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

Area of Science:

• Nanotechnology
• Materials Science
• Biophysics

Background:

• Poly(ethylene glycol) (PEG) ligands are crucial for surface passivation of nanocrystal (NC)-based drug carriers, preventing biomolecule adhesion.
• Understanding PEG ligand shell morphology is vital for controlling drug carrier permeability and efficacy.
• Existing analytical tools struggle to characterize PEG shell structure in situ, limiting knowledge of its barrier properties.

Purpose of the Study:

• To develop and apply a Förster resonance energy transfer (FRET) spectroscopy-based method for assessing molecular permeability through PEG-coated ZnO NCs.
• To investigate the in situ morphology of PEG ligand shells on ZnO NCs as a function of ligand chain length.
• To determine if PEG ligands form an impenetrable barrier or a porous structure on NC surfaces.

Main Methods:

• Synthesis of ZnO NCs coated with variable chain length PEG-silane ligands.
• Utilized FRET spectroscopy with ZnO NCs as donors and diffusing molecules as acceptors.
• Quantified energy transfer (EnT) efficiency using time-resolved photoluminescence lifetime (TRPL) spectroscopy.

Main Results:

• Energy transfer efficiency was measured as a function of PEG ligand chain length.
• Unexpectedly, the longest PEG-silane ligands showed similar EnT efficiency to bare ZnO NCs.
• This indicates that the PEG shell is not a rigid, impenetrable barrier but likely porous or in a 'mushroom state'.

Conclusions:

• The 'rigid shell' model for PEG ligands on NCs is insufficient to explain observed permeability.
• The developed FRET spectroscopy approach provides valuable in situ information on ligand shell permeability under biological conditions.
• Findings suggest a more dynamic and permeable PEG shell structure, impacting the design of NC-based drug delivery systems.