Characterizing the Ligand Shell Morphology of PEG-Coated ZnO Nanocrystals Using FRET Spectroscopy.

Poly(ethylene glycol) (PEG) ligands can inhibit proteins and other biomolecules from adhering to underlying surfaces, making them excellent surface ligands for nanocrystal (NC)-based drug carriers. Quantifying the PEG ligand shell morphology is important because its structure determines the permeability of biomolecules through the shell to the NC surface. However, few in situ analytical tools can reveal whether the PEG ligands form either an impenetrable barrier or a porous coating surrounding the NC. Here, we present a Förster resonance energy transfer (FRET) spectroscopy-based approach that can assess the permeability of molecules through PEG-coated ZnO NCs. In this approach, ZnO NCs serve as FRET donors, and freely diffusing molecules in the bulk solution are FRET acceptors. We synthesized a series of variable chain length PEG-silane-coated ZnO NCs such that the longest chain length ligands far exceed the Förster radius (R0), where the energy transfer (EnT) efficiency is 50%. We quantified the EnT efficiency as a function of the ligand chain length using time-resolved photoluminescence lifetime (TRPL) spectroscopy within the framework of FRET theory. Unexpectedly, the longest PEG-silane ligand showed equivalent EnT efficiency as that of bare, hydroxyl-passivated ZnO NCs. These results indicate that the "rigid shell" model fails and the PEG ligand shell morphology is more likely porous or in a patchy "mushroom state", consistent with transmission electron microscopy data. While the spectroscopic measurements and data analysis procedures discussed herein cannot directly visualize the ligand shell morphology in real space, the in situ spectroscopy approach can provide researchers with valuable information regarding the permeability of species through the ligand shell under practical biological conditions.

Air-stable n-type Fe-doped ZnO colloidal nanocrystals.

The synthesis of Al and Fe codoped ZnO colloidal nanocrystals (NCs) using a modified etching-regrowth-doping method is presented. We show that the spectroscopic signatures associated with Fe3+ in ZnO disappear upon introduction of Al3+ donor defects into the ZnO lattice. The presence of Al3+ is confirmed by the appearance of a localized surface plasmon resonance feature indicating excess free carriers in the codoped NCs. These spectral changes suggest that Al3+ doping results in a reduction of Fe3+ dopants to the electron paramagnetic resonance-silent Fe2+ dopants that are stable under ambient conditions. These colloidal NCs provide a potential building block for manipulating magneto-optical properties and plasmon responses in colloidal NCs and higher-order nanostructures.