Abstract
Synthetic amorphous silica (SAS) constitute a large group of industrial nanomaterials (NM). Based on their different production processes, SAS can be distinguished as precipitated, fumed, gel and colloidal. The biological activity of SAS, e.g., cytotoxicity or inflammatory potential in the lungs is low but has been shown to depend on the particle size, at least for colloidal silica. Therefore, the preparation of suspensions from highly aggregated or agglomerated SAS powder materials is critical. Here we analyzed the influence of ultrasonic dispersion energy on the biologic activity of SAS using NR8383 alveolar macrophage (AM) assay. Fully characterized SAS (7 precipitated, 3 fumed, 3 gel, and 1 colloidal) were dispersed in H2O by stirring and filtering through a 5 µm filter. Aqueous suspensions were sonicated with low or high ultrasonic dispersion (USD) energy of 18 or 270 kJ/mL, respectively. A dose range of 11.25–90 µg/mL was administered to the AM under protein-free conditions to detect particle-cell interactions without the attenuating effect of proteins that typically occur in vivo. The release of lactate dehydrogenase (LDH), glucuronidase (GLU), and tumor necrosis factor α (TNF) were measured after 16 h. Hydrogen peroxide (H2O2) production was assayed after 90 min. The overall pattern of the in vitro response to SAS (12/14) was clearly dose-dependent, except for two SAS which showed very low bioactivity. High USD energy gradually decreased the particle size of precipitated, fumed, and gel SAS whereas the low adverse effect concentrations (LOECs) remained unchanged. Nevertheless, the comparison of dose-response curves revealed slight, but uniform shifts in EC50 values (LDH, and partially GLU) for precipitated SAS (6/7), gel SAS (2/3), and fumed SAS (3/3). Release of TNF changed inconsistently with higher ultrasonic dispersion (USD) energy whereas the induction of H2O2 was diminished in all cases. Electron microscopy and energy dispersive X-ray analysis showed an uptake of SAS into endosomes, lysosomes, endoplasmic reticulum, and different types of phagosomes. The possible effects of different uptake routes are discussed. The study shows that the effect of increased USD energy on the in vitro bioactivity of SAS is surprisingly small. As the in vitro response of AM to different SAS is highly uniform, the production process per se is of minor relevance for toxicity.
Highlights
Synthetic amorphous silica (SAS) form an important group of industrially relevant nanomaterials (NMs)
In this study on seven precipitated, three fumed, three gel and one colloidal synthetic amorphous silica (SAS) nanomaterials we investigated the biological in vitro effects of SAS dispersions prepared with two different ultrasonic dispersion (USD) energies after the removal of large particles (>10 μm) by filtration
The results partially substantiate the hypothesis, namely, that the in vitro responses of NR8383 cells to SAS dispersed with increased USD energy depends on the production process of SAS
Summary
Synthetic amorphous silica (SAS) form an important group of industrially relevant nanomaterials (NMs). Large quantities of SAS are incorporated in plastics, lacquers and car tires [1]. In consumer products such as cosmetics or food [2,3,4,5] they serve, e.g., as stabilizers, thickeners, or flow enhancing agents [2,4,6,7,8]. The manifold industrial applications of SAS are based upon different physico-chemical properties and different production processes: SAS may be synthesized in an aqueous solution of sodium silicate and this leads to colloidal silica (CS), silica gel (SG), or precipitated silica (PS). While CS is usually supplied as a stable aqueous suspension of well-dispersed nanoparticles [11], PS, FS and SG are delivered as dry powders which consist of aggregates or agglomerates, both of which are formed by nanosized primary silica (SiO2 )
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