Highly crystalline N and S co-doped carbon dots as a selective turn off-on sensor for Cr(vi) and ascorbic acid and a turn off sensor for metanil yellow

Highly crystalline N and S co-doped carbon dots (NS CDs) were synthesized by a hydrothermal method using mesaconic acid, ethylenediamine, and sulfuric acid as C, N, and S sources. The prepared NS CDs displayed blue fluorescence emission (lambda(ex) = 350 nm and lambda(em) = 405 nm) and exhibited a high quantum yield of 33%. The synthesized NS CDs were spherical and had a mean particle diameter of 3.5 +/- 0.86 nm. The prepared carbon dots have large functional groups on their surface, which makes them have excellent water solubility. NS CDs were used as a selective and sensitive turn off and turn on sensor for detection of Cr(VI) with a linear response ranging from 3 mu M to 60 mu M with a detection limit of 0.056 mu M and ascorbic acid with a linear response ranging from 3 mu M to 126 mu M with a detection limit of 1 mu M. The NS CDs also served as a selective and sensitive turn-off sensor for metanil yellow with a linear response ranging from 3 mu M to 130 mu M with a detection limit of 0.14 mu M. The Cr(VI) recognition mechanism was suggested to be a combination of static quenching and the inner filter effect. At the same time, for the detection of ascorbic acid and metanil yellow, it was proposed to be the inner filter effect. The actual sample application of NS CDs as a nanosensor was studied for detection of metanil yellow as an adulterant in turmeric powder.