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.