Approach of nitrogen doping on the carbon dots derived from poly(ethylene glycol) and the fluorophore identification

Abstract

Nitrogen-doped carbon nano dots (N-doped CDs) caught extensive interests since they usually own higher fluorescent quantum yield (QY) with red-shift emission compared with CDs. However, how was nitrogen element doped into CDs? Which groups in N-doped CDs were responsible for the fluorescent emission? Why did the doping cause red shift? These questions were the keys for understanding the origin of photoluminescence of N-doped CDs, which was obscure until today according to recent publications. Herein, CDs derived from poly(ethylene glycol) were prepared in the first step (emission wavelength = 452 nm; QY = 2.3%). Nitrogen doping was realized conveniently by just reflux heating on CDs and m-phenylenediamine at 70 °C for 3 h. Interestingly, such a facile treatment on CDs aroused significant improvement of the fluorescence (emission wavelength = 516 nm; QY = 36.7%). These N-doped CDs performed very well in cellular imaging of MCF-7. Experimental facts demonstrated the C=C bond and the aldehyde in both CDs and N-doped CDs were the fluorophore that was identical with the CDs prepared with carbohydrates reported in our previous work. Additionally, computations revealed that the partly conjugation of the amide and aromatic ring with fluorescent skeleton of CDs caused the red shift of the emission.