Development of Size-Tunable Superparamagnetic Iron Oxide/Fluorescent Conjugated Polymer Composite Nanoparticles for Sentinel Lymph Node Biopsy

Sentinel lymph nodes (SLNs) are critical diagnostic targets for detecting breast cancer metastasis. Integrating preoperative magnetic detection and fluorescence-guided surgical resection is a promising strategy for accurate and minimally invasive SLN biopsy. Achieving this requires tracers with strong fluorescence, high superparamagnetism, and precise size tunability within the 10–100 nm range because particle size significantly affects SLN accumulation. Conjugated polymer nanoparticles (Pdots) are expected to be a promising platform for integrating fluorescent and magnetic detections due to their high optical properties and biocompatibility. However, encapsulating superparamagnetic tracers into Pdots while maintaining controlled sizes and shapes remains challenging. In this study, size-tunable magnetic-fluorescent nanoparticles (MFNPs) composed of superparamagnetic iron oxide nanoparticles (SPIONs) and the conjugated polymer poly(9,9-dioctylfluorene- alt -benzothiadiazole) (F8BT) are developed for precise SLN detection. MFNPs are synthesized via the nanoprecipitation method by encapsulating SPIONs within a F8BT matrix. By adjustment of the F8BT concentration and SPIONs/F8BT mass ratio, the MFNPs size is successfully tuned between 34 and 75 nm. Vibrating sample magnetometry and fluorescence spectroscopy confirm that the obtained MFNPs maintain stable magnetic and fluorescent characteristics regardless of their size. In vivo subcutaneous injection of MFNPs demonstrates successful magnetic and fluorescent identification of SLN. Furthermore, MFNPs show size-dependent macrophage uptake in vitro and SLN accumulation in vivo, indicating their usefulness as a size-tunable platform. The size-tunable MFNPs developed in this study are expected to be utilized as a tool to advance the understanding of nanoparticle-based SLN targeting as well as to enable precise, less invasive SLN biopsy.