Ligands that accelerate nanoceria dissolution may greatly affect its fate and effects. This project identified carboxylic acids that contribute to nanoceria dissolution in aqueous, acidic environments. Nanoceria has commercial and potential therapeutic applications. It biotransforms in vivo. Citric acid is commonly used to stabilize nanoceria during synthesis and in aqueous dispersions. In this study, citrate-stabilized nanoceria dispersions (~ 4 nm average primary particle size) were placed in dialysis cassettes whose membranes would pass cerium salts but not nanoceria particles. The cassettes were immersed in isoosmotic baths containing carboxylic acids at pH 4.5 at 37 °C, or select agents. Cerium atom material balances were conducted for the cassette and bath by sampling of each chamber and cerium quantitation by inductively coupled plasma mass spectrometry. Samples were also collected for high-resolution transmission electron microscopy observation of nanoceria size (cassette). In carboxylic acid solutions, nanoceria dissolution increased cerium concentration in the bath and decreased the nanoceria primary particle size in the cassette. In solutions of citric, malic, and lactic acid, and in the ammonium ion, ~ 15 nm nanoceria agglomerates persisted. With other carboxylic acids, nanoceria agglomerates grew to ~ 1 micron. Nanoceria particles were stable in solutions containing citrate (pH 7.4), water, or horseradish peroxidase i.e., the dissolution half-lives were very high. Extending these findings to in vivo and environmental systems, one would expect acidic environments containing carboxylic acids to degrade nanoceria by dissolution; two examples would be phagolysosomes and in the plant rhizosphere.

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Nanoparticle dissolution in local milieu can affect their ecotoxicity and therapeutic applications. For example, carboxylic acid release from plant roots can solubilize nanoceria in the rhizosphere, affecting cerium uptake in plants. Nanoparticle dispersions were dialyzed against ten carboxylic acid solutions for up to 30 weeks; the membrane passed cerium-ligand complexes but not nanoceria. Dispersion and solution samples were analyzed for cerium by inductively coupled plasma mass spectrometry (ICP-MS). Particle size and shape distributions were measured by transmission electron microscopy (TEM). Nanoceria dissolved in all carboxylic acid solutions, leading to cascades of progressively smaller nanoparticles and producing soluble products. The dissolution rate was proportional to nanoparticle surface area. Values of the apparent dissolution rate coefficients varied with the ligand. Both nanoceria size and shape distributions were altered by the dissolution process. Density functional theory (DFT) estimates for some possible Ce(IV) products showed that their dissolution was thermodynamically favored. However, dissolution rate coefficients did not generally correlate with energy of formation values. The surface-controlled dissolution model provides a quantitative measure for nanoparticle dissolution rates: further studies of dissolution cascades should lead to improved understanding of mechanisms and processes at nanoparticle surfaces.

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