Dissolution and Phosphate-Induced Transformation of ZnO Nanoparticles in Synthetic Saliva Probed by AGNES without Previous Solid–Liquid Separation. Comparison with UF-ICP-MS

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Abstract
The variation over time of free Zn2+ ion concentration in stirred dispersions of ZnO nanoparticles (ZnO NPs) prepared in synthetic saliva at pH 6.80 and 37º C was followed in situ (without solid-liquid separation step) with the electroanalytical technique AGNES (Absence of Gradients and Nernstian Equilibrium Stripping). At these conditions, ZnO NPs are chemically unstable due to their reaction with phosphates. The initial stage of transformation (around 5-10 h) involves the formation of a metastable solid (presumably ZnHPO4), which later evolves into the more stable hopeite phase. The overall decay rate of ZnO NPs is significantly reduced in comparison with phosphate-free background solutions of the same ionic strength and pH. The effective equilibrium solubilities of ZnO (0.29 - 0.47 mg·L-1), as well as conditional excess-ligand stability constants and fractional distributions of soluble Zn species were determined in absence and presence of organic components. The results were compared with the conventional ultrafiltration and inductively coupled plasma-mass spectrometry (UF-ICP-MS) methodology. AGNES proves to be advantageous in terms of speed, reproducibility and access to speciation information.
Funding Information
  • Secretar?a de Estado de Investigaci?n, Desarrollo e Innovaci?n (CTM2016-78798)
  • FP7 Nanosciences, Nanotechnologies, Materials and new Production Technologies (310584)
  • Ag?ncia de Gesti? d?Ajuts Universitaris i de Recerca

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