Determination of vitamin B12 via pH-dependent quenching of the fluorescence of nitrogen doped carbon quantum dots

Highly stable, hydrophilic carbon quantum dots (CQDs) with a fluorescence quantum yield of 43% were prepared by dehydration/condensation and aromatization/carbonization of a mixture of diethylenetriamine and trisodium citrate. The CQDs possess a mixed crystalline and polycrystalline structure in the basic and acidic medium respectively. The fluorescence of the CQDs (with excitation/emission peaks at 390/450 nm) arises from both the CQDs and from molecular fluorophores incorporated in the CQDs and is highly pH dependent in the range between pH 2.0 and 9.0. Fluorescence is strongly quenched by vitamin B12 (VitB12). This finding is exploited in a quenchometric method for the quantification of VitB12 that works in the VitB12 concentration range between 1 nM to 20 mu M, and the lower limit of detection is 210 pM.