Photodynamic Applications of Squaraine Dyes

Squaraine dyes constitute a class of organic chromophores characterised by a four-membered central ring conjugated to electron-donating heterocycles. Their sharp and intense absorption bands in the red to near-infrared region, high molar extinction coefficients and strong fluorescence make them attractive as photosensitisers in photodynamic therapy (PDT). Upon light excitation, squaraine molecules in the triplet state transfer energy to molecular oxygen, generating reactive oxygen species that induce oxidative damage in target cells. Recent advances have addressed key limitations such as poor aqueous solubility, self-aggregation and chemical instability by introducing heavy-atom substitution (for enhanced intersystem crossing), tailoring heterocyclic units to extend absorption wavelengths and embedding dyes in nanocarriers. These approaches have broadened the scope of squaraine-mediated PDT beyond oncological applications to include antibacterial photodynamic therapy and wound-healing formulations. Encapsulation strategies—ranging from quatsome vesicles and polymeric nanoparticles to lignin-based hydrogels—have improved biodistribution, reduced off-target toxicity and allowed controlled release. Integration of squaraines into multifunctional platforms that combine photodynamic action with imaging or photothermal effects is emerging as a powerful route towards image-guided treatment and synergistic therapies. Collectively, these developments underline the global significance of squaraine dyes as versatile and tunable agents for light-activated therapies.

Research from Nature Portfolio

Recent studies have elucidated the molecular mechanisms by which halogenated squaraine derivatives exert photodynamic effects in cancer cells. An iodinated squaraine was shown to accumulate preferentially in mitochondria of human breast cancer cells, where light activation disrupted redox homeostasis and triggered mitochondria-mediated apoptosis. Proteomic analyses revealed concurrent activation of protective and death-inducing pathways, highlighting the delicate interplay between oxidative stress response and cell-death signalling. Importantly, the derivative displayed minimal dark toxicity in non-transformed breast cells, underscoring its selectivity.

Photodynamic Applications of Squaraine Dyes publication trend

The graph below shows the total number of articles in photodynamic applications of squaraine dyes across all publications each year (not limited to Nature Index journals).

Technical terms

Photodynamic therapy (PDT): A minimally invasive treatment that uses light-activated photosensitisers to generate reactive oxygen species and destroy target cells.

Squaraine dye: An organic chromophore featuring a central four-membered squaric acid ring linked to electron-rich heterocycles, notable for red/NIR absorption and high extinction coefficients.

Photosensitiser: A molecule that absorbs light and transfers energy to oxygen or other substrates to produce cytotoxic species.

Reactive oxygen species (ROS): Highly reactive oxygen derivatives, including singlet oxygen and free radicals, responsible for oxidative damage in PDT.

Singlet oxygen: An electronically excited form of molecular oxygen that mediates oxidative cell damage in photodynamic applications.

Quatsome: A unilamellar nanovesicle composed of sterols and quaternary ammonium surfactants, used for drug delivery of hydrophobic agents.

Hydrogel: A three-dimensional network of hydrophilic polymers capable of retaining large amounts of water, used for sustained release and wound dressing.

References

1. Quatsomes Loaded with Squaraine Dye as an Effective Photosensitizer for Photodynamic Therapy. Pharmaceutics (2023).
2. Synergistic antibacterial action of lignin-squaraine hybrid photodynamic therapy: advancing towards effective treatment of antibiotic-resistant bacteria. Journal of Materials Chemistry B (2023).
3. Polymethine dyes for PDT: recent advances and perspectives to drive future applications. Photochemical & Photobiological Sciences (2022).
4. Bis(3,5-diiodo-2,4,6-trihydroxyphenyl)squaraine photodynamic therapy disrupts redox homeostasis and induce mitochondria-mediated apoptosis in human breast cancer cells. Scientific Reports (2017).
5. Exploration of NIR Squaraine Contrast Agents Containing Various Heterocycles: Synthesis, Optical Properties and Applications. Pharmaceuticals (2023).