# Aqueous-Organic Phase Transfer of Gold and Silver Nanoparticles Using Thiol-Modified Oleic Acid
## 3 Results
### Aqueous-Organic Phase Transfer of GNP and SNP  
Aqueous solutions of GNP and SNP were transferred to the solvent dichloromethane (DCM) by using OAS ligand. Solutions containing the nanoparticles were mixed with OAS, forming an emulsion. The emulsion was then broken by the sequential addition of DCM and methanol to allow for phase transfer. After a few minutes, the particles were effectively transferred from water to DCM, as can be seen in Figure 5. The metallic particle phase transfer was carried out by ligand exchange, where the thiol from OAS is bound to the particle surface. Thiol groups have shown the highest affinity to noble metal surfaces, particularly to gold. This coupling has been called chemisorption and has been noted as a covalent bond, while the exact processes and the microscopic nature are still subject to research and discussion [13; 22].  
Figure 4: Scanning transmission electron micrographs of nanoparticles in water. (**a**) GNP; (**b**) Particle size distribution histogram of the GNP imaged in the STEM picture (Figure 4a); (**c**) SNP; (**d**) Particle size distribution histogram of the SNP imaged in the STEM picture (Figure 4c). The solid lines in Figure 4b,d are a Gaussian fit to the data.  
Figure 5: (**a**) GNP before phase transfer and (**b**) after phase transfer; (**c**) SNP before phase transfer and (**d**) after phase transfer.  
Figure 6 shows the absorption spectrum of GNP and SNP in aqueous solution and in DCM. Ultraviolet-visible (UV-vis) adsorption spectra documented that the maximum absorption peak of GNP was around 525 nm and around 407 nm for SNP in aqueous solutions. The optical properties such as absorption maxima and absorption intensity are particle-size-dependent [23]. An intense absorption peak at 525 nm is generally attributed to the surface plasmon excitation of small spherical gold particles, which, according to Mie theory for metal nanoparticles, represents a spherical shape and small particle diameter size [24]. The absorption spectrum of GNP and SNP in organic solvents are also shown in Figure 6. After phase transfer, the vanishing of the plasmon signal for GNP and SNP indicates a great transference, around 100%, to the organic phase (results not shown). Methanol is an intermediary solvent to obtain the phase transfer of nanoparticles. According to [PERSON] et al. [25], methanol can induce a full phase transfer, as well as help in the concentration of nanoparticles in the DCM solvent. The UV-vis absorption spectra show that the optical properties of nanoparticles are maintained through the transfer process from water to DCM. The surface plasmon resonance red shift observed in both normalized UV-vis spectra, for gold and silver nanoparticles (Figure 6), could be due to the modification in refractive index of the media.  
The GNP and SNP aqueous-organic phase transfer was performed using the OAS molecule. The morphology of GNP and SNP, analyzed by scanning transmission electron microscopy (STEM), was maintained spherical, as is shown in Figure 7a,c, respectively. The histogram of the particle diameters are plotted in Figure 7b,d, for GNP and SNP, respectively. The solid lines on those figures are a Gaussian fit to the data, leading to the diameter sizes of 15 \(\pm\) 3 nm and 50 \(\pm\) 6 nm for GNP and SNP, respectively, indicating that the particle size is maintained in the phase transfer process. Besides, nanoparticles presented good stability in the organic solvent for several days without precipitation occurring. The metallic nanoparticles dispersed in the non-polar solvent could be transferred again into a polar solvent by using \(\alpha\)-cyclodextrinrin in the polar phase and stirring, according to literature protocols [11; 26].  
Figure 6: (**a**) GNP; (**b**) SNP. Normalized UV–vis spectra of metallic nanoparticles in aqueous solution (black line), and in the solvent DCM (red dashed line).