Application of Upconverting Nanoparticles in Biological Imaging

Optical and Physical Properties of UCNPs
The luminescence of UCNPs originates from the step-by-step energy level transition of lanthanide ions. For example, after Yb³⁺ absorbs NIR photons (980nm), it excites Er³⁺ (emitting green light, 550nm) or Tm³⁺ (emitting blue light, 450nm) through energy transfer, and finally emits visible light through the anti-Stokes effect. By adjusting the ratio of doped ions (such as Yb³⁺/Er³⁺ or Yb³⁺/Tm³⁺), the emission spectrum can be customized (covering the ultraviolet to near-infrared band). For example, Ho³⁺ doping can achieve multicolor imaging. The energy transfer efficiency is affected by the crystal field symmetry. The core-shell structure (such as NaYF4@NaYF4) can increase the quantum efficiency to more than 5% by reducing surface defects.

Unmodified UCNPs are prone to aggregation due to hydrophobicity, and surface treatment is required to improve biocompatibility:

Hydrophilic coating: Polyethylene glycol (PEG) modification can reduce immune clearance (extend blood circulation half-life to more than 6 hours) and enhance passive tumor targeting (EPR effect).

Functional modification: Silica coating (increase water solubility and reduce toxicity) or antibody coupling (such as anti-HER2 antibody to achieve breast cancer targeting) can improve specificity; folate receptor targeted modification achieves rapid tumor aggregation in lymphoma models (development 2 hours after injection).

Core-shell structure optimization: NaYF4@NaYF4 core-shell design can increase the luminescence intensity by 10 times, while the Fe3O4@NaYF4 composite structure integrates magnetism (for MRI imaging) and upconverting luminescence to achieve integrated diagnosis and treatment.

Energy transfer synergy: Upconverting nanoparticles (UCNPs) combined with quantum dots (such as through the FRET mechanism) can achieve multi-color emission with near-infrared excitation, avoiding the interference of spontaneous fluorescence of biological tissues.

In-depth Analysis of UCNPs Core Application Areas

Bio-imaging: From single-mode to multi-modal precision imaging

Deep in vivo imaging: UCNPs use near-infrared (NIR, such as 980 nm or optimized 915 nm) excitation light to penetrate biological tissues to a depth of several centimeters, which is significantly better than traditional visible light-excited fluorescent probes (penetration is only at the millimeter level). For example, NaYbF4:Tm@NaGdF4 core-shell structure UCNPs achieve high-resolution imaging of the liver and spleen in living mice, and are targeted after modification with citric acid ligands. It is worth noting that UCNPs excited at 800 nm can reduce thermal damage caused by water molecule absorption, and the penetration depth is increased to 25 mm, providing a new solution for deep tumor imaging.

Multimodal imaging synergistic technology: Through rare earth ion doping, UCNPs can be used as MRI contrast agents (relaxation rate up to 5.60 s⁻¹·mM⁻¹) and fluorescent probes at the same time, such as NaGdF4:Yb/Er/Tm system combined with SPECT/CT technology to achieve simultaneous visualization of anatomical structure and metabolic function. In addition, ¹⁸F-labeled UCNPs (such as cit-NPs) can integrate PET/MRI/UCL trimodal imaging, with both sensitivity and spatial resolution, suitable for multi-level detection from cells to living bodies.

Nanoscale temperature measurement: from macro to super-resolution thermal imaging

The luminescence intensity ratio (FIR) of UCNPs is highly sensitive to temperature, especially using the ²H₁₁/₂ and ⁴S₃/₂ thermal coupling energy levels of Er³⁺ (energy level difference ~800 cm⁻¹), which can monitor local temperature changes in cells at the nanoscale (<100 nm). For example, the CaF2:Yb/Er@NaGdF4 core-shell structure uses super-resolution microscopy to achieve accurate temperature measurement of tumor metabolic hotspots (such as mitochondria) with a sensitivity of 0.5°C, providing tools for thermal management of electronic devices or research on abnormal cancer metabolism. In addition, Tm³⁺-doped UCNPs (such as Y₂O₃:Yb/Tm) have a smaller energy level difference (~315 cm⁻¹) and show higher temperature measurement accuracy in the low temperature range (25-45°C), which is suitable for real-time temperature feedback in photothermal therapy.