Systematic investigation of functional ligands for colloidal stable upconversion nanoparticles

Area of Science:

• Nanomaterials Science
• Surface Chemistry
• Biomedical Engineering

Background:

• Achieving long-term colloidal stability of upconversion nanoparticles (UCNPs) in physiological buffers is crucial for their biomedical applications.
• Surface functionalization is key, but current methods face challenges with stability.
• Understanding ligand-surface interactions is vital for designing stable UCNP formulations.

Purpose of the Study:

• To quantitatively investigate the competitive adsorption of phosphate, carboxylic acid, and sulfonic acid ligands onto UCNP surfaces.
• To determine the binding strength of different anchoring groups to identify the optimal conjugation strategy for UCNPs.
• To enhance the colloidal stability of UCNPs in physiological environments.

Main Methods:

• Synthesis of di-block copolymers with poly(ethylene glycol) and phosphate, carboxylic, or sulfonic acid anchoring groups using Reversible Addition Fragmentation Chain Transfer (RAFT) polymerization.
• Quantitative analysis of ligand adsorption and replacement of oleic acid capping molecules on UCNP surfaces using analytical tools.
• Simulated quantitative adsorption energy measurements to assess binding strengths.

Main Results:

• Phosphate ligands demonstrated complete replacement of oleic acid capping molecules on UCNP surfaces.
• Carboxylic and sulfonic acid groups showed incomplete ligand exchange.
• Calculated adsorption energy and experimental results indicated higher binding strength for phosphate anchoring ligands, leading to superior colloidal stability, particularly in phosphate buffer solutions.

Conclusions:

• Polymers featuring multiple negatively charged phosphate moieties provide excellent colloidal stability for lanthanide ion-doped luminescent nanoparticles.
• Phosphate-based surface functionalization is a promising strategy for enhancing UCNP stability in physiological conditions.
• This research offers a pathway for developing robust UCNPs for diverse biomedical applications.