# Solvent-Dispersible Nanostructured MIMI: An Experimental and Computational Study
## 3 Results and Discussion  
All the samples were morphologically characterized by TEM and, from a spectroscopic point of view, by collecting the extinction spectra of the aqueous colloidal dispersion. For samples with a core consisting of gold nanorods, the theoretical spectra were calculated. Following Chart 1, we first discuss all nanoparticles having a gold rod-shaped core. Figure 1a reports the TEM images of AuNR@CTAB, showing the gold nanorods coated with CTAB (57 \(\times\) 18 nm), while Figure 2 (blue line) displays the experimental extinction spectrum of its water dispersion. A longitudinal band at 732 nm and a transversal one at 515 nm were observed. According to the FWHM of the longitudinal band and TEM image, the sample was homogeneous in shape and size. By modeling CTAB-covered gold nanorods of 57 \(\times\) 18 nm (Figure 3a), the calculated spectrum fit satisfactorily with the experimental one. In Figure 1b, the TEM image of the AuNR@SiO\({}_{2}\) sample is reported: a thick silica shell of 60 nm formed around the gold core. When the gold nanorods were covered by the silica shell, their longitudinal band red-shifted to 770 nm (Figure 2, red line). The simulated absorption and extinction cross sections for AuNRs in the same conditions as the experimental measurements (Figure 3a,b) showed a good agreement with the experimental results. In particular, with the addition of the silica shell, the two plasmonic peaks red-shifted, appearing at 520 nm and 775 nm, respectively, due to a change in the refractive index of the surrounding medium (from water to silica) around the gold nanorods. The metal-dielectric interface in the case of nanoparticles covered with CTAB consisted of water (\(\mathrm{n_{d}}^{20}=1.33\), water refractive index) embedding CTAB. In the case of nanoparticles covered with the silica shell, the dielectric had a higher refractive index (\(\mathrm{n_{d}}^{20}=1.45\), silica refractive index) [60; 61; 62]. The TEM image of AuNR@SiO\({}_{2}\)@seed (Figure 1c) shows the whole silica surface functionalized with the small gold seeds. After the seed solution addition, the transversal band increased, due to the contribution of the seeds (Figure 2, green line), while the position of the transversal and longitudinal bands remained unvaried with respect to that observed for the previous sample. In the AuNR@SiO\({}_{2}\)@Au sample, the gold seeds gave way to a gold shell with a thickness of about 55 \(\times\) 28 nm, giving rise to MIMI nanoparticles with overall dimensions of about 280 \(\times\) 200 nm (Figure 1d). The spectral behavior (Figure 2, violet line), confirmed by calculations (Figure 3c) dramatically changed with respect to the previous samples: scattering appeared to be predominant over absorption, and a broadened band in the range from 730 nm to 1200 nm was recorded. The transversal band appeared barely visible. Figure 3: Shown is the 3D meshing of the model geometry of the constitutive elements of the nanomatryoshka and simulated absorbance cross sections of **(a)** AuNR, **(b)** AuNR@SiO\({}_{2}\) and scattering cross sections of **(c)** AuNR@SiO\({}_{2}\)@Au and **(d)** AuNR@SiO\({}_{2}\)@Au@SiO\({}_{2}\). Figure 2: Experimental spectra: extinction spectra of the aqueous colloidal dispersions: AuNR (**a**, blue line); AuNR@SiO\({}_{2}\) (**b**, red line); AuNR@SiO\({}_{2}\)@seed (**c**, green line); AuNR@SiO\({}_{2}\)@Au (**d**, violet line) overlapping with AuNR@SiO\({}_{2}\)@Au@SiO\({}_{2}\) (**e**, orange dotted line). Finally, the previous MIMI nanoparticle was covered by a 3 nm thick silica shell, giving rise to the MIMI nano-matryoshka AuNR@SiO2@Au@SiO2, reported in the TEM image of Figure 1e. The outermost shell of silica of a few nm did not make any contribution to the optical response of the nanoparticle, as shown by the spectrum in Figure 2 (orange dotted line) and confirmed by calculation (Figure 3d).