Synthesis and characterization of DNA–quantum dot conjugates for the fluorescence ratiometric detection of unlabelled DNA

A quantum dot-based ratiometrically responsive fluorescent sensor for unlabeled single-stranded DNA (ssDNA) is reported. Several technical issues concerning the development of high yield ssDNA–QD conjugation chemistry were addressed. The DNA sensor was synthesized by conjugating methacrylic phosphoramidite-functional oligonucleotides to water-soluble cadmium zinc sulfide core/zinc sulfide shell quantum dots (CdZnS/ZnS QDs). Duplex DNA was formed when the QD-bound ssDNA was incubated with its complement. Next, titration with PicoGreen resulted in FRET energy transfer from the dot to the dsDNA intercalating dye. The resulting ratio of the dye to QD integrated emissions is a calibratable metric for label-free DNA detection with a LOD of 3.8 nmol.