# Sensing of Transition Metals by Top-Down Carbon Dots
## 3 Results
### TD vs. TDA: Optical Properties  
The normalized optical absorption spectra of TD and TDA samples are reported in Figure 2a. Both spectra appear broad and structureless over the entire wavelength range, suggesting a near continuum of absorption transitions and also a very similar carbon core of the nanoparticles. Small changes are present in the wavelength region around 300 nm, where the TDA features a small shoulder, possibly due to n-\(\pi^{*}\) transitions [25; 46; 51], associated with the surface changes that occurred with functionalization [20; 23; 29; 30; 42]. Figure 1: **(a)** FT-IR spectra of TD (red line) and TDA (purple line) samples. **(b)** Normalized Raman spectra of TD (red line) and TDA (purple line) samples. FT-IR and Raman spectra have been arbitrarily vertically shifted for clarity. The arrow in the top portion of panel **(a)** marks the spectral range of CH\({}_{2}\) bending vibrations. Figure 2a also reports the emission spectra recorded with excitation at 440 nm. Both TD and TDA samples feature a very broad band with a maximum emission efficiency in the green at about 524 nm. The quantum yield (QY) of the two samples at the excitation wavelength of 440 nm was estimated, taking fluorescein as a reference. The respective QYs is 6.1% for TD, and 9.2% for TDA. Such relatively low QY values are known in the literature for carbon dots synthesised via top-down procedures [4, 6, 22, 27, 28, 39, 42]. It is interesting to note that passivation of the surface allows the QY to be increased by adding chemical groups that favour the radiative emission process. The fluorescence of TD and TDA is highly dependent on excitation energy, which is a typical feature observed in the photoluminescence of carbon nanodots. As shown in Figure 2c,d, both TD and TDA show an identical trend in exciting the samples in a wide range of wavelengths, 410-620 nm, and significant emission shift ranging from cyan (480 nm) to red (650 nm) is found, thus maintaining a wide tunability range [13, 28, 39]. This is further evidence that functionalization does not significantly alter the emission properties of the studied CQDs [33, 32, 13, 46]. The steady state optical analysis, as we have seen, does not give much information about the surface change due to functionalization; however, analysis using time-resolved spectroscopy allows us to highlight a quite appreciable difference in lifetimes. Typical PL time decay measurements at 520 nm are reported in Figure 2b under pulsed excitation at 440 nm. A non-single-exponential decay is found for both samples. Although at the beginning of the decay the curves are very similar, it is possible to appreciate that the TD has a faster decay than T  
\(\exp\{-(t-t_{0})/\tau_{2}\}\)), we found the lifetimes for TD and TDA. In particular, the final decay times are \(\tau_{1}\) = 2.3 \(\pm\) 0.2 ns and \(\tau_{2}\) = 6.9 \(\pm\) 0.2 ns for TDA, and \(\tau_{1}\) = 1.8 \(\pm\) 0.2 ns and \(\tau_{2}\) = 5.2 \(\pm\) 0.2 ns for TD. The two samples have a very similar initial decay rate, and the first lifetime \(\tau_{1}\) is almost consistent within experimental uncertainty. However, we find that the second lifetime, \(\tau_{2}\), is appreciably shorter in TD. Evidently, the chemical groups present on the surface after functionalization tend to reduce the effect of non-radiative relaxation in TDA samples, making their decay slower than TD. As far as the optical characterization of the two samples TD and TDA is concerned, it can be stated that their optical properties are very similar, i.e., both samples show very broad absorption spectra with an almost continuous structure, with some differences in the low wavelength region (300 nm). The emissive properties are also very similar, maintaining tunability over a large excitation range. This is a clear sign of a large structural disorder, typical of fluorescent nanocarbons [21; 24; 32; 33].