Sub-10 nm upconversion nanocrystals for long-term single-particle tracking.

Area of Science:

• Nanotechnology
• Biomedical Imaging
• Materials Science

Background:

• Lanthanide-doped upconversion nanoparticles (UCNPs) offer high photostability and anti-Stokes luminescence for bioimaging.
• A key challenge is balancing small particle size with high emission brightness, as smaller UCNPs are often dimmer.
• Surface defects and energy leakage commonly reduce UCNP efficiency, especially in smaller particles.

Purpose of the Study:

• To engineer sub-10 nm upconversion nanoparticles with enhanced brightness and efficiency.
• To investigate methods for minimizing surface quenching and energy loss in UCNPs.
• To demonstrate the utility of these novel UCNPs for long-term single-molecule tracking in live cells.

Main Methods:

• Fabrication of sub-10 nm cascade actively protected UCNPs using NaYbF4 and NaLuF4 layers.
• Utilizing theoretical modeling and time-resolved measurements to analyze energy transfer pathways and loss mechanisms.
• Employing single-particle level efficiency measurements and live-cell imaging of epidermal growth factor receptor (EGFR) molecules.

Main Results:

• Achieved a 33-fold increase in upconversion efficiency at the single-particle level compared to ~19 nm conventional UCNPs.
• Identified and minimized energy leakage from Er3+ ions to surface defects using a NaYbF4 intermediate layer.
• Demonstrated effective suppression of Yb3+ surface quenching with a NaLuF4 monolayer.
• Successfully tracked single EGFR molecules on live cells for up to one hour, observing dynamic diffusion mode switching.

Conclusions:

• Developed ultra-small (<10 nm) and highly bright UCNPs by actively protecting the cascade structure.
• Mitigated surface-related emission quenching, significantly boosting UCNP efficiency.
• Enabled unprecedented long-term single-molecule tracking of cell surface dynamics, providing insights into receptor behavior.