## 3 Results and Discussion
GQDs@SiO\({}_{2}\) NP1 has an average size of 33 nm (\(\sigma\)% = 8%) and a shell with an average thickness of 12 nm (\(\sigma\)% = 15%), while GQDs@SiO\({}_{2}\) NP2 results in larger NPs (average size 38 nm, \(\sigma\)% = 11%) characterized by a thicker silica shell (15 nm, \(\sigma\)% = 13%) as expected from increasing the TEOS volume (50 \(\upmu\)L). In both these synthetic conditions, no appreciable changes in the core multiplicity can be observed, with the final NPs presenting the higher percentage (67%) of monocore NP. The highly monodispersed GQDs@SiO\({}_{2}\) NP1 sample was chosen as a model system to be functionalized with reactive anchoring amino groups by condensation reaction with APS and for further investigation of optical temperature-dependent measurements. Functionalization of the silica NPs with APS leading to surface grafting of amino groups has been extensively investigated [48]. In the late stage of the silica shell growth suitable amount of APS was added and allowed to react overnight. After several purification steps and dispersion in water, concentration of the grafted amino groups was measured by the ninhydrin assay (see Materials and Methods), resulting in nearly 0.75 \(\upmu\)mol/mg. The chemical modification induced on the silica surface by grafted amino groups was also detected by means of \(\xi\)-potential measurements. GQDs@SiO\({}_{2}\) NPs show a \(\xi\)-potential value of \(-\)21 (\(\pm\)1) eV, due to the presence of negatively charged silanol groups,  
Figure 3: TEM images (**A**,**B**) and statistical analysis (**A1,B1**, performed on 200 nanoparticles (NPs) for each sample) of silica coated GQDs NPs (grey bar graph **A1,B1**) and silica shell (red bar graph **A1,B1**), prepared in a microemulsion at different TEOS volume: 20 \(\upmu\)L (**A,A1**) and 50 \(\upmu\)L (**B,B1**) of TEOS. Additionally, there were 80 \(\upmu\)L of OA-treated GQDs, 6 mL of cyclohexane, 350 \(\upmu\)L of Igepal CO-520 and 200 \(\upmu\)L of NH\({}_{4}\)OH. The reaction has been let to proceed overnight at 28 \({}^{\circ}\)C. Scale bar 50 nm. while a positive \(\xi\)-potential value +20.0 (\(\pm\)0.4) eV was measured for the amino-grafted GQDs@SiO\({}_{2}\) NP1, which was accounted by the successful grafting of positively charge amino moieties (pH6). The \(\xi\)-potential values of both the as prepared and amino-functionalized GQDs@SiO\({}_{2}\) NP 1 indicate the good colloidal stability of both samples. UV-Vis absorption and temperature dependent emission spectra of GQDs@SiO\({}_{2}\) NP1 are reported in Figure 4. The absorption spectrum (Figure 4A) clearly shows the characteristic absorption feature of the GQDs, although the exciton transition ascribed to the PbS core can be hardly observed, probably due to the high attenuation caused by the high extinction coefficient of the CdS and by scattering phenomena due to the silica shell. The temperature-dependent fluorescence of GQDs@SiO\({}_{2}\) NP1 was explored by monitoring the emission (wavelength range of 450-750 nm) in the temperature range of 281-313 K, typical for biological applications [9], (Figure 4B). First, the emission spectra shows that, irrespectively from the temperature, the emission line profile is quite similar to that recorded for the GQDs [9]. Two emission bands, in the NIR region, ascribed to the PbS core and centered at 720 nm, and in the visible range, related to the CdS shell, and centered at 480 nm, respectively, are clearly detected. Both the emission bands are red shifted with respect to their position in the pristine GQDs (716 and 465, respectively), however a larger shift was observed for the CdS related signal. Such features can be described considering the structure of the GQDs, where the PbS core is expected to be nearly isolated from the environment due to the presence of the thick CdS shell. Conversely, the CdS shell is the outmost interface undergoing the whole procedure of silica shell formation (including the critical step of ligand exchange reaction).