## 3 Results and Discussion  
Here, the base assisted water-in-oil microemulsion method was exploited to coat organic dispersible OLA-OA capped GQDs with a silica shell, in order to make them dispersible in aqueous media, thus enabling their possible application in the biomedical field. The interest towards such a GQDs system arises from their original fluorescent properties, able to provide an accurate read-out of the temperature with ultrahigh sensitivity [9] in a wide temperature range, thus being promising as fluorescent nanothermometry systems. Prepared by a solution phase approach, GQDs consist of a PbS core, synthesized by using a hot injection procedure and coated with a thin CdS shell obtained by cation exchange, and finally with a thick CdS shell achieved by SILAR in the presence of OLA and Cd(OA)\({}_{2}\) (Figure 1A). As reported by [PERSON] et al. [26], this procedure results in PbS@CdS@CdS structures with a PbS core size estimated to be 1.2 nm, a thin interlayer of zinc blend CdS and a thick outer shell of wurtzite CdS. As shown in Figure 1B, quite monodispersed triangular and asymmetric shaped NPs with an average final size of 15 nm (\(\sigma\)% = 11%, Figure 1A) have been achieved [26]. The UV-Vis absorption profile is mainly dominated by the excitonic transition of the thick CdS shell, which appeared as a shoulder at nearly 470 nm, with an onset at 512 nm (Figure 1C), and a very weak absorption feature at 591 nm (Figure 1C, inset), which could be related to the very small PbS core [26]. A clearer contribution of the PbS domain can be observed in the photoluminescence spectrum of the GQDs (Figure 1D), which shows the typical dual emission characteristic of the nano-heterostructures [9; 26; 27; 28]. The broad and intense band centered at 715 nm in the near infrared (NIR) region can be assigned to the PbS core, usually characterized by a large Stokes shift (here nearly 364 meV), while the band at 465 nm, in the visible region of the spectrum, was generated by the CdS thick shell, being the peak position in agreement with the band gap energy of the bulk CdS. The lower  
Figure 1: Sketch of the giant PbS@CdS@CdS NCs (**A**) and TEM micrograph (**B**, scale bar 50 nm) with size statistical analysis (**B**, inset), UV-Vis absorption (**C**) and photoluminescence (**D**, \(\lambda\)ex = 400 nm, T = 294 K) spectra of the core-shell–shell sample. The absorbance profile in the 540–600 nm wavelength range has been enlarged in the inset of panel (**C**). emission intensity of the CdS domain can be reasonably ascribed to trap band states and thus to the non-radiative recombination processes, increasing with the number of the SILAR deposition cycles [26]. Being these GQDs only dispersible in apolar organic solvents, to allow their application in a biological environment, they need to be made dispersible in aqueous media by means of a suitable surface functionalization. Direct ligand exchange [44], encapsulation in self assembled molecular structures [29; 30; 45], polymer coating [31] and silica shell coating [32; 33; 34] have been reported as a possible strategy for the phase transfer of organic-capped NPs in water. Here, silica shell growth is used. The low toxicity and chemical stability of the silica structure, [46] which undergoes to dissolution only at acidic pH [47], and the easy integration of surface functional groups, needed for bioconjugation and site specific recognition reactions, make silica shell a suited biocompatible coating, able to limit any leakage in the surrounding environment of the heavy metals forming the GQDs. The water-in-oil microemulsion approach, extensively reported as a strategy used to coat with a hydrophilic siloxane network organic capped NPs [32; 33; 34; 41; 42; 48], was exploited to grow the silica shell onto the GQDs. As described in the reaction scheme reported in Figure 2A, the NPs are first dispersed in the organic phase of the water-in-oil microemulsion, obtained by mixing cyclohexane with suitable amount of Igepal CO-520 non-ionic surfactant and water ammonia solution.