Carbon quantum dots doped with phosphorus and nitrogen are a viable fluorescent nanoprobe for determination and cellular imaging of vitamin B 12 and cobalt(II)

Phosphorus and nitrogen dually-doped carbon quantum dots (PN-CQDs) were prepared from sucrose, 85% phosphoric acid and 1,2-ethylenediamine as the sources for carbon, phosphorus and nitrogen, respectively. The PN-CQDs possess good water solubility and favorable biocompatibility. The excitation/emission peaks are at 365/451 nm, but bright blue, green, or red emissions are found depending on whether the excitation wavelengths of the laser are set to 408 nm, 488 nm, or 543 nm, respectively. Fluorescence is quenched by both vitamin B 12 (VB 12 ) and Co(II) by a combination of inner filter effect and static quenching. The PN-CQDs are shown to be useful nanoprobes for determination of VB 12 and Co(II). Response to VB 12 is linear in the range of 2.0–31 μM. The response to Co(II) is linear in two ranges, viz. from 1.7–12 μM and from 28 to 141 μM. The limit of detection of VB 12 and Co(II) are 3.0 nM and 29.4 nM, respectively. The nanoprobe was successfully applied to the analyses of VB 12 in drug samples and of Co(II) in spiked water samples, and it gave satisfactory results. The nanoprobe was also applied to the determination of VB 12 and Co(II) in human hepatocarcinoma cells (type SMMC7721), human pulmonary epithelial cells (type BEAS-2B), human adenocarcinoma cells (type A549), and human pheochromocytoma cells (type PC12), respectively. Graphical abstract Schematic presentation of the quenching of the fluorescence of phosphorus and nitrogen dually-doped carbon quantum dots (PN-CQDs) by vitamin B 12 (VB 12 ) and Co(II).