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NIR-II Fluorescence Imaging Reveals Bone Marrow Retention of Small Polymer Nanoparticles.

The concept that systemically administered nanoparticles are highly accumulated into the liver, spleen and kidney is a central paradigm in the field of nanomedicine. Here, we report that bone is an important organ for retention of small polymer nanoparticles using in vivo fluorescence imaging in the second near-infrared (NIR-II) window. We prepared different sized polymer nanoparticles with both visible and NIR-II fluorescence. NIR-II imaging reveals that the behavior of nanoparticle distribution in bone was largely dependent on the particle size. Small polymer nanoparticles of ∼15 nm diameter showed fast accumulation and long-term retention in bone, while the nanoparticles larger than ∼25 nm were dominantly distributed in liver. Confocal microscopy of bone sections indicated that the nanoparticles were largely distributed in the endothelial cells of sinusoidal vessels in bone marrow. The study provides promising opportunities for bone imaging and treatment of bone-related disease.