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β-phase NaGdF4 nanocrystals doped with Er3+ and Yb3+ possessing diverse morphologies were synthesized from the thermolysis of trifluoroacetate precursors in 1-octadecene and oleic acid by modifying the hot-injection strategy. Modulation of the injection temperature during the hot-injection step was an effective approach to control the size and shape of the prepared nanocrystals and allowed for the direct synthesis of nanorods. Here, we report for the first time the fabrication of monodispersed uniform nanorods through a one-step thermolysis approach. The different supersaturation caused by the different temperatures could directly manipulate the nucleation and growth of α-phase nanoparticles before the α → β phase transition, subsequently influences the Ostwald ripening mode during the α → β phase transition, and consequently affects their morphology (i.e., nanorods, nanospheres, nanoprisms, nanoplates, and nanodisks), uniformity, and monodispersity. The upconversion luminescence intensity decreased with increase of the surface to volume ratio of the upconverting nanocrystals, and a higher ratio of green to red emission was observed when the aspect ratio was close to 1. The negative contrast enhancement on T2-weighted magnetic resonance images caused by the upconverting nanocrystals was increased with increasing size with the exception of the nanorods, which performed the best as T2 contrast agents despite being smaller compared to the nanoplates. Our work provided strong evidence for the use of morphology controlled synthesis in NaGdF4 based upconverting nanocrystals and their implementation in multifunctional nanoplatforms for future biomedical applications.
Wang et al. (Mon,) studied this question.