Quantum Dot Bioconjugation during Core–Shell Synthesis

Abstract

Quantum dots (QDs) have attracted tremendous interest in biological applications, such as bioimaging, biolabeling, and biosensing, because of their advantages over organic fluorophores at high quantum yield (QY), size-tunable narrow emission, photostability, etc. Many of the above applications entail water-soluble and biomolecule-conjugated QDs. To date, various strategies have been developed to obtain QD bioconjugates, either by cross-linking chemistry, 5] biotin–avidin interactions, or ligand-exchange methods, but these commonly require multiple steps to obtain the final products. Herein, we present a simple and robust one-step method for creating stable, water-soluble QD–biomolecule conjugates. We demonstrate the successful implementation of this strategy by using DNA molecules as a model system, but expect this could also be extended to other types of biomolecules. This new strategy was devised so that direct capping of QDs with thiolated DNA oligonucleotides could be achieved during the formation of core–shell QDs. This in situ functionalization of QDs with DNA avoids the cross-linking chemistry or the second-step ligand exchange after core–shell formation. We demonstrate that a high DNA density on the QD surface is achieved and the high QY and stability of the QDs are preserved. Such prepared DNA-capped QDs could serve as excellent candidates for fabrication of biosensors and nanodevices. The process of QD functionalization with DNA is illustrated in Figure 1a. The starting oleylamine-capped CdSe QDs were prepared by our previously reported lowtemperature synthesis method. Coating of the CdSe core with a ZnS shell and capping of the surface with thiolated DNA oligonucleotides are achieved in a single-step one-pot reaction by mixing the oleylamine-capped QD core with Zn, S , and thiolated DNA all dissolved in dimethyl sulfoxide (DMSO). We synthesized two different CdSe@ZnS core–shell QDs with the surface capped with different DNA strands (Figure 2): green QD (lEm= 548 nm) capped with a capture strand C1, denoted G-QD-C1, and red QD (lEm= 617 nm) capped with a capture strand C2, denoted R-QD-C2. These QDs are purified and finally dispersed in 0.5 @Tris/acetic acid/ EDTA (TAE) Mg buffer solution (see Experimental Section for details). The PL spectra of the QDs before and after the ZnS shell growth/surface capping are shown in Figure 1b. For measurement of the spectra, the oleylamine-capped CdSe QDs were dispersed in hexane solution and the obtained DNA-capped CdSe@ZnS core–shell QDs were dispersed in 0.5 @TAEMg buffer solution. As we demonstrated, there were negligible spectral shifts before and after shell formation. The QYs of DNA-capped CdSe@ZnS QDs are significantly increased from those of the QD cores: 8.6 to 41.3% for the green QD and 4.4 to 21.7% for the red QD. It is well-known that passivating CdSe core particles with a larger-bandgap layer of ZnS increases the QYs of QDs. The significant increase in the QYs indicates that the core–shell structures are formed successfully. The excellent water solubility of the DNAcapped QDs also reveals that their surfaces are well-capped with DNA. Transmission electron microscopy (TEM) images (Figure 1c and d) show that both G-QD-C1 and R-QD-C2 have uniform sizes of (4.9 0.2) and (6.9 0.3) nm, respectively, and good crystalline structures (see inset images). Figure 1. a) One-step in situ DNA functionalization of CdSe@ZnS core–shell QDs. b) Photoluminescence (PL) spectra of CdSe core QDs and the DNA-capped CdSe@ZnS core–shell QDs. Both the green and red QDs show a significant increase of the QY after growth of the ZnS shell and DNA capping, simultaneously. The intensities were normalized by green CdSe@ZnS core–shell QDs. c,d) TEM images of green and red core–shell QDs, respectively. Higher-magnification images of individual dots are shown in the insets.