Despite the pressing need to noninvasively monitor transplanted cells in vivo

with fl uorescence imaging, desirable fl uorescent agents with rapid labeling

capability, durable brightness, and ideal biocompatibility remain lacking.

Here, phosphorylcholine-coated near-infrared (NIR) fl uorescent semiconducting

polymer nanoparticles (SPNs) are reported as a new class of rapid,

effi cient, and cytocompatible labeling nanoagents for in vivo cell tracking.

The phosphorylcholine coating results in effi cient and rapid endocytosis and

allows the SPN to enter cells within 0.5 h in complete culture medium apparently

independent of the cell type, while its NIR fl uorescence leads to a tissue

penetration depth of 0.5 cm. In comparison to quantum dots and Cy5.5, the

SPN is tolerant to physiologically ubiquitous reactive oxygen species (ROS),

resulting in durable fl uorescence both in vitro and in vivo. These desirable

physical and physiological properties of the SPN permit cell tracking of human

renal cell carcinoma (RCC) cells in living mice at a lower limit of detection

of 10 000 cells with no obvious alteration of cell phenotype after 12 d.

SPNs thus can provide unique opportunities for optimizing cellular therapy

and deciphering pathological processes as a cell tracking label.