**4. Conclusions**

Electrospun nanobiopaper bilayer coatings of water barrier PCL and PHB biopolymers containing PdNP were here developed for the first time over conventional cellulose paper. Some of the materials were post-processed by annealing, to achieve both better adhesion among layers, and to form a continuous structure to improve the barrier properties to water. SEM and FIB-SEM results

indicated that the PdNP agglomerated in certain areas of the fiber cross-section, but TEM results also indicated that some of the PdNP were dispersed and distributed within the biopolymer matrix. Better distribution of the PdNP was inferred for the PCL matrix. FIB-SEM 3D reconstruction was a very powerful tool for visualizing composites beyond the conventional SEM results, and it seen to exhibit good correspondence with TEM results. However, TEM was further able to resolve better at the nanoscale, and it showed that some of the PdNP were highly dispersed and distributed within the fibers. The water barrier was enhanced, as expected, after annealing of the fiber-based materials, but the annealing process also decreased the oxygen absorption capacity. A previous work indicated that with electrospun PHB fibers containing PdNP, the oxygen scavenging capacity of the films reduced to a significant extent after annealing, compared to the non-heated highly porous electrospun fibers. In this more advanced study on the topic, the oxygen scavenging of the PdNP was largely enhanced, even after annealing, by incorporating these within PCL, a more oxygen permeable material that is still biodegradable.

The fully biodegradable fiber-based multilayered materials developed here show their tremendous potential for becoming the next generation of barrier papers, with demonstrated water barrier and oxygen scavenging capacity that are of interest in, for instance, food packaging applications.

**Author Contributions:** Conceptualization was devised by J.M.L.; Methodology, Validation, and Formal Analysis was carried out by A.C., J.M.L., P.K.S., A.G.; U.S. Investigation, Resources, Data Curation, Writing—Original Draft Preparation and Writing—Review & Editing was performed by A.C., J.M.L., P.K.S., A.G., U.S. Supervision, J.M.L.; Project Administration, J.M.L.

**Funding:** This research was funded by the Spanish Ministry of Economy and Competitiveness (MINECO) project AGL2015-63855-C2-1-R, the H2020 EU YPACK (reference number 773872) and ResUrbis (Reference number 730349).

**Acknowledgments:** A. Cherpinski would like to thank the Brazilian Council for Scientific and Technological Development (CNPq) of the Brazilian Government for her predoctoral gran<sup>t</sup> (205955/2014-2). U. Stachewicz thanks the National Science Centre in Poland for the Sonata Bis 5 project, No 2015/18/E/ST5/00230 allowing this study, and a PhD scholarship for P.K. Szewczyk. The 3D imaging was supported by the infrastructure at the International Centre of Electron Microscopy for Materials Science (IC-EM) at AGH University of Science and Technology.

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