Dissolution Behaviour of Metal-Oxide Nanomaterials in Various Biological Media

Abstract

Toxicological effects of metal-oxide-engineered nanomaterials (ENMs) are closely related to their distinct physical-chemical properties, especially solubility and surface reactivity. The present study used five metal-oxide ENMs (ZnO, MnO₂, CeO₂, Al₂O₃, and Fe₂O₃) to investigate how various biologically relevant media influenced dissolution behaviour. In both water and cell culture medium (DMEM), the metal-oxide ENMs were more soluble than their bulk analogues, with the exception that bulk-MnO₂ was slightly more soluble in water than nano-MnO₂ and Fe₂O₃ displayed negligible solubility across all tested media (regardless of particle size). Lowering the initial concentration (10 mg/L vs. 100 mg/L) significantly increased the relative solubility (% of total concentration) of nano-ZnO and nano-MnO₂ in both water and DMEM. Nano-Al₂O₃ and nano-CeO₂ were impacted differently by the two media (significantly higher % solubility at 10 mg/L in DMEM vs. water). Further evaluation of simulated interstitial lung fluid (Gamble's solution) and phagolysosomal simulant fluid (PSF) showed that the selection of aqueous media significantly affected agglomeration and dissolution behaviour. The solubility of all investigated ENMs was significantly higher in DMEM (pH = 7.4) compared to Gamble's (pH 7.4), attributable to the presence of amino acids and proteins in DMEM. All ENMs showed low solubility in Gamble's (pH = 7.4) compared with PSF (pH = 4.5), attributable to the difference in pH. These observations are relevant to nanotoxicology as increased nanomaterial solubility also affects toxicity. The results demonstrated that, for the purpose of grouping and read-across efforts, the dissolution behaviour of metal-oxide ENMs should be evaluated using aqueous media representative of the exposure pathway being considered.

Keywords: ICP-MS; PSF and Gamble; aluminium oxide; cerium oxide; inhalation pathway; iron oxide; manganese oxide; nanoparticles; zinc oxide.

Figure 1

Influence of particle size (nano vs. bulk) on % solubility of MnO₂, ZnO, CeO₂, Al₂O₃, and Fe₂O₃ after 48 h of incubation in water and DMEM. Initial metal-oxide concentration was 100 mg/L. Results presented as mean (standard deviation) of triplicates. All pH values are reported in Tables S5A and S10, and Table S8 (Supplementary Materials) shows additional information. nd = Not detected. For a given metal-oxide and medium, ‘*’ and ‘***’ indicate significant differences between nano vs. bulk at α = 0.05 and 0.001, respectively, based on the Student’s or Welch’s t-test.

Figure 2

Influence of initial concentrations (10 mg/L vs. 100 mg/L) on % solubility of the ZnO, MnO₂, CeO₂, Al₂O₃, and Fe₂O₃ ENMs in water and DMEM after 48 h of incubation (<LOD = below the limit of detection; nd = not detected). Results are presented as mean (standard deviation) of triplicates, and pH values are reported in Table S5A. For a given metal-oxide and medium, ‘**’ and ‘***’ indicate significant differences between 10 vs. 100 mg/L at α = 0.01 and 0.001, respectively, based on the Student’s t-test. See Figure S2 in the Supplementary Materials for absolute mass dissolved in water and DMEM.

Figure 3

Influence of aqueous medium on % solubility of nano metal-oxides at a 100 mg/L initial concentration (2, and CeO₂, respectively, in water. For a given metal-oxide, values followed by the same letter are not statistically different at α = 0.05. See Table S8 for standard deviations and Table S9B for results of statistical tests.

Figure 4

Evidence of Al (a) and Fe (b) sedimentation during control experiments with soluble salts: analyte loss increases as centrifugation time increases. Results (% recovered in supernatant) are presented as mean (standard deviation) of triplicates. no sep = No centrifugation, 2 × 30 min and 3 × 30 min centrifugation at 20,000× g.