Top-Down Preparation of Nanoquartz for Toxicological Investigations

Abstract

Occupational exposure to quartz dust is associated with fatal diseases. Quartz dusts generated by mechanical fracturing are characterized by a broad range of micrometric to nanometric particles. The contribution of this nanometric fraction to the overall toxicity of quartz is still largely unexplored, primarily because of the strong electrostatic adhesion forces that prevent isolation of the nanofraction. Furthermore, fractured silica dust exhibits special surface features, namely nearly free silanols (NFS), which impart a membranolytic activity to quartz. Nanoquartz can be synthetized via bottom-up methods, but the surface chemistry of such crystals strongly differs from that of nanoparticles resulting from fracturing. Here, we report a top-down milling procedure to obtain a nanometric quartz that shares the key surface properties relevant to toxicity with fractured quartz. The ball milling was optimized by coupling the dry and wet milling steps, using water as a dispersing agent, and varying the milling times and rotational speeds. Nanoquartz with a strong tendency to form submicrometric agglomerates was obtained. The deagglomeration with surfactants or simulated body fluids was negligible. Partial lattice amorphization and a bimodal crystallite domain size were observed. A moderate membranolytic activity, which correlated with the number of NFS, signaled coherence with the previous toxicological data. A membranolytic nanoquartz for toxicological investigations was obtained.

Keywords: crystallinity; fracturing; milling; nanoparticle; quartz; silanol; silica.

Figure 1

Scanning Electron Microscopy (SEM) micrographs of a respirable crystalline silica dust of industrial origin (cQ-f, in this work). Nanometric particles (highlighted by the white arrows) adhere on the surface of micrometric particles.

Figure 2

Field emission SEM (FESEM) micrographs at low and high magnification of synthetic as-grown quartz (gQ, (A)), fine-fractured quartz (gQ-ff, (B)), and nanoquartz samples (gQ-n1, gQ-n2, and gQ-n3, (C), (D), and (E), respectively) obtained by ball milling.

Figure 3

Crystallinity of the nanoquartz samples (gQ-n1, gQ-n2, and gQ-n3) compared with the as-grown micrometric quartz (gQ), the fine-fractured quartz (gQ-ff), and a mined fractured quartz used as reference (cQ-f). Diffractograms obtained by XRPD analysis in the 15–70° region (A). Magnification of the diffractograms in the 10–50° region showing the amorphous broad signal (B).

Figure 4

Crystallite size domains (A) and relative numbers (wt.%, (B)) of submicron (>100 nm), nano (<100 nm), and amorphous phases of nanoquartz samples (gQ-n1, gQ-n2, and gQ-n3), calculated by Rietveld refinement of XRPD diffractograms.

Figure 5

Mechanism of quartz fracturing induced by planetary milling in a wet environment. Prolonged millings cause abrasion of the larger crystals, promoting the formation of nanoparticles, and induce amorphization of the nanoquartz fraction, highlighted with a purple halo. Size of crystallites is not in scale.

Figure 6

Transmission Electron Microscopy (TEM) of gQ-n3 after dispersion in ultrapure water and probe sonication. Large agglomerates of 20–30 nm quartz nanoparticles are evidenced at low magnification (A). The particles in the highlighted white square are imaged at higher magnification (B). High-resolution image of a portion of the agglomerated quartz (C) highlights crystalline core, with several diffraction planes visible, and the amorphous external layer (indicated by the asterisk) formed during high-energy milling. A larger submicrometric highly crystalline quartz particle ((D), SAED in the inset). Nanometric agglomerates of milled quartz at low magnification and their corresponding large-field SAED evidenced multiple reflections and rings that indicate a nanometric size of the primary particles ((E,F), SAED in the inset).

Figure 7

Size distribution of the fine-fractured quartz (gQ-ff) and nanoquartz samples (gQ-n1, gQ-n2, and gQ-n3) that were dispersed in water and probe-sonicated. Size distribution of the four samples analyzed by DCS, expressed as relative wt.% (A). Variation of the particle population in the micrometric, submicrometric, and nanometric domains, reported as relative number of particles detected by DCS (B). Hydrodynamic size expressed as relative intensity distribution (C) and relative number distribution of particles (D) of gQ-n3 dispersed in water (with or without post dispersion filtering through a 220 nm porous membrane), and in different biologically relevant media, i.e., Triton X (TRX), dioctyl sulfosuccinate sodium salt (AOT), bovine serum albumin (BSA), phosphate buffered saline (PBS, 10 mM, pH 7.4), artificial phagolysosome fluid (ALF, pH 4.5). Analysis was performed by DLS. FE-SEM micrographs of gQ-n3 after filtration through 220 nm pore size filter (E,F).

Figure 8

Surface silanol distribution of nanoquartz samples (gQ-n1, gQ-n2, and gQ-n3). Transmittance FTIR spectra in the 2200–2800 cm⁻¹ range (νSi-OD region) were collected at b.t. after H/D isotopic exchange and subsequent outgassing for 120 min. Spectra were normalized by the bulk mode and the SSA of the particles (A) or by the number of interacting silanols (B). Hemolytic activity of fine-fractured (gQ-ff) and nanoquartz samples (gQ-n1, gQ-n2, and gQ-n3), reported as function of the particle mass (C) or particle exposed surface area (D). A mined fractured quartz (cQ-f) was used as positive reference particle for the test. Data are mean ± s.d. of three independent experiments; p values of gQ-n1 and gQ-n2 compared to gQ-n3 determined by two-way ANOVA followed by Tukey’s post hoc test (mean effect): *** p < 0.001.