Hydrothermal synthesis of ionic liquid-capped carbon quantum dots with high thermal stability and anion responsiveness

Ionic liquid (1-aminopropyl-3-methyl-imidazolium bromide, [APMIm][Br])-capped carbon quantum dots (CQDs) were obtained by a hydrothermal method. Characterization results demonstrated that the CQDs possessed an average particle size of 2.9 nm, poor crystallinity, and [APMIm][Br] shell linked by amide bond. In addition, the CQD surfaces were also decorated by some oxygen-containing groups. These surface groups made CQDs have an excellent dispersibility in water. The optical property measurements indicated that CQDs aqueous suspension emitted strongest blue fluorescence at 443 nm under 375 nm excitation with a fluorescence quantum yield of 20.3 %. The luminescence property of CQDs in water exhibited excitation wavelength-, concentration-, and pH-dependences, high photostability, and outstanding salt resistance. More importantly, the [APMIm][Br] shell endowed CQDs with a high thermal stability (>300 °C) and an interesting anion responsiveness, i.e., CQDs spontaneously transferred from aqueous phase to ethyl acetate phase once the anion of N(CF3SO2) 2 − was introduced. These particular properties should provide more possibilities for the utility of CQDs in various fields.