Silver nanoparticles-clays nanocomposites as feed additives: Characterization of silver species released during in vitro digestions. Effects on silver retention in pigs

Two different clay nanocomposites, as sepiolite-Ag and kaolinite-Ag, are studied as carriers for silver nano particles (AgNPs) oral administration as antimicrobial agent in additives for animal feed. A three-step digestibility assay, corresponding to stomach, small and large intestine simulations, has been followed. Ultrafiltration and asymmetrical flow field-flow fractionation (AF4) coupled to UV-Vis absorption and ICPMS detectors have been used for size characterisation of the silver species released during the in vitro digestibility assays. Less than 1% of the total silver is released in the stomach simulation step, probably due to the formation of silver chloride on the nanocomposite surface. In the case of the intestine simulation, silver released increases and tends to form complexes with the enzymes present in the media. A larger amount of silver was released from kaolinite-Ag compared to sepiolite-Ag (17 vs. 7%), probably due to a higher retention rate of silver shown by sepiolite, justified by its sorption capacity and fibrous structure. No evidences were found about a significant release of silver nanoparticles along the in vitro assay from any of the two nanocomposites studied. These results have been compared to the silver levels found in liver and faeces from weaned pigs fed with these supplements for 35 days and followed by basal diet free of silver for 4 weeks in an in vivo assay. Piglets fed with kaolinite-Ag retained more silver in liver than those receiving sepiolite-Ag, and the opposite was observed in faeces, although differences were not statistically significant. Silver levels found in muscles were below the limit of detection (0.009 mu g Ag g(-1)) in all cases.