A comprehensive review on singlet oxygen generation in nanomaterials and conjugated polymers for photodynamic therapy in the treatment of cancer

A key role in lessening humanity's continuous fight against cancer could be played by photodynamic therapy (PDT), a minimally invasive treatment employed in the medical care of a range of benign disorders and malignancies. Cancerous tissue can be effectively removed by using a light source-excited photosensitizer. Singlet oxygen and reactive oxygen species are produced via the photosensitizer as a result of this excitation. In the recent past, researchers have put in tremendous efforts towards developing photosensitizer molecules for photodynamic treatment (PDT) to treat cancer. Conjugated polymers, characterized by their efficient fluorescence, exceptional photostability, and strong light absorption, are currently under scrutiny for their potential applications in cancer detection and treatment through photodynamic and photothermal therapy. Researchers are exploring the versatility of these polymers, utilizing sophisticated chemical synthesis and adaptable polymer structures to create new variants with enhanced capabilities for generating singlet oxygen in photodynamic treatment (PDT). The incorporation of photosensitizers into conjugated polymer nanoparticles has proved to be beneficial, as it improves singlet oxygen formation through effective energy transfer. The evolution of nanotechnology has emerged as an alternative avenue for enhancing the performance of current photosensitizers and overcoming significant challenges in cancer PDT. Various materials, including biocompatible metals, polymers, carbon, silicon, and semiconductor-based nanomaterials, have undergone thorough investigation as potential photosensitizers for cancer PDT. This paper outlines the recent advances in singlet oxygen generation by investigators using an array of materials, including graphene quantum dots (GQDs), gold nanoparticles (Au NPs), silver nanoparticles (Ag NPs), titanium dioxide (TiO2), ytterbium (Yb) and thulium (Tm) co-doped upconversion nanoparticle cores (Yb/Tm-co-doped UCNP cores), bismuth oxychloride nanoplates and nanosheets (BiOCl nanoplates and nanosheets), and others. It also stresses the synthesis and application of systems such as amphiphilic block copolymer functionalized with folic acid (FA), polyethylene glycol (PEG), poly(β-benzyl-L-aspartate) (PBLA10) (FA-PEG-PBLA10) functionalized with folic acid, tetra(4-hydroxyphenyl)porphyrin (THPP-(PNIPAM-b-PMAGA)4), pyrazoline-fused axial silicon phthalocyanine (HY-SiPc), phthalocyanines (HY-ZnPcp, HY-ZnPcnp, and HY-SiPc), silver nanoparticles coated with polyaniline (Ag@PANI), doxorubicin (DOX) and infrared (IR)-responsive poly(2-ethyl-2-oxazoline) (PEtOx) (DOX/PEtOx-IR NPs), particularly in NIR imaging-guided photodynamic therapy (fluorescent and photoacoustic). The study puts forward a comprehensive summary and a convincing justification for the usage of the above-mentioned materials in cancer PDT.

Anthraquinone-Modified Silica Nanoparticles as Heterogeneous Photocatalyst for the Oxidative Hydroxylation of Arylboronic Acids

In this work, the synthesis and characterization of a heterogeneous photocatalyst based on spherical silica nanoparticles superficially modified with anthraquinone 2-carboxylic acid (AQ-COOH) are presented. The nanomaterial was characterized by TEM, SEM, FT-IR, diffuse reflectance, fluorescence, NMR, DLS, XRD and XPS. These analyses confirm the covalent linking of AQ-COOH with the NH2 functionality in the nanomaterial and, more importantly, the photocatalyst retains its photophysical properties once bound. The heterogeneous photocatalyst was successfully employed in the aerobic hydroxylation of arylboronic acids to phenols under sustainable reaction conditions. Phenols were obtained in high yields (up to 100 %) with low catalyst loading (3.5 mol %), reaching TOF values of 3.7 h-1 . Using 2-propanol as solvent at room temperature, the visible light photocatalysis produced H2 O2 as a key intermediate to promote the aerobic hydroxylation of arylboronic acids. The heterogeneous photocatalyst was reused at least 5 times, without modification of the nanomaterial structure and morphology. This simple heterogeneous system showed great catalytic activity under sustainable reaction conditions.

Nonyl Acridine Orange as a Prospective Photocatalyst in Chalcogenylation of Coumarins and Quinolinones

A mild and efficient method for preparation of 3-sulfenyl and 3-selenyl coumarins and quinolinones mediated by artificial light or sunlight is presented. The elaborated protocol highlights the use of nonyl acridine orange as a photocatalyst to generate a sulfenyl radical from thiols that is further trapped by a heterocycle. The utility of the protocol is justified by a diverse scope of thiols, including short cysteine-containing peptides. The same reaction conditions can be applied for preparation of 3-selenyl coumarins and quinolinones. Various protected and unprotected selenocysteine-containing peptides were successfully utilized demonstrating high tolerance for amino acids with sensitive groups (Arg, Lys, Trp, His, and Tyr).

Amphi-Active Superoxide-Solvating Charge Redox Mediator for Highly Stable Lithium-Oxygen Batteries

A multifunctional electrolyte additive for lithium oxygen batteries (LOBs) was designed to have (1) a redox-active moiety to mediate decomposition of lithium peroxide (Li₂O₂ as the final discharge product) during charging and (2) a solvent moiety to solvate and stabilize lithium superoxide (LiO₂ as the intermediate discharge product) in electrolyte during discharging. 4-Acetamido-TEMPO (TEMPO = 2,2,6,6-tetramethylpiperidin-1-yl)oxyl) or AAT was employed as the additive working for both charge and discharge processes (amphi-active). The redox-active moiety was rooted in TEMPO, while the acetamido (AA) functional group inherited the high donor number (DN) of N,N-dimethylacetamide (DMAc). Integrating two functional moieties (TEMPO and AA) into a single molecule resulted in the bifunctionality of AAT (1) facilitating Li₂O₂ decomposition by the TEMPO moiety and (2) encouraging the solvent mechanism of Li₂O₂ formation by the high-DN AA moiety. Significantly improved LOB performances were achieved by the superoxide-solvating charge redox mediator, which were not obtained by a simple cocktail of TEMPO and DMAc.

Correlation between the Fluorination Degree of Perfluorinated Zinc Phthalocyanines, Their Singlet Oxygen Generation Ability, and Their Photoelectrochemical Response for Phenol Sensing

Electron-withdrawing perfluoroalkyl peripheral groups grafted on phthalocyanine (Pc) macrocycles improve their single-site isolation, solubility, and resistance to self-oxidation, all beneficial features for catalytic applications. A high degree of fluorination also enhances the reducibility of Pcs and could alter their singlet oxygen (¹O₂) photoproduction. The ethanol/toluene 20:80 vol % solvent mixture was found to dissolve perfluorinated F_nPcZn complexes, n = 16, 52, and 64, and minimize the aggregation of the sterically unencumbered F₁₆PcZn. The ¹O₂ production ability of F_nPcZn complexes was examined using 9,10-dimethylanthracene (DMA) and 2,2,6,6-tetramethylpiperidine (TEMP) in combination with UV-vis and electron paramagnetic resonance (EPR) spectroscopy, respectively. While the photoreduction of F₅₂PcZn and F₆₄PcZn in the presence of redox-active TEMP lowered ¹O₂ production, DMA was a suitable ¹O₂ trap for ranking the complexes. The solution reactivity was complemented by solid-state studies via the construction of photoelectrochemical sensors based on TiO₂-supported F_nPcZn, F_nPcZn|TiO₂. Phenol photo-oxidation by ¹O₂, followed by its electrochemical reduction, defines a redox cycle, the ¹O₂ production having been found to depend on the value of n and structural features of the supported complexes. Consistent with solution studies, F₅₂PcZn was found to be the most efficient ¹O₂ generator. The insights on reactivity testing and structural-activity relationships obtained may be useful for designing efficient and robust sensors and for other ¹O₂-related applications of F_nPcZn.

Organic dyes supported on silicon-based materials: synthesis and applications as photocatalysts

The most important advance in photocatalysis in the last decade has been the synthesis and application of organic compounds to promote this process.

Pheophorbide A and Paclitaxel Bioresponsive Nanoparticles as Double-Punch Platform for Cancer Therapy

Cancer therapy is still a challenging issue. To address this, the combination of anticancer drugs with other therapeutic modalities, such as light-triggered therapies, has emerged as a promising approach, primarily when both active ingredients are provided within a single nanosystem. Herein, we describe the unprecedented preparation of tumor microenvironment (TME) responsive nanoparticles exclusively composed of a paclitaxel (PTX) prodrug and the photosensitizer pheophorbide A (PheoA), e.g., PheoA≅PTX₂S. This system aimed to achieve both the TME-triggered and controlled release of PTX and the synergistic/additive effect by PheoA-mediated photodynamic therapy. PheoA≅PTX₂S were produced in a simple one-pot process, exhibiting excellent reproducibility, stability, and the ability to load up to 100% PTX and 40% of PheoA. Exposure of PheoA≅PTX₂S nanoparticles to TME-mimicked environment provided fast disassembly compared to normal conditions, leading to PTX and PheoA release and consequently elevated cytotoxicity. Our data indicate that PheoA incorporation into nanoparticles prevents its aggregation, thus providing a greater extent of ROS and singlet oxygen production. Importantly, in SK-OV-3 cells, PheoA≅PTX₂S allowed a 30-fold PTX dose reduction and a 3-fold dose reduction of PheoA. Our data confirm that prodrug-based nanocarriers represent valuable and sustainable drug delivery systems, possibly reducing toxicity and expediting preclinical and clinical translation.

A versatile heterogeneous photocatalyst: nanoporous gold powder modified with a zinc(II) phthalocyanine derivative for singlet oxygen [4 + 2] cycloadditions

Nanoporous gold was functionalized with a photosensitizer, a zinc(II) phthalocyanine derivative. Such systems are active for the generation of reactive singlet oxygen which can be used for photocatalytic oxidation reactions. This study aims to demonstrate the versatility of such an approach, in terms of substrates and the employed solvent, only possible for a truly heterogeneous catalytic system. The activity of the hybrid system was studied for [4 + 2] cycloadditions of three different types of dienes and a total of eight substrates in two organic solvents and once in water. The highest activity was measured for 1,3-diphenylisobenzofuran, which is also highest in terms of sensitivity for the reaction with ¹O₂. Trends in conversion could be anticipated based on reported values for the rate constant for the reaction of ¹O₂. In almost all cases, an amplification of the conversion by immobilization of the sensitizer onto nanoporous gold was observed. The limiting case was ergosterol, which was the largest of all substrates with a van-der-Waals radius of about 2.1 nm. Additional factors such as the limited lifetime of ¹O₂ in different solvents as well as the hampered diffusion of the substrates were identified.

Glass wool supported ruthenium complexes: versatile, recyclable heterogeneous photoredox catalysts

Versatile and recyclable heterogeneous photocatalysts based on the use of glass wool supported ruthenium complexes and organic dyes.

Perylene-Grafted Silicas: Mechanistic Study and Applications in Heterogeneous Photoredox Catalysis

A mechanistic study is herein presented for the use of heterogeneous photocatalysts based on perylene moieties. First, the successful immobilization of perylene diimides (PDI) on silica matrices is demonstrated, including their full characterization by means of electronic microscopy, surface area measurements, powder XRD, thermogravimetric analysis, and FTIR, ²⁹ Si and ¹³ C solid-state NMR, fluorescence, and diffuse reflectance spectroscopies. Then, the photoredox activity of the material was tested by using two model reactions, alkene oxidation and 4-nitrobenzylbromide reduction, and mechanistic studies were performed. The mechanistic insights into their photoredox activity show they have promising dual photocatalytic activity for both organic oxidations and reductions.

Keratin nanoparticles co-delivering Docetaxel and Chlorin e6 promote synergic interaction between chemo- and photo-dynamic therapies

The combination of chemotherapy and photodynamic therapy (PDT) is considered a valuable strategy for increasing therapeutic response in cancer treatment, and the re-formulation of pharmaceuticals in biocompatible nanoparticles (NPs) is particularly appealing for the possibility of co-loading drugs exerting cytotoxicity by different mechanisms, with the aim to produce synergic effects. We report the in-water synthesis of a novel keratin-based nanoformulation for the co-delivery of the antimitotic Docetaxel (DTX) and the photosensitizer Chlorin e6 (Ce6). The drug-induced aggregation method allowed the formation of monodisperse NPs (DTX/Ce6-KNPs) with an average diameter of 133 nm and loaded with a drug ratio of 1:1.8 of Ce6 vs DTX. The efficacy of DTX/Ce6-KNPs was investigated in vitro in monolayers and spheroids of DTX-sensitive HeLa (HeLa-P) and DTX-resistant HeLa (HeLa-R) cells. In monolayers, the cytotoxic effects of DTX/Ce6-KNPs toward HeLa-P cells were comparable to those induced by free DTX + Ce6, while in HeLa-R cells the drug co-loading in KNPs produced synergic interaction between chemotherapy and PDT. Moreover, as respect to monotherapies, DTX/Ce6-KNPs induced stronger cytotoxicity to both HeLa-P and HeLa-R multicellular spheroids and reduced their volumes up to 50%. Overall, the results suggest that KNPs are very promising systems for the co-delivery of chemotherapeutics and PSs, favoring synergic interactions between PDT and chemotherapy.

Photo-induced anticancer activity and singlet oxygen production of prodigiosenes

The photo-induced cytotoxicity of prodigiosenes is reported. One prodigiosene represents a synthetic analogue of the natural product prodigiosin, and two are conjugated to molecules that target the estrogen receptor (ER). A comparison of incubation and irradiation frameworks for the three prodigiosenes is reported, with activity against ER- and ER+ lines explored. Furthermore, the ability of the three prodigiosenes to photosensitise the production of singlet oxygen is demonstrated, shedding mechanistic light onto possible photodynamic therapeutic effects of this class of tripyrroles.

Assessment of encapsulated dyes’ distribution in silica nanoparticles and their ability to release useful singlet oxygen

Working with silica nanoparticle encapsulated BODIPY and xanthene photosensitizers, we have determined that singlet oxygen spends up to 78% of its lifetime inside the nanocarriers.

A photobleaching resistant polymer supported hexanuclear molybdenum iodide cluster for photocatalytic oxygenations and photodynamic inactivation of Staphylococcus aureus

Photoinactivation of Staphylococcus aureus has been achieved using a hexanuclear molybdenum cluster, [Mo₆I₈(CH₃COO)₆]²⁻, supported on a polystyrene matrix.