Migration and characterisation of nanosilver from food containers by AF⁴-ICP-MS

Highlights

• The presence of silver was evaluated in different commercial food containers.

• Dissolved silver and nanosilver were distinguished during migration tests.

• A sensitive method for size characterisation of silver nanoparticles (AgNPs) by AF⁴-ICP-MS was proposed.

• Scanning electron microscopy/energy-dispersive X-ray analysis showed AgNP transformations in the extracts.

Abstract

In this work, silver migration from commercial food containers was evaluated according to European Regulation 10/2011. Several experimental parameters affected silver release: food simulant, temperature, exposition time and sampled bag area. Results demonstrated a significant silver nanoparticle (AgNP) migration into aqueous and acidic simulants. The amount of silver migrated increased with storage time and temperature although, in general, silver showed a low tendency to migrate into food simulants (17 ng/g). However, the food simulant did not seem to be a really outstanding variable for long term storage.

AF⁴-ICP MS was used to confirm the presence of AgNPs in the simulants. The low limit of detection achieved (0.4 μg L⁻¹) allowed the identification of AgNPs and their size characterisation (40–60 nm). Finally, scanning electron microscopy/energy-dispersive X-ray analysis suggested a possible transformation of the AgNPs detected in the extracts, due to association with other ligands, such as chlorine and sulphur, present in the original containers.

Introduction

Nanotechnology is nowadays one of the most active technological research areas. Since its introduction, engineered nanoparticles have entered our daily life. However, their environmental fate and final impact on human health is still widely unknown.

The most active area of food nanoscience is packaging (Duncan, 2011). Nanoparticles are easily incorporated into polymers to produce functional materials (Althues, Henle, & Kaskel, 2007) that contribute to extending and improving food shelf life. Among them, silver nanoparticles (AgNPs) are the most frequently used (Luoma, 2008), due to their recognised antimicrobial properties. In vitro toxicological studies have demonstrated that AgNPs are cytotoxic, genotoxic, antiproliferative and possibly carcinogenic (Asharani, Mun, Hande, & Valiyaveettil, 2009). However, in vivo toxicological studies concerning AgNPs are still scarce in the literature and their conclusions are sometimes contradictory (Ji et al., 2007, Takenaka et al., 2001).

Nowadays, it is widely accepted that the behaviour and toxicity of AgNPs is affected by a wide range of factors, including particle number and mass concentration, surface area, charge, size, state of aggregation, elemental composition, pH, electrolyte composition, solution ionic strength, as well as structure, shape and capping agent (Levard, Hotze, Lowry, & Brown, 2012). Capping agents prevent nanoparticles aggregation and oxidation; however, despite their use, AgNPs undergo several transformations in the environment, which modify their properties and affect their transport, fate and toxicity. It is well-known that metallic silver is not stable enough under most environmental conditions, its oxidation being thermodynamically favoured at room temperature. Once oxidised, silver ions tend to react with sulphide (Levard, Reinsch et al., 2011) chloride (Levard, Michel et al., 2011) and organic matter (Liu & Hurt, 2010). Regarding AgNPs toxicity, two possible mechanisms are considered. The most conservative one proposes that silver atoms detach from the surfaces of AgNPs and cause cellular damage. However, most studies point to the fact that AgNPs act as efficient vehicles to deliver a large quantity of silver ions in a short period of time (Lok et al., 2007).

Because AgNPs can rapidly diffuse, due to their small size, during recent years the increased use of AgNPs in plastic food containers has become a cause for concern and their presence in, for example, food-contact materials has started to be regulated. In the EU, the use of AgNPs in plastic food containers is not allowed (Art. 9, EU/10/201), although the presence of certain silver zeolites is authorised in plastic food containers and rubber seals (Art. 7, EU/10/201).

Despite the relevance of the topic, up to now, only a limited number of studies have reported on the presence and possibility of migration of AgNPs from plastic food containers (Echegoyen and Nerín, 2013, Goetz et al., 2013, Huang et al., 2011, Smirnova et al., 2012). In these studies, food containers were exposed to a number of food-simulating solutions under a variety of experimental conditions according to the European Food Safety Authority (EFSA) (1935) recommendation (Art. 10 EU/1935/2004), in an attempt to determine the possible risks for human health. Several discrepancies were found in these studies; however, in all cases, a proportion of the silver that migrated from food containers was in nanoparticle form, being able to enter the human body.

A wide range of analytical techniques are required for detection and characterisation of nanoparticles because no single technique can provide all relevant information. These include: imaging (scanning electron microscopy, SEM; transmission electron microscopy, TEM; atomic force microscopy, AFM; etc.), characterisation (mass spectrometry, X-ray diffraction, nuclear magnetic resonance, NMR; etc.) and separation techniques (high-performance liquid chromatography, HPLC; hydrodynamic chromatography, HDC; field flow fractionation, FFF; etc.). FFF is a highly promising technique, in which separation relies on physical separation of particles in an open channel under the effect of an applied field, without involving any type of stationary phase. This feature makes FFF particularly suitable for the size separation of nanoparticles in complex mixtures, especially when coupled to a high sensitivity detector, such as inductively coupled plasma mass spectrometry (ICP-MS).

The aim of this study was to evaluate the presence and possible migration of AgNPs from several commercially available plastic food containers. Migration experiments were carried out according to EU 10/2011, to cover a variety of food simulants and use conditions. The presence of silver in the obtained food-simulating solutions was determined by ICP-MS. Furthermore, in order to assess the toxicity of the silver released from food containers, it was necessary to develop a high sensitivity method to determine the nature of the detected silver species. For that reason, the presence and the characterisation of AgNPs in the food-simulating solutions was performed by AF⁴-ICP-MS. To the best of our knowledge, this is the first time AF⁴-ICP-MS has been used in this kind of migration assay.

Finally, further confirmation of the presence and morphology of nanosilver additives was carried out by scanning electron microscopy and energy-dispersive X-ray (SEM/EDX) analysis of both the test plastics and the simulant extracts.