Doping effect and fluorescence quenching mechanism of N-doped graphene quantum dots in the detection of dopamine.

Element doping is recognized as a powerful way to modify surface defect structure and further enhance the fluorescence performance of graphene quantum dots (GQDs). N-doped, S-doped and S, N co-doped GQDs were synthesized to explore the influence of element doping on fluorescence sensing of dopamine (DA) biomolecules. Two interesting works are found, one is that the N-doped GQDs with urea as N source are more effective than the S-doped and S,N co-doped GQDs, characterized by the higher quantum yield (QY) up to 78% and sensitive fluorescence quenching performance to DA. The other is that the N-doped GQDs with ethylenediamine as N source have the highest QY up to 95%, however, exhibits no quenching performance to DA. This abnormal observation is discussed based on the microstructure analysis. Under the optimal reaction condition, the N-doped GQDs exhibit a dual linear relationship of quenching intensity with DA concentration in the range of 10-3000 nM and 3000-7000 nM with detection limits of 3.3 and 611 nM, respectively. The quenching mechanism of N-doped GQDs toward DA is explored from the view of N chemical states, biomolecule structure of DA homologues and redox reaction of DA.