# Sensing of Transition Metals by Top-Down Carbon Dots
## 4 Conclusions  
The study carried out on two families of carbon quantum dots, TD and TDA, both synthesised by a top-down method, provides information on the structural and optical properties of these nanomaterials, and on their suitability as sensors of transition metal ions. Both samples show a sp\({}^{2}\)-hybridized carbon core, confirmed by Raman spectroscopy, which presents the typical D and G bands. The surface changes induced by functionalization, although not enormous, are confirmed by FT-IR analysis, showing slight but significant differences, such as the near disappearance of CH\({}_{2}\) groups in TDA. While the steady-state optical properties are almost identical, both samples showing quasi-continuum absorption spectra and tunable fluorescence, differences are observed between the decay lifetimes of the two samples, further confirming the effect of functionalization. Interestingly, the sensing responses of TD and TDA to metal ions in solution are very different from one another. TD fluorescence shows a remarkable and selective quenching process in presence of cobalt ions, approximately 50% throughout the 0-40 uM concentration range. In contrast, the other transition metals only lead to an increase of the luminescence intensity. TDA, on the other hand, responds to all of the metals taken into consideration, which are found to behave as excellent quenchers. However, an exception is represented by zinc, which induces a continuous increase in the intensity of emission in the whole range of explored concentrations (0-40 uM). Therefore, it can be said that the optical response of TDA is selective to zinc ions due to their unique behaviour in the interaction process. Overall, both TD and TDA are suitable for heavy metal sensing applications, showing sensitivity and selectivity to specific ions over a wide range of concentrations in the micromolar range. These results confirm the potential of carbon quantum dots as a multifunctional platform for optical and sensing applications, and the importance of surface functionalization to control their properties. Conceptualization, F.B., A.S., F.M. and S.A.; investigation, F.B., A.S., F.M. and S.A.; writing--original draft preparation, F.B., A.S. and F.M. ; writing--review and editing, F.B., A.S., G.B., M.C., F.M.G., F.M. and S.A.; supervision, F.M. and S.A.; funding acquisition, F.M. and S.A. All authors have read and agreed to the published version of the manuscript. Figure 4: (**a**) Lifetime analysis for TD solutions with and without metals: pure TD (purple line), Co\({}^{2+}\) (blue line), Cu\({}^{2+}\) (light blue line), Fe\({}^{3+}\) (green line), Ni\({}^{2+}\) (orange line), and Zn\({}^{2+}\) (red line). (**b**) TDA lifetime analysis in solution with and without metals: pure TDA (purple line), Co\({}^{2+}\) (blue line), Cu\({}^{2+}\) (light blue line), Fe\({}^{3+}\) (green line), Ni\({}^{2+}\) (orange line), and Zn\({}^{2+}\) (red line). Lifetime has been always recorded at 520 nm under excitation at 440 nm. **Funding:** This research was funded by industrial PhD grant DOT1320535, \"Sensori ottici a base di carbon nanodots per la rivelazione fluorescente di inquinanti\", provided by MIUR (DDG n.2983) under the call \"PON ricerca e innovazione 2014-2020--Dottorati Innovativi a Caratterizzazione Industriale\", and by the PRIN project \"CANDL2--CARBON NANODOTS FOR LIGHT-EMITITING MATERIALS AND LASING APPLICATIONS\", funded by Ministero dell'Universita e della Ricerca under grant number 2017W75 RAE. **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable. **Data Availability Statement:** The data presented in this study are available on request form the corresponding author. **Acknowledgments:** We acknowledge useful scientific discussion with members of the LaBAM group at the Department of Physics and Chemistry Emilio Segre at University of Palermo, [[https://www.unipa.it/lamp/](https://www.unipa.it/lamp/)]([https://www.unipa.it/lamp/](https://www.unipa.it/lamp/)).