Effects of Electrostatic Repulsion and Steric Exclusion on Single-Stranded DNA Attaching to the ZnS Surface of Quantum Dots

DNA linked quantum dot (QD) assemblies have been applied in biological drug delivery, programmable materials, and photonics. The mechanism of a DNA chain attaching to the QD surface is very important to developing new DNA-QD nanostructures. In this work, we combine all-atom molecular dynamics simulations and experiments to reveal the mechanism of a single-stranded DNA (ssDNA) attaching to the ZnS surface of CdSe-ZnS (core-shell) QDs. We verified the ssDNA model chain conformation through agreement with experimental and theoretical conformations in the literature. We report the ssDNA chain conformation on a flat ZnS surface and on differently sized ZnS QD surfaces. They depend on the combined effects of electrostatic repulsion and steric exclusion from 3-mercaptopropionic acid (MPA) and O-(2-mercaptoethyl)-O'-methyl-hexa (ethylene glycol) (mPEG) capping on ZnS QDs. Both simulations and Förster Resonance Energy Transfer (FRET) experiments indicate that ssDNA tail is closer to the QD surface when the QD size is larger. In addition, we find through both simulations and Transmission Electron Microscopy (TEM) experiments that the maximum valence numbers are 1, 2 and 3 on QDs of 6, 9 and 14 nm in diameter, respectively. We emphasize the critical role of the electrostatic repulsion on the conformation and the number of attached ssDNA on the QD surface.