Biomass-Derived Red–Blue Solvatochromic Carbon Quantum Dots for UV-Blocking and Active Packaging Applications

Abstract

In this research, biomass-derived carbon quantum dots (CQDs) with solvent-dependent emission colors were synthesized from mangosteen leaves via a solvent-mediated hydrothermal route, in order to elucidate the relationship between structure and emission and to evaluate their potential for multifunctional packaging applications. Systematic morphological and structural analyses demonstrated that the solvent polarity governed the degree of carbonization, particle size, and surface chemistry. Red-emissive CQDs (R-CQDs) were formed in a less polar medium and exhibited larger sp² domains enriched with hydroxyl groups, whereas blue-emissive CQDs (B-CQDs) synthesized in a polar solvent had smaller sp² domains with higher values for the content of oxygen- and nitrogen-containing functionalities. These distinctions resulted in excitation-independent red emission for R-CQDs and excitation-tunable blue-green emission for B-CQDs. R-CQDs showed enhanced antioxidant activity (IC₅₀ = 8.84 μg mL^–1) relative to B-CQDs (IC₅₀ = 21.03 μg mL^–1), while antibacterial activity against Escherichia coli and Staphylococcus aureus was observed exclusively for R-CQDs, indicating a structure-dependent antibacterial response. When the CQDs were incorporated into poly­(vinyl alcohol) (PVA) matrices, the nanocomposite films demonstrated enhanced UV-blocking performance, with R-CQD/PVA film giving stronger attenuation due to enhanced π–π* transitions and more efficient interfacial energy transfer. Storage tests also showed that when tomatoes were packaged with R-CQD/PVA film, ripening was delayed by approximately 7–14 days compared to the control films. These findings establish clear correlations between solvent, structure, and function for biomass-derived CQDs and suggest a sustainable strategy for designing multifunctional nanocomposite films for UV-protective and active food-packaging applications.

1. Introduction

Carbon quantum dots (CQDs) are a class of carbon-based nanomaterials that exhibit tunable photoluminescence over a spectral range from blue to red. Their emission behavior is generally considered to arise from a combination of quantum confinement effects within sp²-hybridized carbon domains, surface functional groups, defect-related states, and molecular fluorophores formed during carbonization. Smaller sp² domains are typically associated with higher-energy blue emission, whereas enlarged π-conjugated domains, oxygen-rich surface states, and defect-induced energy levels stabilize the lower-energy emissive centers, resulting in green-to-red emission. Consequently, variations in the sp² domain size, surface oxidation, and compositional heterogeneity often lead to broad, excitation-dependent, or multicolor emission profiles for CQD systems. −

In recent years, CQDs derived from natural biomass have attracted increasing attention due to their low cost, environmental compatibility, favorable biocompatibility, and availability of a diverse range of methods for green synthesis. Biomass precursors of many types, including fruits, leaves, vegetables, and agricultural waste, have been successfully converted into CQDs using hydrothermal, solvothermal, pyrolytic, and microwave-assisted methods. In most cases reported in the literature, biomass-derived CQDs exhibit predominantly blue-to-green photoluminescence, an effect that is commonly attributed to their relatively small sp²-carbon domains, abundant oxygen-containing surface functionalities, and defect-related emissive states. −

However, the problem of achieving long-wavelength emission, and particularly red fluorescence, from biomass-derived CQDs remains challenging, and success has been reported for only a limited number of systems based on specific natural precursors. − Available studies suggest that red emission is correlated with enlarged π-conjugated sp² domains, heterogeneous surface states, and increased aromaticity induced during carbonization and the incorporation of heteroatoms. A systematic understanding of the structure–emission relationships governing the transition from short- to long-wavelength emission in biomass-derived CQDs is lacking, giving rise to a need for well-controlled synthesis strategies and in-depth mechanistic investigations.

In addition to precursor selection, recent studies have demonstrated that biomass waste can serve as a versatile source of carbon for the preparation of CQDs with tunable emission characteristics. For example, solvothermal synthesis using spinach biomass has yielded CQDs that can be made to display blue, crimson, grayish white, and red emissions under a single excitation wavelength by varying the solvent environment. In that system, the boiling point and polarity of the solvent were found to influence the particle size, surface functional groups, and porphyrin-related luminescence centers. ,

Similarly, CQDs synthesized from lignin using mixed solvent systems, such as γ-valerolactone and water, have demonstrated strong luminescence, indicating that even lignocellulosic biomass precursors, which are traditionally regarded as challenging in terms of their use as carbon sources, can yield optically active CQDs when the solvent conditions are appropriately tuned. Additional examples based on watermelon peel and grape pomace provide further evidence for the generality of solvent- and precursor-dependent emission behavior in biomass-derived CQDs.

Taken together, these studies suggest that careful selection of the biomass precursors and control over the solvent environment represent an effective strategy for achieving color tunability and enhanced optical performance without the need to rely on synthetic organic carbon sources.

Of the various parameters affecting the synthesis process, the solvent environment is widely recognized as an important aspect governing structural evolution during carbonization. Variations in solvent polarity can alter dehydration and polymerization pathways, the degree of oxidation, nucleation kinetics, and the stabilization of intermediate components, which in turn influence the distribution of surface states and emissive centers in CQDs, even when synthesized from identical precursors. ,− In addition, the solvent-mediated stabilization of excited states can induce solvatochromic photoluminescence behavior, enabling multiwavelength emission under a single excitation condition. Emission tuning in biomass-derived CQDs can also be achieved through the incorporation of heteroatoms, temperature-controlled carbonization, and surface passivation strategies for the selective modification of emissive sites. These observations collectively highlight the multiplicity of chemical pathways through which the biomass composition and reaction conditions regulate the CQD photophysics. ,,

Despite these advances, however, the issue of systematic solvent-dependent variation using a single biomass precursor has received little attention, and few studies have directly correlated solvent-controlled structural evolution with the emergence of distinct emissive behaviors in CQDs. In this work, solvent-mediated modulation during hydrothermal synthesis is systematically examined using mangosteen (Garcinia mangostana L.) leaves as a precursor with constant reaction parameters. − , Mangosteen leaves are abundant agricultural byproducts that are rich in polyphenolic constituents, representing carbon-rich and oxygen-containing building blocks that facilitate the formation of functionalized carbon nanostructures. By isolating the influence of the solvent environment, this approach allows for the formation of red- and blue-emissive CQDs (R-CQDs and B-CQDs) and offers insight into how solvent-dependent structural variations govern the evolution of emissive centers in biomass-derived nanomaterial systems.

The incorporation of CQDs into polymer matrices such as poly­(vinyl alcohol) (PVA) is attracting interest for use in active and intelligent packaging applications, owing to their potential for ultraviolet (UV) shielding, antioxidant functionality, mechanical reinforcement, and antibacterial efficacy. − In this work, CQDs synthesized under different solvent conditions are systematically compared and incorporated into PVA films to assess their UV-shielding performance, mechanical properties, antioxidant activity, and antibacterial efficacy. The results demonstrate that solvent-mediated structural variation in biomass-derived CQDs governs not only their emissive behavior but also their functional performance in polymer composite films, suggesting that these are promising materials for advanced food-packaging-related applications.

2. Experimental Section

2.1. Synthesis of Carbon Quantum Dots

Mangosteen leaves were collected from a local orchard in Thailand and served as the carbon precursor without further chemical treatment. The leaves were thoroughly washed with deionized (DI) water (17.0 MΩ·cm) to remove surface impurities and then air-dried at room temperature before being chopped into small pieces with a blender.

To synthesize R-CQDs, 10 g of the prepared mangosteen leaves were dispersed in 100 mL of absolute ethanol (AR grade, Sigma-Aldrich, Germany). For B-CQDs, DI water was used as the solvent under otherwise identical conditions. Each suspension was magnetically stirred for 3 h and then filtered through 11 μm filter paper to remove insoluble residues.

The filtrates obtained from this process were transferred to Teflon-lined stainless-steel autoclaves and subjected to hydrothermal treatment at 200 °C for 3 h. After being left to cool naturally to room temperature, brownish (R-CQDs) and yellowish (B-CQDs) dispersions were obtained. These CQD solutions were further purified by sequential filtration through 0.45 and 0.22 μm syringe microfilters to remove residual particulates and large aggregates and then stored in the dark at 4 °C for later characterization.

2.2. Preparation of CQD/PVA Nanocomposite Films

PVA (DAEJUNG Co., Ltd., Korea) was used as the polymer matrix for the fabrication of CQD/PVA nanocomposite films. A brief description of the process is as follows: 1.0 g of PVA powder was dissolved in 15 mL of DI water under continuous stirring at 70 °C for 15 min until a clear and homogeneous solution was obtained, and the solution was then allowed to cool to approximately 30 °C. Following this, 15 mL of the purified CQD aqueous dispersion was added dropwise to the PVA solution under continuous stirring. The mixture was stirred for a further 2 h to ensure uniform dispersion of CQDs within the polymer matrix, resulting in a slightly colored viscous solution. The CQD/PVA nanocomposite films were prepared by casting the mixture into clean Petri dishes, followed by drying at room temperature. The dried films were carefully peeled from the substrates and stored in a desiccator prior to characterization, mechanical testing, UV-blocking evaluation, and food-packaging performance studies.

2.3. Morphological and Structural Characterization

The morphology and microstructure of the samples were examined by using transmission electron microscopy (TEM, JEM-2100, JEOL, Japan). The surface morphology and elemental composition were analyzed by field-emission scanning electron microscopy (FESEM, SIGMA HV, Carl Zeiss) with an energy-dispersive X-ray spectroscopy (EDS) system. X-ray diffraction (XRD) patterns were obtained by using a D2 PHASER diffractometer (Bruker, Germany). Raman spectra were recorded with a DXR3 Raman microscope (Thermo Fisher Scientific, MA, USA) using a 530 nm excitation laser. Fourier transform infrared (FTIR) spectra were collected with an IRSPRINT spectrometer (Shimadzu, Kyoto, Japan). X-ray photoelectron spectroscopy (XPS) measurements were performed on an AXIS Ultra DLD spectrometer (Kratos Analytical Ltd., Manchester, UK).

2.4. Optical Characterization

The optical properties of the samples were characterized through the use of a Duetta spectrofluorometer (Horiba, Japan) to record the UV–visible absorption (UV–Vis) and fluorescence emission spectra. Fluorescence recombination lifetime measurements were conducted using a FluoroMax Plus spectrofluorometer (Horiba, Japan). The absorption spectra were also used to estimate the optical transition energy of the CQDs using the Tauc plot relation, as expressed in eq : ,

$$\alpha h\upsilon =B{(h\upsilon -E_{\mathrm{g}})}^{1/2}$$ 1

where α denotes the absorption coefficient, h is Planck’s constant, v denotes the frequency of the incident photon, B is a proportionality constant, and E _g represents the optical band gap. It should be noted that for CQDs, the extracted values of E _g are interpreted as optical transition edge energies (E _edge) that represent effective transitions rather than a true bandgap, due to the disordered and noncrystalline nature of the material.

The fluorescence quantum yield (ϕ) of the CQDs was determined using a comparative method based on UV–Vis absorption and fluorescence spectrophotometry with appropriate reference standards. Quinine sulfate (QS) and sulfuric acid (H₂SO₄) were purchased from Sigma-Aldrich (Merck, Germany), while Rhodamine B (RhB) was obtained from Loba Chemie (India). QS was used as a reference for B-CQDs and RhB for R-CQDs, to ensure spectral overlap with their respective emission regions. QS was dissolved in 0.1 M H₂SO₄, with a reported quantum yield of 0.54 at an excitation wavelength of 350 nm. RhB (1 mM) was prepared in absolute ethanol, with a reported quantum yield of 0.70 at an excitation wavelength of 543 nm. For both the CQDs and reference solutions, a series of diluted samples was prepared such that the absorbance at the excitation wavelength was maintained below 0.1, to minimize inner filter effects. The integrated fluorescence emission intensity was recorded, and the slopes (K) were obtained from linear plots of the integrated fluorescence intensity versus absorbance.

The fluorescence quantum yields of the CQDs were calculated using eq :

$${\varphi }_{\mathrm{C}\mathrm{Q}\mathrm{D}\mathrm{s}}={\varphi }_{\mathrm{r}\mathrm{e}\mathrm{f}}\times (K_{\mathrm{C}\mathrm{Q}\mathrm{D}\mathrm{s}}/K_{\mathrm{r}\mathrm{e}\mathrm{f}})\times {({\eta }_{\mathrm{C}\mathrm{Q}\mathrm{D}\mathrm{s}}/{\eta }_{\mathrm{r}\mathrm{e}\mathrm{f}})}^2$$ 2

where the subscripts CQDs and ref denote the CQDs and the corresponding reference (QS or RhB), respectively; ϕ is the fluorescence quantum yield; K is the slope of the linear relationship between the integrated fluorescence intensity and absorbance; and η is the refractive index of the solvent. The values of the refractive indices were taken as 1.33 for the aqueous solutions (QS and B-CQDs) and 1.36 for the ethanol-based solutions (RhB and R-CQDs).

2.5. Antioxidant Activity

The antioxidant activity of the CQD samples was evaluated using a 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical-scavenging assay. A 152 μM DPPH solution was prepared in methanol and subsequently mixed with CQD solutions at five different concentrations. For each assay, 100 μL of the DPPH solution was added to 100 μL of the corresponding CQD sample solution. The mixtures were incubated in the dark at room temperature for 30 min, after which the absorbance was measured at 517 nm, with quercetin as the reference antioxidant.

The radical-scavenging efficiency (%) was calculated using eq :

$$\mathrm{Antioxidant\; activity}(\%)=\frac{100\times (\mathrm{Absorbance\; control}-\mathrm{Absorbance\; sample})}{\mathrm{Absorbance\; control}}$$ 3

The value of IC₅₀, defined as the concentration required to achieve 50% inhibition of DPPH radicals, was determined from the plot of antioxidant activity versus the sample concentration using linear interpolation. The experiments were performed in triplicate for each concentration.

2.6. Antibacterial Activity

Escherichia coli (ATCC 25922) and Staphylococcus aureus (ATCC 25923) were used as representative Gram-negative and Gram-positive bacteria, respectively. Bacterial suspensions were prepared at a concentration of 1 × 10⁸ CFU mL^–1 prior to antibacterial evaluation. All microbiological procedures were conducted under aseptic conditions using a Class II biosafety cabinet (Model AC2–4S8-NS, ESCO, Singapore). The culture media and laboratory materials were sterilized by autoclaving (Model ES-315, TOMY, Japan), and dry heat sterilization was carried out using a hot air oven (Model UF-55, Memmert, Germany). Bacterial cultures were incubated in a temperature-controlled incubator (Model BD56, Binder, Germany).

The antibacterial activity of the prepared CQDs was evaluated using an agar disk diffusion (inhibition zone) assay. Bacterial strains were cultured in tryptic soy broth at 37 °C for 24 h, and the turbidity of the suspensions was adjusted to a 0.5 McFarland standard. The standardized bacterial suspensions were uniformly spread onto Mueller–Hinton agar plates using sterile cotton swabs. Sterile paper disks were impregnated with the CQD solutions and placed onto the agar surface to ensure uniform sample loading, and the plates were incubated at 35 °C for 18 h. Antibacterial activity was assessed by measuring the diameter of the inhibition zone formed around each sample.

2.7. Mechanical and UV-Blocking Performance of Films

The mechanical properties and UV-blocking performance of the prepared films were characterized as follows. Tensile measurements were performed using a universal testing machine (Model 5980 Series dual-column floor frames, Instron, USA) at room temperature. Film specimens were cut into rectangular strips (typically 10 × 50 mm) in accordance with ASTM D882. The Young’s modulus was determined from the corresponding stress–strain curves.

The UV-blocking performance was evaluated from the UV–Vis transmittance spectra recorded using a UV–Vis spectrophotometer, with DI water as the reference. The UV-blocking efficiency (B _UV) was calculated according to eq , based on the transmittance spectra T(λ) over the ultraviolet C (UVC, 200–280 nm), ultraviolet B (UVB, 280–315 nm), and ultraviolet A (UVA, 315–400 nm) regions.

$$B_{\mathrm{UV}}=100-\frac{{\int }_{200}^{400}T(\lambda )\mathrm{d}\lambda }{{\int }_{200}^{400}\mathrm{d}\lambda }(\%)$$ 4

2.8. Active Food Packaging Performance

When evaluating the performance of the CQD-based films, tomatoes with similar characteristics, in terms of size, color, and origin, were selected to ensure consistency. The effectiveness of the films in extending the shelf life of tomatoes was evaluated under two storage conditions: (i) a direct-wrapping test, in which tomatoes were individually wrapped with different films; and (ii) a cup-covering test, in which the films were placed over paper cups containing the tomatoes. All samples were stored under ambient conditions.

Appropriate evaluation methods were selected for each storage condition. In the direct wrapping setup, a visual scoring method was employed, as surface changes such as the appearance of black spots, mold growth, and overall skin deterioration could be clearly observed. In the cup-covering experiments, color variations were monitored, with changes in the peel color quantified using a colorimeter (GG-CMS-PRO, GuGear, Thailand). Color parameters were determined according to the CIE (Commission Internationale de l’Éclairage) color system, where L* denotes lightness, a* represents the red–green axis, and b* corresponds to the yellow–blue axis. The total color difference (ΔE) was calculated using eq ,

$$\mathrm{\Delta }E=\sqrt{{(\mathrm{\Delta }{L}^{*})}^2+{(\mathrm{\Delta }{a}^{*})}^2+{(\mathrm{\Delta }{b}^{*})}^2}$$ 5

where ΔL*, Δa*, and Δb* are the differences in lightness, red–green, and yellow–blue color coordinates, respectively, relative to the initial values (day 0).

All measurements were performed in triplicate, and the results are presented here as mean ± standard deviation. Statistical analysis was conducted using a one-way analysis of variance (ANOVA) to evaluate the differences among the sample groups at each time point. Statistical significance of the differences between the groups at each time point is indicated (p < 0.05), while nonsignificant differences are denoted as ns.

3. Results and Discussion

3.1. Morphological and Structural Characterization of CQDs

The morphology and structural characteristics of the CQDs were examined using high-resolution transmission electron microscopy (HRTEM) and XRD. HRTEM images revealed that both R-CQDs and B-CQDs had nearly spherical morphologies, with uniform dispersion and relatively narrow size distributions (Figure a,c). The average particle sizes were determined as approximately 7.10 ± 1.58 nm for the R-CQDs and 3.10 ± 0.28 nm for the B-CQDs (Figure e,f).

1.

(a, c) TEM and (b, d) HRTEM images of R-CQDs and B-CQDs, respectively. (e, f) Corresponding particle size distribution histograms for R-CQDs and B-CQDs. (g) XRD patterns for the prepared CQDs.

Lattice fringes with interplanar spacings of 0.22 ± 0.01 nm and 0.21 ± 0.01 nm were observed (Figure b,d), which were assigned to the (100) plane of graphitic carbon, indicating a partially crystalline carbon framework. The smaller particle size of the B-CQDs was associated with enhanced quantum confinement, which was consistent with their blue-shifted emission relative to the R-CQDs.

These morphological observations were further supported by the XRD patterns shown in Figure g. For the R-CQDs, a prominent broad diffraction peak centered at 2θ = 19.84° was observed, accompanied by two weak peaks at 28.63° and 40.69°, which were assigned to the (002) and (100) planes of graphitic carbon, respectively. The broad nature of these peaks reflected the amorphous and nanocrystalline character of the carbon framework, indicating partially disordered graphitic domains that are typically observed for biomass-derived carbon dots.

The B-CQDs yielded sharper diffraction reflections at 28.63°, 40.69°, 50.47°, and 59.16°, which were indexed to the (002), (100), (102), and (103) planes, respectively. , These well-defined reflections indicated a higher degree of structural ordering and partial graphitization, suggesting that the B-CQDs had a more crystalline carbon structure than the R-CQDs. Furthermore, the calculated d-spacings (0.31, 0.22, 0.18, and 0.16 nm) were consistent with the lattice fringes (0.21–0.22 nm) observed in the HRTEM analysis, thus confirming the existence of nanoscale graphitic domains and validating the quantum confinement effect inferred from the particle size variation.

The Raman spectra displayed characteristic D and G bands at 1351 and 1570 cm^–1, respectively (Figure a). The values of the I _D/I _G ratio of 0.73 for the R-CQDs and 0.50 for the B-CQDs indicate a higher degree of structural disorder for the R-CQDs (Table S1); this is consistent with the predominantly amorphous nature of the R-CQDs as inferred from their XRD patterns, which involved broad diffraction features rather than sharp crystalline reflections.

2.

(a) Raman and (b) FTIR spectra of R-CQDs and B-CQDs. (c) High-resolution XPS spectra of (d, e) C 1s and (f, g) O 1s corresponding to R-CQDs and B-CQDs, respectively. (h) High-resolution N 1s spectrum for B-CQDs.

A lower I _D/I _G ratio is indicative of more ordered sp² carbon domains and is commonly associated with a greater contribution from core-related emissive states and a wider effective band gap. Higher degrees of structural disorder and amorphous character can be associated with enhanced surface-state contributions, corresponding to red-shifted emission.

The surface functionalities of the synthesized CQDs were analyzed by FTIR and XPS, as summarized in Tables S2 and S3, respectively. The FTIR spectra for the R- and B-CQDs indicated the presence of various oxygen- and nitrogen-containing functional groups, including hydroxyl (O–H), aliphatic (C–H), carbonyl (CO), aromatic (CC), and C–O-related functionalities (C–OH, C–O, and C–O–C).

For the R-CQDs, pronounced C–H and C–OH bands were seen at 2922 and 1447 cm^–1, respectively, suggesting a higher abundance of hydroxyl-related surface groups. In contrast, B-CQDs showed relatively intense O–H/N–H stretching bands, together with CN and C–O–C bands (Figure b), indicating a greater contribution from oxygen- and nitrogen-containing surface functionalities and a lower hydroxyl content.

The results of an XPS analysis provided further support for these observations in the form of detailed information about the elemental composition and chemical states of the CQDs. Survey spectra revealed O 1s/C 1s atomic concentration ratios of 0.17 and 0.60 for the R-CQDs and B-CQDs, respectively (Figure c), indicating a significantly higher surface oxygen content in the B-CQDs. In addition, a weak N 1s signal was detected at a binding energy of approximately 400 eV with an atomic concentration of 0.98%, suggesting the presence of nitrogen species in relatively small amounts.

Deconvolution of the high-resolution C 1s and O 1s spectra indicated that the C 1s spectrum of the R-CQDs was dominated by C–C/CC (sp² carbon) contributions at 285.0 eV, accounting for 77.8% of the fitted peak area, with additional contributions from C–O (286.7 eV, 17.3%), CO (287.7 eV, 3.1%), and OCO (289.0 eV, 1.7%) (Figure d). The corresponding O 1s spectrum consisted of multiple oxygen species, including CO (531.4 eV, 6.3%) and C–O–C/C–O (532.4 eV, 34.1%), with a dominant contribution from OCO and surface-adsorbed oxygen species (533.3 eV, 51.1%), along with minor amounts of physically adsorbed species (534.4 eV, 8.6%) (Figure f). The relatively low CO/(C–C/CC) area ratio (0.04) and high sp²/sp³ ratio (3.52) suggest a comparatively high degree of π-conjugation with limited oxygenated functionalities (Table S3).

The C 1s spectrum for B-CQDs had a broader distribution, including a distinct low binding energy component at 283.3 eV (33.5%), which can be associated with electron-rich or defect-related carbon species, along with sp² carbon contributions at 284.2 eV (13.3%) and 285.1 eV (34.5%). Additional peaks corresponding to C–O/C–N (286.4 eV, 6.3%) and CO (287.3 eV, 12.3%) were also observed (Figure e). Compared to R-CQDs, the CO/(C–C/CC) ratio (0.26) was higher and the sp²/sp³ ratio was lower (0.92), indicating a reduced degree of conjugation and a greater contribution from heteroatom-containing functional groups.

The O 1s spectrum for the B-CQDs was primarily composed of C–O–C/C–O (531.3 eV, 42.1%) and CO (530.3 eV, 33.7%) species, with additional contributions from OCO/surface-associated species (532.1 eV, 17.4%) and low binding energy oxygen species (529.5 eV, 6.8%) (Figure g). The N 1s spectrum also revealed the presence of C–N (398.3 eV, 69.6%) and graphitic N (399.8 eV, 30.6%), thus confirming the incorporation of nitrogen into the carbon framework (Figure h).

The combined FTIR and XPS analyses revealed that the R-CQDs had a more conjugated carbon framework with surface-associated oxygen-containing groups, whereas the B-CQDs were enriched with oxygen-containing functional groups with a smaller contribution from nitrogen-related components, which were absent from the R-CQDs. These surface differences were closely associated with the polarity of the solvent used for synthesis: the less polar solvent used for the R-CQDs favored extended dehydration and carbonization, resulting in a lower relative content of oxygenated functionalities and a higher degree of π-conjugation, while still retaining surface hydroxyl-related groups, whereas the more polar solvent used for the B-CQDs promoted carbonization and the formation of smaller particles bearing abundant oxygen- and nitrogen-containing functionalities.

Consistent with these observations, the R-CQDs had a larger average particle size and a higher degree of structural disorder, as evidenced by the higher I _D/I _G ratio in the Raman spectra and the amorphous characteristics indicated by the broad diffraction feature in the XRD patterns. The predominance of defect-rich and amorphous carbon domains, together with the lower oxygen functionalization but sufficient surface groups confirmed by the FTIR and XPS analyses, may facilitate interparticle hydrogen bonding interactions. These interactions can promote the aggregation of neighboring CQDs, leading to the formation of larger CQD assemblies. ,

The solvent environment played an essential role in regulating nucleation, carbon core growth, and surface evolution, which together determined the particle size, structural order, and chemical composition of biomass-derived CQDs. The variations in the structure and surface states were likely to influence their optical properties, providing motivation for the subsequent photophysical characterization.

3.2. Optical Properties of CQDs

In view of these distinct structural and surface characteristics, the two CQD systems were expected to exhibit markedly different optical behaviors. The defect-rich and amorphous carbon domains combined with the more conjugated carbon framework and surface-associated oxygen groups of the R-CQDs were expected to induce red-shifted emission, whereas the oxygen- and nitrogen-enriched surface states of the B-CQDs were expected to favor higher-energy, surface-state-dominated emission. The optical absorption and photoluminescence properties of both CQDs were, therefore, systematically investigated.

The UV–Vis absorption spectra of the R- and B-CQDs (Figure a) revealed distinct optical signatures reflecting their structural differences. For the R-CQDs, bands were seen at 285, 317, and 665 nm, which were assigned to π–π* transitions of aromatic CC domains, n−π* transitions of surface CO groups, and low-energy absorption associated with extended π-conjugation domains, respectively. These features were consistent with the results of the FTIR, XPS, HRTEM, and XRD analyses and matched the observed red-shifted emission. For the B-CQDs, a single band was observed at ∼280 nm, corresponding to a smaller aromatic core. The absence of longer-wavelength absorption indicated limited π-conjugation due to smaller graphitic domains and abundant oxygen- and nitrogen-containing surface groups, favoring emission governed by surface states and quantum-confinement effects.

3.

(a) UV–vis absorption spectra for the CQDs. (b) Optical band gaps estimated using Tauc plots. (c) Fluorescence emission spectra recorded under 365 nm excitation. (d, e) Three-dimensional fluorescence excitation–emission maps for the CQDs. (f) CIE chromaticity diagram. (g) Peak position and (h) intensity of fluorescence as a function of excitation wavelength. (i) Time-resolved photoluminescence decay profiles for the CQDs (FL: fluorescence; Ex.: excitation; Em.: emission).

Tauc plots yielded values for the optical transition edge energy (E _edge) of ∼1.81 eV for the R-CQDs and ∼3.37 eV for the B-CQDs (Figure b), representing effective optical transition energies rather than true bandgaps. The lower value for R-CQDs is consistent with transitions associated with extended sp² domains, whereas the higher value for B-CQDs reflects reduced conjugation and a greater contribution from surface functionalities. These results provide a structural basis for the observed differences in emission behavior.

The fluorescence behavior of the synthesized CQDs exhibited distinct excitation–emission characteristics depending on the emission color and surface chemistry. Figure c presents the fluorescence emission spectra and corresponding photographs of the CQDs under 365 nm excitation, while Figure f shows the CIE chromaticity diagram illustrating the emission coordinates.

The R-CQDs demonstrated two excitation-independent emission peaks centered at 682 and 720 nm over an excitation range of 510–670 nm (Table S4, Figure d,g,h), indicating relatively uniform emissive states associated with a conjugated sp² carbon framework. In contrast, the B-CQDs displayed excitation-dependent emission, with the maximum shifting from 446 to 548 nm as the excitation wavelength was increased from 370 to 495 nm (Figure e,g,h), suggesting the presence of multiple emissive states.

These differences are further supported by the composition-dependent optical response shown in Figure . As the fraction of R-CQDs increased, transmission in the visible region decreased (Figure a), accompanied by the gradual emergence and enhancement of red emission bands at around ∼650–780 nm under an excitation wavelength of 365 nm (Figure b). Concurrently, the intensity of blue emission was progressively suppressed, indicating a systematic shift in the dominant emissive pathways from high-energy to low-energy states. This trend is consistent with the red-shifted optical transition edge, whose close alignment with the excitation energy corresponding to maximum emission suggests efficient population of low-energy emissive states with minimal energy loss during relaxation.

4.

(a) Transmission, (b) fluorescence intensity, (c) fluorescence intensity ratio (I _red/I _blue) as a function of R-CQD fraction, and (d) time-dependent fluorescence intensity fluctuations of R-CQDs and B-CQDs over 60 min (FL: fluorescence).

This behavior is quantitatively reflected in the fluorescence intensity ratio (I _red/I _blue), which increased monotonically with an increase in the R-CQD fraction (Figure c), indicating a continuous transition from blue-dominated to red-dominated emission. The absence of abrupt changes suggests that these two emissive components coexist and evolve proportionally rather than forming entirely new emissive states, supporting the interpretation of mixed or interacting emissive domains.

These differences are consistent with the FTIR and XPS results, which indicate that the R-CQDs possess a higher sp²/sp³ ratio and lower relative oxygen functionalization, while B-CQDs have more oxygen- and nitrogen-containing surface groups. The lower-polarity synthesis environment for R-CQDs favors dehydration and carbonization, promoting the extended π-conjugation, whereas the higher-polarity condition for B-CQDs facilitates the formation of more functionalized surfaces.

Similarly, evidence for the solvent-dependent enhancement of sp² conjugation and corresponding reduction in the effective optical transition energy is provided by both experimental observations and density functional theory (DFT) calculations, which indicate that enlarged sp² domains and reduced surface oxidation can lower emissive energy levels. − The polarity of the solvent has consistently been shown to influence the surface functional groups and induce a redshift in CQD emission through modulation of the main surface functionalities. Furthermore, solvatochromic emission arising from solvent–surface electronic coupling has been experimentally demonstrated, highlighting the role of the solvent environment in tuning the optical properties of the CQDs. Modulation of emission due to solvent-controlled surface states and core electronic structure has also been corroborated through combined spectroscopic characterization and DFT modeling.

In addition, the solvent-induced stabilization of sp²-rich domains, together with controlled interparticle interactions, may contribute to the dominance of low-energy radiative recombination pathways. , As a result, the emissive electronic states become more uniform, leading to stable red-to-near-infrared (NIR) emission. These interpretations are consistent with recent DFT calculations reported by Zhang et al., indicating that red-shifted emission in CQDs was favored by larger, fully conjugated sp² domains with minimal sp³ interruptions and zigzag edges, which generated uniform electronic states, a finding that is in agreement with the stable emission observed in experiments.

Time-resolved photoluminescence (TRPL) measurements provided further support for these observations (Table S5 and Figure i). The R-CQDs exhibited biexponential decay with an average lifetime of 6.56 ns, indicating two dominant emissive components associated with delocalized π–π* transitions within the conjugated domains. One important finding (as illustrated in Figure a,c) was that the excitation energy (1.85 eV) and emission energies (1.82 and 1.72 eV) were closely aligned with the optical transition edge (E _edge = 1.81 eV), suggesting minimal energy loss during radiative recombination. This behavior is consistent with the higher quantum yield (Q = 8.33%) and lower nonradiative decay rate (K _nr = 1.40 × 10⁸ s^–1) for the R-CQDs (Table S6 and Figure d).

5.

Schematic diagrams of optimal emission energy level alignment for (a) R-CQDs and (b) B-CQDs; (c) plot of sp² carbon fraction, I _D/I _G ratio, emission maximum energy, and optical transition edge energy (E _edge) for R-CQDs and B-CQDs, with parameter values to enable a direct comparison of structural and optical properties; (d) linear fits of integrated fluorescence intensity as a function of absorbance for quinine sulfate (y _QS), Rhodamine B (y _RhB), R-CQDs (y _R‑CQDs), and B-CQDs (y _B‑CQDs) for estimation of quantum yield (FL: fluorescence).

In contrast, B-CQDs displayed a biexponential decay with a shorter average lifetime of 2.00 ns, reflecting multiple emissive pathways. The excitation energy (3.35 eV) was close to E _edge (3.37 eV), whereas the emission energy (2.78 eV) was significantly lower, indicating a larger relaxation pathway prior to emission (Figure b). This suggests increased nonradiative relaxation via surface or defect states, consistent with the lower quantum yield (Q = 1.56%) and higher nonradiative decay rate (K _nr = 4.92 × 10⁸ s^–1) for B-CQDs. A rate constant analysis (Table S6) indicated that the radiative decay constants were comparable for both CQDs (K _r = ∼0.1 × 10⁸ s^–1), and that the primary difference arose from nonradiative processes. The significantly higher K _nr for B-CQDs highlights the dominant role of surface-related recombination pathways in reducing emission efficiency. ,

Time-dependent fluorescence measurements (Figure d) revealed that B-CQDs had lower intensity fluctuations (∼11.1%) compared to R-CQDs (∼23.5%) over 60 min, indicating more stable emission under continuous excitation. This suggests that the emissive states for B-CQDs are less sensitive to environmental or dynamic perturbations, whereas the R-CQDs may involve more dynamic or interaction-sensitive emissive domains, consistent with their defect-rich and partially aggregated nature.

Taken together, the absorption, fluorescence, and TRPL results indicate that the polarity of the solvent governed carbonization, the surface chemistry, and the uniformity of emissive states. R-CQDs, formed in a less polar medium, exhibited extended π-conjugation and relatively uniform emissive states, resulting in excitation-independent red–NIR emission with reduced energy loss, whereas B-CQDs, which were synthesized in a more polar solvent, possessed heterogeneous surface states, leading to excitation-dependent emission and enhanced nonradiative relaxation. These findings provide a mechanistic basis for tuning of the photophysical properties of CQDs through controlled synthesis conditions.

3.3. Antioxidant Activity of CQDs

The antioxidant activity of the CQDs was evaluated using the DPPH radical-scavenging assay, with quercetin as a reference standard. As expected, quercetin exhibited strong radical-scavenging capability, with an IC₅₀ value of 4.50 μg mL^–1, reflecting its well-known phenolic antioxidant nature (Table S7, Figure a).

6.

(a) Antioxidant activity of the CQDs evaluated by the DPPH radical-scavenging assay. (b) Linear fitting of concentration-dependent inhibition data used for IC₅₀ determination. (c) Diameters of the inhibition zones against Escherichia coli and Staphylococcus aureus. (d) Representative photographs illustrating the antibacterial activity of the R-CQDs.

Both R-CQDs and B-CQDs demonstrated concentration-dependent DPPH radical-scavenging behavior, confirming their intrinsic antioxidant activity. Notably, the R-CQDs exhibited higher scavenging efficiency across the concentration range examined here, achieving 82.32% inhibition at a concentration of 15.96 μg mL^–1 and an IC₅₀ value of 8.84 μg mL^–1. In comparison, B-CQDs reached a maximum inhibition of 88.80% at a higher concentration (36.75 μg mL^–1), with a corresponding IC₅₀ value of 21.03 μg mL^–1 (Tables S8 and S9, Figure b). The lower IC₅₀ value for the R-CQDs represents superior antioxidant efficiency, as a smaller amount of material was required to achieve 50% radical inhibition.

This enhanced antioxidant performance of the R-CQDs may be associated with their larger sp²-conjugated domains and the higher abundance of surface hydroxyl groups, which could facilitate electron or hydrogen donation to neutralize DPPH radicals, although it should be noted that this interpretation is based on correlations between structural features (e.g., sp² carbon fraction and surface functional groups) and the observed antioxidant activity, rather than direct mechanistic evidence. Enlarged sp² domains may promote more effective charge delocalization and stabilization of radical intermediates, thereby potentially enhancing free-radical scavenging efficiency, as widely reported for carbon-based nanomaterials with extended π-conjugation. In addition, it has been suggested that hydroxyl-rich surface functionalities contribute to hydrogen-donating antioxidant activity in carbon dots. , The lower antioxidant efficiency of B-CQDs may be related to their smaller carbon cores and less-developed sp² conjugation, which could limit charge delocalization and radical stabilization.

These results suggest that solvent-mediated structural variations not only modulate the optical properties of biomass-derived CQDs but also may influence their antioxidant behavior. The pronounced radical-scavenging activity of the R-CQDs, particularly at low effective concentrations, highlights their potential for use in active packaging systems, where antioxidant functionality could be beneficial for suppressing oxidative degradation and extending the shelf life of foods.

3.4. Antibacterial Activity of CQDs

The antibacterial activities of the CQDs were evaluated using an agar diffusion assay against representative Gram-negative and Gram-positive bacteria, namely Escherichia coli and Staphylococcus aureus, respectively. Distinct antibacterial behaviors were observed depending on the emission characteristics of the CQDs. Clear inhibition zones against both bacterial strains were seen for R-CQDs, with average diameters (Table S10, Figure S1) of 10.00 ± 1.00 mm for E. coli and 13.67 ± 0.58 mm for S. aureus. Upon dilution of the R-CQDs to 50%, (7.98 μg mL^–1) antibacterial activity against E. coli was completely suppressed, whereas a measurable inhibition zone against S. aureus (10.33 ± 0.58 mm) was still observed, indicating a concentration-dependent antibacterial response (Figure c,d). In contrast, B-CQDs did not produce detectable inhibition zones against either bacterial strain under identical experimental conditions, demonstrating the absence of intrinsic antibacterial activity for these CQDs. It should be noted that antibacterial activity was evaluated using a qualitative approach to enable comparative assessment among the samples; in other words, the antibacterial performance discussed here should be interpreted as comparative evidence based on inhibition zone observations, rather than a direct measure of bactericidal efficiency.

The marked contrast in antibacterial performance between the R-CQDs and B-CQDs suggests the importance of structural evolution and surface chemistry rather than the photoluminescence color alone. The R-CQDs, which emitted at lower photon energies, were generally associated with a higher degree of carbonization and more extended sp²-conjugated domains. These structural features may contribute to enhanced interactions at the bacterial interface; however, specific mechanisms such as the generation of oxidative stress or membrane disruption were not directly investigated in this study and are proposed only as possible explanations based on prior reports.

The stronger antibacterial efficacy of the R-CQDs against S. aureus compared with E. coli may be attributed to differences in the structure of the bacterial cell envelope. Gram-positive bacteria lack an outer membrane, allowing for direct interaction between CQDs and the peptidoglycan layer and underlying cytoplasmic membrane. The outer membrane of Gram-negative bacteria acts as an additional barrier to diffusion, which may explain the observed reduction in antibacterial activity upon dilution.

The complete absence of antibacterial activity for the B-CQDs indicates that their physicochemical properties are insufficient to induce observable inhibition under the experimental conditions. These findings suggest that antibacterial behavior is not an inherent property of CQDs but instead emerges from specific structural and chemical characteristics, although further mechanistic studies are required to confirm these effects. ,

3.5. Properties and Performance of CQD/PVA Composite Films

3.5.1. Morphological and Elemental Characterization

The SEM images (Figure a) revealed that all films, including pristine PVA, R-CQDs/PVA, and B-CQDs/PVA, had smooth and homogeneous surfaces, suggesting that the CQDs were well dispersed within the PVA matrix. The cross-sectional SEM images also had comparable film thicknesses, with values of approximately 23.2, 22.7, and 17.0 μm for the PVA, R-CQDs/PVA, and B-CQDs/PVA films, respectively, reflecting the fact that the films were prepared under similar casting conditions, although slight variations in thickness cannot be completely excluded.

7.

(a) Cross-sectional SEM images and corresponding EDS elemental maps of C, O, and N for B-CQD/PVA (top row), R-CQD/PVA (middle row), and pristine PVA films (bottom row), indicating the film morphology and elemental distribution. (b) Optical transmittance spectra for PVA and CQD/PVA composite films. (c) UV-blocking performance of R-CQDs, B-CQDs, PVA, R-CQD/PVA film, and B-CQD/PVA film over the UVA, UVB, and UVC regions.

The EDS data (Table S11) showed that the pristine PVA film consisted mainly of carbon (72%) and oxygen (28%), with an O:C ratio of 0.39, reflecting the hydroxyl-rich nature of PVA. Upon incorporation of CQDs, the content of both oxygen and nitrogen increased, and the O:C ratios increased to 0.53 and 0.45 for R-CQD/PVA and B-CQD/PVA films, respectively. The higher O:C ratio for the R-CQD/PVA film suggests the incorporation of oxygen-containing functional groups from the CQD surface and possible chemical interfacial interactions with the PVA matrix.

One interesting finding was that nitrogen (5%) was detected in the R-CQD/PVA film, whereas the XPS spectrum for the pristine R-CQDs revealed only carbon and oxygen. This may indicate that nitrogen-related components, which were not detectable in the pristine R-CQDs, became observable after the formation of the film, possibly because of residue incorporation or processing-induced interfacial interactions.

For the B-CQD/PVA film, the presence of 3% nitrogen was consistent with the XPS results of N-containing B-CQDs, suggesting that the nitrogen-related features were retained after incorporation into the polymer matrix. The increased heteroatom content and O:C ratios may imply enhanced CQD–PVA interfacial affinity.

3.5.2. UV-Shielding and Mechanical Properties

The UV-shielding performance of the pristine PVA, CQD/PVA composite films (containing 25 wt % CQDs), and pure CQDs was evaluated across the UVA (315–400 nm), UVB (280–315 nm), and UVC (200–280 nm) regions (Figure b). As shown in Table S12 and Figure c, pristine PVA offered limited UV protection, blocking only 44%, 18%, and 14% of UVC, UVB, and UVA radiation, respectively. The R-CQD/PVA composite film showed near-complete attenuation of UV across these regions, indicating strong suppression of UV transmission. This performance may be associated with the conjugated sp² carbon domains and π–π* transition in the R-CQDs, which contribute to broadband absorption. Similarly, pure R- and B-CQDs exhibited near-complete UV attenuation (>99%), providing evidence for their intrinsic UV-absorptive capability.

In contrast, the B-CQD/PVA film showed moderate UV-shielding efficiency (approximately 88% for UVC and 74% for UVB) with notably lower protection in the UVA region (approximately 52%). This reduced performance may be related to differences in the extent of the conjugated domains of the B-CQDs, which can influence absorption at longer wavelengths. Nevertheless, the incorporation of 25 wt % B-CQDs resulted in a pronounced improvement compared to pristine PVA, suggesting their potential for partial UV filtration applications.

To further evaluate the optical properties in the visible region, the transmittance at 550 nm was analyzed as a representative indicator of film transparency. The pristine PVA film exhibited a high transparency (86.65%). The B-CQD/PVA film retained comparable transparency (81.23%), whereas the R-CQD/PVA film showed significantly reduced transmittance (40.46%), indicating a lower optical clarity in the visible range (Figure b). These results suggest a trade-off between the UV-blocking performance and visible transparency. Although the films were prepared under similar conditions and exhibited comparable thicknesses, the influence of the film thickness on transmittance cannot be entirely excluded. Hence, the UV-shielding behavior observed here is considered to be primarily associated with the intrinsic absorption properties of the CQDs, although potential thickness effects should also be taken into account.

Taken together, these results suggest that the incorporation of CQDs can enhance UV attenuation in polymeric matrices, probably through combined absorption and interfacial interactions. The stronger performance of the R-CQD/PVA film highlights the potential role of the CQD structure and surface chemistry, suggesting a promising but preliminary strategy for the design of nanocomposite films for UV-protective applications.

The incorporation of CQDs into the PVA matrix led to a notable reduction in the Young’s modulus, with values for the R-CQD/PVA and B-CQD/PVA films of 1490 and 1368 N mm^–2, respectively, compared to 2483 N mm^–2 for pure PVA (Table S13, Figure a). This decrease in modulus was likely to have been caused by the disruption of interchain hydrogen bonding within the PVA matrix by the embedded CQDs. FTIR analysis suggests that for both R-CQDs and B-CQDs, there are increased contributions from oxygen-containing functional groups, including CO carbonyl (1650–1750 cm^–1), C–O/C–N (1100–1400 cm^–1), and C–O–C ether (1060–1095 cm^–1) vibrations, accompanied by a relative reduction in CC aromatic features (1550–1650 cm^–1) (Figure S2). Upon incorporation into the PVA matrix, the N–H/O–H stretching region (3020–3525 cm^–1) appears slightly broadened, suggesting possible interactions between the CQDs and PVA chains, although no distinct peak shift is observed due to the inherently broad nature of hydrogen-bonded bands. More noticeable changes are observed in the C–H aliphatic region (2790–3000 cm^–1), where a C–H-related peak becomes more apparent for the B-CQD/PVA film, whereas R-CQDs already exhibit such features prior to composite formation.

8.

(a) Stress–strain curves of the films, with the inset showing the linear fitting used to determine Young’s modulus. (b) Results of the tomato wrapping test, in which visual spoilage was evaluated daily using a six-point scoring scale. (c) Color difference (ΔE) and (d) a* value as functions of storage time for the control, PVA-, and CQD/PVA-covered samples.

Notably, the R-CQD/PVA film retained a slightly higher modulus than the B-CQD/PVA film; this result was attributed to the larger size and hydroxyl-rich surface of the R-CQDs, which may have promoted additional hydrogen-bonding interactions with PVA chains that partially mitigated the loss of mechanical stiffness. Although the R-CQD/PVA film had a slightly higher Young’s modulus, the B-CQD/PVA film had higher values for the tensile strength and elongation at breaking, suggesting that different reinforcement mechanisms governed the elastic stiffness and fracture behavior.

3.5.3. Active Packaging Performance

The effectiveness of these two different films in extending the shelf life of tomatoes was evaluated under two conditions: (i) direct wrapping and (ii) cup covering, where the films were placed over paper cups containing tomatoes. All samples were stored under ambient conditions.

For the wrapping test, tomato spoilage was assessed visually on a daily basis using a six-point scale, where higher scores indicate greater deterioration (0 = excellent, 1 = good, 2 = slight deterioration, 3 = moderate, 4 = poor, and 5 = severe rotting), as summarized in Table S14. During the initial storage period (days 1 and 2), there was negligible spoilage of all samples (score = 0), confirming a comparable level of initial quality. Distinct differences became evident from day 4 onward. Unwrapped control tomatoes deteriorated rapidly, reaching severe spoilage (score = 5) by day 14, indicative of accelerated moisture loss, oxidation, and microbial growth (Figures b and a). Pure PVA films gave a moderate delay in deterioration, with spoilage increasing more slowly and reaching a score of four on day 14, suggesting that PVA primarily functioned as a passive barrier with limited protective capability.

9.

Photographs of tomatoes under (a) direct wrapping and (b) cup-covering conditions, in which the films were placed over paper cups containing tomatoes.

The CQD/PVA composite films yielded improved preservation performance: tomatoes wrapped with R-CQD/PVA showed no detectable spoilage throughout the entire storage period, whereas the B-CQD/PVA film effectively suppressed deterioration until day 12, followed by only slight spoilage at day 14. These observations indicate that the incorporation of CQDs significantly improved the protective function of the polymer films.

In the cup-covering test, consistent trends were observed based on color difference (ΔE) analysis (Tables S15–S18 and Figures c and b). Although ΔE increased gradually for all samples during storage, the tomatoes covered with the control and PVA films underwent pronounced color changes, reaching ΔE values of 28.0 and 32.1, respectively, by Day 7. In contrast, the R-CQD/PVA and B-CQD/PVA films markedly suppressed discoloration, with substantially lower ΔE values of 17.9 and 26.1, respectively. The superior color stability imparted by the R-CQD/PVA film demonstrated its enhanced ability to inhibit oxidative browning and pigment degradation, a finding that is attributable to the synergistic effects of UV-shielding and antioxidant functionalities. Statistical analysis based on a one-way ANOVA revealed that the differences in ΔE among the sample groups became statistically significant (p < 0.05) from Day 3 onward, with the R-CQD/PVA film showing significantly lower ΔE values compared to the control and PVA films, confirming its superior performance in suppressing discoloration (Figure S3a, Tables S19–S25).

The evolution in the color was also monitored based on variation in the a* coordinate in the CIE Lab color space (Figure d). For the control samples (stored without film), there was a continuous increase in a* from −7.52 to 18.04 over 7 days, reflecting the natural transition from green to red during ripening. Tomatoes sealed with pure PVA film showed a similar trend (a* = 24.21 at Day 7), suggesting that the polymer matrix alone provided limited barrier protection against oxygen and light. A one-way ANOVA analysis indicated that the increase in a* values for the control and PVA samples was significantly higher than for the CQD-containing films (p < 0.05) at later storage times (Day ≥ 3) (Figure S3b, Tables S26–S33).

The R-CQD/PVA films effectively suppressed this color transition, with a* remaining below 3.75 throughout the storage period. This stabilization was likely to have been caused by the combined antioxidant and UV-blocking effects of the R-CQDs, which mitigated photo-oxidative degradation of chlorophyll and retarded anthocyanin accumulation. The B-CQD/PVA films also gave a delay in the color change, with a moderate increase in a* (up to 14.81 at day 7), indicating partial inhibition of ripening due to their oxygen-scavenging and UV-attenuating capabilities. Statistically, the a* values for the R-CQD/PVA films were significantly lower than those for all other groups (p < 0.05). The B-CQD/PVA film showed intermediate behavior, with a significant difference from the control but a less pronounced improvement than the R-CQD/PVA film.

Both visual inspection and colorimetric analyses demonstrated that CQD–PVA nanocomposite films, and particularly those containing R-CQDs, effectively slowed the ripening and color change of the fruit. These findings highlight the potential of CQD-functionalized coatings as active packaging materials for preserving postharvest quality and extending shelf life. The statistical outcomes further supported these observations, confirming that the incorporation of CQDs significantly improved the color stability and delayed ripening compared to the control and pure PVA films (p < 0.05).

4. Conclusion

In this research, CQDs were successfully synthesized from mangosteen leaves via a solvent-controlled hydrothermal method, and clear correlations were revealed among their structure, emission behavior, and functional performance. The polarity of the solvent dictated the degree of carbonization and surface chemistry, yielding R-CQDs with extended sp² domains and B-CQDs with oxygen- and nitrogen-rich surfaces. The R-CQDs were found to have strong antioxidant and antibacterial activities, and their incorporation into PVA matrices enhanced UV shielding while modulating the mechanical properties of the film through interfacial interactions. A notable finding was that the R-CQD-based films delayed the ripening and color change of tomatoes during storage.

These results highlight the pivotal role of surface chemistry in tuning the photophysics and functionality of CQDs and provide a sustainable platform for the design of multifunctional nanocomposite films for UV-protective, antioxidant, and antibacterial food-packaging applications.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsphyschemau.6c00013.

• The results of Raman, FTIR, and XPS analyses of the CQDs (Tables S1–S3); the excitation-dependent photoluminescence properties and time-resolved photoluminescence decay fitting results for R-CQDs and B-CQDs (Tables S4 and S5); a summary of the photophysical parameters (Q, τ_avg, K _r, K _nr, K _total) (Table S6); an evaluation of the antioxidant activity based on the DPPH assay (Tables S7–S9); antibacterial activity data including inhibition zone measurements and representative images (Table S10 and Figure S1); FTIR spectra for pristine PVA, CQDs, and CQD/PVA films (Figure S2); results for the structural, UV-shielding, mechanical, and packaging performance of pristine PVA and CQD/PVA films (Tables S11–S18); and the statistical significance (ANOVA, p-values) of the variations in ΔE and a* (Figure S3), with detailed results in Tables S19–S33 (PDF)

T.L.: Writingreview and editing, Visualization, Validation, Methodology, Investigation, Formal analysis, Data curation. P.P.: Writingreview and editing, Writingoriginal draft, Visualization, Validation, Supervision, Resource, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization. S.A.: Writingreview and editing, Writingoriginal draft, Visualization, Validation, Supervision, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization.

The authors declare no competing financial interest.