Luminescence color regulation of carbon quantum dots by surface modification

Labelling various biological molecules with multi-color carbon quantum dots (CQDs) in the same scene has great potential application for investigating the relationship between those molecules. It is still a great challenge to regulate the color of CQDs. Ample -NH2 groups on the surface of green carbon quantum dots (GCQDs) were characterized and confirmed by 1H nuclear magnetic resonance (1H NMR) data. As-NH2 groups had high organic reactivity, the surface of GCQDs was designed and modified with several organic functional groups through a simple surface modification strategy of organic reactions. The surface band gap states of GCQDs were tuned gradually by the corresponding modified functional groups, and the emission colors of GCQDs were tuned from green to yellow-green, then to orange, and finally to red. All the emission spectra for these multicolor carbon quantum dots (M-CQDs) were excitation independent, and the lifetimes for the M-CQDs were mainly determined by the second electron decay process from the surface defect band states. The red-shift colors for the M-CQDs were ascribed to the emissions from their surface defect band states, and the luminescence mechanism had been investigated. This work opens up a novel synthesis strategy to regulate the emission colors of carbon quantum dots.