## 3 Results and Discussion
### Raman Analysis  
The preparation of the natural nano structures to be applied in the Raman analysis did not require any particular technique as explained in the Section 2.4. These were used as substrates to perform Raman measurements on samples of Rhodamine 6G. Under the action of the laser excitation, Rhodamine 6G showed peaks at 614, 774, 1182, 1362, 1507 and 1648 cm\({}^{-1}\). The 614 cm\({}^{-1}\) peak corresponds to the C-C-C ring in-plane bending mode. The C-H out-of-plane bending mode for R6G was observed at 774 cm\({}^{-1}\) while the C-O-C stretching frequency appeared at 1182 cm\({}^{-1}\). Peaks centred at 1362, 1507, 1571 and 1648 cm\({}^{-1}\) were attributed to the aromatic C-C stretch of the R6G molecule [50]. As is evident from the spectra images (Figure 3), typical characteristic peaks from Rhodamine-6G are observed our measurements. Figure 2: From top left: perspective view of the peacock feather (**A**); opal (**B**) and diatom (**C**) nanostructures. From top right: simulated enhanced electric field of the peacock feather (**D**); opal (**E**) and diatom (**F**) nanostructures. The spectra obtained from each natural structure were processed with the aim of highlighting the qualitative aspects of the results and comparing the measurements. Each sample showed a different behaviour, mainly due to structural reasons. The best response came from the opal (marked in red in the image above). This was due to the small size of its nanospheres, whose diameters are a few dozens of nm, as well as its regular and flat conformation, which allowed, in addition to complete gold coverage, direct interaction between the source laser and the molecule. The spectrum obtained using the peacock feather as support for the Raman analysis, showed visible, but low intensity, peaks (marked in green in the image above). In fact, the irregular structure of the peacock feather does not allow complete gold coverage, and consequently part of the sample was not able to produce the SERS effect. Moreover, since the surface of the feather, at the microscale, was not flat, the deposited solution containing Rhodamine did not stand in position, falling out of the sample in a few minutes. Another limitation was the usable power intensity as high powers can damage the feather. The detected peaks concerning the diatoms (marked in blue in the image above) were indistinguishable from baseline and did not allow the characterization of the molecule. This happened because, while in the peacock feather and the opal, the nanostructures were in direct contact with the analyte and largely exposed to the laser beam, the diatoms 3D tubular structures were casually arranged in the space. In addition, the 3D structure caused the absorption of a portion of the molecule; thus, most of the signal came from diatoms and not from Rhodamine, and that is why the silicon pattern (marked in black in the image above), even without nanostructures, showed peaks higher than those of the diatoms. Furthermore, the irregularity of the diatoms does not allow total gold coverage; the size of the diatom nanostructure is bigger than the nanostructure of the other samples, causing a decrease in the enhancement obtainable. Improvements in the SERS signals were computed as intensity ratios between the natural nanostructures and flat Si-Au samples for the peak at 1358 cm\({}^{-1}\). We found that the SERS improvement for opal was approximately 13.88, peaofowl had a much lower value of 1.88, while for diatom, the value of 0.63 indicated that the Si-Au substrate behaved much better as SERS substrate. Considering that SERS intensities scale like the fourth power of the electric field, the just-mentioned ratios correspond to the following improvements in the enhancing factors: 1.93 for opal, 1.17 for peaofowl, and 0.89 for diatom. It is worth remarking that these values are not the absolute enhancing factors of the electric field, but only the relative values with reference to Si-Au SERS enhancing factor.