Introduction

Carbon is a key element in organic matter, giving carbon-based nanomaterials inherent biocompatibility, sustainability, availability, and low cost1. Advances in recent years have led to the development of diverse carbon nanomaterials such as fullerenes, carbon nanotubes (CNTs), graphene, and nanodiamonds. These materials have gained attention for their nanoscale structures and exceptional physicochemical properties, especially in microelectronics.

However, their practical use is limited by challenges such as difficult synthesis (e.g., nanodiamonds), poor solubility (CNTs, graphene, fullerenes), and weak visible-light luminescence, which restricts their applications in optoelectronics and bioimaging1.

Carbon quantum dots (CQDs) have emerged as a promising alternative. These zero-dimensional, fluorescent nanoparticles (typically \(<10\) nm) exhibit strong luminescence, water solubility, chemical stability, low toxicity, and tunable optical properties2. Discovered in 2004 and formally synthesized in 20063,4, CQDs address many limitations of traditional carbon nanomaterials. Their abundant surface functional groups (e.g., –OH, –COOH, –NH2) and size-dependent electronic properties make them suitable for bioimaging, drug delivery, photocatalysis, energy devices, and more5,6,7.

CQDs are typically synthesized using top-down (e.g., laser ablation, arc discharge) or bottom-up (e.g., hydrothermal, solvothermal) methods8,9,10. While top-down techniques are often costly and complex, bottom-up methods, especially hydrothermal synthesis, are preferred for their scalability, cost-effectiveness, and compatibility with biomass precursors11.

Hydrothermal methods enable the green synthesis of CQDs from eco-friendly sources like banana peels, human hair, and milk, yielding comparable properties to conventionally produced CQDs12,13,14,15. These biomass sources often contain naturally occurring heteroatoms, enriching CQDs with surface functionalities without the need for doping16. Consequently, biomass-derived CQDs are gaining attention for their environmental sustainability and wide-ranging applications in bioimaging, sensing, and energy storage17,18,19,20.

Orange peels (Citrus sinensis), a byproduct of the widely cultivated citrus industry for juice, essential oils, and medicinal products, are rich in bioactive compounds, including vitamin C, phenolic compounds, and polysaccharides21. These abundant agricultural residues have been increasingly repurposed for the synthesis of advanced functional materials, such as activated carbon, nanocarbons, bioplastics, and metal nanoparticles22,23,24,25. The valorization of orange peel waste into high-performance materials not only provides a sustainable approach to agricultural waste management but also advances green material innovation. Notably, the synthesis of CQDs from orange peels has been documented in several studies, demonstrating their potential as fluorescent nanomaterials with applications in sensing, bioimaging, and optoelectronics12,26,27,28.

In this study, CQDs were successfully synthesized from carbonized orange peel waste using a facile, environmentally friendly hydrothermal approach, yielding materials with enhanced quantum efficiency and stable optical characteristics. The structural, chemical, and optical properties of the CQDs were thoroughly characterized using transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectroscopy, ultraviolet-visible (UV-Vis) spectroscopy, steady-state photoluminescence (PL), and time-resolved photoluminescence (TRPL) spectroscopy. Multi-Gaussian fitting was applied to the PL spectra to deconvolute emission profiles, revealing the zero-phonon line (ZPL) and phonon sidebands, which correlate closely with the carbonyl (C=O) stretching mode at approximately 0.21 eV, as identified by FT-IR. The Huang-Rhys factor, calculated through both PL spectral analysis and Stokes shift, quantifies the electron-phonon coupling, providing insights into the vibronic interactions and recombination dynamics of the CQDs.

Results and discussion

Characterization of the CQDs

The morphological characterization of the CQDs was analyzed using TEM, and the particle size distribution is shown in Fig. 1(a–b). The results indicate that the CQDs are primarily monodispersed and spherical, with particle sizes ranging from 2 to 8 nm and the mean particle size of \(4.58\pm 1.37\) nm. In addition, the images reveal a tendency for the CQDs to aggregate, forming clusters that increase their overall size. This is a characteristic behavior commonly observed in carbon-based materials29.

To elucidate the surface chemistry, functional groups, and molecular structure of the CQDs, FT-IR spectroscopy was performed across the wavenumber range of 4000–500 \(\text {cm}^{-1}\). Figure 1(c) displays the FT-IR spectra of the carbonized orange peel precursor and the CQDs obtained via two phase-transfer methods, namely freeze-drying and hot-drying. In this study, hot-drying was selected as the primary method owing to its simplicity and practicality.

The spectra indicate that the synthesized CQDs retain characteristic functional groups originating from the orange peel precursor, including –OH, –NH, and –C=C vibrations at 3600–3200 \(\text {cm}^{-1}\) and 993 \(\text {cm}^{-1}\), consistent with those reported for carbonized biomass and graphitic domains30.

A comparative analysis of the two phase-transfer techniques reveals that freeze-drying enhances the resolution of distinctive CQD absorption bands, notably C-N stretching vibration at 1186 \(\text {cm}^{-1}\), C–O–C at 1351 \(\text {cm}^{-1}\), C=O at 1693 \(\text {cm}^{-1}\), and the broad –OH/–NH stretching at 3600–3200 \(\text {cm}^{-1}\). Nevertheless, the hot-drying method also preserves these functional moieties, yielding spectra that confirm the retention of essential bonding features. Taken together, these results suggest that while freeze-drying may improve spectral clarity, hot-drying offers a straightforward and efficient strategy for phase transfer, with minimal loss of functional group integrity in CQDs.

Fig. 1
Fig. 1
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(a) Representative TEM image of the CQDs. (b) Size histogram of the CQDs with curve fitted to the data using a Gaussian model. (c) FT-IR spectra of carbonized orange peel and respective CQDs.

The experimental UV–Vis absorption spectrum of the CQDs, shown in Fig. 2(a), exhibits two prominent absorption peaks at 240 nm and 338 nm, corresponding to the \(\pi\)–\(\pi ^{*}\) transition of conjugated C=C bonds within the aromatic sp\(^{2}\) domains and the n–\(\pi ^{*}\) transition of carbonyl (C=O) groups on the CQD surface, respectively31,32. A weak shoulder is also observed around 450 nm, a characteristic feature commonly reported for nitrogen- and oxygen-rich CQDs synthesized via hydrothermal carbonization.

In biomass-derived CQDs, particularly those prepared from orange peel with citric acid and ammonia, the shoulder near 450 nm can be attributed to electronic transitions associated with surface defect states and extended \(\pi\)-conjugation networks. Nitrogen incorporation from ammonia leads to surface functionalities such as C–N, C=N, and N–C=O, which introduce mid-gap energy levels within the carbon framework. The presence of the C–N stretching vibration at 1186 \(\text {cm}^{-1}\) in the FT-IR spectrum further confirms successful nitrogen incorporation. These nitrogen-related states facilitate n–\(\pi ^{*}\) transitions at lower energies, resulting in the broad absorption tail in the visible region33,34,35. Additionally, partial carbonization of the orange peel precursor may generate residual aromatic clusters with extended conjugation, further contributing to low-energy optical absorption36.

Therefore, the small shoulder at approximately 450 nm most likely originates from a combination of n–\(\pi ^{*}\) transitions involving nitrogen- and oxygen-containing surface groups and localized \(\pi\)–\(\pi ^{*}\) transitions in residual aromatic domains. Such features are typical of hydrothermally synthesized, biomass-derived CQDs, reflecting their complex hybrid electronic structure governed by both core conjugation and surface chemistry.

Fig. 2
Fig. 2
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(a) UV–Vis absorption spectra of the CQD dispersions. (b) Photoluminescence (PL) spectra recorded at various concentrations under excitation at \(\lambda _\text {exc}=320\) nm. (c, d) Variation of peak absorbance, integrated PL intensity, Gaussian peak energies, and corresponding full width at half maximum (FWHM) parameters—obtained from triple-Gaussian fitting of the PL spectra—as a function of CQD concentration.

The CQD solution exhibited prominent light blue fluorescence under ultraviolet (UV) irradiation at 365 nm, as shown in the inset of Fig. 2(b). The PL spectra at various concentrations are presented in Fig. 2(b), where the emission peak displays a slight redshift from 410 to 418 nm (Fig. 2(d)). However, this shift is marginal and not strongly pronounced.

Figure 2(c) shows the relationship between maximum absorbance and integrated PL intensity of CQDs at different concentrations (0.02–0.24 mg/mL). Varying the concentration allows investigation of how particle density influences optical behavior and helps distinguish intrinsic photophysical properties from concentration-dependent effects. As shown, the peak absorbance (black squares) increases linearly with concentration, consistent with previous studies12,15, while the integrated PL intensity (red triangles) displays a non-monotonic trend—rising to a maximum at 0.1 mg/mL before decreasing at higher concentrations.

This decrease in PL intensity originates from two main mechanisms. First, aggregation-induced quenching (AIQ) occurs through \(\pi\)–\(\pi\) stacking interactions and reduced transition energies in aggregated conjugated domains, leading to fluorescence quenching37. Second, reabsorption (inner-filter) effects become significant at higher concentrations, where photons emitted by smaller CQDs are reabsorbed by larger ones with overlapping absorption spectra. Due to quantum confinement, the larger CQDs subsequently emit lower-energy photons38,39,40. This size-dependent spectral overlap enhances reabsorption and, together with aggregation, accounts for the observed PL quenching at elevated concentrations. Such behavior is consistent with established photophysical trends in both carbon-based and semiconductor quantum dots, including CdTe QDs, where reabsorption rather than intrinsic nonradiative decay dominates the concentration-dependent PL decrease41,42.

The PL and photoluminescence excitation (PLE) spectra of the CQDs were thoroughly analyzed, with excitation and emission wavelengths fixed at \(\lambda _\text {exc}=360\) nm and \(\lambda _\text {em}=432\) nm, respectively, corresponding to the characteristic spectral peaks (Fig. 3). The analysis revealed a notable discrepancy between the absorption profile of the CQDs and their excitation spectrum, suggesting distinct underlying optical behaviors. Specifically, the peak at 240 nm in the PLE spectrum is precisely aligned with the primary absorption peak in Fig. 2(a). However, the secondary emission peak demonstrates a 23 nm blueshift relative to the second absorption peak, indicating subtle variations in the electronic transition mechanisms within the CQDs.

The broad emission and asymmetry in PL spectra of CQDs observed in Figs. 2(b) and 3 are contributed by several factors, such as size heterogeneity, surface states, electron-phonon coupling, and excitation-dependent emission4,31,33,43,44,45,46. To clarify these features, the PL spectrum excited at \(\lambda _\text {exc}=360\) nm is fitted with multi-Gaussian models to determine the contribution of these factors accurately. Table 1 summarizes the peak photon energies obtained from fitting the PL spectrum in the energy domain using double-Gaussian, triple-Gaussian, and quadruple-Gaussian models, as detailed in Figure S1. The double-Gaussian model yields an inadequate fit to the experimental PL spectra of CQDs, achieving an \(R^2\) value of 0.9993. In contrast, the quadruple-Gaussian model provides a near-optimal fit with an \(R^2\) of 0.99997; however, its fourth Gaussian component is negligible in most experimental datasets, suggesting overparameterization. Consequently, the triple-Gaussian model, with an \(R^2\) of 0.99993, is identified as the most suitable for fitting the experimental data due to its balance of accuracy and simplicity, as well as the symmetry of its Gaussian components around the central peak at energy \(E_\text {p2}\), which corresponds to the zero-phonon line (ZPL).

The energy separations between Gaussian components obtained from the double-, triple-, and quadruple-Gaussian fitting models are summarized in Table 1. In the triple-Gaussian model, the energy differences of 0.211 eV (\(\Delta E_{12}=E_\text {p2}-E_\text {p1}\)) and 0.207 eV (\(\Delta E_{23}=E_\text {p3}-E_\text {p2}\)) closely match the local phonon energy of \(\hbar \omega = 0.21\) eV, corresponding to the C=O stretching vibration at 1693 \(\text {cm}^{-1}\) identified in the FT-IR spectrum (Fig. 1(c)). This strong correspondence indicates that the additional peaks at \(E_\text {p1}\) and \(E_\text {p3}\) arise from phonon sidebands associated with electron–phonon coupling, consistent with the model proposed by Han and Bester (2022)47. These sidebands manifest as broader features adjacent to the zero-phonon line (ZPL), reflecting simultaneous electronic transitions accompanied by phonon emission or absorption.

Similar phonon-assisted luminescence behavior has been widely observed in both carbon-based and semiconductor quantum dots. Yu et al. (2012) systematically investigated the temperature-dependent fluorescence of carbon dots and demonstrated that the redshift and quenching of emission intensity with increasing temperature originate from thermally activated nonradiative processes governed by electron–phonon interactions31. Their analysis revealed that the coupling between electronic excited states and phonon modes dictates the thermal modulation of emission energy and linewidth, while the strong electron–electron interactions combined with weak electron–phonon interactions account for the unusually broad PL band observed even at 77 K. This finding highlights the complex interplay between electronic correlation and lattice vibrations in carbon-based systems. Similarly, Wang et al. (2022) attributed the red emission of o-phenylenediamine-derived carbon dots to electron–phonon coupling–assisted radiative recombination, whereas Kelley (2019) and Khosla et al. (2018) reported that exciton–phonon coupling and polaron formation give rise to spectral sidebands and temperature-dependent broadening in II–VI semiconductor nanocrystals46,48,49. Collectively, these studies suggest that electron–phonon coupling, alongside size heterogeneity and surface-related emissive states, plays an intrinsic role in shaping the broad and asymmetric PL profiles of carbon quantum dots, consistent with the observations in this work.

Therefore, the triple-Gaussian model provides a physically meaningful description of the CQDs’ emission behavior, where the central peak (\(E_\text {p2}\)) corresponds to the primary electronic transition and the adjacent peaks (\(E_\text {p1}\), \(E_\text {p3}\)) represent phonon replicas. The close agreement between the experimentally derived energy separations and the characteristic C=O phonon energy further substantiates this interpretation, validating the model’s reliability in capturing the vibronic coupling–driven luminescence characteristics of the CQDs.

Table 1 Photon energies associated with the resolved Gaussian components determined from multi-Gaussian fitting analysis of the PL spectra.
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The Huang-Rhys factor describes the strength of the coupling between electronic transitions and vibrational (phonon) modes, which is particularly relevant for understanding the broad or structured PL spectra of CQDs. The Huang-Rhys factor (S) for the CQDs was calculated using two distinct methods: analysis of the PL spectrum and evaluation of the Stokes shift. In the first method, S was calculated as \(S \approx \frac{I_1 + I_3}{I_2}\), where \(I_1\), \(I_2\), and \(I_3\) represent the integrated intensities of the phonon sidebands and the ZPL at energies \(E_\text {p1}\), \(E_\text {p2}\), and \(E_\text {p3}\), respectively, yielding \(S = 1.5\). In the second method, the Stokes shift (\(\Delta E_\text {Stokes}\)) was derived from the absorption maximum at 340 nm (see Fig. 2(a)) and emission maximum at 419 nm (PL spectrum excited at 340 nm in Fig. 4(a)), corresponding to energies of 3.647 eV and 2.959 eV, respectively, giving \(\Delta E_\text {Stokes}=0.688 \, \text {eV}\). Using the phonon energy \(\hbar \omega = 0.21 \, \text {eV}\), as derived from the carbonyl (C=O) stretching mode at 1693 cm\(^{-1}\), the Huang-Rhys factor was estimated as \(S\approx \frac{\Delta E_\text {Stokes}}{2\hbar \omega }=1.64\). Both methods confirm significant electron-phonon coupling in CQDs, with the triple-Gaussian PL analysis providing a more precise S value due to its direct deconvolution of spectral features, which captures the contributions of both phonon sidebands and the ZPL.

Fig. 3
Fig. 3
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Normalized photoluminescence (PL) spectra of CQDs obtained under an excitation wavelength of \(\lambda _\text {exc} = 360\) nm, together with the corresponding normalized photoluminescence excitation (PLE) spectra measured at an emission wavelength of \(\lambda _\text {em} = 432\) nm.

Notably, the broad and asymmetric features of the PL spectra of CQDs exhibit remarkable consistency across multiple synthesis batches. The normalized PL spectra, excited at 320 nm, for CQD samples obtained at four different synthesis durations are depicted in Figure S2(a). Subsequent analysis demonstrates negligible variations in the peak energies (\(E_\text {p}\)) and FWHM parameters, as shown in Figures S2(b) and S2(c). In particular, the mean central peak energy (\(E_\text {p2}\)) is \(2.917 \pm 0.020\) eV, with a corresponding average FWHM of \(0.508 \pm 0.014\) eV, as summarized in Table S1. The average energy separations of \(E_\text {p1}\) and \(E_\text {p3}\) relative to \(E_\text {p2}\) are \(0.199 \pm 0.005\) eV and \(0.228 \pm 0.007\) eV, respectively. Furthermore, the Gaussian component at higher photon energy is characterized by a larger FWHM, whereas the converse holds for the lower-energy component, indicating an extension of the PL spectra toward the low-photon-energy (long-wavelength) region. These observations also highlight the robustness and reproducibility of the hydrothermal synthesis route employing biomass precursors, such as orange peel, thereby validating its efficacy in yielding CQDs with stable optical properties.

The triple-Gaussian model was employed to deconvolute the PL spectra of CQDs measured across a range of concentrations, providing insights into the concentration-dependent optical properties. Figure 2(d) illustrates the variation of Gaussian peak energies and corresponding FWHM parameters, derived from triple-Gaussian fitting of the PL spectra, as a function of CQD concentration. The peak energies and energy separations between the Gaussian components are detailed in Table S2. Notably, all Gaussian peak energies reach their maximum values at a concentration of 0.08 mg/mL. For instance, the central peak energy (\(E_\text {p2}\)), corresponding to the ZPL, increases from 2.850 eV at 0.02 mg/mL to a maximum of 2.923 eV at 0.08 mg/mL, while the associated FWHM decreases from 0.575 eV to 0.520 eV. The Gaussian 1 component, corresponding to a phonon sideband, demonstrates the most pronounced increase in peak energy (\(E_\text {p1}\)), rising from 2.518 eV at a CQD concentration of 0.02 mg/mL to 2.730 eV at 0.08 mg/mL, while its associated FWHM decreases from 0.862 eV to 0.675 eV. Beyond this concentration, both peak energies and FWHM parameters exhibit minimal variation, suggesting a saturation of concentration-dependent effects. The average energy separations between the phonon sidebands (\(E_\text {p1}\), \(E_\text {p3}\)) and the ZPL (\(E_\text {p2}\)) across all CQD concentrations are determined to be \(\Delta E_{12}=0.212 \pm 0.039 \, \text {eV}\) and \(\Delta E_{23}=0.238 \pm 0.006 \, \text {eV}\), respectively, as derived from the triple-Gaussian fitting results presented in Table S2. These values closely align with the characteristic phonon energy of approximately 0.21 eV, attributed to the carbonyl (C=O) stretching mode observed at 1693 cm\(^{-1}\) via FT-IR spectroscopy. The consistency of these energy separations underscores the role of electron-phonon coupling in shaping the PL spectra.

Further analysis, as depicted in Figure S3, reveals the dependence of the integrated emission intensities of the Gaussian components on CQD concentration. The Gaussian 2 component (ZPL) dominates the overall PL intensity and mirrors the trend of the total integrated measured PL intensity (Fig. 2(c)), indicating its primary role in radiative recombination. In contrast, the intensities of Gaussian 1 and Gaussian 3 (phonon sidebands) exhibit significant increases only within the concentration range of 0.02 mg/mL to 0.08 mg/mL, stabilizing at higher concentrations. This behavior suggests that at lower concentrations, the enhancement of phonon sideband intensities is driven by increased electron-phonon interactions, possibly due to changes in surface state populations or reduced reabsorption effects. The observed peak energy increase and FWHM reduction up to 0.08 mg/mL can be attributed to minimized reabsorption and aggregation effects at this optimal concentration, where the CQDs achieve a balance between radiative efficiency and electronic environment stability.

At higher concentrations (\(>0.08\) mg/mL), the negligible variation in peak energies and FWHM parameters may result from aggregation-induced quenching or saturation of surface state contributions, which limit further changes in the electronic structure50. The dominance of the ZPL intensity and the stability of energy separations highlight the robustness of the triple-Gaussian model in capturing the vibronic structure of CQD PL spectra across varying concentrations. These findings elucidate the complex interplay between concentration-dependent reabsorption, surface chemistry, and electron-phonon coupling.

Fig. 4
Fig. 4
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(a) Normalized photoluminescence (PL) spectra of the CQDs recorded at various excitation wavelengths. (b, c) Dependence of the emission wavelength and emission intensity on the excitation wavelength, respectively. The inset in (b) shows the corresponding CIE chromaticity coordinates for each PL spectrum. (d) Excitation–emission contour map of the CQDs synthesized from orange peels.

The excitation wavelength plays a crucial role in determining the PL characteristics of CQDs, significantly influencing both the emission intensity and wavelength. As illustrated in Fig. 4(a), the normalized PL spectra of CQDs under excitation wavelengths ranging from 320 to 460 nm exhibit a gradual color shift from blue to sky blue and finally to light green as the excitation wavelength increases. This spectral evolution reflects the combined effects of excitation-dependent emission and intrinsic variations in nanoparticle properties51. These variations arise primarily from differences in CQD size distribution, the presence of surface energy trap states, and the excitation-wavelength-dependent population of emissive states resulting from the varying absorption efficiency of CQDs at different excitation wavelengths.

To quantitatively evaluate the correlation between excitation and emission wavelengths, the peak emission wavelengths were plotted against their corresponding excitation wavelengths, as illustrated in Fig. 4(b). The resulting linear relationship, with a coefficient of determination (\(R^2 = 0.992\)) and a slope of 0.77, demonstrates a consistent red-shift in the emission wavelength with increasing excitation wavelength—quantified as \(\Delta \lambda _\text {em}/\Delta \lambda _\text {exc}\)—within the excitation range of 340–500 nm. This linear dependence can be ascribed to both the distribution of CQD particle sizes and the involvement of surface-related trap states and aromatic ring structures, which collectively influence the energy band structure and radiative transitions of CQDs. The observed trend is consistent with previous reports and further emphasizes the crucial role of surface chemistry and structural heterogeneity in governing the optical tunability of CQDs36,52,53,54,55.

In addition to spectral shifts, the intensity of PL emission also demonstrates strong excitation wavelength dependence. As the excitation wavelength increases from 280 nm to 320 nm, the peak emission intensity rises sharply from \(1.63\times 10^6\) counts per second (CPS) to a maximum of \(6.2\times 10^6\) CPS, while the peak emission wavelength increases insignificantly (Fig. 4(b)). Beyond 320 nm, however, the intensity progressively diminishes, reaching as low as \(0.11\times 10^6\) CPS at 500 nm (Fig. 4(c)). This trend further underscores the excitation-wavelength-dependent nature of CQD luminescence.

To further elucidate the influence of excitation wavelength on the broadening of PL spectra in CQDs, triple-Gaussian deconvolution was applied to all PL spectra. The dependence of emission peak energies and corresponding FWHM parameters on excitation wavelength is presented in Figure S4(a,b). The peak energies of the Gaussian components exhibit a pronounced increase as the excitation wavelength rises from 280 nm to 300 nm, followed by a gradual decrease at longer wavelengths. Within the excitation wavelength range of 300 nm to 400 nm, a distinct correlation is observed: Gaussian peaks at higher photon energies correspond to smaller FWHM values, while those at lower energies exhibit broader FWHM values. Specifically, Table S3 illustrates that, within the excitation wavelength range of 300 nm to 400 nm, the average energy separations between the phonon sideband peaks and the central ZPL are \(\Delta E_{12}=0.193 \pm 0.030\) eV and \(\Delta E_{23}=0.210 \pm 0.016\) eV, respectively, which are in close agreement with the characteristic phonon energy of approximately 0.21 eV associated with the carbonyl (C=O) stretching mode4,50,56,57. These findings underscore the significant influence of excitation energy on the luminescence behavior and recombination dynamics of CQDs, highlighting the sensitivity of their optical properties to variations in electronic environment and surface states. This behavior reflects the complex interplay between quantum confinement effects and surface chemistry, affirming the suitability of the triple-Gaussian model for accurately capturing the spectral characteristics of CQDs.

Figure 4(d) presents the excitation–emission contour plot of the CQDs, illustrating the dependence of their PL characteristics on excitation wavelength. The emission intensity is visualized through a color gradient ranging from red (highest intensity) to violet (lowest intensity), corresponding to excitation wavelengths between 300 and 430 nm and emission wavelengths from 360 to 550 nm. The highest PL intensity (\(>9\times 10^6\) CPS), represented by the dark red region, is observed within the excitation range of 320–348 nm and emission wavelengths of 416–446 nm, consistent with the peak identified in the PLE spectrum.

Beyond this optimal excitation–emission region, the emission intensity gradually decreases, indicated by the transition toward cooler color zones. Such excitation-dependent luminescence behavior has been widely reported for CQDs and is primarily governed by their hybrid core–shell structure and surface functionalities. In particular, surface groups such as C=O and C–H introduce localized emissive trap states that can be selectively activated under different excitation energies, resulting in tunable PL emission58,59,60. The broad emission range covering the visible spectrum (380–700 nm) further supports the potential of these CQDs for fluorescence sensing, photocatalysis, and photochemical applications.

The strong and tunable photoluminescence of the CQDs is attributed to the combined effects of sp\(^2\)– and sp\(^3\)–hybridized carbon domains, quantum confinement, and the presence of multiple emissive trap states. These effects become more pronounced with increasing excitation energy or CQD concentration, as electron–electron interactions and trap-state activation influence the recombination dynamics of charge carriers61,62.

To further confirm that the observed PL originates from the CQD structure rather than from residual molecular fluorophores such as citrazinic acid, pH-dependent PL measurements were conducted. As shown in Figure S5, the PL intensity gradually decreases under acidic conditions, retaining approximately 80% of its original value at pH 1–3. This trend contrasts sharply with the behavior of pure citrazinic acid, whose emission is almost completely quenched in strong acid due to protonation of its carboxyl and hydroxyl groups63. The partial retention of luminescence in our samples indicates that the emissive centers are primarily associated with the carbon core and surface passivation states rather than molecular fluorophores. Therefore, even though the CQDs were purified only by centrifugation and not by dialysis, the pH-dependent PL behavior provides strong evidence that the emission originates intrinsically from the carbon quantum dots.

The time-resolved photoluminescence (TRPL) measurement of the CQD sample is presented in Fig. 5, where the decay curves are plotted as normalized intensity. The solid lines in subfigures (a), (b), and (c) correspond to the best fits using single-, bi-, and tri-exponential models, respectively. The corresponding reduced \(\chi ^2\) values are \(9.896\times 10^{-5}\), \(2.343\times 10^{-5}\), and \(2.225\times 10^{-5}\), indicating a progressive improvement in fitting accuracy with the inclusion of additional decay components. As shown in Fig. 5, the tri-exponential model provides the most satisfactory agreement between the experimental data and the fitted curve, with minimal residual deviation. Therefore, the tri-exponential fitting was selected as the most appropriate model to describe the decay behavior, which is consistent with the presence of multiple emissive pathways in the CQD system. In Fig. 5(c), the magenta curve represents the best fit obtained using the tri-exponential decay equation64,65:

$$\begin{aligned} I(t)=A_1 \exp {(-t/\tau _1)}+A_2 \exp {(-t/\tau _2)}+A_3 \exp {(-t/\tau _3)}, \end{aligned}$$
(1)

where I is the fluorescence intensity, \(\tau _i\) and \(A_i\) (\(i=\overline{1,3}\)) represent the decay time constants and respective normalized amplitudes of the time-resolved decay lifetime, respectively. The average lifetime of CQDs was calculated with the equation below:

$$\begin{aligned} \tau _\text {avg}=\frac{A_1\tau _1^2+A_2\tau _2^2+A_3\tau _3^2}{A_1\tau _1+A_2\tau _2+A_3\tau _3}, \end{aligned}$$
(2)

The results of the decay fitting analysis are presented in Table 2. The fastest component, \(\tau _1=0.94\) ns, is attributed to intrinsic excitonic recombination within the sp\(^2\)-hybridized carbon core. Such a rapid decay is characteristic of band-edge transitions, where excitons recombine radiatively with minimal influence from surface or defect states66,67,68. In contrast, the longer-lived components, \(\tau _2=4.43\) ns and \(\tau _3=9.93\) ns, are generally associated with recombination through surface- or defect-related states that arise from oxygen-containing functional groups (e.g., C=O, COOH) identified by FT-IR, heteroatom dopants, or structural vacancies. Carriers trapped in these localized states typically exhibit slower radiative or nonradiative relaxation pathways, giving rise to multi-exponential decay behavior69. The relative contributions of these slow components depend sensitively on the surface chemistry of the CQDs: passivation or functionalization can modulate both their lifetimes and amplitudes by altering the density and nature of trap states64,66,70. In some engineered systems, surface-state stabilization even leads to ultralong delayed emission, such as room-temperature phosphorescence with lifetimes in the millisecond-to-second range71. Thus, while the photoluminescence origin in CQDs is multifactorial, involving both core and surface contributions69, the assignment of the shortest decay channel to intrinsic core emission is well supported by the literature67,68.

The average fluorescence lifetime of the sample was calculated to be 7.97 ns, according to Equation 2. Notably, the shorter lifetime closely aligns with values reported in earlier literature64,65. This lifetime is related to the radiative (\(\tau _r\)) and non-radiative (\(\tau _{nr}\)) lifetimes by:

$$\begin{aligned} \tau _\text {avg}=\frac{1}{\tau _r}+\frac{1}{\tau _{nr}}, \end{aligned}$$
(3)

The photoluminescence quantum yield (QY) is the ratio of radiative recombination events to the total recombination events72:

$$\begin{aligned} \phi =\frac{k_r}{k_r+k_{nr}}=\frac{\tau _\text {avg}}{\tau _r}, \end{aligned}$$
(4)

The QY of CQDs reported in this work is 21.7% (Table 3), according to the measurement method described in the Experimental Section. The radiative (\(\tau _r\)) and non-radiative (\(\tau _{nr}\)) lifetimes are calculated to be: \(\tau _r=36.73\) ns and \(\tau _{nr}=10.18\) ns.

Fig. 5
Fig. 5
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Time-resolved photoluminescence (TRPL) decay profiles of CQDs synthesized from orange peels at room temperature. The normalized PL intensity is plotted as a function of time. The experimental data are fitted using (a) single-, (b) bi-, and (c) tri-exponential decay models, respectively.

The green synthesis of CQDs from orange peels yielded materials with notable optical and surface characteristics governed by quantum confinement and surface-state effects. The optical properties obtained in this study were systematically compared with previous reports on CQDs synthesized from various biomass precursors. As summarized in Table 312,27,28,73,74,75,76,77,78,79,80,81, variations in excitation-dependent emission, emission maxima, and quantum yield reflect the influence of precursor composition and synthesis route on the resulting nanostructures.

Compared with the CQDs reported by Surendran et al. (2020)27, which exhibited a quantum yield of 11.37%, the CQDs obtained in this work show a markedly higher value of 21.7%. This improvement arises mainly from the pre-carbonization of orange peels at 200 \(^{\circ }\)C under inert atmosphere, promoting the formation of ordered aromatic sp\(^{2}\) domains and reducing oxygen-rich surface defects that act as nonradiative centers. Additionally, ammonia serves as both a pH regulator and nitrogen dopant, introducing C–N and C=N groups that enhance radiative recombination through emissive surface states. High-speed centrifugation (10,000 rpm, 10 min) further improves particle uniformity by removing carbonaceous residues.

Overall, these optimized synthesis conditions effectively enhance the optical quality of the CQDs while maintaining their environmentally friendly and cost-efficient advantages. The improved quantum yield and stable emission behavior indicate that the photophysical properties of the CQDs are governed by a favorable balance between quantum confinement and well-passivated surface states, forming the basis for their strong luminescence characteristics discussed above.

In conclusion, comprehensive characterization confirmed that the CQDs synthesized from orange peel waste via a hydrothermal method exhibit robust fluorescence emission, excellent aqueous dispersibility, and high quantum yield. Multi-Gaussian deconvolution of the PL spectra revealed a ZPL and phonon sidebands, with energy separations (\(\sim 0.211\) eV and \(\sim 0.207\) eV) closely aligned with the carbonyl (C=O) phonon energy of approximately 0.21 eV, as identified by FT-IR. The Huang-Rhys factor, calculated through PL spectral analysis (\(S \approx \frac{I_1 + I_3}{I_2}\)) and Stokes shift methods, further elucidated the strong electron-phonon coupling governing the CQDs’ optical properties. These findings underscore the efficacy of biomass-derived precursors in yielding high-performance nanomaterials with distinctive physicochemical properties, highlighting their potential for diverse applications in bioimaging, sensing, and energy storage.

Table 2 Fitting parameters of the corresponding PL decay curve.
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Table 3 Absorption and emission characteristics, along with quantum yields (QY), of CQDs, ordered by QY values. The emission peak reported for this study was obtained at an excitation wavelength of 340 nm.
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Experimental

Materials

Orange peels (Citrus sinensis) were obtained from the local market and processed by cutting them into small pieces and rinsing twice with distilled water. The pieces were then dried in an oven at 60 \(^{\circ }\)C for 2 hours. Subsequently, the dried peels were carbonized in an inert gas furnace at 200 \(^{\circ }\)C for 6 hours82. Citric acid and ammonia, both of AR grade, were obtained from Merck and used without further purification.

CQDs synthesis

The CQD synthesis process from orange peels is shown in Fig. 6, according to the previous study27. Initially, 2 g of carbonized orange peel (OP) and 2 g of citric acid were homogeneously mixed with 30 mL of double-distilled water, followed by adding 5 mL of ammonia. The pH of the solution was adjusted to 7. The dispersed solution was transferred to a Teflon-lined stainless steel autoclave and heated at 200 \(^{\circ }\)C for 6 hours. After 6 hours, the autoclave was allowed to naturally cool to room temperature. The reaction solution was centrifuged at 10,000 rpm for 10 minutes under ambient conditions. The precipitate and the supernatant were collected separately for further characterization.

Fig. 6
Fig. 6
Full size image

Synthesis process of carbon quantum dots (CQDs) from orange peels.

Characterization

UV-Vis absorption spectroscopy

UV-Vis absorption spectroscopy represents a pivotal advancement in analytical techniques, facilitating the identification of various compounds. The absorption properties of synthesized materials were evaluated within the wavelength range of 200–650 nm using a Hitachi UH5300 UV-Vis Spectrometer and a quartz cuvette (10 mm).

Photoluminescence spectroscopy

Photoluminescence (PL) spectra were recorded using a HORIBA FluoroMax-4 spectrometer equipped with a 150 W Xenon lamp as the excitation source. The excitation wavelength range was 280–500 nm, with excitation and emission slit widths of 5 nm, and all measurements were performed at room temperature.

For pH-dependent PL measurements, the CQD dispersion (0.1 mg/mL) was divided into aliquots with pH adjusted between 1 and 14 using 0.1 M HCl or 0.1 M NaOH. Each sample was sonicated for 5 min and equilibrated for 30 min before measurement. The PL spectra were recorded under identical conditions using an excitation wavelength of 320 nm. To facilitate comparison, all PL spectra were normalized to the maximum emission intensity at neutral pH (pH = 7).

Photoluminescence quantum yield (QY) measurement

The relative photoluminescence quantum yield (QY) of CQDs (\(\phi _x\)) was measured using quinine sulfate in a solution of 0.1 M H2SO4 as a reference (\(\phi _{st}=54\%\) at 350 nm excitation)83. To minimize the self-absorption effect, the absorbance of the reference sample solution and the suspensions of CQDs was controlled below 0.1 by dilution at the optimal excitation wavelength. The relative QY can be calculated using the following equation83,84:

$$\begin{aligned} \phi _x = \phi _{st} \left( \frac{I_x}{I_{st}} \right) \left( \frac{A_{st}}{A_x} \right) \left( \frac{\eta _x}{\eta _{st}} \right) ^2, \end{aligned}$$
(5)

The integrated fluorescence intensity (I), the refractive index of the solvent (\(\eta\)), and the absorbance at the optimal excitation wavelength of 310 nm (A) were used for the calculations. The subscripts “st” and “x” refer to the quinine sulfate standard and carbon quantum dots (CQDs), respectively. For all solutions, the ratio of the refractive indices (\(\frac{\eta _x}{\eta _{st}}\)) is approximately equal to 1.

Time-resolved photoluminescence (TRPL) measurement

Time-resolved photoluminescence (TRPL) spectra were acquired using an FLS1000 fluorescence spectrophotometer (Edinburgh Instruments, UK), equipped with a pulsed LED light source operating at a power of 40 \(\mu\)W per pulse. The excitation and emission wavelengths were 320 nm and 417 nm, respectively.

FT-IR analysis

Fourier Transform Infrared (FT-IR) spectroscopy is an analytical technique used to identify components of materials by using infrared radiation to scan test samples and observe their chemical properties, particularly their functional groups. The FT-IR spectrum of the CQD sample was recorded using a Nicolet\(^\text {TM}\) iS50 FT-IR spectrometer, covering a wavenumber range of 4000–500 \(\text {cm}^{-1}\).