Carbon quantum dots-Annexin V probe: photoinduced electron transfer mechanism, phosphatidylserine detection, and apoptotic cell imaging.

An annexin V-based probe is designed and fabricated using carbon quantum dot as highly stable and biocompatible fluorescent crystals for real-time fluorescence imaging of apoptotic cells. Carbon quantum dots were synthesized, characterized, and conjugated to annexin V. The fluorescence of CQDs at 450 nm (excitation at 350 nm) is quenched due to the photoinduced electron transfer between "carbon quantum dots" and two amino acids (tyrosine and tryptophan) in the annexin structure as quencher. The probe shows very strong and bright fluorescence emission in the presence of phosphatidylserine on the outer layer of the apoptotic cell membrane. It was shown that using fluorescence spectroscopy, the probe can be applied to sensitive phosphatidylserine determination and using fluorescence microscopy, it is possible to monitor cell apoptosis in real time.