# Sensing of Transition Metals by Top-Down Carbon Dots
## 3 Results
### Structural Characterization  
We carried out a rich structural characterisation using Raman and FT-IR spectroscopy, in order to understand both the surface and structural properties of the two analysed samples. In Figure 1a, the FT-IR spectra of the two samples are presented. As can be seen, the samples being prepared and purified under the same conditions show very similar characteristic signals.  
As already obtained in a previous study by ATR measurements [39], the main changes on the surface structure of the CQDs induced by functionalization consist in the formation of plentiful C-O-C groups on TDA, absorbing near 1100 cm\({}^{-1}\). The FT-IR measurements reported in Figure 1, acquired in the spectral range 4000-1250 cm\({}^{-1}\), show a variety of signals due to different surface groups, with a few differences in the two samples. These data provide further hints on the success of surface functionalization, although the results, compared with the previous ATR study [39], are harder to trace back to specific chemical structures of functional groups. In Figure 1, we see in both samples a broad band around 3400 cm\({}^{-1}\), which can be attributed to -OH stretching. It is worth noting that the shape of the bands for TD and TDA are slightly different, suggesting different surface interactions which can be related to functionalization. Both samples have bands peaking around 1600 cm\({}^{-1}\), more precisely located at 1656 cm\({}^{-1}\) for TD and 1602 cm\({}^{-1}\) for TDA, which are attributable to vibrational motions of the C=C bond. The spectra evidence a difference between TD and TDA in the peaks in the range 1440-1350 cm\({}^{-1}\), which can be attributed to CH\({}_{2}\) bending. In fact, it is clear that the TD displays more pronounced peaks, unlike the TDA, which has almost no signals in this region. The near disappearance of CH\({}_{2}\) from the TDA can also be attributed to surface functionalization [38; 39; 50].  
To deepen the structural characterisation of the samples, the respective Raman spectra were recorded, using an excitation wavelength of 325 nm. As shown in Figure 1b, the TD spectrum shows the presence of the two main bands D and G, which have peaks at 1388 cm\({}^{-1}\) and 1605 cm\({}^{-1}\), respectively [43; 44]. These features indicate the presence of a carbonaceous core with a sp\({}^{2}\) hybridisation. The analysis carried out using the ratio of I\({}_{\rm D}\)/I\({}_{\rm G}\) intensities shows a value of 0.59, which highlights the presence of a tendentially graphitic core [44]. A similar analysis can be made for the TDA. Its Raman spectrum shows the same D and G bands, but shifted at 1419 cm\({}^{-1}\) and 1599 cm\({}^{-1}\), respectively. The ratio I\({}_{\rm D}\)/I\({}_{\rm G}\) is 0.55. Considering that also in this case the core of the nanoparticles can be described as graphitic with sp\({}^{2}\) hybridisation, the small differences recorded in the Raman band positions can be attributed to some strain of the lattice attributed to the functionalization. In addition to this, the presence of a peak at around 1000 cm\({}^{-1}\) in TDA can be attributed to a Raman vibration of the C-O [44], confirming the successful functionalization of the CDs surface.  
In conclusion, the structural characterisation carried out has provided important information on the type of carbonaceous core present in both samples, which is, however, very similar and with a sp\({}^{2}\) hybridisation [44; 45]. As far as infrared FT-IR is concerned, the analysis carried out allows for the evidencing of small differences in the functional groups present on the surface of the samples, which are the main candidates for the interactions with heavy metals.