Green synthesis and multi-Gaussian analysis of carbon quantum dots from orange peels: optical properties, phonon interactions, and Huang-Rhys factor characterization

Abstract

Carbon quantum dots (CQDs) have garnered considerable interest due to their distinctive optical properties, cost-effectiveness, and eco-friendly synthesis routes. This study investigates CQDs synthesized from orange peels through a sustainable hydrothermal method. The CQDs were characterized using UV-Vis spectroscopy, fluorescence spectroscopy, Fourier Transform Infrared (FT-IR) spectroscopy, and transmission electron microscopy (TEM) to elucidate their optical and structural properties. Multi-Gaussian fitting of the photoluminescence (PL) spectra was employed to deconvolute the emission profiles, revealing the contributions of the zero-phonon line (ZPL) and phonon sidebands, which correlate closely with the characteristic carbonyl (C=O) stretching mode at approximately 0.21 eV, as identified by FT-IR. The Huang-Rhys factor, calculated from both the PL spectral analysis and the Stokes shift, quantifies the electron-phonon coupling strength, providing insights into the recombination dynamics. The synthesized CQDs exhibited robust blue fluorescence under UV excitation, with excitation-wavelength-dependent tunable emission. Moreover, the CQDs achieved a significantly enhanced quantum yield compared with previously reported orange peel–derived counterparts, together with stable radiative lifetimes, demonstrating efficient radiative recombination. These features confirm their strong potential for bioimaging, optoelectronic, and sensing applications and reinforce the viability of fruit waste as a sustainable precursor for high-quality fluorescent nanomaterials with well-defined vibronic characteristics.

Fig. 1

(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.

Fig. 2

(a) UV–Vis absorption spectra of the CQD dispersions. (b) Photoluminescence (PL) spectra recorded at various concentrations under excitation at 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.

Fig. 3

Normalized photoluminescence (PL) spectra of CQDs obtained under an excitation wavelength of nm, together with the corresponding normalized photoluminescence excitation (PLE) spectra measured at an emission wavelength of nm.

Fig. 4

(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.

Fig. 5

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.

Fig. 6

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