Introduction

Recently, the discovery of luminescent carbon nanodots (CDs) has been revolutionary in the field of materials science. Thanks to their superior properties such as high photoluminescence, good water solubility, low toxicity, and excellent biocompatibility, CDs have quickly become a superior alternative to traditional organic dyes and heavy metal-based inorganic semiconductor quantum dots. Therefore, this type of material has been widely applied in many fields such as sensors [1], [2], [ [3], bioimaging [2], [4], [5], [6], drug delivery [7], [8] and photocatalysis [9], [10], [11]. Despite their immense potential, developing efficient and environmentally friendly carbon dot synthesis processes remains a significant challenge. Numerous different techniques for synthesizing carbon dots have been widely published, including ultrasonic-assisted techniques [12], graphite laser etching [13], microwave-assisted techniques [14], strong acid oxidation [15], electrochemical methods [16], glycerol pyrolysis [17]. However, these methods often require complex equipment, consume significant energy, or utilize toxic chemical solvents. Therefore, the trend of utilizing readily available, inexpensive, and natural biomass sources for carbon dot fabrication in a «green chemistry» approach is attracting considerable interest from researchers.

In this study, we present a method for fabricating carbon dots (CDs) using a one-stage hydrothermal technique from a natural precursor, passion fruit juice. To elucidate the structural properties and optical mechanisms of the material, a series of systematic spectroscopic measurements were performed. First, electron absorption spectra (UV-Vis) were analyzed to identify the background energy transition regions. Next, Fourier transform infrared (FTIR) spectra were used to identify the abundant chemical functional groups on the particle surface. The relationship between these functional groups and the electronic structure was then clarified through fluorescence excitation spectra (PLE) to find the optimal trapping energy region. Finally, fluorescence emission spectra (PL) were investigated to evaluate the actual luminescence efficiency of the material in the visible light region. This comprehensive approach provides insight into the surface state-dependent emission mechanisms, laying the foundation for practical applications of passion fruit CDs such as bioimaging, sensing, etc.

Carbon dots (CDs) were synthesized via a one-step hydrothermal carbonization method using passion fruit juice as a natural biomass precursor. Briefly, fresh passion fruits were washed thoroughly, and the seeds were separated using a sieve to obtain pure juice. The pristine passion fruit juice was then diluted with doubly distilled water at a volume ratio of 1:1. The resulting mixture was transferred into a Teflon-lined stainless steel autoclave and heated at 200 °C for 12 h in an electric oven. After the hydrothermal reaction, the autoclave was allowed to cool naturally to room temperature. The obtained dark brown solution was filtered through a 2 µm pore-size filter paper to remove large carbonaceous aggregates and unreacted char, yielding a homogenous CD solution for further investigations.

The surface functional groups and chemical framework of the as-prepared CDs were identified by Fourier-transform infrared (FTIR) spectroscopy using a Thermo Nicolet FTIR 6700 spectrometer in the wavenumber range of 4000–400 cm ^-1 . The optical absorption properties were investigated via ultraviolet-visible (UV-Vis) absorption spectroscopy recorded on an Agilent 8453 UV-Vis spectrophotometer (Agilent Technologies, USA) within a wavelength range of 200–800 nm. The photoluminescence (PL) properties, including emission and excitation (PLE) profiles, were systematically evaluated using a NanoLog fluorescence spectrophotometer (Horiba, Edison, USA).

Results and discussions

Passion fruit juice is predominantly composed of water (~75 %) and carbohydrates (~23 %). The carbohydrate fraction typically comprises dietary fibers and simple sugars (including sucrose, fructose, and glucose), alongside a notable content of ascorbic acid. The formation mechanism of carbon dots (CDs) from this biomass precursor under hydrothermal conditions can be systematically described through three sequential stages: (i) Hydrolysis and degradation: In the presence of naturally occurring organic acids like ascorbic acid, the carbohydrates undergo acid-catalyzed hydrolysis, dehydration, and degradation to generate highly soluble intermediate compounds, such as furan derivatives (e.g., furfural and hydroxymethylfurfural), ketones, and volatile organic acids (acetic, levulinic, and formic acids). (ii) Polymerization and aromatization: Subsequently, these intermediate species experience cross-linking, polymerization, and condensation to form soluble polymeric chains. Aromatization and the generation of aromatic clusters proceed via aldol condensation, cycloaddition, and hydroxymethyl condensation indirectly mediated through furan rings. (iii) Nucleation and growth: Once the concentration of these aromatic clusters surpasses the critical supersaturation threshold, carbon nucleation is triggered. The growth of these carbonaceous nuclei is driven by the diffusion of surrounding molecular species toward the particle surface. Consequently, this process yields CDs that are inherently self-passivated with abundant oxygen- and hydrogen-containing functional groups [5].

Figure 1 shows the Fourier transform infrared (FTIR) spectrum performed to identify the chemical functional groups present on the surface of the CDs material. The spectral results show a broad and strong absorption band in the region from 3200 to 3600 cm ^-1 , characteristic of the stretching vibrations of the -OH functional groups [5], [4]. A strong absorption peak at approximately 1650 cm ^-1 is attributed to the stretching vibrations of the C=O bond in the carboxyl group or the C=C bond in the amorphous carbon structure of the particle core [4]. In addition, absorption peaks in the 1350–1400 cm ^-1 region correspond to the stretching vibrations of the C-H bond and the stretching vibrations of the C-O bond [18]. The abundant presence of these oxygen and hydrogen-containing functional groups not only demonstrates the successful carbonization and surface functionalization processes from passion fruit juice precursors, but also explains the excellent dispersion of CDs in aqueous environments. Furthermore, this oxygen-rich functional group shell acts as surface energy traps, directly influencing the photoluminescence properties of the particles in subsequent measurements.

Fig. 1. Fourier transform infrared spectrum of carbon nanodots

Figure 2 shows the UV-Vis absorption spectra of CDs materials, performed to investigate the background electron energy transition states. The absorption curve displays a distinct peak at approximately 285 nm, which is thought to be related to the   * energy transition of C=C double bond clusters within the amorphous carbon core network. In addition, a broad absorption shoulder peak appears in the 320–350 nm region, characteristic of the n — * energy transition of oxygen-containing functional groups (such as C=O or C-O densely distributed in the surface shell of the particles), as analyzed from the FTIR spectrum in Figure 1. Notably, the UV-Vis spectrum has an absorption tail extending from the ultraviolet to the visible light region (> 400 nm). This characteristic demonstrates the existence of complex surface energy trapping states, which play a decisive role in the photoluminescence mechanism of the material [18].

Fig. 2. UV-Vis absorption spectrum of carbon nanodots fabricated from passion fruit

Fig. 3. Fluorescence excitation spectrum of carbon nanodots fabricated from passion fruit

Figure 3 illustrates the photoluminescence excitation (PLE) spectrum of the CDs, recorded to determine the optimal excitation wavelength and elucidate the underlying energy transitions responsible for emission. The PLE spectrum exhibits a dominant, well-defined excitation peak at approximately 485 nm. This peak position, located in the longer wavelength region, strongly correlates with the extended absorption tail observed in the UV-Vis spectrum (Figure 2), a typical characteristic of carbon dots with a high density of surface defect states or energy-trapping sites. This finding confirms that the photoluminescence of the passion fruit-derived CDs is primarily governed by energy transitions within the oxygen-rich functionalized surface shell, rather than the    ^* transitions of the amorphous carbon core [19]. Consequently, based on the PLE data, an excitation wavelength of 485 nm was selected as the optimal condition to achieve maximum fluorescence emission efficiency in subsequent optical measurements.

Fig. 4. Fluorescence excitation spectrum of carbon nanodots fabricated from passion fruit

Figure 4 displays the photoluminescence emission (PL) spectrum of the CDs, recorded to evaluate their actual emission performance under optimal excitation conditions. Upon excitation at 485 nm (the optimal wavelength determined from the PLE spectrum in Figure 3), the CDs exhibit a strong, symmetrical fluorescence band with a maximum emission peak at approximately 530 nm, falling within the green spectral region. The calculated Stokes shift between the excitation and emission peaks is approximately 45 nm, demonstrating an efficient radiative energy dissipation process. This relatively broad emission band is a hallmark characteristic confirming that the photoluminescence of the passion fruit-derived CDs is strongly governed by the diverse distribution of oxygen-rich surface energy-trapping states (such as C=O) and (-OH) configurations verified by FTIR analysis), rather than quantum size effects originating from the amorphous carbon core [19].

Conclusion

Carbon dots (CDs) fabricated from passion fruit juice via a hydrothermal method possess a surface structure rich in oxygen-containing functional groups. The strong correlation among the absorption, excitation, and green emission peaks confirms that the photoluminescence mechanism of the material is predominantly governed by surface energy-trapping states. These findings open up promising avenues for the potential application of this material in the fields of sensing and bioimaging.

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