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Highly luminescent upconversion nanoparticles (UCNPs) with small sizes are highly desirable for bioapplications. A facile in situ cation exchange strategy has been developed to greatly enhance the UC luminescence of hexagonal phase NaYF4 NPs while maintaining their small particle size and shape. Via a cation exchange treatment by hot-injecting Gd3+ precursors into the as-prepared NPs solution without pre-separation, the naked-eye visible UC emission of the NPs was enhanced about 29 times under 980 nm near infrared (NIR) excitation with unchanged particle size. The cation exchange process was further demonstrated for the case of NaYF4 nanorods (NRs). After the cation exchange, the nanorod was broken into two NPs with stronger emission. The cation exchanged hydrophobic UCNPs were further encapsulated with poly(amino acid) and successfully applied for targeted cancer cell UC luminescence imaging.
This research was supported in part by the National Natural Science Foundation of China (Grant No. 21275015), the State Key Project of Fundamental Research of China (Grant Nos. 2011CBA00503 and 2011CB932403), the Program for New Century Excellent Talents in University of China (No. NCET100213), the Science Foundation of Xinjiang Uygur Autonomous Region (201191170), the Fundamental Research Funds for the Central Universities(ZZ1321), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry.