Scope and limitations of the TEMPO/EPR method for singlet oxygen detection: the misleading role of electron transfer

Use of scavenger agents in heterogeneous photocatalysis: truths, half-truths, and misinterpretations

The use of scavenger agents must be thoughtfully considered in mechanistic investigations of heterogeneous photocatalysis since atypical radicals are produced.

Mn-based catalysts for sulfate radical-based advanced oxidation processes: A review

Sulfate radical-based advanced oxidation processes (AOPs) have drawn increasing attention during the past two decades, and Mn-based materials have been proven to be effective catalysts for activating peroxymonosulfate (PMS) and peroxydisulfate (PDS) to degrade many contaminants. This article presents a comprehensive review of various Mn-based materials to activate PMS and PDS. The activation mechanisms of different Mn-based catalysts (i.e., Mn oxides MnO_x, MnO_x hybrids, and MnO_x‑carbonaceous material composites) were first summarized and discussed in detail. Besides the commonly reported free radicals (SO₄^-• and ^•OH), non-radical mechanisms such as singlet oxygen and direct electron transfer have also been discovered for selected materials. The effects of pH, inorganic ions, natural organic matter (NOM), dissolved oxygen content, temperature, and the crystallinity of the materials on the catalytic reactivity were also discussed. Then, important instrumentations and technologies employed to characterize Mn-based materials and to understand the reaction mechanisms were concisely summarized. Three common overlooks in the experimental designs for examining the PMS/PDS-MnO_x systems were also discussed. Finally, future research directions were suggested to further improve the technology and to provide a guidance to develop cost-effective Mn-based materials to activate PMS/PDS.

New VIVO-complexes for oxidative desulfurization of refractory sulfur compounds in fuel: synthesis, structure, reactivity trend and mechanistic studies

A series of 5-coordinate oxidovanadium(iv) complexes based on 2-(2'-hydroxyphenyl)imidazole (HPIMH), with substituent groups of different electronegativities on the phenolic para position (HPIMX; X = -H, -Br, -OMe and -NO2), were synthesized and characterized. Three of these complexes were characterized by single crystal X-ray diffraction, [VIVO(PIMH)2], [VIVO(PIMBr)2] and [VIVO(PIMNO2)2], as well as a dioxidovanadium(v) compound ([VVO2(PIMH)(PIMH2)]). The complexes were tested for their catalytic activities in the oxidation of dibenzothiophene (DBT), the major refractory organosulfur compound found in fuel. The nitro substituted compound [VIVO(PIMNO2)2] had the highest catalytic oxidation activity followed by: [VIVO(PIMH)2] > [VIVO(PIMBr)2] > [VIVO(PIMMeO)2]. The decrease in activity is attributed to the different electronegativities of the substituent groups, which influence the electron density on the metal center, the V[double bond, length as m-dash]O bond distances and infrared stretching bands. Geometry index (τ) values calculated from single crystal X-ray diffraction (SC-XRD) data and DFT studies provided further insights on the trend in activity observed. SC-XRD, EPR, 51V NMR and UV-Vis spectroscopies, and DFT studies were instrumental in studying the mechanism of the catalyzed reaction and proposal of intermediate species. Both radical and non-radical pathways are plausible for the catalytic oxidation and participation of reactive oxygen species in both pathways is also postulated.

Trace amounts of fenofibrate acid sensitize the photodegradation of bezafibrate in effluents: Mechanisms, degradation pathways, and toxicity evaluation

Effluent organic matter (EfOM), which is composed of background natural organic matter (NOM), soluble microbial degradation products, and trace amounts of organic pollutants, can play an important role in the photodegradation of emerging pollutants in the effluent. In this study, the impact of organic pollutants, using fenofibrate acid (FNFA) as a representative, on the photodegradation of emerging contaminants, using bezafibrate (BZF) as a representative, in effluents was investigated. It is found that BZF undergo fast degradation in the presence of FNFA although BZF is recalcitrant to degradation under simulated sunlight irradiation. The promotional effect of FNFA is due to the generation of singlet oxygen (¹O₂) and hydrated electrons (e^-_aq). Based on the structures of the identified intermediates, ¹O₂ initiated oxidation and e^-_aq initiated reduction reactions were the main photodegradation pathways of BZF in the effluents. The toxicity of the main photodegradation intermediates for BZF and FNFA was higher than that of the parent compounds, and the acute toxicity increased during simulated sunlight irradiation. The results demonstrated that trace amounts of organic compounds in EfOM can play an important role in sensitizing the photodegradation of some emerging pollutants in the effluent.

Edge-nitrogenated biochar for efficient peroxydisulfate activation: An electron transfer mechanism

N-doped biochars (NBCs) were prepared by pyrolyzing corncob biomass and urea in different proportion which manifested superior catalytic performance of peroxydisulfate (PDS) activation for sulfadiazine (SDZ) degradation. Through both dynamic fitting and density functional theory (DFT) calculations, the critical role of edge nitrogenation in biochar (BC) structure was revealed for the first time. The incorporation of edge nitrogen configurations (pyridinic N and pyrrolic N rather than graphitic N) generated reactive sites for the PDS activation. Additionally, a thorough investigation was conducted to explicate the PDS activation mechanism by NBC through chemical quenching experiments, electron spin resonance (ESR) detection, oxidant consumption monitoring and electrochemical analysis. Different from the well-reported singlet oxygen (¹O₂) dominated nonradical mechanism, an electron transfer pathway involving surface-bound reactive complexes was proved to play a major role in the NBC/PDS system. Benefit from the electron transfer mechanism, the NBC/PDS system not only has wide pH adaptation for real application, but also shows high resistance to the inorganic anions in aquatic environment. We believe this study will deepen the understanding of the carbon-driven persulfate activation mechanism and provide strong technical support for the BC-mediated persulfate activation in practical applications.

Emissive Enhancement of the Singlet Oxygen Chemiluminescence Probe after Binding to Bovine Serum Albumin

A chemiluminescence probe for singlet oxygen ¹O₂ (SOCL) was investigated in phosphate buffer saline (PBS), either in the absence of proteins or containing bovine serum albumin (BSA). In the protein-free PBS, the reactivity of SOCL for methylene blue (MB)-photosensitized ¹O₂ was found to be moderate or low. The reaction yield increased with temperature and/or concentration of dissolved molecular oxygen. Unexpectedly, the presence of BSA boosted both the emissive nature and the thermal stability of the phenoxy-dioxetane intermediate formed in the chemiexcitation pathway. Isothermal titration calorimetry showed that SOCL has a moderate binding affinity for BSA and that entropy forces drive the formation of the SOCL-BSA complex. A model with two identical and independent binding sites was used to fit the binding isotherm data. Co-operative binding was observed when MB was present. Local viscosity factors and/or conformational restrictions of the BSA-bound SOCL phenoxy-dioxetane were proposed to contribute to the formation of the highly emissive benzoate ester during the chemically initiated electron exchange luminescence (CIEEL) process. These results led us to conclude that hydrophobic interactions of the SOCL with proteins can modify the emissive nature of its phenoxy-dioxetane, which should be taken into account when using SOCL or its cell-penetrating peptide derivative in living cells.

Cotransformation of Carbon Dots and Contaminant under Light in Aqueous Solutions: A Mechanistic Study

In this study, the photochemistry of carbon dots (CDs) and their effects on pollutant transformation were systematically examined. Diethyl phthalate (DEP) degradation was strongly enhanced by CDs under UV light, with the observed reaction rate constant ( k_obs) increased by 2.4-15.1-fold by CDs at a concentration of 0.5-10 mg/L. Electron paramagnetic resonance (EPR) spectrometry combined with free radical quenching experiments with various chemical probes indicated the production of reactive oxygen species (ROS), including hydroxyl radicals (^•OH), singlet oxygen (¹O₂), and superoxide radical anions (O₂^•-), and these contributed to the enhanced DEP degradation. Meanwhile, CDs were also degraded to low-molecular-weight species and partially mineralized to CO₂ by ROS, as evidenced by Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) and total organic carbon (TOC) analysis, and transformation of CDs was accelerated by DEP. Furthermore, CDs were degraded rapidly under natural sunlight, accompanied by the formation of ^•OH and ¹O₂. Anions such as CO₃^2-, NO₃^-, and Cl^- had limited effects on transformation of CDs, while humic substances greatly inhibited this process. Our results indicate that photoreactions of CDs play an important role in influencing the transformation of pollutants and CDs themselves in the natural aquatic environment. The findings provide invaluable information for evaluating risks associated with the release of CDs into the natural environment.

Probing the Formation of Reactive Oxygen Species by a Porous Self-Assembled Benzophenone Bis-Urea Host

Herein, we examine the photochemical formation of reactive oxygen species (ROS) by a porous benzophenone-containing bis-urea host (1) to investigate the mechanism of photooxidations that occur within the confines of its nanochannels. UV irradiation of the self-assembled host in the presence of molecular oxygen generates both singlet oxygen and superoxide when suspended in solution. The efficiency of ROS generation by the host is lower than that of benzophenone (BP), which could be beneficial for reactions carried out catalytically, as ROS species react quickly and often unselectively. Superoxide formation was detected through reaction with 5,5-dimethyl-1-pyrroline N-oxide in the presence of methanol. However, it is not detected in CHCl₃, as it reacts rapidly with the solvent to generate methaneperoxy and chloride anions, similar to BP. The lifetime of airborne singlet oxygen (τ_Δairborne) was examined at the air-solid outer surface of the host and host·quencher complexes and suggests that quenching is a surface phenomenon. The efficiency of the host and BP as catalysts was compared for the photooxidation of 1-methyl-1-cyclohexene in solution. Both the host and BP mediate the photooxidation in CHCl₃, benzene, and benzene-d ₆, producing primarily epoxide-derived products with low selectivity likely by both type I and type II photooxidation processes. Interestingly, in CHCl₃, two chlorohydrins were also formed, reflecting the formation of chloride in this solvent. In contrast, UV irradiation of the host·guest crystals in an oxygen atmosphere produced no epoxide and appeared to favor mainly the type II processes. Photolysis afforded high conversion to only three products: an enone, a tertiary allylic alcohol, and a diol, which demonstrates the accessibility of the encapsulated reactants to oxygen and the influence of confinement on the reaction pathway.

Local Photothermal/Photodynamic Synergistic Therapy by Disrupting Bacterial Membrane To Accelerate Reactive Oxygen Species Permeation and Protein Leakage

Bacterial infection is still a ticklish clinical challenge even though some advanced antibacterial materials and techniques have been put forward. This work reports that rapid and effective antibacterial performance is achieved by the synergistic local photothermal and photodynamic therapy (PTDT). Within 10 min of light irradiation, both Escherichia coli and Staphylococcus aureus are almost completely eliminated by the action of photothermy (52.1 °C) and limited reactive oxygen species (ROS), the corresponding bacterial killing efficiencies are 99.91 and 99.97%, respectively, which are far higher than single modal therapy, i.e., photothermal therapy or photodynamic therapy with antibacterial efficacy of 50 or 70%, respectively. The mechanism is that bacterial membrane permeation is increased by PTDT because photothermy shows more severe impact only on E. coli by destroying the outmost bacterial panniculus, whereas the inner panniculus of the two kinds of bacteria is more sensitive to ROS. Hence, ROS penetrates the bacterial membrane more easily, and meanwhile, the proteins in the bacteria are severely lost after the bacterial membrane disruption, which leads to bacterial death. In vivo results reveal that rapid and effective sterilization is an important process to accelerate wound healing, and the traumas on the rats' backbones heal well within 12 days by PTDT. Furthermore, the PTDT is friendly to major organs of rats during the therapeutic process. Therefore, the synergistic therapy system can be a safe therapeutic system for clinical sterilization with great potential. More importantly, the antibacterial mechanism presented in this work has great guiding significance for the design of other advanced antibacterial systems and techniques.

The Photodynamic Properties and the Genotoxicity of Heat-Treated Silicalite-1 Films

We investigated the use of a supported silicalite-1 film (SF) as a promising coating for metallic materials used in the fabrication of prostheses. The role of carbonaceous residua present on high-temperature calcined-SF in generating singlet oxygen for future use as a sterilization method has also been addressed, and the potential genotoxicity of these residua in osteoblast-like cells has been investigated. Calcination of as-synthesized SF induced the appearance of a rather complicated mixture of aliphatic and aromatic species on its outer surface. A series of variously volatile polycyclic aromatic hydrocarbons (PAH), including naphthalene, fluorene, phenanthrene, anthracene, fluoranthene, and pyrene, were identified in micromole concentrations. Irradiation of these PAHs on calcined-SF immersed in air-saturated chloroform led to the formation of very low concentrations of singlet oxygen. However, an increased level of DNA damage was observed on calcined-SF by immunofluorescence staining of phosphorylated histone H2AX analyzed by flow cytometry.

Reaction of phenol with singlet oxygen

Photo-degradation of organic pollutants plays an important role in their removal from the environment. This study provides an experimental and theoretical account of the reaction of singlet oxygen O2(1Δg) with the biodegradable-resistant species of phenol in an aqueous medium. The experiments combine customised LED-photoreactors, high-performance liquid chromatography (HPLC), and electron paramagnetic resonance (EPR) imaging, employing rose bengal as a sensitiser. Guided by density functional theory (DFT) calculations at the M062X level, we report the mechanism of the reaction and its kinetic model. Addition of O2(1Δg) to the phenol molecule branches into two competitive 1,4-cycloaddition and ortho ene-type routes, yielding 2,3-dioxabicyclo[2.2.2]octa-5,7-dien-1-ol (i.e., 1,4-endoperoxide 1-hydroxy-2,5-cyclohexadiene) and 2-hydroperoxycyclohexa-3,5-dien-1-one, respectively. Unimolecular rearrangements of the 1,4-endoperoxide proceed in a facile exothermic reaction to form the only experimentally detected product, para-benzoquinone. EPR revealed the nature of the oxidation intermediates and corroborated the appearance of O2(1Δg) as the only active radical participating in the photosensitised reaction. Additional experiments excluded the formation of hydroxyl (HO˙), hydroperoxyl (HO2˙), and phenoxy intermediates. We detected for the first time the para-semibenzoquinone anion (PSBQ), supporting the reaction pathway leading to the formation of para-benzoquinone. Our experiments and the water-solvation model result in the overall reaction rates of kr-solvation = 1.21 × 104 M-1 s-1 and kr = 1.14 × 104 M-1 s-1, respectively. These results have practical application to quantify the degradation of phenol in wastewater treatment.

Synthesis, photodynamic activities, and cytotoxicity of new water-soluble cationic gallium(III) and zinc(II) phthalocyanines

The cationic Ga(III) and Zn(II) phthalocyanines carrying N-methyl-pyridinium groups at eight peripheral β-positionshave been synthesized. These complexes are highly soluble in dimethyl sulfoxide (DMSO) and moderately soluble in water and phosphate buffered saline (PBS); both Ga(III)Cl and Zn(II) complexes have shown no aggregation in water up to 1.2 × 10^-4 and 1.5 × 10^-5 M, respectively. A higher water-solubility of Ga(III)Cl complex as compared to Zn(II) complex is ascribed to the presence of an axially coordinated chloride. The spectroscopic properties, photogeneration of singlet oxygen (¹O₂), and cytotoxicity of these complexes have been investigated. The absolute quantum yields (ΦΔ_absolute) for the photogeneration of singlet oxygen using Ga(III)Cl and Zn(II) complexes have been determined to be 4.4 and 5.3%, respectively, in DMSO solution. The cytotoxicity and intracellular sites of localization of Ga(III)Cl and Zn(II) complexes have been evaluated in human HEp2 cells. Both complexes, localized intracellularly in multiple organelles, have shown no cytotoxicity in the dark. Upon exposure to a low light dose (1.5 J/cm²), however, Zn(II) complex has exhibited a high photocytotoxicity. The result suggests that Zn(II) complex can be considered as a potential photosensitizer for Photodynamic therapy (PDT).

Activation of Anthracene Endoperoxides in Leishmania and Impairment of Mitochondrial Functions

Leishmaniasis is a vector-borne disease caused by protozoal Leishmania. Because of resistance development against current drugs, new antileishmanial compounds are urgently needed. Endoperoxides (EPs) are successfully used in malaria therapy, and experimental evidence of their potential against leishmaniasis exists. Anthracene endoperoxides (AcEPs) have so far been only technically used and not explored for their leishmanicidal potential. This study verified the in vitro efficiency and mechanism of AcEPs against both Leishmania promastigotes and axenic amastigotes (L. tarentolae and L. donovani) as well as their toxicity in J774 macrophages. Additionally, the kinetics and radical products of AcEPs’ reaction with iron, the formation of radicals by AcEPs in Leishmania, as well as the resulting impairment of parasite mitochondrial functions were studied. Using electron paramagnetic resonance combined with spin trapping, photometry, and fluorescence-based oximetry, AcEPs were demonstrated to (i) show antileishmanial activity in vitro at IC₅₀ values in a low micromolar range, (ii) exhibit host cell toxicity in J774 macrophages, (iii) react rapidly with iron (II) resulting in the formation of oxygen- and carbon-centered radicals, (iv) produce carbon-centered radicals which could secondarily trigger superoxide radical formation in Leishmania, and (v) impair mitochondrial functions in Leishmania during parasite killing. Overall, the data of different AcEPs demonstrate that their structures besides the peroxo bridge strongly influence their activity and mechanism of their antileishmanial action.

Structure-activity study of furyl aryloxazole fluorescent probes for the detection of singlet oxygen

In this study, we report the synthesis and the photochemical behavior of a series of new "click-on" fluorescent probes designed to detect singlet oxygen. They include a highly fluorescent chemical structure, an aryloxazole ring, linked to a furan moiety operating as singlet oxygen trap. Their activity depends on both the structure of the aryloxazole fluorophore and the electron-donating and electron-accepting properties of the substituents attached to the C-5 of the furan ring. All probes are selectively oxidized by singlet oxygen to give a single fluorescent product in methanol and produce negligible amounts of singlet oxygen themselves by self-sensitization. The most promising dyad, (E)-2-(2-(5-methylfuran-2-yl)vinyl)naphtho[1,2-d]oxazole, FN-6, shows outstanding reactivity and sensitivity: it traps singlet oxygen with a rate constant (5,8 ± 0.1) x 1(07) M-1 s-1 and its fluorescence increases by a factor of 500 upon reaction. Analysis of the dyads reactivity in terms of linear free energy relationships using the modified Swain and Lupton parameter F and the Fukui condensed function for the electrophilic attack, suggests that cycloaddition of singlet oxygen to the furan ring is partially concerted and possibly involves an exciplex with a "more open" structure than could be expected for a concerted cycloaddition.

Identifying the Nonradical Mechanism in the Peroxymonosulfate Activation Process: Singlet Oxygenation Versus Mediated Electron Transfer

Select persulfate activation processes were demonstrated to initiate oxidation not reliant on sulfate radicals, although the underlying mechanism has yet to be identified. This study explored singlet oxygenation and mediated electron transfer as plausible nonradical mechanisms for organic degradation by carbon nanotube (CNT)-activated peroxymonosulfate (PMS). The degradation of furfuryl alcohol (FFA) as a singlet oxygen (¹O₂) indicator and the kinetic retardation of FFA oxidation in the presence of l-histidine and azide as ¹O₂ quenchers apparently supported a role of ¹O₂ in the CNT/PMS system. However, the ¹O₂ scavenging effect was ascribed to a rapid PMS depletion by l-histidine and azide. A comparison of CNT/PMS and photoexcited Rose Bengal (RB) excluded the possibility of singlet oxygenation during heterogeneous persulfate activation. In contrast to the case of excited RB, solvent exchange (H₂O to D₂O) did not enhance FFA degradation by CNT/PMS and the pH- and substrate-dependent reactivity of CNT/PMS did not reflect the selective nature of ¹O₂. Alternatively, concomitant PMS reduction and trichlorophenol oxidation were achieved when PMS and trichlorophenol were physically separated in two chambers using a conductive vertically aligned CNT membrane. This result suggested that CNT-mediated electron transfer from organics to persulfate was primarily responsible for the nonradical degradative route.

Chemical fingerprints of cold physical plasmas - an experimental and computational study using cysteine as tracer compound

Reactive oxygen and nitrogen species released by cold physical plasma are being proposed as effectors in various clinical conditions connected to inflammatory processes. As these plasmas can be tailored in a wide range, models to compare and control their biochemical footprint are desired to infer on the molecular mechanisms underlying the observed effects and to enable the discrimination between different plasma sources. Here, an improved model to trace short-lived reactive species is presented. Using FTIR, high-resolution mass spectrometry, and molecular dynamics computational simulation, covalent modifications of cysteine treated with different plasmas were deciphered and the respective product pattern used to generate a fingerprint of each plasma source. Such, our experimental model allows a fast and reliable grading of the chemical potential of plasmas used for medical purposes. Major reaction products were identified to be cysteine sulfonic acid, cystine, and cysteine fragments. Less-abundant products, such as oxidized cystine derivatives or S-nitrosylated cysteines, were unique to different plasma sources or operating conditions. The data collected point at hydroxyl radicals, atomic O, and singlet oxygen as major contributing species that enable an impact on cellular thiol groups when applying cold plasma in vitro or in vivo.

Nanoenzyme-Augmented Cancer Sonodynamic Therapy by Catalytic Tumor Oxygenation

Ultrasound (US)-triggered sonodynamic therapy (SDT) can solve the critical issue of low tissue-penetrating depth of traditional phototriggered therapies, but the SDT efficacy is still not satisfactorily high in combating cancer at the current stage. Here we report on augmenting the SDT efficacy based on catalytic nanomedicine, which takes the efficient catalytic features of nanoenzymes to modulate the tumor microenvironment (TME). The multifunctional nanosonosensitizers have been successfully constructed by the integration of a MnO _x component with biocompatible/biodegradable hollow mesoporous organosilica nanoparticles, followed by conjugation with protoporphyrin (as the sonosensitizer) and cyclic arginine-glycine-aspartic pentapeptide (as the targeting peptide). The MnO _x component in the composite nanosonosensitizer acts as an inorganic nanoenzyme for converting the tumor-overexpressed hydrogen peroxide (H₂O₂) molecules into oxygen and enhancing the tumor oxygen level subsequently, which has been demonstrated to facilitate SDT-induced reactive oxygen species production and enhance SDT efficacy subsequently. The targeted accumulation of these composite nanosonosensitizers efficiently suppressed the growth of U87 tumor xenograft on nude mice after US-triggered SDT treatment. The high in vivo biocompatibility and easy excretion of these multifunctional nanosonosensitizers from the body have also been evaluated and demonstrated to guarantee their future clinical translation, and their TME-responsive T₁-weighted magnetic resonance imaging capability provides the potential for therapeutic guidance and monitoring during SDT.

Protein-Based 3D Microstructures with Controllable Morphology and pH-Responsive Properties

The microtechnology of controlling stimuli-responsive biomaterials at micrometer scale is crucial for biomedical applications. Here, we report bovine serum albumin (BSA)-based three-dimensional (3D) microstructures with tunable surface morphology and pH-responsive properties via two-photon polymerization microfabrication technology. The laser processing parameters, including laser power, scanning speed, and layer distance, are optimized for the fabrication of well-defined 3D BSA microstructures. The tunable morphology of BSA microstructures and a wide range of pH response corresponding to the swelling ratio of 1.08-2.71 have been achieved. The swelling behavior of the microstructures can be strongly influenced by the concentration of BSA precursor, which has been illustrated by a reasonable mechanism. A panda face-shaped BSA microrelief with reversible pH-responsive properties is fabricated and exhibits unique "facial expression" variations in pH cycle. We further design a mesh sieve-shaped microstructure as a functional device for promising microparticle separation. The pore sizes of microstructures can be tuned by changing the pH values. Therefore, such protein-based microstructures with controllable morphology and pH-responsive properties have potential applications especially in biomedicine and biosensors.

"Dark" Singlet Oxygen and Electron Paramagnetic Resonance Spin Trapping as Convenient Tools to Assess Photolytic Drug Degradation

Forced degradation studies are an important tool for a systematic assessment of decomposition pathways and identification of reactive sites in active pharmaceutical ingredients (APIs). Two methodologies have been combined in order to provide a deeper understanding of singlet oxygen-related degradation pathways of APIs under light irradiation. First, we report that a "dark" singlet oxygen test enables the investigation of drug reactivity toward singlet oxygen independently of photolytic irradiation processes. Second, the photosensitizing properties of the API producing the singlet oxygen was proven and quantified by spin trapping and electron paramagnetic resonance analysis. A combination of these techniques is an interesting addition to the forced degradation portfolio as it can be used for (1) revealing unexpected degradation pathways of APIs due to singlet oxygen, (2) clarifying photolytic drug-drug interactions in fixed-dose combinations, and (3) synthesizing larger quantities of hardly accessible oxidative drug degradants.

Use of fluorescent probes for ROS to tease apart Type I and Type II photochemical pathways in photodynamic therapy

Photodynamic therapy involves the excitation of a non-toxic dye by harmless visible light to produce a long-lived triplet state that can interact with molecular oxygen to produce reactive oxygen species (ROS), which can damage biomolecules and kill cells. ROS produced by electron transfer (Type 1) include superoxide, hydrogen peroxide and hydroxyl radical (HO), while singlet oxygen (1O2) is produced by energy transfer. Diverse methods exist to distinguish between these two pathways, some of which are more specific or more sensitive than others. In this review we cover the use of two fluorescence probes: singlet oxygen sensor green (SOSG) detects 1O2; and 4-hydroxyphenyl-fluorescein (HPF) that detects HO. Interesting data was collected concerning the photochemical pathways of functionalized fullerenes compared to tetrapyrroles, stable synthetic bacteriochlorins with and without central metals, phenothiazinium dyes interacting with inorganic salts such as azide.