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Fluorescent nanodiamonds (FNDs) are carbon-based nanomaterials that emit bright, photostable fluorescence and exhibit a modifiable surface chemistry. Myeloid-derived suppressor cells (MDSCs) are an immunosuppressive cell population known to expand in cancer patients and contribute to worse patient outcomes. To target MDSC, glycidol-coated FND were conjugated with antibodies against the murine MDSC markers, CD11b and GR1 (dual-Ab FND). In vitro, dual-Ab FND uptake by murine MDSC was significantly higher than IgG-coated FND (94.7% vs. 69.0%, p < 0.05). In vivo, intra-tumorally injected dual-Ab FND primarily localized to the tumor 2 and 24 h post-injection, as measured by in vivo fluorescence imaging and flow cytometry analysis of the spleen and tumor. Dual-Ab FND were preferentially taken up by intra-tumoral MDSC, representing 87.1% and 83.0% of FND+ cells in the tumor 2 and 24 h post-injection, respectively. Treatment of mice with anti-PD-L1 immunotherapy prior to intra-tumoral injection of dual-Ab FND did not significantly alter the uptake of FND by MDSC. These results demonstrate the ability of our novel dual-antibody conjugated FND to target MDSC and reveal a potential strategy for targeted delivery to other specific immune cell populations in future cancer research. [ABSTRACT FROM AUTHOR]