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Transition metal cluster complexes

Contact persons: Kamil Lang, Kaplan Kirakci

We investigate new cluster complexes (e.g., Mo, W, Re, Cu) as biomaterials with photodynamic and radiosensitizing properties (Fig. 1). We were the first to propose that such cluster complexes constitute versatile theranostic tools with desirable features such as high luminescence quantum yields, production of singlet oxygen, and radioluminescent properties attractive for X-ray luminescence computed tomography or X-ray induced photodynamic therapy of cancer.

Figure 1. (A) Typical structure of Cu(I) clusters: Cu4I4(pyridin)4. Color coding: Cu (dark yellow), I (magenta), N (blue), C (black), hydrogen atoms are omitted for simplicity. (B) Typical structure of octahedral molybdenum clusters with adamantane ligands: [Mo6I8(adamantan)6]2-. Color coding: Mo (blue), I (magenta), O (red), C (black), hydrogen atoms are omitted for clarity.

Singlet oxygen

Singlet oxygen O2(1Dg) is a potent mediator of phototoxicity and is typically generated by energy transfer from the excited triplet states of a photosensitizer to molecular oxygen (Fig. 2). This feature, coupled with the limited diffusion length of the O2(1Dg) in cells (d ~ 150-190 nm) due to its short lifetime, leads to a high selectivity in the destruction of targeted problematic cells. In addition, O2(1Dg) has strong bactericidal and virucidal properties, which are the bases for a promising method for fighting microorganisms (often resistant) - antimicrobial photodynamic inactivation. Singlet oxygen interacts with cell structures and interferes with different metabolic pathways, preventing the development of resistance of microorganisms towards photodynamic treatment.

Selected references:
Lang, J. Mosinger, D. M. Wagnerová: Photophysical properties of porphyrinoid sensitizers noncovalently bound to host molecules; models for photodynamic therapy. Coord. Chem. Rev. 248/3-4 (2004) 321-350.
Lang, J. Mosinger, P. Kubát: Nanofibers and nanocomposite films for singlet oxygen-based applications. Chapter 15, pp. 305 – 321. Singlet Oxygen: Applications in Biosciences and Nanosciences (Vol. 1), S. Nonell, C. Flors (Eds.), European Society for Photobiology 2016. The Royal Society of Chemistry, Cambridge, UK.

Figure 2. Production of singlet oxygen by energy transfer from the excited triplet states of a photosensitizer to molecular oxygen. Typical photosensitizers: octahedral molybdenum or tungsten clusters, porphyrins, phthalocyanines.

Clusters as luminescent and radioluminescent materials

Tetranuclear copper(I) iodide complexes with the cubane-like structure (Fig. 1A) have been investigated extensively due to their peculiar photoluminescence properties. As these complexes have affordable synthetic protocols, high photoluminescence yields, and are built from high Z elements, we investigated their radioluminescent properties under X-ray irradiation and suggested that these complexes with the {Cu4I4} cluster core are suitable for the design of scintillating materials (Fig. 3).

Selected reference:
Kirakci, K. Fejfarová, J. Martinčík, M. Nikl, K. Lang: Tetranuclear copper(I) iodide complexes: A new class of X-ray phosphors. Inorg. Chem. 56 (2017) 4610 - 4615.

Figure 3. Photoluminescence of some Cu(I) complexes under 365 nm excitation (A) compared with their radioluminescence under X-rays (CuKα, 40 kV, 30 mA) (B

Clusters as photosensitizers and radiosensitizers of singlet oxygen

The octahedral molybdenum or tungsten clusters (M6) are nanometer sized metallic aggregates of Mo(II) or W(II) atoms stabilized by eight strongly bonded inner ligands (Li), usually halogen atoms, and six labile inorganic/organic apical ligands (La) (Fig. 4, see also Fig. 1B). Upon excitation with X-rays or light from the UV to green spectral region, these complexes form long-lived triplet states that relax via red-NIR luminescence with high quantum yields. This luminescence is efficiently quenched by oxygen, leading to the formation of O2(1Dg) in high yields. In contrast to commonly used organic photosensitizers such as porphyrins or phtalocyanins, which lose their photosensitizing activity upon aggregation, these complexes remain good O2(1Dg) photosensitizers even in their aggregated form. The high versatility, due to the choice of apical ligands, allows for using these complexes in their molecular form or to incorporate them into nanomaterials towards biological applications.

Photosensitizers such as M6 activated by light can be used in dermatology in the frame of photodynamic therapy (PDT). These photosensitizers can be also employed in the photoinactivation of antibiotic-resistant bacteria. Moreover, these clusters can be activated by X-rays in the frame of X-ray induced photodynamic therapy of cancer (Fig. 5). Indeed, X-rays have the advantage to penetrate deeply into tissues and can reach tumours that are not accessible to visible or infrared light. Furthermore, related photo/radioluminescence can be employed for diagnosis or analytical purposes.

Figure 4. Structure of octahedral molybdenum or tungsten clusters (M6). Metal atoms are in blue, Li are inner ligands, usually halogen atoms, and La are labile inorganic/organic apical ligands.

Figure 5. Schematic function of M6 nanoparticles (red ball) in X-ray induced photodynamic therapy of cancer.

Selected references:
K. Kirakci, J. Zelenka, M. Rumlová, J. Cvačka, T. Ruml, K. Lang: Cationic octahedral molybdenum cluster complexes functionalized with mitochondria-targeting ligands: photodynamic anticancer and antibacterial activity. Biomater. Sci. 7 (2019) 1386 – 1392.
K. Kirakci, J. Zelenka, M. Rumlová, J. Martinčík, M. Nikl, T. Ruml, K. Lang: Octahedral molybdenum clusters as radiosensitizers for X-ray induced photodynamic therapy. J. Mater. Chem. B 6 (2018) 4301 - 4307.
K. Kirakci, P. Kubát, K. Fejfarová, J. Martinčík, M. Nikl, K. Lang: X-ray-inducible luminescence and singlet oxygen sensitization by an octahedral molybdenum cluster compound: A new class of nanoscintillators. Inorg. Chem. 55 (2016) 803−809.
K. Kirakci, P. Kubát, J. Langmaier, T. Polívka, M. Fuciman, K. Fejfarová, K. Lang: A comparative study of the redox and excited state properties of (nBu4N)2[Mo6X14] and (nBu4N)2[Mo6X8(CF3COO)6] (X = Cl, Br, or I). Dalton Trans. 42 (2013) 7224-7232.
K. Kirakci, P. Kubát, M. Dušek, K. Fejfarová, V. Šícha, J. Mosinger, K. Lang: A Highly luminescent hexanuclear molybdenum cluster: a promising candidate toward photoactive materials. Eur. J. Inorg. Chem. (2012) 3107-3111.

Clusters as radiocontrast compounds

Octahedral rhenium cluster complexes, e.g., [{Re6Q8}(CN)6]4–, where Q is S, Se, or Te, have very low toxicity, low luminescence quantum yields, however, they and attractive as radiocontrast agents.

Selected reference:
A. Ivanov, D. I. Konovalov, T. N. Pozmogova, A. O. Solovieva, A. R. Melnikov, K. A. Brylev, N. V. Kuratieva, V. V. Yanshole, K. Kirakci, K. Lang, S. N. Cheltygmasheva, N. Kitamura, L. V. Shestopalova, Y. V. Mironov, M. A. Shestopalov: The water-soluble Re6-clusters with aromatic phosphine ligands – from synthesis to potential biomedical applications. Inorg. Chem. Front. 6 (2019) 882-892.

Nanocrystalline reactive metal oxides

Contact person: Jiří Henych

Nanostructured oxides of various metals (e.g., Ti, Ce, Mn, or Fe) show exceptional spontaneous or light-induced reactivity and catalytic activity enabling many chemical reactions and transformations taking place on their surface. This is due to unique physico-chemical properties (e.g., acid-base or redox), which are often very closely related to the size of the individual crystals.

These properties can be used in many environmentally and biologically oriented nanotechnologies, e.g., for fast and effective removal and decontamination of toxic substances from water or air. These oxides show high reactivity, for example, towards organophosphate chemical warfare agents (Soman, Sarin) and pesticides (e.g., chlorpyrifos), but also towards other substances such as drugs in water (e.g., sulfonamide antibiotics, see Fig. 1a) or substances that disrupt the hormonal functions of animals and humans (bisphenol S).

 

Fig. 1: (a) ceria-catalyzed hydrolytic cleavage of sulfonamide antibiotic yielding various degradation products, (b) dephosphorylation of p-nitrophenyl 5'-thymidine monophosphate on the surface of CeO2 nanoparticle.

The high reactivity of nanooxides is also very interesting for biological and medical applications. For example, cerium oxides exhibit so-called multi-enzymatic mimetic activity, i.e., that they are able to catalyze reactions that catalyze natural enzymes in biological systems, such as oxidases, peroxidases, catalases, superoxo-dismutases or phosphatases as shown in Fig. 1b.

For the preparation of reactive nanooxides, we develop undemanding synthesis procedures and study their properties and reactivity in detail with the aim of describing and clarifying the mechanism of selected environmentally and biologically relevant reactions. In addition to single-component and mixed oxides, we also prepare nanocomposite and hybrid materials, such as those with carbon nanostructures (graphene, graphene oxide, nanodiamonds), 2D materials, or photoactive molecular clusters.

Fig. 2: TEM images of CeO2 nanoparticles grown on graphene oxide.

We are fully equipped for hydrothermal, solution, microwave, sonochemical and solid-state laboratory syntheses, but we also use high-volume reactors (15, 50, 100 L) when scaling up selected syntheses. We use two high-power ultrasound systems (1kW and 2kW) with water-cooled reactors and temperature control to prepare low-dimensional structures (including graphene or quantum dots) by the top-down method and sonochemical syntheses.

Fig. 3: (a) 1 KW sonicator with water-cooled reactor with temperature control for sonochemistry and delamination of 2D nanomaterials, (b) TEM of graphene sheet prepared by ultrasonication.

To study nanomaterials, we use high-end electron microscopes (SEM and TEM) with elemental mapping, a modern atomic force microscope (AFM), or various spectroscopic methods (FTIR, Raman, UV-Vis). We combine the methods of HPLC/DAD, GC-MS and in situ/Operando DRIFT spectroscopy to study the reaction mechanisms of (photo)degradation reactions.

Fig. 4: Operando DRIFTS setup, enabling a comprehensive study of catalytic processes and reactions on the surface of solid substances with subsequent analysis and quantification of gaseous products.

We cooperate closely with the Institute of Physics of the Academy of Sciences of the Czech Republic (nanodiamond research), with J.E. Purkyně University in Ústí nad Labem (degradation of environmental pollutants and new applications of cerium dioxide), Military Research Institute in Brno and the National Institute for NBC Protection (degradation of chemical warfare agents), Uppsala University (study of photoinduced surface chemical reactions in situ), Bulgarian Academy of Sciences (nanostructured oxides), University of Alcalá (catalytic decontamination materials) etc.

References

  1. Henych, M. Šťastný, S. Kříženecká, J. Čundrle, J. Tolasz, T. Dušková, M. Kormunda, J. Ederer, Š. Stehlík, P. Ryšánek, V. Neubertová, P. Janoš. Ceria-catalyzed hydrolytic cleavage of sulfonamides. Inorg. Chem.2024, 63, 2298-2309.
  2. Henych, M. Šťastný, Z. Němečková, M. Kormunda, Z. Šanderová, Z. Žmudová, P. Ryšánek, Š. Stehlík, J. Ederer, M. Liegertová, J. Trögl, P. Janoš. Cerium-bismuth oxides/oxynitrates with low toxicity for the removal and degradation of organophosphates and bisphenols. ACS Appl. Nano Mater.2022, 5, 17956-17968.
  3. Henych, M. Šťastný, J. Ederer, Z. Němečková, A. Pogorzelska, J. Tolasz, M. Kormunda, P. Ryšánek, B. Bażanów, D. Stygar, K. Mazanec, P. Janoš. How the surface chemical properties of nanoceria are related to its enzyme-like, antiviral and degradation activity. Environ. Sci.:Nano2022, 9, 3485-3501.
  4. Henych, M. Šťastný, Z. Němečková, K. Mazanec, J. Tolasz, M. Kormunda, J. Ederer, P. Janoš. Bifunctional TiO2/CeO2 reactive adsorbent/photocatalyst for degradation of bis-p-nitrophenyl phosphate and CWAs. Chem. Eng. J.2021, 414, 128822.
  5. J. Henych, Š. Stehlík, K. Mazanec, J. Tolasz, J. Čermák, B. Rezek, A. Mattsson, L. Österlund. Reactive adsorption and photodegradation of soman and dimethyl methylphosphonate on TiO2/nanodiamond composites. Appl. Catal. B-Environ. 2019, 259, 118097.