Synthesis of cationic CdSe/ZnS quantum dots and applications in DNA detection

Abstract

Semiconductor nanoparticles or quantum dots (QDs), especially CdSe/ZnS QDs, are of great interest in bioapplications due to their unique size-dependent optical properties and photo-stability. One of the plausible applications of QDs is DNA detection using QDs as a novel fluorescence tool for sensing of DNA using the electrostatic interaction between negative charge of DNA and positive charge on QDs surface. In this work, CdSe/ZnS QDs were synthesized using a hot-solution decomposition process resulting in hydrophobic CdSe/ZnS QDs. To obtain water-soluble and cationic QDs, the surface of QDs were modified with poly(ethylene)imine (PEI) via micelle formation. Because of hydrophilicity of PEI and hydrophobicity of QDs surface, for the formation of micellar structures, PEI was partially acylated with octanoic acid through EDC/NHS-coupling process to increase the hydrophobicity. The the structure of octanoic acid-modified PEI (Oct-PEI) were studied using infrared spectroscopy, 1H and 13C NMR, and mass spectrometry. TEM results showed that diameter of the synthesized CdSe/ZnS QDs are 4.60 ±0.53 nm and are in spherical shapes. Moreover, XRD results also showed that the synthesized QDs contained both of CdSe and ZnS. To characterized the cationic QDs (Oct-PEI/QDs), the UV-Visible spectroscopy, fluorescent spectroscopy, dynamic light scattering analysis and TEM were used. The size and zeta-potentials of the QDs showed that, the sizes and charges of Oct-PEI/QDs were in the range of 250-400 nm and +36 to +40 mV, respectively. The ratio between of Oct-PEI and QDs was varied and it was found that the higher ratio of QDs and Oct-PEI, the larger size and the higher magnitudes of positive charge. The optical properties of cationic QDs was measured and compared with the original QDs. The absorption and emission results showed that the Oct-PEI/QDs still maintained the properties as the original QDs, and the fluorescent signal lasted for 30 days in samples prepared using the Oct-PEI/QDs ratio of 60:1 by mol. TEM results revealed that the increase in sizes of the particles were likely due to micelles contained several individual particles. Moreover, it was found that the Oct-PEI/QDs interacted with DNA and DNA matched peptide nucleic acid (PNA) for investigating the sensing properties. The results showed that the Oct-PEI/QDs can interact with DNA and become potentially useful for biological studies.