Synthesis of Mercaptoethylamine-coated CdSe/CdS Nanocrystals and Their Use for DNA Probe

Abstract

Luminescent semiconductor nanocrystals (NCs), so-called "quantum dots" (QDs), have attracted increasing interest for bioanalytical labeling applications in recent years.1 NCs can be prepared with adequate homogeneity in both size and shape to allow emission with narrow bandwidths. 2,3 In addition, when their inorganic core is over-coated with a thin layer of a wider band-gap semiconducting material, a substantial enhancement in the NC photoluminescence quantum yield occurs. [4][5][6] For instance, CdSe NCs have a low fluorescent quantum yield, because the optically excited carriers (a positive and negative charge) get trapped at the poorly passivated surface of the nanocrystal. However, when the core is wrapped in a shell of several monolayers of zinc sulfide (ZnS), the core/shell crystals could have a fluorescent quantum yield of 50%. 4 The extinction coefficient of the nanocrystals is several-times larger than that of an organic dye molecule, making the nanocrytals incredibly bright. Finally, the use of appropriate surface-capping ligands permits their dispersion in a variety of solvents, including water, which provides a surface suitable for bioconjugation. 3,7,8 Recently, much research on applying NCs to biolabeling and analysis has been reported. However, most of them have focused on detecting proteins. 9-14For detecting nucleic acid, many studies have been inclined to use electrochemical methods. [15][16][17][18][19] Wang et al. 20 described an electrochemical method for the detection of DNA based on carbon-nanotubes carrying a large number of CdS particle tracers. Cai et al. 21used gold-nanoparticle-modified electrodes to enhance the amount of immobilized probe DNA. Hybridization was induced by the exposure of an ssDNA-containing gold electrode to ferrocenecarboxyaldehyde-labeled complementary DNA in solution. A few references about detecting nucleic acid with other methods have been reported. 22 Hansen et al. 23 applied CdS, ZnS, and PbS nanoparticles conjugated with short DNA sequences, which were immobilized via hybridization with complementary sequences on a gold surface. When a DNA target was added, it could be identified by ousting the existing hybridization between one of the DNA-nanoparticle conjugates and the surface DNA. Thus target DNA could be detected by an electrochemical DNA sensor. Algar et al. 24 detected nucleic acid based on fluorescence resonance energy transfer (FRET) between CdSe/ZnS QDs and a fluorophore. Although these methods are good for detecting DNA, they are too complicated, the probes are unstable, and some of the methods can not be used in quantitative detection.In the present work, highly luminescent CdSe/CdS nanocrystal conjugates were prepared. These water-soluble nanocrystals (coated with mercaptoethylamine) have a homogeneous positive charge density, which allows ultrasensitive quantitative detection of negatively charged DNA. Based on an electrostatic interaction between DNA and functionalized NCs, a novel highly sensitive fluorometric detection for DNA has been develo...