Optical Characterization of Fluorescent Chitosan-Based Carbon Dots Embedded in Aqueous Natural Dye

Abstract

(1) Background: This work evaluated the optical characterization of aqueous fluorescent chitosan-based carbon dots (or carbon nanoparticles CNPs) embedded in natural dye for potential functional packaging applications. Chitosan-based materials are nontoxic, biodegradable, biocompatible, bactericidal, and produced from renewable polymer sources. Anthocyanins are pigments of different colors with a large range of potential applications, such as in bioindicators and biomonitoring; (2) Methods: The CNPs were synthetized in aqueous solutions using chitosan as a carbon source. The natural dye was extracted from the leaves of Tradescantia pallida Purpurea in aqueous solutions. The fluorescence quantum efficiency (η) and fluorescence lifetime (τ) were determined using the mode-mismatched pump–probe thermal lens (TL) technique and time-resolved fluorescence lifetimes (TRFL) measurements, respectively; (3) Results: The η and τ were measured for CNPs embedded in natural dye solution at different concentrations (5.2, 12.09, and 21.57 mass percentage composition). The η and τ photophysical parameters obtained for CNPs embedded in natural dye were compared with those of other CNPs synthesized using different carbon sources, such as leaves, seeds, and protein; (4) Conclusions: Fluorescence spectra and time-resolved fluorescence measurements corroborate the TL results, and relatively high values of η were obtained for the CNP synthesized and embedded in natural dye.

1. Introduction

Chitosan-based materials have been widely reported in different applications in agriculture, pharmacy, biomedical applications, and environmental protection due to chitosan being nontoxic, biocompatible, biodegradable, and exhibiting antimicrobial, bactericidal, and antioxidant properties [1,2,3,4,5]. Chitosan is a polysaccharide biopolymer reported to be acquired through the chemical alkaline deacetylation of chitin present in algae and the exoskeletons of arthropods (such as crustaceans and insects), it is also found naturally in limited quantities in some fungi [2,3,5]. Chitosan solutions are fluorescent materials, and fluorescence quantum efficiency (η) enhancement has been obtained for chitosan-based carbon nanoparticles (CNPs) synthesized using heat treatment [6,7,8,9]. Synthesized fluorescent CNPs exhibit low cytotoxicity, and excellent biocompatibility [10,11]. In addition, CNPs can attach to specific cells and enter cells without or with additional functionality, enhancing research in applications such as biosensing, cell mapping, nanodrug carriers, electrochemical imaging, and antifungal and antibacterial activities [6,9,10,11,12]. Moreover, the antifungal properties of chitosan can be enhanced with different additives, such as essential oils, natural dyes, and/or metallic nanoparticles [13,14].

Anthocyanin is a natural dye belonging to the family of flavonoids that exhibits a variety of colors like orange, pink, violet, and blue, and is found in different plants, flowers, fruits, and tubers [15,16,17]. This natural dye has various possible applications in health, the pharmaceutical industry, food processing, solar cell development, and the potential of hydrogen (pH) indicators [16,18,19,20]. Moreover, anthocyanin is reported to have fungicidal properties on necrotrophic fungi, such as Fusarium and Colletotrichum [21], and bactericidal effects against the proliferation of E. coli [22]. On the other side, the development of biomaterials prepared with carbon dots embedded in natural dye for colorful and functional packaging is in progress, and recently, several films have been proposed as food spoilage indicators and food packaging to extend food shelf life [23,24,25,26,27]. Highly fluorescent carbon dots developed from green synthesis and embedded into natural dye for fluorometric probes show relevant potential for applications in food coatings and functional packaging. Smart food packaging applications have been reported, with carbon dots synthesized using rose petal waste as a carbon source and anthocyanin in carrageenan-based functional films to monitor the freshness of minced pork and shrimp [25]. In addition, carbon dots doped with TiO₂ were used with anthocyanin, obtained from the residue of sweet potato peels, for carrageenan-based active and pH-dependent films. These films were used for packaging and monitoring the freshness of shrimp, highlighting their potential in preserving the quality and indicating shrimp spoilage [23].

The present work reports on the spectroscopic and optical characterizations of the synthesized CNPs embedded in natural dye extracted from leaves of Tradescantia pallida Purpurea [28,29] at different ratios for possible bioapplications. The thermal lens (TL) technique was applied for the determination of fluorescence quantum efficiency (η). Fluorescence spectra and fluorescence lifetime (τ) were also measured to corroborate the TL results obtained for chitosan-based materials. The photophysical parameters τ and η were compared with results obtained for other CNPs synthesized from leaves, seeds, and/or protein for bioapplications. The leaves of Tradescantia pallida Purpurea are an excellent source of anthocyanins [20,28,29], and CNPs are characterized individually and point out possible interactions between natural dye agents with the goal of intensifying their properties as possible antifungal and/or bactericidal agents for potential smart food packaging applications [13,20,21].

2. Materials and Methods

2.1. Sample Preparation

Chitosan solutions were prepared by diluting 2 g of low molecular weight Sigma-Aldrich chitosan in 100 mL of 2% acetic acid aqueous solutions. The mixture was placed in a magnetic stirrer for 6 h and then centrifuged for 30 min, resulting in a liquid with high viscosity and yellowish coloration. Chitosan-based carbon nanoparticles (CNPs) were synthesized by hydrothermal–pyrolysis carbonization method by diluting 2 g of low molecular weight chitosan in 18 mL of 2% acetic acid aqueous solutions. The sample was subjected to magnetic stirring for 6 h and heated in a porcelain crucible at 180 °C for 10 h [7]. The result was a solid, hard mass that was macerated for 20 min. Subsequently, the sample was diluted in 2% acetic acid in a proportion of 6.5 mg of CNP in 9.6 mL of aqueous acetic acid solution. For the analysis, the CNP solutions were passed through a 450 nm filter, followed by another filter with 220 nm pores.

Anthocyanin was extracted from the leaves of Tradescantia pallida Purpurea collected from the city of Uberlândia (state of Minas Gerais, Brazil) with a similar size (11.7 ± 0.2) cm and color. The leaves were washed with tap and distilled water [20,21]. Afterward, approximately 20 leaves were cut into thin strips and dehydrated for 1.5 h at 400 W (54 ± 3) °C. An analytical balance (Model AUW220D—Shimadzu Brazil, Barueri, SP, Brazil) was used to determine the powder mass (8 g). Next, the leaves were manually macerated (at room temperature) using a mortar and pestle for approximately twenty minutes with 20 mL of distilled water. The mixture was then sieved and filtered twice with a paper filter to obtain the dye solution. The aqueous dye was centrifuged for 1.5 h at 7200 rpm (BioPet Model 8011154, Biosigma, SP, Brazil). Then, the samples were stored and refrigerated (~5 °C).

The CNP aqueous solution (0.11 g/mL) was called the S_CNP sample, the S_anth sample is an anthocyanin aqueous solution (0.4 g/mL), and for CNP embedded in anthocyanin in aqueous solutions, the ratios used were 5.2, 12.09, and 21.57 mass percentage composition (% by mass) of CNPs for samples called S₁, S₂, and S₃, respectively.

2.2. Spectroscopic and Optical Techniques

Absorbance measurements were performed with a 2 mm quartz cuvette using an Evolution^TM 201 UV-visible spectrometer (Model UV-1650 PC, Shimadzu). Fluorescence spectra (with excitation wavelength at 514.5 nm from argon laser at power P_e = 40 mW) were obtained using a portable spectrometer from Thorlabs. Fluorescence in the function of wavelength (330–560 nm) was performed using Shimadzu RF-5301 PC Spectrofluorophotometer. Fourier transform infrared (FTIR) spectroscopy was performed using a Perkin-Elmer Frontier spectrometer (Perkin-Elmer, Waltham, MA, USA) with a resolution of 2 cm⁻¹. Atomic force microscopy (AFM) imaging of the CNPs was recorded at room temperature with a scanning probe microscope (SPM-9600, Shimadzu). Time-resolved fluorescence lifetimes (TRFL) measurements were performed using a light source PLS 450 LED with λ = (460 ± 10) nm, 40 μW average power at 40 MHz and 800 ps. TRFL results for liquid samples were analyzed using the FluoTime 100 time-resolved fluorescence spectrometer from PicoQuant (Berlin, Germany) using a 1 cm thick quartz cuvette [20,30]. A Ludox solution was used as a scattering sample for prompt measurement [20,30,31]. X-ray powder diffraction (XRD) measurements (LabX XRD-6000 Shimadzu X-ray diffractometer) were performed using a copper target with a voltage of 40 KV and a current of 30 mA.

2.3. Thermal Lens (TL)

TL behavior occurs when an excitation laser beam (with wavelength λ_e) passes through a liquid sample (cuvette with thickness L), and the absorbed energy is converted into heat. The solution’s refractive index is modified by heating, which causes a thermally induced phase change [32,33]. In the continuous wave (CW) excitation regime, the variation in the probe beam (with wavelength λ_p) on-axis intensity I(t) at the central part of the probe laser beam can be expressed as follows [32]:

$$\mathrm{I}\left(\mathrm{t}\right)=\mathrm{I}\left(0\right){\left[1-{\displaystyle \frac{\mathsf{\theta }}{2}}{\tan }^{-1}\left({\displaystyle \frac{2 \mathrm{m}\mathrm{V}}{\frac{\left[{\left(1+2\mathrm{m}\right)}^2+{\mathrm{V}}^2\right]{\mathsf{\tau }}_{\mathrm{c}}}{2\mathrm{t}}+1+2\mathrm{m}+{\mathrm{V}}^2}}\right)\right]}^2,$$

(1)

where I(0) is the on-axis intensity when t or θ is zero. The parameter τ_c = w_e²/4D is the characteristic thermal time constant, where D = K/ρC is the thermal diffusivity (cm²/s), K is the thermal conductivity (W/cmK), ρ is the density (g/cm³), and C is the specific heat (J/gK). The parameter m = (w_p/w_e)², where w_e and w_p are the excitation and probe beam radii at the sample position, respectively. Here, the parameter V ≈ z₁/z_c when z_c << z₂, where z₂ (cm) is the distance between the sample and TL detector, z₁ is the distance between the sample and probe beam waist, and z_c = πw_op²/λ_p is the confocal distance of the probe beam, where w_op is the probe beam radius at the focus with wavelength λ_p.

In the dual-beam mode-mismatched configuration (pump and probe beams), the TL transient signal amplitude (θ), induced by the pump beam, is approximately the phase difference of the probe beam between r = 0 and r = $\sqrt{2}$w_e, expressed as follows [30,34]:

$$\mathsf{\theta }=-{\displaystyle \frac{{\mathrm{P}}_{\mathrm{e}}\mathsf{\alpha }{\mathrm{L}}_{\mathrm{e}\mathrm{f}\mathrm{f}}}{\mathrm{K}{\mathsf{\lambda }}_{\mathrm{p}}}}\mathsf{\phi }{\displaystyle \frac{\mathrm{d}\mathrm{n}}{\mathrm{d}\mathrm{T}}}$$

(2)

where P_e is the power of the excitation beam, α (cm⁻¹) is the optical absorption coefficient at the excitation wavelength (λ_e), L_eff = (1 − Exp(−αL))/α is the sample effective length, dn/dT is the temperature coefficient of refractive index and φ is the fraction of photon energy that is converted into heat. The propagation of a probe laser beam through the TL will result in either the spreading (dn/dT < 0) or focusing (dn/dT > 0) of the beam. The fluorescence quantum efficiency (η) or quantum yield, could be determined in this form:

$$\mathsf{\eta }=(1-\mathsf{\phi })<{\mathsf{\lambda }}_{\mathrm{e}\mathrm{m}}>/{\mathsf{\lambda }}_{\mathrm{e}}$$

(3)

where <λ_em> is the average emission wavelength. In this study, a diode laser at 532 nm and an He-Ne laser at 632.8 nm were used as excitation and probe beams, respectively. The parameters V = 1.74 and m = 7.87 were used in mode-mismatched configuration.

3. Results and Discussions

Figure 1 presents the absorbance spectra for CNP aqueous solutions (S_CNP), natural dyes inserted in CNP solutions (S₁, S₂, and S₃), and natural dyes (S_anth) extracted from the leaves of Tradescantia pallida Purpurea. For CNP, the band at ~280 nm is typical that of carbon dots obtained from different materials, and this band is attributed to be π-π* conjugate transition of the C=C sp² bond in the carbon core of chitosan-based CNP [7,9]. On the other hand, the typical absorption bands for natural dye were observed at (509 ± 1), (544 ± 1), and (585 ± 1) nm (Figure 1b–e). These bands agree with the values reported for the anthocyanin natural dye extracted from Tradescantia pallida in different solvents, such as aqueous solutions, ethanol, and acetone [21,35,36]. The typical absorption bands observed in the (500–590) nm region, mainly at 585 nm, are attributed to B-ring substituted anthocyanins with a quinonoidal base structure [36,37,38]. The B-ring substituted anthocyanins completely converted to the quinoidal base are reported at pH~5.5 [36,37], and this structure is associated with color stability proprieties due to complex acylation patterns [38]. In addition, Figure 1 presents the fluorescence of the chitosan-based carbon dots nanoparticles S_CNP (Figure 1f), and the peak fluorescence is ~583 nm when the sample is excited at 514.5 nm. Figure 2 presents the fluorescence spectra for S_CNP at different excitation wavelengths (λ_e = 330–560 nm). The inset presents normalized fluorescence spectra, and a typical fluorescence shift is reported for carbon dots [7,9,10,11]. This excitation-dependent relationship of the fluorescence spectra is attributed to defects in the energy levels produced by surface passivation, and the fluorescence due to recombination radiation from excitons trapped by the defects and multiple transition modes [7,9,11].

Figure 3a,b presents the AFM results for the chitosan-based carbon nanoparticles S_CNP and the height distributions of size measured, respectively. The average size distribution of the CNPs obtained by AFM measurement (Figure 3b) is ~6.0 nm. Figure 3c presents the XRD analysis of chitosan powder and chitosan-based carbon nanoparticles (S_CNP) powder. The main crystalline peaks of chitosan occur at 2θ = 20.3° and an amorphous shoulder at 11.7° [7,10]. After hydrothermal–pyrolysis carbonization, a broadened main peak of ~20° is observed, and the crystallinity of the material decreases due to its amorphous nature [7,10]. For carbon dots synthesized using dextrin, a single peak observed at ~24° was attributed to the graphitic nature of the carbon [39]. CNPs obtained from Trapa bispinosa peel as carbon research presented the XRD peaks at 2θ = 24.7° and 2θ = 43.3° that were attributed to (002) and (101) diffraction patterns of graphitic carbon, respectively [40]. For carbon dots obtained from banana juice a broad peak at 21.1° due to (002) peak is reported [41], in this case, the interlayer spacing of the carbon dots ~0.42 nm was obtained, higher than that of the graphitic interlayer spacing (~0.33 nm). A larger spacing interlayer than that of bulk graphite has been attributed to poor crystallization [41,42].

Figure 4 presents the fluorescence spectra at an excitation wavelength of λ_e = 514.5 nm for the S_CNP, S_anth, and S₁, S₂, and S₃ samples. The values of <λ_em> for each natural dye sample (b–e) are presented in

Table 1. TL results for chitosan-based carbon dot nanoparticles embedded in natural dye (S₁, S₂, and S₃) and anthocyanin (S_anth).

Sample	<λem> (±1 nm) 1	D (10−7 m2/s)	Θ (W−1)	η
S1	653	(1.42 ± 0.07)	(180 ± 10)	(0.31 ± 0.02)
S2	650	(1.41 ± 0.09)	(180 ± 10)	(0.31 ± 0.02)
S3	648	(1.4 ± 0.1)	(160 ± 10)	(0.41 ± 0.03)
Santh	655	(1.41 ± 0.03)	(190 ± 20)	(0.26 ± 0.03)

¹ λ_e = 514.5 nm, and the average <λ_em> values were obtained using tree-independent fresh samples measurements.

. For S_CNP, <λ_em> = 618 nm was obtained. The peak emission wavelengths obtained for natural dye (S_anth) are positioned at 615 nm and 650 nm; for samples S₁, S₂, and S₃, the average peak emission values are localized at (613 ± 1) nm and (650 ± 2) nm. The first peak presents a displacement of 614 to 611 nm with an increased CNP amount. The fluorescence spectra of natural dye extracted from Tradescantia pallida Purpurea in aqueous solutions with different potentials of hydrogen (pH~4–8) and <λ_em> = (652 ± 3) nm in this pH range were reported [20,21].

Fourier transform infrared (FTIR) spectra for the CNP derived from chitosan, natural dyes (S_anth) extracted from Tradescantia leaves, and S₁ and S₃ samples are presented in Figure 5. The FTIR spectra show a broad band in the 3500–3100 cm⁻¹ region for the samples analyzed. In this region, the characteristic O−H stretching absorption band (peak at 3342 cm⁻¹) overlaps with the N−H stretching vibrations of the amino group (peak at 3257 cm⁻¹) [7,43,44]. The 2925 and 2880 cm⁻¹ peaks correspond to the symmetric and asymmetric vibrations of CH₂, respectively [45]. These peaks can indicate the presence of carbon chains in the CNP structure [45], and in this region are more evident in the CNP samples than in the anthocyanin sample. The absorption peak at 1638 cm⁻¹ is attributed to the C=O (or C=N) vibrations, characterizing the amide functional group vibration [7,45,46,47,48]. This absorption peak at 1638 cm⁻¹ is only evident in the CNP sample. The absorption peak at 1558 cm⁻¹ corresponds to the stretching vibrations in the aromatic rings of carbonyl groups (C=C) or (C=O) for natural dye and it is attributed to the stretching of (C−N) for CNP [20,21,49]. The peak at 1558 cm⁻¹ appears in both samples but is much more intense in the CNP sample. The peak at (1462–1340) cm⁻¹ corresponds to the C−N stretching vibration [48]. This band indicates the presence of nitrogen on the surface of the CNPs [50]. In our samples, a peak was observed at 1420 cm⁻¹. Additionally, the peak at 1388 cm⁻¹ corresponds to the stretching vibrations of C−H rotation [51]. Finally, the 1081 and 1050 peaks can be attributed to the −C−O group [52,53], and 1023 cm⁻¹ peak to the (C−H or C−O) radical group [45]. These peaks suggest that the CNPs and anthocyanins have excellent hydrophility and that the samples can be well dispersed in water due to the presence of −OH and −COOH groups on their surface [53].

On the other hand, for the S_anth powder extracted in distilled water, the main vibrational modes were observed at 3342, 1558, 1388, and 1081 cm⁻¹. These bands agree with the peaks reported in references [20,21]. The bands at 823 cm⁻¹ and 656 cm⁻¹ are attributed to ring vibrations C=C−C and to the presence of aromatic C−H bonds [54,55]. In addition, the natural dyes extracted from Tradescantia pallida Purpurea were analyzed using a high-performance liquid chromatography system coupled with a quadrupole time-of-flight high-resolution mass spectrometer, as described in reference Lima et al. [21]. The anthocyanin structures consist mainly of cyanidin, three glucose molecules, arabinose and three ferulic molecules [21,28,29].

Figure 6a shows a typical TL transient signal for the S₃ sample that was fitted using Equation (1), which supplies the TL amplitude θ and τ_c values. As dn/dT is negative, the TL effect defocuses the probe beam in the far field. The average thermal parameters Θ = −θ/PαL_eff of the S₃ were calculated, and the results are presented for all samples in Table 1. For S_CNP, the value of the Θ = (59 ± 8) W⁻¹ was determined. Using Equations (2) and (3), the K parameter for aqueous solutions [56], dn/dT = −(0.92 ± 0.03) × 10⁻⁴ K⁻¹ (at 22 °C) determined for water [56,57], <λ_em> values from Table 1, and λ_e = 532 nm, the values of the radiative quantum yield η were determined for all samples (Table 1). Figure 6b presents the Θ and η values in the function of natural dye and CNPs (% in mass). The η value obtained for natural dye is in good agreement with the values reported for anthocyanin extracted as a function of the season and several potential hydrogens [20,21]. For samples S₁–S₃, the average fluorescence quantum yields increased by ~32% concerning the value for natural dye alone. High fluorescence quantum yields were determined for the chitosan-based carbon dot nanoparticle-embedded natural dye (Table 1). Therefore, the improvement in the η value is related to the increase in the carbon dots concentration (5.2–21.6% by mass of CNPs). This result is potentially important, because the increase in fluorescence of the anthocyanin-carbon dots nanoparticles enhances applications involving the fluorescence of these materials and, on the other side, benefits from the eventual fungicidal effect of the natural dye [20,21]. The η values for several carbon-based nanoparticles synthesized using different carbon sources [58,59,60,61] are presented in

Table 2. The τ and η values for several carbon-based nanoparticles, which were synthesized using different carbon sources.

Carbon Source	Sample Name	τ (ns)	η (%)
Bovine Serum Albumin [58]	FC-NPs-2	7.6	16.5
Jinhua Bergamot [59]	C-dots	3.84	50.78
Grape Seed [60]	sGQDs	10.04	31.79
Tamarindus indica Leaves [61]	CQDs	5.98	46.6
Chitosan (this work)	S3 CNPs	(3.65 ± 0.04)	(41 ± 3)

for comparison. However, for chitosan fluorescent nanoparticles subjected to thermal treatment at 150 °C, a value of η = 0.15 was reported [6]; for CNP by hydrothermal carbonization of chitosan at 180 °C, η = 0.43 [7], and η = 0.07 were reported for chitosan-based carbon dots [9]. From τ_c and using τ_c = w_e²/4D, the D values were determined (Table 1). For S_CNP, the value determined was D = (1.42 ± 0.07) 10⁻⁷ m²/s, which is in good agreement with the results for aqueous solutions [30,35]. Furthermore, for S_CNP the estimated value of η using TL is 0.8. For comparison, η higher than 0.7 has been reported for carbon-based dots co-doped with nitrogen and sulfur (N, S-CDs) [62]. According to recent studies, a value as high as η = 0.88 has been reported for orange juice as a carbon source using hydrothermal treatment for the carbon dots synthesis process [63].

Time-resolved fluorescence (TRFL) spectroscopy measurements were also used for chitosan-based carbon dot nanoparticles and natural dyes (Figure 7). TRFL measurements of the excited states were performed, and the experimental results were adjusted for samples S₁, S₂, S₃, and S_anth using the single exponential B + A₁$·$Exp (−t/τ₁), where A₁ and B are constants and τ₁ is the fluorescence lifetime. The TRFL setup was tested using sodium fluorescein (Synth) in an alkaline aqueous solution (pH 11.24), and the transient emission was adjusted with a single exponential (τ₁ = (4.13 ± 0.01) ns and χ² = (1.03 ± 0. 01)) [20,30,31]. For S_CNP, the TRFL measurements were adjusted using two exponential B + A₁$·$exp (−t/τ₁) + A₂$·$exp (−t/τ₂), and the average fluorescence lifetime τ_AVE was determined using the following expression [47,58,64]:

$${\mathsf{\tau }}_{\mathrm{A}\mathrm{V}\mathrm{E}}={\displaystyle \frac{{\mathrm{A}}_1{\mathsf{\tau }}_1^2+{\mathrm{A}}_2{\mathsf{\tau }}_2^2}{{\mathrm{A}}_1{\mathsf{\tau }}_1+{\mathrm{A}}_2{\mathsf{\tau }}_2}}$$

(4)

where τ_AVE is the amplitude-weighted average lifetime of fluorescence, and τ₁ and τ₂ are the time constants of the fast and slow decay processes, respectively. A₁ and A₂ are the corresponding fractional values of the fluorescence intensities. The average TL and TRFL results were obtained using three independent sets of fresh samples, and each set of measurements was repeated approximately seven times.

Figure 7 shows the TRFL for aqueous chitosan-based carbon dots nanoparticles, S_CNP, for CNPs embedded in natural dye in different proportions (S₁, S₂ and S₃), and the natural dye S_anth extracted from ornamental leaves of Tradescantia pallida Purpurea. The fluorescence decays were well fitted with a single exponential for samples S₁, S₂, S_3, and S_anth, and the χ² value was less than 1.20 for all samples (

Table 3. Time-resolved fluorescent results for chitosan-based carbon dot nanoparticles S_CNP, S₁, S₂, S₃, and natural dye S_anth.

Sample	τ1 (ns) (A1 %)	τ2 (ns) (A2 %)	χ2	τAVE (ns)
SCNP	1.4 ± 0.2 (73.68)	6.2 ± 0.3 (26.32)	1.00 ± 0.01	4.34
S1	3.58 ± 0.05 (100)	-	1.01 ± 0.01	-
S2	3.71 ± 0.04 (100)	-	1.18 ± 0.08	-
S3	3.65 ± 0.04 (100)	-	1.20 ± 0.06	-
Santh	3.2 ± 0.2 (100)	-	1.03 ± 0.05	-

). For S_CNP, two exponentials were used for fluorescence decay fitting, and the average fluorescence lifetime τ_AVE was calculated using Equation (4) (Table 3). Three exponentials have been reported in the literature for functionalizing chitosan−carbon dots [47], and the multiple lifetime components were attributed to possible several emission species on the surface of the carbon dots [47]. The fluorescence lifetime and η parameters presented a discrete dependence on the ratios of anthocyanin-carbon dots nanoparticles (5.2–21.6% by mass of CNPs) used. For comparison, typical probes such as fluorescein, rhodamine 6G and bacteriochlorophyll, some fluorescent proteins, and natural dyes exhibit lifetimes in the range 1–4 ns [20,30,31,65]. In addition, τ values are presented in Table 2 for several carbon-based nanoparticles synthesized using different carbon sources, such as bovine serum albumin, Jinhua bergamot, grape seed, and Tamarindus indica leaves [58,59,60,61]. The η values and nanosecond lifetimes of carbon-based nanomaterials have potential for biological and optoelectronic applications [47,66]. However, the green synthesis of fluorescent carbon dots from raw materials used as carbon sources (such as seeds, flowers, fruits, extracts, and foods) has been in increasing development for several applications, including textile engineering and degrading dyes used in textile fabrics [67,68,69]. Other relevant applications of carbon dots synthesized and embedded into curcumin natural dye were reported, including their use as low-cost bioimaging agents and their antimicrobial activity [70]. Carbon dots synthesized from Curcuma longa are reported as probes for detecting triazophos pesticide [71]. In addition, a system based on carbon dots and curcumin was developed for the fluorescence turn-on detection of fluoride ions (F⁻) [72].

4. Conclusions

The fluorescence quantum yield (η) and the fluorescence lifetime (τ) parameters were determined for synthesized carbon-based nanoparticles (CNPs) with an approximate height distribution of ~6 nm and embedded in aqueous natural dye extracted from the leaves of Tradescantia pallida Purpurea (ratios of 5.2, 12.09, and 21.57% by mass of CNPs for S₁, S₂ and S₃, respectively). Up to 32% higher η average values were obtained for CNP embedded in natural dye samples than for anthocyanin (S_anth). The nanosecond lifetime was obtained with an average τ = 3.65 ns for CNP embedded in natural dye. These results highlight the synthesized nanomaterials embedded in anthocyanin for potential applications in enhancing food coloration, smart functional food packaging, and extending food shelf life.