## 4 Bionanosystems
### Bionanosystems for RA
#### 4.1.1. Bionanosystems Based on Organic Carriers
Furthermore, the authors demonstrated that RA-loaded CS nanoparticles were safe without relevant cytotoxicity against retinal pigment epithelium and human corneal cell line, thus making these nanoparticles very promising drug delivery systems for ocular application in oxidative eye conditions. [PERSON] et al. [99] later demonstrated that RA-loaded nanoparticles with zeta potential of 20-30 mV are suitable to adhere to the intestinal epithelium, releasing the antimicrobial compound to the gut and preventing or controlling intestinal disorders provoked by bacterial food pathogens (i.e., _Bacillus cereus_, _Escherichia coli O157_, _Listeria innocua_, _Staphylococcus aureus_, _Salmonella typhimurium_, and _Yersinia enterocolitica_). As an alternative to encapsulation in spherical CS particles, adsorption of RA on CS powder was also studied, obtaining water-insoluble RA-CS complexes characterized by a high amount of immobilized RA [100,101]. From kinetic studies it was established that the adsorption of RA on CS powder occurred in two steps: in the first step RA molecules were attached to the ionized amine groups on the surface of the CS powder due to electrostatic interaction, while in the next step the previously adsorbed RA molecules became the sites for adsorption of additional RA molecules due to hydrophobic interaction. Furthermore, this study revealed an interesting aspect; that of the fluorescence of aqueous dispersions of RA-loaded powders. The authors showed that as the amount of RA adsorbed on the CS powder increases, the fluorescence emission maximum shifts towards shorter wavelengths, suggesting a stronger association of RA molecules on the surface of the CS powder [102]. Interestingly, [PERSON] et al. [103] prepared RA-loaded chitosan encapsulated graphene nanoparticles and demonstrated that this combination has high in vitro antibacterial activity, and it is efficacious for wound healing treatment. Furthermore, chitosan was combined with zinc oxide and the biocompatible nanoparticles were successfully used to preconcentrate and recover RA in complicated matrices, such as the extracts of medical plants [104], by dispersive micro-solid phase extraction. Cyclodext (CDs) are another example of natural polymers successfully used for encapsulating polyphenols [105,106]. CDs are cyclic oligosaccharides soluble in water with an inner hydrophobic core and a hydrophilic exterior, which can be used as host-nanosystems [107]. Indeed, the hydrophobic cavity (diameter about 0.60 nm) can act as a host entrapping polyphenols and terpen, thereby improving solubility in water, increasing chemical and physical stability, and, in a few cases, enhancing the activity of guest molecules. Molecules such as gallic acid, (\(-\))-epigallocatechin, (\(-\))-epigallocatechin-gallate, quercetin, chlorogenic and caffeic acids, and also RA were successfully encapsulated in CDs [108,109,110]. [PERSON] et al. [109] showed that RA can be encapsulated into the cavities of \(\beta\)- and \(\gamma\)-CDs, leading to water-soluble and photo-stable inclusion complexes. In these systems, the complementary use of 1D and 2D-ROESY NMR and capillary electrophoresis demonstrated that RA and \(\beta\)-CD form a stable complex and that catechol subunits of RA are inserted inside the CD hydrophobic cavity, while no interactions of RA with the outside of the CD unit could be reliably detected. The results suggest that this conformation can improve the thermo-oxidative stability of RA and modulate its release as a function of pH [108]. The complexation of RA with \(\alpha\)-CD, \(\beta\)-CD and with differently functionalized CDs revealed an increment of the RA water solubility with an increase in the fluorescence intensity and a blue shift of the emission maximum (Figure 6). In addition,the antioxidant capacity of RA also increased for RA-CD complexes, which was interpreted as the stabilization of the 1-_e_ oxidized catechol moiety of RA by enhanced intramolecular H-bonding [111]. Alginate beads can be also used as RA nanocarriers. For example, [PERSON] et al.