Microscopic time-resolved imaging of singlet oxygen by delayed fluorescence in living cells

Singlet Oxygen Photophysics: From Liquid Solvents to Mammalian Cells

Molecular oxygen, O2, has long provided a cornerstone for studies in chemistry, physics, and biology. Although the triplet ground state, O2(X3Σg-), has garnered much attention, the lowest excited electronic state, O2(a1Δg), commonly called singlet oxygen, has attracted appreciable interest, principally because of its unique chemical reactivity in systems ranging from the Earth's atmosphere to biological cells. Because O2(a1Δg) can be produced and deactivated in processes that involve light, the photophysics of O2(a1Δg) are equally important. Moreover, pathways for O2(a1Δg) deactivation that regenerate O2(X3Σg-), which address fundamental principles unto themselves, kinetically compete with the chemical reactions of O2(a1Δg) and, thus, have practical significance. Due to technological advances (e.g., lasers, optical detectors, microscopes), data acquired in the past ∼20 years have increased our understanding of O2(a1Δg) photophysics appreciably and facilitated both spatial and temporal control over the behavior of O2(a1Δg). One goal of this Review is to summarize recent developments that have broad ramifications, focusing on systems in which oxygen forms a contact complex with an organic molecule M (e.g., a liquid solvent). An important concept is the role played by the M+•O2-• charge-transfer state in both the formation and deactivation of O2(a1Δg).

Identification of excimer delayed fluorescence by Protoporphyrin IX: A novel access to local chromophore concentration?

Protoporphyrin IX (PpIX) is a molecule produced in the mitochondria following the administration of its approved precursor, aminolevulinic acid (ALA). Strong light absorber at different wavelengths in the visible range, PpIX is extensively used as a photosensitizer (PS) for Photodynamic Therapy (PDT). PpIX is also an ideal molecular probe for the quantification of the tissue oxygen partial pressure (pO₂), as its delayed fluorescence (DF) is quenched by oxygen, creating a direct relationship between the DF lifetime and the pO₂. A limitation of both techniques is the ignorance of the PpIX concentration in tissues when the pO₂ is measured or during PDT. In this study, the prompt (PF) and delayed fluorescence of PpIX dissolved in DiMethylFormamide (DMF) were acquired, in absence of oxygen, at different PpIX concentrations. Measurements of the PpIX emission for different excitation energies and temperatures, as well as spectral considerations led to the conclusion that E-type (thermal) DF was the dominant DF mechanism at low PpIX excited states concentrations (density of absorbed energy Hε[PpIX] < 1 μJ. cm^-3, H:excitation radiant exposure per pulse, ε: molar extinction coefficient at excitation wavelength) while P-type (Triplet Triplet Annihilation) DF took place at higher excited states concentrations (Hε[PpIX] > 10 μJ. cm^-3). The gradual development of a strong, red-shifted structureless DF peak at 670 nm, invisible in the PF and absorption spectra, strongly points towards the first observation of PpIX excimer DF (EDF). It appears that, similarly to other aromatic molecules, PpIX excimers can be formed either by the encounter of two molecules in the first excited triplet state T₁, or by the reaction of an excited singlet S₁ with a triplet T₁. Excimer DF could be beneficially used to determine the local concentration of PpIX, as the initial DF intensity ratio I₀⁶⁷⁰/I₀⁶³⁰ is linearly correlated with the local PpIX concentration, and thus rises up to the challenge of PpIX based pO₂ measurement and PDT. This work could also pave the way for a fine comprehension of the production, diffusion and catabolization of PpIX in biological tissues.

Understanding delayed fluorescence and triplet decays of Protoporphyrin IX under hypoxic conditions

Photosensitizers of singlet oxygen exhibit three main types of reverse intersystem-crossing (RISC): thermally activated, triplet-triplet annihilation, and singlet oxygen feedback. RISC can be followed by delayed fluorescence (DF) emission, which can provide important information about the excited state dynamics in the studied system. An excellent model example is a widely used clinical photosensitizer Protoporphyrin IX, which manifests all three mentioned types of RISC and DF. Here, we estimated rate constants of individual RISC and DF processes in Protoporphyrin IX in dimethylformamide, and we showed how these affect triplet decays and DF signals under diverse experimental conditions, such as a varying oxygen concentration or excitation intensity. This provided a basis for a general discussion on guidelines for a more precise analysis of long-lived signals. Furthermore, it has been found that PpIX photoproducts and potential transient excited complexes introduce a new overlapping delayed luminescence spectral band with a distinct lifetime. These findings are important for design of more accurate biological oxygen sensors and assays based on DF and triplet lifetime.

Imaging of singlet oxygen feedback delayed fluorescence and lysosome permeabilization in tumor in vivo during photodynamic therapy with aluminum phthalocyanine

Significance: Singlet oxygen is a key cytotoxic agent in photodynamic therapy (PDT). As such, its imaging is highly desirable, but existing direct imaging methods are still limited by the exceptionally low yield of the luminescence signal. Singlet oxygen feedback delayed fluorescence (SOFDF) of the photosensitizer is a higher yield alternative for indirect measurement of this signal. <p> Aim: The aim was to explore feasibility of SOFDF imaging in vivo in tumor-bearing mice during PDT and investigate how SOFDF images can be transformed into images of singlet oxygen. In addition, we study whether lysosome permeabilization can be visualized through fluorescence lifetime.</p> <p> Approach: Mice were intravenously injected with 2.5 mg/kg of photosensitizer aluminum(III) phthalocyanine tetrasulfonate (AlPcS₄) 20 h prior to experiments, having subcutaneous BxPC3 pancreas tumors. Time-resolved delayed fluorescence and prompt fluorescence (PF) were imaged using an intensified time-gated camera with 10-Hz pulsed laser excitation at 690 nm. </p> <p> Results: Delayed emission from AlPcS₄ was detected with lifetimes 7 to 11  μs, which was attributed to SOFDF and shown to be oxygen-dependent. Singlet oxygen images were approximated by the ratio of SOFDF/PF at each pixel. SOFDF images of a good quality could be captured within several seconds with a radiant exposure of ∼20  mJ  /  cm². In addition, lifetime images of AlPcS₄ PF in ns-time domain enabled us to visualize the event of lysosome permeabilization, as the lifetime increased from ∼4.7 to 5.2 ns. </p> <p> Conclusions: Imaging of SOFDF in vivo in mouse tumor during PDT with AlPcS₄ is feasible, and it is a promising method for singlet molecular oxygen monitoring. Moreover, the time-gated approach also enables visualization of the lysosome permeabilization that alters the PF lifetime. </p>

A fluorescent nanoprobe for real-time monitoring of intracellular singlet oxygen during photodynamic therapy

Sensing of intracellular singlet oxygen (¹O₂) is required in order to optimize photodynamic therapy (PDT). An optical nanoprobe is reported here for the optical determination of intracellular ¹O₂. The probe consists of a porous particle core doped with the commercial ¹O₂ probe 1,3-diphenylisobenzofuran (DPBF) and a layer of poly-L-lysine. The nanoparticle probes have a particle size of ~80 nm in diameter, exhibit good biocompatibility, improved photostability and high sensitivity for ¹O₂ in both absorbance (peak at 420 nm) and fluorescence (with excitation/emission peaks at 405/458 nm). Nanoprobes doped with 20% of DPBF are best suited even though they suffer from concentration quenching of fluorescence. In comparison with the commercial fluorescent ¹O₂ probe SOSG, 20%-doped DPBF-NPs (aged) shows higher sensitivity for ¹O₂ generated at an early stage. The best nanoprobes were used to real-time monitor the PDT-triggered generation of ¹O₂ inside live cells, and the generation rate is found to depend on the supply of intracellular oxygen. Graphical abstract A fluorescent nanoprobe featured with refined selectivity and improved sensitivity towards ¹O₂ was prepared from the absorption-based probe DBPF and used to real-time monitoring of the generation of intracellular ¹O₂ produced during PDT.