The Combined ICP-MS, ESEM-EDX, and HAADF-STEM-EDX Approach for the Assessment of Metal Sub-Micro- and Nanoparticles in Wheat Grain.

Metal sub-microparticles (SMPs) and nanoparticles (NPs) presence in food is attributable to increasing pollution from the environment in raw materials and finished products. In the present study, a multifaceted analytical strategy based on Environmental Scanning Electron Microscopy and High-Angle Annular Dark-Field-Scanning Transmission Electron Microscopy coupled with Energy-Dispersive X-ray Spectroscopy (ESEM-EDX, HAADF-STEM-EDX) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) was proposed for the detection and characterization of metal and metal-containing SMPs and NPs in durum wheat samples, covering a size measurement range from 1 nm to multiple µm. ESEM-EDX and ICP-MS techniques were applied for the assessment of SMP and NP contamination on the surface of wheat grains collected from seven geographical areas characterized by different natural and anthropic conditions, namely Italy, the USA, Australia, Slovakia, Mexico, Austria, and Russia. ICP-MS showed significant differences among the mean concentration levels of metals, with the USA and Italy having the highest level. ESEM-EDX analysis confirmed ICP-MS concentration measurements and measured the highest presence of particles < 0.8 µm in size in samples from Italy, followed by the USA. Less marked differences were observed when particles < 0.15 µm were considered. HAADF-STEM-EDX was applied to a selected number of samples for a preliminary assessment of internal contamination by metal SMPs and NPs, and to expand the measurable particle size range. The multifaceted approach provided similar results for Fe-containing SMPs and NPs. ICP-MS and ESEM-EDX also highlighted the presence of a significant abundance of Ti- and Al-containing particles, while for STEM-EDX, sample preparation artifacts complicated the interpretation. Finally, HAADF-STEM-EDX results provided relevant information about particles in the low nm range, since, by applying this technique, no particles smaller than 50 nm were observed in accordance with ESEM-EDX.

Application of silver-based biocides in face masks intended for general use requires regulatory control.

Silver-based biocides are applied in face masks because of their antimicrobial properties. The added value of biocidal silver treatment of face masks to control SARS-CoV-2 infection needs to be balanced against possible toxicity due to inhalation exposure. Direct measurement of silver (particle) release to estimate exposure is problematic. Therefore, this study optimized methodologies to characterize silver-based biocides directly in the face masks, by measuring their total silver content using ICP-MS and ICP-OES based methods, and by visualizing the type(s) and localization of silver-based biocides using electron microscopy based methods. Thirteen of 20 selected masks intended for general use contained detectable amounts of silver ranging from 3 µg to 235 mg. Four of these masks contained silver nanoparticles, of which one mask was silver coated. Comparison of the silver content with limit values derived from existing inhalation exposure limits for both silver ions and silver nanoparticles allowed to differentiate safe face masks from face masks that require a more extensive safety assessment. These findings urge for in depth characterization of the applications of silver-based biocides and for the implementation of regulatory standards, quality control and product development based on the safe-by-design principle for nanotechnology applications in face masks in general.

Titanium dioxide particles frequently present in face masks intended for general use require regulatory control.

Although titanium dioxide (TiO2) is a suspected human carcinogen when inhaled, fiber-grade TiO2 (nano)particles were demonstrated in synthetic textile fibers of face masks intended for the general public. STEM-EDX analysis on sections of a variety of single use and reusable face masks visualized agglomerated near-spherical TiO2 particles in non-woven fabrics, polyester, polyamide and bi-component fibers. Median sizes of constituent particles ranged from 89 to 184 nm, implying an important fraction of nano-sized particles (< 100 nm). The total TiO2 mass determined by ICP-OES ranged from 791 to 152,345 µg per mask. The estimated TiO2 mass at the fiber surface ranged from 17 to 4394 µg, and systematically exceeded the acceptable exposure level to TiO2 by inhalation (3.6 µg), determined based on a scenario where face masks are worn intensively. No assumptions were made about the likelihood of the release of TiO2 particles itself, since direct measurement of release and inhalation uptake when face masks are worn could not be assessed. The importance of wearing face masks against COVID-19 is unquestionable. Even so, these results urge for in depth research of (nano)technology applications in textiles to avoid possible future consequences caused by a poorly regulated use and to implement regulatory standards phasing out or limiting the amount of TiO2 particles, following the safe-by-design principle.