Leaching mechanisms of PVP coated silver nanoparticles from anti-microbial bioplastics

Abstract

Silver based nanocomposites (Ag NC) show great potential as packaging materials, given their antimicrobial performance, protecting food against microbial degradation. The incorporation of silver nanoparticles (Ag NPs) in biobased food contact materials could be considered as an extra opportunity, contributing to a circular bioeconomy with less food waste. However, the lack of knowledge regarding nanoparticle release mechanisms from (biobased) nanocomposites and the fate on the nano-silver in the end-of-life packaging scenario’s (e.g. waste-disposal, recycling…), leads to restrictive legislation. In the case of silver, excessive NP exposure is undesired in terms of health, safety and environmental considerations. Therefore, this study aims to elucidate the NP migration mechanisms and influential factors such as NP size, matrix material (polyhydroxyalkanoate (PHA) vs cellulose) and external migration conditions on the leaching behaviour of Ag NPs and the antimicrobial performance of Ag NCs.

Commercially available spherical Polyvinylpyrrolidone (PVP) coated (~0.2%) Ag NPs are bulk mixed in the PHA matrices using a solvent-based masterblend approach on the one hand, and deposited on cellulose fiber surfaces on the other hand. The processing of these two models allows to differentiate between desorption and diffusion-based migration mechanisms. The internal structure is analysed using scanning electron microscopy (SEM), and the absolute silver loading is quantified via inductively coupled plasma atomic emission spectroscopy (ICP-AES). Subsequently, the NC models are exposed to food simulants (A, B, D2) via full immersion at standardized conditions (EU Regulation 10/2011). To distinguish the silver release in ionized versus nanoparticulate form, the simulant leachate is followed up longitudinally during the immersion by means of single particle inductively coupled plasma mass spectrometry (SP-ICP-MS). In addition, antimicrobial efficiency is tested on Gram-negative and Gram-positive bacteria.

The silver leaching behaviour was found to be dependent on the incorporation mode (bulk vs fiber surface) of the NPs in the NC matrix, rationalized by their different dominant migration mechanisms. In addition, the time- and size (40 vs. 65 nm) dependency of Ag NP leaching could be established and correlated to the NP mobility in two biodegradable matrix systems. A preferential leaching of the smaller NPs was observed. Ultimately, the physicochemical properties of the leaching medium such as acidity (simulant B) play an important role in the extent of silver release and its physical form (dissolved vs. nanoparticulate) in the leachate. Hence, the diffusion, dissolution and desorption release mechanisms of silver are elucidated.

In conclusion, this methodology can be further optimized to quantify potential NP migration from other biopolymers in order to ensure the safety and application potential of bio-nanocomposites as active packaging material.