X-ray Inducible Luminescence and Singlet Oxygen Sensitization by an Octahedral Molybdenum Cluster Compound: A New Class of Nanoscintillators

Abstract

Newly synthesized octahedral molybdenum cluster compound (n-Bu4N)2[Mo6I8(OOC-1-adamantane)6] revealed uncharted features applicable for the development of X-ray inducible luminescent materials and sensitizers of singlet oxygen, O2((1)Δg). The compound exhibits a red-NIR luminescence in the solid state and in solution (e.g., quantum yield of 0.76 in tetrahydrofuran) upon excitation by UV-vis light. The luminescence originating from the excited triplet states is quenched by molecular oxygen to produce O2((1)Δg) with a high quantum yield. Irradiation of the compound by X-rays generated a radioluminescence with the same emission spectrum as that obtained by UV-vis excitation. It proves the formation of the same excited triplet states regardless of the excitation source. By virtue of the described behavior, the compound is suggested as an efficient sensitizer of O2((1)Δg) upon X-ray excitation. The luminescence and radioluminescence properties were maintained upon embedding the compound in polystyrene films. In addition, polystyrene induced an enhancement of the radioluminescence intensity via energy transfer from the scintillating polymeric matrix. Sulfonated polystyrene nanofibers were used for the preparation of nanoparticles which form stable dispersions in water, while keeping intact the luminescence properties of the embedded compound over a long time period. Due to their small size and high oxygen diffusivity, these nanoparticles are suitable carriers of sensitizers of O2((1)Δg). The presented results define a new class of nanoscintillators with promising properties for X-ray inducible photodynamic therapy.

Superior performance of macroporous over gel type polystyrene as a support for the development of photo-bactericidal materials

“…The hexanuclear molybdenum complex 1 has the valuable property of bright-red room temperature phosphorescence. [24][25][26][27][28][29][30][31][32][33][34][35] In our case solid powder of 1 emitted phosphorescence at λ max = 678 nm, 1@P gel emitted at λ max = 671 nm and 1@P mp displayed emission at λ max = 676 nm. The intensity of both emissions are quenched by oxygen upon exposure of the samples to air.…”

“…For comparison purposes, phosphorescence of 1 in several solvents was recorded at λ max = 671-675 nm (Figure 1d) in accordance with the literature. [24][25][26][27][28][29][30][31][32][33][34][35] The performance of 1@P gel and 1@P mp as bactericidal materials was tested against S.aureus and P.aeruginosa. For both types of microorganisms notable differences were found between the gel-type and macroporous resins, especially in the case of gram positive bacteria.…”

“…22 Recently we have proposed a hexanuclear molybdenum cluster complex (1: [Mo 6 I 8 Ac 6 ] 2-, Ac is acetate; see structure in Figure 1) as photoactive agent for aPDI, 23 owing to its notable photochemical properties. [24][25][26][27][28][29][30][31][32][33][34][35] Regarding the supports for the photoactive agents, a variety of materials have been described in the literature, like polyurethane, cellulose, nylon, chitosan, silica and polystyrene. [13][14][15][16][17][18][19][20] This last polymer is one of the most popular supports employed so far, dating back to the pioneering works of Schaap and Neckers.…”

Cellular internalisation, bioimaging and dark and photodynamic cytotoxicity of silica nanoparticles doped by {Mo6I8}4+ metal clusters

“…Firstly, similarly to their structural analogues, hexarhenium clusters [{Re 6 Q 8 }L 6 ] m (Q = S or Se), these complexes demonstrate high chemical and photostability of the cluster cores {Mo 6 X 8 } 4+ that are primarily responsible for their triplet excited state photoluminescence (i.e. 4,5,11,15 Additionally, the metal cluster complexes can also act as powerful photosensitisers in the singlet oxygen ( 1 O 2 ) generation, [5][6][7][8][16][17][18] which makes them especially interesting for applications associated with the generation of singlet oxygen in vivo, such as, for example, photodynamic therapy (PDT). Secondly, their broad emission spectra extend in the red/near infra-red region (from ∼550 to more than 950 nm) 4-13 overlapping the so called "optical tissue window" (650 − 900 nm), where the boundaries of the region are defined by the minimal light absorption of hemoglobin (< 650 nm) and water (> 900 nm).…”

“…[24][25][26] All together these properties make hexamolybdenum cluster complexes very attractive for biomedical applications (e.g. To avoid the possible exchange of the ligands L by water molecules or OHions and simultaneously achieve a biocompatibility, the molybdenum metal cluster complex can be encapsulated within an inert biocompatible carrier matrix, such as polystyrene (PS) micro-beads 11,18,26 or SiO 2 . 27 Despite the mentioned potential for biomedical applications, the known hexamolybdenum cluster complexes themselves are not ideally suited for these purposes.…”