A chelating dendritic ligand capped quantum dot: preparation, surface passivation, bioconjugation and specific DNA...

Herein we report the synthesis of a new chelating dendritic ligand (CDL) and its use in the preparation a compact, stable and water-soluble quantum dot (QD), and further development of specific DNA sensor. The CDL, which contains a chelative dihydrolipoic acid moiety for strong QD surface anchoring and four dendritic carboxylic acid groups, provides a stable, compact and entangled hydrophilic coating around the QD that significantly increases the stability of the resulting water-soluble QD. A CDL-capped CdSe/ZnS core/shell QD (CDL-QD) has stronger fluorescence than that capped by a monodendate single-chain thiol, 3-mercapto-propionic acid (MPA-QD). In addition, the fluorescence of the CDL-QD can be enhanced by 2.5-fold by treatments with Zn2+ or S2- ions, presumably due to effective passivation of the surface defects. This level of fluorescence enhancement obtained for the CDL-QD is much greater than that for the MPA-QD. Further, by coupling a short single-stranded DNA target to the QD via the CDL carboxylic acid group, a functional QD-DNA conjugate that can resist non-specific adsorption and hybridize quickly to its complementary DNA probe has been obtained. This functional QD-DNA conjugate is suitable for specific quantification of short, labelled complementary probes at the low DNA probe:QD copy numbers via a QD-sensitised dye fluorescence resonance energy transfer (FRET) response with 500 pM sensitivity on a conventional fluorimeter.