# Sensing of Transition Metals by Top-Down Carbon Dots
## 3 Results
### Heavy Metals Sensing  
One possibility to discriminate against and highlight the optical and surface differences in TD and TDA is to evaluate their response to heavy metal ions in aqueous solutions. In particular, the use of heavy metals of the first transition series (Fe, Co, Ni, Cu, and Zn) as potential quenchers has been considered here [34; 35; 36; 37; 38; 40; 49; 50]. Sensing tests were conducted on these two CQDs, assessing through fluorescence and absorption spectroscopy any spectral changes in the presence of different aliquots of heavy metals. The first approach to determine and evaluate the interaction between transition metals and the samples under investigation was to use fluorescence spectroscopy. Figure 3 shows the emission of the two samples TD and TDA in the presence of increasing amounts of metal. In Figure 3a, the interaction between Co\({}^{2+}\) and TD is shown. It can be seen that, as the amount of Co\({}^{2+}\) increases, the emission intensity decreases steadily with each addition of metal, resulting in approximately 50% quenching of the fluorescence intensity at the maximum concentration 40 uM of metal ions. The same effect is found for the interaction between TDA and Co\({}^{2+}\). As reported in Figure 3b, a decrease in emission intensity is observed as the amount of Co\({}^{2+}\) increases, and, also in this case, the quenching reaches 50% for 40 uM of metal ions. The two fluorescence quenching processes appear to be very similar to each other, so the interaction process with cobalt, a likely photoinduced electron transfer, is independent of surface functionalization. In Figure 3c,d, on the other hand, the interaction process between the two CQDs and Zn\({}^{2+}\), is assessed. In this case the process is the reverse, i.e., the zinc causes an increase in fluorescence, and again both samples respond in the same way. This feature could be due to a screening effect by the Zn\({}^{2+}\) ions that cover the surface of the CQDs, thus causing a consistent increase in emission intensity [47]. Since similar results are found for TD and TDA, it can be stated that the interaction process in this case too is independent of the functionalization of the surface. To deepen the dependence on the metal ions concentration, the experiments were repeated also for other ions (Fe\({}^{3+}\), Cu\({}^{2+}\), Ni\({}^{2+}\)), and a Stern-Volmer analysis was performed. Figure 3e,f, show the concentration dependence of the emission in all considered cases. Different interaction processes between TD/TDA and metals ions under the same conditions can be assumed. As far as TD is concerned, the Stern-Volmer graphs in Figure 3e show different trends for the different ions. The interaction process between Co\({}^{2+}\) and TD causes a clear fluorescence quenching with an important response over the whole concentration range, the possible interaction being the formation of a coordination bond between Co\({}^{2+}\) and OH groups present in abundance on the TD surface [50]. With regard to the other metals, Cu\({}^{2+}\) induces an initial very pronounced fluorescence quenching of approximately 10%, but after the first few additions, the interaction process shows a levelling until, at high concentrations, the TD seems to be shielded by Cu, and emission amplitude decreases [47]. For the remaining metals, Ni, Fe, and Zn, there does not seem to be a process of quenching of the fluorescence, but rather a process of intensity increase. It can be assumed that, for these three metals, there is an electronic screening of the surface from PL quenchers over the entire concentration range [48]. Figure 3: Emission spectra recorded with excitation wavelength at 440 nm for (**a**) TD and (**b**) TDA in an aqueous solution in the presence of Co\({}^{2+}\) (0–40 uM). Emission spectra recorded with excitation wavelength at 440 nm for (**c**) TD and (**d**) TDA in aqueous solution in the presence of Zn\({}^{2+}\) (0–40 uM).