of pure agar aerogel is due to the sulfate groups in its structure. The antioxidant activity of the aerogels containing Cl-GQDs increases by Cl-GQDs increasing, so the aerogel containing 20 % Cl-GQDs has the highest antioxidant activity (30.94 %). As aforementioned, the sp² structure of carbon atoms results in the inhomogeneous spreading of electron density in the hexagonal carbon ring, which leads to charge transfer from GQDs to free radicals to passivate them. The charge transfer capability of Cl-GQDs enhances significantly, which leads to their remarkable antioxidant activity (94 %).

Fig. 9.

Antioxidant activity of aerogels comprising various Cl-GQDs contents.

The results of the UV-blocking analysis of aerogels comprising various Cl-GQDs contents are shown in Table 3. Agar aerogel has the lowest UV light absorption in the UVA, UVB, and UVC regions, and the absorption of the UVA region is the lowest. The UV-blocking of agar aerogels increased by Cl-GQDs addition to the aerogels. So, the amounts of UV absorption by aerogel containing 20 % Cl-GQDs are 99.18, 99.51, and 99.64 % for UVA, UVB, and UVC, respectively. The high absorption of UV rays in all three regions in the aerogels containing Cl-GQDs is due to the high ability of Cl-GQDs to absorb UV rays. Also, some researchers have shown that color is one of the most effective variables in protecting against ultraviolet rays. Materials with higher yellowness and redness absorb more UV rays because UV has a very short wavelength. Therefore, the more the red and yellow color of the sample, the higher the absorption of UV rays [45]. According to the results of the colorimetric analysis of the aerogels, the aerogel containing Cl-GQDs has high redness and yellowness, which is another reason for the high UV absorption of these samples. Since UVC rays and most UVB rays are absorbed by the ozone layer, and UVA rays reach the earth's surface, therefore, it is important to find the materials that can absorb UVA. The results of this analysis indicate that agar aerogels containing Cl-GQDs have high absorption for the UVA range. In such a way that aerogel containing 20 % Cl-GQDs has an absorption of up to 100 %.

Table 3.

UV absorption of aerogels containing different amounts of Cl-GQD.

sample	Type of UV ray	Absorption percentage
Agar aerogel	UVA	86.68 ± 2.01
	UVB	91.54 ± 3.02
	UVC	95.46 ± 1.63
Agar/1 % Cl-GQDs aerogel	UVA	91.51 ± 3.1
	UVB	96.78 ± 1.2
	UVC	97.28 ± 1.01
Agar/5 % Cl-GQDs aerogel	UVA	95.52 ± 1.09
	UVB	98.01 ± 0.65
	UVC	98.30 ± 0.58
Agar/10 % Cl-GQDs aerogel	UVA	96.67 ± 1.7
	UVB	98.70 ± 0.78
	UVC	99.02 ± 0.52
Agar/20 % Cl-GQDs aerogel	UVA	99.18 ± 2.3
	UVB	99.51 ± 0.62
	UVC	99.64 ± 0.45

4. Conclusion

In recent decades, growing environmental and health concerns have amplified the demand for biodegradable and active food packaging, more than ever before. To address this, our study aimed to the development of a novel biodegradable and active food packaging material, utilizing agar aerogel films embedded various amounts of Cl-GQDs. It is postulated that these biodegradable films, fortified with Cl-GQDs, will exhibit superior UV-blocking and antioxidant properties for food products compared to their counterparts, the unenhanced agar aerogels. In our methodology, Cl-GQDs were synthesized via the hydrothermal method and subsequently integrated into agar aerogels at different concentrations. UV visibility and antioxidant activity analyses showed pronounced UV-blocking and antioxidant capacities of agar aerogels containing Cl-GQDs. These inherent properties of the film are attributed to the UV absorption and robust free radical scavenging properties of the Cl-GQDs. The developed agar aerogel containing Cl-GQDs has several potential applications in active packaging. It can serve as a moisture absorber, maintaining food freshness by reducing humidity inside the package. Its UV-blocking properties protect sensitive food products from harmful radiation, thereby extending shelf life. The strong antioxidant activity of Cl-GQDs helps preserve food quality. For specific applications, agar aerogel containing 20 % Cl-GQDs can be used for thermal insulation, while 1 % Cl-GQDs are suitable for active film purposes. However, it should be noted that research on GQDS is still ongoing, and further studies are needed to assess their safety for human health and the environment. Additionally, due to the inherent gas and liquid permeability of aerogels, further research is warranted to optimize their structure and composition, striking a balance between functionality and practicality for sustainable food packaging applications.

Data availability statement

Data will be made available on request.

CRediT authorship contribution statement

Kimia Taji: Software, Methodology, Formal analysis, Data curation. Hajar Shekarchizadeh: Writing – review & editing, Validation, Supervision, Investigation, 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.