Multicolored Crystals Synthesized For Bar Codes

Luminescent materials find wide use in areas such as anticounterfeiting and biological sensors, especially when patterned as bar codes. Such materials, however, can be difficult to make or require expensive instrumentation to detect. New microrods can be easily synthesized to emit multiple colors by doping with various lanthanides, reports a group led by Xiaogang Liu of the National University of Singapore (J. Am. Chem. Soc. 2014, DOI: 10.1021/ja5013646). The rods are composed of NaYF₄ and doped with ytterbium, thulium, or erbium in varying proportions to emit blue, green, or red light in response to infrared radiation. The researchers grew single-color rods in solution to dimensions of 1.7 µm long and 0.14 µm in diameter. They then added tips of another color by incubating the rods with nanoparticles containing a different dopant. The rods and their tips can be distinguished by using a standard optical microscope. Dispersing the microrods in dimethyl sulfoxide yielded an ink that remains transparent under ambient light. The researchers used the ink to stamp letters on paper and illuminated them with IR light. In cell culture experiments, the rods penetrated the outer membranes of cancer cells, showing promise for biological applications.