## 3 Results and Discussion
A non-negligible percentage (20%) of empty silica NPs was attained when using 30 \(\mu\)L of the OLA-treated GQDs dispersion (Figure 2B) while, when a larger GQDs volume (up to 80 \(\mu\)L) was used, the amount of empty silica NPs effectively dropped and the procedure results essentially in core-shell type nanostructures (Figure 2C). In this latter case, elongated nanostructures (yield 32%) hosting multiple but well separated GQDs are also detected, which turn into interconnected worm like structures, at further increasing of the GQDs concentration (data not shown). To understand the mechanism that directs core-shell geometry, the role played by the GQD surface ligand stoichiometry and composition has to be considered. The outmost CdS shell, obtained by sulfur injection and dropwise addition of Cd(OA)\({}_{2}\) in OLA and ODE, as reported in the Experimental section, can be described as (CdS)\({}_{n}\)(Cd(OA)\({}_{2}\))\({}_{m}\)(OLA)\({}_{p}\), where \(n\) relates to the size of the shell and \(m\) and \(p\) describe the stoichiometry of the ligand shell, also based on the preservation of the GQD electroneutrality [54]. According to ligand classification described by [PERSON] et al. [56], OLA is an L-type ligand, which is usually not tightly bound to the surface, and Cd(OA)\({}_{2}\) is a (CdX\({}_{2}\)) Z-type ligand. Therefore, a mixed L- and Z-type surface chemistry is expected at the GQD surface. During the purification steps, ethanol added as non-solvent can easily remove OLA L-type ligands, while there is a limit to the amount of oleate X-type ligand that can be removed from the surface due to the charge arising from the excess of residual Cd\({}^{2+}\) ions. Similarly, during the silica shell growth, TEOS and IGEPAL may preferentially exchange loosely bound OLA capping molecules rather than oleate ligand. Therefore, the mixed ligands composition at the surface of pristine GQDs results in a not homogenous silica shell. Conversely, the post synthesis OLA treatment promote the partial displacement of the Cd(OA)\({}_{2}\) Z-type ligands by the excess of OLA added to the GQD sample, finally resulting in a (CdS)\({}_{n}\)(Cd(OA)\({}_{2}\))\({}_{m\text{-}}\)(OLA)\({}_{p\text{+}}\) surface stoichiometry that still preserves the GQD electroneutrality. As a result, a much more uniform OLA ligand layer coordinates the GQDs surface, thus driving the formation of a homogenous silica coating. In addition, the OLA left free in the microemulsion can be assumed to behave as a co-surfactant, lowering the interfacial surface tension and hence the reverse micellar film rigidity, thus energetically favoring the entrapping of the GQDs inside the water pool. The micelle occupancy also depends on the GQDs content. While empty silica NPs, together with core-shell structures, were formed using 30 \(\upmu\)L OLA-treated GQDs (Figure 2C), the increase in OLA-treated GQDs sample volume up to 80 \(\upmu\)L (Figure 2D) improved the yield of core-shell structures. Meanwhile, in this latter case, the excess of OLA, free in microemulsion, can promote micelles coalescence due to interfacial tension reduction, that template the formation of elongated silica structures hosting multiple well-separated cores (Figure 2D). The role of free OLA becomes more prominent further increasing the OLA-treated GQDs concentration, with the formation of worm like structures (data not shown). Figure 3 displays TEM micrographs (Figure 3A,B) and the corresponding statistical analysis (Figure 3A1,B1) of single core nanostructures prepared using 80 \(\upmu\)L of OLA-treated GQDs and two different amounts of TEOS, 20 \(\upmu\)L (Figure 3A, sample GQDs@SiO\({}_{2}\) NP1) and 50 \(\upmu\)L (Figure 3B, sample GQDs@SiO\({}_{2}\) NP2) respectively.