Water-Soluble CdSe Quantum Dots Passivated by a Multidentate Diblock Copolymer

Abstract

We describe a process for transferring trioctylphosphine oxide-stabilized CdSe semiconductor nanocrystals (CdSe/TOPO quantum dots, QDs) from toluene into water through a ligand exchange process with a poly(ethylene glycol-b-2-N,N-dimethylaminoethyl methacrylate) (PEG-b-PDMA) diblock copolymer. In this polymer, the DMA units serve as multidentate ligands for the QD. While we expected the protruding PEG layer to enhance the water-solubility of the polymer/QD adduct, it was not sufficient, upon initial exposure to the QDs, to impart water solubility. Residual TOPO groups remained at the particle surface. Multiple exchanges with the polymer accommodated the slow dissociation of TOPO groups from the CdSe surface, and allowed the particles to achieve water solubility (dispersibility). Transfer to water was accompanied by a 10-fold decrease in the quantum yield of photoluminescence (PL), but this intensity could be recovered by photoactivation. Before photoactivation, the QDs in water showed a small decrease in PL intensity as the solutions were warmed from 25 to 55 °C, which was fully reversible when the solutions were cooled. After photoactivation, the QDs showed a similar decrease in PL intensity upon warming, but only a fraction of this loss of intensity was recovered when the sample was cooled to room temperature. At high ionic strength (0.2−1.0 M NaCl) the PL emission intensity decreased, accompanied by an increase in trap emission at longer wavelength.