Upconversion nanoparticles, a novel class of rare-earth-doped inorganic nanomaterials, can convert low-energy near-infrared light into high-energy visible or ultraviolet light through the anti-Stokes shift effect, showing broad application prospects in biomedicine, optical sensing, and anti-counterfeiting materials. Among them, surface-group-free upconversion nanoparticles, due to the absence of additional functional modifications on their surface, possess advantages such as high purity and good compatibility, making them one of the research hotspots in the field of nanomaterials in recent years.
These surface-group-free upconversion nanoparticles exhibit well-defined and stable optical properties. Their excitation wavelength is fixed in the 975-980 nm range, located in the near-infrared region. This characteristic effectively reduces background fluorescence interference and improves the signal-to-noise ratio in applications. Their emission wavelength has dual emission peaks at 365 nm and 475 nm, corresponding to a violet-blue light emission color. The narrow emission peak characteristic gives them a significant advantage in multi-channel detection, effectively avoiding stray light interference and ensuring detection accuracy.
In terms of physicochemical properties, these nanoparticles use water as a solvent, exhibiting excellent water solubility and stable dispersion in aqueous solutions. This avoids the biotoxicity or environmental hazards associated with organic solvents, laying the foundation for their widespread application in biological systems and aquatic environments. The particle size is precisely controlled at 25 nm, falling within the optimal range of 10-30 nm. This size minimizes luminescence quenching caused by surface defects and avoids aggregation problems due to excessively large particle sizes, balancing luminescence efficiency and dispersibility while also meeting the particle size requirements for nanomaterials in the biomedical field.
Regarding concentration, there are slight variations in concentration between batches of these nanoparticles, with a normal range of 5-10 hm/ml. This concentration fluctuation can be optimized through precise control of the preparation process to meet the differentiated concentration requirements of various application scenarios. Their surface is free of any functional group modifications, exhibiting a smooth surface and complete crystal structure, effectively reducing the risk of non-specific adsorption and simplifying the subsequent functionalization process. Active groups such as amino and carboxyl groups can be flexibly introduced according to actual application needs, expanding their application range.
The nanoparticles can be mass-produced using hydrothermal synthesis and other processes, eliminating the need for complex surface modification steps and reducing preparation costs and process complexity. Their luminescence mechanism is based on the energy level transitions of rare-earth ions, achieving efficient upconversion luminescence by absorbing low-energy near-infrared photons and releasing high-energy violet-blue photons. They also exhibit good photostability and chemical inertness, allowing for stable use over a wide pH range and at temperatures from room temperature to medium-high temperatures.
Currently, these surface-group-free upconversion nanoparticles have shown application potential in multiple fields. In biomedicine, they can be used for cell tracking and deep tissue imaging; in optical sensing, they can serve as highly sensitive detection probes; and in anti-counterfeiting materials, they can be used as fluorescent markers for precise anti-counterfeiting. In the future, further optimization of the preparation process and control of concentration stability will broaden their application scenarios, providing crucial support for technological breakthroughs in related fields.
