Synthesis of near-infrared-fluorophore-loaded microplastics with different compositions for in vivo tracking

Abstract

Microplastics (MPs) are pervasive environmental pollutants of growing concern owing to their potential impact on ecosystems and human health. Although reports of human exposure to MPs are increasing, our understanding of the in vivo behavior of MPs remains limited. This study aimed to enable the observation of the in vivo dynamics of MPs by loading them with a fluorescent dye that emits light in the second near-infrared (NIR-II) biological window, which allows deep-tissue imaging. Plastic granules— composed of polypropylene, polyethylene, or polystyrene—were dispersed in tetrahydrofuran to generate nanosized particles, which were mixed with a solution of IR-1061 dye in acetonitrile and heated at 55 °C (N₂ atmosphere) to promote dye loading. An aqueous solution of bovine serum albumin was then added dropwise and stirred in open air to evaporate the organic solvent, resulting in obtaining water-dispersed IR-1061-loaded MPs that exhibited fluorescence in the NIR-II region. The in vivo behavior of IR-1061-loaded MPs was examined following oral administration to mice. No fluorescence from IR-1061-loaded MPs was observed in tissues outside the gastrointestinal tract, indicating minimal intestinal absorption. At 48 h post-administration, NIR-II fluorescence from IR-1061-loaded MPs was observed in feces but not in vivo, indicating that the MPs were excreted in feces. The extent of intestinal retention was influenced by MP size rather than chemical composition. The development of methods for synthesizing NIR-II-fluorophore-loaded MP models with various chemical compositions will support risk assessments by providing insights into the environmental and biological fates of MPs.