Carboxylic acids accelerate acidic environment-mediated nanoceria dissolution

Ligands that accelerate nanoceria dissolution may greatly affect its fate and effects. This project assessed the carboxylic acid contribution to nanoceria dissolution in aqueous, acidic environments. Nanoceria has commercial and potential therapeutic and energy storage applications. It biotransforms <i>in vivo</i>. Citric acid stabilizes nanoceria during synthesis and in aqueous dispersions. In this study, citrate-stabilized nanoceria dispersions (∼4 nm average primary particle size) were loaded into dialysis cassettes whose membranes passed cerium salts but not nanoceria particles. The cassettes were immersed in iso-osmotic baths containing carboxylic acids at pH 4.5 and 37 °C, or other select agents. Cerium atom material balances were conducted for the cassette and bath by sampling of each chamber and cerium quantitation by ICP-MS. Samples were collected from the cassette for high-resolution transmission electron microscopy observation of nanoceria size. In carboxylic acid solutions, nanoceria dissolution increased bath cerium concentration to &gt;96% of the cerium introduced as nanoceria into the cassette and decreased nanoceria primary particle size in the cassette. In solutions of citric, malic, and lactic acids and the ammonium ion ∼15 nm, ceria agglomerates persisted. In solutions of other carboxylic acids, some select nanoceria agglomerates grew to ∼1 micron. In carboxylic acid solutions, dissolution half-lives were 800–4000 h; in water and horseradish peroxidase they were ≥55,000 h. Extending these findings to <i>in vivo</i> and environmental systems, one expects acidic environments containing carboxylic acids to degrade nanoceria by dissolution; two examples would be phagolysosomes and in the plant rhizosphere.

可加速纳米氧化铈（nanoceria）溶解的配体，会对其归趋与生物效应产生显著影响。本研究旨在评估羧酸对水性酸性环境中纳米氧化铈溶解过程的影响。纳米氧化铈已实现商业化应用，同时具备治疗与储能的潜在应用价值，且可在体内（in vivo）发生生物转化。柠檬酸可在合成阶段与水性分散体系中稳定纳米氧化铈。本研究中，我们将平均一次粒径约4 nm的柠檬酸盐稳定型纳米氧化铈分散液装入透析盒（dialysis cassette），该透析盒的膜可允许铈盐（cerium salts）透过，但可截留纳米氧化铈颗粒。随后将透析盒浸没于pH 4.5、温度37 ℃的含羧酸等渗浴液（iso-osmotic baths），或其他指定试剂的浴液中。通过对透析盒与浴液的两个腔室分别取样，并采用电感耦合等离子体质谱法（ICP-MS）定量铈含量，完成了透析盒与浴液中铈原子的物料衡算。从透析盒中取样，通过高分辨透射电子显微镜（High-Resolution Transmission Electron Microscopy）观测纳米氧化铈的粒径变化。在羧酸溶液中，纳米氧化铈的溶解使浴液中的铈浓度提升至透析盒初始引入铈总量的96%以上，同时降低了透析盒内纳米氧化铈的一次粒径。在柠檬酸、苹果酸、乳酸及铵离子溶液中，纳米氧化铈团聚体仍可维持约15 nm的粒径。在其他羧酸溶液中，部分指定的纳米氧化铈团聚体可生长至约1微米。在羧酸溶液中，纳米氧化铈的溶解半衰期为800~4000小时；而在纯水与辣根过氧化物酶（horseradish peroxidase）溶液中，其溶解半衰期则≥55000小时。将本研究结果外推至体内与环境体系，可以推测：含羧酸的酸性环境可通过溶解作用降解纳米氧化铈，例如吞噬溶酶体（phagolysosome）与植物根际（plant rhizosphere）即为两类典型场景。