Synthesis of pH-Sensitive Nitrogen-Doped Carbon Dots with Biological Imaging Function and Their Application in Cu2+ and Fe2+ Determination by Ratiometric Fluorescent Probes

pH-sensitive nitrogen-doped carbon dots (N-CDs) were synthesized using immature seeds of elm trees as a carbon source and ethylenediamine as a coreactant through a facile one-step hydrothermal method. The N-CDs were characterized using fluorescence spectroscopy, fluorescence lifetime, ultraviolet-visible absorption, X-ray photoelectron spectroscopy, X-ray diffraction, and Fourier transform infrared spectroscopy, as well as transmission electron microscopy. The N-CDs displayed excellent fluorescence properties and responded to pH changes. The N-CDs exhibited low toxicity and good biocompatibility and had the potential to be used for the biological imaging of HeLa cells and mung bean sprouts. Utilizing the mechanism of fluorescence resonance energy transfer, ratiometric fluorescent probes were prepared by simple mixing of N-CDs and fluorexon in a Britton-Robinson buffer solution. The ratiometric fluorescent probe was used to detect Cu2+ and Fe2+. The linear equations were R-Cu = -0.0591[Q] + 3.505 (R-2 = 0.992) and R-Fe = -0.0874[Q] + 3.61 (R-2 = 0.999). The corresponding limits of detection were 0.5 and 0.31 mu M, respectively. The good results had been obtained in the actual samples detection.