We describe a versatile scheme to prepare an array of multidentate surface capping molecules. Such materials permit strong interactions with semiconductor nanocrystals and render them water compatible. These ligands were synthesized by reacting various chain length poly (ethylene glycols) with thioctic acid, followed by ring opening of the dithiolane moiety. Functionalization of CdSe-ZnS quantum dots with these ligands allow processing of the nanocrystals not only in aqueous but in many other polar solvents. Further synthetic processing of the ligands with biotin moieties allowed for investigating assays based on the avidin-biotin interactions. These ligands provide a straightforward means of preparing QDs that exhibit greater resistance to environmental changes, making them more amenable for use in live cell imaging and other biotechnological applications.
Quantum dots (QDs) are a versatile synthetic photoluminescent nanomaterial whose chemical and photo-physical properties suggest that they may be superior to conventional organic fluorophores for a variety of biosensing applications. We have previously investigated QD-fluorescence resonance energy transfer (FRET) interactions by using the E. coli bacterial periplasmic binding protein - maltose binding protein (MBP) which was site-specifically dye-labeled and self assembled onto the QD surface and allowed us to monitor FRET between the QD donor and the acceptor dye. FRET efficiency increased as a function of the number of dye-acceptor moieties arrayed around the QD donor. We used this system to further demonstrate a prototype FRET based biosensor that functioned in the chemical/nutrient sensing of maltose. There are a number of potential benefits to using this type of QD-FRET based biosensing strategy. The protein attached to the QDs surface functions as a biosensing and biorecognition element in this configuration while the QD acts as both nanoscaffold and FRET energy donor. In this report, we show that the sensor design can be extended to target a completely unrelated analyte, namely the explosive TNT. The sensor consists of anti-TNT antibody fragments self-assembled onto the QD surface with a dye-labeled analog of TNT (TNB coupled to AlexaFluor 555 dye) prebound in the fragment binding site. The close proximity of dye to QD establishes a baseline level of FRET and addition of TNT displaces the TNB-dye analog, recovering QD photoluminescence in a concentration dependent manner. Potential benefits of this QD sensing strategy are discussed.
Colloidal semiconductor quantum dots (QDs) have narrow photoemission bandwidths and broad absorption spectra that are ideal for multiplexing applications. In contrast to organic dyes, which require a complex arrangement of excitation sources and filters to generate multiple signals, many populations of QDs can be simultaneously excited with a single excitation source. In a mixed sample, the narrow and symmetric emission profile of QDs allows simple deconvolution of the composite signal to generate individual QD photoluminescence (PL) contributions. We have shown that CdSe-ZnS core-shell QDs function as efficient energy donors in fluorescence resonance energy transfer (FRET) systems. In this study, we tested several QD-protein bioconjugates, each having a unique PL spectrum (or "color") functioning as independent signal channels, to assess the feasibility of a QD FRET-based multiplexing system. Several populations of QDs were self-assembled with labeled and unlabeled proteins, mixed in solution and excited at single wavelength. The resulting spectra were deconvoluted using the known QD emission profiles to reveal individual contributions of each QD population. QDs coated with dye-labeled protein acceptors showed distinct FRET-induced PL quenching due to the presence of proximal dye acceptors. Steady-state fluorescence results were verified by time-resolved spectroscopic data from the mixed samples where a reduced QD lifetime indicated the presence of proximal dye quencher on one or more QD populations. We will discuss how these findings are used to develop QD-based FRET multiplexed biosensors using a similar strategy where each QD population has surface-bound proteins that are sensitive to a unique molecular target.