Branched polyethylenimine-functionalized carbon dots as sensitive and selective fluorescent probes for N-acetylcysteine via an off-on mechanism.

N-Acetylcysteine (NAC) plays an important role in optimizing the protective ability of cells as well as in the treatment of some chronic clinical conditions. Unfortunately, current methods for determining NAC based on fluorescence assay strategies remain poorly investigated. In this study, a new fluorescence method for highly sensitive and selective detection of NAC was developed. The detection method employed the fluorescence (FL) signal of branched polyethylenimine-functionalized carbon dots (PEI-CDs) via an off-on mechanism. The detection system was based on the formation of cupric amine complexes by the reaction of Cu2+ ions with surface amino groups on PEI-CDs. The FL of PEI-CDs at 460.0 nm upon exciting at 360.0 nm was quenched as a result of electron transfer between the complexes and PEI-CDs. Upon addition of NAC to Cu2+-CD solution, electron transfer occurred from the mercapto group on NAC to Cu2+, leading to the formation of Cu+ species and the dissociation of cupric amine complexes. As a result, the FL signal of PEI-CDs was turned on since single excited electrons cannot be transferred from PEI-CDs to the cupric amine complexes. The detection limit of this method was 0.56 μM, while the linear response ranged from 5.56 μM to 277.8 μM.