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

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). The CDL ligand, which contains a chelative dihydrolipoic acid (DHLA) moiety for strong QD surface anchoring and four dendritic carboxylic acid groups, can provide a stable, compact and entangled hydrophilic coating around the QD. We show that a CDL capped CdSe/ZnS core/shell QD (with a relatively thin ZnS shell) can be effectively “annealed” of surface defects by treatments with Zn2+ or S2- ions, leading to a significant fluorescence enhancement of over 250%. Further, by coupling a short DNA target to the QD via the CDL carboxylic acid group, a functional QD-DNA conjugate that can resist the strong, non-specific adsorption of DNA has been prepared. The resulting QD-DNA conjugate hybridizes quickly and specifically to its complementary DNA probe, making it a suitable DNA sensor capable of detecting specific complimentary DNA probe at low DNA probe:QD copy numbers via QD-sensitised dye fluorescence resonance energy transfer (FRET) response with sub-nanomolar detection limit on a conventional fluorimeter.