Two-Photon Excitation Techniques in Photodynamic Therapy

Two-photon excitation (TPE) has emerged as a transformative approach in photodynamic therapy (PDT), harnessing near-infrared light to activate photosensitisers deep within tissue while minimising photodamage to surrounding structures. By requiring the near-simultaneous absorption of two lower-energy photons, TPE confines the generation of cytotoxic reactive oxygen species to the focal volume, enabling highly localised killing of cancerous or diseased cells. The use of photosensitiser molecules with large two-photon absorption cross-sections, optimised singlet oxygen quantum yields and targeted subcellular localisation has driven recent progress. Developments span from small-molecule dyes and porphyrin derivatives to nanomaterials and plasmon-enhanced assemblies, each tuned to exploit the optical window of biological tissue (700–1,000 nm). These advances promise deeper tissue penetration, improved therapeutic indices and the integration of real-time multiphoton imaging with treatment. Applications range from precision tumour ablation and antimicrobial interventions to combined photothermal–photodynamic strategies, all demonstrating the global significance of TPE-PDT in minimising side effects, overcoming hypoxic barriers and extending minimally invasive clinical options.

Research from Nature Portfolio

Recent studies have introduced dipolar and quadrupolar azonia aromatic heterocycles that accumulate selectively in mitochondria and exhibit exceptionally high two-photon brightness under near-infrared excitation. These precision photosensitisers trigger efficient mitochondrial damage and subsequent cell death in live animal cells, opening avenues for targeted anticancer therapy. Complementary work on triphenylamine derivatives with large two-photon absorption cross-sections in the 760–860 nm range has demonstrated mitochondrial uptake, reactive oxygen species production and fast apoptosis, alongside multiphoton fluorescence imaging of subcellular redistribution during cell death. Investigations into structural variants of these compounds have further shown that the number and nature of peripheral branches control endocytic uptake pathways and determine whether cells undergo apoptotic or necrotic processes upon two-photon illumination.

Two-Photon Excitation Techniques in Photodynamic Therapy publication trend

The graph below shows the total number of articles in two-photon excitation techniques in photodynamic therapy across all publications each year (not limited to Nature Index journals).

Technical terms

Two-photon excitation: Simultaneous absorption of two near-infrared photons to reach an excited electronic state, enabling confined excitation and deeper tissue penetration.

Photosensitiser: A molecule activated by light to transfer energy to oxygen, producing reactive oxygen species for cell damage in photodynamic therapy.

Two-photon absorption cross-section: A measure, in Göppert-Mayer units, of the probability that a molecule will absorb two photons simultaneously under two-photon excitation.

Singlet oxygen quantum yield: The fraction of absorbed photons that leads to the generation of singlet oxygen, a key cytotoxic species in PDT.

Reactive oxygen species: Highly reactive oxygen-derived molecules, including singlet oxygen and free radicals, that mediate cellular damage in photodynamic treatments.

References

1. Two-photon small-molecule fluorescence-based agents for sensing, imaging, and therapy within biological systems. Chemical Society Reviews (2021).
2. Nitric oxide activatable photosensitizer accompanying extremely elevated two-photon absorption for efficient fluorescence imaging and photodynamic therapy. Chemical Science (2018).
3. Mitochondria-targeted Triphenylamine Derivatives Activatable by Two-Photon Excitation for Triggering and Imaging Cell Apoptosis. Scientific Reports (2016).
4. Exploring Low-Power Single-Pulsed Laser-Triggered Two-Photon Photodynamic/Photothermal Combination Therapy Using a Gold Nanostar/Graphene Quantum Dot Nanohybrid. ACS Applied Materials & Interfaces (2023).
5. Two-photon activated precision molecular photosensitizer targeting mitochondria. Communications Chemistry (2021).
6. Interplay between Cellular Uptake, Intracellular Localization and the Cell Death Mechanism in Triphenylamine-Mediated Photoinduced Cell Death. Scientific Reports (2020).