# Sensing of Transition Metals by Top-Down Carbon Dots
## 3 Results
### Heavy Metals Sensing
Stern–Volmer analysis at the emission wavelength 520 nm under excitation at 440 nm for (**e**) TD and (**f**) TDA in the presence of the metal ions Co\({}^{2+}\) (blue line), Cu\({}^{2+}\) (green line), Ni\({}^{2+}\) (purple line), Zn\({}^{2+}\) (orange line), and Fe\({}^{2+}\) (red line). All measurements are carried out at a CQD concentration of 5 mg/L. Overall, from the above Stern-Volmer graphs, it appears that TD has a remarkable selectivity and affinity towards the Co\({}^{2+}\) ion over a wide range of concentrations, as it is the only metal that is able to significantly decrease the emission intensity of TD, making this latter sample an ideal candidate for a sensing application towards this metal. By contrast, the other metals have a different interaction process with the TD surface causing essentially the increase in emission. TDA, on the other hand, shows a decidedly different trend in the Stern-Volmer plots: all of the metals are able to reduce the intensity of emissions, and Co\({}^{2+}\), Fe\({}^{3+}\), Ni\({}^{2+}\), and Cu\({}^{2+}\), in particular, seem to be excellent quenchers [34; 35; 36; 37; 38; 40; 49]. Here, the ease of interaction between these metal ions and the surface of the TDA does not allow for discrimination of the metals present in solution, and so in the case of these metals, the quenching selectivity of the carbon nanodots towards them is drastically reduced. However, TDA can be considered selective for Zn\({}^{2+}\) ions, in that the interaction with this ion appears markedly different, and in contrast to the pattern observed with the other metals. Zn\({}^{2+}\) are in fact the only ions which cause an enhancement of TDA fluorescence intensity of approximately 15%. This effect could be due, as in the case of TD, to an electrostatic screening effect by the Zn\({}^{2+}\) ions, which, by attaching themselves to the surface, can increase the PL intensity by limiting the geometric rearrangement of the surface groups [47; 51]. Finally, in order to distinguish the type of quenching and, in general, the type of interaction process between the samples examined, TD and TDA, and the heavy metals, we carried out a study using time-resolved spectroscopy to assess the fluorescence decay kinetics, and thus distinguish whether the type of quenching could be static or dynamic [1; 8; 10; 31; 32; 33; 34; 50]. The study was carried out on both samples, using the maximum concentration of metals used in the previous experiment, i.e., 40 uM, and evaluating the PL time decay in the presence of heavy metals. The results of the experiments are reported in Figure 4. As far as TDA is concerned, Figure 4b, despite the fact that the Stern-Volmer plots (Figure 3f) show an important PL quenching for metals such as Co\({}^{2+}\), Ni\({}^{2+}\), Fe\({}^{3+}\), Cu\({}^{2+}\), and Zn\({}^{2+}\), it is clear that the decay kinetics in the presence and absence of metals remain completely unchanged within experimental uncertainty. This is the fingerprint of so-called static quenching, which is associated to the formation of entirely non-luminescent complexes between TDA and the quenchers, most likely quenched by extremely efficient (sub-nanosecond) photoinduced electron transfer. By analogy, this model can also be applied to the Zn\({}^{2+}\) ion, where the increased fluorescence intensity in steady state measurements can be described as a static enhancement, occurring within stable TDA-Zn complexes. As far as TD is concerned, the exact same thing can be observed: although steady state measurements show a change in the emissive properties in the presence of the metal ions, the lifetime of TD in the presence of the ions does not change within the experimental repeatability, so here too the interaction process with the CQDs can be defined as static, in the same sense explained above. Based on the data in Figure 3, we finally estimated the limit of detection (LOD) of our CQDs for Co\({}^{2+}\) and Zn\({}^{2+}\) respectively.