Highly fluorescent nitrogen-doped carbon dots for the determination and the differentiation of the rare earth element ions.

As indispensable and strategic elements, Rare Earth Elements (REEs) have been found wide high-tech applications in many modern industries. Their mining, extraction and purification address on economic and environmental significances. This study was the first to report the successful determination and differentiation of rare earth metals using the Nitrogen-doped Carbon Dots (NCDs). NCDs were synthesized via the one-step hydrothermal treatment method starting from folic acid. The as-received NCDs can emit bright blue fluorescence at 440 nm in a high quantum yield of 85% due to the fluorescence enhancement effect of N dopant atoms on NCDs' surface. This as-received NCDs can not only determine REE ions based on the fluorescence quenching effect of the newly-formed REE-NCDs complexes, also distinguish two groups of REEs with significantly different electronic structures. It was found REE ions (Group II REEs: Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb) can only quench fluorescence of the initial NCDs solution, but not change its fluorescence wavelengths. This was contrasted to the similar quenching effect of REE ions (Group I REEs: La, Y, Lu) on NCDs, but further 20 nm red-shift of the fluorescence wavelength (from 439 nm to 458 nm). Taking examples of the selected La and Sm, their detection limit were found to be 0.045 μM (the linear range of 0-6 μM, R =0.998) and 0.29 μM (the linear range of 0-10 μM, R =0.993), respectively. The characterizations of NCDs, based on the ultraviolet and visible spectrophotometry (UV-VIS), X-ray photoelectron spectrometer (XPS) and Fourier Transform infrared spectroscopy (FTIR), helped in revealing the coordination status of rare earth ions on the carboxyl group of the NCDs surface and also mechanisms of their electron and energy transfers.