Upconversion nanoparticles: from hydrophobic to hydrophilic surfaces

Area of Science:

• Nanomaterials Science
• Surface Chemistry
• Biotechnology

Background:

• Photon upconversion nanoparticles (UCNPs) exhibit unique near-IR to visible light conversion properties.
• Hydrophobic UCNPs limit applications in aqueous environments like bioimaging and bioassays.
• Developing water-dispersible UCNPs is crucial for expanding their utility.

Purpose of the Study:

• To classify and critically assess strategies for rendering hydrophobic UCNPs water-dispersible.
• To provide insights into the advantages and limitations of various surface modification methods.
• To identify key challenges and future research directions in UCNP surface engineering.

Main Methods:

• Classification of surface modification strategies into four categories: chemical modification, ligand addition, shell coating, and ligand replacement.
• Assessment of methods including oleate ligand oxidation, amphiphile/shell coating (SiO2, TiO2, Au, Ag), and direct/two-step ligand exchange.
• Discussion of challenges in synthesis, surface modification reproducibility, yield, quantum efficiency, workup, characterization, and reagent cost.

Main Results:

• Chemical modification (A) offers limited colloidal stability and functionalization.
• Coating methods (B, C) are versatile but can increase polydispersity.
• Ligand replacement (D) is simple and controllable but may yield particles with limited buffer stability.
• Current research often relies on trial-and-error due to a lack of mechanistic understanding.

Conclusions:

• Various methods exist to achieve water-dispersible UCNPs, each with specific advantages and drawbacks.
• Further research is needed for better control over particle synthesis, surface modification, and enhanced performance in aqueous media.
• Biocompatibility remains a critical factor for UCNP applications, particularly in bioimaging.