## 3 Results and Discussion
Therefore, the possible structural modification induced by the surface manipulation step--surface chemistry modification and change of the dielectric properties of the surrounding medium--may affect its optical properties. Beside the red shift of the CdS emission peak, the quenching of the CdS emission to a higher extent than PbS is also observed. The intensity ratio of the PbS emission band over the CdS emission band is 1.7 for GQDs at 294 K, while it results 2.2 for the GQDs@SiO\({}_{2}\) NP at the same temperature, thus confirming an effect of the silica shell growth on the optical properties of the CdS shell. The temperature dependent fluorescence spectra reported in Figure 4B were recorded in the temperature range purposely selected for biological applications. The results show that, as long as the temperature increased, a decrease of the luminescence intensity is observed, as also reported for the bare GQDs core [9]. Fitting each PL curve with a double Gaussian function corresponding to the CdS and PbS emission, respectively, and the integration of the photoluminescence contribution of both signals, allowed us to measure the integral area of the emission for each material (I\({}_{\text{PbS}}\) and I\({}_{\text{CdS}}\)) and, thus, to evaluate the ratiometric response in the temperature range investigated. Both the PbS (Figure 5A red line and circle symbol) and the CdS (Figure 5A blue line and circle symbol) emission linearly decrease with temperature, with a more sensible emission quenching measured for the PbS signal. Figure 4: UV-Vis absorption (**A**) and fluorescence spectra in a 281–333 K temperature range (**B**, \(\lambda\)ex = 400 nm) of silica coated GQDs NPs. A sketch of the nanostructure is shown in panel (**A**). This result can be explained by taking into account the material's thermal stability. The PbS core [9] are much more sensitive to temperature changes than the CdS, probably due to thermal activated traps/defects. The ratiometric response (Figure 5B) shows that I\({}_{\rm{PbS}}\)/I\({}_{\rm{CdS}}\) monotonically decreased with temperature with a slope of 0.01 K\({}^{-1}\) (R\({}^{2}\) = 0.97), which coincides with that of the bare GQDs, in the same temperature range. The average relative sensitivity is nearly 0.2% K\({}^{-1}\), higher than the value measured for non- ratiometric QDs [57] and within the range measured for rare earth doped NPs (3.7\(-\)0.1% K\({}^{-1}\)) [1,7,8,17,18]. It has to be pointed out that the temperature response relies on mechanisms that are specific for each type of material, temperature range and environment; therefore, a direct comparison among diverse probes is not strictly relevant. In addition, by taking into account each system advantages and limitations, the availability of diverse temperature probes at the nanoscale offers the possibility to select the system most suitable for the specific case of study. Overall, our results clearly indicate that the optical temperature-dependent response of the GQDs was not significantly affected by (i) the whole procedure developed to grow the silica coating, (ii) the presence of the obtained hydrophilic silica shell and (iii) dispersion in aqueous medium. Therefore, these features proved that the silica functionalization represents a reliable strategy to coat GQDs towards their effective use as biological/biomedical temperature probes.