"On-off-on" fluorescence switch of graphene quantum dots: A cationic control strategy

Highlights: • A novel cationic control strategy for the design of "on-off-on" fluorescence switch was proposed. • Cations on the surface graphene quantum dots dominated the quenching and recovery of fluorescence. • Group IIIA ions Al³⁺, Ga³⁺ and In³⁺ were used for fluorescence recovery for the first time. • A kind of anti-counterfeiting stealth ink for information encryption was obtained. Fluorescence switch based on graphene quantum dots (GQDs) is of great interest in the areas of environment, medicine and data security because of their promising applications. However, the known ternary "on-off-on" fluorescent switches of GQDs are very few due to the limited effective fluorescence recovery strategies. In this work, we proposed a new cationic control strategy and designed a novel "on-off-on" fluorescence switch, i.e., GQDs-Cu²⁺/Fe³⁺/Cr³⁺-Al³⁺/Ga³⁺/In³⁺, where the fluorescence of synthesized GQDs was quenched by Fe³⁺, Cu²⁺ and Cr³⁺, and subsequently was recovered by three kinds of Group IIIA ions Al³⁺, Ga³⁺ and In³⁺. The corresponding recovery degree can reach up to 0.81 compared with the pristine GQDs. The XPS results combining with quantum chemical calculations illustrated that the change of π-conjugated region caused by cations dominated the quenching and recovery of fluorescence. Furthermore, based on this "on-off-on" mechanism, a kind of anti-counterfeiting stealth ink for information encryption was also suggested. This work is the first report that Group IIIA ions Al³⁺, Ga³⁺ and In³⁺ can be used for fluorescence recovery, and the cationic control strategy proposed here provides a new and promising strategy for the development of "on-off-on" fluorescent switch in future studies.