## 2 Materials and Methods  
Materials. For the synthesis of the giant PbS@CdS@CdS QDs the following chemicals were used: lead (II) chloride (PbCl\({}_{2}\), 98%), oleylamine (OLA, technical grade, 70%), oleic acid (OA, 90%), cadmium oxide (99%, powder, CdO), cadmium acetate (Cd(OA)\({}_{2}\), 99,9995%) and sulfur (99.9999% powder, S). The silica shell growth and functionalization required tetraethyl orthosilicate (98% d = 0.934 g/mL, TEOS), aqueous ammonia (d = 0.900 g/mL, NH\({}_{4}\)OH), 5-polyoxyethylene nonylphenylether (Igepal CO-520, Mn = 441) and (3-Aminopropyl)triethoxysilane (97%, APS). For the ninhydrin test, ninhydrin and 2,6-lutidine (98%, Alfa Aesar) were used. All chemicals, purchased from Sigma Aldrich @ Merck KGaA, Darmstadt, Germany, if not specified otherwise, were used without purification. All solvents, namely methanol, ethanol, chloroform, cyclohexane, octadecene (ODE) were of the highest purity available. Synthesis of GQDs. Synthesis of PbS@CdS QDs was carried out via a two-step cation exchange reaction, as reported in [26; 40], followed by the growth of a thick CdS shell. In particular, the PbS cores were synthesized by using OLA as ligands [40]. Typically, PbCl\({}_{2}\) (3.6 mmol) in OLA (2.4 mL) and sulfur (0.36 mmol) in OLA (0.24 mL) were purged, respectively, by N\({}_{2}\) at room temperature for 30 min. The PbCl\({}_{2}\)-OLA suspension was heated and kept at 160 \({}^{\circ}\)C for 1 h and then cooled down to 120 \({}^{\circ}\)C under vacuum for 15 min and then put under N\({}_{2}\) flux. The sulfur in OLA at room temperature was quickly injected into the PbCl\({}_{2}\)-OLA suspension, the solution was vigorously stirred and the reaction was conducted at 100 \({}^{\circ}\)C for 10 min, before being quenched by putting the reaction flask into cold water. Ethanol/acetone (1:1) was added, and then the suspension was centrifuged and the QDs pellets were dispersed in toluene. PbS@CdS QDs with a thin shell were synthesized via a cation exchange method [40]. Typically, CdO (2.3 mmol), OA (2 mL) and ODE (10 mL) were heated to 255 \({}^{\circ}\)C under N\({}_{2}\) for 30 min. The clear solution was cooled down to 155 \({}^{\circ}\)C under vacuum for 15 min and then put under N\({}_{2}\). PbS QDs suspension in toluene (1 mL, absorbance = 3 at the first exciton peak) was diluted in 10 mL toluene, degassed for 30 min and then immediately heated to 100 \({}^{\circ}\)C. The Cd/OA mixture was injected and the reaction was let to proceed at 100 \({}^{\circ}\)C for 120 min and at 240 \({}^{\circ}\)C for 2 h. The reaction was quenched by injection of cold toluene (\(-\)20 \({}^{\circ}\)C), and ethanol was added to collect the PbS@CdS QDs finally dispersed in hexane. Deposition of the thick CdS layer on PbS@CdS QDs followed the successive ionic layer absorption and reaction (SILAR) procedure described by [PERSON] et al. [9; 26]. Typically, in a 100 mL round-bottom flask, OLA (5 mL), ODE (3 mL) and PbS@CdS QDs (\(2\times 10^{-7}\) mol in hexane) were degassed at 110 \({}^{\circ}\)C for 20 min. The reaction flask was re-stored with N\({}_{2}\) and the temperature was further raised to 240 \({}^{\circ}\)C with stirring. The sulfur dispersed in ODE (1 mL, 0.2 M) was added dropwise and the mixture allowed to react for 10 min, followed by dropwise addition of 1 mL 0.2 M Cd(OA)\({}_{2}\) in ODE. The shell was then annealed for 90 min. All subsequent shells were annealed at 240 \({}^{\circ}\)C for 1.5 h following the injection of sulfur and 2 h following dropwise addition of the Cd(OA)\({}_{2}\) in ODE. Sulfur/Cd(OA)\({}_{2}\) addition volumes for shell addition cycles 1-8 were as follows: 1, 1, 1.5, 1.5, 2, 2, 3 and 3 mL, respectively. The reaction was quenched by injection of cold toluene (\(-\)20 \({}^{\circ}\)C). Ethanol was added, then the suspension was centrifuged and the supernatant was removed. The QDs were dispersed in hexane. Growth of the silica shell on the GQDs. Silica shell was grown onto the GQDs by using a base assisted water-in-oil microemulsion approach.