Comparison between terbium-doped carbon dots and terbium-functionalized carbon dots: Characterization, optical properties, and applications in anthrax biomarker detection

Rare earth-carbon dots (RE-CDs) nanomaterials combine the merits of both RE and CDs, which have been the promising candidates in the biochemical sensing field. Owing to the diversity of synthetic methods and precursors, the electronic structure, optical properties and sensing performances of RE-CDs vary greatly. Nevertheless, a comparing study focusing on the synthetic methods of RE-CDs using same precursors has not been reported to date. To facilitate the exploration of the interaction mechanisms for RE–CDs, same precursors (folic acid and terbium nitrate) were used to synthesize RE-CDs via two most common fabrication methodologies (doping and direct coordination) in this study. The morphologies, surface groups, and optical properties of the as-prepared terbium-doped CDs (Tb-CDs) and terbium functionalized CDs ([email protected]) were comprehensively compared and analyzed. The anthrax biomarker (dipicolinic acid, DPA) sensing performances of both RE-CDs were also evaluated. Taking the original CDs without terbium as control, the Tb-CDs had higher quantum yield (QY) and longer fluorescence lifetime, and the fluorescence emission peak was red-shifted, suggesting the effective passivation of surface defects. On the contrary, QY and fluorescence lifetime of [email protected] decreased, and the emission peak remained unchanged, indicating that interaction between free Tb3+ and CDs led to the less excited electrons, similar to the fluorescence quenching behavior of metal ions on CDs. Further characterization also helped in demonstrating the different interaction mechanisms. Moreover, for the superior performance in DPA detection, Tb-CDs was chosen as the probe for the follow-up application. The Tb-CDs exhibited a sensitive response to DPA in the concentration range of 0.05–8 μM with the detection limit of 44 nM. In addition, it was successfully applied to real samples of running water and lake water.