Ratiometric ultrasensitive fluorometric detection of ascorbic acid using a dually emitting CdSe@SiO2@CdTe quantum dot hybrid

A dually emitting quantum dot nanohybrid was designed for the fluorometric 2-wavelength determination of ascorbic acid (AA). The nanohybrid consists of red-emitting CdTe quantum dots (QDs) covalently linked to the surface of silica nanoparticles containing green-emitting CdSe QDs. The fluorescence of the red-emitting CdTe QDs (peaking at 616 nm) is first quenched by addition of KMnO4 due to oxidation under formation of CdTeO3 and TeO2. On addition of AA, the red fluorescence is restored due to the reduction of surface CdTeO3/TeO2 to form CdTe. The green fluorescence of the green-emitting CdSe QDs (peaking at 522 nm), in contrast, remains constant and its intensity serves as a reference signal. Both kinds of QDs can be photoexcited at 380 nm. An easily distinguishable transition in fluorescence (from green to orange-red) can be observed on increasing the amount of added AA. Under optimal conditions, the ratio of the intensities of the red and the green fluorescence increases linearly in the 0.1 to 5.0 mu M AA concentration range, and the detection limit is as low as 35 nM. The assay was successfully applied to the quantitation of AA in (spiked) fruit juice samples.