No abstract

The use of quantum dot (qdot) nanomaterials in aqueous media for biosensing, imaging, and energy conversion typically requires multistep phase transfer routes based on tailoring surface chemistry. Such surface modification can lead to instability, and increased hydrodynamic diameters, which affect utility. Thus, the ability to synthesize qdots under aqueous conditions with improved photophysical properties that are comparable to the state of the art would be very beneficial. One limitation to this is the availability of high temperature aqueous protocols, which limits size control and crystalline annealing. Here, we show the ability to fabricate highly emissive CdSe, CdSe/CdS, and CdSe/CdS/ZnS qdots under fine-tuned hydrothermal conditions. The novelty of this approach is the use of a synthetic microwave reactor for dielectric heating that provides both kinetic control, and in situ monitoring of temperature and pressure. Results indicate the dramatic improvement for core and core-shell qdot luminescence at hydrothermal temperatures, as indicated by increased monodispersity, quantum yields, qdot brightness, and lifetimes.

The energy transfer between DNA-linked CdSe/ZnS quantum dots (qdots) and gold nanoparticles (AuNPs) is described. The assembly produced qdot−AuNP clusters with satellite-like morphology. Owing to the programmability of the DNA linkage, both assembly as well as disassembly were used as a tool to probe quenching efficiency. Upon assembly, resonance energy transfer between the qdot donor and AuNP acceptor was measured as photoluminescence (PL) quenching. The magnitude of the quenching was approximated upon measurement of PL recovery once the cluster was disassembled by addition of a ssDNA fuel strand, which effectively displaced the qdot-to-AuNP dsDNA linkage. This controllable assembly/disassembly behavior was then used as a morphological tool to separate PL quenching from an inner filter effect originating from the AuNP's high surface plasmon resonance (SPR) extinction. This corrected quenching value was observed from steady state PL measurements, which were then substantiated by PL decay measurements. Finally, the quenching efficiency was related to cluster spatial properties via use of the nanometal surface resonance energy transfer (NSET) method. The AuNP interface to qdot core distance was estimated at ≈8 nm, which was close to the distances visualized by TEM.

A straightforward functionalization strategy for the direct attachment of single-stranded oligonucleotides (ssDNA) to quantum dot (qdot) interfaces is described. The approach takes advantage of a histidine-mediated phase transfer protocol that results in qdots with high colloidal stability in aqueous buffers. The weakly bound histidine encapsulation facilitates monolayer exchanged with both thiolated ssDNA and polyhistidine-tagged proteins. The successful biomodification at the qdot interface was probed by FRET analysis. The modest FRET efficiencies measured suggest the DNA to be in an extended conformation that is the result of high surface coverage that the direct attachment provides.

The DNA-mediated self-assembly of multicolor quantum dot (QD) clusters via a stepwise approach is described. The CdSe/ZnS QDs were synthesized and functionalized with an amphiphilic copolymer, followed by ssDNA conjugation. At each functionalization step, the QDs were purified via gradient ultracentrifugation, which was found to remove excess polymer and QD aggregates, allowing for improved conjugation yields and assembly reactivity. The QDs were then assembled and disassembled in a stepwise manner at a ssDNA functionalized magnetic colloid, which provided a convenient way to remove unreacted QDs and ssDNA impurities. After assembly/disassembly, the clusters' optical characteristics were studied by fluorescence spectroscopy and the assembly morphology and stoichiometry was imaged via electron microscopy. The results indicate that a significant amount of QD-to-QD energy transfer occurred in the clusters, which was studied as a function of increasing acceptor-to-donor ratios, resulting in increased QD acceptor emission intensities compared to controls.