Near-Infrared II fluorescent dyes refer to fluorescent dyes with emission wavelengths in the 1000–1700 nm range. Compared to visible light (400–700 nm) and Near-Infrared I (700–900 nm), NIR-II imaging offers significant advantages in tissue penetration depth, spatial resolution, and signal-to-noise ratio, achieving centimeter-scale penetration depth and micrometer-scale resolution, making it a cutting-edge research direction in the field of live imaging.
I. Physical Basis and Advantages of the NIR-II Window
| Characteristic | Visible Light (400–700 nm) | NIR-II (1000–1700 nm) | Improvement |
|---|---|---|---|
| Tissue Penetration Depth | <1 mm | >5 mm, up to centimeter scale | NIR-II is 2–3 times that of NIR-I |
| Autofluorescence | High | Extremely low | Optimal signal-to-noise ratio |
| Scattering | High (Rayleigh scattering ∝ 1/λ⁴) | Extremely low | Significantly improved resolution |
| Primary Absorbers | Hemoglobin, melanin | Water (weak absorption), lipids | Minimal background interference |
| Spatial Resolution | Micrometer scale | Micrometer scale (<100 μm vessels) | High resolution at depth |
II. Applications of NIR-II Dyes in Live Imaging
1. Deep Vascular Imaging
NIR-II imaging enables clear visualization of microvessels with diameters <100 μm, achieving high-resolution vascular imaging at centimeter-scale penetration depths.
Typical Applications:
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Cerebral Vascular Imaging: Imaging through intact skull without craniotomy or cranial window
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Tumor Angiogenesis: Real-time monitoring of tumor microvessel density and morphology
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Lower Limb Vascular Imaging: Assessing peripheral artery disease and ischemia models
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Lymphatic Vessel Imaging: High-resolution visualization of lymphatic vessel structure and function
Key Metrics: NIR-II imaging can resolve <10 μm capillaries (in thin tissues such as mouse ear) and <50 μm microvessels in deep tissues.
2. Precise Tumor Localization and Surgical Navigation
The high penetration depth and high signal-to-noise ratio of NIR-II imaging offer unique advantages in tumor imaging:
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Deep Tumor Boundary Identification: Identifying tumor boundaries >5 mm deep under the skin
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Micro-metastasis Detection: Detecting micro-metastases <1 mm in diameter
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Peritoneal Metastasis Imaging: Real-time localization of widespread intraperitoneal metastases
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Surgical Navigation: Assisting surgeons in achieving complete tumor resection
Targeting Strategies:
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Passive Targeting: Enhanced Permeation and Retention (EPR) effect
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Active Targeting: Conjugation of NIR-II dyes to antibodies/peptides (e.g., EGFR, HER2, PD-L1)
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Activatable Probes: Responsive to tumor microenvironment (pH, enzymes, ROS)
3. Lymphatic System Imaging
| Application | Features |
|---|---|
| Sentinel Lymph Node Mapping | Intradermal injection of NIR-II dye for real-time tracking of lymphatic drainage |
| Lymphedema Assessment | Evaluating abnormalities in lymphatic vessel structure and function |
| Lymphatic Metastasis Detection | Detecting tumor lymph node metastasis |
4. Gastrointestinal and Urinary Tract Imaging
NIR-II dyes can be administered orally or intravenously for gastrointestinal and urinary tract imaging:
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Gastrointestinal Motility Monitoring: Oral administration of NIR-II dyes for real-time imaging of gastrointestinal motility
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Renal Function Assessment: Evaluating renal metabolism and excretion of dyes
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Ureter Identification: Intraoperative identification of ureters to prevent injury
5. Immune Cell Tracing
NIR-II dyes label immune cells (e.g., CAR-T cells, macrophages) for dynamic monitoring:
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CAR-T Cell Homing: Tracking migration of CAR-T cells to tumor sites
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Recruitment to Inflammatory Sites: Monitoring immune cell recruitment to inflamed tissues
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Cell Dynamics Studies: Long-term tracking of cell distribution and survival
6. Multichannel NIR-II Imaging
Using NIR-II dyes with different emission wavelengths enables simultaneous multi-target imaging:
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Dual-Color Vascular Imaging: Distinguishing arteries and veins (using different dyes)
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Tumor-Vessel Dual-Modality Imaging: Simultaneously visualizing tumors and surrounding vessels
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Multi-Target Molecular Imaging: Detecting multiple biomarkers simultaneously
III.Advantages and Limitations of NIR-II Dyes
| Dimension | Advantages | Limitations |
|---|---|---|
| Penetration Depth | Up to centimeter scale; suitable for deep organ imaging | Ultra-deep tissues (>1 cm) remain challenging |
| Signal-to-Noise Ratio | Extremely low autofluorescence; optimal SNR | Requires high-sensitivity InGaAs cameras |
| Spatial Resolution | Micrometer scale; resolves <100 μm vessels | Limited by scattering; cannot reach optical diffraction limit |
| Temporal Resolution | Up to video rate (>30 fps) | Fast dynamic imaging requires high-power excitation |
| Clinical Translation | Organic small-molecule dyes show promise | Most dyes still in preclinical research |
| Toxicity | Organic dyes have relatively low toxicity | Inorganic nanoparticles have long-term toxicity concerns |
| Commercialization | Some products commercialized (e.g., IR-1061) | Commercialization far less than NIR-I |
| Instrumentation | Requires specialized InGaAs cameras | High instrument cost; low availability |
IV.Selection Guide
| Application Scenario | Recommended Dye Type | Rationale |
|---|---|---|
| Deep Vascular Imaging (Brain, Lower Limb) | Ag₂S quantum dots, organic small molecules (CH1055) | High penetration depth, high resolution |
| Precise Tumor Localization | Targeted conjugated NIR-II dyes (IRDye series derivatives) | Specific accumulation, high signal-to-noise ratio |
| Surgical Navigation | Organic small-molecule NIR-II dyes | Metabolically excretable, low toxicity, high clinical translation potential |
| Lymphatic Imaging | IR-FE (AIE characteristics), ICG (NIR-I) | High contrast, real-time tracking |
| Immune Cell Tracing | NIR-II dye labeling (lipid-soluble or conjugated) | Long-term tracking, deep tissue imaging |
| Multichannel Imaging | Combination of NIR-II dyes with different emission wavelengths | Simultaneous multi-target detection |
| Time-Gated Imaging | Rare earth-doped nanoparticles (long fluorescence lifetime) | Effective elimination of autofluorescence background |
| Preclinical Research | Organic small-molecule NIR-II dyes | Low toxicity, clear metabolism, high translation potential |
V.Future Trends
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Rapid Development of Organic NIR-II Dyes: High quantum yield (>10%), good water solubility, and metabolically excretable organic small-molecule NIR-II dyes are key research priorities
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Accelerated Clinical Translation: The first NIR-II organic dye has entered clinical trials for tumor surgical navigation
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Multichannel NIR-II Imaging: Development of spectrally separated NIR-II dye libraries for 5–10 color simultaneous imaging
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Smart Responsive NIR-II Probes: Development of activatable probes responsive to pH, enzymes, reactive oxygen species, and hypoxia
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NIR-I/NIR-II Multi-Window Imaging: Combined use of NIR-I and NIR-II dyes for multi-scale imaging from superficial to deep tissues
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Theranostics: Development of probes with both NIR-II imaging and photothermal/photodynamic therapy capabilities
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Instrument Miniaturization: Development of portable and endoscopic NIR-II imaging systems to promote clinical translation
艳 李
