Water-soluble fluorescent dyes refer to fluorescent dyes that incorporate hydrophilic groups such as sulfonate groups, carboxyl groups, quaternary ammonium salts, or polyethylene glycol (PEG) chains into their molecular structure, enabling direct dissolution in water, buffers, or cell culture media. In live imaging, these dyes are among the most widely used fluorescent labeling tools due to their excellent biocompatibility and ease of conjugation to biomolecules.
I. Structural Features and Water-Solubility Mechanism
The hydrophilicity of water-soluble fluorescent dyes primarily derives from polar functional groups in their molecular structure:
| Hydrophilic Group | Mechanism | Representative Dyes |
| Sulfonate Group (-SO₃H) | Strongly acidic group, fully ionized in water, conferring good water solubility and anti-aggregation properties | Sulfonated Cy series, ICG, Alexa Fluor series |
| Carboxyl Group (-COOH) | Ionizes to -COO⁻ at neutral pH, increasing water solubility while providing conjugation sites | FITC derivatives, rhodamine derivatives |
| Quaternary Ammonium Salt (-N⁺R₃) | Positively charged, enhances both water solubility and cell membrane permeability | Zwitterionic rhodamines (OregonFluor) |
| Polyethylene Glycol (PEG) Chain | Interacts with water molecules via hydrogen bonding, enhancing water solubility and reducing non-specific adsorption | PEGylated BODIPY, PEGylated cyanine dyes |
| Hydroxyl Group (-OH) | Interacts with water molecules via hydrogen bonding, assisting solubilization | Some fluorescein derivatives |
II.Applications of Water-Soluble Dyes in Live Imaging
1. Targeted Imaging (Antibody/Peptide Conjugation)
Carboxyl or amino groups on water-soluble dye molecules can be covalently conjugated to amino or sulfhydryl groups of antibodies or peptides to form targeted probes. Common conjugation strategies include:
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NHS Ester Activation: Reacts with lysine residues of antibodies
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Maleimide: Reacts with cysteine residues of antibodies (thiol-directed conjugation)
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Click Chemistry: Azide-alkyne cycloaddition for efficient conjugation
Application Examples:
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HER2 antibody conjugated to near-infrared dyes (e.g., IRDye 800CW) for breast cancer imaging
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PD-L1 antibody conjugated to ICG for immunotherapy response prediction
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RGD peptide conjugated to Cy5 for tumor angiogenesis imaging
2. Live Cell Labeling
| Dye/Probe | Application | Characteristics |
| Calcein AM | Live cell viability assay | Non-polar precursor enters cells; hydrolyzed to water-soluble fluorescent molecule retained in cytoplasm |
| SR101 | Astrocyte labeling | Specifically taken up by astrocytes via OATP1C1 transporter |
| Water-Soluble Chitosan-Cyanine | 3D vascular imaging | Compatible with tissue clearing techniques; enables capillary-level imaging |
| Zwitterionic Dyes | Intracellular target labeling | Combine water solubility with cell membrane permeability |
3. Small-Molecule Fluorescent Probes (Responsive Type)
Water-soluble small-molecule probes can achieve "switch-on/off" responses to specific analytes via intramolecular charge transfer (ICT) or photoinduced electron transfer (PET) mechanisms:
Typical Design Strategies:
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ICT Probes: Analyte binding alters intramolecular charge distribution, causing fluorescence changes
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PET Probes: Analyte binding blocks the PET process, restoring fluorescence
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FRET Probes: Analyte induces changes in donor-acceptor distance
Application Examples:
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Cysteine (Cys)-responsive probe: emission wavelength 755 nm, detection limit 0.14 μM, used for tumor imaging
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pH-responsive probes: monitoring acidic tumor microenvironment
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Enzyme-responsive probes: imaging of cathepsins, matrix metalloproteinases (MMP) activity
4. Vascular and Perfusion Imaging
Water-soluble dyes distribute uniformly in blood vessels and are less prone to lipid adsorption, making them suitable for angiography:
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ICG: Clinical standard angiographic agent; used for fundus angiography and lymphography
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IRDye Series: Used for in vivo vascular permeability assessment
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NIR-II Water-Soluble Dyes: Deep vascular imaging; enables clear visualization of microvessels with diameters <100 μm
5. Multimodal Imaging Probes
Water-soluble dyes can be combined with other imaging modalities to construct multimodal probes:
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Fluorescence-MRI Dual Modality: Conjugation to Gd-DOTA or superparamagnetic iron oxide nanoparticles
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Fluorescence-Photoacoustic Dual Modality: NIR-I/II dyes inherently possess photoacoustic imaging capabilities
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Fluorescence-Nuclear Medicine Dual Modality: Conjugation to radionuclides such as ¹⁸F, ⁶⁴Cu, ¹¹¹In
III.Advantages and Limitations of Water-Soluble Dyes
| Dimension | Advantages | Limitations |
| Solubility | Direct dissolution in aqueous media; no organic solvents required | Some dyes still have insufficient water solubility; may require chemical modification |
| Biocompatibility | Low cytotoxicity; suitable for in vivo applications | Inorganic nanoparticles have long-term toxicity concerns |
| Conjugation Convenience | Contains carboxyl, amino, and other active groups; easy conjugation to biomolecules | Conjugation may affect dye fluorescence properties |
| In Vivo Distribution | Primarily in blood vessels and cytoplasm; controllable distribution | Small-molecule dyes rapidly cleared by kidneys |
| Penetration Depth | Shallow penetration in visible region (<1 mm); significantly improved in NIR-I/II regions | Visible dyes unsuitable for deep imaging |
| Labeling Persistence | Short to medium term (hours to days) | Less persistent than lipid-soluble dyes |
IV.Future Trends
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Development of NIR-II Water-Soluble Dyes: Addressing hydrophobicity issues of traditional NIR-II dyes; developing high-quantum-yield, excretable organic small-molecule water-soluble dyes
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Zwitterionic Dyes: Such as the OregonFluor series, combining water solubility with cell membrane permeability for intracellular target labeling
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Smart Responsive Probes: Activatable probes combining water solubility with responsiveness to disease microenvironment (pH, enzymes, ROS, hypoxia)
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Multimodal Imaging Probes: Integrating water-soluble fluorescent dyes with MRI, PET, photoacoustic, and other imaging modalities for synergistic multimodal imaging
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Accelerated Clinical Translation: Advancing NIR-II water-soluble dyes into clinical trials; expanding applications in surgical navigation and early tumor diagnosis
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