The unique properties of nanomaterials are being leveraged to create textiles with enhanced fabric properties, a trend exemplified within the production of face masks since the onset of the Covid-19 pandemic. The use of silver (Ag) is advertised in several commercially available face masks because of its biocidal properties, claiming a higher form of protection to the wearer. The silver biocide can present itself either in ionic form, nanoparticle (NP) form or as part of a composite material, however, this is not always correctly advertised[1]. In addition, the presence of titanium dioxide (TiO2) nanoparticles, acting as a whitening and mattening agent, in synthetic fibres applied in face masks has been demonstrated[2], even when no specific information was provided on their packaging. Since TiO2 is a suspected carcinogen (IARC 2B) and potential negative health effects are reported for nano-silver[3], their use in masks raises health concerns because of the potential inhalation exposure to nanoparticles. This emerging area of concern lacks methodological development regarding the assessment of inhalation exposure to nanoparticles during mask use. To assess the potential of particle release and associated risks, at first a detailed characterization of the localization and the form of nanomaterials within the mask is necessary. This work applies scanning transmission electron microscopy (STEM) coupled with energy-dispersive x-ray spectroscopy (EDX) on a set of 10 face masks. The method is evaluated in terms of ability to distinguish different nanoforms, to measure particle properties (size/shape/agglomeration state) important in the context of risk assessment in line with the EU’s regulatory framework and to assess potential release of NP.
A set of 10 face masks, including community, surgical and FFP2 masks, was characterized. 6 masks, where the application of biocides (5 Ag, 1 unspecified) was advertised, and 4 masks, demonstrated to contain TiO2 (2 of them also had Ag advertised) were selected to assess the form, localization and measurement of Ag (and other possible biocides) and TiO2, respectively. Since the masks usually consist of multiple layers of fabric, at first the layer with the highest Ag or TiO2 content was identified using inductively coupled plasma assisted optical emission spectroscopy or mass spectroscopy and selected for subsequent STEM analysis. The mask was disassembled and a piece of 1 mm x 5 mm was cut from the selected layer. The preparation of TEM specimens from the pieces of mask followed the methodology described in Wouters et al.[4] Ultra-thin sections of the textiles were prepared by embedding them in epoxy resin, followed by ultra-thin sectioning using ultramicrotomy. Subsequent STEM-EDX analysis was carried out on a Talos F200S transmission electron microscope equipped with a high angle annular dark field detector and a Super-X EDX detector (Thermo Fisher Scientific, Eindhoven, The Netherlands). Imaging and image analysis was done using Velox software (V3.11, Thermo Fisher Scientific).
In 5 face masks, a form of (nano)silver could be identified. In each of those masks, the silver biocide presents itself under a different form: (i) a silver-silica NP composite, (ii) a reaction mass of silica, titanium dioxide and silver chloride NP (depicted in Fig. 1), (iii) a silver-zincoxide NP composite, (iv) nanoscale silver precipitation on a mineral complex and (v) NP containing a combination of silver and sulfur or silver and tin. In 4 out of 5 cases, the identified biocide form is not matching the advertized description, which is often ambiguously reported or lacks the mention of a ‘nanoform’. In cases (i) and (iii) the silver biocide is embedded within the fibres, while in the other three cases it is at least partly present on the surface of fibres. The amount of silver-containting particles detected in each sample is too low to build a statistically relevant size distribution but their sizes range from 5-200 nm and they have a spheroidal shape. Even though only 4 masks were selected for evaluation of incorporated TiO2 particles, TiO2 was detected innine masks. This was notmentionedon the packaging or in the provided technical information. The TiO2 appears in the form of aggregates and agglomerates of spheroidal (nano)particles which are embedded within the fibres, and some are also present on the edge of fibres. A preliminary quantitative analysis of TiO2 in selected samples, reveals an average aggregate/agglomerate size of 186 nm and an average constituent particle size of 100 nm in terms of the minimum Feret diameter, in agreement with the typical textile grade TiO2[5]. Three masks are found to consist of fibres with a (partial) coating of NP. In one mask this coating consists of a mix of TiO2 and AgCl NPs embedded within silica (Fig. 1). The mask with unspecified biocide consists
of fibres with a partial coating of agglomerated silica NP with constituent particle sizes on the order of a few tens of nm. And lastly, one mask, which applies a photocatalytic activation of the biocide, consists of a mix of polymer fibres and silica-containing, fibre-like structures. The latter ones have agglomerated TiO2 particles attached to them, with constituent particle sizes varying from approximately 10 to 50 nm, i.e. smaller than the textile grade TiO2 reported before. These three masks are considered to be most prone to partice release, based on the localization and amount of observed nanomaterial.
STEM-EDX proves itself as a valuable methodology to allow identification of the type of biocide, the localisation of the (nano)particles and their size measurement within fibres of face masks. Our analyses reveal that packaging labels are often not accurately reporting the type of biocide or nanoparticles applied within the product, highlighting the need for regulatory control. The presence of NP on the fibre’s surface presses for investigations of potential release of particles under realistic usage conditions, which will be explored in the next stage of the project.
