Upconverting Nanoparticles (UCNPs) belong to nanomaterials that transform near-infrared light into higher energy visible or ultraviolet light via a multiphoton absorption process. Unique anti-Stokes luminescence emerges from sequential energy transitions within lanthanide ions like Er³⁺/Yb³⁺ which enables near-infrared excitation capabilities along with visible light emission and maintains low background interference. The material demonstrates significant application possibilities within bioimaging, photodynamic therapy, drug delivery systems, energy conversion techniques, and anti-counterfeiting coding. Near-infrared excitation achieves deeper tissue penetration while minimizing photodamage and autofluorescence to enhance the imaging signal-to-noise ratio.
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Synthesis Method of UCNPs
High-temperature coprecipitation method: Mainly used for the synthesis of fluoride matrices (such as NaYF₄), by precisely controlling temperature and time, high-crystalline hexagonal nanoparticles can be prepared, and their luminescence efficiency is significantly better than that of cubic phases. However, the size of nanoparticles synthesized by the coprecipitation method may increase with the increase of reaction time, which limits its application in biomedicine.
Thermal decomposition method: Using metal trifluoroacetate as a precursor, high-temperature decomposition in an oleic acid/oleylamine/octadecene system can prepare monodisperse oil-soluble UCNPs. For example, hexagonal NaYF₄:Yb,Er with a particle size of about 20 nm can be synthesized by reacting at 310°C for 1.5 hours. Although this method can obtain high-quality nanocrystals, it requires an inert atmosphere and high temperature conditions, and the cost is relatively high.
Hydrothermal/solvothermal method: Under mild conditions, by adjusting the solvent ratio (such as ethanol/water) and the concentration of the fluorine source, the morphology (such as disk-like, rod-like) and size of the nanocrystals can be controlled. For example, LaF₃ nanodisks synthesized under hydrothermal conditions at 160°C have a uniform hexagonal phase and strong Upconverting luminescence. Solvothermal method can prepare plate-like UCNPs with higher Upconverting emission intensity. In addition, microwave-assisted solvothermal method can quickly synthesize β-hexagonal NaYF₄, optimize thermal sensitivity, and provide a new approach for biothermal imaging. However, the industrialization of hydrothermal method still faces challenges such as long reaction time, high pressure and poor repeatability.
Core-shell Structure Design
The core-shell structure reduces surface defects by isolating the energy transfer path and significantly improves the luminescence efficiency. For example: NaYF₄@NaYF₄ core-shell structure: the shell passivates surface defects, increases the luminescence intensity several times, and enhances the stability of the water environment.
SiO₂ coating: not only improves hydrophilicity, but also reduces the contact between Er³⁺ and water molecules, reduces fluorescence quenching, and increases the luminescence intensity by 7-12 times. In addition, SiO₂-coated NaYF₄:Yb,Er/Tm nanoparticles have reduced cytotoxicity and are more suitable for bioimaging.
High-throughput combinatorial screening
Optimize doping ratio, morphology and matrix materials through rapid synthesis and characterization (such as fluorescence spectroscopy, XRD). For example: doping with Li⁺ or Zn²⁺ can significantly enhance the luminescence of the Y₂O₃ matrix, and its mechanism is to extend the lifetime of the intermediate state energy level and reduce non-radiative relaxation. Ni²+ doping can increase the luminescence intensity of NaYF₄, but it will cause the morphology to change from hexagonal phase to nanorods.
Surface modification technology
Surface modification can improve the water dispersibility, biocompatibility and functionalization of UCNPs:
Ligand exchange: replace hydrophobic ligands (such as oleic acid) with PEG-phosphate, or encapsulate them with amphiphilic block polymers, and the hydrophilic end carboxyl group can enhance water solubility. The operation only takes 30 minutes.
Charge regulation: polyethyleneimine (PEI) modification gives positive charge, enhances cell uptake efficiency and colloidal stability.
Luminescence regulation: The surface chemical environment changes the luminescence characteristics, such as oleic acid-coated NaYF₄:Yb,Er is mainly green light, while the proportion of red light increases after PEG modification.
Spectral Characteristics and Optimization Strategies
Luminescence mechanism: The multiphoton transition process of lanthanide ions (such as Er³⁺/Yb³⁺) is the core of Upconverting luminescence. Yb³⁺ acts as a sensitizer, transferring energy to activator ions (such as Er³⁺ or Tm³⁺) through 980 nm excitation, triggering multiphoton cascade transitions. For example, the green light (~540 nm) of Er³⁺ corresponds to the ²H₁₁/₂→⁴I₁₅/₂ transition (two-photon process), while the red light (~660 nm) originates from the ⁴F₉/₂→⁴I₁₅/₂ transition (three-photon process). The blue light (~450 nm) emission of Tm³⁺ involves a more complex energy transfer pathway: Yb³⁺ causes Tm³⁺ to transition from ³H₆→³F₂→¹G₄ energy levels through three energy transfers, and finally releases blue light through ¹G₄→³H₆ transition. Studies have shown that temperature changes significantly affect energy level population. For example, the emission spectrum intensity of YbEr@Y nanoparticles in the range of 10-295 K decreases with increasing temperature, which is related to the enhancement of phonon-assisted non-radiative transitions.
Key Challenges and Solutions
Surface defect quenching: Dangling bonds and lattice distortion on the surface of nanocrystals introduce non-radiative recombination centers. Experiments have shown that core-shell structures (such as NaYF₄@NaYF₄) can increase quantum efficiency by 3-5 times. The mechanism is that the outer shell effectively isolates the interaction between surface defects and activator ions. SiO₂ coating passivates the surface dangling bonds by hydroxyl groups, which increases the quenching concentration of Eu³⁺:Y₂O₃ nanoparticles from 5% of the bulk material to 15% of the nanosystem.
Spectral regulation: The synergistic effect of the doping ratio and the matrix lattice field can achieve precise wavelength tuning. For example, Yb³⁺/Tm³⁺ emits blue light (~450 nm) in the NaYF₄ matrix, while the main emission peak of the NaGdF₄ matrix with the same doping ratio is red-shifted to ~475 nm due to the difference in lattice field strength. In addition, the introduction of Gd³⁺ can expand the emission band to the near infrared through the energy transfer Upconverting (EMU) mechanism.
UCNPs Application Fields
Biomedicine: The near-infrared excitation of UCNPs (such as 808/980 nm) can penetrate tissues to a depth of 3-5 cm, and the tissue autofluorescence background is reduced to 1/20 of the visible light excitation, which has been used for deep imaging of liver cancer in situ tumors.
Integrated diagnosis and treatment: UCNPs loaded with zinc phthalocyanine (ZnPc) can synchronously release singlet oxygen (¹O₂, photodynamic therapy) and produce photothermal effect (ΔT≈15℃) under the triggering of 660 nm red light. Animal experiments have shown that the system has a tumor inhibition rate of 92% for breast cancer, and the side effects are significantly lower than traditional chemotherapy.
Energy conversion: UCNPs are integrated into dye-sensitized solar cells (DSSCs) as a spectral conversion layer, which can convert low-energy infrared light (~980 nm) into visible light, extending the light response range of DSSCs to the near-infrared region. Studies have shown that co-sensitization strategies (such as ADEPA-1/LEG4 dye combination) combined with UCNPs can increase battery efficiency from 7.2% to 14.3%. Silver nanoparticles (AgNPs) modified TiO₂ photoanode further enhance light capture through the localized surface plasmon resonance (LSPR) effect, increasing the short-circuit current density by 35%.
Security coding: Time-space resolution coding technology based on multicolor luminescence has been applied to high-end anti-counterfeiting. For example, NaYF₄:Yb/Tm/Er nanoparticles can emit blue-green-red tricolor light by regulating the excitation power density, and can achieve a dynamic coding combination of 10⁶ in combination with pulsed laser modulation, which has been used in drug traceability systems.
Application of Dynamic Light Scattering (DLS) in Surface Modification of UCNPs
Particle size and dispersibility analysis: PEI modification increases the hydrated particle size of UCNPs from 20 nm to 35 nm, and the particle size distribution index (PDI) decreases from 0.25 to 0.12, indicating that surface charge repulsion improves dispersion stability. When pH>8, the deprotonation of PEI causes the particle size to increase sharply to 50 nm and is accompanied by aggregation, which is closely related to the decrease of Zeta potential from +35 mV to +18 mV.
Zeta potential evaluation: Surface positive charge (such as +35 mV modified by PEI) can significantly improve cell uptake efficiency. Flow cytometry showed that the internalization rate of positively charged UCNPs in HepG2 cells was 3.2 times that of neutral particles, but excessive charge (>+40 mV) would trigger lysosomal escape barriers and needed to be regulated to +20-30 mV through PEGylation to balance efficiency and safety.
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