Octahedral molybdenum clusters as radiosensitizers for X-ray induced photodynamic therapy

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).

PEGylated Molybdenum-Iodine Nanocluster as a Promising Radiodynamic Agent against Prostatic Adenocarcinoma

The combination of photodynamic therapy and radiotherapy has given rise to a modality called radiodynamic therapy (RDT), based on reactive oxygen species-producing radiosensitizers. The production of singlet oxygen, O2(1Δg), by octahedral molybdenum (Mo6) clusters upon X-ray irradiation allows for simplification of the architecture of radiosensitizing systems. In this context, we prepared a radiosensitizing system using copper-free click chemistry between a Mo6 cluster bearing azido ligands and the homo-bifunctional linker bis-dPEG11-DBCO. The resulting compound formed nanoparticles, which featured production of O2(1Δg) and efficient cellular uptake, leading to remarkable photo- and radiotoxic effects against the prostatic adenocarcinoma TRAMP-C2 cell line. Spheroids of TRAMP-C2 cells were also used for evaluation of toxicity and phototoxicity. In vivo experiments on a mouse model demonstrated that subcutaneous injection of the nanoparticles is a safe administration mode at a dose of up to 0.08 g kg-1. The reported results confirm the relevancy of Mo6-based radiosensitizing nanosystems for RDT.

Molecularly Engineered Room-Temperature Phosphorescence for Biomedical Application: From the Visible toward Second Near-Infrared Window

Phosphorescence, characterized by luminescent lifetimes significantly longer than that of biological autofluorescence under ambient environment, is of great value for biomedical applications. Academic evidence of fluorescence imaging indicates that virtually all imaging metrics (sensitivity, resolution, and penetration depths) are improved when progressing into longer wavelength regions, especially the recently reported second near-infrared (NIR-II, 1000-1700 nm) window. Although the emission wavelength of probes does matter, it is not clear whether the guideline of "the longer the wavelength, the better the imaging effect" is still suitable for developing phosphorescent probes. For tissue-specific bioimaging, long-lived probes, even if they emit visible phosphorescence, enable accurate visualization of large deep tissues. For studies dealing with bioimaging of tiny biological architectures or dynamic physiopathological activities, the prerequisite is rigorous planning of long-wavelength phosphorescence, being aware of the cooperative contribution of long wavelengths and long lifetimes for improving the spatiotemporal resolution, penetration depth, and sensitivity of bioimaging. In this Review, emerging molecular engineering methods of room-temperature phosphorescence are discussed through the lens of photophysical mechanisms. We highlight the roles of phosphorescence with emission from visible to NIR-II windows toward bioapplications. To appreciate such advances, challenges and prospects in rapidly growing studies of room-temperature phosphorescence are described.

Graphene Oxide Sheets Decorated with Octahedral Molybdenum Cluster Complexes for Enhanced Photoinactivation of Staphylococcus aureus

The emergence of multidrug-resistant microbial pathogens poses a significant threat, severely limiting the options for effective antibiotic therapy. This challenge can be overcome through the photoinactivation of pathogenic bacteria using materials generating reactive oxygen species upon exposure to visible light. These species target vital components of living cells, significantly reducing the likelihood of resistance development by the targeted pathogens. In our research, we have developed a nanocomposite material consisting of an aqueous colloidal suspension of graphene oxide sheets adorned with nanoaggregates of octahedral molybdenum cluster complexes. The negative charge of the graphene oxide and the positive charge of the nanoaggregates promoted their electrostatic interaction in aqueous medium and close cohesion between the colloids. Upon illumination with blue light, the colloidal system exerted a potent antibacterial effect against planktonic cultures of Staphylococcus aureus largely surpassing the individual contributions of the components. The underlying mechanism behind this phenomenon lies in the photoinduced electron transfer from the nanoaggregates of the cluster complexes to the graphene oxide sheets, which triggers the generation of reactive oxygen species. Thus, leveraging the unique properties of graphene oxide and light-harvesting octahedral molybdenum cluster complexes can open more effective and resilient antibacterial strategies.

Catalytic nanotechnology of X-ray photodynamics for cancer treatments

Photodynamic therapy (PDT) has been applied in cancer treatment because of its high selectivity, low toxicity, and non-invasiveness. However, the limited penetration depth of the light still hampers from reaching deep-seated tumors. Considering the penetrating ability of high-energy radiotherapy, X-ray-induced photodynamic therapy (X-PDT) has evolved as an alternative to overcome tissue blocks. As the basic principle of X-PDT, X-rays stimulate the nanoparticles to emit scintillating or persistent luminescence and further activate the photosensitizers to generate reactive oxygen species (ROS), which would cause a series of molecular and cellular damages, immune response, and eventually break down the tumor tissue. In recent years, catalytic nanosystems with unique structures and functions have emerged that can enhance X-PDT therapeutic effects via an immune response. The anti-cancer effect of X-PDT is closely related to the following factors: energy conversion efficiency of the material, the radiation dose of X-rays, quantum yield of the material, tumor resistance, and biocompatibility. Based on the latest research in this field and the classical theories of nanoscience, this paper systematically elucidates the current development of the X-PDT and related immunotherapy, and highlights its broad prospects in medical applications, discussing the connection between fundamental science and clinical translation.

Phosphorescent Metal Halide Nanoclusters for Tunable Photodynamic Therapy

Photodynamic therapy (PDT) is currently limited by the inability of photosensitizers (PSs) to enter cancer cells and generate sufficient reactive oxygen species. Utilizing phosphorescent triplet states of novel PSs to generate singlet oxygen offers exciting possibilities for PDT. Here, we report phosphorescent octahedral molybdenum (Mo)-based nanoclusters (NC) with tunable toxicity for PDT of cancer cells without use of rare or toxic elements. Upon irradiation with blue light, these molecules are excited to their singlet state and then undergo intersystem crossing to their triplet state. These NCs display surprising tunability between their cellular cytotoxicity and phototoxicity by modulating the apical halide ligand with a series of short chain fatty acids from trifluoroacetate to heptafluorobutyrate. The NCs are effective in PDT against breast, skin, pancreas, and colon cancer cells as well as their highly metastatic derivatives, demonstrating the robustness of these NCs in treating a wide variety of aggressive cancer cells. Furthermore, these NCs are internalized by cancer cells, remain in the lysosome, and can be modulated by the apical ligand to produce singlet oxygen. Thus, (Mo)-based nanoclusters are an excellent platform for optimizing PSs. Our results highlight the profound impact of molecular nanocluster chemistry in PDT applications.

Flexible and Transparent Luminescent Cellulose-Transition Metal Cluster Composites

Red-NIR luminescent polymers are principally obtained from petroleum-based derivatives in which emitters, usually a critical raw material such as rare-earth or platinum group metal ions, are embedded. Considering the strong ecological impact of their synthesis and the major risk of fossil fuel energy shortage, there is an urgent need to find alternatives. We describe a luminescent nanocomposite based on red-NIR phosphorescent molybdenum nanoclusters, namely Cs₂Mo₆I₈(OCOC₂F₅)₆, embedded in an eco-friendly cellulose biopolymer matrix that is obtained by a simple solvent casting technique. While homogeneity is kept up to 20 wt% of cluster complex doping, annealing hybrids leads to a large increase of their emission efficiency, as demonstrated by quantum yield measurements.

Molybdenum-Iodine Cluster Loaded Polymeric Nanoparticles Allowing a Coupled Therapeutic Action with Low Side Toxicity for Treatment of Ovarian Cancer

The ability of cancer cells to develop resistance to anti-cancer drugs, known as multidrug resistance, remains a major cause of tumor recurrence and cancer metastasis. This work explores the double mechanism of toxicity of (D, L-lactide-co-glycolide) acid (PLGA) nanoparticles encapsulating a molybdenum cluster compound, namely Cs₂[{Mo₆I₈}(OOCC₂F₅)₆] (CMIF). Hemocompatibility and biocompatibility assays show the safe potential of CMIF loaded nanoparticles (CNPs) as delivery systems intended for tumor targeting for PDT of ovarian cancer with a slight hemolytic activity and a lack of toxicity up to 50 µM CMIF concentration. Cellular uptake shows a preferential uptake of CNPs in lysosomes, which is not interfering with CMIF activity. The double mechanism of CNPs consists in a production of ROS and a DNA damage activity, from 5 µM and 0.5 µM respectively (CMIF concentration). The cellular death mechanism comprises 80% of necrosis and 20% of direct apoptosis by direct DNA damages. This work confirms CMIF loaded PLGA nanoparticles as an efficient and relevant delivery system for PDT.

Octahedral Molybdenum Cluster-Based Nanomaterials for Potential Photodynamic Therapy

Photo/radiosensitizers, such as octahedral molybdenum clusters (Mo₆), have been intensively studied for photodynamic applications to treat various diseases. However, their delivery to the desired target can be hampered by its limited solubility, low stability in physiological conditions, and inappropriate biodistribution, thus limiting the therapeutic effect and increasing the side effects of the therapy. To overcome such obstacles and to prepare photofunctional nanomaterials, we employed biocompatible and water-soluble copolymers based on N-(2-hydroxypropyl)methacrylamide (pHPMA) as carriers of Mo₆ clusters. Several strategies based on electrostatic, hydrophobic, or covalent interactions were employed for the formation of polymer-cluster constructs. Importantly, the luminescent properties of the Mo₆ clusters were preserved upon association with the polymers: all polymer-cluster constructs exhibited an effective quenching of their excited states, suggesting a production of singlet oxygen (O₂(¹Δ_g)) species which is a major factor for a successful photodynamic treatment. Even though the colloidal stability of all polymer-cluster constructs was satisfactory in deionized water, the complexes prepared by electrostatic and hydrophobic interactions underwent severe aggregation in phosphate buffer saline (PBS) accompanied by the disruption of the cohesive forces between the cluster and polymer molecules. On the contrary, the conjugates prepared by covalent interactions notably displayed colloidal stability in PBS in addition to high luminescence quantum yields, suggesting that pHPMA is a suitable nanocarrier for molybdenum cluster-based photosensitizers intended for photodynamic applications.

Enhancement of photoactivity and cellular uptake of (Bu4N)2[Mo6I8(CH3COO)6] complex by loading on porous MCM-41 support. Photodynamic studies as an anticancer agent

The incorporation by ionic assembly of the hexanuclear molybdenum cluster (Bu₄N)₂[Mo₆I₈(CH₃CO₂)₆] (1) in amino-decorated mesoporous silica nanoparticles MCM-41, has yielded the new molybdenum-based hybrid photosensitizer 1@MCM-41. The new photoactive material presents a high porosity, due to the intrinsic high specific surface area of MCM-41 nanoparticles (989 m² g^-1) which is responsible for the good dispersion of the hexamolybdenum clusters on the nanoparticles surface, as observed by STEM analysis. The hybrid photosensitizer can generate efficiently singlet oxygen, which was demonstrated by using the benchmark photooxygenation reaction of 9,10-anthracenediyl-bis(methylene)dimalonic acid (ABDA) in water. The photodynamic therapy activity has been tested using LED light as an irradiation source (λ_irr ~ 400-700 nm; 15.6 mW/cm²). The results show a good activity of the hybrid photosensitizer against human cervical cancer (HeLa) cells, reducing up to 70 % their viability after 20 min of irradiation, whereas low cytotoxicity is detected in the darkness. The main finding of this research is that the incorporation of molybdenum complexes at porous MCM-41 supports enhances their photoactivity and improves cellular uptake, compared to free clusters.

Avenue to X-ray-induced photodynamic therapy of prostatic carcinoma with octahedral molybdenum cluster nanoparticles

X-Ray-induced photodynamic therapy represents a suitable modality for the treatment of various malignancies. It is based on the production of reactive oxygen species by radiosensitizing nanoparticles activated by X-rays. Hence, it allows overcoming the depth-penetration limitations of conventional photodynamic therapy and, at the same time, reducing the dose needed to eradicate cancer in the frame of radiotherapy treatment. The direct production of singlet oxygen by octahedral molybdenum cluster complexes upon X-ray irradiation is a promising avenue in order to simplify the architecture of radiosensitizing systems. One such complex was utilized to prepare water-stable nanoparticles using the solvent displacement method. The nanoparticles displayed intense red luminescence in aqueous media, efficiently quenched by oxygen to produce singlet oxygen, resulting in a substantial photodynamic effect under blue light irradiation. A robust radiosensitizing effect of the nanoparticles was demonstrated in vitro against TRAMP-C2 murine prostatic carcinoma cells at typical therapeutic X-ray doses. Injection of a suspension of the nanoparticles to a mouse model revealed the absence of acute toxicity as evidenced by the invariance of key physiological parameters. This study paves the way for the application of octahedral molybdenum cluster-based radiosensitizers in X-ray-induced photodynamic therapy and its translation to in vivo experiments.

A Cell Membrane Targeting Molybdenum-Iodine Nanocluster: Rational Ligand Design toward Enhanced Photodynamic Activity

The development of singlet oxygen photosensitizers, which target specific cellular organelles, constitutes a pertinent endeavor to optimize the efficiency of photodynamic therapy. Targeting of the cell membrane eliminates the need for endocytosis of drugs that can lead to toxicity, intracellular degradation, or drug resistance. In this context, we utilized copper-free click chemistry to prepare a singlet oxygen photosensitizing complex, made of a molybdenum-iodine nanocluster stabilized by triazolate apical ligands. In phosphate-buffered saline, the complex formed nanoaggregates with a positive surface charge due to the protonatable amine function of the apical ligands. These nanoaggregates targeted cell membranes and caused an eminent blue-light phototoxic effect against HeLa cells at nanomolar concentrations, inducing apoptotic cell death, while having no dark toxicity at physiologically relevant concentrations. The properties of this complex were compared to those of a negatively charged parent complex to highlight the dominant effect of the nature of apical ligands on biological properties of the nanocluster. These two complexes also exerted (photo)antibacterial effects on several pathogenic strains in the form of planktonic cultures and biofilms. Overall, we demonstrated that the rational design of apical ligands toward cell membrane targeting leads to enhanced photodynamic efficiency.

Progress in Nanocarriers Codelivery System to Enhance the Anticancer Effect of Photodynamic Therapy

Photodynamic therapy (PDT) is a promising anticancer noninvasive method and has great potential for clinical applications. Unfortunately, PDT still has many limitations, such as metastatic tumor at unknown sites, inadequate light delivery and a lack of sufficient oxygen. Recent studies have demonstrated that photodynamic therapy in combination with other therapies can enhance anticancer effects. The development of new nanomaterials provides a platform for the codelivery of two or more therapeutic drugs, which is a promising cancer treatment method. The use of multifunctional nanocarriers for the codelivery of two or more drugs can improve physical and chemical properties, increase tumor site aggregation, and enhance the antitumor effect through synergistic actions, which is worthy of further study. This review focuses on the latest research progress on the synergistic enhancement of PDT by simultaneous multidrug administration using codelivery nanocarriers. We introduce the design of codelivery nanocarriers and discuss the mechanism of PDT combined with other antitumor methods. The combination of PDT and chemotherapy, gene therapy, immunotherapy, photothermal therapy, hyperthermia, radiotherapy, sonodynamic therapy and even multidrug therapy are discussed to provide a comprehensive understanding.

Nanomaterials for Deep Tumor Treatment

According to statistics, cancer is the second leading cause of death in the world. Thus, it is important to solve this medical and social problem by developing new effective methods for cancer treatment. An alternative to more well-known approaches, such as radiotherapy and chemotherapy, is photodynamic therapy (PDT), which is limited to the shallow tissue penetration (< 1 cm) of visible light. Since the PDT process can be initiated in deep tissues by X-ray irradiation (X-ray induced PDT, or XPDT), it has a great potential to treat tumors in internal organs. The article discusses the principles of therapies. The main focus is on various nanoparticles used with or without photosensitizers, which allow the conversion of X-ray irradiation into UV-visible light. Much attention is given to the synthesis of nanoparticles and analysis of their characteristics, such as size and spectral features. The results of in vitro and in vivo experiments are also discussed.

Photodynamic Therapy of Inorganic Complexes for the Treatment of Cancer†

Photodynamic therapy (PDT) is a medicinal tool that uses a photosensitizer and a light source to treat several conditions, including cancer. PDT uses reactive oxygen species such as cytotoxic singlet oxygen (¹ O₂ ) to induce cell death in cancer cells. Chemotherapy has historically utilized the cytotoxic effects of many metals, especially transition metal complexes. However, chemotherapy is a systemic treatment so all cells in a patient's body are exposed to the same cytotoxic effects. Transition metal complexes have also shown high cytotoxicity as PDT agents. PDT is a potential localized method for treating several cancer types by using inorganic complexes as photosensitizing agents. This review covers several in vitro and in vivo studies, as well as clinical trials that reported on the anticancer properties of inorganic pharmaceuticals used in PDT against different types of cancer.

From molecules to nanovectors: Current state of the art and applications of photosensitizers in photodynamic therapy

Photodynamic therapy (PDT) is a concept based on a selective activation by light of drugs called photosensitizers (PS) leading to reactive oxygen species production responsible for cell destruction. Mechanisms of photodynamic reaction and cell photo-destruction following direct or indirect mechanisms will be presented as well as PS classification, from first generation molecules developed in the 1960 s to third generation vectorized PS with improved affinity for tumor cells. Many clinical applications in dermatology, ophthalmology, urology, gastroenterology, gynecology, neurosurgery and pneumology reported encouraging results in human tumor management. However, this interesting technique needs improvements that are currently investigated in the field of PS excitation by the design of new PS intended for two-photon excitation or for X-ray excitation. The former excitation technique is allowing better light penetration and preservation of healthy tissues while the latter is combining PDT and radiotherapy so that external light sources are no longer needed to generate the photodynamic effect. Nanotechnology can also improve the PS to reach the tumor cells by grafting addressing molecule and by increasing its aqueous solubility and consequently its bioavailability by encapsulation in synthetic or biogenic nanovector systems, ensuring good drug protection and targeting. Co-internalization of PS with magnetic nanoparticles in multifunctional vectors or stealth nanoplatforms allows a theranostic anticancer approach. Finally, a new category of inorganic PS will be presented with promising results on cancer cell destruction.

Synthesis, Structure, and Spectroscopic Study of Redox-Active Heterometallic Cluster-Based Complexes [Re5MoSe8(CN)6]n

The heterometallic cluster-based compound K₅[Re₅MoSe₈(CN)₆] was obtained by high-temperature reaction from a mixture of ReSe₂ and MoSe₂ in molten potassium cyanide. The redox behavior of the [Re₅MoSe₈(CN)₆]^5- cluster anion was studied by cyclic voltammetry in aqueous and organic media showing two reversible one-electron-redox transitions with E_1/2 of -0.462 and 0.357 V versus Ag/AgCl in CH₃CN. Aqueous media potentials were found to be noticeably shifted to higher values because of solvation. Chemically accessible potentials allowed us to structurally isolate and characterize the [Re₅MoSe₈(CN)₆]ⁿ (n = 3-, 4-, and 5-) cluster complex in several charge states with corresponding cluster skeletal electron (CSE) numbers ranging from 24 to 22. The electronic absorption of the [Re₅MoSe₈(CN)₆]ⁿ cluster complex varies significantly upon a change of the CSE number, especially in the visible and near-IR regions. The local cluster core distortion upon electron removal was confirmed by density functional theory calculation, while the overall geometry of the cluster anion remained practically unaltered.

Emerging hybrid biomaterials for oxidative stress induced photodynamic therapy

Cancer therapy has undergone tremendous advancements in the past few years. The drawbacks of most of these therapies have encouraged researchers to obtain further insight into the complex chemical, biochemical and biological processes ongoing in the evolving cancer cells. These studies have led to an advent of reactive oxygen species mediated therapies to target and disrupt the cancer pathology. Photodynamic therapy (PDT) has emerged as a potent candidate for oxidative stress mediated non-invasive technique for rapid diagnosis and treatment of cancer. Towards this, biomacromolecules derived hybrid nanomaterials have contributed largely in the development of various therapeutics and theranostics for efficacious cancer management that can assist PDT. This review summarizes various hybrid biomaterials and advanced techniques that have been explored widely in the past few years for PDT application. The article also mentions some of the important in-vitro and in-vivo developments and observations explored by employing these materials for PDT application. The article also describes the interactions of these materials at the biological interface and the probable mechanism that assist in generation of oxidative stress and subsequent cell death.

Recent research progress in the construction of active free radical nanoreactors and their applications in photodynamic therapy

Photodynamic therapy is the most important treatment strategy in free radical therapy. However, tumor microenvironment hypoxia is a key obstacle in PDT. In order to overcome this obstacle, the strategy of in situ production of O₂/radicals by catalytic reaction in solid tumors was proposed. In recent years, it has been found that there are many oxygen-independent carbon-based free radicals that can generate toxic active free radicals under laser irradiation and lead to tumor cell death. Based on the rational design of multifunctional nano-medicine, the active free radical nano-generator has opened up a new way for the highly developed nanotechnology and tumor cooperative therapy to improve the therapeutic effect. In this paper, the research status of active free radical nano-generators, especially reactive oxygen species, including the construction mechanism of active free radical nanomaterials, is reviewed and the application of free radical nano-generators in tumor therapy is emphasized.

A water-soluble octahedral molybdenum cluster complex as a potential agent for X-ray induced photodynamic therapy

X-ray-induced photodynamic therapy (X-PDT) has recently evolved into a suitable modality to fight cancer. This technique, which exploits radiosensitizers producing reactive oxygen species, allows for a reduction of the radiation dose needed to eradicate cancer in the frame of the radiotherapy treatment of deep tumors. The use of transition metal complexes able to directly produce singlet oxygen, O₂(¹Δ_g), upon X-ray irradiation constitutes a promising route towards the optimization of the radiosensitizer's architecture. In our endeavour to conceive pertinent agents for X-PDT, we designed an octahedral molybdenum cluster complex (Mo₆) with iodine inner ligands, and carboxylated apical ligands bearing ethylene oxide organic functions. The sodium salt of this complex is highly soluble in aqueous media and displays red luminescence which is efficiently quenched by oxygen to produce O₂(¹Δ_g) in a high quantum yield. Furthermore, due to its high radiodensity, the complex exhibits radioluminescence in aqueous media, with the same spectral features as for photoluminescence, indicating the production of O₂(¹Δ_g) upon X-ray irradiation. The uptake of the complex by Hep-2 and MRC-5 cells is negligible during the first hours of incubation, then considerably increases in connection with the hydrolysis of the apical ligands. The complex exhibits low toxicity in vitro and induces a radiotoxic effect, noticeable against cancerous Hep-2 cells but negligible against normal MRC-5 cells, at X-ray doses that do not affect cell viability otherwise. The first evaluation of in vivo toxicity of an Mo₆ complex on a mouse model evidences a moderate and delayed toxic effect on kidneys, with an intravenous LD₅₀ value of 390 ± 30 mg kg^-1, possibly connected with hydrolysis-induced aggregation of the complex. Overall, this complex displays attractive features as a singlet oxygen radiosensitizer for X-PDT, highlighting the potential of transition metal cluster complexes towards this modality.

Electrophoretically Deposited Layers of Octahedral Molybdenum Cluster Complexes: A Promising Coating for Mitigation of Pathogenic Bacterial Biofilms under Blue Light

The fight against infective microorganisms is becoming a worldwide priority due to serious concerns about the rising numbers of drug-resistant pathogenic bacteria. In this context, the inactivation of pathogens by singlet oxygen, O₂(¹Δ_g), produced by photosensitizers upon light irradiation has become an attractive strategy to combat drug-resistant microbes. To achieve this goal, we electrophoretically deposited O₂(¹Δ_g)-photosensitizing octahedral molybdenum cluster complexes on indium-tin oxide-coated glass plates. This procedure led to the first example of molecular photosensitizer layers able to photoinactivate bacterial biofilms. We delineated the morphology, composition, luminescence, and singlet oxygen formation of these layers and correlated these features with their antibacterial activity. Clearly, continuous 460 nm light irradiation imparted the layers with strong antibacterial properties, and the activity of these layers inhibited the biofilm formation and eradicated mature biofilms of Gram-positive Staphylococcus aureus and Enterococcus faecalis, as well as, Gram-negative Pseudomonas aeruginosa and Escherichia coli bacterial strains. Overall, the microstructure-related oxygen diffusivity of the layers and the water stability of the complexes were the most critical parameters for the efficient and durable use. These photoactive layers are attractive for the design of antibacterial surfaces activated by visible light and include additional functionalities such as the conversion of harmful UV/blue light to red light or oxygen sensing.

Octahedral Molybdenum Cluster Complexes with Optimized Properties for Photodynamic Applications

Two new octahedral molybdenum cluster complexes act as an efficient singlet oxygen supplier in the context of the photodynamic therapy of cancer cells under blue-light irradiation. These complexes integrate the {Mo₆I₈}⁴⁺ core with 4'-carboxybenzo-15-crown-5 or cholate apical ligands and were characterized by ¹H NMR, HR ESI-MS, and CHN elemental analysis. Both complexes display high quantum yields of luminescence and singlet oxygen formation in aqueous media associated with a suitable stability against hydrolysis. They are internalized into lysosomes of HeLa cells with no dark toxicity at pharmacologically relevant concentrations and have a strong phototoxic effect under blue-light irradiation, even in the presence of fetal bovine serum. The last feature is essential for further translation to in vivo experiments. Overall, these complexes are attractive molecular photosensitizers toward photodynamic applications.

Nanocomposites for X-Ray Photodynamic Therapy

Photodynamic therapy (PDT) has long been known as an effective method for treating surface cancer tissues. Although this technique is widely used in modern medicine, some novel approaches for deep lying tumors have to be developed. Recently, deeper penetration of X-rays into tissues has been implemented, which is now known as X-ray photodynamic therapy (XPDT). The two methods differ in the photon energy used, thus requiring the use of different types of scintillating nanoparticles. These nanoparticles are known to convert the incident energy into the activation energy of a photosensitizer, which leads to the generation of reactive oxygen species. Since not all photosensitizers are found to be suitable for the currently used scintillating nanoparticles, it is necessary to find the most effective biocompatible combination of these two agents. The most successful combinations of nanoparticles for XPDT are presented. Nanomaterials such as metal-organic frameworks having properties of photosensitizers and scintillation nanoparticles are reported to have been used as XPDT agents. The role of metal-organic frameworks for applying XPDT as well as the mechanism underlying the generation of reactive oxygen species are discussed.

Advances in nanomaterials for photodynamic therapy applications: Status and challenges

Photodynamic therapy (PDT), as a non-invasive therapeutic modality that is alternative to radiotherapy and chemotherapy, is extensively investigated for cancer treatments. Although conventional organic photosensitizers (PSs) are still widely used and have achieved great progresses in PDT, the disadvantages such as hydrophobicity, poor stability within PDT environment and low cell/tissue specificity largely limit their clinical applications. Consequently, nano-agents with promising physicochemical and optical properties have emerged as an attractive alternative to overcome these drawbacks of traditional PSs. Herein, the up-to-date advances in the fabrication and fascinating applications of various nanomaterials in PDT have been summarized, including various types of nanoparticles, carbon-based nanomaterials, and two-dimensional nanomaterials, etc. In addition, the current challenges for the clinical use of PDT, and the corresponding strategies to address these issues, as well as future perspectives on further improvement of PDT have also been discussed.

PEGylated Purpurin 18 with Improved Solubility: Potent Compounds for Photodynamic Therapy of Cancer

Purpurin 18 derivatives with a polyethylene glycol (PEG) linker were synthesized as novel photosensitizers (PSs) with the goal of using them in photodynamic therapy (PDT) for cancer. These compounds, derived from a second-generation PS, exhibit absorption at long wavelengths; considerable singlet oxygen generation and, in contrast to purpurin 18, have higher hydrophilicity due to decreased logP. Together, these properties make them potentially ideal PSs. To verify this, we screened the developed compounds for cell uptake, intracellular localization, antitumor activity and induced cell death type. All of the tested compounds were taken up into cancer cells of various origin and localized in organelles known to be important PDT targets, specifically, mitochondria and the endoplasmic reticulum. The incorporation of a zinc ion and PEGylation significantly enhanced the photosensitizing efficacy, decreasing IC50 (half maximal inhibitory compound concentration) in HeLa cells by up to 170 times compared with the parental purpurin 18. At effective PDT concentrations, the predominant type of induced cell death was apoptosis. Overall, our results show that the PEGylated derivatives presented have significant potential as novel PSs with substantially augmented phototoxicity for application in the PDT of cervical, prostate, pancreatic and breast cancer.

Phosphinate Apical Ligands: A Route to a Water-Stable Octahedral Molybdenum Cluster Complex

Recent studies have unraveled the potential of octahedral molybdenum cluster complexes (Mo₆) as relevant red phosphors and photosensitizers of singlet oxygen, O₂(¹Δ_g), for photobiological applications. However, these complexes tend to hydrolyze in an aqueous environment, which deteriorates their properties and limits their applications. To address this issue, we show that phenylphosphinates are extraordinary apical ligands for the construction of Mo₆ complexes. These new complexes display unmatched luminescence quantum yields and singlet oxygen production in aqueous solutions. More importantly, the complex with diphenylphosphinate ligands is the only stable complex of these types in aqueous media. These complexes internalize in lysosomes of HeLa cells, have no dark toxicity, and yet are phototoxic in the submicromolar concentration range. The superior hydrolytic stability of the diphenylphosphinate complex allows for conservation of its photophysical properties and biological activity over a long period, making it a promising compound for photobiological applications.

Enhancement of Photodynamic Cancer Therapy by Physical and Chemical Factors

The viable use of photodynamic therapy (PDT) in cancer therapy has never been fully realized because of its undesirable effects on healthy tissues. Herein we summarize some physicochemical factors that can make PDT a more viable and effective option to provide future oncological patients with better-quality treatment options. These physicochemical factors include light sources, photosensitizer (PS) carriers, microwaves, electric fields, magnetic fields, and ultrasound. This Review is meant to provide current information pertaining to PDT use, including a discussion of in vitro and in vivo studies. Emphasis is placed on the physicochemical factors and their potential benefits in overcoming the difficulty in transitioning PDT into the medical field. Many advanced techniques, such as employing X-rays as a light source, using nanoparticle-loaded stem cells and bacteriophage bio-nanowires as a photosensitizer carrier, as well as integration with immunotherapy, are among the future directions.

Cationic octahedral molybdenum cluster complexes functionalized with mitochondria-targeting ligands: photodynamic anticancer and antibacterial activities

Octahedral molybdenum cluster complexes have recently come forth as pertinent singlet oxygen photosensitizers towards biological applications. Still, their phototoxic efficiency in the absence of nanocarriers remains limited due to their poor cellular uptake. Here, two cationic octahedral molybdenum cluster complexes, bearing carboxylate ligands with triphenylphosphonium (1) or N-methyl pyridinium (2) mitochondria-targeting terminal functions, have been designed and synthesized. Their photophysical properties in water and in vitro biological activity were investigated in the context of blue-light photodynamic therapy of cancer and photoinactivation of bacteria. Upon blue light irradiation, complex 1 displays red luminescence with a quantum yield of 0.24 in water, whereas complex 2 is much less emissive (ΦL < 0.01). Nevertheless, both complexes efficiently produce singlet oxygen, O2(1Δg). Complex 1 is rapidly internalized into HeLa cells and accumulated in mitochondria, followed by relocation to lysosomes and clearance at longer times. In contrast, the more hydrophilic 2 is not internalized into HeLa cells, highlighting the effect of the apical ligands on the uptake properties. The treatment with 1 results in an intensive phototoxic effect under 460 nm irradiation (IC50 = 0.10 ± 0.02 μM), which exceeds by far those previously reported for octahedral cluster-based molecular photosensitizers. The ratio between phototoxicity and dark toxicity is approximately 50 and evidences a therapeutic window for the application of 1 in blue-light photodynamic therapy. Complex 1 also enters and efficiently photoinactivates Gram-positive bacteria Enterococcus faecalis and Staphylococcus aureus, documenting its suitability as a blue-light photosensitizer for antimicrobial applications.

X-ray-activated nanosystems for theranostic applications

X-rays are widely applied in clinical medical facilities for radiotherapy (RT) and biomedical imaging. However, the sole use of X-rays for cancer treatment leads to insufficient radiation energy deposition due to the low X-ray attenuation coefficients of living tissues and organs, producing unavoidable excessive radiation doses with serious side effects to healthy body parts. Over the past decade, developments in materials science and nanotechnology have led to rapid progress in the field of X-ray-activated tumor-targeting nanosystems, which are able to tackle even systemic tumors and relieve the burden of exposure to large radiation doses. Additionally, novel imaging contrast agents and techniques have also been developed. In comparison with conventional external light sources (e.g., near infrared), the X-ray technique is ideal for the activation of nanosystems for cancer treatment and biomedical imaging applications due to its nearly unlimited penetration depth in living tissues and organisms. In this review, we systematically describe the interaction mechanisms between X-rays and nanosystems, and provide an overview of X-ray-sensitive materials and the recent progress on X-ray-activated nanosystems for cancer-associated theranostic applications.

Water-soluble Re6-clusters with aromatic phosphine ligands – from synthesis to potential biomedical applications

New hexarhenium clusters exhibit radio- and photoluminescence, have low cytotoxicity, are capable of penetrating into cells and exhibit photodynamic toxicity.

The Ouzo effect to selectively assemble molybdenum clusters into nanomarbles or nanocapsules with increased HER activity

Molybdenum clusters assemble spontaneously into nanocapsules or nanomarbles depending on their solubility in a water/THF mixture.