## 3 Results and Discussion
### Surface Functionalization of Nanostructured Porous Silicon  
The surface engineering of nPSi is an important step in using this material in biomedical applications, such as drug delivery carriers. Modifying the chemical surface of the nPSi structure allows the control of some parameters, including surface charge, hydrophilic behavior, and biocompatibility, which may have a considerable influence on managing essential oil load and release. Figure 3 depicts the reaction scheme for each surface functionalization of the nPSi layers studied in this work. By oxidizing the nPSi structure (nPSi-Ox), hydrophilic behavior is obtained, which enhances the loading of positively charged compounds and the degradation rate and allows further functionalization. Undecylenic acid functionalization increases the mechanical stability and solubility of nPSi (nPSi-UAc) due to the contribution of amphipathic chains characterized by their apolar hydrocarbon chains and their polar acidic head. On the other hand, APTS (nPSi-APTS) and chitosan (nPSi-Chi) functionalization give a positive surface charge due to the amino groups, increasing the biocompatibility and load of negatively charged compounds. Finally, \(\beta\)-cyclodextrin-trim polymerization (nPSi-\(\beta\)CD) enhances loading of insoluble compounds and controls kinetic release due to their cavity-like structure with a lipophilic cavity and hydrophilic surface. To monitor the success of each process, FTIR measurements were carried out (Figure 4). The nPSi-Ox reaction was chemically induced by H\({}_{2}\)O\({}_{2}\). The FTIR spectrum shows a characteristic strong absorption band at 1040 cm\({}^{-1}\) and a stretching mode shoulder at 1170 cm\({}^{-1}\), which are related to asymmetric stretching in Si-O from the Si-O-Si group, confirming the success of the process [22]. Additionally, weak absorbance bands appeared at 615 cm\({}^{-1}\) and 790 cm\({}^{-1}\) related to the Si-H and Si-OH bonds, respectively. Furthermore, a weak O-H stretching band from Si-OH groups appears at 3261 cm\({}^{-1}\) and the surface signal assigned to O\({}_{y}\)-Si-H\({}_{\rm x}\) stretching mode can be observed at 885 cm\({}^{-1}\) and 2100 cm\({}^{-1}\)[22; 23]. Undecylenic acid was grafted onto the nPSi layer according to the reaction shown in Figure 3B. During this process, the nPSi surface was hydrocarbonized. The FTIR spectrum demonstrated this state. Two absorption peaks were detected at 625 cm\({}^{-1}\) and 2088 cm\({}^{-1}\) corresponding to Si-H and Si-H\({}_{\rm x}\) groups stretching vibration and deformation mode, respectively. In addition, two absorption peaks appeared at 660 cm\({}^{-1}\) and 912 cm\({}^{-1}\), related to Si-H\({}_{2}\) scissors bending and scissors mode [31]. On the other hand, the absence of a C=C band at 1645 cm\({}^{-1}\) indicated the rupture of the double bond by union with a silicon surface [31]. An additional band is presented at 1713 cm\({}^{-1}\) attributed to the C=O carboxyl group stretching vibration mode [32]. Furthermore, two additional bands were presented at 2855 and 2925 cm\({}^{-1}\) described as the C-H stretching mode of an aliphatic chain [32]. 3-aminopropyl-triethoxysilane was employed to functionalize the nPSi layers (nPSi-APTS) to incorporate amino groups on the surface (Figure 3C). The FTIR spectrum showed three different absorption bands at 3204 cm\({}^{-1}\), 1636 cm\({}^{-1}\), and 790 cm\({}^{-1}\) caused by N-H stretching, and NH\({}_{2}\) bending, respectively. Moreover, two weak signals were observed at 2870 and 2938 cm\({}^{-1}\) which could be assigned to the C-H bond due to asymmetrical and symmetrical stretching vibrations of CH\({}_{2}\) groups [33]. In addition, a strong band appears at 1035 cm\({}^{-1}\) which may be attributed to the Si-O stretching band related to the Si-O-Si group formed between nPSi-Ox and APTS [33]. Chitosan was also utilized to functionalize the nPSi structure, and its chemical reaction is shown in Figure 3D.