Nitrogen-doped (N-doped) photoluminescent carbon dots (CDs) were prepared by a one-pot microwaveassisted hydrothermal treatment using histidine as the sole carbon source in the absence of acid, alkali, or metal ions. With a diameter of 2 S nm, the synthesized CDs had apparent lattice fringes and exhibited an excitation-dependent photoluminescent behavior. The CDs were highly yielded, welldispersed in aqueous solution, and showed high photostability in the solutions of a wide range of pH and salinity. They were used as probes to identify the presence of Fel'. ions with a detection limit of 10 riM. With confirmed nontoxicit-y, these CDs could enter the cancer cells, indicating a practical potential for cellular imaging and labeling.