Abstract
One of the principle hurdles to wider incorporation of semiconductor quantum dots (QDs) in biology is the lack of facile linkage chemistries to create different types of functional QD--bioconjugates. A two-step modular strategy for the presentation of biomolecules on CdSe/ZnS core/shell QDs is described here which utilizes a chemoselective, aniline-catalyzed hydrazone coupling chemistry to append hexahistidine sequences onto peptides and DNA. This specifically provides them the ability to ratiometrically self-assemble to hydrophilic QDs. The versatility of this labeling approach was highlighted by ligating proteolytic substrate peptides, an oligoarginine cell-penetrating peptide, or a DNA-probe to cognate hexahistidine peptidyl sequences. The modularity allowed subsequently self-assembled QD constructs to engage in different types of targeted bioassays. The self-assembly and photophysical properties of individual QD conjugates were first confirmed by gel electrophoresis and Forster resonance energy transfer analysis. QD-dye-labeled peptide conjugates were then used as biosensors to quantitatively monitor the proteolytic activity of caspase-3 or elastase enzymes from different species. These sensors allowed the determination of the corresponding kinetic parameters, including the Michaelis constant (K(M)) and the maximum proteolytic activity (V(max)). QDs decorated with cell-penetrating peptides were shown to be successfully internalized by HEK 293T/17 cells, while nanocrystals displaying peptide--DNA conjugates were utilized as fluorescent probes in hybridization microarray assays. This modular approach for displaying peptides or DNA on QDs may be extended to other more complex biomolecules such as proteins or utilized with different types of nanoparticle materials.