## 3 Results and Discussion
### Spectroscopic Characterization  
The UV-Vis absorption profile of TCDs is consistent with that expected for graphene-based carbon dots, with a shoulder at 265 nm that arises from \(\pi\)-\(\pi^{\star}\) transitions derived from the aromatic sp\({}^{2}\) domain [24] and/or of C=C and C=N bonds.  
The TCDs resulted in having excitation-independent emissions, with the maximum emission at 426 nm and the maximum excitation at 343 nm in aqueous media and 430 nm and 344 nm, respectively, in PBS, suggesting a slight stabilization of the surface-excited states due to the presence of the buffer. This absorption wavelength is attributable to n-\(\pi^{\star}\) transitions from C=O bonds [23]. This excitation independency has already been described in carbon dots and is explained by the conjugated \(\pi\)-system acting as a center for quantum confinement and the absorption of photons, whereas N-containing and O-containing surface groups provide different vibration relaxations [25]. The large Stokes shifts obtained are also a consequence of surface self-trapping for vibration relaxation.  
The influence on the luminescence of these N-containing or O-containing groups traps was evaluated by studying the luminescence with a different degree of surface protonation. Then, photoluminescence was measured at different pH values. Both emission intensity and emission wavelength suffer alterations at pH 4 and pH 10, being more or less constant at intermediate values (Figure 4).  
Figure 3: FTIR spectra of (black solid line) tomato juice, (blue dashed line) GCD and (red dotted line) TCD.  
This behavior suggests the presence of two different surface functionalities, an acidic one with pK around 4 and an alkaline one with pK near 10. These values agree with the typical pK for carboxylic acid and amine moieties, whose presence is also supported by FTIR data. In fact, the lower pH is close to one of the pK values of ascorbic acid (4.2), one of the most important nutrients in tomato [26].  
Quantum yields were measured with an absolute methodology, finding values of 1.03% (in water) or 1.24% (in PBS). These poor quantum yields are common in carbon dots obtained from natural carbon sources, which are usually lower than 13% [27], probably because of the inherent complexity of a natural source.  
Absorption data were also analyzed using [PERSON]'s law for direct transitions [28]:  
\[(\alpha\mathrm{hv})^{2}=\mathrm{k}\;(\mathrm{hv}+\mathrm{E_{g}}) \tag{4}\]  
where \(\alpha\) is the absorption coefficient, hv is the energy of the incident light and k is a constant. According to this, estimated, bandgap values (E\({}_{\mathrm{g}}\)) were 3.43 eV for TCD and 3.25 eV for GCD (Figure 5a); these are consistent with other already-published values of E\({}_{\mathrm{g}}\) for carbon dots [18].  
Structural disorders and/or defects drive poorly defined limits in the valence and conduction bands, which causes a tail for values below E\({}_{\mathrm{g}}\) known as '[PERSON]'s tail' (Figure 5b). The energy associated with this zone can be evaluated by linearizing Equation (2):  
\[\alpha\mathrm{hv}\;=\;\alpha_{0}\;\mathrm{e}^{(\mathrm{hv}/\mathrm{E_{u}})} \tag{5}\]  
where \(\alpha_{0}\) is a constant and E\({}_{\mathrm{u}}\) is the Urban's energy whose values are calculated from the linearization, being 344 meV for TCD and 329 meV for GCD, respectively. This is an indication that GCD shows a structure with fewer defects than TCD, which is understandable according to the complex matrix of tomato juice. Nevertheless, despite this fact, E\({}_{\mathrm{u}}\) values of TCDs are lower than those reported for other carbon dots synthesized from vegetable wastes such as grape pomace and tea residues [Error!Bookmarknotdefined.], indicating a more ordered structure.  
Figure 4: Emission wavelength and intensity of TCDs at different pH values.