**4. Conclusions**

In this study, hydrophilic AgNPs were loaded in two different niosomes, producing two systems, namely NioTw20 + AgNPs and NioSp20 + AgNPs. A deep physical–chemical characterization was

carried out to obtain information on the influence of AgNPs on the preparation and features of niosomal formulations. First of all, the DLS studies confirm the nanosize and stability of both systems in water. Moreover, the entrapment e fficiency for the two systems was investigated, and it was more e fficient for Span 20 than Tw20 niosomes, which was probably related to their di fferent internal structures. Microviscosity and polarity investigations demonstrated that no interactions occurred between the niosomal double layer and the AgNPs, which were probably located inside aqueous compartments. The SAXS data confirmed the presence of the AgNPs located inside the aqueous compartment of the two niosomal systems, and also allowed highlighting the di fferent structures of their double layers. The morphological characterization indicates that the niosomes maintained spherical shapes. Moreover, stability was confirmed in water, bovine serum, and human serum. Moreover, hydrophilic and lipophilic probe release profiles were obtained in HEPES and in human serum. In conclusion, both systems evidenced the entrapment of AgNPs: NioTw20-AgNPs and NioSp20-AgNPs. The two systems are stable in water, bovine serum, and human serum, and maintain the ability to entrap also hydrophilic or lipophilic model molecules. This work demonstrates that the niosomes' features are not altered by AgNPs loading, and confirms that these niosomal formulations are good candidates for the delivery of AgNPs together with other drugs, opening new promising ways for their biotechnological applications.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2079-4991/9/8/1177/s1, Figure S1: (**a**) Synthetic scheme for AgNPs-3MPS; (**b**) Uv-vis spectrum of AgNPs-3MPS in water with SPR at λ = 425 nm; (**c**) <2RH> = 8 ± 3 nm of AgNPs-3MPS in water; (**d**) ζ-potential = −35 ± 2 mV of AgNPs-3MPS in water; Figure S2: Calibration curve of AgNPs; SI Details about Small angle X-ray Scattering (SAXS).

**Author Contributions:** Conceptualization, C.M., M.C., I.V. and I.F.; Chemical synthesis and characterizations of AgNPs: I.V. and I.F.; Preparation of niosomes and silver doped niosomes C.M., F.R., P.N.H., M.C.; SAXS investigations: E.d.F., J.M.; AFM investigations: D.P., M.R., L.A.; All authors reviewed and approved the entire manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** The Grant of Excellence Departments, MIUR (ARTICOLO 1, COMMI 314–337 LEGGE 232/2016), is gratefully acknowledged by Iole Venditti. This work has been supported by Italian Ministry of Education, Universities and Research—Dipartimenti di Eccellenza—L. 232/2016 (Department of Drug Chemistry and Technologies, Carafa and Marianecci).

**Conflicts of Interest:** The authors declare no conflict of interest.
