UCNPs and QDs are two important types of fluorescent nanomaterials, widely used in bioimaging, sensing, treatment and other fields. UCNPs are based on the nonlinear optical process of lanthanide elements, which can convert low-energy near-infrared light into high-energy visible light; QDs rely on the quantum confinement effect and control the emission wavelength by size (such as CdSe QDs can be adjusted to the visible light range).
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Luminescence Mechanism and Excitation Mode: Upconversion Nanoparticles (UCNPs) vs. Quantum Dots
UCNPs
Mechanism: Based on the sequential multiphoton absorption process of lanthanide ions (such as Yb3+/Er3+), anti-Stokes luminescence (near-infrared → visible light) is achieved through energy transfer upconversion (ETU) or excited state absorption (ESA).
Excitation advantage: Near-infrared excitation (980 nm) penetrates deeper into biological tissues (up to several centimeters) and almost does not cause tissue autofluorescence and phototoxicity.
QDs
Mechanism: Quantum confinement effect causes the energy band gap to change with size. Small-sized QDs emit blue shift and large-sized QDs emit red shift.
Excitation limitation: UV or visible light excitation is required, which is easy to cause interference from spontaneous fluorescence of biological tissues, and the penetration depth is limited (<1 mm) and may cause photodamage.
Applicable Scenario Analysis: Upconversion Nanoparticles (UCNPs) vs. Quantum Dots
UCNPs' advantages
Deep tissue imaging: Near-infrared excitation has strong penetration and is suitable for deep tissue imaging such as brain and tumors.
Long-term tracking: No light flickering and photobleaching, suitable for long-term tracking of single molecules.
Integrated treatment: For example, near-infrared excitation can activate photosensitizers to kill tumor cells in photodynamic therapy (PDT).
Multimodal imaging: Can be combined with MRI, PET and other technologies to improve diagnostic accuracy.
QDs' advantages
Multiple detection: The emission wavelength can be precisely controlled by size, suitable for multicolor labeling.
Real-time monitoring: Short fluorescence lifetime (nanosecond level) is suitable for high-speed dynamic process imaging.
In vitro detection: High quantum yield (up to 80%) improves in vitro detection sensitivity.
Spectral Characteristics: Upconversion Nanoparticles (UCNPs) vs. Quantum Dots
UCNPs
Through 4f-4f electron orbital transitions lanthanide ions generate narrow-band emission peaks with a half-width less than 20 nm and UCNPs allow ions like Er3+ to emit red and green light and Tm3+ to produce blue light. The emission spectrum of materials can be controlled through these specified methods.
Ion doping: To produce multi-color light, adjust the combination of doped ions like Yb3+/Er3+ or Yb3+/Tm3+ so that NaYF4:Yb,Er emits green light and NaYF4:Yb,Tm emits blue light.
Core-shell structure: The NaYF4@NaYF4 core-shell arrangement reduces surface defects while boosting luminescence efficiency by over ten times. NaYF4 shells serve as inert barriers to isolate lattice defects while NaGdF4:Yb/Er active shells promote energy transfer.
Spectral purification: Design non-stoichiometric crystals by adjusting the Na/Ln ratio or apply surface coatings that absorb unwanted emissions to suppress non-target emission peaks.
QDs
QDs emission spectrum exhibits broadband features with a half-maximum width that ranges from 20 to 50 nm and regulation strategies encompass:
Size and composition regulation: Researchers control nanocrystal size and material composition (such as CdSe/ZnS) to adjust emission wavelength but need to prevent peak broadening from non-uniform size distribution.
Environmental sensitivity: The fluorescence properties of QDs are highly sensitive to variations in pH levels and temperature changes as well as the presence of divalent ions which can result in fluorescence quenching or aggregation. In physiological environments QDs containing Cd become toxic because they release heavy metal ions.
Biocompatibility and Photostability: Upconversion Nanoparticles (UCNPs) vs. Quantum Dots
UCNPs
Deep tissue imaging: Near-infrared excitation at 980 nm achieves centimeter-level penetration depth in tissues which minimizes scattering and light damage while enabling deep in vivo imaging. UCNPs show detection sensitivity one order of magnitude greater than QDs during mouse experiments and eliminate background fluorescence interference.
Photostability: UCNPs exhibit stable luminescence because their fluorescence intensity remains unaffected by continuous light exposure due to the absence of organic chromophores.
No photoblinking: Long-term biomolecule tracking is feasible with single particle tracking as it avoids signal switching phenomena.
Optimized biocompatibility: PEGylation of surfaces minimizes immune system clearance while extending the duration of blood circulation; carboxyl and amino modifications enable precise molecular binding which decreases nonspecific interaction. Ultra-small UCNPs with sizes less than 15 nm have the ability to enter cells while presenting substantially reduced toxicity compared to Cd-containing quantum dots.
QDs
Toxicity issues: The release of heavy metal ions like Cd²⁺ from Cd-based QDs leads to cell apoptosis which restricts their usage in living organisms. Although surface coating methods like ZnS shells or polymer layers reduce toxicity levels they do not fully remove the risk.
Photostability defects: The measurement of individual QDs during single particle imaging faces random signal disruptions which complicate quantitative analysis.
Photobleaching sensitivity: Extended exposure to illumination reduces fluorescence intensity by over 50%, which necessitates regular calibration efforts.
Quantum Yield and Efficiency Comparison: Upconversion Nanoparticles (UCNPs) vs. Quantum Dots
UCNPs
Quantum yield: usually low, with the literature reporting a range of 0.005%-3% (NaYF4:Er3+/Yb3+ nanoparticles), but by optimizing the core-shell structure (such as LiLuF4:Ln3+), the absolute quantum yield can be increased to 5.0% (Er3+) and 7.6% (Tm3+). Dye sensitization strategies can significantly improve efficiency, with brightness increased by thousands of times.
Power dependence: significantly better than QDs at low power density (1-103 W/cm2), especially suitable for multiphoton imaging. Its nonlinear upconversion process is several orders of magnitude higher than the multiphoton absorption efficiency of QDs, and only low-cost continuous wave lasers are required for excitation.
QDs
Quantum yield: up to 80%, but susceptible to environmental interference (such as quenching effect).
Excitation conditions: pulsed laser excitation with high power density (104-106 W/cm2) is required. It is easily interfered by the spontaneous fluorescence of biological tissues in in vivo imaging, and the detection sensitivity is lower than that of UCNPs.
Comparison of Application Fields: Upconversion Nanoparticles (UCNPs) vs. Quantum Dots
Bio-imaging
UCNPs
Near-infrared excitation (~980 nm) has a large penetration depth and no tissue spontaneous fluorescence background. It has been used for whole-body phosphorescence imaging, optical tomography and multimodal imaging (such as UPL/MRI, UPL/PET). The luminescence intensity is highly dependent on temperature, which can achieve nanoscale temperature measurement.
QDs:
Narrow emission bandwidth (20-50 nm) supports multi-color imaging. QDs are suitable for dynamic tracking of cell surface receptors.
Therapy and sensing
UCNPs
Near-infrared light excitation can activate surrounding photosensitizers to produce singlet oxygen and kill deep tumor cells. Near-infrared light triggers drug release.
QDs
Fluorescence resonance energy transfer technology is used for high-sensitivity biological detection.
Synergistic application
UCNPs and QDs composite system: Through the FRET mechanism, UCNPs act as energy donors and QDs act as receptors to achieve high-sensitivity biological detection (such as the detection limit of β-hCG marker is 3.8 ng/mL). The hybrid structure of dye-sensitized UCNPs and QDs can further optimize the energy transfer efficiency.
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