Quantitative ROI analysis of the PET imaging data and ex vivo biodistribution studies. Time–activity curves of the liver, blood, 4T1 tumor, and muscle upon intravenous injection of (a) UCNP@[89Zr]HMSN-PEG5k-TRC105, (b) UCNP@[89Zr]HMSN-PEG5k, or (c) UCNP@[89Zr]HMSN-PEG5k-TRC105 after a blocking dose (1 mg per mouse) of TRC105. (d) Comparison of 4T1 tumor uptake among the three groups. The difference in 4T1 uptake between UCNP@[89Zr]HMSN-PEG5k-TRC105 and the two control groups was statistically significant (**p < 0.005). (e) Biodistribution comparison in 4T1 tumor-bearing mice at 48 h p.i. All data represent three mice per group.