Huan Ge

Many methods have been reported for synthesizing graphene oxide (GO) and graphene oxide quantum dots (GOQDs) where a tedious operational procedure and long reaction time are generally required. Herein, a facile one‐pot solvothermal method that allows selective synthesis of pure GO and pure GOQDs, respectively is demonstrated. What is more, the final product of either GO or differently sized GOQDs can be easily controlled by adjusting the reaction temperatures or reactant ratios, which is also feasible when enlarged to gram scale. The 2.5 nm GOQDs show excellent photoluminescence that can be utilized for bioimaging or distinctive detection of Eu 3+ and Tb 3+ from their respective mixtures with other rare earth and/or transition metal ions, at sub‐ppm level.

The perfect energy level overlap of 2H11/2, 4S3/2, and 4F9/2 in Er3+ ions with those of 5F3, 5F4/5S2, and 5F5 in adjacently codoped Ho3+ ions allows efficient interenergy transfer. Therefore, in addition to routine activators, Er3+ or Ho3+ can further act as sensitizers to transfer the upconverted energy to nearby Ho3+ or Er3+, resulting in enhanced upconversion luminescence due to the emission overlap. Proper codoping of Er3+/Ho3+ or Ho3+/Er3+ obviously elevates the maximum doping concentration (thus producing additional upconverted photons) to a level higher than that causing luminescence quenching and significantly enhances upconversion emissions compared with those of singly Er3+ or Ho3+-doped host materials. Indeed, the so-far strongest red upconversion emission under 1532 nm excitation was obtained in LiYF4:Er/Ho@LiYF4 nanoparticles and Ho3+-sensitized Er3+ upconversion emissions excited by 1150 nm laser was simultaneously discovered. With great enhancement compared with that of singly Ho3+ doped counterparts, this work demonstrates the generality and rationality of our design strategy.