## 3 Results and Discussion  
### The Coatings of Nanoparticle and Their Characterization  
PAA2K coating is the first to be taken into consideration. This coating was well described in previous articles, and gave a good stability to nanoparticles [22, 23]. Its thickness was estimated by light scattering, comparing the hydrodynamic diameters from bare and coated nanoparticles (Table 1). Bare nanoparticles were measured at 11.6 nm, while PAA-coated nanoparticles measured 20.5 nm. The difference in size is due to the PAA2K layer that corresponds to 4.45 nm. This thickness is almost compatible with the length of the PAA2K chains with 22 monomers (for 2.1 kg/mol PAA2K polymer). NP PAA were then coated with the PDMS-_co_-APMS copolymer. The method described here, consisting of extracting nanoparticles from water to an organic solvent, was successfully achieved by other teams using surfactants [29, 30]. [PERSON] _et al._ used sodium dodecyl sulfate (SDS) molecules, which interacted with nanoparticles by electrostatic interactions. However, one main drawback is that SDS is soluble in both water and organic solvents. In this case, it is very difficult to establish whether all SDS molecules are involved in the nanoparticle coating or not. In our case, the PDMS-_co_-APMS copolymer selected is only soluble in organic solvents, so that we can easily manage the quantity of the copolymer added to the diethyl ether solution. The whole copolymer is attached to the nanoparticles when the aqueous phase conserves only a little orange color, indicating the presence of a small amount of nanoparticles. [PERSON] _et al. [29]_ also shed light on the fact that the coating of nanoparticles is complete when no emulsion is left in the vial after mixing. Because emulsions are always difficult to destabilize and wash, we operated with gentle mixing and adjusted the amount of PDMS-_co_-APMS to coat the whole nanoparticle surface. As a consequence, either small or no polymer excess is obtained during this synthesis. This reaction is conducted in a few minutes and the purification process is achieved in less than one hour. To understand the efficiency of this coating, one can consider the interaction that occurred between carboxylate functions of the PAA2K chains and the amine functions of the PDMS-_co_-APMS copolymer. These amine functions can react with the negative carboxylate functions by electrostatic interaction only if they are positively charged. This is facilitated by decreasing the pH of the water phase lower than their pKa. Typical pKa of amine are in the range of 9-11. As an example, this pKa is measured above 9 for amine containing polymers, but decreased to 7.2 when the amines are close to the surface [31]. For the copolymer scrutinized in this study, the pKa should be at a comparable value. Reactivity of such an amine is maximum for 2 pH units above its pKa. The reaction can be achieved \begin{table}
\begin{tabular}{l c c c c c c c c} \hline \hline  & \multicolumn{3}{c}{Bare nanoparticles} & \multicolumn{3}{c}{SC-PAA} & \multicolumn{3}{c}{SC-PAA-APMS} \\ \hline amine & Diameter & PDI & Diameter [nm] & PDI & Thickness of & Diameter & PDI & Thickness of \\ content & [nm] & & & & coating [nm] & [nm] & & coating [nm] \\ \hline
2–3\% & 11.6 & 0.16 & 20.5 & 0.17 & 4.5 & 41.6 & 0.14 & 10.6 \\
4–5\% & 11.6 & 0.16 & 20.5 & 0.17 & 4.5 & 84.2 & 0.28 & 32 \\
6–7\% & 11.6 & 0.16 & 20.5 & 0.17 & 4.5 & 66.9 & 0.27 & 23.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Hydrodynamic diameters of bare, PAA and PAA/PDMS-_co_-APMS coated nanoparticles. only if the pH is lower than 6-7. This was experimentally observed in our case. The interaction occurred at a pH lower than 6. However, the pH must not be too low to ensure nanoparticle stability. Indeed, the PAA2k-coated nanoparticles are stable with pH higher than 4.5 [22]. For this reason, the pH was adjusted to 5.5 during the experiment by adding small amounts of 0.1 M nitric acid solution. At this value, nanoparticles change spontaneously from the aqueous phase to the organic one (Figure 2).